Sources of Mistakes in PFD Calculations for Safety-Related Loop Typicals

In order to prevent any harm for human beings and environment, IEC 61511 imposes strict requirements on safety instrumented functions (SIFs) in chemical and pharmaceutical production plants. As measure of quality a safety integrity level (SIL) of 1, 2, 3 or 4 is postulated for the SIF. In this context 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 approach suffers from many uncertainties. Especially a lack of reliable failure rate data causes many problems. Reference data for different environmental conditions are available to solve this situation. However, this pragmatism leads to a PFD bandwidth, not to a single PFD value as desired. In order to make a decision for a numerical value appropriate for plant applications in chemical industry, a data ascertainment has been initiated by the European NAMUR within its member companies. Combined with statistical methods their results display large deficiencies for the bottom-up approach. As one main source of mistakes the distribution of the loop PFD has been identified. The well known percentages for sensor, logic solver and final element part often cited in literature could not be confirmed.