U.S. patent application number 11/902642 was filed with the patent office on 2009-07-30 for diaphragm and blades for turbomachinery.
This patent application is currently assigned to Alstom Technology Ltd. Invention is credited to Richard Martin Bridge, Philip David Hemsley.
Application Number | 20090191053 11/902642 |
Document ID | / |
Family ID | 34531746 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090191053 |
Kind Code |
A1 |
Bridge; Richard Martin ; et
al. |
July 30, 2009 |
Diaphragm and blades for turbomachinery
Abstract
A diaphragm for an axial flow turbomachine, in which outer
shrouds of adjacent fixed blades in a row of blades contact each
other circumferentially to form a circumferentially continuous load
path. Inner shrouds of the blades only contact each other on
contact faces oriented to transmit loads in the radial and/or axial
directions of the turbomachine.
Inventors: |
Bridge; Richard Martin;
(Whitchurch, GB) ; Hemsley; Philip David; (Rugby,
GB) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Alstom Technology Ltd
Baden
CH
|
Family ID: |
34531746 |
Appl. No.: |
11/902642 |
Filed: |
September 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2006/060937 |
Mar 22, 2006 |
|
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|
11902642 |
|
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Current U.S.
Class: |
415/208.2 |
Current CPC
Class: |
F05D 2260/94 20130101;
F01D 9/041 20130101; F05D 2240/80 20130101; F01D 5/225 20130101;
F05D 2260/941 20130101; F05D 2250/00 20130101 |
Class at
Publication: |
415/208.2 |
International
Class: |
F01D 9/04 20060101
F01D009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2005 |
GB |
0505978.7 |
Claims
1. A blade for use in a row of fixed blades in an axial flow
turbomachine, comprising: (a) a radially outer shroud portion, (b)
a blade aerofoil portion, and (c) a radially inner shroud portion
having two opposed side edges for contacting corresponding side
edges of adjacent inner shroud portions of adjacent blades in a row
of such blades, wherein each opposed side edge comprises a
projecting step portion, a recessed step portion and a chamfered
step portion that joins the projecting step portion to the recessed
step portion, the projecting step portions being at opposite ends
of their respective side edges and configured to project into
co-operating recessed step portions of adjacent inner shroud
portions of adjacent blades, the chamfered step portions being
arranged to transfer forces between adjacent inner shroud portions
transversely of the circumferential direction in the row of blades
and to prevent circumferential transmission of loads between
adjacent inner shroud portions.
2. A blade according to claim 1, wherein each opposed side edge of
the inner shroud portion further comprises a planar portion, and
wherein the projecting step portions comprise parts of the side
edges that jut out relative to the planar portions, and the
recessed step portions comprise parts of the side edges that are
undercut relative to the planar portions.
3. A blade according to claim 2, wherein contact faces of the
planar portions, the projecting step portions and the recessed step
portions are arranged to radially abut each other in the row of
blades, thereby to transfer radial forces between adjacent inner
shroud portions.
4. A blade according to claim 1, the blade being a steam turbine
blade.
5. A blade according to claim 1, the blade being a gas turbine
blade or a compressor blade.
6. A diaphragm for an axial flow turbomachine, comprising: outer
shrouds of adjacent fixed blades in a row of blades which contact
each other circumferentially to form a circumferentially continuous
load path; and inner shrouds of the blades which only contact each
other on contact faces oriented to transmit loads in a radial
and/or axial direction of the turbomachine.
7. A diaphragm according to claim 6, in which there is an
interference fit between adjacent inner shrouds on their contact
faces, and the interference fit applies sufficient torque forces to
the shrouds to ensure that the contact faces remain in contact with
each other throughout operation of the turbomachine.
8. A diaphragm according to claim 6, wherein the contact faces for
transmitting loads in radial directions contact each other when the
diaphragm is in the as-assembled cool condition and throughout all
operating conditions of the turbine, but the contact faces for
transmitting loads in axial directions only contact each other when
the diaphragm reaches an operating temperature.
9. A diaphragm according to claim 6, wherein opposed side edges of
the inner shrouds contact corresponding side edges of adjacent
inner shrouds of adjacent blades and each opposed side edge
comprises a projecting step portion, a recessed step portion and a
chamfered step portion that joins the projecting step portion to
the recessed step portion, the projecting step portions being at
opposite ends of their respective side edges and configured to
project into co-operating recessed step portions of adjacent inner
shrouds of adjacent blades, the chamfered step portions comprising
contact faces operative to transmit loads in axial directions
between adjacent inner shroud portions and to prevent
circumferential transmission of loads between adjacent inner shroud
portions.
