What does M(C)/M(O) in a logic diagram mean?
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In the logic diagram, what does M(C)/M(O) denote? Does it refer to the control valve being in a manually fully closed/fullly open position, or is it necessary to set it at a specific valve position as defined by the logic, such as 20% closed or 80% open, etc.? When the program condition is no longer valid, to what position does the control valve return? Does it remain in the manual closed or manual open position, or does it go back to its previous automatic position and switch to automatic mode?
M(C) means fully closed, M(O) means fully open, and M(X) is used to set it to a certain degree of opening. What do you mean by the program conditions disappearing? Chain? All actions require permission from the program; when you switch to manual mode, it won’t return to automatic mode on its own. The program must issue commands to control this, otherwise things will get chaotic.
This is the first time I’ve seen this notation; I understand it as MC for manual close and MO for manual open. In interlock systems, control valves can be adjusted to a certain degree of opening or closing using solenoids or mA values. This takes into account process requirements, analytical considerations, as well as the practices followed by enterprises. Regulating valves used in interlocks may be equipped with solenoid valves as required by standards, but a one-size-fits-all approach is not recommended. Interlocks should also be divided into safety interlocks and non-safety interlocks, which in turn affects the accessories of control valves and the methods used to operate them. Control valves can be integrated into both DCS systems and SIS interlocks, but I believe they should be used as little as possible in SIS interlocks, especially those related to SIL2 levels. (Not only do issues related to the instruments themselves and their protection layers come into play, but when control valves are involved in interlocks, additional position feedback is also required.) The priority of interlock actions is such that interlocks take precedence over control functions; in other words, the interlock setting takes precedence over the normal control setting. As per the standards, SIS systems require manual confirmation for resetting, while DCS systems do not require such confirmation. However, in practice, human intervention is still needed after an interlock is triggered, i.e., a person must adjust the valve opening until it stabilizes before switching to automatic mode. Of course, non-safety interlocks can also be restored to the previous valve state, which is related to the company’s practices and the system’s design.