Project Application Guide

Copper Nickel Alloy 90/10 vs 70/30 in Offshore, Marine, and Desalination Projects: A Scenario-Based Selection Walkthrough for Specifiers

Specifying copper nickel alloy for a real project rarely starts with a chemistry handbook. It starts with a question from the shipyard or the EPC: "we have a 4 m/s seawater line at 65 °C carrying 200 ppm sand — which grade do you actually recommend?" This walkthrough answers that question by walking through five live project scenarios, comparing 90/10 with 70/30 in each, and showing how the surrounding pressure boundary, tube bundle, and valve choice must move with the alloy decision.

Why the Project Scenario Drives the Alloy Decision

Copper-nickel is the default seawater material for a reason: the protective film that forms on a 90/10 or 70/30 surface in clean seawater is thin, adherent, and self-healing. Once matured, it holds general corrosion rates in the 0.02 to 0.002 mm/yr range for the rest of the piping's service life. But the design window that allows that film to mature is different on every project. The shape of that window — not the alloy brochure — is what decides whether 90/10 is enough, whether 70/30 is justified, or whether the line has stepped out of copper-nickel territory altogether.

The three most common design variables that move a project from 90/10 to 70/30 (or beyond) are velocity, sand loading, and the presence of refinery or desalination brine. The two variables that push a project from copper-nickel to Monel or Inconel are temperature above roughly 200 °C and chemistry containing strong acids or alkalis. Reading these five numbers correctly, on the very first datasheet, is what separates a 30-year line from a 3-year replacement.

Scenario 1: FPSO Seawater Injection and Ballast Lines

Offshore Oil & GasFPSO — 90/10 in the main ring, 70/30 in the high-velocity manifolds

A typical FPSO carries 8 to 14 km of seawater piping split between fire main, ballast, cooling, and produced-water reinjection. The bulk of that ring runs at 2 to 3 m/s with 50 to 200 ppm sand and tops out near 50 °C — the exact envelope where 90/10 (UNS C70600, CW352H) earns its place. It is weldable with the 70/30 consumable, available in seamless and welded pipe to ASTM B466 / B467, and pairs naturally with matching copper nickel flanges (ASME B16.5 Class 150 or 300) on every equipment nozzle.

The exception is the seawater lift manifold and the fire-main branch, where pump discharge pushes steady-state velocity past 3.5 m/s and fire-fighting peaks reach 12 to 15 m/s. In those locations, an upgrade to 70/30 (UNS C71500, CW354H) is justified: the higher iron and nickel content keeps the film intact at the higher wall shear, and the higher strength allows a marginal wall-thickness reduction. The material cost premium is small against the cost of a single dry-dock intervention.

Scenario 2: Naval Frigate and Commercial Vessel Seawater Cooling

ShipbuildingWarship and merchant vessel — 90/10 with selective 70/30 for scoop circuits

A modern frigate or a large commercial vessel typically runs 1.5 to 2.5 km of seawater cooling pipe below the waterline. Most of it sees 2.0 to 2.8 m/s of clean harbor or coastal seawater at 10 to 35 °C — again, the canonical 90/10 service. The compact, vibration-loaded environment below the engineroom also benefits from the alloy's natural biofouling resistance, which keeps chlorination systems lightly loaded and reduces the maintenance burden on the crew.

The two locations where 70/30 earns the upgrade are the scoop discharge branch and any section that runs at sustained velocity above 3.0 m/s, particularly where the line passes through a duplex stainless or aluminum-bronze fitting with a small internal diameter. The transition from Cu-Ni to a dissimilar material also calls for a 65% nickel-copper weld consumable (ECuNi-7 / ERNiCu-7) to absorb the iron dilution and avoid HAZ cracking.

Scenario 3: Multi-Stage Flash (MSF) and Reverse Osmosis (RO) Desalination

DesalinationMSF heat-rejection section — 90/10 shell, 70/30 or modified 70/30 tubes

A 50,000 m³/day MSF plant has roughly 12,000 heat-rejection tubes carrying warm brine at 90 to 110 °C. The shell side is conventionally 90/10 to BS MA18, but the tube side uses 70/30 (UNS C71500) for higher velocity tolerance, or the modified 70/30 grade C71640 (2% Mn, 2% Fe) for the most turbulent heat-rejection stages where local sand loading peaks.

