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Stub Ends & Lap Joints
A comprehensive engineering reference for EPC contractors, piping engineers and procurement teams — covering stub end geometry, Type A/B/C classification per MSS SP-43, lap joint flange system mechanics, bi-metallic assembly economics, collar face finish requirements, dimensional data, material grades, NACE compliance and full project documentation for all process piping lap joint applications.
Stub End Design, MSS SP-43 Type Classification
& Lap Joint Flange System
Bi-Metallic · Free Rotation · ASME B16.9 · Collar Face
Definition and Engineering Function
A stub end (also called a lap joint or Van Stone lap) is a butt weld fitting consisting of a short pipe section with a factory-formed lapped collar at one end. The collar face is the sealing surface — it seats the gasket and transmits the bolt load from the backing (lap joint) flange into the gasket. The stub end is butt-welded to the pipe at its plain end; a free-rotating backing flange slides over the pipe before welding and then bears against the back of the stub end collar to complete the lap joint flange connection.
The stub end is always used in conjunction with a lap joint backing flange — the two components together form the complete lap joint flange assembly. The critical engineering advantage is that the backing flange can rotate freely around the pipe, allowing bolt holes to be aligned in any angular position without rotating the pipe spool. Additionally, when the backing flange is manufactured from carbon steel and only the stub end is the expensive alloy, the assembly provides significant cost savings over a full-alloy weld neck flange.
The most significant economic benefit of the stub end / lap joint system is the ability to use a low-cost carbon steel backing flange with a high-alloy stub end. Only the stub end contacts the process fluid — so only the stub end needs to be the expensive alloy (SS 316L, duplex 2205, Inconel 625, titanium). The large, heavy backing flange — which contacts only the bolt load and the outside of the stub end collar — is manufactured from carbon steel A105. For large-bore systems in duplex, Inconel or titanium, this reduces the flange material cost by 50–90% per connection compared to full-alloy weld neck flanges, while maintaining full corrosion resistance at the fluid contact surface.
Stub End Anatomy
MSS SP-43 Type Classification
Stub End vs Weld Neck Flange — Engineering Comparison
MSS SP-43 Dimensional Data
& Collar Face Finish Requirements
Total Length · 125–250 AARH · Squareness ≤0.5 mm TIR
Critical Collar Face Finish Requirement
The collar face of the stub end is the gasket seating surface — it is the equivalent of the raised face on a weld neck flange. The collar face finish requirement is therefore identical to that of a standard raised face flange: 125–250 AARH (approximately 3.2–6.3 µm Ra) serrated (phonographic) finish for spiral-wound gaskets. This finish must be verified with a surface profilometer during QC inspection. A finish that is too smooth (below 125 AARH) does not provide adequate gasket seating stress in the spiral-wound gasket seating serrations; a finish that is too rough (above 250 AARH) may shear off the soft gasket inner layers rather than compress them. For pharmaceutical and food-grade applications, the collar face finish is Ra ≤ 0.8 µm (smooth) to comply with 3-A hygienic standards.
The collar face must also be checked for squareness — the collar face must be perpendicular to the pipe axis within 0.5 mm TIR (Total Indicator Reading) across the collar face diameter. A non-square collar produces a non-uniform gasket load distribution — the gasket will be overloaded on one side and under-loaded on the opposite side, leading to either gasket crushing or inadequate seating and potential leak. Squareness is verified using a dial gauge swept across the collar face while rotating the stub end on a lathe or V-blocks.
| NPS | Pipe OD (mm) | Collar OD (mm) | Collar Thick. (mm) | Short Length (mm) | Long Length (mm) | Bevel End ID — Sch 40 (mm) |
|---|---|---|---|---|---|---|
| ½" | 21.34 | 34.9 | 4.8 | 25 | 38 | 15.8 |
| 1" | 33.40 | 50.8 | 6.4 | 38 | 51 | 26.6 |
| 1½" | 48.26 | 63.5 | 7.9 | 51 | 64 | 40.9 |
| 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 |
| 16" | 406.40 | 469.9 | 17.5 | 152 | 178 | 387.4 |
| 20" | 508.00 | 584.2 | 19.1 | 178 | 203 | 478.8 |
MSS SP-43-2018 Type A. Collar OD matches the lap joint backing flange bore clearance — the backing flange bore is typically 0.5–2.0 mm larger than the collar OD to allow free rotation. Bevel end ID is for Schedule 40 pipe — this changes with pipe schedule and must be specified to match the actual pipe schedule in service. Stub end collar face finish: 125–250 AARH serrated for SWG; squareness ≤ 0.5 mm TIR mandatory. All dimensions in mm.
