What are control valves and how do they work?

To begin with, Control Valves are used to control the flow of fluids such as liquid, gas, and steam by partially opening or fully opening or vice-versa. 

The control valve consists of three main parts each of which available in different types and designs 

  •  Valve body 
  •  Valve Actuator 
  •  Valve positioner

The opening and closing are usually done by actuators. The actuators can be operated electrically or pneumatically or hydraulically.

Pneumatic actuators are most commonly used in industrial automation because of their simplicity. It just requires compressed dry air to work.

The standard input pressure range to the actuator is 3 to 15 psi. We have all come across this terminology 3 to 15 psi in automation here is where it is used.

Types of Actuators:

1) single acting( spring return ) actuators 

  • Air to open actuators
  • Air to close actuators 

           The single-acting actuators need input air supply either to open or close depending on the type of the actuator.

2) Double acting actuators: 

               The double-acting actuators need input air supply for both the actions (to close or open ).

Actuators are driven by the output of the positioners based on the input electrical signal ( 4 to 20 mA ) or pneumatic signal ( 3 to 15 psi ) to the positioners.

Types of positioners :

             1) pneumatic positioners

            2)electro-pneumatic positioners

Electro-pneumatic positioners - 

                   In this case, the positioner is a built-in I/P converter. Thus the electrical signal from the controller such as PLC or DCS is directly given to the positioners. It is also called smart positioners.

The control valve in the image is a single-acting air to open with a smart positioner. 

Pneumatic positioners -  

                It has two input air supply. One is regulated air supply from the main air header and another one is from the I/p converter. The signal from a controller such as PLC or DCS is an electrical signal ( 4 to 20 mA ) and I/p converts are used to convert that electrical signal into a pneumatic air signal for the pneumatic positioners.

Positioners that are available with one output are used to control single-acting actuators and two outputs are used to control double-acting actuators.