| Ultra-supercritical steam turbine is one of the most advanced power generation equipments at current world due to its high efficiency, safety and cleanness, so ultra-supercritical power generation technology guides future development of coal-fired power. As working parameters keep on increasing, many high-temp components run in creep regime, frequent startups & shutdowns and peak regulations result in thermal fatigue of the components, furthermore, excellent controllability requires high ability of closing valve. So far, the maximum steam parameters reach 620 ℃/13 MPa for intermediate pressure valve of domestic turbines, accordingly, the structural damage or failure caused by high temperature can not be neglected. Steam valve serves as one of critical components in regulation system of coal-fired power unit, it withstands high parameter steam from boiler. Whether the valve normally operates or not determines the safety of whole unit. Because the steam pressure and temperature approach the utmost utilization of valve material GX12 Cr and FB2 alloy, the components of inlet valve in term of valve body, U-type seal, valve disc used to suffer failure and damage. Therefore, the structural strength analysis and life assessment are performed on the three components according to their failure characteristics.Firstly, through thorough illustration of theoretical method on creep-fatigue, the theoretical method was constructed on the thick-wall structures, which is fit for calculating and analyzing high-temp strength. Comparing with convectional method as linear damage accumulation rule(LDA), the methodology based on continuum damage mechanics(CDM) has three advantages of nonlinearity, coupling, multiaxiality. By using these two methods, the creep-fatigue damage was calculated and analyzed on the object of newly-designed valve(new) and in-service valve(old) under 22 normally operational years. The results demonstrate that the LDA overestimates the creep-fatigue damage of the valve body. Because the interaction effect and multiaxial state of stress exacerbate the creep-fatigue damage in some extent, the damage assessment on valve body by CDM is much better than that by LAD. The result shows that the maximum damage of new valve is 0.286, while the maximum damage of old valve is 0.267. Furthermore, comparing with creep, the fatigue contributes more to total damage.Secondly, the multiaxial creep damage coefficient was introduced in the paper on calculating and analyzing the multiaxial creep equivalent strain of the U-type seal, basing on Cocks-Ashby cavity growth theory. The coefficient not only microscopically reflects the essence of creep failure induced by cavity growth in seal material, but also mascopically characterizes multiaxial creep failure under complex stresses. The multiaxial creep design criterion was employed to assess the high-temp strength of U-type seal. The contact stress variation of U-type seal was obtained and analyzed under various operating condition and design parameters. The result shows that the maximum multiaxial creep equivalent strain is 0.5% on U-type seal under thin-wall flange, while the one is 0.8% on U-type seal under thick-wall flange. Regarding that the allowable strain is limited at 1% in related criterion, the high-temp strength of U-type seal is satisfied accordingly. Besides, the maximum contact stress of U-type seal is 280 MPa in initial creep stage and decrease down to 82 MPa in final creep stage of 0.2million hours. If the bolt relaxation does not take into account during the creep, the assessment result is more conservative on the high-temp strength of U-type seal.Thirdly, a combination method of visco-elasto-plastic body impact and contact was developed in the paper, and subsequently an explicit dynamic constitutive model was employed in impact damage prediction for valve disc and seat, which included modified damage term. The impact dynamic stress is a highly nonlinear function, and it includes many factors of plastic strain, strain rate, multiaxial effect, adiabat heat, damage. Accordingly it is able to reflect energy conversion and dissipation, strain rate hardening, thermal softening effect. In order to obtain optimum impact and geometry parameters, comparative studies of influence on impact damage were carried out among disc obliquity, impact area ratio, ascpect ratio, impact velocity. The impact behaviors of 1m/s, 3m/s, 6m/s were studied in the paper. Studies suggest the maximum allowable impact number reaches 4000 corresponding to impact damage of 0.0025 under the optimum parameters within 1m/s.In summary, the paper illustrates technical route and flowchart on structural strength analysis. Steam valve of a 1000 MW ultra-supercritical turbine was chosen for study, furthermore, design criterion and assessment methodology for structural strength on creep, fatigue, impact were developed in the paper, which were widely suitable for thick-wall structures. Multiaxial creep-fatigue assessment program was developed by adopting ABAQUS Python language. The research provides valuable theory foundation for the strength design of high-temp turbine components, and it is meaningful for safety operation. Works present in paper have reference value, so it guides further safety assessment and life prediction in the future. |
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