Direct-Acting and Reverse-Acting Positioners
     The terms "direct" and "reverse" are frequently used when discussing control valves, positioners, and controllers. While the definitions of direct and reverse seem pretty straightforward, they cause quite a bit of confusion - especially when split-ranging is done.
     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
 
Equals
Increasing Signal from Controller
Increasing Output from Positioner
 
 
Input Decreases
Output Decreases
 
Equals
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
 
Equals
Increasing Signal from Controller
Decreasing Output from Positioner
 
 
Input Decreases
Output Increases
 
Equals
Decreasing
Signal From 
Controller
Increasing 
Output From Positioner
 
 
 
Direct-Acting and Reverse-Acting Controllers
     Controllers can be set up in either direct or reverse modes. It was stated that 99% of the positioners are direct acting, and it follows that if a balance is to be maintained in the control loop that 99% of the controllers will be reverse acting. If the control valve and its controller are not in balance, the control valve will either go to the wide-open position and stay there, or it will stay closed and act as though it is not responding. This situation can normally be corrected by reversing the action of the controller.
Direct-Acting Controller 
Setpoint Increases
Output Increases
 
Equals
Increase
in
Setpoint
Increase
in
Output
 
 
Setpoint Decreases
Output Decreases
 
Equals
Decrease 
in 
Setpoint
Decrease 
in 
Output
 
 
 
Reverse-Acting Controller
Setpoint Increases
Output Decreases
 
Equals
Increase
in
Setpoint
Decrease
in
Output
 
 
Setpoint Decreases
Output Increases
 
Equals
Decrease 
in 
Setpoint
Increase 
in 
Output
 
 
 
     Two of the more common control valve uses are for pressure control. In both instances, the controllers are reverse acting. Most pressure-reducing valves will be fail-closed and most back-pressure control valves will be fail-open. If the pressure-reducing valve were fail-open or the back-pressure valve fail-closed, then the controllers would have been direct acting.
The key is to start with the fail-safe mode of the control valve.
 Control Valve Concepts / Actuator Operating Modes / Positioners / Flow Characteristics / Seat Leakage / Packing / Helpful Hints / Cashco Terminology / Advanced Topics
 
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