10. A diaphragm according to claim 9, wherein each opposed side
edge of the inner shroud portion further comprises a planar
portion, and wherein the projecting step portions comprise parts of
the side edges that jut out relative to the planar portions, and
the recessed step portions comprise parts of the side edges that
are undercut relative to the planar portions.
11. A diaphragm according to claim 10, wherein contact faces of the
planar portions, the projecting step portions and the recessed step
portions are arranged to radially abut each other, thereby to
transfer radial forces between adjacent inner shroud portions.
12. A steam turbine diaphragm according to claim 6, in combination
with a steam turbine.
13. A diaphragm according to claim 6, in combination with a gas
turbine or compressor.
14. A turbine including a diaphragm according to claim 6.
15. A compressor including a diaphragm according to claim 6.
Description
FIELD
[0001] Exemplary embodiments relate to the arrangement of turbine
blades to form a turbine diaphragm of fixed blades that can operate
at high temperatures and that results in a reduction in working
fluid leakage in turbines caused by distortion of the blade rows
due to changes in the operating temperature.
BACKGROUND
[0002] Steam turbines convert the energy in steam firstly into
mechanical energy, in the form of rotational energy, and then into
electrical energy. Multiple rows, which are termed stages, of
turbine blades are used to rotate a turbine shaft. Each steam
turbine stage alternately consists of stationary and rotating
components: the stationary components are rows of turbine blades
mounted to the inside of the casing of the turbine, and are herein
referred to as `fixed blades`; and the rotating components are rows
of turbine blades mounted to a turbine rotor, and are herein
referred to as `moving blades`.
[0003] The pressurised steam enters the turbine axially and first
impinges on the blade surfaces of a row of fixed blades. The blades
deflect the steam onto a row of moving blades which in turn also
deflect the steam back to the axial direction, causing themselves
move in the opposite direction to the deflected steam. This causes
the turbine rotor to rotate and the steam to expand slightly. The
next stage of fixed and moving blades, repeats the process. This
process continues through the turbine until the steam is completely
expanded.
[0004] Each successive stage of blades is optimised to deal with
the pressure and volume of the steam expected at the blades'
location in the turbine, as the steam will become successively less
pressurised as it moves through the successive rows of turbine
blades.
[0005] As shown in FIGS. 1 and 2, the fixed turbine blades 103, 203
can be mounted either directly in the turbine casing 100, 200 or in
separate diaphragms 202. The blades making up a turbine stage are
interconnected to provide damping, thereby avoiding possible
vibrations which could damage the turbine.
[0006] Referring to FIG. 1, there are small axial clearances
between the fixed blades 103 and the moving blades 105 to prevent
the blades contacting each other. There are also small radial
clearances between the fixed casing 100 and the rotating components
105, 108; and between the rotor 101 and the stationary components
103, 109. These clearances must be made as small as possible to
avoid steam leakage, as steam flow through the clearances does not
pass through the blading and so is unable to produce any power.
Sealing fins 104 are provided in the radial clearances to reduce
the amount of steam passing through them. The sealing fins 104 may
be fixed either to the rotor 101, to the casing 100 or to the ends
of the blades 103, 105.
[0007] In the case where the fixed blades 103 are mounted in the
casing 100, as shown in FIG. 1, any distortion of the casing 100
due to thermal effects will affect the radial clearance between the
ends of the blades 109 and the rotor 101 as the row of blades 103
will no longer form an accurate circle. This can result in some of
the ends of the turbine blades 109 contacting the sealing fins 104,
whilst the rotor 101 is rotating, and as a result the sealing fins
104 becoming damaged. Once the distortions in the casing 100
disappear, this damage to the sealing fins 104 leads to increased
leakage of the steam, because the sealing fins 104 are less able to
inhibit steam leaking through the radial clearance between the end
of the turbine blade 109 and the rotor 101.
