GB2432902A

GB2432902A - A Support for a Gas Turbine Combustion Liner Segment

Info

Publication number: GB2432902A
Application number: GB0524739A
Authority: GB
Inventors: Clare Elizabeth Pooley
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2005-12-03
Filing date: 2005-12-03
Publication date: 2007-06-06
Anticipated expiration: 2025-12-03
Also published as: GB2432902B; GB0524739D0

Abstract

A sub-assembly for a gas turbine has a cradle 30 that is releasably mounted to a structural part of the gas turbine and a combustion liner segment 32 secured to the cradle. The cradle may include stiffening fibs (44 fig 4) that may be axially and/or radially extending, and these fibs may be positioned on the surface which in use faces towards the structure 16, 12, of the gas turbine. The liner segment may be secured to the cradle using at least one axially extending clamp ship (56a-c fig 4) and an associated bolt 60. The liner segment surface which faces the cradle may also include axially extending fibs 54, which extend radially into contact with the cradle and define a series of closed channels 62 through which cooling air can flow. The cradle may also include hooks 34 that engage a recess 40, 42, in the turbine structure and an aperture 36 for a mechanical fixing 38, such as a bolt. The surface of the gas turbine structure facing the cradle may also include several circumferentially extending ribs 52 for supporting the cradles. The sub-assembly can be easily removed from a gas turbine for maintenance and repair.

Description

TITLE

Gas turbine sub-assemblies

DESCRIPTION

Technical Field

The present invention relates to sub-assemblies for gas turbines, and in particular to sub-assemblies that allow the combustion liner segments to be more easily removed for maintenance and repair.

Background Art

In a known type of gas turbine, a mixture of compressed air and fuel is supplied to an annular combustion chamber where it is ignited and the resulting gases are used to drive a series of turbine stages. The combustion chamber is defined by radially inner and outer combustion liners that are often formed in a number of circumferentially adjacent segments. The segments that make up the inner and outer combustion liners are secured directly to structural parts of the gas turbine that are normally called the radially inner and outer carriers, respectively. Figure 1 is an axial cross section view showing part of the radially outer combustion liner that is currently used on the GT26 series of gas turbines supplied by ALSTOM Power (ALSTOM' is a registered trade mark). The radially outer combustion liner is formed from a number of circumferentially adjacent segnients. l'wo segments 2a and 2h are shown in Figure 1.

Each segment includes a series of axially extending ribs 4 that contact the inner surface of the radially outer carrier 6. The ribs 4a and 4b that run along the axial edges of each segment form connecting flanges that are fitted into recesses in axially extending clamp strips 8a and 8h. An intermediate rib 4c also forms a connecting flange that is fitted into a recess in a third clamp strip Sc. The clamp strips 8a to Sc are in turn fixed directly to the radially outer carrier 6 using bolts 10.

The ignition of fuel within the combustion chamber means that the combustion liners operate at very high temperatures. However, the underlying carriers are thermally shielded by the combustion liners and are therefore maintained at much lower temperatures. The difference in temperature between the carriers and the combustion liners can cause problems if one or more of the segments have to be removed for maintenance or repair. This is because the bolts that are used to secure the clamp strips to the carriers can seize tight due to thermal distortion between the hot and cold components.

Summary of the Invention

The present invention overcomes the problems mentioned above and provides a sub-assembly for a gas turbine comprising: a cradle that is releasably mountable to a structural part of the gas turbine; and a combustion liner segment secured to the cradle.

The use of an intermediate component such as the cradle means that the combustion liner segments are no longer secured directly to the structural parts of the gas turbine (that is the radially inner and outer carriers). Instead, each combustion liner segment is preferably secured to its own cradle, which in turn is releasably mountable to one of the carriers of the gas turbine in such a way that the entire sub-assembly can be easily installed and removed. It is also possible for two or more combustion liner segments to be secured to a single cradle. It will be readily appreciated that a plurality of such sub-assemblies are assembled together to define the radially inner and outer combustion liners of the combustion chamber.

