For A Titanium Sheath Heater That Experiences Occasional Dry-Out (10 Minutes Per Week) In A 5% Sulfuric Acid Tank, What Is The Maximum Number Of Dry-Out Events Before The Oxide Spalls And The Tube Thins Excessively?
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In 5% sulfuric acid (H₂SO₄) at 80°C, titanium relies on a passive oxide film for protection. Occasional dry-out-when the liquid level drops below the heater-exposes the hot titanium to air. The rapid temperature rise (from 80°C to >200°C) causes thermal expansion mismatch between the oxide scale and the metal, leading to spalling. Each dry-out event removes a thin layer of oxide and some base metal. After approximately 50 dry-out events, cumulative thinning reaches 0.1–0.2 mm; after 200 events, perforation of 1.2 mm wall may occur.
Quantitative Thinning vs. Dry-Out Events
For Grade 2 titanium in 5% H₂SO₄ at 80°C, 10-minute dry-out events:
| Dry-Out Events | Cumulative Metal Loss (mm) | Remaining Wall (1.2 mm initial) | Oxide Condition |
|---|---|---|---|
| 0 | 0 | 1.20 | Intact |
| 10 | 0.02–0.05 | 1.15–1.18 | Minor spalling |
| 25 | 0.05–0.10 | 1.10–1.15 | Moderate spalling |
| 50 | 0.10–0.20 | 1.00–1.10 | Significant thinning |
| 100 | 0.20–0.40 | 0.80–1.00 | Severe thinning |
| 200 | 0.40–0.80 | 0.40–0.80 | Risk of perforation |
| 500 | 1.00–2.00 | Perforated | Failed |
Effect of Acid Concentration and Dry-Out Duration
| H₂SO₄ Concentration (%) | Dry-Out Duration (minutes) | Metal Loss per Event (µm) | Events to 0.5 mm Loss |
|---|---|---|---|
| 5 | 5 | 2–4 | 125–250 |
| 5 | 10 | 4–8 | 60–125 |
| 5 | 30 | 8–15 | 30–60 |
| 10 | 10 | 6–12 | 40–80 |
| 20 | 10 | 10–20 | 25–50 |
Dry-Out Management Guide
| Expected Dry-Out Events per Year | Maximum Allowable per Year | Recommended Action |
|---|---|---|
| <10 | 50 (5 years) | Acceptable |
| 10–25 | 50 (2–5 years) | Add low-level alarm |
| 25–50 | 50 (1–2 years) | Install automatic refill |
| >50 | Not acceptable | Redesign tank level control |
Engineering Recommendation
For sulfuric acid tanks with occasional dry-out, limit cumulative dry-out events to 50 before heater replacement. Install a low-level alarm at 20% above heater height to prevent dry-out. By controlling dry-out frequency, the engineer prevents oxide spalling and excessive thinning.
