INSTRUMENTATION QUESTION AND ANSWERS

INTRODUCTION :

It is the branch of engineering which deals with the measurement, monitoring,  display etc. of the various of energy exchanges which take place during process operations. "In short Instrumentation is the study of Instrument."

INSTRUMENT :

Instrument is a devices which is used to measure, monitor, display etc. of a process variable.

                                                                                                                                                                                                                                                                                                                                  

1. What are the process Variable ?

The process Variable are :

·        Flow.

·        Pressure.

·        Temperature.

·        Level.

·        Quality i. e. % O₂, CO₂, pH etc.

2. Define all the process Variable and state their unit of measurement. ?

Flow : Defined as volume per unit of  time  at specified temperature and pressure                                                                                              

           Conditions, is generally measured by positive-displacement or rate meters.

Units : kg / hr , litter / min, gallon / min , m³ / hr , Nm³ / hr. ( gases )

Pressure : Force acting per unit Area. P = F/A

Units      : bar , pascal , kg / cm² , lb / in².

Level : Different between two heights.

Units : Meters, mm, cm, percentage.

Temperature : It is the degree of hotness or coldness of a body.

Units              : Degree Centigrade, Degree Fahrenheit, Degree Kelvin, Degree Rankine.

Quality : It deals with analysis.( pH, % CO₂, % 0₂, Conductivity, Viscosity )

3. What are the primary elements used for flow measurement ?

The primary elements used for flow measurement are :

·        Orifice Plate.

·        Venturi tube.

·        Pitot tube.

·        Annubars.

·        Flow Nozzle.

·        Weir & Flumes.

4. What are the different types of orifice plates and state their uses ?

The different types of orifice plates are :

·        Concentric.

·        Segmental.

·        Eccentric.

·        Quadrant Edge.

Concentric :

The concentric orifice plate is used for ideal liquid as well as gases and steam service. This orifice plate  beta  ratio fall between of 0.15 to 0.75 for liquids and 0.20 to 0.70 for gases, and steam. Best results occur between value of 0.4 and 0.6. beta ratio means ratio of the orifice bore to the internal pipe diameters.

Eccentric :

The eccentric orifice plate has a hole eccentric. Use full for measuring containing solids, oil containing water and wet steam. Eccentric plates can use either flange or vena contracta taps, but the tap must be at 180º or 90º to the eccentric opening.

Segmental :

The segmental orifice place has the hole in the form segment of a circle. This is used for colloidal and slurry flow measurement. For best accuracy, the tap location should be 180º from the center of tangency.

Quadrant Edge :

It common use in Europe and are particularly useful for pipe sizes less than 2 inchs.

 

5. How do you identify an orifice in the pipe line. ?

Gas Service :On gas service the transmitter is mounted above the orifice plate because Gases have a property  of self venting and secondly condensate formation.

An orifice tab is welded on the orifice plate which extends our of the line giving an indication of the orifice plate.

6. Why is the orifice tab provided. ?

The orifice tab is provided due to the following reasons.

·        Indication of an orifice plate in a line.

·        The orifice diameter is marked on it.

·        The material of the orifice plate.

·        The tag no. of the orifice plate.

·        The mark the inlet of an orifice.

7.What is Bernoulli's theorem and where it is applicable. ?

Bernoulli's theorem states the "total energy of a liquid flowing from one point to another remains constant." It is applicable for non compressible liquids.

 

8. How do you identify the H. P. side or inlet of an orifice plate in line. ?

The marking is always done H. P. side of the orifice tab which gives an indication of the H. P. side.

9. How do you calibrate a D. P. transmitter. ?

The following steps are to be taken which calibrating :

1. Adjust zero of the Xmtrs.

2. Static pressure test    : Give equal pressure on both sides of the transmitter.  Zero should not shift. If it is shifting carry out static alignment.

3. Vacuum test : Apply equal vacuum to both the sides. The zero should not shift.

4. Calibration Procedure:

5.     Give 20 psi air supply to the transmitter.

6.    Vent the L.P. side to atmosphere.

7.    Connect output of the Instrument to a standard test gauge. Adjust zero.

8. Apply required pressure to high pressure side of the transmitter and adjust the span.

9. Adjust zero again if necessary.

10. What is the seal liquid used for filling impulse lines on crude and viscous liquid ?

Glycol.

11. How  do  you  carry  out  piping for  a  Different pressure flow transmitter on liquids, Gas and steam services ? Why ?

Liquid lines : On liquid lines the transmitter is mounted below the orifice plate because liquids  have a  property  of  self draining.

                                                                                            

Steam Service :On steam service the transmitter is mounted below the orifice plate with condensate pots.  The  pots should be at the same level.

