1. Why the Service Envelope Decides Everything

Copper-nickel is not one alloy. It is a family, and the family is graded by the working fluid, the velocity, the temperature, and the biofouling regime. Before any line is sized, five questions have to be answered: chloride concentration, design temperature, design velocity, biofouling risk, and the presence of ammonia or sulfide contamination. If those five are wrong, no grade of Cu-Ni will save the line.

The two grades that cover 90% of seawater and marine service are C70600 (90-10 Cu-Ni, 1.5-2.0% Fe, 0.5-1.0% Mn) and C71500 (70-30 Cu-Ni, 0.4-1.0% Fe, 1.0-1.5% Mn). The 90-10 grade is the default for shipboard seawater cooling, firewater, and sanitary lines at moderate velocities. The 70-30 grade earns its place on higher-temperature condensers, brine lines, and the more aggressive brine-side service in MSF and MED desalination plants. The standard that ties the chemistry, the heat treatment, and the test scope together is EEMUA 144 / EEMUA 234, with the corresponding UNS numbers and the ASTM B466 / B467 / B111 specifications carrying the procurement detail.

A common mistake is to specify C70600 tubing and then push it into a 5.0 m/s brine line where the design limit is closer to 3.5 m/s. The alloy is right; the operating point is wrong; the wall loss shows up in year 3. The fix is to size the line for the velocity, then choose the alloy, not the other way round.

2. Choosing Between 90-10 and 70-30 for Real Service

The 90-10 / 70-30 choice is a mechanical and a corrosion decision taken together, not a corrosion decision alone. On a 4-inch ship cooling line at 2.5 m/s and 50°C, 90-10 is the right answer. On the same diameter at 3.5 m/s and 80°C, the corrosion film on 90-10 starts to destabilize, and 70-30 is the upgrade. The pick that almost no one talks about is the Fe/Mn ratio — iron at 1.5-2.0% in 90-10 and 0.4-1.0% in 70-30 is what makes the protective oxide film stick in moving seawater, and the spec has to call the ratio out, not just the UNS number.

Where 90-10 Is the Correct Pick

• Shipboard seawater cooling (main engine, auxiliary engine, generators).
• Firewater and bilge lines on commercial vessels and offshore platforms.
• Brine recirculation on small- to medium-scale RO desalination trains.
• Sanitary and utility lines below 60°C and below 3.0 m/s.

Where 70-30 Is the Correct Pick

• Multi-stage flash (MSF) and multi-effect distillation (MED) brine heaters and evaporator shells.
• Condenser and heat-exchanger water boxes where inlet water exceeds 60°C.
• High-pressure piping on offshore platforms with chloride-rich produced-water exposure.
• Any service where the spec calls for longer design life and where the velocity is at the upper end of the 90-10 envelope.

Both grades are available in the same EEMUA-234 and ASTM B466 supply chain, and both ship with matching companion products: the copper nickel flanges for the joint terminations, the BW and SW fittings to EN 10253 / ASME B16.9 for the size transitions, and the Gasket, Stud Bolt & Nut kits to ASME B16.5 / B16.21 for the bolted joints. The risk in procurement is not picking the wrong alloy — it is picking the right alloy and a mismatched flange or fitting material from a different supply chain.

3. The Standards Stack for a Cu-Ni Bundle

A seawater bundle has to be specified as a coherent stack, not a list of independent parts. A spec that calls EEMUA 234 for the tube but ANSI B16.5 for the flanges without addressing the facing and the gasket style is asking for a receiving-floor debate that ends in rework. The table below is the standards stack that holds together on a typical shipboard, offshore, or desalination supply.

EEMUA 234 is the document that most European shipyards and offshore operators insist on; the US Navy and the chemical side tend to call ASTM B466 directly. The procurement should write both — EEMUA 234 as the acceptance standard, ASTM B466 as the material reference — and let the certificate reconcile. The opposite stack (ASTM only, no EEMUA) is the stack that gets challenged at the FAT.

4. The Joint: Flanges, Gaskets, and Bolting Done Right

A copper-nickel flange is not a carbon-steel flange with a Cu-Ni sticker on the cover page. The bore, the gasket face, the bolt pattern, and the stud length are all matched to a softer, ductile alloy and to the gasket style that survives seawater. The most common receiving-floor finding on a Cu-Ni bundle is a flange that was machined to a carbon-steel drawing and is now out of flatness by 0.2 mm — enough to leak at the first hydrostatic test.

The right pipe flanges for the seawater side are weld-neck Cu-Ni to ASME B16.5 Class 150, with raised face (RF) or flat face (FF) as the service dictates. Slip-on flanges are acceptable on low-pressure utility lines but are not the choice for the main seawater cooling or firewater riser. On the bolting side, Cu-Ni stud bolts to ASTM B151 / B552 or, where higher strength is required, B7 / B7M with a PTFE or graphite gasket — never a fiber gasket, which loses integrity in seawater service above 60°C.

The gasket stud bolt nut package has to be quoted as one decision, not three. The flange facing, the gasket style, and the bolt grade are matched at the drawing, not at the receiving floor. A buyer who sources the Cu-Ni flanges from one supplier and the stud bolts from another is buying a 30-day receiving delay and a high probability of bolt-length mismatch on the bolted-up joint.

