From crate to spool: what to check before the joint is even made

Most seawater leaks are not "weld failures" – they are assembly failures that show up after a season of thermal cycling. The first hour on site sets the tone. When a pallet of copper nickel flanges is craned onto the quay, the supervisor should run a four-point pre-fit check before any bolt goes in:

• Heat number on the flange matches the MTC and the line spool it will be welded to. No heat mixing on a single bolted joint.
• Sealing face (RF or FF) is free of radial nicks, gouges and sticker residue. Even a 0.2 mm radial score will leave a leak path under torque.
• Bolt circle, bore and PCD are dimensionally checked against ASME B16.5 or EEMUA 234, depending on the marine class the project is built to.
• Gasket, stud bolt and nut set is the one paired in the shop – not a "similar" set pulled from another pallet. Gasket stud bolt nut kits are matched to the flange class and facing; mixing them up is one of the top three causes of premature joint rework on board ship.

Aligning the pair: the small mistakes that cause big rework

Cu-Ni is softer than carbon steel, so it is far less forgiving of misalignment. A pair of copper nickel flanges that is "almost aligned" will crush unevenly on one side, leave a gap on the other, and the joint will weep from day one. The three alignment moves that protect the seal:

• Set the pipe supports first, then bring the flange in – never force the pipe to "find" the flange.
• Check parallel across the bolt circle with a feeler gauge at four points. Target gap is uniform within 0.5 mm before any bolt is torqued.
• Confirm the gasket sits centred on the bolt circle, not offset toward the bore. An off-centre gasket is the single most common cause of a Cu-Ni joint re-make at the quayside.

Bolt-up sequence: pass, pattern, and final torque

The bolt-up sequence is the most documented step on a Cu-Ni joint and the most often done wrong. A correct sequence loads the gasket evenly and keeps the flange faces parallel through every pass. The pattern we use on 4-, 8- and 12-bolt flanges is below.

For Cu-Ni threads, always use a PTFE or Xylan-based anti-seize on the stud and the nut face. Dry torque on Cu-Ni threads will gall within the first 24 hours and the joint will lock up – a frequent reason crews cannot re-tighten a leaking joint during the dock trial.

Welded-in flanges: protecting the heat-affected zone

When the copper nickel flanges are butt-welded to the pipe spool rather than slip-on, the welded joint itself becomes the long-term reliability question. Cu-Ni is sensitive to overheating, so the field WPS has to be tuned, not borrowed from the carbon steel procedure on the same vessel.

• Use a low-heat-input TIG process with Cu-Ni filler (typically ERCuNi for 90/10, ERCuNi30 for 70/30).
• Keep interpass temperature below 65 °C – measure with a temperature stick, not by feel.
• Back-purge the bore with argon to 8–10 m³/h until the root pass is complete. Cu-Ni oxidises from the inside out, and a stained root pass is a corrosion initiation point in service.
• After fit-up, run dye-penetrant on the root and cap – any indication is repair territory, not "acceptable" territory.

Site inspection: what the QC engineer should sign for

The QC sign-off on a Cu-Ni flange pair is not a "look-and-leave" moment. The evidence the inspector should see, in writing, before marking the joint as complete:

• MTC with heat number, full chemistry (Cu, Ni, Fe, Mn, Pb, Zn) and mechanical results for the specific flange batch.
• Dimensional record: bolt circle, bore, facing finish against the class drawing.
• PMI test result on a sample per heat, confirming Ni content matches 90/10 or 70/30.
• Torque log with values per bolt, per pass, signed by the fitter and the supervisor.
• Final visual on the gasket seating area – no radial marks, no crushed fibre, no missing witness marks on the bolts.

When this record is kept, any future failure analysis on the joint is anchored to a known heat, a known torque history and a known gasket batch. That is what turns a "we think the joint leaked" call into a "we know the joint leaked because…" report.

Bundling the flange: why the spool behind it matters as much

A seawater line is only as strong as its weakest component. A correctly assembled Cu-Ni flange paired with a carbon-steel spool one heat away will still drive galvanic corrosion, and no amount of gasket tightening will fix it. This is why procurement and assembly should be planned together, not in silos.

A complete seawater train release from EZ Steel Industrial typically ships as one bundled package – Cu-Ni pipe to EEMUA 234 / ASTM B466, Cu-Ni weld-neck or slip-on flanges to ASTM B151, Cu-Ni butt-weld fittings to ASME B16.9, matched industrial valves with bronze or super-duplex trim, and the gasket stud bolt nut set already paired to the flange class. The fitter on site then bolts, welds and torques against one heat trail, not three or four – and the joint history stays intact for the next twenty years of dock trials.