U.S. patent application number 12/706866 was filed with the patent office on 2010-09-16 for steam turbine.
This patent application is currently assigned to HITACHI, LTD.. Invention is credited to Shunsuke MIZUMI, Kenichi MURATA, Hideki ONO.
Application Number | 20100232966 12/706866 |
Document ID | / |
Family ID | 41849497 |
Filed Date | 2010-09-16 |
United States Patent
Application |
20100232966 |
Kind Code |
A1 |
ONO; Hideki ; et
al. |
September 16, 2010 |
STEAM TURBINE
Abstract
In a steam turbine including a turbine rotor 7; a moving blade 1
secured to the turbine rotor 7; a shroud 3 provided on an outer
circumferential side distal end of the moving blade 1; and an outer
circumferential side stationary wall 4 internally embracing the
turbine rotor 7 and forming an outer circumferential side passage
wall of a steam path, the shroud 3 has an inner circumferential
surface 14 so formed that a moving blade outlet flare angle
.alpha..sub.3 is greater than a moving blade inlet flare angle
.alpha..sub.2; an angle .alpha..sub.1 formed between the outer
circumferential side stationary wall 4 and the turbine central axis
50 is generally equal to the moving blade inlet flare angle
.alpha..sub.2; and an angle .alpha..sub.4 formed between the outer
circumferential side stationary wall and an turbine central axis 50
is generally equal to the moving blade outlet flare angle
.alpha..sub.3.
Inventors: |
ONO; Hideki; (Mito, JP)
; MIZUMI; Shunsuke; (Hitachinaka, JP) ; MURATA;
Kenichi; (Hitachi, JP) |
Correspondence
Address: |
MATTINGLY & MALUR, P.C.
1800 DIAGONAL ROAD, SUITE 370
ALEXANDRIA
VA
22314
US
|
Assignee: |
HITACHI, LTD.
Tokyo
JP
|
Family ID: |
41849497 |
Appl. No.: |
12/706866 |
Filed: |
February 17, 2010 |
Current U.S.
Class: |
416/179 |
Current CPC
Class: |
F05D 2220/31 20130101;
F05D 2250/232 20130101; F01D 5/143 20130101; F01D 5/225
20130101 |
Class at
Publication: |
416/179 |
International
Class: |
F01D 5/22 20060101
F01D005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2009 |
JP |
2009-062229 |
Claims
1. A moving blade for a steam turbine, the moving blade having a
shroud formed at an external circumferential side distal end,
wherein the shroud has an inner circumferential surface so formed
that: a moving blade outlet flare angle is greater than a moving
blade inlet flare angle; the moving blade inlet flare angle is
generally equal to a moving blade upstream side flare angle of an
outer circumferential side stationary wall adjacently provided on
an upstream side of the shroud; and the moving blade outlet flare
angle is generally equal to a moving blade downstream side flare
angle of the outer circumferential side stationary wall adjacently
provided on a downstream side of the shroud.
2. The moving blade according to claim 1, wherein an outer
circumferential surface of the shroud has a surface including a
downstream side distal end and being parallel to a turbine central
axis, and a distance between an upstream side distal end of the
outer circumferential surface and the turbine central axis is
smaller than a distance between the downstream side distal end of
the outer circumferential surface and the turbine central axis.
3. The moving blade according to claim 1, wherein an outer
circumferential surface of the shroud is formed of a surface
generally parallel to a turbine central axis, and the shroud is
hollow.
4. The moving blade according to claim 2, wherein the moving blade
constitutes a final stage of a low-pressure turbine.
5. The moving blade according to claim 3, wherein the moving blade
forms an extraction stage adjacently provided on an upstream side
of an extraction channel adapted to extract steam from a steam
passage.
6. A steam turbine comprising: a turbine rotor; a moving blade
secured to the turbine rotor; a shroud provided on an outer
circumferential side distal end of the moving blade; and an outer
circumferential side stationary wall internally embracing the
turbine rotor, the stationary wall forming an outer circumferential
side passage wall of a steam path; wherein the shroud has an inner
circumferential surface so formed that: a moving blade outlet flare
angle is greater than a moving blade inlet flare angle; the moving
blade inlet flare angle is generally equal to a moving blade
upstream side flare angle of an outer circumferential side
stationary wall adjacently provided on an upstream side of the
moving blade; and the moving blade outlet flare angle is generally
equal to a moving blade downstream side flare angle of the outer
circumferential side stationary wall adjacently provided on a
downstream side of the moving blade.
