Iterative algorithm of failure rate and proof test cycle of chemical safety instrumented systems

Key words:  safety instrumented system  maximum likelihood estimation  proof test cycle  update of failure rate  Monte Carlo simulation

Abstract:

It is always assumed that the failure rate is positive, when the current domestic and foreign standards are used to calculate the average failure probability on demand (PFD_avg) of the safety instrumented system (SIS). However, due to random factors such as aging, corrosion, and wear of the equipment, the failure rate will slowly increase over time, leading to increasing errors in the calculation model. In addition, the safety instrumented system is in a dormant state, and it needs to be proof test regularly to discover possible hidden faults. However, under the influence of uncertain factors in actual production, the proof test may not be carried out according to the scheduled cycle, resulting in an increase in PFD_avg than expected. In the current work, we draw on the proof test process of safety valve (PSV) in the API581 standard, and combine it with the failure rate update method of OLF070. Taking the emergency pressure relief valve subsystem of the wax oil hydrogenation unit as a case, and based on the proof test results in the previous cycle, the failure rate of the equipment and the next proof test cycle are iteratively updated while ensuring the availability of the subsystem. The result shows that, the difficulty of risk control under the time-varying failure rate and non-periodical detection of the SIS has been solved, and the theoretical basis for rational decision-making is provided.