US20140322007A1

US20140322007A1 - Turbine diaphragm construction

Info

Publication number: US20140322007A1
Application number: US13/848,199
Authority: US
Inventors: Angus Robert Brummitt-Brown; Adrian Clifford LORD
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2012-03-21
Filing date: 2013-03-21
Publication date: 2014-10-30
Also published as: JP2013194742A; EP2642079A1; CN103321694A

Abstract

A turbine diaphragm has radially inner and outer hollow diaphragm rings (22, 24) comprising box structures that include the radially inner and outer platform portions (261, 262) of static blade units (26) as part of their structures. The box structures are strengthened by the incorporation of ribs or struts (28, 30) which extend radially and axially between the walls of the box structures.

Description

TECHNICAL FIELD

This disclosure relates to the construction of diaphragms for turbines, and in particular, to a novel structure and manufacturing process for diaphragms in the high or intermediate pressure (HP or IP) stages of axial flow steam turbine power plants.

TECHNICAL BACKGROUND

A known way of constructing a turbine diaphragm is to mount an annulus of static guide blade units between an inner ring and an outer ring. Each such blade unit comprises an aerofoil portion that extends between an inner platform and an outer platform, the blade unit normally being manufactured as a single integral component. This is known as the “platform” type of construction. Each platform is in the form of a segment of a cylinder so that when the annulus of blade units is assembled the inner platforms combine to create an inner cylinder and the outer platforms combine to create an outer cylinder. The inner platforms are welded to an inner ring that retains the blade units and provides a mount for a sealing arrangement, such as a labyrinth seal, that acts between the inner ring and a rotor shaft of the turbine. The outer platforms are welded to an outer ring that provides support and rigidity to the diaphragm. Each of the inner and outer rings comprises two semi-circular halves which are joined along a plane that contains the major axis of the diaphragm and passes between blade units so that the entire diaphragm can be separated into two parts for assembly around the rotor of the turbo-machine.
Existing platform constructions for large HP or IP steam turbine diaphragms generally comprise solid inner and outer rings cut from thick metal plate, or forged, or formed from bar stock. Since such rings have substantial dimensions in the axial direction of the turbine, e.g., 100 mm to 200 mm, the cost of such rings is a significant factor in the ex-works price of a large steam turbine. If the rings are cut from thick plate, there is a high proportion of waste (e.g., the centre cut-out portion of an inner ring), whereas forging or other forming operations add more expense and become more difficult to control as greater thicknesses and weights of stock are manipulated.