7. The steam turbine according to claim 6, wherein an outer
circumferential surface of the shroud has a surface including a
downstream side distal end and being parallel to a turbine central
axis, and a distance between an upstream side distal end of the
outer circumferential surface and the turbine central axis is
smaller than a distance between the downstream side distal end of
the outer circumferential surface and the turbine central axis.
8. The steam turbine according to claim 6, wherein an outer
circumferential surface of the shroud is formed of a surface
generally parallel to a turbine central axis, and the shroud is
hollow.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a turbine moving blade
applied to a steam turbine.
[0003] 2. Description of the Related Art
[0004] In general, a steam turbine includes a plurality of stages
each composed of a moving blade and a stator vane axially provided
on a turbine rotor. In addition, the steam turbine is provided, on
an outer circumferential portion of an outlet of its final stage,
with a flow guide portion adapted to lead steam into an exhaust
hood. Such a steam turbine is operated such that the stator vane
formed as a restrictive passage accelerates steam to increase its
kinetic energy and the moving blade converts the kinetic energy
into rotational energy to generate power. Then, some of the steam
is turned in an extraction channel in a rotor-radial direction and
rest of the steam is discharged into the exhaust hood. See
JP-2003-27901-A.
[0005] Since shortening the length of a turbine shaft in such a
steam turbine can reduce a difference in an axial thermal extension
of the turbine rotor, the effects of reducing loss resulting from
leakage flow and improving reliability in turbine shaft vibration
can be expected.
SUMMARY OF THE INVENTION
[0006] The axial length of a low-pressure turbine depends on a
position at which the radial turning of a flow guide provided at
the downstream end of an external stationary wall of a final stage
terminates. Therefore, if the curvature of the flow guide portion
is increased, the radial turning of the flow guide portion can be
terminated on the more upstream side in a steam flow direction
(hereinafter, simply described as the upstream side). Thus, the
length of the turbine shaft can be reduced. However, the exhaust
chamber is formed as a diffuser path, which has an inverse pressure
gradient. Because of this, if the curvature of the flow guide
portion is increased to increase a flare angle of the flow guide
portion, separation of a steam flow from the flow guide portion is
likely to occur, which may probably cause a flow loss.
Incidentally, the above-mentioned flare angle means an angle formed
between a steam passage outer circumferential wall and a turbine
central axis.
[0007] In addition, it is necessary to radially turn the stream
flow in a shorter shaft length by reducing the turbine shaft
length. Therefore, in an extraction stage provided on the upstream
side of the low-pressure turbine, a deviation is increased between
a flare angle of a shroud inner circumferential surface of a blade
constituting the extraction stage and a flare angle of an outer
circumferential side stationary wall inner circumferential surface
adjacently provided on the moving blade downstream side. In
addition, a distance between the moving blade outlet and the
extraction path is reduced and the steam flow is radially turned in
a shorter distance between the moving blade outlet and the
extraction path inlet. Thus, a separation swirl is likely to occur
at the extraction path inlet, which may probably cause a flow
loss.
[0008] Accordingly, it is an object of the present invention to
provide a steam turbine moving blade that can reduce the length of
a turbine shaft while suppressing occurrence of a loss resulting
from flow separation and from a secondary flow to suppress a
decrease in turbine efficiency.
[0009] To solve the above object, according to an aspect of the
present invention, there is provided a moving blade for a steam
turbine, the moving blade having a shroud formed at an outer
circumferential side distal end, wherein the shroud has an inner
circumferential surface so formed that a moving blade outlet flare
angle is greater than a moving blade inlet flare angle, the moving
blade inlet flare angle is generally equal to a moving blade
upstream side flare angle of an outer circumferential side
stationary wall adjacently provided on an upstream side of the
shroud; and the moving blade outlet flare angle is generally equal
to a moving blade downstream side flare angle of the outer
circumferential side stationary wall adjacently provided on a
downstream side of the shroud. More specifically, the moving blade
and the steam turbine are each configured as recited in
corresponding claims.
[0010] The present invention can reduce the length of a turbine
shaft while suppressing occurrence of a loss resulting from flow
separation and from a secondary flow to suppress a decrease in
turbine efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A and 1B are cross-sectional views of an essential
portion of a final stage of a steam turbine according to a first
embodiment of the present invention.
[0012] FIGS. 2A and 2B are cross-sectional views of an essential
portion of a final stage of a steam turbine according to a second
embodiment of the present invention.
[0013] FIGS. 3A and 3B are cross-sectional views of an essential
portion of an extraction stage of a steam turbine according to a
third embodiment of the present invention.
[0014] FIGS. 4A, 4B and 4C are cross-sectional views of an
essential portion of an extraction stage of a traditional steam
turbine.
