Study On Thermal Fatigue Damage Of The Valve On The Intermediate-pressure Section Of Steam Turbine In Power Plants

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.