New quantitative safety standards : Different techniques, different results?

Safety Instrumented Systems (SIS) are used in the process industry to perform safety functions. Many parameters can influence the safety of a SIS like system layout, diagnostics, testing and repair. In standards like the German DIN [DIN19250, DIN0801] no quantitative analysis was demanded. The analysis according to these standards is based on expert opinion and qualitative analysis techniques. New standards [IEC61508, ISA-S84] require quantitative risk analysis but do not prescribe how to perform the analysis. Earlier publications [ROU95, ROU96] have shown that different analysis techniques cover different aspects of system behaviour. This paper shows by means of a case study, that different (quantitative) analysis techniques may lead to different results. Consequence is that the application of the standards to practical systems wil! not always lead to unambiguous results. The authors therefor propose a technique to overcome this major disadvantage.