[0015] FIGS. 5A and 5B are cross-sectional views of an essential
portion of an extraction stage of a steam turbine according to a
fourth embodiment of the present invention.
[0016] FIGS. 6A and 6B are cross-sectional views of an essential
portion of an extraction stage of a steam turbine according to a
fifth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Preferred embodiments of the present invention will
hereinafter be described in detail with reference to the drawings.
Incidentally, like or corresponding elements are denoted with like
reference numerals over the drawings.
[0018] A description is given of a first embodiment of the present
invention. FIG. 1A illustrates a configuration of an essential
portion of a final stage and an exhaust hood of a low-pressure
turbine as viewed from the side. A stator vane 2 and a moving blade
1 are paired to constitute a turbine final stage. An outer
circumferential end of the stator vane 2 is supported by an outer
circumferential side stationary wall 4 and an inner circumferential
end is supported by an inner circumferential side stator wall 5. A
plurality of the stator vanes 2 are provided in a circumferential
direction. On the other hand, a plurality of the moving blades 1
are circumferentially secured to a turbine rotor 7. A shroud 3 is
provided at outer circumferential side distal ends of the moving
blades 1 so as to connect together the plurality of moving blades
provided in a rotor circumferential direction. Types of the shroud
3 include a type in which a plurality of moving blades are
assembled and secured by a single member and a type in which covers
each having an inter-blade pitch are arranged for the respective
moving blades and circumferentially press fitted thereto by torsion
of the blade due to rotation. The shroud 3 used in the present
embodiment may be any one of these types.
[0019] Arrow 51 in FIG. 1A denotes a flow direction of steam in a
steam path 19 defined between the outer circumferential side
stationary wall 4 and the inner circumferential side stationary
wall 5. In the following, a downstream side in a flow direction of
steam is simply called the downstream side and an upstream side in
the flow direction of steam is simply called the upstream side.
[0020] A casing 9 for covering the outer circumferential side
stationary wall 4 is provided on a turbine-radially outer
circumferential side (hereinafter, simply described as the outer
circumferential side) of the outer circumferential side stationary
wall 4. An exhaust hood 12 is defined between the outer
circumferential side stationary wall 4 and the casing 9. A flow
guide portion 11 adapted to lead steam leaving the moving blade 1
to the exhaust hood 12 is formed at the downstream side end portion
of the outer circumferential side stationary wall 4. A bearing cone
10 is provided on a turbine-radially inner circumferential side
(hereinafter, simply called the inner circumferential side) of the
flow guide portion 11. In this way, an annular diffuser path 18 is
defined between the bearing cone 10 and the flow guide portion
11.
[0021] The flow guide portion 11 and the bearing cone 10 are each
bent in the turbine-radial direction. The diffuser path 18
communicates with the exhaust hood 12. Thus, the steam having
passed through the final stage moving blade 1 passes through the
diffuser path 18. While the flowing direction is turned from the
axial direction to the radial direction, the steam decelerates so
that energy according to the deceleration is converted to pressure
to recover pressure. Then, the steam is led to the exhaust hood.
After having led to the exhaust hood 12, the steam is introduced
into a condenser (not illustrated) communicating with the exhaust
hood.
[0022] A description is next given of a structure of the shroud 3.
Hereinafter, a flare angle is defined as an angle formed between an
outer circumferential wall of a steam path 19 and a turbine central
axis 50. The outer circumferential wall of the steam path 19 means
e.g. an inner circumferential wall surface 13 of the outer
circumferential side stationary wall 4, an inner circumferential
surface 14 of the shroud 3 or an inner circumferential surface 15
of the flow guide portion 11.
[0023] Referring to FIG. 1B, the inner circumferential surface 14
of the shroud 3 is formed to be radially smoothly bent so that the
flare angle is gradually increased from the upstream side toward
the downstream side. An angle of a tangential line A (indicated
with a broken line) located at an upstream side end of the inner
circumferential surface 14 with respect to the turbine central axis
50 is referred to as a moving blade inlet flare angle
.alpha..sub.2. An angle of a tangential line B (indicated with a
broken line) extending from a downstream side end of the inner
circumferential surface 14 with respect to the turbine central axis
50 is referred to as a moving blade outlet flare angle
.alpha..sub.3. The inner circumferential surface 14 of the shroud 3
in the present embodiment is formed such that the moving blade
outlet flare angle a.sub.3 is greater than the moving blade inlet
flare angle .alpha..sub.2.
