Overview
The heat exchanger, as a heat transfer device that transfers some of the heat from the hot fluid to the cold fluid between materials, has a wide range of applications in people's daily lives and in industries such as petroleum, chemical, power, pharmaceutical, atomic and nuclear industries. It can be used as independent equipment, 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, etc.
Especially in the larger amount of energy consumption in the chemical industry, heat exchanger in the chemical production of heat exchange and transfer process is indispensable equipment, in the entire chemical production equipment also occupies a considerable proportion.
Heat exchanger from its function, on the one hand, is to ensure that the industrial process of the medium required by the specific temperature, on the other hand, is also to improve the energy utilization rate of the main equipment. According to its structure, there are mainly plate heat exchangers, floating head heat exchangers, fixed tube plate heat exchangers and U-shaped tube heat exchangers and so on. In addition to the plate heat exchanger, the rest of the several belong to the
Shell And Tube Heat Exchanger.
As a result of shell and tube heat exchanger has a larger heat transfer area per unit volume, and good heat transfer effect, while having a strong structure, adaptability, manufacturing technology and other advantages, has become the most commonly used a typical heat exchanger.
Shell and tube heat exchanger in the heat exchanger tube and tube plate connection
In the shell and tube heat exchanger heat exchanger tube and tube plate is the only barrier between the heat exchanger tube and shell process, heat exchanger tube and tube plate connection between the structure and connection quality determines the quality and service life of the heat exchanger, is a vital part of the heat exchanger manufacturing process.
Most of the heat exchanger damage and failure are occurring in the heat exchanger tube and tube plate connection parts, the quality of the connection joint also directly affects the safety and reliability of chemical equipment and devices, so for shell and tube heat exchanger in the heat exchanger tube and tube plate connection process has become the most critical control link in the heat exchanger manufacturing quality assurance system. At present, in the heat exchanger manufacturing process, the heat exchanger tube and tube plate connections are mainly: welding, expansion, expansion plus welding and gluing plus expansion and other methods.
1. Welding
Heat exchanger tube and tube plate using welding connection, due to the tube plate processing requirements are low, simple manufacturing process, there is a better sealing, and welding, appearance inspection, maintenance are very convenient, is currently the shell and tube heat exchanger in the heat exchanger tube and tube plate connection is the most widely used a connection method. In the use of welded connections, there to ensure that the strength of the welded joint sealing and resistance to pull off the strength of the welding and only to ensure that the heat exchanger tube and tube plate connection sealing sealing welding. For strength welding its use performance is limited, only for vibration and no gap corrosion occasions.
When using welding connection, the distance between the heat exchanger tube can not be too close, otherwise the heat influence, the quality of the weld is not easy to ensure, while the end of the tube should be left a certain distance, in order to help reduce the mutual welding stress. The length of the heat exchanger tube sticking out of the tube plate should meet the specified requirements to ensure its effective bearing capacity. In the welding method, according to the material of the heat exchanger tube and the tube plate can be welded by welding rod arc welding, TIG welding, CO2 welding and other methods. For heat exchanger tube and tube plate connection requirements between the high heat exchanger, such as design pressure, design temperature, high temperature changes, and bear alternating load heat exchanger, thin tube plate heat exchanger, etc. should be used TIG welding.
Conventional welding connection method, due to the tube and tube plate hole between the gap, easy to produce gap corrosion and overheating, and welding joints at the thermal stress may also cause stress corrosion and damage, these will make the heat exchanger failure. Currently in the domestic nuclear industry, power industry and other industries using heat exchangers, heat exchanger tube and tube plate connection has begun to use the inner hole welding technology, this connection method will heat exchanger tube and tube plate end welding to tube bundle inner hole welding, the use of full fusion through the form, eliminating the gap in the end welding, improve the ability to resist gap corrosion and stress corrosion, the ability to resist stress corrosion.
Its high vibration fatigue strength, can withstand high temperature and pressure, the mechanical properties of the welded joint is better; the joint can be internal non-destructive testing, the internal quality of the weld can be controlled to improve the reliability of the weld. But the bore welding technology assembly is more difficult, high requirements for welding technology, manufacturing and inspection complex, and manufacturing costs are relatively high. With the heat exchanger to high temperature, high pressure and large-scale development, its manufacturing quality requirements are increasingly high, the hole welding technology will be more widely used.
2. Expansion joint
Expansion is a traditional heat exchanger tube and tube plate connection method, the use of expansion equipment to make the tube plate and tube elastic plastic deformation and close fit, the formation of a solid connection, that is, sealing and can resist pulling off the purpose. In the manufacturing process of heat exchangers, expansion is suitable for no severe vibration, no excessive temperature changes, no serious stress corrosion occasions.
