Welding issues of the tube sheet in fixed-tube-sheet heat exchangers and the shell
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For fixed-tube-sheet heat exchangers, we generally use structure (b) from Appendix G of GB151 for the weld between the tube sheet and the shell. However, for some reason in practice, this weld passes the 20% UT non-destructive testing but fails when subjected to 100% UT testing. Could it be due to that stress groove? Is it still due to the misalignment amount during the connection? Or is there something wrong with our manufacturing process? I hope those who are from Sichuan can provide an answer
Personally, I think the structure choice is not appropriate; if filming is intended, it is recommended to choose D
But ordinary fixed tube sheets use that structure anyway. If you have a better one, could you please introduce it? It’s difficult to attach plates in the case of RT
Can radiography be performed using the (b) structure in Appendix G of GB151? How to apply the patch? Use a γ source? According to clause 3.16.1 of GB151, it is not necessary to take X-ray photos for this welded joint; PT or RT inspection can be used instead.
Double-sided welding is used for welding, and the structure selection also needs to be taken into consideration; it has little to do with edge misalignment.
Can this structure use double-sided welding? Can people go in? This structure can only be constructed using TIG welding for the root pass or a fully penetrative welding method with backing plates
Generally, before welding the tube sheet to the shell, the heat exchange tubes have already been inserted, so it’s not possible for anyone to get inside. This is even more difficult in the case of tubes with small diameters. The structure with gaskets you mentioned corresponds to structure C in 151, but we usually use structure B, as it’s simpler. We also use TIG welding for the root pass. But as I asked in my question, we’re not sure what the issue is; the structure specified in the standards should be fine, so it’s unclear where the problem lies in the manufacturing process. I seek advice from those who are more experienced.
Stress grooves do not cause the aforementioned problems; their main function is to ensure thorough welding between the tube sheet and the cylinder, as well as to relieve some of the stress. Regarding the issue you raised, I believe there could be several reasons for it: 1. Whether the winding of the cylinder body and the alignment of the blunt edges with the tube sheet flanges are proper; in other words, there may be a misalignment issue. If such an issue exists, the X-ray images will show defects that resemble cracks. 2. TIG welding should be used for the root pass, with argon shielding applied to the back side; otherwise, the tissue on that side will take on a slag-like appearance, and defects similar to pores will be visible in the X-ray images.
You are right; I think the misalignment issue is the main problem, and sometimes there is also a problem with incomplete welding. We generally do not use RT because it’s difficult to apply the probes, so we usually use UT. Do you have any good suggestions regarding the method of aligning the cylinder and the tube sheet? For example, regarding what kind of tooling to use, thank you.
This post was last edited by ztr1118 on 2012-4-28 at 15:11. 1. For fixed-tube-sheet heat exchangers, the structure at the weld between the tube sheet and the shell adopts structure (b) in Appendix G of GB151. This structure is a butt-welded structure. Since it is not possible to enter the interior of the housing, single-sided welding can only be performed from the outside. If the assembly quality is good and backside argon shielding is available, full penetration and double-sided formation can be achieved ； Otherwise, it is difficult to achieve double-sided forming, and the root area may not be fully welded. Therefore, the assembly quality of this structure is very important, as it directly affects the welding quality. Typically, for such structures, it is only required to ensure the welding quality from a technical standpoint; surface penetrant testing is carried out after bottom welding and full welding, while radiographic testing or ultrasonic testing of the welds is not required. 2. This structure cannot be patterned, and the weld joints cannot be radiographically inspected. 3. This structure cannot also undergo effective ultrasonic testing. Regulations and standards stipulate that the technical grade for ultrasonic testing of butt joints shall be no lower than Grade B. The acceptable grade is Level I (full non-destructive testing) or Level II (partial non-destructive testing). According to JB/T4730.3, for technical grade B, inspection must be carried out on one side on both sides or on one side on both sides. Due to structural reasons, this weld can only be inspected on one side only. Strictly speaking, this weld does not meet the conditions for ultrasonic testing. If limited one-sided ultrasonic testing is carried out, setting aside the requirements of specifications and standards, only incomplete inspection data can be obtained. Only the weld quality at the local area of the joint can be detected, and it is difficult to assess the quality grade. It can only be said that some defects can be detected; testing is better than not testing, which gives one some idea at least. …… I’m not a specialist in non-destructive testing. I can only share my views on this issue based on the depth of my understanding. If there are any mistakes, please point them out.
Thank you for your comprehensive answers; I’ve learned a lot from them. It seems I need to focus more on the alignment process. Indeed, the standards do not require UT testing for this weld, so as you said, performing such testing is merely to give everyone a sense of assurance!