Lap Joint Flanges — ASME B16.5 Lapped Joint Flange & Stub End Manufacturer
Tesco Steel & Engineering manufactures lap joint (lapped joint) flanges with matched stub ends — the two-piece flange system: the stub end butt-welds to the pipe and provides the sealing face, while the loose backing flange spins free behind the lap, carrying the bolt load without ever touching the fluid. Two virtues follow: bolt holes always align — the erector's and maintainer's flange — and in alloy piping only the stub end needs the expensive metal: a 316L, duplex or Hastelloy line can run carbon-steel backing flanges at a fraction of solid-alloy cost — the same idea shipbuilding standardised as the EEMUA 145 copper-nickel collar. Made per ASME B16.5 in ½″–24″ NB, Classes 150–2500, with stub ends per ASME B16.9 (Type A/B) and MSS SP-43, flange and stub end certified together as a matched pair. The honest boundaries are stated: high bending loads, severe cycling and crevice-alert services read the weld neck bench. Every family supplies both halves: stub ends from A105 to Hastelloy, backing flanges in alloy, carbon or galvanised. Every lot with EN 10204 3.1/3.2 MTC. ISO 9001:2015, made in Mumbai, India — exported to 50+ countries.
ASME B16.5 · Classes 150–2500½″–24″ NB · Larger to B16.47Two-Piece System — Flange Never WettedBolt Holes Always AlignThe Alloy Economy — CS Backing on Alloy LinesStub Ends per B16.9 & MSS SP-43Matched Pairs, Certified TogetherEN 10204 3.1 / 3.2 · ISO 9001:2015
Lap Joint Flanges with Matched Stub Ends — The Two-Piece System
What is a Lap Joint Flange?
The two-piece flange system. A lap joint flange is the loose half of a pair: the stub end butt-welds to the pipe and its flared lap provides the gasket face; the backing flange slides over the pipe, sits loose behind the lap, and carries the entire bolt load — never touching the fluid. Because it is never fixed, the flange rotates freely: bolt holes always align. Because it is never wetted, it can be cheaper metal than the line — carbon-steel backing on alloy systems. Per ASME B16.5, ½″–24″ NB, Classes 150–2500; stub ends per ASME B16.9 / MSS SP-43. EN 10204 3.1 on the matched pair.
Also searched as: lapped joint flange, LJ flange, LJF, loose flange, backing flange, stub end flange, lap flange, Van Stone flange (the flared-pipe cousin) — all the same product family. Related pages: the type bench — slip-on (the fixed cousin) / weld neck (where loads rule) / socket weld / threaded — the references — face types guide / dimension charts / weight chart — and the marine cousin: the EEMUA 145 CuNi collar story.
The Two-Piece System — Who Does What
Stub End (the lap)
Backing Flange
Fixed to pipe?
Yes — butt-welded
No — slides free, rotates
Touches fluid?
Yes — the wetted part
Never
Provides
The gasket sealing face
The bolt load, via the lap's shoulder
Material follows
The line — alloy per service
The economics — alloy, carbon or galvanised
Standard
ASME B16.9 (Type A/B) · MSS SP-43
ASME B16.5 lap joint pattern
Backing Flange + Stub End + Assembled Joint — the flange bears on the lap, never the fluid
The site lesson every fitter learns once: slide the flange onto the pipe first, then weld the stub end — a stub end welded before its flange is on the pipe means cutting it off again.
What the Loose Flange Buys
Bolt Holes Always Align
The flange spins until the bolts enter — erection is forgiving, prefab spools meet site equipment without grinding out welds, and awkward orientations are trivial.
The Alloy Economy
Only the stub end needs the line's metallurgy — a Hastelloy system with carbon-steel backing flanges saves the price difference at every single joint.
Built for Dismantling
Strainers, filters, exchanger channels and rotating equipment reconnect without fighting orientation — the maintainer's flange by design.
The Marine Pedigree
EEMUA 145's copper-nickel collars with galvanised backing flanges are this idea in shipbuilding dress — a whole marine standard built on the two-piece split.
