Stress And Life Analysis Of Supercritical Boiler Steam-water Separator Under Variable Operating Conditions

Supercritical units operate with once-through boiler with the advantage of high start-up speed and it is suitable for variable load operation. The boiler components’s stress are varied frequently under variable operating conditions and its life loss are accelerated that can affect the safe operation of the unit. The steam-water separator which is the biggest thick wall pressure components of supercritical boiler is chosen as the research object. Its stress field and life loss are calculated and analysed under variable operating conditions.In this dissertation, a comparison is conducted among several steam-water separators used in ultra supercritical boiler that are in service in our country now. Based on the variable condition dynamic mathematical model, the dynamic characteristics of steam-water separator is analysed during wet state and wet-dry state transfer state and dry state.The steam-water separator of a1000MW unit is chosed as the research object. The mathematical models of transient temperature field and transient stress field are set up, and through reasonably simplifying the structure of the separator, a three-dimensional finite element model is set up by ANSYS software. In this model, by applying boundary conditions, constraint conditions and initial conditions, it is possible to stimulate the transient temperature field, transient thermal stress field, transient mechanic stress field and transient total stress field in all kinds of operating conditions including cold start-up, hot and warm start-up, shut down, etc.. Thus the temperatures of separator inner and outer walls can be obtained and the regularities of temperature variation can be observed. And eventually the position of the maximum thermal stress and its change law can be concluded. Through analyzing the distribution of mechanical stress and its change law, proportional relationship between mechanical stress and medium pressure, and the total stress field of the separator under temperature load and pressure load act together, the position of dangerous point and the distribution and change law of total stress can be known.In this dissertation, Finite Element method is combined with Artifical Neural Network method. Using the dynamic mapping function of Elman Neural Network, a dynamic soft sensor model of steam-water separator total stress is set up. The medium pressure and the temperature of the inner and outer walls of the separator are determined as the input variables of the model while the total stress is chosed as the output variable. The train samples are trained while the model structure parameters are determined at the same time. It is verified by the measured data from power plants that this model has satisfactory prediction performance. And this work provides a theoretical basis for on-line stress monitoring and life assessment.During the operational conditions, the main failure modes of the steam-water separator are fatigue failure caused by alternating loads, creep failure caused by high temperature and failure caused by creep-fatigue interaction. Based on continuum damage mechanics and with multi-axial load taken into consideration, a nonlinear damage model of life analysis of steam-water separator is set up. The results provided by this model are compared to the ones calculated by traditional models. It turns out that the damage result from multi-axis continuum damage model is bigger than that figured out by single axis model and linear cumulative damage theory, which shows that the multi-axial complex stress state has accelerated the accumulation of material damage, thus shortening the service life of steam-water separator. The load spectrum of the steam-water separator is analysed and the service life is evaluaed with the nonlinear multi-axis damage model.