[0008] To protect the fixed blades, and therefore the sealing fins
104, from the above distortion of the casing, without having to
increase the radial clearances between the ends of the blades and
the rotor, the fixed blades can be mounted in a diaphragm as shown
in FIG. 2. The diaphragm 202, 203, 204 is usually a welded
structure, in two halves to allow it to fit around the turbine
shaft, with an outer 202 or inner 204 ring with sufficient mass to
ensure that radial distortion is minimised and the blades 203 thus
remain in an accurate circle. The outer ring of the diaphragm 202
is mounted to the inner surface of turbine casing 200 in a groove
201, and the inner ring of the diaphragm 204 fits within a groove
205 in the rotor 207. The inner ring of the diaphragm 204 does not
contact the rotor 207, creating a clearance in-between, but a
finned seal 206 is provided in the groove 205 in the rotor 207 to
reduce steam flow through the clearance. The moving blades 209 are
positioned axially adjacent to the fixed blades 203 provided in the
diaphragm 202, 203, 204 and are fixed to the rotor by a moving
blade root 208. At the end of the moving blades 209 there is
provided a moving blade shroud portion 210, creating a clearance
between the moving blade shroud portion 210 and the turbine casing
200. This clearance is likely to be provided with another finned
seal to reduce steam flow through the clearance.
[0009] However, recent designs of diaphragm are much more compact,
as shown in FIGS. 3, 4 and 5. In the arrangement shown in FIG. 3,
the fixed blades 303 are mounted in a compact diaphragm 302, 303,
309 with an outer ring 302 and an inner ring 309. Seals 306B are
provided to reduce steam flow through the clearance between the
inner ring of the diaphragm 309 and the rotor 301. The moving
blades 304 have blade roots 305 mounted in the rotor 301. Seals
306A are provided in the clearance between the outer shroud 307 of
the moving blade 304 and the inside surface of an axially
projecting portion 310 of the outer ring of the diaphragm 302. The
axially projecting portion 310 of the outer ring of the diaphragm
302 lies radially between the turbine casing 300 and the outer ring
of the diaphragm 302.
[0010] This design of diaphragm allows for advantageous rotor
construction, such as allowing the use of a drum rotor and t-root
fixings. However, this means that the thermal inertias of the outer
ring 401, 402 and inner ring 405, 406 of the diaphragm 400, 500
shown in FIGS. 4 and 5 differ. The result of this is that the outer
401, 402 and inner rings 405, 406 heat up, and cool down, at
different speeds to each other.
[0011] As shown in FIG. 4, the outer and inner rings of the
diaphragm 400 must be split at 403, 407 into two halves, so
splitting the diaphragm across its diameter, to allow it to be
positioned around a rotor. The differing thermal expansion
resulting from the difference in temperatures can cause the two
halves of the diaphragm to distort as shown in exaggerated form in
FIG. 5, so that together they form a figure of 8 or oval shape.
This means that in some regions of the circumference the stationary
parts move closer to the moving parts, closing up the clearance
between them which can then cause damage when the fins contact the
blades or the rotor, resulting in a permanent increase in leakage
as described above.
[0012] An exemplary purpose of the invention is, therefore, to
reduce or eliminate the problem of a compact diaphragm containing a
row of turbine blades suffering thermal distortion which results in
increased steam leakage and damage to the turbine.
SUMMARY
[0013] In brief, the invention provides a turbine diaphragm for an
axial flow turbomachine, in which outer shrouds of adjacent fixed
blades contact each other circumferentially to form a
circumferentially continuous load path, but in which inner shrouds
of the blades only contact each other on contact faces oriented to
transmit loads in the radial and/or axial directions. This
arrangement can avoid circumferential load paths through the inner
shrouds and thereby ameliorates the stated problem of thermal
distortion.
[0014] To achieve this result consistently, an interference fit can
be provided between adjacent inner shrouds on their contact faces,
and the interference fit can apply sufficient torque forces to the
shrouds to ensure that the contact faces remain in contact with
each other throughout the temperature range of operation of the
turbomachine.
[0015] In a preferred exemplary embodiment, the contact faces for
transmitting loads in radial directions contact each other when the
diaphragm is in the as-assembled cool condition and throughout all
operating conditions of the turbine, but the contact faces for
transmitting loads in axial directions only contact each other when
the diaphragm reaches an operating temperature.
[0016] Opposed side edges of the inner shrouds contact
corresponding side edges of adjacent inner shrouds of adjacent
blades and each opposed side edge comprises a projecting step
portion, a recessed step portion and a chamfered step portion that
joins the projecting step portion to the recessed step portion, the
projecting step portions being at opposite ends of their respective
side edges and configured to project into co-operating recessed
step portions of adjacent inner shrouds of adjacent blades, the
chamfered step portions comprising contact faces operative to
transmit loads in axial directions between adjacent inner shroud
portions and to prevent circumferential transmission of loads
between adjacent inner shroud portions.