SUMMARY OF THE DISCLOSURE

In its broadest aspect, the present disclosure provides a high or intermediate pressure, axial flow, turbine diaphragm comprising a radially inner ring and a radially outer ring, at least one such ring being hollow and having axially opposed walls comprising plate material.
Preferably, the turbine diaphragm comprises: an annulus of static blade units, each such blade unit having an aerofoil portion, a radially inner platform portion and a radially outer platform portion; and a radially inner hollow diaphragm ring and/or a radially outer hollow diaphragm ring, the or each such hollow ring comprising a box structure that includes the blade units as part of the box structure.
As implied above, it is not always necessary for both the inner and outer rings to comprise hollow structures. Thus, it would be possible for one of the rings to be hollow, but the other ring could be a known type of solid construction.
In one embodiment, the box structure of the inner hollow diaphragm ring comprises: a pair of axially opposed side walls; a radially inner circumferentially extending wall; and a radially outer circumferentially extending wall formed by the radially inner platform portions of the static blade units.
Similarly, the box structure of the outer hollow diaphragm ring may comprise: a pair of axially opposed side walls; a radially outer circumferentially extending wall; and a radially inner circumferentially extending wall formed by the radially outer platform portions of the static blade units.
We prefer that at least one, and preferably both, of the radially inner and outer platform portions of the static blade units are integrally formed with the aerofoil portion, e.g. by machining from a single piece of material. However, it would also be possible to fabricate the static blade units by attaching one or both of the platform portions to the aerofoil portion.
To give the box structures of the inner and outer hollow diaphragm rings good rigidity, we propose to strengthen them by incorporating a plurality of reinforcing ribs in their interiors, the ribs being aligned to the radial and axial directions and circumferentially spaced-apart from each other. Thus, the ribs extend across the box structures between the axially opposed side walls and the radially inner and outer circumferentially extending walls. It should be understood that when the word “ribs” is used throughout this description and accompanying claims, it includes structurally equivalent reinforcing features such as struts and the like.
Optionally, the radially outer wall of the outer diaphragm ring may be divided into a plurality of separate circumferentially extending sections by radially outward projections of at least one of the reinforcing ribs. Such an arrangement could facilitate the fitting of load lifting features to the upper and lower halves of the outer ring, and/or engagement of the outer ring with corresponding location features in a turbine casing surrounding the diaphragm. Engagement of the outer ring with such location features could enable cross-key location of the diaphragm within the turbine casing and/or prevent rotation of the diaphragm within the casing as the static blades deflect the steam flow during operation of the steam turbine.
The radially inner side of the radially inner wall of the radially inner ring may comprise a circumferentially extending recess configured to retain a seal against leakage between relatively high and low pressure sides of the diaphragm.
To enable assembly of the diaphragm within the turbine, the diaphragm is fabricated as two halves which upon assembly are united with each other on a diametric joint line. The joint may comprise a bolted joint provided at each end of the diametric joint line of the outer diaphragm ring. Alternatively, the diametrically opposed ends of the upper and lower halves of the diaphragm may be held in registration with each other by means of location features that support the outer diaphragm ring within the surrounding turbine casing.
In some embodiments, it is envisaged that only one of the inner and outer diaphragm rings will be a hollow structure, the other ring being solid.
The present disclosure also provides methods of manufacture of the hollow diaphragm rings and the diaphragm, as described herein and claimed in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the concept disclosed herein will now be described, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic radial section through the upper half of a known type of HP, axial flow, steam turbine diaphragm;

FIG. 2 is a diagrammatic representation of a radial section through the upper half of an HP or IP steam turbine diaphragm constructed in accordance with the concept disclosed herein;

FIG. 3 is a three-dimensional perspective view of a partially de-constructed embodiment of the upper half of an HP or IP steam turbine diaphragm;

FIG. 4 is a view similar to FIG. 3 but of the lower half of the diaphragm;

FIG. 5 is a three-dimensional perspective view of an inner ring upon which a set of static blade units has been mounted; and

