Why copper nickel flanges are still the seawater default

For shipbuilding, offshore platforms, desalination trains, and coastal power stations, copper nickel flanges remain the most predictable choice when the line carries raw seawater, brine, or deaerated seawater. The alloy forms a thin, self-healing oxide film that resists chloride pitting, tolerates biofouling, and sheds marine growth far better than carbon or standard stainless steel.

Two grades dominate the spec sheet: C70600 (90/10 Cu-Ni, 1–1.8% Fe) for general seawater service, and C71500 (70/30 Cu-Ni, 0.4–1.0% Fe) for higher velocities, sand-laden water and more aggressive chemistry. They fit cleanly into the same flanged bolt pattern as their pipe flanges siblings, so designers do not need to redraw the line just to switch alloy.

Pick the right grade for the service envelope

A buyer who treats Cu-Ni as a single material usually over-specs or under-specs. The table below is the working shortcut our engineers use when reviewing a piping class.

Both grades fall under the broader copper nickel alloy family and are delivered against ASTM B151 (forgings) or B466 (seamless pipe) chemistry. The chemistry limits below are the ones to enforce on the MTC, not just the descriptive brochure:

Bolting, facing and gasket choices that actually seal

A Cu-Ni flange pair is only as good as the joint. For class 150 seawater service we ship weld-neck or slip-on flanges faced to a 125–250 RMS finish, paired with spiral-wound gaskets with graphite or PTFE filler and matching gasket stud bolt nut sets in B7/B8 with PTFE or Xylan coating to prevent galling on the Cu-Ni bolt holes.

For higher pressures, raised face (RF) and ring-type joint (RTJ) facings are both available. The bolted joint checklist we apply to every release:

• Match flange pressure class (150 / 300 / 600) to the line design pressure, with margin for water hammer.
• Use EEMUA 234 dimensions when the line interfaces with marine pipe classes – this avoids flange-to-pipe mismatch on retrofits.
• Specify bolt torque values on the data sheet; Cu-Ni threads gall easily if torqued dry.
• Keep gasket seating surface clean and free of radial scratches before final torque.

Procurement: bundle the flange with the rest of the line

A frequent failure mode is buying Cu-Ni flanges from one supplier, the matching Cu-Ni pipe spool from another, and the industrial valves from a third. When each batch arrives with a different chemistry lot, traceability breaks, and welding procedures have to be re-qualified mid-project.

Bundling solves this. At EZ Steel Industrial, a typical seawater train release packages:

• Cu-Ni 90/10 pipe spool to EEMUA 234 / ASTM B466
• Cu-Ni weld-neck and slip-on flanges to ASTM B151
• Cu-Ni butt-weld fittings to ASME B16.9
• Bronze or Cu-Ni body industrial valves with monel or super-duplex trim where velocity is high
• Matched stud-bolt, nut and spiral-wound gasket set

Because all items come from one mill with one heat-number trail, the MTC chain stays intact from receiving inspection to final torque-up.

MTC and inspection: what to verify before sign-off

For a 90/10 or 70/30 flange release, the receiving inspector should look beyond the certificate header. The minimum evidence we ask for on every Cu-Ni flange shipment:

• Mill Test Certificate (MTC) with heat number, full chemistry, and mechanical results (tensile, hardness).
• Hydrostatic test report for the flange body when required by class.
• Dimensional check on bolt circle, bore and facing finish against ASME B16.5 / B16.47.
• PMI (Positive Material Identification) on a sample per heat to confirm Ni content matches the spec.
• Visual and dye-penetrant check on the sealing face – any radial nick is a reject.

When the flange arrives with full traceability, on-site installation is faster, and any future failure analysis is anchored to a known heat rather than a guess.

Common mistakes and how to avoid them

After two decades of supplying seawater systems, the same handful of issues appear on most failed joints. None of them are mystery problems – they are specification gaps.

• Mixing Fe content. A C70600 flange with 0.6% Fe next to one with 1.7% Fe will behave differently in service. Lock Fe in the spec, not just Ni and Cu.
• Substituting aluminium-bronze. Al-bronze flanges look similar and cost less, but they are not interchangeable with Cu-Ni in sulphide-containing or deaerated water. Keep the alloy line unbroken.
• Ignoring weld prep on flanges that will be welded in. A forged Cu-Ni flange has different heat input limits than the pipe. ｕｐｄａｔｅ the WPS for the joint, do not reuse the carbon-steel procedure.
• Skipping the gasket coating spec. Uncoated graphite on a Cu-Ni flange face can stick and damage the surface at the next overhaul.