Electro-Pneumatic Valve Positioners: Function & Features

Valve positioners are motion-control devices that are made in such a way to actively compare valve stem position against the control signal, adjusting the actuator diaphragm or piston till the correct stem position is achieved, that is, in a nutshell a valve positioner’s goal is to ensure the mechanical valve’s position matches the command signal at all times. One common type of a positioner is the electro-pneumatic valve positioner; this valve positioner is used in electronic control loops to operate pneumatic diaphragm control valve actuators. The figure below illustrates an electro-pneumatic positioner on diaphragm actuator.

The positioner receives a 4-20 mA DC input signal, and uses a current/pressure (I/P) converter, nozzle-flapper, and a pneumatic relay to convert the input signal to a pneumatic output signal. The output signal is then applied directly to the actuator diaphragm, producing valve plug position that is proportional to the input signal. Valve plug position is mechanical fed back to the torque comparison of plug position and input signal. Split-range operation capability provides full travel of the actuator with only a portion of the input signal range.

Control valve positioners are typically constructed in such a way to source and vent high air flow rates; hence they also fulfill the functionality of a volume booster. Therefore, they do not only ensure more precise valve stem positioning, but also faster stem velocity (and shorter time delays) than if the valve actuator were directly powered by a current/pressure transducer and lastly, adding a positioner to a pneumatically actuated control valve ensures tight shutoff.

Related articles:

• Directional Control Valves: Function & Principle of Operation
• Valve Positioners: Function & Types
• Pressure Control Valve Operation
• The Parts and Functions of a Valve
• Pressure Regulators: Function & Types
• Pneumatic Actuators