Control valve

A control valve is a pneumatic and power-operated (driven by an external energy source) device that manipulates/regulates fluid (gas, steam, water, or chemical compounds) flows in a process system to keep a desired operating variable, like pressure, temperature, or flow rate.

[2] It's a critical part of a control loop and is a common example of a final control element (it takes the output signal from the controller anddirectly change sht ephysical process) in modern industries.

Operation

[2] A control valve can receives voltage from a controller like a PLC or DCS (distributed control system) to operate. It compares the actual flow rate (process variable) to the desired flow rate (set point). The controller outputs a signal to the valve valve to bring the flow rate to the set point value.

[2] Since a PLC is electrical, a control valve may need a control device like a I/P (current-pressure) transducer to convert that electric signal for it to operate.

Fail-safe mode

[2] A control valve may fail if the loss from the control system occurs, a position known as the "fail-safe" mode. As the supply air pressure is raised, the rubber diaphragm pushes against the spring and mvoes the valve stem down to the valve body. Reducing the supply air pressure makes the spring to move out of the body.

[2] The type of "fail-safe" mode depends on the application the control valve it's used for.

The actuator causes the valve to close in a "fail-closed" (FC) control valve.

The actuator causes the valve to open in a "fail-open" (FO) control valve.

An ATO control valve in fail-safe mode would be a "fail-closed" valve and its valve would fail in close position, without air pressure supplied.
An ATC control valve in fail-safe mode would be a "fail-open" valve and its valve would fail in open position, without air pressure.
- mnm: Remember that
  "fail-open" means it fails in open position and
  "fail-closed" means it fails in closed position.

[2] In cases of a "power failure" to the actuator, the electric/pneumatic force is gone and the spring pressure forces the valve to open.

ATO operation mode

The air-to-open (ATO) mode is one of the 2 main operation modes (fail-safe mode) of a pneumatic control valve actuator. The valve's operation mode depends entirely on how the actuator and spring's arrangement.

In an ATC control valve, in failed-closed mode, applying air pressure opens the valve. Increasing air pressure pushes the valve open. If air pressure is removed/lost, the spring automatically pushes the valve fully closed.

ATC operation mode

The air-to-close (ATC) mode is the .

In an ATC control valve, applying air pressure closes the valve. Increasing air pressure forces the valve shut. With air pressure removed/lost, the internal spring automatically pushes the valve fully open.

Control air ranges

The control air ranges are standard industrial signal spans, often ranging from 3 (20 kPa) to 15 PSI (100 kPa). The pressure span is defined mathematically as the difference between the max and min pressure: Span = Pmax - Pmin.

In an ATO control valve, its opening span is calculated as

Assuming the standard range:

E.g., At 3 PSI, the ATO valve is (3PSI - 3PSI)/(15PSI - 3PSI) x 100% = 0% open and 100% closed.

In an ATC control valve, its opening span is calculated as

E.g., At 3 PSI, the ATC valve is 100% - (3PSI - 3PSI)/(15PSI - 3PSI) x 100% = 100% open and 0% closed.

[2]

Components

A control valve has 2 main components:

The actuator provides the force to move by driving the valve stem or shaft between to the desired position. It receives a signal from the controller and converts it (pneumatic air, electricity, or hydraulic fluid) into mechanical motion. In normal operations, the spring's force must be overcome by the actuator.
The valve (valve body or just "body") controls the fluid flow, being the physical housing and internal mechanism in the pipe. As the actuator moves the valve's stem, the internal plug alters the opening's size in the pipe to regulate throttle or stop fluid flow.

[4]

[5]

[3]

The following components are typically found in a control valve's actuator:

A control valve's spring balances pneumatic or fluid pressure against mechanical force to position the valve stem and provide a safe default state upon power loss, working in 2 main actions: force opposition and fail-safe positioning.

Air or fluid pressure pushes against a diaphragm or piston, creating a force that compresses the internal spring. The valve moves until the pressure force equals the opposing spring force.

The following components are typically found in a control valve's valve body:

The valve stem is the rod moving up and down connected to the actuator and valve plug to regulate flow through the valve, connected betwen the actuator and valve plug. It transfers mechanical motion (push/pull or rotation) generated by the actuator directly down to the valve body to move the valve plug up and down (or rotate it).
- stem: rod/central axis
- plug: anything used to block/fill/seal hole/gap
The valve plug is the moveable throttling element in the body, connected at the end of the plug.

Types

A control valve can be operated electrically, pneumatically, or hydrolically, [2] but the most reliable and used are the pneumatic types.

[2]

[2] Valve bodies are of various types depending on service conditions, design applications, layouts.

Control valves can also be categorized as linear motion and rotary motion:

[2]

Linear and rotary motion

[2] A linear motion.

In a linear motion valve, the stem and valve move up and down.

A rotary motion valve often rotates 90° from open to the closed position. A common type of rotary motion valve is the butterfly valve, as shown on the left.

[2] Sliding control valves are control valves that , like the gate valve.

Applications

References

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Control valve

Operation

Fail-safe mode

ATO operation mode

ATC operation mode

Control air ranges

Components

Types

Linear and rotary motion

Applications

References

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