WO2026017692A1

WO2026017692A1 - A ring assembly for a power-generating turbomachine, turbomachine comprising same, and method of assembling

Info

Publication number: WO2026017692A1
Application number: PCT/EP2025/070238
Authority: WO
Inventors: Sambandam MANOHARAN; Leonardo Tognarelli
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2024-07-17
Filing date: 2025-07-15
Publication date: 2026-01-22
Anticipated expiration: 2027-01-17

Abstract

The ring assembly comprises a first ring component and a second ring component, arranged coaxial to one another and extending around an axis of the ring assembly. The two ring components are constrained to one another along an interface formed by mutually matching surfaces of revolution of the first ring component and second ring component. The two ring components are coupled by interference fitting. Disclosed herein is also a turbomachine including one or more ring components and a method of assembling.

Description

A ring assembly for a power-generating turbomachine, turbomachine comprising same, and method of assembling

DESCRIPTION

TECHNICAL FIELD

[0001] Exemplary embodiments of the present disclosure pertain to turbomachines. Specifically, embodiments disclosed herein, pertain to power-generating turbomachines, i.e. expanders or turbines and components thereof.

BACKGROUND ART

[0002] Turbines or expanders include combustors which ignite a pressurized gaseous mixture containing fuel and oxidant. The resulting pressurized flow of hot combustion gas is expanded in an expansion flow path, including one or more expansion stages. Each expansion stage includes at least one annular row or array of stationary vanes, aka stationary blades, and one annular row or array of rotor blades, which form part of a turbine rotor, arranged for rotation in a turbine casing.

[0003] The annular row of rotor blades is often surrounded by a shroud, which prevents or limits leakages of expanding gas radially outside the expansion flow path and towards the casing in which the rotor is housed.

[0004] A maj or concern regarding combustion of fossil fuels relates to the production of carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0005] In recent years, thermal cycles have been developed, aimed at reducing the environmental impact of the power generation cycles using fossil fuels. For this purpose, the option of post combustion capture of carbon dioxide has been investigated. Carbon dioxide capture facilities have been developed, to process flue gas exhausted from gas turbines and remove carbon dioxide therefrom, prior to discharging the flue gas in the environment. The cost of a carbon dioxide capturing facility is high, both in terms CAPEX, as well as in terms of energy required to run the facility, which reduces the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in the flue gas is low. This requires large volumes of flue gas to be processed through the carbon dioxide capturing facility and renders the capturing process particularly inefficient.

[0006] In recent years, oxy-combustion cycles, also known as oxy-fuel cycles, have been developed, wherein fuel, such as natural gas, is blended into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2) at high pressure. The blend of fuel, oxygen and carbon dioxide burns in a combustor of an expander, thus producing a pressurized combustion gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0007] The combustion gas is expanded in the expander or turbine to generate mechanical power, which can eventually be converted into electric power by an electric generator driven into rotation by the expander or can be used for mechanical drive purposes. The exhausted combustion gas, i.e., the flue gas discharged at the discharge side of the expander, is cooled in a regenerative heat exchanger and further chilled to condensate water, which is removed from the chilled flue gas. A main part of the low- temperature flue gas, consisting mainly, or exclusively, of carbon dioxide, is pressurized and recycled through the regenerative heat exchanger towards the combustor of the expander. A remaining part of the flue gas is removed and carbon dioxide contained therein is captured.

[0008] Oxygen supplied to the combustor of the expander can be obtained by separation from ambient air, removing nitrogen therefrom, such that the working fluid supplied to the combustor mainly consists of oxygen and carbon dioxide and does not include nitrogen. The resulting combustion gas does not include nitrogen and the percentage carbon dioxide content thereof is substantially higher than in combustion gas from a standard gas turbine cycle. The higher carbon dioxide percentage in the combustion gas renders carbon capture more efficient and less expensive

[0009] Oxy-fuel cycles, such as those described above, are particularly promising in terms of efficiency, reduction of noxious emissions, and efficient carbon capture. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by elevated pressure and temperature values, as well as by high pressure drops in each stage of the expander and strong temperature gradients across machine components facing the expansion flow path.

[0010] These critical aspects affect the design of machine components facing the expansion flow path. Specifically, stationary shrouds surrounding the annular rows of rotary blades experience high thermal gradients and operate under severe pressure conditions, in contact with a process gas (high-pressure and high-temperature carbon dioxide), which may be aggressive. The selection of materials and the technology adopted for assembling a stationary shroud component may become critical in case of supercritical CO2 expanders, or other turbomachines operating under similar thermodynamic conditions.

[0011] Similar issues may arise in the design of components supporting stationary blades, aka stationary vanes, or nozzles.

[0012] An object of embodiments disclosed herein is to improve the design of shroud assemblies, particularly for use in supercritical carbon dioxide expanders or other power-generating turbomachines operating under severe thermodynamic conditions and chemical environments, or analogous mechanical components, for instance rings configured to support stationary machine components facing the hot expansion flow path, such as stationary blades, i.e. stationary vanes.

