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Tech Info > Seal Material Selection

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.

Recommended:
Most services
Marginal lubrication
Not recommended:
Severe abrasive service


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 for mating rings. Graphite-loaded self-sintered SiC has greatly enhanced tribological performance for SiC/SiC pairs for abrasive service, and is AST's standard grade for primary rings for hard face/hard face pairs.

Recommended:
Most services
High speed and pressure
Not recommended:
Caustics (reaction-bonded only)
Hydrofluoric acid (reaction-bonded only) 

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 graphite-loaded silicon carbide primary rings in severe abrasive services, but is not as resistant to corrosion as silicon carbide.

Recommended:
Abusive service
Not recommended:
Most metal salts, acids

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.  Alumina should not be used as either a primary or mating ring in a hard face/hard face pair.  Silicon carbide has replaced alumina in most applications.

Recommended:
Low speeds and presssures
Not recommended:
Dry running and marginal lubrication 
 


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.

Recommended:
Oils
Water, including wastewater
Acids (many)
Benzene, toluene
Chlorine, chlorinated solvents
Ethylene glycol

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.

Recommended:
Steam, hot water
Caustics
Ketones (acetone, MEK)
Acetic acid
Ethylene glycol
Ethanol, methanol
Not recommmended:
Oils, most lubricants
Acids
Benzene, toluene
Hydrocarbons (aromatic and aliphatic)
Ethers


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.

Recommended:
Sour oil and gas
Oils, lubricants
Amines, ammonia
Steam, hot water
Acids
Caustics
Not recommmended:
Toluene
Non-polar solvents
Ethers
Ketones
Chlorinated solvents
Acetic acid


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.

Recommended:
Almost all chemicals (depending on compound)
Hot services
Not recommmended:
Where other elastomers can be used



"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