Process for connecting the heat exchange tubes to the tube sheet in shell-and-tube heat exchangers

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Overview: As a heat transfer device used to transfer part of the heat from a hot fluid to a cold fluid between materials, heat exchangers are widely applied in people’s daily lives as well as in industries such as petroleum, chemicals, power generation, pharmaceuticals, atomic energy, and nuclear industry. It can function as a standalone device, such as a heater, condenser, cooler, etc ； It can also serve as a component of certain process equipment, such as heat exchangers in some chemical processing equipment. Especially in the chemical industry, which has high energy consumption, heat exchangers are essential devices for heat exchange and transfer during chemical production, and they account for a significant proportion of all the equipment used in such production. From a functional perspective, heat exchangers serve to ensure the specific temperature required for certain media in industrial processes, and they are also key devices for improving energy efficiency. Based on their structural format, they mainly include plate heat exchangers, floating-head heat exchangers, fixed-tube-sheet heat exchangers, and U-tube heat exchangers, among others. Among them, except for the plate heat exchanger, the rest are shell and tube heat exchangers. Due to its large heat exchange area per unit volume, excellent heat exchange efficiency, as well as advantages such as a robust structure, strong adaptability, and mature manufacturing processes, the shell-and-tube heat exchanger has become one of the most commonly used types of heat exchangers. The connection between the heat exchange tubes and the tube sheet in shell-and-tube heat exchangers: In such exchangers, the heat exchange tubes and the tube sheet constitute the only barrier between the tube side and the shell side. The structure of the connection between these elements, as well as the quality of that connection, determine the quality and service life of the heat exchanger; it is therefore a crucial aspect in the manufacturing process of heat exchangers. Most failures and damages to heat exchangers occur at the junctions where the heat exchange tubes are connected to the tube sheet. The quality of these connections also has a direct impact on the safety and reliability of chemical processing equipment and systems. Therefore, the process used to connect the heat exchange tubes to the tube sheet in shell-and-tube heat exchangers constitutes the most critical control aspect within the quality assurance system for heat exchanger manufacturing. Currently, in the manufacturing process of heat exchangers, the methods used to connect the heat exchange tubes to the tube sheet include welding, expansion joining, a combination of expansion joining and welding, as well as adhesive bonding combined with expansion joining. 1. Welding: When the heat exchange tubes and the tube sheet are connected by welding, it requires fewer processing steps for the tube sheet, the manufacturing process is simple, good sealing performance is achieved, and welding, visual inspection, and maintenance are all easy to carry out. This is currently the most widely used method for connecting heat exchange tubes to tube sheets in shell-and-tube heat exchangers. When using welding for connections, there is strength welding, which ensures the sealability and tensile pull-out strength of the weld joint, and seal welding, which only ensures the sealability of the connection between the heat exchange tubes and the tube sheet. For high-strength welding, its performance is limited, and it is only suitable for applications with low vibration and no gap corrosion. When welding is used for connection, the distance between the heat exchange tubes should not be too small; otherwise, due to the effects of heat, it becomes difficult to ensure the quality of the welds. Additionally, a certain distance should be maintained between the tube ends to help reduce the welding stresses between them. The length of the heat exchange tube extending beyond the tube sheet must meet specified requirements to ensure its effective load-bearing capacity. In terms of welding methods, depending on the materials of the heat exchange tubes and tube sheets, processes such as shielded metal arc welding, TIG welding, and CO2 welding can be used. For heat exchangers with high requirements for the connection between the heat exchange tubes and the tube sheet, such as those with high design pressures, high design temperatures, large temperature variations, as well as heat exchangers subjected to alternating loads or those with thin tube sheets, TIG welding is recommended. Conventional welding methods, due to the gap between the tubes and the holes in the tube sheet, are prone to gap corrosion and overheating. Moreover, the thermal stresses generated at the weld joints can also lead to stress corrosion and failure, all of which can render the heat exchanger ineffective. In heat exchangers currently used in industries such as the domestic nuclear industry and power industry, internal hole welding technology is being employed for the connection between the heat exchange tubes and the tube sheet. This connection method replaces end welding of the tubes and sheet with welding inside the tube bundle, using a full penetration approach that eliminates gaps at the ends; as a result, it improves resistance to gap corrosion and stress corrosion. It also possesses high resistance to vibration fatigue, can withstand high temperatures and pressures, and offers good mechanical properties for the welded joints ； Internal non-destructive testing can be performed on the joints, allowing control over the quality inside the welds and thereby enhancing their reliability. However, internal hole welding technology is difficult to assemble, requires high standards for welding skills, involves complex manufacturing and inspection processes, and has relatively high production costs. As heat exchangers evolve toward higher temperatures, higher pressures, and larger sizes, the requirements for their manufacturing quality increase, and internal hole welding technology will be used more extensively. 