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									                    Industrial Instrumentation




Dr. –Ing. Naveed Ramzan

 PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   1
                       Pressure Sensors
             “In any given plant, the number of
              pressure gauges used is probably
              larger than all other instruments
                        put together”




PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   2
Practical Considerations in Pressure Measurements




  In industrial applications, good repeatability often is more
  important then absolute accuracy.


  If process pressures vary over a wide range, transducers with good
  linearity and low hysteresis are the preferred choice.


  Ambient and process temperature variations also cause errors in
  pressure measurements, particularly in detecting low pressures and
  small differential pressures. In such applications, temperature
  compensators must be used




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   3
Practical Considerations in Pressure Measurements

                                           Selection Criteria

Pressure transducers usually generate output signals in the millivolt range (spans of
100 mV to 250 mV). When used in transmitters, these are often amplified to the
voltage level (1 to 5 V) and converted to current loops, usually 4-20 mA dc. The
transducer housing should be selected to meet both the electrical area classification
and the corrosion requirements of the particular installation.

If the installation is in an area where explosive vapors may be present, the
transducer or transmitter and its power supply must be suitable for these
environments. This is usually achieved either by placing them inside purged or
explosion proof housings, or by using intrinsically safe designs.


The single most important decision in selecting a pressure transducer is the
                                          range
When high process temperatures are present, one can consider the use of various
methods of isolating the pressure instrument from the process. These include loop
seals, siphons, chemical seals with capillary tubing for remote mounting, and
purging.
   PEC – UET Lahore   Dr. Shahid Naveed        29. November 2011 /   Dr. –Ing Naveed Ramzan   4
Practical Considerations in Pressure Measurements

                                         Maintenance
 Without exception, pressure sensors require scheduled, periodic
 maintenance and/or recalibration.




  PEC – UET Lahore   Dr. Shahid Naveed    29. November 2011 /   Dr. –Ing Naveed Ramzan   5
Practical Considerations in Pressure Measurements


                                         Calibration

 Pressure transducers can be recalibrated

                            1. on-line or
                             2. in a calibration laboratory.


 Laboratory recalibration typically is preferred, but often is not possible or
 necessary.


 In the laboratory, there usually are two types of calibration devices:

 1. deadweight testers that provide primary, base-line standards, and
 2. laboratory or field standard calibration devices


  PEC – UET Lahore   Dr. Shahid Naveed     29. November 2011 /   Dr. –Ing Naveed Ramzan   6
                   Self Assessment Test




PEC – UET Lahore    Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   7
Question No. 1



 The difference between gauge and absolute pressure is




 A. a Vacuum

 B. 0.433 psia

 C. atmospheric pressure

 D. zero




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   8
Question No. 2



 Pressure is defined as




 A. Force per unit area

 B. F. A

 C. A/F

 D. None of the above




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   9
Question No. 3



 Metals used in the bourdon tubes should not undergo




 A. Fatigue

 B. hysteresis

 C. creep

 D. All of the above




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   10
Question No. 4



 A capsule diaphragm is made by welding two diaphragms




 A. to a solid base

 B. together at the center

 C. together around the edges

 D. To two other diaphragms




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   11
Question No. 5



 Dead weight tester is used for




 A. testing dead weights

 B. measuring process pressures accurately

 C.     producing high pressure

 D.       calibrating pressure instruments




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   12
Question No. 6



 One torr is defined as




 A. One mm Hg

 B. One inch Hg

 C.     One atmosphere

 D.     One kilopascal




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   13
Question No. 7



 Which gauge measures pressure by sensing changes in the thermal
 conductivity of the gas




