Aeroengine Safety

- 7.3.1 Fundamental Operating Behavior of Brush Seals
- 7.3.2 Brush Seal Damage
- 7.3.3 Remedies for Brush Seal Damage

Brush seals are moving seals and have similar potential applications to labyrinth seals. The elements of these seals are brushes made from many fibers/bristles opposite a (usually) smooth glide surface. The brush is the static part of the seal, since the bristles could be overstressed and break off due to centrifugal forces as are common during engine operation. Depending on the radial gap, the sealing power of a brush seal is roughly the same as that of a five-fin labyrinth (Ref. 7.3.1-18). However, compared with a labyrinth, the brush seal only takes up a very short axial space. On the other hand, the radial height necessary for a brush seal is considerably larger than for a labyrinth seal.
The maximum pressure difference at a single-stage brush seal is about 10 bar (Ref. 7.3.1-13) and is determined by the strength and stiffness properties of the brush.
Because brush seals are installed and replaced as machine parts, similar to roller bearings, they have advantages over labyrinths during installation and overhaul. Whereas labyrinths are usually an integral part of a larger expensive part and its fins are repaired on the part (e.g. deposit welding and reworking), brushes can be quickly and easily replaced.
The special advantage of brush seals is their operating behavior. If the brush seal is properly designed, clearance losses during operation cause the brush bristles to elastically deflect with minimal material
removal and no damaging heat development. This ensures the effectiveness of the seal is better maintained over more stress cycles than in comparable labyrinths, where rubbing causes material removal and quickly results in a larger seal gap.
The use of brushes also has a safety advantage, compared with labyrinths. In principle, the self-increasing, often catastrophic rubbing typical for labyrinths (Fig. "Labyrinth failure by rubbing") should not occur with brush seals.