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					 Overview of Pressure Vessel Design




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                                            2
Overview of Pressure Vessel Design
                    By:

          Vincent A. Carucci
      Carmagen Engineering, Inc.




             Copyright © 1999 by




              All Rights Reserved




                      3
                                  TABLE OF CONTENTS

PART 1:   PARTICIPANT NOTES .....................................................................................5


PART 2:   BACKGROUND MATERIAL ..............................................................................64




                                                  4
 Part 1:
Workbook




  5
           OVERVIEW OF
      PRESSURE VESSEL DESIGN
                       By: Vincent A. Carucci
                     Carmagen Engineering, Inc .




 1




Notes:




                Course Overview
     • General
     • Materials of Construction
     • Design
     • Other Design Considerations
     • Fabrication
     • Inspection and Testing
 2




Notes:




                                                   6
              Pressure Vessels
     • Containers for fluids under pressure
     • Used in variety of industries
       – Petroleum refining
       – Chemical
       – Power
       – Pulp and paper
       – Food
 3




Notes:




            Horizontal Drum on
             Saddle Supports              Nozzle




                                            A
                                                   Shell




                 Head                                                      Head




                    Saddle Support                         SaddleSupport
                       (Sliding)                               (Fixed)


                                            A




                                        Section A-A



                                     Figure 2.1
 4




Notes:




                                                                                  7
           Vertical Drum
          on Leg Supports
                 Head




             Shell                                Nozzle




                                                           Head




                                                       Support
                                                       Leg




 5                            Figure 2.2



Notes:




         Tall Vertical Tower
                        Nozzle
                                  Head



                     Trays
                                  Shell



                     Nozzle




                                    Cone




                                         Nozzle


                      Shell




                Nozzle                    Head
                                    Skirt
                                    Support




 6                       Figure 2.3



Notes:




                                                                  8
            Vertical Reactor
                                                Inlet
                                                Nozzle
                          Head




                      Upper
                    Catalyst
                       Bed                      Shell




                Catalyst Bed
                Support Grid




                        Lower
                    Catalyst
                       Bed
                                                  Outlet
                                                  Collector

                         Head

                                                Outlet
                                                Nozzle
             Support
                Skirt



 7                                 Figure 2.4



Notes:




         Spherical Pressurized
            Storage Vessel
                                                           Shell




                                                                  Support
                                                                  Leg




                                                        Cross
                                                        Bracing




                                Figure 2.5
 8




Notes:




                                                                            9
              Vertical Vessel on
                Lug Supports




  9
                        Figure 2.6




Notes:




            Scope of ASME Code
                Section VIII
      • Section VIII used worldwide
      • Objective: Minimum requirements for safe
        construction and operation
      • Division 1, 2, and 3




 10




Notes:




                                                   10
           Section VIII Division 1
  • 15 psig < P ≤ 3000 psig
  • Applies through first connection to pipe
  • Other exclusions
      – Internals (except for attachment weld to vessel)
      – Fired process heaters
      – Pressure containers integral with machinery
      – Piping systems


 11




Notes:




          Section VIII, Division 2,
            Alternative Rules
  • Scope identical to Division 1 but
    requirements differ
      – Allowable stress
      – Stress calculations
      – Design
      – Quality control
      – Fabrication and inspection
  • Choice between Divisions 1 and 2 based on
 12
    economics


Notes:




                                                           11
       Division 3, Alternative Rules
          High Pressure Vessels

      • Applications over 10,000 psi
      • Pressure from external source, process
        reaction, application of heat, combination
        of these
      • Does not establish maximum pressure
        limits of Division 1 or 2 or minimum limits
        for Division 3.
 13




Notes:




          Structure of Section VIII,
                 Division 1
  • Subsection A
       – Part UG applies to all vessels
  • Subsection B
       – Requirements based on fabrication method
       – Parts UW, UF, UB
  • Subsection C
       – Requirements based on material class
       – Parts UCS, UNF, UHA, UCI, UCL, UCD, UHT,
         ULW, ULT
  • Mandatory and Nonmandatory Appendices
 14




Notes:




                                                      12
          Material Selection Factors

      •   Strength
      •   Corrosion Resistance
      •   Resistance to Hydrogen Attack
      •   Fracture Toughness
      •   Fabricability


 15




Notes:




                         Strength
      • Determines required component thickness
      • Overall strength determined by:
          – Yield Strength
          – Ultimate Tensile Strength
          – Creep Strength
          – Rupture Strength


 16




Notes:




                                                  13
            Corrosion Resistance
      • Deterioration of metal by chemical action
      • Most important factor to consider
      • Corrosion allowance supplies additional
        thickness
      • Alloying elements provide additional
        resistance to corrosion


 17




Notes:




                 Resistance to
                Hydrogen Attack

      • At 300 - 400°F, monatomic hydrogen
        forms molecular hydrogen in voids
      • Pressure buildup can cause steel to crack
      • Above 600°F, hydrogen attack causes
        irreparable damage through component
        thickness
 18




Notes:




                                                    14
              Brittle Fracture
          and Fracture Toughness
      • Fracture toughness: Ability of material to
        withstand conditions that could cause
        brittle fracture
      • Brittle fracture
         – Typically at “low” temperature
         – Can occur below design pressure
         – No yielding before complete failure

 19




Notes:




            Brittle Fracture and
        Fracture Toughness, cont’d

      • Conditions required for brittle fracture
        – High enough stress for crack initiation and
          growth
        – Low enough material fracture toughness at
          temperature
        – Critical size defect to act as stress
          concentration

 20




Notes:




                                                        15
          Factors That Influence
           Fracture Toughness
 • Fracture toughness varies with:
      - Temperature
      - Type and chemistry of steel
      - Manufacturing and fabrication processes

 • Other factors that influence fracture
   toughness:
      - Arc strikes, especially if over repaired area
      - Stress raisers or scratches in cold formed thick
        plate
 21




Notes:




      Charpy V-Notch Test Setup


                                           Scale


                                                         Starting Position
                                                                       Hammer
                                 Pointer



                  End of swing                                   h'

                                                   Specimen


                                                    h'

                                 Anvil




 22




Notes:




                                                                                16
              ASME Code and
        Brittle Fracture Evaluation
      • Components to consider
       – Shells              – Nozzles
       – Manways             – Tubesheets
       – Heads               – Flanges
       – Reinforcing pads    – Flat cover plates
       – Backing strips      – Attachments essential
         that remain in        to structural integrity
         place                 that are welded to
                               pressure parts
 23




Notes:




        Temperatures to Consider
      • Minimum Design Metal Temperature
        (MDMT)
        – Lowest temperature at which component has
          adequate fracture toughness
      • Critical Exposure Temperature (CET)
        – Minimum temperature at which significant
          membrane stress will occur


 24




Notes:




                                                         17
                  Simplified ASME
                Evaluation Approach
 • Material specifications classified into
   Material Groups A through D
 • Impact test exemption curves
       – For each Material Group
       – Acceptable MDMT vs. thickness where impact
         testing not required
 • If combination of Material Group and
   thickness not exempt, then must impact test
 25
   at CET


Notes:




                     Material Groups
  MATERIAL
   GROUP                             APPLICABLE MATERIALS
      Curve A   •   All carbon and low alloy steel plates, structural shapes, and bars not
                    listed in Curves B, C & D
                •   SA-216 Gr. WCB & WCC, SA-217 Gr. WC6, if normalized and tempered
                    or water-quenched and tempered

      Curve B   •   SA-216 Gr. WCA, if normalized and tempered or water-q u e n c h e d a n d
                    tempered
                •   SA-216 Gr. WCB & WCC for maximum thickness of 2 in., if produced
                    to fine grain practice and water-quenched and tempered
                •   SA-285 Gr. A & B
                •   SA-414 Gr. A
                •   SA-515 Gr. 60
                •   SA-516 Gr. 65 & 70, if not normalized
                •   Except for cast steels, all materials of Curve A if produced to fine
                    grain practice and normalized which are not included in Curves C & D
                •   All pipe, fittings, forging, and tubing which are not included in Curves
                    C&D

                                 Table 3.1 (Excerpt)
 26




Notes:




                                                                                                18
                Material Groups, cont’d
  MATERIAL
   GROUP                                                                        APPLICABLE MATERIALS
      Curve C    •   SA-182 Gr. 21 & 22, if normalized and tempered
                 •   SA-302 Gr. C & D
                 •   SA-336 Gr. F21 & F22, if normalized and tempered
                 •   SA-387 Gr. 21 & 22, if normalized and tempered
                 •   SA-516 Gr. 55 & 60, if not normalized
                 •   SA-533 Gr. B & C
                 •   SA-662 Gr. A
                 •   All material of Curve B if produced to fine grain practice and
                     normalized which are not included in Curve D

      Curve D    •   SA-203                                                                                              •       SA-537 Cl. 1, 2 & 3
                 •   SA-508 Cl. 1                                                                                        •       SA-612, if normalized
                 •   SA-516, if normalized                                                                               •       SA-662, if normalized
                 •   SA-524 Cl. 1 & 2                                                                                    •       SA-738 Gr. A

       Bolting   •   See Figure UCS-66 of the ASME Code Section VIII, Div. 1, for impact
      and Nuts       test exemption temperatures for specified material specifications

                                                                        Table 3.1 (Excerpt)
 27




Notes:




  Impact Test Exemption Curves
  for Carbon and Low-Alloy Steel
                                                          140

                                                          120

                                                          100
                      F




                                                                            A                        B
                      Minimum Design Metal Temperature,




                                                           80

                                                           60
                                                                                                         C
                                                           40

                                                                                                             D
                                                           20

                                                            0

                                                          -20


                                                          -40
                                                          -55
                                                          -60                   Impact testing required

                                                          -80
                                                                0.394   1            2           3               4           5

                                                                                     Nominal Thickness, in.
                                                                            (Limited to 4 in. for Welded Construction)



                                                                                   Figure 3.1
 28




Notes:




                                                                                                                                                         19
      Additional ASME Code Impact
            Test Requirements
      • Required for welded construction over 4 in.
        thick, or nonwelded construction over 6 in.
        thick, if MDMT < 120°F
      • Not required for flanges if temperature
        ≥ -20°F
      • Required if SMYS > 65 ksi unless
        specifically exempt


 29




Notes:




           Additional ASME Code
                Impact Test
           Requirements, cont’d
      • Not required for impact tested low
        temperature steel specifications
        – May use at impact test temperature
      • 30°F MDMT reduction if PWHT P-1 steel
        and not required by code
      • MDMT reduction if calculated stress <
        allowable stress
 30




Notes:




                                                      20
                   Fabricability

      • Ease of construction
      • Any required special fabrication practices
      • Material must be weldable




 31




Notes:




        Maximum Allowable Stress
 • Stress: Force per unit area that resists loads
   induced by external forces
 • Pressure vessel components designed to
   keep stress within safe operational limits
 • Maximum allowable stress:
       – Includes safety margin
       – Varies with temperature and material
 • ASME maximum allowable stress tables for
   permitted material specifications
 32




Notes:




                                                     21
              Maximum Allowable
                Stress, cont’d
                         ALLOWABLE STRESS IN TENSION FOR CARBON AND
                                          LOW-ALLOY STEEL
          Spec No.  Grade         Nominal       P-No. Group No. Min. Yield                      Min. Tensile
                                Composition                       (ksi)                            (ksi)
             Carbon Steel Plates and Sheets
         SA-515       55             C-Si         1       1        30                                 55
                      60             C-Si         1       1        32                                 60
                      65             C-Si         1       1        35                                 65
                      70             C-Si         1       2        38                                 70

         SA-516             55              C-Si               1          1           30              55
                            60            C-Mn-Si              1          1           32              60
                            65            C-Mn-Si              1          1           35              65
                            70            C-Mn-Si              1          2           38              70

         Plate - Low Alloy Steels
         SA-387        2 Cl.1     1/2Cr-1/2Mo                  3          1           33              55
                       2 Cl.2     1/2Cr-1/2Mo                  3          2           45              70
                      12 Cl.1      1Cr-1/2Mo                   4          1           33              55
                      12 Cl.2      1Cr-1/2Mo                   4          1           40              65
                      11 Cl.1 1 1/4Cr-1/2Mo-Si                 4          1           35              60
                      11 Cl.2 1 1/4Cr-1/2Mo-Si                 4          1           45              75
                      22 Cl.1     2 1/4Cr-1Mo                  5          1           30              60
                      22 Cl.2     2 1/4Cr-1Mo                  5          1           45              75



      ASME Maximum Allowable Stress (Table 1A Excerpt)
                       Figure 3.2
 33




Notes:




              Maximum Allowable
                Stress, cont’d
                       ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL
                            Max Allowable Stress, ksi (Multiply by 1,000 to Obtain psi)
                                    for Metal Temperature, °F, Not Exceeding
                                                                                                        Spec
             650     700         750    800    850    900    950   1000    1050 1100     1150    1200    No.
                                                                          Carbon Steel Plates and Sheets
             13.8    13.3    12.1      10.2   8.4    6.5    4.5    2.5    --     --     --      --     SA-515
             15.0    14.4    13.0      10.8   8.7    6.5    4.5    2.5    --     --     --      --     SA-515
             16.3    15.5    13.9      11.4   9.0    6.5    4.5    2.5    --     --     --      --     SA-515
             17.5    16.6    14.8      12.0   9.3    6.5    4.5    2.5    --     --     --      --     SA-515

             13.8    13.3    12.1      10.2   8.4    6.5    4.5    2.5    --     --        --    --        SA-516
             15.0    14.4    13.0      10.8   8.7    6.5    4.5    2.5    --     --        --    --        SA-516
             16.3    15.5    13.9      11.4   9.0    6.5    4.5    2.5    --     --        --    --        SA-516
             17.5    16.6    14.8      12.0   9.3    6.5    4.5    2.5    --     --        --    --        SA-516

                                                                          Plate-Low Alloy Steels (Cont'd)
             13.8    13.8    13.8      13.8   13.8   13.3   9.2    5.9    --      --    --      --      SA-387
             17.5    17.5    17.5      17.5   17.5   16.9   9.2    5.9    --      --    --      --      SA-387
             13.8    13.8    13.8      13.8   13.4   12.9   11.3   7.2    4.5     2.8   1.8     1.1     SA-387
             16.3    16.3    16.3      16.3   15.8   15.2   11.3   7.2    4.5     2.8   1.8     1.1     SA-387
             15.0    15.0    15.0      15.0   14.6   13.7   9.3    6.3    4.2     2.8   1.9     1.2     SA-387
             18.8    18.8    18.8      18.8   18.3   13.7   9.3    6.3    4.2     2.8   1.9     1.2     SA-387
             15.0    15.0    15.0      15.0   14.4   13.6   10.8   8.0    5.7     3.8   2.4     1.4     SA-387
             17.7    17.2    17.2      16.9   16.4   15.8   11.4   7.8    5.1     3.2   2.0     1.2     SA-387


                  ASME Maximum Allowable Stress (Excerpt), cont'd
                               Figure 3.2, cont'd
 34




Notes:




                                                                                                                    22
            Material Selection Based
            on Fracture Toughness
      Exercise 1
        •   New horizontal vessel
        •   CET = - 2°F
        •   Shell and heads: SA-516 Gr. 70
        •   Heads hemispherical: ½ in. thick
        •   Cylindrical shell: 1.0 in. thick
        •   No impact testing specified
        •   Is this correct?
        •   If not correct, what should be done?
 35




Notes:




              Exercise 1 - Solution
  • Must assume SA-516 Gr. 70 not normalized.
    Therefore, Curve B material (Ref. Table 3.1).
  • Refer to Curve B in Figure 3.1.
       – ½ in. thick plate for heads: MDMT = -7°F
       – ½ in. thick plate exempt from impact testing since
         MDMT < CET
  • 1 in. shell plate: MDMT = +31°F
       – Not exempt from impact testing



 36




Notes:




                                                              23
      Exercise 1 - Solution, cont’d
 • One approach to correct: Impact test 1 in. plate
   at -2°F. If passes, material acceptable.
 • Another approach: Order 1 in. plate normalized
      – Table 3.1: normalized SA-516 is Curve D material
      – Figure 3.1: 1 in. thick Curve D, MDMT = -30°F
      – Normalized 1 in. thick plate exempt from impact testing




 37




Notes:




      Exercise 1 - Solution, cont’d
      • Choice of option based on cost, material
        availability, whether likely that 1 in. thick non-
        normalized plate would pass impact testing




 38




Notes:




                                                                  24
           Design Conditions
             and Loadings
  • Determine vessel mechanical design
  • Design pressure and temperature, other
    loadings
  • Possibly multiple operating scenarios to
    consider
  • Consider startup, normal operation,
    anticipated deviations, shutdown
 39




Notes:




            Design Pressure
                                               PT = D e s i g n P r e s s u r e a t
                                                    Top of Vessel




                                             γ = Weight Density of
                                                 Liquid in Vessel


                H = Height
                    of Liquid




                                             P BH = D e s i g n P r e s s u r e o f
                                                   Bottom Head



                                Figure 4.1
 40




Notes:




                                                                                      25
              Temperature Zones
                in Tall Vessels
                     Section 4
                      (T-Z)




                     Section 3
                      (T-Y)




                     Section 2
                      (T-X)




                     Section 1
                      (T) F

                                              Support Skirt

                                              Grade




                                 Figure 4.2
 41




Notes:




             Additional Loadings
      • Weight of vessel and normal contents
        under operating or test conditions
      • Superimposed static reactions from weight
        of attached items (e.g., motors, machinery,
        other vessels, piping, linings, insulation)
      • Loads at attached internal components or
        vessel supports
      • Wind, snow, seismic reactions
 42




Notes:




                                                              26
      Additional Loadings, cont’d
 • Cyclic and dynamic reactions caused by
   pressure or thermal variations, equipment
   mounted on vessel, and mechanical loadings
 • Test pressure combined with hydrostatic
   weight
 • Impact reactions (e.g., from fluid shock)
 • Temperature gradients within vessel
   component and differential thermal
   expansion between vessel components
 43




Notes:




         Weld Joint Categories

                                                           C           C
                 C
                                                                       A
                                           A                               C
                               B
                     A D

                                                       D   A                   B
                                       B                           D
                           D                   B
                                                               A
         B   A                                                 C
                                   C               D




                               Figure 4.3
 44




Notes:




                                                                                   27
                                Weld Types
                                                          Butt joints as attained by double-welding or by other
                           1                              means which will obtain the same quality of deposited
                                                          weld metal on the inside and outside weld surface.
                                                          Backing strip, if used, shall be removed after
                                                          completion of weld.

                                                         Single-welded butt joint with backing strip which
                           2                             remains in place after welding.




