(82c) Field-Based Evaluation of PFD Values for Safety-Related Loop Typicals

According to IEC 61511, each safety instrumented function (SIF) in chemical production plants has to be assigned a safety integrity level (SIL). Thereby, for every SIF realization, i.e. safety-related loop, a SIL-specific probability of failure on demand (PFD) must be proven. Usually, the PFD calculation is performed based on the failure rates of each loop component, aided by commercial software tools. But this bottom-up SIL proof suffers from many uncertainties. Especially a lack of reliable failure rate data causes many problems. Reference data acquired in different en-vironments are available to solve this situation. However, this pragmatism leads to a PFD bandwidth. In order to make a decision for a single PFD value appropriate for plant applications in chemical industry, a data ascertainment has been initiated by the European NAMUR within its member companies in 2002. Since 2003 more than 12,000 single-channel and 10,000 dual-channel safety-related loops have been observed in detail. Combined with statisti-cal methods their results display large deficiencies of the bottom-up ap-proach.