Large Diameter Flanges
Big-bore flanges for the lines that outgrow ASME B16.5. From NPS 26 through 60 in B16.47 Series A and B, out to custom sizes past 200 inches, here is how large diameter works and how to spec it right.
What Counts as a Large Diameter Flange?
Large diameter begins where ASME B16.5 leaves off. B16.5 dimensions run through NPS 24. Past that, from NPS 26 to 60, weld neck and blind flanges are governed by ASME B16.47. Above 60 inches there is no standard dimensional table, so the flange is engineered and built to the job.
That single jump, from 24 to 26 inches, changes the whole conversation. Outside diameters, bolt patterns, and weights climb fast, tolerances get less forgiving, and a flange that shows up wrong is not a small mistake to eat. Getting the standard, series, and material right the first time is the whole game.
Texas Flange has worked this end of the size range since 1986, in carbon steel, stainless, and exotic alloys, and produces custom sizes well beyond the standard tables.
ASME B16.47: Series A vs. Series B
Once a line passes NPS 24, ASME B16.47 takes over for weld neck and blind flanges through NPS 60. It splits into two series, A and B, that came from different legacy standards and are not interchangeable. Picking the wrong one is a real problem on the install, not a paperwork detail.
Series A traces to MSS SP-44. It is the heavier option: a larger outside diameter, fewer but larger bolts, and ring-type joint facing from Class 300 through 900. That extra metal is what handles external loads, bending moments from equipment nozzles, thermal stress, and the sheer weight of big-bore pipe, so Series A is the usual pick for new construction and transmission.
Series B traces to API 605. It is lighter: a smaller outside diameter and bolt circle, but more bolts packed closer together, which stiffens the joint against gasket creep and relaxation. It also adds a Class 75 rating that Series A does not have. Series B is common on retrofit and replacement work and on pressure-driven lines where external loading is manageable.
| Attribute | Series A (MSS SP-44) | Series B (API 605) |
|---|---|---|
| Legacy standard | MSS SP-44 | API 605 |
| Outside diameter | Larger | Smaller |
| Bolting | Fewer, larger bolts | More, smaller bolts |
| Bolt circle | Larger | Smaller |
| RTJ facing | Supported, Class 300 to 900 | Not covered |
| Class 75 rating | Not available | Available |
| Relative weight | Heavier | Lighter |
| External load rating | Higher | Lower |
| Typical use | New construction, high external loading | Retrofit and replacement, internal-pressure focus |
The two series will not bolt to each other. Match the series already in the line or follow the spec, and when a job sits on the edge, our team can confirm the right call before material ships. Full dimensions by class are on our Series A and Series B dimension pages.
How a Large Flange Joins the Pipe
Two questions get mixed up at this size and they are not the same. How a large flange is made, a forged weld neck, a flange cut from plate, or a rolled ring, is a manufacturing question. How it joins the pipe, butt weld, fillet weld, or bolted, is an installation question. Here is the installation side.
| Type | How It Joins the Pipe | Welding Note |
|---|---|---|
| Weld Neck | Butt weld to matching pipe bevel | Full-penetration circumferential butt weld; preferred for high pressure, temperature, and fatigue |
| Slip-On / Ring-Type | Pipe inserted, then fillet welded | Typically two fillet welds (inside and outside) when the hub or plate allows; ring flanges may be single fillet per design and code |
| Blind | No pipe weld | Bolted joint only; face and gasket rules still apply |
| Lap Joint / Stub End | Stub end welded; flange floats | The flange itself is not structurally welded to the pipe |
At this size, weight and handling drive the install as much as the weld. Flanges are rigged and aligned before bolt-up, large butt welds are commonly inspected by RT or UT, and bolt-up torque is specified rather than guessed. Large slip-on and plate flanges tend to serve lower-pressure duty, while weld neck carries the severe-service, high-pressure work.
Size and Standard Coverage
Large diameter is not one standard but several, each owning a size band and a service world. Here is how the coverage lines up, and what we provide across it.
| Standard | Size Range | Flange Types | Notes |
|---|---|---|---|
| ASME B16.5 | NPS 1/2 to 24 | All standard types | Standard flanges up to 24 inches |
| ASME B16.47 Series A | NPS 26 to 60 | Weld Neck, Blind | MSS SP-44 lineage; Classes 150 to 900; RTJ 300 to 900 |
| ASME B16.47 Series B | NPS 26 to 60 | Weld Neck, Blind | API 605 lineage; Classes 75 to 900 |
| AWWA C207 | NPS 4 to 144 | Ring and Hub (steel) | Waterworks; lettered Classes B, D, E, F |
| Custom / plate | Beyond NPS 60, to 200-plus | Plate, Weld Neck, Blind, special | Engineered and built to the print |
For exact dimensions and weights across Series A and Series B pressure classes, see our flange dimensions and weights reference. Large-diameter waterworks flanges are detailed on our AWWA C207 page. AWWA C207 covers carbon and alloy steel, with Classes B, D, and E running to NPS 144 and Class F to NPS 48. Stainless-steel flanges for water service follow AWWA C228.
Materials for Large Diameter Service
Material choice at large diameter carries an extra wrinkle: forging size. Standard A105 forgings top out around 10,000 pounds, so past a certain size a large-diameter flange is either cut from A516 Grade 70 plate or supplied as a large forging rather than standard A105. Stainless, alloy, and high-yield pipeline grades scale up the same way.
| Material | Grade | Typical Service |
|---|---|---|
| Forged Carbon Steel | ASTM A105 | General service within standard forging weight limits |
| Plate Carbon Steel | ASTM A516 Gr. 70 | Large-diameter plate flanges above the A105 forging limit |
| Low-Temp Carbon Steel | ASTM A350 LF2 | Cold-service transmission and process lines |
| Stainless Steel | ASTM A182 F304/F316 | Corrosive, water treatment, and chemical service |
| Chrome-Moly Alloy | ASTM A182 F11/F22/F5/F9/F91 | High-temperature power and refining service |
| High-Yield Pipeline | ASTM A694 F52/F60/F65 | High-pressure oil and gas transmission |
MTRs are available on all grades. For domestic or AIS-compliant material on infrastructure work, note the requirement on the RFQ and we will quote to it.
Where Large Diameter Flanges Go
Large diameter flanges show up wherever the pipe gets big: oil and gas transmission pipelines, water and wastewater transmission mains, power generation, and petrochemical process headers. Oil and gas leans on B16.47 Series A and high-yield A694 for transmission, municipal water runs on AWWA C207, and power and refining pull in chrome-moly alloys for temperature. Different worlds, same problem: the connection has to hold at size.
Since 1986, Texas Flange has supplied and produced large-diameter flanges across ASME B16.47, AWWA C207, and custom prints, in carbon, stainless, and exotic alloys, from NPS 26 to 200 inches and beyond. Past NPS 60 or off-standard OD, we build to your drawing. Send us the size, class, series, and material, or the print, and we will quote it.
