The cylindrical shell and head components are joined together with girth welds (Category B welds). These welds are usually made using a welding procedure employing a combination of automatic SAW and manual SMAW that is very similar to those described for longitudinal welds.
The cylindrical shell (or head) components to be joined together are positioned horizontally on rolls that can revolve the mating components in unison for girth welding. ASME Code, Section VIII, Divisions 1 and 2, requires obtaining the following tolerances for misalignment of the girth welds (Paragraphs UG-80 and AF-142):
Section Thickness (t), in. Misalignment Tolerances
Up to 1/2, incl. 1/4 t
Over 1/2 to 3/4, incl. 1/4 t
Over 3/4 to 1 1/2, incl. 3/16 in.
Over 1 1/2 to 2, incl. 1/8 t
Over 2 1/8 t (3/4 in. max.)
These tolerances are somewhat larger than required for longitudinal welds (see Section 651). Nevertheless, it is almost always necessary to use clips or strong backs (temporary attachments) welded to the mating components to obtain fit-up within these limits, and to assure that they are maintained during welding. The company inspector should observe the fit-up of several of these welds to make certain that the fabricator is exercising the good workmanship practices necessary to achieve fit-up within the required limits.
As for longitudinal welds, most of the welding is performed by SAW from the outside or inside of the shell. The mating shell (or head) components being welded are tacked together, and are then revolved in unison under the welding head, so the weld metal is deposited in the groove between the two components. Subsequently, the unsound SAW weld metal in the root pass is removed by back gouging or grinding, and the weld is completed by manual SMAW from the opposite side of the shell. The discussion for welding of longitudinal joints (see Section 651) also applies to girth welding.
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