Lap Joint vs Slip-On Flange
Same B16.5 dimensions, very different assembly: one flange is welded to the line, the other floats behind a stub end and never sees the process.
- Dimensions
- Same B16.5 table as slip-on
- Wetted?
- Lap joint never touches the fluid
- Extra component
- Lap joint needs a stub end
- Bolt hole alignment
- Lap joint rotates freely
Short answer. A lap joint flange is a loose backing ring that never contacts the process — a separate stub end provides the bore and the gasket face. A slip-on is welded directly to the pipe and is fully wetted. The lap joint costs an extra component but lets you put a cheap carbon steel flange behind an expensive alloy stub end, and it spins freely so bolt holes always line up.
ASME B16.5 tabulates one set of outside diameters, thicknesses, bolt circles and bolt patterns per class per size, and every flange type in that class uses them. A lap joint and a slip-on of the same class and size have identical bolting and are fully interchangeable at the joint — the difference is entirely in how each attaches to the pipe.
Two components instead of one
The lap joint arrangement splits the flange's two jobs. The stub end — a short pipe section with a flared lap — is butt-welded to the line, provides the bore, and forms the gasket surface. The backing flange slides on behind the lap and does nothing but deliver bolt load. Stub end dimensions come from ASME B16.9, not B16.5.
Because the backing flange is never wetted, it can be a different material from the line. On a stainless or duplex system that turns a large, expensive alloy flange into a small alloy stub end plus an ordinary carbon steel ring — a genuine saving that grows with size and alloy cost.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Flange OD | Bolt circle | Bolting | Cl 150 hub | Cl 300 hub |
|---|---|---|---|---|---|
| NPS 1/2 | 3.5088.90 mm | 2.3860.45 mm | 4 × 1/2" | 0.6215.75 mm | 0.8822.35 mm |
| NPS 3/4 | 3.8898.55 mm | 2.7569.85 mm | 4 × 1/2" | 0.6215.75 mm | 1.0025.40 mm |
| NPS 1 | 4.25107.95 mm | 3.1279.25 mm | 4 × 1/2" | 0.6917.53 mm | 1.0626.92 mm |
| NPS 1 1/4 | 4.62117.35 mm | 3.5088.90 mm | 4 × 1/2" | 0.8120.57 mm | 1.0626.92 mm |
| NPS 1 1/2 | 5.00127.00 mm | 3.8898.55 mm | 4 × 1/2" | 0.8822.35 mm | 1.1930.23 mm |
| NPS 2 | 6.00152.40 mm | 4.75120.65 mm | 4 × 5/8" | 1.0025.40 mm | 1.3133.27 mm |
| NPS 2 1/2 | 7.00177.80 mm | 5.50139.70 mm | 4 × 5/8" | 1.1228.45 mm | 1.5038.10 mm |
| NPS 3 | 7.50190.50 mm | 6.00152.40 mm | 4 × 5/8" | 1.1930.23 mm | 1.6942.93 mm |
| NPS 3 1/2 | 8.50215.90 mm | 7.00177.80 mm | 8 × 5/8" | 1.2531.75 mm | 1.7544.45 mm |
| NPS 4 | 9.00228.60 mm | 7.50190.50 mm | 8 × 5/8" | 1.3133.27 mm | 1.8847.75 mm |
| NPS 5 | 10.00254.00 mm | 8.50215.90 mm | 8 × 3/4" | 1.4436.58 mm | 2.0050.80 mm |
| NPS 6 | 11.00279.40 mm | 9.50241.30 mm | 8 × 3/4" | 1.5639.62 mm | 2.0652.32 mm |
| NPS 8 | 13.50342.90 mm | 11.75298.45 mm | 8 × 3/4" | 1.7544.45 mm | 2.4461.98 mm |
| NPS 10 | 16.00406.40 mm | 14.25361.95 mm | 12 × 7/8" | 1.9449.28 mm | 2.6266.55 mm |
| NPS 12 | 19.00482.60 mm | 17.00431.80 mm | 12 × 7/8" | 2.1955.63 mm | 2.8873.15 mm |
| NPS 14 | 21.00533.40 mm | 18.75476.25 mm | 12 × 1" | 3.1279.25 mm | 3.0076.20 mm |
| NPS 16 | 23.50596.90 mm | 21.25539.75 mm | 16 × 1" | 3.4487.38 mm | 3.2582.55 mm |
| NPS 18 | 25.00635.00 mm | 22.75577.85 mm | 16 × 1 1/8" | 3.8196.77 mm | 3.5088.90 mm |
| NPS 20 | 27.50698.50 mm | 25.00635.00 mm | 20 × 1 1/8" | 4.06103.12 mm | 3.7595.25 mm |
| NPS 24 | 32.00812.80 mm | 29.50749.30 mm | 20 × 1 1/4" | 4.38111.25 mm | 4.19106.43 mm |
Lap joint flanges use the slip-on dimensions. The mating stub end is dimensioned by ASME B16.9.
The rotation advantage
A welded flange's bolt holes are fixed the moment the weld cools. Get the orientation wrong on a long spool with a flange at each end and the correction is a cut and a reweld. A lap joint flange spins on the stub end until the moment the bolts go in, which is why they show up on large fabricated spools, on tank nozzles, and on anything that has to mate with equipment whose bolt pattern is not certain until it arrives.
Lap joint
- Spins freely, so bolt holes always line up — ideal for large or awkward spools
- Cheap carbon steel flange behind an expensive alloy stub end
- Easy to dismantle for cleaning or inspection
- Weakest of the six in fatigue
- Requires a matching stub end, adding a component
Slip-on
- Cheaper than weld neck and tolerant of small alignment errors
- One bore fits every schedule in the size
- Shorter than a weld neck, useful in tight spaces
- Two fillet welds instead of one butt weld; harder to inspect
- Roughly a third of the fatigue life of a weld neck
Where not to use one
Lap joints are the weakest of the six types in fatigue. The lap is a stress riser and the backing flange bears on it unevenly. Keep them out of cyclic service, out of high-temperature work, and off lines subject to significant external moment.
Dimensions: lap joint flanges · slip-on flanges · B16.9 fittings and stub ends · weld neck vs slip-on.
Common questions
What is a stub end?
A short butt-welding component with a flared lap at one end, dimensioned by ASME B16.9. It provides the bore and the gasket surface for a lap joint flange, which supplies only bolt load.
Do lap joint flanges use the same dimensions as slip-on flanges?
Yes. ASME B16.5 publishes one set of dimensions covering both, so the outside diameter, bolt circle and bolting are identical.
Why use a lap joint flange on stainless piping?
Because the backing flange is never wetted, so it can be carbon steel. On large alloy lines that removes most of the flange's alloy content and a large part of its cost.
Are lap joint flanges suitable for high pressure?
They carry the same class rating, but they have the poorest fatigue performance of the six types, so most specs exclude them from cyclic and high-temperature service regardless of class.