NC stands for normally closed. It describes an actuated valve that uses a mechanical spring to drive the valve to the fully closed position upon loss of motive power, such as compressed air or electricity. This is a primary fail-safe state designed to stop the flow of hazardous fluids or isolate systems during a plant emergency.
Check the P&ID again, that feed valve is supposed to be NC so we don't flood the vessel if we lose instrument air.
Always verify the failure mode of a valve before removing the actuator, as an NC valve stores significant spring energy that can cause injury if released improperly.
Also heard as
- normally closed
Do not mix it up with
- Fail-open: NC valves close on power loss, while fail-open valves spring open to vent or relieve pressure.
Related terms
Fail-closed
Describes a valve that springs shut when it loses air, power or its control signal.
Fail-open
Describes a valve whose actuator drives it fully open when air, power or signal is lost.
Spring-return
Describes a single-acting actuator that uses air to move one way and a spring to move back and fail safe.
Solenoid valve
An electrically operated valve in which a coil pulls a plunger to open or close an orifice.