[0017] Preferably, each opposed side edge of the inner shroud
portion further comprises a planar portion, the projecting step
portions comprise parts of the side edges that jut out relative to
the planar portions, and the recessed step portions comprise parts
of the side edges that are undercut relative to the planar
portions. To transfer radial forces between adjacent inner shroud
portions, it is arranged that contact faces of the planar portions,
the projecting step portions and the recessed step portions
radially abut each other.
[0018] In another aspect, exemplary embodiments provides a blade
for use in a row of fixed blades in an axial flow turbomachine,
comprising:
[0019] (a) a radially outer shroud portion,
[0020] (b) a blade aerofoil portion, and
[0021] (c) a radially inner shroud portion having two opposed side
edges for contacting corresponding side edges of adjacent inner
shroud portions of adjacent blades in a row of such blades,
[0022] wherein each opposed side edge comprises a projecting step
portion, a recessed step portion and a chamfered step portion that
joins the projecting step portion to the recessed step portion, the
projecting step portions being at opposite ends of their respective
side edges and configured to project into co-operating recessed
step portions of adjacent inner shroud portions of adjacent blades,
the chamfered step portions being arranged to transfer forces
between adjacent inner shroud portions transversely of the
circumferential direction in the row of blades and to prevent
circumferential transmission of loads between adjacent inner shroud
portions.
[0023] An exemplary turbine blade can interconnect on its inner
edge with neighbouring blades, but not transmit circumferential
tensile and compressive forces to these neighbouring blades. This
can be accomplished by an arrangement that ensures each blade
remains free to expand in the circumferential direction whilst
keeping contact between the blades. With a small circumferential
clearance, for example of less than 0.5 mm, neighbouring blades no
longer transmit the tensile or compressive forces that cause the
diaphragm to distort under heating or cooling. The blades are held
in position through fixing to the outer ring of the diaphragm.
[0024] Further aspects of the invention will be apparent from a
perusal of the following description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Exemplary embodiments of the invention will now be
described, with reference to the accompanying drawings, in which
like reference symbols indicate the same or similar components.
[0026] FIG. 1 is a partial section taken in a radial plane
coincident with the rotational axis of a turbine showing the
arrangement of a fixed blade mounted in the casing and a moving
blade mounted in the rotor;
[0027] FIG. 2 is a partial section taken in a radial plane
coincident with the rotational axis of a turbine showing the
arrangement of a fixed blade mounted in a massive diaphragm and a
moving blade mounted in the rotor;
[0028] FIG. 3 is a partial section taken in a radial plane
coincident with the rotational axis of a turbine showing the
arrangement of a fixed blade mounted in a compact diaphragm and a
moving blade mounted in the rotor;
[0029] FIG. 4 is an end view along the turbine's rotational axis
showing a row of fixed blades mounted in a compact diaphragm seen
in isolation from other turbine structure;
[0030] FIG. 5 is a view similar to FIG. 4, but showing, in
exaggerated form, the row of fixed blades undergoing distortions
caused by inner and outer rings of the diaphragm being at different
temperatures due to the different thermal inertias of the inner and
outer rings;
[0031] FIG. 6 is a perspective view of three neighbouring turbine
blades according to the preferred embodiment of the invention;
[0032] FIGS. 7a to 7c are perspective views of a turbine blade
according to the preferred embodiment of the invention, each view
being on a different side of the blade;
[0033] FIG. 8a is a perspective view of the three neighbouring
turbine blades of FIG. 6 being mounted on an outer ring of the
diaphragm;
[0034] FIG. 8b is a partial section taken on line B-B of FIG. 8a
showing the outer shroud portion of one of the turbine blades
contacting the inner surface of the diaphragm's outer ring before
welding has occurred;
[0035] FIGS. 9a and 9b are enlarged cross-sectional views of a
stepped edge joint on the inner shroud portions of the neighbouring
turbine blades of FIG. 6, showing the joint before undergoing
heating (FIG. 9a) and after undergoing heating (FIG. 9b);
[0036] FIG. 10 is a view similar to FIG. 9b, showing the forces
acting on the joint when a full set of turbine blades are inserted
into a turbine diaphragm; and
[0037] FIG. 11 is a perspective view of a turbine blade according
to an alternative embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] A turbine blade according to the preferred embodiment and a
diaphragm containing a row of such turbine blades, will now be
described with reference to FIGS. 6 to 10.
