Note: Descriptions are shown in the official language in which they were submitted.
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ASPIRATING LABYRINTH SEAL
BACKGROUND OF THE INVENTION
The present invention relates generally to face seals for rotating machinery,
and more
particularly to aspirating or gas-balanced face seals.
Face seals are used to minimize leakage through a gap between two components,
where such leakage is from a higher pressure area to a lower pressure area.
Such seals
have been used in rotating machinery, for example steam turbines and gas
turbines.
In gas turbines, face seals are used between static hardware, between rotor
and stator
components, and may be used between different rotating components. The gaps or
leakage paths between these different components must be sealed, and the seals
applied need to be able to compensate for variations in the gaps due to
differential
thermal and mechanical component growths during the machine operating cycle.
The variable gap to be sealed is commonly accommodated by either providing a
compliant contact that is maintained between the components, for example using
a
brush seal or a leaf seal, or by creating a complex leakage path which results
in
pressure losses, and thus a reduced flow, for example with a labyrinth seal.
In a
labyrinth seal between rotating and static components, extremes in closure of
the gap
may be accommodated by allowing a rub of rotor labyrinth teeth against a
softer
stator matrix ("abradable"). Due to the initial gaps, and due to contact of
the seals
with adjacent surfaces, none of these seals may meet all performance and
durability
requirements.
As an example, Figure 1 illustrates a portion of a gas turbine engine
including the aft
end of a compressor 10, a diffuser 12, an annular combustor 14, and a high-
pressure
turbine 16 which includes a stationary nozzle 18 and rotating turbine blades
20 carried
by a turbine rotor 22. The compressor 10 is driven by the turbine 16 through a
shaft
24. The space between the hot gas primary flowpath "F" and the shaft 24
defines a
secondary flowpath. For various reasons including maximization of efficiency
and
avoidance of wear, it is desired to control leakage through the secondary
flowpath as
much as possible. This is done by including one or more seal assemblies which
reduce
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or block flow therethrough. In the illustrated example, a compressor discharge
pressure (CDP) seal assembly 26 including a rotating member 28 having a
plurality of
radially-outwardly extending annular seal teeth 30 positioned opposite a
stationary
abradable member 32 is disposed inboard of the diffuser 12. A forward outer
seal
(FOS) 34 including a rotating member 36 having a plurality of radially-
outwardly
extending annular seal teeth 38 positioned opposite a stationary abradable
member 40
is disposed inboard of the turbine nozzle 18. Face seals are used in other
locations in
the engine as well. Both the CDP seal assembly 26 and the FOS 34 are subject
to
deterioration over extended operation as described above.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a face seal with low leakage and high
durability. The
clearance between the seal elements is controlled so that the seal teeth will
not rub
against the seal rotor. This offers efficient sealing both at the time of
manufacture of
the engine and also after extended time in service. It is estimated that
leakage across
the primary seal face may be reduced by approximately 25% compared to prior
art
face seals.
According to one aspect, the invention provides a seal assembly, having: a
first
annular component defining a generally axially facing first primary sealing
surface;
and a second annular component defining a generally axially facing second
primary
sealing surface, the second annular component being mounted in an axially
moveable
relationship to a seal support such that the second primary sealing surface is
disposed
facing the first primary sealing surface; wherein at least one of the first
primary
sealing surface and second primary sealing surface has at least one annular
seal tooth
extending axially therefrom.
According to another aspect of the invention, a seal body for a seal assembly
disposed
about an axis includes: an annular, axially extending portion; a radially
extending
portion defining a primary sealing surface, and cooperating with the axially
extending portion to define a generally L-shaped cross section; and at least
one
annular seal tooth extending axially from the primary sealing surface.
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According to another aspect of the invention, a seal assembly disposed about
an axis
for a gas turbine engine includes: a rotor having an axially facing first
primary sealing
surface; a stationary seal support disposed adjacent the rotor; an annular
seal body
attached to the seal support and disposed between the rotor and the seal
support, the
seal body being axially movable with respect to the seal support, the annular
seal
body including a generally radially extending portion which defines an axially
facing
second primary sealing surface which faces the first primary sealing surface,
and a
generally axially extending portion; wherein at least one of the first primary
sealing
surface and second primary sealing surface has at least one annular seal tooth
extending axially therefrom.