When NOT to Use Lap Joint Flanges — The Honest Boundaries
Three boundaries, stated plainly.High external loads and bending: the loose flange bears on a narrow lap shoulder and nothing ties it to the pipe structurally — nozzle-load-heavy and high-moment positions belong to the weld neck bench, whose integral hub reinforces the joint. Severe cyclic and vibrating service: the lap-to-flange bearing surface is a fretting interface — pulsating lines read the welded patterns. And crevice-alert duty: the annular space at the flange bore and the bearing face behind the lap are crevices by construction — harmless in utility service, but in wet chloride duty the stainless bench's crevice warnings apply, and critical corrosive service often welds for exactly this reason. Inside its honest territory — dismantleable, moderate-load, alloy-economical piping — the system serves for decades.
How Our Lap Joint Assemblies Are Manufactured
1
Backing flange machining — from certified forgings: bore and shoulder to suit the lap, bolt circle drilled to class, the shoulder radius dressed to bear evenly on the back of the lap — the detail that spreads bolt load instead of concentrating it.
2
Stub end production — forged or wrought per ASME B16.9 (Type A/B, long or short) or MSS SP-43; the lap faced with the gasket-surface finish, the weld end bevelled for the line butt weld.
3
Pairing — flange shoulder checked against lap OD; supplied as matched pairs (or separate line items to the same order).
4
Testing & marking — PMI on every alloy piece, both halves; heat-number marking on flange and stub end alike.
5
Certification — one EN 10204 3.1 document covering the pair (3.2 witnessed on request) — the traceability arrives together, as the joint installs together.
6
Packing — laps and bevels protected, galvanised or coated backing finish as ordered, packed sea-worthy.
Where Lap Joint Flanges Are Used
The pattern's honest territory: alloy process piping where the economy pays at every joint — stainless, duplex and nickel-alloy chemical lines with carbon-steel backing — systems that dismantle often (strainer stations, filter housings, exchanger channels, dosing skids), prefabricated spools meeting site-installed equipment, copper-nickel marine systems in the collar tradition, lined and glass-lined piping with Van Stone flared laps, and the utility corners of every plant where rotation at erection saves hours. Production and supply below:
Backing Flanges — Production Batch at Our WorksCarbon Steel Backing Flanges — The Alloy Economy's Cheap HalfStainless Pairs — Flange & Stub End Certified Together
Lap Joint Flange Dimensions — ASME B16.5
Flange dimensions are class-governed per ASME B16.5; stub end dimensions per B16.9 / MSS SP-43. Full class-by-class charts:
Lap Joint vs Slip-On — The Loose-vs-Fixed Decision
Lap Joint
Slip-On
Welded to pipe?
No — the stub end takes the weld
Yes — two fillet welds
Wetted?
Never — only the stub end
Yes — bore sees fluid
Rotation
Free until bolted
Fixed at welding
Material split
Flange can be cheaper than line
Must match the service
Typical duty
Alloy economy, dismantling, prefab
Simple fixed carbon-steel process piping
The shared boundary: neither pattern reinforces like a weld neck's tapered hub — heavily-loaded, high-bending and severe cyclic services read the weld neck bench regardless.
How to Specify & Order a Lap Joint Flange
Seven elements — remember the system is a pair:
1
Size & standard — e.g. 3″ NB ASME B16.5.
2
Pressure class — 150#–2500#.
3
Flange material — matching alloy, carbon steel or galvanised, per the alloy-economy decision; finish stated for the environment's outside.
4
Stub end — material to match the line, schedule, and pattern: B16.9 Type A/B (long or short) or MSS SP-43 — or ‘flange only’ where the lap already exists.
5
Service — named where it helps the crevice and load checks of the boundaries section.
6
Certification — EN 10204 3.1 (our standard) / 3.2 witnessed — flange and stub end certified together.
Example: “Lap Joint Flange, 3″ NB, ASME B16.5 Class 150, backing flange A105 galvanised, stub end ASTM A403 316L Sch 40S B16.9 Type A long, matched pairs, EN 10204 3.1 — 20 sets.” Quotations normally within 24 hours with price, unit weights (both parts) and delivery.
