Views: 10 Author: Monica Publish Time: 2026-03-05 Origin: Site
Lap joint flange — also called a loose flange, a loose ring flange, or a stub end assembly — is a two-piece pipe flange: a stub end that is butt-welded to the pipe, and a loose backing ring that slides over the pipe and spins freely around the stub end. The stub end creates the sealing face and is the only part that ever touches the process fluid; the ring only supplies the bolt clamping force.
Because the ring is never welded or wetted, you can spin it by hand to line up bolt holes, break and remake the joint as often as you like, and make the ring out of cheap carbon steel while the wetted stub end stays 316L, duplex, or a nickel alloy. In large-bore or high-alloy piping, that material split is usually the single biggest reason to specify a lap joint instead of a one-piece flange.
| Item | Answer |
|---|---|
| Also called | Loose flange, loose ring flange, lap joint stub end assembly, Van Stone assembly (FRP) |
| Main standard | ASME B16.5 (NPS 1/2 to 24); ASME B16.47 for larger sizes |
| Pressure classes | 150, 300, 400, 600, 900, 1500, 2500 |
| Components | Stub end (wetted) + loose backing ring + gasket + bolts and nuts |
| Welds required | One butt weld — stub end to pipe. The ring is never welded. |
| Wetted parts | Stub end and gasket only |
| Rotation | Ring rotates 360 degrees before the bolts are tensioned |
| Best for | Frequent disassembly, hard alignment, expensive wetted alloys, lined or FRP pipe |
| Avoid when | Severe cyclic service, high vibration, heavy bending moments |
A lap joint flange works by splitting two jobs that an ordinary flange does at the same time: the stub end seals against the gasket, while the loose ring supplies the bolt clamping force. Because the ring is not welded to the pipe, it can rotate freely until the bolts are tightened.
Picture a washer sitting behind a short lip. The stub end is welded to the pipe and ends in a flat, wide lip — the lap. The gasket sits on the face of that lip. The loose ring drops over the pipe and presses against the back of the lip. When you tension the bolts, the ring squeezes the stub-end lip forward onto the gasket, and the gasket seals against the mating flange. Nothing rotates the pipe, and nothing welds the ring.
Two practical consequences follow directly from that load path. First, bolt-hole alignment stops being a fitting problem: you rotate the ring, not the spool. That is a real saving on large diameters, on spools that are hard to swing, and on tie-ins into existing headers. Second, the ring never sees the fluid, so it does not need the fluid's corrosion resistance — only the strength to carry the bolt load. That is what makes the alloy-stub-end-plus-carbon-steel-ring combination possible.
The same mechanism also sets the limit. Bolt load travels through a comparatively thin lap rather than through a heavy tapered hub, so a lap joint has less fatigue strength and less resistance to bending moments than a weld neck flange. It is an excellent maintenance-and-alignment joint, not a high-integrity structural one.
A lap joint assembly has four parts: the stub end, the loose backing ring, the gasket, and the bolting. Only the first of these is welded, and only the first is wetted by the process fluid.
Stub end — the short, wide-lipped fitting that is butt-welded to the pipe. It carries the sealing face, so its material must match the service. Supplied to ASME B16.9 or MSS SP-43; see our lap joint stub end product page and the ASME B16.9 lap joint stub end dimensions chart.
Loose backing ring — the flat flange ring with the bolt holes. It is not welded and does not touch the fluid. See the lap joint loose flange product page.
Gasket — normally a spiral wound gasket sized to the stub-end facing and pressure class, per ASME B16.20.
Bolts and nuts — stud bolts with two nuts, torqued in a cross pattern. Because the ring can rotate, bolt-hole orientation is set at the last moment rather than at fabrication.
Ordering note: a lap joint flange and a stub end are two separate line items. They are sized to the same NPS and pressure class, but the stub end must additionally match the pipe wall schedule, because the weld is a butt weld to the pipe itself.
