Research On The Spurious Trip Model And The Testing Effect Of Safety Instrumented Systems

Safety Instrumented Systems (SIS), as one of the most important safety mechanisms in petrochemical installation, are widely applied. It is a pity, however, that the spurious trip of the SIS may lead to unplanned partial or full process shutdown. The effects of spurious trip include unnecessary production termination and potential risks caused by the stresses and hazards brought during system restoration. In order to improve the safety performance of SIS, the mechanism and characteristics of spurious trip of SIS system were systematically studied in this paper.Aim at providing a fit for service method for the assessment of Safety Integrity Level of SIS, Markov Process (MP) and Fault Tree Analysis (FTA) were used in the construction and modeling of reliability of SIS. The model is then applied in the analysis of spurious trip performance and system optimization. The main works and findings of this paper are briefly summarized as follows:1. Based on the synthetically analysis of the cause and influence of spurious trip, together with the experience obtained in assessment practice, a novel precise spurious trip definition was provided in this paper. The spurious trip model of classic redundancy systems was constructed based on Markov process, the model is then applied in the analysis of the anti-spurious trip performance of SIS and proved helpful for the study of safety and reliability of SIS.2. Based on the model introduced in this paper, a novel Mean Time to Failure Spurious ( MTTF spurious) calculation algorithm was proposed and proved to be feasible by comparing it with classic spurious trip calculation model. Take advantage of the newly proposed algorithm, the relation between probability of fail safe (PFS) and spurious trip rate (STR) was also studied in this paper. The feasibility of using PFS in evaluating spurious trip was also studied and the characteristics of these two reliability index compared.3. The spurious trip rate and dangerous failure rate of two periodical repair strategies, namely exponential and determination distribution were studied in this paper. The influences of maintenance period and maintenance efficiency on system reliability were also studied.4. In order to identify the actual logical which appeared in complex safety instrumented systems (SIS), a new actual logical identification method was proposed based on Fault Tree Analysis (FTA) and Failure Mode and Effect Analysis (FMEA) methods. An associated simplify algorithm was also put forwarded to solve the complex problem encountered during reliability analysis of hybrid and voting system.