A mechanical seal that has to run dry is built around a carbon graphite face, usually an electrographite or impregnated grade, running against a hard, finely lapped silicon carbide mate, with secondary seals chosen for the heat the dry faces generate. How dry the duty is shifts the choice: carbon copes with intermittent dry running and with running dry in ordinary air, but a seal in bone-dry gas or vacuum needs a molybdenum disulfide grade.
A plain hard-against-hard pair survives dry contact only with a diamond coating. Material choice is enough for a contacting seal that runs dry at intervals; a seal that must run dry continuously at high speed or pressure usually also needs gas-lubricated, non-contacting face designs.
Carbon Graphite Faces
Carbon graphite is the usual dry-running face because it lubricates itself, which no uncoated hard face material does. In a dry-running seal the pair is usually carbon against silicon carbide.
Why Carbon Can Run Dry
A carbon graphite face runs dry because its graphite grains are stacks of graphene layers held together by weak forces, and those layers slide over each other when the face rubs.
As the seal turns, the carbon lays a burnished graphite film onto the mating face. The film fills the roughness of the mate, so after break-in the carbon runs on a layer of graphite laid over the hard surface.
The mechanism has one condition: graphite needs a small amount of adsorbed moisture or gas to slide easily. Ordinary air carries enough. Bone-dry gas and vacuum do not, and in those duties even the best carbon grade stops lubricating.
Carbon Graphite or Electrographite
Electrographite is the better dry-running grade when heat limits the seal: high speed, high face temperature, or hot air that oxidizes carbon. Carbon graphite stays the better grade when face load or face flexure is the limit.
Graphitizing a carbon face into electrographite can raise its temperature resistance in oxidizing conditions by more than 200 °F (110 °C).
Impregnated Grades
For dry running, salt-impregnated graphite is the specialist grade, and among the common metal and resin grades, antimony-impregnated carbon does best.
Salt makes the burnished film more robust than any other impregnant does, so the film holds on rougher mating faces. Salt impregnation can also raise a carbon face’s temperature resistance by more than 100 °F (55 °C).
Salt grades come either partly porous or fully impervious. Specify an impervious grade when the seal has to maintain an aerodynamic film between the faces.
Carbon Grades for Dry Gas and Vacuum
Where the gas around the seal is bone-dry, or the seal runs in vacuum, molybdenum disulfide-based grades replace carbon graphite.
A rule of thumb puts the switch at the dew point of the seal environment: below −30 °F (−35 °C), the atmosphere is likely too dry for any carbon grade, and a molybdenum disulfide grade is the only feasible face material. Vacuum seals, dry mixer seals and nitrogen-purged dry gas seals are the usual duties below that dew point.
Moly grades belong only in genuinely dry environments. Moisture that helps graphite raises the friction of molybdenum disulfide, so a moly face is no upgrade for a seal that runs dry in ordinary humid air.
Mating Face Material and Finish
The face a dry-running carbon runs against must be hard, with a surface roughness of 16 µin (0.4 µm) or less, and sintered silicon carbide is the usual choice.
Of the two common hard mates, silicon carbide and tungsten carbide, silicon carbide copes better with the heat of a dry run. Tungsten carbide is tougher against mechanical shock but more prone to heat checking than the advanced ceramics.
Hard Face Pairs and Diamond Coatings
Silicon carbide against silicon carbide, the hard pair usually chosen for abrasive service, cannot run dry uncoated; a plain hard pair runs dry only when a diamond coating is on at least one face, and then only for a limited time.
Silicon carbide with free graphite in it, sold as graphite-loaded or hybrid silicon carbide, improves dry-run survival over plain sintered silicon carbide. It does not lubricate itself the way carbon does.
In a 2023 study of a silicon carbide ring coated with nanocrystalline diamond, run dry at room temperature against an uncoated silicon carbide ring, the coated pair’s dry life fell sharply as speed and load rose, and at the highest speed the coating wore through. Diamond also adds considerable cost to a face, and coatings can delaminate under high stress. Weigh that cost against the current seal’s MTBR before specifying it.
Secondary Seal Materials
FFKM and FKM are the O-ring compounds that best withstand the heat of dry running, and past what any elastomer takes, a metal bellows seal with flexible graphite static seals removes elastomers from the seal altogether.
From lower to higher temperature capability, the common elastomers rank:
- Nitrile (NBR) and neoprene (chloroprene, CR): low-cost, general-purpose compounds for lower-temperature water and oil.
- FKM: a higher operating temperature range with good chemical compatibility, and widely used in mechanical seals.
- FFKM: high operating temperatures with excellent chemical compatibility, provided the grade is matched to the duty.
PTFE or carbon/graphite wedges are the other replacement for O-rings where temperature rules elastomers out. Carbon/graphite static seals are used in hot oil service at temperatures approaching 750 °F (398 °C).