Control valve

A Control valve is a special type valve which is used to control fluid flow by changing the size of flow passage as controlled by a signal from a controller.
Control valve is a power operated device that is used to regulate or manipulate the flow of fluid such as gas, oil, water, steam.

Figure: Typical Control Valve

A valve with pneumatic, Hydraulic, Electric or other externally powered actuator that automatically opens or closes the valve fully or partially by the signals received from a controlling instrument is called control valve.

What are the parts of a control valve?

Any control valve can be divided into two sections

Here we will learn the parts related to the actuator only. The valve part is completely mechanical item which will be discussed on another post.

Figure: Cross-section of Control Valve

Also read: What is droop speed control?

The actuator is the device connected to the valve through the valve-stem that provides the force required to change the size of flow passage of that valve.
Suppose a valve is now 30% open, you want to open it more like 50%, If you give the command to do this, The actuator will provide the driving force to the valve for opening it from 30% to 50%.
In case of manual valve, there is not any actuator like this, wheel or handle is there to do the work of actuator.

There are many types of actuator available:

Hydraulic actuators are used on that places where large amount of force is required to operate the valve.

For example, the main steam injection valve of any steam turbine.

Although hydraulic actuators come in many designs, piston type is the most common.
A typical piston type hydraulic actuator is shown in below figure.

Figure: Piston type Hydraulic actuator

It consists of a cylinder, spring, piston, valve-stem and a hydraulic fluid chamber.
Whenever the hydraulic fluid is allowed to go inside this chamber it forces the piston to move upward against the spring force on the top of piston.
If the fluid force is greater than spring force, the piston begins to move upward, spring compress as a result the valve begins to open.
Oppositely if we allow the fluid to go outside the spring will press the piston to go downward as a result the valve will be close.
We can operate the valve at fully open, fully close or partially open/close.
By regulating the amount of fluid inside the chamber, the valve can be operated at partially open/close condition.

Normally if you want to operate any manual valve you have to rotate the hand-wheel of that valve, right?
Now you are thinking......you will not rotate the wheel or handle, you will use an electric motor to rotate that wheel/handle, then it will be motorized actuator.

"The actuator in which motor is used for operating the valve is called motorized actuator"

More read: What happens when a motorized valve fails?

Figure: Motorized Control Valve

This types of valve used in such an application where the control of fluid temperature is desired.