[0024] An angle formed between the inner circumferential surface 13
of the outer circumferential side stationary wall 4 constituting
the final stage and the turbine central axis 50 is referred to as a
moving blade upstream side flare angle .alpha..sub.1. The flow
guide portion 11 is formed to be radially smoothly bent so that the
flare angle is gradually increased from the upstream side toward
the downstream side. An angle formed between the turbine central
axis 50 and a tangential line (indicated with a broken line)
extending from a curvature start point C of the inner
circumferential surface 15 of the flow guide portion 50 is referred
to as a moving blade downstream side flare angle .alpha..sub.4. In
the present embodiment, the inner circumferential surface 14 of the
shroud 3 is formed as below. The moving blade inlet flare angle
.alpha..sub.2 is generally equal to the moving blade upstream side
flare angle .alpha..sub.1 of the outer circumferential side
stationary wall 4 adjacently provided on the moving blade upstream
side. In addition, the moving blade outlet flare angle
.alpha..sub.3 is generally equal to the moving blade downstream
side flare angle .alpha..sub.4 of the floor guide portion 11 of the
outer circumferential side stationary wall adjacently provided on
the moving blade downstream side.
[0025] On the other hand, the outer circumferential surface of the
shroud 3 has an inclination surface 16 on the upstream side and a
parallel surface 17 parallel to the turbine central axis 50 on the
downstream side. A shroud upstream side outer diameter is made
smaller than a shroud downstream side outer diameter. The shroud
upstream side outer diameter is a distance from the turbine central
axis 50 to the upstream side end of the outer circumferential
surface of the shroud 3. The shroud downstream side outer diameter
is a distance from the turbine central axis 50 to the downstream
side end of the outer circumferential surface of the shroud 3.
Incidentally, the inclination angle of the inclination surface 16
is set such that the shroud 3 has a thickness generally uniform
from the upstream side to the downstream side.
[0026] Seal fins 6 are provided on the outer circumferential side
stationary wall 4 opposite the parallel surface 17 of the outer
circumferential surface of the shroud 3. This narrows a gap between
the outer circumferential side stationary wall 4 and the shroud 3
to suppress leakage of a steam flow getting around the moving blade
1. Incidentally, in the turbine final stage, a leakage passage area
defined between the seal fins and the shroud is smaller than the
passage area of the moving blade. Therefore, the seal fins 6 may be
provided only on the moving blade outlet side.
[0027] A description is given of a function and effect of the
present embodiment. The inner circumferential surface 14 of the
shroud 3 is formed such that the moving blade inlet flare angle
.alpha..sub.2 is generally equal to the moving blade upstream side
flare angle .alpha..sub.1 and the moving blade outlet flare angle
.alpha..sub.3 is generally equal to the moving blade downstream
side flare angle .alpha..sub.4. Therefore, steam flows parallel to
the inner circumferential surface from the outer circumferential
side stationary wall 4 to the shroud 3. The steam flows parallel to
the inner circumferential surface from the shroud 3 to the flow
guide portion 11. The flow of steam is radially smoothly turned
between the stator vane and the moving blade and between the moving
blade and the flow guide portion. Thus, occurrence of a loss
resulting from flow separation and from a secondary flow can be
suppressed, which can suppress the lowering of turbine
efficiency.
[0028] In the present invention, it is preferred that the moving
blade inlet flare angle .alpha..sub.2 be equal to the moving blade
upstream side flare angle .alpha..sub.1 and the moving blade outlet
flare angle .alpha..sub.3 be equal to the moving blade downstream
side flare angle .alpha..sub.4. However, if respective deviations
of the flare angles are generally equal to each other, i.e., if
each of the deviations falls within 5.degree., achievement of the
effect of the present invention can be expected.
[0029] The internal circumferential surface 14 of the shroud 3 is
formed such that the moving blade outlet flare angle .alpha..sub.3
is greater than the moving blade inlet flare angle .alpha..sub.2 so
as to allow also the moving blade 1 to take on a share of the
radial turning of steam. The steam on the moving blade shroud side
of the low-pressure turbine final stage has high-speed and large
kinetic energy. Conventionally, this high-speed steam has radially
been turned mainly by the diffuser path 18 and the exhaust hood 12
having an inverse pressure gradient. In the present invention, the
radial turning of steam is shared by the inside of the moving blade
at which steam speed is slower and the kinetic energy is smaller
than at the outlet of the moving blade. Therefore, a radially
turning amount of steam flow in the exhaust hood 12 can be made
smaller than ever before. Consequently, if the radius of curvature
of the inner circumferential surface 15 of the flow guide portion
11 is equal to the traditional one, i.e., if separation occurrence
potential of the passage shape is generally equal to the
traditional one, the shaft length to a position where steam is
radially turned can be reduced.