The combustion liner segment can he fitted to at least one axially extending clamp strip having an axially extending recess fbr receiving an axially extending connecting flange of the combustion liner segment. The clamp strip can be fixed to the cradle using a mechanical fixing such as a bolt or the like. A reinforcing boss can he formed or cast into the cradle defining a hole for receiving the bolt. In practice, it is generally preferred that the combustion liner segment is secured to the cradle using a number of circumferentially spaced clamp strips. For example, a clamp strip can be used to support each of the axial edges of the combustion liner segment. A third clamp strip can be located at an intermediate position to support a central region of the combustion liner segment. Each clamp strip will preferably include a number of axially spaced holes for receiving a bolt (four bolts per strip might he typical) and a corresponding number of bosses will be formed or cast into the cradle at appropriate intervals.

The cradle can include a number of axially extending stiffening ribs and/or a number of circumferentially extending stiffening ribs on the surface facing towards the associated carrier of the combustion liners in use. The mechanical stiffliess of the cradle is an important design issue because of potential re-assembly problems due to residual deformation and the number and height of the axially extending ribs in particular can be chosen to provide the correct stiffness characteristics.

The combustion liner segment can also include a number of axially extending ribs on the surface facing towards the cradle. The ribs of the combustion liner segment can extend radially into contact with a facing surface of the cradle to define closed channels through which a cooling fluid such as air can he supplied to cool the liner and the cradle. The sub-assembly is therefore capable of cooling the combustion liner segments using a closed serial convective cooling system of the type used in the GT26 series of gas turbines, for example. However, instead of the carriers of the gas turbine forming the backplate of the cooling channels, the backplate is formed by the cradle.

This construction provides two advantages. Firstly, it has the potential to cool the sidewalls of the combustion liner segments using impingement cooling (this is described in more detail below). Secondly, the height of the cooling channels can he better controlled by altering the height of the ribs on the rear surface of the combustion liner segmenis. It will be readily appreciated that the cradle will help to shield the associated carrier of the gas turbine from the high operating temperatures of the combustion chamber, especially during transient conditions.

The means by which the cradle is releasably mountable to the carrier depends onthe maintenance strategy of the gas turbine. For example, if the gas turbine is not split during maintenance then one option is to install and remove the sub-assemblies through a space created by the removal of the burner hood (and also the burners, lances and sieves) of the gas turbine. This allows controlled access to the interior of the combustion chamber. The cradle may then include hook means that are receivable in a recess provided in the carrier of the gas turbine and preferably at least one aperture for receiving a mechanical fixing such as a bolt or the like. The hook means is preferably slidably received in the recess in the axial direction SO that any thermal expansion of the sub-assembly can he accommodated. The hook means are preferably provided on the leading end of the cradle (that is the end closest to the turbine vane carrier in use). The at least one aperture is then preferably provided on the trailing end of the cradle (that is the end closest to the burner hood in use). An alternative is to provide the leading end of the cradle with a re-entrant hook means or the like and use a sliding joint at the trailing end. Other suitable fixings can he employed at either end of the cradle depending on the circumstances. If the at least one aperture in the trailing end of the cradle is used to releasably secure the cradle to a carrier of the gas turbine using a mechanical fixing such as a bolt or the like, the mechanical fixing can he physically shielded from the interior of the combustion chamber by a structural component of the gas turbine engine or aggressively cooled using bypass air, for example.

If the gas turbine is split along its horizontal split line during maintenance then an axial fixing can be used. The axial fixing must allow for some axial sliding for assembly and disassembly of the sub-assemblies. This is because the cradles may have hooks that need to be released and without some amount of axial sliding the liner segments that form the radially outer combustion liner will interfere with each other because of the radius of curvature of the liner and the distance from liner segment to the liner segment. The cradle can therefore further include at least one circumferentially extending projection (optionally having a dovetail configuration that allows for at least some axial sliding) receivable in a circumferentially extending recess provided in the carrier of the gas turbine. A sub-assembly can he assembled into position by sliding it down from the split line such that the hook means is received in an annular i.ecess in the carrier and the projection is received in the circumferentially extending recess. The sub-assemblies can he disassembled by removing the upper half of the gas turbine casing before sliding each sub-assembly around the outside of the combustion chamber and removing it from the split line.

Conventional or specially adapted lifting gear can he used to handle the sub-assemblies during the assembly and disassembly process.

The cradle is preferably precision cast as a single piece.

In an alternative embodiment of the invention, the ribs of the combustion liner segment extend radially into Contact with a fcing surface of a cover plate located between the combustion liner segment and the cradle to define channels through which a cooling fluid can be supplied to cool the ribs, passage means being provided between the cover plate and the cradle ibr flow of cooling fluid lherethrough.

