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ASME B31.3 Limitations on Bolting

Requirements for bolting are provided in para. 309. Listed bolting is acceptable. The only relevant component standards are ASME Bl.l, Unified Inch Screw Threads (UNand UNR Thread Form). ASME B 18.2.1, Square and Hex Bolts and Screws (inch series), and ASME B 18.2.2, Square and Hex Nuts (inch series). Table A-2 in ASME B31.3, Appendix A provides listed boiling material specifications. Unlisted bolting (i.e., not listed in Table A-2) are subject to the requirements of para. 302.3.3 (per para. 309.1.2).

Requirements for flanged joints relative to bolt strength are provided as follows. Low strength is considered to refer to bolts with a specified minimum yield strength of 207 MPa (30 ksi) or less:

  1. Low-strength bolts are not permitted for flanges with ASME B16.5 Class 400 or higher ratings or with flanged joints using metallic gaskets, unless design calculations have been made to show the bolts have sufficient strength to maintain joint tightness. Although not specifically stated in ASME B31.3, this should be done per the bolting design requirements in ASME BPVC, Section VIII, Division 1, Appendix 2.

  2. Certain flanges are considered to be weaker due to the material of construction or the flange design. These are flanges per ANSI B16.1 (cast iron), ANSI B16.24 (bronze), MSS SP-42, and MSS SP-51. These arc required to use low-strength bolting to avoid overloading the flange, unless one of the following is satisfied:

  • The flanges are flat face and use a full face gasket. This essentially prevents excessive flange rotation due to bolt load;

  • The sequence and torque limits for the boltup are specified, giving appropriate consideration to the loads and the strength of the flange.

  1. Low-strength bolting is prohibited from flange joints under severe cyclic conditions. However, since the stresses in a flanged joint due to piping thermal expansion loads are not normally calculated, it is unclear as to when this would ever be implemented.

The depth of lapped holes is required to be sufficient to provide a length of thread engagement at least seven-eighths times the nominal thread diameter. #Little_PEng.

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