Submit an R.F.Q. — Lap Joint Flanges
Technical sales team responds within 24 hours. All pressure classes, materials and stub end combinations available.
R.F.Q. Submitted Successfully
Our technical team will contact you within 24 hours.
Reference: sales@rrhydraulics.com
Lap Joint Flanges
A comprehensive engineering reference for EPC contractors, piping engineers and procurement teams — covering lap joint flange mechanics, stub end pairing, bi-metallic assembly economics, ASME B16.5 dimensional data, P-T ratings, material grades, NACE compliance, and full project documentation for all industrial piping applications requiring frequent dismantling or bi-metallic connections.
Lap Joint Flange Design Principle,
Stub End System & Engineering Advantages
Bi-Metallic · Free Rotation · Bolt Hole Alignment · Frequent Dismantling
Definition and Engineering Principle
A lap joint flange (also called a Van Stone flange or loose flange) is a two-component flange system consisting of: (1) a loose backing flange — a flat-faced ring with bolt holes that slides freely over the pipe and is not welded to the pipe; and (2) a stub end — a short pipe nipple with a factory-formed lapped collar (the sealing face) that is butt-welded to the pipe. The backing flange bears against the back of the stub end collar and transmits bolt load from the bolts into the stub end face, which in turn seats the gasket. The backing flange itself does not contact the process fluid — only the stub end does.
This two-component design delivers three unique engineering advantages not available with any other flange type: (1) Free rotation of the backing flange — the loose flange can rotate freely around the pipe before bolt-up, enabling bolt holes to be aligned in any angular position without rotating the pipe itself; (2) Bi-metallic assembly economics — the backing flange (in contact only with the bolt load, not the fluid) can be low-cost carbon steel, while only the stub end (in contact with the process fluid) needs to be the expensive corrosion-resistant alloy; and (3) Easy dismantling — the pipe can be removed from the flange connection by unbolting without disturbing the welded stub end, making lap joint connections the most maintainable flange type for frequent-access connections.
The most significant economic benefit of the lap joint flange system is the ability to use a low-cost carbon steel backing flange with a high-alloy stub end. In a standard weld neck or slip-on flange, the entire flange — including the large, heavy forged flange body — must be manufactured from the expensive corrosion-resistant alloy (SS 316L, duplex 2205, Inconel 625, etc.). In a lap joint assembly, only the small stub end — which is the fluid-contact component — needs to be the expensive alloy. The large backing flange, which contacts only the bolt load on its face and the outside of the stub end collar on its bore, is manufactured from standard carbon steel A105. For large-bore, high-alloy systems (duplex, Inconel, titanium), this can reduce the flange material cost by 50–80% per connection compared to full-alloy weld neck flanges.
Lap Joint Flange System Anatomy
Stub End Types — MSS SP-43 Classification
Lap Joint vs Weld Neck — Engineering Comparison
ASME B16.5 Dimensional Data
& Pressure-Temperature Ratings
Flange OD · Bolt Circle · Stub End Collar · P-T Rating
Critical Dimensional Requirement — Backing Flange Bore vs Stub End OD
The backing flange bore must be machined to a close sliding clearance over the stub end OD — typically +0.5 to +2.0 mm clearance depending on the NPS. If the clearance is too tight, the backing flange will not rotate freely and the bolt hole alignment advantage is lost. If the clearance is too large, the backing flange will not be adequately supported by the stub end collar and may rock under bolt-up, producing non-uniform gasket loading. The ASME B16.5 backing flange bore is specified as a specific dimension (not a standard pipe OD) that matches the stub end OD per MSS SP-43. Always verify that the backing flange bore matches the stub end OD from the same dimensional standard — mixing stubs from different standards can cause fit issues.
| NPS | Flange OD (mm) | Bolt Circle (mm) | No. Bolts | Bolt Size | Stub End Collar OD (mm) | Backing Bore (mm) | Flange Thick. (mm) |
|---|---|---|---|---|---|---|---|
| ½" | 88.9 | 60.3 | 4 | M12 | 34.9 | 35.3 | 11.2 |
| 1" | 108.0 | 79.4 | 4 | M12 | 50.8 | 51.3 | 14.2 |
| 1½" | 127.0 | 98.4 | 4 | M16 | 63.5 | 64.0 | 15.9 |
| 2" | 152.4 | 120.6 | 4 | M16 | 92.1 | 92.7 | 17.5 |
| 3" | 190.5 | 152.4 | 4 | M16 | 127.0 | 127.8 | 19.1 |
| 4" | 228.6 | 190.5 | 8 | M16 | 157.2 | 158.0 | 19.1 |
| 6" | 279.4 | 241.3 | 8 | M20 | 215.9 | 216.9 | 22.4 |
| 8" | 342.9 | 298.4 | 8 | M20 | 269.9 | 271.3 | 25.4 |
| 10" | 406.4 | 362.0 | 12 | M20 | 323.8 | 325.4 | 28.4 |
| 12" | 482.6 | 431.8 | 12 | M20 | 381.0 | 382.8 | 31.8 |
ASME B16.5-2017 Class 150. Backing flange bore is the critical clearance fit dimension — the bore is slightly larger than the stub end collar OD to permit free rotation while maintaining adequate support for the gasket load. Stub end collar OD and lap face dimensions per MSS SP-43 (Type A or B) or ASME B16.9. The backing flange does not have a raised face — it is flat-faced on the gasket side, as the sealing face is on the stub end collar only. All dimensions in mm.
