Note: Descriptions are shown in the official language in which they were submitted.
A shroud assemblv for a turbine
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Technical Field
This invention relates generally to a shroud
assembly for a turbine and more particularly to an
expansion control ring mounted in the assembly.
Gas turbine engines, in order to operate
efficiently, must maintain a relatively close tolerance
between the turbine blades and the surrounding engine.
This is necessary in order to~minimize the loss of power
due to expanding gases passing between the end of the
turbine blade and the surrounding shroud. In any turbine
engine, expansion of the turbine wheel relative the engine
; shroud is a problem which must be dealt with during design
and in some cases, during engine operation. While this
invention deals specifically with a gas turbine environ-
ment, it would be equally applicable to a steam turbine
environment.
As noted above, the efficiency of a gas turbine
engine to a great extent depends on the "tightness" of the
` 20 gas stream passing the gasifier turbine. When the hot
gases impinge on the turbine wheel and the extending tur-
bine blades, there is considerable expansion of the turbine
- wheel. In order to maintain the close fit between the
turbine wheel and the surrounding turbine shroud structure,
several actions can be taken. First, it is appropriate to
attempt to ensure that the rate of expansion of the turbine
shroud structure is essentially the same or is
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slightly greater than the rate of expansion of the
turbine wheel. In order ~o achieve this, it is
necessary to design the turbine shroud structure so it
expands equally throughout its entire circumerence.
This type of structure is taught in U.S. Ratent
4,251,185 for "An Expansion Control Ring for a Turbine
: Shroud Assembly" issued to Karl W. Karstensen on
February 17, 1981. While the structure disclosed in
the aforesaid patent permits expansion of the shroud
assembly concentric with the engine axis, it does not
; eliminate the necessity for some sort of additional
clearance between the turbine blades and the shroud to
compensate for transient inconsistencies in the
expansion of the turbine wheel that result in
temporary eccentricities of the turbine wheel during
acceleration or deceleration of the engine.
Accordinyly, in the aforesaid patent an abradable
shroud structure is also utilized. Such a shroud
structure permits contact of the various turbine
blades with the shroud structure without undue injury
to the turbine blades. Specifically, the abradable
portion of the shroud structure is made softer than
the turbine blade so that upon such contact, the
shroud structure is worn away.
In other applications, the tip of the
turbine blade is designed so that a certain amount of
a turbine blade tip will wear away. These type
turbine blades are commonly called "squealer tips"
stemming from the act that the contact of the blade
with the turbine shroud causes a "squeal." A unique
disadvantage of a "squealer tip" blade is that the
cooling passages normally ound in gasi-Eier turbine
blades are easily blocked by the abrasion of the
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turbine tip. Once the cooling passages are blocked in the
turbine blade t the turbine blade is subject to overheating
and subsequent Eailure.
In both the abradable shroud type structure and
the "squealer tip" type structure, the clearance between
the turbine wheel and the shroud structure i9 permanently
increased every time there is contact between the turbine
wheel and the shroud structure that is sufficient to cause
abrading of the metal, either on the shroud structure or
the turbine tip. Not only is there a permanent increase
in clearance; there is a possibility that the abraded
particles may cause damage to the power turbine downstream
of the gasifie turbine.
The foregoing illustrates limitations of the
lS known prior art. Thus, it is apparent that it would be
advantageous to provide an alternative to the prior art.
Disclosure of the Invention
In one aspect of the present invention, there is
provided apparatus for a turbine having a housing defining
~0 an axis and a turbine wheel mounted for rotation in the
housing substantially about the axis; said apparatus
comprising a shroud assembly mounted concentrically about
the axis and around the turbine wheel, and including an
- expansion control ring, and an improvement comprising means
for permitting the expansion control ring to float freely
perpendicularly to the axis to maintain general concen
; tricity with the turbine wheel as the turbine wheel moves
between positions eccentric and concentric to the axis.
The invention also consists of a turbine shroud
assembly having a pair of manifold rings; a spacer ring
positioned between the manifold rings and defining a
plurality of notches on its inner perimeter which has
radius R2, each notch having a depth D and a width W~,
the improvement comprising an expansion control ring which
fLoats between the manifold rings and which has an outer
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perimeter of radius Rl where Rl is less than R2; and
the expansion control ring further having a plurality of
lugs extending outwardly at the outer perimeter thereof,
each of the lugs having a width Wl and a height of H;
where Wl is less than W2 and H is less than or equal
to D.
