Nozzle welds (Category D welds) can be the most difficult welds to make. More flaws are encountered in these welds than in longitudinal welds (Category A welds) and girth welds (Category B welds). Furthermore, nozzle welds are subjected to relatively high and complex stresses, especially if piping connections apply external loads and moments. The pressure vessel specifications PVM-MS-4749 and PVMMS-4750 recommend NDE of these welds, exceeding ASME Code requirements for vessels that will be 100% radiographed, in order to assure that they will provide adequate integrity and reliability for severe service conditions.
Drop-outs (i.e., holes) are cut into the shell or head components of a vessel, usually using oxyfuel gas cutting, and the weld bevels (joint preparations) are also oxyfuel gas cut or machined. The nozzle barrel is then inserted into the opening, and the proper fit-up for welding is maintained by the use of clips (temporary attachments) welded to the shell component and nozzle. It is advisable for the company inspector to observe the fit-up of some nozzles prior to welding to assure that the fabricator is exercising good workmanship practices, that full penetration is being achieved, and that the specified tolerances for orientation and elevation will be achieved.
Nozzles can be welded into shell and head components either before or after the various components are joined together depending on the fabricator’s shop facilities and the most efficient plan for final assembly. Welding the nozzles into the shell or head component is generally accomplished by manual SMAW. Automatic SAW is sometimes used for large diameter nozzles in relatively thick shells. Some fabricators like to use semiautomatic FCAW, which can be more efficient than manual welding due to the continuous wire feed. However, a high incidence of underbead cracking can occur with FCAW unless low-hydrogen electrodes are used. This concern is addressed in the Company specifications. Also see Section 660 for more details.
It is essential that the fabricator use only welding procedures that have been properly qualified according to ASME Code, Section IX. The Company inspector should maintain adequate surveillance on the shop floor to assure that the fabricator is strictly adhering to the qualified procedures, including preheat.
When integrally reinforced nozzle forgings are used (see Figure 600-4), double-V joint preparations are generally employed to permit welding from both the inside and outside surfaces, similar to that described for the welding of longitudinal joints (see Section 651). However, when “built-up” nozzles with a reinforcing pad are used (see Figure 600-5), a single-V joint preparation is more commonly used for installation of the nozzle barrel, with all of the welding performed from the inside. The outside surface of the weld is subsequently gouged or ground to remove
unsound weld metal, and the reinforcing pad is then welded to the nozzle barrel and shell component from the outside.
The company inspector should determine that all of the specified tolerances for orientation and elevation have been obtained after the welding is completed, because some distortion can occur during welding. Refer to Standard Drawings GA-C1266 (horizontal vessels) or GA-C1267 (vertical vessels) for recommended tolerances.
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