TA2 90 Degree LR Titanium Elbow: Extrados Thinning

Опубликовано Engineering Team, Shaanxi Qianyi Tuoda Technology Co., Ltd от 2026-11-12

TA2 90 degree long radius titanium elbows to ASTM B363 are stocked in common NPS sizes, but on a welded elbow the longitudinal seam must never sit on the extrados. The outer radius is where forming stretches and thins the wall — commonly around 0.875 times nominal before the mill tolerance is even applied, and roughly 0.79 times nominal once a -12.5% incoming tolerance and 10–15% forming loss are combined. It is also where the highest flow velocity and the highest bending strain coincide. Putting a weld and its heat-affected zone in that band is an invitation to a pinhole at the outlet tangent.

This guide is for buyers specifying elbows for chemical process piping. It covers the long-radius versus short-radius decision, extrados thinning arithmetic, weld seam placement in welded elbows, erosion at the outer radius, the B363/B16.9/B16.28 boundary, the NDT that buyers forget to ask for, the service limits of TA2, the cost of an elbow leak, and the eight fields that belong on the RFQ. For the wider family see our titanium pipe fittings category and the R-value and pressure-drop article.

Standards	ASTM B363 (wrought Titanium Fittings); ASME B16.9 (LR dimensions); B16.28 (short radius); B16.25 (bevel)
Grades	TA2 / Gr2 / UNS R50400; match elbow grade and schedule to adjoining pipe; Gr7, Gr12 available
Service	Chemical process piping turns, pump discharge, heat exchanger inlet and outlet, header routing
Core distinction	The outer radius is the thinnest, fastest and most strained part of the elbow. Specify its wall and, on welded elbows, where the seam goes.

The Seam on the Outer Radius: A Pump Discharge Elbow That Perforated

A chemical plant ordered 4 inch Sch 10S TA2 90 degree long radius elbows for a pump discharge line in 2023. The purchase order read "ASTM B363 Gr2 90 deg LR elbow 4 in Sch 10S, welded". The supplier delivered welded elbows — two pressed half-shells joined by two longitudinal seams — which is a legitimate construction when properly specified. One of those seams had been positioned along the extrados. During installation, a short-radius elbow was substituted at one location to make the spool fit a tight space.

Fourteen months later the outer radius perforated just downstream of the elbow crown. Sectioning showed three things at the same band: the wall was measurably thinner than the schedule, the longitudinal weld's heat-affected zone ran through the thinnest region, and the flow leaving the short-radius bend was still separating at the outlet tangent. The weld had been radiographically acceptable at manufacture; the failure was not a weld defect in isolation but a weld placed in the worst possible location and then exposed to accelerated flow.

The rule: the extrados of an elbow is simultaneously the thinnest wall, the highest local velocity and the highest bending strain. On a welded elbow, specify the seam position — at or near the intrados / neutral axis, never on the outer radius — and verify the outer-radius wall by UT. A schedule number alone does not describe either.

The repair required replacing the elbow, re-welding with full argon backing purge and adding a straight run downstream to let the flow reattach. Our 90 degree seamless elbow stock page and the B363 fitting guide describe the constructions and inspection packages available.

Long Radius vs Short Radius: R = 1.5D Is Not a Preference

A long radius 90 degree elbow has a centreline bend radius of 1.5 times the nominal pipe diameter (R = 1.5D). A short radius elbow has R = 1.0D and is covered by ASME B16.28 rather than the standard B16.9 LR dimensions. The shorter bend saves space, but it turns the flow more abruptly: pressure drop rises, the boundary layer separates more strongly at the outlet tangent, and erosion at the outer radius increases. On a corrosive or solids-bearing line that difference shows up as wall loss at exactly the location that is already thinnest.

Configuration	When to use	Watch out for
Seamless LR, R = 1.5D	Default for chemical, erosive, cyclic and high-velocity lines	Still requires extrados minimum wall and UT mapping
Welded LR (pressed half-shells)	Larger diameters, cost-sensitive steady lines	Seam position, full-penetration weld, weld NDT, pickling
Short radius, R = 1.0D	Tight space, clean low-velocity fluid, non-erosive duty	Higher pressure drop and outlet turbulence; do not substitute on site
Mitered / fabricated bend	Very large diameter or special angles	Multiple girth welds, lower fatigue life, crevices at weld lands

Write the radius on the PO and on the isometric, and treat any on-site substitution of SR for LR as a design change requiring approval. The two fittings are not dimensionally interchangeable either — centre-to-end dimensions differ — so a substitution usually also forces a spool modification, which is exactly how the change at the opening case happened. For a fuller treatment of the pressure-drop side see our R-value and pressure-drop notes.

