Heat exchanger tube and tube plate connection

Heat exchanger, as the heat transfer equipment that transfers part of the heat from the hot fluid to the cold fluid between materials, has a wide range of applications in people's daily life and in petroleum, chemical, power, pharmaceutical, atomic energy and nuclear industries. It can be used as independent equipment, such as heaters, condensers, coolers, etc.; it can also be used as a component of some process equipment, such as heat exchangers in some chemical equipment, etc.

Especially in the larger amount of energy consumption in the chemical industry, heat exchangers 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, to ensure that the industrial process of the medium required by the specific temperature, on the other hand, is also the main equipment to improve energy utilization. According to its structure, there are mainly plate heat exchanger, floating head heat exchanger, fixed tube plate heat exchanger and U-shaped tube heat exchanger and so on. In addition to the plate heat exchanger, the rest of the several belong to the shell and tube heat exchanger.

As the shell and tube heat exchanger has a larger heat transfer area per unit volume, and good heat transfer effect, while having a strong structure, adaptable, mature manufacturing process and other advantages, has become the most common use of a typical heat exchanger.

The connection between heat exchanger tube and tube plate in shell and tube heat exchanger

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 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 occur 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 plate connections are mainly: welding, expansion, expansion plus welding and gluing plus expansion and other methods.

Welding

Heat exchanger tube and tube plate using welded connection, due to the lower processing requirements of the tube plate, manufacturing process is simple, there is a better sealing, and welding, appearance inspection, maintenance are very convenient, is currently the shell and tube heat exchanger tube and tube plate connection is the most widely used in a connection method. In the use of welded connections, there to ensure the strength of the welded joint sealing and pull-off strength and only to ensure that the heat exchanger tube and tube plate connection sealing sealing welding. For strength welding its performance is limited, only for the vibration is small 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 extending 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 heat exchanger tube and 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, as well as the heat exchanger subjected to alternating loads, thin tube plate heat exchanger, etc. TIG welding is appropriate.

Conventional welding connection method, due to the existence of gaps between the tube and tube plate hole, prone to gap corrosion and overheating, and the thermal stress generated at the welded joint may also cause stress corrosion and damage, which can cause the heat exchanger failure. At present, in the domestic nuclear industry, power industry and other industries using the heat exchanger, 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 penetration 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 development of heat exchangers to high temperature, high pressure and large-scale, the manufacturing quality requirements are increasingly high, the bore welding technology will be more widely used.

Expansion joint

Expansion joint is a traditional method of connecting heat exchanger tubes to the tube plate, using expansion apparatus to make the tube plate and tube to produce elastic-plastic deformation and close fit, forming a solid connection, which is sealed 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.

The current expansion process used 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; using liquid bag type hydraulic expansion by computer-controlled operation, high precision, and can ensure that the expansion tightness uniformity, the reliability of the connection is better than mechanical expansion. However, the processing accuracy requirements are strict, and it is difficult to ensure the success of expansion of dense joints, and it is also more difficult to repair if they fail to expand again.

Expansion and 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, the use of 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. 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, thereby improving the service life of the heat exchanger.

This improves the service life of the heat exchanger, and has higher strength and sealability than simple expansion or strength welding. For ordinary heat exchangers usually use the "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 expansion of two kinds.

(1) first expansion after welding expansion when the use of lubricating oil 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, it is not appropriate to use mechanical expansion of the way. The use of paste expansion, although not pressure-resistant, but 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 pipe mouth.

But the use of conventional manual or mechanically controlled expansion method can not achieve uniform paste expansion requirements, while the use of computer-controlled expansion pressure of the liquid bag type expansion method can be convenient and uniform to achieve the 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 gases with high temperature and pressure in the external leakage of the strength of the expansion of the sealing performance will cause some damage.

(2) first welding and then expansion for the first welding and then expansion 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 damage to the welded joint.

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 gap between the outer diameter of each heat exchanger tube and the tube plate tube hole is uneven along the axial direction. When the welding is completed after expansion, the tube centerline must coincide with the tube plate hole centerline to ensure the quality of the joint, if the gap is large, due to the greater rigidity of the tube, excessive expansion deformation will produce damage to the welded joint, or even cause the weld off the weld.

Glued and expanded joints

The use of gluing and expansion process can help solve the heat exchanger tube and tube plate connections often appear in the problem of leakage and leakage, 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, size to choose a good process parameters, mainly 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, has the value of promotion.

Conclusion

(1) in the shell and tube heat exchanger heat exchanger tube and 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 plate, and 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 heat exchanger tubes and plates, 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 and application of high-end products.