[0030] The increase in flare angle due to the reduced length of the
turbine shaft increases the curvature of the shroud 3, i.e.,
increases the difference between the moving blade outlet flare
angle .alpha..sub.3 and the moving blade inlet flare angle
.alpha..sub.2. However, the inclination surface 16 is formed on the
upstream side of the outer circumferential surface of the shroud 3
and the parallel surface 17 is formed on the downstream side in
parallel to the central axis. This makes the upstream side outer
diameter smaller than the downstream side outer diameter. The
above-mentioned upstream side outer diameter corresponds to the
distance from the turbine central axis 50 to the upstream side
distal end of the outer circumferential surface of the shroud 3. In
addition, the above-mentioned downstream side outer diameter
corresponds to the distance from the turbine central axis 50 to the
downstream side distal end of the outer circumferential surface of
the shroud 3. Thus, the weight increase of the shroud per se can be
suppressed. As a result, it is possible to prevent the strength
reliability of the turbine blade from lowering.
[0031] A description is next given of a second embodiment of the
present invention. FIG. 2A illustrates a structure of an essential
portion of a final stage and an exhaust hood of a low-pressure
turbine as viewed from the side. Incidentally, the same elements as
in the first embodiment are denoted with like reference numerals
and their explanations are omitted.
[0032] In the present embodiment, an inner circumferential surface
downstream side end portion of an outer circumferential side
stationary wall 4 supporting a stator vane 2 of a final stage is
formed parallel to a turbine central axis 50. In addition, a moving
blade upstream side flare angle .alpha..sub.1 is formed at an
approximately 0 degree at an outlet of the outer circumferential
side stationary wall 4. On the other hand, an inner circumferential
surface of a shroud 3 is composed of an upstream side parallel
surface 21 parallel to the turbine central axis 50 and a downstream
side inclination surface 22. Incidentally, the upstream side end
portion of the inner circumferential surface is included in the
parallel surface 21 and the downstream side end portion is included
in the inclination surface 22.
[0033] A moving blade inlet flare angle and a moving blade outlet
flare angle are defined as below. If the upstream side end portion
of the inner circumferential surface is included in a plane
(precisely, curve-shaped in a circumferential direction and
straight line-shaped in an axial direction, hereinafter, simply
described as the plane), an angle formed between the plane
including the upstream side end portion and the central axis, i.e.,
an angle formed between a cross-line which the plane including the
upstream side end portion crosses with a turbine meridian plane and
the central axis, is referred to as the moving blade inlet flare
angle. If the downstream side end portion of the inner
circumferential surface is included in the plane, an angle formed
between the plane including the downstream side end portion and the
central axis i.e., an angle formed between a cross-line which the
plane including the downstream side end portion crosses with a
turbine meridian plane and the central axis, is referred to as the
moving blade outlet flare angle. Thus, in the present embodiment,
an angle formed between the parallel surface 21 and the turbine
central axis 50 is defined as a moving blade inlet flare angle
.alpha..sub.2. An angle formed between the inclination surface 22
and the turbine central axis 50 is defined as a moving blade outlet
flare angle .alpha..sub.3.
[0034] In the present embodiment, the inner circumferential surface
of the shroud 3 is such that the moving blade inlet flare angle
.alpha..sub.2 is generally equal to a moving blade upstream side
flare angle .alpha..sub.1 and the moving blade outlet flare angle
.alpha..sub.3 is generally equal to a moving blade downstream side
flare angle .alpha..sub.4 of a floor guide portion 11. In addition,
the moving blade outlet flare angle .alpha..sub.3 is formed to be
greater than the moving blade inlet flare angle .alpha..sub.2.
Incidentally, as shown in FIG. 2A, the inner circumferential
surface 15 of the floor guide portion 11 is formed to be tilted at
a given angle relative to the central axis from the upstream side
toward the downstream side without being radially bent. In this
case, the tilted angle of the inner circumferential surface 15 of
the floor guide portion 11 is formed as the moving blade downstream
side flare angle .alpha..sub.4.
[0035] The outer circumferential surface of the shroud 3 has an
upper stream side parallel surface 23 being parallel to the turbine
central axis 50 and including an upstream side end, a downstream
side parallel surface 25 being parallel to the turbine central axis
50 and including a downstream side end, and an inclination surface
24 inclined relative to the turbine central axis 50 and connecting
the upstream side parallel surface with the downstream side
parallel surface. The shroud 3 is formed such that its upstream
side outer diameter is smaller than its downstream side outer
diameter. The upstream side outer diameter is a distance from the
turbine central axis 50 to the upstream side distal end of the
outer circumferential surface of the shroud 3. The downstream side
outer diameter is a distance from the turbine central axis 50 to
the downstream side distal end of the outer circumferential surface
of the shroud 3. In addition, the shroud 3 is formed to have a
generally constant thickness from the upstream side to the
downstream side.