                               For circumferential
                               joint only



                           3                             Single-welded butt joint without backing strip.



                           4                             Double-full fillet lap joint.



                           5                             Single-full fillet lap joint with plug welds.


                           6                             Single-full fillet lap joint without plug welds.




                                                     Figure 4.4
 45




Notes:




              Weld Joint Efficiencies

      Joint          Acceptable Joint Categories                                                   Degree of
      Type                                                                                 Radiographic Examination
                                                                                         Full                Spot   None
       1      A, B, C, D                                                                 1.00                0.85   0.70
       2      A, B, C, D (See ASME Code for limitations)                                 0.90                0.80   0.65
       3      A, B, C                                                                     NA                  NA    0.60
       4      A, B, C (See ASME Code for limitations)                                      NA                NA     0.55
       5      B, C (See ASME Code for limitations)                                         NA                NA     0.50
       6      A, B, (See ASME Code for limitations)                                        NA                NA     0.45




                                                     Figure 4.5
 46




Notes:




                                                                                                                           28
                Summary Of ASME
                 Code Equations
                                          Thickness,               Pressure,                              Stress,
               Part                         t , in.                  P, psi                               S, psi
                                             p

                                        Pr                        SE 1t                             P (r + 0 .6 t )
         Cylindrical shell                                       r + 0. 6t
                                     SE1 − 0 .6P                                                         tE1
                                                                  2S E t                            P (r + 0 .2 t )
                                                                 r + 0. 2t                              2 tE
                                          Pr
          Spherical shell
                                      2SE1 − 0 .2 P
                                          PD                        2 SEt                           P (D + 0. 2t )
                                      2 SE − 0 .2P                 D + 0 .2 t                           2tE
               2:1
         Semi - Elliptical
                                          0. 885 PL                   SEt                      P (0 .885 L + 0. 1t )
              head
                                          SE − 0 .1P            0 .885 L + 0 .1 t                       tE

        Torispherical head                                      2 S E t cos α                  P (D + 1. 2t cos α )
         with 6% knuckle               PD                   D + 1. 2t cos α                       2 tE cos α
                              2 cos α (SE − 0 .6 P )
         Conical Section
            ( = 30°)
             α


                                                     Figure 4.6
 47




Notes:




      Typical Formed Closure Heads                                                         t



                                      t

                                                                             R

                                                      sf
                                                                                                         sf

                                ID                                               ID

                             Flanged                                     Hemispherical

                                                 t
                                                                                           t
                      h
                                                            h
                                                       sf                                            sf


                             Elliptical                              Flanged and Dished
                                                                        (torispherical)




                                  α          t                                   α         t




                                                                                                    sf
                                                                                       r
                                ID                                               ID
                             Conical                                     Toriconical



 48
                                                     Figure 4.7




Notes:




                                                                                                                       29
          Hemispherical
      Head to Shell Transition
                   t                                                                       th
                   h




                                                       Thinner Part




                                                                                                                    Thinner Part
         l ≥ 3y
                                                                      l ≥ 3y

                                                Tangent Line
          y                                                                                                y
                                         Length of required taper, l,
                                              may include the width
                                                   of the weld
                                         ts                                                                    ts




                                               Figure 4.8
 49




Notes:




                  Sample Problem 1
                           Hemispherical


                                                                      DESIGN INFORMATION
                                                                      Design Pressure = 250 psig
                                                                      Design Temperature = 700° F
                                                                      Shell and Head Material is SA-515
                                                                       Gr. 60
                                                    4' - 0"           Corrosion Allowance = 0.125"
                                                                      Both Heads are Seamless
                          60' - 0"                                    Shell and Cone Welds are Double
                                                                       Welded and will be Spot
                                                                       Radiographed
                                                                      The Vessel is in All Vapor Service
                                                                      Cylinder Dimensions Shown are
                                                                       Inside Diameters



                         10' - 0"




                                                   6' - 0"
                         30' - 0"




                   2:1 Semi-Elliptical




                                              Figure 4.9
 50




Notes:




                                                                                                                                   30
       Sample Problem 1 - Solution
      • Required thickness for internal pressure of cylindrical
        shell (Figure 4.6):
                                 Pr
                       tp =
                              SE1 − 0 .6P
      • Welds spot radiographed, E = 0.85 (Figure 4.5)

      • S = 14,400 psi for SA- 515/Gr. 60 at 700°F (Figure 3.2)

      • P = 250 psig

 51




Notes:




                    Sample Problem 1
                     Solution, cont’d
       • For 6 ft. - 0 in. shell

          r = 0.5D + C = 0.5 × 72 + 0.125 = 36.125 in.
                  Pr              250 × 36.125
        tp =               =
               SE1 − 0 .6 P 14,400 × 0 .85 − 0.6 × 250   = 0.747 in.

          t = tp + c = 0.747 + 0.125

          t = 0.872 in., including corrosion allowance

 52




Notes:




                                                                       31
                   Sample Problem 1
                    Solution, cont’d
      • For 4 ft. - 0 in. shell


         r = 0.5 × 48 + 0.125 = 24.125 in.

                                250 × 24 . 125
                  tp =                                  = 0.499 in.
                         14, 400 × 0. 85 − 0 .6 × 250


         t = 0.499 + 0.125


         t = 0.624 in., including corrosion allowance
 53




Notes:




                   Sample Problem 1
                    Solution, cont’d
      Both heads are seamless, E = 1.0.
      Top Head - Hemispherical (Figure 4.6)

         r = 24 + 0.125 = 24.125 in.

               Pr               250 × 24 . 125
 tp =                   =                             = 0.21 in.
          2 SE 1 − 0. 2P 2 × 14 ,400 × 1 − 0 .2 × 250

         t = tp + c = 0.21 + 0.125

         t = 0.335 in., including corrosion allowance
 54




Notes:




                                                                      32
                 Sample Problem 1
                  Solution, cont’d
      • Bottom Head - 2:1 Semi-Elliptical (Figure 4.6)


         D = 72 + 2 × 0.125 = 72.25 in.
           PD                250 × 72 . 25
 tp =               =                             = 0.628 in.
        2SE − 0 .2 P 2 × 14 ,400 × 1 − 0 .2 × 250

         t = 0.628 + 0.125
         t = 0.753 in., including corrosion allowance


 55




Notes:




           Design For External
        Pressure and Compressive
                Stresses
      • Compressive forces caused by dead
        weight, wind, earthquake, internal vacuum
      • Can cause elastic instability (buckling)
      • Vessel must have adequate stiffness
         – Extra thickness
         – Circumferential stiffening rings


 56




Notes:




                                                                33
              Design For
         External Pressure and
      Compressive Stresses, cont’d
      • ASME procedures for cylindrical shells,
        heads, conical sections. Function of:

         – Material                        – Temperature
         – Diameter                        – Thickness
         – Unstiffened length


 57




Notes:




                  Stiffener Rings
                                              Moment Axis of Ring

                                                                    h/3




                      L   L         L           L           L




                      L   L         L           L           L



            h/3

                                        h = Depth of Head




                              Figure 4.10
 58




Notes:




                                                                          34
                 Sample Problem 2

                                                          DESIGN INFORMATION
                                                          Design Pressure = Full Vacuum
                                                          Design Temperature = 500° F
                                4' - 0"                   Shell and Head Material is
                                                           SA-285 Gr. B, Yield Stress = 27 ksi
                                                          Corrosion Allowance = 0.0625"
                                                          Cylinder Dimension Shown
                    150' - 0"
                                                            is Inside Diameter




                                          2:1 Semi-Elliptical
                                              (Typical)



                                   Figure 4.11
 59




Notes:




       Sample Problem 2 - Solution
      • Calculate L and Do of cylindrical shell.

        L = Tangent Length + 2 × 1/3 (Head Depth)
        L = 150 × 12 + 2/3 × (48/4) = 1,808 in.
        Do = 48 + 2 × 7/16 = 48.875 in.

      • Determine L/D o and Do /t
        Account for corrosion allowance:

         t = 7/16 – 1/16 = 6/16 = 0.375 in.
         Do /t = 48.875 / 0.375 = 130
         L/Do = 1808 / 48.875 = 37

 60




Notes:




                                                                                                 35
                           Sample Problem 2
                            Solution, cont’d
      • Determine A.
      • Use Figure 4.12, Do /t, and L/Do .


        Note: If L/Do > 50, use L/D o = 50. For L/Do < 0.05, use
              L/Do = 0.05




 61




Notes:




                           Sample Problem 2
                            Solution, cont’d
                                                                                                                                                   A = 0.000065



                                  D o/t = 100
                                                                                                                                                                     .0001
                                                                                                                                                         5 6 7 8 9




                                  D o/t = 125
             D o/t = 130
                                  D o/t = 150
                                                                                                                                                         4




                                  D o/t = 200
                                                                                                                                                         3




                                  D o/t = 250                                                    0
                                                                                              4 0 500       0            0         000
                                                                                            =           = 60         = 80      = 1,
                                                                                                                                                         2




                                                                                                  =
                                                                                                                                                                     .00001




                                  D o/t = 300                                   D o /t       Do/t    /t
                                                                                                    Do        D o /t    D o /t
                                                                                                                                             1.8
                           50.0

                                    40.0
                                           35.0
                                                  30.0

                                                         25.0

                                                                20.0
                                                                18.0
                                                                       16.0
                                                                              14.0


                                                                                     10.0




                                                                                                                     5.0

                                                                                                                           4.0
                                                                                                                                 3.5
                                                                                                                                 3.0

                                                                                                                                       2.5


                                                                                                                                             2.0

                                                                                                                                                   1.6

                                                                                                                                                   1.2
                                                                              12.0


                                                                                             9.0
                                                                                                   8.0
                                                                                                         7.0
                                                                                                               6.0




                                                                                                                                                   1.4




                                                                                     Length + Outside Diameter = L/Do


                                            L/D = 37
                                               o




                                                                                       Factor A
                                                                                      Figure 4.12
 62




Notes:




                                                                                                                                                                              36
                            Sample Problem 2
                             Solution, cont’d
                                                                                                                              20,000
                  GENERAL NOTE: See Table CS-1 for tabular values                                                             18,000
                                                                                                          up to 300°F         16,000
                                                                                                                  500°F       14,000
                                                                                                                  700°F       12,000
                                                                                                                  800°F       10,000




                                                                                                                                       FACTOR B
                                                                                                                  900°F        9,000
                                                                                                                               8,000

                                 E=29.0 x 106
                                                                                                                               7,000

                                            6
                                                                                                                               6,000
                                 E=27.0 x 10

                                 E=24.5 x 10
                                            6                                                                                 5,000

                                 E=22.8 x 10
                                            6
                                                                                                                               4,000
                                 E=20.8 x 10
                                            6
                                                                                                                               3,500
                                                                                                                               3,000

                                                                                                                               2,500


                                                                                                                               2,000
                    2   3 4 5 6 7 8 9           2     3   4   5 6 7 8 9     2   3 4 5 6 7 8 9         2   3   4   5 6 7 8 9
         .00001                     .0001                           .001                        .01                       1
                                                                                                                          .

                                                                 FACTOR A
                          A=0.000065

                                                           Factor B
 63                                                       Figure 4.13



Notes:




                            Sample Problem 2
                             Solution, cont’d
      • Calculate maximum allowable external pressure

                                                       2 AE
                                  Pa =
                                                    3 (Do / t )
        Where:
        E = Young's modulus of elasticity
                  E = 27 × 106 psi (Figure 4.13) at T = 500°F
                  Pa = 9 psi

 64




Notes:




                                                                                                                                                  37
                    Sample Problem 2
                     Solution, cont’d
          Since Pa < 15 psi, 7/16 in. thickness not sufficient
      • Assume new thickness = 9/16 in.,
        corroded thickness L = 1/2 in.

           Do 48 . 875                       L
              =        = 97 .75                = 37 (as before )
            t   0 .5                        Do
          A = 0.000114

                 2 × 0. 000114 × 27 × 10 6
          Pa =                             = 15 .7 psi
                         3 × 130 .33
 65




Notes:




      Exercise 2 - Required
 Thickness for Internal Pressure
      •   Inside Diameter        - 10’ - 6”
      •   Design Pressure        - 650 psig
      •   Design Temperature - 750°F
      •   Shell & Head Material - SA-516 Gr. 70
      •   Corrosion Allowance - 0.125 in.
      •   2:1 Semi-Elliptical heads, seamless
      •   100% radiography
      •   Vessel in vapor service
 66




Notes:




                                                                   38
                  Exercise 2 - Solution
  • For shell                             Pr
                         tp =
                                 SE   1   − 0 . 6P

    P = 650 psig
    r = 0.5 × D + CA
        = (0.5 × 126) + 0.125 = 63.125 in.
  • S = 16,600 psi, Figure 3.3 for SA-516 Gr. 70
  • E = 1.0, Figure 4.8 for 100% radiography

                               650 × 63 . 125
                tp =                                     = 2 .53 in.
                       (16 ,600 × 1. 0 ) − (0. 6 × 650 )
 67




Notes:




       Exercise 2 - Solution, cont’d
       Add corrosion allowance
              tp = 2.53 + 0.125 = 2.655 in.
      • For the heads
                  PD
       tp =
              2 SE − 0. 2P

                 650 (126 × 0 . 9) + 0 .250
         tp =                                   = 2 .23 in .
                (2 × 16 ,600 ) − (0 . 2 × 650 )

       Add corrosion allowance

 68
              tp = 2.23 + 0.125 = 2.355 in.


Notes:




                                                                       39
        Reinforcement of Openings

      • Simplified ASME rules - Area replacement
      • Metal used to replace that removed:
        -   Must be equivalent in metal area
        -   Must be adjacent to opening




 69




Notes:




             Cross Sectional View of
                Nozzle Opening
                                                                   Dp
                                        tn                    Rn
                                                  tr n

                                                                                                         te
        2.5t or 2.5t + t
                   n   e
        Use smaller value                                tr




              t             c




          2.5t or 2.5t          h
                    n                                                   d
        Use smaller value


                                      d or R + t n + t
                                            n                               d or R + t n + t
                                                                                  n

                                      Use larger value                      Use larger value

                                For nozzle wall inserted                      For nozzle wall abutting
                                through the vessel wall                       the vessel wall


                                                   Figure 4.14
 70




Notes:




                                                                                                              40
      Nozzle Design Configurations
                              (a)
                   Full Penetration Weld
                 With Integral Reinforcement         (a-1)                    (a-2)                         (a-3)

                                                               Separate Reinforcement Plates Added




                             (b)                        (c)                         (d)                           (e)
                                     Full Penetration Welds to Which Separate Reinforcement Plates May be Added




                                           (f-1)                (f-3)




                                          (f-2)
                                                                (f-4)                        (g)


                                                        Self - Reinforced Nozzles




                                                              Figure 4.15
 71




Notes:




          Additional Reinforcement
      • Necessary if insufficient excess thickness
      • Must be located within reinforcement zone
      • Allowable stress of reinforcement pad
        should be ≥ that of shell or head
      • Additional reinforcement sources
        – Pad
        – Additional thickness in shell or lower part of
          nozzle

 72




Notes:




                                                                                                                        41
                 Sample Problem 3
                  DESIGN INFORMATION
                  Design Pressure = 300 psig
                  Design Temperature = 200° F
                  Shell Material is SA-516 Gr. 60
                  Nozzle Material is SA-53 Gr. B, Seamless
                  Corrosion Allowance = 0.0625"
                  Vessel is 100% Radiographed
                  Nozzle does not pass through Vessel Weld Seam




                                                                  NPS 8 Nozzle
                                                                  (8.625" OD)
                                                                   0.5" Thick




                     0.5625" Thick Shell, 48" Inside Diameter



                                        Figure 4.16
 73




Notes:




      Sample Problem 3 - Solution
  • Calculate required reinforcement area, A
      A = dtrF
  Where:
      d =Finished diameter of circular opening, or
         finished dimension of nonradial opening in
         plane under consideration, in.
      tr = Minimum required thickness of shell using
           E = 1.0, in.
      F = Correction factor, normally 1.0
 74




Notes:




                                                                                 42
                Sample Problem 3 -
                 Solution, cont’d
      • Calculate diameter, d.
        d = Diameter of Opening – 2 (Thickness +
            Corrosion Allowance)
         d = 8.625 – 1.0 + .125 = 7.750 in.
      • Calculate required shell thickness, tr (Figure 4.6)
         tr = 0.487 in.
      • Assume F = 1.0
 75




Notes:




                Sample Problem 3 -
                 Solution, cont’d
      • Calculate A
        A = dtrF
        A = (8.625 - 1.0 + 0.125) × 0.487 × 1
          = 3.775 in.2
      • Calculate available reinforcement area in vessel
        shell, A1 , as larger of A1 1 or A 12
         A11 = (Elt - Ftr)d

 76
         A12 = 2 (Elt-Ftr)(t + tn)


Notes:




                                                              43
                Sample Problem 3 -
                 Solution, cont’d
 Where:
  El = 1.0 when opening is in base plate away from welds,
        or when opening passes through circumferential joint
        in shell (excluding head to shell joints).
  El = ASME Code joint efficiency when any part of opening
        passes through any other welded joint.
  F = 1 for all cases except integrally reinforced nozzles
        inserted into a shell or cone at angle to vessel
        longitudinal axis. See Fig. UG-37 for this special
        case.
  tn = Nominal thickness of nozzle in corroded condition, in.
 77




Notes:




                Sample Problem 3 -
                 Solution, cont’d
 A11 = (E lt - Ftr)d = (0.5625 - 0.0625 - 0.487) × 7.75 = 0.1 in. 2

 A12 = 2 (E lt - Ftr) (t + tn )

      = 2(0.5625-0.0625-0.487) × (0.5625-0.0625+0.5 -0.0625)

      = 0.0243 in.2

 Therefore,
    A1 = 0.1 in.2 available reinforcement in shell

 78




Notes:




                                                                      44
                   Sample Problem 3 -
                    Solution, cont’d
  • Calculate reinforcement area available in nozzle wall, A 2 ,
    as smaller of A21 or A 22 .

        A21 = (tn -trn) 5t

        A22 = 2 (tn -trn) (2.5 tn + te )




 79




Notes:




                  Sample Problem 3 -
                   Solution, cont’d
      Where:

      trn =   Required thickness of nozzle wall, in.

      r =     Radius of nozzle, in.

      te = 0 if no reinforcing pad.

      te = Reinforcing pad thickness if one installed, in.

      te = Defined in Figure UG-40 for self-reinforced
           nozzles, in.
 80




Notes:




                                                                   45
                   Sample Problem 3 -
                    Solution, cont’d
      • Calculate required nozzle thickness, trn (Figure 4.6)


                           Pr
              t rn =
                       SE 1 − 0 .6 P

                  300 ( 3 .8125 + 0 .0625 )
         t rn =                             = 0 .0784 in.
                   15 ,000 × 1 − 0 .6 × 300


 81




Notes:




                  Sample Problem 3 -
                   Solution, cont’d
 • Calculate A2 .
 A21 = (tn - trn )5t = (0.5 - 0.0625 - 0.0784) × 5 (0.5625 - 0.0625)
       = 0.898 in.2
 A22 = 2 (tn - trn ) (2.5 tn + te )
    = 2 (0.5 - 0.0625 - 0.0784) [2.5 × (0.5 - 0625) + 0]
       = 0.786 in.2
 Therefore,
   A2 = 0.786 in.2 available reinforcement in nozzle.

 82




Notes:




                                                                       46
                 Sample Problem 3 -
                  Solution, cont’d
      • Determine total available reinforcement area, AT;
        compare to required area.
         AT = A1 + A2 = 0.1 + 0.786 = 0.886 in.2
        AT < A, nozzle not adequately reinforced, reinforcement
        pad required.
      • Determine reinforcement pad diameter, D p.
        A5 = A - AT
        A5 = (3.775 - 0.886) = 2.889 in.2
 83




Notes:




                 Sample Problem 3 -
                  Solution, cont’d
      • Calculate D p
         te = 0.5625 in. (reinforcement pad thickness)
         A5 = [Dp - (d + 2 tn)] te
         2.889 = [Dp - (7.75 + 2(0.5 - 0.0625)] 0.5625
         Dp = 13.761 in.
      • Confirm D p within shell reinforcement zone, 2d
        2d = 2 × 7.75 = 15.5 in.
        Therefore, D p = 13.761 in. acceptable
 84




Notes:




                                                                  47
                           Flange Rating
 • Based on ASME B16.5

 • Identifies acceptable pressure/temperature combinations

 • Seven classes
   (150, 300, 400, 600, 900, 1,500, 2,500)

 • Flange strength increases with class number

 • Material and design temperature combinations without
   pressure indicated not acceptable

 85




Notes:




      Material Specification List

            Material Groups                                 Product Forms
          Material    Nominal
          Group      Designation      Forgings               Castings               Plates
          Number        Steel
                                   Spec. No.    Grade   Spec. No.   Grade   Spec. No.        Grade
           1.1        Carbon        A105         --      A216       WCB      A515             70
                                    A350        LF2       --         --      A516             70
                      C-Mn-Si        --          --       --         --      A537             Cl.1
           1.2        Carbon         --          --      A216       WCC       --              --
                                     --          --      A352       LCC       --              --
                       2 ½ Ni        --          --      A352       LC2      A203             B
                       3 ½ Ni       A350        LF3      A352       LC3      A203              E



                 ASME B16.5, Table 1a, Material Specification List (Excerpt)




                                               Figure 4.17

 86




Notes:




                                                                                                     48
  Pressure - Temperature Ratings
           Material
                             1.1               1.2                1.3
          Group No.
           Classes     150   300   400   150   300   400    150   300   400
          Temp., °F
          -20 to 100   285   740   990   290   750   1000   265   695   925
             200       260   675   900   260   750   1000   250   655   875
             300       230   655   875   230   730    970   230   640   850
             400       200   635   845   200   705    940   200   620   825
             500       170   600   800   170   665    885   170   585   775
             600       140   550   730   140   605    805   140   534   710
             650       125   535   715   125   590    785   125   525   695
             700       110   535   710   110   570    755   110   520   690
             750       95    505   670    95   505    670   95    475   630
             800       80    410   550    80   410    550   80    390   520
             850       65    270   355    65   270    355   65    270   355
             900       50    170   230    50   170    230   50    170   230
             950       35    105   140    35   105    140   35    105   140
             1000      20     50   70     20   50     70    20    50     70

                                   Figure 4.18
 87




Notes:




                 Sample Problem 4
      Determine Required Flange Rating

      Pressure Vessel Data:
           Shell and Heads:                    SA-516 Gr.70
           Flanges:                            SA-105
           Design Temperature: 700°F
           Design Pressure:                    275 psig



 88




Notes:




                                                                              49
       Sample Problem 4 - Solution
      • Identify flange material specification
        SA-105
      • From Figure 4.17, determine Material Group No.
        Group 1.1
      • From Figure 4.18 with design temperature and
        Material Group No. determined in Step 3
         – Intersection of design temperature with Material
           Group No. is maximum allowable design pressure for
           the flange Class
 89




Notes:




               Sample Problem 4 -
                Solution, cont’d
        – Table 2 of ASME B16.5, design information for all
          flange Classes
        – Select lowest Class whose maximum allowable
          design pressure ≥ required design pressure.

