Why Copper-Nickel Flanges Earn Their Place on Seawater Piping

On a ship, in a coastal power plant, or on a desalination skid, copper nickel alloy has built a thirty-year track record of resisting seawater, brine, and biofouling. The flange is the most-stressed interface in that system, so the alloy selection that works for the pipe has to work just as hard for the copper nickel flanges that join it.

Two grades do most of the work. C70600 (90/10 Cu-Ni) covers the majority of shipboard and cooling-water service. C71500 (70/30 Cu-Ni) is the upgrade you reach for when velocity, temperature, or contamination pushes the system beyond the 90/10 envelope. Picking the wrong one is rarely catastrophic on day one, but it usually shows up as accelerated erosion, pinhole leaks, or unexpected galvanic attack two to four years into service.

90/10 vs 70/30: The Decision That Drives the Specification

Before you write a flange datasheet, the project answers three questions for you. Each answer nudges you toward one grade or the other.

1. What is the design flow velocity?

EEMUA 234 guidance treats 3.5 m/s as the upper limit for sustained flow in 90/10 systems. Where design velocities sit between 3.5 and 5 m/s, or where designers want extra margin against future capacity creep, 70/30 enters the conversation. In practice, the 70/30 grade extends the upper velocity limit to roughly 5 m/s, which is what makes it the default for main seawater lift lines and larger-diameter cooling circuits.

2. What is the operating temperature and water quality?

90/10 holds up well in ambient seawater up to about 50 °C. Above that, or in polluted harbours with elevated sulphide content, 70/30 resists sulphide attack and erosion-corrosion noticeably better. If the system touches brackish water with high chlorine residual, the same logic applies.

3. What standard is the engineer calling out?

EEMUA 234 covers 90/10 for shipbuilding and offshore piping. ASME B16.24, ASTM B151, and EN 1254 cover both grades, but project specifications often lock in one composition for traceability. Get the spec right first; the flange face, bore, and class follow.

Matching the Flange to the Pipe Schedule

A common procurement mistake is ordering a flange rated to one class while the mating pipe is welded to a different schedule. The flange must match the bore of the pipe, the facing of the connected equipment, and the pressure class the system actually runs at, not the class written on an old datasheet.

• For shipboard systems, the typical pairing is a Class 150 weld-neck or slip-on flange with a Schedule 40 90/10 pipe.
• For desalination and high-pressure cooling water, move to Class 300 and verify the gasket seating surface (RF, FF, or RTJ) against the connected valve or equipment face.
• Where space is tight, slip-on flanges are easier to align, but weld-neck is the safer choice for cyclic temperature service and for any line that will see future disassembly.

A Side-by-Side View of 90/10 and 70/30 Flanges

Sourcing Cu-Ni Flanges as Part of a Bundled Project Package

Flanges almost never arrive on site alone. They sit inside a bolted assembly that includes the pipe, the gaskets, and the stud bolts and nuts that actually hold the joint together. Sourcing those items separately is one of the most common reasons small procurement mistakes turn into flange-replacement delays on site.

A complete Cu-Ni bolted-joint package typically bundles:

• The Cu-Ni pipe itself, cut to spool drawings, often delivered with matching butt weld fittings for directional changes.
• The Cu-Ni flanges in the right class and facing, matched to the connecting equipment.
• gasket stud bolt nut sets rated for seawater, usually with PTFE or graphite gaskets and matched bolt materials to avoid galvanic mismatch.
• Where the line transitions to a different alloy, isolation gaskets and dielectric kits to keep the galvanic couple under control.

When this package arrives as one shipment, traceability is much easier: one set of mill certificates, one heat-number family, and one QC chain from raw material to bolted joint. That is the main reason pipe fittings, flanges, and bolting are increasingly procured as a single engineering package rather than three separate purchase orders.

Procurement Checklist Before You Sign the PO

A short, disciplined checklist catches most of the errors that show up months later in a drydock or a plant turnaround. Before you release the order, confirm the following.

• Alloy grade is called out by UNS number (C70600 or C71500), not just by a generic "Cu-Ni" label.
• Standard reference is in the line: EEMUA 234, ASME B16.24, ASTM B151, or EN 1254.
• Pressure class and facing are fixed: Class 150 RF, Class 300 RF, and so on, matched to the pipe schedule.
• Mill test certificates are required, with traceability to the heat number that will be welded to the pipe.
• Bolting and gaskets are included, or are already specified in a sibling purchase order, to avoid a stalled installation on arrival.

Where EZ Steel Industrial Fits Into This Picture

EZ Steel Industrial supplies 90/10 and 70/30 copper-nickel flanges from its ISO 9001-certified mill in Changsha, with full EN, ASME, and EEMUA traceability. Flanges can be ordered standalone for an existing project, or bundled with Cu-Ni pipes, industrial valves, and gasket stud bolt nut sets so a single QC chain runs through the whole piping system. The 480,000-ton annual capacity and 30 years of export experience make it practical to support both shipyard deliveries and power-plant retrofit orders at the same time.