Instrumentation

Control Valve positioner or Pneumatic positioner

In this lesson, we will study Control Valve Positioner or Pneumatic positioner.

  • Pneumatic Valve Positioners are the most commonly used valve accessories.
  • A pneumatic positioner is a device which will accurately position a control valve in accordance with the pneumatic control system.
  • The control signal is routed to the positioner where the comparison of the valve position (actual) to the control signal (desired) is used to develop an output pneumatic signal which operates the valve actuator.
valve positioner
Valve positioner
  • The valve positioner allows valve flow characteristics to be changed. Most valve positioners employ a rotating cam in the feedback system. This cam can be changed to simulate different valve flow characteristics. A linear globe can be used to respond in an equal percentage manner.
  • The pneumatic positioner allows split range operation. In a split range control loop, one controller used to drive two control valves.
  • The pneumatic positioner compares the control signal with the actual valve position through the mechanical feedback linkage.
  • If the valve position is inaccurate or incorrect, the positioner will load or exhaust air from the valve actuator until the correct valve position is obtained.
  • A pneumatic positioner requires both a control signal and an instrument supply air for normal operation.
Pneumatic positioner
Pneumatic positioner

Most positioners come equipped with three gauges to indicate supply air pressure, control signal pressure (instrument air) and actuator diagram signal (signal to valve or output signal).

There are 3 pressure gauges in the control valve positioner.

1.Instrument (Input).

2.Valve (Output).

3.Supply.

Instrument air comes from i/p converter(0.2 to 1.0 kg/cm^2)

Valve air supplied to the control valve.

Supply Air is an air supply for pneumatic positioners. Supply air is in the range of (0 to 6.0 kg/cm^2).

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