Actuation & Instrumentation
Fail-Safe Action
Fail-safe action is the position a valve moves to when its power source is lost: fail-closed (FC), fail-open (FO), or fail-last (FL) — also written fail-in-place. It is a process-safety decision made at design time, and it is delivered by the actuator: a spring (or a stored pneumatic/hydraulic charge) drives the valve to the safe position on air or power failure.
Choosing the direction: ask "what does the process need on loss of control?" Fuel to a burner → fail-closed. Cooling medium → fail-open. A vent to flare protecting against overpressure → fail-open. Some services (e.g. a large valve where spring size is impractical) use fail-last with the risk assessment that justifies it.
Terminology caution: air-to-open equals fail-closed and air-to-close equals fail-open only for spring-diaphragm actuators; with piston actuators the same behaviour must be engineered (spring, air-to-both-sides with trip tanks, or stored-energy accumulators). The site's air-to-open entry explains the signal-side view.