SUMMARY

[0013] According to one aspect, disclosed herein is a shroud assembly for a powergenerating turbomachine, for instance an expander, such as a supercritical CO2 expander, wherein the shroud assembly comprises a ring and a plurality of shroud segments arranged generally continuously around the ring and retained thereto. The ring comprises a first ring component and a second ring component, coaxial to one another and constrained to one another. In the first ring component and the second ring component form a seat extending circumferentially around an axis of symmetry of the shroud assembly. The seat comprises a first circular, i.e. annular, slot formed between the first ring component and the second ring component; and wherein each shroud segment comprises a first hook engaging the first circular slot. The ring components are preferably coupled to one another by interference-fitting or shrink-fitting. In other embodiments, the two ring components, which form the seat where the shroud segments are retained, can be coupled to one another in a different way, for instance by welding.

[0014] By splitting the ring of the shroud assembly into two ring components several advantages are achieve. For instance, each ring component is smaller and lighter than the entire ring and is therefore easier and less expensive to manufacture, for instance by forging.

[0015] Moreover, assembling of the shroud segments is easier, each ring component can be monolithic, i.e. can be formed by a continuous body extending around 360°. Since the shroud segments are mounted in the seat formed by coupling the first and second ring component to one another, no notch is required to introduce the shroud segments into the seat.

[0016] In some embodiments the two ring components can be coupled to one another, with the shroud segments mounted therein, by welding.

[0017] However, in particularly advantageous embodiments the two ring components are mounted by shrink-fit. Mounting is made simpler and the structural resistance of the ring is higher.

[0018] According to a further aspect, disclosed herein is a power-generating turbomachine, comprising a casing and a rotor housed in the casing for rotation therein around a rotation axis. The rotor includes: at least one annular row of rotor blades extending radially outwardly from the rotor and positioned in an expansion flow path; and at least one stationary shroud assembly surrounding the rotor blades. The shroud assembly is configured as outlined above and is stationarily housed in the casing.

[0019] According to a yet further aspect, disclosed herein is a method for assembling a shroud assembly for a turbomachine, the shroud assembly comprising a plurality of shroud segments coupled to a ring, the ring comprising a first ring component and a second ring component, coupled to one another. The method comprises a step of assembling together a plurality of shroud segments and one of a first ring component and a second ring component. The method further comprises a step of bringing the first ring component and the second ring component together, thus forming a circumferential seat, in which the shroud segments are engaged. Furthermore, the method also includes the step of coupling to one another the first ring component and the second ring component with the shroud segments locked in the circumferential seat.

[0020] Further features and embodiments of the ring assembly, of the turbomachine and of the method for mounting the ring assembly are described below with reference to the enclosed drawings, and are outlined in the attached claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Reference is now made briefly to the accompanying drawings, in which:

Fig.1 illustrates a sectional view of an embodiment of an expander according to the present disclosure;

Fig.2 illustrates a sectional view of the ring assembly of the first expansion stage of the expander of Fig.1 in one embodiment;

Fig.3 illustrates a sectional view of the ring assembly of the first expansion stage of the expander of Fig.1 in another embodiment;

Fig.4 illustrates a sectional view of the ring assembly of the second expansion stage of the expander of Fig.1;

Fig.5 illustrates an axonometric view of the second stage ring assembly;

Fig.6 illustrates an enlargement of the detail VI of Fig.5;

Fig.7 illustrates a sectional view of a portion of one ring component of the ring assembly of Fig.4;

Fig.8 illustrates a sectional view of a portion of another ring component of the ring assembly of Fig.4;

Fig.9 illustrates a sectional view of an array of shroud segments of the ring assembly of Fig.4;

Fig.10 illustrates a sectional view, similar to Fig.3, of a ring assembly in a further embodiment;

Fig.11 illustrates a sectional view of a ring assembly in a yet further embodiment;

Fig.12 illustrates a sectional view according to line XII-XII of Fig.11; and

Figs. 13 and 14 illustrate sectional view similar to Fig.3, of a ring assembly in further embodiments.

DETAILED DESCRIPTION

[0022] A sectional view of one embodiment of an expander 1 according to the present disclosure is shown in Fig.1. The section is taken along a plane containing a rotation axis A-A of the expander. The sectional view shows only half expander, which is axial-symmetrical.

[0023] The expander 1 includes an outer casing 3 and an inner casing 5. The outer casing 3 can include a main body 3 A and a closure 3B on the aft side of the expander. The main body 3A and the closure 3B are coupled, through respective flanges, along a plane orthogonal to the rotation axis of the expander. In this embodiment, therefore, the outer casing 3 is a so-called vertically split casing. Conversely, in the embodiment disclosed herein, the inner casing 5 is a horizontally split casing.

