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Control valves can be built from various combinations of valve actuator and valve body. The combination of actuator and valve body is usually chosen to provide a particular failure mode should the instrument air supply fail for any reason.
The most common control valve actuator used in the industry is the diaphragm actuator. Diaphragm actuators, as in the case of valve bodies, can be classified as either direct or reverse acting.
Read Also : Control Valve Actuators: Basic Types and Designs
Any failure mode can be obtained with a combination of direct or reverse acting
actuator and direct or reverse acting valve body. The two most common failure mode of control valves are :
1. Fail Open
2. Fail Close
These two failure modes can be achieved by an Air to Close Valve (ATC) and an Air to Open valve (ATO).
Air to Close Control Valve (ATC)
An air to close (ATC) valve and therefore fail open valve, can be obtained with the combination of a reverse acting actuator and a reverse acting valve body or a direct acting actuator and a direct acting valve body.
Air to Open Control Valve (ATO)
An air to open (ATO) valve and therefore fail close valve, can be obtained with a combination of direct actuator and reverse body or reverse actuator and direct Body.
Valve Body and Actuator Combination and Their Failure Modes
The action of an actuator can easily be determined (usually by whether the air is supplied to the upper or lower half of the housing). Direct or reverse acting valve bodies are not always readily identifiable. Most often, reference to the nameplate or flow sheet is usually necessary to correctly identify the action of a valve body – reverse acting or direct acting.
Listed in the table below are all possible combinations of valve body and actuator and their failure modes:
Valve Actuator
|
Valve Body
|
Valve Action
|
Failure Mode
|
Direct
|
Direct
|
Air to Close
|
Fail Open
|
Reverse
|
Reverse
|
Air to Close
|
Fail Open
|
Direct
|
Reverse
|
Air to Open
|
Fail Closed
|
Reverse
|
Direct
|
Air to Open
|
Fail Closed
|