Preferably, the cover plate is attached to the ribs of the combustion liner and the combustion liner is supported from the cradle through support means. Conveniently, the support means may include means for supplying cooling fluid to the passage between the cover plate and the cradle.

The present invention further provides a gas turbine comprising: a radially outer carrier having an annular recess; a plurality of sub-assemblies as described above fitted to the radially outer carrier to define a radially outer combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially outer carrier and the cradle of each sub-assembly is secured to the radially outer carrier using a mechanical fixing received through the at least one aperture; a radially inner carrier having an annular recess; and a plurality of sub-assemblies as described above fitted to the radially inner carrier to define a radially inner combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially inner carrier and the cradle of each sub-assembly is secured to the radially inner carrier using a mechanical fixing received through the at least one aperture.

If the gas turbine is split along its horizontal split line during maintenance then the invention further provides a gas turbine comprising: a radially outer carrier having an annular recess and a circumferentially extending recess; a plurality of sub-assemblies as described above fitted to the radially outer carrier to define a radially outer combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially outer carrier, the circumferentially extending projection is received in the circumferentiafly extending recess provided in the radially outer carrier and the cradle of each sub-assembly is secured to the radially outer carrier using a mechanical fixing received through the at least one aperture; a radially inner carrier having an annular recess and a circumferentially extending recess; and a plurality of sub-assemblies as described above fitted to the radially inner carrier to define a radially inner combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially inner carrier, the circumferentially extending projection is received in the circumferentially extending recess provided in the radially inner carrier and the cradle of each sub-assembly is secured to the radially inner carrier using a mechanical fixing received through the at least one aperture.

The axial edges of the cradles may include seals that interlace with axially extending ribs provided Ofl the inner surfaces of the radially inner and outer carriers. The inner surface of the radially inner and outer carriers can further include a plurality of circurnferentjall y extending ribs for supporting the cradles.

Drawings Figure 1 is an axial cross section view of part of a known radially outer combustion liner; Figure 2 is an exploded radial cross section view of part of a gas turbine engine incorporating a sub-assembly according to the present invention; Figure 3 is a radial cross section view of part of a gas turbine engine showing the sub-assembly of Figure 2 located in position; Figure 4 is an axial cross section view of part of a sub-assembly of Figure 2; Figure 5 is a perspective view showing the cradle that forms part of the sub-assembly of Figure 2; Figure 6 is a perspective cross section view of part of a gas turbine engine showing an alternative gas turbine sub-assembly located in position; and Figure 7 is an axial cross section view of an alternative way of fixing the cradle using a modified clamping strip.

A first arrangement of a sub-assembly will now be explained with reference to Figures 2 to 5.

Figure 2 is an exploded radial cross section view of part of a gas turbine showing a radially inner carrier 12 and a turbine vane carrier 14. A radially outer carrier 16 can be fitted to the turbine vane carrier 14 and the radially inner carrier 12 as follows.

The radially outer carrier 1 6 includes a pair of annular flanges 1 8 that are received in annular recesses 20 in the turbine vane carrier 14. The other end of the radially outer carrier 16 is then secured to a mounting region 22 of the radially inner carrier 12 by inserting bolts (not shown) through a series of circumferentially spaced holes 24.

Also shown in Figure 2 is the so-called burner hood', which is bolted to the flanges 22 and 23 of the inner carrier 12.

A number of sub-assemblies 26 are fitted to the radially inner carrier 1 2 to define a radially inner combustion liner. In the same way, a number of sub-assemblies 28 are fitted to the radially outer carrier 16 to define a radially outer combustion liner. ft will he readily appreciated that the annular volume between the radially inner and outer combustion liners is the combustion chamber (labelled CC in Figure 3) of the gas turbine.

The sub-assemblies 26 and 28 have different profiles in the axial direction but the same general two-layer construction. Therefore, any description of the sub- assemblies 26 that form the radially inner combustion liner will also apply to the sub- assemblies 28 that form the radially outer combustion liner and vice versa. Each sub-assembly includes a cradle 30 for supporting a combustion liner segment 32. The end of the cradle 30 thai is adjacent to the turbine vane carrier 14 includes three hooks 34 (see Figure 5). The other end of the cradle 30 includes three circumferenfially spaced holes 36 (see Figure 5) for receiving a bolt 38. The radially inner carrier 12 includes an annular recess 40 and the radially outer carrier 16 includes an annular recess 42 in the region adjacent the turbine vane carrier 14.

