Carbon Mechanical Seals

Topics Covered

Why use Carbon?

Operating Mechanism

Environmental Factors

Why use Carbon?

Carbon is used as one of the faces on many seal applications because of its self-lubricating properties, chemical inertness and relatively low cost. Silicon carbide and tungsten carbide seal faces account for the majority of the other applications. Carbon seal grades are available in a wide range of abrasion resistances. The softer, low hardness grades are preferred where the mating face is a relatively soft metal. Harder, more abrasion resistant grades are preferred where the fluid sealed contains suspended or dissolved solids.

Depending on the mechanical seal type, there is often additional assembly work such as placing the carbon primary ring into an adapter or assembling carbon components into a seal head. Other types of seals use carbon as a loose component that is combined with a retainer, disk, springs and the secondary seal such as a bellows or O-ring.

Operating Mechanism

When carbon is rubbed against metal or ceramics, a microscopically thin transfer film is quickly laid down. This low friction film plays a vital role in controlling temperature rise if the interface becomes dry during rubbing. The formation of a proper transfer film requires the presence of absorbable vapours such as water. Seal grades with special impregnants are available for those rare applications in which no suitable vapours are present.

Environmental Factors

General service carbons can be used up to 260°C (500°F) and special grades are available that provide resistance up to 540°C (1004°F). While carbon is one of the most chemically inert materials available to the design engineer, highly corrosive chemicals such as hydrofluoric acid can attack some grades. Thermoset resins, selected for their chemical resistance, are used in many carbon grades to render the seal ring impervious. With few exceptions, carbon and the resin sealer are completely unaffected by solvents, acids or caustics. Only strong oxidising agents such as boiling, concentrated nitric acid or hot mixed nitric and hydrofluoric acid affect carbon. Thermoset resins slowly degrade at temperatures ranging from 200 to 300°C. Degradation of the impregnant may cause the permeability to increase but other physical properties are rarely affected. Special carbons and other impregnants such as metals and inorganic salts are used for seal applications in the 300‑500°C (572-932°F) range.

Primary author : Lawrence Thorwart

Source : Abstracted from Materials World, vol. 2, pp. 519-21, 1994 “The materials key to seal performance”

For more information on Materials World please visit The Institute of Materials