A simplified procedure for the analysis of Safety Instrumented Systems in the process industry application

International Standards and Guidelines propose qualitative and quantitative methodologies for the safety assessment of the Safety Instrumented System (SIS). However, some of these methodologies are often complex and not very easy to apply. In fact, some criticalities are found by technicians voted to the functional safety such as the study of SIS for complex architectures, calculation of safety parameters, difficulties in the identification of the SIS subsystem during the design review to guarantee the safety requirements, and so on. The aim of this paper is to propose a simplified and more efficient methodology for safety assessment of electromechanical SIS in compliance with the Standards IEC 61508 and IEC 61511. The proposed technique is based on an alternative implementation of the Reliability Block Diagram (RBD) approach for the performance analysis of the Safety Instrumented System. In order to demonstrate the advantages of the proposal, a case study of some of the safety functions is considered. With respect to other methodologies normally used for safety analysis, the results proved the proposed approach both easier in the application and time-saving. In addition, such results are comparatively close to those obtained by using the Standard methods.