ASME B16.5 vs B16.47 Series A vs Series B Large Flanges

Why this matters

For any procurement engineer issuing an MR for piping above NPS 24, the choice between ASME B16.5, B16.47 Series A and B16.47 Series B is one of the most frequently confused points on a bid sheet. The three standards cover different size ranges, the two series under B16.47 are NOT interchangeable, and a wrong call results in flange faces that cannot mate with the mating equipment on site. This guide walks through the practical procurement-side decisions for the ASME B16.5 vs B16.47 Series A vs Series B family.

Scope comparison

Face types in B16.5 include raised face (RF), flat face (FF) and ring-type joint (RTJ). Series A defines RTJ from Class 300 through Class 900; Series B does not include RTJ in the same scope.

Key differences

• Heritage: B16.47 Series A flanges are essentially the former MSS SP-44 dimensions; Series B comes from the former API 605 standard.
• Geometry: For the same NPS and class, Series A is heavier and has a larger outside diameter with fewer, larger bolts. Series B is more compact, with a smaller OD, tighter bolt circle and more bolts of smaller diameter.
• Strength: The thicker hub and ring of Series A give it higher load capacity than Series B.
• Interchangeability: Series A and Series B flanges of the same NPS and class will not mate with each other. Bolt circle, bolt size and OD all differ.
• Class 75: Only Series B includes the low-pressure Class 75; Series A does not.
• Below NPS 24: B16.47 does not apply. Use B16.5 for these sizes.

When to choose which

• NPS 1/2 to NPS 24: ASME B16.5 is the default. It is the most widely stocked standard worldwide and matches the bulk of valve, pump and vessel nozzle drilling.
• NPS 26 and above, new project: B16.47 Series A is generally preferred for greenfield work because the heavier construction tolerates higher bending and bolt-up loads, and the supply chain is broader.
• Replacement / refurbishment: If the existing flanges on site were originally API 605 (now Series B), replacement must remain Series B. Ask the operator for as-built drawings or measure OD and bolt circle before placing the order.

We supply both series via our forged flanges line.

Procurement / spec checklist

1. State NPS, pressure class, face type and standard explicitly: e.g. "WN, NPS 36, Class 300, RF, ASME B16.47 Series A, ASTM A105N".
2. For B16.47, ALWAYS specify A or B. Never assume.
3. Confirm the bore is to match the connecting seamless steel pipe schedule.
4. Specify gasket type and bolt material consistent with face type.
5. For sour or low-temperature service, link the flange spec to a material standard (e.g. NACE MR0175 / ISO 15156, or ASTM A350 LF2).
6. Reference the latest published edition of the ASME standard rather than fixing a year, unless the project is locked to a specific edition.

If you need a quick spec check before issuing the MR, send the line list via our inquiry form and ask for our QA certificates.

Sources

• https://whatispiping.com/difference-asme-b-16-47-series-a-and-series-b-flanges/
• https://www.texasflange.com/blog/b16-47-series-a-vs-series-b-whats-the-difference/
• https://www.asme.org/codes-standards/find-codes-standards/b16-5-pipe-flanges-flanged-fittings-nps-1-2-nps-24-metric-inch-standard
• https://apiint.com/blog/understanding-ansi-asme-b16-5-and-b16-47-standards/