Direct and Reverse Action Positioners

The key to working with control valves and controllers is to remember that there must always be a balance maintained in the system. "Direct" and "reverse" are kind of like "positive" and "negative" in that where you find one you will usually find the other.

While control valve bodies and control valve actuators can be described as being direct acting or reverse acting, thinking about such things when working through a system problem only adds to the confusion. Therefore, it is always best to consider the FAIL SAFE mode of the valve and simply let the control valve be what it may be.

Positioners, 99% of the time, will usually mimic the input signal from the controller. That is, they will be DIRECT ACTING.

Direct-Acting Positioner

Input Increases Output Increases Increasing Signal from Controller  = Increasing Output from Positioner Input Decreases Output Decreases Decreasing Signal From                     Controller =  Decreasing  Output From Positioner

Another reason the direct-acting pneumatic positioner is so popular is that it can be by-passed and the control valve will respond to the input signal from the controller as though the positioner were in the control loop. If a positioner malfunction occurs or if the positioner causes the control valve to become unstable, it can be easily by-passed. Many control valves in the field are operating with a by-passed positioner.

Reverse-acting positioners are sometimes used on control valves, but their appearance is rare. Occasionally one will be found in a split-ranging sequence.

Reverse-Acting Positioner

Input Increases Output Decreases Increasing Signal from Controller  = Decreasing Output from Positioner Input Decreases Output Increases                       Decreasing Signal From           Controller =  Increasing  Output From Positioner