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This post was last edited by ztr1118 on 2012-4-1 20:13. Topic: Analysis and discussion on hydraulic strength expansion joining alone for shell-and-tube heat exchangers. Keywords: heat exchanger, strength expansion joint, hydraulic expansion joint, analysis and discussion. Thread link: http://bbs.hcbbs.com/thread-959889-1-1.html (1) ztr1118: According to GB151, a strength expansion joint is defined as one that ensures the sealing performance and tensile strength of the connection between the heat exchanger and the tube sheet. The applicable range of strength expansion joining is: (1) design pressure less than or equal to 4 MPa ； ⑵ Design temperature is less than or equal to 300℃ ； ⑶ There is no severe vibration during operation, no excessive temperature changes, and no significant stress corrosion. To simplify the variables in the discussion, the following assumptions are made: Heat exchanger tube material: 00Cr19Ni10, specification Φ19×2. Tube sheet material: 16MnⅢ with a surfacing layer of 00Cr19Ni10; the thickness of the surfacing layer is 14 mm, and the total thickness of the tube sheet is 120 mm. Connection method: Hydraulic strength expansion joining is used exclusively. How can we ensure the sealing performance and connection strength of joints from the perspectives of design, manufacturing, and inspection? The seminar covers the following topics: design of the expansion groove, the part that determines the sealing performance, the part that ensures connection strength, the issues to be addressed in expansion welding process evaluation tests, and how to detect and verify the quality of expansion welded products……… ztr1118: We sincerely invite members from the heat exchange equipment discussion forum to participate in this discussion as well. wangjianwei86: It’s a good question. This issue has been bothering me for a long time; I just can’t figure out why, with such high pressure – it’s less than or equal to 4.0 MPa – there is no leakage once the connections are sealed ； Grooved expansion joints are a bit better: what’s really important is the expansion joints without grooves, those with flanges ； Regarding the few questions you raised: What is primarily used for sealing? For strength-based expansion joining, slots are usually created first, and then the tubes can be expanded into those slots; it’s hydraulic expansion, I guess – I’ve never seen it done otherwise ； Then there’s also a section that expands to fill the gap; it’s these two parts that serve to create a seal! ! What determines the strength of the link? It mainly depends on the slots, after all, there are two slots! ! There’s also that flanging structure, right? ； One more question: do all heat exchange tubes that are joined by strength expansion need to have their edges flanged? ! ! ztr1118: Flanged structures are generally used in the boiler industry; pressure vessel industries usually do not employ this type of structure. Yokoso: Let’s talk about the inspection of crimping: In most cases, it is done by using water pressure or air leakage testing, right? The usual inspection process is as follows after crimping is completed ； Magnetic particle inspection and penetrant testing are no longer effective ； So I think water pressure is the best method for detecting leaks! ! When the pressure on the tube side is high and the pressure on the shell side is low, it’s necessary to conduct an ammonia leakage test. I’ve heard of it, but I’ve never done it myself; it’s said to be quite expensive! ! Hehe (2) ztr1118: Let’s first talk about the setup of the tube expansion groove. Though I’m not very skilled at walking in the snow, I hope this can serve as a starting point to inspire better ideas! In accordance with the requirements of GB151, a expansion groove must be provided for strength expansion joining. However, the dimensions of the expansion groove specified in GB151 are: width of 3 mm and depth of 0.5 mm. I believe that this type of expansion groove is only suitable for mechanical expansion joining, not for hydraulic expansion joining. According to the specifications of version 99, the width of the expansion groove is equal to (1.1–1.3) times the square root of the product of the diameter of the heat exchange tube and its wall thickness. According to other literature, similar regulations and requirements also exist. According to this example, if the coefficient is set at 1.2, the width of the tube expansion groove is 8.14 mm, which is rounded to 8.0 mm, and the groove depth is set at 0.6 mm. Widening the expansion groove helps to force the tube wall metal into the expansion groove during expansion, thereby enabling the expansion joint to achieve a higher pull-out strength that meets the design requirements for tensile resistance. The number of tube expansion grooves is 3. A groove is provided at the surfacing layer; considering that the thickness of this layer is only 14 mm, the width of the tube expansion groove is reduced to 6.5 mm ； Two expansion groove slots are provided in the middle part of the tube sheet, with a slot width of 8 mm. Subsequent expansion joint process qualification tests showed that the pulling stress between the tube and the tube sheet fully met the design requirements, with a considerable margin of safety. After dissecting the test specimen for evaluation, the metal in the tube wall was forced into the expansion grooves, with a filling degree of over 80%. aljs: We use expansion welding for everything here. After all, it’s a pressure vessel, so it’s better to be cautious. 