[0036] In the present embodiment, the inner circumferential surface
14 of the shroud 3 is formed as below. The moving blade inlet flare
angle .alpha..sub.2 is generally equal to the moving blade upstream
side flare angle .alpha..sub.1. The moving blade outlet flare angle
.alpha..sub.3 is generally equal to the moving blade downstream
side flare angle .alpha..sub.4. Further, the moving blade outlet
flare angle .alpha..sub.3 is greater than the moving blade inlet
flare angle .alpha..sub.2. Therefore, steam 20 flows between the
inner circumferential surface 14 of the shroud 3 and the inner
circumferential surface 13 of the outer circumferential side
stationary wall 4 and between the inner circumferential surface 14
of the shroud 3 and the inner circumferential surface 15 of the
flow guide portion 11, in general parallel to the inner
circumferential surfaces. Thus, occurrence of a loss resulting from
steam flow separation and from a secondary flow can be suppressed,
which can suppress the lowering of turbine efficiency.
[0037] Incidentally, also in the present embodiment, it is
preferred that the moving blade inlet flare angle .alpha..sub.2 be
equal to the moving blade upstream side flare angle .alpha..sub.1
and the moving blade outlet flare angle .alpha..sub.3 be equal to
the moving blade downstream side flare angle .alpha..sub.4.
However, if respective deviations of the flare angles are generally
equal to each other, i.e., if each of the deviations falls within
5.degree., achievement of the effect of the present invention can
be expected.
[0038] It is possible to turn the steam flow in the radial
direction between the inlet and outlet of the moving blade 2. It is
possible to make the amount of radially turning steam flow smaller
than ever before in the exhaust hood 12 including the flow guide
portion 11 having high separation potential due to inverse pressure
gradient. As a result, if the curvature radius of the inner
circumferential surface 15 of the flow guide portion 11 is equal to
the traditional one, i.e., if the separation occurrence potential
of the passage shape is general equal to the traditional one, it is
possible to reduce the shaft length to the radial turn.
[0039] The upstream side outer diameter which is the distance from
the turbine central axis 50 to the upstream side distal end of the
outer circumferential surface of the shroud 3 is made smaller than
the downstream side outer diameter which is the distance from the
turbine central axis 50 to the downstream side distal end of the
outer circumferential surface of the shroud 3. Therefore, even if
the tilted angle of the inclination surface 22 is increased, it is
possible to suppress the increase in the weight of the shroud per
se. Consequently, it is possible to prevent the strength
reliability of the turbine blade from lowering.
[0040] A description is given of a third embodiment of the present
invention. FIG. 3 illustrates a configuration of an essential
portion of an extraction stage and of an extraction channel on the
upstream side of a low-pressure turbine as viewed from the side.
Incidentally, the same constituent elements as in the first
embodiment are denoted with like reference numerals and their
explanations are omitted.
[0041] Referring to FIG. 3A, an extraction port 29 is provided
between an outer circumferential side stationary wall 27 supporting
a stator vane 26 and an outer circumferential side stationary wall
28 constituting part of the next stage so as to circumferentially
open and communicate with an extraction channel 30. The extraction
channel 30 communicates with an extraction chamber (not shown)
circularly provided to circumferentially extend toward the outer
circumferential side of the outer circumferentially side stationary
wall. A portion of steam 20 flowing in the steam passage 19 is
extracted from the extraction port 29 through the extraction
channel 30 to the extraction chamber to form an extracted steam
flow 40. Further, the extracted steam flow 40 is taken out to the
outside of the turbine through an extraction pipe circumferentially
provided at a single or plurality of positions to connect with the
extraction chamber.
[0042] A plurality of moving blades 31 are secured to a turbine
rotor 7 between the stator vanes 26 and the extraction port 29. The
moving blades 31, along with the stator vanes 26, constitute an
extraction stage. A shroud 32 is mounted on the outer
circumferential ends of the moving blades 31. An internal
circumferential surface 33 of the shroud 32 is radially smoothly
bent to gradually increase a flare angle from the upstream toward
the downstream. In addition, the internal circumferential surface
33 is formed such that a moving blade outlet flare angle
.alpha..sub.3 is greater than a moving blade inlet flare angle
.alpha..sub.2. The moving blade outlet flare angle .alpha..sub.3 is
an angle formed between a tangential line F (indicated with a
broken line) at a downstream side distal end of the shroud inner
circumferential surface 33 and a turbine central axis 50. The
moving blade inlet flare angle .alpha..sub.2 is an angle formed
between a tangential line E (indicated with a broken line) at an
upstream side distal end of the shroud inner circumferential
surface 33 and the turbine central axis 50.
