Carbon Graphite Rings for Dry Gas Seals in Chemical Vacuum Pumps: 80℃ Oil-Free Screw Vacuum Pump Service with Condensable Vapors

Solvent recovery processes in chemical and pharmaceutical industries extensively use oil-free screw vacuum pumps, and their Dry Gas Seals (DGS) are the core sealing components. The service is characterized by extracting condensable organic solvent vapors such as methanol, acetone, ethyl acetate, and dichloromethane, with frequent vacuum pump start/stop cycles (2-3 times per day, switching between different solvent batches). At the moment of startup, a stable gas film has not yet formed on the seal face, putting it in dry friction state, requiring seal materials resistant to dry wear, thermal crack resistance, and fast thermal conduction. Based on the application experience of Huoqiu County Huahao Sealing Co., Ltd., this article introduces key points for selecting carbon graphite rings for dry gas seals.

1. Service Characteristics

1.1 Typical Service Parameters

• Suction media: methanol (boiling point 64.7℃), acetone (56℃), ethyl acetate (77℃), dichloromethane (39.6℃)
• Working pressure: inlet 1-100 kPa (absolute), outlet 101-200 kPa (discharged to condenser)
• Working temperature: seal chamber 60-95℃ (condensable vapor compression heat release), max 105℃
• Pump speed: 2950-3500 r/min (4-pole motor)
• Start-stop cycles: ≥ 1000 times/year, frequent start-stop

1.2 Failure Modes

Conventional resin-impregnated graphite dry gas seal failure statistics:

1.Startup dry friction wear (50%): Gas film disappears at shutdown, at the next startup instant (<0.5 s) faces directly contact dry friction. Resin-impregnated graphite has poor thermal conduction (15-25 W/m·K), face instantaneous temperature can reach 300℃ → resin carbonization → seal face shows "hot spots" → wear spots → 0.5 mm+ wear in 3-6 months causing leakage.

2.Condensable liquid film lubrication breakdown (25%): Part of solvent vapor condenses into liquid film (1-5 μm) on seal faces, but polar solvents (methanol) will swell ordinary resin impregnant → impregnant precipitation → porosity increase → strength decrease.

3.Seal face thermal cracks (15%): Frequent start-stops cause temperature shocks (ambient → 80℃ → ambient), thermal expansion coefficient mismatch between resin graphite and SSiC (resin graphite 4.5×10⁻⁶/℃ vs SSiC 3.2×10⁻⁶/℃) → thermal stress → microcracks.

4.O-ring aging (10%): Solvent vapor causes FKM O-ring swelling (FKM swelling rate in methanol 10-15%), elasticity decreases, compensation fails.

2. Material Selection

2.1 Antimony-Copper Bimetal Impregnated Graphite

Huahao Sealing M280Sb-Cu antimony-copper bimetal impregnated graphite is recommended for oil-free vacuum pump dry gas seal rotating rings:

• Base material: fine-grain high-strength graphite (grain 10 μm, density 1.85 g/cm³, porosity 14-17%)
• Impregnant: antimony (Sb) 12% + copper (Cu) 18% bimetal vacuum pressure impregnation (impregnation rate ≥ 95%, residual porosity < 1%)
• Thermal conductivity: 120 W/m·K (5-8 times that of resin-impregnated graphite; startup dry friction heat rapidly conducts away, avoiding hot spots)
• Linear expansion coefficient: 2.1×10⁻⁶/℃ (close to SSiC's 3.2×10⁻⁶/℃, frequent start-stop thermal stress reduced by 60%)
• Mechanical strength: compressive 380 MPa, flexural 95 MPa, Shore HS 92 (dry friction wear resistance 3-4 times that of resin-impregnated graphite)
• Solvent resistance: bimetal impregnant does not swell in any organic solvent (fully stable in methanol, acetone, ethyl acetate, toluene, etc.)

2.2 Mating Material and Auxiliary Seals

• Mating ring: SSiC pressureless sintered silicon carbide (R_a 0.05-0.1 μm mirror polish, flatness ≤ 0.0009 mm, PV value paired with antimony-copper graphite can reach 25 MPa·m/s)
• O-rings: FFKM (Kalrez Spectrum 7075, resistant to all organic solvents, methanol swelling rate < 1%) or FEPM (Aflas 150P, resistant to polar solvents)
• Metal parts: 316L stainless steel (suitable for non-corrosive solvents; for halogenated hydrocarbons select 2205 duplex steel)

