U.S. patent application number 09/731907 was filed with the patent office on 2002-06-13 for bucket tip clearance control system.
Invention is credited to Schroder, Mark Stewart.
Application Number | 20020071762 09/731907 |
Document ID | / |
Family ID | 24941404 |
Filed Date | 2002-06-13 |
United States Patent
Application |
20020071762 |
Kind Code |
A1 |
Schroder, Mark Stewart |
June 13, 2002 |
Bucket tip clearance control system
Abstract
A bucket tip clearance control system forms part of a
turbomachinery apparatus including a casing, an outer shroud
coupled with the casing, and an inner shroud coupled with the outer
shroud. The tip clearance control system includes a flow circuit
for a thermal medium defining a flow path within the outer shroud.
A thermal medium source delivers the thermal medium to the flow
circuit in a predefined condition according to operating parameters
of the turbomachinery apparatus. The temperature of the outer
shroud is controlled according to the predefined condition of the
thermal medium. By accurately controlling the temperature of the
outer shroud, bucket tip clearance can be controlled and optimized
during all of the various operation stages of turbomachinery.
Inventors: |
Schroder, Mark Stewart;
(Hendersonville, NC) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201
US
|
Family ID: |
24941404 |
Appl. No.: |
09/731907 |
Filed: |
December 8, 2000 |
Current U.S.
Class: |
415/173.2 ;
415/173.3 |
Current CPC
Class: |
F01D 11/24 20130101 |
Class at
Publication: |
415/173.2 ;
415/173.3 |
International
Class: |
F01D 011/24 |
Claims
What is claimed is:
1. A bucket tip clearance control system that forms part of a
turbomachinery apparatus including a casing, an outer shroud
coupled with the casing, and an inner shroud coupled with the outer
shroud, the tip clearance control system comprising: a flow circuit
for a thermal medium, the flow circuit defining a flow path within
the outer shroud; and a thermal medium source in fluid
communication with the flow circuit, the thermal medium source
delivering the thermal medium to the flow circuit in a predefined
condition according to operating parameters of the turbomachinery
apparatus, wherein a temperature of the outer shroud is controlled
according to the predefined condition of the thermal medium.
2. A bucket tip clearance control system according to claim 1,
wherein the outer shroud of the turbomachinery apparatus comprises
an upper half secured to a lower half at a split line, and wherein
the flow circuit comprises at least two cavities in the outer
shroud, one of the cavities being disposed in a vicinity of the
split line.
3. A bucket tip clearance control system according to claim 2,
wherein the flow circuit comprises a first flow path within the
upper half of the outer shroud and a second flow path within the
lower half of the outer shroud, and wherein the flow circuit
comprises at least two cavities in each of the first flow path and
the second flow path, one of the cavities in each of the first and
second flow paths being disposed in a vicinity of the split
line.
4. A bucket tip clearance control system according to claim 1,
wherein the flow circuit comprises four cavities in the outer
shroud.
5. A bucket tip clearance control system according to claim 4,
wherein the four cavities communicate via at least one hole from
cavity to cavity.
6. A bucket tip clearance control system according to claim 5,
wherein the four cavities communicate via a series of holes from
cavity to cavity.
7. A bucket tip clearance control system according to claim 1,
wherein the operating parameters comprise steady state
turbomachinery operation and transient state turbomachinery
operation.
8. A turbomachinery apparatus comprising: a first stage bucket
without a bucket shroud; an inner stator shroud disposed adjacent
the first stage bucket defining a bucket tip clearance between the
inner stator shroud and the first stage bucket; an outer stator
shroud supporting the inner stator shroud for relative radial
movement; an outer casing coupled with the outer stator shroud; and
a bucket tip clearance control system for controlling the bucket
tip clearance, the tip clearance control system comprising (1) a
flow circuit for a thermal medium, the flow circuit defining a flow
path within the outer stator shroud, and (2) a thermal medium
source in fluid communication with the flow circuit, the thermal
medium source delivering the thermal medium to the flow circuit in
a predefined condition according to operating parameters of the
turbomachinery apparatus, wherein a temperature of the outer stator
shroud is controlled according to the predefined condition of the
thermal medium.
9. A turbomachinery apparatus according to claim 8, wherein the
outer stator shroud comprises an upper half secured to a lower half
at a split line, and wherein the flow circuit comprises at least
two cavities in the outer stator shroud, one of the cavities being
disposed in a vicinity of the split line.
