Valve Positioners are devices used to put the valve in the correct position by increasing or decreasing the air load pressure on the actuator.
Valve positioners are used for controlling valve where accurate and rapid control is required without error or hysteresis.
Usually, an I/P converter sends the opening-closing request to the valve but cannot confirm the position of the valve actuator. The process controller tells the required position to the positioner, and the positioner senses the current position of the actuator and compares with the required setpoint. Then give the control action through the flapper-nozzle amplification system.
A positioner can reduce the effect of many dynamic variations. This include change in packing friction due to dirt, corrosion, lubrication, or lack of lubrication.
The above shown is a pneumatic valve positioner. The current position of the actuator is transferred through beam and cam. The control signal is given through a bellow. The supply pressure to the valve is controlled by the flapper-nozzle system. The flapper nozzle controls the supply pressure by referring the feedback from the actuator and setpoint signal.
There are other types of valve positioners:
Electro-pneumatic positioner: Perform the same function as the pneumatic type, but uses an electrical current of 4-20mA instead of air as the input signal which uses i/p converter.
Electronic positioner: This positioner replaces the i/p converter with a microprocessor inside the positioner itself to perform the same function as the above positioner.
Digital positioner: This type of positioner to position the valve actuator and monitor and record data. They are very accurate, use less air than analogue positioner.
Application:
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Limiting control valve dead, band
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Change valve response
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Control double acting actuator
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Increase shut off force
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Split-ranging
Advantages:
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Rapid and precise positioning
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Remove stiction and friction effect of gland
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Eliminates hysteresis
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Remove effect of large distance between valve and positioner.