 A. Pirani gauge

 B. Slack diaphragm gauge

 C. Mcleod gauge

 D. None of them




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   14
Question No. 8



 A thermocouple gauge is one type of




 A. Ionization gauge

 B. Thermal conductivity gauge

 C. Mcleod gauge

 D. None of these




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   15
Question No. 9



 Diaphragms used in pressure applications are




 A. light

 B. Small in size

 C. slack

 D. bimetallic




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   16
Question No. 10



 A Mcleod gauge can measure pressure as low as




 A. 0.05 torr

 B. 0.005 torr

 C. 0.0005 torr

 D. 0.00005 torr




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   17
Question No. 10



 What type of manometer is best for measuring low pressures




 A. Well

 B. Inclined

 C. U-Tube

 D. Multiple tube




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   18
Question No. 11



 A capacitance pressure transducer indicates changes in pressure by
 changing the




 A. Voltage output of an ac circuit

 B. frequency

 C. capacitance

 D. alternating current




  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   19
                                       Home Work

           Make similar 5 MCQs from
       Temperature and Pressure Sensors
               by each member.
       Solve Them and attached the sheet




PEC – UET Lahore   Dr. Shahid Naveed     29. November 2011 /   Dr. –Ing Naveed Ramzan   20
A Quick View! What we have covered?

Total Lectures held 9
 Fundamentals of Electrical Technology and
  digital logic employed in the measurement
 Review of Scientific principles employed in instruments
 Parts of Instrument
 Performance Characteristics of Instruments
 Selection and Calibration of Instruments
 Instruments Identification and Line Symbols
 Principle measurements desired in industry
  (a) Temperature
  (b) Pressure, Load
  (c) Level
                                                 Assignment
  (d) Flow                                         due on
  (e) Others ( Weight, Composition, pH etc.)     2-11-2010
   (f) Transducers
 Installation and Installation Costs
                                                 Marks : 20
 Case Studies
Quiz Held = 1      : Next Test due on = 19-10-2010
                                  Level Sensors




PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   22
Level Measurement

       Level is another common process variable that is
         measured in many industries. The method used will
         vary widely depending on the nature of the industry,
         the process, and the application.

    Inventory:
    -- a constant supply or storage of material
    Control:
    -- continuous, batch, blending, and mixing control
    -- stabilize flow to the next process
    Alarming:
    -- hi/lo limits, safety shut down
    Data Logging:
    -- material quantities for inventory and billing
       purposes and where regulatory requirements are
       necessary
  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   23
What is measured?

       The measured medium can be liquid, gas or solid
         and stored in vessels (open/closed tanks),
         silos, bins and hoppers.

                    Units of level can be expressed in:
                     feet (meters)
                     gallons (liters)
                     pounds (kilograms)
                     cubic volume (ft3, m3)
Methods ---- Direct or Indirect (inferential)



                                                                            Hydrostatic Head
                                                                            Float
                                                                            Load Cells
                                                                            Magnetic Level
                                                                             Gauge
                                                                            Capacitance
                                                                             Transmitters
                                                                            Magnetostrictive
                                                                            Ultrasonic
                                                                            Microwave
                                                                            Laser
                                                                            Radar
                                                                            Guided Wave
                                                                             Radar
                                                                            Dip Stick
                                                                            Vibration

   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   25
Direct Methods

       Direct methods sense the surface or
          interface of the liquid and is not
          affected by changes in material density
          (Specific Gravity)

             Examples:
                    Dip Stick
                    Resistance Tapes
                    Sight Glass
                    Floats
                    Ultrasonic
Indirect Methods (Inferential)




       Indirect methods “infer” liquid level by measuring
         some other physical parameter such as pressure,
         weight, or temperature.
       Changing materials means a corrective factor must
         be used or recalibrating the instrument.

                    Examples:
                     Hydrostatic head methods
                     Load Cells
                     Capacitance
                     Conductivity
Selection Criteria


   When determining the type of level sensor that
    should be used for a given application, there are
    a series of questions that must be answered:

    Open tank or closed tank?
    Can the level sensor be inserted into the tank or
     should it be completely external? Contact or non-
     contact?
    Continuous measurement or point measurement?
    Direct or Indirect measurement?
    What type of material is being measured? Liquid
     or Solid? Clean or Slurry?


