Page 269 - The Tribology Handbook

B22 Labyrinths, brush seals and throttling bushes

BRUSH SEALS
Brush seals are an alternative for labyrinths in gas turbine
engine applications, reducing leakage by a factor up to five
or tenfold, although relatively expensive. The brush seal
comprises a bundle of metal filaments welded at the base.
The filaments are angled circumferentially at about 45
degrees, filament length is chosen to give an interference of
0.1-0.2 mm with the sealing counterface. Filaments are
typically about 0.7 mm diameter and manufactured from
such materials as high temperature alloys of nickel or
cobalt. Suitable counterface materials include hardfacings
of chromium carbide, tungsten carbide or alumina
rnrn

0.7 rnrn

Figure 22.5 Brush seal with typical dimensions

BLADE SPACING, in

LABYRINTHS

A labyrinth [Figure 22.61 can reduce leakage below that of
a bush seal by about half. This is because eddies formed
in the grooves between the vanes increase the flow res-
istance. Labyrinths are commonly used for gas and steam
turbines.
Typically, the vane axial spacing might be twenty times
the vane lip clearance. If the spacing is too small eddy
formation is inefficient, if too large then the seal becomes
unduly long.

0 1.0 2.0 3.0 4.0 5.0 6.0
BLADE SPACING, mm
Ambient pressure = 1 bar; blade thickness = 0.14 mm
radial clearance = 0.127 rnrn pressure ratio = 0.551;
temperature 310 K
Figure 22.7 Performance of a typical rabyrinth seal

Materials
Vane and rotor must be compatible bearing materials, in
case of rubbing contact, but in turbines stresses are high
and creep is a critical factor. Metal foil honeycomb is a
convenient form of material for labyrinths since its integ-
rity is retained if rubbing occurs, and yet it deforms
readily. Suitable honeycomb is produced in 18/8 stainless
steel and Nimonic 75, 80a and 90. Another convenient
combination comprises metal fins and a carbon bush. The
Figure 22.6 Typical labyrinth arrangements latter can be segmented if necessary for large diameters.
B22.4