Normally Closed

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A valve that is designated on the P&IDs as normally closed, abbreviated as NC,  is one that is closed during normal operating conditions.  A normally closed valve might be one that is used on the bypass of a piece of equipment or an isolation valve for a spare piece of equipment.  Under typical operation, this valve would be closed but during abnormal conditions, such as a maintenance event or shutdown, this valve might be opened up.

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Tags: Valve Safety Instrumentation and Controls