A brief discussion on the manufacturing process for the combined expansion welding connection of heat exchanger tube sheets and heat exchange tubes

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Standard GB151－1999 specifies that strength expansion joining is suitable for applications with a design pressure of ≤4 MPa, a design temperature of ≤300°C, no severe vibrations, no significant temperature fluctuations, and no stress corrosion； Strength welding is suitable for applications with low vibration and no gap corrosion ； The combination of expansion and welding is suitable for applications requiring high sealing performance, exposure to vibration or fatigue loads, susceptibility to gasket corrosion, and the use of composite tube sheets. It can be seen that the application conditions for connection methods such as simple expansion joining or strength welding are limited. The expansion-welding combined structure is widely used because it can effectively dampen the vibrations of the tube bundle, thereby preventing damage to the weld seams and avoiding gap corrosion; moreover, it offers higher strength and sealing performance compared to expansion bonding or pure welding. Currently, for conventional heat exchange tubes, the \"expansion fitting + strength welding\" method is commonly used ； For heat exchangers that are important or operate under severe conditions, a \"strength expansion + sealing welding\" approach is required. The expansion-welding combined structure can be divided into two types based on the sequence of expansion and welding in the manufacturing process: expansion first followed by welding, and welding first followed by expansion. 1 Expand first then weld: After the tube and the tube sheet are expanded together, a 15 mm long section of the tube that has not been expanded should remain at the tube end, in order to avoid the accumulation of expansion stress and welding stress, thereby reducing the impact of welding stress on the expansion process. There is a gap between this 15 mm long unexpanded section of the tube and the holes in the tube sheet (see Figure 1). During welding, due to the effect of the high-temperature molten metal, the gas in the gap is heated and expands rapidly. According to foreign sources, the pressure in the gap chamber can reach an ultra-high level of 200–300 MPa at the weld joint. The high-temperature, high-pressure gas in the clearance chamber causes fatal damage to the sealing performance of the gasket when it escapes, and pinholes that are difficult to detect with the naked eye also form at the weld seams. The mechanical expansion jointing method commonly used today is highly sensitive to welding cracks and pores; as lubricant penetrates into these gaps, defects arising during welding become even more severe. These oil stains that have penetrated into the gaps are difficult to remove completely; therefore, the expansion followed by welding process should be used, rather than mechanical expansion. Since gasket expansion is pressure-intolerant, it can nevertheless eliminate the gap between the tube and the tube sheet holes, thereby effectively damping the vibration of the tube bundle at the welded areas at the tube ends. However, conventional manual or mechanically controlled mechanical expansion cannot meet the requirements for uniform expansion, whereas a liquid-bag type tube expander that uses computer-controlled expansion pressure can easily and uniformly achieve such expansion requirements. When using a liquid-bag type expansion tube machine for expansion joining, in order to achieve ideal results, the dimensional fit between the tube and the holes in the tube sheet prior to expansion joining must meet quite strict requirements in terms of design and manufacturing. Only in this way can, for the conventionally designed \"expansion + strength welding\", the method of expansion followed by welding be adopted; whereas for the specially designed \"strength expansion + strength welding\", the method of expansion first, then strength welding, and finally strength expansion can be used. http://www.nmtech.com.cn/jishuwang/upload1/0706201121122138.jpg2 Weld first and then expand. During the manufacturing process, a heat exchanger contains a considerable number of heat exchange tubes; there is a large gap between the outer diameter of these tubes and the diameter of the holes in the tube sheet. Moreover, the gap between the outer diameter of each heat exchange tube and the holes in the tube sheet varies along the axial direction (see Figure 2). When expansion jointing is performed after welding, the center line of the tube must coincide with the center line of the tube hole in the tube sheet. When the gap is small, the 15 mm unexpanded section at the upper end will help reduce the impact of expansion joint deformation on welding. When the gap is large, due to the high rigidity of the tube, excessive expansion deformation will exceed the cushioning effect of the 15 mm unexpanded area, causing damage to the welded joint and even leading to weld separation. Therefore, for the welding-first then expansion process, controlling the accuracy of the tubes and tube sheet holes as well as their fit is the primary issue. When the gap between the tube and the tube sheet cavity becomes small enough, the expansion jointing process will not compromise the quality of the welded joint. According to available data, the welded joints at the pipe ends have a considerable capacity to withstand axial forces; even in the case of sealed welding, when a static pull-out test is conducted, the pipe breaks, but the weld joint does not separate. However, the weld joint has a relatively poor capacity to withstand tangential shear forces; therefore, after welding, due to inadequate control, it may lead to failure due to excessive expansion or damage to the welded joint as a result of the expansion process. http://www.nmtech.com.cn/jishuwang/upload1/0706201121449127.jpg3 Rational manufacturing processes 3.1 Tolerance control for tubes and tube holes (1) Heat exchange tubes: When purchasing heat exchange tubes, it is necessary to ensure that those used in each heat exchanger are made from the same raw material batch obtained through cold drawing, and that they are produced on the same tube-drawing machine that has passed verification tests. This ensures that each heat exchange tube has the same material properties, specifications, and precision. Uniformity in the outer diameter of