Copper-Nickel Seamless Tube Under ASME SB111 / B111M
ASME SB111 and its metric companion B111M are the specification for seamless copper and copper alloy tube for surface condensers, evaporators and heat exchangers, and they are technically identical to ASTM B111 and B111M. The standard covers both the copper-nickel grades used for marine and power plant service and the brasses and aluminium brasses used where water chemistry is less demanding.
The copper-nickel alloys in the specification are the 90/10 and 70/30 compositions, together with the chromium-bearing grade developed for higher velocity seawater. Iron and manganese are deliberate additions in these alloys rather than residuals: without them the protective film that keeps the alloy passive in flowing seawater does not form properly, and the corrosion resistance that the grade is chosen for is lost. This is why the iron range in the standard is a controlled window and not simply a maximum.
Grades and Chemical Requirements
| Grade | Family | Controlled additions | Typical duty |
|---|---|---|---|
| C70600 | 90/10 copper-nickel | Ni 9.0–11.0 %, Fe 1.0–1.8 %, Mn up to 1.0 % | seawater piping, condensers, heat exchangers |
| C70620 | 90/10 copper-nickel, welded-tube version | as C70600 with tighter residuals and no lead addition | systems where low-lead chemistry is required |
| C71500 | 70/30 copper-nickel | Ni 29.0–33.0 %, Fe 0.4–1.0 %, Mn up to 1.0 % | higher velocity and more aggressive seawater |
| C72200 | copper-nickel-chromium | Cr addition with nickel and iron | high-velocity seawater, sand-bearing water |
| C68700 | aluminium brass | Al 1.8–2.5 % with arsenic inhibitor | clean seawater condensers, cost-sensitive duty |
Composition limits and the permitted tolerances for each element are listed in the standard, and each grade is supplied in several tempers. The annealed tempers carry a maximum grain-size requirement in addition to the tensile requirements, because grain size governs the ability of the tube to be expanded into a tube sheet and bent into a U-shape without cracking.
Seamless copper-nickel pipe - larger diameters in heavier walls and with pipe-type dimensions - is specified separately under ASTM B466 / B466M. Product forms other than tube, including plate, sheet and strip for tube sheets and water boxes, are covered by the copper-nickel plate and sheet standards, so a complete seawater system is usually built from more than one specification.
Seawater Corrosion Performance
Protective film formation: in flowing seawater the alloy develops a thin, adherent corrosion-product film containing iron, nickel and copper, which limits further metal loss. The iron content of the alloy is what makes this film stable, and the film needs a minimum flow velocity and adequate oxygen to form and remain intact.
Velocity range: copper-nickel tube has a design velocity window. Below it, deposits form and create under-deposit attack; above it, the film is mechanically removed and erosion-corrosion begins. The window is set by the alloy and the water chemistry and is specified by the plant designer.
Sulphide and pollution resistance: copper-nickel alloys tolerate polluted and sulphide-bearing seawater far better than the brasses, which is the main reason a power station with estuary cooling water specifies them instead of admiralty brass.
Ammonia and stress-corrosion: the copper-nickel alloys are substantially more resistant to ammonia-related stress-corrosion cracking than the high-zinc brasses, which matters in plants where ammoniacal compounds are present on the steam side.
Biofouling: the alloys release copper ions at a low rate and inhibit marine growth, keeping heat transfer closer to design value between cleanings.
Applications
Power plant condensers: main condensers and auxiliary coolers in coastal and estuarine stations, where the cooling water is seawater or brackish and the tube must last the life of the unit.
Marine and offshore: seawater piping, cooling systems, air coolers, lubricating oil and jacket water coolers, and large-volume seawater systems on vessels and platforms.
Desalination: brine heaters, heat recovery stages and condenser sections in multi-stage flash and reverse osmosis plants.
Shipbuilding hardware: tube sheets, water boxes, salt-water fittings, propeller sleeves, hull and piling components where a copper-nickel surface is required.
Process and refinery: exchangers and coolers in refineries and chemical plants where the cooling side is seawater and the process side is hydrocarbon or process fluid.
Heat-exchanger tube: the dominant use of the standard, in straight lengths for shell-and-tube exchangers and in U-bent form for removable bundles.
Fabrication, Installation and Inspection Points
Annealed tube before expansion: tube expanded into a tube sheet must be in the annealed condition with grain size inside the standard limit, otherwise cracking appears at the rolled joint.
Stress relief after U-bending: a stress-relief anneal after cold bending reduces residual stress and the risk of service cracking in the bend.
Do not mix galvanically active materials: copper-nickel tube in a steel water box creates a galvanic couple, so the water box and tube sheet are normally protected with a coating or with sacrificial anodes, and the design follows the plant's corrosion-protection specification.
Cleanliness during installation: embedded iron particles from steel tools or a dirty rack destroy the protective film locally and start pitting; use non-ferrous or stainless tooling and cover the tube ends.
Non-destructive examination: eddy-current testing of the full length after the final anneal is the primary acceptance test, supplemented by tensile and grain-size checks for the ordered temper and a hydrostatic or pneumatic test when required by the order.
Storage and transport: tube is end-capped, dried and bundled on wooden pallets or in reinforced crates so the bore is free of moisture and contamination on arrival.
FAQ
Q: What is the difference between ASME SB111 and ASTM B111?
The two documents are technically identical. ASTM B111 is the original materials specification and ASME SB111 is the ASME edition adopted for use with the boiler and pressure vessel and piping codes.
Q: Why do 90/10 and 70/30 copper-nickel alloys contain iron?
Iron is added deliberately, within a controlled range, because it is needed to form and stabilise the protective film that limits corrosion in flowing seawater. Without adequate iron the alloy loses much of its seawater performance.
Q: When should C71500 be chosen instead of C70600?
The 70/30 grade is selected for higher seawater velocities, more aggressive or polluted water and where greater resistance to erosion-corrosion is needed. The 90/10 grade is adequate and more economical for clean seawater at moderate velocity.
Q: Can copper-nickel tube be welded?
Yes, it welds well by gas tungsten arc and gas metal arc processes with a matching filler, and welded tube in the C70620 chemistry is produced for particular service requirements. Seamless tube remains the norm for critical condenser duty.
Q: What temper should a condenser retube order specify?
Annealed tube with grain size within the standard limit for the alloy, which allows roller expansion into the tube sheet and cold forming of a U-bend without cracking. The certificate should state both tensile results and grain size.
Q: How is tube for a seawater exchanger tested before shipment?
Full-length eddy-current examination after the final anneal, tensile testing of a specimen, grain-size measurement on annealed tube, dimensional verification of outside diameter, wall thickness and ovality, and a pressure test where the purchase order requires one.
