Article
The weld neck flange is the workhorse of oil and gas, refinery, petrochemical and power piping. Its long tapered hub gives it the best strength and fatigue performance of all common flange types, which is why piping specifications call for it in demanding service.
How the weld neck design works
The hub tapers from the flange ring down to the pipe wall thickness, and the flange is butt-welded to the pipe. This taper moves the weld away from the flange ring and spreads stress gradually, so the joint handles bending, thermal cycling and vibration far better than a slip-on flange.
Where weld neck flanges are used
- High pressure and high temperature lines
- Lines with cyclic loads, vibration or temperature swings
- Hazardous, flammable or toxic fluids
- Lines that must be fully radiographed
Advantages and limitations
| Advantages | Limitations |
|---|---|
| Highest strength and fatigue life | Heavier and more expensive than slip-on |
| Butt weld can be radiographed | Needs accurate pipe cutting and fit-up |
| Smooth bore, low turbulence | Bore must be ordered to match the pipe |
The bore must match the pipe
A weld neck flange is bored to the inside diameter of the pipe it joins. A 6" Class 300 flange for SCH 40 pipe has a different bore from one for SCH 80 pipe. If the bore is not stated, the flange may not match and a step will be left inside the weld. Always state the schedule, or give the pipe wall thickness in millimetres.
Classes and facings
Under ASME B16.5, weld neck flanges are made in Classes 150, 300, 400, 600, 900, 1500 and 2500 for sizes 1/2" to 24". Larger sizes come under ASME B16.47 Series A or B. The usual facing is raised face (RF); ring type joint (RTJ) is common from Class 600 upwards; flat face (FF) is used against cast iron equipment.
Materials
- Carbon steel: ASTM A105, A350 LF2 for low temperature
- Stainless steel: ASTM A182 F304/F304L, F316/F316L, F321, F347
- Duplex: ASTM A182 F51 and F53
- Alloy steel: ASTM A182 F11, F22, F91
A complete description
Weld Neck Flange, 6" Class 300 RF, bore to SCH 40S, ASTM A182 F316L, ASME B16.5 – 16 nos
Frequently asked questions
Why is the weld neck flange the strongest type?
Its tapered hub transfers stress smoothly into the pipe and the butt weld is full penetration, giving excellent fatigue resistance.
What does WNRF mean?
Weld neck, raised face – the most common weld neck flange combination.
What happens if the flange bore does not match the pipe?
A step forms inside the joint, which complicates welding, causes turbulence and can lead to rejection.
Key takeaways
- Weld neck flanges are the standard for severe service.
- Always give the bore or pipe schedule.
- Specify size, class, facing, bore, material and standard.
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