ASME B16.47 Flanges Explained: Series A vs. Series B
If you’re working with large-diameter piping involving anything above 24 inches, ASME B16.5 is no longer your primary standard. That’s where ASME B16.47 takes over. And once you’re in B16.47 flange territory, you’ve got a choice to make: Series A or Series B.
We get this question regularly. The short answer is that they’re not interchangeable, they come from different legacy standards, and picking the wrong one can create real problems on an installation. Here’s what you actually need to know.
What Is ASME B16.47?
ASME B16.47 is the standard covering large-diameter steel pipe flanges, specifically weld neck flanges and blind flanges in sizes 26″ through 60″. It covers pressure classes 75, 150, 300, 400, 600, and 900, and its materials align with ASME B16.5 (with the exception of nickel alloys), so the same pressure-temperature tables generally apply.
The reason B16.47 exists is straightforward: once you get past NPS 24″, the dimensions in B16.5 simply don’t extend far enough as they weren’t designed by the committee. Large-diameter piping in pipelines, petrochemical plants, power generation facilities, and water infrastructure needs its own dimensional standard, and that’s ASME/ANSI B16.47.
What makes B16.47 different from most other flange standards is that it’s divided into two categories, Series A and Series B, each with distinct dimensional requirements, bolt counts, and design philosophies.
Series A: The Heavier-Duty Option
ASME B16.47 Series A flanges trace their lineage to MSS SP-44, a standard developed by the Manufacturers Standardization Society. If you’ve ever worked with large size “MSS SP-44 flanges,” those are now designated as ASME B16.47 Series A. Same reference dimensions, with an updated designation.
Series A flanges are built heavier. They have a larger outside diameter and use fewer bolt holes than their Series B counterparts at the same size and pressure class. That larger OD and bolt circle, combined with bigger individual bolts, makes Series A the preferred choice for applications with higher external loading demands — situations where the flange assembly needs to handle not just internal pressure but also bending moments, thermal stress, and the weight of large-bore pipe. When defaulting to one or the other, Series A is the industry standard.
Series A also has broader face type coverage. Ring-type joint (RTJ) flanges are defined under Series A from Class 300 through Class 900. If your project spec calls for RTJ in large-diameter service, Series A is where you’re working.
Where Series A flanges are commonly specified:
- New pipeline construction, particularly in oil and gas transmission
- High-pressure or high-temperature process piping where external loading is a consideration and body weight matters
- Applications that have historically used MSS SP-44 and are maintaining dimensional continuity, including high yield material reference grades
- Projects requiring RTJ face type in large diameters for optimal sealing
Series B: The Leaner, More Bolt-Dense Option
ASME B16.47 Series B flanges come from API 605, the American Petroleum Institute standard for large-diameter carbon steel flanges. Like Series A, the designation has since been incorporated into B16.47 — “API 605 flanges” and “B16.47 Series B” refer to the same dimensions.
Series B flanges are dimensionally lighter than Series A and are sometimes called “compact flanges” by industry regulars. They have a smaller outside diameter and a smaller bolt circle, but they compensate by using more bolts at a smaller size. More fasteners, closer together, distributed around a tighter circle. The result is less flange rotation after installation and a stiffer bolted joint, which matters in systems where gasket creep and joint relaxation are concerns.
The tradeoff is that Series B doesn’t support RTJ face types within the B16.47 standard, and it isn’t necessarily rated for the same level of external loading as Series A. For applications where the primary concern is internal pressure rather than external forces, that’s often a reasonable tradeoff for a lighter, more cost-effective flange.
Series B also includes Class 75, a pressure class not found in Series A, which is relevant for certain lower-pressure large-diameter applications. This is a popular option for vacuum applications where weight of an assembly is a primary concern.
Where Series B flanges are commonly specified:
- Refurbishment and replacement work on existing systems originally built to API 605
- Large-diameter piping systems where external loads are manageable and internal pressure performance is the priority
- Applications requiring Class 75 pressure rating
- Projects where the smaller bolt circle reduces flange movement after installation
Series A vs. Series B: The Key Differences at a Glance
| Series A (MSS SP-44) | Series B (API 605) | |
| Legacy standard | MSS SP-44 | API 605 |
| Outside diameter | Larger | Smaller |
| Bolt count | Fewer, larger bolts | More, smaller bolts |
| Bolt circle diameter | Larger | Smaller |
| RTJ face type | Supported (Class 300–900) | Not covered |
| Class 75 available | No | Yes |
| Flange weight | Heavier | Lighter |
| External load rating | Higher | Lower |
| Typical use | New construction, high external loading | Replacement/retrofit, internal pressure focus |
| Cost | Generally higher | Generally lower |
So Which One Do You Need?
The most straightforward rule: follow the project spec. Although Series A is more common, if your engineering documents call out “ASME B16.47 Series A” or “Series B,” that’s your answer. Series A and Series B flanges are not interchangeable — the bolt patterns are different, the outside diameters are different, and mating one to the other in a bolted joint is not going to work without modifications.
If you’re selecting a series for new design work, the deciding factors are usually:
External loading. If your large-bore line is subject to significant bending moments, equipment nozzles, compressor inlets, or systems with thermal expansion, Series A’s heavier construction handles those loads better.
System continuity. Replacement or maintenance work on an existing line should match whatever series was originally installed. Mixing series in the same bolted joint is not an option.
Face type requirements. If the application calls for ring-type joint facing, you need Series A.
Weight and cost constraints. If your application is primarily pressure-driven with manageable external loads, Series B delivers a lighter, more economical flange that still meets ASME B16.47 requirements.
When in doubt, talk to your piping engineer before you order. Large-diameter flanges aren’t small-dollar items, and getting the series wrong means starting over with new material.
A Note on Flange Types Within B16.47
ASME B16.47 covers weld neck flanges and blind flanges. That’s it. If your project needs slip-on flanges or other configurations in large diameters, you’re technically outside of B16.47 and looking at other standards such as B16.1 for cast iron, or custom-designed flanges per process requirements. Large diameter slip-ons do exist in our catalog to an “industry standardized design” but outside of ASME code, in which case it will require your engineer to sign off on its use for your applications. For large-diameter weld neck and blind flange dimensions and weights across Series A and Series B pressure classes, we have full dimensional data available on our website, as well as templates for you to use to design your own custom flanges.
The Bottom Line
ASME B16.47 exists specifically for large-diameter applications that outgrow B16.5, and the Series A vs. Series B distinction reflects two different engineering philosophies that developed in parallel before being unified under a single standard. Series A is built heavier with more capacity for external loading; Series B is leaner, more bolt-dense, and more commonly used for replacement work on legacy API 605 systems.
Both series follow ASME B16.47, both are acceptable under the same pressure-temperature ratings, and neither is categorically “better” — the right answer depends entirely on what your project spec, your piping engineer, and the existing system require.
If you’re working through a large-diameter flange requirement and need help confirming the right series, pressure class, or dimensional data, reach out to the Texas Flange team. We’ve been navigating these specifications since 1986 and we can help you get to the right answer before material hits your dock.