FIG. 6 is a view similar to FIG. 5, in which an incomplete outer ring has been mounted on the set of static blade units. The drawings are not to scale.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a known “platform” type of construction for a large, heavy duty, axial flow, steam turbine diaphragm 1, and shows a view on a radial section of the upper half of a diaphragm during manufacture, the centreline of the diaphragm 1 being shown as dash-dotted line C. The view includes a static blade unit 2, comprising radially inner and outer platforms 3, 4, and an aerofoil portion 5 whose radially inner and outer ends are integral with the aerofoils and platforms, the entire blade unit being machined from solid. When the assembly is finished, the inner and outer platforms 3, 4 form the inner and outer port walls for the flow of steam through the diaphragm. A complete diaphragm 1 is built up by assembling successive blade units 2 into an annular array between inner and outer diaphragm rings 6, 7 and attaching the platforms to the diaphragm rings by means of deep filler welds 8 to 11. The inner and outer diaphragm rings and platforms are further machined as appropriate to accommodate turbine sealing features and to fit adjacent turbine features. For example, a sealing feature has been machined on the inside circumference of the inner ring 6, comprising a channel 12 with re-entrant hook features 13, this being provided to hold a labyrinth seal or the like (not shown) to seal between the inner ring 6 and a turbine shaft 14, which is indicated by dashed lines.
It is known practice for HP steam turbines employing platform construction to build the blade units 2 onto the inner diaphragm ring 6 and then to shrink the outer diaphragm ring 7 on to the blade units 2. The inner and outer diaphragm rings 6, 7 are required to support the blade units, to give the diaphragm rigidity against forces that tend to distort it during assembly and operation of the turbine, and, with respect to the outer diaphragm ring, to locate the diaphragm securely within a turbine casing (not shown) that surrounds the diaphragm in the finished turbine construction.
FIG. 2 is a diagram of a radial section through the upper half of an HP or IP steam diaphragm 20 constructed in accordance with the concept disclosed herein, in which the radially inner and outer diaphragm rings 22, 24, are hollow, being fabricated from plate material as box structures. Thus, the box structure of the inner hollow diaphragm ring 22 comprises a pair of axially opposed side walls 221, 222, a radially inner circumferentially extending wall 223, and a radially outer circumferentially extending wall 261 formed by radially inner platform portions of static blade units 26. Similarly, the box structure of the outer hollow diaphragm ring 24 comprises a pair of axially opposed side walls 241, 242, a radially inner circumferentially extending wall 262 formed by radially outer platform portions of the static blade units 26, and a radially outer circumferentially extending wall 243.
As an example, the plate material thickness of each side wall 221, 222, 241, 242 and the outer wall 243 could be less than 35%, (say, 20% to 30%) of the axial thickness of the ring of which it is a part. However, the radially inner most wall 223 is shown as having a greater thickness than the other walls because a recess 228 is machined into it, as explained later. One example of a suitable plate material for fabrication of the hollow diaphragm rings is chrome molybdenum steel plate, such as specification ASTM A 387 Gr.22 C12. Such steels are commonly used to fabricate pressure vessels and there are many suppliers of chrome molybdenum steel plate worldwide. Other high alloy steels may also be suitable.
In this embodiment, the radially inner and outer platform portions 261, 262, respectively, of the static blade units 26 are integrally formed with their aerofoil portions 263. Alternatively, it would also be possible to fabricate the static blade units 26 by joining one or both of the platform portions 261, 262 to the aerofoil portions 263 before incorporating the platform portions into the inner and outer rings. In either case, the blade units may comprise, for example, the well-known and readily available 12Cr alloy steel, although other materials, such as nickel-base superalloys, could also be used
To give the box structures of the inner and outer hollow diaphragm rings 22, 24 good rigidity, we propose to strengthen them by incorporating a plurality of stiffening ribs 28, 30, respectively, in their interiors, the ribs 28, 30 being aligned in the radial and axial directions and circumferentially spaced-apart from each other. Thus, in the axial direction the ribs 28, 30 extend across the box structures between the axially opposed side walls 221, 222 and 241, 242 of the inner and outer rings 22, 24, and in the radial direction the ribs 28, 30 extend across the box structures between the radially inner and outer circumferentially extending walls 223, 261 and 262, 243 of the rings.
To obtain the maximum cost benefit from the present concept, it is assumed above that both the inner and the outer diaphragm rings will comprise a hollow box construction, but it would nevertheless be possible for only one of the rings to be hollow, the other ring being of a solid construction.
As will be explained in more detail below, the box structures may be fabricated by attaching the side walls 221, 222 and 241, 242 to the reinforcing ribs 28 and 30, then attaching the inner platforms 261 of the blade units 26 to the side walls 221, 222 of the inner ring 22. After this, the side walls 241, 242 of the outer ring 24 may be attached to the outer platforms 262 of the blade units 26. The box structures of the inner and outer rings may be completed by attaching the inner circumferentially extending wall 223 to the side walls of the inner ring 22 and attaching the outer circumferentially extending wall 243 to the side walls of the outer ring 24. The attachments mentioned above may be made by tack welding the components together, but permanent welds are made to finish the diaphragm as indicated in FIG. 2, where circumferentially extending weld lines 224 to 227 and 244 to 247 are shown. Thus, for inner ring 22, welds 226 and 227 attach the side walls 221, 222 to the platform portions 261 and welds 224 and 225 attach the side walls 221, 222 to the radially inner wall 223, whereas for outer ring 24, welds 244 and 245 attach the side walls 241, 242 to the platform portions 262 and welds 246 and 247 attach the side walls 241, 242 to the radially outer wall 243.
