Analysis of Expansion Joint Process of Heat Exchanger Tube and Tube Sheet

According to the different use conditions and processing conditions of the heat exchanger, the connection methods are basically divided into three types: expansion joint, welding and expansion welding. Because the expansion joint method can withstand high pressure, it is especially suitable for materials with poor weldability and A situation in which the welding workload in the manufacturing plant is excessive. Therefore, this method is widely used in practical production. With the continuous development of technology, new technologies such as roller expansion, explosion expansion and hydraulic, liquid bag and rubber expansion have been developed successively. This paper intends to compare these several expansion processes, and provide a reference for choosing a reasonable expansion process for actual production.

1 Traditional expansion joint process

1.1 Roller expansion method

The method is to embed three small-diameter rollers on a frame, with a tapered mandrel in the middle, as shown in Figure 1. When expanding the pipe, the cylindrical part of the expander is inserted into the pipe hole, the mandrel is rotated by electric, pneumatic and other power, and the roller is rotated along the circumference of the mandrel, so that the mandrel is squeezed into the inner surface of the pipe and the pipe is forced to expand to a certain degree. Expansion tightness, so that the tube is tightly expanded to the hole of the tube sheet. The tube expansion operation can be divided into two types: forward type and backward type. The forward type is to insert the frame into the tube, rotate the mandrel, and squeeze forward. After reaching the set tightening degree, the motor reverses, and the tube is pulled out from the tube to complete the expansion process. . The reverse type rotates the mandrel forward like the forward type. After reaching the original tightening degree, the motor stops. At the same time, the clutch of the reverse device is engaged and reversed. The relative position of the roller and the mandrel remains unchanged. Parallel expansion is carried out continuously and uniformly from the depth to the inlet. Since this expansion process is from the inside to the outside, the elongation of the tube occurs on the outside of the tube sheet, which can eliminate the stress state of the tube bundle and improve the quality of the product [2], so it is used for expansion connections with a length greater than 60cm.

1.2 Explosion expansion process

The method is to use high-energy explosives to make the huge pressure of the shock wave generated at the moment of explosion (10 × 10-6 ~ 12 × 10-6 s) to force the tube to produce high-speed plastic deformation, thereby expanding the tube and the tube sheet. Together, the connection between the tube and the tube sheet is realized. Figure 2 is a schematic diagram of explosion expansion. In the figure, the cylindrical explosive is placed in the center of the pipe end. In order to prevent the shock wave from damaging the pipe wall, there is a tubular buffer packing (viscous or plastic) around the explosive, so that the pressure can be evenly transmitted to the pipe. on the wall.

2 New expansion technology

2.1 Hydraulic expansion process

The hydraulic expansion process is also called soft expansion, which can expand more pipe joints at one time. Hydraulic expansion is a new expansion technology, which is expanded to the inner surface of the plate hole by applying high hydraulic pressure to the inner surface of the tube to plastically deform the tube. The expansion head of the hydraulic expansion joint is a section of mandrel with a diameter slightly smaller than the inner diameter of the tube. There are multiple seals on the outer surface of the two ends of the mandrel, and an oil inlet hole is arranged in the middle of the mandrel. The high pressure is applied to the pipe section to make the pipe plastically expand and deform to realize the expansion joint.

2.2 Rubber tube expansion process

The new rubber expansion technology is developed on the basis of the deformation of rubber under force. It uses the radial pressure generated by the axial compression of the rubber elastic body to expand the tube to the tube sheet. The working principle of the rubber expander is shown in Figure 4. When the loading rod exerts a tensile force, the expanding rubber is axially compressed and radially expanded at the same time, and the expanding force is sufficient to deform the tube material, thereby realizing the connection between the tube and the tube sheet. In order to prevent the 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 the rubber expansion tube is made of high-strength steel. It is loaded on the tie rod through the pressure water or oil of about 20MPa. Since the pulling force is balanced against the pressure ring, it forms an internal force system without other supports or constraints. Expanding rubber is made of materials with high elasticity and high strength.

