A spring in the actuator pushes the valve open as the air bleeds off, so these valves are also called air-to-close. They are chosen where an open valve is the safe state: cooling water to an exothermic reactor, pump minimum-flow recirculation, or a vent that must relieve. The required fail action is fixed during process hazard review and appears on the P&ID as FO.
Cooling water valve is fail-open, so if you lose instrument air the reactor still gets full flow.
Also heard as
- FO
- air-to-close
- ATC
Do not mix it up with
- Fail-closed: The fail position is the opposite; which one is right depends on which state is safe for the process.
Related terms
Fail-closed
Describes a valve that springs shut when it loses air, power or its control signal.
Spring-return
Describes a single-acting actuator that uses air to move one way and a spring to move back and fail safe.
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.