Finite Element Modeling And Multi Field Coupling Stress Analysis Of High-Mid Pressure Cylinder-Bolt System Of A Combined Cycle Steam Turbine

The finite element stress analysis of the cylinder-tightening bolt system and localized bolts under typical service conditions was carried out for the localization of fastening bolts for high and medium pressure cylinder of a 390 MW gas-steam combined cycle unit introduced by a power plant.From the stress point of view,the service safety of the cylinder-bolt system was studied after the localization of fastening bolts.First of all,the three-dimensional solid model of the cylinder-bolt system was set up by using equal-scale survey paper based on the cylinder-bolt assembly drawing.The high-mid pressure cylinder-bolt system(the overall-model)and the key bolts(the sub-model)were constructed using the method of joint modeling.The overall and local finite element stress analysis of the cylinder-bolt system was carried out for the typical service conditions of the turbine(cold start,hot start and steady state operation).By introducing an artificial neural network and using the measured temperature of the cylinder inner wall as the input to find the convective heat transfer coefficient reversely,the nonlinear relationship between the transient temperature of the cylinder inner wall and the heat transfer coefficient was established.the transient temperature field of the cylinder-bolt system under cold and hot start conditions was simulated based on it.The comparison of the simulated temperature value and the measured value of the key parts of the cylinder proved that the temperature field simulation results in this paper are generally accurate and reliable(the maximum error between simulated and measured temperature was less than 8%).Considering the coupling effects of cylinder field weight,bolt preload,steam pressure and temperature stress,the overall stress field of the cylinder-bolt system under typical service conditions was studied systematically.The results showed that the maximum equivalent stress of the cylinder increased from 200 MPa to 527 MPa under cold-start conditions,and changed from 200 MPa to 384 MPa under hot-start conditions.The above-mentioned maximum equivalent stress occurs at the joint surface of the nut and the cylinder body,and its value was below the yield strength of cylinder materials.According to the analysis of the contact stress in the cylinder,the pre-tightening force of the tightening bolt could meet the sealing requirements of the cylinder after the localization of fastening bolts.Based on the overall stress analysis of the cylinder-bolt system,the sub-model of tightening bolts was used to study the local stress field of the tightening bolts(HJ23-HJ31)in key parts of the cylinder under typical service conditions.The results showed that: under the action of preload,the maximum equivalent stress of the 5 bolts was between 682?747 MPa,which appeared at the root of the first thread tooth;the maximum equivalent stress value of the bolts decreased to 614?659 MPa during the startup to steady state.The maximum equivalent stress was transferred from the first tooth to the second tooth.And the mechanical factors of the maximum equivalent stress transfer in the bolts were analyzed from the change of the equivalent stress component with the service conditions.For the typical working conditions,the maximum equivalent stress of the tightening bolts in the key parts of the cylinder was below the yield strength of the bolt material at the corresponding temperature,which indicated that the combined cycle unit cylinder-bolt system can be safely put into service after localization of fastening bolts in key parts of high and medium pressure cylinders.