Bakhmutska Y. Steam Turbine Rotors Lifetime Increase by Improving Thermal and Thermo-stress-strain State during Start-up Events

The thesis is devoted to the study of factors and mechanisms that influence the thermal and thermal stress state of rotors of high power steam turbines during start-ups from different thermal states, as well as the analysis of the possibility of increasing the turbine lifetime (number of allowable start-ups) by design modification and start-up technology change. The methodological approach for the transient thermal and thermo-stress-strain state of steam turbine rotors have been developed. The approach allows to increase the accuracy of the analysis results by taking into account the process of steam film condensation on the rotor elements and the physics of the jet stream of steam and turbulization of the flow in the cavities of the end seals, use of the refined approach for the definition of thermal boundary conditions, improved calculation model of the rotor and elastoplastic statement for the thermo-structural task, which is beneficial for the rotor material low cycle fatigue estimation. Thermal boundary conditions were determined and the transient thermal and thermo-stressed states at pre-heating phase, cold and hot start-ups and shut down were studied, applying the developed approach to the K-325-23.5 steam turbine high-pressure rotor. The analysis of film condensation and jet nature of the flow in the seals accounting influence on the thermal and thermo-stress state of the rotor is performed. The results of the thermo-stress state analysis showed overloaded rotor zones in terms of high stresses and the risk of low cycle fatigue cracks initiation. The new design of front-end seal carrier and technology for rotor heating at the pre-heating phase was proposed. The proposed actions result in the decrease of the level of thermos-stresses and the decrease in an allowable number of start-ups for the steam turbine.