Systematic approach for the SIL-proof of non-standard safety-loops

Safety instrumented functions (SIFs) in the process industry are subject to regulations stated by international standards such as IEC 61508 and 61511. To be compliant with these standards a safety integrity level (SIL) assessment has to be carried out. The main goals are numerical measures for the amount of reliability that the SIF offers. One of these is the probability of failure on demand (PFD) i.e. the probability of the SIF to have failed at the time a demand occurs. The standards provide simple calculation formulas to describe standard safety systems (e.g. 1oo2). Complex shutdown structures can not be treated like the standardized MooN-systems. This paper provides an extended Markovian approach to calculate inhomogeneously equipped safety loops with very special structures. It is based on a specific class of piping-setups where several combinations of tripping final elements lead to a safe state of the process. An example is used to demonstrate the advantages of the approach compared with the standard calculation methods. Cumulated NAMUR data is used for numerical proof. The approach can directly be applied to complex sensor structures as well as to systems where e.g. one sensor triggers a huge amount of actuators.