Views: 1 Author: Monica Publish Time: 2026-03-05 Origin: Site
A lap joint flange is a two-component piping fitting that consists of a loose flange ring and a stub end welded to the pipe.
This design allows the flange ring to rotate freely around the stub end for easy bolt hole alignment. The stub end is butt-welded to the pipe and provides the sealing surface, while the loose ring clamps against it with a gasket. Lap joint flange enables quick disassembly, reduces installation stress, and suits systems with minor misalignments or space limits.
Lap joint flange dimension specifications follow the ASME B16.5 standard for nominal pipe sizes from ½ inch to 24 inches and pressure classes 150 to 2500.
Dimensions match those of slip-on flanges except for the internal radius that fits the stub end lap. Each pressure class level is specified separately below with representative key dimensions (outside diameter, minimum thickness, bolt circle diameter, number of bolt holes, and approximate weight).
Class 150 Lap Joint Flange Dimensions
Class 150 Lap Joint Flanges are suitable for general service applications with lower pressure requirements. The table below provides complete dimensions chart according to ASME B16.5.
NPS | Outside Diameter (O) | Thickness (T) | Length Thru Hub (Y) | Hub Diameter (X) | Bore (B) | Bolt Circle (W) | Number of Holes | Hole Diameter | Approx Weight (lbs) |
1/2 | 3.50 | 0.44 | 0.62 | 1.19 | 0.90 | 2.38 | 4 | 0.62 | 1 |
3/4 | 3.88 | 0.50 | 0.62 | 1.50 | 1.11 | 2.75 | 4 | 0.62 | 2 |
1 | 4.25 | 0.56 | 0.69 | 1.94 | 1.36 | 3.12 | 4 | 0.62 | 2 |
1¼ | 4.62 | 0.62 | 0.81 | 2.31 | 1.70 | 3.50 | 4 | 0.62 | 3 |
1½ | 5.00 | 0.69 | 0.88 | 2.56 | 1.95 | 3.88 | 4 | 0.62 | 3 |
2 | 6.00 | 0.75 | 1.00 | 3.06 | 2.44 | 4.75 | 4 | 0.75 | 5 |
2½ | 7.00 | 0.88 | 1.12 | 3.56 | 2.97 | 5.50 | 4 | 0.75 | 7 |
3 | 7.50 | 0.94 | 1.19 | 4.25 | 3.60 | 6.00 | 4 | 0.75 | 8 |
3½ | 8.50 | 0.94 | 1.25 | 4.81 | 4.10 | 7.00 | 8 | 0.75 | 11 |
4 | 9.00 | 0.94 | 1.31 | 5.31 | 4.60 | 7.50 | 8 | 0.75 | 13 |
5 | 10.00 | 0.94 | 1.44 | 6.44 | 5.69 | 8.50 | 8 | 0.88 | 15 |
6 | 11.00 | 1.00 | 1.56 | 7.56 | 6.75 | 9.50 | 8 | 0.88 | 19 |
8 | 13.50 | 1.12 | 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 |
14 | 21.00 | 1.38 | 3.12 | 15.75 | 14.18 | 18.75 | 12 | 1.12 | 105 |
16 | 23.50 | 1.44 | 3.44 | 18.00 | 16.19 | 21.25 | 16 | 1.12 | 140 |
18 | 25.00 | 1.56 | 3.81 | 19.88 | 18.20 | 22.75 | 16 | 1.25 | 160 |
20 | 27.50 | 1.69 | 4.06 | 22.00 | 20.25 | 25.00 | 20 | 1.25 | 195 |
24 | 32.00 | 1.88 | 4.38 | 26.12 | 24.25 | 29.50 | 20 | 1.38 | 275 |
Class 300 Lap Joint Flange Dimensions
For higher pressure applications, Class 300 lap joint flanges feature increased thickness and bolt circle dimensions.
