Stress Analysis And Structural Optimization Design Of Flat Head With Jacket Under Thermal-mechanical Coupling Field

It is very difficult to design the non-standard pressure vessel structure using the design by rule (DBR) according to the existing pressure vessel code, which requires the detailed stress analysis and structural design through design by analysis (DBA) method. DBA is based on elastic stress analysis, plastic failure criteria, and elastic-plastic failure criteria. It is more stringent than the traditional DBR method on material selection, fabrication process, inspection and acceptance of pressure vessel. With the rapid development of computer technology and numerical theory, DBA method has been widely used in pressure vessel design. In this work, based on elastic-perfectly plastic material model, the mechanical properties of flat head with jacket were investigated under mechanical load, thermal load, and thermal-mechanical coupling loads by finite element method (FEM). The optimization design method was employed to rationally design the top and bottom panel’s wall thicknesses of flat head with jacket to satisfy the process requirements. The stress categorization method was conducted for evaluating the reliability of the optimal structure based on the stress linearization technology. Furthermore, the welding residual stress numerical simulation and assessment of flat head with jacket structure were carried out. The results provided valuable reference for the scientific design and strength assessment of such jacket structure. The main simulation results are as follows:(1) The model was simplified according to the structural features of the flat head with jacket. Finite element model of the flat head with jacket structure was established using solid elements, shell elements, and multipoint constraint (MPC) method, respectively. By comparison and analysis, it was found that the calculation displacement of MPC method were in good agreement with that of solid elements, demonstrating that the MPC method had advantages of high calculation accuracy and short calculation time.(2) The wall thickness distribution of top and bottom panels had little influence on the displacements of the structure when mechanical load was applied to the flat head with jacket with the total thickness of the top and bottom panels keeping at118mm while each layer thickness of panel keeping not less than25mm. If each layer thickness of panel is less than25mm, the stress of the most area of panel will reach the material yield strength and lead to the plastic deformation, further leading significant increase of the structural displacement. Besides, the influence of supporting forms and cavity height on mechanical properties of flat head with jacket structure was investigated. It was found that the supporting forms have significant influence on the mechanical properties of the structure, and addition the nozzles in the larger displacement and stress area can play the enhancing effect. The displacement and stress of overall structure were decreased with the increase of the cavity height, which is helpful to improve the mechanical properties of the structure, but increased the manufacturing costs with larger material to be used. Taking the heat exchange, mechanical properties, manufacturing cost and other factors into consideration,202mm was chosen as the proper cavity height.(3) The flat head with jacket structure was analyzed under thermal and thermal-mechanical loads. The simulation results indicated that the thermal load has prominent effect on the displacement of the structure. The maximum displacement occurred in the center and the maximum equivalent stress happened in the connection region of the panels and nozzles when the thermal-mechanical loads was applied to the structure. Large area of plastic deformation occurred at the upper surface of the top panel and the stress reached the yield strength of the material, which fit the performance characteristics of the elastic-perfectly plastic material.(4) The wall thickness distribution of top and bottom panels of flat head with jacket significantly affected the mechanical properties of the structure. The optimization mathematical model of the top and bottom panels was established, the corresponding program was developed using APDL language to obtain the optimal wall thicknesses of the top and bottom panels. Based on the structural optimization, the stress on the dangerous section was classified using the stress linearization technology, and compared with the corresponding allowable stress value. The assessment results showed that the structure satisfied the strength requirements.(5) The top and bottom panels of the flat head with jacket were connected to the nozzles by welding, and the welding zone is usually the weakest link of the structural failure. Therefore, based on APDL language, the simulation of welding residual stress of the connection structure between top and bottom panels with nozzles was conducted using the element birth-death technology and indirect thermal-structural coupling method. The distribution of welding residual stress in the welding zone was obtained. The results could provide an effective method for evaluating the welding strength and life of flat head with jacket structure, as well as guidance to eliminate the welding residual stress.