ASME B16.5 covers lap joint flanges from NPS 1/2 through NPS 24 in pressure classes 150 to 2500. Their envelope dimensions match slip-on flanges, except that the bore is machined with a radius to sit over the stub-end lap.
All dimensions below are in inches and follow the ASME B16.5 tables for lap joint flanges. The critical dimension to check against your stub end is the bore (B) and the corner radius at the lap: if the radius is wrong, the ring will not seat squarely and the gasket will not load evenly.
Class 150 Lap Joint Flange Dimensions
Class 150 is the general-service class for water, low-pressure process lines, and utility piping. The table below gives the full NPS 1/2 to 24 chart.
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
Class 300 adds thickness and a larger bolt circle for higher-pressure process duties.
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 shares the same outside diameter, thickness, and bolt circle as Class 300 for these flange types in the B16.5 tables; the higher rating comes from the pressure–temperature rating of the material group, not from a heavier section. Note that bolt-hole diameter and mass still differ. Confirm against the current edition of ASME B16.5 before ordering.
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 is heavy-duty high-pressure service. The chart below covers NPS 1/2 to 12; sizes above NPS 12 in Class 1500, plus full Class 600, 900, and 2500 data, are in the complete ASME B16.5 flange dimension chart. Class 2500 is 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.
Use ASME B16.5 for NPS 1/2 to 24, and ASME B16.47 for NPS 26 to 60. Outside the ASME system, EN 1092-1 and JIS B2220 are the common regional equivalents.
| Standard | Size range | Classes / ratings | Typical use |
|---|---|---|---|
| ASME B16.5 | NPS 1/2 – 24 | Class 150 – 2500 | Global default: oil and gas, chemical, power |
| ASME B16.47 Series A (MSS SP-44) | NPS 26 – 60 | Class 150 – 900 | Large bore, heavier and stiffer |
| ASME B16.47 Series B (API 605) | NPS 26 – 60 | Class 75 – 300 | Large bore, lighter and more compact |
| EN 1092-1 (Type 02 / 04 loose plate) | DN 10 – DN 4000 | PN 2.5 – PN 400 | Europe; Type 04 is the closest match to a lap joint |
| JIS B2220 | 10A – 1500A | 5K – 30K | Japan and parts of Asia |
| ASME B16.1 | NPS 1 – 24 | Class 125 / 250 | Cast iron flanges, water and legacy systems |
For large-diameter work, see the ASME B16.47 Series A flange dimensions and Series B dimensions charts. Note that Series A and Series B flanges are not interchangeable: bolt circles and thicknesses differ.
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.
A lap joint flange is rated by pressure class, but the allowable working pressure falls sharply as temperature rises and depends on the material group of both the ring and the stub end. The joint can never be stronger than the stub end and its weld to the pipe.
The table below is indicative, at ambient temperature, for ASME B16.5 material Group 2.2 (for example, ASTM A182 F316). Treat it as a screening guide only — always confirm the actual number against ASME B16.5 Table 2 for your material group and design temperature.
| Pressure class | Approx. working pressure | Approx. metric | Typical service |
|---|---|---|---|
| 150 | 275 psi | 19 bar | Water, utilities, low-pressure process |
| 300 | 720 psi | 50 bar | General process and hydrocarbon duty |
| 400 | 960 psi | 66 bar | Higher-pressure process |
| 600 | 1440 psi | 99 bar | High-pressure process and steam |
| 900 | 2160 psi | 149 bar | High-pressure, high-integrity duties |
| 1500 | 3600 psi | 248 bar | Severe high-pressure service |
| 2500 | 6000 psi | 414 bar | Extreme pressure; NPS 12 and smaller |
Three points matter more than the numbers themselves. First, a rating is a system property: the gasket, the bolting, and the stub-end-to-pipe weld all have to be qualified for the same condition. Second, the stub end is usually the weakest link, because it is a relatively thin section carrying the full bolt load through its lap. Third, most piping codes restrict lap joint assemblies in severe cyclic service, where repeated thermal or pressure cycling can fatigue the lap — in those duties a weld neck flange is normally specified instead.
