| LNG vaporizing station is an important part of urban natural gas supply engineering, and plays an important role in gas supply and emergency peak shaving, etc. LNG vaporizing station with a large amount of LNG stored commonly belongs to the major hazard. Once the leakage and diffusion of gas occurs in these places, it will be easy to cause fires, explosions and other disastrous consequences. Safety instrumented system is one of the effective measures to ensure the safe and reliable operation of the vaporizing station. Currently, the assessment of functional safety is in its infancy in our country. In addition, there is no a method which can systematically assess the functional safety of LNG vaporizing station safety instrumented system. There are a series of problems (such as whether the safety instrumented function is sufficient, whether the redundant structure is appropriate, and whether the safety integrity level meets the requirements.) in practical application. Therefore, the present situation of safety instrumented system of LNG vaporizing station has been studied, and the existing problems have been analyzed, a method for classification and verification of safety integrity level for LNG vaporizing station has been established, the functional safety assessment of safety instrumented system in LNG vaporizing station has been formed. It has an important guiding significance for LNG vaporizing station safety functions identification, safety integrity level classification and verification, redundant structure determination, instrument type selection and functional test cycle. In this regard, the research works carried out are as follows:(1) Through the detailed investigation of the design code and data of LNG vaporizing station, the requirements of design code for safety instrumented system, the configuration and interlocking logic relations of safety instruments in actual station field have been summarized and analyzed. Existing outstanding problems are as follows:1) relevant design codes have only some principled and instructive provisions on safety instruments. What kind of redundancy structure the safety instrumented system should be and what level the SIL level should achieve are not involve; 2) existing safety instrumented functions only used lool redundant structure. Whether the target SIL level meets the requirements, whether need to add other safety instrumented functions have not been verified.(2) In order to ensure the high efficiency of the subsequent SIL classification work, vaporizing station of different grades use compatible classification method, LNG vaporizing stations were divided into five grades according to the total storage capacity. The total capacity Vi>5000m3 for the first grade,2000m3<Vi≤5000m3 for the second, 1000m3<V1≤2000m3 for the third,200m3<V1≤1000m3 for the fourth, Vi≤200m3 for the fifth.(3) The advantages and disadvantages of common process hazard analysis (PHA) methods have been compared and analyzed. Combined with the characteristics of a range of process equipment, process complexity of LNG vaporizing station, hazard and operability analysis (HAZOP) method was used to analyze the process risk of LNG vaporizing station, and safety instrumented functions were determined on the basis of the analysis report.(4) Risk matrix, risk graph and LOPA analysis, those three common methods of SIL classification have been studied. Through comparing and analyzing the advantages, disadvantages and scope of application of each method, a method for classifying LNG vaporizing station level was proposed, which is applicative for different levels. The main contents of this method were summarized as follows:LOPA analysis is applicative for the first or second level of LNG vaporizing station; and LOPA analysis or risk graph are applicative for the third or fourth level, and the risk matrix or risk graph are applicative for the fifth level. Finally, a process of SIL classification of LNG vaporizing station is proposed.(5) Three common methods (fault tree analysis, reliability block diagram and Markov model) of SIL verification have been studied, and the advantages, disadvantages and scope of application of each other were compared and analyzed. The key factors that affect the accuracy and complexity of reliability modeling were discussed, such as failure mode, redundant structure, common cause failure and equipment self-diagnosis function, etc. The β model was selected as common cause failure model, a Markov model of 1oo 1, 1oo2,2oo2 and 2oo3 redundant structure and a SIL verification process for LNG vaporizing station were established. Moreover, based on mixed Programming of C# and Matlab, a STL verification calculation program of the LNG vaporizing station was prepared.(6) Functional safety assessment of the vaporizing station with LNG storage amount of 900m3 has been carried out. Results showed as follows:1) Through HAZOP analysis, seven Safety instrumented functions were identified. Five of them are extant safety instrumented function and two of them need to add. The added two safety instrumented functions are overpressure interlock, discharge of storage tanks and overvoltage protection of gasifier; 2) Through the LNG vaporizing station classification method, it is determined that the station belongs to the four level station. According to SIL classification process, LOPA analysis of higher accuracy was selected for SIL classification of these seven safety instrumented functions. Classification results show that:one SIF no security requirements, three SIF for the SIL1 level, three SIF for the SIL2 level; 3) Using the established Markov verification model and written verification calculation program, seven safety instrumented functions were verified. All the SIF can only meet the requirements of SIL1 level, which three of them could not meet target SIL2 level.4) Taking emergency shutdown system (SIF-5) as an example, the effects of various measures (redundant structure, function test cycle and instrument self-test) on the SIL level were discussed. Finally, An improved method with combination of a variety of measures were put forward, which is applicative for these ones whose safety instrumented system are not enough. |
PDF Full Text Request