Studies Of The Sealing Mechanism Of The Hydraulically Expanded Joint

Shell-and-tube heat exchangers are widely used in petrochemical industries. The connection joints between tubes and tube-sheets turn to be the weakest parts in a exchanger construction, which often lead to the leakage of the joints and finally the uselessness of the exchanger. Owing to many advantages, hydraulic expansion technique is increasingly employed for the connection of tubes and tube-sheets of shell-and-tube heat exchangers. Sealing is one of the important parameters reflecting the quality of the hydraulically expanded joint.A 3-D parameterized finite element model is established for the non-linear analysis of the hydraulically expanded joint. Based on this model, influence of the different combinations of tube and tube-sheet materials with coupling of expansion pressures on the sealing of the joint is analyzed. For the thermal loading resulting from the temperature differences between the tube-side fluid and shell-side fluid, the thermal analysis is performed and the temperature distribution in the joint is obtained, thus, the thermal effect on the sealing of the joint is investigated. Finally, the mutual influence of expanded joints is studied.Results show that whether the tube hole on the tube-sheet is grooved or not, sealing circular-bands exist on the contact surface between the tube and the tube-sheet under the action of the expansion pressures. The residual contact pressures on the sealing circular-bands are higher than on other positions, which enhances the sealing of the joint. The sealing circular-bands are located close to the two ends of the hole when it is not grooved, but they are located at the two brinks of the groove for a grooved hole and in latter case, the residual contact pressures on them are even higher, reflecting that the joint with a grooved tube-sheet hole is more capable of sealing. When the material of the tube is softer than that of the tube-sheet, the residual contact pressures are higher than the otherwise case, implying that the sealing of the joint for a softer tube material is better. For a given material combination of the joint, there is an optimum value of the expansion pressures under which the minimum residual contact pressure on the sealing circular-bands is highest, and when exceeding this value, the minimum residual contact pressure will decrease with the increase of expansion pressures, weakening the sealing ability of the joint. Under the thermal loading resulting from the temperature differences between the tube-side fluid and shell-side fluid, the temperature distribution in the joint is not uniform and no radial temperature gradient across the contact surface is found. As a result, the effect of the temperature differences on the sealing of the joint is small. When the expansion pressures are small, the mutual effect of the expanded joints is significant, but with increasing the expansion pressures, the mutual effect is decreased.