Lap joint flanges appear wherever lines are opened often, are hard to align, are made of an expensive alloy, or are built from non-metallic or lined pipe.
Chemical and petrochemical processing — corrosive lines that need regular inspection and cleaning. The rotatable ring turns a bolt-up into a one-person job. See our chemical equipment page.
Oil and gas — moderate-pressure piping, tie-ins into existing headers, and skids where spools are awkward to rotate. See oil and gas.
Water and wastewater treatment — large layouts with frequent modification, where bolt-hole alignment on big diameters is the practical problem. See water treatment.
Marine and offshore — seawater systems where the wetted path needs duplex, super duplex, or a nickel alloy but the ring does not. Watch galvanic corrosion closely. See marine engineering and nickel alloy flanges for subsea service.
Food, beverage, and pharmaceutical — hygienic lines that are stripped for cleaning between batches; the stub-end face is easy to inspect and refinish.
FRP, GRP, and lined pipe — the Van Stone style stub end with a rotatable backing ring is the standard way to flange brittle or plastic-lined pipe without over-stressing it.
Mining and slurry — abrasive lines that are rotated or replaced on a wear schedule.
Lap joint flanges are classified two ways: by the sealing face on the stub end (raised face, flat face, or ring-type joint), and by the pattern of the stub end itself (MSS SP-43 Type A, B, or C). Both classifications must be specified when you order.
| Type | Sealing surface | Where it is used | Why it is chosen |
|---|---|---|---|
| Raised face (RF) | Raised ring on the stub end: 1/16 in for Class 150 and 300, 1/4 in for Class 400 and above | Default for most process piping | Concentrates the gasket load on a narrow ring, giving a tighter seal at lower bolt load |
| Flat face (FF) | Completely flat stub-end face | Mating to cast iron, brittle, or FRP flanges | Spreads the bolt load over the full face so the mating flange will not crack |
| Ring-type joint (RTJ) | Groove machined into the stub end, holding a metal ring gasket | High-pressure and high-temperature service, typically Class 900 and above | Metal-to-metal seal that survives conditions a soft gasket cannot |
The stub end does the sealing, so its pattern matters as much as the ring. MSS SP-43 classifies wrought stainless stub ends into three types; ASME B16.9 covers the equivalent wrought steel patterns in long and short lengths.
| Type | How it is made | Pairs with | Typical application |
|---|---|---|---|
| Type A | Machined from bar or forging, with a generous radius at the lap | Lap joint (loose) flange | Standard choice for a lap joint assembly |
| Type B | Machined, with a shorter lap | Slip-on flange used as the backing ring | Where an existing slip-on acts as the loose ring |
| Type C | Formed from pipe, with the lap flared or rolled | Lap joint flange or slip-on flange | Cost-sensitive, light-wall stainless systems |
Long pattern (LP) is the standard stub-end length and is what most ASME B16.5 lap joint assemblies use.
Short pattern (SP) is used where space between the flange face and the pipe is tight; it is lighter and cheaper but gives the ring less bearing area.
Confirm type, pattern, and dimensions against the current MSS SP-43 or ASME B16.9 tables before ordering — see our stub end types, installation and uses guide for detail.
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.
