METHODS OF TUBE ATTACHMENT IN WATER TUBE BOILER



Tubes can be attached to drums and headers by expanding or by welding. In most cases the generating, screen, and water walI tubes are expanded into plain seats, and then bell-mouthed. The tube ends must be cleaned, and then carefully drifted, or roller expanded into the holes in the tube plate. 
They must project through the tube plate by at least 6 mm. To prevent tubes pulling out of the tube plate, they must be bell-mouthed. This bell-mouthing is to be I mm for every 25mm of outside diameter
plus 1·5mm. 
Superheater tubes are also usually expanded and bell-mouthed up to steam temperatures of about 450°C; 

In the case of tubes with larger diameters, such as downcomers, it is usual to use grooved seats. The tube material flowsinto the grooves during the expansion process so helping to form a tight seal.




Temperatures of about 450°C; above this value the tubes are often attached by welding. As can be seen in  a stub tube is interposed between the actual tube and the header. Two methods of welding are shown. 

In the gas-welded tube a backing ring may be required to prevent weld metal breaking through into the tube bore. 
In the other type, backing rings are not necessary due to the specially shaped tube end. 

Electric welding may be used. However, the tube must make a ninety degree bend close to the joint in order to prevent direct tensile stresses coming onto the weld.

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