According to the functional safety standard IEC 61508 (2010), requirements apply to Safety Instrumented Functions (SIFs) for achieving a certain Safety Integrity Level (SIL). A typical SIF comprises of initiators (e.g. sensors), logic solvers and final elements (e.g. valves). A common method for SIF equipment vendors to document compliance with the standard is the use of SIL product certificates. This can be an efficient way of demonstrating functional safety compliance, but also involves certain challenges which will be discussed in this paper.
A study has been undertaken to identify recurring issues in functional safety compliance. This study is based on experience with SIL projects undertaken for various vendors, integrators/engineering companies and operators. The projects comprise SIL Failure Mode, Effects and Diagnostic Analyses (FMEDAs), safety manuals, certifications and SIL compliance reports. The main aim of the study is to identify challenges and pitfalls and to provide solutions for preparing traceable and unambiguous compliance documentation. During the study it became evident that final elements presented unique challenges to SIL compliance, so this paper particularly focuses on documenting compliance for valves.
The study reveals challenges related to the methodology used to reach a rational conclusion to the integrity of SIF elements covered in each certificate. This especially relates to definition of safe state for the relevant SIFs. It is important to have a clear and unambiguous definition of safe state, as this might give a different result regarding compliance. Clearly stating the application where the element is going to be used is important; this includes operational and environmental conditions. Challenges are also related to the failure rate derivation and whether the scope of the FMEDA is the element isolated or the element in a given application/system. The paper presents concrete examples of challenges, describes the impact of erroneous analyses and proposes solutions for each challenge.
This paper considers best practices for issuing SIL certificates, limitations and advantages of SIL certification, and methods to ensure efficient compliance documentation, reliable results, and avoidance of the challenges discussed.