What Is a Hub Flange?

A hub flange is any flange with an integral tapered hub that transitions from the flange body to the pipe wall thickness. The hub distributes bending stress gradually from the stiff flange ring into the more flexible pipe, reducing stress concentration at the weld joint. Weld neck flanges are the most common example—their long, tapered hub is the defining feature that makes them suitable for high-pressure, high-temperature, and cyclic-loading applications.

Hub Geometry per ASME B16.5

Parameter	Description
Hub length	Distance from flange back face to weld end
Hub taper	Gradual reduction from flange ring thickness to pipe wall thickness
Bore	Matches pipe ID (schedule-specific)
Weld end	Butt-weld bevel per ASME B16.25
Hub OD (at flange)	Larger than pipe OD; tapers down to pipe OD at weld end

Hubbed vs Non-Hubbed Flanges

Feature	Hubbed Flange (WN)	Non-Hubbed Flange (SO, Blind)
Stress distribution	Gradual transfer through hub taper	Abrupt transition; higher stress concentration
Fatigue resistance	Excellent (smooth stress path)	Lower (stress riser at weld)
Pressure rating	Full ASME B16.5 rating	Same, but with lower fatigue life
Temperature cycling	Superior performance	More prone to fatigue cracking
Weld type	Single butt weld	Fillet weld(s) (SO) or no weld (blind)
Radiographic inspection	Butt weld can be RT inspected	Fillet welds harder to inspect
Cost	Higher (forging + machining)	Lower

Hub Height by NPS and Class

NPS	Class 150 Hub (mm)	Class 300 Hub (mm)	Class 600 Hub (mm)	Class 900 Hub (mm)
4	55.6	60.5	73.2	82.6
6	62.0	69.9	82.6	95.3
8	68.3	76.2	91.9	108.0
12	76.2	88.9	108.0	127.0

Approximate hub heights for weld neck flanges per ASME B16.5, A105 carbon steel.

Common Mistake

The hub is integral to the forging—it cannot be added later. A slip-on flange with a butt-welded extension pipe is not equivalent to a weld neck flange. The stress distribution characteristics are entirely different, and the fabricated version does not meet ASME B16.5 requirements.

Why the Hub Matters

The hub serves three engineering functions:

1. Stress transition: Smoothly transfers bending and axial loads from the rigid flange body to the flexible pipe wall
2. Reinforcement: Adds material at the flange-to-pipe junction where stresses are highest
3. Alignment: Provides a long straight bore for accurate pipe-to-flange fit-up during welding

Pro Tip

For critical service applications—lethal fluids, cyclic pressure, and temperatures above 400degF—always specify weld neck flanges over slip-on flanges. The hub design provides a significantly longer fatigue life and is easier to inspect by radiographic testing.

Read the full guide to flanges