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The process of connection between heat exchange tube and tube plate in shell and tube heat exchanger

One An overview of the

     Heat exchanger, as a heat transfer device that transfers some heat of hot fluid between materials to cold fluid, is widely used in People's Daily life and industries such as petroleum, chemical industry, power, medicine, atomic energy and nuclear industry.It can be used as an independent equipment, such as heater, condenser, cooler, etc.It can also be used as part of some process equipment, such as heat exchanger in some chemical equipment.

     Especially in the chemical industry which consumes a large amount of energy, heat exchanger is an indispensable equipment in the heat exchange and transfer process of chemical production, and occupies a considerable proportion in the whole chemical production equipment.

     In terms of its function, heat exchanger is on the one hand to ensure the specific temperature required for the medium in the industrial process, on the other hand, it is also the main equipment to improve the energy efficiency.According to its structure, there are plate heat exchanger, floating head heat exchanger, fixed tube-plate heat exchanger and u-shaped tube heat exchanger.Except plate heat exchanger, the others belong to shell heat exchanger.

     Because shell and tube heat exchanger has larger heat transfer area per unit volume, good heat transfer effect, strong structure, strong adaptability, mature manufacturing process and other advantages, has become the most commonly used a typical heat exchanger.

Two The connection between the heat exchange tube and the tubesheet in a shell and tube heat exchanger

     In shell and tube heat exchanger, the heat exchange tube and tube plate are the only barrier between the tube side and shell side of the heat exchanger.

     Most of the heat exchanger of the damage and failure occurred in the connection position of the heat exchange tube and tube plate, the joint quality has a direct impact on the safety and reliability of the chemical equipment and devices, so the heat exchange tube and tube plate in heat exchanger tube connection technology has become a heat exchanger manufacturing quality assurance system to control the key link in.At present in the heat exchanger manufacturing process, the connection between the heat exchange tube and the tube plate is mainly: welding, expansion, expansion and welding, and plastic expansion and expansion and other methods.

  1. welding

         When the heat exchange tube and tube plate are connected by welding, due to the lower processing requirements for the tube plate, simple manufacturing process, good sealing, and easy welding, appearance inspection and maintenance, it is the most widely used connection method of heat exchange tube and tube plate in the shell heat exchanger.When welding connection is used, there are strength welding to ensure the sealing of welding joint and tensile strength, and only to ensure the sealing of heat exchange tube and tube-plate connection.The performance of strength welding is limited, and it is only suitable for low vibration and no gap corrosion.

         When using welding connection, the distance between the heat exchange tubes should not be too close, otherwise affected by heat, weld quality is not easy to be guaranteed, and the pipe end should have a certain distance, in order to reduce the welding stress between each other.The length of the protruding tube plate of the heat exchange tube should meet the specified requirements to ensure its effective carrying capacity.In terms of welding method, according to the material of heat exchange tube and tube plate, welding can be carried out by welding electrode arc welding, TIG welding, CO2 welding and other methods.TIG welding is suitable for heat exchangers with high requirements for connection between heat exchange tubes and tubeplates, such as heat exchangers with high design pressure, high design temperature and large temperature variation, as well as heat exchangers with alternating loads and thin tubeplates.

         Due to the gap between the pipe and the tubesheet hole, the gap corrosion and overheating are easy to occur in the conventional welding connection method, and the thermal stress generated at the welding joint may also cause stress corrosion and damage, which will make the heat exchanger fail.At present, in the domestic industry, power industry and other industries in the use of the heat exchanger, heat exchange tube and tube plate connection has started to use inner hole welding technology, this kind of connection method will end of heat exchange tube and tube plate welded to the tube bundle bore welding, in the form of full penetration, eliminates the end weld cracks, improve the ability to resist corrosion and stress corrosion resistance of the gap, the vibration fatigue strength is high, can bear high temperature and high pressure, the mechanical properties of welded joint is better;The internal quality of weld can be controlled and the reliability of weld can be improved.However, the inner hole welding technology is difficult to assemble, has high requirements on welding technology, complex manufacturing and inspection, and relatively high manufacturing cost.With the development of heat exchanger to high temperature, high pressure and large size, the requirement for its manufacturing quality is higher and higher, the inner hole welding technology will be more widely used.

