Definition Reverse / Direct Acting Valves

See eplaination on following link.

Mercor

http://www.maintenanceresources.com/referencelibrary/controlvalves/cashcopositandcontr.htm

in general, the term direct acting is applied to the control algorithm, more often than to the valve. although, some manufacturer uses this term interchangeably.

the control algorithm, said to be direct acting, meaning the output of the controller (the manipulated variable), like valve opening position or motor speed, etc..., will follow the input (the measured variable), like measured flow or level or pressure or temperature, etc... that is, increase input will result in increase output. (Ignore the fail-closed/ fail-open or air-to-close/air-to-open for now).

vice versa, the control algorithm, said to be reverse acting, meaning the output of the controller will move in the opposite direction with respect to the change in the input. that is, increase input will result in decrease output. for example, flow control is always reverse acting algorithm.

Now, the air-to-open valve or fail-closed valve, sometimes is referred to as direct acting valve, is originated from the fact that when you apply more air to the actuator, the valve is open more. also from the old school: increasing signals to the valve will result in the valve open more. therefore, this type of valves is actuated to open.

the air-to-close valve or fail-open valve, sometimes is referred to as reverse acting valve, is originated from the fact that, it is normally open. when you apply more air or other actuation signal to the actuator, the valve will close more.

Thanks Khangnguyenca that agrees perfectly with the result that was agreed to at work, which gives me confidence that the right decision was made. The "old school" explanation explains how we all got a little confused with it.

Cheers again!

I like your explanation.