Valve positioners are used on controlling valves where accurate and rapid control is required without error or hysterises.
- Precise postioning
- can cope with large variations in forces acting on plug
- Rapid postioning
- Removes stiction and friction effects of gland
- Removes effects of large distances between vale and postioner
- Elimates hysterises
Assembly
A valve positioner consists of a very high gain amplifier- this may be pneumatic, electricpneumatic etc, and a feed back link which detects the actual position of the valve.
The required movement is for the valve to close. The input pressure from the controller to the bellows falls. The flapper moves away from the nozzle and the presure after the orifice falls. The pressure to the diaphragm falls and the valve begins to close. The feed back arm moves up rotating the cam clockwise. This raises the beam increasing back pressure in the nozzle until equilibrium is again achieved.
The change over cock allows the signal from the controller to be placed directly on the diaphragm
Force balanced Valve Positioner
Motion balanced Valve Positioner