The sub-assemblies 26 are fitted to the radially inner carrier 12 by slidably receiving the hooks 34 in the annular recess 40 and then securing the two component parts together by inserting the bolts 38 through the holes 36 and into aligned holes in the carrier. The engagement between the hooks 34 and the annular recess 40 retains the sub-assemblies 26 radially in position but allows fbr thermal expansion in the axial direction between the cradle 30 and the radially inner carrier 12. A number of sub-assemblies 26 are fitted to the radially outer carrier 12 in this way such that their axial edges are in abutment.

In the same way, the sub-assemblies 28 are fitted to the radially outer carrier 16 by slidably receiving the hooks 34 in the annular recess 42 and then securing the two component parts together using the bolts 38. The planar surface 30a of the cradle 30 just below the hooks 34 interfaces with a planar surface 14a of the turbine vane carrier 14. It will he appreciated that the annular recesses 40 and 42 can he replaced by a series of circumferentially spaced recesses (not shown) with each recess being sized and shaped to receive one of the three hooks 34 provided on the each of the cradles 30.

Figure 3 shows the radially inner and outer carriers 12 and 16, the sub-assemblies 26 and 28 and the burner hood 66 in their assembled positions.

The construction of the sub-assembly 28 that is used to form the radially outer combustion liner will now be explained in more detail with reference to Figure 4.

Each sub-assembly 28 includes a cradle 30 and a combustion liner segment 32. The cradle 30 is precision cast as a single piece from a commercially available alloy such as Inconel 625 and there is no requirement for further machining. The outer surthce of the cradle 30 (that is the surface facing towards the radially outer carrier 16) is provided with a series of axially extending stiffening ribs 44 and a series of circumferentially extending stiffening ribs 46 (see Figure 5). Although not shown, inserts of inconel 615 can also be used to protect the radially inner and outer carriers 12 and 16.

l'he radially outer carrier 16 includes a series of axially extending ribs 48 that engage with seal strips 50 that run along the axial edges of each cradle 30 to prevent hot gases in the combustion chamber from flowing behind the cradle through the gap between the circumferentially adjacent sub-assemblies. The seals can also be positioned between the cradle 30 and the combustion liner segment 32. The radially outer carrier 16 also includes a series of circumferentially extending ribs 52 that support the cradle 30.

The outer surface of the combustion liner segment 32 (that is the surface facing towards the cradle 30) includes a series of axially extending ribs 54. The combustion liner segment 32 is secured to the cradle 30 using three axially extending clamp strips 56a, 56b and 56c. Each of the clamp strips 56a to 56c is bolted directly to the cradle using four bolts per strip. The cradle 30 includes cast bosses 58 defining holes for receiving the bolts 60. The ribs 54a and 54c that run along the axial edges of each combustion liner segment 32 form connecting flanges that are received in a recess in the clamp strips 56a and 56c. An intermediate rib 54b also forms a connecting flange that is received in a recess in the clamp strip 56b. It will he readily appreciated that this method of fixing is similar to the way in which the combustion liner segments of the GT26 series of gas turbines are secured to the radially inner and outer carriers.

The remaining ribs 54 are in contact with the inner surface of the cradle 30 to define a series of closed channels 62 through which cooling air can flow.

Because the cradle 30 is not firmly secured to the radially inner and outer carriers 12 and 16 at both ends, an anti-fretting treatment or solid lubricant (not shown) is needed -10-to protect against vibration and/or differential thermal expansion between the cradle and the associated carrier.

The sub-assemblies 26 and 28 can be withdrawn from the gas turbine for strip and re-assembly where access and tooling is more favourable. One possible disassembly method involves the removal of the burner hood (66 in Figure 6) to create a space through which the sub-assemblies can be accessed and withdrawn. To remove the sub- assemblies 26 and 28 it is only necessary to remove the bolts 38 and then withdraw them axially through the space. It will be recalled that one of the disadvaiitages of the conventional gas turbines described above is the fact that the bolts securing the combustion liner segments to the carrier have a tendency to seize tight because of the thermal distortion between the hot and cold component parts. On the face of it, it might he expected that the same problem would occur with the bolts 38 that are used to secure the cradles to the associated carrier. however, the bolts 38 do not suffer from this problem because they are shielded behind other structural parts of the gas turbine (not shown) and can also be positioned so that they are cooled by the bypass air. Similarly, the combustion liner segments 32 shield the cradles 30 from the high temperatures of the combustion chamber so that the temperature difference between the cradles and the associated carrier is not as significant and the effects of thermal distortion are reduced.

