Page 254 - The Tribology Handbook

Selection of seals B19

(di (e) (f)

Figure 19.10 Examples of clearance seals: (a) labyrinth; (b) viscoseal; IC) centrifugal seal; (d) fixed bushing;
(e) floating bushing; (f) polymeric-bushing seal

Multiple seals

One seal or several in series may be used, depending on
the severity of the application. Table 19.3 shows six basic
dynamic sealing problems where two fluids have to be
separated. Since contact seals rely on the sealed fluids for
lubrication of the sliding parts it is essential that the
seal(s) chosen should be exposed to a suitable lubricating
liquid. Where thus is not already so, a second seal enclos-
ing a suitable ‘buffer’ liquid must be used Multiple seals
are also used where the pressure is so large that It must be
broken down in stages to comply with the pressure limits
of the individual seals, or where severe limitations on
contamination exist. Table 19.3 lists the procedures for
dealing with these various situations. Where a buffer fluid
is used, care should be taken to ensure proper pressure
control, especially when exposed to temperature variation. BUFFER FLUID
The pressure drop across successive seals will not be
identical unless positive control is provided. Figure 19.11 Multiple seals, with buffer fluid

Terminodolg y:
‘Tandem seals’ multiple seals facing same direction, used to stage the pressure drop of the system. Inter-stage pressures
progressively lower than sealed pressure.
‘Double seals’ pair of seals facing opposite directions, used to control escape of hazardous or toxic sealed fluid to
environment, or to permit liquid lubrication of the inner seal. The buffer pressure is normally higher than
the sealed pressure.

B19.3