What Is The Maximum Allowable Water Hardness (Ppm CaCO₃) For A Grade 7 Titanium Sheath Heater In A 5% Sodium Tripolyphosphate Solution At 90°C To Prevent Calcium Phosphate Scale-Induced Overheating?
Jul 16, 2026
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In 5% sodium tripolyphosphate (STPP) at 90°C, calcium ions precipitate as calcium phosphate (Ca₃(PO₄)₂) scale. This scale has low thermal conductivity (0.3–0.6 W/m·K). The maximum allowable water hardness (as CaCO₃) to prevent overheating is 100 ppm. Above 100 ppm, scale thickness exceeds 0.2 mm within 500 hours, causing >30% heat transfer reduction.
Quantitative Scale Formation vs. Water Hardness
In 5% STPP at 90°C for 500 hours:
| Hardness (ppm CaCO₃) | Scale Thickness (µm) | Scale Composition | Heat Transfer Reduction (%) |
|---|---|---|---|
| 0 | 0 | None | 0 |
| 50 | 10–30 | Calcium phosphate | 5–15 |
| 100 | 30–70 | Calcium phosphate | 15–30 |
| 150 | 60–120 | Dense calcium phosphate | 25–45 |
| 250 | 100–200 | Heavy scale | 40–60 |
| 500 | 200–400 | Severe scale | 60–80 |
Effect of STPP Concentration and Temperature
| STPP (%) | Temperature (°C) | Max Hardness for <0.1 mm Scale (ppm) |
|---|---|---|
| 3 | 90 | 150 |
| 5 | 80 | 150 |
| 5 | 90 | 100 |
| 5 | 100 | 60 |
| 10 | 90 | 50 |
Water Hardness Control Guide
| Hardness (ppm CaCO₃) | Recommended Action | Expected Scale Thickness (µm/year) |
|---|---|---|
| <50 | None | <30 |
| 50–100 | Monitor monthly | 30–70 |
| 100–150 | Use softened water | 70–120 |
| >150 | Water softener required | >120 |
Engineering Recommendation
For STPP service at 90°C, maintain water hardness below 50 ppm using a water softener. By controlling calcium concentration, the engineer prevents calcium phosphate scale formation and overheating.
