In abrasive slurry service-mining, pigment production, catalyst handling-solid particles entrained in the fluid impact the PFA heater sheath, gradually eroding the wall thickness. For a 1.5 mm PFA sheath (common in general industrial heaters), the maximum particle velocity without erosive failure depends on particle hardness, size, concentration, and impact angle. For silica sand (Mohs hardness 7, 200 µm mean diameter, 5% concentration by weight), the maximum safe velocity is 2.5–3.0 m/s for a service life of 2 years (assuming 8,000 operating hours per year). Above 3.5 m/s, erosion exceeds 0.5 mm/year, reducing the 1.5 mm wall to below 1.0 mm within 2 years, at which point permeation and mechanical failure become likely. For harder particles (alumina, Mohs 9), the maximum velocity drops to 1.5–2.0 m/s. For softer particles (limestone, Mohs 3), velocities up to 5.0 m/s are acceptable. The erosion rate follows E = k × v^n, where n = 2.5–3.5 for PFA.
Erosion Rate Calculation and Velocity Limits
The erosion rate of PFA is described by E = C × (v)^3.2 × (d)^0.7 × (H_p/H_t)^1.2, where E is mm/year, v is velocity (m/s), d is particle diameter (mm), H_p is particle hardness (Mohs), H_t is target hardness (PFA ~2.5 Mohs), and C is a concentration-dependent constant. For silica sand (d=0.2 mm, H_p=7) at 2% weight concentration, C ≈ 0.015. At v=2.5 m/s, E = 0.015 × (2.5)^3.2 = 0.015 × 22.6 = 0.34 mm/year. Over 2 years (16,000 hours), erosion = 0.68 mm. Starting from 1.5 mm, remaining thickness = 0.82 mm-at the limit for reliable service (minimum 0.8–1.0 mm recommended). At v=3.0 m/s, E = 0.015 × (3.0)^3.2 = 0.015 × 39.8 = 0.60 mm/year, 1.20 mm over 2 years, leaving only 0.30 mm-unacceptable. The maximum safe velocity for 2-year life with 1.5 mm wall is therefore 2.7 m/s for this particle set.
For alumina (H_p=9, d=0.1 mm, 1% concentration), C ≈ 0.025. At v=1.8 m/s, E = 0.025 × (1.8)^3.2 = 0.025 × 10.2 = 0.255 mm/year, 0.51 mm over 2 years, remaining 0.99 mm-acceptable. At v=2.0 m/s, E = 0.025 × (2.0)^3.2 = 0.025 × 12.1 = 0.30 mm/year, 0.60 mm over 2 years, remaining 0.90 mm-marginal but acceptable. At v=2.5 m/s, E = 0.025 × 22.6 = 0.565 mm/year, 1.13 mm over 2 years, remaining 0.37 mm-unacceptable.
Maximum Particle Velocity by Material (1.5 mm Wall, 2-Year Life)
| Particle Material | Hardness (Mohs) | Typical Size (µm) | Max Velocity (m/s) for 2-Year Life | Erosion Rate at Max Velocity (mm/year) | Remaining Thickness After 2 Years (mm) |
|---|---|---|---|---|---|
| Polymer pellets (HDPE, PP) | 1–2 | 2,000–5,000 | >5.0 | <0.10 | >1.3 |
| Calcium carbonate (limestone) | 3 | 100–300 | 4.5–5.0 | 0.18–0.22 | 1.06–1.14 |
| Sodium chloride (salt crystals) | 2.5 | 200–500 | 4.0–4.5 | 0.20–0.25 | 1.00–1.10 |
| Copper ore (chalcopyrite) | 3.5–4 | 100–200 | 3.5–4.0 | 0.25–0.30 | 0.90–1.0 |
| Iron ore (hematite) | 5–6 | 100–200 | 2.8–3.2 | 0.35–0.45 | 0.80–0.90 |
| Silica sand (quartz) | 7 | 100–200 | 2.5–3.0 | 0.34–0.50 | 0.80–1.00 |
| Silica sand | 7 | 200–400 | 2.2–2.7 | 0.40–0.60 | 0.70–0.90 |
| Glass beads | 5–6 | 100–300 | 3.0–3.5 | 0.30–0.45 | 0.80–1.00 |
| Alumina (Al₂O₃) | 9 | 50–100 | 1.8–2.2 | 0.30–0.50 | 0.80–1.00 |
| Silicon carbide (SiC) | 9.5 | 50–150 | 1.2–1.5 | 0.45–0.70 | 0.60–0.80 |
| Zirconia (ZrO₂) | 8 | 50–100 | 1.5–2.0 | 0.40–0.60 | 0.70–0.90 |
| Fly ash (mixed) | 5–6 | 50–150 | 3.0–3.5 | 0.30–0.45 | 0.80–1.00 |
Practical Implications and Mitigation
For existing installations, measure local flow velocity across the heater using a pitot tube or ultrasonic flow meter. Position the measurement point within 50 mm of the heater surface on the upstream side, as velocity here is 1.5–2.5× higher than bulk tank velocity due to flow constriction. If measured velocity exceeds the maximum for the particle type, take corrective action: (1) Install a flow deflector (perforated plate) 50–100 mm upstream of the heater to reduce localized velocity. A well-designed deflector can reduce velocity at the heater by 40–60%. (2) Increase heater wall thickness to 2.5–3.0 mm, which increases allowable velocity by 30–50% for the same erosion allowance. (3) Reduce pump speed or install a variable frequency drive to lower circulation velocity. (4) Change heater orientation: a vertical heater with flow parallel to its axis experiences much lower erosion (by a factor of 5–10) than a horizontal heater perpendicular to flow. For slurry service, always mount heaters vertically with flow moving axially along the heater length, not crosswise.
For 1.5 mm PFA sheaths already in abrasive service, annual thickness measurement is mandatory. Use an ultrasonic gauge at three heights and four circumferential positions. If thinning exceeds 0.2 mm/year, the velocity is above the safe limit. Schedule replacement when remaining thickness reaches 1.0 mm, and implement velocity reduction measures for the replacement heater. Operating a 1.5 mm sheath below 1.0 mm thickness risks sudden perforation, bath contamination, and ground fault.
Conclusion: 2.5–3.0 m/s Maximum for Silica Sand
For a 1.5 mm PFA sheath in silica sand slurry (Mohs 7, 100–200 µm, 2–5% concentration), the maximum particle velocity without erosive failure over 2 years of continuous operation is 2.5–3.0 m/s. At higher velocities, erosion exceeds 0.5 mm/year, reducing wall thickness below the safe minimum of 0.8–1.0 mm within 2 years. For harder particles (alumina, SiC), maximum velocity drops to 1.5–2.0 m/s. For softer particles (limestone, salt), velocities up to 4.5 m/s are acceptable. Engineers must measure local velocity, not rely on bulk flow rates. If the required process velocity exceeds the maximum for the particle type, specify a thicker PFA wall (2.5–3.0 mm) or implement velocity reduction measures. A 1.5 mm sheath is suitable for low-velocity, low-concentration, or soft-particle slurries only. For high-velocity hard-particle service, 1.5 mm is inadequate regardless of other design features. The 1.5 mm wall is the most common industrial thickness, but in abrasive service, it is often the wrong choice. Calculate erosion rate before specifying. If in doubt, go thicker. Erosion does not forgive.