As also shown in FIG. 2, the radially inner side of the radially inner wall 223 of the radially inner ring 22 may comprise a recess or hook arrangement 228 configured to retain and hold a shaft seal, such as a labyrinth or brush seal (not shown), in sealing engagement with a shaft 14 to minimise leakage between relatively high and low pressure sides of the diaphragm.
To enable assembly of such diaphragms into a turbine, the diaphragms are fabricated in two halves, an upper half and a lower half, which are united with each other on a horizontal diametric joint line J. For example, a bolted joint can be provided at each end of the joint line J. Alternatively, the top and bottom halves of the diaphragm can each be independently supported by means of cross-key location features, as known in the industry.
To illustrate fabrication of a diaphragm in more detail according to the present concept, we refer to FIGS. 3 and 4. FIG. 3 is a three-dimensional pictorial view of a partially de-constructed embodiment of the upper half of an HP or IP steam turbine diaphragm, whereas FIG. 4 is a similar view of the lower half of the diaphragm. In both views, only one static blade unit 26 is shown in position between upper and lower, inner and outer half- rings 22U, 24U and 22L, 24L, the other blade units having been omitted to show the box construction of the half-rings more clearly. As far as possible, FIGS. 3 and 4 use the same reference numbering scheme as FIG. 2.
One possible manufacturing sequence starts with the side walls for the upper half- rings 22U, 24U, the lower half- rings 22L, 24L, and the stiffening ribs 28, 30, being cut from plate material to the sizes required for the specific turbine in which the diaphragm is to be installed. Additionally, joint blocks 32U, 34U and 32L, 34L for the above-mentioned bolted joints (which also act to strengthen the box structure) are produced from thicker gauge plate for incorporation in the upper and lower outer half- rings 24U, 24L.
Next, the stiffening ribs 28 are attached to one of the side walls of each of the inner half- rings 22U, 22L, and the stiffening ribs 30, together with the joint blocks 32U, 34U, are attached to one of the side walls of the outer half-ring 24U, the joint blocks 32U, 34U being positioned at diametrically opposite ends of the outer half-ring 24U. Similarly, stiffening ribs 30, together with the joint blocks 32L, 34L are attached to one of the side walls of the outer half-ring 24L. After this, the other side walls of the inner and outer half-rings are attached to the ribs 28 and 30 and—in the case of the outer half-rings—to the joint blocks. At this stage (or alternatively, before incorporation of the joint blocks into the box structure), threaded bolt holes 36U, 36L can be machined into their respective joint blocks.
The above sequence produces inner and outer half-rings that are not yet complete, in that the upper and lower inner half rings 22, 22L, comprise only their side walls and their stiffening ribs, whereas upper and lower outer half rings 24, 25L, comprise only their side walls, their stiffening ribs and their joint blocks.
The inner half rings for the upper and lower halves of the diaphragm can now be placed together on a jig to produce an inner ring. Similarly, the outer half rings for the lower and upper halves of the diaphragm can be joined together to produce an outer ring. Whereas the inner half-rings may be joined together on the jig by temporary tack welding, the outer half-rings may be joined together by temporary bolts which extend through the joint blocks provided at the diametrically opposed ends of the upper and lower, outer half-rings.
Having joined the two inner half- rings 22, 22L together, the inner platforms 261 of the static blade units 26 can be tack welded to the side walls 221, 222 of the inner ring to produce the assembly shown in FIG. 5. While still on the jig, a machining operation is performed to produce even, circular outer surfaces on the outer platforms 262 of the static blade units 26. After this, the outer ring is heated up and then shrunk on to the blade units 26 to bring the side walls 241, 242 into engagement with the outer platforms 262, thereby producing the assembly shown in FIG. 6.
At this stage in the production of the diaphragm, the circumferential welds 226, 227 and 244, 245, which were noted in connection with FIG. 2, are performed on the assembly of FIG. 6 to securely bond the inner and outer rings 22, 24 to the static blade units 26. The circumferential welds can be interrupted at the joint line J between the upper and lower halves of the diaphragm. Alternatively, the circumferential welds can be continuous over the joint lines, the weld bead being subsequently machined away at the joint line to enable separation of the upper and lower halves of the diaphragm from each other.
Following the above circumferential welding operation, and referring back to FIGS. 3 and 4, the final steps of the diaphragm fabrication process are to attach the radially outer wall 243 of the outer ring 24 to the side walls 241, 242, and to attach the radially inner wall 223 of the inner ring 22 to the side walls 221, 222, using circumferential welds 224, 225 and 246, 247 as indicated in FIG. 2. It will be evident that these welds may be interrupted if necessary to take account of the joints between the upper and lower half-rings and any sections of the outer wall 243 divided from each other by the ribs 30, for the reasons explained below.
In variations of the fabrication processes outlined above, the radially inner wall 223 may be attached to the hollow inner ring 22 before the inner platforms 261 of blade units 26 are attached to the side walls 221, 222 of the inner ring. Furthermore, the radially outer wall 243 may be attached to the hollow outer ring 24 either before or after the side walls 241, 242 of the outer ring 24 are attached to the outer platforms 262 of blade units 26.
