Regarding the setup of SIS interlock
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The company’s tank area was originally equipped with an emergency interlock system that was configured within the DCS: when the liquid level became too high, the feed supply was interrupted and the feed pump was stopped; when the liquid level dropped too low, the discharge process was halted and the discharge pump was stopped as well. Now, it is required that major hazard sources be equipped with SIS, because SIS is static and valves cannot be operated on a daily basis; in that case, would another set of such interlocks need to be installed within the SIS?
Strictly speaking, DCS and SIS systems are independent systems. First, it is necessary to determine the SIL level; if it reaches SIL2 or higher, the SIS requires the independent installation of relevant detection instruments, control valves, and other interlock systems.
If SIS is required, it must be set up independently, because in principle, your BPCS represents one layer of protection, while SIS is another separate layer of protection; the two need to be completely independent of each other. If calculations show that without SIS the security requirements cannot be met, then SIS must be used... After all, **regulations regarding this are quite strict these days. I don’t know whether what was said above is aimed at wasting funds, but **the push for this measure is likely intended to raise the entry barriers for new companies, so that not just anyone can get into this field – at least they need to have sufficient funds. It’s just a guess!
SIS is an independent safety interlock system that must be separate from the DCS system. As you described, there are two sets of interlocks required: one for high liquid level and another for low liquid level.