A lap joint flange looks like a slip-on flange from a distance, and on the drawing it is easy to confuse the two. The difference matters more than it looks: a lap joint flange does not seal against the pipe wall like a slip-on flange. It sits loose on a welded stub end, and that one design choice is what makes it the standard way to join stainless steel piping at a fraction of the cost. This article breaks down the two parts, the long and short pattern options, and where the arrangement stops being the right answer.
How a Lap Joint Joint Works
The joint is two pieces working together. The stub end is butt-welded to the pipe end, with a flange (lap) turned outward at the end. The lap joint flange is a loose ring that slides over the pipe before the stub end is welded, and backs against that turned flange face. When you tighten the bolts, the two stub end faces on either side of the joint squeeze a gasket between them - the loose flange carries the bolt load but never touches the process fluid. Because the flange rotates freely on the pipe, the bolt holes can be lined up against the mating flange before the bolts go in, which is the whole reason the design exists for aligned assembly.
| Part | What it is | Standard | Contacts the fluid |
| Stub end | Welded to pipe end, turned flange at face | Long pattern: ASME B16.9 / MSS SP-43; Short pattern: ASME B16.5 | Yes |
| Lap joint flange | Loose ring that backs the stub end lap | ASME B16.5 | No |
Long Pattern vs Short Pattern Stub End
Buyers often specify "stub end" without saying which pattern, and the two are not interchangeable in long-run service. The long pattern (type L) follows ASME B16.9 and MSS SP-43: it is longer, with the turned face set well back from the weld, so the heat of welding does not disturb the sealing face. The short pattern (type S) follows ASME B16.5: it is shorter and cheaper, but the weld sits closer to the face. Long pattern is the default for stainless piping that sees repeated assembly and disassembly; short pattern is the economy choice for static, non-critical joints.
| Feature | Long pattern (L) | Short pattern (S) |
| Standard | ASME B16.9, MSS SP-43 | ASME B16.5 |
| Length | Longer; weld further from sealing face | Shorter; more compact |
| Best for | Repeated dismantling, stainless piping | Static, low-cost joints |
| Cost | Higher | Lower |
Why It Saves Money on Stainless Steel Pipe
This is the reason the design exists. On a stainless steel line, a normal raised-face flange must be made of stainless steel all the way through - A182 F304 or F316 forged flange, which is expensive. With a lap joint arrangement, only the stub end touches the fluid, so the loose flange can be plain carbon steel A105. The flange never sees the process, so its corrosion resistance does not matter. On a large-bore stainless line the forged flange is the costlier component by far, and swapping it for carbon steel is where the saving comes from. The pipe itself and the stub end are still stainless, so the corrosion performance of the system is unchanged.
When It Is the Right Choice
- Stainless or alloy piping where the flange cost would dominate the joint cost
- Joints that must be dismantled often - pump, valve and equipment flanges in maintenance zones
- Installations where bolt holes must be aligned at assembly, and rotating a loose flange helps
- Large-bore lines where the forged flange saving is largest
Where It Does Not Belong
The arrangement has real limits. The sealing face is the turned lap of the stub end, which is weaker than a solid forged flange face - high vibration, cyclic loading or severe bending load will distort it. It is not the first choice for high-pressure, high-temperature critical service where a weld neck flange is specified for a reason. It also needs a properly selected gasket on the lap face; the geometry of the turned face does not accept every gasket type. And on small lines the cost saving disappears, because the flange is no longer the expensive item.
Installation and Ordering Notes
- Order the stub end and the flange together - the bore and lap dimensions must match, and the lap diameter must match the flange radius.
- Specify long or short pattern explicitly on the inquiry; do not assume the mill will choose.
- Material: stub end A403 WP304/WP316 for stainless lines; loose flange A105 unless the flange itself needs to resist external corrosion.
- After welding the stub end, dress the turned face before assembly - heat tint or weld spatter on the lap ruins the seal.
- Confirm the pressure class (ASME B16.5 Class 150 to 2500) on both the stub end and the flange, and match the mating flange.