 

Gas Service :On gas service the transmitter is mounted above the orifice plate because Gases have a property  of self venting and secondly condensate formation.

12. An operator tells you that flow indication is more ? How would you start checking ?

·        First   flushing  the  transmitter. Flush  both  the  impulse  lines. Adjust  the  zero by equalizing  if necessary. If still the indication is more then.

·        Check L.P. side for choke. If that is clean then.

·        Check the leaks on L.P. side. If not.

·        Calibrate the transmitter.

13. How do you do a zero check on a D.P. transmitter ?

Close one of the valve either H.P. or L.P. open the equalizing valve. The O/P should read zero.

14. How would you do Glycol filling or fill seal liquids in seal pots 7 Draw and explain.

·        The procedure for glycol filling is :

·        Close the primary isolation valves.

·        Open the vent on the seal pots.

·        Drain the use glycol if present.

·        Connect a hand pump on L.P. side while filling the H.P. side with glycol.

·        Keep the equalizer valve open.

·        Keep the L.P. side valve closed.

·        Start pumping and fill glycol.

·      Same repeat for L.P. side by connecting pump to H.P. side, keeping equalizer open and  H.P. side isolation valve closed.

·        Close the seal pot vent valves.

·        Close equalizer valve.

·        Open both the primary isolation valves.

 

15. How will you vent air in the D.P. cell ? What if seal pots are used ?

·        Air is vented by opening the vent plugs on a liquid service transmitter.

·        On services where seal pots  are  used  isolate  the  primary  isolation  valves  and  open  the vent valves. Fill the line from the transmitter drain plug with a pump.

16. Why flow is measured in square root ?

Flow varies directly as the square root of different pressure F = K square root of  AP. Since this flow varies as the square root of differential  pressure the pen  does  not  directly  indicate flow. The flow can be determined by taking the square root of the pen. Say the pen reads 50% of chart.

17. What is absolute pressure ?

Absolute pressure is the total pressure present in the system

Abs. pressure = Gauge pressure + Atm. pressure.

18. What is absolute zero pressure ?

Absolute zero = 760 mm Hg Vacuum.

19. What is the maximum Vacuum ?

The maximum Vacuum = 760 mm Hg.

20. What is Vacuum ?

Any pressure below atmospheric pressure is vacuum.

                  

21. What are the primary elements for measuring pressure ?

The primary elements used for measuring pressure are :

·        Bourdon tube.

·        Diaphragm.

·        Capsule.

·        Bellows.

·        Pressure Springs.

The above are known as elastic deformation pressure elements.

Type of Bourdon tubes.

·        ' C ' type.

·        Spiral.

·        Helix.

Diaphragm : The diaphragm is best suited for low pressure measurement.

Capsules     : Two circular diaphragms are welded together to form a pressure capsule.

Material Used : phosphor bronze, Ni-spanc stainless steel.

Bellows       : Bellows is a one - piece, collapsible, seamless metallic unit with deep folds    

                       formed from very tin walled tubing.

                      Material used : Brass, phosphor bronze, stainless steel. Used for high   

                       pressure.

Pre.spring : Pressure springs of helical or spiral shape used for measuring high pressures.

24.How will you calibrate an absolute pressure transmitter using vacuum manometer  Range 0-400mm abs. ?

The procedure for calibration is as follows :

·        Connect air supply to the transmitter.

·        Connect a test gauge of  0-1.4 Kg/cm2 to the output.

·        Connect Vacuum pump with tee off to the manometer.

·        Apply 760 mm Vacuum ( or nearest ) and adjust zero.

·        Apply 360 mm Vacuum adjust span. ( 760 - 360 = 400 mm abs. )

25. You are given a mercury manometer range 0 -760 mm ? A vacuum gauge reads 60 mm vacuum. The test manometer reads 50 vacuum ? Which of the two in correct.

The transmitter is correct because 760 - 50 = 710 mm abs.

26. Why is an inclined manometer used ?

It is used to extend the scale of the instrument. Because the manometer is at an angle to the vertical.

27. What is the principle of a pressure gauge ?

Pressure works on Hooks law.

Principle : "Measuring the stress in an elastic medium"

28. Draw and explain a pressure gauge ? What is the used of a Hair spring ?

The parts of a pressure gauge are :

1.      'C' type Bourdon tube.

2.      Connecting link.

3.      Sector gear.

4.      Pinion gear.

5.      Hair Spring.

6.      Pointer.

7.      Dial.

Uses of Hair Spring : Hair spring serves two purposes namely

·        To eliminate any play into linkages.

·        It serves as a controlling torque.