  • At 700°F, Material Group 1.1: Lowest Class that
    will accommodate 275 psig is Class 300.
  • At 700°F, Class 300 flange of Material Group
    1.1: Maximum design pressure = 535 psig.

 90




Notes:




                                                                50
                      Flange Design
      • Bolting requirements
        – During normal operation (based on design
          conditions)
        – During initial flange boltup (based on stress
          necessary to seat gasket and form tight seal

                 W
          Am =
                 S
 91




Notes:




               Flange Loads and
                 Moment Arms
                                                           Flange
                                                           Ring
                 Gasket

                                     t                h



                 A        hG                  W


                                                                    C
                     hT                               hD

                                g1
               HT
                          G
                               HG        HD       B                     g0

                                                      Flange Hub

                                     Figure 4.19
 92




Notes:




                                                                             51
         Stresses in Flange Ring
                and Hub
      • Calculated using:
        – Stress factors (from ASME code)
        – Applied moments
        – Flange geometry
      • Calculated for:
        – Operating case
        – Gasket seating case
 93




Notes:




              Flange Design and
           In-Service Performance
       Factors affecting design and performance
      • ASME Code m and y parameters.
      • Specified gasket widths.
      • Flange facing and nubbin width, w
      • Bolt size, number, spacing



 94




Notes:




                                                  52
      ASME Code m and y Factors
                                                                                                                                 Min.
                                                                                                                                                      Facing Sketch
                                                                                           Gasket                               Design
                                                                                                                                                      and Column in
              Gasket Type and Material                                                     Factor,                             Seating
                                                                                                                                                     ASME Table 2-5.2
                                                                                             m                                 Stress y,
                                                                                                                                                       (Figure 4.21)
                                                                                                                                  psi

       Flat metal, jacketed asbestos filled:
          Soft aluminum                                                                            3.25                               5,500
          Soft copper or brass                                                                     3.50                               6,500
                                                                                                                                                  (1a), (1b), (1c),
          Iron or soft steel                                                                       3.75                               7,600
                                                                                                                                                  (1d); (2);
          Monel                                                                                    3.50                               8,000
                                                                                                                                                  Column II
          4-6% chrome                                                                              3.75                               9,000
          Stainless steels and nickel-base alloys                                                  3.75                               9,000


       Solid flat metal:
          Soft aluminum                                                                            4.00                           8,800
          Soft copper or brass                                                                     4.75                           13,000          (1a), (1b), (1c),
          Iron or soft steel                                                                       5.50                           18,000          (1d); (2), (3), (4),
          Monel or 4-6% chr ome                                                                    6.00                           21,800          (5); Column I
          Stainless steels and nickel-base alloys                                                  6.50                           26,000




                                                                       Figure 4.20
 95




Notes:




       ASME Code Gasket Widths
                                          Facing Sketch                                        Basic Gasket Seating Widthb o
                                          (Exaggerated)
                                                                                               Column I                                  Column II


                                      N
                                                             N

                       (1a)                                                                N                                            N
                                      N
                                                                                           2                                            2
                                                          N

                       (1b)
                        w
                                      T

                              N                                                 w + T ;  w + N max 
                                                                                 2       4         
                                                                                                                              w + T w + N  
                       (1c)
                          w
                                                  w ≤N                                                                            2 ;  4 max 
                                           T                                                                                                 
                                  N



                       (1d)                        w ≤N
                                                              HG                               H   G
                                                         G             h                   G                  h
                                                                           G                                      G
                                               O.D. Contact Face
                                                                                                           Gasket
                                                                   b                                   C
                                                                                                       L
                                                                                                           Face




                                                                       For b o > ¼ i n .                          For b o < ¼ i n .



                                           ASME Code Gasket Widths (Table 2-5.2 excerpt)



                                                                   Figure 4.21
 96




Notes:




                                                                                                                                                                         53
                     Gasket Materials
                   and Contact Facings
                                     Gasket Materials and Contact Facings
            Gasket Factors m for Operating Conditions and Minimum Design Seating Stress y
                     Gasket Material                  Gasket     Min.       Sketches     Facing
                                                      Factor    Design                 Sketch and
                                                        m      Seating                 Column in
                                                               Stress y,               Table 2-5.2
                                                                  psi
        Flat metal, jacketed asbestos filled:          3.25     5500                    (1a), (1b),
          Soft aluminum                                3.50     6500                   (1c),2 , (1d)2,
          Soft copper or brass                         3.75     7600                        (2)2,
          Iron or soft steel                           3.50     8000                    Column II
          Monel                                        3.75     9000
          4% - 6% chrome                               3.75     9000
          Stainless steels and nickel-base alloys




                                                    Figure 4.22
 97




Notes:




          Maximum Allowable
        Working Pressure (MAWP)
        Maximum permitted gauge pressure at top of
        vessel in operating position for designated
        temperature
      • MAWP ≥ Design Pressure
      • Designated Temperature = Design Temperature
      • Vessel MAWP based on weakest component
        – Originally based on new thickness less corrosion
          allowance
        – Later based on actual thickness less future corrosion
 98       allowance needed



Notes:




                                                                                                         54
                     Local Loads
       • Piping system

       • Platforms, internals, attached equipment

       • Support attachment




  99




Notes:




           Types of Vessel Internals
  • Trays
  • Inlet Distributor
  • Anti-vortex baffle
  • Catalyst bed grid and support beams
  • Outlet collector
  • Flow distribution grid
  • Cyclone and plenum chamber system
 100




Notes:




                                                    55
                  ASME Code and
                  Vessel Internals
   • Loads applied from internals on vessel to be
     considered in design

   • Welding to pressure parts must meet ASME
     Code




 101




Notes:




              Corrosion Allowance
              For Vessel Internals
       • Removable internals: CA = CA of shell
         – Costs less
         – Easily replaced


       • Non-removable internals: CA = 2 (CA of shell)
         – Corrosion occurs on both sides

 102




Notes:




                                                         56
       Head-to-Shell Transitions
                             t                                                           t
                             h                                                               h




                                          Thinner part




                                                                                                              Thinner part
                         l                                                       l
                                                                  Tangent
                                                                    Line
                     y
                                                                                                      y




                                      t                                                               t
                                      s                                                                   s



                                                                             t
                         t
                         h                                                   h




                     y
                                 Tangent                                             y
                                  Line




                                                         Thinner part
                     l




                                                                                                                             Thinner part
                                                                             l




                                  t                                                                   t
                                      s                                                                   s




                                                                                             Fillet
                                                                                             Weld




                                                                        Butt Weld


                                 Intermediate Head Attachment



                                      Figure 6.1
 103




Notes:




       Typical Shell Transitions

                 C                                                                                                                          CL
                 L               In all cases, l shall not
                                     be less than 3y.
             y


                                                                                                                                                 l

         l




                                                                                                                                                     C
                                                                                                                                                     L




                                      Figure 6.2
 104




Notes:




                                                                                                                                                         57
                     Nozzle Neck
                  Thickness Tapers




                               Figure 6.3

 105




Notes:




                      Stiffener Rings




            In-Line                            Continuous Fillet Weld On
       Intermittent Weld                       One Side, Intermittent Weld
                               Staggered            On Other Side
                           Intermittent Weld



                               Figure 6.4
 106




Notes:




                                                                             58
         Post Weld Heat Treatment
   • Restores material properties
   • Relieves residual stresses
   • ASME Code PWHT requirements
        – Minimum temperature and hold time
        – Adequate stress relief
        – Heatup and cooldown rates




 107




Notes:




             Inspection and Testing
       Inspection includes examination of:

         •   Base material specification and quality

         •   Welds

         •   Dimensional requirements

         •   Equipment documentation

 108




Notes:




                                                       59
          Common Weld Defects

                  Between Weld Bead and Base Metal                                Between Adjacent Passes


                                                         Lack of Fusion




               Incomplete Filling at Root on One Side Only                        Incomplete Filling at Root


                                                    Incomplete Penetration


                                                                        External Undercut




                                                    Internal Undercut


                                                              Undercut


                                                    Figure 7.1
 109




Notes:




                           Weld Defects
   Presence of defects:

       • Reduces weld strength below that required
       • Reduces overall strength of fabrication
       • Increases risk of failure




 110




Notes:




                                                                                                               60
                        Types of NDE
             NDE TYPE              DEFECTS               ADVANTAGES             LIMITATIONS
                                   DETECTED
         Radiographic          Gas pockets, slag      Produces               Expensive.
                               inclusions,            permanent record.      Not practical for
                               incomplete             Detects small flaws.   complex shapes.
                               penetration, cracks    Most effective for
                                                      butt-welded joints.
         Visual                Porosity holes, slag   Helps pinpoint         Can only detect
                               inclusions, weld       areas for additional   what is clearly
                               undercuts,             NDE.                   visible.
                               overlapping
         Liquid Penetrant      Weld surface-type      Used for ferrous    Can only detect
                               defects: cracks,       and nonferrous      surface
                               seams, porosity,       materials. Simple   imperfections.
                               folds, pits,           and less expensive
                               inclusions,            than RT, MT, or UT.
                               shrinkage
         Magnetic Particle     Cracks, porosity,      Flaws up to ¼ in.      Cannot be used on
                               lack of fusion         beneath surface can    nonferrous
                                                      be detected.           materials.
         Ultrasonic            Subsurface flaws:      Can be used for        Equipment must be
                               laminations, slag      thick plates, welds,   constantly
                               inclusions             castings, forgings.    calibrated.
                                                      May be used for
                                                      welds where RT not
                                                      practical.




                                               Figure 7.2
 111




Notes:




                  Typical RT Setup
                        X-Ray Tube




                                                         X-Ray




                                                                        Film


                             Test Specimen



                                          Figure 7.3
 112




Notes:




                                                                                                 61
             Pulse Echo UT System
                      Cathode Ray Tube (CRT)




                            A        C
                                 B              Read Out

                                                Base Line


                          Input-Output                            Cable
                           Generator
                                         Transducer


                                           A

                                Couplant

                        Test Specimen




                                                  B

                                            C




                                                           Flaw


                                           Figure 7.4
 113




Notes:




                  Pressure Testing
       • Typically use water as test medium
       • Demonstrates structural and mechanical
         integrity after fabrication and inspection
       • Higher test pressure provides safety margin
       • P T = 1.5 P (Ratio)




 114




Notes:




                                                                          62
          Pressure Testing, cont’d
   Hydrotest pressures must be calculated:
   • For shop test. Vessel in horizontal position.
   • For field test. Vessel in final position with
     uncorroded component thicknesses.
   • For field test. Vessel in final position and with
     corroded component thicknesses.
   • PT ≤ Flange test pressure
   • Stress ≤ 0.9 (MSYS)
   • Field test with wind
 115




Notes:




                          Summary
       • Overview of pressure vessel mechanical design
       • ASME Section VIII, Division 1
       • Covered

          – Materials              – Design
          – Fabrication            – Inspection
          – Testing




 116




Notes:




                                                         63
      Part 2:
Background Material




       64
I.       INTRODUCTION
Pressure vessels are used in many industries (e.g., hydrocarbon processing,
chemical, power, pharmaceutical, food and beverage). The mechanical design
of most pressure vessels is done in accordance with the requirements contained
in the ASME Boiler and Pressure Vessel Code, Section VIII. Section VIII is
divided into three divisions. This course provides an overview of pressure vessel
mechanical design requirements. It focuses on Division 1, highlights the
differences in scope among the three divisions of Section VIII, and discusses
several factors related to pressure vessel design that the ASME Code does not
cover. The following summarizes the main sections of the course:

•    General

     -   Main Pressure Vessel Components
     -   Primary Process Functions of Pressure Vessels
     -   Scope of ASME Code Section VIII
     -   Structure of Section VIII, Division 1
•    Materials of Construction

     -   Material Selection Factors
     -   Maximum Allowable Stress
•    Design

     -   Design Conditions and Loadings
     -   Weld Joint Efficiency and Corrosion Allowance
     -   Design for Internal Pressure
     -   Design for External Pressure and Compressive Stresses
     -   Reinforcement of Openings
     -   Flange Rating
     -   Flange Design
     -   Maximum Allowable Working Pressure
•    Other Design Considerations

     -   Vessel Support
     -   Local Loads



                                            65
    -   Vessel Internals
•   Fabrication

    -   Acceptable Welding Details
    -   Postweld Heat Treatment Requirements
•   Inspection and Testing
    -   Inspection
    -   Pressure Testing

This course is nominally only ½ day in length. Therefore, it cannot provide an in-
depth treatment of all aspects of pressure vessel design. However, the topics
are covered in sufficient depth to provide participants with a general
understanding of pressure vessel design requirements, to design pressure vessel
components to a limited extent, or to review pressure vessel designs prepared by
others. It also prepares individuals who require a more thorough understanding
of pressure vessel design to attend a more in-depth course or to acquire the
necessary knowledge on their own.




                                       66
II.   General
This section describes the various components of pressure vessels through the
use of conceptual drawings. It also describes the scope of the ASME Boiler and
Pressure Vessel Code Section VIII, and the basic structure of Section VIII,
Division 1.

A.    Main Pressure Vessel Components

      Pressure vessels are containers for fluids that are under pressure. They
      are used in a wide variety of industries (e.g., petroleum refining, chemical,
      power, pulp and paper, food, etc.)

      1.0    Shell

            The shell is the primary component that contains the pressure.
            Pressure vessel shells are welded together to form a structure that
            has a common rotational axis. Most pressure vessel shells are
            either cylindrical, spherical, or conical in shape.

             •   Figure 2.1 illustrates a typical horizontal drum. Horizontal
                 drums have cylindrical shells and are fabricated in a wide range
                 of diameters and lengths.
             •   Figure 2.2 illustrates a small vertical drum. Small vertical drums
                 are normally located at grade. The maximum shell length-to-
                 diameter ratio for a small vertical drum is about 5:1.
             •   Figure 2.3 illustrates a typical tall, vertical tower. Tall vertical
                 towers are constructed in a wide range of shell diameters and
                 heights. Towers can be relatively small in diameter and very tall
                 (e.g., a 4 ft. diameter and 200 ft. tall distillation column), or very
                 large in diameter and moderately tall (e.g., a 30 ft. diameter and
                 150 ft. tall pipestill tower).

                 A tower typically contains internal trays in the cylindrical shell
                 section. These internal trays (noted in Figure 2.3) are needed
                 for flow distribution. Several types of tower trays are available,
                 such as the bubble -cap, valve, sieve, and packed. The
                 particular type of tray used depends on the specific design
                 conditions and process application.

                 The shell sections of a tall tower may be constructed of different
                 materials, thicknesses, and diameters. This is because
                 temperature and phase changes of the process fluid – two of


                                         67
          the factors that affect the corrosiveness of the process fluid -
          vary along the tower’s length. Alloy material or a corrosion-
          resistant lining are sometimes used in sections of a vertical
          tower where corrosion is a critical factor.

      •   Figure 2.4 illustrates a typical reactor vessel with a cylindrical
          shell. The process fluid undergoes a chemical reaction inside a
          reactor. This reaction is normally facilitated by the presence of
          catalyst which is held in one or more catalyst beds.

2.0   Head

      All pressure vessel shells must be closed at the ends by heads (or
      another shell section). Heads are typically curved rather than flat.
      Curved configurations are stronger and allow the heads to be
      thinner, lighter, and less expensive than flat heads. Figures 2.1
      through 2.4 show heads closing the cylindrical sections of the
      subject pressure vessels. Heads can also be used inside a vessel.
      These “intermediate heads” separate sections of the pressure
      vessel to permit different design conditions in each section.




                                 68
                                Nozzle




                                  A
                                         Shell




Head                                                              Head




       Saddle Support                            Saddle Support
          (Sliding)                                 (Fixed)


                                  A




                              Section A-A




                  Horizontal Drum on Saddle Supports
                               Figure 2.1




                                  69
    Head



Shell                          Nozzle




                                        Head




                                    Support
                                    Leg




        Vertical Drum on Leg Supports
                   Figure 2.2




                     70
         Nozzle
                       Head


  Trays
                       Shell



   Nozzle




                         Cone




                              Nozzle

    Shell




Nozzle                           Head
                         Skirt
                         Support



         Tall Vertical Tower
              Figure 2.3




                  71
                                                    Inlet
                                                    Nozzle
                        Head




                     Upper
                    Catalyst
                       Bed                          Shell



             Catalyst Bed
             Support Grid




                     Lower
                    Catalyst
                       Bed
                                                      Outlet
                                                      Collector

                       Head

                                                    Outlet
                                                    Nozzle
          Support
            Skirt

                               Vertical Reactor
                                  Figure 2.4

3.0   Nozzle

      A nozzle is a cylindrical component that penetrates the shell or
      heads of a pressure vessel. The nozzle ends are usually flanged to
      allow for the necessary connections and to permit easy disassembly
      for maintenance or access. Nozzles are used for the following
      applications:

      •   Attach piping for flow into or out of the vessel.
      •   Attach instrument connections, (e.g., level gauges, thermowells,
          or pressure gauges).


                                      72
      •   Provide access to the vessel interior at manways.
      •   Provide for direct attachment of other equipment items, (e.g., a
          heat exchanger or mixer).

      Nozzles are also sometimes extended into the vessel interior for
      some applications, such as for inlet flow distribution or to permit the
      entry of thermowells.

      Figure 2.5 shows a pressurized storage vessel with a spherical
      shell.

