Skirt Attachment Weld
Attaching a support skirt to a pressure vessel does not involve a pressure containing weld. Nevertheless, the skirt attachment weld must be made properly to provide the integrity required to reliably support the vessel, including the maximum wind and earthquake loads that might occur, and in a manner that does not jeopardize the pressure containing integrity of the vessel. The skirt is usually fabricated by rolling and welding, similar to the formation of the cylindrical shell components of the vessel (see Section 630).
ASME Code, Section VIII, Divisions 1 and 2 (Paragraphs UW-28c and AF-210.4), requires that the welding procedure used for the attachment of a support skirt to a pressure vessel be qualified according to ASME Code, Section IX. The welding procedure should be very similar to those used for pressure- containing welds in the vessel, including requirements for preheat and postweld heat treatment.
A support skirt can be directly welded to the bottom head of a pressure vessel, as shown in Figure 600-7a. Automatic SAW or manual SMAW is normally used to make this type of skirt attachment weld. This type of skirt attachment cannot be readily nondestructively examined, and flaws almost always exist in the “crotch.” These flaws could propagate by fatigue if they are subjected to thermal cycling.
Attachment of the support skirt to the bottom head employing a weld pad as shown in Figure 600-7b provides a higher integrity attachment that is preferred for heavy and thick wall vessels, or those that will be subjected to thermal cycling that could initiate fatigue cracks. The weld pad can be deposited on the head by either manual SMAW or automatic SAW. The weld pad is subsequently machined to a smooth contour that incorporates a weld bevel (joint preparation) for attachment of the skirt. The skirt is then welded to the machined weld pad, and this joint preparation permits either RT or UT examination.
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