Direct and reverse action of control valves

The structure of a control valve consists of an actuator and a valve body. A control valve changes the flow rate passing through it in response to a signal; therefore, considering the control valve as a whole, an increase in the signal leads to an increase in flow rate or a decrease in flow rate, which is known as direct action, while an increase in the signal resulting in a decrease in flow rate or a decrease in the signal leading to an increase in flow rate is known as reverse action. The signal mentioned here is clearly a pneumatic signal; therefore, its positive effect is to open the valve (A.O.), that is, an increase in the pneumatic signal leads to an increase in the valve’s opening degree ； The reaction is an air-lock valve (A.C); that is, an increase in the air signal leads to a decrease in the valve opening ； Another term commonly used in engineering is that the valve closes when the control signal is lost (FC), which means failure-to-open; this is equivalent to an air-operated open valve (A.O) ； When the control valve loses the air signal, the valve opens (FO), which is known as fault-open valve operation; this is equivalent to an air-close valve (A.C.).

Regulators have positive and negative actions; when the value of the controlled variable increases, an increase in output corresponds to a positive action. Positioners have positive and negative actions; an increase in the signal leads to an increase in the output pressure of the positioner, which is considered a positive action. Control valves also have positive effects; an increase in air pressure causes the push rod of the control valve to move downward, resulting in a positive action.

First, it is necessary to determine the forward and reverse action of the control valve! Some valves are reactive.