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Determination of Coke Drum Fitness for Service _amp; Remaining Life

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					Determination of Coke Drum
     Fitness for Service
              &
      Remaining Life
        Richard Boswell, P.E.
    Stress Engineering Services, Inc.
              Houston, TX
                and
            Tom Farraro
  CITGO Refinery Lake Charles, LA
          CITGO F-201 COKE DRUMS
n   Built 1968               n   ASME VIII/ Div 1
n   Diameter = 21 feet       n   7 Plate Rings
n   Cyl Length = 70 feet     n    Top : 1” Clad plate
n   Top : 60 psi at 836 oF   n   Bottom : 1.64” Clad
n   Bottom : 60 + Hydro      n   Mat’l : SA-515 Gr. 55
    at 899 oF                n   Clad : 7/64” 405 SS
n   18 hr Coking Cycle       n   Replaced in 1996 after
                                 28 years
      How Do Coke Drums Fail?
n   Primary failure mechanisms are:
    – Bulging and distortion of shell plates typically
      20 - 40 ft above the skirt attachment weld.

    – Circumferential cracking adjacent to welds and
      bulges both (OD and ID initiated).

    – Cracking and bulging in area of skirt to shell
      attachment weld.
      Why do Coke Drums Fail?
n   Typically coke drums are designed as
    pressure vessels utilizing the ASME
    Pressure Vessel Code.

n   The ASME code assumes that internal
    pressure is the primary (largest) stress to
    which a pressure vessel is subjected.
Actual Measured Coke Drum Shell Stress




   5


Maximum Hoop Stress
due to Pressure
Erroneous Design Assumptions
n   Erroneous Assumption # 1

    – Primary stress is a coke drum is caused by internal
      pressure.

n   Erroneous Assumption # 2

    – The 3 to 1 safety factor built into the ASME code is
      sufficient to accommodate “secondary” stresses
      caused by thermal gradients, fatigue and bending.
How Can We Identify the Source of
the Primary Stress in Coke Drums?
n   Dynamic modeling of coke drum operation using
    the finite element method (FEM)was utilized to
    determine source of the high measured stresses.


n   Analysis was based on actual shell stress/strain
    data obtained by monitoring 300 + coke drum
    cycles 1.5 (Years)
       Results of FEM Analysis
n   FE analysis indicated 3 sources of high
    stresses measured on coke drums:

    – Localized hot spot (thermal gradient stress)

    – Coke/shell differential shrinkage stress

    – Skirt/shell attachment (thermal gradient stress)
    FEM Results (bulge analysis)
n   Corrugations influence membrane and surface
    stress across bulge.

n   Higher stress due to larger diameter.

n   Higher stress due to ring bending moments.

n   Larger membrane stress at “valley” of bulge.

n   Circ. welds are typically at “valley”.
          Life Limiting Factors
n   Low Cycle Fatigue Life.

n   Reduction of wall thickness in bulge
    “valleys”.

n   “Squatting” and leaning of drum due to
    bulge collapse.
        Low Cycle Fatigue Life
n   Hoop and axial stresses are cyclic and
    average 40-60 ksi.

n   Yeild strength of the base metal was found
    to be 32ksi.

n   Low cycle fatigue was determined to be a
    life limiting factor.
                      Stages of Fatigue Life in Steel

                      0.80
                      0.70
Length of Crack in.




                      0.60
                      0.50
                      0.40
                      0.30
                      0.20
                      0.10
                      0.00
                                                                                              3rd
                                        1st Stage                                2nd Stage   Stage
                      (0.10)
                               0   10     20    30      40    50    60    70       80   90    98
                                                     Fatigue Life Expended (%)
      Fatigue Life Determination
n   Drums operated at 24hr cycle for 20yrs and 19
    hour cycle for 8 years.

n   Total cycles on drums at present is 1845 + 3652 =
    5497 cycles.

n   Using 50 ksi as average cyclic stress fatigue life
    from ASME VIII Div 2 curves = 5000 Cycles.
                       Fatigue Limits for Carbon and
                             Low Alloy Steels
               10,000,000
Applied Stress (psi)




                       1,000,000



                        100,000
                           50 ksi


                         10,000



                          1,000
                                    10   100   1,000      10,000   100,000   1,000,000
                                                       5000
                                               # of Cycles
Laser Profile Map of Coke Drum
Bulge Growth Rate




       Shell ID 126”
Wall Thickness in Valley Between Bulges



                          1.64 “nom.



                           1.44” nom   .
        STRUCTURAL
     CHARACTERIZATION
n Thermal gradients during quench are not
  sharp
n Cylindrical flexibility accommodates
  shrinkage without high stress
n Thermal gradients through wall are reduced
  by inertia and insulation
    FITNESS FOR SERVICE ISSUES

n   How do bulges affect structural integrity?


n   How predictable is crack/bulge growth?


n   Can End of life be predicted?

n   Can we continue to operate these vessels in a
    reliable, and predictably safe manner?
        STRUCTURAL
     CHARACTERIZATION
n Over 300 cycles monitored with Strain
  Gages and Thermocouples
n Hoop and Axial stress range +/- Yield
n When range is higher, defect damage can be
  accelerated.
n Limited monitoring of a few cycles could
  capture minimum loading : false security
      MAXIMUM BULGING
n F-201 B Nominal inside radius = 126”
n November ‘94 survey : 8.4% bulge
n May ‘95 survey : 9.5 % bulge
n Other profiles did not increase this much
      THICKNESS SURVEY
n Vertical and Circumferential surveys
n At circ. weld, wall thinned by 15% for 6”
  both above and below
n No significant thinning at bulges
n Drums are shortening
        BULGE ANALYSIS
n A series of finite element models made for
  selected contours
n Drum is a corrugated cylinder
n Manually digitized at 1 foot intervals
n Axisymmetric shell model in ABAQUS
n Loaded by internal pressure with
  hydrostatic gradient to bottom
       STRESS ANALYSIS
n Corrugated Cylinders Have Higher Stress
  than Straight Cylinders
n Consider Ring Bending Moment Stress as a
  Primary Stress
n Compare Pressure + Ring Bending to
  Primary Bending Stress Allowable
n New Pressure Rating Should be Considered
             SUMMARY
n Awareness of operation and response is
  essential for FFS evaluation
n Drums experience very high short duration
  stress during quench
n Occasionally this stress is very large
n Inspection is justified after large stress
  cycles

                Stress Engineering Services    11
              SUMMARY
n Long term results defined load conditions
  for new drums
n Designed beyond the scope of a pressure
  vessel
n Improved economics and safety
n Greater reliability
n Less maintenance, longer life
TYPICAL COKING CYCLE
Typical Stresses During Quench Cycle
OPERATING HOOP STRESS




                        18
OPERATING AXIAL STRESS




                         19
OPERATING MEMBRANE
       STRESS




                     20

				
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posted:8/18/2011
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