SIL loop: certified or on certified equipment - the way to go!

摘要

Presumably, having started to read this article, you have heard of; been involved with or been panicked by IEC61508 and need to either understand the process of SIL assessment of safety related instrument loops or even perform such calculations. A safety instrumented loop is any loop whose failure to operate could realize a hazard to life, the environment or to Asset Management. Hopefully, you will be fully aware that the new standard consists of seven parts. Parts 1 to 4 are normative and as such constitute the main requirements whilst parts 5 to 7 provide guidance and supporting information. Of specific importance to the assessment of instrument loops are parts 2, 3 and 6. Part 2 addresses the requirements for assessment including both qualitative and quantitative, the LOWEST of these two assessments will apply! Part 3 addresses the issue of software in programmable electronic systems (PES) and part 6 provides formulae and guidance to support the quantitative analysis. However, don't forget Part 1 as this lays down the requirements for documentation, responsibility and competence. OK that's the background, so what is the best way to set about designing safety loops to IEC 61508? Is it possible to just use any instrument that satisfies the functional specification, OR is there an advantage in choosing certified components from accredited companies? Lets look at a typical basic safety loop.