Under what circumstances are the allowable signals used in instrument interlocks?

FYR: Allow start permissive – the conditions that must be met before starting. Interlock refers to the interlocked shutdown that occurs when the triggering conditions are met. Difference: It’s sufficient to meet the requirement before startup ； Interlocking is a permissible condition during standstill, and becomes an interlocking condition after operation.

If you’re drawing a logic diagram, one part is written in the activation condition and the other in the deactivation condition. Although the same result can be achieved in some cases, strictly speaking, their meanings are still different.

Your logic isn’t entirely accurate. According to your literal interpretation, there seems to be no difference, as the prerequisite in both cases is that the pump is running. In the first scenario, opening the valve is not allowed; this can be achieved in several ways: one method is to make the valve-opening signal inactive so that it cannot be operated, while another method is to issue a command to close the valve. In this way, even if someone tries to open the valve, the command to close it remains active, preventing the valve from opening； The second one: when the pump is running, close the valve; this is a manifestation of the first situation

Conditions that permit startup; these are not necessarily conditions for shutdown. For example, regarding the lubricating oil pressure in the compressor: if it’s greater than 3 kg, the compressor can be started; if it’s less than 2.5 kg, an auxiliary pump is activated to increase the pressure; if it’s less than 1.5 kg, the compressor shuts down. Starting is allowed when it is placed in the driving circuit; this should accurately reflect the actual on-site condition. A value greater than 3 means permission, while a value less than 3 means prohibition. If the condition permitting startup is not a stop condition, then the interlocked result must be used to unlock it, with the compressor’s operation signal bypassing the permission for startup.

In terms of conditions, allowing initiation and enabling interlock stop are two different things: the former allows action (operation or interlock) only when the conditions are met, while the latter triggers an interlock directly when the conditions are satisfied. From a management perspective, the conditions that permit startup generally cannot be bypassed, whereas interlock conditions can usually be bypassed as long as they are not safety-related interlocks. In practical applications, the conditions that permit startup are the same as those for interlocked shutdown; in such cases, it is possible to combine them into a single condition in the absence of specific conditions (such as manual/automatic mode switching). But personally, I think if it really has to be integrated into one entity and a comprehensive assessment is required, my suggestion is that it’s better to keep them separate.