The O-ring is rated for the gas. The pressure is within spec. Six months into service on a chlorine line, the O-ring has swelled 15% and the fitting leaks. The elastomer was compatible at ambient conditions. It wasn't compatible at process conditions over time.
Two Fundamentally Different Sealing Mechanisms
Metal seals achieve leak tightness through plastic deformation. A soft metal gasket is compressed between two precision-machined faces. The gasket yields locally, conforming to the surface geometry. The seal is the deformed metal itself — no organic compounds, no polymers, no materials that absorb or react with the process gas.
Elastomer seals achieve leak tightness through elastic recovery. An O-ring compressed into a groove exerts a continuous restoring force against the sealing faces. The seal depends on the O-ring maintaining its physical properties: hardness, elasticity, and dimensional stability. If the O-ring swells, hardens, softens, or takes a permanent compression set, the sealing force changes and leak integrity is compromised.
| | Metal seal (VCR) | Elastomer seal (VCO) | |---|---|---| | Failure mechanism | Mechanical damage to gasket or sealing face | Material degradation of O-ring | | Failure progression | Usually sudden or detectable | Can be gradual (swelling, compression set) | | Affected by gas chemistry | Minimal | Yes — absorption, swelling, chemical attack | | Affected by temperature | Minimal within rated range | Yes — accelerated aging, compression set | | Maintenance | Replace gasket | Replace O-ring |
Why Gas Chemistry Matters for Elastomers
Absorption and swelling: Many gases dissolve into elastomer materials under pressure. When pressure is released, the absorbed gas expands within the elastomer, causing swelling and dimensional change. A swollen O-ring no longer fits its groove correctly.
Chemical attack: Oxidizing gases attack elastomer polymer chains directly. Halogens and their compounds are particularly aggressive. The attack degrades mechanical properties over time, reducing elasticity and increasing brittleness.
Outgassing: Elastomers contain plasticizers and processing aids. Under process conditions, these compounds outgas into the process stream, contributing to baseline impurity levels.
Gas Services Where Metal Seal Is Required or Strongly Preferred
Fluorine and highest-reactivity fluorine compounds (F₂, ClF₃) These gases attack virtually all elastomers, including fluorocarbon materials that are otherwise chemically resistant. All-metal sealing is required.
NF₃ service NF₃ is significantly more stable than F₂. High-performance elastomers — Kalrez, Chemraz, perfluoroelastomers — are used in NF₃ service in many applications. Compatibility verification against specific service conditions, concentration, temperature, and pressure is required rather than automatic exclusion of elastomer seals.
Highly corrosive halide gases (Cl₂, HCl, BCl₃) For these services, metal seals are frequently preferred because they eliminate long-term elastomer degradation concerns. High-performance elastomers may be used in certain configurations, but require verification of long-term compatibility under actual service conditions.
Ultra-high-purity applications with sub-ppb impurity requirements Elastomer outgassing contributes measurable impurities. For processes where acceptable impurity levels are below what elastomer outgassing produces, metal seals are required throughout.
Where Elastomer Seals Are Acceptable
- Inert gas service (N₂, Ar, He) at moderate purity requirements
- Instrument connections where frequent maintenance access is required and gas compatibility is confirmed
- CDA and pneumatic actuator supply lines
- Applications where the process gas is compatible with the specific elastomer and purity requirements do not exclude outgassing contribution
The key verification step: confirm compatibility not just at ambient conditions, but at actual process temperature and pressure, over the intended service life.
Bottom Line
The choice between metal and elastomer seals is about the chemical interaction between the seal material and the process gas over the service life of the connection — not primarily about pressure rating or assembly convenience.
For reactive, corrosive, or ultra-high-purity gas service, metal seals are the default. For inert gas service with confirmed compatibility, elastomer seals offer legitimate advantages. The specification decision requires knowing both the gas chemistry and the purity requirements through the full operating life of the system.
Need VCR metal gasket fittings or VCO O-ring face seal fittings? View KTIC's fittings or contact us for material compatibility support.
Related: VCR vs. VCO — Choosing the Right Face-Seal Fitting Related: 316L vs. 304 — Why Material Grade Cannot Be Substituted