What Is a Long Weld Neck Flange?

A long weld neck (LWN) flange is a standard weld neck flange with an extended hub—essentially a flange and pipe nozzle machined from a single forging. The hub length can range from 150 mm to over 600 mm depending on the application. LWN flanges replace a separate nozzle-plus-flange assembly with one integral piece, eliminating a butt weld and reducing potential leak points.

Key Specifications

Property	Detail
Base standard	ASME B16.5 (flange dimensions)
Hub length	Custom—specified per project (typically 150-600+ mm)
Size range	NPS 1/2 to NPS 24
Pressure classes	150 through 2500
Face types	RF, RTJ
Common materials	A105 (CS), A182 F316 (SS), A182 F11/F22 (alloy)
Manufacturing	Machined from a single forging (not welded extension)
Bore	Custom—matches vessel nozzle or pipe ID

When to Use LWN Flanges

Long weld neck flanges are not a standard catalog item. They are custom-manufactured for specific applications:

• Vessel and tank nozzles: The extended hub passes through the vessel wall or insulation, providing a flanged connection at the outer surface
• Column nozzles: Distillation columns and reactors use LWN flanges to reach through thick insulation
• Wellhead connections: Upstream oil and gas applications where a one-piece forging provides higher integrity than a welded assembly
• Equipment with thick walls: Where the nozzle must penetrate a reinforced section

Pro Tip

Because LWN flanges are custom forgings, lead times are significantly longer than standard flanges—typically 8-16 weeks versus 2-4 weeks for stock WN flanges. Order early in the project schedule to avoid delays.

LWN vs Standard WN + Pipe Nozzle

Feature	Long Weld Neck	WN Flange + Pipe Nozzle
Pieces	1 (single forging)	2 (flange + pipe section)
Welds	1 (hub-to-vessel)	2 (pipe-to-flange + pipe-to-vessel)
Integrity	Higher (no mid-nozzle weld)	Standard
NDE requirement	1 weld to inspect	2 welds to inspect
Cost	Higher per piece	Lower per piece
Lead time	8-16 weeks (custom)	2-4 weeks (stock)
Weight	Heavier (solid forging)	Lighter

Common Mistake

Ordering LWN flanges with standard ASME B16.5 dimensions only. The hub length, bore, and OD at the base of the hub must all be specified. Provide a detailed dimensional sketch with the purchase order, including the bore at the flange face and at the hub base (weld end).

The one-piece design eliminates the weakest point in a nozzle assembly—the butt weld between the nozzle pipe and the flange. For critical services, this additional integrity justifies the cost premium and extended lead time.

Read the full guide to flanges