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Control Valve Basics

Control Valve

 What is a control valve

A control valve is an instrument that regulates the flow of fluids (oil, water, or gases) as the valve position is changed by moving the stem with help of an actuator.
This actuator may be a motor of a pneumatic or hydraulic jack.
 The control valve is the most commonly used final control equipment in the control loop of an industrial process.
The control valve receives a control signal as input from plc or DCS, a controller is used to compare the setpoint and control valve actual position and then decides to open or close the valve according to match the set point.

Classes of  Control Valve on the base of motion

Two main types of valves are according to the motion of the stem.

1. Linear motion 

2. Rotary motion

1. Linear motion 

Linear motion is a movement of valve stem up and down, a common type of linear motion control valve is the Gate valve.

2. Rotary motion

 A rotary motion valve moves or rotates at an angle usually 90 degrees, common type of rotary motion valve is a Butterfly valve.

Parts of control valve

Control valves are made up of the following main parts.




1. actuator
2. Body
3. Bonnet
4. Stem
5. Seat
6. Packing

1. actuator

  An actuator is a device that is used to move or operate something.
In the case of a control valve, the actuator is a part of a valve that is used to move the valve through a stem.
Actuators used in the control valve are mostly of three types.

a. Electrical actuator
b. Pneumatic actuator
c. Hydraulic actuator

a. Electrical actuator

These are actuators that drive through electrical energy, for example, a motor.



b. Pneumatic actuator

Pneumatic actuators are operated through pneumatic or compressed air by using a pneumatic cylinder.

Following is a linear motion pneumatic actuator.



Following is a rotary motion pneumatic actuator.



c. Hydraulic actuator

Hydraulic actuators are driven through hydraulic energy using a hydraulic cylinder.




2. Body

The valve body is a part of the control valve that allows transmission of flow.
 Depending upon the applications of the valve, the valve body may be of a different type.




3. Bonnet

A bonnet is a cover on the valve body. Bonnets are available in many designs. Bonnet is usually a forged or cast part.

4. Stem

This is a part connecting between the actuator and valve body plug

5. Seat

This a part of the valve where the plug rests to stop the flow of fluid.

6. Packing

This is used to avoid leakage of fluid outside the valve.

Types of control valves according to working

On-off services:

Gate valve, Ball valve, Diaphragm valve, Plug valve, Butterfly valves are classified as an on-off servicing valve.

Throttling services

Globe valve, Butterfly valve, Diaphragm valve, pinch valve are classified as a throttling valve

Non-return

Check valves are also classified as a non-return valves.

Fail-Open Safe Mode

The actuator moves the valve to the open position in case of a power or signal failure, typically using a spring inside the pneumatic actuator to drive the stem to that position once air or electrical supply is lost. Fail-open is specified where an open valve is the safer condition — for example, a cooling water valve that should stay open to prevent overheating if control is lost.

Fail-Close Safe Mode

The actuator moves the valve to the closed position in case of a power or signal failure, again typically driven by a spring inside the pneumatic actuator. Fail-close is specified where a closed valve is the safer condition — for example, a fuel supply valve that should shut off automatically if control power or air supply is lost.

Selecting the correct fail-safe mode is a process safety decision, not just a mechanical preference — it should always be based on which position (open or closed) leaves the process in its safest state during a loss of power or signal, not on which is more convenient for normal operation.




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Comments

  1. JOHN MULINDI27 January 2021 at 13:10

    Well explained. As an instrument engineer, knowing about control valves helps your work in the industry.

    ReplyDelete
  2. Jagan Mohan Rao9 February 2021 at 14:40

    Very good article with lot of information, thanks for knowledge sharing.

    ReplyDelete
  3. Aamir malik creative eee17 February 2021 at 08:03

    thanks

    ReplyDelete

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