Prevention of tube sheet welding deformation

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Prevention of welding deformation in tube sheets: In heat exchanger manufacturing, welding deformation of tube sheets often occurs due to improper sequencing of assembly and welding, as well as inappropriate selection of welding parameters. This deformation worsens the stress conditions on the tube sheets, leading to poor sealing and even the detachment of tubes. This article provides a brief overview of common types of welding deformation in tube sheets and methods for preventing them. I. Common forms of tube sheet welding deformation: The two most common types of tube sheet deformation after welding are shown in Figure 1. In the manufacture of U-tube heat exchangers, the two types of deformation depicted in Figure 1(A) occur frequently. Figure 1: (A) Arch-shaped deformation; (B) Wave-like deformation. The reasons for the above deformations are as follows: 1. Improper sequencing of assembly and welding; 2. Incorrect welding sequence ； 3. Incorrect welding direction ； 4. Unreasonable welding parameters ； Causes local overheating ； 5. Appropriate auxiliary measures were not adopted. II. Welding of the tube sheet 1. Fixed-tube-sheet heat exchangers: Since welding is required at both ends of the heat exchange tubes in this type of heat exchanger, and the tubes become rigidly fixed after welding, welding one end of the tube sheet affects the other end. Therefore, the welding is carried out following the specified sequence and based on the principles for selecting welding parameters, with necessary auxiliary measures taken as well. (1) Assemble the tie rods, spacers, and baffle plates onto the tube sheet, then pass the tubes through; install the tube bundle inside the cylinder and align the tube sheet with the cylinder. Next, align the tube sheet at the other end with the cylinder as well, guide the tubes out of the tube sheet, and adjust their protruding length. (2) First, weld the pipes in the central area of the tube sheet; the number of pipes to be welded should not be less than 1/4 of the total number of pipes. After welding 1/2 of the pipes at one end, weld all the pipes at the other end, and then complete welding of the remaining 1/2. The order of welding the pipes to the tube sheet should be radial, from the inside outward, and in a symmetrical manner, as shown in Figures 2 and 3. (3) Welds between the welded tube sheet and the cylinder. Each layer of weld shall be constructed in segments using symmetric operations, as shown in Figure 4. The welds in each layer should be offset by 180°; after welding one layer at one end, a layer is welded at the other end, and this process is repeated until welding is complete. Figure 2: Tubes in the middle section; Figure 3: Soldering of the tubes in the middle section – schematic diagram (within the circle). Sequence: Subsequent steps follow the principles shown in the diagram. Figure 4: Welds between the tube sheet and the cylinder body; Figure 5: Welding sequence for each layer of tubes. As shown in Figure 4, each layer is welded according to the principles indicated in the diagram, while in Figure 3 the same sequence for each layer should be shifted by 180°. (4) For welding the remaining tube ends, start by completing the welds at the ring joint between the tube sheet and the cylinder body, then proceed layer by layer outward, alternating between the two tube sheets in a symmetrical manner, as shown in Figure 5. (5) While ensuring the quality of the weld joint, it is advisable to use welding parameters with low values; this helps to avoid deformation caused by local overheating. CO2 welding should be used for the root pass, followed by TIG welding for the cover pass. (6) Supplementary measures. There are two common methods: one is to tighten a part with high rigidity (such as a matching flange) in order to resist deformation ； Second, enhance the cooling effect to avoid deformation caused by uneven heating. 2. U-tube heat exchangers: Since U-tube heat exchangers have only one tube sheet and the rigidity of their tubes is also lower than that of fixed-tube heat exchangers, they can generally be handled in the following sequence and using the corresponding measures. (1) Assemble the tie rods, spacing tubes, baffle plates, etc., then pass the tubes through; after all the tubes have been inserted, perform tack welding. Next, install the tube bundle inside the cylinder and assemble the tube sheet with the cylinder. (2) Weld 1/4 of the total number of tubes to the tube sheet in the sequence shown in Figure 6. Figure 6: Welding sequence of pipes on the U-shaped tube sheet (all subsequent sequences follow the principles shown in this figure). (3) Weld the joints between the tube sheet and the cylinder in accordance with the sequence in Figure 4, with each layer being offset by 180° from the previous one. (4) Weld the remaining pipe joint welds in the order shown in Figure 6. (5) The principles for selecting welding parameters and auxiliary measures are the same as those for fixed-tube-sheet heat exchangers. III. Conclusion Through production experience, the aforementioned welding sequence and principles for selecting welding parameters, combined with appropriate auxiliary measures, can effectively control the welding deformation of the tube sheet.