The stub end is chosen for corrosion resistance because it touches the fluid; the ring is chosen for strength and cost because it does not. That asymmetry is what lets you put an expensive alloy only where it is needed.
| Material family | Common specification | Typical grades | Why it is chosen |
|---|---|---|---|
| Carbon steel | ASTM A105 | A105 | Non-corrosive utility service, lowest cost ring material |
| Low-temperature carbon steel | ASTM A350 | LF2 | Services down to about -46 °C |
| Austenitic stainless steel | ASTM A182 | F304/F304L, F316/F316L, F321, F347 | General corrosion resistance; food, pharma, water — see [[316L stainless steel|/stainless-steel-316l.html]] |
| Duplex and super duplex | ASTM A182 | F51 (S31803/S32205), F53 (S32750), F55 (S32760) | Chloride resistance plus roughly double the yield strength of 316 — see [[S32750|/super-duplex-s32750.html]] and [[S32760|/super-duplex-s32760.html]] |
| Super austenitic | ASTM A182 | 254 SMO (S31254), 904L (N08904), AL-6XN (N08367) | Seawater and aggressive acids beyond 316 — see [[904L|/stainless-steel-904l.html]] |
| Nickel alloys | ASTM B564 | N06625 (Inconel 625), N10276 (Hastelloy C276), N08825 (Incoloy 825), N04400 (Monel 400), N08020 (Alloy 20) | Severe acid, seawater, and high-temperature duty — see [[Inconel 625|/inconel-625.html]] and [[Hastelloy C276|/hastelloy-c276.html]] |
Because the ring is never wetted, the most common way to control cost on an alloy line is to specify an alloy stub end with a carbon steel backing ring. The saving grows with diameter, because ring mass grows far faster than stub-end mass.
| Stub end (wetted) | Backing ring (not wetted) | What you gain | What to watch |
|---|---|---|---|
| 316L | Carbon steel A105 | Lowest-cost assembly with a stainless wetted path | Galvanic corrosion if the joint is regularly wetted externally |
| Duplex F51 / F53 | Carbon steel A105 | Chloride-resistant wetted path at a fraction of a solid-alloy flange | Ring must be coated and kept dry in marine atmospheres |
| Inconel 625 / Hastelloy C276 | Carbon steel or 316 | Severe-service wetted path without paying for a full alloy flange | Galvanic potential is larger; insulation and coating matter |
| Same material both parts | Matches stub end | Maximum compatibility, no galvanic couple | Highest cost; justified in seawater immersion or where inspection is impossible |
The trade-off is galvanic corrosion. A carbon steel ring pressed against a stainless or nickel-alloy stub end forms a galvanic couple whenever an electrolyte bridges them — salt spray, rain, washdown water, or immersion. Mitigate it by coating or painting the ring, using insulating sleeves and washers on the bolts, detailing the joint so water does not sit in the crevice, and inspecting marine and washdown locations on a fixed schedule.
Choose a lap joint flange when alignment, disassembly, or wetted-alloy cost dominates. Choose a weld neck when strength, fatigue life, or cycling dominates. Choose slip-on, socket weld, or threaded only where their specific constraints are acceptable — and note that socket weld and threaded flanges are generally limited to small bore.
| Criterion | Lap joint (LJ) | Slip-on (SO) | Weld neck (WN) | Socket weld (SW) | Threaded (THD) |
|---|---|---|---|---|---|
| Parts required | Stub end + loose ring | One flange | One flange | One flange | One flange |
| Welds required | 1 butt weld (stub end to pipe) | 2 fillet welds | 1 butt weld | 1 fillet weld | No weld |
| Weld quality | Butt weld, can be fully radiographed | Fillet welds, not easily radiographed | Butt weld, can be fully radiographed | Fillet weld, crevice remains | No weld; thread is the seal risk |
| Bolt-hole alignment | Excellent — ring rotates freely | Good — can rotate before welding | Poor — fixed once welded | Good — can rotate before welding | Good — can rotate before tightening |
| Ease of disassembly | Excellent, and the ring is reusable | Poor — the flange is welded on | Poor — the flange is welded on | Poor — welded on | Good — can be unscrewed |
| Strength and fatigue resistance | Low to moderate | Moderate | Highest | Moderate | Lowest |
| Resistance to bending moments | Low | Moderate | Highest | Moderate | Low |
