Control Valve Positioner
A Control Valve Positioner is a device used to increase or decrease the air load pressure driving the actuator until the valve’s stem reaches a “POSITION” balanced to the output SIGNAL from the process variable instrument controller.
Valve positioners control valves where accurate and rapid control is required without error or hysteresis.
Positioners are generally mounted on the side- yoke or top casing of the pneumatic actuator for linear sliding stem control valves and at or near the end-of- shaft for rotary control vales.
For either primary design type, “mechanical feedback linkage” connected directly to the valve’s stem provides feedback to the controller.
The process controller tells the positioner to “change” position; the feedback linkage reports back to the positioner confirming that a change has occurred and gives a “sense” of the magnitude of the change in status.
Producers typically install valve positioners on a pneumatic actuator’s yolk or top casing for linear control valves.
On rotary control valves, the valve positioner is installed in line with the valve and actuator stems on top of the actuator or the side of the actuator. Installation depends on what type of actuator one uses.
Affixing the positioner to the actuators allows the positioner to measure the stem travel (linear valves) or degree of rotation (rotary valves). This also changes the position of the valve as required based on the input signal from the instrument controller.
Product Description
When the process variable differs from the desired setpoint, the instrument controller sends an electrical or pneumatic signal to the positioner. This varies its pneumatic output to the actuator to move the valve open or closed accordingly. This occurs until the process variable reaches the desired set point.
How many types of positioners are? There are three primary types of positioners:
⦿ Pneumatic Valve Positioner
⦿ Electro-Pneumatic (EP) Valve Positioner
⦿ Digital Valve Positioner
Pneumatic control valve positioners use a pneumatic input signal to pressure opening or closing the valve. Others use analog signals to do the same thing. Smart control valve positioners can even change analog signals to digital to control your valves.
Product Feature
What are the reasons for using valve positioners? There are a few of them, like:
⦿ Increase control system resolution;
⦿ Allow the use of characteristic cams in rotary valves;
⦿ Increase speed of response to a change in process; allows faster loading and venting;
⦿ Allow distances between controller and control valve;
⦿ Permit use of piston actuators with high instrument air supply pressures.
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