Study Of The Strength And Tightness Of Tube-to-Tubesheet Connection Joints

Presently, China has become a big manufacturing country, but not a manufacturing power, especially in the fields of petroleum, chemical and energy industries etc. Shell-and-tube heat exchangers are widely used in these industries. The tube-to-tubesheet connection joints (including expanded joint, welded joint, and expanded-and-welded joint) are the places which are large in number and complicated in deformation in the manufacturing process, and easiest in failure during the application process. Therefore, the research on the strength and tightness of tube-to-tubesheet connection joints is of considerable industrial application value and relatively significant theoretical importance.Firstly, in this study, the stress-strain curves for relevant materials were experimentally obtained, which provided material property data for succeeding nonlinear numerical simulation, and assured the correctness of calculation results from the material aspect.Secondly, experimental research and numerical simulation were conducted for the strength of connection joints of hydraulically expanded joints, welded joints, and expanded-and-welded joints. Emphasis was placed on the pull-out forces and the forms of failure of these joints under room temperature. Under this research conditions it was found that the pull-out force of expanded joint is far less than that of the welded joint, the former is about 1/6～1/5 of the latter, implying that the load-carry ability of welded joint is much larger than that of the expanded joint. For the welded, expanded and the combined expansion-weld joints the pull-out forces of welded joint is slightly bigger than that of the other two. This indicated that there is no big difference among the strengths of the three joints in the axial load-carry ability. The value of the expanding pressure in the expanded-and-welded joints has very little influence on the pull-out force. For the welded joint, the tube was pulled out by forming a crack closed to the surface of the tube in the axial direction, indicating that the pull-out failure is a shear break of the weld. For the expanded-and-welded joint, the pull-out failure is the separation of the contact surfaces of the tube and tubesheet followed by the shear break of the weld. The results of numerical simulation are quite close to that of the experimental measurements.Then, simulation for the formation of expanding prior to welding joint was conducted. There will exit residual contact pressure on the contact surface between the tube and tubesheet hole after expansion. In the succeeded welding process, heat coming from the welding will bring influence on the residual contact pressure. At the same time, the contact status on the contact surface will also affect the distribution and transmission of welding heat, i.e. their influences are mutual. In this study, by performing heat and structure combining analysis, the residual contact pressures on the contact surface during the process of heating from 20℃to 1500℃and then cooling down to 20℃were calculated. Radiation and convection were considered in the thermal analysis, with the thermal properties of the metallic materials the same but changed with the temperature. In the structural analysis, the experimentally measured the stress-strain curves of the materials under room temperature were employed as the basis and modified for the materials under other temperaturers. The calculation results showed that during the process of joint formation, if the metallurgical changes of materials caused by welding were neglected, within the thermal elasto-plasticity range, big changes took place on the residual contact pressures between tube and tubesheet during the welding process, especially, when the temperature of the weld is very high, the contact pressure on the upper sealing circular-bands close to the welded seam was nearly disappeared, and the contact pressure on the lower sealing circular-bands was mostly lost. But when the weld cooled back to room temperature, the residual contact pressure almost recovered. This indicated that the tightness and anti-pull-out ability relied on residual contact pressure formed during expanded connection remain no significant change. The result implied that the joint connection technique of expansion-and-welding is rational. This form of connection can increase the reliability and endurance of the joint in application.Finally, in order to simulate the tightness of hydraulically expanded joints under working conditions, the influences of different pressures and temperatures of tube-side and shell-side fluid, and the overall temperatures (high or low) on the minimum residual contact pressure on the sealing circular-bands were also calculated in this study. The results showed that under the same expanding pressure, if the tube-side pressure is higher than the shell-side pressure, the tightness of the joints increases, and the influence of the pressure difference on the upper sealing circular-bands is greater than the influence of the pressure difference on the lower sealing circular-bands. Under the same expanding pressure, if the tube-side working temperature is higher than the shell-side working temperature, as the upper and lower surfaces of the tubesheet contacted respectively with tube-side fluid and shell-side fluid, the values of the minimum residual contact pressure on the upper sealing circular-bands decrease considerably, and the values of the lower sealing circular-bands increase significantly, the tightness of the connection joint increases. Conversely, if the tube-side temperature is lower than shell-side temperature, the tightness of the connection joint decreases slightly. Under the same expanding pressure and the temperature difference between the tube-side and shell-side fluids, the tightness of the connection joint decreases slightly with the fluid temperature.