Slip-On vs Lap Joint Flange
Slip-on (SO) and lap joint (LJ) flanges are both low-cost alternatives to weld neck flanges. They share a similar appearance but differ in how they connect to the pipe and how the sealing surface is formed.
Comparison Table
| Feature | Slip-On (SO) | Lap Joint (LJ) |
|---|---|---|
| Connection | Slides over pipe, double fillet welded (front + back) | Slides over pipe behind a stub end; no weld to flange |
| Stub end required | No | Yes (stub end provides the sealing face) |
| Welds required | 2 fillet welds (flange to pipe) | 1 butt weld (stub end to pipe) |
| Sealing face | On the flange itself (raised face) | On the stub end (not the flange) |
| Rotation | Fixed after welding | Free to rotate for bolt hole alignment |
| Flange material | Must match pipe (CS, SS, alloy) | Can be carbon steel (stub end matches pipe) |
| Cost (flange only) | Low | Low (but requires stub end) |
| Installed cost | Low (two simple fillet welds) | Moderate (butt weld + stub end cost) |
| Alignment ease | Moderate (must align holes before welding) | Excellent (rotate flange after stub end is welded) |
| Pressure rating | Class 150-600 (limited by fillet weld) | Class 150-300 (limited by stub end design) |
| Fatigue resistance | Moderate | Lower (stress concentration at stub end) |
| Standard | ASME B16.5 | ASME B16.5 (flange) + MSS SP-43 or ASME B16.9 (stub end) |
When to Use Slip-On
Slip-on flanges are the default for non-critical, low-pressure services where cost savings over weld neck flanges are important. They are common in:
- Utility piping (water, air, nitrogen)
- Low-pressure process lines (Class 150 and Class 300)
- Applications where pipe stress analysis shows low bending loads at the flange connection
- Large-bore, low-pressure systems where weld neck flanges are prohibitively expensive
The double fillet weld (one on the pipe OD at the flange hub, one on the pipe end at the bore) provides adequate strength for most moderate services.
When to Use Lap Joint
Lap joint flanges are specified when:
- Bolt hole alignment is difficult (crowded pipe racks, tight spaces). The flange rotates freely around the stub end after welding.
- Exotic pipe materials are used (stainless steel, duplex, nickel alloys). The rotating flange can be carbon steel, which saves significant cost. Only the stub end, which contacts the process fluid, needs to be the exotic alloy.
- Frequent disassembly is expected (systems requiring regular cleaning or inspection).
- Galvanic isolation is needed between dissimilar metals in the piping system.
For guidance on flange face types and gasket selection, see the flange face finish guide.
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