Tuesday, December 11, 2012

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



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