                                                       Shell




                                                             Support
                                                             Leg




                                                   Cross
                                                   Bracing

              Spherical Pressurized Storage Vessel
                           Figure 2.5

4.0   Support

      The type of support that is used depends primarily on the size and
      orientation of the pressure vessel. In all cases, the pressure vessel
      support must be adequate for the applied weight, wind, a nd
      earthquake loads. The design pressure of the vessel is not a
      consideration in the design of the support since the support is not


                                  73
pressurized. Temperature may be a consideration in support design
from the standpoint of material selection and provision for differential
thermal expansion.
4.1 Saddle Supports

    Horizontal drums (See Figure 2.1) are typically supported at two
    locations by saddle supports. A saddle support spreads the
    weight load over a large area of the shell to prevent an
    excessive local stress in the shell at the support points. The
    width of the saddle, among other design details, is determined
    by the specific size and design conditions of the pressure
    vessel. One saddle support is normally fixed or anchored to its
    foundation. The other support is normally free to permit
    unrestrained longitudinal thermal expansion of the drum.
4.2 Leg Supports

    Small vertical drums (See Figure 2.2) are typically supported on
    legs that are welded to the lower portion of the shell. The
    maximum ratio of support leg length to drum diameter is
    typically 2:1. Reinforcing pads and/or rings are first welded to
    the shell to provide additional local reinforcement and load
    distribution in cases where the local shell stresses may be
    excessive. The number of legs needed depends on the drum
    size and the loads to be carried. Support legs are also typically
    used for spherical pressurized storage vessels (See Figure 2.5).
    The support legs for small vertical drums and spherical
    pressurized storage vessels may be made from structural steel
    columns or pipe sections, whichever provides a more efficient
    design. Cross bracing between the legs, as shown in Figure
    2.5, is typically used to help absorb wind or earthquake loads.
4.3 Lug Supports

    Lugs that are welded to the pressure vessel shell (See Figure
    2.6) may also be used to support vertical pressure vessels. The
    use of lugs is typically limited to vessels of small to medium
    diameter (1 to 10 ft.) and moderate height-to-diameter ratios in
    the range of 2:1 to 5:1. Lug supports are often used for vessels
    of this size that are located above grade within structural steel.
    The lugs are typically bolted to horizontal structural members to
    provide stability against overturning loads; however, the bolt
    holes are often slotted to permit free radial thermal expansion of
    the drum.




                            74
           4.4 Skirt Supports

               Tall, vertical, cylindrical pressure vessels (e.g., the tower and
               reactor shown in Figures 2.3 and 2.4 respectively) are typically
               supported by skirts. A support skirt is a cylindrical shell section
               that is welded either to the lower portion of the vessel shell or to
               the bottom head (for cylindrical vessels). Skirts for spherical
               vessels are welded to the vessel near the mid-plane of the shell.
               It is normally not necessary for the skirt bolt holes to be slotted
               (as with lug supports). The skirt is normally long enough to
               provide enough flexibility so that radial thermal expansion of the
               shell does not cause high thermal stresses at its junction with
               the skirt.




                      Vertical Vessel on Lug Supports
                                 Figure 2.6

B.   Scope of the ASME Code Section VIII

     Pressure vessels are typically designed in accordance with the ASME
     Code Section VIII, even for locations outside the US. Section VIII is
     divided into three divisions: Division 1, Division 2, and Division 3. Division
     1 is used most often since it contains sufficient requirements for the
     majority of pressure vessel applications.




                                       75
The main objective of ASME Code rules is to establish the minimum
requirements that are necessary for safe construction and operation. The
ASME Code protects the public by defining the material, design,
fabrication, inspection, and testing requirements that are needed to
achieve a safe design. Experience has shown that the probability of a
catastrophic pressure vessel failure is reduced to an acceptable level by
use of the ASME Code.

The ASME Code is written to apply to many industries. Accordingly, it
cannot anticipate and address every possible design requirement or
service application. Therefo re, users must supplement the ASME Code
by specifying additional requirements that are appropriate for their
particular industry and applications.

1.0   Division 1

      The ASME Code Section VIII, Division 1 applies for pressures that
      exceed 15 psig and through 3,000 psig. At pressures below 15 psig,
      the ASME Code is not applicable. At pressures above 3,000 psig,
      additional design rules are required to cover the design and
      construction requirements that are needed at such high pressures.

      The ASME Code is not applicable for piping system components
      that are attached to pressure vessels. Therefore, at pressure vessel
      nozzles, ASME Code rules apply only through the first junction that
      connects to the pipe. This junction may be at the following
      locations:

      •   Welded end connection through the first circumferential joint.

      •   First threaded joint for screwed connections.
      •   Face of the first flange for bolted, flanged connections.

      •   First sealing surface for proprietary connections or fittings.

      The Code also does not apply to no n pressure-containing parts that
      are welded, or not welded, to pressure-containing parts. However,
      the weld that makes the attachment to the pressure part must meet
      Code rules. Therefore, items such as pressure vessel internal
      components or external supports do not need to follow Code rules,
      except for any attachment weld to the vessel.

      The ASME Code identifies several other specific items where it does
      not apply. These include:



                                  76
      •   Fired process tubular heaters (e.g., furnaces).
      •   Pressure containers that are integral parts mechanical devices
          (e.g., pump, turbine, or compressor casings).
      •   Piping systems and their components.

      Note that all detailed design requirements discussed in this course
      are based on Division 1. Refer to Divisions 2 and 3 for comparable
      information in those documents

2.0   Division 2, Alternative Rules

      The scope of Division 2 is identical to that of Division 1; however,
      Division 2 contains requirements that differ from those that are
      contained in Division 1. Several areas where the requirements
      between the two divisions differ are highlighted below.

      •   Stress. The maximum allowable primary membrane stress for
          a Division 2 pressure vessel is higher than that of a Division 1
          pressure vessel. The Division 2 vessel is thinner and uses less
          material. A Division 2 vessel compensates for the higher
          allowable primary membrane stress by being a more stringent
          than Division 1 in other respects.
      •   Stress Calculations. Division 2 uses a complex method of
          formulas, charts, and design by analysis that results in more
          precise stress calculations than are required in Division 1.
      •   Design. Some design details are not permitted in Division 2
          that are allowed in Division 1.
      •   Quality Control. Material quality control is more stringent in
          Division 2 than in Division 1.
      •   Fabrication and Inspection. Division 2 has more stringent
          requirements than Division 1.

      The choice between using Division 1 and Division 2 is based on
      economics. The areas where Division 2 is more conservative than
      Division 1 add to the cost of a vessel. The lower costs that are
      associated with the use of less material (because of the higher
      allowable membrane stress) must exceed the increased costs that
      are associated with the more conservative Division 2 requirements
      in order for the Division 2 design to be economically attractive.




                                  77
             A Division 2 design is more likely to be attractive for vessels that
             require greater wall thickness, typically over approximately 2 in.
             thick. The thickness break point is lower for more expensive alloy
             material than for plain carbon steel, and will also be influenced by
             current market conditions. A Division 2 design will also be attractive
             for very large pressure vessels where a slight reduction in required
             thickness will greatly reduce shipping weights and foundation load
             design requirements.

     3.0      Division 3, Alternative Rules For Construction of High
              Pressure Vessels

             Division 3 applies to the design, fabrication, inspection, testing, and
             certification of unfired or fired pressure vessels operating at internal
             or external pressures generally above 10,000 psi. This pressure
             may be obtained from an external source, a process reaction, by the
             application of heat, or any combination thereof. Division 3 does not
             establish maximum pressure limits for either Divisions 1 or 2, nor
             minimum pressure limits for Division 3.

C.   Structure of Section VIII, Division 1

     The ASME Code, Section VIII, Division 1, is divided into three subsections
     as follows:

     •     Subsection A consists of Part UG, the general requirements that
           apply to all pressure vessels, regardless of fabrication method or
           material.
     •     Subsection B covers requirements that apply to various fabrication
           methods. Subsection B consists of Parts UW, UF, and UB that deal
           with welded, forged, and brazed fabrication methods, respectively.
     •     Subsection C covers requirements that apply to several classes of
           materials. Subsection C consists of Parts UCS (carbon and low-alloy
           steel), UNF (nonferrous metals), UHA (high-alloy steel), UCI (cast
           iron), UCL (clad and lined material), UCD (cast ductile iron), UHT
           (ferritic steel with properties enhanced by heat treatment), ULW
           (layered construction), and ULT (low-temperature materials).

     Division 1 also contains the following appendices:

     •     Mandatory Appendices address subjects that are not covered
           elsewhere in the Code. The requirements that are contained in these
           appendices are mandatory when the subject that is covered is included



                                         78
           in the pressure vessel under consideration. Examples of Mandatory
           Appendices are:
            -    Supplementary Design Formulas
            -    Rules for Bolted Flange Connections with Ring Type Gaskets
            -    Vessels of Noncircular Cross Section
            -    Design Rules for Clamped Connections
       •   Nonmandatory Appendices provide information and suggested good
           practices. The use of these nonmandatory appendices is not required
           unless their use is specified in the vessel purchase order. Examples of
           nonmandatory appendices are:
            -    Basis for Establishing Allowable Loads for Tube-to-Tubesheet
                 Joints
            -    Suggested Good Practice Regarding Internal Structures
            -    Rules for the Design of Tubesheets
            -    Flanged and Flued or Flanged Only Expansion Joints
            -    Half-Pipe Jackets

III.   Materials of Construction
This section discusses the primary factors that influence material selection for
pressure vessels and the maximum allowable material stresses specified by the
ASME Code. The mechanical design of a pressure vessel can proceed only
after the materials have been specified. The ASME Code does not state what
materials must be used in each application. It specifies what materials may be
used for ASME Code vessels, plus rules and limitations on their use. But, it is up
to the end user to specify the appropriate materials for each application
considering various material selection factors in conjunction with ASME Code
requirements.

A.     Material Selection Factors

       The main factors that influence material selection are:

       •   Strength

       •   Corrosion Resistance
       •   Resistance to Hydrogen Attack

       •   Fracture Toughness


                                        79
•     Fabricability

Other factors that influence material selection are cost, availability, and
ease of maintenance.

1.0      Strength

        Strength is a material's ability to withstand an imposed force or
        stress. Strength is a significant factor in the material selection for a
        particular application. Strength determines how thick a component
        must be to withstand the imposed loads.

        The overall strength of a material is determined by its yield strength,
        ultimate tensile strength, creep and rupture strengths. These
        strength properties depend on the chemical composition of the
        material. Creep resistance (a measure of material strength at
        elevated temperature) is increased by the addition of alloying
        elements such as chromium, molybdenum, and/or nickel to carbon
        steel. Therefore, alloy materials are often used in elevated
        temperature applications.

2.0      Corrosion Resistance

        Corrosion is the deterioration of metals by chemical action. A
        material's resistance to corrosion is probably the most important
        factor that influences its selection for a specific application. The
        most common method that is used to address corrosion in pressure
        vessels is to specify a corrosion allowance. A corrosion allowance
        is supplemental metal thickness that is added to the minimum
        thickness that is required to resist the applied loads. This added
        thickness compensates for thinning (i.e., corrosion) that will take
        place during service.

        The corrosion resistance of carbon steel could be increased through
        the addition of alloying elements such as chromium, molybdenum,
        or nickel. Alloy materials, rather than carbon steel, are often used in
        applications where increased corrosion resistance is required in
        order to minimize the necessary corrosion allowance.

3.0      Resistance to Hydrogen Attack

        At temperatures from approximately 300°F to 400°F, monatomic
        hydrogen diffuses into voids that are normally present in steel. In
        these voids, the monatomic hydrogen forms molecular hydrogen,
        which cannot diffuse out of the steel. If this hydrogen diffusion


                                    80
      continues, pressure can build to high levels within the steel, and the
      steel can crack.

      At elevated temperatures, over approximately 600°F, monatomic
      hydrogen not only causes cracks to form but also attacks the steel.
      Hydrogen attack differs from corrosion in that damage occurs
      throughout the thickness of the component, rather than just at its
      surface, and occurs without any metal loss. In addition, once
      hydrogen attack has occurred, the metal cannot be repaired and
      must be replaced. Thus, it is not practical to provide a corrosion
      allowance to allow for hydrogen attack. Instead, materials are
      selected such that they are resistant to hydrogen attack at the
      specified design conditions.

      Hydrogen attack is a potential design factor at hydrogen partial
      pressures above approximately 100 psia. Material selection for
      these hydrogen service applications is based on API 941, Steels for
      Hydrogen Service at Elevated Temperatures and Pressures in
      Petroleum Refineries and Petrochemical Plants. API 941 contains a
      family of design curves (the Nelson Curves) that are used to select
      appropriate material based on hydrogen partial pressure and design
      temperature.

4.0   Fracture Toughness

      Fracture toughness refers to the ability of a material to withstand
      conditions that could cause a brittle fracture. The fracture toughness
      of a material can be determined by the magnitude of the impact
      energy that is required to fracture a specimen using Charpy V-notch
      test. Generally speaking, the fracture toughness of a material
      decreases as the temperature decreases (i.e., it behaves more like
      glass). The fracture toughness at a given temperature varies with
      different steels and with different manufacturing and fabrication
      processes.

      Material selection must confirm that the material has adequate
      fracture toughness at the lowest expected metal temperature. It is
      especially important for material selection to eliminate the risk of
      brittle fracture since a brittle fracture is catastrophic in nature. It
      occurs without warning the first time the necessary combination of
      critical size defect, low enough temperature, and high enough stress
      occurs.




                                 81
4.1 ASME Code and Brittle-Fracture Evaluation

   The following pressure vessel components must be considered
   in brittle fracture evaluations:

     •   Shells

     •   Manways
     •   Heads

     •   Reinforcing pads
     •   Nozzles

     •   Tubesheets
     •   Flanges

     •   Flat cover plates
     •   Backing strips that remain in place
     •   Attachments that are essential to the structural integrity of the
         vessel when welded to pressure-containing components
         (e.g., vessel supports)

   The Minimum Design Metal Temperature (MDMT) is the lowest
   temperature at which the component is designed to have
   adequate fracture toughness. It is a function of the component’s
   material specification and thickness. The Critical Exposure
   Temperature (CET) is the minimum metal temperature that can
   occur at the same time as a significant membrane stress in the
   vessel (e.g., at a pressure that is greater than 25% of the design
   pressure). The CET is determined by either ambient conditions
   or process conditions, whichever results in the lowest metal
   temperature. While the terms MDMT and CET are often used
   interchangeably, they are separate parameters.

   Each component must be evaluated separately for impact test
   requirements based on its material, thickness, and MDMT. In
   all cases, the MDMT must be no greater than the CET.

   Division 1 contains a simplified approach to evaluate the
   potential for brittle fracture in carbon and low-alloy steel.
   Material specifications are classified within Material Groups A
   through D for the purpose of brittle fracture evaluation (See
   Table 3.1, excerpted from Figure UCS-66 of Division 1). The
   Code contains exemption curves for these Material Groups that


                             82
identify the acceptable MDMT versus thickness (0 in. through 6
in.) where impact testing (Charpy V-notch) is not required. The
curves shown in Figure 3.1 are excerpted from Figure UCS-66.
If the design conditions do not permit exemption in accordance
with this basis, then material impact testing at the specified CET
is required to permit its use. The Code specifies the necessary
impact test procedure and acceptance criteria.




                       83
Material Group Applicable Materials
    Curve A         •   All carbon and low alloy steel plates, structural shapes, and bars not listed in Curves B, C,
                        and D.
                    •   SA-216 Gr. WCB and WCC, SA-217 Gr. WC6, if normalized and tempered or water-quenched and
                        tempered.
     Curve B        •   SA-216 Gr. WCA if normalized and tempered or water-quenched and tempered
                    •   SA-216 Gr. WCB and WCC for maximum thickness of 2 in., if produced to fine grain practice and
                        water-quenched and tempered
                    •   SA-217 Gr. WC9 if normalized and tempered
                    •   SA-285 Gr. A and B
                    •   SA-414 Gr. A
                    •   SA-515 Gr. 60
                    •   SA-516 Gr. 65 and 70 if not normalized
                    •   SA-612 if not normalized
                    •   SA-662 Gr. B if not normalized
                    •   Except for cast steels, all materials of Curve A if produced to fine grain practice and normalized which
                        are not included in Curves C and D
                    •   All pipe, fittings, forgings, and tubing which are not included in Curves C and D
                    •   Parts permitted under Para. UG-11 shall be included in Curve B even when fabricated from plate that
                        otherwise would be assigned to a different curve
     Curve C        •   SA-182 Gr. 21 and 22 if normalized and tempered
                    •   SA-302 Gr. C and D
                    •   SA-336 Gr. F21 and F22 if normalized and tempered
                    •   SA-387 Gr. 21 and 22 if normalized and tempered
                    •   SA-516 Gr. 55 and 60 if not normalized
                    •   SA-533 Gr. B and C
                    •   SA-662 Gr. A
                    •   All material of Curve B if produced to fine grain practice and normalized which are not included in
                        Curve D
    Curve D         •   SA-203
                    •   SA-508 Cl. 1
                    •   SA-516 if normalized
                    •   SA-524 Cl. 1 and 2
                    •   SA-537 Cl. 1, 2, and 3
                    •   SA-612 if normalized
                    •   SA-662 if normalized
                    •   SA-738 Gr. A
 Bolting and Nuts   See Figure UCS-66 of Division 1for impact test exemption temperatures for specified material
                    specifications.
                           Material Groups for Impact Test Exemptions
                                            Table 3.1




                                                            84
                                          140

                                          120

                                          100
Minimum Design Metal Temperature, F

                                                            A                           B
                                          80

                                          60
                                                                                            C
                                          40
                                                                                                D
                                          20

                                           0

                                          -20

                                          -40
                                          -55
                                          -60
                                                                   Impact testing required

                                          -80
                                                0.394   1               2           3               4        5

                                                                         Nominal Thickness, in.
                                                                (Limited to 4 in. for Welded Construction)


                                       Impact Test Exemption Curves for Carbon Steels
                                                         Figure 3.1

                                      A capital letter that designates the corresponding Material Group
                                      appears above each curve in Figure 3.1. If the CET of a pressure
                                      vessel is equal to or above that shown by the intersection of the
                                      Material Group curve and component thickness, then impact testing
                                      is not required. For example, a Group B material that is 1.5 in. thick
                                      does not require impact testing as long as the CET of the vessel is
                                      approximately 50°F or higher.

                                      Division 1 has additional impact test requirements, some of which
                                      are highlighted below. Refer to the code for additional information.

                                      •    Impact testing is required for all welded construction that is over
                                           4 in. thick if the MDMT is below 120°F.
                                      •    Impact testing is required for non-welded construction (e.g., a
                                           seamless, bolted heat exchanger cover plate) if the component
                                           is over 6 in. thick and the MDMT is below 120°F.


                                                                            85
      •   Impact testing is not required for ASME B16.5 or B16.47 ferritic
          steel flanges if the design metal temperature is no colder than -
          20°F.

      •   Unless specifically exempt by Fig. UCS-66, materials with a
          minimum yield strength greater than 65 ksi must be impact
          tested.

      •   Low temperature grades of steel that are impact tested to
          conform to the particular material specification (e.g., SA-333 or
          SA-350) may be used at design metal temperatures as low as
          the impact test temperature.
      •   If PWHT is done on P-1 material when it is not required by
          ASME Code rules, its impact test exemption temperature may
          be reduced by 30°F from that provided in Fig. UCS-66 (Ref.
          Para. UCS-68), as long as the resulting exemption temperature
          is no lower than -55°F. This recognizes the fact that a material’s
          fracture toughness is improved after stress relief.
      •   The MDMT of a vessel component may be further reduced if the
          general primary membrane stress in the vessel component is
          less than the design allowable stress. This could occur in
          situations where the nominal thickness of the component is
          greater than that required for the design conditions plus
          corrosion allowance (Ref. Fig. UCS-66.1).

      Division 1 also contains impact-testing procedures and impact-
      energy requirements for cases that are subject to impact testing.
      Refer to Division 1 for details.

5.0   Fabricability

      Fabricability refers to the ease of construction and to any special
      fabrication practices that are required to use the material. Of special
      importance is the ease with which the material can be rolled or
      otherwise shaped to conform to vessel component geometry
      requirements.

      Pressure vessels commonly use welded construction. Therefore, the
      materials used must be weldable so that individual components can
      be assembled into the completed vessel. The material chemistry of
      the weld area must be equivalent to that of the base material so that
      the material properties and corrosion resistance of the weld area will
      be the same as those of the base material.




                                 86
B.   Maximum Allowable Stress

     One of the major factors in the design of pressure vessels is the
     relationship between the strength of the components and the loads (i.e.,
     pressure, weight, etc.) imposed upon them. These loads cause internal
     stresses in the components. The design of a pressure vessel must ensure
     that these internal stresses never exceed the strength of the vessel
     components.

     Pressure vessel components are designed such that the component
     stresses that are caused by the loads are limited to maximum allowable
     values that will ensure safe operation. Maximum allowable stress is the
     maximum stress that may be safely applied to a pressure vessel
     component. The maximum allowable stress includes a safety margin
     between the stress level in a component due to the applied loads and the
     stress level that could cause a failure.

