Construction and the Fluid Film

A mechanical seal contains process fluid where a rotating shaft passes through a stationary housing. It replaced gland packing in most services because it leaks far less, needs no adjustment, and does not score the shaft.

The four sealing points

• The seal faces — one rotating, one stationary, lapped extremely flat and held together by springs and hydraulic pressure. This is the dynamic seal and the only one that moves.
• Between the rotating face and the shaft — a static elastomer, O-ring or wedge.
• Between the stationary face and the housing — a static elastomer or gasket.
• Between the gland plate and the pump housing — a gasket.

The fluid film

The faces do not run dry against each other. A film of process fluid, typically less than a micron thick, separates them, providing lubrication and carrying away heat. A properly running seal therefore leaks a small amount, usually vaporising before it is visible.

Everything about seal reliability follows from protecting that film. If it is lost — through dry running, flashing, solids, or excessive face load — the faces contact, heat rapidly and fail within seconds to minutes.

Face materials

A hard face runs against a softer one: silicon carbide against carbon is the common general combination; silicon carbide against silicon carbide is used for abrasive duties but has no tolerance for dry running; tungsten carbide is used where strength matters. The elastomer must suit the fluid and temperature, and elastomer incompatibility is a frequent and easily avoided failure cause.