What are fail-safe positions for control valves?

Fail-safe position is the valve position on loss of signal or air supply. Common configurations: Fail-Closed (FC or Air-to-Open, ATO) - valve closes on air failure, used when closed position is safer (fuel supply valves). Fail-Open (FO or Air-to-Close, ATC) - valve opens on air failure, used when open position is safer (cooling water valves). Fail-Last (FL) - valve stays in last position using lock-up valve, used when either extreme is undesirable. Fail-safe selection is critical for process safety and depends on hazard analysis.