Bolted Flanged Connections

The Code covers the design of flanges in (mandatory) Appendix 2. The scope of the rules apply to gaskets contained entirely within the bolt circle. According to the rules, acceptable flanged nozzles may be attained by the use of either standard rated flanges or by flange calculations. Normally, the standard ANSI B16.5 “Pipe Flanges” or API 605 “Large Diameter Carbon Steel Flanges” will be used, as they can be selected with very little design effort, “off the shelf.” When a standard flange is selected from these specifications, no additional calculations are required.

The following are typical flange standards:
• MSS SP-44, Classes 300, 400, 600, and 900 in sizes 26-36 inches
• Standard 605, “Large Diameter Carbon Steel Flanges,” 75, 150, and 300-pound rating in sizes 26-60 inches inclusive
• Taylor Forge Standard, Classes 75, 175, and 350 in sizes 26-72 inches, 92 and 96 inches
• AWWA (American Water Works Association) Standard C207-55, Classes B, D, and E, in sizes 6-96 inches

Follow the rules of Appendix 2 for the following conditions: (1) when it is necessary to calculate a flange because one of the standard flanges in the correct size is not available; (2) when the pressure temperature ratings are not adequate; or (3) when special design considerations are to be addressed. The design of flanges is largely a trial and error process where the flange thickness is varied. Stresses in the flange and hub are calculated, and if they exceed the allowable tensile stress a new thickness is chosen until the stresses are within the allowed amount. The code covers integral, loose, and optional flange designs. Integral means the pipe, hub, and ring are one continuous piece. An example of this type of flange is a welding neck flange. Loose means no attachment of the assembly to the pipe, such as slip-on, lap joint, and threaded flanges. Optional flanges are integral flanges by construction, but analysis is permitted by the simpler method for loose flanges.

The calculation of flanges requires:
• Selection of materials for flange, bolts, and gaskets
• Determination of facing and gasket details so that the bolt loading may be determined and bolt sizes selected
• Determination of the bolt circle and the loads, moment arms and moments due to gasket seating and operating conditions
• Determination of stresses

Manufacturers like Taylor Forge provide manuals devoted to the design of flanges and include a one page summary giving step by step calculation sheets.

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