Process for connecting heat exchange tube and tube plate in shell and tube heat exchanger
What is a shell and tube heat exchanger?
Shell and tube heat exchangers are also known as tube and tube heat exchangers. The partition wall heat exchanger is a heat transfer surface which is closed on the wall surface of the tube bundle in the casing. The heat exchanger has a simple structure and reliable operation, can be manufactured by using various structural materials (mainly metal materials), can be used under high temperature and high pressure, and is the most widely used type at present.
|Structure And Type||Types Of Shell And Tube Heat Exchangers|
|Structure And Manufacturing Standard||Process For Connecting|
The shell and tube heat exchanger consists of a housing, a heat transfer tube bundle, a tube sheet, a baffle, and a tube box. The casing is mostly cylindrical, and a tube bundle is installed inside, and both ends of the bundle are fixed on the tube sheet. The two types of hot and cold fluids for heat exchange, one flowing in the tube, called the tube flow fluid, and the other flowing outside the tube, called the shell side fluid. In order to increase the heat transfer coefficient of the fluid outside the tube, a number of baffles are usually installed in the housing. The baffle increases the speed of the shell side fluid, forcing the fluid to pass laterally through the tube bundle multiple times over a specified path, enhancing fluid turbulence. The heat exchange tubes can be arranged on the tube sheets in an equilateral triangle or square. The equilateral triangles are arranged in a compact manner, the fluid outside the tube is highly turbulent, and the heat transfer coefficient is large; the square arrangement is convenient for cleaning outside the tube, and is suitable for fluids that are easy to scale. Each time the fluid passes through the tube bundle, it is called a tube process; each time it passes through the shell, it is called a shell side.
The shell and tube heat exchanger has different temperatures of the fluid and the tube bundle due to the different temperatures of the fluid inside and outside the tube. If the two temperatures differ greatly, a large thermal stress will be generated in the heat exchanger, causing the tube to bend, break, or pull off the tubesheet. Therefore, when the temperature difference between the tube bundle and the casing exceeds 50 °C, appropriate compensation measures should be taken to eliminate or reduce the thermal stress. According to the compensation measures adopted, shell and tube heat exchangers can be divided into the following main types:
- The tube plate at both ends of the fixed tube plate heat exchanger tube bundle is integrated with the shell, and the structure is simple, but it is only suitable for the heat exchange operation when the temperature difference between the hot and cold fluids is not large, and the shell side does not need mechanical cleaning. When the temperature difference is slightly larger and the shell side pressure is not too high, an elastic compensation ring can be mounted on the housing to reduce thermal stress.
- The tube plate at one end of the tube head of the floating head heat exchanger can float freely, completely eliminating the thermal stress; and the whole tube bundle can be taken out from the housing for mechanical cleaning and maintenance. The application of the floating head heat exchanger is relatively wide, but the structure is relatively complicated and the cost is high.
- U type tubular heat exchanger: each heat exchange tube is bent into a U shape, and the two ends are respectively fixed in the lower two areas of the same tube plate, and are divided into two chambers by means of a partition in the tube box. The heat exchanger completely eliminates thermal stress, and the structure is simpler than the floating head type, but the tube process is not easy to clean.
- Stuffing type heat exchanger: the structural function of the stuffing type heat exchanger is that only one end of the tube plate is fixedly connected to the casing, and the other end is sealed by a stuffing box. The tube bundle can be freely stretched and does not cause temperature difference stress caused by the temperature difference between the shell wall and the tube wall. The advantage of the stuffing type heat exchanger is that the structure is simpler than the floating head heat exchanger, the manufacture is convenient, the consumables are few, and the cost is low; the tube bundle can be taken out from the casing, and the tube and the tube can be cleaned and the maintenance is convenient. The disadvantage is that the stuffing box pressure is not high, generally less than 4.0 MPa; the shell side medium may leak through the stuffing box, which is not suitable for flammable, explosive, toxic and valuable media. The stuffing box heat exchanger is suitable for occasions where the temperature difference between the tube and the shell wall is large or the medium is easy to scale, which needs to be cleaned frequently and the pressure is not high.
- kettle heat exchanger: the structure of the kettle heat exchanger is to set up an appropriate evaporation space in the upper part of the casing, and at the same time has the function of a steam chamber. The tube bundle can be a fixed tube plate type, a floating head type or a U shaped tube type. The kettle heat exchanger is easy to clean and maintain, can handle unclean, easy to scale media, and can withstand high temperature and high pressure. It is suitable for liquid vapor heat exchange and can be used as the simplest structure waste heat boiler.
In general, the shell and tube heat exchanger is easy to manufacture, has low production cost, wide selection of materials, convenient cleaning, strong adaptability, large processing capacity, reliable operation, and can adapt to high temperature and high pressure. Although it is not comparable to plate and plate fin heat exchangers in terms of compactness, light heat transfer and unit metal consumption, it has some of the aforementioned advantages and is therefore used in chemical, petroleum and other industries. Still in a dominant position.
The shell and tube heat exchanger connects the tube to the tube sheet and is then fixed by the housing. Its types are roughly divided into fixed tube plate type, kettle type floating head type, U tube type, sliding tube plate type, stuffing box type and casing type, etc., which we briefly introduced. According to the type of medium, pressure, temperature, dirt and other conditions, the various structural types of the connection between the tube sheet and the shell, the shape and heat transfer conditions of the heat transfer tube, the cost, the convenience of maintenance and inspection, etc. Shell and tube heat exchangers.
Shell and tube heat exchanger: is a partition wall heat exchanger which is closed on the wall surface of the tube bundle in the casing as a heat transfer surface. The heat exchanger has a simple structure and reliable operation, and can be made of various structural materials (mainly metal materials). ) It can be used at high temperature and high pressure. It is the most widely used type. (Design and manufacturing follow standards: foreign TEMA ASME domestic GB151, GB150)
Process for connecting heat exchange tube and tube plate in 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.
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 convenient. 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 the successful expansion of the joints of the dense cloth. It is also difficult to repair the joint after failure.
3. Expansion and welding
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.
- (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.
The main points of the process:
- (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 expansion is helpful to solve the leakage and leakage problems when the heat exchange tube is connected with the tube sheet, 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 is firstly more suitable for promotion in high end products.
Source: China Tube Plate Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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