Direct and reverse action of control valves
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Hello everyone. What do positive and negative action for control valves mean? And what is their relationship with FC (fail-closed) and FO (fail-open)? Thank you
This is for safety reasons; some valves need to remain open in the event of a failure, such as in the venting system. Some requirements are to be off in the event of a fault, such as the heat transfer oil control valve.
For now, understand that the direct action of a valve is FO, while its reverse action is FC. Go back and study carefully the section on the actuator of control valves.
Direct and reverse action of control valves: An increase in the input signal causes the valve stem to extend, which is direct action; the opposite is reverse action. Air-on/air-off for control valves: An increasing input signal causes the valve to open; it is an air-on valve, also known as fault closed (FC) ； The opposite is a wind shut-off valve, also known as Fault Open (FO).
Normal operation, reverse operation, fault off, and fault on are different meanings. Direct action means that an increase in the input signal causes the valve stem to move downward, while reverse action causes it to move upward ； Fault close refers to the valve being in a closed state when a fault occurs (loss of air pressure); the opposite is fault open.
By the backreaction I’m referring to, I think it’s the backreaction of the actuator – is that correct? You are referring to the forward and reverse action modes of the control loop, right? My answer did easily mislead the original poster!
Positive action refers to the downward movement of the valve stem as the signal pressure increases. The opposite is called reaction. FC means the valve closes in case of a gas supply failure; otherwise, it is FO.
One refers to control and regulation, and the other refers to the valve mechanical mechanism.
What I’m referring to is also the direct and reverse action of control valves: when the input signal to the control valve increases, a valve stem that moves downward corresponds to a direct-acting control valve, while the opposite case applies to reverse-acting control valves.
The direct and reverse action of a control valve refer to the direction in which the valve moves under automatic operation, when there is a discrepancy between the instrument’s measured value and the set value. For example, a flow control valve operates on the principle of feedback: when the measured value is higher than the set value, the opening degree of the control valve should be reduced. FC or FO refers to the safe position that the control valve should be in in the event of a power loss or loss of instrument air, and it is related to the process design requirements.
Normal operation, reverse operation, fault off, and fault on are different meanings. The direct/indirect action of a control valve is related to the positioner, actuator, and valve body; it can be understood that when the input signal to the positioner increases, the valve opens more, which represents direct action, while the opposite situation represents indirect action ； Fault closed FC refers to the condition in which the valve is in a closed position when there is a failure in the instrument air supply (no air available); conversely, Fault open FO is the opposite situation, and this is determined from a safety perspective.