A new proposal for the analysis of safety instrumented systems

In the field of standardization a lot of International Standards and Guidelines concerning qualitative and quantitative methodologies for safety assessment of Safety Instrumented System (SIS) in function of operative environment are available. Such documents consider different techniques but often they are complex and not very plain in their application; in particular, some criticalities are found by technicians voted to the functional safety et. al.. The aim of this paper is to propose a simplified and more efficient methodology for the safety assessment of an electromechanical SIS in compliance with the standards IEC 61508 and IEC 61511. The proposed approach is based on an alternative application of the Reliability Block Diagram (RBD) technique in the performance analysis of safety instrumented system. In order to prove the advantages of the application, a case study of some safety functions is considered. In comparison with other methodologies traditionally used for safety analysis, the proposed approach gives advantages in terms of time-saving and conservative results. Finally an evaluation of the impact of the common cause failures are also given.