US20090191053A1

US20090191053A1 - Diaphragm and blades for turbomachinery

Info

Publication number: US20090191053A1
Application number: US11/902,642
Authority: US
Inventors: Richard Martin Bridge; Philip David Hemsley
Original assignee: Alstom Technology AG
Current assignee: GE Vernova GmbH
Priority date: 2005-03-24
Filing date: 2007-09-24
Publication date: 2009-07-30
Also published as: JP2008534837A; GB0505978D0; CN101146980A; WO2006100256A1; DE112006000603T5

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.

Description

FIELD

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

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’.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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

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.
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.
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.
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.
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.
In another aspect, exemplary embodiments provides 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.
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.
Further aspects of the invention will be apparent from a perusal of the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

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;

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;

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;

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;

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;

FIG. 6 is a perspective view of three neighbouring turbine blades according to the preferred embodiment of the invention;

FIGS. 7 a to 7 c 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;

FIG. 8 a is a perspective view of the three neighbouring turbine blades of FIG. 6 being mounted on an outer ring of the diaphragm;

FIG. 8 b is a partial section taken on line B-B of FIG. 8 a 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;

FIGS. 9 a and 9 b 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. 9 a) and after undergoing heating (FIG. 9 b);

FIG. 10 is a view similar to FIG. 9 b, showing the forces acting on the joint when a full set of turbine blades are inserted into a turbine diaphragm; and

FIG. 11 is a perspective view of a turbine blade according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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.
FIGS. 7 a and 7 b 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.
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.
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. 8 a and 8 b and described later.
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.
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.
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.
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 ¹of 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 ¹on 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.
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 ¹to 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.
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.
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.
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.
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 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 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. 7 c) of the left hand inner shroud projects into co-operating recessed step portion 736 (FIG. 7 b) of the right hand inner shroud, with the radially inward facing contacting face 735 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 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 ¹. This applies sufficient torque to the shrouds to ensure that the contact faces 735, 735 ¹remain in hard contact with each other during operation of the turbine.
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. 9 a, 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. 9 b, 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.
In an alternative non-preferred embodiment shown in FIG. 11, the expansion joint mechanism between the inner shroud portions 730 a of neighbouring blades 700 a 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 ¹of 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 730 a in the as-assembled cool condition of the diaphragm occurs on the faces of the chamfered step portions 743 ¹and 744 ¹of 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 730 a in the circumferential direction because the chamfered step portions 7431, 744 ¹react loads in a generally axial direction.
As can be seen, the circumferentially facing edges 731 a, 733 a of the inner shroud portion 730 a are each provided with a projecting step portion 733 a 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 ¹, 744 ¹forms an angled face between a recessed step portion 731 a and the projecting step portion 733 a. 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.
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.
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.
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.
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.
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.
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”.

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.