BRIEF DESCRIl'TION OF THE DRAWINGS
The invention may be best understood by reference to the following description
taken
in conjunction with the accompanying drawing figures in which:
Figure 1 is a schematic side view of a portion of a prior art gas turbine
engine;
Figure 2 is a schematic side cross-sectional view of a face seal assembly
constructed
in accordance with an embodiment of the invention;
Figure 3 is a front view of a portion of the face seal assembly of Figure 2;
Figure 4 is an enlarged sectional view of a portion of the face seal assembly
of Figure
2;
Figure 5 is another enlarged sectional view of a portion of the face seal
assembly of
Figure 2; and
Figure 6 is a schematic side cross-sectional view of another exemplary face
seal
assembly.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings wherein identical reference numerals denote the same
elements throughout the various views, Figures 2 and 3 show an exemplary seal
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assembly 42 which seals leakage between an area of relatively high pressure
P(high)
and an area of relatively low pressure P(low). In this particular example, the
seal
assembly 42 takes the place of a compressor discharge pressure (CDP) seal as
described above and is disposed between a core shaft 24' and a diffuser casing
12',
however it is to be understood that the features of the seal assembly 42 may
be used in
any application where a face seal is required. The basic components of the
seal
assembly 42 can include a rotor 44, a stationary seal support 46, and a seal
body 48
(sometimes referred to as a "slider"), all disposed about a longitudinal axis
of the
engine. The rotor 44 is generally disk-shaped and defines a first axially
facing primary
seal surface 50.
The seal support 46 is a nonrotating, axially-extending component and defines
a
radially-facing secondary sealing surface 47. In the illustrated example it is
a
continuous 360 ring, but it could be configured as a segmented annular
structure, or
an array of individual supports. Its aft end 52 has a radially-extending
flange 54
which is secured to the diffuser 12' by one or more fasteners 56. Its forward
end 58
carries one or more spring seats 60. In this example there are five spring
seats equally-
spaced around the perimeter of the seal support 46; however the spring seats
60 could
alternatively be configured as a continuous or segmented annular structure. As
best
seen in Figure 3, the spring seat 60 has a generally cylindrical body 62 and
an arcuate
flange 64 which defines a pair of laterally-extending mounting ears 63. The
spring
seat is secured to the seal support 46 by one or more fasteners 65. The body
62
includes a radially-inwardly extending alignment rail 66 which is received in
a
complementary alignment slot 68 of the seal body 48 to maintain the desired
relative
angular alignment or "clocking" of the seal support 46 and the seal body 48.
The seal
body 48 is thus carried by the seal support 46 such that it can move axially
but not
laterally.
The seal body 48 is an annular component which may be continuous or segmented,
and has a generally L-shaped cross section with a radially extending portion
70 and
an axially extending portion 72.
A plurality of pullback springs 73 are disposed between the spring seats 60
and a
radially-outwardly extending flange 74 of the seal body 48. The aft end of
each
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pullback spring 73 is located in a spring pocket 76 of the flange 74, or other
suitable
locating feature. The pullback springs 73 serve to displace the seal body 48
away
from the rotor 44. This function is described in more detail below. As
illustrated,
there are five compression-type coil springs, but other types and numbers of
springs
may be used.
A secondary seal 78, for example a piston ring of a known type, is disposed in
a
groove 80 in the flange 74, and seals against the axially facing secondary
sealing
surface 47 of the spring support assembly 46. The piston ring may be of a
known
type which provides a continuous (or nearly continuous) circumferential seal.
The
purpose of the secondary seal 78 is to prevent leakage through a path between
the
seal body 48 and the seal support 46, which is subject to the same pressure
differential
as the primary seal, while allowing axial movement of the seal body 48. It
should be
noted that the specific configuration of the seal components and mounting
structure
described above is not critical and may be varied to suit a particular
application
without affecting the functional aspects of the seal assembly 42.
As shown in Figures 4 and 5, the radially extending portion 70 of the seal
body 48
defines an axially facing second primary sealing surface 82. This second
primary
sealing surface 82 is disposed in close proximity to the rotor 44 and faces
the first
primary sealing surface 50. A circumferential seal tooth 84, commonly referred
to as a
"starter seal", extends axially from the outer end of the radially extending
portion 70,
outboard of the rotor 44, and in this particular example, is angled radially
inward.
Fluid passages 86, 87 are formed through the radially extending portion 70 in
a
known manner as required for hydrostatic balancing of the seal body 48 in
operation
(described in more detail below).
The second primary sealing surface 82 includes a planar inner portion 82A and
an
outer portion 82B, separated by an annular groove 83. The outer portion 82B
includes
at least one, and optionally a plurality of annular, radially spaced-apart,
axially-
extending seal teeth 88 which are intended to form a circuitous or tortuous
flow path
for radial fluid flow, to limit flow from the primary flow path to the
secondary flow
path. In the illustrated example, there are two seal teeth 88A and 88B with
tapered
cross-sectional profiles, separated by annular, rounded-bottom grooves 90. The
teeth
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88 could also protrude from a planar surface if desired. It should also be
understood
that the seal tooth configuration could be reversed, i.e. the seal teeth 88
could be
formed on the first primary sealing surface 50 instead.