| NPS | Pipe OD (mm) | Collar OD (mm) | Collar Thickness (mm) | Type A Length Short (mm) | Type A Length Long (mm) | Bevel End ID (Sch. 40) |
|---|---|---|---|---|---|---|
| 1" | 33.40 | 50.8 | 6.4 | 38 | 51 | 26.6 |
| 2" | 60.33 | 92.1 | 7.9 | 51 | 64 | 52.5 |
| 3" | 88.90 | 127.0 | 9.5 | 64 | 76 | 77.9 |
| 4" | 114.30 | 157.2 | 9.5 | 76 | 89 | 102.3 |
| 6" | 168.28 | 215.9 | 11.1 | 89 | 102 | 154.1 |
| 8" | 219.08 | 269.9 | 12.7 | 102 | 127 | 202.7 |
| 10" | 273.05 | 323.8 | 14.3 | 127 | 152 | 254.5 |
| 12" | 323.85 | 381.0 | 15.9 | 127 | 152 | 303.2 |
MSS SP-43-2018 Type A (long pattern) stub ends. Collar OD matches the backing flange bore minus the clearance gap. Bevel end ID is for Schedule 40 pipe — must match the actual pipe schedule used in the piping system. Stub end collar face finish: 125–250 AARH serrated for spiral-wound gaskets. All dimensions in mm.
θ_WNF = ± (d_hole / BC) × (180/π) // WNF: angular tolerance limited by bolt hole clearance on bolt circle
// WORKED EXAMPLE: 8-bolt NPS 4" Class 150, BC=190.5mm, d_hole=19.1mm
θ_WNF_max = ±(9.55/190.5)×(180/π) = ±2.87° // WNF: only ±2.87° angular tolerance within the bolt hole clearance
// Lap joint: ±180° (any orientation) — eliminates piping spool twist and angular misalignment entirely
Material Grades, Bi-Metallic Design
& NACE Compliance
NACE MR0175 · PWHT · Galvanic Isolation · Charpy Impact
| Backing Flange | Stub End Material | Application | Savings vs Full Alloy | Galvanic Risk? |
|---|---|---|---|---|
| CS A105 | CS A105 / A234 WPB | General CS service, utilities | — | None |
| CS A105 | SS 304L (A403 WP304L) | Chemical, food, pharma piping | 40–60% vs SS WNF | Moderate — isolate |
| CS A105 | SS 316L (A403 WP316L) | Offshore, chloride, chemical | 50–65% vs SS 316L WNF | Moderate — isolate |
| CS A105 | Duplex 2205 (A815 WP-S31803) | Offshore sour, seawater | 60–75% vs Duplex WNF | Moderate — isolate |
| CS A105 | Inconel 625 (B705 N06625) | High-temp corrosive, acid service | 70–85% vs Inconel WNF | Moderate — isolate |
| CS A105 | Titanium Gr.2 (B861 Gr.2) | Seawater, chlorine, desalination | 75–90% vs Ti WNF | High — must isolate |
| CS A350 LF2 | SS 304L (cryogenic) | LNG, ethylene, cryogenic service | 45–60% vs SS 304L WNF | Moderate — isolate |
| SS 316L A182 F316L | SS 316L (same alloy) | Full SS 316L — severe corrosion | — | None |
When a carbon steel backing flange is used with a stainless steel, duplex or titanium stub end in a bi-metallic assembly, galvanic corrosion will occur at the contact interface between the CS backing flange bore and the alloy stub end collar in any wet or process environment. The carbon steel is anodic relative to the stainless/duplex/titanium and will corrode preferentially at the contact zone. In dry or non-corrosive service environments, this risk is low. In wet process or outdoor environments, the backing flange bore must be protected by: (1) applying a corrosion-resistant coating (e.g. fusion-bonded epoxy or zinc-rich primer) to the bore and contact face of the backing flange; (2) inserting a non-metallic isolation sleeve between the backing flange bore and stub end OD; or (3) specifying the entire backing flange in the same alloy as the stub end (eliminating the cost saving but eliminating galvanic risk). For titanium stub ends, galvanic isolation is mandatory — the potential difference between titanium and carbon steel in seawater is among the most aggressive possible.
