Why It Matters:
It is important to understand what SIL certification means – and what it does not mean. SIL (Safety Integrity Level) ratings were established to define a metric for evaluating a system’s level of operational reliability with regard to safety, as defined by IEC 61508. As it applies to machine monitoring systems, a SIL rating refers to the probability of failure on demand of the protection system. A key point to remember is that SIL ratings have nothing to do with monitoring precision, which is represented by false trips and missed detects. Keep in mind that, before a SIL-rated safety system comes into play, operators have to determine the appropriate SIL rating for the machinery that has to be safety protected. In other words, IEC 61508 is a risk-based standard and, in order to apply it, criteria for the tolerability of risks must be established for the machine, e.g. a HAZOP study must be carried out (Hazard and Operability).
The Best Approach:
Some marketing phrasing, such as “SIL ready” or “Equates to SIL,” can be puzzling. However, there are two ways to clarify the confusion. First, look for a monitoring system whose vendors provide genuine SIL certificates issued by recognized certification institutions. Keep in mind that the SIL rating must cover not only the safety system itself, but also the inherent com-
ponents in the safety loop from sensor to the ESD (Emergency Shutdown Device). Second, be aware that SIL ratings should not only be high, but relevant to your application. For instance, SIL certification for monitoring over-speed protection is of no significance to a reciprocating compressor user, but a rating for a safety system that performs segmented RMS vibration analyses may be significant for your machine. You can expect a protection system with specialized capabilities for reciprocating machinery to be rated as high as SIL 2.