     1.0   Maximum Allowable Stress Criteria

           The ASME Code Section II, Part D, Appendix 1 discusses the basis
           used to establish maximum allowable stress values for materials
           other than bolting for Division 1 vessels. A similar discussion is
           contained in Section II, Part D, Appendix 2 for bolting, and Section
           VIII, Division 1, Appendix P for low-temperature, cast or ductile iron
           materials. Refer to these appendices for the specific safety margins
           and other considerations used in determining the maximum
           allowable stresses.

           Two sets of allowable stress values are provided in Division 1 for
           austenitic materials and for specific non-ferrous alloys. The higher
           alternative allowable stresses exceed two-thirds but do not exceed
           90% of the minimum yield strength of the material at temperature.
           The higher allowable stress values should be used only where
           slightly higher deformation of the component is not in itself
           objectionable (e.g., for shell and head sections). These higher
           allowable stresses are not recommended for the design of flanges or
           other strain-sensitive applications. In the case of flanges, for
           example, the larger deformation that would be expected if the higher
           allowable stresses were used could cause flange leakage problems
           even though a major flange failure would not occur.

     2.0   ASME Maximum Allowable Stress Tables

           Tables in the ASME Code Section II, Part D contain the maximum
           allowable tensile stresses of materials that are acceptable for use in


                                      87
ASME Code Section VIII pressure vessels. The maximum allowable
stress varies with temperature because material strength is a
function of temperature.

Figure 3.2 (adapted from Table 1A of the ASME Code Section II,
Part D) shows examples of maximum allowable Division 1 tensile
stress for three different material specifications.

The first part of Figure 3.2 identifies the Spec. No. (i.e., material
specification number), the grade (a material specification may have
multiple strength grades), the nominal chemical composition, the P-
No. and Group No., and the minimum yield and tensile strengths in
thousands of pounds per square inch (ksi). This first part of Figure
3.2 also helps identify similarities that may exist among the material
specifications (e.g., nominal alloy composition, yield strength, and
tensile strength). In some cases, these similarities may help select
the material to use for pressure vessel fabrication, given specific
process conditions. The maximum allowable stress va lues as a
function of temperature are presented in the second part of Figure
3.2.

The information that is contained in the ASME Code Table 1A has
been condensed and reorganized in Figure 3.2 in two parts to help
Participants compare the material types and to note variances in
maximum allowable stress that are determined by temperature and
alloy composition.




                           88
                   ALLOWABLE STRESS IN TENSION FOR CARBON AND
                                    LOW-ALLOY STEEL
 Spec No.   Grade           Nominal       P-No.   Group No. Min. Yield   Min. Tensile
                          Composition                          (ksi)         (ksi)
     Carbon Steel Plates and Sheets
SA-515        55              C-Si          1         1         30           55
              60              C-Si          1         1         32           60
              65              C-Si          1         1         35           65
              70              C-Si          1         2         38           70

SA-516        55             C-Si             1      1          30           55
              60           C-Mn-Si            1      1          32           60
              65           C-Mn-Si            1      1          35           65
              70           C-Mn-Si            1      2          38           70

         Plate - Low Alloy Steels
SA-387        2 Cl.1       ½ Cr-½ Mo          3      1          33           55
              2 Cl.2       ½ Cr-½ Mo          3      2          45           70
             12 Cl.1        1Cr-½ Mo          4      1          33           55
             12 Cl.2        1Cr-½ Mo          4      1          40           65
             11 Cl.1     1 ¼ Cr-½Mo-Si        4      1          35           60
             11 Cl.2     1 ¼ Cr-½Mo-Si        4      1          45           75
             22 Cl.1       2 ¼ Cr-1Mo         5      1          30           60
             22 Cl.2       2 ¼ Cr-1Mo         5      1          45           75

            ASME Maximum Allowable Stress (Table 1A Excerpt)
                             Figure 3.2




                                         89
         ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL
               Max Allowable Stress, ksi (Multiply by 1,000 to Obtain psi)
                       for Metal Temperature, °F, Not Exceeding
                                                                                       Spec
650    700    750    800    850    900    950    1000   1050 1100 1150 1200             No.
                                                            Carbon Steel Plates and Sheets
13.8   13.3   12.1   10.2   8.4    6.5    4.5    2.5     --      --      --      --   SA-515
15.0   14.4   13.0   10.8   8.7    6.5    4.5    2.5     --      --      --      --   SA-515
16.3   15.5   13.9   11.4   9.0    6.5    4.5    2.5     --      --      --      --   SA-515
17.5   16.6   14.8   12.0   9.3    6.5    4.5    2.5     --      --      --      --   SA-515

13.8   13.3   12.1   10.2   8.4    6.5    4.5    2.5     --     --      --     --    SA-516
15.0   14.4   13.0   10.8   8.7    6.5    4.5    2.5     --     --      --     --    SA-516
16.3   15.5   13.9   11.4   9.0    6.5    4.5    2.5     --     --      --     --    SA-516
17.5   16.6   14.8   12.0   9.3    6.5    4.5    2.5     --     --      --     --    SA-516

                                                            Plate-Low Alloy Steels (Cont'd)
13.8   13.8   13.8   13.8   13.8   13.3    9.2   5.9     --       --     --      --   SA-387
17.5   17.5   17.5   17.5   17.5   16.9    9.2   5.9     --       --     --      --   SA-387
13.8   13.8   13.8   13.8   13.4   12.9   11.3   7.2    4.5      2.8    1.8     1.1   SA-387
16.3   16.3   16.3   16.3   15.8   15.2   11.3   7.2    4.5      2.8    1.8     1.1   SA-387
15.0   15.0   15.0   15.0   14.6   13.7    9.3   6.3    4.2      2.8    1.9     1.2   SA-387
18.8   18.8   18.8   18.8   18.3   13.7    9.3   6.3    4.2      2.8    1.9     1.2   SA-387
15.0   15.0   15.0   15.0   14.4   13.6   10.8   8.0    5.7      3.8    2.4     1.4   SA-387
17.7   17.2   17.2   16.9   16.4   15.8   11.4   7.8    5.1      3.2    2.0     1.2   SA-387

              ASME Maximum Allowable Stress (Excerpt), cont'd
                           Figure 3.2, cont'd

       Note that the allowable stresses at temperatures between
       -20°F and 650°F are the same as the allowable stress at 650°F for each
       material presented in Figure 3.2 (except for SA-387, Grade 22 Cl. 2). The
       allowable stress increases for SA–387, Grade 22 Cl. 2 material at
       temperatures below 650°F to a maximum of 18.8 ksi at 100°F and below.
       Note that each material specification has different Types, Grades, and/or
       Classes within it. In some cases, these differences are due to different
       chemical compositions, while in other cases they may be due to the
       particular steel making process that is employed. Higher strength grades
       of a particular material specification have higher maximum allowable
       stresses.




                                           90
Exercise 1
Material Selection Based on Fracture Toughness
A new horizontal pressure vessel is being designed for an application where the
CET is -2°F. The material being used for the shell and heads is SA-516 Gr. 70
plate. The heads are hemispherical in shape and are ½ in. thick. The cylindrical
shell is 1.0 in. thick. The supplier has not specified any impact testing for the
shell and head plate. Is this correct? If this is not correct, what should be done
to correct the situation?




                                        91
IV.   Design

A.    Design Conditions and Loadings

      The mechanical design of a pressure vessel begins with specification of
      the design pressure and design temperature. Pressure imposes loads
      that must be withstood by the individual vessel components. Temperature
      affects material strength and, thus, its allowable stress, regardless of the
      design pressure. Some pressure vessels have multiple sets of design
      conditions that correspond to different modes of operation. For example,
      during its operating cycle, a reactor may have a high pressure and
      moderate temperature during normal operation, but it may operate at a
      much lower pressure and a very high temperature during catalyst
      regeneration. Both sets of design conditions must be specified because
      either one or the other may govern the mechanical design.

      All pressure vessels must be designed for the most severe conditions of
      coincident pressure and temperature that are expected during normal
      service. Normal service includes conditions that are associated with:

      •   Startup.

      •   Normal operation.
      •   Deviations from normal operation that can be anticipated (e.g., catalyst
          regeneration or process upsets).
      •   Shutdown.

      Pressure vessels must also be designed for other loading conditions and
      service factors that may apply in particular situations. These are
      highlighted later.




                                       92
1.0   Pressure
      1.1   Operating Pressure

            The operating pressure must be set based on the maximum
            internal or external pressure that the pressure vessel may
            encounter. The following factors must be considered:

            •     Ambient temperature effects.
            •     Normal operational variations.

            •     Pressure variations due to changes in the vapor pressure
                  of the contained fluid.

            •     Pump or compressor shut-off pressure.
            •     Static head due to the liquid level in the vessel.

            •     System pressure drop.
            •     Normal pre-startup activities or other operating conditions
                  that may occur (e.g., vacuum), that should be considered
                  in the design.
      1.2       Design Pressure

            Generally, design pressure is the maximum internal pressure (in
            psig), that is used in the mechanical design of a pressure
            vessel. For full or partial vacuum conditions, the design
            pressure is applied externally and is the maximum pressure
            difference that can occur between the atmosphere and the
            inside of the pressure vessel. Some pressure vessels may
            experience both internal and external pressure conditions at
            different times during their operation. The mechanical design of
            the pressure vessel in this case is based on which of these is
            the more severe design condition.

            The specified design pressure is based on the maximum
            operating pressure at the top of the vessel, plus the margin that
            the process design engineer determines is suitable for the
            particular application. A suitable margin must also be provided
            between the maximum operating pressure and the safety relief
            valve set pressure. This margin is necessary to prevent
            frequent and unnecessary opening of the safety relief valve that
            may occur during normal variations in operating pressure. The
            safety relief valve set pressure is normally set equal to the
            design pressure.



                                   93
Pressure vessels, especially tall towers, may have liquid in them
during normal operation. The maximum height of this liquid
normally does not reach the top of the vessel. The liquid level
that is required for design is specified by the process design
engineer.

The hydrostatic pressure that is exerted by the liquid must be
considered in the design of vessel components upon which it
acts. Therefore, the pressure that is used to design a vessel
component is equal to the design pressure at the top of the
vessel, plus the hydrostatic pressure of the liquid in the vessel
that is above the point being designed (i.e., P BH = P T + γH). See
Figure 4.1.




                       94
                                                  PT = Design Pressure at
                                                       Top of Vessel




                                                γ = Weight Density of
                                                    Liquid in Vessel



      H = Height
          of Liquid




                                                PBH = Design Pressure of
                                                      Bottom Head

                            Design Pressure
                               Figure 4.1



2.0       Temperature
          2.1  Operating Temperature

              The operating temperature must be set based on the maximum
              and minimum metal temperatures that the pressure vessel may
              encounter. The operation and vertical length of tall towers, and
              the presence of liquid in the bottom section, sometime result in
              large temperature reductions between the bottom and top of the
              vessel. It is permissible to specify different operating


                                    95
      temperatures at different elevations of such a pressure vessel,
      as long as the temperatures can be accurately predicted. This
      approach results in dividing the vessel into sections along its
      vertical length. Each section is designed for the temperature
      that it will encounter, rather than for the most severe condition at
      the bottom of the vessel. Figure 4.2 illustrates this concept.




         Section 4
           (T-Z)




         Section 3
           (T-Y)




        Section 2
          (T-X)




        Section 1
         (T) F

                                           Support Skirt

                                            Grade

           Temperature Zones in Tall Vessels
                      Figure 4.2

2.2      Design Temperature

      The design temperature of a pressure vessel is the maximum
      fluid temperature that occurs under normal operating conditions,
      plus an allowance for variations that occur during operation.
2.3      Critical Exposure Temperature (CET)

      The CET must also be specified for pressure vessel design to
      ensure that materials that have adequate fracture toughness are
      selected for construction (i.e., MDMT ≤ CET). Fracture
      toughness was previously discussed.


                             96
     3.0   Other Loadings

           Paragraph UG-22 of Division 1 specifies the loadings that must be
           considered to determine the minimum required thicknesses for the
           various vessel components. These design loadings are as follows:

           •   Internal or external design pressure.
           •   Weight of the vessel and its normal contents under operating or
               test conditions.
           •   Superimposed static reactions from the weight of attached
               equipment (e.g., motors, machinery, other vessels, piping,
               linings, insulation).
           •   Loads at attached of internal components or vessel supports.

           •   Wind, snow, and seismic reactions.
           •   Cyclic and dynamic reactions that are caused by pressure or
               thermal variations, or by equipment that is mounted on a vessel,
               and mechanical loadings.
           •   Test pressure combined with hydrostatic weight.

           •   Impact reactions such as those that are caused by fluid shock.
           •   Temperature gradients within a vessel component and
               differential thermal expansion between vessel components.

B.   Weld Joint Efficiency and Corrosion Allowance

     The weld joint efficiency and corrosion allowance are additional design
     parameters that are required to calculate vessel component thicknesses.

     1.0   Weld Joint Efficiency

           Weld joint efficiency (E) accounts for the quality of a welded joint
           and for the concentration of local stress. This higher local stress is
           due to local material or structural discontinuities.

           Paragraph UW-12 of Division 1 specifies weld joint efficiencies to be
           used to calculate component thicknesses. Figure 4.3 identifies weld
           joint categories, Figure 4.4 identifies weld types, and Figure 4.5
           defines weld joint efficiencies based on the type of weld and degree
           of radiographic examination.




                                       97
               The majority of pressure vessel welds use a Type 1 joint design. A
               Type 1 joint has an efficiency of either 0.85 or 1.00, corresponding
               with spot or full radiographic examination, respectively.



                                                                            C           C
                               C
                                                                                        A
                                                            A                               C
                                               B
                                     A D

                                                                        D   A                   B
                                                        B                           D
                                           D                    B

                                                                                A
           B               A                                                    C
                                                    C               D

                               Weld Joint Categories
                                    Figure 4.3

     2.0       Corrosion Allowance

               Corrosion, erosion, or abrasion causes vessel components to thin
               during their operating life. To compensate for this thinning,
               components must have their thicknesses increased over those that
               are calculated using the ASME Code design formulas. Internal
               corrosion/erosion-resistant linings are sometimes used as an
               alternative to the use of greater component thicknesses.

               Process design and materials engineers typically specify the
               corrosion allowance. The corrosion allowance is based on the
               expected corrosion rate for the vessel material in the anticipated
               process environment. The corrosion rate is multiplied by the
               nominal design life of the vessel (normally 20 years) to determine
               the corrosion allowance.

C.   Design for Internal Pressure

     1.0       Cylindrical Shells

               The idealized equations for the calculation of hoop and longitudinal
               stresses, respectively, in a cylindrical shell under internal pressure
               are as follows:
                                          Pr          Pr
                                   σθ =      and σ1 =
                                           t          2t


                                               98
    These equations assume a uniform stress distribution through the
    thickness of the shell. Note that the longitudinal stress is half the
    hoop stress. Since this is an idealized state, the ASME Code
    formulas (See Figure 4.6) have been modified to account for non-
    ideal behavior.

                                        Butt joints as attained by double-welding or by other
1                                       means which will obtain the same quality of deposited
                                        weld metal on the inside and outside weld surface.

                                        Backing strip, if used, shall be removed after
                                        completion of weld.


                                        Single-welded butt joint with backing strip which
2                                       remains in place after welding.




          For circumferential
          joint only




3                                       Single-welded butt joint without backing strip.



4                                       Double-full fillet lap joint.



5                                       Single-full fillet lap joint with plug welds.


6                                       Single-full fillet lap joint without plug welds.




                         Types of Welded Joints
                               Figure 4.4




                                   99
Joint             Acceptable Joint Categories                 Degree of
Type                                                   Radiographic Examination
                                                     Full       Spot        None
 1      A, B, C, D                                   1.00       0.85         0.70
 2      A, B, C, D (See ASME Code for limitations)   0.90       0.80        0.65
 3      A, B, C                                      NA          NA         0.60
 4      A, B, C (See ASME Code for limitations)      NA          NA         0.55
 5      B, C (See ASME Code for limitations)         NA          NA         0.50
 6      A, B, (See ASME Code for limitations)        NA          NA         0.45
                           Maximum Weld Joint Efficiency
                                   Figure 4.5

             Longitudinal stress can govern the design of a cylindrical section
             when loadings other than internal pressure induce longitudinal
             stresses that are greater than one half of the hoop stress due to
             internal pressure. One example where this could occur is in the
             lower section of a ta ll tower where wind or earthquake loading could
             cause high longitudinal stresses. In these cases, the longitudinal
             stress that is due to these other loads is added to the longitudinal
             stress due to internal pressure. The total combined longitudinal
             stress is then limited to the maximum allowable stress.

             Figure 4.6 summarizes the ASME Code equations used to calculate
             the minimum required thickness for common pressure vessel
             components. The equations have also been rearranged to calculate
             pressure and stress as a function of thickness.




                                            100
                             Thickness,               Pressure,             Stress,
        Part                   tp, in.                  P, psi               S, psi

                                Pr                       SE1 t            P (r + 0.6t )
  Cylindrical shell
                            SE1 − 0.6P                 r + 0.6t                tE 1

                                Pr                      2SEt              P (r + 0.2t )
  Spherical shell
                            2SE1 − 0.2P                r + 0.2t               2tE
         2:1                   PD                      2SEt               P (D + 0.2t )
Semi -Elliptical head
                            2SE − 0 .2P               D + 0.2t                2tE
                             0.885PL                    SEt            P (0.885L + 0.1t )
Torispherical head
 with 6% knuckle             SE − 0.1P              0.885L + 0.1t             tE

                                PD                   2SEt cos α        P (D + 1.2t cos α )
                        2 cos α(SE − 0.6P )
  Conical Section
     (α = 30°)                                      D + 1.2t cos α         2tE cos α
                         Summary of ASME Code Equations
                                    Figure 4.6
          Where:

                 P      =       Internal design pressure, psig. When used in the
                                pressure calculation equations, this is the MAWP.
                 r      =       Internal radius, in. Add corrosion allowance to
                                specified uncorroded internal radius.
                 S      =       Allowable Stress, psi. When used in the thickness
                                calculation equations, this is the allowable stress for
                                the material used. When used in the stress
                                calculation equations, this is the calculated stress for
                                the given pressure and thickness.
                 E1, E =        Longitudinal weld joint efficiency
                 tp     =       Required wall thickness for internal pressure of the
                                part under consideration, in.
                 t      =       Actual wall thickness (less corrosion allowance) of the
                                part under consideration, in.
                 D      =       Inside diameter, in. Add twice the corrosion
                                allowance to specified uncorroded inside diameter.
                 DL     =       Cone inside diameter at large end, in. Add twice the
                                corrosion allowance to specified uncorroded inside
                                diameter.


                                              101
      DS     =      Cone inside diameter at small end, in. Add twice the
                    corrosion allowance to specified uncorroded inside
                    diameter.
      L      =      Inside crown radius of torispherical head, in. Add
                    corrosion allowance to specified uncorroded inside
                    crown radius.
      α      =      One half of the apex angle of the cone at the
                    centerline, degrees.

                                          0.5(D L − Ds )
                            α = tan −1
                                         (Cone Length)

2.0   Heads

      Figure 4.7 shows typical types of closure heads. Elliptical,
      hemispherical, and torispherical are the most commonly used head
      types. Note in Figure 4.7 that all head types but the conical have a
      straight flange (sf) section, which simplifies welding the head to the
      adjacent cylindrical shell section. The elliptical and torispherical
      heads have an indicated head depth (h), which is measured from
      the straight flange to the maximum point of curvature on the inside
      surface.




                                 102
                                                                       t


                       t

                                                          R
                                    sf
                                                                                 sf
                ID                                            ID
             Flanged                                   Hemispherical

                               t
                                                                       t
h
                                            h
                                     sf                                     sf

             Elliptical                            Flanged and Dished
                                                      (torispherical)




                   α       t                                   α       t




                                                                            sf
                                                                   r
                 ID                                           ID
             Conical                                    Toriconical


               Typical Formed Closure Heads
                         Figure 4.7

    As with shells, the internal head dimensions that are used to
    calculate the required thicknesses must first be increased to account
    for the corrosion allowance. The corrosion allowance must then be
    added to the calculated thicknesses. See Figure 4.6 for the ASME
    Code equations that are used to calculate the wall thickness of each
    head type.