NPS | Outside Diameter (O) | Thickness (T) | Length Thru Hub (Y) | Hub Diameter (X) | Bore (B) | Bolt Circle (W) | Number of Holes | Hole Diameter | Approx Weight (lbs) |
1/2 | 3.75 | 0.56 | 0.88 | 1.50 | 0.90 | 2.62 | 4 | 0.63 | 2 |
3/4 | 4.62 | 0.62 | 1.00 | 1.88 | 1.11 | 3.25 | 4 | 0.75 | 3 |
1 | 4.88 | 0.69 | 1.06 | 2.12 | 1.38 | 3.50 | 4 | 0.75 | 3 |
1¼ | 5.25 | 0.75 | 1.06 | 2.50 | 1.72 | 3.88 | 4 | 0.75 | 4 |
1½ | 6.12 | 0.81 | 1.19 | 2.75 | 1.97 | 4.50 | 4 | 0.88 | 6 |
2 | 6.50 | 0.88 | 1.31 | 3.31 | 2.46 | 5.00 | 8 | 0.75 | 7 |
2½ | 7.50 | 1.00 | 1.50 | 3.94 | 2.97 | 5.88 | 8 | 0.88 | 10 |
3 | 8.25 | 1.12 | 1.69 | 4.62 | 3.60 | 6.62 | 8 | 0.88 | 13 |
3½ | 9.00 | 1.19 | 1.75 | 5.25 | 4.10 | 7.25 | 8 | 0.88 | 17 |
4 | 10.00 | 1.25 | 1.88 | 5.75 | 4.60 | 7.88 | 8 | 0.88 | 22 |
5 | 11.00 | 1.38 | 2.00 | 7.00 | 5.69 | 9.25 | 8 | 0.88 | 28 |
6 | 12.50 | 1.44 | 2.06 | 8.12 | 6.75 | 10.62 | 12 | 0.88 | 39 |
8 | 15.00 | 1.62 | 2.44 | 10.25 | 8.75 | 13.00 | 12 | 1.00 | 58 |
10 | 17.50 | 1.88 | 3.75 | 12.62 | 10.92 | 15.25 | 16 | 1.13 | 91 |
12 | 20.50 | 2.00 | 4.00 | 14.75 | 12.92 | 17.75 | 16 | 1.25 | 140 |
14 | 23.00 | 2.12 | 4.38 | 16.75 | 14.18 | 20.25 | 20 | 1.25 | 190 |
16 | 25.50 | 2.25 | 4.75 | 19.00 | 16.19 | 22.50 | 20 | 1.38 | 250 |
18 | 28.00 | 2.38 | 5.12 | 21.00 | 18.20 | 24.75 | 24 | 1.38 | 295 |
20 | 30.50 | 2.50 | 5.50 | 23.12 | 20.25 | 27.00 | 24 | 1.38 | 370 |
24 | 36.00 | 2.75 | 6.00 | 27.62 | 24.25 | 32.00 | 24 | 1.63 | 550 |
Class 400 Lap Joint Flange Dimensions
Class 400 lap joint flanges are designed for medium-high pressure services with correspondingly heavier dimensions.
NPS | Outside Diameter (O) | Thickness (T) | Length Thru Hub (Y) | Hub Diameter (X) | Bore (B) | Bolt Circle (W) | Number of Holes | Hole Diameter | Approx Weight (lbs) |
1/2 | 3.75 | 0.56 | 0.88 | 1.50 | 0.90 | 2.62 | 4 | 0.62 | 2 |
3/4 | 4.62 | 0.62 | 1.00 | 1.88 | 1.11 | 3.25 | 4 | 0.75 | 3 |
1 | 4.88 | 0.69 | 1.06 | 2.12 | 1.38 | 3.50 | 4 | 0.75 | 4 |
1¼ | 5.25 | 0.75 | 1.06 | 2.50 | 1.72 | 3.88 | 4 | 0.75 | 5 |
1½ | 6.12 | 0.81 | 1.19 | 2.75 | 1.97 | 4.50 | 4 | 0.88 | 7 |
2 | 6.50 | 0.88 | 1.31 | 3.31 | 2.46 | 5.00 | 8 | 0.75 | 8 |
2½ | 7.50 | 1.00 | 1.50 | 3.94 | 2.97 | 5.88 | 8 | 0.88 | 12 |
3 | 8.25 | 1.12 | 1.69 | 4.62 | 3.60 | 6.62 | 8 | 0.88 | 15 |
3½ | 9.00 | 1.19 | 1.75 | 5.25 | 4.10 | 7.25 | 8 | 1.00 | 20 |
4 | 10.00 | 1.25 | 1.88 | 5.75 | 4.60 | 7.88 | 8 | 1.00 | 25 |
5 | 11.00 | 1.38 | 2.00 | 7.00 | 5.69 | 9.25 | 8 | 1.00 | 32 |
6 | 12.50 | 1.44 | 2.06 | 8.12 | 6.75 | 10.62 | 12 | 1.00 | 45 |
