Interlocking and coercion

This post was last edited by ZhiGE on 2018-12-14 at 19:31. 1. Simply put, BYPASS means disabling that interlock logic; such logic no longer functions. Its value changes as the process conditions change. However, even if the interlock threshold is reached, the interlock will not activate anymore； 2. Forced (SIMULATION) means assigning a fixed value (either digital or analog) to the conditions or outputs of that logic circuit; this value does not change as the process evolves, and it certainly does not trigger any interlock actions ； 3. If configuration has been done and authorization is available, bypassing can be performed on the upper-level system (operation interface); for example, starting up in bypass mode is allowed to be done through the upper-level system ； 4. It is mandatory that only instrument control system engineers perform relevant tasks on the lower-level machine (within the logic configuration program); once this requirement is enforced, the display of that parameter can no longer reflect changes in the process conditions. This is very dangerous, and operations within the system of the operating equipment also carry significant risks ； 5. Therefore, it is recommended to carry out proper bypass operations after submitting the interlock release request and the work order request. At the same time, it is necessary to avoid disabling interlocks under abnormal operating conditions, as interlocks are there to ensure safety. Moreover, it should be restored in a timely manner once the conditions are met. 6. For example, if a pump’s inlet pipeline is equipped with a fork-type interlock, will the pump stop running when the fork-type interlock is bypassed and it can no longer detect the liquid level? (No) When the level cannot be detected by the same fork, the interlock bypass pump still cannot start; in such a case, a fake signal must be sent to the fork for the pump to start. So what is the difference between interlock bypass and sending a fake signal? (See 1, 2) As I understand it, once the tuning fork interlock is bypassed, the pump starts and stops normally regardless of whether the tuning fork can detect the liquid level; then why is it still necessary to provide fake signal values? (Forcing and bypass are generally not used together.) If a fake signal is sent to the tuning fork, will the interlock function properly once it is activated? (It doesn’t work.) Hope this helps you.

The answer on the second floor was excellent; I’ve learned a lot!

Everything said on the 2nd floor is correct, but it may not hit the mark. I feel that some logic like reset should be included in it. You have bypassed the tuning fork, but the logic is still functioning normally; there may be things that require resetting, and since you haven’t bypassed those elements, it’s necessary to provide a fake value for the tuning fork. It would be better if you took a look at the logic diagram.

As I understand it, once the tuning fork interlock is bypassed, the pump starts and stops normally regardless of whether the tuning fork can detect the liquid level; then why is it still necessary to provide fake signal values? (Forcing and bypassing are generally not used together); this part is not entirely clear to me. After bypassing the tuning fork interlock, since there is no logic in place, why can’t the tuning fork detect the liquid level? The pump must be given a false signal in order to start

Is this the kind of logic diagram you're talking about?

Yes, it is clearly visible from the diagram that there is a ready output in the interlock system; whenever the pump stops, it can only be started again once it is in a ready state. It’s essentially a reset logic. The bypass tuning fork can be seen as a mechanism that keeps a 1 value supplied to the second input of the \"AND\" module (ignoring the actual liquid level), but it is not possible to avoid the second interlock. That’s why the situation you mentioned occurs.