Overview
Heat Exchangers, as heat transfer equipment that transfers part of the heat from the hot fluid between materials to the cold fluid, have a wide range of applications in people's daily lives and industries such as petroleum, chemical, power, medicine, atomic energy, and nuclear industry. It can serve as an independent device, such as a heater,
Condenser, cooler, etc; It can also be used as a component of certain process equipment, such as heat exchangers in some chemical equipment.
Especially in the chemical industry with high energy consumption, heat exchangers are indispensable equipment in the heat exchange and transfer process of chemical production, and they also occupy a considerable proportion in the entire chemical production equipment.
From the perspective of its function, heat exchangers are not only responsible for ensuring the specific temperature required by industrial processes for the medium, but also the main equipment for improving energy utilization efficiency. According to their structural forms, there are mainly plate heat exchangers, floating head heat exchangers, and fixed tube plates
Type heat exchangers and U-tube heat exchangers, etc. Except for plate heat exchangers, the other types belong to shell and tube heat exchangers.
Due to its large heat exchange area per unit volume, good heat exchange efficiency, and advantages such as sturdy structure, strong adaptability, and mature manufacturing process, shell and tube heat exchangers have become the most commonly used typical heat exchanger.
Connection between heat exchange tubes and tube sheets in shell and tube heat exchangers
In a
Shell And Tube Heat Exchanger, the heat exchange tube and tube plate are the only barriers between the tube and shell sides of the heat exchanger. The connection structure and quality between the heat exchange tube and tube plate determine the quality and service life of the heat exchanger, which is a crucial link in the manufacturing process of the heat exchanger.
The damage and failure of most heat exchangers occur at the connection between the heat exchange tubes and tube sheets, and the quality of the connection joints directly affects the safety and reliability of chemical equipment and devices. Therefore, the connection process between the heat exchange tubes and tube sheets in shell and tube heat exchangers is crucial
It has become the most critical control link in the quality assurance system of heat exchanger manufacturing. At present, in the manufacturing process of heat exchangers, the connection between heat exchange tubes and tube sheets mainly includes welding, expansion joint, expansion joint with welding, and adhesive joint with expansion joint.
1. Welding
When the heat exchange tube and tube plate are connected by welding, due to the low requirements for tube plate processing, simple manufacturing process, good sealing, and convenient welding, appearance inspection, and maintenance, it is currently the application of connecting heat exchange tubes and tube plates in shell and tube heat exchangers
The most widely used method of connection. When using welding connections, there is strength welding that ensures the sealing and tensile strength of the welded joint, and sealing welding that only ensures the sealing of the heat exchange tube and tube plate connection. For strength welding, its performance is limited and only suitable for
Used in situations with low vibration and no gap corrosion.
When using welding connections, the distance between heat exchange tubes should not be too close, otherwise it will be affected by heat and the quality of the weld seam will not be easily guaranteed. At the same time, a certain distance should be left at the tube ends to reduce welding stress between them. The length of the heat exchange tube protruding from the tube plate must meet the requirements
The specified requirements are required to ensure its effective bearing capacity. In terms of welding methods, welding can be carried out using methods such as electrode arc welding, TIG welding, CO2 welding, etc. based on the material of the heat exchange tube and tube plate. For heat exchangers with high requirements for the connection between heat exchange tubes and tube sheets, such as those with high design pressure, high design temperature, large temperature changes, and those that withstand alternating loads, thin tube sheet heat exchangers, etc., TIG welding is recommended.
The conventional welding connection method, due to the gap between the pipe and the tube plate hole, is prone to gap corrosion and overheating, and the thermal stress generated at the welding joint may also cause stress corrosion and damage, which can lead to the failure of the heat exchanger. Currently in China
In heat exchangers used in industries such as nuclear and power industries, the connection between heat exchange tubes and tube sheets has begun to use inner hole welding technology. This connection method changes the end welding of heat exchange tubes and tube sheets to inner hole welding of tube bundles, using a full penetration form, eliminating the need for internal hole welding
The gap welded at the end enhances the ability to resist gap corrosion and stress corrosion,
Its anti vibration fatigue strength is high, it can withstand high temperature and high pressure, and the mechanical properties of welded joints are good; Internal non-destructive testing can be performed on the joint, and the internal quality of the weld can be controlled, improving the reliability of the weld. But the assembly of inner hole welding technology is difficult,
High requirements for welding technology, complex manufacturing and inspection, and relatively high manufacturing costs. With the development of heat exchangers towards high temperature, high pressure, and large-scale, the requirements for their manufacturing quality are becoming increasingly high, and the inner hole welding technology will be more widely used.
2. Expansion joint
Expansion joint is a traditional method of connecting heat exchange tubes and tube sheets, which uses expansion instruments to cause elastic-plastic deformation between the tube sheets and tubes, forming a firm connection and achieving the goal of both sealing and anti pulling. During the manufacturing process of heat exchangers, expansion occurs
Suitable for situations without severe vibration, significant temperature changes, and severe stress corrosion.
