Cu-Ni 90/10 vs. 70/30: A Velocity and Sand Content Decision Matrix for Piping Engineers

If your seawater piping design has entrained sand in the flow, the velocity limit you used for clean seawater is wrong. It is too high. And the gap between what 90/10 and 70/30 can tolerate under those conditions is not as wide as the price premium suggests.

This article gives you a practical decision framework based on two variables that actually drive the 90/10 vs. 70/30 selection in sandy seawater service: flow velocity and sand content. No alloy metallurgy lecture. Just the numbers and the reasoning you need to size a line correctly.

Start With the Clean Seawater Baseline

Before sand enters the picture, know your clean-seawater velocity ceiling. The table below consolidates the most widely cited engineering limits for piping greater than 80mm diameter:

| Alloy | Piping Max Velocity (m/s) | Heat Exchanger Tubing Max (m/s) | Source |
|---|---|---|---|---|
| 90/10 CuNi (C70600) | 3.0–3.5 | 2.0–3.0 | Nickel Institute / CDA 225 |
| 70/30 CuNi (C71500) | 3.5–4.0 | 2.5–3.5 | Nickel Institute / CDA 225 |

These are sustained design velocities, not short-duration excursion limits. For brief events — hydrotesting, flushing, startup surge — both alloys tolerate 10–15 m/s without damage.

Critical flow velocity research supports the same ordering. In a controlled loop study, the 90/10 tube maintained a continuous protective film below 3 m/s, while the 70/30 tube maintained a continuous film in the range of 0.5–4.7 m/s. The critical velocity for 90/10 fell between 3 and 4 m/s; for 70/30, it was beyond 4.7 m/s.

So the baseline gap is roughly 0.5 m/s in piping and 0.5 m/s in tubing. That is the entire clean-water advantage of 70/30. Now see what sand does to it.

The Sand Penalty: Subtract 1–1.5 m/s

This is the single most important number for sandy service design. Copper Development Association guidance is explicit: in seawater containing entrained sand, the flow rate should be decreased by 1–1.5 m/s.

Apply that penalty to the baseline:

One independent source is more conservative still: for piping below 25mm, guidance suggests about 2.5–3.0 m/s for 90/10 and 70/30 in clean seawater, reduced to 2.0 m/s for 90/10 and 2.4 m/s for 70/30 with entrained sand. Another specification states that if sand exceeds 0.2% by weight, line velocity must be reduced to ≤3 m/s even for C71500.

The pattern is consistent: sand compresses the velocity envelope, and it compresses it for both alloys. The 0.5 m/s advantage of 70/30 survives the penalty, but it no longer looks decisive.

Why the Sand Penalty Exists: Film Destruction, Not Just Abrasion

The mechanism is not simple mechanical abrasion, where harder sand grinds away softer metal. It is erosion-enhanced corrosion, and it works like this:

1. Cu-Ni alloys rely on a protective corrosion product film — primarily cuprous oxide with incorporated iron and nickel — for corrosion resistance. Stabilised corrosion rates of both alloys are around 0.002 mm/year once that film matures.

2. Sand particles impacting the surface remove this film locally. The bare metal beneath is exposed to oxygenated seawater and corrodes rapidly until a new film forms.

3. At low sand impact frequencies, the film can repair itself between impacts. Above a threshold, the removal rate exceeds the formation rate and corrosion accelerates.

Research on 90/10 Cu-Ni with sand particles confirms that adding sand increases corrosion current density without changing the fundamental effects of flow velocity and impact angle. All three parameters — velocity, impact angle, and sand impact frequency — show strong effects on erosion-enhanced corrosion.

The practical implication: sand does not simply add a mechanical wear term on top of the corrosion rate. It amplifies the existing velocity-driven corrosion mechanism. That is why the penalty is a velocity reduction, not a separate wall-thickness allowance.

The Sand Content Threshold That Matters

Not all sand concentrations are equal. The literature points to a meaningful threshold:

• Below approximately 300 ppm: erosion-corrosion attack is present but generally manageable within the reduced velocity envelope.

• Above approximately 300 ppm: attack becomes most virulent, and the velocity reduction must be applied conservatively or exceeded entirely.

At 1% sand content (10,000 ppm), controlled testing on B10 copper-nickel (a 90/10 variant) identified a critical flow velocity range of 2.5–3.5 m/s at which the protective film undergoes complete removal. Note that this is the film destruction threshold — not a wall-loss threshold. Once the film is destroyed, corrosion rates climb sharply.

