ASME B16.5 vs B16.47 Flanges: Sizes, Series and How to Choose

The Short Answer

ASME B16.5 and ASME B16.47 are two different flange standards that cover different size ranges. B16.5 is the everyday standard for flanges and flanged fittings from NPS 1/2 through NPS 24 (DN 15–600). B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60 (DN 650–1500), and it contains two series: Series A (based on MSS SP-44) and Series B (based on API 605). The practical question is almost always size: up to NPS 24 you use B16.5; above that you move to B16.47 and then decide between Series A and Series B based on bolting, face-to-face dimensions and existing plant stock.

What Each Standard Covers

ASME B16.5 Basics

ASME B16.5 — Pipe Flanges and Flanged Fittings — covers flanges from NPS 1/2 to NPS 24 in pressure classes 150, 300, 400, 600, 900, 1500 and 2500. It specifies dimensions, tolerances, facing types (flat face, raised face, ring joint), bolting, and pressure-temperature ratings for materials such as carbon steel, stainless steel and alloy steels. It is the standard most plant engineers encounter daily, and it is the basis for most small-to-medium valve flanges.

ASME B16.47 Basics

ASME B16.47 — Large Diameter Steel Flanges — covers NPS 26 through NPS 60. Because large flanges were historically built to two different industry patterns, B16.47 codified both: Series A follows the MSS SP-44 pattern (generally larger bolt circles and heavier bolting), and Series B follows the API 605 pattern (generally a lighter, compact design). The two series are dimensionally different and are not interchangeable, so the standard you choose must be stated clearly on the datasheet.

Key Differences at a Glance

Aspect	ASME B16.5	ASME B16.47
Size range	NPS 1/2 to NPS 24 (DN 15–600)	NPS 26 to NPS 60 (DN 650–1500)
Pressure classes	150 to 2500	150 to 900 (typical classes; verify per edition)
Series / patterns	Single pattern per class	Series A (MSS SP-44) and Series B (API 605)
Typical use	Most process piping and valve flanges	Large-bore piping, big valve flanges, tank nozzles
Interchangeability	Widely stocked and standardized	Series A and B are not interchangeable with each other
Bolting pattern	Defined per class and size	Differs between Series A and B

Which One Should You Specify?

For flanges up to NPS 24, B16.5 is the default. It is what most valve datasheets, piping specs and stock programs assume, and it gives you the broadest sourcing options. Above NPS 24, move to B16.47 and make an explicit decision between Series A and Series B: choose Series A where you need the heavier, MSS SP-44 style bolting typical of some process and marine installations; choose Series B where the compact API 605 style matches the existing plant standard or the connected equipment. The wrong series is a real problem — a Series A flange will not bolt to a Series B companion flange, and correcting this on site is costly.

Common Confusions

One frequent confusion is thinking B16.47 Series A and B are just “old and new” versions of the same flange — they are parallel patterns, not revisions, and both remain current. Another is mixing the size ranges: a 24-inch flange is B16.5 territory, while a 26-inch flange jumps to B16.47, so the boundary is right at NPS 24/26. A third is assuming the pressure-temperature ratings of the two standards are identical for the same class; ratings depend on the material and the standard edition, so always check the rating tables rather than carrying values over by habit. When large valve flanges are involved, confirm the standard and series with the valve vendor before the piping design is finalized — the flange data on the valve datasheet must match the piping spec exactly, a point echoed in our valve selection guides.

Application Example: Upgrading a Water Line

A utility is installing a 36-inch isolation valve in a raw water line. The piping design calls for ASME B16.47 flanges, and the team initially assumes any B16.47 flange will work. During review they discover the plant standard is Series A while the valve vendor quoted Series B; the bolt circles do not match. Catching this in the datasheet review stage — before fabrication — saves a long delivery delay. The lesson is to state “ASME B16.47 Series A” (or B) explicitly in the specification, never just “B16.47.” For smaller branch lines on the same project, the 6-inch and 12-inch flanges follow B16.5 as usual, which is why both standards coexist on one project without conflict.

Practical Recommendation

Write the flange standard, series (when applicable) and facing type explicitly on every datasheet and spec. For NPS 1/2–24, specify ASME B16.5 and confirm the facing and rating class against the piping spec. For NPS 26–60, specify ASME B16.47 plus Series A or B, and double-check bolting dimensions with the mating flange. When valves are involved, cross-check the valve’s flange data against the companion flanges before ordering; the cost of a mismatch is a full re-manufacture. For a broader view of how standard compliance fits into valve specification, the how to choose a valve process covers datasheet checks step by step.

Conclusion

B16.5 handles the everyday small-to-medium flanges; B16.47 takes over above NPS 24 with its two parallel series. Know your size range, state the series explicitly, and verify companion flanges before fabrication. Flange standards are unforgiving of ambiguity, but they are easy to get right with a disciplined datasheet.

This article provides general educational knowledge only and is not professional engineering advice.

Sources & Further Reading

ASME, API, MSS

Tags: Valve Standards