Chemical Compatibility of Elastomers in Valve Seals

Elastomeric seals and diaphragms in valves are vulnerable to swelling, softening, hardening, and stress cracking when exposed to incompatible process chemicals. Selecting the correct elastomer for each application is critical to preventing premature seal failure and maintaining the valve's pressure containment capability.

Common Elastomer Types and Base Chemical Resistance

• Nitrile (NBR): excellent oil and petroleum hydrocarbon resistance; temperature range minus 40 to 120 degrees C; poor resistance to ozone, aromatic hydrocarbons, and ketones

• EPDM: excellent water, steam, and polar solvent resistance; temperature to 150 degrees C; not compatible with petroleum products or aromatic hydrocarbons

• Fluorocarbon (FKM/Viton): outstanding chemical resistance including hydrocarbons, acids, and aromatics; temperature to 200 degrees C; poor in steam above 150 degrees C and amines

• Perfluoroelastomer (FFKM/Kalrez): universal chemical resistance including steam, amines, and ketones; temperature to 327 degrees C; extremely high cost limits use to critical applications

• Neoprene (CR): good ozone, weathering, and moderate oil resistance; used in refrigeration and outdoor applications

Swelling and Hardness Change Testing

ASTM D471 provides a standard method for measuring volume change and hardness change of elastomers after immersion in a test fluid. Acceptable swell is typically less than 10 to 15 percent volume increase; excessive swell causes the seal to extrude from its groove. Hardness reduction above 10 Shore A points indicates significant plasticization. Test at the maximum operating temperature for at least 70 to 168 hours to simulate long-term service conditions.

Temperature Effects on Elastomer Performance

High temperature accelerates chemical attack and thermal aging. At elevated temperature, polymer chain scission leads to softening and loss of elasticity, while oxidation cross-linking can cause hardening and cracking. Use the Arrhenius equation to estimate elastomer life at service temperature: reducing the temperature by 10 degrees C typically doubles the service life. Dynamic seals that cycle should be tested at temperature and frequency representative of actual service.

Special Considerations for Hydrogen Service

• High-pressure hydrogen can permeate elastomers and cause rapid gas decompression (RGD) damage when pressure is released quickly

• Test per NORSOK M-710 or ISO 10423 Annex F for materials intended for gas service above 70 bar

• FFKM and specially formulated FKM grades with increased filler content show better RGD resistance

• Avoid O-rings in dynamic high-pressure hydrogen service; consider energized PTFE lip seals or metal C-rings

Material Traceability and Documentation

Valve datasheets should specify elastomer compounds by ASTM D1418 type designation (NBR, FKM, EPDM) and hardness (Shore A). For critical or regulated applications, require the elastomer compound number, manufacturer, and chemical resistance qualification data. Retain records of elastomer material certifications, test reports, and any service incidents involving seal failures to build a database of validated material-service combinations.