Lap Joint Flanges — Frequently Asked Questions
What is a lap joint flange?
A lap joint flange is the loose half of a two-piece flange system. The working piece is the stub end — a short fitting that butt-welds to the pipe, whose flared lap provides the gasket sealing face. The lap joint flange itself slides over the pipe before the stub end is welded and sits loose behind the lap: when the joint is bolted, the flange presses the lap against the mating gasket and carries the entire bolt load — without ever touching the line fluid. Two virtues follow directly: the flange spins freely, so bolt holes always align no matter how the pipe was welded; and since only the stub end is wetted, the flange can be a cheaper material than the line. Manufactured per ASME B16.5 in ½" to 24" NB, Classes 150 to 2500, with stub ends per ASME B16.9 or MSS SP-43, in every material family this site carries — EN 10204 3.1/3.2 certification on flange and stub end alike.
How does the two-piece system work — stub end and backing flange?
By splitting the two jobs a flange normally does. In a weld neck or slip-on, one forging both seals the fluid and carries the bolt load. The lap joint system separates them: the stub end — welded to the pipe, its lap faced like a gasket surface — does all the sealing and all the fluid contact; the backing flange — never welded, never wetted — does all the bolting. Assembly order matters and is the classic site lesson: slide the flange onto the pipe first, then butt-weld the stub end; a stub end welded before its flange is on the pipe means cutting it off again. In service the joint behaves like any flanged connection of its class — the gasket seats on the lap face, the bolts load through the flange's shoulder onto the back of the lap — and at dismantling the flange simply slides back, leaving the welded stub end in place for the next gasket.
Why do bolt holes always align — the rotation franchise?
Because the flange is never fixed to the pipe. On welded flange types, bolt-hole orientation is set forever the moment the welder tacks the flange — and a two-hole misalignment discovered at erection means grinding out a weld. The lap joint flange spins freely on the pipe until the bolts enter, so alignment is automatic at every joint, every time. The consequences run through a piping system's whole life: erection is faster and more forgiving, especially on prefabricated spools meeting site-installed equipment; frequent-dismantling services — strainers, filters, exchanger channels, rotating equipment — reconnect without fighting hole orientation; and awkward orientations (valves that must sit at an angle, instruments that must face the walkway) are trivial. Where a system is dismantled often or erected fast, the rotation alone justifies the pattern — the alloy economics of the next FAQ are a second, independent reason.
What is the alloy economy — carbon steel flanges on alloy lines?
The lap joint system's famous cost trick, and the reason it dominates expensive-alloy small piping. Because the backing flange never touches the fluid, it does not need the line's metallurgy: a 316L, duplex or Hastelloy system can run its stub ends in the line alloy — the only wetted part — with plain carbon steel or galvanised backing flanges carrying the bolts, at a fraction of the cost of solid-alloy flanges. On nickel-alloy systems the arithmetic is dramatic: a solid C-276 flange costs many times its carbon-steel twin, and every lap joint substitutes one. The same logic built an entire marine standard: EEMUA 145's copper-nickel welding-neck collars with galvanised backing flanges — described on our cupronickel long weld neck pages — are the lap joint idea in shipbuilding dress. The one discipline: the backing flange still needs coating or material adequate for the environment's outside (a carbon flange on an offshore deck still rusts), so specify the flange finish alongside the stub-end alloy.
What are stub ends — Type A, Type B and MSS SP-43?
The welded half of the system, standardised in its own right. ASME B16.9 covers forged and wrought stub ends in two machining patterns: Type A, whose lap face is machined to suit the serrated gasket surface a lap joint flange expects, and Type B, machined flat for use with slip-on-style backing; both come in the standard long pattern and a short pattern where space is tight. MSS SP-43 covers the lighter-schedule stainless stub ends common in corrosion-resistant, lower-pressure service — the economical partner for the alloy-economy systems of the previous FAQ. The specification rules: the stub end's material follows the pipe (it is the wetted, welded component — its certificate matters exactly as pipe does); its schedule matches the line bore; and its lap outside diameter must suit the backing flange's shoulder, which is why flange and stub end are best bought as a matched pair — as we supply them, certified together.