                                   103
2.1   Elliptical Heads - The 2:1 semi-elliptical head is the most
      commonly used head type. Half of its minor axis (i.e., the inside
      depth of the head minus the length of the straight flange
      section) equals one-fourth of the inside diameter of the head.
      The thickness of this type of head is normally equal to the
      thickness of the cylinder to which it is attached.

2.2   Hemispherical Heads - The required thickness of a
      hemispherical head is normally one-half the thickness of an
      elliptical or torispherical head for the same design conditions,
      material, and diameter. Hemispherical heads are normally
      fabricated from segmented sections that are welded together,
      spun, or pressed. Hemispherical heads are an economical
      option to consider when expensive alloy material is used. In
      carbon steel construction, hemispherical heads are generally
      not as economical as elliptical or torispherical heads because of
      higher fabrication cost. Carbon steel hemispherical heads may
      be economical for thin, very large-diameter vessels, or in thick,
      small-diameter vessels.

      The thickness transition zone between the hemispherical head
      and shell must be contoured to minimize the effect of local
      stress. Figure 4.8 shows the thickness transition requirements
      that are contained in the ASME Code.

2.3   Torispherical Heads - A torispherical (or flanged and dished)
      head is typically somewhat flatter than an elliptical head and can
      be the same thickness as an elliptical head for identical design
      conditions and diameter. The minimum permitted knuckle
      radius of a torispherical head is 6% of the maximum inside
      crown radius. The maximum inside crown radius equals the
      outside diameter of the head.




                            104
                      th                                             th




                                             Thinner Part




                                                                                    Thinner Part
        l ≥ 3y
                                                            l ≥ 3y

                                        Tangent Line
            y                                                              y
                                  Length of required taper, l,
                                    may include the width
                                         of the weld
                                  ts                                           ts




 Thickness Transition Between Hemispherical Head and Shell
                         Figure 4.8

      2.4       Intermediate Heads – An intermediate head may be installed
                inside a pressure vessel to separate two sections that can have
                different design conditions. Most head types can be used as
                intermediate heads. Intermediate heads are evaluated for
                internal pressure in the same way as external heads.

3.0         Conical Sections

        Tall towers may have sections with different diameters along their
        length. The transition between the different diameters is made in a
        conical section. The most common design for a conical transition
        does not have formed knuckles at the ends of the cone. The
        cylindrical sections of different diameter are welded to each end of
        the cone. The required thickness for internal pressure of a conical
        shell without transition knuckles is calculated using the equation
        shown in Figure 4.6. This equation assumes that half of the cone-
        apex a ngle is no greater than 30°.

        Formed knuckles are sometimes used at the cone-to-cylinder
        transition in order to reduce localized stresses. When knuckles are
        used, the transition is called toriconical. The use of knuckles is

                                       105
mandatory when the cone half-apex angle exceeds 30°. Knuckles
are also sometimes used for smaller angles when there is concern
about potentially high local stresses at the cone -to-cylinder junction.
The ASME Code has design procedures for toriconical sections.




                           106
Sample Problem 1 – Design for Internal Pressure
The geometry and design data of a vertical cylindrical pressure vessel are
specified in Figure 4.9. Cost estimates are being prepared for this vessel. It is
your job to estimate the required component thicknesses.

A. What are the minimum required thicknesses for the two cylindrical sections?


              Hemispherical


                                                  DESIGN INFORMATION
                                                  Design Pressure = 250 psig
                                                  Design Temperature = 700° F
                                                  Shell and Head Material is SA-515
                                                   Gr. 60
                                                  Corrosion Allowance = 0.125"
                                  4' - 0"
                                                  Both Heads are Seamless
            60' - 0"                              Shell and Cone Welds are Double
                                                   Welded and will be Spot
                                                   Radiographed
                                                  The Vessel is in All Vapor Service
                                                  Cylinder Dimensions Shown are
                                                   Inside Diameters


            10' - 0"




                                 6' - 0"
            30' - 0"




     2:1 Semi-Elliptical




                               Sample Problem 1
                                  Figure 4.9




                                            107
Solution

1. The required wall thickness for internal pressure of a cylindrical shell is
      calculated using the following equation from Figure 4.6:

                                              Pr
                                   tp =
                                          SE1 − 0.6P

2. Since the welds are spot radiographed, E = 0.85 (from Figure 4.5)

3. S = 14,400 psi for SA-515/Gr. 60 at 700°F (from Figure 3.2)

4. P is given as 250 psig.

5. For the 6 ft. - 0 in. shell, calculate r (including corrosion allowance)

                    r = 0.5D + CA = 0.5 x 72 + 0.125 = 36.125 in.

                      Pr           250 × 36.125
           tp =             =                          = 0.747 in.
                  SE1 − 0.6P 14,400 × 0.85 − 0.6 × 250

                               t = tp + c = 0.747 + 0.125

                  t = 0.872 in. required including corrosion allowance

6. For the 4 ft. - 0 in. shell, calculate r (including corrosion allowance)

                            r = 0.5 x 48 + 0.125 = 24.125 in.

                                250 × 24.125
                   tp =                                  = 0.499 in.
                          14,400 × 0.85 − 0.6 × 250

                                   t = 0.499 + 0.125

               t = 0.624 in. required (including corrosion allowance)




                                           108
B.    For the same vessel, what are the minimum required thicknesses for the
      top and bottom heads?

Solution

1. Since both heads are seamless, E = 1.0.

2. Top Head - Hemispherical head (Equation from Figure 4.6)

                              r = 24 + 0.125 = 24.125 in.

                      Pr            250 × 24.125
           tp =              =                           = 0.21 in.
                  2SE1 − 0.2P 2 × 14,400 × 1 − 0.2 × 250

                               t = tp + c = 0.21 + 0.125

                  t = 0.335 in. required including corrosion allowance

3. Bottom Head - 2:1 Semi-Elliptical Head (Equation from Figure 4.6)

                            D = 72 + 2 x 0.125 = 72.25 in.

                     PD             250 × 72.25
           tp =             =                           = 0.628 in.
                  2SE − 0.2P 2 × 14,400 × 1 − 0.2 × 250

                                   t = 0.628 + 0.125

                  t = 0.753 in. required including corrosion allowance




                                          109
D.   Design for External Pressure and Compressive Stresses

     Pressure vessels are subject to compressive forces such as those caused
     by dead weight, wind, earthquake, and internal vacuum. Pressure vessel
     components behave differently under these compressive forces than when
     they are exposed to tensile forces (e.g., from internal pressure). This
     difference in behavior is due to elastic instability, which makes shells
     weaker in compression than in tension. In failure by elastic instability, the
     vessel is said to collapse or buckle. The paragraphs that follow discuss
     buckling of cylindrical shells due to external pressure. These basic
     principles also apply to other forms of shells as well as to heads and to
     compressive loads other than external pressure.

     1.0    Overview

           The critical pressure that causes buckling is not a simple function of
           the stress that is produced in the shell, as is true with tensile loads.
           An allowable stress is not used to design pressure vessels that are
           subject to elastic instability. Instead, the design is based on the
           prevention of elastic collapse under the applied external pressure.
           This applied external pressure is normally 15 psig for full vacuum
           conditions.

           The maximum allowable external pressure can be increased by
           welding circumferential stiffening rings (i.e., stiffeners) around the
           vessel shell. The addition of stiffening reduces the effective buckling
           length of the shell, and this length reduction increases the allowable
           buckling pressure. These stiffener rings may be welded on either
           the inside or the outside of the shell. Figure 4.10 illustrates the use
           of stiffeners on a pressure vessel cylinder.

           Other factors also affect the design of a pressure vessel for external
           pressure since they also influence its resistance to buckling.

            •   At elevated temperature, the material stress-strain curves are
                nonlinear with no definite yield point and with a variable
                modulus of elasticity.
            •   The shell diameter and thickness are additional geometric
                parameters that affect shell stiffness.

           Paragraphs UG-28 and UG-33 of Division 1 contain procedures to
           calculate the allowable external pressure on cylindrical shells and




                                       110
        heads, respectively. These calculation procedures use an iterative
        approach.



                                                   Moment Axis of Ring

                                                                         h/3



                      L        L         L            L          L



                      L        L         L            L          L


      h/3
                                             h = Depth of Head

                Stiffener Rings on Pressure Vessel Cylinders
                                 Figure 4.10


        The maximum allowable compressive stress in a pressure vessel
        component that is due to loads other than external pressure is
        limited to the lower of the following:

            •   The allowable tensile stress, or
            •   A value, Factor B (See Figure 4.13), determined using the
                external pressure calculation procedure.

2.0         Shells

        The allowable external pressure of a cylindrical shell is a function of
        material, design temperature, outside diameter, corroded thickness,
        and unstiffened length. See Division 1 for procedural details.

3.0         Heads

        The allowable external pressure of a head is a function of material,
        design temperature, outside radius, head depth, and corroded
        thickness. Stiffening rings a re not used to increase the allowable
        external pressure of heads. The head thickness is increased as
        required to achieve the required external pressure. When an
        intermediate head is installed inside a pressure vessel, it may be


                                      111
      necessary to design it for an external pressure that is higher than 15
      psig. See Division 1 for procedural details.

4.0   Conical Sections

      The allowable external pressure of a conical section is a function of
      material, design temperature, outside diameters at the small and
      large ends, conical section length, apex angle, and corroded
      thickness. The allowable external pressure may be increased by the
      addition of stiffener rings, or by increasing the cone thickness. See
      Division 1 for procedural details.




                                112
Sample Problem 2 - External Pressure Calculation
This Sample Problem demonstrates the external pressure design procedure for
one example of a cylindrical pressure vessel shell. Refer to Division 1 for
additional details and procedures to use for heads and conical shells.

A tall cylindrical tower is being supplied. The geometry and design conditions
are specified in Figure 4.11. The vendor has proposed that the wall thickness of
this tower be 7/16 in., and no stiffener rings have been specified. Is the 7/16 in.
thickness acceptable for external pressure? If it is not acceptable, what minimum
thickness is required? Round your answer upward to the nearest 1/16 in.




                                                   DESIGN INFORMATION
                                                   Design Pressure = Full Vacuum
                                                   Design Temperature = 500° F
                          4' - 0"                  Shell and Head Material is
                                                    SA-285 Gr. B, Yield Stress = 27 ksi
                                                   Corrosion Allowance = 0.0625"
                                                   Cylinder Dimension Shown
        150' - 0"
                                                    is Inside Diameter




                                    2:1 Semi-Elliptical
                                         (Typical)

                          Sample Problem 2 - Solution
                                 Figure 4.11

Solution

1.    First, calculate the unstiffened design length, L, and the outside diameter,
      Do, of the cylindrical shell, both in inches.




                                        113
                   L = Tangent Length + 2 × 1/3 (Head Depth)

     The tangent length is given as 150 ft.

     Since the heads are semi-elliptical, the depth of each head is equal to ¼
     the inside diameter of the shell.

                   Head Depth = 48 /4 = 12 in.

                   L = 150 × 12 + 2/3 × 12 = 1,808 in.

     Calculate outside diameter D o, in.

                   Do = 48 + 2 × 7/16 = 48.875 in.

     Next, determine the ratios L/D o and D o/t.

     Accounting for the corrosion allowance,
     t = 7/16 – 1/16 = 6/16 = 0.375 in.

                   Do/t = 48.875 / 0.375 = 130

                   L/D o = 1808 / 48.875 = 37

2.   Determine the value of A using Figure 4.12 and the calculated D o/t and
     L/D o.

     Note: If L/D o > 50, use L/D o = 50. For L/D o < 0.05, use L/D o = 0.05.




                                       114
                                                                                                                                             A = 0.000065


                    Do/t = 100




                                                                                                                                                                     .0001
                                                                                                                                                         4 5 6 789
                    D o/t = 125
Do/t = 130
                    D o/t = 150


                    Do/t = 200




                                                                                                                                                         3
                    D o/t = 250                                                        0    0    0         0        00
                                                                                     40 = 50 = 60        80      1,0




                                                                                                                                                         2
                                                                                 =                    t=      t=




                                                                                                                                                                     .00001
                                                                              /t       / t   /t      /       /
                    D o/t = 300                                        D     o       Do    Do      Do     Do
                                   30.0

                                          25.0

                                                 20.0
                                                 18.0


                                                               14.0


                                                                              10.0
                                                                                     9.0
                                                                                           8.0
                                                                                                 7.0
                                                                                                       6.0




                                                                                                                                           2.0
                                                                                                                                           1.8
                                                                                                                                                 1.6
                                                                                                                                                 1.4
                                                                                                                                                       1.2
                                                        16.0


                                                                      12.0




                                                                                                             5.0

                                                                                                                   4.0
                                                                                                                         3.5
                                                                                                                               3.0

                                                                                                                                     2.5
             50.0

                     40.0
                            35.0




                                                                             Length + Outside Diameter = L/D
                                                                                                          o

                             L/Do = 37




                                                            Factor A
                                                           Figure 4.12


3.    Move horizontally to the line for the value of D o/t = 130 determined in Step
      2. Use interpolation for intermediate values of D o/t. Move vertically
      downward from this intersection point to determine Factor A.

                                   A = 0.000065

4.    Using the value of A from Step 4, enter the applicable material chart. For
      this case, the applicable material chart is Figure CS-1, excerpted in Figure
      4.13. Move vertically in this chart to the intersection with the correct
      design temperature line. Use interpolation for intermediate temperatures.
      Note that in this case, the value of A is to the left of all the temperature
      curves.




                                                                       115
                                                                                                                         20,000
         GENERAL NOTE: See Table CS-1 for tabular values                                                                 18,000
                                                                                                     up to 300°F         16,000
                                                                                                            500°F        14,000
                                                                                                            700°F        12,000
                                                                                                            800°F        10,000




                                                                                                                                  FACTOR B
                                                                                                            900°F         9,000
                                                                                                                          8,000
                                                                                                                          7,000
                      E-29.0 = 106
                                                                                                                          6,000
                      E-27.0 = 106

                      E-24.5 = 106                                                                                       5,000

                      E-22.8 = 106                                                                                        4,000
                      E-20.8 =   106                                                                                      3,500
                                                                                                                          3,000

                                                                                                                          2,500

                                                                                                                          2,000
          2    3 4 5 6 7 8 9              2      3   4     5 6 7 8 9   2   3 4 5 6 7 8 9         2      3     4     5 6 7 8 9
.00001                            .0001                       .001                         .01                            .1

                                                           FACTOR A
                A=0.000065

                                                           Figure CS-1
                                                           Figure 4.13

         5.      Calculate maximum allowable external pressure for the value of t, psi.

                                                2AE
                                       Pa =
                                              3(D o / t)

                 Where:

                 E=      Young's modulus of elasticity at design temperature for the
                         material, psi. Do not confuse this parameter with the weld joint
                         efficiency, E, that is used elsewhere.

                                       E = 27 x 106 psi from Figure CS-1 (Figure 4.13) at T = 500°F

                                              2 × 0.000065 × 27 × 10 6
                                       Pa =
                                                     3 × 130.33

                                       Pa = 9 psi




                                                               116
     Since the calculated P a < 15 psi, the proposed 7/16 in. shell thickness is
     not sufficient.

     Note: In cases where A is located under the temperature curves,
     determine the Factor B by reading horizontally across from the
     intersection point. Then determine the maximum allowable external
     pressure, P a, from the following equation:

                            4B
                   Pa =
                          3(D o /t )

6.   Now determine how thick the shell must be in order to have P a ≥15 psi.
     This is a trial-and-error process, by which the thickness is increased until
     an acceptable value is found. The intent is to use the thinnest shell that
     will meet the requirement. Without going through all the iterations, we will
     assume a new shell thickness of 9/16 in. and thus a corroded thickness of
     ½ in.

                   D o 48.875                         L
                      =       = 97.75                    = 37 (as before)
                    t   0.5                           Do

                   A = 0.000114

                          2 × 0.000114 × 27 × 10 6
                   Pa =                            = 15.7 psi
                                 3 × 130.33




                                       117
Exercise 2
Required Thickness for Internal Pressure
Determine the minimum required thickness for the cylindrical shell and heads of
the following pressure vessel:

•   Inside Diameter           -       10’ - 6”
•   Design Pressure           -       650 psig

•   Design Temperature            -   750°F
•   Shell & Head Material         -   SA-516 Grade 70

•   Corrosion Allowance           -   0.125”
•   2:1 Semi-Elliptical heads, seamless

•   100% radiography of cylindrical shell welds
•   The vessel is in an all vapor service (i.e., no liquid loading)




                                           118
E.    Reinforcement of Openings

      Calculation of the required wall thickness of a nozzle is one step in the
      design of openings in pressure vessels. This is done in the same manner
      as for any other cylindrical shell. There is more to the design of openings
      than calculating the nozzle thickness, cutting a hole in the vessel, and
      welding the nozzle in.

      The ASME Code uses simplified rules to ensure that the membrane
      stresses are kept within acceptable limits when an opening is made in a
      vessel shell or head.

                                                                      Dp
                                           tn                    Rn
                                                     trn

                                                                                                             te
      2.5t or 2.5tn + t e
     Use smaller value                                      tr




              t               c




       2.5t or 2.5tn               h
     Use smaller value                                                     d



                                        d or Rn + t n + t                      d or Rn + tn + t

                                        Use larger value                       Use larger value

                                  For nozzle wall inserted                        For nozzle wall abutting
                                   through the vessel wall                        the vessel wall

                            Cross-Sectional View of Nozzle Opening
                                         Figure 4.14

      When the opening is made, a volume of material is removed from the
      pressure vessel. This metal is no longer available to absorb the applied
      loads. The ASME Code simplifies the design calculations by viewing the
      nozzle -to-vessel junction area in cross section (See Figure 4.14). This
      simplification permits the nozzle reinforcement calculations to be made in
      terms of metal cross-sectional area rather than metal volume. The ASME
      Code requires that the metal area that is removed for the opening must be
      replaced by an equivalent metal area in order for the opening to be
      adequately reinforced. The replacement metal must be located adjacent




                                                     119
to the opening within defined geometric limits. The replacement metal
area may come from two sources:

•   Excess metal that is available in the shell or nozzle neck that is not
    required for pressure or to absorb other loads.

•   Reinforcement that is added to the shell or nozzle neck.

Figure 4.15 shows several typical nozzle design configurations including
examples of inserted versus abutted nozzles, pad reinforcement versus no
reinforcement, and self-reinforced nozzles. Self-reinforced nozzles are
forged fittings that have extra thickness in the nozzle-to-vessel junction
area to provide reinforcement.

Additional reinforcement must be provided if the vessel shell and nozzle
do not have sufficient excess thickness that is not required for pressure or
other loads. Additional reinforcement can be in one of the following forms:

•   A reinforcement pad.

•   Additional thickness in the vessel shell or head.
•   Additional thickness in the nozzle near its attachment to the vessel.

The reinforcement must be located within defined boundaries in order for it
to be considered effective.




                                  120
             (a)
   Full Penetration Weld
With Integral Reinforcement          (a-1)                    (a-2)                         (a-3)

                                               Separate Reinforcement Plates Added




            (b)                         (c)                         (d)                           (e)

                     Full Penetration Welds to Which Separate Reinforcement Plates May be Added




                          (f-1)                 (f-3)




                         (f-2)
                                                (f-4)                        (g)



                                        Self - Reinforced Nozzles
                          Typical Nozzle Design Configurations
                                       Figure 4.15

      If a reinforcement pad is used, its material should have an allowable
      stress that is at least equal to that of the pressure vessel shell or head
      material to which it is attached. No credit can be taken for the additional
      strength of any reinforcement that has a higher allowable stress. If
      reinforcement material with a lower allowable stress is used, the
      reinforcement area must be increased to compensate for this.


