【作者】 陈忠兵；

【导师】 陈传尧； 李国清；

【作者基本信息】 华中科技大学 ， 固体力学， 2009， 博士

【摘要】 火力发电机组中使用大量的阀门,但工程实际中阀门壳体的失效时有发生,对火电机组的安全、稳定和经济运行带来极大影响。本论文首先通过专项工业调查,获得了有关电站阀壳热疲劳开裂的翔实数据。调查发现,阀门壳体失效是具有普遍性的问题,热疲劳是阀门失效的主要原因;裂纹出现的时间、存在的部位具有统计上的规律性,与机组的运行工况、服役历程、阀壳的制造质量和缺陷有着密切的关系。其次采用有限元方法,分析了与电站运行工况相对应的阀壳温度场和应力场,联合各工况下阀壳有限元机械应力的计算结果,绘制了与机组一次完整启停相对应的组合应力时程图。本文所研究的阀门壳体是大型铸造件,表面不可避免地存在凹坑或凸起等缺陷,在国内电站行业还缺乏对材料缺陷与疲劳寿命之间关系的定量研究。本文根据材料的典型缺陷和尺寸,建立材料的缺陷模型,从而实现了材料缺陷与疲劳寿命之间的定量化研究路线,进而首次编制出了电站阀门的疲劳载荷谱。随后,按照上述建立的缺陷模型,采用Neuber方法,获得了代表阀壳真实应力-应变循环的疲劳载荷。随后,针对每年的电站运行记录,编制了电站阀门壳体的年疲劳载荷块谱。所编制的疲劳载荷谱为阀门疲劳寿命的标准化设计提供了一种定量的参考。根据疲劳载荷谱,采用损伤的线性累积原则,估算出机组一个运行周期内阀门的疲劳寿命,并与工业调查结果进行对照。研究结果表明:所预测的裂纹萌生部位与实际工业调查结果相符合;所预测的疲劳寿命(15年)与工业调查结果(12～14年阀壳出现大量可见裂纹)也基本一致。研究还发现:事故工况是导致阀壳裂纹萌生的重要因素,事故工况导致的热疲劳损伤减少阀壳寿命34%以上。最后,论文研究了阀壳表面缺陷与疲劳寿命的定量关系。研究结果表明,在正常运行周期内,严重的表面缺陷将导致阀门寿命下降约30%,而轻微的表面缺陷对正常的阀门寿命影响很小。由于表面缺陷的影响,每年含5次事故运行周期的阀门寿命将减少约50%。这一研究结论为大型火电站阀门的设计、制造、检测、运行和维护提供了一种定量化的参考。本文所取得的研究结果对大型火电站阀门的设计、制造、运行检验和检修具有一定的参考价值;本文所提出的研究路线和方法可供电站其它厚壁汽室件的疲劳研究时参考与借鉴。更多还原

【Abstract】 The valves are the most important components of the steam turbine and are used to control the steam flow. Failures of turbine valves frequently occurred in many power plants due to serve uneven temperature change and high pressure working environment. Thus, it brings great threat to the economic and safe services of thermal power plants.Recently, the author has done an industry inspection on the Chinese-made turbine valves in power plants located on Hubei Provinces, China. It is reported that cracks are the main cause to the failure of valves. Since 2000, many turbine valves are found with short cracks or even much long propagation cracks. In the inspection report, we collected data of crack locations and crack sizes in components of turbines. By analyzing the failure, the cracks are believed to result from cyclic stresses on the valve body during the turbine services. Therefore, thermal stress analysis and fatigue life estimation of the valve body are necessary. This is the original motivation of the study in this dissertation.Firstly, the geometrical model of the valve body is created using the CAD program UG. The interactive mesh generation algorithm included in Hyper mesh is used to realize the meshing of the computational domain and thereafter to build a three dimensional finite element model by using a Hyper mesh-ansys translator. The practical records of the turbine operation are used as input to FEA procedures. The properties of the valve material varying with temperature are considered by using material constant table and linear interpolation method. Therefore, five cases including cold-startup, hot-startup, and extremely-hot-startup, shutdown and emergence, are carefully studied in this study.The finite analysis of the temperature fields and thermal stress fields were carried out in this paper.The temperature results obtained by FEA show that temperature variety with time on the key nodes is sharper than that in elsewhere, then the key nodes are selected as the dangerous zone where the initial cracks are spotted. The thermal stress results obtained sequentially using ANSYS are shown that much higher stresses experienced on the wall near the intersections between the steam inlet or outlet and the main body of the valve during tur- bine cold-startup. A maximum circumferential stress of 293 MPa value experienced during emergence shutdown. Considering a complete turbine operation from startup to shutdown and summarizing the above analysis, the fatigue is mainly induced by the circumferential transient thermal stress variation at a key point (or ’dangerous’ point) near the stiffening rib in the valve body.Since the valve was made of cast steel, the defect casting around the inner wall of the valve inevitably existed. Using the results of stress fields obtained above, the local stress-strain loops are obtained by using Neuber’s method and hysteresis loop equation . The rain-fall method is used in the data transform process and three typical stress loops are used to define the loops of fatigue loading pattern for the valve.Following the analytical process, the cumulative fatigue damage is calculated according to the Miner’s law, and the number of stress loops to produce failure is obtained for each fatigue load pattern. The results for fatigue with or without considering of emergence shutdown are obtained respectively. The steam turbine is assumed to have ten times cold-startups, 40 times extreme-hot-startups and 240 times warm-startups in one year, then the fatigue life to produce crack will be 22.8 years. On the other hands, the fatigue life will be reduced to about 15 years, provided that 5 accidents are added to the yearly operation process. It is also found that, the thermal fatigue damage of the valve induced by emergence shutdown increase as much as twenty times than that during normal turbine operations. On the other hand, the effect of surface defects or defect casting is quantitative studied. The results for turbine normal process show that, the severe surface defects will reduce the life of the valve about 30% while tiny defects almost do not affect the fatigue life. In contrast to this result,the result for turbine process including 5 accidents per year show that, the severe surface defects will reduce the life of the valve more than one time. The above conclusions provide information for adjustment of turbine operation and for maintance and inspection of steam valves in power plants.Although this dissertation is mainly focused on fatigue life prediction of regulating valves on the intermediate-pressure section of the steam turbine, it takes a reference for study on thermal fatigue of other thick-wall steam room components in China-made thermal power plant in the future.更多还原