3. Comparison of several expansion techniques 3.1 Analysis of advantages and disadvantages of mechanical expansion techniques

The mechanical expansion is done by using a tube expander. The tube expander is divided into two types: forward expansion and backward expansion according to the feeding direction. The former is suitable for the expansion of general heat exchangers and pipes with a diameter of less than 38mm. Its expansion rod has a taper of 1:25 to 1:50, so that the circumferential component force is smaller than the friction force, thereby avoiding the roller and the roller. The relative sliding between the expansion rods is to facilitate the introduction of the expander, and the roller is designed with a certain taper head end; the latter is usually used for deep expansion and expansion with a diameter greater than 38mm, and the radial expansion of the rollers It is realized by the retraction pulling force of the expansion rod. When the pipe is expanded, the pipe is axially elongated, so the outer end can be freely deformed, so the axial compressive stress and deformation caused by the advancing pipe expander to the pipe connection are avoided. Mechanical expansion joints can not only withstand certain axial force, thermal shock and repeated thermal cycles, but also are easy to operate and flexible to use, and are widely used in manufacturing and maintenance. However, the mechanical expansion pipe also has the following shortcomings: the degree of expansion between the pipes is different, and the connection strength and tightness are uneven; the inner surface of the expansion pipe interface is hardened, which brings difficulties to repeated expansion. There are certain restrictions on the compatibility of the expansion joint of the pipe and the tube sheet material, such as the expansion joint of titanium tube and carbon steel, the expansion joint of aluminum tube and carbon steel, etc. The expansion of thick-walled pipes is more difficult.

3.2 Comparison and Analysis of Other Expansion Processes and Mechanical Expansion Processes

In addition to the advantages of uniform force on the pipe wall, less axial elongation of the pipe and uniform work hardening, the hydraulic expansion pipe has a large axial pull-off force and a large axial pull-off force because the metal of the pipe wall can almost completely fill the pipe hole and groove. Good sealing. The advantages of rubber expansion pipe: it belongs to the expansion process of soft characteristics, the junction between the expansion area and the unexpanded area is not obvious, the transition is smooth, the residual stress is small, and the performance of stress corrosion resistance and fatigue resistance is good; no axial extension of the pipe, Same as explosion expansion, it is suitable for welding first and then expansion; the pipe diameter deviation is not strict, and it is suitable for the expansion of elliptical pipes; the scope of application is large, and the pipe diameter of 10～100mm and the thin-walled pipe of 1mm can be carried out well The expansion joint; hydraulic control rod, easy to control the quality of the expansion pipe and adjust the expansion tightness. The basic requirement of explosion expansion is only to enable the pipe to be expanded to the tube sheet and to ensure sufficient connection strength. Therefore, for the expansion of pipes of common specifications of heat exchangers, most of them use low-explosive explosives such as ammonium nitrate, and the dosage is small, so ordinary expansion pipes can also directly use detonators or detonating cords for explosive expansion. The tube wall is made of kraft paper as a protective layer. Because the tube is in an instantaneous superplastic state under huge pressure during explosion, and can fully fill the hole and groove, the explosion expansion tube has high connection strength and reliable sealing; and because the explosion expansion tube has better material Compatibility, so it is suitable for the expansion of different materials and various pipe diameters; in addition, the explosive expansion pipe has a significant advantage, that is, the process tooling is simple and the production efficiency is extremely high. However, the compatibility of the expansion joint between the tube and the tube sheet material has certain limitations, such as the expansion joint of titanium tube and carbon steel, which is not applicable.

4 Conclusion

(1) To ensure the reliability of the connection between the tube and the tube sheet, it is not only required to select the connection form correctly and reasonably according to the design conditions (such as temperature, pressure, joint connection strength, fatigue and corrosiveness of the medium, etc.), but also in the manufacturing and construction. There should be a reasonable process system and inspection system applicable to different connection forms. Practice has proved that the reliability of connection joints is not only determined by design conditions, but also often affected by the degree of feasibility of manufacturing and construction.

(2) Among several expansion processes, rubber expansion and hydraulic expansion are the best in terms of expansion performance; the production efficiency of explosive expansion is the highest; due to the limitation of production conditions, mechanical expansion is currently the most commonly used.

(3) During the processing of the connection joint between the heat exchanger tube and the tube sheet, if only expansion is required, the expansion process method with better expansion performance should be selected as far as possible according to the actual production conditions to ensure excellent expansion quality. .

(4) For occasions with harsh conditions of use, such as heat exchangers that are resistant to high temperature and high pressure, dynamic load and corrosion resistance, a combination of expansion and welding should be used. Many experimental data show that no matter which type of expansion welding connection is used, the tensile strength and sealing performance of the joint are higher than those of single expansion or welding, and even exceed the strength of the pipe material to some extent.