The tube ends are linished smooth before insertion into the boiler drums and expanded into the drum. Care is to be taken that the ends are free of any score marks. These joints are mechanically held by the expansion of the tube in the drum holes. The tube deforms to a plastic condition and the drum hole, in elastic condition, exerts the required sealing force for the joint against the drum operating pressure.
Dimensional tolerances are very important. Typically the bank tubes project into the steam drum and water drum by 3/8 inch or 10mm. So one cannot make the tube shorter or longer without causing problems for expansion. As the convection bank tubes are nested inside each other, bending of the tube must be to exacting tolerances. UNEEK bend all convection banks on CNC bending machines for repeatability. Every tube is then checked on a full size laser etched template to ensure dimensional accuracy.
The tube ends may be swaged down in size. This reduces the required tube hole size in the boiler drum and hence drum thickness can be minimised. During the swaging process, the tube material flows in a plastic manner causing an increase in the tube wall at the swaged end. This increase in thickness needs to be controlled to effectively expand and seal the tube into the boiler drum. Another issue is an increase in hardness of the cold worked swaged end. The best option is to heat treat the ends to soften them for expansion into the drum, otherwise we run the risk of splitting the tube during expansion.
In order to control the parameters for tube expansion of swaged ends, the best option is to specify the requirements at the time of order placement with regard to:
Convection bank tubes connect the upper drum and lower drum in bi-drum boilers. These bank tubes not only act as raisers and down comers between the drums but also carry the weight of the bottom drum in the case of top drum supported boilers. Any failure of these tubes will result in damage to the nearby tubes or drum or both, depending on the location of the leak. Therefore any repair and plugging of tubes need to be carried out with extreme care.