[0039] FIGS. 7a and 7b show a single turbine blade 700 according to
the preferred embodiment of the invention. The blade 700 is formed
as a single solid part, forged and machined from a metal block, in
three portions: an outer shroud portion 710, a blade portion 720
and an inner shroud portion 730.
[0040] The outer shroud portion 710 is formed as a substantially
rectangular or parallelogram-shaped plate having four edge faces
711, 712, 714, 716. It is curved in the circumferential direction
of the turbine diaphragm so that when all the blades are assembled
into the diaphragm, adjoining outer shroud portions 710 form a ring
whose centre of curvature coincides with the turbine's rotational
axis.
[0041] The radially inner surface 713 of the outer shroud portion
710 forms a flow surface of the turbine passage. During manufacture
of the turbine diapragm, the radially outer surface 715 of the
outer shroud portion 710 is fixed by welding to an inwardly
projecting flange 805 of an outer ring 800 of the diaphragm, as
shown in FIGS. 8a and 8b and described later.
[0042] Circumferentially facing edges 712, 714 of the outer shroud
portion 710 extend generally in the axial direction of the turbine
and are substantially planar surfaces. When the blades 700 are
assembled into the diaphragm, there is circumferential contact
between the shroud edges 712, 714 of neighbouring shrouds to form a
circumferentially continuous load path, but as explained below
there is no circumferential contact between the inner shroud
portions 730. The axially facing, circumferentially extending edges
711, 716 of the outer shroud portion 710 are also substantially
planar surfaces and the distance between them is the same axial
width as the outer ring 800.
[0043] The blade portion 720 comprises an aerofoil 721 that
connects the outer shroud portion 710 to the inner shroud portion
730. The inner shroud portion 730 is formed as a substantially
rectangular or parallelogram-shaped plate that is curved in the
circumferential direction of the turbine diaphragm so that when all
the blades are assembled into the diaphragm, adjacent inner shroud
portions 730 form a ring whose centre of curvature coincides with
the turbine's rotational axis. In the assembled turbine diaphragm,
the radially outer surface 740 of the outer shroud portion 710
forms a flow surface of the turbine passage and the radially inner
surface 738 seals against the rotor, e.g., by means of sealing fins
mounted on the rotor, similar to fins 306 in FIG. 3.
[0044] Like the outer shroud portion 710, the inner shroud portion
730 has substantially planar axially facing, circumferentially
extending edges 741, 742. However, unlike outer shroud portion 710,
each circumferentially facing, generally axially extending edge of
the inner shroud portion 730 has a projecting step portion 732,
734, a complementary recessed step portion 736, 737, a chamfered
step portion 743, 744 that joins the projecting step portion to the
recessed step portion, and a planar portion 731, 733. The planar
portions 731, 733 occupy half the height of their shroud edges,
extend the full axial extent of the inner shroud 730 and are
located radially outward of the recessed and projecting step
portions. The projecting step portions 732, 734 comprise parts of
the shroud edges that jut out relative to the planar portions 731,
733 of the shroud edges, whereas the recessed step portions 736,
737 comprise parts of the shroud edges that are undercut relative
to the planar portions 731, 733. Half the axial extent of the inner
shrouds is occupied by the projecting step portions 732, 734, which
occupy axially opposed positions on the opposed circumferentially
facing edges of the inner shroud. Similarly, the recessed step
portions extend over the remaining half of the axial extent of the
inner shrouds and occupy axially opposed positions on their
respective shroud edges. Hence, when the blades are assembled into
the turbine diaphragm, the projecting step portions 732, 734 of
each inner shroud mate with the recessed step portions 736, 737 of
the neighbouring inner shrouds, to form sliding differential
expansion joints between the inner shrouds, as explained in more
detail below.
