A flange on a seawater line is rarely the most expensive line item in a project, yet it is one of the first components to fail when the metallurgy, the facing, or the bolting is wrong. A ship cooling loop, a ballast line, a desalination intake, and an offshore firewater ring main all share the same enemy: chloride-rich water that punishes carbon steel and most stainless grades. The material that has held the line against this for more than fifty years of real service is not exotic, it is a copper nickel flange in 90/10 or 70/30 grade, specified to the right standard and integrated into the rest of the piping package.

This guide is written for project buyers and piping engineers who already know the basics and need a sharper way to specify copper nickel alloy pipe flanges for real seawater service. The goal is to keep the bolted joint tight, the alloy family consistent, and the supply package traceable from mill to ship.

Start with the service environment, not with the alloy

Alloy selection is a consequence, not a starting point. Before 90/10 or 70/30 is written into the datasheet, four things should be locked down: chloride content, water velocity, temperature, and the presence of sulphides or ammonia. Each of these nudges the choice in a different direction.

• Standard seawater cooling at ≤ 3.5 m/s velocity? 90/10 Cu-Ni (UNS C70600) is the default workhorse for ship side, heat exchangers and desalination intake.
• Higher velocity, hotter service, or polluted / sulphide-bearing water? Move up to 70/30 Cu-Ni (UNS C71500) for better erosion and stress-corrosion resistance.
• Sour service (H₂S) or ammonia contamination? Cu-Ni is generally acceptable, but specify NACE MR0175 limits on the bolting and review the operating envelope with the design team.
• Firewater or occasional flow lines? 90/10 remains the most economical choice and is field-proven over decades.

Doing this exercise before the RFQ saves a lot of paperwork later. Many field failures attributed to "bad flanges" actually started as bad specifications.

Match the flange type and facing to the duty

Cu-Ni flanges are commonly produced as slip-on (SO), weld-neck (WN), blind (BL), and lap-joint configurations, all to ASME B16.5 dimensional envelopes in Class 150 and Class 300 for most marine duties. The facing is almost always raised face (RF); flat face (FF) is reserved for cast iron or non-metallic connections where bolt loading needs to be spread.

Two practical notes from real shipyard work. First, butt-welding to a Cu-Ni pipe requires the matching Cu-Ni filler (AWS A5.7 ERCuNi or ERCuNiAl); do not let the shop substitute a Cu-Al or Cu-Si wire just because it is on the shelf. Second, weld-neck flanges are almost always the better choice for vibration and fatigue duty, even when slip-on flanges are dimensionally acceptable on paper.

Standards, traceability, and what to put on the RFQ

Most ship and offshore projects are written against EEMUA Publication 144, ASME SB171 / SB466, EN 12451 / EN 1653, or the corresponding national equivalents. A flange that quotes only "Cu-Ni" without a standard is not a flange you can sign off on. Ask the mill to confirm the UNS number, the heat treatment condition, and the dimensional standard on the data sheet, not only on the certificate.

A clean, project-ready RFQ for Cu-Ni flanges should list:

• UNS alloy (C70600 for 90/10, C71500 for 70/30) and the manufacturing standard (ASME SB171, EEMUA 144, EN 1653).
• ASME B16.5 size, class, facing (RF / FF), and bore schedule matched to the mating pipe.
• Dimensional tolerances where they matter: bore, facing finish, bolt circle.
• Heat treatment: most Cu-Ni flanges are supplied in the as-fabricated or annealed condition; state which is required.
• Testing: hydrotest, 100% PMI, and any third-party inspection (EN 10204 3.1 or 3.2 MTC).
• Marking: heat number, UNS alloy, standard, and size on every flange, not only on the crate.

The flange never travels alone: gaskets, stud bolts, and valves

A Cu-Ni line is a system. The flange, the gasket, the stud bolts and nuts, and the connecting industrial valves must be specified as one corrosion package. Mismatched materials are the single most common source of crevice corrosion and bolt failure on seawater systems. Two details to lock in before placing the order:

• Gasket. Use a non-asbestos compressed fibre or a flexible graphite with a stainless or alloy ｉｎｓｅｒｔ. Avoid rubberised or PTFE gaskets on hot or firewater duty.
• Stud bolt and nut. Specify ASTM A193 B8M (AISI 316) or B8 class for marine service, with ASTM A194 grade 8M or 8 nuts. If the project calls for super-duplex bolting, change the entire alloy family to match; do not mix stainless bolts with Cu-Ni flanges on hot lines without confirming galvanic compatibility.

If valves sit in the same seawater loop, their bodies and trim should be consistent with the flange and pipe metallurgy. Specifying the flange, the gasket, the bolt set, and the valve as one item on the same purchase order line keeps the supply chain tight and the MTC file consistent.

Installation points that decide service life

Most premature Cu-Ni flange failures we see on site are not material failures; they are installation issues. A short list to give to the yard or site team:

• Use the correct Cu-Ni weld procedure and qualified welders (ASME IX or EN ISO 9606). Cu-Ni welds look easy and are very easy to get wrong.
• Apply bolt torque in a star pattern, in two or three passes, to the gasket manufacturer's recommended value. Over-torque distorts the Cu-Ni flange face.
• Keep the bolted joint free of carbon steel filings. Cut carbon steel pipework on a separate bench, and avoid grinding sparks near the Cu-Ni surface.
• Protect the flange faces during storage and transport with end caps and a vapour-phase inhibitor. Cu-Ni resists seawater, but it does not resist mishandling.
• Flush and pickle the system before commissioning to remove mill scale, weld oxide, and construction debris. A clean start is the cheapest corrosion protection you can buy.

A short checklist before you release the PO

Run the requisition through this list before sending it out. Items that survive a quick review almost never come back as field complaints.

• UNS alloy + manufacturing standard + dimensional standard all written out.
• Size, class, facing, and bore schedule spelled out per ASME B16.5.
• Gasket, stud bolt, nut, and valve materials fixed in the same PO line.
• MTC level set: EN 10204 3.1 for commercial, 3.2 for classed service.
• Heat number marking on each flange, not only on the bundle.
• Export packaging: seaworthy crate, end caps on flange faces, VCI paper, IPPC mark.
• Lead time matched to the project's dry-dock or hook-up window.