It will be seen from FIGS. 3 and 4 that the radially inner wall 223 of the inner ring is continuous over the entire circumferential extent of each of the upper and lower half- rings 22U, 22L, whereas the radially outer wall 243 is discontinuous, being divided into a plurality of separate circumferentially extending sections by radial outward projections of the reinforcing ribs 30. Although the ribs 30 project radially beyond the radial extent of the side walls 241, 242, the ribs do not extend all the way through the radial thickness of the sections of the radially outer wall 243, thereby creating rectangular recesses in the radially outer wall 243. These recesses may aid the addition of features to facilitate handling of the assembled diaphragm, in that threaded holes (not shown) may be drilled in the exposed ends of ribs 30 to accommodate crane lifting eyes. Alternatively, or additionally, the recesses may be used to enable engagement of the outer ring 24 with correspondingly projecting anti-rotation keys and/or cross-key location features in the turbine casing (not shown) that surrounds it in the fully assembled turbine.
Note also that instead of being recessed radially inwardly of the outer surface of the outer wall 243, it would be possible for the ribs 30 to project radially beyond the outer surface of the outer wall 243. This arrangement could also be used to provide anti-rotation key and cross-key location features in conjunction with the surrounding turbine casing.
Although it is suggested above that the outer wall 243 may be divided by a plurality of the ribs 30, it should be understood that the number of ribs 30 selected to project beyond the radial extent of the walls 241, 242 is at the option of the designer. For example, a single rib projecting at top dead centre, in conjunction with a complementary feature in the turbine casing, would be sufficient to create an anti-rotation feature for the diaphragm.
It should be noted that although it may be convenient for some purposes, it is not necessary for the strengthening ribs to extend radially beyond the side walls 241, 242 in order to facilitate the formation of location or lifting features. For example, in FIG. 6 it will be seen that a rib 301 has a radially outer edge surface that is flush with the radially outer edge surfaces of the side walls, so that an unbroken section of the outer wall 243 can be welded into place over the rib 301, see FIG. 3. In fact, all the ribs 30 in the outer ring 24 could be like rib 301, so that the radially outer wall 243 would not be divided into sections, except by the joint blocks 32L, 34L and 32U, 34U. Any required lifting or location features could then be machined into or welded onto the outer wall 243.
It will be noticed from FIG. 3 that the ends of the sections of the radially outer wall 243 that are nearest to the joint blocks 32U, 34U, are tilted inwards relative to the side walls 241, 242 of the upper half-ring 24U in order to allow access to the bolts (not shown) which hold the upper and lower halves of the finished diaphragm together.
As mentioned previously, it would be possible to construct a diaphragm in which only one of the rings would be a hollow box structure, the other ring being of a solid construction. In the case where only the inner diaphragm ring is a hollow box structure, it could be fabricated and assembled on the jig as described above, complete with the static blade units, and the two halves of the solid outer ring could be joined together and shrunk onto the outer platforms of the blade units, followed by welding as appropriate. In the case where only the outer diaphragm ring is hollow, the two halves of the solid inner diaphragm ring could be assembled onto the jig and the inner platforms of the blade units could be tack welded onto the inner solid ring. The outer ring could be manufactured as described previously and could then be shrunk onto the outer platforms of the blade units and welded as appropriate.
Following fabrication of the diaphragm as described above, it is necessary for it to undergo a stress-relieving heat-treatment and final machining procedures, e.g., machining of the recess 228 in the inner wall 223 of the inner ring 22, see FIG. 2. The diaphragm can then be split into its upper and lower halves, ready for assembly into a turbine.
Although the above description has mentioned the present concept in the context of large, heavy-duty steam turbines, it could also be applicable to lighter-weight turbines, such as those used in ships or as auxiliary turbines in industrial plants. Possible application of the concept to gas turbine diaphragms should also not be ignored.
Adoption of the concept proposed herein confers a number of advantages. Firstly, the amount of material used in manufacture of the diaphragm is reduced, without a proportional reduction in strength. This is because the loadings experienced by the diaphragm rings are primarily bending loads, and material near to their neutral axes makes a relatively small contribution to bending strength. Secondly, the static blade unit platforms are utilised as integral parts of the inner and outer rings, thereby optimising material usage. Thirdly, manufacture of the bolted joint between the upper and lower halves of the diaphragm is simplified relative to the procedure used for solid outer rings. This is because the hollow box construction of the rings according to the present concept allows insertion of pre-configured bolt-receiving blocks between the side walls of the rings. Fourthly, and following from the above, the reduction in the amount of costly high grade material used in the inner and outer rings, a reduction in the amount of waste material produced during machining of the components, and reduced complexity in machining processes with regard to production of the bolted joint, leads to a significant reduction in the cost of a finished diaphragm, e.g., a reduction of up to 40% according to our initial studies. Fifthly, the strengthening ribs can be used to simplify the provision of anti-rotation keys and location keys acting between the diaphragm and the turbine casing.
The above embodiments have been described above purely by way of example, and modifications can be made within the scope of the appended claims. Thus, the breadth and scope of the claims should not be limited to 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.
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; i.e., in the sense of “including, but not limited to”.

