What Are ASME B16.47 Series B Flanges?
ASME B16.47 Series B flanges are the compact large-diameter flange design derived from API 605. Covering NPS 26 through NPS 60, Series B flanges use a smaller outside diameter and bolt circle than Series A, with fewer, larger-diameter bolts. Series B is the only series that includes the low-pressure Class 75 rating.
Series B Range
| Parameter | Series B (API 605 Heritage) |
|---|---|
| Size range | NPS 26 – NPS 60 |
| Pressure classes | Class 75 / 150 / 300 / 400 / 600 / 900 |
| Dimensional tables (SI) | Table 37 (Class 75), Table 38 (150), Table 39 (300), Table 40 (400), Table 41 (600), Table 42 (900) |
| Raised face | 1.5 mm on Classes 75/150/300; 6.4 mm on Classes 400/600/900 |
| Flange types | weld neck, blind, hub (slip-on / threaded hub) |
Class 75 — Series B Only
Class 75 flanges appear only in Series B (Table 37). They follow the ANSI/API 605 heritage and are intended for low-pressure large-diameter service such as cooling water, ventilation and air. Class 75 pressure-temperature ratings terminate at 350°C.
Why Series B vs Series A?
Series B (API 605) is more compact — smaller flange OD and bolt circle — which saves weight on large NPS piping. Series A (MSS SP-44) is heavier with a larger drilling. They are not interchangeable: always match the series of the existing piping.
Materials
| Material | Grade | Temperature Range |
|---|---|---|
| ASTM A105 | Carbon steel forging | -29°C to 425°C |
| ASTM A182 | F304 / F316 | Up to 538°C |
| ASTM A350 | LF2 | -46°C to 425°C (low-temperature service) |
Dimensions
Full dimensional tables for Series B are in ASME B16.47-2025 Tables 37 through 42 (SI units): Table 37 Class 75, Table 38 Class 150, Table 39 Class 300, Table 40 Class 400, Table 41 Class 600 and Table 42 Class 900, covering NPS 26 through NPS 60.
Related Standards
- ASME B16.47 — Large-Diameter Flanges Overview (NPS 26–60)
- ASME B16.47 Series A Flanges (MSS SP-44)
- ASME B16.5 — Flanges NPS 1/2–24
- ASME B16.21 — Nonmetallic Gaskets (incl. Class 75 sizes)
- Our Flange Range
- ASTM A105 Carbon Steel Forgings
- ASTM A182 F304 Stainless Steel
FAQ
What is the difference between Series A and Series B?
Series A follows MSS SP-44 (larger OD and bolt circle); Series B follows API 605 (more compact, fewer larger bolts). They are dimensionally different and cannot be interchanged.
Does Series B include Class 75?
Yes — Class 75 exists only in Series B, given in ASME B16.47 Table 37. It is a low-pressure rating for large-diameter water and air service, with ratings terminating at 350°C.
What pressure classes are available in Series B?
Class 75, 150, 300, 400, 600 and 900, NPS 26 through NPS 60. Dimensional tables are ASME B16.47 Tables 37 through 42.
When would I choose Series B?
Where the existing system calls for API 605 drilling, or where the compact weight advantage of Series B on large NPS piping matters. Always confirm the series before ordering.
What materials do you supply?
A105 carbon steel, A182 F304/F316 stainless steel and A350 LF2 low-temperature steel, forged and machined to the Series B tables.