Study on the Process of Connecting Heat Exchange Tubes to Tubesheets in Shell and Tube Heat Exchangers
Yu Bin Yang Honglan Abstract: The quality of the connection between the heat exchange tube and the tube sheet in the shell-and-tube heat exchanger directly affects the quality and service life of the heat exchanger. Through systematic research on the process of connecting heat exchange tubes with tube sheets, and comprehensive evaluation, the application of these studies in actual production has certain guiding significance and applicable value. When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, hot corrosion and fluid pressure, the connection between the heat exchange tube and the tube sheet is easily destroyed, and it is difficult to ensure the joint strength and sealing by expansion or welding. Requirements. At present, the method of expanding and using is widely used. The expansion joint and the welded structure can effectively dampen the damage of the tube bundle vibration to the weld seam, can effectively eliminate the stress corrosion and the gap corrosion, improve the fatigue resistance of the joint, thereby improving the service life of the heat exchanger, and simply expanding or Strength welding has higher strength and tightness. For ordinary heat exchangers, the form of "expansion-intensity-strength welding" is usually adopted; and the use of the heat exchangers with severe conditions requires the form of "strength expansion and sealing". Expansion joint and welding can be divided into two types: first expansion and post welding and first expansion after welding in the order of expansion and welding.
Yantai Engineering Vocational and Technical College (Shandong 264006)
I. Overview
As a heat transfer device that transfers part of the heat of the hot fluid between the materials to the cold fluid, the heat exchanger has a wide range of applications in people's daily life and petroleum, chemical, power, pharmaceutical, atomic energy and nuclear industries. It can be used as a stand-alone device, such as heaters, condensers, coolers, etc.; it can also be used as part of certain process equipment, such as heat exchangers in some chemical equipment. Especially in the chemical industry with large energy consumption, heat exchangers are indispensable equipment in the process of heat exchange and transfer of chemical production, and also occupy a considerable proportion in the entire chemical production equipment.
From the perspective of its function, the heat exchanger is on the one hand ensuring the specific temperature required by the industrial process for the medium, and on the other hand is the main equipment for improving energy efficiency. According to its structure, there are mainly plate heat exchangers, floating head heat exchangers, fixed tube plate heat exchangers and U-tube heat exchangers. In addition to the plate heat exchanger, the other several belong to the shell-and-tube heat exchanger. Because the shell-and-tube heat exchanger has a large heat exchange area per unit volume, and the heat exchange effect is good, and has the advantages of strong structure, strong adaptability, mature manufacturing process, etc., it has become the most commonly used typical change. Heater.
Second, the connection between the heat exchange tube and the tube sheet in the shell-and-tube heat exchanger
In the shell-and-tube heat exchanger, the heat exchange tube and the tube sheet are the only barrier between the tube and shell side of the heat exchanger. The connection structure and connection quality between the heat exchange tube and the tube sheet determine the quality of the heat exchanger. The advantages and disadvantages and service life are a vital part of the heat exchanger manufacturing process. The destruction and failure of most heat exchangers occur at the connection between the heat exchange tube and the tube sheet. The quality of the joints also directly affects the safety and reliability of chemical equipment and equipment, so it is changed in the shell-and-tube heat exchanger. The connection process between the heat pipe and the tube sheet becomes the most critical control link in the heat exchanger manufacturing quality assurance system. At present, in the heat exchanger manufacturing process, the connection between the heat exchange tube and the tube sheet mainly includes: welding, expansion joint, expansion joint and welding, and glue and expansion joint.
Welding
When the heat exchange tube and the tube sheet are welded, the processing is simple, the manufacturing process is simple, the sealing is good, and the welding, visual inspection and maintenance are very convenient, and the current shell-and-tube heat exchanger is The medium heat exchange tube and tube sheet are the most widely used one. When the welded joint is used, there is a strength weld which ensures the sealing property and the tensile strength of the welded joint, and a seal weld which only ensures the sealing property of the heat exchange tube and the tube sheet. There is a limit to the performance of the strength weld, which is only suitable for applications with less vibration and no crevice corrosion.
When welding is used, the distance between the heat exchange tubes should not be too close. Otherwise, the quality of the welds is not easily ensured by heat, and the pipe ends should be kept at a certain distance to reduce the welding stress between them. The length of the heat transfer tube extending from the tube sheet must meet the specified requirements to ensure its effective bearing capacity. In the welding method, according to the material of the heat exchange tube and the tube sheet, welding by arc welding, TIG welding, CO2 welding or the like may be employed. For heat exchangers with high requirements for connection between heat exchange tubes and tube sheets, TIG welding should be used for heat exchangers and thin tube sheet heat exchangers with large design pressure, high design temperature, large temperature variation, and alternating load. .
