Article
A stub end is a short length of pipe with a flared lip at one end. It is welded to the pipe, and a loose lap joint flange slides behind the lip to bolt the joint together. Because the flange never touches the process fluid, the combination is a smart way to save cost in stainless and alloy piping.
Why use a stub end and lap joint flange?
- Lower cost – only the stub end needs to be in expensive stainless or alloy steel; the flange can be carbon steel.
- Easy bolt alignment – the flange rotates freely, so bolt holes line up without twisting the pipe.
- Frequent dismantling – useful where lines are opened often for cleaning or inspection.
Long and short pattern
| Pattern | Standard | Length | Typical use |
|---|---|---|---|
| Long (ASA / ANSI) | ASME B16.9 | Longer barrel | Process piping, more room for welding |
| Short (MSS) | MSS SP-43 | Shorter barrel | Thin-wall stainless lines, compact layouts |
Type A, B and C
- Type A – the standard stub end with a radiused lip, made to fit a standard lap joint flange. The most common type.
- Type B – designed for use with a standard slip-on flange used as a backing flange; it has a sharper corner.
- Type C – fabricated from pipe with a lap welded on or machined; it can be used with either lap joint or slip-on backing flanges.
Gasket and facing
The gasket sits on the face of the stub end lip, so the lip acts as the flange facing. The lip finish and flatness therefore need the same care as a raised face on a weld neck flange.
Limitations
Lap joint assemblies have lower fatigue resistance than weld neck flanges and are generally not chosen for severe cyclic or very high-pressure service. They are ideal for low to moderate pressure stainless systems in chemical, food and water plants.
A complete description
Stub End 4" SCH 10S, MSS SP-43 Type A, ASTM A403 WP316L-W + Lap Joint Flange 4" Class 150, ASTM A105, ASME B16.5
Frequently asked questions
What is the difference between MSS and ASA stub ends?
MSS SP-43 stub ends are the short pattern, while ASME B16.9 (ASA) stub ends are the long pattern with a longer barrel.
Can the lap joint flange be carbon steel on a stainless line?
Yes. The flange does not touch the fluid, so a carbon steel or coated flange is often used with a stainless stub end to save cost.
Which stub end type is most common?
Type A, used with a standard lap joint flange.
Key takeaways
- Stub end + lap joint flange = low cost in stainless systems.
- Choose long (B16.9) or short (MSS SP-43) pattern.
- Type A suits standard lap joint flanges; the stub lip is the gasket face.
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