   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   28
Selection Criteria


   For all liquids you will need:

    The system operating temperature with max. and
     min. excursions?
         two wide range – expensive the sensor

    The system operating pressure?

    Check that system ‘T’ and ‘P’ do not conflict with
     the materials of construction?




   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   29
Selection Criteria

   For Solids:

    Bulk density
         Be careful with very large silos as compaction
     at the bottom can greatly change assume bulk
     densities

    Flow characteristics?


    Expected particle size distribution?

    Is solid abrasive and/or corrosive and what is the
     moisture/solvent content?
   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   30
For Liquids

Dip Stick

                                           Simple and cheap
                                           Can be used with any wet
                                            material and not affected
                                            by density.
                                           Can not be used with
                                            pressurized tanks
                                           Visual indication only
                                            (electronic versions are
                                            available)

          RodGauge - similar to a dipstick found in a car, it has weighted
                  line markings to indicate depth or volume
   PEC – UET Lahore   Dr. Shahid Naveed      29. November 2011 /   Dr. –Ing Naveed Ramzan   31
For Liquids

Sight Glass
                                                                      Another simple
                                                                       direct method
                                                                       of measuring
                                                                       liquids.
                                                                      Can be used in
                                                                       pressurized
                                                                       tanks (as long
                                                                       as the glass or
                                                                       plastic tube can
                                                                       handle the
                                                                       pressure)
                Good for applications where non-contact measurement is needed (like beverages)


  PEC – UET Lahore     Dr. Shahid Naveed        29. November 2011 /    Dr. –Ing Naveed Ramzan    32
For Liquids

Floats
        Float rides the surface level to provide the
          measurement. Many different styles are
          available. Usually used for pump control,
          high/low level alarms and emergency shut-off




                      Liquid density does not affect measurement
   PEC – UET Lahore      Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   33
For Liquids

Conductivity Level Measurement




                  Point Level Measurement                            Continuous Level Measurement




                                       Advantages and disadvantages

                                                     Low Cost
                                       Conductive, non-coating liquids only
                                     Insulating coatings can cause problems



   PEC – UET Lahore    Dr. Shahid Naveed       29. November 2011 /       Dr. –Ing Naveed Ramzan   34
For Liquids

Resistance Tape
      The pressure of the fluid in the tank causes the
        tape to short-circuit, thus changing the total
        resistance of the measuring tape. An electronic
        circuit measures the resistance; it's directly
        related to the liquid level in the tank.




   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   35
For Liquids

Bubblers                                                                                 Instrument input
                                                                                          does not matter

  Bubblers allow the                                               P


    indicator to be                                                                       Regulated purge

    located                                                                                   system
                                                                                          (air or nitrogen)

    anywhere.
  The air pressure in
    the tube varies
    with the head
    pressure of the                                                                         Bottom of tube
                                                                                         determines reference
    height of the                                                                                point


    liquid.
 Can’t be used in closed tanks or where purging a liquid is not allowed (soap). Very
 popular in the paper industry because the air purge keeps the tube from plugging.
   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan          36
For Liquids

Bubblers                                                                                Instrument input
                                                                                         does not matter

 Advantages:                                                      P



                                                                                         Regulated purge
 -- Easy installation                                                                        system
                                                                                         (air or nitrogen)

 -- Continuous reading
    providing analogue or
    digital signal

 -- No moving parts
                                                                                           Bottom of tube
                                                                                        determines reference
 -- Good accuracy and                                                                           point
    repeatability



  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan          37
For Liquids

Bubblers                                                                                Instrument input
                                                                                         does not matter

 Limitations:                                                     P



                                                                                         Regulated purge
 -- Not suitable for                                                                         system
    pressurized tanks                                                                    (air or nitrogen)




 -- Sediments may block
    tube or probe

                                                                                           Bottom of tube
                                                                                        determines reference
 -- Tanks must be freely                                                                        point
    vented



  PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan          38
Hydrostatic Head Level Sensors


        These methods infer level by measuring
         the hydrostatic head produced by the
         liquid column.