the heat exchange tubes ensures an appropriate gap between the tubes and the tube holes in the tube sheet, while uniformity in the inner diameter ensures compatibility with the expansion heads of the liquid-bag type tube expansion machines, thereby extending the service life of these expansion heads. The gap between the tube and the tube sheet holes is generally required to be within the range of (0.3±0.05) mm, and the tolerance between the outer diameter of the expansion head of a liquid-bag type tube expander and the inner diameter of the tube should also be kept within the range of (0.3±0.05) mm. (2) Tube sheet: To ensure that the bore diameter of the heat exchanger tube sheet is within the same tolerance range as the outer diameter of the tubes, it is first necessary to determine the machining precision of the tube sheet bores based on the actual precision dimension of the outer diameter of the tubes upon arrival. As mentioned above, the gap between the tube sheet bores and the actual uniform outer diameter of the arrived tubes should still be kept within the range of (0.3 ± 0.05) mm. 3.2 Processing and acceptance of heat exchange tubes and tube sheets (1) Heat exchange tubes ① The heat exchange tubes purchased as required shall be inspected item by item in accordance with relevant standards before being stored, with precise measurement of their inner and outer diameters as well as their tolerance ranges. ②Before inserting the heat exchange tubes, cut them to the appropriate length based on the actual measured length of the shell side, to avoid having to use a grinder to trim them after insertion. When grinding is carried out using a grinder, the abrasive particles from the wheel can easily get into the gaps between the tubes and the tube sheet holes. Silicate abrasive particles can cause slag inclusions during welding, posing a risk to the welded joint. ③Before the heat exchange tubes are inserted, the tube section within the expansion range should be degreased, and any burrs on the inside and outside of the tube ends must be removed; this is particularly important when using liquid-pocket expansion heads. (2) Tube sheet ① The tube sheet shall be a qualified forged piece, with uniform internal material and no defects on the expansion joint surface that could affect the quality of expansion joining. For the critical heat exchangers in the unit, high-grade forgings should be used as much as possible; in addition to being inspected in accordance with relevant standards, these forgings should also undergo ultrasonic testing. ②The coaxiality of the tube holes in the tube sheet and baffle plates must be ensured. Drill using the same template to ensure that the tube holes in the tube sheet and baffle through which each heat exchange tube passes are on the same center line; otherwise, it will cause great difficulties in inserting the tubes. ③The drilling finish of the tube sheet and the width of the tube bridge on the tube sheet are both to be inspected in accordance with the Grade I requirements of GB151-1999. ④The accuracy of the tube holes is checked using self-made go and no-go gauges, and records are kept. If a drilling diameter of φ(25.4 ± 0.05) mm is required, use a plug gauge of φ25.45 mm and a go gauge of φ25.3 mm; each hole should be checked individually, and holes that fall outside the specified range should be marked so that special measures can be taken to address them. ⑤In the case of strength expansion, the depth of the expansion groove should be maintained within the range of (0.5 ± 0.05) mm. For the liquid bag expansion joint method, based on current research results, it is recommended that the groove width be 8 mm and the groove spacing also be 8 mm; a dual-groove structure is generally used. ⑥Before expansion joining, the pipe holes must be thoroughly cleaned to remove burrs from the edges of the grooves; any impurities that could affect the tightness of the expansion joint are not allowed. 3.3 Connection of pipes and tube sheets (1) Expansion joining: The liquid-bag type hydraulic expansion joining method is recommended to ensure uniform tightness in the expansion joint. Because in the liquid-bag type expansion tube machine, the expansion pressure is set manually, with computer-controlled operation, ensuring high precision. For carbon steel heat exchange tubes with dimensions of 25×2.5, the pressing pressure for bonding is typically 110–120 MPa, while the strength pressing pressure is 170–180 MPa. When using heat exchange tubes with special specifications, theoretical calculations can be carried out first, followed by simulation tests to determine the appropriate hydraulic range for expansion and strength expansion, in order to ensure the reliability of the expansion joint. (2) Welding generally employs tungsten inert gas welding with filler wire. The weld height H is ensured to be no less than 1.4 times the wall thickness of the tube. Double-layer TIG welding is employed, with the start point of the second weld pass being at least 15° away from the start point of the first weld pass, in order to eliminate defects that may occur in the first weld pass, especially at the start and end points. (3) Connection method: When the drawing specifies “expansion fitting + strength welding”, the following two methods can be used: ① Expansion fitting (water filling for leak testing) ； Strength welding (hydrostatic test). ②Strength welding (pressure test) ； Pressurization (hydrostatic test). When the tube sheet holes exceed the specified dimensions, expansion bonding should be carried out first, followed by welding, to prevent the expansion bonding process from affecting the quality of the welds. When the drawing is designed with \"strength expansion + seal welding\", it is recommended to use the following method: expansion bonding (water filling for leak testing) ； Seal welding (pressure test) ； Strength expansion (hydrostatic test). 4 Conclusion: Domestic heat exchangers have a relatively reduced service life due to the fact that their basic materials, processing precision, and manufacturing methods have not been optimized in combination. Heat exchangers with a expansion welding and brazing combination structure, which are currently widely used, can, taking into account China’s national conditions and through a series of quality control measures, be manufactured to be of high quality, long-lasting, and satisfactory to users. This post was last edited by eddy-8280103 on 2009-2-21 07:56.]