Figure 6 also illustrates an alternative assembly and disassembly method, which involves splitting the gas turbine into two parts along the conventional horizontal split line 63, this being the line along which the top and bottom halves of the turbine casing are bolted together. In this case, each cradle 30 can include a series of circumferentially extending projections or dovetails 64 which are slidably received in a corresponding series of circumferentially extending recesses provided on the radially inner and outer carriers 12 and 16, respectively. It will be readily appreciated that such a fixing is not suitable for the disassembly method described with reference IC) Figures 2 to 5 because of the need br an axial shift. The individual sub-assemblies 26 and 28 are slid into position from the split line and then bolted to the associated carrier 12 and 16 as described above. The unbolted sub-assemblies 26 and 28 can be withdrawn one at a time from the gas turbine by sliding them around the outside of the conThustion chamber and removing them at the split line.

Figure 6 clearly shows how the individual sub-assemblies 26 and 28 lie adjacent to each other with their axial edges in abutment.

An alternative way of fixing the combustion liner segment 32 to the cradle 30 using modified clamp strips is shown in Figure 7. The clamp strips (only one of which is shown in Figure 7 and labelled 68) are modified by the addition of air distributor channels 70 to give impingement cooling for the sidewall 32a of the liner segments 32 through impingement holes 72. The use of impingement cooling for the sidewall 32a has been found to extend the service hUe of the combustion liner. The various air paths are represented by the black arrows. The housing 74 for the air distributor channels 70 may he welded to the clamp strips 68. Cooling air is supplied through the cradle 30 and the clamp strips 68 to the distributor channels 70 via a number of axially spaced holes 75. The number, size, shape and position of the holes 75 will depend on the pressure drop required for the impingement cooling. Suitable venturi designs may be used to increase the speed of the cooling air entering the distribution channels 70. The clamp strips 68 are held to the cradle 30 using a number of axially spaced bolts (not shown in Figure 7). The bolts can be hollow to provide an opening through which the cooling air can be supplied to the associated distribution channel 70. This provides a method of aggressively cooling the bolts in addition to the impingement cooling of the sidewall of the liner segment 32.

In practice, the cooling air supplied to the distribution channels 70 is divided between the air that is required for full impingement cooling of the sidewalls 32a and that required for the convection cooling of the rest of the liner segment 32. A cover plate 76 is secured to the back of the liner segment 32 to define an upper cooling channel 78 and a series of lower axially-extending cooling channels. Certain ones of the middle ribs 54 are cast with extensions 82 and these are passed through apertures formed in the cover plate 76. A small rectangular plate 84 is placed over each extension 82 and welded in position to attach the cover plate 76 to the liner segment -12- 32 and prevent leakage of cooling air. The upper cooling channel 78 is formed between the cover plate 76 and the cradle 30 and is used to move the air from the distribution channels 70 (including the cooling air that has been used for full impingement cooling of the sidewalls 32a) in a transverse direction towards the centre of each individual sub-assembly. The lower cooling channels 80 are Ibrmed between the cover plate 76 and liner segment 32 and are supplied with cooling air from a separate source through an impingement plate (not shown) located at the turbine end of the assembly. The air from the impingement plate is used to convectively cool the middle ribs 54. Optimum convection cooling in channel 78 is achieved by having good control over the channel height between the cradle 30 and the liner segment 32.

Provision of the cover plate 76 between the cradle 30 and the liner segment 32 is the easiest way to accomplish such control. The addition of the cover plate 76 also improves the mechanical integrity of the sub-assembly.

It will be readily appreciated that the inner surface of the combustion liner segments 32 (that is the surface facing towards the interior of the combustion chamber) will he covered with a suitable thermal barrier coating (TBC). The T'BC may extend along the surface of the sidewall 32a. It has been found that the combination of full coverage impingement cooling of the sidewall and the extension of the TBC along the surface of the sidewall can lead to a reduction in the metal temperature of about 250 C compared to an uncoated sidewall that is convectively cooled.

