Functional Safety Specialists May be Stuck in the Past – Do our SIL calculations Reflect Reali

by Keith Brumbaugh

One obvious problem with incorporating systematic failures is their non-random nature, hence the difficulty in including them in standard calculations. Many functional safety practitioners claim that systematic errors are addressed (i.e., minimized or eliminated) by following all the procedures in the ISA/IEC 61511 standard. Yet even if the standard were strictly adhered to, could anyone realistically claim a 0% chance of a SIF failing due to a systematic issue? Some will say that systematic errors cannot be predicted, much less modeled. But is that true?

Traditional PFD calculations are a useful starting point, but it is possible to incorporate systematic errors into a SIF’s real-world performance model. One can use Bayes’ theorem to capture data after a SIF has been installed — either through operating experience or incidents — and update the function’s predicted performance. This methodology can incorporate both objective and subjective observations. It can also be used to justify prior use of existing and non-certified equipment.

To learn more about the use of Bayes’ theorem in SIF performance evaluations, read the full paper here.

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