Redundancy with multiple equipments or subsystems is usually used to improve the safety and reliability of the safety instrument system. Common redundancy structures are 1oo2,2oo2,1oo3 ,2oo3 etc,there are even large redundancy structures in devices. Finding a appropriate method to model those large redundancy structures is very important for the quantitative reliability a-nalysis. In this paper,Fault Tree Method was used to model a large redundancy structure MooN under certain conditions. After a iterative calculation,when N-M + 1≥4,the random hardware failure can be ignored to simplify the calculation model. Finally a large redundancys average PFD calculations of a large scale ethylene plant safety instrument systems prove the simplified model have a high precision.%安全仪表系统经常采用多个设备或子系统构成冗余结构来提高自身的可靠性.常见的冗余结构有1002,2002,2003等,在大型装置中还会遇到更为复杂的冗余结构.为定量分析大型冗余结构的可靠性,选择合适的方法对其进行建模至关重要.本文用故障树方法对大型冗余结构进行可靠性建模,并对假设参数条件下MooN结构的故障树模型进行简化分析,经过迭代计算,当M和N满足N-M+l≥4时,可以忽略系统的随机硬件失效,从而简化了计算模型.最后通过计算某大型乙烯装置安全仪表系统中大型冗余结构的危险失效概率,证明简化后的模型具有较高的精确度.
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