What CuNi 70/30 Copper Nickel Pipe Is
CuNi 70/30 is a wrought copper nickel alloy containing roughly 70 % copper and 30 % nickel, standardised as UNS C71500 in ASTM B111/B111M. In European practice it is written CuNi30Mn1Fe or CuNi30Fe1Mn, and the Chinese designation is BFe30-1-1 under the national copper alloy designation system. It was developed for seawater service where 90/10 cupro nickel reaches its limits: high flow velocity, sand laden water, polluted harbour water and systems where the fluid contains residual sulphide or ammonia.
The alloy relies on the same protective oxide film mechanism as 90/10, but the higher nickel content makes that film more stable and more tolerant of disturbance, so the wall survives longer where the water is aggressive. The price premium over 90/10 is justified whenever the cost of retubing a condenser exceeds the metal cost difference.
Chemical Composition of UNS C71500
| Element | Requirement |
|---|---|
| Nickel (Ni) | 29.0 to 33.0 % |
| Iron (Fe) | 0.4 to 1.0 % |
| Manganese (Mn) | 1.0 % maximum |
| Zinc (Zn) | 0.50 % maximum |
| Carbon (C) | 0.05 % maximum |
| Lead (Pb) | 0.02 % maximum |
| Sulphur (S) | 0.02 % maximum |
| Phosphorus (P) | 0.02 % maximum |
| Copper (Cu) | Remainder |
Residual elements other than those listed are held to 0.50 % in total. Carbon, sulphur and phosphorus are kept low because they form inclusions that act as initiation points for localised corrosion in seawater, and the iron range is narrow because iron controls the durability of the protective film at the tube inlet.
Mechanical Properties
| Temper | Grain size | Tensile strength, minimum | Yield at 0.5 % extension, minimum |
|---|---|---|---|
| Annealed OS025 | 0.025 mm nominal | 414 MPa (60 ksi) | 172 MPa (25 ksi) |
| Annealed OS035 | 0.035 mm nominal | 372 MPa (54 ksi) | Specified by the product specification |
Typical service data for the alloy include a tensile strength range of 350 to 550 MPa with 20 to 40 % elongation, depending on temper, and a design temperature capability up to about 371 C in the annealed condition without loss of toughness at cryogenic temperature. The alloy retains good ductility at low temperature, which is why it also appears in liquefied gas and cold seawater systems.
Sizes, Delivery Forms and Inspection
| Outside diameter | Wall thickness | Typical application |
|---|---|---|
| 12.70 mm (1/2 in) | 0.9 to 1.65 mm | Small seawater coolers |
| 15.88 mm (5/8 in) | 1.24 to 1.65 mm | Power station condensers |
| 19.05 mm (3/4 in) | 1.24 to 2.11 mm | Desalination heat exchangers |
| 25.40 mm (1 in) | 1.65 to 2.77 mm | Seawater pipework and large exchangers |
| 38.10 to 76.20 mm | 2.0 to 3.0 mm | Piping headers, risers, ship systems |
Tube and pipe are supplied annealed to a controlled grain size for roller expansion and bending, or in drawn tempers where stiffness is wanted. Random and fixed straight lengths up to 12 m, U-bent tube and coiled small diameter tube are all produced. Testing follows ASTM B111/B111M with tension, flattening, flaring and expansion tests, plus eddy current examination to ASTM E243 and hydrostatic or pneumatic proof testing when specified.
Applications in Seawater Environments
Multi-stage flash and multi-effect desalination heat exchanger bundles.
Surface condensers and feedwater heaters in coastal power stations.
Shipboard seawater systems, ballast and firewater heat exchangers.
Offshore platform cooling circuits and process coolers with residual sulphide.
Heat exchangers in refineries and chemical plants handling brackish or polluted water.
Marine air conditioning condensers and central cooling systems.
In flowing seawater the alloy typically corrodes at a rate below 0.003 mm per year, and design velocity limits are higher than for 90/10, commonly taken as about 4.5 m/s for clean seawater with a reduction where solids are present.
Working and Joining Notes
CuNi 70/30 is readily cold formed, deep drawn and bent, and it machines to a good surface finish, with polished surfaces reaching fine finishes when required. Thermal conductivity is lower than that of 90/10, so welding heat stays close to the joint; gas tungsten arc and gas metal arc welding with matching copper nickel filler are the preferred processes, and oxyacetylene welding is not recommended. Stress relief after heavy cold work is advisable where the environment is aggressive.
Cleanliness controls performance. Grease, carbon steel particles and iron oxide deposits left in the bore during fabrication break down the protective film and start pitting, so the water side should be flushed and, for power plant condensers, checked for carbon film before the unit enters service. Ferrous sulphate dosing is a separate system decision made by the plant operator rather than by the tube supplier.
FAQ
Q: When should 70/30 be chosen instead of 90/10?
Choose 70/30 when the water is faster than about 3 m/s, carries sand, or contains sulphide or ammonia, and when the exchanger is difficult or expensive to retube. For clean seawater at moderate velocity, 90/10 is the more economical grade.
Q: What are the equivalent grade names of C71500?
The alloy is designated CuNi30Mn1Fe in European standards, BFe30-1-1 under Chinese designation practice, and appears as C71500 in ASTM B111/B111M. All describe the same copper nickel family with about 30 % nickel.
Q: What velocity is allowed in seawater service?
Clean seawater designs commonly allow about 4.5 m/s for 70/30, which is higher than the limit used for 90/10. Where the water carries suspended solids, the velocity and the inlet geometry need to be reviewed together.
Q: Is 70/30 tube suitable for U-bend bundles?
Yes. Order soft annealed tube to a nominal grain size so that the bend develops without cracking, and check that the bend radius is at least twice the outside diameter.
Q: How does the alloy perform at high temperature?
The annealed alloy is used up to around 371 C in heat exchanger and process service and keeps its toughness at cryogenic temperature, so it suits cold seawater and liquefied gas duties as well as hot condensers.
Q: What causes early failure of copper nickel tube?
The usual causes are deposits and crevices from poor cleaning, incorrect iron content in the alloy, excessive water velocity at the inlet, and contamination by sulphide or ammonia. All four are controllable by specification, fabrication and water treatment.