| Pressure capability | Moderate; limited by the stub-end lap | Moderate | Highest | Moderate, small bore | Low, small bore |
| Suitable for cyclic service | Generally avoided | Limited | Preferred | Limited | Avoided |
| Typical relative cost | Low ring cost; two line items | Lowest | Highest | Low to moderate | Low |
| Where the cost advantage comes from | Cheap ring + alloy stub end | Simple, light flange | None — pay for integrity | Compact, no weld prep | No welding at all |
| Typical size range | NPS 1/2 – 24+ | NPS 1/2 – 24+ | NPS 1/2 – 24+ | NPS 1/2 – 3 typically | NPS 1/2 – 4 typically |
| Best for | Frequent strip-down, hard alignment, costly wetted alloys, lined or FRP pipe | Low-cost fixed low-pressure lines | High pressure, high temperature, cycling, severe duty | Small-bore, leak-tight, non-cyclic lines | Temporary, non-hazardous, no-hot-work lines |
| Avoid when | Severe cycling, high vibration, heavy bending loads | Cyclic or high-pressure duty | Cost is the only driver | Cyclic duty or corrosive crevice risk | Toxic, flammable, or cyclic service |
Will the joint be broken open repeatedly? If yes, a lap joint is usually the cheapest option over the plant's life, because the ring is reused and the gasket faces are easy to inspect and re-machine.
Is the wetted alloy expensive? If yes, a lap joint with a carbon steel ring puts the alloy only where the fluid touches it.
Is the service cyclic, high-vibration, or carrying heavy bending loads? If yes, choose a weld neck flange.
Is the line small bore, low pressure, and non-hazardous? Then socket weld or threaded may be adequate — see the socket weld flange guide and the threaded flange guide.
Is cost the only driver and the joint will stay fixed? Then a slip-on flange is usually the lowest-cost answer.
For a broader view of the range we supply, start from the main flanges hub page.
Slide the ring on first, then butt-weld the stub end to the pipe. The single most common installation error is forgetting the ring before welding — after the stub end is welded, the ring can no longer be fitted.
Confirm the parts match. The stub end must match the pipe NPS and wall schedule; the ring must match the NPS and pressure class and have a bore radius that seats on the stub-end lap.
Slide the loose ring onto the pipe with the bolt-hole side facing the joint, then slide the stub end on behind it.
Butt-weld the stub end to the pipe using a qualified procedure (ASME Section IX WPS/PQR). Stagger the weld start and stop, and keep heat input controlled to limit distortion of the sealing face.
Inspect the weld as the service requires — radiography or dye penetrant for alloy and high-pressure lines. Verify the stub-end face is flat and undamaged after welding.
Check the sealing face and clean it. Any weld spatter, scratch, or radial score across the stub-end face is a potential leak path.
Fit the gasket of the correct type and size for the facing and pressure class, without sealant unless the gasket manufacturer requires it.
Bring the two flanges together and rotate the ring until the bolt holes line up. Insert all bolts and run the nuts up by hand.
Torque in a cross pattern in three steps — typically 30%, 60%, then 100% of target torque — and re-torque after the first thermal cycle, since gaskets creep and relax.
Inspect the crevice between the ring and the stub end, keep chlorides away from the stainless, and keep the bolts free. Most lap joint leaks start at the crevice or at a damaged stub-end face, not in the ring.
Inspect the crevice. The gap between the backing ring and the stub-end lap traps moisture and chlorides and is the classic crevice-corrosion site. Look for rust staining, pitting, or deposits weeping from the joint.
Check for galvanic corrosion. A carbon steel ring against a stainless or nickel-alloy stub end is a galvanic couple. Look for preferential attack of the carbon steel, especially in marine, washdown, or buried service, and renew coatings before they fail.
Clean with non-chloride products. Chloride-bearing cleaners and bleach can trigger chloride stress corrosion cracking on stainless stub ends, particularly on 304 and 316 above roughly 50–60 °C.
Lubricate the bolts. Use a quality anti-seize compound so torque is repeatable and the nuts can be removed at the next outage. Never re-use damaged or necked studs.
