Control Valve Interview Questions and Answers

Q. What is a control valve?

The control valve is the final component that directly changes the value of the manipulated variable by changing the rate of flow of the control agent.

A control valve is a consists of an the operator and the valve body. The operator provides the power to change the position of the valve plug inside the body. The plug is connected to the operator by a stem, which slides through the stuffing box. An air signal from the controller is applied above the diaphragm. An increasing air signal from the controller is applied above the diaphragm. The rising air signal will push down the operator's stem against the force exerted by the spring on the diaphragm plate. The valve is adjusted so that the plug starts moving when 3 psi is applied to the diaphragm and touches the seat when 15 psi is applied to the diaphragm. As the air pressure increases, the valve will close. So the house "want to close". Another type is "air to open", such that a value of 3 psi on the diaphragm closes and 15 psi indicates air to open fully.

This allows for direct flow rate control and consequent control of process parameters such as pressure, temperature and liquid volume.

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Q. What are the different types of control valves?

Basically the control valve is divided into two categories first depending on the action and second depending on the body.

Depending on the action: Control valves in this category are of two types firstly air to close and air to open.

Depending on body: The type of control valve in this category are globe valve, angle valve, butterfly valve and three way valve.

Q. What is the air of a closed control valve?

Basically this control valve closes due to increase in air pressure is called air to close control valve.

Q. Where is air to close control valve used?

Basically air to closed control valve is used for cooling water line, tide lines and safety relief line.

Q. What is the air required to open the control valve?

Basically this control valve is opened due to increase in air pressure, it is called air to open control valve.

Q. Where is air used to open a control valve?

Basically steam line and feed line are used for air to open control valve.

Q. What is the part of control valve?

Control valve parts are actuator, positioner, tension nut, gland packing, bonnet, scale, feed back linkage, grease nipple, spring, plug, seat, body, diaphragm, diaphragm case, booster, limit switch.

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Q. What is the CV of a control valve?

Basically the capacity of a control valve which indicated the number of US gallons of water flowing through a fully open valve at 15°C at a pressure drop of 1 PSI/m.

Q. Where is on/off control valve used?

On/off control valves are commonly used when precise control is not required, processes with sufficient capacity to allow the end operator to continue the measurement cycle, and on/off control valves is primarily used in the  refrigeration system as well as air conditioning system.

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Q. Why does the control valve operate at 15 PSI?

Basically at higher pressure the size of the actuator is larger in area.

Force = Pressure * Area

Q. How can a control valve be opened quickly?

Positioners have different types of cams. By selecting the correct cam in it and replacing the valve plug we can open the control valve quickly.

Q. What material should be used for gland packing of high pressure and temperature control valves?

Graphite.

Q. What materials are commonly used for the construction of control valve diaphragms?

Rubber.

Q. What materials are commonly used for the construction of control valve bodies?

mild steel.

Q. What are the valve characteristics of a control valve?

Valve characteristics of control valves are linear, quick opening and equal percentage.

Q. What is a bench set in a control valve?

The control valve has a bench set spring range seating.

Q. What are valve accessories in control valves?

Positioner, Regulator, I/P and Pressure Gauge.

Q. When specifying a control valve, where would you specify the extension bonnet?

Extension bonnets are used for low temperature service, i.e. for fluid temperatures below -20 degrees Fahrenheit. Its purpose is to prevent the packing box from freezing. Heating coils are sometimes wrapped around the bonnet to achieve this.

Q. How do you work on a control valve line?

When the control valve is in line or in service, it must be bypassed and secondarily the line must be depressed and drained.

Q. How do you change the action of a control valve?

• If the control valve is without a bottom cap. A real need to change.
• If the bottom cap is provided.
• Disconnect the stem from actuator stem.
• Separate the body from the bonnet.
• Remove the bottom cap and plug from the body.
• Separate the plug from the stem by removing the pin.
• Fix the stem to the other end of the plug and fix the pin back.
• Turn the body upside down. After inserting the plug and stem attach it to the bonnet.
• Reconnect the stem to the actuator stem.
• Fix the bottom cap back.
• Calibrate the valve.

Q. What effect does a pipe reducer have on valve capacity?

When control valves are installed in pipe reducers, the actual valve capacity is reduced. Reduces excess pressure in the system by acting as a riser contraction in series with the reducer valve.

Q. A control valve in stuck. How to start an inspection?

• First of the all get is control valve is passed from operation.
• Check the tongue of the diaphragm of the control valve.
• Disconnect the actuator stem from the control valve stem as far as possible.
• Stroke the actuator and see if the actuator moves. Otherwise the diaphragm may be punctured.
• If the actuator is running, connect the plug stem back to the stroke control valve. If that doesn't work, loosen the gland nut a bit and see if that works. If not, the control valve will have to be removed from the line to the shop.