SEALING, A hypothetical turbine cooling and sealing arrangement,A generator cooling system,Labyrinth seals.

Fig. 9-5 A hypothetical turbine cooling and sealing arrangement.

SEALING :
12. Seals are used to prevent oil leakage from the engine bearing chambers, to control cooling airflows

and to prevent ingress of the mainstream gas into the turbine disc cavities.
13. Various sealing methods are used on gas turbine engines. The choice of which method is
dependent upon the surrounding temperature and pressure, wearability, heat generation, weight, space
available, ease of manufacture and ease of installa- tion and removal. Some of the sealing methods are
described in the following paragraphs. A hypothetical turbine showing the usage of these seals is shown in
fig. 9-5.


Labyrinth seals :
14. This type of seal is widely used to retain oil in bearing chambers and as a metering device to
control internal airflows. Several variations of labyrinth seal design are shown in fig. 9-7.
15. A labyrinth seal comprises a finned rotating member with a static bore which is lined with a soft
abradable material, or a high temperature honeycomb structure. On initial running of the engine
the fins lightly rub against the lining, cutting into it to

Fig. 9-6 A generator cooling system.

give a minimum clearance. The clearance varies throughout the flight cycle, dependent upon the
thermal growth of the parts and the natural flexing of the rotating members. Across each seal fin there is a

pressure drop which results in a restricted flow of sealing air from one side of the seal to the other.
When this seal is used for bearing chamber sealing, it prevents oil leakage by allowing the air to flow from
the outside to the inside of the chamber. This flow also induces a positive pressure which assists the oil
return system.

16. Seals between two rotating shafts are more likely to be subject to rubs between the fins and
abradable material due to the two shafts deflecting simultaneously. This will create excessive heat which
may result in shaft failure. To prevent this, a non-heat producing seal is used where the abradable lining is
replaced by a rotating annulus of oil. When the shafts deflect, the fins enter the oil and maintain the seal
without generating heat (fig. 9-7).