        A pressure sensing element is installed
         at the bottom of the tank and pressure
         is converted to level.

        Different liquid densities or closed tank
         applications must be accounted for.


   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   39
Hydrostatic Head Level Sensors

    General Theory for Head Measurement

                                                        The Pressure exerted by
                  Liquid Density (D)
                                                          the Height of the liquid
 Height (H)
                                                          is:
                                                                  P = H x Density*
                                         Pressure PSI


                                                        If the Density of the
                                                           liquid is known then
                                                        H=        Pressure
                                                                  Density*
              *Note: For liquids other than water, use the density of water 0.0361 lb/in3 as a reference and
                                             multiply by the SG of the liquid.
Example
     A dip stick measurement of the level of these 2 tanks
     indicates 30 feet of liquid in both tanks. Calculate the
      pressure that each gauge will read if tank 1 contains
     water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85)

                                                     Oil
                Water
                                                  Density (D)
              Density (D)                Height
     Height
                                          (H)
      (H)

                                                                   Tank 2
                                Tank 1                              PSI
                                 PSI




                    P = ? psi                          P = ? psi
Example
     A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks.
    Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2
                                      contains oil (S.G. = 0.85)




                                                                      Oil
                  Water
                                                                   Density (D)
                Density (D)                              Height
      Height
                                                          (H)
       (H)

                                                                                    Tank 2
                                     Tank 1                                           PSI
                                         PSI




                                                                        P = ? psi
                      P = H x Density
                  = 30 ft x 0.0361 lbs/in3
                   = (30 x 12) x 0.0361
                              = 13 psi
Example
    A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks.
   Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2
                                     contains oil (S.G. = 0.85)




                                                                     Oil
                 Water
                                                                  Density (D)
               Density (D)                              Height
     Height
                                                         (H)
      (H)

                                                                                     Tank 2
                                    Tank 1                                            PSI
                                        PSI




                 P = H x Density x SG                                 P = H x Density x SG
               = 30 ft x 0.0361 lbs/in3 x 1                       = 30 ft x 0.0361 lbs/in3 x 0.85
                  = (30 x 12) x 0.0361                             = (30 x 12) x 0.0361 x 0.85
                             = 13 psi                                           = 11 psi
Hydrostatic Head Level Sensors (Cont‘d)


   Practical Considerations when using head type instruments
        The reference point of the tank vs
          instrument input must be considered.

                      Liquid Density                                                       Liquid Density
                           (D)                                                                  (D)
           Height                                                        Height
            (H)                                                           (H)                               Pressure
                                                                                                                 PSI



                                          Pressure PSI




  This may not be practical in some applications where the tank elevation
       is below grade or where a remote visual reading is required.

   PEC – UET Lahore   Dr. Shahid Naveed            29. November 2011 /            Dr. –Ing Naveed Ramzan    44
Hydrostatic Head Level Sensors (Cont‘d)


 Tank Elevations

          Vertical rises and drops contribute to the
            overall height and therefore head pressure.
            Horizontal runs have no effect.

                     Water
                   Density (D)
         Height
          (H)




                                                  P




                                 P
Hydrostatic Head Level Sensors (Cont‘d)

  Closed Tank Applications
            P   (atmospheric)




                                                 Open tanks are vented to
                                                   atmosphere so the pressure at
                                                   the bottom of the tank is only
                                                   due to the head pressure of the
                                                   liquid.
                                          P   Head   =hxD




                                                       Closed tanks are not vented to
                                                         atmosphere so the pressure at
                                P
                                                        the bottom of the tank is due to
                                    vapour



                                                       the head pressure of the liquid +
                                                         the vapour pressure above the
                                                                    surface.
                                     P   Head   = (h x D) + P vapour
Magnetic Level Sensor




                                                         Used where the sight
                                                           glass level gauge can
                                                           not be used.
                                                         Magneto-resistive types
                                                           can provide an
                                                           electrical output.




