CONTROL VALVE

1. What is a ‘control valve’? 

A control valve is a final control element in a process line to control the process.

2. Explain the types of control valves?

1.Globe valve

2.Butterfly valve

3.Ball vale

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

A control valve whose trim has two seats n plugs. A double seated control valve is commonly used in high differential, huge capacity, turbulent flow lines.

CONTROL VALVE

ON-OFF VALVE

4.    What is a single seated balanced trim?

It is a trim having a single seat n plug 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.

ON-OFF VALVE

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

Direct acting : Stem extends when air pr in actuator increases. Air to extend.

Reverse acting : Stem retracts when air pr in actuator increases. Air to retract.

SMALL TYPE ON-OFF VALVE

STEM,PLUG,SEAT AND GAGE

6.    Explain the following on a control valve:

Trim, CV, Actuator, Stem travel, Valve body, Yoke, Gland packing

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

CV      : It is a valve coefficient, defined as the capacity index of a valve

as the number of u.s. gallons per minute of water that will flow through a

valve in the wide open position with a ressure drop of a valve in wide open

position with a pressure drop of a 1psi.it is used in the valve sizing

calculations.

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.

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

A control valve is said to be over sized 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 ensure actuator stem takes a position proportional to the input signal. It positions the diffrent types of valve actuators of different capacities and spring ranges by converting the low pr/capacity signal 3-15psi or 4-20a into high pr and large capacities. Iie to increase the speed of the response of the valve. To compensate the forcing effects of the fluids causing an unbalanced valve plug. To minimize frictional effects of glanding packing actuator stem packing. To split the valve travel and for reversing the valve action to the output of the controller by reversing the action, and change the valve characteristics. In case the require stroke length isn't achieved, then the positioner either increases or decreases its output until the valve achieves the desired stroke length.

11.Why is a valve positioner preferred even on a control 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 as 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 packings?

A sealing system in the valve body which prevents the process fluid coming out through the valve stem. Commonly used gland packing 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 it travels up and down.