Welding and inspection between pipe and tube sheet

　　In the manufacturing process of heat exchangers, the quality of tube-tubesheet welds has always been one of the key issues in heat exchanger quality control. According to my country's existing technical standards and specifications for heat exchanger manufacturing, surface magnetic particle or penetrant testing is used to evaluate the final quality of heat exchanger tube-tube sheet welds. Due to the inherent limitations of these two methods, magnetic particle testing can only detect surface and near-surface defects of ferromagnetic material welds, and penetrant testing can only detect surface opening defects of non-porous materials, and neither can detect internal defects of welds. Therefore, it is a very important method to use radiography technology to detect internal defects of the pipe-tube sheet weld, improve the quality of the pipe-tube sheet weld, and reduce the leakage rate of the pipe mouth. A company manufactured heat exchangers for Germany's BAYER and other companies. According to the technical requirements, radiographic inspection of the tube-to-tube-sheet welds was performed, which improved the welding quality of the tube-tube-sheet welds. No leakage at the tube mouth was found after use. 　　 1. Radiation source size In radiographic inspection, the clarity of the film should be improved as much as possible to improve the imaging quality of the film and thereby increase the defect detection rate. There are two main aspects: first, select film with fine grain size and low background haze; second, in terms of detection technology, try to reduce the geometric unclarity of the film as much as possible. In the radiographic inspection of pipes and tubesheet welds, AgfaD4 model is used for the film; in terms of inspection technology, the geometric ambiguity of the film depends on the relationship between the size of the radiation source F, the distance D from the radiation source to the transilluminated weld or other workpiece, and the distance d from the radiation source side of the transilluminated weld or workpiece to the film, see Figure 1 and formula (1). In formula (1), the size of Ug is directly proportional to the size of F and d, and inversely proportional to the size of D, because d is constant. To make Ug as small as possible, the only way is to make F as small as possible and D as large as possible. 　　After comparison, the Ir-192 radioactive source produced by the American AEA Company was selected. Its focus size is 0 5 × 0 5 mm, which is currently the smallest focus size civilian radioactive source. After being used to meet the requirements, the film imaging reaches the predetermined standard. The selection of D, that is, the determination of the focal length, will be discussed below. 　 　2. Determination of focal length. Due to structural reasons, only the radioactive source can be used for off-center diameter transillumination of the connecting weld between the tube and the tube sheet, as shown in Figure 2. In formula (1), since Ug is inversely proportional to D, according to convention, to make Ug as small as possible, D should be made as large as possible. However, when the deviation distance is large, the imaging quality of the film will be poor due to factors such as scattered rays. To this end, two specifications of specimens, 25×2 and 20×2, were produced, and artificial defects of different sizes were made on the same weld, with diameters and depths of 0.8, 1.0 and 1.5mm respectively. Different focal lengths were used for transillumination tests. After repeated comparison tests, the focal length was selected between 15 and 25mm. The imaging quality of the film was ideal. When the focal length exceeded 25mm, the imaging quality of the film gradually decreased. Since it is difficult to process hole-shaped artificial defects smaller than 0.8×0.8mm, no transillumination test of hole-shaped artificial defects smaller than 0.8×0.8mm has been conducted. Therefore, the minimum size at which hole-shaped defects can be identified on the film is not yet clear. 　　 3 Protection of scattered rays In the process of radiographic inspection of the welds connecting pipes and tube sheets, the shielding and protection of scattered rays is an important issue. Since the range of the weld between the tube and the tube sheet is very small, without appropriate scattered radiation protection measures, the clarity of the film cannot meet the film evaluation requirements, and artificial defects on the workpiece cannot be identified at all on the film. At the beginning, a relatively thick intensifying screen was used, and 1.6mm lead screens were used for both the front and rear screens. As a result, the imaging quality of the film was not ideal. Later, a commonly used lead intensifying screen with a thickness of 1.0 mm was used, and a shielding block was added to the tube. In this way, the shielding of scattered rays was very effective, and the imaging quality of the film was significantly improved. Since the shielding block actually plays the role of a compensation block, the material and thickness of the shielding block were selected. A 21×10mm steel block and a 21×2mm lead block were used to test the shielding and compensation effects. After experimental comparison, it was proved that the effect of using the 21×2mm lead block as the shielding and compensation block was ideal. The film taken with the 21×2mm lead block as the shielding and compensation block had better imaging quality than the film taken with the 21×10mm steel block as the shielding and compensation block. In addition, lead blocks are easier to make and easier to place during operation. Therefore, in actual transillumination, lead blocks are used to protect and compensate for scattered rays. 　 　