A Tale of Two BPCS Credits, A Bayesian Case Study

Have you ever had the problem of having a perfectly functional BPCS* interlock that you know is highly failure immune, yet when it comes time for the Hazard Analysis, you may only take one credit? Unfortunately, for facilities following the IEC 61511 safety lifecycle, any interlock not designed according to the standard is limited to one risk reduction credit. This can make meeting extremely low total mitigated event likelihood targets (such as 1x10-5 or 1x10-6) exceedingly difficult.

What can you do if you do not want to redesign your BPCS interlock to meet the IEC 61511 requirements? The only thing left is to seek out a deviation, though you better have good justification.

This case study will examine the approach used for one client to justify two risk reduction credits on their robust BPCS interlock in two basic steps. The first step was to decide a reasonable probability of failure using a Failure Mode and Effects Analysis technique (FMEA). All relevant failure modes including the ubiquitous human component were examined. Next, plant operating history was reviewed and applied in a Bayesian analysis to determine the upper credibility (confidence) limit. The overall FMEA and Bayesian analysis process, including the "why," "how," and results will be provided.

As a bonus, the methods used in this case study can be directly translated into a case for Prior Use Justification, data collection, and user-customized and maintained failure rate data.



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