In a causal logic diagram, should the internal sequences within a single machine be represented?

I’m not sure if I’ve understood correctly... Do you mean that if there’s a small air pump used as the air source for the instrument, the pump operates within a range of 6 to 8 kg; it runs when the pressure is below 6 kg and stops when it exceeds 8 kg... Should this be reflected in the cause-and-effect table? It depends on whether the control of this air pump is determined by the controller specified in the cause-and-effect table. In other words, if the cause-and-effect table relates to the DCS system, and the small air pump has its own functions for starting, stopping, and overpressure protection, then there is no need to include these functions in the cause-and-effect table, as the DCS system has no control over the starting and stopping of the small air pump. If you are a PLC system and the air pump itself does not have automatic start/stop functionality, requiring the PLC to control it based on pressure sensor data, then a cause-and-effect table needs to be created. (The above cause-and-effect table = logic diagram; DCS and PLC are essentially the same, used merely for differentiation.)

It’s not necessary. Because the purchased compressor was not designed by you, nor produced by you, it is not within your scope of responsibility.

That’s what I mean; generally, there is a PID diagram first, and then the causal logic table, right?

You keep dealing with this cause-and-effect logic every day – I wonder what your job actually is? Design? Party A? If you’re the client, then you should compile all the interlock logic related to the equipment. In the future, even if the operators are unaware of it, you—as the person in charge of control—must be well-informed. For instance, if the equipment suddenly stops running, the workers won’t understand what caused it; they won’t be able to figure it out. But you must know the reason. If you’re a designer, you might not need to write this down. However, if the equipment is purchased by the client, shouldn’t you first prepare a technical specification document? You should also have some basic requirements for this equipment; even if the process does not require interlocks, you should include them in the blueprints after the equipment is returned.

The designer puts forward requirements for the compressor, and Party A orders a solution from the manufacturer based on those requirements. The manufacturer will provide specific equipment instructions and instrument interlock logic; however, this logic and information should be included in the equipment diagram.

Are you asking whether this logic should be included in the PID? I think it can be withdrawn.