Design optimization of a safety-instrumented system based on RAMS+C addressing IEC 61508 requirements and diverse redundancy |
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Authors: | A.C. Torres-Echeverrí a,S. Martorell |
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Affiliation: | a Department of Automatic Control and System Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b Department of Chemical and Nuclear Engineering, Polytechnic University of Valencia, Camí de Vera sn, 46022 Valencia, Spain |
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Abstract: | This paper presents the design optimization by a multi-objective genetic algorithm of a safety-instrumented system based on RAMS+C measures. This includes optimization of safety and reliability measures plus lifecycle cost. Diverse redundancy is implemented as an option for redundancy allocation, and special attention is paid to its effect on common cause failure and the overall system objectives. The requirements for safety integrity established by the standard IEC 61508 are addressed, as well as the modelling detail required for this purpose. The problem is about reliability and redundancy allocation with diversity for a series-parallel system. The objectives to optimize are the average probability of failure on demand, which represents the system safety integrity, Spurious Trip Rate and Lifecycle Cost. The overall method is illustrated with a practical example from the chemical industry: a safety function against high pressure and temperature for a chemical reactor. In order to implement diversity, each subsystem is given the option of three different technologies, each technology with different reliability and diagnostic coverage characteristics. Finally, the optimization with diversity is compared against optimization without diversity. |
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Keywords: | Safety system IEC 61508 Common cause failure Diversity Safety integrity level Optimization Genetic algorithm Safety Reliability |
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