The current expansion joint processes mainly include mechanical rolling and hydraulic expansion. Uneven mechanical rolling and expansion joints make it very difficult to repair them with expansion pipes once the connection between the pipe and the tube plate fails; Adopting liquid bag hydraulic expansion joint controlled by computer, with high accuracy and the ability to
Ensure that the tightness of the expansion joint is uniform and consistent, and the reliability of the connection is better than that of mechanical expansion joint. However, strict requirements are placed on machining accuracy, and it is also difficult to ensure successful expansion of densely packed joints. If they fail, it is also difficult to repair them through expansion.
3. Expansion joint and welding
When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion, and fluid pressure, the connection between the heat exchange tube and the tube plate is extremely easy to be damaged, and using expansion or welding is difficult to ensure the requirements of connection strength and sealing. Currently widely adopted
It is a method of expansion welding combined with other methods. The expansion joint and welding structure can effectively damp the damage of tube bundle vibration to the weld seam, eliminate stress corrosion and gap corrosion, improve the fatigue resistance of the joint, and thus improve the service life of the heat exchanger
Simple expansion or strength welding has higher strength and sealing performance. For ordinary heat exchangers, the form of "adhesive expansion% strength welding" is usually adopted; However, heat exchangers with strict usage conditions require the use of "strength expansion%"
The form of seal welding. Expansion and welding can be divided into two types according to the order of expansion and welding in the process: first expansion and then welding, and first welding and then expansion.
(1) The lubricating oil used during the first expansion and then welding expansion joint will penetrate into the joint gap, and they have strong sensitivity to welding cracks, pores, etc., which makes the phenomenon of defects during welding more serious. These oil stains that penetrate into the gaps are difficult to remove
Clean, so the process of first expanding and then welding is adopted, and mechanical expansion joint is not suitable. Although the use of adhesive expansion is not pressure resistant, it can eliminate the gap between the pipe and the pipe plate hole, so it can effectively damping the vibration of the pipe bundle to the welding part of the pipe mouth.
However, conventional manual or mechanically controlled expansion methods cannot achieve uniform expansion requirements, while the liquid bag expansion method with computer-controlled expansion pressure can conveniently and uniformly achieve expansion requirements. During welding, due to the high temperature melting of the metal
The impact is that the gas inside the gap is heated and rapidly expands, causing certain damage to the sealing performance of the strength expansion when these gases with high temperature and pressure leak out.
(2) For the welding followed by expansion process, the primary issue is to control the accuracy and fit of the pipe and tube plate holes. When the gap between the tube and the tube plate hole is reduced to a certain value, the expansion process will not damage the quality of the welded joint. But the welded joint bearing
The ability to withstand shear force is relatively poor, so if the control during strength welding does not meet the requirements, it may cause over expansion failure or damage to the welded joint due to expansion.
During the manufacturing process, there is a significant gap between the outer diameter of the heat exchange tube and the tube plate hole, and the gap between the outer diameter of each heat exchange tube and the tube plate hole is uneven along the axial direction. When expanding after welding is completed, the centerline of the pipe must be aligned with the center of the pipe plate hole
The overlapping of lines is necessary to ensure the quality of the joint. If the gap is large, due to the high rigidity of the pipe, excessive expansion deformation will cause damage to the welded joint, and even lead to weld detachment.
4. Adhesive and expansion joints
The use of adhesive and expansion joint processes helps to solve the common problems of leakage and leakage at the connection between heat exchange tubes and tube sheets in heat exchangers. It is important to select the appropriate adhesive according to the working conditions of the bonded parts. In the process of process implementation, heat exchange should be combined
The structure and size of the device should be selected with good process parameters, mainly including curing pressure, curing temperature, swelling force, etc., and strictly controlled during the production process. This process is simple, easy to implement, and reliable, and has been recognized in practical use by enterprises. It has
Promotion value.
Conclusion
(1) In the connection method between heat exchange tubes and tube sheets in shell and tube heat exchangers, conventional welding or expansion alone is difficult to ensure the connection strength and sealing requirements.
(2) The use of expansion joint and welding method is conducive to ensuring the connection strength and sealing between the heat exchange tube and the tube plate, and improving the service life of the heat exchanger.
(3) The method of using adhesive and expansion joints helps to solve the problems of leakage and leakage when connecting heat exchange tubes and tube sheets, and the process is simple, feasible, and reliable.
(4) Inner hole welding technology, as a fully penetrated welding method, has excellent resistance to gap corrosion and stress corrosion, vibration fatigue strength, and mechanical properties of welded joints; The internal quality of the weld can be controlled, improving the reliability of the weld
Sex is more suitable for promotion and application in high-end products.
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