Article
Slip-on and weld neck flanges are the two most ordered flange types. Both connect a pipe to a bolted joint, but they do it in very different ways, and the choice affects cost, inspection and long-term reliability.
How each one is attached
- Slip-on (SO) – the pipe slides into the flange bore. The flange is fillet welded on the outside at the hub and on the inside at the face.
- Weld neck (WN) – the flange hub is butt-welded end to end with the pipe, giving one full-penetration weld.
Comparison
| Point | Slip-on | Weld neck |
|---|---|---|
| Flange price | Lower | Higher |
| Fit-up | Easy, tolerant of pipe length | Needs accurate cutting and alignment |
| Welds | Two fillet welds | One butt weld |
| Radiography | Not practical on fillet welds | Butt weld can be radiographed |
| Strength under internal pressure | About two-thirds of a weld neck | Full strength |
| Fatigue life | Roughly one-third of a weld neck | Best of the common flange types |
| Typical service | Low pressure water, air, utilities | High pressure, high temperature, hazardous fluids |
When a slip-on flange makes sense
- Cooling water, fire water and utility lines
- Low pressure, ambient temperature service
- Where site cutting and fit-up need to be simple
- Class 150 and 300 lines where the piping specification allows it
When to insist on a weld neck flange
- Class 600 and above, or high temperature
- Thermal cycling, vibration or heavy external loads
- Flammable, toxic or lethal fluids
- Lines requiring 100% radiography
Total installed cost
A slip-on flange is cheaper to buy, but it needs two welds instead of one. On small sizes the saving is clear; on larger sizes the extra welding time can narrow the gap. The decision should follow the piping specification first and price second.
Ordering notes
Slip-on flanges are bored slightly larger than the pipe OD, so they do not need a schedule. Weld neck flanges must be bored to the pipe schedule. Example: Slip-On Flange 4" Class 150 RF, ASTM A182 F304L, ASME B16.5.
Frequently asked questions
Is a slip-on flange weaker than a weld neck?
Yes. Its strength under internal pressure is about two-thirds of a weld neck flange and its fatigue life is much lower.
Does a slip-on flange need a pipe schedule in the order?
No. It is bored to fit over the pipe OD, unlike a weld neck flange which must match the pipe bore.
Can slip-on flanges be used in Class 600?
Some codes and specifications allow it, but most plant specifications restrict slip-on flanges to lower classes and non-critical service.
Key takeaways
- Slip-on: cheaper and easier, for low pressure utility service.
- Weld neck: stronger, radiographable, for severe service.
- Follow the piping spec first, then compare installed cost.
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