The bevel end bore of a stub end must be specified to match the inside diameter of the mating pipe per the actual pipe schedule in service. MSS SP-43 does not mandate a specific bore — it is a purchaser-specified dimension. Ordering a stub end without specifying the bore (or relying on a default bore) produces a bore mismatch at the butt weld root that creates a step at the weld root. This step acts as both a flow restriction and a fatigue crack initiation site, particularly in cyclic service. Always specify the pipe schedule (e.g. "Sch 80 bore per ASME B36.10M") explicitly in every stub end purchase order — it is the most commonly overlooked dimension and the most common cause of lap joint assembly non-conformance at site.
Material Grades, Bi-Metallic Combinations
& NACE Compliance
CS Backing + Alloy Stub · NACE MR0175 · Charpy
| Backing Flange | Stub End Material | Application | Savings vs Full Alloy WNF | Galvanic Risk? |
|---|---|---|---|---|
| CS A105 | CS A234 WPB | General CS service, utilities | — | None |
| CS A105 | SS 304L (A403 WP304L) | Chemical, pharma, food piping | 40–60% vs SS WNF | Moderate — protect bore |
| CS A105 | SS 316L (A403 WP316L) | Offshore, chloride, chemical | 50–65% vs SS 316L WNF | Moderate — protect bore |
| CS A105 | Duplex 2205 (A815 S31803) | Offshore sour, seawater | 60–75% vs Duplex WNF | Moderate — protect bore |
| CS A105 | Inconel 625 | High-temp corrosive, acid | 70–85% vs Inconel WNF | Moderate — protect bore |
| CS A105 | Titanium Grade 2 | Seawater, chlorine, desalination | 75–90% vs Ti WNF | High — mandatory isolation |
| A350 LF2 | SS 304L (cryogenic) | LNG, ethylene, cryogenic | 45–60% vs SS WNF | Moderate — protect bore |
| SS 316L A182 F316L | SS 316L (same alloy) | Full SS 316L severe corrosion | — | None |
| Material | ASTM Grade | Yield (MPa) | Temp Range (°C) | Corrosion | Application |
|---|---|---|---|---|---|
| CS Standard | A234 WPB | ≥240 | −29 to +538 | Low | General CS stub ends, utilities, oil & gas |
| LTCS −46°C | A420 WPL6 | ≥240 | −46 to +343 | Low | LNG, ethylene, cryogenic stub ends |
| 1.25Cr-0.5Mo | A234 WP11 | ≥205 | −29 to +593 | Moderate | High-temp refinery, H₂ service |
| SS 304/304L | A403 WP304/304L | ≥205 | −196 to +816 | High | Chemical, pharma, food, cryogenic |
| SS 316/316L | A403 WP316/316L | ≥205 | −196 to +816 | Very High | Offshore, chloride, chemical, pharma |
| SS 321 | A403 WP321 | ≥205 | −196 to +816 | High | High-temp SS, sensitisation resistance |
| Duplex 2205 | A815 WP-S31803 | ≥448 | −50 to +315 | Very High | Offshore sour, seawater, chloride |
| Super Duplex | A815 WP-S32750 | ≥550 | −50 to +300 | Extreme | Subsea, HPHT, severe sour service |
| Inconel 625 | B705 N06625 | ≥276 | −196 to +980 | Extreme | High-temp corrosive, acid, offshore |
| Titanium Grade 2 | B861 Gr.2 | ≥275 | −196 to +315 | Extreme | Seawater, wet chlorine, desalination |
When a carbon steel backing flange is used with an alloy stub end, galvanic corrosion will occur at the contact interface between the CS backing flange bore and the alloy stub end collar in any wet environment. The carbon steel is anodic and corrodes preferentially. In outdoor or wet service, protect the backing flange bore by: (1) fusion-bonded epoxy or zinc-rich primer on the bore and contact face; (2) non-metallic isolation sleeve between backing flange bore and stub end OD; or (3) specify the full backing flange in the same alloy (eliminating the cost saving). For titanium stub ends, galvanic isolation from carbon steel backing flanges is mandatory — titanium is highly cathodic relative to carbon steel in any aqueous environment.
Applications, QC Protocols
& Export Documentation
EN 10204 3.2 · PMI · Charpy · Collar Face AARH · Squareness
When to Specify Stub Ends
Stub ends are the correct specification in the following conditions: (1) Bi-metallic 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 and economically compelling justification; (2) Frequent dismantling — where pipe sections must be regularly disconnected for maintenance, cleaning, equipment removal or spool replacement; the free-rotating backing flange allows the pipe to slide out axially; (3) Difficult bolt hole alignment — where pipe routing makes correct bolt hole alignment by pipe rotation impossible; and (4) Low-pressure service (Class 150/300) — where the lower structural requirements allow the lap joint assembly to be used without sacrificing integrity.
Stub ends in SS 316L (A403 WP316L) with carbon steel A105 backing flanges are used throughout chemical plant piping — acid transfer lines, solvent systems, chemical reactor nozzle piping and heat exchanger connections. For pharmaceutical piping, electropolished SS 316L stub ends with Ra ≤ 0.8 µm collar face finish comply with 3-A hygienic standards. Frequent CIP/SIP maintenance access requirements make the lap joint the preferred design for pharmaceutical branch and header connections.