The 0.788 x Nominal Problem: Where the Wall Goes

Forming a bend stretches the outer radius and compresses the inner radius. The extrados is thinned, the intrados is slightly thickened, and the neutral axis stays roughly at nominal. Typical forming loss at the extrados is around 10–15%. If the starting tube or plate was already at the low end of a permitted -12.5% wall tolerance, the two effects multiply: approximately 0.875 x 0.90 = 0.788 times nominal at the thinnest point. These are approximate planning figures, not design values — but they explain why a perfectly compliant Sch 10S elbow can arrive with a wall well below what the designer assumed.

Three clauses fix it. First, state a minimum wall at the extrados in millimetres, with no minus tolerance at any measured point. Second, require an ultrasonic thickness map at a defined grid — for example five points from tangent to tangent along the outer radius, plus the crown and both intrados quadrants — with the report tied to each fitting's heat number. Third, require dimensional checks for bend radius, ovality, centre-to-end dimensions and bevel geometry to B16.9 (or B16.28) and B16.25.

The schedule on both ends must match the adjoining pipe. A heavier elbow creates an internal step at the weld, which trips the flow and creates a crevice; a lighter elbow becomes the weakest pressure boundary in the line. This is the same mismatch problem described for elbow, tee and reducer sets, and it is much cheaper to prevent at the PO than to grind out after installation.

Welded Elbows: Seam Position, Weld Quality and NDT

A welded elbow is made from pressed half-shells or from a formed plate with one or two longitudinal seams. It is a legitimate and often economical product, particularly at larger diameters, but it is a different specification from a seamless elbow. The PO should name the construction, the seam location, the welding procedure, the weld NDT and the post-weld treatment.

Seam position is the distinctive requirement. On a two-seam half-shell elbow, the seams should sit at or near the intrados / neutral axis, where forming strain and flow velocity are lowest. A seam on the extrados combines a heat-affected zone, a possible weld reinforcement or mismatch, and the thinnest wall in one band. Specify "longitudinal seams located at the intrados / neutral axis, minimum X degrees away from the extrados centreline" and ask for a photograph showing the seam orientation before shipment. It takes one image to catch.

The NDT package should include: radiographic or ultrasonic examination of the longitudinal welds to a named acceptance standard; UT thickness mapping of the extrados; dye penetrant on formed surfaces and both bevel ends; dimensional inspection; PMI on every fitting; and controlled HF/HNO3 pickling followed by a thorough rinse and a free-iron test. Add hydrostatic testing if the project code or a cyclic duty requires it. Ask for the reports to ship with the parts rather than being "available on request". The default B363 inspection is a floor; it does not include an extrados thickness survey of every elbow.

Where TA2 Elbows DO Fail: Read This Before You Approve the Bend

Erosion-corrosion at the extrados and outlet tangent

Flow accelerates around the outer radius and separates at the outlet tangent, especially with entrained solids, two-phase flow or flashing. The passive film is removed faster than it reforms, and the thinnest wall is right there. Set a velocity limit, keep LR on erosive duties, provide a straight downstream run, and specify the extrados minimum wall. See the tube bundle failure cases for the same inlet-turbulence mechanism on tubes.

Hot chloride crevices at weld lands and internal steps

Incomplete-penetration welds, unremoved backing strips, weld reinforcement and schedule mismatch create stagnant pockets. Above roughly 60–70 C in chloride service, those crevices acidify and attack the film. Design them out with full-penetration welds, matching schedules and crevice-free geometry before considering a more expensive grade; see the Gr2 vs Gr7 guide.

Reducing acids, fluoride and dry chlorine

TA2 has no useful resistance to HF or fluoride contamination, degrades in hot concentrated HCl and H2SO4, and needs trace water to repassivate in dry chlorine and water-free organics. Check cleaning chemicals and upset cases as well as normal operation. These are absolute exclusions rather than grade-selection questions.

