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A valve positioner is a device equipped on the pneumatic actuator that to increase or decrease the inlet air pressure to move the valve towards to the correct position. When there is no positioner, the control signal goes directly to the actuator. When a positioner is there, it receive this signal and then outputs a different signal to the actuator.

Valve positioners are usually mounted on the yolk or top casing of a pneumatic actuator (for linear movement control valves such as globe control valve), or near the end of the shaft (for rotary movement control valves such as ball valves). For either set-up, the positioner is connected mechanically to the valve stem or valve shaft. This let the valve’s position to be controlled with the position requested by the controller.

Globe control valve diagram

A valve positioner responds to a signal from some type of master control system, typically a distributed control system (DCS), a programmable logic controller (PLC), or PID controller. The control system reads a signal from a process sensor (flowmeter, temperature sensor, pressure sensor, etc...) and compares that reading to the desired setpoint. A corrective signal, based on the difference, is provided to the valve positioner which re-adjusts (if necessary) the valve position to bring the system in to equilibrium. Pneumatic-pneumatic valve positioners

Valve positioners are available with pneumatic, electrical, electro-pneumatic, and digital operation. Here is a brief description of each:

Pneumatic
Pneumatic positioners receive pneumatic signals (3-15 or 6-30 PSIG). The positioner then throttles supply air to the valve actuator to move the valve to the required position. Pneumatic positioners are intrinsically safe and can provide a large amount of force to close a valve.

Electric:
Electric valve positioners receive an electric signal, usually 4-20 mA, 1-5 VDC, 2-10 VCD or 0-10 VDC and generally drive the motors in electric actuators. They perform the same function as pneumatic positioners do, but use electricity instead of air pressure as an input signal. Electro-pneumatic valve positioners

Electro-Pneumatic
Electro-pneumatic valve positioners contain internal I/P (current to pneumatic) modules that converts the electrical input signal to a pneumatic output (4-20 mA to 3-15 PSIG for instance). Very similar to a pneumatic positioner except that its input is electrical.

Positioner Advantages