Many requirements for welds in the ASME code depend on a weld’s category and its type, but those two classifications often get mixed up. So what are they, and what is the difference between them? A weld’s category is based on its location on the vessel, and a weld’s type is determined by how it is welded. For example, longitudinal shell seams are category A, circumferential seams are category B, and other categories are shown in Figure UW-3:
A Type 1 vessel weld is a butt joint, welded from both sides to achieve full penetration. A Type 2 weld is a butt joint welded from one side, with a permanent backing strip left in place. Other types are defined in Table UW-12.
The joint efficiency of a weld is determined by its type and the amount of non-destructive examination of the joint. The weld’s category will determine which design formulas should be used to calculate stress in the weld.
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