Transducers convert a current signal to a pneumatic signal.The most common transducer converts a 4-20 ma electric signal to a 3-15 psig pneumatic signal.
Volume boosters are used on throttling control valves to provide fast stroking action with large input signal changes. At the same time,the flow boosters allow normal positioner air flow with small changes in the positioner input signal.Depending on actuator size,packing set and the number used boosters can decrease valve stroking times up to 90 percent.
A valve of positioner is like a proportional controller.The set point is control signal from the primary controller and the controlled variable is the valve position. The positioner compensate for disturbances and nonlinearities.
The use of positioner are as follows
*When signal pressure is not enough to operate the control valve
*To make split range between valve
*It can be used to reverse the action of the actuator from air to open to open to air to close and vice versa.
*To minimize the effect of hysteresis effect
*To minimize the response time for the valve.
*If the actuator is spring less Positioner will be used
*If the valve has high friction
Quick exhaust valves allow the cylinder actuator to quickly vent one side to atmosphere,resulting in an almost immediate full-open or full-closed position. This sudden movement generally limits quick exhaust applications to on/off services where positioners are not used.
It is designed to hold the actuator in the last operating position upon air failure.
Used to open the valve or close it.
Limit switches are available to indicate a valve open or closed position.
It is used in fail lock up system for to set the air pressure in required level for lock the valve.