ABSTRACT
A tube-plate is a thick disc pierced with an array of holes to accept heat exchanger tubes. The tubes are sealed into the plate by expansion, by welding or by a combination of both techniques. The oldest, and most convenient, method is roller expansion, which uses three angled rollers that are rotated in the tube by a tapered mandrel. As they rotate, the rollers create zones of elastic–plastic expansion in the tube and in the surrounding area of the tube-plate. In essence, the tubes are expanded to be larger than the holes in the tube plate and, hence, are gripped by the tube-plate to create the desired sealing action. Ideally, one wants the tube plate to expand elastically, with no permanent deformation, since it is the elastic accommodation of the tube plate that is the source of the sealing force. The process is reliable and long lasting. The problem with all expansion processes, however, is that the interfacial force between the tube and tube plate, after only a small amount of expansion quickly, reaches a peak, and then falls away. Further expansion does not achieve a better seal—in fact the situation gets worse, since the plastic zone of expansion propagates into the tube plate. When adjacent or even remote holes are now rolled, any tube with a less-than-ideal seal may be found to be leaking, even when the seal had previously been tested and found to be sound. The problem is caused by the complexity of the disturbance caused to the elastic accommodation by over-enthusiastic rolling, so that even remote rolling may reduce the interfacial pressure between a tube and the tube plate to the point where the join will leak. Attempts to recover the situation by further rolling have mostly been found to be less than a brilliant success, since without a detailed understanding of the overall situation, it is unclear which tube to roll and by how much. It has to be born in mind that what is desired at the end of the day is a whole tube-sheet of non-leaking joints, not simply individual sealed pipes.
