Can the detection instruments of the SIS system and the DCS system be used redundantly together?
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As the title suggests, let’s take an example: a storage tank is equipped with two level gauges, one of which is a measuring instrument integrated into the DCS system. Another level gauge is a sensing instrument of the SIS system. So, can the high liquid level interlock signal of the SIS system use a 2-out-of-1 approach (with both level gauges having high interlock settings), or must it be based solely on the high level detection value from the SIS system? .
If you want both instruments to be able to trigger interlocks via SIS, the signal from the DCS instrument must be fed into the SIS system. In your example, assuming the interlock action is the closing of the inlet valve, the ESD (the logic within SIS) should be programmed as follows: LLHS-1 (from local) ------- OR ------- inverse -------- LEV(A.O.) │ LLHS-2 (from DCS) --------- │ The signal from LLHS-2 that enters the DCS, if it is a 4-20mA AI signal, must be converted into a 0 or 1 DO signal before it can be fed into the SIS system (at least, the SIS systems I’ve seen use 0 or 1 DI and DO signals). There are actually many ways to do this; this is just a relatively common approach. Additionally, the SIS system is already part of the DCS system; by “DCS” in your case, you’re probably referring only to the US-related systems within the DCS system (the human-operated systems)
The SIS system is independent of the DCS system. For example, I determine that the safety instrument level of the SIS system is SIL3. Are the two systems completely independent of each other, to the point where communication between them is not possible? . . .
In principle, this is not allowed, but there are some situations with lower requirements where DCS and SIS systems share detection sensors; that is, the sensor signals are sent to both the DCS and the SIS. However, this situation strictly limits its use, and it basically does not occur in practice.
For process engineering professionals, the SIS system is truly a major challenge – it’s very difficult to understand. . . . .
I’ve also never been able to figure this out. Mark, come and take a look in a few days. Did SIS only become popular in the past decade or so? Many people don’t understand it.