Detecting instrument redundancy in the SIS system
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Should the detection instruments in the SIS interlock circuits, such as three-to-two, two-to-one, and two-to-two configurations, be specified by the safety assessment company in the LOPA report? Isn’t this outside the scope of a design institute?
Firstly, LOPA analysis is a protection layer analysis, a type of safety analysis at the process level, which can only determine the safety level required for each unit and each circuit. Only after a HAZOP analysis (not limited to this method) can it be determined whether your current instrument configuration meets the required safety level. The three-out-of-two and two-out-of-one choices you mentioned are both methods to improve the security level (there are many such methods). Secondly, several valves and instruments are required; the design institute is the ultimate authority in this regard. If your institute has the capability, then it can carry out the analysis itself (currently, no special qualifications are needed to conduct such analyses – as long as you can produce a report). If it lacks such capabilities, then external experts can be hired to do the analysis (with the owner covering the costs). However, the design institute remains the final authority, and only the design drawings constitute the official documents; the safety assessment company merely provides the basis for the design.
Both Lopa and Hazop were carried out by the same safety assessment company, but the interlock methods for the instruments were not specified in either case. For example, whether it is two-out-of-three or one-out-of-two.
LOPA only performs grading. NooM is determined by the design institute (it can be changed as long as the usage conditions are met).