| Material | ASTM Grade | Application | Temperature Range | Notes |
|---|---|---|---|---|
| CS Backing — A105 | ASTM A105 N | Standard backing flange, all service | −29 to +538°C | Most common backing flange material; fluid does not contact |
| CS Backing — A181 Gr.II | ASTM A181 Gr.II | Class 150/300 backing flange | −29 to +427°C | Lower strength alternative; Class 150 and 300 only |
| LTCS Backing — A350 LF2 | ASTM A350 LF2 | Cryogenic / low-temp service | −46 to +343°C | Impact tested; used where stub end is also LTCS or SS |
| SS 304L Stub End | A403 WP304L / A182 F304L | Chemical, pharmaceutical, food | −196 to +816°C | Butt weld stub end per MSS SP-43 or ASME B16.9 |
| SS 316L Stub End | A403 WP316L / A182 F316L | Offshore, chloride, chemical | −196 to +816°C | Mo-bearing; superior chloride resistance |
| Duplex 2205 Stub End | A815 S31803 / A182 F51 | Offshore sour, seawater | −50 to +315°C | NACE MR0175; ferrite content per EFC 16 |
| Inconel 625 Stub End | B705 N06625 / B564 N06625 | High-temp corrosive, acid | −196 to +980°C | Highest corrosion resistance; high-cost alloy |
| Titanium Gr.2 Stub End | B861 Gr.2 / B381 F-2 | Seawater, chlorine, desalination | −196 to +315°C | Mandatory galvanic isolation from CS backing flange |
Applications, QC Protocols
& Export Documentation
Bi-Metallic Package · EN 10204 3.2 · PMI · Charpy · TPI
When to Specify a Lap Joint Flange
The lap joint flange is the correct specification in the following conditions: (1) Bi-metallic piping systems — where the process fluid requires an expensive corrosion-resistant alloy but the system economics demand cost reduction on the large forged flange bodies; this is the most common application and the primary economic justification for the lap joint design; (2) Frequent dismantling — where pipe sections must be regularly disconnected for maintenance access, cleaning, equipment removal or spool replacement — the free-rotating backing flange allows the pipe to be slid out axially without needing to cut the pipe or rotate it; (3) Difficult bolt hole alignment — where pipe routing makes it impossible to achieve correct bolt hole alignment by rotating the pipe spool, the free-rotating backing flange allows perfect bolt alignment regardless of pipe orientation; (4) Class 150 and 300 low-pressure service — where the lower structural requirements allow the more economical lap joint design to be used without sacrificing integrity.
Lap joint flanges with carbon steel backing flanges and SS 316L or duplex 2205 stub ends are extensively used in chemical plant piping for corrosive service — acid transfer lines, solvent systems, chemical reactor nozzle piping, and heat exchanger connections. The bi-metallic assembly provides the corrosion resistance of high-alloy material at the fluid contact surface while avoiding the cost of full-alloy forged flanges throughout the system. Frequent-dismantling requirements for equipment cleaning turnarounds make the lap joint the preferred design for chemical plant connections where quarterly or annual maintenance access is required.
On offshore platforms, seawater service piping in duplex 2205 or super duplex frequently uses lap joint assemblies (carbon steel backing flange + duplex stub end) for large-bore cooling water, fire water, seawater lift and ballast piping. The large-bore duplex stub ends are a fraction of the cost of the equivalent full-duplex weld neck flanges — particularly significant for NPS 8" through 24" where forged duplex flange weight and material cost are substantial. All offshore lap joint assemblies require EN 10204 3.2 MTC with TPI countersignature on both the backing flange and stub end, PMI on the stub end alloy, NACE hardness mapping (duplex) and ferrite content measurement.
SS 316L lap joint assemblies (both backing flange and stub end in 316L, or CS backing with 316L stub) are used in pharmaceutical and food processing piping where frequent cleaning and equipment access are mandatory — typically quarterly or monthly clean-in-place (CIP) and steam-in-place (SIP) cycles that require piping sections to be disconnected, cleaned externally and reinstalled. The lap joint backing flange allows the pipe spool to be slid out axially for cleaning without the need to unscrew any welded connection. Stub end collar face finish: 0.8 µm Ra maximum for pharmaceutical contact; 3-A hygienic standards compliance required.
A350 LF2 backing flanges with SS 304L or 316L stub ends are used in LNG plant piping and cryogenic process systems where the low-temperature impact requirements of the backing flange must be met and the fluid contact material must be austenitic stainless (which does not require Charpy impact testing due to its inherently good toughness at cryogenic temperatures). The bi-metallic combination provides the Charpy-tested LTCS backing flange for structural integrity at MDMT and the 304L stub end for the cryogenic fluid service without requiring Charpy testing on the stub end.
