Difference between lockable and non-lockable valves, as well as normally open and normally closed valves
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On normal PID diagrams, there are valves marked as open or closed; some also have valves designated as always open or always closed. What are the principles behind setting these valves? What is the difference between a lockable valve and a normally open valve?
The lock opens after the interlock action occurs; it opens under normal operating conditions as well
Valves that can be locked open or locked closed require a higher level of approval in the management process compared to valves that remain open or closed at all times; the former need authorization from higher-ranking supervisors.
Valves that open and close automatically are interlocked valves, while valves that are always open or always closed are valves that are manually operated on a regular basis
For valves that are to be kept open or closed, lead seals are usually applied to the valve handles and stems to ensure that the valve cannot be operated. This applies to valves such as those located before and after safety valves, as well as bypass valves for important flow meters. There are also valves that remain either always open or always closed, such as instrument connection valves and the handles before and after control valves. This is part of the requirements related to management. For valves that are part of interlock mechanisms and have upstream and downstream manual valves, if it is specified that they should remain in the open position at all times, then sealing measures should be taken to meet the higher management requirements; otherwise, it should be specified explicitly that they remain in the open position.