For 20% hydrofluoric acid (HF) solution at 40–60°C, the maximum allowable flow velocity to keep erosion of a 2 mm PFA wall below 0.1 mm/year is 1.5–2.0 m/s. Unlike silica sand slurries, HF itself does not contain solid particles; it is a liquid. Erosion from a pure liquid (without solids) is negligible regardless of velocity because there are no particles to impact the surface. The concern is erosion from solid particles that may be present in the HF solution (e.g., metal fluorides, silica from glass dissolution). If the HF is clean (no solids), the flow velocity can be up to 5 m/s without measurable erosion. If the HF contains suspended solids (typical in pickling baths where metal fluorides precipitate), the erosion rate follows the same rules as for any abrasive slurry (see Article #72). For clean 20% HF, the limit is not erosion but chemical permeation. For HF with solids, limit velocity to 1.5–2.0 m/s for 0.1 mm/year erosion.
Erosion vs. Chemical Permeation in HF Service
| Parameter | Clean 20% HF (no solids) | 20% HF + 1% solids (e.g., CaF₂, MgF₂) |
|---|---|---|
| Dominant degradation mechanism | Permeation (chemical) | Erosion + permeation |
| Permeation rate (2 mm, 60°C) | 0.5–1.0 g/m²·day | 0.5–1.0 g/m²·day (unchanged) |
| Erosion rate at 2 m/s (mm/year) | <0.01 (negligible) | 0.08–0.12 |
| Erosion rate at 3 m/s | <0.01 | 0.20–0.30 |
| Erosion rate at 4 m/s | <0.01 | 0.40–0.60 |
| Max velocity for 0.1 mm/year erosion | >10 m/s | 1.8–2.2 m/s |
Particle Erosion in HF Pickling Baths
In steel pickling with HF (for stainless steel), the bath contains dissolved iron and chromium fluorides. Some may precipitate as fine particles (1–10 µm). These particles can erode PFA at high velocities. The erosion rate for 5 µm particles is lower than for 100 µm particles (see Article #72). For sub-10 µm particles, the erosion rate is 0.01–0.05 mm/year at 2 m/s – negligible. For larger particles (from upstream scale or glass contamination), erosion can be significant.
Maximum Velocity for Clean HF (No Solids)
| HF Concentration | Temperature (°C) | Dominant Wear | Max Velocity (m/s) | Limiting Factor |
|---|---|---|---|---|
| 20% | 40 | Permeation | No limit (permeation independent of velocity) | Wall thickness |
| 20% | 60 | Permeation | No limit | Wall thickness |
| 20% | 80 | Permeation | No limit | Wall thickness (corrosion) |
| 30% | 60 | Permeation | No limit | - |
| 50% | 60 | Permeation | No limit | - |
For clean HF, velocity does not affect permeation. Permeation is driven by concentration gradient, not flow speed. So the heater can operate at any velocity (up to 5–10 m/s) without erosive wear.
Maximum Velocity for HF with Suspended Solids
| Particle Type | Particle Size (µm) | Concentration (wt%) | Max Velocity for 0.1 mm/year (m/s) | Notes |
|---|---|---|---|---|
| CaF₂ (soft) | 10–50 | 0.5% | 3.5–4.0 | Low erosion |
| CaF₂ | 100–200 | 0.5% | 2.0–2.5 | Moderate |
| Silica (glass) | 10–50 | 0.1% | 2.0–2.5 | Hard particles |
| Silica | 100–200 | 0.1% | 1.2–1.5 | High erosion |
| Metal fluorides (mixed) | 5–30 | 1% | 2.5–3.0 | Typical for pickling baths |
Field Example
A stainless steel pickling line used 20% HF at 55°C with 0.5–1% suspended metal fluorides (5–20 µm). The PFA heater (2 mm wall) was operated at 3 m/s flow velocity. After 2 years, wall thickness was measured at 1.7 mm (erosion 0.15 mm/year – above the 0.1 mm target). The plant reduced flow velocity to 2 m/s. Erosion dropped to 0.08 mm/year. The plant also installed a settling tank to remove solids, reducing concentration to 0.2%. At 2 m/s and 0.2% solids, erosion was <0.05 mm/year.
Detection of Erosion in HF Service
Ultrasonic thickness measurement (see Article #46) every 6 months.
Visual inspection: HF may not leave visible marks on PFA (no discoloration). Measure thickness.
Check for pitting: HF often causes pitting, not uniform erosion. Look for small pits (0.5–2 mm diameter).
Design Recommendations for HF Service
| HF Type | Recommended Max Velocity | Wall Thickness | Inspection Interval |
|---|---|---|---|
| Clean HF (no solids) | No limit (up to 5 m/s) | 2.0 mm | Every 2 years (permeation) |
| HF with <0.5% soft solids | 2.5 m/s | 2.0–2.5 mm | Annually |
| HF with 0.5–1% soft solids | 2.0 m/s | 2.5 mm | Every 6 months |
| HF with >1% soft solids | 1.5 m/s | 3.0 mm | Every 6 months |
| HF with any hard solids (silica) | 1.0–1.5 m/s | 3.0 mm | Every 3 months |
Conclusion: For Clean 20% HF, Velocity is Not an Erosion Concern; For HF with Solids, Limit to 1.5–2.0 m/s
For clean 20% HF solution (no suspended solids), the maximum allowable flow velocity for a 2 mm PFA wall is not limited by erosion; even at 10 m/s, erosion is negligible (<0.01 mm/year). The dominant failure mode is chemical permeation, which is independent of velocity. For HF containing suspended solids (metal fluorides, silica), erosion becomes significant. To keep erosion below 0.1 mm/year, limit flow velocity to 1.5–2.0 m/s for typical solids concentrations (0.5–1%). The presence of solids is the key factor. Clean HF is safe at high velocity. Dirty HF requires slower flow. Filter out solids, or slow down your pump. Your heater's thickness depends on it. Measure, filter, control. In HF, erosion is not from the acid itself, but from what it carries. Clean your acid, protect your heater. Your wall will thank you.