[0043] An angle formed between an inner circumferential surface 34
of the outer circumferential side stator blade wall 27 and the
turbine central axis 50 is defined as a moving blade upstream side
flare angle .alpha..sub.1. An angle formed between an upstream side
lateral wall of the extraction port 29 and the turbine central axis
50 is defined as a moving blade downstream side flare angle
.alpha..sub.4. The inner circumferential surface 33 of the shroud
32 in the present embodiment is formed such that the moving blade
inlet flare angle .alpha..sub.2 is generally equal to the moving
blade upstream side flare angle .alpha..sub.1. In addition, the
moving blade outlet flare angle .alpha..sub.3 is generally equal to
the moving blade downstream side flare angle .alpha..sub.4.
[0044] An outer circumferential surface of the shroud 32 is
composed of an upstream side parallel surface 37 being parallel to
the turbine central axis 50 and including an upstream side end; a
downstream side parallel surface 38 being parallel to the turbine
central axis 50 and including a downstream side parallel surface
38; and a curved surface 39 radially bent relative to the turbine
central axis 50 so as to connect the upstream side parallel surface
37 with the downstream side parallel surface 38. Also in the
present embodiment, the shroud upstream side outer diameter is made
smaller than the shroud downstream side outer diameter. This
intends to reduce the weight of the shroud.
[0045] The low-pressure turbine upstream stage may suffer from a
significant influence on performance degradation caused by steam
leakage. Therefore, seal fins 6 are arranged on the inner
circumferential surface of the outer circumferential side
stationary wall 27 opposite the upstream side parallel surface 37
and the downstream side parallel surface 38 so as to be
circumferentially extended. A step is provided on a seal fin
installation portion of the outer circumferential side stationary
wall in order to make a distance between the seal fins and the
parallel surfaces constant. The positional relationship between the
seal fins 6 and the shroud 32 is axially shifted due to thermal
extension difference resulting from high temperature during the
operation. However, even in such a case, since the seal fins are
arranged on the parallel surfaces of the shroud outer
circumferential surface, a gap between the seal fins 6 and the
shroud 32 can be allowed to remain unchanged during operation to
maintain sealing performance.
[0046] As shown in FIG. 3A, a portion, on the outer circumferential
side, of the steam flow leaving the moving blade 31 has a radial
component and is introduced into the extraction channel 30.
[0047] FIG. 4A is a schematic view illustrating an axially
shortened extraction stage on the upstream side of a traditional
low-pressure turbine. An upstream stage of the low-pressure turbine
has a blade shorter than that of the downstream stage and a large
seal gap relative to the blade length. Therefore, the upstream
stage has a relatively more significant leakage loss than the
downstream stage. Thus, it is necessary to enhance a seal effect by
arranging a plurality of fins 61 from the inlet to outlet of the
moving blade 31. However, the low-pressure turbine has a large
thermal extension difference. It is necessary, therefore, to
arrange a shroud 41 in parallel to a turbine central axis 50 in
order to maintain a radial gap. In other words, both a moving blade
inlet flare angle and a moving blade outlet flare angle in the
shroud 41 are 0 degree. The low-pressure turbine upstream stage has
a blade smaller than, thus circumferential velocity lower than
those of the downstream stage, which leads to a low flow velocity
at a blade distal end. Further, the low-pressure turbine upstream
stage has an accelerated flow with normal pressure gradient;
therefore, separation is unlikely to occur. However, the extraction
stage is provided with an extraction channel 30 adjacently to a
downstream side steam path outer circumferential wall to allow a
portion of steam to escape. If a shaft is reduced in length, a
deviation between a moving blade outlet flare angle .alpha..sub.3
and a moving blade downstream flare angle .alpha..sub.4 is
increased. In addition, a distance between the moving blade outlet
and the extraction port 29 is reduced to shorten a shaft span.
Therefore, steam is radially turned in a short shaft span between
the moving blade outlet and the extraction port 29. Thus, a
separation swirl 42 may occur close to the extraction port 29 in
some cases.
[0048] Returning to FIG. 3A and 3B, in the present embodiment, the
inner circumferential surface 33 of the shroud 32 is radially
smoothly bent so as to gradually increase the flare angle from the
upstream toward the downstream. In addition, the moving blade inlet
flare angle .alpha..sub.2 is generally equal to the moving blade
upstream side flare angle .alpha..sub.1 and the moving blade outlet
flare angle .alpha..sub.3 is generally equal to the moving blade
downstream side flare angle .alpha..sub.4. Therefore, the steam
flow is radially turned between the inlet and outlet of the moving
blade 2, which can radially turn the steam flow on the upstream
side of the extraction port 29. Thus, the axial length of the
turbine can be reduced while suppressing the lowering of turbine
efficiency resulting from flow separation at the extraction channel
inlet portion.
