ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings. Included are:
(1) flanges with rating class designations 150, 300, 400, 600, 900, and 1500 in sizes NPS 1/2 through NPS 24 and flanges with rating class designation 2500 in sizes NPS 1/2 through NPS 12, with requirements given in both metric and U.S. Customary units with diameter of bolts and flange bolt holes expressed in inch units;
(2) flanged fittings with rating class designation 150 and 300 in sizes NPS 1/2 through NPS 24, with requirements given in both metric and U.S. Customary units with diameter of bolts and flange bolt holes expressed in inch units;
(3) flanged fittings with rating class designation 400, 600, 900, and 1500 in sizes NPS 1/2 through NPS 24 and flanged fittings with rating class designation 2500 in sizes NPS 1/2 through NPS 12 that are acknowledged in Non-Mandatory Appendix E in which only U.S. Customary units are provided.

B16.5 is limited to flanges and flanged fittings made from cast or forged materials, and blind flanges and certain reducing flanges made from cast, forged, or plate materials. Also included in this Standard are requirements and recommendations regarding flange bolting, flange gaskets, and flange joints.
This Standard is to be used in conjunction with equipment described in other volumes of the ASME B16 Series of Standards as well as with other ASME standards, such as the Boiler and Pressure Vessel Code and the B31 Piping Codes.

Careful application of these B16 Standards will help users to comply with applicable regulations within their jurisdictions, while achieving the operational, cost and safety benefits to be gained from the many industry best-practices detailed within these volumes.

Intended for manufacturers, owners, employers, users and others concerned with the specification, buying, maintenance, training and safe use of valves with pressure equipment, plus all potential governing entities.

What's the difference between ANSI B16.5 and ASME B16.5 standards?

The B16.5 standard was first published by ANSI in 1973.  Prior to that it was an ASA standard.  In 1996 the standard changed to ASME B16.5.   For 10 years prior to the change, the standard included both and was listed as ANSI/ASME B16.5. 

So what's the difference?  The answer is there is no difference.  The ASA, ANSI and ASME have continually refined and broadened the B16.5 standard.  I'm sure there is a committee working on future enhancements right now. 

The current and correct standard is ASME B16.5.  ANSI B16.5 is common industry lingo and is still referred to extensively by architects, engineers, manufacturers and suppliers.  And although the ANSI B16.5 standard does not technically exist, it its understood as another way to say ASME B16.5. 

What is the difference between ASME B16.5, ASME B16.47 Series A, Series B and API 605 Flanges
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.

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.

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.