For SWRO plants, the high-pressure train is dominated by super-duplex stainless (ASTM A790 / A928) and is outside the copper-nickel space, but the seawater intake, the low-pressure booster train, and the energy-recovery device shell are still 90/10 territory. The booster train and the cartridge filter housing benefit from coordinated 90/10 pipe plus matching flanges plus a non-metallic gasket (because Cu-Ni is sensitive to spiral-wound graphite at high chloride).

Scenario 4: Offshore Wind Substation and Tidal Stream Cooling

Renewable OffshoreOffshore wind OSS and tidal platforms — 90/10 for the seawater ring, 70/30 in splash-zone risers

An offshore wind substation (OSS) and a tidal stream platform both run auxiliary seawater cooling for the HVDC converter station or the generator housing. The piping is short, the velocity window is narrow, and the splash zone puts the alloy under alternating wet-dry conditions. 90/10 with a marine-grade coating is the default. The riser sections in the tidal zone — alternately wetted and dried by the tide — are better specified in 70/30, where the higher nickel content slows the long-term atmospheric corrosion of the splash zone.

A common oversight in this scenario is the cathodic-protection interaction. Cu-Ni pipe must be electrically isolated from the structure's sacrificial-anode system with isolating flanges or insulating gaskets, or the structure's anodes will be consumed protecting the pipeline instead of the jacket. Specifying the alloy is only half the decision; the joint detail is the other half.

Scenario 5: Coastal Power Plant Condenser and Auxiliary Cooling

Power GenerationOnce-through cooling and condenser — 70/30 tubes, 90/10 piping, Monel 400 for the hot well

A 1,000 MW coastal power plant typically carries 18,000 to 22,000 condenser tubes in 70/30 (UNS C71500) to ASTM B395, with the tube sheet in naval brass or aluminum bronze. The condenser water box and the connecting circulating-water pipe are 90/10 to BS MA18 / EEMUA 234. The hot well and the condensate line — which can carry sulfide-bearing water at temperatures above 80 °C — frequently step up to Monel 400 (UNS N04400) to ASTM B165 because copper-nickel starts to lose its protective film above 200 °C and in sulfidic environments.

This is also the scenario where the heat efficiency tubes in the auxiliary systems — the closed-loop cooling side and the lube-oil coolers — come into view. Precision finned tubes in aluminum or copper-nickel clad aluminum are the typical choice for the air-cooled side, while the shell-side uses U bend tubes in 90/10 or 70/30 to allow the bundle to expand without high thermal stress at the tube sheet.

Decision Matrix: 90/10 vs 70/30 vs Higher-Nickel Alloys

Bringing the five scenarios together, the alloy decision can be condensed into a single table that any specifier can read against a line class. The table below is the working document our engineering team uses to anchor the conversation with shipyards, EPCs, and refinery mechanical leads.

Service Window	90/10 (C70600)	70/30 (C71500)	Move Beyond Cu-Ni
Seawater ≤ 2.5 m/s, ≤ 50 °C, clean	Default — lowest cost, easiest weld	Over-spec	—
Seawater 2.5–3.5 m/s, ≤ 80 °C	Acceptable, watch sand	Preferred	—
Seawater > 3.5 m/s or with refinery brine	Not recommended	Default	—
Seawater with 200–1,000 ppm sand	Only for short spool pieces	Modified 70/30 (C71640) for tubes	—
Hot well or condensate with sulfides	—	Limited	Monel 400 (N04400) per ASTM B165
Above 200 °C, or strong acid / alkali	—	—	Inconel 600/690, or titanium
Aerospace / nuclear primary loop	—	—	RCC-M Cu-Ni or Inconel per project spec

How the Pressure Boundary Has to Move With the Alloy

Every alloy decision above leaves a footprint on the four adjacent items: the pipe flanges, the gasket, the stud bolt and nut, and the industrial valves. Mismatches in this trio are the largest single source of marine Cu-Ni leaks on commissioning.

Flanges: ASME B16.5 / B16.47 raised face, matching the body material. Copper nickel flanges (Class 150 and 300) in 90/10 or 70/30 body, with the same facing as the mating valve or fitting.
Gaskets: non-asbestos compressed fiber for low-pressure, spiral-wound with graphite or PTFE filler for ASME Class 150/300 service. Avoid graphite in high-chloride service on Cu-Ni bodies; specify PTFE or a flexible graphite with inhibiter.
Stud bolts and nuts: aluminum-bronze (ASTM F468) or Monel (ASTM F468 Alloy 400) for Cu-Ni flanged joints; ASTM A193 B7 / A194 2H for steel flanges; never mix bolting families on a single joint.
Valves: the industrial valves in the seawater ring should have Cu-Ni or aluminum-bronze bodies, monel or super-austenitic trim, and ISO 15848 Class AH or API 624 stem sealing for low-emission service.

