Analysis Of Low Cyclic Fatigue Life Of The Valves On The Secondary Unit Of Steam Turbine

As the most important component of the turbine, failure accidents of valves frequently occur due to its high temperature and high pressure work environment. And be short of research on designing, manufacturing, and running of the valves, failure accidents bring great incipient fault to the economic running of power plant and personal safety. Therefore in order to solve the practical problem in power generation, it is necessary to study the fatigue life of the turbine valves.In this paper the 3D model of the turbine valve is generated using the commercial software, HyperMesh. The model is discreted by the eight nodes hexahedron elements. Based on the technique of finite element, the analytical calculation was performed for the transient temperature fields and stress fields of the turbine middle pressure main steam valve. The cracked region's load spectrums are plotted and the fatigue life of the valve is predicted by local stress-stain approach. Firstly, this paper analyzes especially the transient temperature fields and stress fields under various operating conditions including cold startup, shutdown, strong hot startup, limit hot startup and accidental operation. Secondly, the change of convection heat transfer and material property following the temperature variation is considered. Thirdly, the standpoint which the cracked region's circumferential stress is regarded as the important factor effecting on the fatigue life is presented. Fourthly, contrast the computational results to the failure investigation data of the valve.The results indicate that the circumferential stress concentration region is corresponded to the cracked region and the credibility of the fatigue life prediction is proved by the investigative data. This paper concluded that the accidental operation is the principal reason for the crack initiation and the ratio of fatigue damage will be up to twenty because of the accidental operation.