10. A turbomachinery apparatus according to claim 9, wherein the
flow circuit comprises a first flow path within the upper half of
the outer stator shroud and a second flow path within the lower
half of the outer stator shroud, and wherein the flow circuit
comprises at least two cavities in each of the first flow path and
the second flow path, one of the cavities in each of the first and
second flow paths being disposed in a vicinity of the split
line.
11. A turbomachinery apparatus according to claim 8, wherein the
flow circuit comprises four cavities in the outer stator
shroud.
12. A method of controlling bucket tip clearance in a
turbomachinery apparatus including a casing, an outer shroud
coupled with the casing, and an inner shroud coupled with the outer
shroud, the method comprising: providing a flow circuit for a
thermal medium, and defining a flow path via the flow circuit
within the outer shroud; delivering the thermal medium to the flow
circuit in a predefined condition according to operating parameters
of the turbomachinery apparatus; and controlling a temperature of
the outer shroud according to the predefined condition of the
thermal medium.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to land-based, i.e.,
industrial gas turbines and, more particularly, to a gas turbine
bucket tip clearance control system including a flow circuit within
a turbine outer shroud that controls a temperature of the outer
shroud via a thermal medium.
[0002] Hot gas path components in gas turbines typically employ air
convection and air film techniques for cooling surfaces exposed to
high temperatures. High pressure air is conventionally bled from
the compressor, and the energy of compressing the air is lost after
the air is used for cooling. In current heavy duty gas turbines for
electric power generation applications, the stationary hot gas path
turbine components are attached directly to massive turbine housing
structures, and the shrouds are susceptible to bucket tip clearance
rubs as the turbine casing thermally distorts. That is, the thermal
growth of the turbine casing during steady state and transient
operations is not actively controlled, and bucket tip clearance is
therefore subject to the thermal characteristics of the turbine.
Bucket tip clearance in these heavy duty industrial gas turbines is
typically determined by a maximum closure between the shrouds and
the bucket tips (which usually occurs during a transient) and all
tolerances and unknowns associated with steady state operation of
the rotor and stator.
[0003] In some turbine designs, the stage 1 bucket is unshrouded
because of complex aerodynamic loading and the stress carrying
capability of the bucket. That is, the stage 1 bucket tip has no
sealing mechanisms to prevent hot gas from flowing over the bucket
tip. It is desirable to maintain a minimum clearance between the
bucket tip and the turbine inner shroud so that an amount of hot
gas flow that bypasses the turbine (and therefore is not expanded
for work) is minimized.
BRIEF SUMMARY OF THE INVENTION
[0004] In an exemplary embodiment of the invention, a bucket tip
clearance control system forms part of a turbomachinery apparatus
including a casing, an outer shroud in a slip fit configuration
with the casing, and an inner shroud coupled to the outer shroud.
The tip clearance control system includes a flow circuit for a
thermal medium, wherein the flow circuit defines a flow path within
the outer shroud. A thermal medium source is provided in fluid
communication with the flow circuit and delivers the thermal medium
to the flow circuit in a predefined condition according to
operating parameters of the turbomachinery apparatus, such as
steady state operation and transient state operation. The
temperature of the outer shroud is controlled according to the
predefined temperature conditioning of the thermal medium.
[0005] Preferably, the outer shroud of the turbomachinery apparatus
includes an upper half secured to a lower half at the horizontal
engine split line. In this context, the flow circuit may include at
least two cavities in the outer shroud, one of the cavities being
disposed adjacent the split line. The flow circuit may include a
first flow path within the upper half of the outer shroud and a
second flow path within the lower half of the outer shroud. In this
context, the flow circuit preferably includes at least two cavities
in each of the first flow path and the second flow path, one of the
cavities in each of the first and second flow paths being disposed
adjacent the split line. In one arrangement, the flow circuit
includes four cavities in the outer shroud. These cavities
preferably communicate via at least one hole from cavity to cavity
or via an array of metering holes from one cavity to another
cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a cross-sectional view through a portion of a gas
turbine, showing the turbine outer casing, outer shroud, inner
shroud and first stage bucket tip;
[0007] FIG. 2 is a schematic illustration of the tip clearance
control system of the invention;
[0008] FIG. 3 is a schematic illustration of an upper half flow
circuit; and
[0009] FIGS. 4 and 5 illustrate the upper half flow circuit shaped
corresponding to an upper half of the outer shroud.