Claims

1. A high or intermediate pressure, axial flow, turbine diaphragm (20) comprising a radially inner ring and a radially outer ring (22, 24), at least one such ring being hollow and having axially opposed walls (221, 222 and/or 241, 242) comprising plate material.

2. A turbine diaphragm according to claim 1, comprising:

(a) an annulus of static blade units (26), each such blade unit having an aerofoil portion (263), a radially inner platform portion (261) and a radially outer platform portion (262); and

(b) a radially inner hollow diaphragm ring (22) and/or a radially outer hollow diaphragm ring (24), the or each such hollow ring comprising a box structure that includes the blade units (26) as part of the box structure.

3. A turbine diaphragm according to claim 2, in which a box structure of the inner hollow diaphragm ring (22) comprises:

(a) a pair of axially opposed side walls (221, 222);

(b) a radially inner circumferentially extending wall (223); and

(c) a radially outer circumferentially extending wall formed by the radially inner platform portions (261) of the static blade units (26).

4. A turbine diaphragm according to claim 2, in which a box structure of the outer hollow diaphragm ring (24) comprises:

(a) a pair of axially opposed side walls (241, 242);

(b) a radially outer circumferentially extending wall (243); and

(c) a radially inner circumferentially extending wall formed by the radially outer platform portions (262) of the static blade units (26).

5. A turbine diaphragm according to claim 2, in which one or both of the radially inner and outer platform portions (261, 262) of the static blade units are formed integrally with the aerofoil portion (263).

6. A turbine diaphragm according to claim 2, in which the box structure of the or each hollow diaphragm ring (22, 24) is strengthened by incorporating a plurality of reinforcing ribs (28, 30) into the box structure, the ribs being aligned to the radial and axial directions and circumferentially spaced-apart from each other.

7. A turbine diaphragm according to claim 6, in which the radially outer wall (243) of the outer diaphragm ring (24) is divided into separate circumferentially extending sections by at least one of the reinforcing ribs (30).

