The core difference between Fail Open (FO) and Fail Close (FC) valve actuators lies in the direction of valve movement under fault conditions. The specific differences are as follows:
1. Fail Open (FO): When the actuator loses power (such as power failure, gas failure, signal loss), the valve automatically opens fully.
Its working principle: Usually relying on mechanical structures such as springs, gas tanks or gravity to drive the valve to the open position in the event of a fault.
Typical applications: Safety discharge: such as the emergency discharge valve of the reactor, it needs to be opened in the event of a fault to avoid overpressure.
Process protection: Prevent pipeline blockage or equipment dry burning (such as maintaining flow when the cooling water valve fails).
Fire protection system: Ensure that the water source is unobstructed in the event of a fault.

2. Fail Close (FC): In the event of a fault, the valve automatically closes fully.
Its working principle: Through the reverse action of the spring or energy storage device, the valve is forced to close in the event of a fault.
Typical applications: Hazard isolation: such as the gas shut-off valve, it is closed in the event of a fault to prevent leakage.
Medium protection: Avoid leakage of precious or hazardous media (such as chemical pipelines).
System shutdown: Protect key equipment (such as steam valve failure closure to avoid overheating).

3. Other considerations
1) Failure lock (FL, Fail Locked): The valve remains in place when it fails, suitable for processes that do not allow sudden switching.
2) Energy type: Pneumatic actuators are commonly FO/FC, and electric actuators require additional energy storage mechanisms.
3) Selection basis: The failure mode needs to be determined based on process safety analysis (such as HAZOP), and the safest state after failure is given priority.