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The function of a valve positioner
Release time:
2026-04-07 15:22
(1) Improving the steady-state characteristics of the valve: After the installation of a valve positioner, even a slight change in the controller output signal will, through the nozzle–flapper mechanism and the amplifier, result in a substantial variation in the air pressure supplied to the control valve diaphragm chamber. This adjustment overcomes stem friction and eliminates the effects of unbalanced forces acting on the control valve, thereby ensuring that the valve position accurately follows the signal from the controller. With improved steady-state performance, the control valve becomes suitable for the following applications: ① situations requiring precise valve-position control; ② applications involving large-diameter valves with high differential pressures and significant unbalanced forces; ③ applications where tight packing gland compression is necessary to prevent leakage, such as in high-pressure, high-temperature, or cryogenic service; ④ processes in which solid particles in the process fluid may become lodged, or where the fluid exhibits high viscosity.
(2) Improving the dynamic characteristics of the valve: The positioner modifies the original first-order lag characteristic of the valve, reducing the time constant and transforming it into a proportional characteristic. In general, when the pneumatic signal transmission line exceeds 60 meters in length, a valve positioner should be used.
(3) Altering the valve’s flow characteristic: By modifying the shape of the positioner’s feedback cam, the control valve’s linear, logarithmic, and quick-opening flow characteristics can be interchanged.
(4) For split-range control, a single controller is used to operate two or more control valves, such that each valve completes its full stroke within a specific segment of the control signal. For example, one valve may be configured to complete its full stroke in the 4–12 mA DC current range, while the other valve completes its full stroke in the 12–20 mA DC current range.
(5) Valve positioners for reverse-action valves are classified as either direct-acting or reverse-acting. In a direct-acting positioner, an increase in the input signal results in an increase in the output air pressure; in a reverse-acting positioner, an increase in the input signal results in a decrease in the output air pressure. By using a reverse-acting positioner, a normally open valve can be converted into a normally closed valve, and a normally closed valve can be converted into a normally open valve.
2020-01-19
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