Risk reduction factor

Risk Reduction Factors Coutrol/ rator responses. Alarms, Control system response. Manual anti automatic ESD, Fire/gas detection system Sa/ety System Responses Relief valves. Depressurization system. Isolation systems, High reliability trips. Back-up systems... 

Minimum two SIL2 SIFs (or equivalent safeguards to achieve this Risk Reduction Factor, RRF) are required. 

Table 3.8 Safety integrity levels probability of failure on demand, target risk reduction factor and target frequency of dangerous failure to perform the SIF.

Safety Integrity Level Probability of Failure on Demand (PFDavg.) Low Demand Mode Risk Reduction Factor (RRF)... 

For case three, a risk reduction factor of 17 is obtained. This is almost SIL1 borderline. It is the final element configuration that causes this reduction in RRF. 

Most designers would choose a completely separate BPCS and SIS as shown in Figure 15-1. In this design option, the BPCS could be used as an independent layer of protection, assuming that a shutdown function was included in the control logic. Although the BPCS can only provide a risk reduction factor of 10, it effectively lowers the SIL requirement for the SIF. The SIF itself can be analyzed for risk reduction. For comparison, a combined risk reduction factor can be calculated. The results are shown in Table 15-5. 

The results of the SIF analysis show a PFDavg that equals 0.0229. This provides a risk reduction factor of 44. Given the BPCS layer of protection, the total risk reduction factor of the equipment set is 440. 

Table 2. Total risk reduction factors for each Case Study Showing Individual Risk Reduction Factors for Safe Place, Safe Person and Safe Systems Components. 

Finally, the risk reduction factor given to the HIPPS is determined as the ratio of the total unmitigated event fiequency to the tolerable fiequency. 

Risk Reduction Factor = Total unmitigated event frequency/ Tolerable frequency = 3.72E-3/5.0E-6 = 743... 

The risk reduction factor for a BPCS (which does not conform to tEC 61511 ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) or lEC 61508) used as a protection layer shall be below 10. 

If a risk reduction factor of greater than 10 is claimed for the BPCS, then it shall be designed to the requirements within this standard. 

NOTE - Layer of protection analysis (LOPA) is often implemented as an order-of-magnitude assessment. Consequently, it is typical for the purpose of the LOPA calculation to assume a rounded off risk reduction factor of 10 for an operator action IPL implemented in the BPCS layer when it has met the other criteria discussed in this Annex and in Annex F. 

According to lEC 61508-4 [lEC, 1998c], low demand mode safety functions are those where the frequency of demands on safety-related system is no greater than one per year and no greater than twice the proof test frequency . The measure of safety performance of a demand mode safety function is the risk reduction factor, AR ... 

The risk reduction factor gives the required average probability of failure on demand, PFDavg(target), of the safety function according to ... 

RR, risk reduction factor (highest and lowest RR shoum in bracket). 

According to lEC 61511, an IPL must have the same inherent characteristics. As per lEC 61511 part 1, BPCS can he considered as one protection layer with a risk reduction factor <10. Also as per lEC 61511, IPL must provide at least 100-fold of risk reduction (10 for a protection layer) and have functional availability of at least 0.9 (lEC 61511,2003). According to lEC 61511, an IPL is just a protection layer with stricter requirements to availahility and degree of risk reduction. When this is considered with respect to previous discussions isn t it little confusing Protection layers may work in concert with other layers, but for IPL this is missing ... 

Allocation of SIL and risk reduction factor for each SIF with details as mentioned in Table Vl/4.0.6-1 SIL assignment table... 

SIS consists of a number of SIFs. Each SIF will have some risk reduction factor and this indicated by safety integrity level (SIL). 

LOPA INDEPENDENT PROTECTION LAYER, EACH HAS OWN RISK REDUCTION FACTOR. 

Various probable IPLs are shown in Fig. V/4-0 l. Each IPL has a risk reduction factor (RRF), which is an effective index for safety integrity. Higher SILs have higher RRFs. Our concentration will be on SIS. The overall process for SIS determination is shown in Fig. XII/1.0-2. 

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