Analysis of the expansion jointing process for heat exchanger tubes and tube sheets

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This post was last edited by Yixin Electromechanical on 2017-5-1 20:05. Analysis of the expansion jointing process for heat exchanger tubes and tube sheets. Wang Gang (School of Chemical Engineering, Qinghai University, Xining 810016, Qinghai). The connection between tubes and tube sheets is one of the most important processes in the production of shell-and-tube heat exchangers. Since such projects require a large amount of labor time, and more importantly, the joints are prone to failure during operation. Therefore, developing high-efficiency and high-quality connection technologies has become a key research topic in manufacturing. Depending on the operating conditions of the heat exchanger and the processing requirements, the connection methods mainly fall into three categories: expansion joining, welding, and a combination of expansion joining and welding. Since expansion joining can withstand high pressures, it is particularly suitable for situations where the weldability of the materials is poor or where the welding workload in the manufacturing facility is too large. Therefore, this method is widely used in actual production. With the continuous advancement of technology, new expansion techniques such as roller expansion, explosive expansion, as well as hydraulic, fluid-bag, and rubber expansion have been developed. This paper aims to compare these various tube expansion processes, providing a reference for selecting the appropriate one for actual production. 1 Traditional expansion joining processes 1.1 Roller tube expansion method This method involves inserting three small-diameter rollers into a frame, along with an expansion tool that features a conical mandrel, as shown in Figure 1. During tube expansion, the cylindrical part of the expander is inserted into the tube hole; electric or pneumatic power is used to rotate the mandrel, and the rollers rotate around the mandrel, forcing it to press against the inner surface of the tube and causing the tube to expand to a certain degree, thereby securing the tube tightly to the holes in the tube sheet. The tube expansion process can be divided into forward and reverse types. In the forward type, the frame is inserted into the tube; the mandrel is rotated to push it forward and expand it, and once the desired level of tightening is achieved, the motor reverses direction to pull the frame out of the tube, thus completing the tube expansion process. In the reverse mode, the spindle rotates forward just like in the forward mode; once the desired level of tightening is achieved, the motor stops. At the same time, the clutch of the retraction mechanism engages to cause reversal, and the relative position between the roller and the spindle remains unchanged. While rotating in reverse, the tube is expanded uniformly from that depth all the way to the entrance. Since this expansion joining process proceeds from the inside out, with the tube expanding on the outside of the tube sheet, it allows the stress conditions on the tube bundle to be eliminated and improves product quality; therefore, it is used for connections with an expansion length of more than 60 cm. 　file:///C:/Users/Administrator.I76565M50M3621G/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg　　1.2 Expansion jointing using explosives　　　This method makes use of high-energy explosives; the immense pressure generated by the shock wave at the moment of explosion (10×10-6 to 12×10-6 seconds) is used to force the tube to undergo rapid plastic deformation, thereby joining the tube to the tube sheet and achieving a connection between them. Figure 2 shows a schematic diagram of explosive expansion joining. In the figure, the cylindrical explosive is placed at the center of the pipe end. To prevent shock waves from damaging the pipe wall, a tubular cushioning filler (a viscous material or plastic) surrounds the explosive, allowing the pressure energy to be distributed evenly across the pipe wall. 2 New Expansion Joining Processes 2.1 Hydraulic Tube Expansion Process The hydraulic tube expansion process, also known as soft expansion joining, allows for the expansion of a large number of pipe joints in a single operation. Hydraulic expansion is a new type of expansion joining technique; it involves applying high hydraulic pressure to the inner surface of the tube, causing the tube to plastically deform and thus expand into the inner surface of the plate hole. The expansion head used in hydraulic expansion joining is a mandrel with a diameter slightly smaller than that of the pipe’s inner diameter. Multiple seals are present on the outer circular surfaces at both ends of the mandrel, and an oil inlet hole is located in the middle of the mandrel. High pressure is applied to the section of pipe between the two seals, causing the pipe to deform plastically and thus enabling expansion joining. The structure of the hydraulic tube expander is shown in Figure 3. 　 file:///C:/Users/Administrator.I76565M50M3621G/AppData/Local/Temp/msohtmlclip1/01/clip_image004.gif　　2.2 Rubber Expansion Tubing Process　　　The new rubber expansion technology was developed based on the principle of deformation under stress; it utilizes the radial pressure generated by the axial compression of rubber elastomers to expand the tube and attach it to the tube sheet. The working principle of the rubber expansion tube machine is shown in Figure 4. When a pulling force is applied via the tie rod, the expansion rubber is subjected to axial compression while also experiencing radial expansion; this expansion force is sufficient to deform the pipe material, thereby achieving the connection between the pipe and the tube sheet. To prevent axial movement of the rubber under high pressure, special hard rubber sealing rings are installed at both ends of the expansion head. The tie rod of rubber expansion tubes is made of high-strength steel. It is loaded on the tie rod by pressurized water or oil at around 20 MPa; since the tensile force is balanced by the compressive force from the ring, an internal force system is formed, without the need for any additional support or constraints. The expansion tube rubber is made from materials with high elasticity and strength. 