ASME B16.5 standard covers Steel Pipe Flanges and Flanged Fittings from NPS 1/2 through NPS 24 Metric/Inch in pressure class 150 to class 2500. It covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings. Two very important flanges such as weld neck flange and blind flanges are very commonly used in piping systems. The term "B16.5" or "B16 5" is used interchangeability and refers to same standard. However, the standard ASME B16 5 (ANSI B16 5) only covers size up to 24 inches. For bigger sizes, ASME B16.47 standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, and testing for pipe flanges in sizes NPS 26 through NPS 60 and in ratings Classes 75, 150, 300, 400, 600, and 900.
Standard ASME B16.5 weldneck flanges can be bought online here
Standard ASME B16.5 blind flanges can be bought online here
However, the standard ASME B16.47 is further divided into ASME B16.47 series A and ASME B16.47 series B flange for Blind flange and Weldneck flange. Series B flange was also formerly known as API 605 flange. Term B16.47 or B 16 47 or ANSI B 16 47 are all used interchangeabily and refers to the same standard.
ASME B16.47(Large Diameter Steel Flanges) is a standard for large diameter steel pipe 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 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.
So what is the difference between ASME B16.47 Series A and Series B (API605) flanges?
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. ASME B16.47 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 bolt circle diameter results in less flange movement after installation. Both Series A and Series B standards define weld neck flanges and blind flanges in raised face type (RF) but only Series 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 Series A flanges are more expensive than ASME B16 47 Series B flanges.
Most piping 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.
See documents below for comparison of flange sizes in two main standards (B16.5 and B16.47)
Flange Dimensions as per ASME B 16 5 standard for 150# Welding Neck Flange, Blind Flange, Slipon, Socketweld & Lapjoint flange is here
Flange Dimensions as per ASME B16 47 series A standard for 150# weldneck and blind flange is here
Flange Dimensions as per ASME B 16.47 series B standard for 150# weld neck and blind flange is here
Conclusions: Standard weld neck, slip on, blind etc. flanges are covered by standard B16.5 for sizes up to 24" nominal pipe. Sizes 26" and bigger are covered by B16.47 series A or series B for weldneck and blind flanges. Series A flange is heavier, thicker and cost more whereas series B flange has more bolt holes and is lighter cheaper than series A. API605 flange and series B flanges are same thing.