| A 660 MW supercritical steam turbine suffered an early fracture accident in its high pressure internal cylinder.In this paper,finite element analysis was carried out based on transient heat transfer and multi-physics coupling theory,aiming at the service safety assessment requirements of replacing the accident stud with other material stud.Through the combination of high and high pressure internal cylinder--integral modeling of fastening stud and local modeling of fastening stud,the transient temperature field and coupling stress field of the steam turbine's high and medium pressure internal cylinder and its key fastening stud are systematically studied under the typical service conditions of the power plant(starting,rated and variable load,etc.),and the safety of the replacement scheme of the steam turbine's high and high pressure internal cylinder fastening stud is preliminarily evaluated from the point of stress.In view of the complexity of the high pressure internal cylinder and tightening bolt system and the need for the accuracy of thread tooth local stress analysis,this paper introduces the "sub-model method" and combines the cylinder and bolt integral modeling with the bolt local modeling to build the three-dimensional finite element model of the above complex system.All bolt polished rod model is adopted in the whole model and binding contact simulation of thread connection,the four key bolt in the child model using three-dimensional symmetric thread model,and to give full consideration to the contact(including thread,flange nut and cylinder surface contact,upper and lower cylinder block combined with surface contact,etc.)of the nonlinear friction between contact.The finite element modeling method mentioned above not only provides a basis for accurately obtaining the stress distribution of the tightening bolt of the high and high pressure internal cylinder of steam turbine,but also provides a new idea for the optimization modeling of the finite element analysis of complex structure.According to the cold tightening torque and hot tightening elongation of the bolt in the middle of the cylinder,the integral and local stress analysis of the cylinder-bolt system in the pre-tightening state is firstly carried out in this paper.The results show that the bolt bears axial tensile stress and the flange bears compressive stress.The maximum stress of the bolt occurs at the variable section of the screw,which is much lower than the yield strength of the material.In the sub-model,the maximum stress of the bolt occurs at the root of the first thread engaged with the nut or flange thread,but the maximum equivalent stress is still slightly less than the yield strength of the material.Combined with the design parameters of steam turbine and the time history curve of steam parameters under typical working conditions,the transient temperature field analysis of the cylinder-bolt system under typical working conditions was carried out using the third type of thermal boundary conditions,and the distribution rules of the axial and radial transient temperature fields of the cylinder were obtained.The results show that the temperature distribution of the cylinder-bolt system is very uneven under start-up condition.Under the loads of 660 MW and 330 MW,the axial and radial temperature difference of the cylinder is alleviated,and the flange near the air inlet of the high pressure inner cylinder bears the maximum temperature difference and thermal deformation.The transient curves of the temperature difference between inner and outer walls of the flange and the radial temperature difference between the bolt and the flange indicate that the most dangerous moment of the cylinder is before and after the rotor speed rises to full speed.The coupled stress field analysis of the cylinder-bolt system under typical service conditions(start-up,660 MW rated,330 MW low load,etc.)was carried out,and the overall stress field distribution law of the cylinder-bolt system at various characteristic moments was obtained.The results show that: during the start-up process,due to the uneven distribution of cylinder body temperature,flange and bolt temperature changes,the thermal stress caused by thermal load dominates,so the coupling stress distribution of cylinder body is very uneven;The maximum stress of cylinder body occurs at the joint of flange and nut,while the maximum stress of cylinder wall is always lower than the yield strength of material.Under the loads of 660 MW and 330 MW,the overall stress of the cylinder decreased,most of the stress was lower than 200 MPa,and the local stress reached 400 MPa.Under the above characteristic working conditions,the maximum stress of the polished rod screw always occurs at the initial position where the nut is fitted,and the maximum equivalent stress is far lower than the yield strength of the material.The sub-model method was used to analyze the local coupling stress field of the fastening bolt under start-up condition,660 MW steady-state condition and 330 MW low-load condition.The results show that under various working conditions,the stress of the first thread is the highest,and the stress distribution in the thread meets the attenuation rule along the axial direction.In the sub-model,the maximum stress at the root of the screw thread is still slightly lower than the yield strength of the material at the corresponding service temperature,which meets the relevant standards of pre-tightening design at home and abroad.In this paper,the transient change process of equivalent stress of cylinder body and contact stress of central parting surface was comprehensively investigated,and the relationship between maximum equivalent stress of thread root and yield strength of four key bolts was compared under typical working conditions,so as to analyze cylinder strength,sealing property of central parting surface and service condition of bolt after replacement.The results show that the overall and local stress of the cylinder and bolt meet the requirements of safe service,but there is a risk of steam leakage in the middle section,so further sealing measures should be taken. |
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