Weld Neck vs Slip-On Flange

Quick Answer

Weld neck flanges use a single butt weld and a tapered hub for maximum strength. Slip-on flanges slide over the pipe and require two fillet welds. WN flanges are stronger, more expensive, and mandatory for high-pressure service. Slip-on flanges cost less and are acceptable for low-to-moderate pressure utility lines.

Side-by-Side Comparison

The differences between these two flange types affect design, cost, and installation time.

Feature	Weld Neck (WN)	Slip-On (SO)
Hub	Long tapered hub	Short, flat hub
Weld type	Single butt weld	Two fillet welds (front + back)
ASME B31.3 SRF	1.0	0.87
Fatigue strength	~100% of pipe	~67% of WN per ASME tests
Bore	Matches pipe ID (schedule-specific)	Slightly larger than pipe OD
Radiography	Fully radiographable	Not radiographable (fillet welds)
Approximate cost	Higher (20-40% more)	Lower
Alignment ease	Requires precise fit-up	Easier—slides over pipe
Typical class range	150-2500	150-600
Standard	ASME B16.5	ASME B16.5

Strength and Fatigue

ASME B16.5 rates WN and SO flanges at the same pressure-temperature values. The difference appears in the piping code. ASME B31.3 assigns weld neck connections a strength reduction factor (SRF) of 1.0, meaning the joint is as strong as the pipe itself. Slip-on connections receive an SRF of 0.87, reflecting the lower fatigue resistance of fillet welds.

In cyclic service (frequent thermal expansion, pressure surges, or vibration), this 13% strength reduction matters. That is why most project piping specifications default to WN flanges for process lines and allow SO flanges only on utility services.

Cost and Installation

Slip-on flanges are cheaper to buy and faster to install. They slide over the pipe with no need for precise end preparation. Two fillet welds—one on the inside, one on the outside—complete the connection. Total fabrication time is shorter than a WN butt weld, which requires bevel preparation, fit-up, root pass, and fill passes.

Pro Tip

For large-diameter piping (NPS 16+), the cost gap between WN and SO flanges narrows because material cost dominates over fabrication labor. Specifying WN at larger sizes adds little to the budget but significantly improves joint integrity.

When to Specify Each

Use weld neck flanges for:

• Process piping above Class 300
• Lines requiring 100% radiographic examination
• Cyclic or high-temperature service
• Lethal and Category M fluid service (ASME B31.3)
• All sizes NPS 16 and above (common project practice)

Slip-on flanges are acceptable for:

• Utility water, low-pressure air, and drain lines
• Class 150 and Class 300 non-critical services
• Temporary connections and tie-ins
• Applications where gasket and bolt selection is straightforward

When in doubt, default to weld neck. The upfront cost premium is small insurance against joint failure.

Read the full guide to flanges