Explain the air to open and air to close control valve
Air to open valves are normally held closed by the spring and require air pressure (a control signal) to open them – they open progressively as the air pressure increases. Air to close valves are valves that are held open by the valve spring and require air pressure to move them towards the closed position. Fail safety of the valve. Air to open is when the valve malfunction the diaphragm position is open, and air to close the diaphragm closes when malfunction.
For process safety and as per process requirement and application whether it’s in the cooling circuit or fuel control circuit. To ensure always fail-safe operation in case extreme emergencies like power failure and airflow get cut off