what is a normally close / normally open pneumatic valve

Pneumatically actuated valves such as butterfly valves or ball valves, the pneumatic actuator comes in a single action mode or double action mode. Single action mode has a normally close position or normally open position.

Normally closed pneumatic valve means that when there is no air supply to the air actuator; the valve is in a closed position, when the air supply is connected, the valve is in an open position. Normally open pneumatic valve means that when there is no air supply to the air actuator, the valve is in the open position; when the air supply is connected, the valve is in the closed position.

The air-actuated normally closed valve is also called air to open and spring to close/ spring return. Explanation: spring-to-close: Close by spring force when the driving compressed air is cut off or when compressed air supply failure occurs.

The air-actuated normally open valve is always in the open position, using air to close.

A double acting ball valve means air to open and air to close the valve position.