Research On Function Safety Of Heterogeneous Redundant Systems With KooN Configuration

The Safety Instrumented System(SIS)is a safety barrier which is specially designed to detect abnormal movements of the equipment and to deal with the possible potential hazards,enabling controlled safe parking of the entire device and avoiding accidents happened.At present,it has been widely used in most petrochemical plants.However,due to the influence of the structure and hardware of the safety instrumented system itself,it may not be able to perform the appropriate safety functions when it needs to perform safety functions,resulting in serious accidents.Therefore,how to evaluate the reliability and availability of safety instrumented system reasonably and accurately so as to ensure the safe implementation of safety instrumented system is an urgent problem to be solved.Based on the research of functional safety of safety instrument system both at home and abroad,this paper optimizes the reliability and availability quantitative evaluation formula of heterogeneous redundant systems with KooN structure.At the same time,consider the "special" situation that SIS with multi-voting structure in practical applications.The Probability of Failure on Demand(PFD)and Suprious Trip Rate(STR)calculation models were further studied.The reliability optimization assessment is based on the IEC61508 standard and the PFD principle,according to the failure probability model of single channel,a model was built for calculating generalized PFD considering the influence of different failure rate,channel failure sequence and common cause failure(CCF)on the on the failure probability of the system circuit.In addition,the safety instrumented systems which needs to consider the partial testing and higher calculation accuracy.As the analysis process for the conventional multi-phase Markov(CMM)model is complicated and the dimension explosion may occur,a simplified multi-phase Markov(SMM)approach is proposed in the present study.For the availability optimization evaluation,the different failure mechanism and calculation model of spurious trip for the SIS subsystems sensors,logic solvers and final elements are discussed.A new STR formula SNRS(STR of Non-identical Redundant System)is given.In view of the problem that multiple voting confugurations reliability assessment results are non-conservative and the results conservative calculation process complicated,the traditional calculation model is improved by determining the correction factor RF of the PFD calculation.A new PFD computing model of SIS with multiple voting structures is established.In addition,considering the lack of availability calculation model of SIS with multiple voting structures,the STR correction factor AF is determined,and the availability evaluation calculation model of the SIS multi-voting structure is proposed innovatively.The above SIS quantitative evaluation optimization formula in the corresponding case study results shows that through the comparison between the current mainstream calculation methods,and the effectiveness of the proposed method is verified.While improving the model not only make calculation results more accurate,but also solve the problem of SIS functional safety irrational assessment.Furthermore,it can avoid the complicated modeling process while maintaining the calculation accuracy,which greatly reduces the amount of calculation.Therefore,reduce the occurrence of the accident and the probability of reducing the risk of the hazard to a certain degree.