SEAL RING MATERIALS

Carbon-graphite is a self-lubricating material, and is a versatile primary ring material used with mating rings of harder materials such as silicon carbide, alumina, or tungsten carbide.  AST's standard carbon-graphite is a premium redensified, resin impregnated grade which has superior wear characteristics and broad chemical resistance.  For dry running and for severe corrodents including hydrofluoric and nitric acids, special carbon grades are available.

Silicon carbide (SiC) resists a wide range of chemicals. It is also the hardest of seal ring materials, giving it excellent abrasion resistance. Silicon carbide's high thermal conductivity and low coefficient of friction are essential for the effective sealing of volatile liquids and high pressure services. Reaction-bonded SiC can be corroded by caustics and hydrofluoric acid.  Self-sintered SiC resists all chemicals, and is our standard grade. Graphite-loaded self-sintered SiC has greatly enhanced tribological performance for SiC/SiC pairs for abrasive service.

Alumina ceramic is inert in pure form and has been used in chemical and water applications as a mating ring for carbon-graphite primary rings. Alumina is limited to moderate speeds and pressures. Alumina is susceptible to thermal shock, and is not recommended for services involving periods of dry running or for petroleum refinery services.  Silicon carbide has replaced alumina in most applications.

Tungsten carbide is a popular mating ring for carbon-graphite primary rings in hydrocarbon process applications and rugged applications involving vibration and mechanical shock. Tungsten carbide is a good mating ring for silicon carbide primary rings in severe abrasive services, but is not as resistant to corrosion as silicon carbide.

O-RING ELASTOMERS

To make a reliable selection of O-ring compound for use in a mechanical seal, all constituents of the sealed fluid must be known.  O-rings can also be degraded by fluids used periodically to clean the pumping system.   Although detailed checking of each constituent is required to ensure proper elastomer selection, a few simple guidelines can provide a starting point.

Fluoroelastomer (Viton®, Fluorel™, FKM) O-rings have excellent physical properties and compression set resistance.

Not recommmended:
Caustics, alkalis
Ketones (acetone, MEK)
Amines, ammonia
Ethers
Acetic acid
Steam
Ethanol, methanol

Ethylene Propylene (EP, EPT, EPDM) resists caustics, and is commonly used in pulp and paper processing.  EP is the first choice for hot water and steam service.  EP is rapidly attacked by petroleum oils and greases, so be sure to use silicone lubricant when installing EP O-rings.

Aflas™ (tetrafluoroethyene/propylene dipolymer) resists combinations of amines and oils, and is therefore used in sour oil and gas and in refrigeration applications.   Cost is comparable to fluoroelastomers.

Perfluoroelastomers (Isolast®, Kalrez®, FFKM) resist high temperatures and a wide range of chemicals.  Compression set resistance is inferior to fluoroelastomers.   Cost is high, approaching 100 times that of fluoroelastomers.  Compounds optimized for broad-spectrum chemical resistance (Isolast J9503) or for high-temperature performance (Isolast J8325) are available.

"Aflas" is a registered trademark of Asahi Glass Co., Ltd.
"Fluorel" is a registered trademark of 3M
"Isolast" is a registered trademark of Trelleborg Sealing Solutions Inc
Viton® and Kalrez® are registered trademarks of Du Pont Dow Elastomers