Valve action defines whether the valve will be fully open, or fully closed in the event of any process failure.
The engineer must define whether the safest condition for each valve is fully open or fully closed. This will be the failure position, and the combination of the actuator and valve body must achieve this position upon loss of power.
We must analyze the entire process, including integrated units to identify the safest conditions. In a few cases, the failure condition is “unchanged”. If the air power is lost, air leakage will result in a slow drift to either open or closed.