Lap joint flanges, also known as backup flanges or loose ring flanges, are used in piping systems that require frequent disassembly or alignment flexibility. They consist of two components: a stub end welded to the pipe and a loose backing flange that slips over the stub and bolts to a mating flange. This design allows the backing flange to rotate, which makes it easier to align bolt holes during installation. Lap joint flanges do not seal by themselves; they rely on a gasket, which creates a secure seal between two surfaces.
These flanges work perfectly for low to moderate-pressure applications. They are commonly used where access is limited or regular maintenance is needed. They can accommodate different types of pipe and flange materials, which makes them suitable for systems carrying corrosive or high-temperature fluids. In addition to being a cost-effective option, lap joint flanges are valued for their ease of installation, alignment flexibility, and ability to be reused without the need to cut or re-weld pipe connections.
Lap joint flange dimensions are defined by ANSI/ASME B16.5 for sizes NPS ½″ to 24″ and pressure classes from 150 to 2500. For larger pipe sizes, NPS 26″ to 60″, dimensions follow ASME B16.47 standards. They are commonly manufactured from materials such as A105 forged carbon steel, A182, 304/316 forged stainless steel, and alloy steel to suit a wide range of service conditions.
| Nominal Pipe Size Range | Outside Diameter (A) | Flange Thickness (C) | Bolt Hole Diameter (H) |
|---|---|---|---|
| 1/2″ – 1 1/2″ | 3.5″ – 5″ | 0.44″ – 0.69″ | 0.62″ – 0.88″ |
| 2″ – 4″ | 6″ – 9″ | 0.75″ – 1″ | 0.75″ |
| 5″ – 6″ | 10″ – 11″ | 1.06″ – 1.12″ | 0.88″ |
| 8″ – 10″ | 13.5″ – 16″ | 1.31″ – 1.56″ | 0.88″ – 1″ |
| 12″ – 16″ | 19″ – 23.5″ | 1.75″ – 2″ | 1″ – 1.12″ |
| 18″ – 24″ | 25″ – 32″ | 2.06″ – 2.38″ | 1.25″ – 1.38″ |
The diagram below illustrates the typical components and key features of a lap joint flange connection used with a stub end.
Used in systems that require frequent disassembly, such as for maintenance, inspection, or cleaning.
Ideal for pipelines operating under moderate pressure, where heavy-duty sealing isn’t required.
Paired with corrosion-resistant stub ends to handle aggressive liquids while keeping material costs low.
Well-suited for installations that need to be assembled and dismantled quickly, like test setups or pilot systems.
Widely used on ships and offshore structures due to easy bolt alignment and corrosion resistance.
Used in oil, gas, chemical, and water systems that may need quick adjustments, expansions, or reconfiguration.
Raised face lap joint flanges have a small raised area around the ID. It helps center the gasket and improves sealing by compression during bolt tightening. Ideal where perfect sealing is required.
Flat face lap joint flanges have smooth surfaces with no raised areas. They work best with full-face gaskets and are usually used in low-pressure systems where sealing demands are low.
These flanges have machined grooves for metal ring gaskets, which create a strong, high-pressure seal. Common in extreme conditions like oil, gas, and chemical processing systems.
Lap Joints will have a radius on the ID. In some cases, you can modify an ANSI flange to accommodate a stubend.
Lap joint flanges are two-piece components used with a stub end, which allows the flange to rotate for easy bolt alignment.
The key difference lies in construction and use:
Lap joint flanges are not suitable for high-pressure or high-vibration systems. Their loose design can compromise sealing integrity in dynamic conditions, and they generally provide less structural strength than welded flange types.
To order lap joint flanges from API International, determine your required pipe size, pressure class, material, and any special standards (such as ANSI/ASME B16.5). Contact API International’s experienced team, and we will help you select the right flange for your application.
• Custom Bolt Holes, Overbore, Counterbore, Tapped, Welds, O-ring Grooves, Threads, Special Serrations, Bevels, Chamfers
• Metric Parallel, Metric Tapered, Parallel (BSPP), Taper Thread (BSPT), UN/UNF/UNC Thread, NPT (National Pipe Thread)
• MWD
• Contact API’s engineering department for exact specifications and tolerances.
