These seals are quite similar to rod and piston seals, but are adapted to rotary motion. Circumferential seals are high-speed, low-leakage seals that require less space than face seals. The contacting types undergo high-velocity rubbing at their primary sealing surfaces, and often require cooling.
Hydrostatic and magnetic seals avoid this difficulty. Hydrostatic seals may also suffer from instability in the gas film.
Some circumferential seals inhibit leakage in both directions. Primary sealing takes place at the cylindrical surface where high-speed relative motion takes place. A secondary leakage path exists at the housing surface where the seal is seated.
Contacting circumferential seals are highly effective for sealing gases, but less effective with liquids, although they are used in some liquid applications. Hydrostatic seals can only be used with gases, while magnetic seals can be used with both liquids and gases.