The configuration of the second primary sealing surface 82 may be defined in
part by
various characteristics of the seal teeth 88, including the number of seal
teeth 88,
their height "H" in an axial direction, their tip width "W", their included
angle in
cross-section "A", their divergence in or out from an axial direction,
referred to as a
slant angle "S" (note that this angle is very small in the illustrated
example), their
radial spacing or pitch "P", and the total radial extent or length of the seal
teeth 88,
denoted "L". Nonlimiting examples of these dimensions are as follows: tooth
height
H about 0.38 mm (0.015 in.), tooth angle A is about 100, the slant angle S
about 00 to
about 450, tip width W about 0.13 mm (0.005 in.) to about 0.76 mm (0.030 in.)
, and
pitch P about 1.3 mm (0.05 in.) to about 3.8 mm (0.15 in.). These values may
be
altered to suit a specific application.
In the illustrated example, the first primary sealing surface 50 has an inner
portion
50A, and an outer portion 50B which is offset axially forward of the inner
portion
50A by a distance "D" (see Figure 2) which is substantially equal to the axial
distance
from the tips of the seal teeth 88 to the inner portion 82A of the second
primary
sealing surface 82. With this configuration, a "step" which resists radial
flow is
defined at the juncture of the inner and outer portions 50A and 50B of the
first
primary sealing surface 50, and during operation distal portions of seal teeth
88 are
disposed axially between the inner and outer portions 50A and 50B of the first
primary sealing surface 50.
In operation, the seal body 48 forms a seal in cooperation with the rotor 44.
The
pullback springs 73 hold the seal body 48 away from the rotor 44 to prevent
contact
between the two components when the engine is stopped. As the engine operating
speed increases, the fluid pressures in the engine's primary and secondary
flowpath
areas increase, and accordingly the seal assembly 42 is subjected to
increasing
pressures acting on its axially facing surfaces, the effect of which is to
cause the seal
body 48 to move towards the rotor 44. By choosing the relative surface areas
of the
different portions of the seal body 48, the number and dimensions of passages
86, 87,
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and the dimensions of the pullback springs 73 in a known manner, the seal
assembly
42 is hydrostatically pressure balanced at a selected operating condition.
Accordingly,
the second primary sealing surface 82 never contacts the first primary sealing
surface
50, but operates with a small axial clearance, for example about 0.05 mm
(0.002 in.)
to about 0.13 mm (0.005 in.). The low operating clearance of the aspirating
seal
assembly 42 combined with the complex flow path through the seal teeth 88 and
between the first and second primary sealing surfaces 50 and 82 minimizes
leakage.
Figure 6 illustrates another seal assembly 142 including a rotor 144, seal
support 146,
and seal body 148 with radially and axially extending portions 170 and 172,
respectively. This seal assembly 142 is similar in construction to the seal
assembly
described but differs in the configuration of the first and second primary
sealing
surfaces 150 and 182. A circumferential starter seal 184, extends axially from
the
outer end of the radially extending portion 170. Fluid passages 186, 187 may
be
formed through the radially extending portion 170 in a known manner as
required for
hydrostatic balancing of the seal body 148.
The second primary sealing surface 182 includes at least one, and optionally a
plurality of annular, radially spaced-apart, axially-extending seal teeth 188
which are
intended to form a circuitous or tortuous flow path in a radial direction. In
the
illustrated example, there are three seal teeth 188A, 188B, and 188C with
tapered
cross-sectional profiles, separated by annular, rounded-bottom grooves 190.
The characteristics of the seal between the first and second primary sealing
surfaces
150 and 182 may be altered to suit a specific application in a manner similar
to that
described above for the seal assembly 48.
An annular seal groove 92 with a rounded bottom is formed in the first primary
sealing surface 150. The corresponding seal tooth 188C has a greater height in
the
axial direction than the other seal teeth 188A and 188B, and will protrude
into the seal
groove 92 during operation to further reduce leakage.
These seal assemblies offer the complex leakage path of a labyrinth seal, and
thus
reduce leakage compared to a flat face seal. However, in contrast to prior art
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labyrinth seals which can rub against the adjacent components, the clearance
between
the seal elements is controlled so that the seal teeth will not rub against
the seal rotor.
This offers efficient sealing both at the time of manufacture of the engine
and also
after extended time in service.
The foregoing has described a face seal assembly. While specific embodiments
of the
present invention have been described, it will be apparent to those skilled in
the art
that various modifications thereto can be made without departing from the
spirit and
scope of the invention.
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