What is the difference between a lap joint flange and a slip-on flange?
Both slide over the pipe; the weld decides everything after that. A slip-on flange is welded to the pipe — two fillet welds, front and back — becoming a fixed, wetted part of the pressure boundary: its bore sees fluid, its orientation is set at welding, and its material must match the service. A lap joint flange is never welded: the stub end takes the weld and the fluid, the flange stays loose, rotates freely and can be a different material. Cost runs close between the two patterns in plain carbon systems — the lap joint adds the stub end but simplifies the flange — so the choice follows the virtues: slip-on for simple, fixed, carbon-steel process piping; lap joint wherever rotation, frequent dismantling or the alloy economy matter. One boundary is shared honestly: neither pattern reinforces the joint the way a weld neck's tapered hub does, so heavily-loaded, high-bending and severe cyclic services read the weld neck bench regardless.
Where should lap joint flanges NOT be used?
Three boundaries, stated plainly. High external loads and bending first: the loose flange transmits bolt load to a relatively narrow lap shoulder, and nothing ties the flange to the pipe structurally — nozzle-load-heavy connections, long unsupported spans and high-moment positions belong to weld neck flanges, whose integral hub reinforces the joint. Severe cyclic and fatigue service second: the lap-to-flange bearing surface is a fretting interface under vibration, and codes treat the joint's fatigue class accordingly — pulsating and vibrating lines read the welded bench. Crevice-alert services third: the annular space between the loose flange bore and the pipe, and the bearing face behind the lap, are crevices by construction — harmless in utility duty, but in wet chloride service the stainless bench's crevice warnings apply, and critical corrosive duty often prefers a welded pattern for exactly this reason. Inside its honest territory — dismantleable, moderate-load, alloy-economical piping — the system serves for decades.
What materials are lap joint flanges and stub ends made in?
Every family on the site's bench, with the two-piece split working in the buyer's favour. Stub ends — the wetted part — follow the line: A105 carbon steel, A350 LF2 for low temperature, the A182 stainless wing from 304L/316L through 321, 347, 310 and 904L, duplex F51 and the super duplex twins, and the nickel bench — Monel, Inconel 600/625, Incoloy 800/825, Alloy 20, Hastelloy C-276/C-22 — plus the copper-nickel pair with their EEMUA heritage. Backing flanges follow the economics: matching alloy where a specification insists, carbon steel (plain, primed or galvanised) where the alloy economy is wanted, and stainless where the environment's outside demands it. Every grade page on this site has a lapped joint sibling — the range table on this page links them — and PMI, heat-number marking and EN 10204 certification travel with stub end and flange alike, certified together as a matched pair.
What are the pressure-temperature ratings of lap joint flanges?
The B16.5 class tables govern, exactly as for every other pattern — a Class 300 lap joint carries the Class 300 rating of its material group — with two honest footnotes the standard itself makes. First, the rating is taken at the weaker of the pair: the joint's pressure boundary is the stub end, so the stub end's material group and schedule set the wetted rating, while the flange's class must match mechanically. Second, B16.5 notes that lap joint assemblies are commonly applied at the moderate end of their ratings in practice: the pattern's virtues — rotation, dismantling, alloy economy — belong mostly to Class 150 and 300 service, and while lap joint flanges exist through Class 2500 on paper, high-energy joints overwhelmingly specify weld necks for the structural reasons the boundaries FAQ gives. The practical rule: choose the class from the line specification as always, and let the pattern question — lap joint or welded — be decided by loads and service, not by rating tables.
What sizes do lap joint flanges come in — and what about Van Stone laps?