For design purposes, treat sand content in three bands:

The Decision Matrix

Here is the matrix you can take to a design review. The cells show recommended alloy based on sustained velocity and sand content.

| Sustained Velocity | Clean (<50 ppm sand) | Moderate Sand (50–300 ppm) | Heavy Sand (>300 ppm) |
|---|---|---|---|---|
| < 2.0 m/s | 90/10 — adequate and cost-effective | 90/10 — within reduced envelope | 90/10 — at conservative limit; verify with supplier |
| 2.0–2.5 m/s | 90/10 — at or near clean-water limit | 90/10 borderline; 70/30 preferred | 70/30 — at conservative limit |
| 2.5–3.0 m/s | 70/30 — 90/10 exceeds clean-water limit | 70/30 required | 70/30 borderline; consider Cr-bearing Cu-Ni |
| 3.0–3.5 m/s | 70/30 — at clean-water limit | 70/30 exceeds sandy limit; reduce velocity or change material | Not recommended for either grade |
| > 3.5 m/s | 70/30 exceeds standard limit; consider Cu-30Ni-2Cr (up to 6–8 m/s) | Not recommended | Not recommended |

The Cr-bearing alloy Cu-30Ni-2Cr is worth noting as a third option in the high-velocity sandy service range. Data predictions for this wrought alloy indicate maximum velocities of 6–8 m/s, substantially above either standard Cu-Ni grade. It costs more than 70/30, but less than titanium, and it eliminates the velocity compromise entirely for most sandy seawater piping.

Three Engineering Scenarios

Scenario 1: Cooling water intake, 2.2 m/s, 150 ppm sand

90/10 at 2.2 m/s with 150 ppm sand is right at the edge of the reduced envelope (2.0–2.5 m/s). It will likely perform, but the margin is thin. If the sand concentration spikes seasonally or during storm events, the film will not recover between impact events.

Recommendation: Either reduce design velocity to 2.0 m/s and stay with 90/10, or step up to 70/30 and run at 2.2 m/s with genuine margin. The 70/30 premium buys operational tolerance to sand spikes.

Scenario 2: Firewater system, intermittent flow, 3.0 m/s, trace sand

Firewater systems are typically stagnant and then flow at high velocity during test or emergency. The sand penalty for intermittent service is less severe than for continuous flow because the exposure duration is short. At 3.0 m/s, 90/10 in clean or trace-sand seawater is within its clean-water limit of 3.5 m/s.

Recommendation: 90/10 is appropriate. The velocity is within limits, and the intermittent duty reduces cumulative film damage. Verify that strainers are installed upstream to keep sand out of the system during the stagnant periods when deposits could form.

Scenario 3: Produced water disposal, 3.2 m/s, 500 ppm sand

This is heavy sand service at a velocity that exceeds the sandy limit for both grades. Neither 90/10 nor 70/30 is the right answer at this velocity and sand load.

Recommendation: Reduce velocity to 2.5 m/s and use 70/30, or specify Cu-30Ni-2Cr and retain 3.2 m/s. The velocity reduction approach increases pipe diameter and therefore cost; the alloy upgrade approach increases material cost per meter. Run both options against your project economics.

What the Cost Difference Actually Buys You

At current pricing, 70/30 carries a 30–50% material premium over 90/10. In sandy service, that premium buys you approximately 0.5 m/s of additional velocity headroom — the difference between designing at 2.0 m/s and designing at 2.5 m/s.

Whether that 0.5 m/s is worth the cost depends entirely on whether your system needs it. If your hydraulic design can be satisfied at 2.0 m/s with 90/10, the premium is wasted. If it cannot — if reducing to 2.0 m/s means a pipe diameter increase, a pump change, or a rework of the layout — then 70/30 is cheaper than the alternative.

Two Design Habits That Matter More Than Alloy Choice

Minimise local turbulence. Weld bead protrusion, misaligned flanges, and poorly radiused fittings create local velocities far above the bulk flow. A system running at a comfortable 2.0 m/s bulk velocity can still fail at a weld bead where local velocity reaches 4 m/s. Maximum weld bead protrusion limits exist for this reason; enforce them during fabrication inspection.

Control sand entry at the source. Strainers, sea chest screens, and settling basins reduce the sand load reaching the piping. A system designed for 300 ppm sand with adequate screening may effectively operate at 100 ppm — well within the 90/10 envelope. The cheapest sand mitigation is preventing the sand from entering the line.

The Bottom Line

In sandy seawater service, the velocity penalty of 1–1.5 m/s applies to both 90/10 and 70/30. The clean-water advantage of 70/30 — roughly 0.5 m/s — survives, but it is narrow.

Use 90/10 when sustained velocity stays below 2.0 m/s with sand present, or below 3.5 m/s in clean water. Use 70/30 when velocity falls between 2.0 and 2.5 m/s with sand present, or between 3.5 and 4.0 m/s in clean water. Above those ranges, neither standard grade is correct — reduce velocity, upgrade to Cu-30Ni-2Cr, or reconsider the material family.

The matrix is not a substitute for a project-specific erosion assessment, particularly in heavy sand service. But it gives you a defensible starting point that prevents both over-specification and the more expensive mistake of under-specification.