2.3 Why Not Select Other Impregnants

| Impregnant Type | Thermal Conductivity | Startup Dry-Wear Life | Solvent Resistance | Cost | Suitability |

|----------------|---------------------|----------------------|-------------------|------|-------------|

| Epoxy Resin | 15-25 | 6 mo - 1 yr | Poor (methanol/acetone swelling) | Low | ✗ Not recommended |

| Furan Resin | 18-30 | 8-14 mo | Fair-Good | Medium | ✗ Fails at >300 cycles |

| Babbitt (Cu-Sn) | 60-85 | 1.5-2.5 yr | Fair | Med-High | △ Fair conduction, medium life |

| Pure Sb | 80-100 | 2.5-3.5 yr | Excellent | High | ✓ Excellent but lower HT strength |

| Sb-Cu Bimetal | 110-130 | 4-6 yr | Excellent | High | ✓ Best |

3. Spiral Groove Dry Gas Seal Structural Design

3.1 Spiral Groove Geometric Parameters (Hydrodynamic Effect Design)

The core of a dry gas seal is the spiral grooves on the rotating ring end face that generate a hydrodynamic effect, making the seal faces non-contact during normal operation (gas film thickness 3-5 μm).

• Groove type: logarithmic spiral (satisfies Archimedean spiral equation r = r₀·e^(θ·tanα)), α = 15-18° (optimal helix angle, balancing hydrodynamic stiffness and leakage)
• Groove depth: 3-5 μm (±0.3 μm, precision laser etching, groove depth tolerance within ±5%)
• Groove count: 12-16 (diameter 60-120 mm), groove width:land width = 1:1 (symmetric structure)
• Groove depth ratio (gas film stiffness key): groove depth h / gas film thickness t ≈ 1:1 → h ≈ 3-5 μm, at which gas film stiffness is maximized (K_gas ≥ 200 N/μm)

3.2 Face Specific Pressure Control

• Face specific pressure P_c: designed at 0.2-0.3 MPa (only 1/3-1/4 of contact seals, non-contact gas film bears the main load)
• Closing force calculation: P_c = P_spring + P_medium × A_med / A_seal
• P_spring (spring specific pressure): 0.08-0.12 MPa (multiple sets of small springs, uniformly loaded)
• A_med / A_seal (medium action area coefficient): 0.75-0.85 (balanced design, avoiding excessive specific pressure at increased pressure)

3.3 Gas Film Thickness Verification

Verify gas film thickness through hydrodynamic simulation + field measurement:

• Simulation (Reynolds equation CFD): at 3000 r/min, 0.1 MPa differential pressure, gas film thickness 4.2 μm, leakage rate 8.5×10⁻⁷ Pa·m³/s
• Actual measurement (capacitance displacement sensor): 4.0-4.5 μm during stable operation, startup instantaneous dry friction contact time < 0.3 s
• Helium leak test: leakage rate ≤ 1×10⁻⁶ Pa·m³/s, better than API 682 Type A dry gas seal leakage standard

4. Huahao Sealing Case Study

A fine chemical company under a large petrochemical group has 4 sets of 50000 t/year ethyl acetate solvent recovery units, with a total of 14 oil-free screw vacuum pumps (flow 1500 m³/h, ultimate vacuum 50 Pa, speed 2950 r/min, seal diameter 90 mm). Originally, there were the following problems with an imported brand resin-impregnated graphite dry gas seal:

1.Average seal life 11 months, frequent startup (switches every 18 hours, about 1500 start-stops/year), teardown found seal faces covered with 0.3-0.5 mm deep wear spots, 3-5 thermal cracks

2.About 13 seal replacements per year, spare parts cost + shutdown maintenance cost about 1.95 million CNY

3.Startup transient seal face dry friction produced smoke, solvent vapor plus wear debris mixture presented explosion risk (one near-miss flash explosion incident in 2018)

After switching to Huahao Sealing's M280Sb-Cu antimony-copper bimetal impregnated graphite rotating ring + SSiC mating ring + spiral groove (α=16°, groove depth 4 μm) + Kalrez 7075 O-rings + small spring balanced type solution in 2019:

• First pump continuous operation 4.2 years (2019.08-2023.11), teardown inspection seal face wear 0.08 mm, no thermal cracks, gas film thickness still stable at 3.8 μm
• Cumulative 3800+ starts, seal life 50.4 months (+358%), improved from 11 months
• Startup dry friction smoke phenomenon completely eliminated, zero flash explosion risk incidents 2019-2023
• Annual spare parts cost + maintenance cost reduced from 1.95 million to about 0.22 million CNY, cumulative savings 7+ million CNY

Selection core: start-stop frequency determines impregnant (>500 times/year requires bimetal impregnation), medium solvent polarity determines O-ring grade (polar solvents select FFKM), thermal conductivity determines dry friction life (Sb-Cu 120 W/m·K is optimal).