Specifying Grade 7 titanium tube for a corrosive heat exchanger starts before the tube schedule. The material reviewer needs to know which fluid is inside the tube, which fluid is outside, the normal and upset temperatures, pressure context, velocity, solids, aeration and cleaning chemistry. A Grade 7 callout without that exposure summary may identify UNS R52400 correctly while leaving the actual corrosion decision unsupported.
Grade 7 is a palladium-bearing commercially pure titanium grade. The palladium addition gives it a different corrosion-response envelope from ordinary commercially pure grades in selected reducing, crevice-forming and aggressive environments. It does not make the grade immune to every acid, fluoride-bearing solution, deposit or galvanic condition. A corrosion specialist should approve the alloy against the complete process, including startup, shutdown and cleaning, rather than against the fluid name alone.
Once the grade is justified, the purchasing problem becomes geometric and procedural. Tube OD must fit the tubesheet strategy; wall must satisfy the equipment design and survive forming; length must suit bundle assembly; ends must support expansion or welding; inspection must occur at a useful stage; and packing must protect long tubes and clean bores. A practical RFQ connects those decisions instead of listing unrelated test names.
A Grade 7 heat-exchanger tube specification should connect one service envelope to one fabrication route and one acceptance plan. Define the process and cooling media, temperatures, shutdown chemistry, OD and wall, joint method, straight or bent delivery, inspection stage and bore protection before asking price or lead time.
First decide whether Grade 7 belongs in the service
Describe both sides of the heat-transfer surface
A shell-and-tube exchanger exposes the tube to two environments. The process side may contain acids or contaminants, while the utility side may contain seawater, brine, cooling water, steam or cleaning solution. State which side enters the bore and which contacts the outside surface. Include concentration ranges, dissolved oxygen, pH, temperature, flow, suspended solids and expected deposits. A material decision based on only the more aggressive-sounding fluid can overlook the condition that actually controls.
Add idle, startup and cleaning conditions
Continuous operation may keep a titanium surface passive while a stagnant shutdown creates a different crevice, deposit or concentration condition. Acid cleaning, chlorination, fluoride contamination or cathodic interaction can also change the exposure. Primary research on palladium additions in fluoride-containing environments is a reminder that alloy additions cannot be discussed apart from chemistry. The study does not provide a universal exchanger limit; it shows why a specific environment must be reviewed.
Compare Grade 7 against a defined alternative
If Grade 2, Grade 12, Grade 16 or another material is being considered, put each option beside the same service envelope, mechanical design and fabrication scope. The ATI corrosion-resistant titanium alloy data explains the role of palladium-alloyed commercially pure grades, while the International Titanium Association's desalination material discusses titanium in seawater heat-exchanger and piping systems. These are screening resources, not substitutes for project corrosion approval.
Turn the equipment drawing into a tube purchase description
| Equipment decision | Tube requirement it creates | Information to place on the RFQ |
|---|---|---|
| Tubesheet joint | Functional OD range, end surface, projection and prepared length. | Hole basis, expanded or welded joint, engagement, end-zone drawing and qualification owner. |
| Pressure and corrosion allowance | Design wall and permitted negative variation. | Nominal or minimum wall convention, tolerance and any mapping requirement. |
| Bundle layout | Straight length, straightness, support contact and insertion condition. | Cut-length tolerance, usable end zones, baffle spacing context and handling limits. |
| U-bend or coil | Centerline geometry, ovality, leg alignment and post-forming condition. | Radius, tangent points, leg lengths, orientation, end alignment and final inspection stage. |
| Clean service | Controlled bore condition, caps and compatible separators. | Cleaning method, acceptance, preservation, opening procedure and prohibited packing materials. |
Choose the dimensional hierarchy
Outside diameter and wall are common purchasing dimensions, but the order should say whether wall is nominal, average or minimum and which tolerance basis governs. For an expanded joint, OD may be the critical assembly interface. For pressure or corrosion design, minimum wall may carry more weight. Trying to make every dimension exceptionally tight can produce a costly tolerance stack without improving exchanger performance.
