What is the principle of operation of a pneumatic positioner on a control valve and why is it used?

A pneumatic positioner is a device mounted on a control valve that ensures the valve stem position accurately corresponds to the control signal it receives. Its principle of operation involves a closed-loop feedback system that compares the desired valve position (as indicated by the control signal) with the actual valve position. The positioner then adjusts the pneumatic pressure applied to the valve actuator until the desired position is achieved. The main components of a pneumatic positioner include a beam or lever mechanically linked to the valve stem, a flapper-nozzle mechanism, a pilot valve or booster relay, and a bellows or diaphragm. The control signal, typically a 3-15 psi pneumatic signal or an electronic signal converted to a pneumatic signal, is applied to the bellows or diaphragm. This creates a force that moves the beam or lever. The movement of the beam or lever changes the distance between the flapper and the nozzle. This change in nozzle backpressure is sensed by the pilot valve or booster relay, which then adjusts the air pressure supplied to the valve actuator. The actuator pressure changes until the valve stem reaches the position corresponding to the control signal, at which point the flapper-nozzle mechanism reaches equilibrium, and the actuator pressure stabilizes. A feedback linkage connects the valve stem to the beam or lever, providing feedback on the actual valve position. Positioners are used to overcome problems associated with valve hysteresis (the difference in valve position for the same control signal depending on whether the valve is opening or closing), valve stiction (static friction that prevents the valve from moving until a certain force is applied), and actuator friction. They also improve valve response time and accuracy, particularly for large valves or valves operating with high pressure drops. A positioner ensures the valve accurately reflects the desired control action, improving overall process control stability and performance.