½" to 24" NB per ASME B16.5 as standard, with larger diameters to B16.47 or drawing — and the full class-by-class dimension charts are on this site. Small and mid bore dominates the order book: the dismantling and alloy-economy virtues concentrate in 1"-8" process and utility piping, while large-bore lap joints serve mostly in copper-nickel and lined systems where the collar tradition rules. The Van Stone name deserves its footnote: a Van Stone flange is the same loose-flange idea with the lap formed by flaring the pipe itself rather than welding on a stub end — common in plastic-lined and glass-lined piping, and the historical ancestor of the pattern. We machine backing flanges to suit flared-lap and lined-pipe systems to drawing, alongside the standard B16.9 stub-end pattern. State size, class, and whether the lap is a stub end or a flared pipe end, and the quotation follows.
How much does a lap joint flange weigh?
Slightly less than the equivalent slip-on flange — the loose flange has no hub weld prep and a plain shouldered bore. Representative Class 150 carbon steel figures for the flange alone: 1" roughly 0.8 kg, 2" roughly 2 kg, 4" roughly 5.5 kg, 8" roughly 14 kg — with the stub end adding its own mass by schedule and pattern (a 2" Sch 40 long-pattern stub end adds roughly 0.5 kg). Weights climb steeply with class, and for the two-piece system the freight arithmetic counts both parts — our quotations state unit and consignment weights per line item, flange and stub end separately, and the site's flange weight chart carries the full reference tables. On alloy systems the split works for the buyer here too: the heavy part (the flange) is the cheap metal, and the expensive alloy travels only in the light stub end.
How are lap joint flange assemblies manufactured and supplied?
As a matched pair, certified together. The backing flange is machined from certified forgings: bore and shoulder to suit the lap, bolt circle drilled to class, and the shoulder radius dressed so it bears evenly on the back of the lap — the detail that decides whether bolt load spreads or concentrates. The stub end is forged or wrought per B16.9 (or MSS SP-43), its lap faced with the gasket-surface finish of the flange standard, its weld end bevelled for the line butt weld. Both parts carry heat-number marking; PMI runs on every alloy piece; and the EN 10204 3.1 certificate (3.2 witnessed on request) covers flange and stub end together so the pair's traceability arrives as one document. Supplied assembled or as separate line items to the same order, threads of the erection sequence — flange on first, then weld — noted on the packing advice where buyers ask.
What details are needed to get an accurate lap joint flange quotation?
Seven elements plus commercial terms: (1) size and dimensional standard — e.g. 3" NB ASME B16.5; (2) pressure class — 150 to 2500; (3) the flange material — matching alloy, carbon steel, or galvanised, per the alloy-economy decision; (4) the stub end — material to match the line, schedule, pattern (B16.9 Type A/B long or short, or MSS SP-43), or 'flange only' where the lap already exists; (5) facing finish on the lap where non-standard; (6) certification — EN 10204 3.1 (our standard) or 3.2 witnessed, flange and stub end certified together; (7) quantity and destination — noting that the two-piece system ships as matched pairs unless ordered separately. Add the service where it helps the crevice and load checks of the boundaries FAQ, and quotations normally return within 24 hours with price, unit weights (both parts) and delivery.
Who manufactures lap joint flanges in India?
Tesco Steel & Engineering is an ISO 9001:2015 certified flange manufacturer based in Mumbai, India, producing lap joint (lapped joint) flanges per ASME B16.5 from ½" to 24" NB (larger to B16.47 or drawing) in Classes 150-2500, with matched stub ends per ASME B16.9 and MSS SP-43 — the backing flange machined from certified forgings with its shoulder dressed to bear evenly on the lap, the stub end faced and bevelled for the line weld, both parts heat-number marked, PMI-checked on alloy grades, and certified together on one EN 10204 3.1/3.2 document. The full material range is supplied on both sides of the two-piece split: stub ends from A105 through stainless, duplex, Monel, Inconel, Incoloy, Alloy 20, Hastelloy and copper-nickel; backing flanges in matching alloy, carbon steel or galvanised finish per the alloy economy. Alongside the complete flange range — weld neck, slip-on, socket weld, threaded, blind and long weld neck. Exported to more than 50 countries.
