As you know most industries had used MSS SP-44 before 1990 and API 605 before 1996 for large flanges because the ASME (former ANSI) B16 series did not have the large size flange standard (> 24 NPS). But now we are using the ASME B16.5 (</24 NPS) and ASME B16.47 (NPS 26 through 60 for WN type and blind flange). API 605 was withdrawn and superseded by ASME B16.47 in 1996.

The Series A (MSS SP-44 base) or Series B (API 605 base) in ASME B16.47 depends on the requirements of the end-user. Here is an example of the requirements in end-user spec.

; Steel flanges NPS 26 through NPS 60,, up to and including Class 900, shall be specified as conforming to ASME B16.47, Series B (same as API 605), except Series A flanges (same as MSS SP 44) shall be specified, as required, to mate with valve or equipment flanges.

; In the case of ASME B16.47 Series B (API Std 605) flanges, care should be exercised when specifying these for valves or fittings on lines which may require to be pigged, since when used as an integral flange (e.g., on a valve) the smaller diameter of the bolt circle as compared with the other two flange standards may result in a reduced bore valve.

; It should be noted that the bolt circle diameters are different for ASME B16.47 Series B (API Std. 605) versus ASME B16.47 Series A (MSS SP 44) Flanges, particularly NPS 24 and above hence they are not dimensionally compatible.


In addition, please think about the field tie-in condition, such as new flanges per ASME standard flange on the existing flange standards (MSS or API). Absolutely the companion flanges shall have the same standard.

Therefore if you want to make your company catalogue for flange materials, you may have to consider both standards of Series A and Series B.

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