Weld Neck vs Long Weld Neck
A standard weld neck (WN) flange per ASME B16.5 has a tapered hub that transitions from the flange face to the pipe wall thickness over a short distance. A long weld neck (LWN) flange extends the hub significantly, creating an integrated pipe nozzle and flange in a single forging. The LWN eliminates the need for a separate pipe stub between the flange and the vessel or equipment nozzle.
Comparison Table
| Feature | Weld Neck (WN) | Long Weld Neck (LWN) |
|---|---|---|
| Hub length | Standard per ASME B16.5 (short taper) | Extended (custom length, typically 150-300 mm) |
| Standard | ASME B16.5 | ASME B16.5 + MSS SP-97 (dimensions); custom specs |
| Manufacturing | Forged and machined | Forged from a single billet, machined to nozzle length |
| Function | Pipe-to-pipe or pipe-to-equipment connection | Direct vessel/column nozzle connection |
| Bore | Matches pipe ID (standard bore) | Custom bore to match vessel nozzle ID |
| Pipe stub needed | Yes (welded to pipe) | No (hub acts as the pipe stub) |
| Stress distribution | Good (tapered hub absorbs bending) | Excellent (longer taper, no girth weld near flange) |
| Weld joints | 1 butt weld to pipe + 1 butt weld to nozzle | 1 butt weld to vessel shell (no intermediate joint) |
| Weight | Standard | Heavier (extra forged material) |
| Cost | Lower | Higher (more material, custom machining) |
| Lead time | Stock availability common | Longer (custom forging) |
| Applications | General piping systems | Vessel nozzles, column nozzles, heat exchangers |
Key Differences
Elimination of pipe-to-flange weld. In a standard arrangement, a nozzle consists of a pipe stub welded to a WN flange. This creates two butt welds (pipe-to-flange and pipe-to-vessel). An LWN flange eliminates the pipe-to-flange weld by integrating the stub into the forging. This reduces the number of welds on the nozzle, which reduces NDE costs, fabrication time, and potential failure points.
Stress performance. The longer hub of an LWN distributes bending moments and thermal stresses more gradually than a standard WN. This is critical for nozzles on vessels subjected to cyclic loading, thermal gradients, or high external piping loads.
Custom bore. LWN flanges can be machined with a bore that matches the vessel nozzle ID rather than the standard pipe schedule. This provides a smooth internal flow path without steps or mismatches at the nozzle-to-pipe transition.
Cost and lead time. LWN flanges are more expensive than standard WN flanges because they require larger forgings and more machining. They are typically made to order rather than stocked. However, the savings from eliminating a butt weld, its associated NDE, and PWHT can offset the higher flange cost.
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