5. Where U-Bend and Heat-Exchanger Integration Enters the Picture

Most copper-nickel bundles do not stop at the pipe rack. They feed a heat exchanger, and a heat exchanger means a U-bend tube. Specifying the tube separately from the bend is the second most common mistake in Cu-Ni procurement — the bend radius, the wall-thickness thinning allowance, and the heat-treatment state have to be matched to the exchanger, not chosen off a catalog page.

The relevant standard for U-bend Cu-Ni tubes is EN 10253-3 / ASTM B395, with the bend radius typically 1.5 × to 3 × the tube OD. The U bend tubes for a 90-10 seawater cooler will run at 1.5 × OD with a wall-thinning allowance of around 10-12% and a stress-relief heat treatment at 550-650°C after bending. The 70-30 grade, used on the higher-pressure brine side, is usually supplied in the annealed condition with a tighter thinning allowance and a tighter bend-radius window.

A practical point: a U-bend order is not a line item on a piping PO. It is a sub-package with its own drawing, its own dimensional report, and its own hydrostatic test. The MTC for the U-bend has to reconcile to the MTC for the straight tube on the same exchanger. A bundle that arrives with the right straight tube and a U-bend from a different heat is a bundle the QA team will not close out.

6. Welding, Filler Metal, and What the WPS Has to Call Out

Cu-Ni is welded differently from carbon steel and from stainless. The filler metal, the joint preparation, the preheat, and the interpass temperature all sit in a narrow window that the WPS has to reflect, not approximate. The default filler for 90-10 to 90-10 is ERCuNi (UNS C71580) or a Cu-Ni-Si-Mn filler; for 70-30, the same filler is used but with tighter interpass control. No bronze filler on a Cu-Ni line — the galvanic jump is real and the failure shows up at the first chloride cycle.

The pipe fittings for Cu-Ni service are predominantly butt-weld to ASME B16.9 / EN 10253, with a small amount of socket-weld on instrument connections. Threaded fittings are not used on a Cu-Ni line — the wall is too thin and the thread galling in seawater is the wrong place to be. Where the spec calls for a transition fitting (Cu-Ni to stainless, or Cu-Ni to Cu-Ni of a different grade), the WPS has to address the bi-metallic joint explicitly, including the gasket isolation and the bolt torque, otherwise the line fails at the transition.

7. Inspection, Testing, and Traceability

A seawater bundle is judged on the certificate, not the catalog. The four documents a serious procurement team insists on for every shipment are:

• Mill Test Report (MTR) per heat, with the actual chemical analysis matching the UNS number and the EEMUA 234 limits.
• Hydrostatic test report for the tube and the fittings, at the pressure required by ASTM B466 / B467.
• Eddy-current or ultrasonic test report for seamless tube, covering 100% of the length.
• Dimensional and visual report for the flanges, fittings, and U-bends, including the wall-thinning figure on every bend.

A Cu-Ni bundle that arrives without a heat-number index, or with the flange MTRs separated from the tube MTRs, is a bundle that takes a week longer on receiving and is open to argument for the next inspection cycle. The traceability requirement is the cheapest line item on the RFQ and the most expensive one to add after the fact.

8. The Bundle: Why Cu-Ni Tubing, Flanges, Fittings, and Bolting Belong on One PO

A seawater line in service is a Cu-Ni tube welded to a Cu-Ni fitting, bolted to a Cu-Ni flange, with a matched gasket and a matched stud bolt. Sourcing these as four separate POs from four separate suppliers is normal. Sourcing them as one bundle is the upgrade that pays for itself on the first receiving shift and on the first inspection cycle five years later.

A bundled Cu-Ni package collapses five receiving problems into one:

• One MTR file. One heat-number index, one cross-reference to the line list, one set of certificates to file. The QA team reads one document, not four.
• One production slot. One heat-treatment campaign, one delivery date, one container. The site does not receive the tube on Monday and the flanges the following Friday.
• One compatibility check. The flange facing, the gasket style, the bolt grade, and the tube chemistry are chosen together by one supplier, not matched together on the floor.
• One warranty. One supplier owns the joint performance. When a leak shows up in service, the phone call goes to one number.
• One freight. Bundled lots cut packaging, changeover, and freight per kilogram. The savings show up in the PO total, not just the unit price.

For a shipboard cooling set, a desalination train, or an offshore seawater injection package, the bundle is the only practical way to keep the MTRs, the heat numbers, and the delivery sequence straight.

9. A Short Checklist Before You Issue the Cu-Ni PO

• Service envelope locked: chloride level, design temperature, design velocity, biofouling regime, contamination.
• Alloy grade chosen by service (90-10 vs 70-30), with the Fe/Mn ratio called out in the spec.
• Standards stack written together: EEMUA 234 + ASTM B466 + ASME B16.5 / B16.9 + EN 10253 where applicable.
• Joint package decided as one: flange facing, gasket style, stud grade, bolt length.
• U-bend and heat-exchanger integration called out separately, with bend radius, thinning allowance, and heat treatment specified.
• Filler metal and WPS scope written for the welds, including any bi-metallic transition joints.
• Traceability scope written: MTR, hydrostatic, NDT, dimensional report — one set per heat.
• Cu-Ni tube, fittings, flanges, and bolting sourced as a single bundled PO from one supplier.