ASME B31.3 Limitations on Flanges

ASME B31.3 Limitations on Flanges

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

  • ANSI B16.1, Cast Iron Pipe Flanges and Flanged Fittings

  • ASME B16.5, Pipe Flanges and Flanged Fittings

  • ASME B16.24, Cast Copper Alloy Pipe Flanges and Flanged Fittings: Classes 150, 300, 600,

  • 900, 1500, and 2500

  • ASME B 16.36, Orifice Flanges, Classes 300, 600, 900, 1500, and 2500

  • ASME B 16.42, Ductile Iron Pipe Flanges and Flanged Fittings, Classes 150 and 300

  • ASME B 16.47, Large Diameter Steel Flanges, NPS 26 Through NPS 60

  • ASME B 16.48, Steel Line Blanks

  • AWWA Cl 15, Flanged Ductile-Iron Pipe with Ductile-Iron or Gray-Iron Threaded Flanges

  • AWWA C207, Steel Pipe Flanges for Water Works Service, Sizes 4 Inch Through 144 Inch

  • (100 mm Through 3,600 mm)

  • MSS SP-44, Steel Pipe Line Flanges

  • MSS SP-51, Class 150LW Corrosion Resistant Cast Flanges and Flanged Fittings

  • MSS SP-65, High Pressure Chemical Industry Flanges and Threaded Stubs for Use with Lens

  • Gaskets

Listed flanges are accepted for their specified pressure ratings. Some of these pressure ratings are tabulated in Appendix 1. Flanges that are not in accordance with one of the listed standards can be accepted as unlisted components in accordance with para. 302.2.3. If the Designer is satisfied that the composition, mechanical properties, method of manufacture, and quality control are comparable to the corresponding characteristics of a listed component, pressure design can be performed in accordance with ASME BPVC, Section VIII, Division 1, Appendix 2 or Y, as applicable. Otherwise, qualification of the flange in accordance with para. 304.7.2 is required. Paragraph 308 provides additional restrictions for flanges.

Slip-on flanges arc required to be double-welded when the service is (1) subject to severe erosion, crevice corrosion, or cyclic loading, (2) flammable, toxic, or damaging to human tissue, (3) under severe cyclic conditions, or (4) at temperatures below -101°C (-150°F). Further, the Designer is cautioned that the use of slip-on flanges should be avoided where many large temperature cycles are expected, particularly if the flange is not insulated.

A double-welded slip-on flange has a weld between the pipe and the flange hub and between the pipe and the bore of the flange. A single-welded slip-on flange only has the weld to the flange hub.For double-welded slip-on flanges in hydrogen service, where hydrogen can diffuse into the annulus between the inner and outer welds, collect, and pressurize it, the flange should be drilled to vent this space. A precautionary consideration, F308.2, mentions this concern.

Slip-on flanges have been substituted for lap joint flanges (both ASME B16.5). However, the flange hub or some sizes is undersized for lap joint applications. As a result, Table 308.2.1 was added to the Code. It limits substitution of slip-on for lap joint flanges to DN 300 (NPS 12) or smaller for PN 20 (Class 150) flanges, and DN 200 (NPS 8) for PN 50 (Class 300) flanges, unless the pressure design has been qualified in accordance with the ASME BPVC, Section VIII, Division 1 rules. In addition, other requirements are provided relative to the lap joint in para. 308.2.1 (c).

For flanges attached to the pipe using expanded joints, threaded joints, and socket weld joints, the restrictions on those joints, discussed later in this chapter, apply. #Little_PEng.

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