Address
304 North Cardinal St.
Dorchester Center, MA 02124
Work Hours
Monday to Friday: 7AM - 7PM
Weekend: 10AM - 5PM
Specify a flange at NPS 30 and write “ASME B16.5” on the purchase order and you have specified a document that does not cover that size. B16.5 stops at NPS 24. Above that, ASME B16.47 takes over, and it does so with two dimensional families — Series A and Series B — that are not interchangeable with each other, let alone with B16.5.
This is a short, practical guide to where the boundary sits, what each standard actually covers, and the three places the distinction most often causes a procurement problem. Both standards are current in their 2025 editions, published 30 May 2025.
Where the boundary sits
| Parameter | ASME B16.5 | ASME B16.47 |
|---|---|---|
| Full title | Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard | Large Diameter Steel Flanges: NPS 26 through NPS 60, Metric/Inch Standard |
| Size range | NPS 1/2 to NPS 24 | NPS 26 to NPS 60 |
| Flange classes | 150, 300, 400, 600, 900, 1500 (NPS 1/2–24); 2500 (NPS 1/2–12) | 75, 150, 300, 400, 600, 900 |
| Flange types | Weld neck, slip-on, socket-weld, threaded, lap-joint, blind | Weld neck and blind |
| Series | Not applicable | Series A and Series B |
| Edition published | 30 May 2025 (2025 edition) | 30 May 2025 (2025 edition) |
| Supersedes | 2020 edition | 2020 edition |
Note that there is no NPS 25. The transition from B16.5 to B16.47 is a step, not a slope: B16.5 ends at NPS 24 and B16.47 begins at NPS 26. So a buyer who “usually orders B16.5” and suddenly needs a larger size is not making a small jump — they are crossing a standard boundary, with everything that implies.
Series A and Series B are not interchangeable
This is the part that generates the claims. B16.47 defines two dimensional families. At the same nominal size and class, a Series A flange and a Series B flange have different outside diameters, different bolt circles and different bolt hole counts. They do not mate with each other, and neither will mate with the B16.5 pattern of the same nominal size.
The published Class 150 examples below make the scale of the difference obvious — and they are worth noting because the size relationship runs counter to intuition: Series A is the heavier, larger-OD pattern, and Series B is the more compact one, even though Series B often carries more bolts because the bolt circle is smaller and the holes are smaller.
| NPS (in) | Series | Outside diameter (in) | Weld neck thickness (in) | Bolt circle (in) | Bolt holes (no. × dia., in) |
|---|---|---|---|---|---|
| 26 | A | 34.25 | 2.69 | 31.75 | 24 × 1.38 |
| 26 | B | 30.94 | 1.62 | 29.31 | 36 × 0.88 |
| 36 | A | 46.00 | 3.56 | 42.75 | 32 × 1.62 |
| 36 | B | 41.62 | 2.06 | 39.75 | 44 × 1.00 |
| 48 | A | 59.50 | 4.25 | 56.00 | 44 × 1.62 |
| 48 | B | 54.81 | 2.56 | 52.56 | 44 × 1.25 |
Class 150 NPS 26, 36 and 48 dimensions as published by a flange manufacturer; confirm against the current B16.47 tables before ordering.
Two practical consequences. First, a purchase order reading “B16.47 Class 150” is incomplete — it must state the series. Second, flanges on an existing line must be identified by their actual mating interface rather than assumed from nominal size and class. A nominal size match does not prove compatibility, and equipment supplied by an original equipment manufacturer may use proprietary drilling.
Material is a separate question from size
Choosing between the two standards settles dimensions. It does not settle material, and the two decisions are independent. For forged stainless flanges and fittings, the material specification is SA-182/SA-182M. Reading the chemical requirements directly from that specification:
| Grade | UNS | C (max, %) | Ni (%) | Cr (%) | Mo (%) |
|---|---|---|---|---|---|
| F304L | S30403 | 0.030 | 8.0–13.0 | 18.0–20.0 | — |
| F316L | S31603 | 0.030 | 10.0–15.0 | 16.0–18.0 | 2.00–3.00 |
| F316N | S31651 | 0.08 | 11.0–14.0 | 16.0–18.0 | 2.00–3.00 |
| F51 | S31803 | 0.030 | 4.5–6.5 | 21.0–23.0 | 2.5–3.5 |
| F53 | S32750 | 0.030 | 6.0–8.0 | 24.0–26.0 | 3.0–5.0 |
| F55 | S32760 | 0.030 | 6.0–8.0 | 24.0–26.0 | 3.0–4.0 |
SA-182/SA-182M chemical requirements. Note this specification lists nickel before chromium — the reverse of the SA-312 pipe table.
A word of caution on what a grade name does and does not guarantee. A matching grade name is not a match. Verify the specification, the grade, the heat treatment condition, the dimensional standard and the pressure-temperature material group. And note that “Class 300” is a rating class, not a pressure: the permissible pressure depends on the material and the temperature, taken from the standard’s rating tables. A flange rated Class 300 does not thereby permit 300 psi. We do not reproduce derating tables here because they belong to the current standard and the applicable material group, and should be read from it directly.
Three checks before you order
- Confirm the size standard first. NPS 24 or below → B16.5. NPS 26 and above → B16.47. NPS 25 does not exist in either.
- If B16.47, state the series. Series A and Series B do not mate. “B16.47 Class 150” without a series is not a complete description.
- State the material specification and grade explicitly — for example SA-182/SA-182M F316L — rather than a generic “stainless”, and confirm the rating against the current tables for your design temperature.
If you need large-diameter stainless flanges quoted, send us the nominal size, class, series, material grade and quantity and we will price it against the current B16.47 tables. Message us on WhatsApp or via our contact page and we will respond the same working day. You can also browse our current flange product range and the wider pipe fittings selection we hold in stock.