Coordinating With the Tube Bundle: Where Heat Efficiency Tubes Meet the Alloy

In a condenser or a large heat exchanger, the tube bundle is the part of the pressure boundary that lives inside the shell. The same alloy logic applies, but with one additional constraint: tube-side velocity and shell-side cross-flow have to stay inside the EEMUA 144 / BS MA18 window, or the film on the inside of the tube will not mature. A 90/10 tube bundle in clean seawater is fine at 1.5 to 2.0 m/s; the same bundle in 70/30 tolerates 2.5 to 3.0 m/s; modified 70/30 (C71640) goes to 3.5 m/s in the most demanding heat-rejection stages.

For the auxiliary side — the lube-oil cooler, the closed-loop cooler, the air-fin cooler — the design moves to heat efficiency tubes. Finned tubes in aluminum or Cu-Ni clad aluminum raise the outside heat-transfer coefficient without changing the inside corrosion story. U bend tubes in 90/10 or 70/30 are the standard answer for floating-head and kettle reboilers where the bundle has to expand thermally without overstressing the tube sheet.

Specifier's rule of thumb

If the design sits inside 90/10 territory on every parameter, stay with 90/10. It is the cheapest Cu-Ni grade, the easiest to weld, and the most widely stocked. Move to 70/30 only when one specific design parameter (velocity, sand, temperature, or brine chemistry) crosses the 90/10 line. Move beyond Cu-Ni only when more than one parameter moves out of the Cu-Ni window at the same time.

Field Checks Before Signing the Purchase Order

A short checklist that has prevented rework on every project our team has shipped in the last three years:

Service envelope (water chemistry, design temperature, velocity window, sand loading, duty) on a one-page datasheet, signed by the project engineer.
Alloy grade and UNS designation called out on every line item, with EEMUA 144 / 234, BS MA18, or ASTM B466 reference noted.
Filler metal qualified before fabrication: 70/30 consumable (AWS A5.6 ECuNi / A5.7 ERCuNi) for Cu-Ni to Cu-Ni; 65% Ni-Cu (ECuNi-7 / ERNiCu-7) for Cu-Ni to steel transitions.
End connection, facing, and bolting matched to the mating flange. No mixed facing types on a single joint.
Cathodic-protection isolation detail agreed with the structural team before spool fabrication starts.
Hydrotest fluid specified (clean, sulfide-free, low-ammonia). Lay-up procedure agreed for any period longer than two weeks.
EN 10204 3.1 mill test certificate on file for every heat, traceable from the spool back to the heat number.
Coordinated quotation for pipe + flanges + gaskets + bolting + valves from a single source, so the entire pressure boundary lives under one quality file.

From a Single Source: How the Bundle Is Actually Assembled

EZ Steel Industrial has been assembling pressure-boundary bundles for marine, offshore, and desalination projects since 1994 out of a single manufacturing and inventory base in Changsha, China. With eight product lines covering carbon, stainless, copper-nickel, and heat-efficiency tubes alongside pipe fittings, pipe flanges, gasket and bolting kits, and industrial valves, the engineering team can hold the entire pressure boundary — pipe, fittings, flanges, tube bundle, bolting, and valves — under one quality file, one MTR trail, and one point of accountability.

The typical project bundle that leaves the Changsha dock includes 90/10 or 70/30 copper nickel alloy pipe to ASTM B466 / B467, matching copper nickel flanges to ASME B16.5, butt-weld or socket-weld fittings to ASME B16.9 / B16.11, gaskets and stud bolt kits to ASME B16.20 / B16.34, and marine-grade industrial valves with ISO 15848 low-emission trim. For the auxiliary side, the same shipment typically includes U bend tubes and finned tubes in matching Cu-Ni or aluminum-fin construction, with full heat-treatment and NDT records.

Send Us Your Line List and We Will Return a Scenario-Matched Quotation

If your next project is a vessel, an FPSO, an offshore wind substation, a coastal power plant, a desalination plant, or a refinery cooling loop, send the line class and the service envelope to EZ Steel Industrial. The engineering team will return a scenario-matched quotation that names the alloy, the standard, the joint, and the valve for every line — with one quality file, one delivery, and one point of accountability.

Reach the team at export@ezsteelpipe.com or +86 731 8870 6116 to start a project conversation.