Thermal cycling and fatigue at the girth welds

An elbow is the flexible element in a piping system and absorbs thermal expansion, so its two girth welds see repeated bending strain. Short-radius and mitered bends concentrate it further. Fabricated or mitered constructions have lower fatigue life than seamless LR elbows — commonly 30–50% lower as an approximate planning figure. For cyclic duty, specify seamless LR, full-penetration welds with argon backing and a flexibility analysis.

Iron contamination from forming tools

Carbon steel dies, shared grinding wheels and steel benches embed free iron in the surface, producing local galvanic pitting in wet chloride service. Dedicated titanium or stainless tooling, a final pickle and a free-iron test to ASTM A380 practice are the standard controls, as described for chemical piping fittings.

Do not accept a welded elbow with a seam on the extrados, do not let short-radius elbows be substituted for LR on site, do not order elbows without an extrados minimum wall, and do not assume the default B363 inspection includes a thickness survey of the outer radius.

Cost Economics: What an Elbow Leak Costs

A 4 inch titanium elbow is a modest line item. Replacing one on a pump discharge line requires isolation, draining, cutting out the bend, a full argon backing purge for two new welds, radiography, hydrotest and recommissioning — and if the line has no bypass, production stops. The premium for a seamless LR elbow, an extrados UT map and a proper weld NDT package is small against that.

Where the cheaper option is genuinely correct: a welded LR elbow with seams at the intrados, full weld NDT and a clean steady low-velocity fluid. That is a rational specification, not a compromise. The error is not buying welded elbows; it is buying them without a seam-position clause and then installing them on an erosive cyclic line. For current base material context see the price index and the latest daily price report. All ratios and multiples here are approximate planning figures, not quotations or design values.

Seven Procurement Mistakes That Show Up at the Outer Radius

1. No extrados minimum wall. A schedule number does not describe the thinnest formed point.
2. No seam-position clause on welded elbows. A seam on the outer radius combines the weld HAZ with the thinnest wall.
3. Allowing SR substitution for LR on site. It changes pressure drop, turbulence and centre-to-end dimensions.
4. Citing B16.9 without B363. You have specified a shape, not titanium or a forming route.
5. Skipping weld NDT and UT mapping. Default B363 inspection is a floor.
6. Mismatched schedules and bevels. Internal steps create turbulence and crevices.
7. No pickling or free-iron test. Forming tools leave iron that pits after hydrotest.

Stock, Small Batch and Lead Time

We stock TA2 90 degree long radius elbows in common NPS sizes and Sch 10S / Sch 40S, in both seamless and welded constructions, with 45 degree and short-radius options and matching tees and reducers from the same grade-controlled lot; see the stock listing and our inventory page. Custom angles, heavy walls, special radii and cut-to-spec ends are supplied through our cut-to-spec service.

Small-batch orders are normal: one replacement elbow for a shutdown repair or a few bends for a pilot plant is as valid as a full piping bill of materials, and small-batch supply is where a stockholder beats a mill minimum. We can also supply the matching pipe, flanges and welding filler for the same repair.

Lead time depends on size, construction, schedule and the inspection or third-party witnessing required. We quote a firm date with the offer. Send the size, angle, radius (LR or SR), schedule, seamless or welded construction, seam position, extrados minimum wall, NDT scope and document set to our quotation desk; our enquiry response procedure lists what we need to reply within 24 hours. For export orders, state the certificate language and packing — bevel ends need caps and protection.

Supplier Verification: What to Ask Before the PO

Ask whether the elbow is seamless or welded, and if welded, where the longitudinal seams will be positioned relative to the extrados — then ask for a photograph. Ask what the measured wall is at five points along the outer radius, not only the nominal schedule, and how it was measured. Ask for the weld WPS/PQR and the radiographic or UT acceptance criteria. Ask which NDT is performed on what percentage of parts and ask to see a sample report. Ask whether PMI is done on every fitting and how the heat number is marked.

Then ask the layout question: is this on a pump discharge or an erosive line, and would you recommend LR with a straight downstream run? A supplier who engages with the flow conditions has understood the failure mode. Our certifications page, materials overview and B363 reference give the document set and grade range.

Frequently Asked Questions

Q1: How thin can the extrados legally be?

