Digital control valve positioners have begun to take over, but you should still know how a pneumatic control valve positioner works. Luckily, you only need some basic physics and mechanical knowledge to understand this device.
The pneumatic control valve positioner’s working principle
The pneumatic control valve positioner and its valve actuator are powered by an air supply. There is an instrument input, with a range of 3 to 15 pounds per square inch (psi), and the air output to move the valve actuator.
In the instrument input, the air pressure will act on the bellows, making it expand or compress. The bellows connect to the beam, which measures the feedback from the valve stem through the cam. It also moves the flapper-nozzle system.
As the beam pivots, it shifts the flapper closer to or further from the nozzle, causing the pneumatic relay to increase or decrease the pressure on the valve actuator. When the valve stem moves, the rotary shaft arm will make the cam rotate, sending feedback to the beam. That, in turn, causes the flapper to move toward or away from the nozzle, and that increases or decreases the air in the actuator through the pneumatic relay. They work in tandem, you see? Hope you guys get the gist of pneumatic control.
This process continually adjusts the valve position based on the input signal. You can also have direct or reverse action on the valve positioner. Usually, you have the left side of the beam as the reverse action and the right side the direct action. On a side note by moving the flapper assembly, you can change this action, but you should follow the proper procedure.
This video will show you how a pneumatic control valve positioner works