Reliability Simulation Of Complex Structures Of Safety Instrumented System (SIS)

Safety Instrumented System(SIS) is an instrumented system used to implement one or more safety instrumented functions.SIS is composed of any combination of sensor(s),logic solver(s), and final elements(s),which is the last barrier to avoid production accidents.According to IEC 61508 and IEC 61511 standards,Safety Integrity Level(SIL) technology is born for quantitative risk- assessing of SIS reliability.However,the Markov model recommended in two main standards can not be used to calculate complex structures,such as 10oo15 appeared in engineering application.The main contents are 3 parts as following:1.Based on Monte Carlo Method,a new reliability simulation model is presented to calculate two factors,Mean Time To Failure Safe(MTTFS) and Mean Time To Failure Danger (MTTFD) of complex structure in SIS.This new model referenced some advantages in Markov model such as failure modes,Common Cause Failure(CCF) and repair rate;2.Improved the multipleβmodel in CCF considering the impact of CCF sensitivity and components number in every CCFG on system reliability;3.Develped SIL_MC software to calculate complex structure's reliability using Delphi 7.0 and simulated several factors' effect on system reliability.Simulation results show that:(1) When using this new model to calculate simple structure such as 1oo2,the difference between this new model's result and the strict Markov model calculation result is less than 5%. Further more,with the structure's complexity increasing,the difference will decrease;(2) The number of Common Cause Failure Group(CCFG) has greater impact on MTTFS and MTTFD parameters.And the more components in a CCFG,the greater its effect;(3) When the components numbers in every CCFGs are similar,the system is more reliable;(4) CCF factorβcan be chosed in a range of 0.020 to 0.040 for petrochemical plants.