DETAILED DESCRIPTION OF THE INVENTION
[0010] Different gas turbine models incorporate different
components for desired results, operation and the like. One design
includes inner and outer shells with four stages of the inner shell
mounting the first and second stage nozzles as well as the first
and second stage shrouds, while the outer shell mounts the third
and fourth stage nozzles and shrouds. An example of such a turbine
design is described in U.S. Pat. No. 6,082,963. An alternative
turbine design, which is the subject of the present invention, does
not include inner and outer shells, but rather includes an outer
casing, an outer stator shroud, and an inner stator shroud disposed
adjacent a first stage bucket, which in this design is unshrouded.
With reference to FIG. 1, the unshrouded first stage bucket is
shown at 12. The gas turbine 10 includes an inner stator shroud 14
disposed adjacent the first stage bucket 12 defining a bucket tip
clearance 16 between the inner stator shroud 14 and the first stage
bucket 12. An outer stator shroud 18 supports the inner stator
shroud 14 radially and axially by hooks 24 and circumferentially by
pins 20 or the like. An outer casing 22 is coupled with the outer
stator shroud 18. One method of coupling the outer shroud 18 to the
turbine casing 22 is a pin scheme similar to that of the
inner/outer shell design noted in the patent referenced above.
Using this method, the first stage turbine nozzle and shroud can be
removed and replaced without removing the entire rotor structure.
Another method of attaching the outer shroud 18 to the turbine
casing uses transverse hooks 24 in the turbine case 22 and the
outer shroud 18. These hooks 24 have ample clearance to accommodate
the radial and circumferential relative motion between the casing
22 and the shroud 18. This method allows radial expansion with ease
of assembly and attachment. Small spring-loaded pins 26 can be
installed through the turbine casing 22 to hold down the outer
shroud 18 and reduce vibrations. The assembly process would be to
install a stage 2 nozzle hanger 28 into the turbine casing 22, then
lower an outer shroud ring assembly of the outer shroud 18 over the
transverse hooks 24 until it rests on the nozzle hanger 28. Of
course, the turbine casing can be coupled with the outer shroud,
and similarly the outer shroud coupled with the inner shroud, in
any known manner accommodating relative radial and circumferential
motion between the casing 22 and the shroud 18. Since the specific
coupling between these components does not form part of the present
invention, additional details thereof will not be further
described.
[0011] The outer shroud 18 of the invention is modified from its
known construction to accept externally conditioned air (or other
suitable fluid medium) flow. As shown in FIG. 2, the external
source of air flow comprises a clearance control skid 30 that
includes heat exchange components and the like to effect
temperature conditioned fluid flow. In this context, the heat
exchange components of the clearance control skid 30 can supply
cooled air flow or heated air flow according to turbine operating
conditions (discussed below). The air flow is conditioned to
control the temperature of the outer shroud 18 and thus its radial
growth. When the radial position of the outer shroud 18 and thus
its attached inner shroud 14 can be externally controlled
independent of gas turbine operation, the resulting tip clearance
16 can be chosen to provide optimum turbine efficiency and power
generation with minimum risk of rubbing during transient operation
(start-up, cool-down, hot restart, etc.).
[0012] The outer shroud 18 is preferably formed of two half ring
pieces that are bolted together at each horizontal joint and
include cloth seals or the like for preventing leakage to form a
complete ring encircling the bucket tip circumference. The outer
shroud 18 may be fabricated from machined forged plates that are
welded together. As an alternative, the outer shroud can be cast,
which would minimize machining costs. The size, material and ease
of core access makes the outer shroud 18 suitable for a casting
process.
[0013] High pressure air bled from the compressor existing above
the stage 1 nozzle inlets provides flow into tubes 32 via scallops
34 machined into the side of the outer shroud 18. A metering
orifice (not shown) may be disposed at the bottom of the supply
holes just prior to entering the inner shroud supply plenum 36.
Preferably, the size and number of scallops 34, flow tubes 32 and
the subsequent metering orifice diameter are optimized to closely
match design requirements. An upper leaf seal 38 covers most of the
circumference of the outer shroud 18, except locally at the
horizontal engine split line joint, where bolting of the two halves
of the outer shroud 18 occurs, thus sealing compressor discharged
air from leaking aft.
[0014] Externally supplied flow from the clearance control skid 30
provides temperature conditioned air into the outer shroud 18 from
suitable connectors that enable fluid flow between components. One
such suitable connector is a so-called "spoolie" that is described
in, for example, commonly owned U.S. Pat. No. 5,593,274, the
contents of which are hereby incorporated by reference. The
spoolies 40 or like connectors penetrate the turbine casing 22 at
or near a top dead center (TDC) position and a bottom dead center
(BDC) position of the engine. In a preferred configuration, four
spoolies 40 are included, one at each inlet and exit at both TDC
and BDC.
