ASME B31.3 Appendix L provides the pressure and temperature ratings of forged aluminum flanges. The flanges are from NPS 1/2 to NPS 24 in three rating Classes 150, 300 and 600 with two grades of aluminum alloys: ASTM B247 3003-H112 and 6061-T6. However, B31.3 does not provide any technical information on the basis of the pressure and temperature ratings. A review of the historical development of ASME B16.5 indicated that the aluminum flanges had the same technical basis for pressure and temperature ratings as the ferrous alloy flanges in ASME B16.5. The 1960 Addenda of the 1957 Edition B16.5 included both aluminum flanges and ferrous alloy flanges. A new Code Case 2905 has been recently approved to allow B31.3 Appendix L aluminum flanges in fabricating Section VIII Division 1 pressure vessels as B16.5 flanges on the basis that both flange specifications have the same safety margin. In this paper, the technical basis of the pressure and temperature rating of aluminum flanges is revisited. Based on the same principle, the pressure ratings are extended to Class 900 and Class 1500 for the two aluminum alloys using the same analysis. Since ASTM B247 5083-H112 is another common grade of aluminum forging alloy, the pressure and temperature ratings are proposed for 5083-H112.

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