Duplex 2205 and super duplex stub ends with carbon steel backing flanges are used on offshore platforms for seawater service, cooling water, firewater and produced water piping. The large-bore duplex stub ends are a fraction of the cost of full-duplex weld neck flanges — particularly significant for NPS 8" through 24". All offshore stub end assemblies require EN 10204 3.2 MTC with TPI on both the stub end and backing flange, PMI on the stub end alloy, NACE hardness, ferrite content (duplex) and Charpy impact.
A420 WPL6 backing flanges with A403 WP304L stub ends are used in LNG plant piping where the low-temperature impact requirements of the backing flange must be met while the 304L stub end provides the cryogenic fluid contact surface with inherent toughness. The Charpy impact programme must include specimens from the stub end collar zone — it is the thickest section and may cool at a different rate during heat treatment.
Quality Control — Stub End Specific Requirements
QC for stub ends covers: (1) Collar OD and thickness — verified against MSS SP-43 for the specified NPS; (2) Total length — overall stub end length confirmed as short or long per the order; (3) Bevel end bore — bore ID verified against the specified pipe schedule — the most commonly missed QC point; (4) Collar face finish — 125–250 AARH measured with a profilometer on the collar face; (5) Collar squareness — perpendicularity to pipe axis ≤ 0.5 mm TIR; (6) PMI — 100% on all alloy stub ends; (7) Charpy impact — from collar zone for LTCS and duplex; (8) Ferrite content — on duplex collar.
Export Packaging and Preservation
- Stub ends wrapped individually in VCI poly bags (CS/alloy) or clean poly bags (SS/duplex/Ti); segregated from carbon steel items to prevent ferrous contamination
- Collar face protected with foam or cardboard face pad retained by strapping; bevel end with plastic cap — collar faces must not be stacked face-to-face without protection
- Bi-metallic packages (backing flange + stub end) shipped as matched sets, tagged together with the same PO line; separating during transit causes assembly errors on site
- Each stub end tagged: NPS, schedule (bore), MSS SP-43 type (A/B/C), length (short/long), material grade, heat/lot number and PO reference
- MTC (EN 10204 3.1/3.2) for stub end and backing flange (separate), collar AARH report, squareness report, dimensional report (collar OD, thickness, length, bevel ID), PMI (alloy stub ends), Charpy impact (LTCS/duplex), NACE hardness (sour), ferrite content (duplex) in waterproof sealed envelope
| # | Document | Standard / Reference | Component | Minimum Requirement |
|---|---|---|---|---|
| 01 | MTC — Stub End | EN 10204 3.1 / 3.2 | Stub end | 3.2 (TPI co-signed) for offshore / NACE / alloy |
| 02 | MTC — Backing Flange | EN 10204 3.1 | Backing flange | 3.1 standard; 3.2 if full SS or duplex backing |
| 03 | Dimensional Inspection — Stub End | MSS SP-43 / ASME B16.9 | Stub end | Collar OD, collar thickness, total length, bevel end ID — all mandatory |
| 04 | Collar Face Finish Report | ASME B16.5 / B46.1 | Stub end | 125–250 AARH measured with profilometer on collar face |
| 05 | Collar Squareness Report | Project specification | Stub end | ≤ 0.5 mm TIR perpendicularity to pipe axis — mandatory |
| 06 | Dimensional Inspection — Backing Flange | ASME B16.5 | Backing flange | OD, bolt circle, bolt holes, bore ID (clearance over stub end OD) |
| 07 | PMI Report (XRF / OES) | Project specification | Stub end | 100% SS, duplex, Inconel, Ti and all alloy stub ends |
| 08 | Charpy Impact Report | ASTM A370 / EN ISO 148 | Stub end | Mandatory for A420 WPL6 and duplex; collar zone included; ≥27 J at MDMT |
| 09 | NACE Hardness Report | ASTM E10 / NACE MR0175 | Both | Sour service: ≤22 HRC cross-section; duplex: ≤28 HRC |
| 10 | Ferrite Content Report (Duplex) | ASTM E562 | Stub end | Mandatory for duplex 2205 and super duplex; FN 35–65%; collar zone included |
| 11 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | Packaging | All wood packing for international export |
RR Hydraulics manufactures and exports stub ends and lap joint packages in all MSS SP-43 types (A, B, C) and ASME B16.9, in A234 WPB / A420 WPL6, A403 WP304L/316L/321, A815 WP-S31803/S32750, Inconel 625 and Titanium Grade 2. NPS ½"–24", schedules Sch 10S to XXS. Bi-metallic packages with matched carbon steel backing flanges supplied as sets. Collar face AARH report, squareness TIR report, bore schedule-match confirmation, PMI, Charpy impact (collar zone), NACE hardness, ferrite content (duplex), EN 10204 3.1/3.2 MTC, TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.