Define usable length rather than total mill length
A long tube can meet overall length while losing useful material to end trimming, surface damage or preparation allowances. Identify finished straight length, permitted trim, any sacrificial ends and the zones that must remain suitable for welding or expansion. If several bundle lengths are required, provide a cut plan and quantity for each instead of one total mass.
Select the product route before fixing the inspection plan
ASTM B338-17(2026) covers both seamless and welded titanium tube for condenser and heat-exchanger service. Its route-specific provisions mean that procurement should not use "ASTM B338 tube" as though the manufacturing history were irrelevant. State seamless, welded/cold-worked, or buyer-approved alternative routes. If more than one route is acceptable, request separate offers and compare their records and downstream qualification implications.
Straight tube is the correct boundary when the buyer owns forming
When the fabricator already has qualified U-bending or coiling procedures, straight Grade 7 titanium tube may be the cleanest purchase boundary. The tube order should still communicate the planned bend radius and forming sequence if they affect condition, surface or length allowance. A small trial quantity can confirm springback and ovality before the full bundle is released.
A formed tube is a finished geometry, not a material option
If U-tubes or coils are purchased, the supplier needs a centerline drawing and post-forming acceptance criteria. Define radius, tangents, leg lengths, spacing, rotation between bends, end alignment, ovality and any minimum wall at the bend. Also state whether cleaning, end preparation and integrity testing occur before or after forming. The same straight starting tube can generate different commercial risks under different bend designs.
Enhanced surfaces require a separate product definition
Integral enhanced surfaces are covered by a distinct framework such as ASTM B891/B891M-19(2024). That standard discusses unenhanced ends, close-fitting tubesheet holes and formed configurations within its scope. A buyer should not add "enhanced" to an ordinary B338 line and expect the same dimensions, test plan or tooling. Define the enhancement profile, active length, plain end zones and measurement method.
Make the joint strategy visible to the tube seller
Expanded joints
The tube seller does not set the expansion procedure, but the supplied OD range, condition, end surface and straightness influence assembly. Provide the tubesheet hole basis, engagement length and end zone required for qualification. If tubes are expanded and seal welded, distinguish the functions of each operation and identify which stage controls acceptance.
Welded joints
For welded tube-to-tubesheet joints, state end projection, cut squareness, deburring, cleanliness and any surface preparation. The equipment fabricator owns weld design, shielding, procedure qualification, personnel qualification and examination. Do not ask the raw tube certificate to stand in for these records. If end preparation is performed after delivery, reserve enough length and protect the bore during shop handling.
Related Grade 7 forms must be purchased under their own product standards. A Grade 7 titanium plate used for a tubesheet or channel component falls under a flat-product basis such as ASTM B265, while a Grade 7 titanium flange may begin from a forging basis such as ASTM B381. Coordinating grade identity is useful; merging their certificates is not.
Build inspection around the supply stage
A straight mill tube, a finished U-tube and an assembled exchanger do not need the same evidence. Start with the ordered standard and route, then add project-specific checks only where they close a real risk. Name the method, procedure, coverage, acceptance basis, stage and report. A phrase such as "100% NDT" is incomplete because it does not identify what is being detected or how acceptance is decided.
| Evidence | Useful question it answers | Boundary it does not cross |
|---|---|---|
| Material test report | Was the delivered tube identified and reported against the ordered grade and product specification? | It does not approve the corrosion selection or completed pressure equipment. |
| Dimensional log | Do critical OD, wall, length and straightness results meet the agreed sampling plan? | It does not prove fit unless the tubesheet and bend interfaces were defined correctly. |
| Integrity or NDE report | Did the named tube condition pass the specified examination at the stated stage? | A pre-bend result does not automatically cover damage introduced during later forming. |
| Packing photographs | Were ends, labels, supports and bundle separation arranged as approved? | Photographs do not replace receiving inspection after transport. |
Place hold points before the evidence disappears
If third-party witnessing, surface review before cleaning or dimensional checks before bending are required, identify those hold points before production. A request added after forming may be impossible to reconstruct. The inspection and test plan should show which party releases each stage and what record is retained.
