Analysis Of High Temperature Strength For Ultra Supercritical Steam Turbine High Pressure Inner Casing

Currently in research and development35MPa/600℃/620℃/620℃double reheat unit, which has high inlet steam parameters, high thermalefficiency, low carbon dioxide emissions and low pollution characteristics.As the high pressure steam parameters into the module increased to35MPa/600℃, significantly beyond the existing ultra-supercritical highpressure module limit. In order to meet the improved steam pressure, thewall thickness of high pressure inner casing increased, changing the size ofthe structural design, which is bound to affect the start-up and shutdowntransient stress and fatigue life of the unit. Therefore, it is necessary tocalculate the startup、shutdown transient stress and creep fatigue damage ofnew design inner casing. And the calculated results will be referred whenimprove and optimize the structure and temperature rise rate of the startupprocess to ensure the safe operation of the unit in service period specified.In this paper, the high pressure inner casing of1000MW a doublereheat domestic ultra-supercritical unit will be analysis object. InABAQUS finite element analysis software platform, conduct comparativeanalysis of steady creep behavior under steady-state operating conditionsas well as start and stop conditions of low-cycle fatigue damage for doublereheat new design high pressure inner casing and now running on activeduty inner casing. And give the structure and start curves improvementsuggestions.By introducing the Norton-Bailey creep constitutive equations andCocks-Ashby multiaxial toughness factor, comparative analysis of creep temperature field, stress field, CA equivalent creep strain distribution ofnew design high pressure inner casing and now running on active dutyinner casing under stable operation of200,000hours. The results disclosed:the maximum stress of inner casing is lower than the material yieldstrength with the corresponding temperature, not reaching the yield state;creep strain significantly varying within the high pressure inner casingareas is mainly concentrated in the steam inlet and balance piston;multiaxial stress significantly influences the creep assessment，thus it ismore realistic and accurate to use multiaxial creep strain to assess creepstrength; After200,000hours, stress relaxation due to the bolt creepbehavior decreases the bolt tightening force, and further affects the sealingperformance in the middle of the high pressure inner casing.By using the same start and stop curve on the new design and hasserved inner casing, the start and stop stress and strain fields are analyzedto Manson-Coffin formula and Miner’s rule is based on the strain obtainedlow cycle fatigue strength result. The results disclosed: the new designinner casing low cycle fatigue damage is far greater than has been servedinner casing. To ensure the safe operation of the new design unit, improvethe start curve and shorten the holding time, reduce the temperature riserate to0.524℃/min, and It’s significant effect to reduce fatigue damage inthe new unit, extend the service life of the unit.Finally, use the Python to do the post-treatment of the FEA. Conductthe creep and fatigue coupled damage analysis based on the theory ofcontinuum damage mechanics, assess the unit’s life, ensure its safeoperation in service period specified.