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ASME B31.3 Limitations on Welded Joints

Welded joints are covered by para. 311 and are required to follow the ASME B31.3 rules for fabrication and examination. In addition, the following specific rules are also provided.

Weld backing rings are generally permitted to be left in. However, if the resulting crevice would be detrimental (e.g., subject to corrosion, vibration, or severe cyclic conditions), para. 311.2.3 recommends removing the ring and grinding the internal joint face smooth. Split-backing rings are prohibited under severe cyclic conditions.

Socket-welded joints are generally permitted, with the following exceptions. They should not be used in services where crevice corrosion (due to the crevice between the pipe and socket) or severe erosion (which can cause local erosion at the gap between the pipe end and socket) may occur. Socket welds larger than DN 50 (NPS 2) are not permitted under severe cyclic conditions. Note that socket welds are generally limited in practice to sizes of DN 50 (NPS 2) or less in any case.

The dimensions of the socket joint are required to conform to either ASME B 16.5 for flanges or ASME B16.11 for other socket welding components (except for socket welding drain or bypass attachments to components that conform to Fig. 4 of ASME B16.5).

Weld dimensions are required to comply with the fabrication rules of ASME B31.3 (Figs. 328.5.2B and 328.5.2C). Socket joints are generally part of listed components, so their pressure design is satisfied by the component standard and compliance with the fillet weld size requirements of the fabrication rules of ASME B31.3.

Paragraph 311.2.5 permits fillet welds to be used for the primary welds to attach socket welding components and slip-on flanges. It also permits fillet welds to be used to attach reinforcement and structural attachments and to supplement the strength or reduce the stress concentration of other welds. Note, however, that the wording does not prohibit the use of fillet welds for other applications, and it has been the interpretation of the Committee that they are not prohibited from use in other joints, such as lined pipe sections connected by a metallic sleeve with fillet welds. This is reflected in Interpretation 10-04:

“Question: Does ASME/ANSI B31.3-1987 Edition and its Addenda permit the use of fillet welds as the primary pressure retaining welds for installation of a full encirclement tee type branch connections?”

“Reply: ASME/ANSI B31.3-1987 Edition and its Addenda do not specifically address rules for fillet welding of full encirclement tee type branch connections.”

Seal welds are permitted to be used to prevent leakage of threaded join. However, the seal weld is not permitted to be considered as contributing to the strength of the joint. #Little_PEng.

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