Process Requirements for Shell-and-Tube Heat Exchangers

In the manufacturing process of shell-and-tube heat exchangers, three aspects-, connection between tubes and tubesheets of the shell-and-tube heat exchanger-have high requirements for welding processes.

Hydrogen removal treatment and post-weld heat treatment must be performed on welds. During the welding process, hydrogen from electrodes, fluxes, and the air decomposes into atomic form at high temperatures and dissolves in the molten metal. When the weld cools, the solubility of hydrogen in steel decreases sharply. Due to the rapid cooling of the weld, hydrogen cannot escape in time and remains in the weld metal, forming delayed cracks after a period of time. After welding, the welds should be heated to 200℃ for 16 hours to conduct hydrogen removal treatment.

Post-weld heat treatment involves heating the entire or local part of the welded workpiece to a temperature below the Ac1 line (phase transformation point) for heat preservation, followed by furnace cooling or air cooling. Its main purposes are to eliminate and reduce the stress generated during the welding process, prevent cracks (hydrogen-induced cracks) in the welded structure, and restore the mechanical properties lost due to cold working. It should be noted that for the channel box equipment flange, to ensure its sealing performance, it is often required to machine the sealing surface after integral heat treatment.

, . Different groove types and welding processes are adopted according to different materials and thicknesses.

Shielded metal arc welding is the most widely used welding method. It has flexible operation and simple equipment, and can be used for all-position welding. However, the welding quality largely depends on the welder's technical level.

Submerged arc welding has high arc heat utilization rate, fast welding speed, and high productivity. It can save metal and improve working conditions, but due to its limitations, it is generally only used for welding straight welds and large circumferential girth welds.

For example: For the longitudinal and circumferential welds of the cylinder (when δ >= 18mm), shielded metal arc welding can be used for back-gouging first; after passing the test and inspection, submerged arc welding is used to fully weld the joint. Since the heat exchange tubes are relatively thin, gas tungsten arc welding (GTAW) is used for welding between the tubesheets and heat exchange tubes, followed by expansion using a tube expander.

There are several common connection methods:a. Strength welding plus seal expansionb. Strength expansion plus seal weldingc. Strength weldingd. Strength expansion

Currently, the combination of expansion and welding is widely used. This method can improve the fatigue resistance of the joint and eliminate stress corrosion and crevice corrosion. For the inspection of welded tube joints, liquid penetrant testing (LPT) or magnetic particle testing (MPT) is generally used to control the joint quality, and there are no inspection requirements for internal defects of the welded joints.

Tubesheets are manufactured by machining, and their hole diameter and hole pitch have tolerance requirements according to different tube bundles. Drilling can be done by scribing drilling, jig drilling, or numerically controlled machine tools. However, when scribing drilling is used, the accuracy is relatively low. Therefore, when drilling the tube holes of baffles, the tubesheets and baffles must be overlapped for match drilling.

After drilling, the baffles should be numbered in sequence and marked with an orientation diagram to facilitate assembly.

Baffles should be drilled as entire circular plates; after drilling, they are scribed and cut into the required shape.

In the manufacturing of heat exchangers, the manufacturing processes of parts such as cylinders and heads are the same as those of general vessels, but with different requirements. Key points include strict material inspection, match drilling of tube holes in tubesheets and baffles, welding of cylinders, and machining of flanges. Looking at the overall manufacturing process, most of them use traditional processes, among which welding accounts for a relatively high proportion. Therefore, welding must be performed in strict accordance with the welding process, and weld flaw detection must be carried out.

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