Lap joint flanges are widely used in instrumentation take-off connections (NPS ½" to 2") on process vessels and piping where instrument connections must be broken and remade frequently for calibration, replacement and maintenance. The free rotation and easy dismantling features of the lap joint design are particularly valuable in congested instrument connection zones where pipe rotation is impossible. Carbon steel backing flanges with alloy stub ends are standard for instrument root valve connections in corrosive service.
Quality Control — Specific Lap Joint Requirements
QC for lap joint flange assemblies covers both the backing flange and the stub end as separate inspected items: (1) Backing flange bore — the bore diameter must be verified to confirm it provides the correct clearance over the stub end OD; an undersized bore prevents free rotation; an oversized bore reduces support for the gasket load; (2) Stub end collar face finish — the sealing face of the stub end collar must be verified to 125–250 AARH for spiral-wound gaskets (same verification as a raised face weld neck flange); (3) Stub end collar squareness — the collar must be perpendicular to the pipe axis within 0.5 mm TIR across the collar face diameter — a non-square collar produces a non-uniform gasket load and potential leak; (4) PMI of stub end — 100% PMI on all alloy stub ends to confirm the correct alloy before installation, as stub ends from different alloys can be difficult to distinguish visually once cut to length.
Export Packaging
- Backing flanges wrapped in VCI film; stub ends in individual sealed poly bags — SS/duplex/alloy stub ends segregated from carbon steel items to prevent ferrous contamination
- Stub end collar face and weld bevel end protected with removable foam or plastic caps; collar faces must not be stacked or placed face-down without protection
- Bi-metallic packages (backing flange + stub end) shipped as matched sets, tagged with the same PO line and tagged together in the same package — separating backing flanges from their matched stub ends during transit causes assembly errors on site
- MTC for backing flange (EN 10204 3.1) and stub end (EN 10204 3.1 or 3.2 as specified), dimensional inspection reports for both, stub end collar face AARH report, PMI report for alloy stub ends, Charpy impact report (A350 LF2/LF3 and duplex grades), NACE hardness report (duplex), ferrite content report (duplex)
- Pallets on ISPM-15 heat-treated timber with stretch wrap; gross/net weight, project tag numbers and country of origin labelled per crate
| # | Document | Standard / Reference | Component | Minimum Requirement |
|---|---|---|---|---|
| 01 | Material Test Certificate — Backing Flange | EN 10204 3.1 | Backing flange | 3.1 for standard; 3.2 if full SS or duplex backing |
| 02 | Material Test Certificate — Stub End | EN 10204 3.1 / 3.2 | Stub end | 3.2 (TPI co-signed) for offshore / NACE / alloy stub ends |
| 03 | Dimensional Inspection — Backing Flange | ASME B16.5 | Backing flange | OD, bolt circle, bolt holes, bore ID — all mandatory |
| 04 | Dimensional Inspection — Stub End | MSS SP-43 / ASME B16.9 | Stub end | Collar OD, collar thickness, total length, bevel end ID |
| 05 | Stub End Collar Face Finish Report | ASME B16.5 / B46.1 | Stub end | 125–250 AARH confirmed; perpendicularity ≤0.5 mm TIR |
| 06 | PMI Report (XRF / OES) | Project specification | Stub end | 100% alloy stub ends (SS, duplex, Inconel, Ti etc.) |
| 07 | MT / PT Surface Exam — Stub End | ASME V Art.7 | Stub end | MT for CS; PT for SS/duplex/exotic; collar face mandatory |
| 08 | Charpy Impact Report | ASTM A370 / EN ISO 148 | Both | Mandatory for A350 LF2/LF3 backing flange and duplex stub |
| 09 | Hardness Survey (NACE) | ASTM E10 / E18 | Both | Sour service: ≤22 HRC full cross-section; duplex ≤310 HB |
| 10 | Ferrite Content Report (Duplex) | ASTM E562 | Stub end | Mandatory duplex 2205 / super duplex; FN 35–65% |
| 11 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Both | Current; scope covers lap joint flange and stub end manufacture |
| 12 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | Packaging | All wood packing for international export |
RR Hydraulics manufactures and exports lap joint flanges and stub end packages in all pressure classes (150–2500) per ASME B16.5, with backing flanges in A105, A181 Gr.II and A350 LF2, and stub ends in A182/A403 F304L and F316L, A182 F51/F53/F55, Inconel 625, Titanium Grade 2, and all standard alloy grades per MSS SP-43 and ASME B16.9. NPS ½"–24" metric and inch. Bi-metallic package supply with matched backing flange and stub end sets. EN 10204 3.1/3.2 MTC, PMI, Charpy impact, NACE hardness mapping, ferrite content (duplex), collar face AARH report, TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.