Field instrumentation interview Questions and answer

What are the different process variables?

  • Flow.
  • Pressure.
  • Temperature.
  • Level.
  • Quality i.e. % O2, CO2, pH etc

Flow, pressure, temperature is the basic three variables.

Define all the process Variable and state their unit of measurement

Flow: It is 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.

Pressure: It is defined as Force per unit Area. P = F/A
Units : bar, Pascal, kg / cm2, lb / in2.

Level: The height of the water column, liquid and powder etc., at the desired measurement of height between minimum level points to maximum level point is called level.

Units: Meters, mm, cm, percentage.

Temperature: It is the degree of hotness or coldness of a body is called temperature.
Units : Degree Centigrade, Degree Fahrenheit, Degree Kelvin, Degree Rankin.

What are the primary elements used for flow measurement?

The primary elements used for flow measurement are:

How do you identify an orifice in the pipeline?

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

Why flow is measured in square root?

The differential pressure measured has nonlinear relation with flow through the pipeline. Differential pressure developed by a venturiorifice platepitot tube, or any other acceleration-based flow element is proportional to the square of the flow rate. The square root extractor is used to convert the measured differential pressure into the flow rate.

How do you calibrate a D. P. transmitter?

The following steps are to be taken which calibrating:
1. Adjust zero of the Tx’r.
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:
* Give 20 psi air or 24V DC supply to the transmitter

* Vent the L.P. side to atmosphere.
* Connect the output of the Instrument to a standard test gauge or Multimeter and adjust zero.
5. Apply required pressure to high-pressure side of the transmitter and adjust the span.
6. Adjust zero again if necessary.

How do you do zero checks on a D. P. transmitter?

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


Explain direct and indirect level measurement:

Direct level measurement:

Bob and tape: A bob weight and measuring tape provide the most simple and direct method of measuring liquid level.

Sight glass: This consists of a graduated glass tube mounted on the side of the vessel. As the level of the liquid in the vessel change, so does the level of the liquid in the glass tube.

Indirect level measurement:

Pressure gauge: This is the simplest method, for pressure gauge, is located at the zero level of the liquid in the vessel. Any rise in level causes an increase of pressure which can be measured by a gauge.

Purge system: In this method, a pipeline is installed vertically with the level open and zero. The other end of the tube is connected to a regulated air supply and to a pressure gauge. To perform a level measurement, the air supply is adjusted so that the pressure is slightly higher than the pressure due to the height of the liquid. This is achieved by regulating the air pressure until bubbles are seen in the cabin leaving the open end of the pipe.

Differential pressure meter: Connections are made at the vessel top and bottom and to the two columns of the D.P. meter. The top connection is made to the L.P. column of the transmitter and the bottom to H.P. column of the transmitter. The difference in pressure in the vessel is balanced out since it is fed to both the column of the meter. The difference in pressure deducted by the meter will be due only to the changing, level of the liquid.

Displacer type level measurement: The level trol is one of the most common instruments used measuring level in closed tanks. This instrument works of Archimedes principle. The displacer in immersed in the liquid due to which there is loss of weight depending on the specified gravity of the liquid. This displacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart at the other end.

How will you reverse an action of the level trol?

The reversing are serves as motion take-off arm from the torque tube. It is provided with a slot on each side of the centre so that link can be connected either for reverse or direct action.

What will happen if the displacer has fallen down while in line?

The output will be maximum.

What will happen if the displacer has a hole in it while in line?

The output will be minimum

What is Pt 100 mean?

Pt100 means 100 OHMS at 0 degrees C for a platinum resistance bulb.

What are two wire and three wire R.T.D. system?

Two wire R .T .D. system: Two wire RTD system use for short distance like a compressor field local panel.
Three wire System: Three wire systems use for long distance coke a field to control Run. The third wire is used for compensation of lead wire resistance.

Why is a converter used in a temp. recorder ?

The converter is designed to convert D. C. input voltage into an A. C. input voltage proportional in amplitude to the input.

Why are Thermowells used ?

In numerous application it is neither desirable nor practical to expose a temperature sensor directly to a process material. Wells are therefore used to protect against damage corrosion, erosion, abrasion and high pressure processes. A thermowell is also useful in protecting a sensor from physical damage during handling and normal operation.

What is the specialty of thermocouples lead wires ?

They should be of the same material as the thermocouple

What is the difference the Wheatstone bridge and a potentiometer?

The difference between a potentiometer and a Wheatstone bridge measuring instrument is that potentiometer is a voltage measuring instrument and Wheatstone bridge is a current measuring instrument.

Explain tuning of controllers

Tuning basically involves adjustment of proportional. Integral and derivative parameters to achieve good control. The gain, time constants, and dead times around the loop will dictate the settings of various parameters of the controller.
Tuning methods are broadly classified into two :
1. Closed Loop Method: e.g. Ultimate Gain Method
2. Open Loop Method: e.g. process Reaction curve

What is the use of single seated valve?

The single seated valve is used on smaller sizes, and in a valve of larger sizes, where an absolute shut off is required. The use of a single seated valve is limited by the pressure drop across the valve in the closed or almost closed position.

What is the use of double seated valve?

In double seated valves the upward and downward forces on the plug due to the reduction of fluid pressure are nearly equalized. It is generally used on bigger size valves and high pressure systems. Actuator forces required are less i.e. a small size actuator.

What types of bonnets would you use of high temp? and very low temp.?

High temperature: Bonnets are provided with radiation fins to prevent glad packing from getting damaged. On very low temperature: Extended bonnets are used to prevent gland packing from getting freeze.

How will you work on a control valve while it is line?

While the control valve is in line or in service, it has to be bypassed and secondly the line to be depressurized and drained.

What is the use of a valve positioner?

  • Quick Action control valve.
  • Valve hysteresis.
  • Valves used on viscous liquids.
  • Split Range.
  • Line pressure changes on valve.
  • Valve Bench set not standard.
  • Reversing valve operation.




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