Nearly every copper-nickel flange enquiry comes down to this choice. C70600 is roughly 90 percent copper and 10 percent nickel. C71500 is roughly 70 percent copper and 30 percent nickel. The 70/30 is the better alloy on most technical axes and costs more, which tempts people to treat it as the safe default. It is not always the right answer.
Velocity, the usual deciding factor
Both alloys protect themselves with a cuprous oxide film, and both lose it if flow shears it away faster than it rebuilds. That limit is higher for 70/30.
Commonly quoted orientation figures put 90/10 near 10 feet per second in pipe and 70/30 nearer 14. Treat those as direction only. The real limit depends on geometry, and local velocity at the outside of a short radius elbow, downstream of a partially closed valve, or at a sudden expansion can be far above the line average. Entrained air makes it much worse, because bubble collapse at the wall is mechanically aggressive.
Design for the local velocity, not the average. A line that averages 8 feet per second can be well past the 90/10 limit at specific fittings. That is where erosion-corrosion shows up, typically as horseshoe shaped pits pointing downstream.
Temperature
70/30 holds strength better as temperature rises and is the choice for the hot stages of a desalination plant, for condenser service, and anywhere the fluid is well above ambient. 90/10 is a cold and warm water alloy.
Both derate steeply compared with steel. Neither is a high temperature material, and the rating must be worked from the design code allowables rather than read off a steel flange table.
Strength
70/30 runs roughly 40 to 50 percent higher in tensile strength. On a large diameter header that can mean a thinner wall and a lighter flange, which occasionally pays for the alloy premium on its own, particularly offshore where weight is costed.
Where 90/10 is the better answer
This is the part that gets missed. 90/10 is not merely the budget option.
- Sulfide tolerance. 90/10 is somewhat more forgiving of sulfide-polluted water than 70/30. In harbour water, or where commissioning conditions cannot be fully controlled, that is a real advantage.
- Higher iron content. The ASTM range for 90/10 runs roughly 1.0 to 1.8 percent iron against 0.4 to 1.0 for 70/30, and iron is what makes the film erosion resistant. Within its velocity envelope, 90/10 is not a compromise.
- Cost and availability. Cheaper, and generally easier to get in the sizes and forms you want, which matters on a schedule.
- Most seawater distribution does not need more. Firewater ring mains, ballast, bilge and general service seawater typically run at velocities well inside the 90/10 range.
Direct comparison
| Factor | C70600, 90/10 | C71500, 70/30 |
|---|---|---|
| Nickel | 9 to 11% | 29 to 33% |
| Iron | 1.0 to 1.8% | 0.4 to 1.0% |
| Velocity tolerance | Lower | Higher |
| Elevated temperature | Lower | Higher |
| Tensile strength | Lower | Roughly 40 to 50% higher |
| Sulfide-polluted water | Somewhat better | More sensitive |
| Cost | Lower | Higher |
| Typical duty | Seawater distribution, firewater, ballast | Pump discharge, condensers, hot brine, high velocity |
Check the iron on the MTR
Whichever grade you buy, read the iron line on the material test report, not just the nickel line. A heat that sits at the bottom of the iron range will pass a chemistry check and still underperform at flow, because iron is what makes the protective film resist erosion. Hold the supplier to the specified range.
Deciding quickly
- Work out the local velocity at the worst fitting, not the line average
- Inside the 90/10 envelope with margin, take 90/10
- Above it, or hot, or weight critical, take 70/30
- Well above either, look at aluminum bronze or a superduplex
- Sand in the water, look at aluminum bronze regardless of velocity
See copper-nickel flanges for material specifications and flange types, and copper alloy grades for the alternatives.