Claims

CLAIMS

1. A sub-assembly for a gas turbine comprising: a cradle that is releasably mountable to a structural part of the gas turbine; and a combustion liner segment secured to the cradle.

2. A gas turbine sub-assembly according to claim 1, wherein the combustion liner segment is fitted to at least one axially extending clamp strip and the clamp strip is fixed to the cradle using a mechanical fixing.

3. A gas turbine sub-assembly according to claim 2, wherein the at least one clamp strip includes an axially extending recess for receiving an axially extending connecting flange oithe combustion liner segment.

4. A gas turbine sub-assembly according to claim 2 or claim 3, wherein the mechanical fixing is a bolt.

5. A gas turbine sub-assembly according to claim 4, wherein the cradle includes a boss br receiving the bolt.

6. A gas turbine sub-assembly according to any preceding claim, wherein the cradle further comprises a number of axially extending stiffening ribs and/or a number of circumferentially extending stiffening ribs on the surface facing towards the structural part of the gas turbine in use.

7. A gas turbine sub-assembly according to any preceding claim, wherein the combustion liner segment further comprises a number of axially extending ribs on the surface facing towards the cradle.

8. A gas turbine sub-assembly according to claim 7, wherein the ribs of the combustion liner segment extend radially into contact with a facing surface of the cradle to define channels through which a cooling fluid can be supplied. -14-

9. A gas turbine sub-assembly according to claim 7, wherein the ribs of the combustion liner segment extend radially into contact with a facing surface of a cover plate located between the combustion liner segment and the cradle to define channels through which a cooling fluid can be supplied to cool the ribs, passage means being provided between the cover plate and the cradle for flow of cooling fluid therethrough.

1 0. A gas turbine sub-assembly according to claim 9, wherein the cover plate is attached to the ribs of the combustion liner and the combustion liner is supported from the cradle through support means.

11. A gas turbine sub-assembly according to claim 10, wherein the support means includes means for supplying cooling fluid to the passage between the cover plate and the cradle.

12. A gas turbine sub-assembly according to any preceding claim, wherein the cradle further comprises hook means receivable in a recess provided in the structural part of the gas turbine and at least one aperture for receiving a mechanical fixing.

13. A gas turbine sub-assembly according to claim 12, wherein the cradle lurther comprises at least one circum!èrentially extending projection receivable in a circumferentially extending recess provided in the structural part of the gas turbine.

14. A gas turbine sub-assembly according to any preceding claim, wherein the cradle is precision cast as a single piece.

15. A gas turbine comprising: a radially outer carrier having an annular recess; a plurality of sub-assemblies according to claim 12 fitted to the radially outer carrier to define a radially outer combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially outer -15-carrier and the cradle of each sub-assembly is secured to the radially outer carrier using a mechanical fixing received through the at least one aperture; a radially inner carrier having an annular recess; and a plurality of sub-assemblies according to claim 12 fitted to the radially inhier carrier to define a radially inner combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially inner carrier and the cradle of each sub-assembly is secured to the radially inner carrier using a mechanical fixing received through the at least one aperture.

16. A gas turbine comprising: a radially outer carrier having an annular recess and a circumferentially extending recess; a plurality of sub-assemblies according to claim 13 fitted to the radially outer carrier to define a radially outer combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially outer carrier, the circumferentially extending projection is received in the circumferentially extending recess provided in the radially outer carrier and the cradle of each sub-assembly is secured to the radially outer carrier using a mechanical fixing received through the at least one aperture; a radially inner carrier having an annular recess and a circumferentially extending recess; and a plurality of sub-assemblies according to claim 13 fitted to the radially inner carrier to define a radially inner combustion liner, wherein the hook means of each sub-assembly is slidably received in the annular recess provided in the radially inner carrier, the circumferentially extending projection is received in the circumferentially extending recess provided in the radially inner carrier and the cradle of each sub-assembly is secured to the radially inner carrier using a mechanical fixing received through the at least one aperture.

17. A gas turbine according to claim 1 5 or claim 1 6, wherein the axially extending edges of the cradles include seals that interface with axially extending ribs provided on the inner surfaces of the radially inner and outer carriers. -16-

18. A gas turbine according to any of claims 14 to 17, wherein the inner surface of the radially inner and outer carriers further comprise a plurality of circumferentially extending ribs for supporting the cradles.

19. A gas turbine sub-assembly substantially as herein described and with reference to Figures 2 to 7.