8 | 15.00 | 1.62 | 2.44 | 10.25 | 8.75 | 13.00 | 12 | 1.12 | 67 |
10 | 17.50 | 1.88 | 3.75 | 12.62 | 10.92 | 15.25 | 16 | 1.25 | 105 |
12 | 20.50 | 2.00 | 4.00 | 14.75 | 12.92 | 17.75 | 16 | 1.38 | 160 |
14 | 23.00 | 2.12 | 4.38 | 16.75 | 14.18 | 20.25 | 20 | 1.38 | 215 |
16 | 25.50 | 2.25 | 4.75 | 19.00 | 16.19 | 22.50 | 20 | 1.50 | 280 |
18 | 28.00 | 2.38 | 5.12 | 21.00 | 18.20 | 24.75 | 24 | 1.50 | 350 |
20 | 30.50 | 2.50 | 5.50 | 23.12 | 20.25 | 27.00 | 24 | 1.62 | 435 |
24 | 36.00 | 2.75 | 6.00 | 27.62 | 24.25 | 32.00 | 24 | 1.88 | 650 |
Class 1500 Lap Joint Flange Dimensions
Class 1500 lap joint flanges are heavy-duty components designed for high-pressure applications. Note that Class 2500 dimensions are available but limited to NPS 12 and smaller.
NPS | Outside Diameter (O) | Thickness (T) | Length Thru Hub (Y) | Hub Diameter (X) | Bore (B) | Bolt Circle (W) | Number of Holes | Hole Diameter | Approx Weight (lbs) |
1/2 | 4.75 | 0.88 | 1.25 | 1.50 | 0.90 | 3.25 | 4 | 0.88 | 4 |
3/4 | 5.13 | 1.00 | 1.38 | 1.75 | 1.11 | 3.50 | 4 | 0.88 | 5 |
1 | 5.88 | 1.13 | 1.63 | 2.06 | 1.38 | 4.00 | 4 | 1.00 | 8 |
1¼ | 6.25 | 1.13 | 1.63 | 2.50 | 1.72 | 4.38 | 4 | 1.00 | 9 |
1½ | 7.00 | 1.25 | 1.75 | 2.75 | 1.97 | 4.88 | 4 | 1.13 | 12 |
2 | 8.50 | 1.50 | 2.25 | 4.13 | 2.46 | 6.50 | 8 | 1.00 | 25 |
2½ | 9.63 | 1.63 | 2.50 | 4.88 | 2.97 | 7.50 | 8 | 1.13 | 35 |
3 | 10.50 | 1.88 | 2.88 | 5.25 | 3.60 | 8.00 | 8 | 1.25 | 47 |
4 | 12.25 | 2.13 | 3.56 | 6.38 | 4.60 | 9.50 | 8 | 1.38 | 75 |
5 | 14.75 | 2.88 | 4.13 | 7.75 | 5.69 | 11.50 | 8 | 1.63 | 140 |
6 | 15.50 | 3.25 | 4.69 | 9.00 | 6.75 | 12.50 | 12 | 1.50 | 170 |
8 | 19.00 | 3.63 | 5.63 | 11.50 | 8.75 | 15.50 | 12 | 1.75 | 285 |
10 | 23.00 | 4.25 | 7.00 | 14.50 | 10.92 | 19.00 | 12 | 2.00 | 485 |
12 | 26.50 | 4.88 | 8.63 | 17.75 | 12.92 | 22.50 | 16 | 2.13 | 630 |
Important Notes on Lap Joint Flange Dimensions
Standard Applicability: ASME B16.5 covers flange dimensions from NPS 1/2 through NPS 24. For sizes larger than 24 inches, refer to ASME B16.47 Series A or B standards.
Dimensional Tolerances: Key tolerances per ASME B16.5 include:
Outside diameter up to 24 inches: ±1/16 inch
Outside diameter over 24 inches: ±1/8 inch
Thickness for 18 inches and smaller: +1/8 inch, -0 inch
Bolt circle diameter: ±1/16 inch
Stub End Compatibility: Lap joint flanges are designed to be used with stub ends. Type A stub ends are machined specifically for lap joint flanges, while Type C ends are fabricated from pipe and can be used with either lap joint or slip-on flanges.
