The Importance of Fluid Film

A single mechanical sealing device incorporates two flat
faces, one fixed and one rotating, running against each other
with a liquid film between them providing lubrication. This
liquid film is commonly known as the fluid film. Without a
stable fluid film between the faces they would be in full
contact causing frictional heat build up and dry running,
leading to excessive wear and component damage.

The key to successful sealing is the maintenance of a cool,
clean and stable fluid film between the sealing faces.

This very thin, 2 - 3 micrometer, fluid film can be difficult to
maintain and this is the primary challenge when sealing a
pump application.

If a single mechanical seal is used then the process fluid
becomes the fluid film. As long as the process fluid is an
acceptable lubricant, is at an acceptable temperature and is
not a dangerous substance, a single seal can be used.

Many process liquids are not, however, acceptable
for cooling and lubricating the seal faces.


These fluids fall into the following categories:

- Slurries.

- Hot liquids that can evaporate as the pressure drops across the faces.

- Cold liquids that can become very viscous or solidify.

- Liquids that crystallize as they contact atmospheric conditions.

- Liquids that coke as they contact atmospheric conditions.

- Liquids that leave a film or a deposit across the seal faces due to the 
heat generated between the faces.

All of the above conditions require a double mechanical seal
with an external barrier fluid between the two sets of seal
faces. The external barrier fluid must be at a higher pressure
than the sealing chamber pressure to drive the clean, cool
barrier fluid across the inboard seal faces rather than allowing
the process fluid to cross the faces.