Lap joint flange dimensions are standardized by ASME B16.5 for sizes NPS ½” to 24″. For larger diameters (NPS 26″ to 60″), dimensions follow ASME B16.47 standards . A complete dimension set includes the backing flange’s outer diameter, bolt circle, and thickness, as well as the lap (or hub) dimensions that interface with the stub end.
These flanges are unique because they form a two-part connection with a separate stub end. The stub end is welded to the pipe, while the lap joint flange (or backing flange) remains loose, allowing it to rotate for easy bolt hole alignment.
Core Dimension Tables (ASME B16.5)
The following tables list the standard dimensions for common sizes and pressure classes. All dimensions are in inches unless noted.
Key Dimension References:
- O / A: Outside Diameter of Flange
- B: Bore (Minimum)
- C / K: Bolt Circle Diameter
- T / B: Flange Thickness
- X: Hub Diameter
- Y: Length Through Hub
ASME B16.5 Class 150 Lap Joint Flange Dimensions
| NPS | O (in) | T (in) | Y (in) | X (in) | Bore B (in) | Bolt Circle C (in) | No. of Holes | Hole Dia (in) | ~Weight (lb) |
| 1/2 | 3.50 | 0.44 | 0.63 | 1.19 | 0.90 | 2.38 | 4 | 0.63 | 1 |
| 3/4 | 3.88 | 0.50 | 0.63 | 1.50 | 1.11 | 2.75 | 4 | 0.63 | 2 |
| 1 | 4.25 | 0.56 | 0.69 | 1.94 | 1.38 | 3.13 | 4 | 0.63 | 2 |
| 2 | 6.00 | 0.75 | 1.00 | 3.06 | 2.46 | 4.75 | 4 | 0.75 | 5 |
| 3 | 7.50 | 0.94 | 1.19 | 4.25 | 3.60 | 6.00 | 4 | 0.75 | 8 |
| 4 | 9.00 | 0.94 | 1.31 | 5.31 | 4.60 | 7.50 | 8 | 0.75 | 13 |
| 6 | 11.00 | 1.00 | 1.56 | 7.56 | 6.75 | 9.50 | 8 | 0.88 | 19 |
| 8 | 13.50 | 1.13 | 1.75 | 9.69 | 8.75 | 11.75 | 8 | 0.88 | 30 |
| 10 | 16.00 | 1.19 | 1.94 | 12.00 | 10.92 | 14.25 | 12 | 1.00 | 43 |
| 12 | 19.00 | 1.25 | 2.19 | 14.38 | 12.92 | 17.00 | 12 | 1.00 | 64 |
ASME B16.5 Class 300 Lap Joint Flange Dimensions (Select Sizes)
| NPS | O (in) | T (in) | Y (in) | Bore (in) | Bolt Circle (in) | No. of Holes | Hole Dia (in) |
| 1/2 | 3.75 | 0.56 | 0.88 | 0.90 | 2.75 | 4 | 0.63 |
| 2 | 6.50 | 0.88 | 1.25 | 2.44 | 5.00 | 8 | 0.75 |
| 3 | 7.88 | 1.06 | 1.50 | 3.57 | 6.62 | 8 | 0.75 |
| 6 | 11.50 | 1.31 | 2.00 | 6.72 | 10.62 | 12 | 0.88 |
| 10 | 17.00 | 1.56 | 2.62 | 10.88 | 15.25 | 16 | 1.00 |
Critical Dimension Categories
When inspecting or ordering a lap joint flange, you must verify several key dimension groups.
| Dimension Category | Specific Measurements to Check | Why It’s Important |
| Flange Body | Outside Diameter (O), Flange Thickness (T) | Ensures the flange fits in the space and meets the pressure class strength requirement. |
| Bolt Connection | Bolt Circle Diameter (C), Number & Diameter of Bolt Holes | Critical. Must match the mating flange exactly for bolts to align and create proper seal pressure. |
| Hub / Lap Interface | Hub Diameter (X), Length Through Hub (Y), Bore (B) | Ensures proper fit with the stub end. The hub’s inner corner has a rounded radius (r) to accommodate the stub end’s flare . |
| Facing | Face Type (Flat Face is standard), Raised Face Height (if applicable) | Determines gasket type and sealing method. The sealing surface is actually provided by the stub end . |
Important Considerations for Accurate Specification
- Specify “True Lap Joint”: For strict adherence to ASME B16.5 length-through-hub (Y) dimensions, you must specify a “True Lap Joint”. Otherwise, a supplier may provide a more common slip-on flange that has been machined to a lap joint face, which can have a shorter hub length.
- Match the Stub End: Lap joint flanges are useless without the correct stub end. The stub end’s lap dimensions must match the flange’s hub dimensions (X, Y, r). Standards like MSS (regular length) and ASA (longer length) exist for stub ends .
- Understand Tolerances: Manufacturing has allowed variances. For example, the thickness tolerance for a flange 18 inches and under is +3.2mm / -0mm . Knowing these is crucial for quality control.
- Pressure Class Changes Everything: As the tables show, a Class 300 flange has a larger outer diameter, bolt circle, and thickness than a Class 150 flange of the same NPS. They are not interchangeable .
Applications and Limitations
Lap joint flanges are ideal for systems requiring frequent disassembly, inspection, or cleaning, as the connection can be taken apart without cutting the pipe . A major cost benefit is that the backing flange can be made from a less expensive material (like carbon steel) than the corrosion-resistant stub end (like stainless steel) that contacts the process fluid .
They are generally not recommended for high-pressure or high-vibration services, as the loose, non-welded connection provides less structural strength and can be prone to crevice corrosion .
Getting lap joint flange dimensions right is critical for a functional, leak-free connection that allows for easy maintenance. The specifications must include both flange and matching stub end details.
Need to ensure your lap joint flanges and stub ends meet exact specifications? We can provide True Lap Joint flanges and matching stub ends in any material combination for your application. Submit your project requirements for a precise quote: http://texasflange.com/lp12/
Frequently Asked Questions
Q: Are lap joint flange dimensions the same as slip-on flange dimensions?
A: They are similar but not identical. The key difference is in the length through the hub (Y), which is typically longer on a true lap joint flange . The bore of a lap joint flange also has a distinctive rounded corner (radius).
Q: How do I know if I need a MSS or ASA stub end?
A: The MSS standard is the most common. The ASA-type stub end is longer. Your system specifications or the need for a longer butt-weld landing area will dictate the choice .
Q: Can I use a flat face lap joint flange with a raised face flange?
A: No. The lap joint flange typically has a flat face, and the sealing surface is created by the raised face of the stub end. You should not mix flat and raised face flanges in a bolted connection, as it will not seal properly.
Q: What materials are available for lap joint flanges?
A: The backing flange can be made from virtually any flange material, from carbon steel (A105) to various stainless steels (304/316), alloys, and exotic metals like Inconel or Hastelloy. This flexibility is a primary advantage of the design.