        Liquid/liquid interface (such as water and oil) can be measured by
                    changing the buoyancy of the magnetic float

   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   47
Displacers

     Not the same as a float.
     The displacer is immersed in the tank and the
      buoyant force of the liquid produces a torque
      which is proportional the amount of liquid level.



                                                                The output force
                                                                     can be
                                                                  converted to
                                                                    provide a
                                                                  proportional
                                                                 pneumatic or
                                                                    electrical
                                                                   continuous
                                                                 output of tank
                                                                      level.

   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /     Dr. –Ing Naveed Ramzan   48
Displacers

   Displacer – side mounted unit

                                   The displacer float size and
                                     diameter is selected
                                     according to the size of
                                     tank and the height of the
                                     level to be measured.
                                   The output force can be
                                     converted to provide a
                                     proportional pneumatic or
                                     electrical continuous
                                     output of level.

                                   Advantages and
                                     disadvantages:
                                    Very little movement of
                                     displacer float
                                    Affected by different liquid
                                     densities
                                    Limited to smaller tanks
                                     sizes
UltraSonic Level Measurement




                                           Non-Contact direct level
                                            sensor
                                           Level is a function of the
                                            time it takes an ultrasonic
                                            pulse to hit the surface and
                                            return


                                  Limitations include:
                  • Surface foam absorbs signal, agitation create reflections
                 • High Pressure & High Temperatures affect the signal speed
                        • Vapour and condensate create false echo’s

   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   50
Radar Level Sensors (Microwave)

Similar to ultrasonic but at a much higher frequency (6.3 GHz)
Various designs
-- Frequency Modulated
  Continuous Wave
-- Pulsed Wave
-- Guided Wave




These sensors have better performance in applications where vapour, dust or uneven
                                 surfaces exist.
   PEC – UET Lahore   Dr. Shahid Naveed   29. November 2011 /   Dr. –Ing Naveed Ramzan   51
Load Cells




                                          Tank level is determined by the
                                            weight of the quantity of
                                            material
                                          Load Cells (strain gauge
                                            transducers) placed at the
                                            bottom of the tank measure
                                            the weight and then convert it
                                            to an electrical signal.




   PEC – UET Lahore   Dr. Shahid Naveed     29. November 2011 /   Dr. –Ing Naveed Ramzan   52
Capacitance Level Detection (RF Level Probes)


         RF Capacitance level sensors are similar in operation to
         conductivity sensors and are typically used when non-
                    conductive liquids are measured.




         The only variable is the dielectric of the tank material that varies in proportion to the level.
               Conductive liquids like mineral water will short out the probe to the tank wall.
                                                 Good for solids
Can be used for non-liquids
                              RF capacitance
                                probes are used
                                in a wide range
                                of applications
                                and are
                                subjected to
                                certain
                                limitations such
                                as:
                                 Electrode
                                  Coating
                                 Low sensitivity
                                 Temperature
                                 Different
                                  materials
                                 Shielding &
                                  noise
Summary



      • Level is measured by locating the
      boundary between two media, called
                  the interface
      • Level can be measured directly or
                    indirectly
      • Noninvasive devices are preferred
         when the material is corrosive,
         hazardous, sterile, or at a high
            temperature or pressure
Creative Assignment!   HAZOP?????

Batch application using point level measurement
                                                                 Detect if the level
                                                                    is at a
                                                                    predetermined
                                                                    point
                                                                 Generally used to
                                                                    detect either a
                                                                    low-level
                                                                    condition or high-
                                                                    level condition
                                                                 The output of a
                                                                    point level sensor
                                                                    is on-off, or 1 or
                                                                    0 state digital
A batch process does NOT use a continuous level measurement device. signal
                      Discussion & Questions?




Dr. Shahid Naveed   29. November 2011 /        Dr. –Ing Naveed Ramzan
                                          57

								
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