3 Comparison of Several Tube Expanding Processes 3.1 Analysis of the Advantages and Disadvantages of Mechanical Tube Expanding Process Mechanical tube expanding is carried out using a tube expander, which can be divided into two types based on the direction of feeding: forward expanding and backward expanding. The former is suitable for ordinary heat exchangers and the expansion fitting of pipes with a bore diameter of less than 38 mm; its expansion rod has a taper of 1:25 to 1:50, which reduces the circumferential force to below the frictional force, thereby preventing relative sliding between the rollers and the expansion rod. To facilitate the insertion of the expansion tool, the rollers are equipped with tapered ends. The latter is generally used for deep expansion fitting and the expansion fitting of pipes with a diameter greater than 38 mm; the radial expansion of its rollers is achieved through the pulling force exerted by the retracting expansion rod. During expansion, the pipe elongates axially, allowing it to deform freely toward the outer end, thus avoiding the axial compressive stress and deformation caused by forward-moving expansion tools in pipe connections. Mechanical expansion joining can not only withstand certain axial forces, thermal shocks, and repeated thermal cycles, but it is also simple to operate and flexible to use, making it widely applied in manufacturing and maintenance. However, mechanical tube expansion also has the following disadvantages: the degree of expansion varies among different tubes, resulting in uneven connection strength and tightness; the inner surface of the expansion joint becomes hardened, which makes it difficult to perform repeated expansion operations. There are certain limitations regarding the compatibility of materials for expansion jointing of pipes and tube sheets; for example, expansion jointing of titanium pipes with carbon steel, as well as aluminum pipes with carbon steel, is subject to such restrictions. Labor productivity is low, and it is difficult to perform expansion jointing on pipes with small diameters or thick walls. 3.2 Comparative analysis of other tube expansion processes with mechanical tube expansion process: In addition to advantages such as uniform stress distribution on the tube wall, minimal axial elongation of the tube, and uniform work hardening, hydraulic tube expansion also features a high axial pulling force and good sealing performance, as the metal in the tube wall can almost completely fill the tube cavity. Advantages of rubber expansion tubing: It belongs to a expansion process with soft characteristics; the boundary between the expanded area and the unexpanded area is not distinct, resulting in a smooth transition, low residual stress, and good resistance to stress corrosion and fatigue. There is no axial extension of the tube, and it can be used for welding first followed by expansion, just like explosive expansion. The requirements regarding tube diameter tolerance are not strict, and it is suitable for expanding elliptical tubes as well. It has a wide range of applications – tubes with diameters ranging from ?10 to ?100 mm, as well as thin-walled tubes with a diameter of 1 mm, can all be expanded effectively. Hydraulic control of the pulling rod makes it easy to control the quality of expansion and adjust the degree of tightening. The basic requirement for explosive expansion joining is merely to enable the tube to be expanded onto the tube sheet and to ensure sufficient connection strength. Therefore, for the expansion joining of pipes of the common specifications used in heat exchangers, **explosives with a low explosion velocity are generally used, in small quantities. As a result, for ordinary pipe expansion joining, detonators or blasting cords can also be employed for explosive expansion joining, while kraft paper is used as a protective layer for the pipe wall. Since the tube is in a state of instantaneous superplasticity under high pressure during the explosion, it can fill the grooves completely, which results in high connection strength and reliable sealing for expansion joining. Moreover, due to its good compatibility with different materials, expansion joining using this method is suitable for tubes of various diameters made from different materials. Another significant advantage of expansion joining is that the required processing tools are simple, leading to extremely high production efficiency. However, the compatibility of expansion bonding between tube and tube sheet materials has certain limitations; for example, it is not applicable to the expansion bonding of titanium tubes with carbon steel. 4 Conclusion (1) To ensure the reliability of the connection between the pipe and the tube sheet, it is necessary not only to select the appropriate connection type based on design conditions such as temperature, pressure, joint strength, fatigue, and the corrosiveness of the medium, but also to have reasonable manufacturing and inspection procedures suitable for each type of connection. Practice has shown that the reliability of connection joints is determined not only by design factors but also often by the feasibility of manufacturing and installation. 　　(2) Among several tube expansion processes, in terms of expansion performance, rubber expansion and hydraulic expansion are the best; explosive expansion offers the highest production efficiency; due to limitations in production conditions, mechanical expansion is currently the most widely used. 　　(3) During the processing of the joints between the heat exchanger tubes and the tube sheet, if expansion joining is the only method required, it is necessary to choose a tube expansion technique with good expansion performance based on the actual production conditions, in order to ensure excellent quality of the expansion joining. 　　(4) For applications with harsh operating conditions, such as heat exchangers that must withstand high temperatures and pressures, dynamic loads, and corrosion, a connection method that combines expansion joining with welding should be used. Many experimental results show that, regardless of the type of expansion welding used, the tensile strength and sealing performance of the joint are higher than those obtained with expansion welding or welding alone, and to some extent even exceed the strength of the pipe material itself.