What Is a Stub End? Lap Joint Connection

Quick Answer

A stub end is a short fitting with a flanged lip (hub) on one end and a butt weld end on the other. It pairs with a lap joint flange to create a bolted connection. The stub end contacts the process fluid; the flange does not. This allows the flange to be made from a cheaper material while only the stub end uses the expensive alloy.

Stub ends are covered by ASME B16.9 (Type A, standard wall) and MSS SP-43 (Type B, lighter wall for stainless steel). The lip OD matches the flange bore, and the hub length provides the weld connection to the pipe.

Types of Stub Ends

Type	Standard	Hub Length	Wall Thickness	Application
Type A	ASME B16.9	Standard (longer)	Matches pipe schedule	Carbon/alloy steel, high-pressure
Type B	MSS SP-43	Shorter	Schedule 5S or 10S	Stainless steel, low-pressure
Type C	MSS SP-43	Shortest	Light wall	Utility services

How Stub Ends Work

The stub end is butt welded to the pipe. The lap joint flange slips over the hub before welding and rests against the lip face. Bolts pass through the flange and compress a gasket between the stub end lip face and the mating flange face.

Key advantage: the lap joint flange can rotate freely around the stub end hub. This makes bolt-hole alignment trivial, which is valuable on large-diameter piping where rotating the entire pipe spool is impractical.

Cost Savings on Exotic Materials

The primary economic benefit appears in systems using expensive alloys. For a duplex stainless steel or nickel alloy piping system, only the stub end (which contacts the fluid) needs to be the exotic material. The lap joint flange can be carbon steel ASTM A105, which costs a fraction of the alloy equivalent.

Component	Material (Exotic System)	Approximate Relative Cost
Weld neck flange	Alloy (e.g., Inconel 625)	100%
Lap joint flange	Carbon steel A105	15-25%
Stub end	Alloy (e.g., B366 WPNIC2)	20-30%
Stub end + LJ flange	Combined	35-55%

For a 10-inch Class 300 connection in Inconel 625, switching from weld neck flanges to stub ends with carbon steel lap joints can save 45-65% on each flanged joint.

Dimensions

Stub end dimensions include the OD (matching pipe), hub length, lip OD (matching flange bore), and lip thickness. For a complete dimensional reference, see the stub ends guide and ASME B16.9 size charts.

Stub ends are not suitable for high-pressure services where flange rigidity and gasket compression are critical. Above Class 600, weld neck flanges are typically preferred over lap joint assemblies.

Read the full guide to pipe fittings