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When and When Not to Use Failure Rates Based on Cycle Testing Data

When and When Not to Use Failure Rates Based on Cycle Testing Data in the Computation of Safety Metrics for SIF Elements

One data source for predicting failure rates for final elements used in safety instrumented functions (SIF) is cycle testing. In cycle testing, the mechanical equipment’s moveable components are exercised rapidly through part or all of their range of motion. Any observed failures are recorded, and the mechanical equipment failure rate is calculated based on the number of failures observed and the number of cycles of motion completed.

These failure rate predictions are necessarily based on several assumptions. It is important to understand these assumptions because, safety metrics, e.g., safety integrity level (SIL), computed using these failure rates are only valid if, in actual equipment use, all the assumptions are met. This paper addresses these issues and details when and when not to use failure rate estimates based on cycle testing data.

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