Connection of heat exchanger tubes and tube sheets: Expansion before welding and welding before expansion

Connection of heat exchanger tubes and tube sheets: Expansion before welding and welding before expansion

^{In the manufacturing process of heat exchangers and other equipment, ‘Expansion before welding’ and ‘Welding before expansion’ are two different processing sequences, which differ in the following ways:}

^{1. Processing sequence and principle}

^{-Expansion before welding:}

^{First of all, the expansion. Expansion is the use of expanders to produce plastic deformation of the tube, while the tube plate hole wall elastic deformation, the tube and the tube plate are closely connected together. There is a good fit between the tube and the tube plate after expansion. Then weld, through welding to further strengthen the connection strength between the tube and tube plate, and at the same time can be sealed after the expansion of the possible small gaps.}

^{-Welding before expansion:}

^{The tube and tube sheet are first welded to form a strong connection. Welding is mainly done by fusing metals, which provides a high level of strength. Afterwards, the expansion is carried out, and the purpose of expansion is to eliminate the stress concentration that may arise during the welding process, and at the same time further enhance the sealing between the tube and the tube plate, so that the connection between the tube and the tube plate is more tight.}

^{2. Connection quality}

^{(1) Sealing}

^{-Expansion before welding:}

^{The expansion process can pre-fill the gap between the tube and the tube plate, reducing the possibility of defects such as porosity during welding, which is helpful in improving the sealing. However, if the surface is not cleaned of impurities such as oil and dirt after expansion, it may affect the quality of the weld, which in turn affects the sealing.}

^{-Welding before expansion:}

^{Expansion after welding can calibrate the welding part to a certain extent, make the tube and tube plate fit more closely, effectively eliminate defects such as tiny cracks that may arise in the welded heat affected zone, and enhance the sealing. However, if the welding quality is poor, there are serious welding defects, expansion may not be able to fully compensate for the impact of these defects on the sealing.}

^{(2) Connection strength}

^{-Expansion before welding:}

^{Expansion joint can make a certain preload between the tube and the tube plate, which can better ensure the relative position of the tube and the tube plate during welding, and make the welding stress distribution more uniform, which helps to improve the strength of the connection. Moreover, the fitting surface after expansion provides a good foundation for welding, so that the welding melt pool can be better filled, enhancing the quality of the weld, thus improving the overall strength of the connection.}

^{-Welding before expansion:}

^{Welding itself provides high joint strength, firmly connecting the tube to the tube sheet. Expansion is carried out after welding, mainly to provide additional reinforcement to the welded connection and further improve the connection strength. However, if expansion is not performed correctly, it may cause damage to the welded part, for example, over-expansion may lead to cracking of the weld and reduce the strength of the connection.}

^{3. Stress distribution}

^{-Expansion before welding:}

^{The expansion process generates certain residual stresses. During welding, due to heat input, new thermal stresses are produced at the already expanded areas. These two types of stresses superimpose on each other. If not properly controlled, they may lead to significant residual stress concentration, adversely affecting the long-term service performance of the equipment, such as causing fatigue cracks.}

^{-Welding before expansion}

^{The thermal stresses generated by welding are mainly concentrated in the weld seam and its heat-affected zone. The expansion process can, to some extent, release and redistribute the welding stresses, resulting in a more uniform stress distribution in the connection area between the tube and the tubesheet, which is beneficial for reducing stress concentration and enhancing the equipment's fatigue resistance.}

^{4.Scope of application}

^{-Expansion before welding:}

^{Applicable to the sealing requirements are very high, and the tube and tube plate material welding performance is good. For example, some small high-precision heat exchangers, in the expansion can ensure a good fit on the basis of welding to further ensure the sealing and connection strength.}

^{-Welding before expansion:}

^{Suitable for equipment subjected to high pressure and vibration loads. By first welding to ensure the basic strength of the connection, and then use the expansion to optimise the distribution of stress and improve the sealing, such as large-scale chemical plants in the heat exchanger equipment, to ensure reliable operation in complex operating conditions.}