                                                    121
      The ASME Code specifies circumstances under which no nozzle
      reinforcement eva luations are needed. It also provides rules to evaluate
      the reinforcement of openings that are located near each other. These
      situations are not discussed in this course. Refer to the ASME Code for
      details. Sample Problem 3 illustrates the procedure used to evaluate
      nozzle reinforcement.

Sample Problem 3 - Reinforcement of Openings
You are reviewing the nozzle design details that are proposed by a vendor for a
new drum and have selected an NPS 8 nozzle into the shell for detailed
evaluation. The vendor has not provided any reinforcement for this nozzle, and
he has not provided any calculations to verify that use of the nozzle without
reinforcement is acceptable.

Determine if this nozzle requires additional reinforcement. If it does, assume that
a 0.5 i n. thick reinforcement pad of SA-516, Gr. 60 material is used. What must
the minimum pad diameter be? Neglect any contribution of weld areas in these
calculations since they are insignificant. The information that is needed to
perform your evaluation is in Figure 4.16. Use Figure 4.14 as a reference.




                                       122
DESIGN INFORMATION
Design Pressure = 300 psig
Design Temperature = 200°F
Shell Material is SA-516 Gr. 60
Nozzle Material is SA-53 Gr. B, Seamless
Corrosion Allowance = 0.0625"
Vessel is 100% Radiographed
Nozzle does not pass through Vessel Weld Seam




                                                NPS 8 Nozzle
                                                 (8.625" OD)
                                                  0.5" Thick




   0.5625" Thick Shell, 48" Inside Diameter


                      Sample Problem 3
                         Figure 4.16




                                123
Solution

Calculate the required reinforcement area, A

                     A = dtrF

Where:

                     d =     Finished diameter of circular opening, or finished
                             dimension (chord length at mid surface of thickness
                             excluding excess thickness available for
                             reinforcement) of nonradial opening in the plane
                             under consideration, in.
                     tr =    Minimum required thickness of the shell using
                             appropriate ASME Code formula and a weld joint
                             efficienc y of 1.0, in.
                     F =     Correction factor normally equal to 1.0.

Calculate the diameter, d.

                     d = Diameter of Opening – 2 (Thickness + Corrosion Allowance)

                     d = 8.625 – 1.0 + .125 = 7.750 in.

Calculate the required thickness of the shell, tr (See Figure 4.6)

                                Pr      300 × (24 + 0.0625)
                     tr =             =                       = 0.487 in.
                            SE1 − 0.6P 15,000 × 1 − 0.6 × 300

Assume a value of 1.0 for F.

Calculate the required reinforcement area, A

                     A = dtrF

                     A = (8.625 - 1.0 + 0.125) × 0.487 × 1 = 3.775 in.2 required area

Calculate the available reinforcement area in the vessel shell, A 1, as the larger of
A11 or A 12

                     A11 = (E lt - Ftr)d



                                           124
                    A12 = 2 (E lt-Ftr )(t + tn)

Where:

                    El =     1.0 when the opening is in the base plate away from
                             the welds, or when the opening passes through a
                             circumferential joint in the shell (excluding head to
                             shell joints).

                    El =     The ASME Code joint efficiency when any part of the
                             opening passes through any other welded joint.

                    F =      1 for all cases except integrally reinforced nozzles
                             that are inserted into a shell or cone at an angle to the
                             vessel longitudinal axis. See Fig. UG-37 for this
                             special case.

                    tn =     Nominal thickness of the nozzle in the corroded
                             condition, in.

          A11 = (E lt - Ftr)d = (0.5625 - 0.0625 - 0.487) x 7.75 = 0.1 in.2

          A12       = 2(E lt - Ftr ) (t + tn)

                    = 2(0.5625 - 0.0625 - 0.487) (0.5625 - 0.0625 + 0.5 - 0.0625)

                    = 0.0243 in.2

   Therefore, A1= 0.1 in.2 available reinforcement in shell

Calculate the reinforcement area that is available in the nozzle wall, A2,
as the smaller of A21 or A22.

                    A21 = (tn-trn)5t

                    A22 = 2(tn-trn)(2.5 tn + te)

      Where:

            trn =   Required thickness of the nozzle wall, in.



                                            125
                        r   = radius of the nozzle, in.

                        te = 0 if there is no reinforcing pad.

                        te = Reinforcing pad thickness if one is installed, in.

                        te = As defined in Figure UG-40 of the ASME Code for
                             self-reinforced nozzles, in.

Calculate the required thickness of the nozzle, trn (See Figure 4.6)

                                    Pr
                        t rn =
                                 SE1 − 0. 6P

                      300 (3.8125 + 0.0625 )
               tm =                          = 0.0784 in.
                      15,000 × 1 − 0.6 × 300

Calculate the available reinforcement in the nozzle neck, A 2, as the smaller of
A21 or A 22.

         A21     = (tn - trn)5t = (0.5 - 0.0625 - 0.0784) x 5(0.5625 - 0.0625)

         A21     = 0.898 in.2

         A22     = 2(tn - trn) (2.5 tn + te)

                 = 2(0.5 - 0.0625 - 0.0784) [2.5 x (0.5 - 0.0625) + 0]

                 = 0.786 in.2

               Therefore, A2 = 0.786 in.2 available reinforcement in nozzle

Determine the total available reinforcement area, A T, and compare it to the
required area.

                        AT = A 1 + A 2 = 0.1 + 0.786 = 0.886 in.2

Since A T < A, the nozzle is not adequately reinforced, and a reinforcement pad is
required.



                                               126
Determine the required reinforcement pad area, A 5, and pad diameter, D p.

Since the required reinforcement area is 3.775 in.2 and the available
reinforcement area is 0.886 in.2 , we need to calculate the required area for the
reinforcement pad.

              A5 = A - AT

              A5 = (3.775 - 0.886) = 2.889 in.2 required area in reinforcement pad.

Now, calculate D p

              te     = 0.5625 in. (reinforcement pad thickness)

              A5     = [D p - (d + 2 tn)] te

              2.889 = [D p - (7.75 + 2(0.5 - 0.0625)] 0.5625

              5.136 = [D p - 8.625]

              Dp = 13.761 in.

Therefore, the minimum required reinforcement pad diameter is 13.761 in.

Confirm that this diameter does not extend beyond the outer limit of the permitted
reinforcement zone in the shell, 2d.

              2d     = 2 x 7.75 = 15.5 in.

              Therefore, D p = 13.761 in. is acceptable.


F.     Flange Rating

       ASME B16.5, Pipe Flanges and Flanged Fittings, provides steel flange
       dimensional details for standard pipe sizes through NPS 24. ASME B16.5
       flanges are acceptable for most pressure vessel nozzles and for shell
       flanges when the vessel diameter corresponds to a standard pipe size.
       Specification of an ASME B16.5 flange involves selection of the correct
       material and flange "Class." The paragraphs that follow discuss the flange
       specification process in general terms.



                                           127
           Flange material specifications are listed in Table 1A in ASME B16.5, a
           portion of which is excerpted as Figure 4.17. The material specifications
           are grouped within specific Material Group Numbers. For example, if the
           pressure vessel is fabricated from carbon steel, ASTM A105 is an
           appropriate flange material specification in most applications. ASTM A105
           material is in Material Group No. 1.1. Refer to ASME B16.5 for additional
           acceptable material specifications and corresponding Material Group
           Numbers.

   Material Groups                                      Product Forms
Material       Nominal
 Group        Designation       Forgings                Castings                Plates
Number           Steel
                            Spec. No.   Grade     Spec. No.    Grade    Spec. No.        Grade
  1.1           Carbon       A105        --           A216     WCB       A515             70
                             A350       LF2            --       --       A516             70
               C-Mn-Si        --         --            --       --       A537            Cl.1
  1.2          Carbon         --         --           A216     WCC        --              --
                              --         --           A352     LCC        --              --
                2 ½ Ni        --         --           A352     LC2       A203             B
                3 ½ Ni       A350       LF3           A352     LC3       A203             E



             ASME B16.5, Table 1a, Material Specification List (Excerpt)
                                   Figure 4.17

           Table 2 of ASME B16.5 is used to select the appropriate flange Class for
           the specified design conditions and Material Group Number. ASME B16.5
           has seven Classes: 150, 300, 400, 600, 900, 1,500, and 2,500. Each
           Class specifies the design pressure and temperature combinations that
           are acceptable for a flange that has that designation. As the number of
           the Class increases, the strength of the flange increases for a given
           Material Group. Figure 4.18 is an excerpt from Table 2 and shows the
           temperature and pressure ratings for three carbon steel Material Groups.




                                                128
Material Group
                       1.1                 1.2                 1.3
      No.
   Classes       150   300   400    150    300   400    150    300    400
  Temp., °F
  -20 to 100     285   740   990    290   750    1000   265    695    925
      200        260   675   900    260   750    1000   250    655    875
      300        230   655   875    230   730     970   230    640    850
      400        200   635   845    200   705     940   200    620    825
      500        170   600   800    170   665     885   170    585    775
      600        140   550   730    140   605     805   140    534    710
      650        125   535   715    125   590     785   125    525    695
      700        110   535   710    110   570     755   110    520    690
      750         95   505   670     95   505     670    95    475    630
      800         80   410   550     80   410     550    80    390    520
      850         65   270   355     65   270     355    65    270    355
      900         50   170   230     50   170     230    50    170    230
      950         35   105   140     35   105     140    35    105    140
     1000         20    50    70     20    50      70    20     50     70

   ASME B16.5, Table 2, Pressure-Temperature Ratings (Excerpt)
                           Figure 4.18

   Specification of the size, material, and Class completes most of the
   selection requirements for flanges. Flange type and gasket material must
   also be specified. Discussion of these factors is beyond the scope of this
   course.




                                   129
Sample Problem 4 – Determine Required Flange Rating
For the pressure vessel described below, use the following procedure to
determine the required flange rating (or Class) in accordance with ASME B16.5.

Pressure Vessel Material Specifications:

Shell and Heads:            SA-516 Gr.70
Flanges:                    SA-105
Design Temperature:         700°F
Design Pressure:            275 psig

1. Identify the material specification of the flange.

       SA-105

2. Go to Figure 4.17 (Table 1A of ASME B16.5) and determine the Material
   Group No. for the selected material specification.

       Group 1.1

3. Go to Figure 4.18 (Table 2 of ASME B16.5) with the design temperature and
   Material Group No. determined in Step 3.

       •      The intersection of design temperature with Material Group No. is
              the maximum allowable design pressure for the flange Class.
       •      Table 2 of ASME B16.5 contains design information for all seven
              possible flange Classes (i.e., 150, 300, 400, 600, 900, 1500, 2500).

       •      Select the lowest Class whose maximum allowable design pressure
              is equal to or greater than the required design pressure.

At 700°F, for Group 1.1 flange material, the Lowest Class that will accommodate
a design pressure of 275 psig is Class 300. At 700°F a Class 300 flange of
Material Group 1.1 can have a design pressure up to 535 psig.




                                         130
G.   Flange Design

     For some pressure vessel applications, it is advantageous to have one or
     more flanged joints in the vessel shell to facilitate entry, removal, and/or
     replacement of internal components (e.g., cartridge trays). In most
     applications such as these, the shell diameter is of a size that standard-
     sized flanges designed in accordance with either ASME B16.5 or ASME
     B16.47 may be used. Mechanical design calculations for these standard
     flanges are not necessary.

     Flanges must be custom-designed in situations where standard-sized
     flanges are not appropriate. The most common application for custom-
     designed flanges is for the girth flanges of shell-and-tube heat
     exchangers. All custom-designed flanges must meet the requirements of
     Appendix 2 of Division 1. The Appendix 2 design procedure is
     complicated and is best done using a computer program. The following
     paragraphs briefly describe:

           •   The main steps in the ASME flange design procedure.
           •   The parameters that affect flange design and in-service
               performance.

     1.0       ASME Flange Design Procedure

               The ASME flange design procedure consists of determining the:

               •   Bolting requirements.

               •   Flange design loads and moments.
               •   Stresses in the flange ring and hub.

               The first step is usually to determine the required number and size
               of bolts. Bolting requirements are determined by calculating the
               loads on the bolts for two separate cases:

               •   Normal operation
               •   Initial flange boltup

               The bolt load during normal operation, W m1, is based on the design
               conditions. The bolt load during initial flange boltup, W m2, is based
               on the load (or stress) necessary to seat the gasket and form a tight
               seal.



                                           131
The bolt area that is required for each of these loads is then
calculated by dividing each bolt load by the bolt allowable stress at
design temperature and room temperature, respectively. Either the
operating case or the gasket seating case may result in the
minimum required bolt area, A m ; therefore, both cases must be
checked. Since bolts come in standard sizes, and there are
limitations on the spacing between bolts, the actual bolt area, A b, is
usually greater than the required bolt area.

The next step is to determine the design loads and moments on the
flange. These loads include the:

•   Design bolt load on the flange (W).
•   Hydrostatic pressure loads that act on the flange (HD and HT).

•   Gasket sealing force (H G).

These loads do not all act at the same location on the flange,
therefore, effective moment arms (hD, hT , and hG) are calculated
based on the locations of the bolts and gasket, and on the flange
geometry (See Figure 4.19). The appropriate loads are then
multiplied by the effective moment arms to determine flange design
moments for the operating and gasket seating cases.

                                                 Flange
                                                 Ring
    Gasket

                         t                  h


    A        hG                     W

                                                          C
        hT                                  hD

                    g1
    HT
             G
                             HD         B                     g0
                  HG

                                            Flange Hub
              Flange Loads and Moment Arms
                        Figure 4.19



                              132
      The stresses in the flange ring and hub are then calculated using
      stress factors specified in the ASME Code (based on flange
      geometry), the applied moments, and the flange geometry. The
      stresses are calculated for both the operating case and gasket
      seating cases and are then compared to the appropriate Code
      allowable stresses.

      All flange stresses will be lower than the appropriate allowable
      stresses if the flange is designed properly. It may be necessary to
      increase the flange thickness, change the hub dimensions, or make
      other changes to the flange design parameters to keep flange
      stresses within their allowable limits. The computer programs that
      suppliers use for flange design use iterative calculation procedures
      to optimize flange design. In this sense, the goal of optimization,
      from the supplier’s viewpoint, is to design a “least weight” (i.e.,
      lowest cost) flange that will satisfy the design requirements.

2.0   Parameters That Affect Flange Design and In-Service
      Performance

      The following parameters affect flange design and in-service
      performance:

      •   ASME Code m and y parameters.
      •   Specified gasket widths.

      •   Flange facing and nubbin width, w.
      •   Bolt size, number, and spacing.

      The gasket factor, m, determines the amount of force required to
      keep the gasketed joint tight. The minimum design seating stress, y,
      determines how much gasket stress is required to initially seat or
      deform the gasket. Both parameters are used in the flange design
      calculations.

      The ASME Code specifies m and y based on gasket type in its
      Table 2 -5.1 (excerpted in Figure 4.20). Higher values of m and y
      typically indicate that a gasket is harder to seal or seat. While this is
      a consideration in gasket selection, gasket type and material are
      usually selected based on historical service experience and the
      corrosion resistance of the gasket material in the process
      environment.




                                  133
               Heat exchanger flanges sometimes have leakage problems during
               operation. When this occurs, there is often the tendency to change
               the gasket to a different type that has provided leak-free
               performance in other applications. This problem-solving method
               should always be approached with caution because the flanges
               were designed for a specific gasket type with its associated m and y
               values. Therefore, the existing bolting may either impose too high a
               load on the gasket (and possibly crush it) or the new gasket may
               require a higher load to seat it (which might not be possible with the
               existing bolting).

                                                               Min.
                                                                         Facing Sketch and
                                                  Gasket      Design
                                                                          Column in ASME
          Gasket Type and Material               Factor, m   Seating
                                                                             Table 2-5.2
                                                             Stress y,
                                                                            (Figure 4.21)
                                                                psi

Flat metal, jacketed asbestos filled:
     Soft aluminum                                   3.25     5,500
     Soft copper or brass                            3.50     6,500
                                                                         (1a), (1b), (1c), (1d);
     Iron or soft steel                              3.75     7,600
                                                                         (2);
     Monel                                           3.50     8,000
                                                                         Column II
     4-6% chrome                                     3.75     9,000
     Stainless steels and nickel-base alloys         3.75     9,000


Solid flat metal:
    Soft aluminum                                    4.00      8,800
    Soft copper or brass                             4.75     13,000     (1a), (1b), (1c), (1d);
    Iron or soft steel                               5.50     18,000     (2), (3), (4), (5);
    Monel or 4-6% chrome                             6.00     21,800     Column I
    Stainless steels and nickel-base alloys          6.50     26,000


                              ASME Code m and y Factors
                                    Figure 4.20

               The TEMA standard for shell-and-tube heat exchangers specifies a
               minimum required width for the peripheral ring gaskets at external
               joints (3/8 in. or ½ in. depending on shell size) and for pass partition
               gaskets (¼ in. or 3/8 in. depending on shell size). These minimum
               gasket widths are typically used over a wide range of service
               conditions.

               The gasket widths referred to in TEMA are actual minimum widths,
               N. In addition to N, two other gasket widths are referred to in the
               ASME Code: the basic seating width, b o, and the effective seating
               width, b. The effective seating width is a function of the basic
               seating width, and the basic seating width is a function of the actual
               width and the type of flange face. See Table 2-5.2 in the ASME


                                               134
Code (excerpted in Figure 4.21). In general, wider gaskets provide
better sealing, but a wider gasket also requires a larger bolt load
(i.e., more bolt area) to seat and seal the gasket. The required
flange thickness increases as the bolting area increases.




                          135
               Facing Sketch                                          Basic Gasket Seating Width bo
               (Exaggerated)
                                                                  Column I                             Column II




               N                        N
(1a)
                                                                       N                                  N
               N                                                       2                                  2


(1b)                                    N



       w
                   T

(1c)       N               w≤N
                                                          w+ T w+N                              w + T  w +N     
                                                              ;    max                               ;      max 
                                                           2    4                                2     4        
       w
                    T

(1d)       N               w≤N
                                        HG                            HG
                                    G            hG               G             hG
                        O.D. Contact Face
                                             b                             C Gasket
                                                                           L Face




                                                 For bo > ¼ in.                  For bo < ¼ in.


                        ASME Code Gasket Widths (Table 2 -5.2 excerpt)
                                       Figure 4.21

                   The effective seating width, b, is also a function of the flange facing
                   type and the nubbin width, w, for flat metal gaskets. Table 2 -5.1 in
                   the Code (excerpted in Figure 4.22) indicates which facing sketch is
                   applicable for a given gasket type and material.




                                                           136
                                 Gasket Materials and Contact Facings
         Gasket Factors m for Operating Conditions and Minimum Design Seating Stress y
              Gasket Material                  Gasket     Min. Design   Sketches      Facing
                                               Factor      Seating                 Sketch and
                                                 m         Stress y,                Column in
                                                              psi                  Table 2-5.2
Flat metal, jacketed asbestos filled:              3.25      5500                    (1a), (1b),
                                                                                         2     2
  Soft aluminum                                    3.50      6500                  (1c), , (1d) ,
                                                                                      2
  Soft copper or brass                             3.75      7600                  (2) , Column
  Iron or soft steel                               3.50      8000                          II
  Monel                                            3.75      9000
  4% - 6% chrome                                   3.75      9000
  Stainless steels and nickel-base alloys

          Gasket Materials and Contact Facings (Table 2-5.2 Excerpt)
                                 Figure 4.22

               The equations for determining b are based on w, N, and the type of
               flange facing. Note that b is used in the Code equations to
               determine the bolt load required for sealing the gasket during
               operation, Wm1, and the bolt load required for seating the gasket
               initially, Wm2. Once a gasket type, material, width, and facing are
               selected, the required bolting area can be determined.

                •   The bolt size, number, and spacing that are used to clamp the
                    flanges together are interrelated parameters that affect their
                    overall design.
                •   The number of bolts multiplied by the bolt root area of a single
                    bolt must be greater than the minimum required bolt area, A m .

