What Is the Realistic Maximum Flow Velocity for a PFA Heater Submerged in a Slurry of Glass Beads Without Erosive Wear?

The PFA heater sheaths are a little bit abrasive to glass beads (diameter 200 µm, Mohs hardness 5–6). Sharp silica sand cuts more, round glass beads less cutting and more surface fatigue. The critical velocity for the commencement of detectable erosion (wall thinning >0.1 mm/year) is 3.5-4.0 m/sec for a 2 mm PFA wall. For a longer service life (5 years, 40,000 h) the highest feasible flow velocity is 2.5–3.0 m/s, which limits erosion to 0.2–0.3 mm/yr. Above 4.5 m/s, erosion rates increase fast to 0.8-1.2 mm/year, eroding the wall to 1 mm in 1-2 years. Glass bead slurries are less erosive than silica sand and more erosive than polymer pellets. For velocities above 3.5 m/sec, specify a thicker PFA wall (3 mm) or a replaceable wear sleeve.

Spherical vs. angular particles Erosion Mechanism
The stress concentration on the PFA surface induced by spherical glass beads is smaller than that of angular particles. But the round shape spreads the power of impact across a broader region, limiting the amount of local cutting. Erosion happens by fatigue. Repeated impacts lead to microcracks that join together to form pits. The erosion rate is proportional to E = k v^n, with n = 2.5-3.0 for spherical particles (vs. 3.0-3.5 for angular). At 3 m/s, glass beads erode at 0.15–0.25 mm/year, silica sand at the same velocity erodes at 0.4–0.6 mm/year. Glass beads are 2-3× less destructive.

This makes PFA heaters more suitable for glass bead slurry service (e.g. agitator bead mills, fluidised bed classifiers) than for silica sand service.

Erosion Rate vs Flow Velocity (2mm PFA wall, 80°C, 10% solids)
Flow Velocity (m/s) Particle Shape Erosion Rate (mm/yr)Time to 1 mm (of 2 mm) Left5-Year Thinning (mm) Recommended Max Velocity, 5-Year Life
1.0 Spherical 0.03-0.05 20-30 years 0.15-0.25 Safe 1.5 Spherical 0.05-0.08 12-20 years 0.25-0.40 Safe 2.0 Spherical 0.08-0.12 8-12 years 0.40-0.60 Safe 2.5 Spherical 0.12-0.18Acceptance 0.60–0.90 5–8 years
3.0 Spherical 0.18-0.25 4-6 years 0.90-1.25 Marginal (10% thinning in 5 years)
3.5 Spherical 0.25–0.35 3–4 years 1.25–1.75Not recommended.
4.0 Spherical 0.35-0.50 2-3 years 1.75-2.50 No
4.5 Spherical 0.50–0.80 1.2–2 years >2.0 (perforation)No.
5.0 Spherical 0.80–1.20 0.8–1.2 yrsPerforation No
For comparison: silica sand (angular, 200 µm) at 3 m/s erodes at 0.5-0.8 mm/year - 2-3× faster.

Influence of Solids Concentration
At constant velocity, erosion rate is an essentially linear function of solids content (wt%). For 3 m/s:

Concentration of Solids (wt%) Erosion Rate (mm/year)5-Year Thinning (mm) Suggestion
1% 0.03–0.05 0.15–0.25 Safe Any speed up to 4 m/s 5 % 0.10-0.15 0.50-0.75Safe to 3.5 m/s.
10% 0.18–0.25 0.90–1.25 Maximum 3.0 m/sec.
20% 0.30-0.45 1.50-2.25 Max. 2.5 m/s
30% 0.45–0.65 2.25–3.25 Avoid PFA Use metal or replacement sleeve
Practical Ideas
Life with 2 mm PFA wall for 5 years :

Maximum velocity = 2.5 m/s for 10% glass beads.

Max. velocity = 3.0 m/sec with 5% glass beads

Maximum velocity = 3.5 m/s for 2% glass beads

To prolong life:

Thicker wall: 3 mm PFA erodes 0.12–0.18 mm/year at 3 m/s → 0.6–0.9 mm thinning in 5 years → acceptable.

Lower velocity: Reduce pump speed or install flow straighteners to reduce velocity to 2 m/sec.

Orientation: Vertical flow parallel to heater axis lowers erosion 5-10× vs. cross-flow (see Article #72).

Wear sleeve: A replaceable PTFE sleeve, 1 mm thick, is fitted over the heater. Change sleeve every 2–3 years The PFA beneath is left intact.

Example from the Field
The PFA heaters with 2mm wall were utilised with a glass bead slurry loop (200 µm, 15% solids, 3.2 m/s, 80°C). After 18 months the heaters had corroded to a thickness of 1 mm (a thinning rate of 0.7 mm per year). The plant installed a wearsleeve(2mmthicknessPTFE, 50each)over the PFA heater.The sleeve wore out after 12 months and was replaced.ThePFAheaterwasoforiginalthickness.The PFA heater was still functioning after 5 years.The 50 each) on PFA heater.The sleeve was worn out after 12 months and was replaced.ThePFAheaterwasoforiginalthickness.The PFA heater was still in service after 5 years.The sleeve cost The250 saved $2,000 in heating replacement over 5 years.

Detect and monitor
For existing slurry service, measure PFA wall thickness annually at five sites (upstream, downstream and sides) with an ultrasonic gauge. Velocity is too high if thinning is >0.2 mm/year. Choices:

20% less velocity (50 to 60% less erosion).

Accepted 2-year heater life.

Add sleeve wear.

Conclusion. 2.5-3.0 m/s Max. for 5-Year Life with 10% Glass Beads
The highest flow velocity for a PFA heater immersed in a slurry of 200 µm glass beads (10% solids, 80°C), without erosive wear, is 2.5-3.0 m/s for 5 years of service life with a 2 mm wall. At 3.5 m/s erosion speeds up and life is reduced to 3–4 years. Glass beads are less erosive than silica sand (2-3× lower erosion rate) although velocity control is still required. Specify thicker walls (3 mm) or interchangeable wear sleeves for higher velocity or extended service life. Thickn. measure Annually; replace if rem. wall ≥1 mm "Round" is possible for spherical beads, although they still wear. Control velocity . Measure erosion . Replace in time . It depends on the heater.