Replacing Metal Seals with Rubber Seals: Critical Considerations and Guidelines

Replacing metal seals with rubber seals is common in equipment maintenance or retrofits (e.g., to reduce costs, simplify installation, or adapt to specific media). However, the significant differences in physical and chemical properties between metal and rubber can lead to seal failure or safety hazards if not properly addressed. This article outlines key precautions to ensure reliability and safety.

I. Feasibility Assessment: ​Not All Scenarios Are Suitable​

1. ​Operating Condition Compatibility​
   • ​Temperature Range:
     • Rubber has lower heat resistance (NBR: ~100°C; FKM: ~250°C) vs. metal (800°C+).
     • ​Verify: Operating/peak temperatures must align with rubber limits.
   • ​Pressure & PV Value:
     • Rubber withstands lower pressure (<10 MPa); friction heat at high speeds accelerates aging.
     • ​Check: System pressure/sliding speed must not exceed rubber design limits.
   • ​Media Compatibility:
     • Rubber may swell/degrade in oils, solvents, or chemicals (e.g., NBR fails with ketones; FKM with amines).
     • ​Critical Step: Cross-reference media composition with rubber compatibility charts (ISO 6072/ASTM D471).
   • ​Abrasive Environments:
     • Particles accelerate rubber wear (metal resists abrasion better).
     • ​Evaluate: Media cleanliness; add wipers if needed.
2. ​Safety & Regulatory Limits​
   • ​Hazardous Media​ (e.g., LNG, liquid oxygen, acids):
     • Rubber has higher permeability → leakage risk.
     • ​Comply: API 682 (mechanical seals), ASME B31.3 (piping) standards.
   • ​Fire Safety:
     • Rubber burns (metal does not); use flame-retardant rubber (e.g., FEPM) or avoid replacement in petrochemical/aviation.

​Conclusion: Do ​not replace​ if conditions exceed rubber limits (high T/P, hazardous media, abrasion).

II. Structural Adaptation: ​Bridging Metal-to-Rubber Differences​

• ​Key Actions:
  • Measure original metal seal dimensions (ID, cross-section).
  • ​Redesign grooves: Calculate compression/gap per rubber standards (e.g., AS 568A).

III. Material Selection: ​Compensating for Performance Gaps​

• ​Special Requirements:
  • ​Conductivity: Add carbon/metal fillers for static dissipation (explosive environments).
  • ​Vacuum Sealing: Use low-outgassing rubber (e.g., FKM).

IV. Installation & Maintenance: ​Preventing Human Error​

1. ​Installation Adjustments​
   • ​No sharp tools: Rubber cuts easily (use nylon tools).
   • ​Lubricant: Silicone grease (avoid petroleum-based oils with NBR).
   • ​Even Compression: Prevent twisting during installation.
2. ​Lifecycle Management​
   • ​Shorter service intervals: Rubber ages faster than metal.
   • ​Leak monitoring: Install sensors for early failure detection.

V. Replacement Scenarios & Risk Cases

Conclusion: ​Three Replacement Principles​

1. ​Conditions first: Never replace if T/P/media are incompatible.
2. ​Structural redesign: Modify grooves/add rings—never a direct swap.
3. ​Lifecycle cost: Frequent rubber maintenance may offset savings.

​Final Warning: In nuclear, aerospace, or high-risk chemical sectors, metal seals are often irreplaceable. Always consult OEM/seal engineers before switching.