A valve positioner is a final control device used to position the valve actuators correctly at the control position. In applying a force to an actuator, there is no guarantee that the actuator is in the correct position. Positioners are used to feedback position information and ensure that the valve is in the correct position regardless of the opposing forces.
The purpose of the valve positioner is to improve the accuracy of the valve action. The positioner can reduce the effect of many dynamic variations. Positioners are used where rapid control is required without error.
Valve Positioner working:
For pneumatic control, the positioner attempts to put the valve into the correct position. The output of the control device is not related to the input signal, but relies on the positioner to achieve the correct valve position.
The performance of the positioner is dependent on the accuracy of the position feedback and linkage used. For critical control applications, the linkage needs to be more accurate and robust. Control pressures are generally 3 to 15 psi, but positioners can operate up to 100psi which provide a greater force and a stiffer action that is less sensitive to load changes. Although, high supply pressures can affect stroking response time.
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.
The 3-15 PSI signal from the I/P and using it as a command (setpoint) for valve stem position, applying as much or as little pressure to the diaphragm as necessary to achieve the desired stem position. The flapper nozzle is controlled by the 3-15 psi pressure signal, the valve actuator is stable at postion maintained by a spring. For higher control action as per the pneumatic signal flapper-nozzle opens and close, which give a corresponding pneumatic pressure to the diaphragm.
At an applied actuator pressure of 3 PSI, the diaphragm generates just enough force to exactly overcome the actuator spring’s pre-load force, but not enough force to actually move the plug off the seat.
There are different types of valve positioner based on the control methods used:
- Force balance positioner
- Motion balance positioner
- Electro-pneumatic postioner
- Digital to pneumatic positioner
Advantages:
- Assist in overcoming friction.
- Greater actuating pressures available
- Accurate valve action
- Feedback provided