[0015] With continued reference to FIG. 1 and with reference to
FIGS. 3 and 4, a closed circuit 42 for conditioned air from the
clearance control skid 30 is defined by a plurality of cavities
within the outer shroud 18. The flow circuit 42 defines a flow path
within the outer shroud for the conditioned flow from the clearance
control skid 30. As discussed above, since the outer shroud 18
includes an upper half secured to a lower half at a split line,
each half of the outer shroud 18 includes a separate inlet and
outlet for conditioned flow and separate flow paths, respectively.
Although the inlets to the upper and lower halves of the outer
shroud 18 are separate, all conditioned flow is preferably provided
by a single clearance control skid 30, ensuring that uniform
temperature conditioned flow is supplied to both halves of the
shroud 18. This prevents detrimental distortion of the shroud 18
due to non-uniform temperature conditioning fluid medium.
Alternatively, multiple clearance control skids 30 could be used to
supply each of the upper or lower halves of the shroud 18. Since
the respective flow circuits of the upper and lower halves of the
outer shroud 18 are substantially identical, the flow circuit 42 in
the upper half of the outer shroud 18 only will be described.
[0016] The conditioned flow from the clearance control skid 30
enters the flow circuit through the spoolie 40 at TDC (and BDC).
The flow is split at the inlet 50 (FIGS. 4 and 5) by a component 51
that extends from the inlet 50 locally to the bottom inlet cavity
of 52. The conditioning flow is then sent circumferentially via 52
nearly to each horizontal joint within each outer shroud half. The
flow is ported through one or more holes from a first end cavity 54
to a second end cavity 56. More than one hole may be used for
porting flow between cavities along with other small diameter holes
farther circumferentially back in the flow path to accommodate
casting core support. Alternatively, a large slot may connect the
two end cavities. The flow in the second end cavity 56 is then
directed circumferentially back toward TDC via 58 to a third cavity
60 at TDC again through one or more large holes or series of
smaller holes. The flow path continues from TDC back to the
horizontal split line of the engine within the third cavity 60 via
62 and passes from the third cavity 60 to a fourth cavity 64. The
flow travels back up to TDC in the fourth cavity 64 via 66, which
acts as a heat exchanger to the first cavity 54, the second cavity
56 and the third cavity 60 to minimize thermal gradients and
overall fluid heat up. Thermal gradients would cause detrimental
distortions in the shroud 18 and defeat the purpose of creating a
uniformly round static structure to encircle the rotating blades or
buckets, and provide an optimized, performance enhancing tip
clearance. Finally, the flow exits the outer shroud 18 through a
slot outlet 68 that is circumferentially out of plane with the
inlet spoolies at TDC, i.e., at the same radial diameter and axial
station, just moved circumferentially (e.g., 15 degrees) from TDC.
The flow is collected in an outlet spoolie and then piped back to
the clearance control skid 30 where the closed loop flow circuit
starts over. When the flow in the second cavity 56 follows
circumferentially back to TDC, the flow acts as a log mean
temperature difference heat exchanger within the outer shroud 18.
That is, the small higher velocity center cavities act as buffering
cavities between the large low velocity cold cavity at the back and
the low velocity hot cavity at the front, which if adjacent each
other could create large thermal gradients within the shroud
structure. In flowing back and forth (i.e., top to horizontal) and
back and differing velocities the heat of the internal flow in each
cavity will conduct to the adjacent cavity creating a heat
exchanger between the two cavities and minimizing the given heat up
in any one cavity. The method of calculating these fluid heat ups
is known as log mean temperature difference.
[0017] With the structure of the present invention, internal
passages within the outer shroud define a flow path of a flow
circuit that condition the outer shroud for minimum thermal
gradients (stress) and optimum uniform growth. By assembling the
outer shroud in halves, the occurrences of leakage is reduced as
compared to existing components while allowing the inner shroud to
be positioned optimal to the bucket tip. The clearance control skid
communicating with the flow circuit can provide heated flow during
transients to move the inner shroud away from the rotor.
Subsequently, during steady state operation, the clearance control
skid can controllably supply cooling flow to shrink the tip
clearance thereby improving efficiency and output.
[0018] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiments, it is to be understood that the invention is not to be
limited to the disclosed embodiments, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *