Discussion on the issue of sharing safety instrumented loops in SIS systems
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The SIF loop enters the SIS system in a 2oo3 configuration; a two-to-one safety barrier (with SIL certification and powered by the SIS system) divides the signals before they are sent to the DCS system for use in three-way voting control. Is such a setup feasible? This setup has the following advantages: 1. Three signals are required to access the DCS system, and using a triple-redundancy design with an additional independent instrument that is not certified to SIL standards enhances safety. 2. It reduces capital costs. This setup is both secure and cost-effective; experts and teachers, please share your opinions
It mainly depends on whether it is implemented as an effective measure to reduce the impact after being incorporated into the DCS; if not, there is no problem. If it is implemented, then it is necessary to consider whether the reduction requirements are still met in the event of common cause failures in the circuit.
GB50770-2013 6.2 specifies clearly how instruments of different SIL levels should be configured. For SIL1 level, it is permissible to share instruments between DCS and SIS; for SIL2 and levels that fall in the borderline range, it is better to use separate systems; SIL3 level prohibits such sharing. The way in which the safety barriers divide signals here is essentially the same; the compliance issue remains a common one.