Points for Attention in Welding Process of Heat Exchanger

        Heat exchangers are widely used in various fields as heat transfer equipment. The connection methods of heat exchange tubes and tube sheets include expansion, welding, and expansion welding. Among them, welding is the most commonly used connection method, and the welding process is good or bad. , Often determines the quality and service life of the heat exchanger.

• Common quality defects of heat exchanger tube sheet welding：

         The stress of the welded joints of the tube sheet and the heat exchange tube is complicated, and the heat exchanger itself has many processes and high requirements in the manufacturing process. Some aspects are often overlooked, and defects are prone to occur. It is difficult to find in a few overpressure inspections. Since welding defects can easily induce expansion, welding quality problems in the manufacturing process are extremely important for heat exchangers. Common quality problems fall into the following categories:

1.The welding length does not meet the requirements

        Tube sheet processing grooves are often too small during manufacturing. When the wall thickness increases, it must be increased appropriately, but it is often not achieved in practice. In addition, the extension length of the heat exchange tube cannot accurately meet the design requirements. In the actual manufacturing process, due to factors such as poor assembly and blanking control, and even due to welding habits of welders, the required size is often not achieved. In this way, the welding length must be less than the specified requirements, and its carrying capacity will decrease.

 2.The processing method before welding is not good

        In the manufacturing process, it is common that the end of the carbon steel heat exchange tube is incompletely cleaned or the tube head is left for a long time after cleaning, resulting in rust, which leads to an increase in impurities in the weld, which greatly affects the welding quality.

3.Improper welding method

        When manual arc welding is used, arc ignition and arc extinction occur directly on the fillet weld, or welding at the vertical position of the tube sheet to form the weld at one time will directly lead to slag inclusion and porosity defects.

• The key points of the heat exchanger tube sheet welding process construction:

1.When performing the bottom welding of the tube-sheet combined joint of the welding cylinder, the principle should be to prevent molten iron from flowing into the cylinder. To implement effective welding measures, the welding method and operation method with weak arc penetrating power should be selected for bottoming:

         If there is no gap between the tube-sheet and cylinder pair, manual arc welding with Φ3.2 electrode should be used for bottom welding, and the welding current should not exceed 120A.

         When there is a gap (greater than 1mm) between the tube sheet and the cylinder, manual arc welding or argon arc welding should be used before the assembly, and the inner backing plate of the cylinder should be used for manual arc welding or argon arc welding to prevent molten iron from flowing into the heat exchange tube. The bottoming current does not exceed 120A.

         For tube sheets with a gap of less than 1mm, the cylinder joints can be welded by manual arc welding vertical welding or argon arc welding. During the bottom welding of the combined welds of the tube-to-tube plate, it is strictly forbidden to use welding rods with diameters above Φ4.0mm for manual arc welding or gas shielded welding. Because of the high current and strong arc penetration, it is easy to weld through.

         After the bottom welding, the heat input should be strictly controlled during the welding of other layers, and the welding heat input should be reduced to reduce tube sheet deformation, resulting in insufficient tube length, deformation of the tube sheet sealing surface, and poor sealing with the tube box assembly.

2.The welding sequence adopts symmetrical and uniform welding; some anti-deformation measures are appropriately adopted, and the tube sheet is pre-compressed inward before welding. For the butt welding of non-ferrous metal sheets, the straight-line gun method should be used, and the lateral swing is not allowed.

Source: "Protection Project" in 2018 on 21 of: Wang Xiaolong