[0049] Incidentally, also in the present embodiment, it is
preferred that the moving blade inlet flare angle .alpha..sub.2 be
equal to the moving blade upstream side flare angle .alpha..sub.1
and the moving blade outlet flare angle .alpha..sub.3 be equal to
the moving blade downstream side flare angle .alpha..sub.4.
However, if respective deviations of the flare angles are generally
equal to each other, i.e., if each of the deviations falls within
5.degree., achievement of the effect of the present invention can
be expected.
[0050] The shroud shown in FIGS. 4B and 4C is formed of a parallel
surface parallel to the central axis from the upstream end to
downstream end of the outer circumferential surface. In contrast to
this, the shroud 32 of the present embodiment is formed such that
the outer circumferential surface includes the upstream side
parallel surface, the downstream side parallel surface, and the
curved surface radially bent to connect the upstream side parallel
surface with the downstream side parallel surface. In addition, the
shroud downstream side outer diameter is made greater than the
shroud upstream side outer diameter. Therefore, even if the
curvature of the shroud 32 is increased, i.e., even if the
difference between the moving blade inlet flare angle .alpha..sub.2
and the moving blade outlet flare angle .alpha..sub.3 is increased,
it is possible to suppress an increase in the weight of the shroud
per se. As a result, it is possible to prevent the lowering of the
strength reliability of the turbine blade.
[0051] A description is given of a fourth embodiment of the present
invention. FIGS. 5A and 5B are schematic views illustrating a
configuration of an essential portion of an extraction stage and of
an extraction channel on the upstream side of a low-pressure
turbine as viewed from the side. Incidentally, the same constituent
elements as in the third embodiment are denoted with like reference
numerals and their explanations are omitted.
[0052] The present embodiment has a structure different from the
third embodiment in a shroud. An inner circumferential surface of
the shroud 43 of the present embodiment is composed of an upstream
side inner circumferential surface 45 including an upstream end and
a downstream side inner circumferential surface 46 including a
downstream end. Also in the present embodiment, the inner
circumferential surface of the shroud 43 is formed as below. A
moving blade inlet flare angle .alpha..sub.2 is generally equal to
a moving blade upstream side flare .alpha..sub.1. The moving blade
inlet flare angle .alpha..sub.2 is an angle formed between the
upstream side inner circumferential surface 45 and the turbine
central axis 50. The moving blade upstream side flare angle
.alpha..sub.1 is an angle formed between the inner circumferential
surface 33 of the outer circumferential side stationary wall 27 and
the turbine central axis 50. In addition, a moving blade outlet
flare angle .alpha..sub.3 is generally equal to a moving blade
downstream side flare angle .alpha..sub.4. The moving blade outlet
flare angle .alpha..sub.3 is an angle formed between the downstream
side inner circumferential surface 46 and the turbine central axis
50. The moving blade downstream side flare angle .alpha..sub.4 is
an angle formed between an inner circumferential surface 36 of an
outer circumferential side stationary wall 28 adjacently provided
on the downstream side. Further, the moving blade outlet flare
angle .alpha..sub.3 is formed greater than the moving blade inlet
flare angle .alpha..sub.2.
[0053] In contrast, the outer circumferential surface 47 of the
shroud 43 is composed of a parallel surface parallel to the turbine
central axis 50 from the upstream end to the downstream end. In
addition, seal fins 6 are provided on the outer circumferential
side stationary wall 27 opposite the parallel surface.
[0054] The present embodiment provides the same effect as that of
the third embodiment shown in FIGS. 3A and 3B. The shroud 43 of the
present embodiment is formed with a hollow internal portion, which
intends weight reduction. In this way, it is possible to suppress
an increase in the weight of the shroud per se while keeping the
shroud outer circumferential surface 47 and the outer
circumferential side stationary wall parallel to each other. As a
result, it is possible to prevent the lowering of strength
reliability of the turbine blade while maintaining sealing
performance.
[0055] A description is given of a fifth embodiment of the present
invention. FIG. 6A illustrates a configuration of an essential
portion of an extraction stage and of an extraction channel on the
downstream side of a low-pressure turbine as viewed from the side.
An extraction portion is the same as that of the third embodiment.
The present embodiment is different from the third embodiment in
that a shroud and seal fins are configured to have the same shapes
as those of the first embodiment. Also the present embodiment
provides the same effect as that of the third embodiment.
* * * * *