                •   The bolts must be far enough away from the shell or hub of the
                    flange, and be far enough apart circumferentially, so that there
                    is adequate clearance to permit access for a wrench.
                •   There must be adequate distance to other flange or vessel
                    surfaces to ensure adequate clearance for standard wrenches.

               It may appear that maintaining these minimum bolt dimensions can
               be easily achieved if a few large bolts are used. However, the bolts
               should also be spaced as close together as practical for several
               reasons.

                •   Having fewer bolts increases the bolt load moment arms.
                    Larger moment arms increase the bending moments for which
                    the flange must be designed and thus increase the required
                    flange thickness.


                                             137
           •   TEMA requires that the flange design moment be increased if
               the bolts are widely spaced. This results in a thicker flange.

           •   Excessive bolt spacing could make the flange more prone to
               leakage. The portions of the gasket located between the bolts
               might not be compressed sufficiently to maintain a tight seal.

H.   Maximum Allowable Working Pressure (MAWP)

     The MAWP of a pressure vessel is the maximum permissible gauge
     pressure in the vessel. It is determined as the lowest MAWP of all the
     vessel's components based on their actual supplied thicknesses. The
     MAWP is specified at the top of the vessel when the vessel is in its
     operating position. The MAWP is also specified at a "designated
     temperature" (i.e., the design temperature) that is coincident with the
     MAWP. The material thicknesses used in these calculations do not
     include any excess thickness that was added for corrosion allowance or to
     absorb loadings other than pressure.

     The MAWP may be used later if a change in operation is being considered
     that requires a more severe design pressure and/or temperature than
     what were originally specified. The MAWP shows whether the same
     pressure vessel may be used at the new design conditions.




                                     138
V.   Other Design Considerations

A.   Vessel Support

     The type of support that is used for a pressure vessel depends primarily
     on the vessel’s size and orientation.

     Shown in Figure 2.1, a saddle support spreads the weight load of a
     horizontal drum over a large area of the shell. This prevents excessive
     local stress in the shell at the support points. The size and design details
     used for the saddle depend on the diameter and thickness of the drum
     and the imposed load.

     As shown in Figure 2.2, small vertical drums are typically supported on
     legs that are welded to the lower portion of the shell. Support legs are
     also typically used for spherical pressurized storage vessels (See Figure
     2.5). The support legs for small vertical drums and spherical pressurized
     storage vessels may be made from structural steel columns or pipe
     sections, whichever provides a more efficient design. Cross bracing
     between the legs (See Figure 2.5) is typically used to help absorb wind or
     earthquake loads.

     Lugs may also be used to support vertical pressure vessels. As shown in
     Figure 5.1, the lugs are typically bolted to horizontal structural members.
     It is common for a reinforcement pad to be first welded to the vessel shell,
     and then the lugs welded to it.

     A support skirt (See Figures 2.3 and 2.4) is a cylindrical shell section that
     is welded either to the lower portion of the vessel shell or to the bottom
     head. Support skirts are commonly used for tall towers.

B.   Local Loads

     It is common for external loads to be applied to nozzles or lugs that are
     attached to pressure vessel shells or heads. External loads cause local
     stresses that are in addition to those caused by pressure, weight, and
     wind loads. External loads may be caused by the following:

     •   Piping system weight, wind, and thermal expansion loads that are
         applied at vessel nozzles.




                                       139
•   Loads from platforms, internal or external piping, internal components,
    or equipment items supported from a vessel shell by lugs or clips
    attached to the shell.
•   Loads at vessel supports, such as columns or lugs.




                 Vertical Vessel on Lug Supports
                            Figure 5.1


The total stress in the vessel shell, including that caused by locally applied
loads, must be kept to within allowable limits. Division 1 does not contain
detailed procedures for evaluating these local loads. Other industry
practices (e.g., Welding Research Council Bulletins 107 and 297) and
Division 2 are commonly used to evaluate local loads.




                                 140
C.   Vessel Internals

     1.0   Types of Internals

           There are many different types of vessel internals used to perform
           various process functions. The following highlights several (but not
           all) of these types:

           •   Trays. Located at various elevations along the length of a
               tower. Provide liquid/vapor flow distribution and separation
               along the length. Various tray types are available to suit specific
               process needs.

           •   Inlet distributor. Installed as an internal extension to the inlet
               nozzle. Used to direct the inlet flow stream and properly
               distribute it within the vessel.
           •   Anti-vortex baffle. Installed at vessel outlet to prevent the
               formation of flow vortices at the exit from the vessel.

           •   Catalyst bed grid and support beams. An open steel gridwork
               may be used to support one or more intermediate catalyst beds
               installed inside fixed bed reactors. The gridwork is typically
               covered by wire mesh screen to prevent the solid catalyst from
               passing through the grid, while the gridwork permits process
               flow. Supplementary beams are typically used to support the
               grid from the vessel shell.

           •   Outlet collector. Typically placed at the outlet of fixed bed
               reactors. Designed to allow process flow while preventing
               catalyst from passing into the downstream system.
           •   Flow distribution grid. In fluidized solids processes (e.g., FCCU,
               Fluid Coker, etc.), a flow distribution grid is used to direct and
               distribute the fluidization media that is needed to keep the solids
               (i.e., catalyst or coke) in a fluidized state inside a vessel.

           •   Cyclone and plenum chamber system. In fluidized solids
               processes (e.g., FCCU, Fluid Coker, etc.), a cyclone and
               plenum chamber system separates entrained catalyst from
               process vapor before the vapor exits the vessel through the
               overhead line.

           ASME Code design requirements only apply to the external,
           pressure-containing “envelope” of the vessel (i.e., shell, heads,
           nozzles, etc.) and not to items contained inside it. The only
           exceptions to this are:



                                       141
      •   Loads that are applied from the internals to pressure-containing
          parts must be considered in the vessel design.

      •   All welding to pressure-containing parts must meet ASME Code
          requirements.

      The end-user, vessel vendor, internals supplier, prime contractor,
      and/or a combination of these entities must develop the detailed
      design requirements for all vessel internals.

2.0   Treatment of Corrosion Allowance.

      Removable pressure vessel internals that are subject to corrosion
      should typically have a corrosion allowance equal to that of the shell.
      In this way, the design of removable internals considers only half of
      the expected total corrosion. The rationale for this approach is that
      removable internals that are designed for only the expected total
      corrosion will cost less initially and can easily be replaced later,
      based on the actual corrosion that occurs.

      Most pressure vessel internals can corrode on both sides. From a
      strength-design viewpoint, corrosion from both sides should be
      considered with regard to non-removable internals. Non-removable
      internals, and those that are major load-bearing members (e.g.,
      catalyst bed supports), must typically have a total corrosion
      allowance that is equal to twice that of the shell.




                                 142
VI.   Fabrication

A.    Acceptable Welding Details

      All pressure vessel welds, including the welds that attach heads, nozzles,
      small fittings, and nonpressure components to a shell, must conform to
      ASME Code requirements. Details that are used for the primary
      circumferential and longitudinal welds were discussed earlier in
      conjunction with weld joint categories.

      The ASME Code specifies weld detail requirements for vessel fabrication
      (e.g., type and size of weld, weld locations, etc.). It also specifies welder
      and welding procedure qualification requirements. The paragraphs that
      follow highlight several of the ASME Code requirements. Refer to the
      ASME Code for further information related to these and other weld details.

      1.0   Thickness Transitions

            The thickness of a pressure vessel head sometimes differs from the
            thickness of the shell it is attached to (e.g., when a hemispherical
            head is attached to a cylindrical shell). The transition between the
            component thicknesses must be made in a taper to avoid excessive
            local stress. Head-to-shell thickness transitions are illustrated in
            Figure 6.1.

      2.0   Intermediate Heads

            An intermediate head is attached to the inside of a cylindrical shell
            when it is needed to separate two sections of the vessel. The butt
            weld between shell sections also attaches to the head, and a fillet
            weld is also located between the head and shell. The ASME Code
            permits elimination of the fillet weld if there is no access and if the
            service is noncorrosive. However, the fillet weld should generally be
            used for all refinery applications to avoid the potential for
            accelerated corrosion due to process fluid getting between the head
            and shell. The attachment of an intermediate head to a cylindrical
            shell is illustrated in Figure 6.1.




                                       143
                 th                                                          th




                              Thinner part




                                                                                                Thinner part
            l                                                        l
                                                      Tangent
            y                                           Line
                                                                                       y



                         ts                                                                ts


            th                                                  th




       y               Tangent                                           y
                         Line
        l
                                             Thinner part




                                                                                                               Thinner part
                                                                l




                         ts                                                                ts




                                                                              Fillet
                                                                              Weld




                                                            Butt Weld


                        Intermediate Head Attachment

                 Typical Head-to-Shell Transitions
                            Figure 6.1

3.0   Openings

      Fabrication details for various types of openings are specified.
      These include unreinforced nozzles (e.g., a nozzle neck welded
      directly to the vessel shell or head), a nozzle with a reinforcing pad
      added, and a self-reinforced nozzle (i.e., where extra thickness is


                                                144
      provided in the nozzle neck to provide the necessary reinforcement).
      These were illustrated in Figure 4.15.

      In some cases, a nozzle neck that has a weld-end may be attached
      to a pipe that is thinner. This attachment between components of
      different thicknesses could occur if extra thickness was included in
      the nozzle neck for reinforcement or if the pipe and nozzle materials
      and/or allowable stresses differ. In such cases, the nozzle neck
      must be tapered to the pipe thickness. Tapers are also used to join
      shell sections that are of different thicknesses. Shell thickness and
      nozzle thickness tapers are illustrated in Figures 6.2 and 6.3,
      respectively.




                    C                                      C
                                                           L
                    L          In all cases, l shall not
                                   be less than 3y.
                y


                                                               l

          l



                                                                   C
                                                                   L




                        Typical Shell Transitions
                               Figure 6.2




              Nozzle Neck Attachment to Thinner Pipe
                            Figure 6.3

4.0   Stiffener Rings

      Stiffener rings may be attached to the vessel shell by continuous,
      intermittent, or a combination of continuous and intermittent welds.


                                     145
           Intermittent welds must be placed on both sides of the stiffener and
           may be either staggered or in-line. The ASME Code specifies
           acceptable spacing, size, and length of the welds. Stiffener ring
           attachment weld options are illustrated in Figure 6.4.




          In-Line                                  Continuous Fillet Weld On
     Intermittent Weld                             One Side, Intermittent Weld
                                  Staggered              On Other Side
                              Intermittent Weld


                         Stiffener Ring Attachment
                                 Figure 6.4

B.   Postweld Heat Treatment Requirements

     Welding heat changes the crystal structure and grain size of the weld heat
     affected zone (HAZ). Postweld heat treatment (PWHT) may be necessary
     to restore the material structure to the required properties. The need for
     PWHT for these metallurgical reasons depends on the materials involved
     and the service conditions that they are exposed to. PWHT requirements
     for these metallurgical or process reasons are not included in the ASME
     Code. They must be specified by the user based on the service and
     materials involved.

     As the weld metal and HAZ cool from the very high welding temperatures,
     the thermal contraction that occurs in the locally heated area is resisted by
     the cooler base metal that surrounds it. This resistance results in residual
     stresses that remain in the structure. For thicker plates, these residual
     stresses must be removed by PWHT. PWHT requirements based on
     stress relief considerations are contained in the ASME Code, Section VIII.




                                       146
The ASME Code contains the temperature and hold time requirements
when PWHT is needed for stress relief considerations. These ASME
Code PWHT requirements are based on material type and thickness, as
specified in Paragraph UCS-56 for carbon and low-alloy steels. The
ASME Code specifies the minimum PWHT temperature and the minimum
holding time at temperature based on the material P-No. and thickness.
Acceptable PWHT procedures are also specified to ensure that adequate
stress relief will occur. Heatup and cooldown rates must be controlled
within specified limits in order to avoid excessive local thermal stresses
during PWHT.




                                147
VII.   Inspection and Testing

A.     Inspection

       Overall inspection of completed pressure vessels includes an examination
       of the following:

       •   Base material specification and quality
       •   Welds

       •   Dimensional requirements
       •   Equipment documentation

       The most common defects for which welds are examined are as follows:

       •   Poor weld shape due to part misalignment.
       •   Cracks in welds or HAZ of the base metal.

       •   Pinholes on the weld surface.
       •   Slag inclusions or porosity in the form of voids.

       •   Incomplete fusion between weld beads or between the weld and the
           base metal.

       •   Lack of penetration or an insufficient extent of penetration of the weld
           metal into the joints.

       •   Undercut, an intermittent or continuous groove that is located adjacent
           to the weld and that is left unfilled by weld metal.

       Several of these common weld defects are illustrated in Figure 7.1.




                                         148
    Between Weld Bead and Base Metal                                Between Adjacent Passes

                                              Lack of Fusion




Incomplete Filling at Root on One Side Only                         Incomplete Filling at Root

                                       Incomplete Penetration


                                                          External Undercut




                                      Internal Undercut

                                                   Undercut



                                 Typical Weld Defects
                                      Figure 7.1

The presence of defects reduces the strength of the weld below that
required by the design calculations, reduces the overall strength of the
fabrication, and increases the risk of failure. Weld inspection must be
performed in a manner that will detect unacceptable defects while not
damaging the vessel material. This type of inspection is called
nondestructive examination, or NDE.

The five primary weld NDE methods are as follows:

•   Radiographic examination (RT)
•   Visual Inspection (VT)
•   Liquid penetrant examination (PT)

•   Magnetic particle test (MT)


                                                 149
         •   Ultrasonic examination (UT)

         The choice of which weld examination method or methods to use depends
         on the weld quality required of the joint, the position of the weld, the
         material to be joined, and the particular defects that are most likely to
         occur. These weld NDE methods are briefly discussed in the paragraphs
         that follow. Figure 7.2 summarizes the types of NDE, the defects typically
         found by each, and the advantages and limitations of each process.

     NDE TYPE                DEFECTS                  ADVANTAGES              LIMITATIONS
                            DETECTED
Radiographic           Gas pockets, slag          Produces permanent       Expensive.
                       inclusions, incomplete     record.                  Not practical for
                       penetration, cracks        Detects small flaws.     complex shapes.
                                                  Most effective for butt-
                                                  welded joints.
Visual                 Porosity holes, slag       Helps pinpoint areas    Can only detect what
                       inclusions, weld           for additional NDE.     is clearly visible.
                       undercuts,
                       overlapping
Liquid Penetrant       Weld surface-type          Used for ferrous and    Can only detect
                       defects: cracks,           nonferrous materials.   surface imperfections.
                       seams, porosity, folds,    Simple and less
                       pits, inclusions,          expensive than RT,
                       shrinkage                  MT, or UT.
Magnetic Particle      Cracks, porosity, lack     Flaws up to ¼ in.       Cannot be used on
                       of fusion                  beneath surface can     nonferrous materials.
                                                  be detected.
Ultrasonic             Subsurface flaws:          Can be used for thick   Equipment must be
                       laminations, slag          plates, welds,          constantly calibrated.
                       inclusions                 castings, forgings.
                                                  May be used for
                                                  welds where RT not
                                                  practical.
                               Summary of NDE types
                                   Figure 7.2




                                                150
1.0   Radiographic Examination (RT)

      The most important NDE method is radiographic examination. In
      radiographic examination, a ray is emitted from a controllable
      source, penetrates a test specimen, and leaves an image on a strip
      of film that is mounted behind the test specimen. This is illustrated
      in Figure 7.3.

               X-Ray Tube




                                         X-Ray




                                                      Film


                  Test Specimen



                         Typical RT Setup
                            Figure 7.3

2.0   Visual Inspection (VT)

      A thorough visual inspection is usually satisfactory for minor
      structural welds. All weld surfaces that will be examined by more
      extensive means are first subject to VT. VT provides an overall
      impression of weld quality and helps to locate areas where
      additional NDE should be performed.

3.0   Liquid Penetrant Examination (PT)

      A liquid penetrant examination involves applying a penetrant which
      contains a fluorescent or visible dye to mark potential defect areas.
      The liquid penetrates into defects by capillary action. Then, by using


                                  151
           a developing procedure, the liquid bleeds out through a capillary
           action at surface flaws and makes them visible.

     4.0   Magnetic Particle Test (MT)

           MT examination is based on the magnetic lines of flux (or force
           lines) that can be generated within a test piece. These force lines
           are parallel if no defects are present. If there is a defect, a small
           break in the force lines appears at the defect location. In MT
           examination, iron powder is applied to the surface and then the test
           piece is magnetized. If there are no defects, the iron powder is
           aligned in straight lines along the North-South magnetic flux lines. If
           there is a defect, the iron powder alignment is disturbed and flows
           around the defect.

     5.0   Ultrasonic Examination (UT)

           In UT examination, sound waves are generated by a power source
           and applied to the test piece through a transducer. Figure 7.4
           shows a pulse echo ultrasonic examination system. The sound
           waves pass through the test piece and are reflected back to the
           transducer either from the far side of the test piece or from a flaw
           that is located at an intermediate position within the test piece. By
           careful calibration, the UT operator knows if a flaw has been
           detected and knows its location and its size.

B.   Pressure Testing

     All pressure vessels that are designed to ASME Code requirements must
     be pressure tested after fabrication and inspection to demonstrate their
     structural integrity before they are placed into operation. The pressure
     test is made at a pressure that is higher than the design pressure. This
     excess pressure provides a safety margin since the vessel component
     stress levels during the test will be higher than those that will occur during
     operation. The objective of the pressure test is to bring the vessel to a
     high enough internal pressure, under controlled conditions, to demonstrate
     its mechanical integrity. Successful completion of the pressure test
     signifies that the vessel is acceptable for operation.




                                      152
         Cathode Ray Tube (CRT)




                A        C
                     B              Read Out

                                     Base Line


              Input-Output                            Cable
                Generator
                             Transducer

                               A

                    Couplant

            Test Specimen



                                          B

                               C



                                               Flaw
                         Pulse Echo UT System
                               Figure 7.4

Pressure tests are typically made using water as the test medium because
of the relative safety of water compared to a pneumatic test. The ASME
Code permits a pneumatic pressure test as an alternative to a hydrostatic
test under certain circumstances. However, a pneumatic test should only
be considered on an exception basis due to the increased safety risks
involved.

Since the hydrostatic test will almost always be used, only the hydrostatic
test will be discussed here. Refer to the ASME Code for pneumatic test
requirements.

The standard hydrotest pressure at the top of the vessel is calculated as
follows:

             PT = 1.5P (Ratio)




                                     153
Where:

               PT     =      Hydrotest pressure at the top of the vessel, psig
               P      =      Vessel MAWP (use vessel design pressure if
                             the MAWP was not determined), psig
               Ratio =       The lowest ratio of the allowable stress at the
                             test temperature to that at the design
                             temperature for the vessel materials used.

The following points must also be considered:

•   Hydrotest pressures must be calculated for the shop test with the
    vessel in the horizontal position, for the field test with the vessel in the
    final position and with uncorroded component thicknesses, and for the
    field test with the vessel in the final position and with corroded
    component thicknesses.
•   The calculated shop hydrotest pressure cannot exceed the test
    pressure of the flanged connections.
•   During the pressure test, the stress at any section of the vessel cannot
    exceed 90% of the material minimum specified yield strength (MSYS),
    based on use of the design weld joint efficiency (E).
•   Vessels also must typically be designed to permit a hydrotest in the
    field at a wind velocity that is typically 25-35% of the design wind
    velocity for the site.

During a field hydrotest, water at a specific gravity of 1.0 is used, and the
vessel is filled to the top. The larger specific gravity and fill height of
hydrotest water results in a higher weight and hydrostatic head load than
occurs during normal operation. Therefore, thicker plates are sometimes
required for lower sections of a tall tower than would be required for the
operational loads.




                                   154
VIII. Summary
This course provided an overview of pressure vessel mechanical design
requirements. It summarized the main components of pressure vessels and
discussed the scope of the ASME Code Section VIII, structure of Division 1,
materials of construction, design requirements and considerations, fabrication,
inspection and testing. Participants now have a good overall understanding of
pressure vessel mechanical design requirements, are prepared to use this
knowledge in their jobs, and have sufficient prerequisite information to take more
detailed pressure vessel courses.




                                       155
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