Important Interview Questions based on CONTROL VALVE


#1

1. What is a control valve?

A control valve is the final control element, which directly changes the valve of the manipulated variable by changing the rate of flow of control agent. A control valve consists of an operator and valve body. The operator provides the power to vary the positionof the valve plug inside the body. The plug is connected to the operator by a stem, which slides through astuffing box.

##2. What are the different types of control valves?

The commonly used control valves can be divided as follows.

1.Depending on action:

Depending on action there are two types of control valves, (1) Air to close, (2) Air to open.

2.Depending on body:

  • Globe valves single or double seated.
  • Angle valves.
  • Butterfly valves.
  • Three way valves.

3. What is Cv of a valve?

Cv is the capacity of a valve and is defined as “No of gallons per minute of water which passes through a fully open valve at a pressure drop of 1 psi.

Cv = Q (ΔP/G)

Where: Cv = Valve co-efficient
Q = Volumetric flow rate (gallons/ minute)
ΔP = Pressure drop across the valve in psi.
G = Specific gravity of flowing fluid.

The valve coefficient Cv is proportional to the area ‘A’ between the plug and valve seat measured perpendicularly to the direction of flow.

4. What are valve positioners?

Valve Positioners are devices used to put the valve in the correct position by increasing or decreasing the air load pressure on the actuator.

Valve positioners are used for controlling valve where accurate and rapid control is required without error or hysteresis.

5. What is the use of a valve positioner?

The valve positioner is used for following reasons:

  • 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.

##6. What is the use of single seated valve and double seated valve?

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

In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure ai 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.

7. What are actuators?

Actuators are the devices which drive the valve stem. There are different types of valve actuators used in valves according to different purposes and based on actuating principle.

There are actuators such as:

  • Pneumatic actuators

  • Electrical actuators

8. What are the different types of plugs?

The different types ofplugs are generally used are:

  • V. port plug: Ported plug are generally used on double seated valves. This is because ported plugs, have a more constant offbalance area.

  • Contoured plug: Contoured plugs are generally used on single seated valve with small trim sizes.

9. What is a cage valve?

A cage valve uses a piston with piston ring seal attached to the sing1e seated valve “plug”. Here the hydrostatic forces acting on the top or the piston or below the valve plug tend to cancel out. The seat ring is clamped in by a cage. Cage valves are generally used for noise reduction.

10. What is a solenoid valve ?

A solenoid is electrically operated valve. It consist of a solenoid (coil ) in which a magnetic plunger moves which is connected to the plug and tends to open or close the value. There are two types of solenoid valves, Normally open and Normally Close.

11. How will you change the valve characteristics with positioner?

The positioner contains different types of came in it. selection of the proper cams in it.By selection of the proper cam the valve opening characteristics can be changed.

##12. How will you change the action of a control valve?

1 . If the control valve is without bottom cap. The actuator needs to be changed.
2. If bottom cap is provided:

a) Disconnect the stem from the actuator stem.

b) Separate the body from the bonnet.

c) Remove the bottom cap and the plug from body.

d) Detach the plug from the stem by removing the pin.

e) Fix the stem at the other end of the plug and fix the pin back.

0 Turn the body upside down. Connect it to the bonnet after inserting the plug and stem.

g) Connect back the stem to the actuator stem.

h) Fix back the bottom cap.

i) Calibrate the valve.

##13. Why does control valve operate at 15 psi?

On higher pressure the actuator sizes becomes bigger in area. The actual force produced by the actuator.

Force = Pressure x Area.

= 15psi xArea, If Area= 15”

Force produced = 15 psi x 25 in2 = 375 pounds.
Actual force acting on a control valve = 375 pounds.

14. What are the different valve characteristic?

Linear: The valve opening to flow rate is a linear curve

Equal percentage: For equal increments of valve opening it will give equal increment in flow rate range. At small opening the flow will also be small. …

Quick opening: At small opening the increments in flow rate is more. At higher opening the flow rate becomes steady.

15. An operator tells you that a control valve in a stuck ? How will you start checking?

  • First of all get the control valve is passed from operation.

  • Check the lingual to the diaphragm of the control valve.

  • Disconnect it possible the actuator stem from the control valve stem.

  • Stroke the actuator and check whether the actuator operates or not. It not then the diaphragm may be punctured.

  • If the actuator operates connect it back to the plug stem stroke the control valve. If it does not operate loosen the gland nuts a bit and see if it operates. If it does not then the control valve has to be removed from the line to w/shop.