Comment on fail safe action i.e Air to Open (ATO) and Air to Closed (ATC) of control valve.

Air to Open (ATO) of Control Valve (Failure Close (FC) - Air to Open)

The actuator's benchset range spring "opens" the valve plug when the loading air pressure drops to or near 0 psig. (Air to Closed Fail Open “Direct”) Same scheme as I.a. A back to air to closed fail open arrangement can be applied to a fail open control valve to eliminate transient effects.

Fail close means the valve will close when the signal is interrupted or lost. This is the opposite of the above scenario. Since air pressure is required to keep the valve open, it will automatically close when the power goes out because there is no longer a working air source.

Actuated or automatic valves that return to a pre-set position after removal of actuating force are called "fail-safe" valves. The most common type is "fail-safe normally-closed." On the other hand, "fail-safe normally-open" valves are much less common, but just as important.

Air to closed (ATC) control valve. (Fail Open (FO) - Air to Close)

The loading air pressure is at or near 0 psig when the benchset range spring of the actuator "closes" the valve plug. (Air to Open Fail Close “Reverse”) Because the instrument air supply piping system can be extensive (large), the instrument air supply can deteriorate very slowly causing operational problems. In such cases it may be desirable to use a 3-way pilot switching valve that "predicts" eventual loss of instrument air supply and quickly strokes the control valve to its fail-safe position, eliminating the transient operational effects of slow decay.

Fail open means the valve will open when signal or power is lost. These types of valves require air pressure to remain closed. After the required air pressure is exhausted, the valve will open naturally. Under a power outage, the source of air pressure will be lost and the valve will "fail".

Normally-closed valve

The valve opens when activated and when the flow is shut off, a separate spring inside the valve forces it to close. The magnetic force of the solenoid coil must overcome the force of the spring to open the valve. Unless actuated, the spring force keeps the valve closed. In case of power failure, the valve will close automatically.

Normally-open valve

The valve closes when energized, and when the current is turned off, a separate spring inside the valve forces it to open. The magnetic force of the solenoid coil must overcome the force of the spring to close the valve. Unless actuated, the spring force keeps the valve open. In case of power failure, the valve will open automatically.