Study Of Inlet Parameter Fluctuations Effect On Creep-fatigue Strength Of Ultra-supercritical Steam Turbine Intermediate-pressure Valve

With the increase of the inlet steam parameters of turbine units,the strength analysis of the components in steam turbine under cyclic loading is significant to improve the life of steam turbine.At present,most of studies on the cyclic load problems of steam turbine components under high temperature and high pressure conditions concentrated in the start and stop phases.However,due to the use of new energy or power grid peak-shaving in recent years,the steam parameters also fluctuate after the unit enters operation.The intermediate-pressure valve is one of the main high temperature components of the steam turbine.It has thick wall,very complicated structure,many connections and corners,making it easily to generate thermal stress during unsteady operation.During the long-term operation of the unit,the stress fluctuation caused by the steam parameters fluctuation may execute an additional impact on the creep of the valve.The alternating stress itself will also aggravate the fatigue failure.Therefore,the failure problem brought by the inlet parameters fluctuation of the valve under operating conditions cannot be ignored.In this thesis,the object is an intermediate-pressure valve of ultra-supercritical steam turbine with a steam pressure of 600℃/5MPa in its medium pressure module.The structural strength,creep damage,fatigue damage of its key parts under regular fluctuations of inlet steam temperature and pressure were calculated in Abaqus platform,and the effects of inlet steam parameter fluctuations on the key parts were summarized based on those results.By using Norton-Bailey creep constitutive equation and steam parameters measured on-site,the high temperature structural strength of intermediate-pressure valve was analyzed.The results showed: the influence of different steam parameters’ fluctuation on the stress distribution were not identical and cannot be ignored.The Mises stress at the thick wall of the diffuser is significantly affected by the fluctuation of the temperature difference between the inner and outer walls.Based on the measured parameters,the fluctuation periods and amplitudes of the highest occurrence were summarized and were taken as input parameters after combination.The results showed that the fluctuations have a non-negligible influence on the overall stress and strain distribution,temperature fluctuation amplitudes,stress fluctuation amplitudes,strain,stress relaxation at the key points.And the influence effect varied with the inlet steam fluctuation amplitude and period.By introducing the theory of Continuous Damage Method and using Matlab,the creep damage and fatigue damage of selected key points were calculated by taking the creep-fatigue interaction into consideration.The laws of influence effects of various parameters on the damage were analyzed to give effective feedback to on-site controlling.