Verify alignment and flatness. The stub end must sit flush against the gasket. If the backing ring is warped, bent, or its bore radius is worn, replace it — a distorted ring loads the gasket unevenly and will leak.
Re-torque after the first heat-up and after any outage that involved breaking the joint, then record the torque used so the next crew works to the same value.
The advantages centre on alignment, maintenance, and material cost. The disadvantages all trace back to one fact: the load path runs through a thin lap instead of a heavy hub.
| Advantages | Disadvantages |
|---|---|
| The ring rotates, so bolt-hole alignment is fast and does not require moving the pipe | Lower fatigue strength than a weld neck; generally avoided in severe cyclic service |
| Only the stub end is wetted, so the ring can be a cheaper material | Two line items to specify, stock, and match (stub end plus ring) |
| The ring is reusable, so replacement cost is limited to the stub end and gasket | The stub end and its weld govern joint strength and usually limit the rating |
| No weld on the ring, so there is no weld-related distortion or heat-affected zone in the bolting component | A crevice exists between the ring and the stub-end lap, creating a crevice-corrosion site |
| Ideal for lined, FRP, and brittle piping that cannot be over-tightened | An alloy stub end against a carbon steel ring creates a galvanic couple if wetted externally |
| Straightforward to inspect and refinish the sealing face, since it sits on the stub end | Lower resistance to bending moments and to external pipe loads |
Is a lap joint flange raised face?
Not by itself. The loose backing ring is flat, and the sealing face is defined by the stub end, not the ring. If your stub end is machined with a raised ring, the assembly behaves as a raised face joint; if the stub end is flat, it behaves as a flat face joint. This matters because gasket selection, bolt load, and the mating flange all follow the stub-end face.
What type of face do lap joint flanges have?
The face is on the stub end, and it can be flat face (FF), raised face (RF), or ring-type joint (RTJ). Raised face is the default for process piping — 1/16 in for Class 150 and 300, and 1/4 in for Class 400 and above. Flat face is used when mating to cast iron, brittle, or FRP flanges so the bolt load spreads over the full face. RTJ is reserved for high-pressure and high-temperature duty, typically Class 900 and above.
When should you use a lap joint flange?
Use one when the line is opened frequently, when bolt-hole alignment is difficult, or when the wetted alloy is expensive and you want a low-cost carbon steel backing ring. It is also the standard choice for lined, FRP, and GRP pipe. The reason is that the ring rotates freely, is reusable, and never touches the fluid — so you pay for alloy only on the stub end.
How do you weld a lap joint flange?
You never weld the ring. Slide the loose ring onto the pipe first, then butt-weld the stub end to the pipe using a qualified ASME Section IX procedure. Welding the ring, or forgetting to fit it before the stub end is welded, are the two most common installation errors. After welding, inspect the joint as the service requires and confirm the stub-end sealing face is flat and free of spatter.
Can you use a carbon steel backing ring with a stainless steel or nickel alloy stub end?
Yes, and it is one of the main reasons to choose a lap joint flange, because the ring is never wetted by the process fluid. The saving grows with pipe diameter. The trade-off is galvanic corrosion: if salt spray, washdown water, or immersion bridges the two metals, the carbon steel ring will corrode preferentially. Control it by coating the ring, using insulating bolt sleeves and washers, detailing the joint to shed water, and inspecting marine and washdown locations regularly.
What is the difference between a lap joint flange and a slip-on flange?
A lap joint flange is two pieces — a stub end welded to the pipe plus a loose ring that rotates — while a slip-on flange is one piece that slides over the pipe and is fixed with two fillet welds. The lap joint can rotate freely before bolting, can be disassembled and reused, and lets you use a cheaper material for the ring. The slip-on is simpler and slightly cheaper to install but is permanently welded in place, and its fillet welds are harder to examine than the lap joint's single butt weld.
