ASME B16.47(Large Diameter Steel Flanges) is a standard for large diameter steel flanges sized from NPS 26 through NPS 60. The latest version provides dimensions and ratings in both metric and inch units. The ASME B16.47 inherently incorporate MSS SP-44: Steel Pipeline Flanges and API 605: Large Diameter Carbon Steel Flanges. Therefore, the MSS SP-44 flanges are designated as ASME B16.47 Series A flanges, while API 605 flanges are designated as ASME B16.47 Series B flanges within this standard. Materials covered in this standard are as that in ASME B16.5 except for nickel alloys which means they share the same pressure-temperature chart for the selection of flange materials.

Generally speaking, ASME B16.47 Series A flanges are much thicker, heavier and stronger than Series B flanges in the same size and pressure rating thus can withstand more external loading than Series B type. Series B flanges need more but smaller fasteners(bolts & nuts) than Series A flanges and have a smaller bolt circle diameter than that of Series A flanges. The smaller the bolt circle diameter is, the less flange movement tends to occur. Both Series A and Series B define weld neck flanges and blind flanges in raised face type(RF) but only Sereis A defines ring type joint(RTJ) flanges from Class 300 through Class 900 within this standard. From a commercial point of view, ASME B16.47 Sereis A flanges are more expensive than Series B flanges. Most piping end users or designers may choose Series A flanges for the new pipeline or equipment. But if you are doing some refurbishment/replacement job or non-critical piping design, Series B flanges are more popular.