[0045] The sliding differential expansion joints between
neighbouring inner shrouds 730 have contact faces comprising the
radially outward facing surfaces 735 of the projecting step
portions 732, 734, the radially inward facing surfaces 735.sup.1 of
the recessed step portions 736, 737 (which may also be
characterised as overhanging surfaces of the planar edge portions
731, 733), and the surfaces of chamfered step portions 743, 744
that form angled faces between the recessed and projecting step
portions. Hence, when the turbine diaphragm is in the fully
assembled condition, the contact faces 735 on any given inner
shroud 730 radially abut the contact faces 735.sup.1 on the
neighbouring inner shrouds to transmit radial loads between the
inner shrouds. Furthermore, at operational temperatures of the
turbine, the chamfered step portions 743, 744 on any given inner
shroud 730 also abut each other to transmit loads between the inner
shrouds transversely of the circumferential direction, i.e., in a
generally axial direction. However, the circumferential facing
surfaces 733, 734, 736; 731, 732, 737 of the inner shroud portions
730 do not contact each other, but remain separated by a small gap
of about 0.1 mm to 0.5 mm to prevent the transmission of tensile or
compressive forces in the circumferential or tangential direction.
Transmission of these forces in the circumferential direction, as
mentioned previously, would lead to the diaphragm being pulled out
of axi-symmetry around the rotor, with the consequences mentioned
in relation to the prior art. Therefore, the above-mentioned small
circumferential gap is left between neighbouring inner shroud
portions 730 to allow for thermal expansion.
[0046] It should be understood that in this embodiment, the design
is such that when a full row, or stage, of blades 700 is assembled
as a diaphragm, internal twisting forces are produced by flexing
the aerofoil portion 721 of the blade during insertion of the blade
into the diaphragm. It is arranged that in the as-assembled cool
condition, the internal twisting forces cause the abutting contact
faces 735, 735.sup.1 to be forced together. As every abutting
contact face has the same amount of internal twisting force acting
on it, the net force is zero when all the neighbouring inner
shrouds are in mating contact.
[0047] To recap, the inner shroud portion 730 of the blade 700 is
adapted to interlock with the neighbouring inner shroud portions
without the inner shroud portions coming into contact in the
circumferential/tangential direction, i.e., there is no appreciable
load transmission between the inner shrouds in directions
perpendicular to a plane coincident with the turbine axis.
[0048] To construct a turbine diaphragm containing a row or stage
of fixed blades 700 for incorporation in a turbine, the blades 700
are inserted into a T-shaped outer ring 800. The radially outer
faces 715 of the outer shrouds 710 abut the radially inner face 808
of a radially inwardly projecting flange 805 that forms the stem of
the T-shaped outer ring 800. The abutment of the outer shroud
portions 710 and the flange 805 creates two nominally cylindrical
channels 804 between the main portion 801 of the outer ring 800 and
the interconnected outer shroud portions 710 of the blades 700. To
secure the blades 700 within the outer ring 800, a welding head is
inserted into the channels 804 and the outer shrouds are fillet
welded to the flange 805 in an automated welding process, as
known.
[0049] Once the diaphragm is constructed as detailed above, it is
cut across its diameter at the outer ring 800 into two semicircular
sections. The outer shrouds 710 of the blades 700 are not fixed to
each other, so the outer ring 800 is cut at a point where two outer
shrouds meet. This allows the two parts of the diaphragm to be
placed around the rotor in the turbine when the turbine is being
assembled. The two semicircular sections of the outer ring 800 can
then be secured together again, e.g., by means of inserting strong
bolts through pre-existing bolting flanges of the outer ring 800,
as known, causing the complete circumferential load path in the
outer shrouds to be restored.
[0050] With reference to FIGS. 7 and 10, the forces acting on two
neighbouring inner shrouds 730 when the diaphragm is assembled will
now be further described. As already mentioned, during assembly of
the blades into the diaphragm, the aerofoils 721 are twisted
slightly out of their natural alignment with respect to the outer
shroud portions, with the result that the inner shroud portions are
forced into contact with each other on contact faces 735,
735.sup.1. As seen in FIG. 10, a projecting step portion 734 of the
right hand inner shroud 730 projects into a co-operating recessed
step portion 737 of the left hand inner shroud, with the radially
inward facing contact face 735.sup.1, formed by the recessed step
portion 737 abutting the radially outward facing contact face 735
of the projecting step portion 734. Similarly, a projecting step
portion 732 (FIG. 7c) of the left hand inner shroud projects into
co-operating recessed step portion 736 (FIG. 7b) of the right hand
inner shroud, with the radially inward facing contacting face
735.sup.1, formed by the recessed step portion 736, abutting the
radially outward facing contact face 735, formed by the projecting
step portion 732. Equal and opposite forces F act radially at the
abutting contact faces 735, 735.sup.1, creating a zero net force
when the entire row of blades 700 is assembled. In essence, when
assembled into the turbine diaphragm, there is an interference fit
between adjacent shrouds on their radial contact faces 735,
735.sup.1. This applies sufficient torque to the shrouds to ensure
that the contact faces 735, 735.sup.1 remain in hard contact with
each other during operation of the turbine.
