Safety instrumented functions and safety integrity levels (SIL)

This paper discusses two performance-based standards, ANSI/ISA S84.01 and IEC d61508, and the requirements they place upon user companies of electrical, electronic and programmable electronic safety related systems (E/E/PE SRS) or Safety Instrumented Systems (SIS). To comply to the requirements of the standards, a user company would have to: (a) identify the safety target level of the process; (b) evaluate the hazardous events that pose a risk higher than the safety target level; (c) determine the safety function(s) that must be implemented in an SIS to achieve the safety target level; (d) implement the safety functions in an SIS and evaluate its safety integrity level (SIL); (e) install, test and commission the SIS; and (f) verify that the installed SIS does in fact reduce the process risk to below the safety target level. Several risk analysis techniques that can be used to comply with the aforementioned requirements are discussed and a simple example is used to illustrate the use, advantages and disadvantages of the techniques. The evaluation of SIL of the SIS (probability to fail to respond to a process demand) is outside the scope of this paper.