4 Selection of image quality meter Since there are currently no corresponding standards and specifications for radiographic inspection of welds connecting pipes and tube sheets, we therefore selected two image quality meters, hole-shaped and linear, for testing based on the principles of simplicity, reliability and practicality. On the 21×10mm steel block that serves as shielding and compensation, a set of hole-shaped artificial defects of 0.8×08mm, 10×10mm and 15×15mm are processed. The test results show that the three hole-like artificial defects can be clearly identified on the film, which is consistent with the transillumination results of the artificial defects processed on the pipe and tube plate connection welds. The linear image quality meter uses the JB4730 standard. The image quality meter labels are 9# and 7#, and the wire diameters are 0.5mm and 0. The 8mm steel wires were tested separately. The steel wires were bent into a ring shape, with a diameter about the diameter of the weld connecting the pipe and tube plate, and placed exactly in the middle of the weld. As a result of the transillumination, the film can clearly show the 9# and 0.5mm image quality meter. Therefore, in actual use, 9#, 0.5mm steel wire is used as the image quality meter as the basis for evaluating film quality. 　 　5. Placement of the ray source As can be seen from Figure 2, there are two ways to place the ray at this position. First, penetrate it from the back of the tube; second, place it from the front. Both methods can be used, but since the first method requires a relatively long transmission tube of the radiation source, the positioning of the radiation source is also more difficult, especially for U-shaped tube bundles. This problem is more prominent; while the second method can avoid these two problems, but the shielding lead plate, film and shielding block must leave penetration holes for the radiation source. This brings some trouble to the packaging of the film. To this end, some simple tooling is designed, which not only makes the cutting and packaging of the film faster and more convenient, but also enables the packaging of the film, the placement of the shielding lead plate and shielding block, and the positioning of the radiation source to be solved at the same time. In order to make the placement of the radiation source more convenient and faster, the delivery tube at the front end of the radiation source is processed into a stainless steel hard straight tube, making the placement of the radiation source easier and the focal length easier to control. Compared with the first method, * * Improved work efficiency. 　 　6 Evaluation of negative films If there are no special provisions on the blackness of negative films, it can be controlled with reference to relevant standards. Due to domestic film viewing light brightness restrictions and the special requirements for this type of weld film evaluation, the blackness range of the film is controlled at 1.8 to 2.5. For the assessment of weld defects in the connection between pipes and tube sheets, since there are no relevant standards or specifications to refer to, the assessment can only be based on the customer's product requirements. For example, the German BAYER company's requirements for the assessment of defects in the welds connecting tubes and tube sheets of its heat exchangers are: (1) No unfusion, cracks and long pores; (2) Single pore or slag inclusion: no more than 0.5×S (S is the wall thickness of the tube); (3) The outer diameter of the tube ≤ 20mm: less than 0 There should be no more than 3 single pores or slag inclusions of 5×S, and the defects must not be at the root of the weld bead; (4) Pipe outer diameter >20mm: less than 0. There should be no more than 5 single pores or slag inclusions of 5×S, and the defects must not be at the root of the weld bead. 　 　7 Conclusion There is no corresponding radiographic inspection of the welds connecting pipes and tube sheets. * * Standards and specifications can be followed. Therefore, problems encountered during the detection process can only be tested one by one, and the good ones can be saved and the bad ones can be eliminated. However, due to limitations of human and financial resources, it is impossible to conduct in-depth tests and research on some issues, such as the relationship between the type and size of defects on the strength (pull-off force) and leakage of the weld connecting the pipe and the tube plate. This has a very important impact on the quality control of the weld connecting the pipe and the tube plate, as well as the process requirements for radiographic inspection technology, and the classification and grade assessment of defects on the film. As radiographic testing technology is increasingly used in the detection of such welds, relevant standards and specifications should be formulated as soon as possible to standardize the radiographic testing of welds connecting pipes and tube sheets. ——Source of information: China Pharmaceutical Machinery and Equipment Network

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There is a mobile tube-sheet all-position argon arc welding technology. The machine head is hand-held. But if the tube-sheet all-position argon arc welding is used on site, will the site conditions be met? Moreover, this method requires higher processing requirements for the welding parts, including cleaning and polishing. RT testing can also be performed after welding, but the cost is high and can the construction period be guaranteed? Since it is a temporary repair and replacement on site, the first priority is to ensure the quality and construction period. In fact, it is completely possible to use manual welding in this case. The key is to clean the welding parts before welding.