A lap joint is a flange split into two parts that do different jobs. A stub end is butt welded to the pipe and provides the gasket face and all the fluid contact. A loose backing ring sits behind the stub end's lap and carries the bolt load. It never touches the process.
That separation of duties is the whole point, and it buys two things that no single-piece flange can offer.
It rotates
Because the backing ring is not attached to anything, it spins freely on the pipe until the bolts go in. Bolt holes can be brought into alignment at assembly rather than being fixed when the flange was welded.
That matters more often than it sounds:
- Connecting to equipment whose flange orientation is fixed and not negotiable
- Spool pieces that have to drop into an existing run with tight tolerances
- Systems dismantled and reassembled frequently, where fighting bolt hole alignment every time is a real cost
- Field fabrication where nobody wants to depend on the welder having got the rotation right
It saves alloy money
This is usually the bigger driver. The backing ring is never wetted, so it does not need to be the corrosion resistant material.
On a stainless, duplex or nickel alloy system, a lap joint lets you buy an alloy stub end and a carbon steel backing ring. The stub end is a relatively small piece of expensive metal. The backing ring is the large heavy piece, and it is cheap steel.
The saving scales with alloy cost and size. On a large diameter nickel alloy line, the difference between an alloy weld neck flange and an alloy stub end with a carbon steel backing ring is substantial. On a small carbon steel line it is not worth the extra part.
One caution: the backing ring sits in whatever the external environment is. In a marine or coastal plant, a carbon steel ring on a stainless system will corrode externally even though it never sees the process. Coat it, or accept replacing it.
What you give up
Lap joint is not as strong as a weld neck, and it is noticeably worse in cyclic service.
- Fatigue life. The load path runs through the lap, which is a geometric discontinuity. Lap joints are generally rated for lower fatigue duty than weld necks and are not the choice for vibrating or thermally cycling lines.
- Bending. The arrangement does not carry bending moment as well as a one-piece flange.
- Two parts to order and track. Stub end and backing ring are separate line items, often in different materials, and they have to arrive together.
- The lap radius must match. The backing ring's inner corner radius has to suit the stub end lap. Mismatch means the ring bears on the wrong place.
Stub ends: long and short
Stub ends come in two lengths, and they are covered by ASME B16.9 along with other butt welding fittings.
| Type | Characteristic | Notes |
|---|---|---|
| Long pattern, Type A | Longer barrel | Standard ASME lap joint stub end. The usual specification |
| Short pattern, Type B | Shorter barrel | MSS pattern, used where space is tight. Check ring compatibility |
The lengths are not interchangeable in a fixed space, and a short stub end may need a different backing ring bore. Specify which pattern you want.
Ordering a lap joint arrangement
- Nominal size and pressure class for the backing ring, which sets the drilling and thickness
- Schedule or wall thickness for the stub end, so it matches the pipe
- Stub end material and specification, the wetted alloy
- Backing ring material, which can be cheaper, plus any coating requirement
- Stub end pattern, long or short
- Face finish on the stub end lap, because that is the gasket surface
- Certification, noting that MTRs are usually needed on the wetted part and sometimes on both
Where it fits against the alternatives
| Requirement | Choose |
|---|---|
| High pressure, fatigue, thermal cycling | Weld neck |
| Rotational alignment needed at assembly | Lap joint |
| Expensive alloy, low to moderate duty | Lap joint, cheap backing ring |
| Frequent dismantling | Lap joint |
| Cheapest carbon steel joint, moderate duty | Slip-on |
| Bolt pattern of a larger size, smaller bore | Reducing flange |
See ring spacers and blinds for the rest of the loose ring family.