Calculation and analysis of thermal stress for high pressure rotor of steam turbine

Abstract

According to the flow operating parameters of turbine during the downtime process , the convective heat transfer coefficient values between rotor surface and steam are calculated. The FEM transient temperature analysis model is setup to obtain the temperature distribution of rotor during the downtime process. Considering the centrifugal force and temperature load of rotor during the downtime process, the high temperature strength finite element model is setup using the dual linear intensive model. Then, the stress distribution of turbine rotor during the downtime process is obtained. Introduction With the development of economy, the ammount of electricity demand and structure are continuously changing. The task of peak adjustion of electrical system has been more and more severe. At present, fossile power plant undertakes the main peak adjustion task. During the process of peak adjustion, the load of turbine changes dramatically and large temperature gradient is existed inside the rotor which intenses the lifetime wastage. However, the lifetime of turbine rotor determines the lifetime of the whole power plant [1]. As a result, the accurate estimation of turbine lifetime wastage of turbine rotor under real start-stop curve is essential. It is quite important to set up the optimization stretegy to make the power plant more safe and economical [2-4]. Finite element model The investigated objective is the high pressure rotor of a steam turbine whose material is 28CrMoNiV. Because the object is an integral-disk rotor, simplification of rotor rim is conducted and the force of blade to the rotor is replaced by equivalent load [5]. Considering the axial symmetry of computational model, two-dimensional model is adopted to simulate the problem to decrease the calculation amount. Two dimensional Plane 77 elements are adopted to fulfill the mesh as indicated in Fig. 1. The total node number is 21876 and the element number is 21118. Because large stress concentration is existed in the rotor round corner, rim round corner, and the temperature change of control stage is intense, the thermal stress of these spots will be intense which even exceeds the yield limit resulting in large low cycle fatigue wastage. In this case, the meshes near these areas are made denser to improve the calculation accuracy. Fig. 1 Overall mesh indication of rotor International Conference on Information Sciences, Machinery, Materials and Energy (ICISMME 2015) © 2015. The authors Published by Atlantis Press 277 Boundary condition for temperature field calculation The initial temperature field during rotor downtime process is the steady-state temperature field. The thermal boundary condition is setup as below: 1. The heat tansfer between outer surface of rotor and steam is assumed as the third bounday condition. The steam temperature is calculated according to sliding operation regulation. The heat transfer coeffient is calculated according the empirical formula by former Soviet Union [6]. 2. The side surface of rotor is the truncation surface of rotor where the heat transfer coefficient is very small. So adibatic condition is assumed in these surfaces. 3. Due to the axial symmetry of rotor, the boundary condition of center line of rotor is assumed as adiabatic. 4. The bearing of rotor is assumed as constant temperature which is the first boundary condition type. Boundary condition for stress field calculation The boundary condition of stress field calculation is set up as below: 1. The mechanical property of material is set up. Elastic-Plastic calculation adopt the dual linear intensive model 2. Displacement constraint is set up. Axial displacement constrains are set up at the rotor left surface and axial displacement coupling is set up at the rotor right surface. 3. Centrifugal loads of rotor and blade are introduced. 4. Temperature loads are introduced. Monitoring points Analysis on temperature field and stress field are conducted at different start-stop condition. According to the results, some stress concentration areas are selected as main monitoring point for subsequent analysis. The detailed monitoring points are defined as Fig. 2. 1. A1 is the rotor outer side transitional round corner before the governing stage; A2 is the corresponding axial node. 2. B1 is the rotor outer side transitional round corner after the governing stage; B2 is the corresponding axial node. 3. C1 is the circular section at the front shaft seal; C2 is the corresponding axial node.