Forming commonly reduces the outer-radius wall by roughly 10–15%, and if the incoming tube was at the low end of a -12.5% mill tolerance, the thinnest point can be near 0.79 times nominal. That is often compliant with the fitting standard. It is not acceptable for a design that assumed full schedule wall, so specify a minimum wall at the extrados and require UT mapping there.

Q2: Where should the weld seam be located?

On a welded elbow, the longitudinal seams should sit at or near the intrados / neutral axis, well away from the extrados centreline. The outer radius is the thinnest wall, the highest local velocity and the highest bending strain; putting a weld and its heat-affected zone there concentrates all three risks. State the seam position in the PO and verify by photograph before shipment.

Q3: Is long radius always better than short radius?

For chemical, erosive, two-phase or cyclic service, yes — LR (R = 1.5D) gives lower pressure drop, less outlet turbulence and less wall loss at the outer radius. Short radius (R = 1.0D, B16.28) is acceptable in tight spaces with clean, low-velocity, non-erosive fluid. Never substitute one for the other on site without a design review, since centre-to-end dimensions also differ.

Q4: Does B363 require extrados UT?

No, not as a routine requirement. B363's baseline inspection is a floor aimed at gross discontinuities. An extrados thickness survey of every elbow is a purchase specification. Add it explicitly, with a defined measurement grid, a minimum wall and a report tied to each fitting.

Q5: What causes pinhole leaks at the outer radius?

A combination of forming thinning, high local flow velocity, outlet-turbulence erosion, and sometimes a weld seam or internal step in the same band. Deposits and hot chloride crevices accelerate it further. Address all of them together: LR geometry, extrados minimum wall, seam away from the outer radius, matched schedules and a velocity limit.

Q6: Can elbows be cut to a custom angle?

Yes. We supply standard 90 and 45 degree elbows plus custom angles, special radii, heavy walls and cut-to-spec ends. Send the size, angle, radius, schedule, construction and inspection scope to the quotation desk and we will confirm feasibility with a firm date.

Q7: Do you stock Sch 10S elbows?

Yes, in common NPS sizes in both seamless and welded constructions. Check the 90 degree elbow stock page and our inventory list, then confirm live availability with your enquiry.

Choose a TA2 Seamless LR Elbow if / Choose Something Else if

Choose a TA2 seamless long radius B363 elbow if the line carries corrosive, erosive, two-phase or cyclic flow; it is a pump discharge or heat exchanger outlet; you can specify the extrados minimum wall and UT mapping; and schedules and bevels match the adjoining pipe. It is also the right choice for small-batch repairs where mill minimums are impractical.

Choose something else if space genuinely requires a short radius on a clean low-velocity line (specify B16.28 SR explicitly); if a welded elbow is chosen for cost or large diameter (specify seam position, WPS/PQR and full weld NDT); if hot chloride crevices, fluoride, hot concentrated reducing acid or dry chlorine are present (redesign or move to Gr7/Gr12, zirconium or a lined system); or if the bending cycle is severe enough to require a flexibility analysis and a heavier wall.

Summary: The Eight Fields That Belong on the RFQ

1. Standards: ASTM B363, dimensions per ASME B16.9 (or B16.28 for SR), bevel per B16.25, with grade TA2 / Gr2 / UNS R50400.
2. Geometry: NPS, angle, bend radius (LR R = 1.5D or SR R = 1.0D), schedule, centre-to-end dimensions, ovality limit.
3. Construction: seamless, or welded with seam location relative to the extrados and the applicable WPS/PQR.
4. Minimum wall: stated in millimetres at the extrados, with no minus tolerance at any measured point.
5. NDT: UT thickness map of the outer radius, RT or UT of longitudinal welds, PT on formed surfaces and bevels, dimensional check, PMI on every fitting.
6. Cleanliness: dedicated tooling, controlled pickling and rinsing, free-iron test, clean oxide-free bevels with protective caps.
7. Documentation: MTR with heat number, UT/RT/PT reports, dimensional report, welding records, certificate of origin, third-party witnessing if required.
8. Commercial terms: quantity (single fittings accepted), export packing with end protection, certificate language, target date, incoterms.

Send those eight items with your line list, design pressure, temperature, medium, velocity and upset cases to our quotation desk, or use the contact page to discuss the bend geometry first. Related reading: the pipe fittings category, the fitting decision tree, and the full product range.