  2. expanding

         Expansion joint is a traditional connection method between heat exchange tube and tube plate, which USES expansion instrument to make the tube plate and tube produce elastic-plastic deformation and closely fit, form a firm connection, and achieve the purpose of sealing and anti-tension.In the heat exchanger manufacturing process, expansion is suitable for no severe vibration, no excessive temperature change, no serious stress corrosion occasions.

         At present, the expansion technology mainly includes mechanical expansion and hydraulic expansion.It is very difficult to repair the failure of connection between the tube and the tube plate when the mechanical rolling and swelling is not evenly connected.It adopts the liquid bag type hydraulic expansion joint controlled by computer, with high precision, and can ensure the uniformity of expansion joint tightness, and the reliability of connection is better than that of mechanical expansion joint.However, strict requirements on machining accuracy, dense joints to ensure the success of expansion also have some difficulties, if failure is more difficult to repair expansion.

  3. Expansion 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 easy to be destroyed, and it is difficult to ensure the connection strength and sealing requirements by expansion or welding.At present, expansion welding is widely used.Expansion welding structure can effectively damp the damage of tube bundle vibration to welding seam, can effectively eliminate stress corrosion and gap corrosion, improve the fatigue resistance of the joint, so as to improve the service life of the heat exchanger, has higher strength and sealing than simple expansion welding or strength welding.For ordinary heat exchangers usually adopt the form of "paste expansion % strength welding";And the use of harsh conditions of heat exchanger is required to use the "strength expansion % seal welding" form.According to the sequence of expansion and welding in the working procedure, expansion and welding can be divided into two types: first expansion and then expansion.

           (1) the lubricating oil used in welding expansion after expansion will penetrate into the gap of the joint, and they are highly sensitive to welding cracks, pores, etc., thus making the phenomenon of defects during welding more serious.These infiltration into the clearance of oil is very difficult to clean, so the use of first expansion welding process, should not be used mechanical expansion of the way.Although sticking expansion is not pressure resistant, it can eliminate the gap between tube and tube plate and tube hole, so it can effectively damp the vibration of tube bundle to the welded part of tube mouth.

    However, the conventional manual or mechanical expansion method can not reach the requirements of uniform expansion, and the computer controlled expansion pressure of liquid bag expansion method can be convenient and uniform to achieve the requirements of expansion.During welding, due to the influence of high-temperature melting metal, the gas in the gap is heated and rapidly expands. These gases with high temperature and high pressure will cause certain damage to the sealing performance of strength expansion during the leakage.

           (2) welding before expansion. For the welding before expansion process, the primary problem is to control the precision and matching of the hole between the pipe and the tube plate.When the gap between tube and tube plate is small to a certain value, the quality of the welded joint will not be damaged in the expansion process.However, the ability of welding joints to withstand shear force is relatively poor, so when the strength welding, if the control does not meet the requirements, may cause over-expansion failure or expansion of the welding joint damage.

    In the manufacturing process, there is a large gap between the outer diameter of the heat exchange tube and the hole of the tube plate, and the gap between the outer diameter of each heat exchange tube and the hole of the tube plate is uneven along the axial direction.When the welding is completed, the center line of the pipe must coincide with the center line of the tube plate and hole to ensure the quality of the joint. If the gap is large, due to the rigidity of the pipe is large, excessive expansion deformation will cause damage to the welded joint, or even cause welding joint disconnection.

  4. Glue joint and expansion joint

         The process of gluing and expanding is helpful to solve the problems of leakage and leakage in the connection between the heat pipe and the tube-plate in the heat exchanger.In the process of the implementation of the heat exchanger structure, size to choose a good process parameters, mainly including curing pressure, curing temperature, expansion force, and strict control in the production process.The process is simple, easy and reliable, and has been recognized in practical use in enterprises.

Three conclusion

  1. in the connection method between the heat exchange tube and the tubesheet, it is difficult to guarantee the connection strength and the requirements of tightness by conventional welding or expansion joint alone.

  2. the method of expansion joint and welding is beneficial to ensure the connection strength and tightness between the heat exchange tube and the tubeplate, and improve the service life of the heat exchanger.

  3. the method of glue connection and expansion connection is helpful to solve the leakage and leakage problems when the heat exchange tube is connected with the tube-board. The process is simple, easy and reliable.

  4. as a full penetration welding method, the inner hole welding technology has good anti-gap corrosion and stress corrosion resistance, anti-vibration fatigue strength and mechanical properties of welded joints.The internal quality of welding seam can be controlled, which improves the reliability of welding seam. Firstly, it is more suitable for promotion and application in high-end products.

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