What is LO or LC valves?

The “Lock open/closed Valve or car seal open/closed Valve” is not a type of valve. It is a “Condition” of a valve. Any valve can have a lock or a Car-Seal added to it.

These devices would be considered “after-marker add-on”.

Any standard valve (Gate, Globe, Ball, Plug, Needle, Diaphram, Orbit, Butterfly, etc) can have a locking device or Car-Seal added to it. Conversely any valve that had these devices on them, can have them removed and then they are just valves.

LO or LC

The use of LO / LC valves is often safety related. This system provides a means to ensure that some critical manual valves remain in their desired position during normal operation and are not mistakenly operated.

Maintaining such a system requires strict and stringent procedures to manage the position of the valves and to manage their locks. In my experience, in also takes a strong safety culture within the organisation.

An example of Locked Open (LO) valves would be on the inlet and the outlet of Pressure Safety Valves (PSVs) or relief valves. These isolation valves on the inlet and outlet of a PSV must indeed remain open during normal operation in order that they can protect the equipment on which they are mounted. Mal-operation / accidental closure of those valves during normal operation could have catastrophic consequences. Hence the requirement to lock them in their open position.

An example of a Locked Closed (LC) valve would be on a line to the maintenance closed drains system. Ingress of flammable hydrocarbons in the maintenance closed drains system during normal operation is hazardous, therefore the isolation valve on the drain line must be closed during normal operation. Hence the requirement to lock them in their closed position.

Locked open / closed means exactly that, i.e. you need a key to open a padlock which is kept secure and only released against work permits etc. Tends to be for safety critical valves - isolation valves for Pressure relief valves for example - to keep them open or closed and stop someone changing the position without authorisation.


Pennpiper has it nailed. The “car seal” is normally just a small seal, often plastic now, but sometimes still metal. This is often installed by Quality inspectors when sealing off tanks or ops guys arranging valves in a particular mode for a specific operation. Removing these seals normally requires very little force or simple cutters.


Always my question every time I am seeing these words CSO/ CSC in the P&ID, I even ask mechanical people the meaning of this but no one got the answer. Thanks for sharing…


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It means car seal open and car seal closed. it is the same function of pad lock.