A lap joint flange is never ordered on its own. It slides over a stub end that is butt-welded to the pipe, and the two parts are usually in different materials. If they are quoted separately without a common set of data, the flange bore may not clear the stub end, or the stub end may not match the pipe wall.
How the Assembly Works
The stub end (lap joint stub end) is welded to the pipe and carries the gasket face. The lap joint flange is loose and rotates on the pipe, which makes bolt-hole alignment easy. Because only the stub end touches the process fluid, it is normally in the pipe material – for example stainless steel or a nickel alloy – while the flange can be carbon steel.
What to Specify Together
| Item | Stub end | Lap joint flange |
|---|---|---|
| Standard | ASME B16.9 (Type A, long or short pattern) or MSS SP-43 (Type A/B/C) | ASME B16.5 (lap joint) or B16.47 |
| Size | Nominal pipe size | Same nominal pipe size |
| Wall / class | Pipe schedule, e.g. Sch 10S or 40S | Pressure class, e.g. Class 150 or 300 |
| Material | Pipe material, e.g. ASTM A403 WP316L | Often ASTM A105 or A182 – process-wetted parts excluded |
| Pattern | Long (ASME) or short (MSS) pattern | Bore and corner radius to suit the stub end |
| Facing | Lap face of the stub end acts as the gasket seat | No raised face – bears on the stub end lap |
Checks Before Approving the Quote
- Stub end pattern (long or short) matches the space available and the flange.
- Flange bore clears the stub end outside diameter and corner radius.
- Stub end wall matches the pipe schedule for welding.
- Pressure class of the flange suits the line class; the joint rating is limited by the stub end and flange together.
- Gasket is sized for the stub end lap face.
Related: flange facing guide and flange RFQ template.
Frequently Asked Questions
Why are lap joint flanges used with stub ends?
The stub end is welded to the pipe and carries the gasket face, while the loose lap joint flange provides the bolting. This lets a carbon steel flange be used with a stainless or alloy pipe system.
What is the difference between long and short pattern stub ends?
Long pattern stub ends are made to ASME B16.9 and short pattern stub ends to MSS SP-43. They have different lengths, so the pattern must suit the piping layout and be stated on the order.
Does a lap joint flange have a raised face?
No. It bears on the lap of the stub end, which acts as the gasket seating surface.