For accurate flange selection, always verify the specific pressure class and facing type required for your application, and consult the complete ASME B16.5 standard for detailed specifications.
While the basic two-piece flange design is standard, lap joint flanges can be categorized by the facing on the stub end and the specific design of the backing flange.
Type 1: Raised Face (RF) Lap Joint Flange. This is the most common type, where the stub end features a raised ring that concentrates gasket pressure for a tighter seal.
In an RF lap joint flange, the stub end is machined with a raised face of either 1/16" or 1/4", depending on the pressure class. This raised surface is where the gasket sits. Because the stub end facing is the flange facing, mating a lap joint flange to another raised face flange is standard practice.
Type 2: Flat Face (FF) Lap Joint Flange. This type has a stub end with a completely flat sealing surface, used primarily when the mating flange is made of a brittle material like cast iron.
A flat face ensures full contact with the mating flange, distributing the bolt load evenly. This prevents cracking that could occur if a raised face flange were bolted to a flat face cast iron component.
Type 3: Ring-Type Joint (RTJ) Lap Joint Flange. Designed for high-pressure and high-temperature services, this type uses a stub end with a groove machined into it to accept a metal ring gasket.
The RTJ design provides a very robust metal-to-metal seal. When the bolts are tightened, the metal gasket is compressed into the groove, creating a seal that can withstand extreme conditions. This type ensures good alignment of the connection.
Type 4: Globack Lap Joint Flanges. This specialized type features spherical surfaces on the stub end and the backing flange to provide greater strength and stiffness.
The Globack design helps accommodate misalignment and offers improved load-bearing capacity compared to standard lap joint flanges, making them suitable for more demanding applications where some angular deflection might occur.
Lap joint flanges serve chemical processing plants. Their rotatable design supports rapid disassembly for cleaning and inspection of corrosive lines.
Petrochemical and oil refining systems rely on them for moderate-pressure piping. The assembly eases alignment during thermal expansion and routine maintenance.
Water treatment facilities apply lap joint flanges where frequent repairs or expansions occur. Rotation simplifies bolt-up in large layouts.
Food and beverage production lines use them to meet strict sanitation standards. Easy access reduces downtime during cleaning cycles.
Lap joint flange vs. slip-on flange
The primary difference is that a lap joint flange rotates freely and requires a stub end, whereas a slip-on flange is fixed after being welded directly to the pipe with two fillet welds. The lap joint is less robust than a weld neck flange but superior to slip-on and threaded connections. Slip-on types offer simpler, lower-cost installation for fixed low-pressure lines but limit disassembly.
Full face vs. lap joint FRP flange
Full face FRP flanges form a single rigid piece with bolt holes across the entire face for direct mating to flat surfaces. Lap joint FRP flanges use a Van Stone stub end with a rotatable backing ring. Full face suits have rigid connections; lap joints improve alignment and reduce stress in fiberglass piping systems.
Lap joint flange vs. weld neck
Lap joint flanges deliver easy assembly and disassembly at moderate cost. A weld neck flange is a one-piece, tapered hub flange that is butt-welded to the pipe, offering the highest integrity and strength for severe services.
Weld neck types provide greater strength and fatigue resistance for high-pressure, high-temperature, or cyclic service. Lap joint types excel in alignment and maintenance but offer lower overall joint integrity.
Proper maintenance ensures the longevity of the connection and prevents leaks.
Visual Inspection: Regularly check for signs of crevice corrosion between the backing flange and the stub end.
Cleaning: Use non-chloride cleaners on the stainless steel stub end to prevent stress corrosion cracking.
Bolt Lubrication: Apply a high-quality anti-seize compound to the bolts to ensure consistent torque and easy removal during future maintenance.
Alignment Check: Ensure the stub end remains flush against the gasket; if the backing flange becomes warped, replace it immediately to maintain a proper seal.
Is a lap joint flange with a raised face?
No. The loose flange ring is flat; the stub end provides any raised sealing surface.
What type of face do lap joint flanges have?
They have a flat face on the ring. The sealing face is on the stub end and can be flat or raised.
When to use a lap joint flange?
Use them when frequent disassembly, bolt alignment, or material cost savings are required in low- to moderate-pressure systems.
How to weld a lap joint flange?
Do not weld the flange ring. Butt-weld the stub end to the pipe after sliding the ring into place.