In previous gas turbine shroud assemblies, it has
- been necessary to include either abradable
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shroud segments or abradable tips on the turbine
blades in order to overcome contact between the
turbine blades and the shroud structure. This
invention permits the shroud structure through a
floating expansion control ring to move eccentrically
to the axis of the engine thereby maintaining
concentricity with the turbine wheel.
The foregoing and other aspects will become
apparent from the following det~iled description of
the invention when considered in conjunction with the
accompanying drawings. It is to be expressly
understood, however, that the drawings are not
intended as a definition of the invention, but are for
the purposes of illustration only.
Brief Description of the Drawin~
Figure 1 i5 a portion of a turbine shroud
assembly which forms an embodiment of the present
` invention.
Figure 2 is a cross sectional view of the
turbine shroud assembly with the associated mounting
elements which form an embodiment of the present
invention along with a turbine blade.
~, Figure 3 is a detailed view of a portion of
the spacer ring and the expansion control ring that
form an embodiment of the present invention.
` Figure 4 includes a sequence of schematic
views of a turbine wheel and the associated shroucl
structure in the present embodiment and the prior art.
Best Mode for Carrying Out the Invention
! 30 Referring to Figure 1, a portion of a
turbine shroud assembly 10 is illustrated. ~ turbine
shroud assembly such as the shroud assembly 10 shown
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in Figure 1 is applicable for use in a turbine engine,
in particular in a gas turbine engine, to surround the
turbine wheel 11 Ishown schematically in Figures 4A E)
which has extending outwardly therefrom a plurality of
blades 12 upon which a stream of hot gases, as
indicated by the arrow G, impinge.
~ s shown in Figure 1, the shroud assembly 10
is comprised of three major portions, a spacer ring
14/ a pair of manifold rings 28 and 30, and an
expansion control ring 20. The spacer ring 14 is the
outermost portion and has formed therein a plurality
of nothces 16, each adapted to receive a corresponding
lug 18 formed on an outwardly extending web 32 of
expansion control ring 20. Expansion control ring 20
has positioned thereon a plurality of shroud segments
22. Shroud segments 22 and expansion control 20
provide the floating interface between the spacer ring
14 which is fixed to the surrounding engine and the
rotating turbine blades 12.
Referring now to Figure 2, it can be seen
; how expansion control ring 20 and the associated
shroud segments 22 are floatlngly affixed to the
engine casing 24. Spacer ring 14 is formed witA a
plurality of holes 26 through which a corresponding
plurality of bolts 27 may be passed. On either side
of spacer ring 14 are manifold rings 28 and 30,
respectively. The purpose of these manifold rings is
more clearly set forth in U.S. Patent No. 4,251,185.
r Suffice it to say these rings in addition to providing
support for the shroud assembly direct cooling air to
the shroud assembly.
As can be seen in Figure 2, bolt 27 passes
;~ through and thus affixes the manifold rings 28 and 30
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and the spacer ring 14 to the engine casing 24.
Spacer ring 14 has a slightly thicker
dimension Tl than the thickness T2 of the web 32 of
expansion control ring 20. This greater thickness
permits movement of expansion control ring 20 relative
the manifold rings 28 and 30 and spacer ring 14.
Referring now to Figure 3, an enlarged view
of a portion of expansion control ring 20 is shown
: relative spacer ring 14. Expansion control ring 20
has an outer perimeter 34 which has a radius R1 while
spacer ring 14 has an inner perimeter 36 having a
. radius R2. It should be apparent that radius R2 is
` greater than radius Rl by a predetermined amoun~.
Similarly, in Figure 3 it can be seen that each lug 18
has a width Wl while each notch 16 has a width W2.
~ Further, the depth of each notch is equal to a
: predetermined amount D, while the height of each lug
is equal to H.
The relationship between each notch and each
lug may now be defined as:
W2 Wl < [(R2 ~ D) - (Rl + H)]
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This clearance relationship results in the
clearance (W2 minus Wl) controlling the radial
movement of the expansion control ring.
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Industrial Applicability
With the parts assembled as set forth above,
the expansion control ring 20 in a gas turbine engine
may move eccentrically as described in the following
paragraphs.