Conventional welding connection methods, due to the gap between the tube and the tube plate hole, are prone to crevice corrosion and overheating, and the thermal stress generated at the welded joint may also cause stress corrosion and damage, which will invalidate the heat exchanger. At present, in the domestic heat exchangers used in the nuclear industry and the power industry, the connection between the heat exchange tubes and the tube sheets has begun to use the internal hole welding technology. This connection method changes the ends of the heat exchange tubes and the tube sheets to one another. The inner hole of the tube bundle is welded, adopting the full penetration form, eliminating the gap of the end welding, improving the resistance to crevice corrosion and stress corrosion resistance, high vibration fatigue resistance, high temperature and high pressure resistance, and mechanical properties of the welded joint. Good; the joint can be internally non-destructively inspected, the internal quality of the weld can be controlled, and the reliability of the weld is improved. However, the inner hole welding technology is difficult to assemble, requires high welding technology, is complicated to manufacture and inspect, and has a relatively high manufacturing cost. With the development of heat exchangers to high temperature, high pressure and large scale, the manufacturing quality requirements are getting higher and higher, and the internal hole welding technology will be more widely used.
2. Expansion joint
Expansion joint is a traditional connection method between heat exchange tube and tube sheet. The tube tube and tube are elastically deformed and closely adhered by the expansion tube device to form a firm connection, which is the purpose of sealing and resisting pull-off. In the manufacturing process of the heat exchanger, the expansion joint is suitable for occasions without severe vibration, no excessive temperature change, and no serious stress corrosion. At present, the expansion joint process mainly includes mechanical bulging and hydraulic expansion. The mechanical bulging expansion is uneven. Once the pipe and the tube sheet are connected, it is very difficult to repair with the expansion tube. The liquid bag type hydraulic expansion is controlled by the computer, the precision is high, and the tightness of the expansion is uniform. The reliability of the connection is better than the mechanical expansion. However, the processing precision is strict, and it is difficult to ensure 100% expansion of the joints of the dense cloth. It is also difficult to repair the joint after failure.
3. Expansion and welding
(1) The lubricating oil used in the first expansion and then the expansion joint will penetrate into the joint gap, and they have strong sensitivity to weld cracks, pores, etc., so that the phenomenon of defects during welding is more serious. These oils that penetrate into the gap are difficult to remove, so the first expansion and post-welding process is not suitable for mechanical expansion. Although the sticking is not resistant to pressure, the gap between the tube and the tube hole of the tube sheet can be eliminated, so that the tube bundle can be effectively damped to the welded portion of the nozzle. However, the conventional manual or mechanically controlled expansion method cannot achieve uniform swelling requirements, and the liquid bag expansion method using the computer to control the expansion pressure can conveniently and uniformly achieve the swelling requirement. During welding, due to the influence of high-temperature molten metal, the gas in the gap is heated and rapidly expanded. These high-temperature and high-pressure gases may cause certain damage to the sealing performance of the strength expansion when leaking.
(2) First post-weld expansion For the first post-weld expansion process, the primary problem is to control the accuracy of the tube and tube plate holes and their cooperation. When the gap between the tube and the tube tube hole is small enough, the expansion process will not damage the quality of the welded joint. However, the ability of the welded joint to withstand shearing force is relatively poor. Therefore, if the control fails to meet the requirements during the strength welding, the over-expansion failure or the expansion joint may damage the welded joint. In the manufacturing process, there is a large gap between the outer diameter of the heat exchange tube and the tube hole of the tube sheet, and the outer diameter of each heat exchange tube and the tube tube hole gap are not uniform in the axial direction. When the expansion is completed, the center line of the pipe must coincide with the center line of the tube hole of the tube to ensure the joint quality. If the gap is large, the expansion of the tube will be large due to the rigidity of the tube. Damage, and even cause the weld to be desoldered.
4. Glue and expansion joint
The process of bonding and expansion is helpful to solve the problems of leakage and leakage frequently occurring in the heat exchanger tube and tube sheet joints. It is important to select the glue according to the working conditions of the glued parts. . In the process of process implementation, the process parameters should be selected in combination with the structure and size of the heat exchanger, including curing pressure, curing temperature, expansion force, etc., and strictly controlled in the production process. The process is simple, easy and reliable, and has been recognized in the actual use of the enterprise, with promotional value.
Third, the conclusion
(1) In the connection method of the heat exchange tube and the tube sheet of the shell-and-tube heat exchanger, it is difficult to ensure the joint strength and the sealing property by using conventional welding or expansion joint alone.
(2) The method of expanding and welding is beneficial to ensure the connection strength and sealing between the heat exchange tube and the tube sheet, and to improve the service life of the heat exchanger.
(3) The method of bonding and expanding the joints helps to solve the leakage and leakage problems when the heat exchange tubes are connected with the tube sheets, and the process is simple and reliable.
(4) Inner hole welding technology as a fully penetration welding method, the ability to resist crevice corrosion and stress corrosion resistance, vibration fatigue strength, and mechanical properties of welded joints are good; the internal quality of welds can be controlled and improved The reliability of the weld. First of all, it is more suitable to promote the application in high-end products.