Study On Life Of Marine Steam Turbine Valve Subjected To Thermal Shock

Steam turbine is the key part of dynamic system in warships,and plays a dicisive role in the steady service.Marine steam turbine works in elevated temperature and pressure,and steam is supplied by high-temperature vessels.Valve is usually the typical part of high-temperature vessels for marine steam turbines,the reliability of which will affect whether ships can work properly,which determines fighting capabilities of warships.Therefore,it is of key significance to predict the life of high-temperature vessels represented by valve.Methods for predicting the damage of valve in terms of fatigue,creep and fatigue-creep coupled effect are studied,and the factors affecting the life will be investigated.After the crack initiated,methods for3 D crack propagation are also studied.Major works are listed as follow:The temperature and stress field of the valve in the startup and stop processes are analysed.Coefficients of heat transfer are extracted by calculating the flow field of the valve,using which the temparature field is obtained.After that,the sequential coupling is utilized to compute the thermal transient stress field of the valve,and then the critical locations of the valve are investigated.For the startup-stop cyclic fatigue of the valve,methods of damage evolution of the valve are established based on multiaxial nonlinear damage Chaboche model,and considering the stochastic startup rate of the valve,the life evaluation method is also provided.Based on Chaboche model,the damage coupled elastoplasticity constitutive model is given,of which the implicit numerical method is provided and encoded into subroutine Umat in ABAQUS to compute the damage evolution of the valve.After that,the effect of the sequence of startup rates on the fatigue life is discovered.Considering the stochasticity of the valve startup rate due to the mobility of marine steam turbines,the stochastic damage evolution is simulated with Monto Carlo method,then the result is compared to conventional singlecondition life design(SCLD),which shows the prediction by SCLD may overestimate the life of valve.For the fatigue-creep coupled effect of the valve,methods for predicting the damage of the valve are established based on fatigue-creep coupled Chaboche model,and the effect of the stochastic work time on valve life is studied.Based on fatigue-creep coupled Chaboche model,the damage coupled viscoplasticity constitutive model is given,of which the implicit numerical method is induced and encoded into subroutine Umat in ABAQUS to compute fatigue-creep damage,and the critical locations are found.The effect of the work time on valve life is investigated,and the life of the stochastic work time is studied further considering the the mobility of marine steam turbines.It is found that life of schochastic work time is lower than that predicted by conventional SCLD.The method of 3D crack propagation after crack initiated is established for fatigue-creep crack growth.Cracks in microstructures of valve material are studied via crystal plasticity theory,which shows the similarity of crack density for volume elements with different grains.A predefined crack size is calculated,which correlates damage method and fracture method.Using fatigue-creep fracture mechanics,the 3D crack growth simulation method is established based on remeshing method,then the dynamic simulation of 3D crack growth is realized by combining ABAQUS with Python program.The effects of startup rate and work time on the crack shape and crack life are also studied.