Control valve interview questions and answer


#1

What is a Control valve?

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A control value is the final control element, which directly changes the valve of the manipulated variable by changing the flow rate of the control agent.

A control valve consists of an operator and a valve body. The operator provides the power to vary the position of the valve plug inside the body. The plug is connected to the operator by a rod, which slides through a packing box. The air signal from the controller is applied to the diaphragm.

What are the different types of control valves ?

The commonly used control valves can be divided as follows.

  1. Depending on Action.

  2. Depending on the Body.

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:

  1. Globe valves single or double seated.

  2. Angle valves.

  3. Butterfly valves.

  4. Three-way valves.

What is the use of single seated valve?

The single seat valve is used in smaller sizes and in larger sized valves, where absolute closure is required. The use of a single seat valve is limited by the pressure drop across the valve in the closed or nearly closed position.

What is the use of double seated valve?

In double-seat valves, the forces up and down in the plug due to the reduction in fluid pressure are almost equal. It is generally used in larger valves and high pressure systems. The required actuator forces are less, that is, a small-sized actuator.

What is Cv of a control 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 ( ▲P / G )

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

How to define seat leakage class for control valves?

A control valve’s ability to shut off has to do with many factors. Seat material, actuator thrust, pressure drop, and the type plug (balanced or unbalanced) can all play a part in how well a particular control valve shuts off. Seat Leakage Classifications.

There are actually six different seat leakage classifications as defined by IEC 60534-4 std or ANSI/FCI 70.2. They commonly range from CLASS IV to CLASS Vl CLASS IV is also known as metal to metal.

What is the difference between flashing and cavitation?

Cavitation is a condition that occurs in the liquid flow where the internal pressure of the liquid, at some point, falls below the vapour pressure and vapour bubbles formation takes place. After this point, the pressure rises above the vapour pressure again and during this pressure recover stage (recall also the concept of FL) the bubbles collapse, and cavitation takes place.

Flashing is a condition that occurs with the liquid flow where the pressure falls below the vapour pressure (same as cavitation) and remains below it. There are then two phases flowing (i.e. liquid and vapour) downstream and no collapsing of bubbles takes place.

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.

When can a bypass be not used on a positioner ?

A by pass on a positioner cannot be used when :

  1. Split Range operation.

  2. Reverse Acting Positioner.

  3. Valve bench set not standard.

What is the use of three-way valves?

Three-way control valves are only used on special systems, where a dividing or mixture of flows according to a controlled ratio is required.

Why the orifices of Pressure Safety Valves indicated by manufacturers are with non-API dimensions, even though designation is as per API?

The API 520 recommends making a preliminary dimensioning of the safety valves with the proposed flow coefficients 0.975 for gas/steam, 0.65 for liquids and the effective orifice areas written in API 526 (D to T). Then, the API uses effective discharge coefficients and effective areas. In the end, this standardizes flow capacity and allows engineers to select valve sizes in the initial stages.

What valves are stop valves?

Stop valves are used to shut off or, in some cases, partially shut off the flow of fluid.

Stop valves are controlled by the movement of the valve stem.

Stop valves can be divided into four general categories: globe, gate, butterfly, and ball valves.

What is the use of link connected to the valve positioner?

The link serves as the feedback to the value. The movement of the ant valve is detected by this link. Sometimes, due to pressure changes in the line in H.P. the position of the valve can be changed, the link, in turn, detects this change and the positioner will produce an output that will operate the valve to the original position.

How will you change the action of a control valve?

  1. If the control valve is without bottom cap. The actual needs to be changed.

  2. If bottom cap is provided.

  • Disconnect the stem from the actuator stem.

  • Separate the body from the bonnet.

  • Remove the bottom cap and the plug from body.

  • Detach the plug from the stem by removing the pin.

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

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

  • Connect back the stem to the actuator stem.

  • Fix back the bottom cap.

  • Calibrate the valve.

When can a bypass be not used on a positioner?

A by pass on a positioner cannot be used when :

  1. Split Range operation.

  2. Reverse Acting Positioner.

  3. Valve bench set not standard