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ASME B31.3 Limitations on Fittings, Bends, Miters, and Branch Connections

Most fittings in ASME B31.3 piping systems are in accordance with the following listed standards:

  • ASME B16.3, Malleable Iron Threaded Finings

  • ASME B16.4, Gray Iron Threaded Fittings

  • ASME B16.9, Factory-Made Wrought Steel Buttwelding Finings

  • ASME B 16.11, Forged Fittings, Socket-Welding and Threaded

  • ASME B16.14, Ferrous Pipe Plugs, Bushings, and Locknuts with Pipe Threads

  • ASME B16.15, Cast Bronze Threaded Fittings, Classes 125 and 250

  • ASME B16.18, Cast Copper Alloy Solder Joint Pressure Fittings

  • ASME B 16.22, Wrought Copper and Copper Alloy Solder Joint Pressure Fittings

  • ASME B 16.26, Cast Copper Alloy Fittings for Flared Copper Tubes

  • ASME B 16.39, Malleable Iron Threaded Pipe Unions, Classes 150, 250, and 300

  • AWWA C110, Ductile-Iron and Gray-Iron Finings, 3 Inch Through 48 Inch (75 mm Through 1200 mm), for Water and Other Liquids

  • AWWA C208, Dimensions for Fabricated Steel Water Pipe Fittings

  • MSS SP-43, Wrought Stainless Steel Buttwelding Fittings

  • MSS SP-73, Brazing Joints for Copper and Copper Alloy Pressure Fittings

  • MSS SP-75, Specifications for High Test Wrought Buttwelding Fittings

  • MSS SP-79, Socket-Welding Reducer Inserts

  • MSS SP-83, Class 3000 Steel Pipe Unions, Socket-Welding and Threaded

  • MSS SP-95, Swage(d) Nipples and Bull Plugs

  • MSS SP-97, Integrally Reinforced Forged Branch Outlet Fittings—Socket Welding, Threaded, and Buttwelding Ends

  • MSS SP-119, Factory-Made Wrought Belled End Socket Welding Fittings (class MP only)

  • SAE J513, Refrigeration Tube Fittings—General Specifications

  • SAE J514, Hydraulic Tube Fittings

  • SAE J518, Hydraulic Flange Tube, Pipe, and Hose Connections, Four-Bolt Split Flanged Type

Listed fittings are accepted for their specified pressure ratings. Note that some fittings are simply specified to have equivalent pressure ratings to matching straight seamless pipe. As discussed in Section 4.1, ASME B31.3 bases this on matching straight seamless pipe with 87.5% of the nominal wall thickness. Some ratings are tabulated in Appendix I. Fittings that are not in accordance with one of the listed standards can be accepted as unlisted components in accordance with para. 302.2.3. The relevant option of this paragraph for fittings is typically para. 304.7.2, because the pressure design equations in para. 304 typically are not relevant.

Branch connections are required to be listed components (para. 306.1.1); fabricated branch connections that are designed per para. 304.3 and welded per para. 311.1 (para. 306.5.1); fittings that are qualified by proof testing per ASME B 16.9, MSS SP-97, or BPVC, Section VIII, Division 1, UG-101 [para. 306.1.3(a)]; or unlisted components that are qualified per para. 304.7.2 (para. 306.1.2).

The following fittings are prohibited from use under severe cyclic conditions.

  • MSS SP-43 fittings (para. 306.1.4)

  • Proprietary “Type C” lap-joint stub-end welding fittings (para. 306.1.4)

  • Fittings (para. 306.1.4) other than forged, wrought, with Ej S 0.90, or cast, with Ec > 0.90.

  • Corrugated or creased bends (para. 306.2.3)

  • Miter bends with angle a > 22.5° (para. 306.3.3)

  • Fabricated branch connections with the branch set on (rather than set in) the run pipe [i.e., Fig. 328.5.4D, sketches (2) and (4)] are permitted (para. 306.5.2)

The following fittings are only permitted for Category D Fluid Service:

  • Miter bends with a single joint angle a > 45°.

Limitations for fabricated and flared laps for use with lap joint flanges are provided in para. 306.4. Note that these limitations do not apply to laps that are in accordance with listed standards, such as ASME B16.9. Fabricated laps are constructed by welding; flared laps are constructed by flaring a pipe section. The outside diameter of the lap is required to comply with the dimension specified in ASME B16.9. The lap thickness is required to be at least equal to the nominal wall thickness of the pipe for fabricated laps, and, for flared laps, 95% of the minimum pipe wall thickness (measured or minimum per purchase specification, considering mill tolerance) multiplied by the ratio of the pipe outside radius to the radius at which the lap thickness is measured (this essentially permits thinning due to the flaring process).

For a fabricated lap, the material for the lap must have an allowable stress at least as great as the pipe material and the material must be listed in ASME B31.3, Table A-l, the allowable stress table. Welding must be per ASME B31.3 and typical details are provided in Fig. 328.5.5. Full penetration welds are required (by reference to the applicable requirements of para. 328.5.4, which covers branch connections).

Flared laps are required to have their pressure design qualified in accordance with para. 304.7.2. Thus, they are essentially treated as unlisted components in this regard. In addition, the radius of the fillet of the flared lap is not permitted to exceed 3 mm (% in.), and the mating flange is required to be beveled or rounded approximately 3 mm ('/g in.) (by reference to para. 308.2.5). This prevents interference between the lap and the flange at the pipe/lap junction. #Little_PEng.

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