Referring now to Figure 4A, the expansion
control ring as shown schematically as a circle at 20,
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while the spacer ring is also shown schematically as a
circle at 14. A schematic turbine wheel 11 with
associated turbine blades 12 is shown in an at rest
position at Figure 4A. Prior art devices having
abradable shroud segments, such as is described in
U.S. Patent 4,251,185 noted above, will take the
configuration shown in Figure 4B should there be a
lack of concentricity between the turbine shroud 20'
and the turbine wheel 11' upon start up or during
operation of the engine. In particular, in Figure 4B
` it can be seen that the turbine wheel 11' has moved
downwardly so that the associated turbine blades .12'
will come in contact with expansion control ring 20'.
With an abradable structure, the expansion control
ring then becomes out of round as indicated in Figure
4C. This out of round condition will result in loss
of efficiency in the turbine engine due to the
increased gap between the blades and the turbine
wheel, as shown in Figure 4C. It is understood that
the illustrations in Figure 4 are exagge~ated to
better define the problem.
Should there be ~Isquealer tip"-type blades
as described in the background of this invention, the
results will be best illustrated by referring to
~igure 4D wherein the actual blades 12'' are reduced
in length about the entire turbine wheel. Should this
occur, the loss of efficiency is much greater than in
the embodiment shown in Fiyure 4C. In Figure 4D, the
"squealer tip" blades 12 " have been worn away so that
~he clearance between the blades 12'' and the
expansion control ring 20 " is increased
concentrically. Of course, the spacer ring 14 " and
the turbine wheel 11" remain unaffected.
The present invention is shown schematically
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in ~igure 4E wherein the turbine wheel 11 is eccentric
to the housing as represented by spacer ring 14. As a
result of the eccentricity of the turbine wheel 11,
initial contact is made between the blades 12 and the
S shroud segment 22 best illustrated in Figure 2 with a
resultant movement of the expansion control ring 20
from its concentric position represented by the
phantom circle 200, to an eccentric position relative
the engine axis ~0, but maintaining concentricity with
the turbine wheel 11. As the turbine wheel moves back
into a concentric relationship with the turbine axis
40, the expansion control ring 20 is free to move or
float relative the turbine wheel. Thus, lugs 18 and
notches 20 provide means for e~pansion control ring 20
to maintain general concentricity with turbine wheel
11, even though turbine wheel 11 is running
eccentrically to the engine axis ~0.
~ eferring now to Fig~res 1 and 3, it can be
seen that the expansion control ring 20 is free to
move in a radial direction and to a certain extent, to
rotate axially. Concurrently, sufficient gap is
maintained between the expansion control ring 20 and
- the spacer ring 1~ so that any variation in the rate
of expansion between the expansion control ring and
the spacer ring 14 may take place without serious
injury to the internal portions of the engine. It is
appropriate to make the clearances between the sides
of the lugs 18 and notches 16 less than the clearances
between the end of the notches 16 and lugs 18 so that
3~ the radial movement of the expansion ring will be
controlled by the side clearance. It is also
important to understand that the clearance between the
sides of the lugs and notches is not made so large so
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that there will be excessive movement, rather the fit
should be sufficiently tight so that movement is
; dampened between the two portions.
~he aforedescribed invention has particular
advantages in that it overcomes the necessity for
using either an abradable shroud or a "squealer tip"
as described in the background of this invention. In
the past, abradable shrouds have been subject to
oxidatîon and corrosion due to the high gas
temperatures and have ~urther suffered from erosion
due to the high gas velocities after a short period of
time. Thus, in addition to the possibility of the
- permanen~ eccentricity as shown in Figure 4C above,
the actual clearance between the turbine wheel and the
expansion control ring 20 will increase due to this
erosion and corrosion problem. When the "squealer
tips" are used on all blades of the turbine wheel,
there is a permanent increased clearance between all
the blades and the shroud structure as shown in Figure
~0 ~D. Furthermore, when there are cooling passages
located in the turbine blades 12, the abrasion around
- the end of the "squealer tip" may block off the air
flow through the turbine blade, thereby having a
deleterious effect on the cooling of the engine
itself.
While the present invention has been
described with reference to a preferred embodiment, it
will be understood by those skilled in the art that
various changes may be made and equivalents may be
substituted for elements thereof without departing
from the scope of the invention. In addition, many
modifications may be made to adapt a particular
si-tuation or material to the teachings of the
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invention without departing from the scope thereof.
Therefore, it is intended that the invention not be
limited to the particular embodiments disclosed, but
that the invention will include all embodiments
falling within the intended scope of the claims.
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