2 F1 I0 X- Q* N: l" ? I’ve never worked with a single expansion joint, so it’s hard for me to give an opinion; I need to learn about it first. ztr1118: In actual engineering projects, structures that combine expansion welding and other methods are quite common. Here, the sealing performance and connection strength of hydraulic expansion joining are primarily discussed. Welcome to join the discussion and share your insights. (III) ztr1118: For a tube/tube sheet joint that uses strength expansion bonding alone, which part is primarily responsible for ensuring the sealing performance? In my opinion, it is provided by the following connection points: 1. The open area of the tube sheet hole — this is the point where the tube and tube sheet are directly expanded and connected. The surface roughness of the aperture area, as well as the presence or absence of longitudinal grooves, have a significant impact on the sealing performance of the joint ； 2. Bottom of the expansion groove – the seal strip formed by the tube wall being pressed into the expansion groove ； 3. The junction of the bulged tube groove and the optical hole — at this location, the tube and the tube sheet form several line contacts (or very narrow surface contacts) to create sealing zones. It is difficult to determine which of the aforementioned locations plays the primary sealing role, as no individual tests have been conducted. Based on the degree of validity of these individual, immature opinions, the order should be 1., 3., 2. The sealing function of the smooth part of the tube sheet holes should be the primary one. (IV) ztr1118: Which part is primarily responsible for the connection strength? For hydraulic strength expansion joining, the design of the expansion groove is very important; for details, please refer to my post on floor 6. Previously, I conducted a series of expansion testing on different materials, specifications, and expansion groove types. Based on the results of the tube/tube sheet pull-out test, the presence of expansion grooves has the most direct and significant impact on the hydraulic strength of the expansion joint. The most typical example is the double-shell and plate heat exchanger, in which the intermediate shell and plate can only be connected by expansion jointing, and this must be a strong expansion jointing; the design of the expansion groove is particularly important. Many technical documents on tube/tube sheet expansion welding also confirm this. (5) ztr1118: Expansion welding process qualification test – This is primarily aimed at determining the hydraulic pressure required for expansion welding of the product through experiments. Expansion joint specimens usually require 8-10 tube holes, which are divided into 3-4 groups. Based on the pipe specifications and existing expansion jointing experience, an approximate range for the hydraulic pressure is determined, such as 140–180 MPa. The expansion jointing pressure can be divided into 3 groups: 140, 160, 180 MPa. Assuming each group consists of 3 tubes, two of them are subjected to a pulling force test, while the other is subjected to an anatomical test. After the expansion joining is completed, the expansion ratio of the smooth bore section is calculated by measuring the dimensions of the heat exchange tube before and after expansion joining. The tube expansion ratio should meet the requirements for strength expansion, primarily to satisfy the sealing requirements. Pull-out force tests were conducted on the two heat exchange tubes in each group. The pull-out stress for strength expansion joining should be greater than 4 MPa. For the other heat exchange tube in each group, it was cut using wire cutting to examine the degree to which the heat exchange tube was pressed into the expansion groove; the filling degree was required to be no less than 80%. Finally, the test results of the three groups are compared and analyzed to determine the hydraulic pressure required for product expansion joining. (VI) ztr1118: How to test and inspect the quality of product crimping. After the expansion hydraulic pressure is determined through expansion process evaluation tests, the products can be subjected to strength expansion joining. Thanks to the support provided by the expansion jointing process evaluation tests, and since the degree of tightness in hydraulic expansion jointing is largely unaffected by tube hole deviations or heat exchange tube deviations, the quality of the expansion joints in these products remains stable with a high level of confidence. The only inspections that need to be carried out are: checking for any leaks or over-sealing, verifying that there is no sealing in areas where it should not be, and inspecting the appearance of the sealed areas. In product expansion joining, in principle, it is no longer necessary to use the expansion ratio to control and verify the degree of tightening during expansion joining. It greatly simplifies the numerous measurements and calculations required in mechanical expansion joining, thereby improving work efficiency. Finally, a pressure test (and sometimes a tightness test) is used to check for any leaks in the connections. If there is slight leakage at the joints, and it is determined that this is not caused by over-inflation, then increasing the hydraulic pressure slightly and inflating again usually resolves the issue.