P&ID Abbreviations List with Meanings
A searchable list of the abbreviations written beside lines, valves and equipment on P&IDs: fail positions, locked and car sealed valves, utility codes, drains, tracing, flanges and control systems.
Fail Positions and Valve Locks
Fail positions (FC, FO, FL) tell you what a control or shutdown valve does when air or power is lost, and are chosen so the process goes to a safe state. Locked (LO, LC) and car sealed (CSO, CSC) marks apply to manual valves whose position must not change without authorisation, such as block valves under relief valves.
Frequently Asked Questions
What does CSO mean on a P&ID?
Car sealed open: the valve is sealed in the open position and may only be closed with authorisation, common for block valves under PSVs.
What is the difference between FC and NC?
FC is the position the valve goes to on loss of air or power; NC is its normal position in operation. A valve can be NC and FO, or NO and FC.
What does NNF mean?
Normally no flow: a line that is normally empty or idle, such as a start up or bypass line.
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