SIL Assessment of Mechanics

Abstract

Failure probability of mechanical components

How to treat nonelectrical components is always a tricky question when safety equipment has to be assessed according SIL. As the scope of EN 61508 is limited to electrical components, the answer is still open. An obvious approach is to deal with mechanical components in the same manner as already done with electronical equipment. And, if difficulties occur it is reasonable to apply the requirements of the standard analogously. This approach shows that in most cases no "special treatment" of mechanics is necessary. Only when the failure probability has to be calculated mechanical parts need to be considered separately. A further complication is that in mechanical components, the failure behaviour depends to a very great extent on the mode of operation. When frequently used the main reason for a failure is wear out. Such failures can be considered as random events. Thus, it is possible to describe these events with the Weibull distribution. Based on the B10 value a fictitious constant failure rate can be determined as an approximation. If mechanics that is operated in the low demand mode no meaningful failure rate can be specified. Therefore, deterministic methods are necessary to achieve the safety target.