Currently used in the expansion process mainly mechanical rolling expansion and hydraulic expansion. Mechanical roll expansion expansion is not uniform, once the tube and tube plate connection failure and then use expansion to repair very difficult; the use of liquid bag hydraulic expansion by computer-controlled operation, high precision, and can ensure that the expansion of the degree of tightness of uniformity, the reliability of the connection is better than mechanical expansion. However, the processing accuracy requirements are strict, to ensure the success of the dense joint expansion also has some difficulties, if the failure of re-expansion repair is also more difficult.
3. Expansion plus welding
When the temperature and pressure is high, and in the thermal deformation, thermal shock, thermal corrosion and fluid pressure, heat exchanger tube and tube plate connection is very easy to be destroyed, using expansion or welding are difficult to ensure the strength of the connection and sealing requirements. At present, the widely used method is the expansion and welding. The expansion and welding structure can effectively damp the vibration of the tube bundle damage to the weld, can effectively eliminate stress corrosion and gap corrosion, improve the fatigue resistance of the joint, thus improving the service life of the heat exchanger.
This improves the service life of the heat exchanger and provides higher strength and sealing than simple expansion or strength welding. For ordinary heat exchangers are usually used "paste expansion % strength welding" form; and the use of harsh conditions of heat exchangers require the use of "strength expansion % sealing welding" form. Expansion plus welding according to expansion and welding in the sequence of the process can be divided into first expansion after welding and first welding after the expansion of two.
(1) expand first and then weld expand when the use of lubricants will penetrate into the joint gap, and they have a strong sensitivity to welding cracks, porosity, etc., thus making the phenomenon of defects in welding more serious. These penetrate into the gap of the oil is difficult to remove clean, so the use of expansion first and then welding process, should not be used mechanical expansion of the way. The use of paste expansion, although not pressure resistant, can eliminate the gap between the tube and the tube plate tube hole, so it can effectively damp the tube bundle vibration to the welded part of the orifice.
However, the use of conventional manual or mechanically controlled expansion methods cannot achieve uniform paste expansion requirements, while the use of computer-controlled expansion pressure of the liquid bag type expansion method can be easily and uniformly achieved paste expansion requirements. In welding, due to the influence of high temperature molten metal, the gas in the gap is heated and expanded sharply, these have high temperature and high pressure gas in the external leakage of the strength of the sealing performance of the expansion will cause some damage.
(2) first weld and then expand for first weld and then expand process, the primary problem is to control the accuracy of the tube and tube plate hole and its fit. When the gap between the tube and the tube plate tube hole is small to a certain value, the expansion process will not damage the quality of the welded joint. But the ability of the weld mouth to withstand shear forces is relatively poor, so the strength of the weld, if the control does not meet the requirements, may cause over-expansion failure or expansion of the welded joint damage.
In the manufacturing process, there is a large gap between the outer diameter of the heat exchanger tube and the tube plate tube hole, and the outer diameter of each heat exchanger tube and the tube plate tube hole gap along the axial direction is not uniform. When the welding is completed after expansion, the tube centre line must coincide with the tube plate hole centre line, in order to ensure the quality of the joint, if the gap is large, due to the greater rigidity of the tube, too much expansion deformation will produce damage to the welded joint, or even cause the weld off.
4. Glue joint plus expansion
The use of gluing and expansion of the process to help solve the heat exchanger in the heat exchanger tube and tube plate connections often appear leakage and leakage problems, it is important to be glued according to the working conditions of the correct choice of glue jointing agent. In the process of implementation of the process should be combined with the structure of the heat exchanger, the size of a good choice of process parameters, including curing pressure, curing temperature, expansion force, etc., and in the production process strictly controlled. This process is simple, easy to implement, reliable, has been recognized in the actual use of enterprises, with the promotion of value.
Conclusion
(1) in the shell and tube heat exchanger tube and tube plate connection method, the use of conventional welding or expansion alone are difficult to ensure the strength of the connection and the requirements for sealing.
(2) The use of expansion and welding method is conducive to ensuring the strength and sealing of the connection between the heat exchanger tube and the tube plate, to improve the service life of the heat exchanger.
(3) The use of gluing and expansion method helps to solve the problem of leakage and seepage when connecting the heat exchanger tube to the tube plate, and the process is simple, easy and reliable.
(4) bore welding technology as a full penetration welding method, the ability to resist gap corrosion and stress corrosion, vibration fatigue strength, mechanical properties of welded joints are very good; internal quality of the weld can be controlled to improve the reliability of the weld, the first more suitable for the promotion of high-end products in the application
Source: Reproduced from the web
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