Plastic: Delrin, POM, Lexan, Nylon 66, Nylon 6, PEEK, PP, PE, Acetal
Rubber: EPDM, Neoprene, Buna-N, Viton
Aluminum: A2011, A2024, A2017, A5052, A5056, A6061, A6063, A6463, A6082, A7075
Copper: 3600, 3604, 3771
Titanium: 1-28
Steel Series:
• Ductile Iron: ASTM A536
• Carbon/Mild Steel: A105, A36, A516-70, A572-50, etc.
• Stainless Steel:
- 300 Series – 303, 304, 304L, 316, 316L
- 400 Series – 420J2, 430, 434, 442
Special Metals: Hastelloy, Inconel, Incoloy, Duplex 2205, Super Duplex 2507, Chrome Steel, alloys, or their equivalents
Stock Finish, Spiral Serrated (Phonographic), Concentric Serrated, Smooth Finish, Cold Water Finish, MWD, Phonographic
Dipped, Nickel Plating, Zinc Plating (clear, yellow, black, red, blue), Super Agitine, Powder, Painted, Galvanized, Epoxy, Lacquer
Water/Wastewater, Irrigation, Agriculture, HVAC, Marine, Exhaust, Mining, Energy/Power Generation/Oil
• Forged:
- Ring Rolling:
• Materials: Carbon Steel, Stainless Steel, Duplex
• Size: 2–202”
• Cast:
- Centrifugal Casting (Industrial):
• Material: Stainless Steel
- Continuous Casting:
• Material: Stainless Steel
- Sand Casting:
• Material: A536, Stainless Steel, Ductile Iron
- Investment/Lost-Wax/Precision Casting
• Material: Stainless Steel
Machining: Milling, Turning, Facing, Boring, Drilling, Tapping, Counter Sinking, Laser Cutting, Water Jet, Plasma Cutting
| Products | Type | Materials | Size Range |
|---|---|---|---|
| Flanges | Flat-Face, Raised-Face, & Plate: Slip-On/Ring Weld, Neck, Blind, Socket Weld, Orifice, Reducing, Threaded | See Materials List | 5–202″ |
| Gaskets | Molded, Extruded, sheet | EPDM, Buna-N, NBR, Non-Asbestos (GNA), Red Rubber (GRR), Viton | 5–60″ |
| Screens For Filters | Basket Screens and Cone Screens (Perforated, Mesh) | Galv. Carbon Steel, Carbon Steel, Stainless Steel | 6”D x 8’-H |
| Rings | Exhaust Rings (Marmon Flanges for DPF Filters) | Carbon Steel, Stainless Steel, Aluminum | 1–16” |
| Seamless Rolled Rings | Forged | Carbon Steel, Stainless Steel, Aluminum | 1–165” |
| Square Hole Rings | Repair Couplers | Carbon Steel, Stainless Steel, Galvanized, Epoxy Coated | 3–32” |
| Fabricated Assemblies | Weldments, Pipe Spools, Valve Bodies, Silencer Housings | Carbon Steel, Stainless Steel | 2–24” |
| Long/Short Radius Elbows | EN 10253-2–S235, P235GH–P265GH–P265NL and ASTM A234, WPB, WPC/ A420 WPL6, WPL3/ A860 WPHY52/ ASTM A234 WP11, WP22, WP5, WP9–16Mo3 | 304L & 316L Stainless Steel | (45° & 90°): OD: 21.3mm (1/2”)–3048mm (120”) WT: Up to SCH. XXS/160 |
| Miscellaneous | Tubesheets / Diffuser Plates / Baffles / Ferrules / Sleeves / Pins | Carbon Steel, Stainless Steel, Duplex | 5–104” |
Who we are
API International is a manufacturer and wholesale distributor of flanges, expansion joints, valves, other pipe fittings and custom fabricated parts. We offer a wide variety of AIS & imported flanges along with other industrial pipe fitting and accessories. Customers turn to us when they need quick delivery of parts and are looking for a higher level of customer service. Standard parts are stocked in one of our conveniently located warehouses for quick distribution throughout the United States.
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