The controller compares the actual flow rate to the desired flow value called the setpoint.

The controller will produce an output to move the valve

to bring the flow rate to the setpoint value.

A control valve is a power-operated device

used to regulate or manipulate the flow of fluids,

such as gas, oil, water, and steam.

It is a critical part of a control loop

and is an example of a final control element.

The Control Valve is by far the most common final control element used in industry today.

A control valve can be operated electrically, pneumatically, or hydraulically.

A control valve receives a signal from a controller

such as a PLC in order for it to move resulting in a change in flow.

Because the PLC signal is electrical,

the control valve may require a device

to convert that electrical signal so that it can operate.

A control valve has two separate components:

the “Valve” and the “Actuator”.

Valve bodies are of different types

or styles depending on service conditions,

piping layout, and desired application.

The two classes of control valves are “Linear Motion” and “Rotary Motion”.

On a linear motion valve,

the stem and valve movement is up and down.

A common type of sliding control valve is the “Gate Valve”.

A rotary motion valve in almost all cases

rotates 90° from open to the closed position.

A commonly used rotary valve is the “Butterfly Valve”.

The actuator is the device connected to the valve through the valve stem

that provides the force required to move the valve.

As we said earlier, the actuator can be controlled electrically,

pneumatically, or hydraulically.

The most common and the most reliable is the “Pneumatic Actuator”.

The control valve receives a signal from a controller

such as a PLC or a DCS in order to operate.

The controller compares the actual flow rate

to the desired flow value called the setpoint.

The controller will produce an output to move the valve

to bring the flow rate to the setpoint value.

Because of how pneumatic actuators are built,

a control valve will fail to a specific position

when a loss of the control signal occurs.

As the supply air pressure is increased,

the rubber diaphragm pushes against the spring

and moves the valve stem down into the valve body.

As the supply air pressure is reduced,

the spring will move the valve stem out of the body.

The position to which the control valve is moved

if a loss of signal occurs is referred to as the “fail-safe” mode.

The type of “fail-safe” mode depends on the application

for which the control valve is used.

The actuator causes the valve to close

in a “Fail-Closed Control Valve”.

The actuator causes the valve to open

in a “Fail-Open Control Valve”.

In normal operation, the force of the spring

must be overcome by the electrical or pneumatic actuator.

In the event of a “power failure” to the actuator,

the electrical or pneumatic force is no longer present

and the spring pressure forces the valve to open.

You might wonder how much the valve opens or closes

when the flow rate changes.

Well, that depends on a number of things

such as the “percentage of flow rate change”

and the “diameter of the pipe”.

Regardless, the Controller must be tuned

so that it produces the correct response to any change in flow rate.

This is most commonly achieved by using “PID” control

within the PLC or Controller.

We have already created several easy to follow videos

on what a PID Controller is.

If you want to learn more about this topic,

just head on over to “Realpars.com/blog”.

Type in “PID” in the search bar

and all the relevant articles and videos will pop up.

We will also put a link in the description

if you are watching this on YouTube or any other social media platform.

In this video, we have learned that a Control Valve

can vary the flow of a liquid or gas used in a process.

We also learned that a Control Valve can be one of two main types,

“Linear” or “Rotary”.

We can use a “fail-safe” type of Valve in critical areas.

Finally, we introduced you to how Control Valves are controlled

by the use of PID instructions in the PLC or DCS system.

Want to learn PLC programming in an easy to understand format

and take your career to the next level?

Head on over to realpars.com