Reliability Analysis And Maintenance Optimization Algorithm Of Time-varying Failure Rate Interlocking System

Aiming at the problem of deviation in the risk assessment of safety instrumented system（SIS）,the unavailability of SIS is misjudged,which leads to the increase of unnecessary expenses when the chemical enterprises formulate maintenance strategies.This paper studies the calculation deviation of the unavailability of emergency blow-down valve in safety interlocking system between exponential distribution calculation and Weibull distribution calculation.According to the reason of deviation,the Markov model of safety interlocking system is optimized.On the basis of the optimization model,the periodic and aperiodic algorithms of optimal maintenance cost are studied.The main research methods are as follows:（1）Using the failure data of emergency blow-down valve in OREDA database,Monte Carlo（MC）simulation was carried out to expand the samples.exponential distribution and weibull distribution were used to estimate the failure parameters,and the interval estimation was carried out under 70%and 90%confidence.The calculation results and safety integrity level（SIL）verification results were compared between exponential distribution calculation and weibull distribution calculation.（2）In order to reduce the deviation of SIS unavailability calculation,the Markov model of SIS with complex redundant structure is optimized.Combined with time-varying failure rate,common cause failure（CCF）,on-line test,off-line test,overhaul and hot and cold reserve mode,the three-dimensional particle swarm optimization（PSO）algorithm is used to solve the optimal cost maintenance strategy of each model.The periodic maintenance strategy under the lowest maintenance cost is formulated.（3）For the formulation of non periodic maintenance strategy of unkuown failure parameter,based on API-581 standard,this paper updates the experiential parameters of the equipment after each inspection,and deduces the recommended maintenance interval for the next maintenance of the equipment.Considering that the process of parameter update is tedious,which is not conducive to security engineers’on-site calculation,this paper develops an Android application software based on Android Studio application development platform to assist parameter update.The results show that,at 70%confidence level,the results of PFD_avg calculated by exponential distribution algorithm are larger than the results of PFD_avg calculated by weibull distribution algorithm,and the average relative deviation of PFD_avg calculated between exponential distribution algorithm and weibull distribution algorithm is 68.01%;At 90%confidence level,the average relative deviation is 42.63%,which indicates that the constant failure rate calculation of IEC61508 will significantly overestimate the unavailability of SIS.The deviation cause unnecessary rectification and corresponding maintenance costs.In the time-varying failure rate Markov model based on deviation optimization,the particle swarm optimization algorithm is used to solve the problem iteratively.The stable optimal solution is output,and the minimum cost maintenance strategy is formulated according to the optimal solution.Based on api-581 standard,the algorithm of maximum maintenance interval is deduced.By the the algorithm,maximum maintenance interval based on the upper limit of unavailability is obtained.