Key Takeaways
1.PTFE-impregnated graphite bushings in 5.0 MPa, 1,480 r/min seawater typically wear ~0.005 mm/yr — an order of magnitude below copper alloys.
2.Aluminum brass bushings in the same service wear roughly 6× faster than PTFE-impregnated graphite.
3.PTFE impregnation fill ratio ≥94%, open porosity reduced from 15.2% to 0.6%. Seawater permeability <1.2×10⁻⁸ cm/s — no chloride pitting risk.
4.Duplex stainless steel 2205 sleeve PREN = 42.5, annual corrosion rate <0.002 mm in 55°C / 19,000 ppm Cl⁻ seawater vs. 316L at 0.037 mm.
5.Retrofits typically cut bushing replacement frequency to once a year or less
6.Post-upgrade RO feed Cu²⁺ from 28 ppb to <1 ppb; membrane flux decline from 11.3%/yr to 5.8%/yr.
1. Pain Points of HP Pump Bushing in RO Desalination
RO technology accounts for >65% of global desalination capacity. RO HP pumps 5.0–7.0 MPa discharge, bushing service is severe: Cl⁻ 18,000–20,000 ppm, sand <50 ppm, ΔP 4.5–5.5 MPa gives PV 2.5–3.5 MPa·m/s.
Huahao Sealing 2024 survey of 21 plants:
| Scheme | Avg. Life (mo) | Main Failure |
|---|---|---|
| Tin-bronze ZCuSn10Pb1 | 2.3 | Corrosion-wear 72% |
| Al-bronze CuZn35Al2FeMn | 3.1 | Dezincification 65% |
| 316L + PTFE liner | 4.8 | PTFE cold-flow 41% |
| Standard resin-imp. graphite | 6.7 | Pore water-swelling 48% |
Most insidious: copper leaching 15–45 ppb Cu²⁺, RO membrane salt rejection drops 3–5% in 3 months at Cu²⁺ >10 ppb.
2. Materials Design of PTFE-Impregnated Graphite Bushing
2.1 Carbon Matrix
Fine-particle petroleum-coke carbon: aggregate ≤8 μm calcined coke, modified pitch + graphene reinforcement, 1,250°C sinter + 2,800°C graphitization. Properties: 1.72 g/cm³, open porosity 15.2%, Shore 82, CS 215 MPa, FS 78 MPa.
2.2 Three-Stage Impregnation-Sinter Cycle
| Stage | Parameters | Fill |
|---|---|---|
| 1st vacuum imp. + sinter | 60% PTFE, 12 MPa, 380°C × 4 h | ~65% |
| 2nd vacuum imp. + sinter | 40% PTFE + silane, 370°C × 3 h | ~88% |
| 3rd pressure imp. + sinter | 25% PTFE, 18 MPa, 365°C × 2 h | ≥94% |
Validation: 0.6% open porosity, boiling water 200 h absorption 0.12%, crosshatch class 0, 6.5 MPa × 24 h permeability <1.2×10⁻⁸ cm/s.
2.3 Tribology Pairing Test (seawater, 5 MPa × 2.5 m/s, 500 h)
| Mating Material | PTFE-impregnated graphite Wear (mm/1000h) | Mating Wear | Friction μ |
|---|---|---|---|
| 2205 DSS (Ra 0.2) | 0.0062 | 0.0018 | 0.052 |
| 2507 Super DSS | 0.0058 | 0.0014 | 0.055 |
| 316L | 0.0091 | 0.0145 | 0.068 |
| Hastelloy C-276 | 0.0053 | 0.0012 | 0.049 |
Recommended: PTFE-impregnated graphite + 2205 for best cost/performance.
3. Surface Engineering on 2205 DSS Sleeve
UNSM nanocrystallization: 20 μm deep nanograin layer (<50 nm), hardness HV280→HV480, -650 MPa compressive residual stress, Ra 0.2→0.08 μm.
W-doped DLC coating (2–3 μm): HV2200, dry μ=0.08, scratch Lc=68 N, after 1,000 h seawater Lc=62 N.
4. Retrofit Implementation Essentials
- Check interference fit and shrink-fit temperature against the duplex steel housing
- Verify face compression and spring pressure against the allowable PV of PTFE-impregnated graphite
- Monitor leakage and bushing temperature during commissioning, then add to routine inspection
- Set replacement intervals by running hours and measured wear
5. Applicability Scope
| Service | Conditions | Recommendation | Life |
|---|---|---|---|
| RO HP 5–8 MPa | Cl⁻ ≤22,000 ppm, T ≤45°C | PTFE-graphite+2205+DLC | >10 yr |
| RO HP 5–8 MPa | T 45–60°C | PTFE-graphite+2507+DLC | >10 yr |
| Seawater intake | Cl⁻ ≤20,000 ppm | PTFE-graphite+2205 | 8–10 yr |
| Brine discharge | Cl⁻ ≤40,000 ppm | PTFE-graphite+2507 | 6–8 yr |
PTFE-impregnated graphite bushings with duplex steel sleeves are an established selection for desalination high-pressure pumps.