Protect the bore and geometry through export delivery
Long thin-wall tubes can be damaged without obvious carton failure. Use distributed support, prevent tube-to-tube rubbing, control bundle mass and protect ends from impact. Straightness should not depend on forcing the bundle into a crate. If multiple heats, sizes or cut lengths share a shipment, maintain physical separation and visible identification.
Bore protection depends on service. Caps can limit loose contamination but may trap moisture if applied after unsuitable cleaning or drying. Define preparation, dryness, cap material, bagging and opening procedure where cleanliness matters. Avoid labels, tapes or separators that leave residue on joining zones. Packing requirements should be measurable rather than expressed as "export standard."
A practical RFQ sequence
Release the service summary first
Provide both media, concentration ranges, temperatures, pressure context, flow, deposits, cleaning and upset conditions. Mark confidential values with controlled ranges if necessary, but do not remove the variables needed for material review.
Then release the geometry and fabrication boundary
State B338 edition, Grade 7 or approved alternatives, route, OD, wall convention, length, tolerances, straight or formed supply, ends and joint method. Attach the tube or U-bend drawing and identify which dimensions are critical to assembly.
Close with evidence, quantity and delivery
List inspection methods and stages, certificate and report content, witness points, traceability, cleaning, packing, quantity by size, destination and required date. Ask for exceptions in a separate schedule so commercial comparison does not hide technical differences.
The titanium rods, wires and tubes category provides related product forms, but the tube order should remain tied to the exchanger drawing and process duty.
For the current standard-edition and quotation comparison context, read Does ASTM B338-17(2026) Make a Grade 7 Titanium Tube RFQ Complete?. It separates the B338 baseline from the buyer-specific items that cause nominally identical tube offers to diverge.
Frequently asked heat-exchanger tube questions
Is Grade 7 always better than Grade 2 for seawater?
No. The correct grade depends on the complete environment, temperature, crevice conditions, contaminants, cleaning, design and commercial priorities. Grade 7 should be justified against a defined service, not selected by rank.
Should the RFQ specify seamless tube only?
Only when the project design, qualification or owner specification requires it. B338 includes seamless and welded routes. If both are acceptable, request separately identified alternatives and compare their compliance and records.
What dimensions matter most for an expanded tube joint?
Tube OD and its relationship to the tubesheet hole are central, along with engagement length, end condition and expansion procedure. Wall and material condition also influence the qualified joint.
When should U-bent tubes be inspected?
Define checks before and after forming according to the risk. Post-bend geometry, ovality, surface and any required integrity examination cannot be inferred solely from straight-tube inspection.
Does a B338 certificate cover a Grade 7 tubesheet?
No. B338 is a tube specification. Plate used for a tubesheet requires its own flat-product basis, material record, dimensions, machining and fabrication documentation.
What packing details should an overseas buyer specify?
State bundle mass, support spacing, tube separation, end protection, bore preservation, label and traceability method, moisture controls, crate limits, destination and any restrictions on contact materials.
References
- ASTM B338-17(2026), Titanium and Titanium Alloy Tubes for Condensers and Heat Exchangers.
- ASTM B891/B891M-19(2024), Enhanced-Surface Titanium Heat Exchanger Tubes.
- ATI CP Grade 7 Titanium, UNS R52400.
- ATI Corrosion-Resistant Titanium Alloys technical data.
- International Titanium Association, titanium in desalination and seawater service.
- Peer-reviewed study of palladium additions and titanium corrosion in fluoride-containing environments.
For an engineering quotation, send the service summary, approved grade and B338 edition, route, OD and wall convention, straight or formed drawing, tube-to-tubesheet joint, inspection stages, certificate requirements, cleaning and bore preservation, quantity, destination and schedule. Identify unresolved design variables instead of hiding them inside a nominal tube size.