| Tube-to-tubesheet joints are critical to the reliability of tubular heat exchangers such as steam generators, industrial coolers and condensers. In the fabrication of heat exchangers, the holes in the tubesheet are drilled slightly larger than the outside diameter of the tube in order to ease the installation process. The tubes are then attached to the tubesheet by an expansion process which creates an interference fit between them. The resulting joint must be tight enough to prevent leakage and restrain tube movement. Hydraulic expansion is becoming one of the most common ways of achieving the interference fit between the tubes and the tubesheet. Due to the expansion process, a high level of residual stresses is created in the tube wall. When tensile, these stresses increase the susceptibility of the tube to Stress-Corrosion Cracking (SCC) which is a major cause of failure of heat exchangers. SCC is further enhanced by the presence of a crevice between the outer tube wall and the hole in the tubesheet, near the shell-side surface.; On the other hand, the stability of the tubes depends greatly on the level of their pull-out force. Therefore, a combination of minimum tensile residual stresses with a maximum residual interference pressure between the tubes and the tubesheet would be the most desirable design.; In this thesis, the nonlinear finite element method was used to investigate the problem. Due to the many dimension-, fabrication- and material-related parameters, the orthogonal design method was used to minimize the number of analyses while providing an accurate understanding of the influence of all parameters involved. Based on the results, some empirical equations for determining maximum residual stresses, residual contact pressure and apparent wall reduction values, are proposed. These equations could provide guidance to the designers and manufacturers of the tube-tubesheet assemblies. |
