#WishIknew – What’s a control valve positioner?


When you have a question about process automation that needs a short answer, let us know with #WishIknew and @Visaya! We’ll reply with a #WishIknew post. It’ll give a quick explanation, then some related articles, videos, or reviews if you want to know more.

I wish I knew what a control valve positioner is!

Control valves have three parts – actuator, valve body, and positioner. I explained this assembly earlier if you want to read that here you go. Anyway, the control valve positioner does exactly that. It positions the valve according to the commands from the control system.

So let me give you a quick rundown of how positioners vary by signal.

Courtesy of GlobalSpec
  • Pneumatic valve positioner: This unit uses air to receive the signal and move the valve to change the product output. The signal follows the standard 3 to 15 psi. First, the air supply goes to the relay and the input to the bellows. Then the bellows move the beam in the flapper assembly. Thus, the nozzle will allow more or less air through the relay, which moves the valve stem down or up. To change the action of the valve, you change the flapper assembly position in the beam.
  • Pneumatic valve positioner with transducer: The valve itself works the same, but it receives a different signal from an analog system. Therefore, you need the transducer to convert the analog signal to 3 to 15 psi to operate the positioner.
  • Smart or digital valve positioner: This unit has a microprocessor to communicate using digital field protocols. It checks the position of the valve using the feedback arm, either with the Hall effect or change of resistance with a potentiometer. A relay controlled by the electronics increases or reduces the air output to the valve. A big plus here is that smart positioners offer diagnostics for you to check.

This video goes over a digital valve positioner:

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