Step 2 Allocation of Safety Functions

Safety lifecycle phase or activity Objectives Requirements Clause or Subclause of ISA-84.01-2004 Inputs Outputs 

Using the proposed list of SIFs, a PHA team meeting is held to determine the required safety integrity levels for the SIFs. In this section, the Layer of Protection Analysis (LOPA) method will be used. For a description of the LOPA method, refer to Annex F in Part 3 of ISA-84.01-2004. Additional guidance is provided in AlChE, COPS, Layer of Protection Analysis, Simplified Process Risk Assessment, 2001. 

2 Layer of Protection Analysis (LOPA) Applied to Example 

This clause explains the transition from data shown in the partial summary of hazard assessment information (Table 5) to LOPA (Table 7). 

This upset initiates a runaway reaction that can catastrophically rupture the reactor. The impact of this event was judged to be extensive, which, as discussed in Table 6 Note 1, leads to a tolerable frequency of 10 /year for a single scenario. Several failures in the control system could cause this upset, with operating experience indicating that this type of upset occurs about once every 10 years. Protection per Table 5 was the Shortstop addition, but the runaway reaction may be too fast for the operator to respond to an alarm. This protection layer is not included for risk reduction. The area is normally occupied, so it was assumed that personnel could be impacted by the event. The pressure safety valves (PSVs) are only estimated to be 90% effective, since plugging is a common problem in this service. Since the PSVs share a common relief line, they are conservatively considered to be a single Independent Protection Layer. This led to an intermediate event likelihood of a 10 per year. Per the conservative assumptions used in this example, only the PSVs qualified as an IPL. The PHA team reviewed all the process safety risk issues and decided that a SIF was appropriate. As shown in Table 7, this requires a SIL 3 SIF. 

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