2. Expansion joining: Expansion joining is a traditional method for connecting heat exchange tubes to tube sheets. It involves using expansion tools to cause elastic-plastic deformation in both the tube sheet and the tube, thereby ensuring a tight fit and a strong connection that provides both sealing properties and resistance to pulling apart. In the manufacturing process of heat exchangers, expansion jointing is suitable for applications with no severe vibrations, no excessive temperature changes, and no serious stress corrosion. The main expansion joining processes currently in use are mechanical rolling expansion and hydraulic expansion joining. Mechanical expansion jointing is uneven; once the connection between the tube and the tube sheet fails, it is very difficult to repair it using expansion joints ； Liquid-bag hydraulic expansion joining is operated under computer control, offering high precision; it ensures uniform tightness in the expansion process, and its reliability in terms of connections is better than that of mechanical expansion joining. However, strict requirements are placed on processing precision, and it is somewhat difficult to ensure successful expansion joining for densely arranged joints; moreover, it is challenging to repair them by expansion joining if they fail. 3. Expansion joining and welding: When temperatures and pressures are high, and under the effects of thermal deformation, thermal shock, thermal corrosion, and fluid pressure, the junction between the heat exchange tubes and the tube sheet is highly susceptible to damage. Neither expansion joining nor welding can ensure the required strength and sealing performance of such connections. Currently, the method of combining expansion welding is widely used. The expansion jointed and welded structure can effectively dampen the damage caused by tube bundle vibrations to the welds, it can effectively prevent stress corrosion and crevice corrosion, and improves the fatigue resistance of the joints; as a result, the service life of the heat exchanger is increased. It offers higher strength and sealing performance compared to expansion joints alone or simple welding methods. For ordinary heat exchangers, the “fit-up % strength welding” method is typically used ； Heat exchangers operating under harsh conditions require a \"strength expansion % seal welding\" approach. Expansion joining followed by welding can be divided into two types based on the sequence of expansion joining and welding in the process: expansion joining first and then welding, or welding first and then expansion joining. (1) The lubricants used in expansion before welding penetrate into the joint gaps. These lubricants are highly sensitive to welding cracks, porosity, etc., thereby exacerbating the occurrence of defects during welding. These oil stains that have penetrated into the gaps are difficult to remove completely; therefore, the expansion followed by welding process should be used, rather than mechanical expansion. Although expansion joints are not pressure-resistant, they can eliminate the gap between the tube and the pipe plate holes, thereby effectively damping the vibration of the tube bundle at the welded sections at the tube ends. However, conventional manual or mechanically controlled expansion bonding methods cannot meet the requirements for uniform expansion, whereas a liquid-bag expansion bonding method that uses computer-controlled expansion pressure can achieve these requirements in a convenient and uniform manner. During welding, due to the effect of the high-temperature molten metal, the gas within the gap is heated and expands rapidly. This high-temperature, high-pressure gas can cause certain damage to the sealing performance of the expansion joint **. (2) Weld first then expand: For the weld-first-then-expand process, the primary issue is controlling the precision of the tubes and the holes in the tube sheet, as well as their fit together. When the gap between the tube and the tube sheet holes becomes small enough, the expansion jointing process will not compromise the quality of the welded joint. However, the weld joint has a relatively poor capacity to withstand shear forces; therefore, during strong welding, if the control parameters are not met, it may lead to failure due to excessive expansion or damage to the welded joint as a result of that expansion. During the manufacturing process, there is a large gap between the outer diameter of the heat exchange tubes and the tube holes in the tube sheet, and this gap varies axially for each heat exchange tube. When expansion joining is performed after welding, the center line of the tube must coincide with the center line of the tube hole in the tube sheet in order to ensure the quality of the joint. If there is a large gap, due to the high rigidity of the tube, excessive expansion deformation can damage the welded joint, and may even lead to the separation of the weld. 4. Adhesive bonding and expansion joining: The use of adhesive bonding and expansion joining techniques helps to address the problems of leakage that often occur at the joints between the heat exchange tubes and the tube sheet in heat exchangers. It is important to select the appropriate adhesive based on the operating conditions of the components to be bonded. During the implementation of the process, it is necessary to select the appropriate process parameters based on the structure and dimensions of the heat exchanger; these parameters mainly include curing pressure, curing temperature, and tightening force, and strict control must be exercised over them during production. This process is simple, easy to implement, and reliable; it has been recognized in actual industrial applications and holds great potential for wider adoption. Conclusion (1) Among the methods for connecting the heat exchange tubes to the tube sheet in shell-and-tube heat exchangers, relying solely on conventional welding or expansion jointing is insufficient to ensure both the required strength of the connection and the necessary level of sealing. (2) Using the expansion jointing and welding method helps to ensure the connection strength and sealing performance between the heat exchange tubes and the tube sheet, thereby extending the service life of the heat exchanger. (3) The use of bonding combined with expansion joining helps to address leakage and seepage issues that occur when connecting heat exchange tubes to tube sheets; this process is simple, easy to implement, and reliable. (4) As a fully penetrative welding method, the internal hole welding technique exhibits excellent resistance to gap corrosion and stress corrosion, as well as high vibration fatigue strength and good mechanical properties of the welded joints ； The internal quality of the weld can be controlled, thereby improving the reliability of the weld; it is therefore more suitable for adoption in high-end products.