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Novel Failure Model for the Purpose of Modeling the Imperfect Proof-Testing

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The maintenance of Safety Instrumented System (SIS), involving the proof test procedure, is a very important phase of maintaining the functional safety of a plant all over the life time. The proof testing gives the possibility of revealing the undetected dangerous failure of a SIS. The safety standards give a draft guideline for this activity but this guideline is problematical in everyday practice because it assumes that the proof test is always perfect. However the proof test is not always perfect in practice. The imperfectness of proof testing can be especially important for valves and other mechanical parts of SIS which have great influence on the performance of safety functions in process industry. This paper suggests a new failure model which takes into account that a proof test can be imperfect not just because it does not cover every possible failure modes but because a proof tested equipment cannot be always considered “as new” as inspired by the standard. This new model gives more detailed information about the different failure modes of valves under operation, maintenance and proof testing; and it makes the SIL calculation more accurate by modeling the degradation failures. It will be showed that the new model also may have influence on the proof testing strategy.
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Proof Test; Proof Test Coverage Factor; Dangerous Undetected Failures; Degraded Failures; Final Element; Markov Model; Probability of Failure on Demand

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