Study On Some Problems Of Elastic-plastic Finite Element Analysis For Shell-and-tube Heat Exchanger

Shell-and-tube heat exchanger, widely used in petroleum and petrochemical process, is one of the most malfunctioned equipments. Especially in the method of design by analysis and expanded joint process, there are still problems in complex stress analysis and strength evaluation, in the equivalent solid plates like opening tube-sheet, in the expanded joint between tube-sheet and heat exchange tubes and in the effect of various working conditions. Therefore, methods of elastic-plastic finite element analysis and experiment are used in this paper to research those problems. In the aspect of complex stress analysis and strength evaluation of heat exchanger, half of the inlet and outlet channel headers, tube-sheets, cylinder cases, tubes and other equipments suffered loads are selected as research objects, according to the structure of heat exchangers and the symmetry of loads. Three-dimensional non-linear stress analysis finite element model of heat exchanger is established. Also by using sub-structure model, assuming the displacement boundary is given in hypothetical cross section, the analysis of elastic-plastic is carried out in the region of shell and nozzle. The heat exchangers studied have been working safely for many years, according to the plastic limit load evaluation method, which fully supports the reasonability of the calculation model and provides a computational model and method for complexity stress analysis as well as strength evaluation of such equipments. In the aspect of equivalence of open tube-sheet to solid plate, the three-dimensional finite element model for the stress analysis of the heat exchanger open tube-sheet structure is established. We studied the numerical simulation method of the expanded and welding between tube plate and heat exchangering tube. Through the numerical simulation result comparison between opening tube plate model and equivalent solid plate model, the stress concentration coefficien of the open tube-sheet area is calculated. By using multipoint constraint method to simulate the connection between tube-sheet and heat exchanging tube, an improved method of equivalence of open tube-sheet to solid plate, which applies to finite element analysis is established. In aspect of the expanded between tube-sheet and heat-exchanging tubes as well as affect of working conditions: Firstly, by using elastic-plastic analysis method of thick wall cylinder, the residue contact pressure calculation fomula of no slot expaned joints under processing load or temperature load is derived. This provides a way for mechanical calculation in the expanded process under working temperature and a basis for numerical simulations of working conditions, and the expanded between slotted tube plate and heat exchanging tube. Secondly, through numerical simulation of slotted joint under various expanded pressures, the contact pressure and residue contact pressure both increases with the elevation of expansion pressure, and when expanded pressure reaches a certain value, the residue contact pressure keeps unchanged. Also the larger residue contact pressure near the slot can appears a sealed ring. Then, through the numerical simulation and experimental research of slotted expanded joint, the error of pull-off force between numerical simulation and experimental results at different temperatures is less than 9.6%, the residue contact pressure first increases and then decreases with the elevation of temperature, and gradually extends along the axial direction. Finally, considering the working medium temperature and pressure, the numerical simulation method of residue contact pressure between expanded tube sheet and heat exchanging tube under working conditions is built. The maximum residue contact pressure gradually decreases with the tube pressure increases, first stays unchanged and then rapidly drops with the shell pressure increases to a certain threshold limited value; along with axial direction, the residue contact pressure zone changes a little with tube pressure, but changes a lot with shell pressure and the zone gradually shrinks when shell pressure surpasses the threshold value; as for pull-off force, it gradually increases with the raise of tube pressure. It also experiences a process of slightly decreases with the raise of shell pressure and then rapidly drops as soon as the pressure surpasses the threshold; when the expanded pressure of the expanded tube sheet and heat exchanging tube has 160MPa~200MPa, the threshold shell pressure increases from 5MPa to 25MPa; when shell pressure has 5MPa below, the strong expanded process is recommend.