I have a question regarding the valve legend.
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When looking at the PID diagram, I encountered the legends related to valves and found some of them a bit confusing. I would like to ask fellow sailors for their advice. For example, I understand that \"locked open\", \"locked closed\", \"seal open\", and \"seal closed\" are related to preventing accidental operations. Could everyone please explain the rest to me?
Locking and unlocking refer to the state of the valve when there is no pressure, or in other words, the state of activation or deactivation when there is no pressure – it is the position of the valve under such conditions. Sealed switches are similar, but they are sealed to prevent accidental operation, making it impossible to operate them by hand. Emergency shutdown is the rapid operation of a valve within a short period of time.
Functions such as valve lock open, valve lock closed, seal open, and seal closed are all applicable to manually operated valves. The proper way to achieve lock open or lock closed is to use a lock to secure the valve’s handwheel or handle in the fully open or fully closed position, as required by the process, thereby preventing unauthorized persons from operating the valve accidentally； Opening and closing a lead seal generally refers to locking the valve’s handwheel or handle with a plastic or metal seal (lead seal); to open or close the valve, it is necessary to break this seal. This is done to prevent accidental operation by personnel. However, such valves are designed to be accessible by anyone in emergency situations, and they are commonly used in safety valve inlets and outlets, shut-off valves, and bypass valves. Emergency shut-off valves are used to cut off flow in case of accidents or operational fluctuations (such as shutting off the fuel supply during a fire). They can be manual or automatic; when automatic valves are used, they should be on/off type valves. Leakage classes V or VI are commonly used for automatic emergency shut-off valves. The choice depends on the process medium and operating conditions, with class V seals being most frequently used, as class VI essentially results in zero leakage – it is difficult to achieve this without soft seals, or such seals are too expensive. These are just my own thoughts; please feel free to correct me.
As a supplementary point, emergency shut-off valves should generally have the capability to open and close quickly.
Valves usually have CSO (CAR SEAL OPEN) or CSC (CAR SEAL CLOSE) markings, which can be used to determine the valve’s status.