Weld neck vs slip-on flange: which one to specify
A weld neck flange has a long tapered hub that is butt-welded to the pipe — the choice for high pressure, high or low temperature and cyclic service, and it can be radiographed. A slip-on flange slides over the pipe and is fillet-welded inside and out — cheaper and easier to fit, but weaker and with much lower fatigue life, so it is kept to lower-pressure, non-cyclic lines.
How each flange is attached
| Weld neck (WN) | Slip-on (SO) | |
|---|---|---|
| Joint to pipe | One full-penetration butt weld | Two fillet welds, inside and outside |
| Bore | Matches the pipe ID — specify the schedule | Slightly larger than the pipe OD |
| Inspection | Radiography or UT of the butt weld | Surface methods (PT / MT) on fillet welds |
| Fit-up | Pipe cut and bevelled to length | Pipe length is more forgiving |
Strength and fatigue
The tapered hub of a weld neck flange moves stress away from the weld and gives a smooth bore, which is why it is used in high-pressure, high-temperature and cyclic service. Commonly quoted design guidance puts the strength of a slip-on flange under internal pressure at around two-thirds of a weld neck flange, and its fatigue life at around one-third. Piping codes and most company specifications restrict slip-on flanges in severe cyclic service.
Cost and weight
Slip-on flanges are lighter and cheaper to buy, and quicker to align on site. Weld neck flanges cost more and need accurate pipe cutting, but the single butt weld is often the cheaper joint to inspect on critical lines.
Dimensions: they bolt together
For the same size and class, ASME B16.5 gives weld neck and slip-on flanges the same outside diameter, bolt circle and bolting, so they mate with each other and with blind flanges. See the flange dimensions chart.
Which to specify
- High pressure (Class 600 and above), high or low temperature, cyclic or vibrating lines: weld neck.
- Utility, cooling-water and low-pressure lines in Class 150 / 300: slip-on is usually acceptable.
- Expensive alloys (825, 2507, C276) on low-pressure lines: consider a lap joint flange with an alloy stub end.
- Small bore, high pressure: socket weld flange.
Materials we supply
Both types in stainless (ASTM A182 F304L / F316L / F904L), duplex (F51 / F60), super duplex (F53 / F55), nickel alloys (ASTM B564) and carbon steel (A105, A350 LF2). See weld neck flanges and slip-on flanges.