How Direct Acting and Reverse Acting Control Valve Loops Work ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How Direct Acting and Reverse Acting Control Valve Loops Work

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A typical control valve loop consists of four basic elements for consideration:
(a) The process under control mainly flow or pressure applications
(b) The process controller that initiates control action to the control valve
(c) A valve positioner (almost always required in most applications)
(d) Control valve action. See How a typical Control Valve Loop Works

These basic elements work in coordination to produce the desired control outcome. Control valve loops can be operated in two basic modes :
(a) Direct acting loops
(b) Reverse acting loops

Direct Acting Control Valve Loops:
In a direct acting control valve loop shown below:
Direct Acting Control Valve Loop
 The controller, positioner and the control valve acting in the following ways:
(a) Controller Action:
As the process variable (flow or pressure) increases numerically, the output signal from the controller increases in proportion and vice versa

(b) Valve Positioner Action:
As the input signal to the valve positioner from the controller increases, the output load or air pressure from the positioner increases

(c) Control Valve Action:
As the air pressure or load increases on the valve actuator, the valve’s plug moves towards the close position for a valve that is ATC – FO (Air-to-Close, Fail Open). 

Reverse Acting Control Valve Loops:
In a reverse acting control valve loop shown below:
Reverse Acting Control Valve Loop
 The controller, positioner and the control valve act in the following ways:
(a) Controller Action:
As the process variable (flow or pressure) increases numerically, the output signal from the controller decreases in proportion and vice versa

(b) Valve Positioner Action:
As the input signal to the valve positioner from the controller decreases, the output load or air pressure from the positioner increases

(c) Control Valve Action
As the air pressure or load increases on the valve actuator, the valve’s plug moves towards the open position for a valve that is ATO - FC (Air-to-Open, Fail Close). 
For different control valve applications, the controller, the positioner and the control valve can be configured for direct or reverse or both direct and reverse in the same control loop depending on the requirement of the various applications.