What Is an ASME B16.5 Lap Joint Flange?
A lap joint (LJ) flange is a loose, rotatable flange that bears against the machined lap of a stub end welded to the pipe. Covered by ASME B16.5, it does not contact the process fluid directly and can be freely rotated around the pipe for easy bolt alignment — a great advantage on large or hard-to-align piping and on corrosion-resistant lines where the stub end can be stainless and the loose flange carbon steel.
How It Works
- A stub end (short flanged spigot per ASME B16.9) is butt-welded to the pipe
- The loose lap joint flange slides over the pipe and bears against the machined lap of the stub end
- Bolt load is transferred through the flange to the stub end lap — the flange itself sees no process pressure
- The flange can rotate, so bolt hole alignment is trivial even on large NPS sizes
Size & Pressure Class Range
- Sizes: NPS 1/2 through NPS 24, in Classes 150 through 2500
- Matched component: lap joint stub end per ASME B16.9 (Type A / B / C per MSS SP-43)
- Minimum lap joint flange thickness: tf listed separately in each dimensional table
Pressure-Temperature Ratings (Group 1.1 Carbon Steel, at 38°C)
| Class | 150 | 300 | 400 | 600 | 900 | 1500 | 2500 |
|---|---|---|---|---|---|---|---|
| Rating, bar | 19.6 | 51.0 | 68.0 | 102.0 | 153.1 | 255.1 | 425.1 |
Materials & Temperature Limits
| Component | Material Options | Notes |
|---|---|---|
| Lap joint flange (loose) | A105, A182 F304/F316, A350 LF2 | -29°C to 425°C (A105); stainless up to 538°C |
| Stub end | A403 WP304/WP316, A234 WPB | Wetted part — choose for corrosion service |
Dimensions
Lap joint flange dimensions — outside diameter O, minimum lap-joint thickness tf, hub diameter X, lapped hub length Y, bore B1 and corner radius r — are listed per class in the dimensional tables of ASME B16.5-2025: Table 8 (Class 150), Table 11 (Class 300), Table 14 (Class 400), Table 16 (Class 600), Table 18 (Class 900), Table 20 (Class 1500) and Table 22 (Class 2500), SI units. The lap outside diameter K that bears against the stub end is given in Table 5.
Related Standards
- ASME B16.5 — Pipe Flanges Overview (NPS 1/2–24)
- ASME B16.9 — Lap Joint Stub Ends
- ASME B16.5 Weld Neck Flanges
- ASME B16.5 Slip-On Flanges
- ASME B16.21 — Nonmetallic Flat Gaskets
- Our Flange Range
- ASTM A182 F316 Stainless Steel
FAQ
What is the advantage of a lap joint flange?
The loose flange rotates freely, so bolt holes can be aligned without rotating the pipe — ideal on large NPS sizes, vessels and tanks. It also lets you use a cheaper carbon steel loose flange with a stainless wetted stub end.
What is a stub end?
A short flanged spigot butt-welded to the pipe, with a machined lap that the loose flange bears against. Lap joint stub ends are covered by ASME B16.9 (Table T6.1-9). You must order both the lap joint flange and the matching stub end.
Are lap joint flanges pressure rated themselves?
The flange follows the same Class pressure-temperature rating table as other B16.5 flanges, but the sealing surface is the stub end lap and gasket — the loose flange only provides the bolt load.
What sizes are available?
NPS 1/2 through NPS 24, Classes 150 through 2500 per ASME B16.5. For NPS 26 and above, lap joint construction follows ASME B16.47.
Can I reuse a lap joint flange?
Yes — because the loose flange is not welded and does not contact the process, it can be unbolted and reused, while the welded stub end stays on the pipe.