| With the increases of the installed capacity and power generation process risk in thermal power plants, more strict safety requirements for power production will be faced. Functional safety analysis of safety instrumented systems becomes a new hotspot in the power industry. As the protection system of the boiler, FSSS (Furnace Safety Supervisory System) is one category of safety instrumented systems, and has been widely used in thermal power plants. In this paper, based on international functional safety standards IEC61508and IEC61511, combined with the design standards, procedures and regulations of thermal power plants, and taking Safety Integrity Level (SIL) as a guiding index, hazard and risk analysis of boiler in thermal power plant through a structured analysis is achieved, the target SIL of FSSS is determined, and functional safety assessment of an in-service FSSS is made. The main results of this study are as follows:(1) The evaluation method of availability based on continuous-time Markov model is proposed. Continuous-time Markov models for four redundant structures of industrial controllers are established, curves of probability of failure on demand, probability of fail safely and availability are obtained, which directly reflect performances of different redundant structures, and simplify the solving process.(2) Reliability and safety assessment methods considering common cause failure are improved. Reliability block diagram models and fault tree models are built for typical industrial architectures-series, parallel, k out of n, hybrid, et al. Based on the improved multiple ? factor model, formulas of reliability, mean time to failure, probability of failure on demand, and probability of fail safely are obtained. Compared with formulas proposed by IEC61508, which is based on simple ? factor analysis method, this method is confirmed more general. By this method, reliability and safety assessments of different structures for manual trip button are evaluated, and the best solutions in two modes-positive and negative are got.(3) Explosion consequence analysis method of the boiler furnace in thermal power plant is proposed, which is based on the boiler and pressure vessel explosion models. Furnace blast energy and shockwave overpressure are quantified by mathematical analysis model. Blast energy, shockwave overpressures, explosion damage extents and risk indices for different explosion extents of the in feed water preheat evaporation system in one-through boiler are confirmed. Quantitative analysis of the explosion possibility for thermal power plant boiler furnace has been made, using layer of protection analysis method. The target SIL of FSSS has been determined through risk diagram and risk matrix method.(4) For the most important safety function of FSSS-Master Fuel Trip (MFT), safety and reliability analysis of hard trip circuit and functional safety assessment of MFT are achieved. Possible corrective measures are made by comparing the actual SIL with target SIL. The results show that:actuator and sensor have greater effects on the SIL index of MFT, compared with logic control section. To raise the functional safety level of FSSS, more importance should be attached to the actuator, and full consideration must be made in determining the functional test cycle, and the structure of instruments. |
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