8. A turbine diaphragm according to claim 3, in which the radially inner side of the radially inner wall (223) of the radially inner ring (22) comprises a circumferentially extending recess (228) configured to retain a seal against leakage between relatively high and low pressure sides of the diaphragm (20).

9. A turbine diaphragm according to claim 1, in which to enable assembly of the diaphragm (20) within a turbine, the diaphragm is fabricated as two halves which upon assembly are united with each other on a diametric joint line (J).

10. A turbine diaphragm according to claim 9, in which a bolted joint is provided at each end of the diametric joint line (J) of the outer diaphragm ring.

11. A turbine diaphragm according to claim 9, in which diametrically opposed ends of the upper and lower halves of the diaphragm are held in registration with each other by location features that support them within a surrounding turbine casing.

12. A turbine diaphragm according to claim 1, in which only one of the inner and outer diaphragm rings is a hollow structure, the other ring being solid.

13. A method of fabricating the turbine diaphragm of claim 6 in which a box structure of the inner hollow diaphragm ring (22) comprises:

(a) a pair of axially opposed side walls (221, 222);

(b) a radially inner circumferentially extending wall (223); and

(c) a radially outer circumferentially extending wall formed by the radially inner platform portions (261) of the static blade units (26), and

in which the inner hollow diaphragm ring (22) is fabricated by the steps of:

(a) attaching the side walls (221, 222) to the reinforcing ribs (28);

(b) attaching the inner platforms (261) of the blade units (26) to the side walls (221, 222) of the inner ring (22); and

(c) attaching the inner circumferentially extending wall (223) to the side walls of the inner ring (22).

14. A method of fabricating the turbine diaphragm of claim 6 in which a box structure of the outer hollow diaphragm ring (24) comprises:

(a) a pair of axially opposed side walls (241, 242);

(b) a radially outer circumferentially extending wall (243); and

(c) a radially inner circumferentially extending wall formed by the radially outer platform portions (262) of the static blade units (26), and

in which the outer hollow diaphragm ring (24) is fabricated by the steps of:

(a) attaching the side walls (241, 242) to the reinforcing ribs (30);

(b) attaching the outer platforms (262) of the blade units (26) to the side walls (241, 242) of the outer ring (24);

(c) attaching the outer circumferentially extending wall (243) to the side walls of the outer ring (24).

15. A method of fabricating the turbine diaphragm of claim 6 in which a box structure of the inner hollow diaphragm ring (22) comprises:

(a) a pair of axially opposed side walls (221, 222);

(b) a radially inner circumferentially extending wall (223); and

(c) a radially outer circumferentially extending wall formed by the radially inner platform portions (261) of the static blade units (26), and in which a box structure of the outer hollow diaphragm ring (24) comprises:

(a) a pair of axially opposed side walls (241, 242);

(b) a radially outer circumferentially extending wall (243); and

(c) a radially inner circumferentially extending wall formed by the radially outer platform portions (262) of the static blade units (26), the method comprising the steps of:

(a) attaching the side walls (221, 222) and (241, 242) to the reinforcing ribs (28) and (30);

(b) attaching the inner platforms (261) of the blade units (26) to the side walls (221, 222) of the inner ring (22);

(c) attaching the side walls (241, 242) of the outer ring (24) to the outer platforms (262) of the blade units (26);

(d) attaching the inner circumferentially extending wall (223) to the side walls of the inner ring (22) and attaching the outer circumferentially extending wall (243) to the side walls of the outer ring (24).

16. A variation of the method of claim 13, in which the radially inner wall (223) is attached to the inner ring (22) before the inner platforms (261) of blade units (26) are attached to the inner ring (24).

17. A variation of the method of claim 14, in which the radially outer wall (243) is attached to the outer ring (24) before the side walls (241, 242) of the outer ring (24) are attached to the outer platforms (262) of blade units (26).

18. A variation of the method of claim 15, in which the radially inner wall (223) is attached to the inner ring (22) before the inner platforms (261) of blade units (26) are attached to the inner ring (24).

19. A variation of the method of claim 15, in which the radially outer wall (243) is attached to the outer ring (24) before the side walls (241, 242) of the outer ring (24) are attached to the outer platforms (262) of blade units (26).