What is the valve positioner?
A valve positioner is a proportional controller that adjusts the output to the actuator based on control system input and feedback from the valve stem position. Pneumatic and electro-pneumatic positioners are used with pneumatic actuators to control infinite valve positioning and/or to provide the greater force.
Types of Positioner:
These are the three most commonly used valve positioners, electro-pneumatic, pneumatic, double-acting rotary valve positioners.
Pneumatic valve positioner:
The figure shows the typical positioner and actuator interface for a single-acting pneumatic positioner.
Supply pressure is supplied to the relay supply valve and fixed restriction. The diameter of this restriction orifice is less than the diameter of the nozzle so that air can bleed out faster than it is being supplied when the flapper is not restricting the nozzle.
When the input signal increases, the bellows expands and moves the beam.The beam pivots the flapper and restricts the nozzle. The nozzle pressure increases and moves the relay diaphragm assembly to open the supply port. Output pressure to the diaphragm actuator increases, moving the actuator stem downward.
Stem movement is fed back to the beam by means of a cam that causes the flapper to pivot slightly away from the nozzle. Nozzle pressure decreases, and the relay supply valve closes to prevent any further increases in output pressure. The positioner is once again in equilibrium but at a higher input signal and a slightly different flapper position.
When the input signal decreases, the bellows contracts to move the beam and pivot the flapper slightly further from the nozzle. The nozzle pressure and exhaust pressure decreases and the exhaust port in the relay opens to release diaphragm casing pressure to the atmosphere, permitting the actuator stem to move upward.
Stem movement is fed back to the beam by the cam to reposition the beam and flapper. When equilibrium conditions are obtained, the exhaust valve closes to prevent any further decreases in diaphragm case pressure.
The electro-pneumatic positioner is similar to the previous pneumatic positioner, here the input signal is an electrical signal instead of a pneumatic signal. This current becomes a pneumatic pressure that is fed to the bellows. Otherwise, this type of positioner works in the same way as a pneumatic positioner.
Each of the positioners can be configured to be direct-acting or reverse-acting. The cam can also be changed to modify the performance of the valve from linear action to equal percentage or quick opening.
Double-Acting Rotary Valve Positioner:
The double-acting positioner is shown here with a current-to-pressure (I/P) converter.
The positioner receives an input signal from a controller that is converted into a signal pressure that is directed to cavity A in the input module. An increase in the pressure of the input signal results in a downward force in the addition beam, with the addition beam turning counterclockwise. This moves the fin slightly toward the nozzle, increasing the pressure of the nozzle. As the nozzle pressure increases, the relay beam rotates clockwise, which causes relay B to increase the pressure of the upper cylinder and relay A to exhaust the lower pressure of the actuator cylinder.
As a result, the actuator rod extends and the actuator rotary shaft rotates clockwise. This causes the feedback lever to pivot clockwise, and the force that is applied to the summing beam by the range spring increases.
This force, which opposes the downward force of the summation beam caused by the increasing pressure of the input signal, continues to increase until the twisting moments of the summation beam are in equilibrium. At this point, the valve shaft is in the correct position for the specific input signal applied.
For the inverse action, the pressure of the input signal is channeled to both cavities A and B. An increase in the signal pressure results in an ascending force in the summing beam, turning the addition beam in the needle direction of the clock. and relay A to increase the lower pressure of the actuator cylinder. As a result, the actuator rod retracts, and the rotary axis of the actuator rotates counterclockwise. This causes the feedback arm to pivot to the left, reducing the force applied to the summation beam by the range spring.