What Is a Weld Neck Flange?

A weld neck flange (WN) is a pipe flange with a long, tapered hub that is butt-welded to the pipe. The hub geometry transfers stress away from the flange face, distributes it across the pipe wall, and reduces turbulence at the joint. This makes WN flanges the go-to choice for high-pressure and high-temperature piping systems.

Property	Detail
Standard	ASME B16.5 (NPS 1/2-24), ASME B16.47 (NPS 26-60)
Pressure classes	150, 300, 600, 900, 1500, 2500
Face types	RF, RTJ, FF
Common materials	A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy)
Connection	Single V-groove butt weld to matching pipe
Bore	Must match pipe ID (specify pipe schedule on PO)
Hub shape	Tapered, gradually transitions from flange to pipe OD

How the Tapered Hub Works

The hub’s gradual taper creates a smooth transition from the thick flange body to the thinner pipe wall. This geometry eliminates abrupt changes in cross-section, which reduces stress concentrations at the weld joint. A single full-penetration butt weld joins the flange to the pipe, and this weld is fully radiographable—critical for lines requiring 100% NDE per ASME B31.3.

Pro Tip

Always specify the pipe schedule when ordering weld neck flanges. The flange bore must match the pipe ID. A Class 300, NPS 6, SCH 80 WN flange has a different bore than a Class 300, NPS 6, SCH 40 WN flange.

When to Use Weld Neck Flanges

WN flanges are specified where mechanical integrity and leak-tightness are critical:

• High-pressure process piping (Class 600 and above)
• High-temperature lines (above 400degF / 200degC)
• Cyclic service where thermal expansion and contraction stress the joints
• Lines requiring full NDE (radiographic examination of the butt weld)
• Lethal service and Category M fluid service per ASME B31.3

For lower-pressure utility services (cooling water, low-pressure steam), slip-on flanges are a more economical alternative. But wherever the piping specification calls for butt-weld connections, the weld neck flange is standard.

Common Mistake

Ordering WN flanges without specifying the schedule leads to bore mismatches. A NPS 8 Schedule 40 pipe has an ID of 7.981 inches; Schedule 80 has an ID of 7.625 inches. Wrong bore = misalignment, turbulence, and potential weld defects.

WN Flange vs Other Types

Compared to slip-on flanges, WN flanges cost more (typically 20-40% higher) but offer superior fatigue resistance. ASME B31.3 assigns a strength reduction factor of 1.0 to butt-welded WN connections versus 0.87 for slip-on flanges with double fillet welds. For stud bolt selection, bolt sizes and quantities are identical between WN and slip-on flanges of the same NPS and class.

Read the full guide to flanges