What Is a Lap Joint Flange?
A lap joint flange consists of two separate components: a loose ring that slides over the pipe and a separately fabricated stub end that is welded to the pipe. The stub end provides the gasket sealing surface while the ring provides the bolting force. The ring remains free to rotate around the pipe, which means bolt holes can be aligned independently of the pipe orientation. No welding is performed on the flange ring itself, which allows the ring and stub end to be made from different materials for cost optimization.
Design and Dimensions
The ring dimensions follow ASME B16.5 for sizes NPS 1/2 through 24. The ring has a flat face on both sides with no raised face since the sealing surface is provided by the stub end. The inner diameter of the ring is larger than the pipe outer diameter to provide clearance for free rotation. Ring thickness matches the standard flange dimensions for each pressure class. The stub end provides the raised face sealing surface and acts as the backing for the ring to bear against during bolt tightening.
| NPS | Ring OD (in) | Ring Thickness (in) | Ring ID (in) | Stub End Lap Thickness (in) |
|---|---|---|---|---|
| 2 | 6.00 | 0.69 | 3.06 | 0.44 |
| 4 | 9.00 | 0.94 | 5.31 | 0.56 |
| 6 | 11.00 | 1.00 | 7.56 | 0.62 |
| 8 | 13.50 | 1.12 | 9.69 | 0.69 |
| 10 | 16.00 | 1.19 | 12.00 | 0.75 |
| 12 | 19.00 | 1.25 | 14.38 | 0.81 |
Stub End Types
There are two standard stub end designs per ASME B16.9. Type A is a short pattern with a straight hub designed for butt-welding to the pipe. Type B is a long pattern with a tapered hub that provides smoother stress transition, similar to a weld neck flange. The stub end bore is machined to match the pipe wall thickness, and the lap thickness provides the gasket contact surface. A radius at the lap-to-hub transition reduces stress concentration at this critical point.
Pressure Ratings
The pressure rating of a lap joint assembly is limited by the stub end material and design rather than the ring. Since the ring does not contact the process fluid, it can be made from a lower grade material than the stub end. Lap joint flanges are typically not used above Class 600 because the two-piece design has less rigidity than one-piece flanges. The temperature rating follows the stub end material limits, which may be higher than the ring material limits since the ring operates at a lower temperature than the process fluid.
Materials
The two-piece design allows significant cost optimization through material selection. The ring can be made from a less expensive material such as carbon steel even when the piping system uses expensive alloys. The stub end material matches the pipe material to maintain corrosion resistance and mechanical properties. Common ring materials include ASTM A105 carbon steel, A350 LF2 low-temperature carbon steel, and A182 F304/F316 stainless steel. Stub end materials match the piping material specification, which may include duplex, nickel alloy, or other specialty grades.
Applications
Lap joint flanges are ideal for piping systems requiring frequent disassembly for cleaning and inspection, such as in chemical and food processing plants. They are commonly used in applications where bolt-hole alignment would be difficult with fixed flanges. Glass-lined or plastic-lined piping systems frequently use lap joint flanges to avoid welding on the lined pipe. Systems using expensive pipe materials benefit from the cost savings of using a less expensive ring material. Cooling water and utility services that require periodic maintenance are also common applications.
Lap Joint vs Other Flange Types
The primary advantage of lap joint flanges is the elimination of bolt-hole alignment issues during installation. The rotating ring can be positioned to match any bolt pattern without rotating the pipe. The two-piece design also reduces cost when mixing material grades. However, lap joint flanges have less strength and rigidity than one-piece flanges and present more potential leak paths because of the separate stub end joint. They also require longer overall assembly length compared to slip-on flanges due to the stub end design.
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