Interview Questions on Control Valves

1. What is a ‘control valve’?

A control valve is a final control element (end element) used in a process line to control the process.

 

Interview Questions on Control Valves

2. Explain the types of control valves?

Commonly used control valve types are:

Globe valve

   Butterfly valve

Ball vale

3.What is a double seated control valve? Where in general is it used?

It is a control valve whose trim (plug and seat) has two seats (contact points). A double seated control valve is commonly used in high differential, huge capacity, turbulent flow lines.

4.What is a single seated balanced trim?

It is a trim having a single seat and balance holes on its plug. A single seated balanced trim is used in a process line where the DP across the valve is high. These kind of trims are useful in reducing the vibration on the valve body and also assists in closing the valve.

Also Read: DCS Interview Questions with Answers

5.What is a ‘direct acting’ and ‘reverse acting’ control valve?

Direct acting control valve    : The valve port closes on air failure.

Reverse acting control valve: The valve ports on an air failure.

6. Explain the following on a control valve:

 Trim                : Trim is a matched pair of ‘plug’ and ‘seat’

CV                  : The amount of water flow in gallons through the control valve when the valve port is                           fully open and the pressure differential across the valve is 1 Psi.

Actuator         : The drive unit having a diaphragm and a piston that operates the valve stem.

Stem travel: The scale that shows the stem movement in inches or centimeter.

Valve body: The bottom portion of a control valve installed on a pipeline to control a process. The valve body contains a trim, pipe flange, bonnet, gaskets, guide bush, gland packing, lantern ring, grease, cooling fins…etc.

Yoke               : A portion of the actuator which connects the actuator to the valve body carrying a                                 stem travel plate.

Gland packing:   A sealing system in the valve body which prevents the process fluid coming out                                   through the valve stem.

Also Read: Valves Interview Questions with Answers

7.What is an ‘over sized’ control valve? How to solve an oversize problem?

A control valve is said to be oversized when a minimum signal to the valve (minimum opening of the valve) brings process to the set point rapidly. This situation leads to an imbalance and high gain in the control loop. The problem can be solved by changing the valve trim to a reduced trim size or by replacing the control valve for the correct size.

8. What is a bench set on a control valve actuator?

On a pneumatic control valve, this is the minimum and maximum air pressure to be applied to achieve the full stroke length of the actuator.

9.Why does a bench set differ on actuators of the same size?

A higher bench set actuator is selected when the control valve is operating on a high pressure process line.

10.What is a valve positioner? How does it work?

Valve Positioner is a unit used on a control valve to keep the valve in position.It works as a booster relay with a valve stroke feed back. A controller output is fed to the valve positioner and the valve position provides an output to the control valve actuator to achieve the pre-calibrated stroke length. In case the require stroke length isnt achieved, then the positioner either increases or decreases its output until the valve achieves the desired stroke length. This situation may occur when there is a great change in the process pressure, gland is too tight…etc.

11.Why is a valve positioner preferred even on a control valve having a bench valve having a bench set 20-100 kPa?

It makes the valve response (movement) quicker.

When there is a load change in the process, the positioner assists the valve to achieve the stroke length per the controller’s instruction.

12.What are the possible reasons if a control valve fails to open/close?

Gland packing is too tight

Actuator bench set is not proper or suitable

Process load change is too high

Trim is an unbalanced type

Actuator spring is broken

Air supply to the valve positioner / actuator is close or high ..etc

13.What is ‘gland packing’? Name the types of commonly used gland packing in P.D.O.?

A sealing system in the valve body which prevents the process fluid coming out through the valve stem.

Commonly used gland packing in PDO are Teflon, Graphite Asbestos..etc

14.What is a ‘lantern ring’? Where is it located on a control valve?

A greasing ring placed in the valve bonnet across the gland packing. The unit provides lubrication to the valve stem when lit travels up and down.

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