The plate materials for cylindrical shell components are rolled (formed) into cylindrical shapes with the required diameters, and the bevels (joint preparations) for the longitudinal welds are made as described in Section 620. The pressure vessel Specification PVM-MS-4749 recommends MT examination of weld bevels for shell components 2-inches thick and greater, to prevent flaws in the plate from compromising the integrity of the weld joint. ASME Code, Section VIII, Divisions 1 and 2, requires misalignment of the mating joint preparations for longitudinal welds to be within the following limits (Paragraphs UW-33 and AF-142):
Section Thickness (t), in. Misalignment Tolerances of Mating Joint
Up to 1/2, incl. 1/4 t
Over 1/2 to 3/4, incl. 1/8 in.
Over 3/4 to 1 1/2, incl. 1/8 in.
Over 1 1/2 to 2, incl. 1/16 t (3/8 in. max.)
It is usually necessary to use clips (temporary attachments) welded to the shell to obtain alignment within these limits, and to maintain the same tolerances during welding. It is advisable for the Company inspector to observe the fit-up of some welds prior to welding to make certain that the fabricator is practicing the workmanship required to obtain alignment within these tolerances.
Longitudinal welds in cylindrical shell components are commonly made by automatic submerged arc welding (SAW) from either the outside or inside of the shell, followed by back gouging and manual shielded arc welding (SMAW) from the opposite side to complete the weld, as illustrated in Figure 600-6. The joint preparation is configured to perform most of the welding with the more efficient automatic SAW process, and SMAW from the opposite side is only employed to fill and cover the back-gouged groove.
The rolled shell component is positioned horizontally with the joint preparation on top for the automatic SAW from the outside, or on the bottom when the automatic welding is performed on the inside. Some fabricators have welding machines that can perform automatic SAW from both the outside and inside. The weld joint must be free of all deleterious material (dirt, grease, etc.) that could contaminate the weld metal, resulting in flaws. The Company inspector should make certain that the fabricator is adhering to this requirement.
The groove for the longitudinal weld is generally completely filled with SAW weld metal from the land to the outside surface of the shell before work is initiated on the inside surface. However, the first layer of SAW weld metal deposited (root pass) generally contains considerable unsound material. This unsoundness could create unacceptable flaws in or below weld metal deposited on the opposite side. Therefore, it must be removed by gouging or grinding prior to beginning manual SMAW from the inside.
Some fabricators propose making the first pass with the gas metal arc welding (GMAW) process before the groove is filled with SMAW weld metal, claiming that GMAW weld metal exhibits less unsoundness than SAW weld metal and that back gouging will not be necessary. However, substituting GMAW for backgouging or grinding can lead to a high incidence of rejectable welds, and is not acceptable.
When it is not possible to weld from the inside, such as with rolled and welded nozzle barrels, it is acceptable to employ a single-V weld preparation with gas Tungsten arc welding (GTAW) of the first layer of weld metal (root pass), followed by filling the remainder of the groove with SAW or SMAW weld metal. Careful GTAW is capable of producing a root bead contour on the inside surface that is equivalent to a Type 1 double-welded joint. The company inspector should visually examine the inside surface of the GTAW root beads to make certain that the fabricator is obtaining good contours.
It is essential that all of the welding procedures used by the fabricator are properly qualified according to ASME Code, Section IX, and that these procedures are strictly adhered to during production welding. The Company inspector should assure that the fabricator is adhering to the qualified welding procedures, including preheat wherever required, by surveillance on the shop floor during fabrication.
Some distortion (or “Peaking”) can occur during welding of the longitudinal joint, and, consequently, the cylindrical shell components may have to be rerolled after welding to be within the ASME Code tolerances for out-of-roundness. Alternatively, some fabricators control distortion by joint configuration and weld bead sequence, balancing welding from the outside and inside. ASME Code, Section VIII, Divisions 1 and 2, prohibits out-of-roundness from exceeding 1% of the nominal diameter for which the component is designed (Paragraphs UG-80 and AF-130). The tolerances for alignment of girth welds must also be obtained.
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