Actuation & Instrumentation
Air-to-Open / Air-to-Close
These terms describe the fail-safe direction of a spring-opposed pneumatic actuator. Air-to-open (increasing air pressure opens the valve) means the spring closes it when air is lost — the valve is fail-closed (FC). Air-to-close means the spring opens it on air failure — fail-open (FO).
The choice is a safety decision, not a convenience one: pick the failure position that leaves the process in its safest state. Fuel gas to a burner is normally air-to-open (fail-closed); cooling water is often air-to-close (fail-open); a vent or relief path may be arranged to fail in the position that protects the vessel.
The action also interacts with the control signal: many controllers output increasing signal = increasing process demand, and the valve action must make that physical sense, which is why the positioner is configurable for direct or reverse action.