[0051] It should be understood that in the as-assembled condition,
when the turbine is not operating and the blades 700 are at ambient
temperature, the chamfered step portions 743, 744 do not contact
each other. This is because, as shown in FIG. 9a, the gap between
the planar portions 731, 733 of neighbouring shroud edges is
relatively wide. However, on heating, the shrouds expand such that
the projecting step portions 734, 735 extend further into the
respective co-operating recessed step portions 736, 737, until the
faces of the chamfered step portions 743, 744 come into contact
with each other. This prevents the projecting step portions 732,
734 from projecting all the way into the recessed step portions
736, 737 and preserves a small inter-shroud gap as shown in FIG.
9b, to ensure there is no circumferential load path through the
inner shrouds. Further thermal expansion of the inner shrouds
generates equal and opposite forces on the abutting chamfered
contact faces 743, 744, which contribute a zero net force in the
assembled operating turbine diaphragm. The forces at the chamfered
step portions act transversely of the circumferential/tangential
direction.
[0052] In an alternative non-preferred embodiment shown in FIG. 11,
the expansion joint mechanism between the inner shroud portions
730a of neighbouring blades 700a differs from that shown in FIGS. 6
to 10. In the preferred embodiment as described in relation to
FIGS. 6 to 10, the contact faces 735, 735.sup.1 of the inner
shrouds, when assembled, contact each other in a radial direction
relative to the axis of the turbine, but the chamfered step
portions 743, 744 only contact each other when the turbine reaches
an operating temperature. However, in the alternative embodiment of
FIG. 11, the radial contact faces are omitted and interference
contact between the inner shrouds 730a in the as-assembled cool
condition of the diaphragm occurs on the faces of the chamfered
step portions 743.sup.1 and 744.sup.1 of the inner shroud edges,
leaving a small circumferential gap between the inner shroud edges
of adjacent blades, as was the case in the preferred embodiment.
This again can avoid transferring forces between inner shrouds 730a
in the circumferential direction because the chamfered step
portions 7431, 744.sup.1 react loads in a generally axial
direction.
[0053] As can be seen, the circumferentially facing edges 731a,
733a of the inner shroud portion 730a are each provided with a
projecting step portion 733a occupying substantially half the axial
length of each circumferentially facing edge, the projecting step
portions being at axially opposite ends of their respective
circumferential facing edges. Each chamfered step portion
743.sup.1, 744.sup.1 forms an angled face between a recessed step
portion 731a and the projecting step portion 733a. These angled
faces of the chamfered step portions 743, 744 contact the faces of
the chamfered step portions of neighbouring inner shrouds in
substantially axially abutting relationship when the turbine
diaphragm is assembled.
[0054] Although the above description mentions welding of the outer
shrouds 710 to the outer ring 801, other ways of connecting blades
700 to the outer ring of the diaphragm are available, such as
through a T-root type of fixing, or similar.
[0055] In still further embodiments the expansion joint mechanism
can be used in other types of turbines, such as gas turbines.
Furthermore, the invention could also be applicable to fixed blades
in compressors.
[0056] Exemplary embodiments can be used over a wide range of
temperatures and pressures experienced by turbines, e.g., 150 to
600 degrees Celsius and 5 to 300 bars. Steel and/or nickel alloys
or other appropriate materials can be used in the fabrication of
the turbine components described here.
[0057] The present invention has been described above purely by way
of example, and modifications can be made within the scope of the
invention as claimed. The invention also consists in any individual
features described or implicit herein or shown or implicit in the
drawings or any combination of any such features or any
generalisation of any such features or combination, which extends
to equivalents thereof. Thus, the breadth and scope of the present
invention should not be limited by any of the above-described
exemplary embodiments. Each feature disclosed in the specification,
including the claims and drawings, may be replaced by alternative
features serving the same, equivalent or similar purposes, unless
expressly stated otherwise.
[0058] Any discussion of the prior art throughout the specification
is not an admission that such prior art is widely known or forms
part of the common general knowledge in the field.
[0059] Unless the context clearly requires otherwise, throughout
the description and the claims, the words "comprise", "comprising",
and the like, are to be construed in an inclusive as opposed to an
exclusive or exhaustive sense; that is to say, in the sense of
"including, but not limited to".
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