[0024] This combination of inner horizontally split casing 5 and outer vertically split casing 3 is particularly beneficial in case of a supercritical carbon dioxide expander, or another expander which processes a gas under similar thermodynamic conditions involving high temperature gradients and elevated pressure drops across some at least of the expansion stages. Nevertheless, in other embodiments, the novel features disclosed herein can be used in a different expander or turbine structure, for instance including a single casing, or a vertically split inner casing and a horizontally split outer casing, for instance.

[0025] A combustor, such as a can combustor including a plurality of combustion chambers 7, is positioned at the forward side of the expander. An annular chamber 6 is positioned between the outer casing 3 and the inner casing 5.

[0026] As used herein “forward” and “aft” are referred to the direction of flow of the process gas through the expander 1. Therefore, “forward” indicates a position on the side of the combustor chambers 7 and “aft” indicates a position on the side opposite the combustor chambers 7, i.e., the discharge side of the expander 1.

[0027] The expander 1 further comprises a rotor 11 housed in the inner casing 5 and adapted to rotate around the rotation axis A-A. The rotor 11 comprises a rotor shaft 13 and a plurality of annular rows, arrays, or sets of rotor blades. In the exemplary embodiment of Fig.1, the rotor 11 comprises eight annular arrays of rotor blades 15. Each array of rotor blades comprises rotor blades circumferentially arranged around the rotation axis A-A of the rotor 11.

[0028] An annular row, array, or set of stationary blades, aka stationary vanes, 17 is positioned upstream of each annular array of rotor blades 15. The blades of the annular arrays of stationary blades 17 are circumferentially arranged around the rotation axis A-A.

[0029] Pairs of adjacent stationary blades or vanes define respective nozzles which orient the expanding gas in the correct orientation with respect to the downstream annular row or rotor blades 15. [0030] Each annular array of stationary blades 17 and respective annular array of rotor blades 15 form together a stage of the expander 1.

[0031] In the embodiment shown in Fig. l all stationary blades 17 of each annular array or set of stationary blades, except those of the last stage, are supported by a respective ring assembly 18. The ring assemblies 18 will be referred to herein after also simply as “rings”. The most downstream annular array of stationary blades 17 is mounted on an aft portion 5B of the inner casing 5. The aft portion 5B of the inner casing is coupled to a main body 5 A of the inner casing 5. The main body 5 A can in turn be formed by a plurality of casing portions. Each casing portion can be split along a plane containing the rotation axis A-A of the rotor 11, i.e., the inner casing 5 is a so- called horizontally split casing.

[0032] The ring assemblies 18 can be centered and locked against rotation in the casing by means of pins, keys or other locking features, to prevent rotation of the ring assemblies 18 during operation of the expander.

[0033] Each annular array of rotor blades 15 is surrounded by a respective shroud 19. In the embodiment of Fig.1, each shroud 19 is supported by a respective one of the above-mentioned ring assemblies, or rings, 18. The unit including the shroud 19 and the ring assembly, or ring, 18 whereto the shroud 19 is coupled and by which the shroud is supported, is referred to herein as a “shroud assembly 20”. The most upstream ring 18 supports the forward-most annular row of stationary vanes, or stationary blades 17 and does not support the shroud 19 which surrounds the respective most upstream annular row of rotor blades. A separate ring supports the shroud 19 of the first expander stage.

[0034] In the embodiment of Fig. l the shroud assemblies of all stages, except the first and the last, include respective rings, i.e. ring assemblies, which also support the respective stationary vanes or stationary bladesl7.

[0035] The rings 18 form a structure which supports the stationary blades 17 and the shrouds 19 and which separates the expansion flow path of the hot expanding gas from the inner casing 5. The expansion flow path extends through the stationary vanes or stationary blades 17 and the rotor blades 15 and is surrounded by the shrouds 19.

[0036] The rings 18 decouple the expansion flow path from the inner casing 5 and form with the latter a gap 62 for a pressurized cooling or purging fluid, between the rings 18 and the inner casing 5. Calibrated flow passages (not shown) can be foreseen in the structure formed by the rings, such that a controlled amount of cooling gas can flow from the pressurized fluid gap towards the expansion flow path.

[0037] As will be described in detail below, some of the ring assemblies 18, i.e. rings 18, comprise a first ring component and a second ring component. The first and second ring components forming the ring assembly 18 are coaxial and are coupled to one another. In some embodiments, the first ring component and the second ring component forming the ring assembly are coupled to one another along an interface, formed by mutually contacting surfaces of the first ring component and second ring component.

[0038] The ring components of a ring assembly, or ring, 18, can be coupled by interference-fitting, in particular by shrink-fitting. Interference between the two ring components of a single ring assembly 18 is generated at the interface between the ring components, for instance along mutually contacting cylindrical or conical surfaces. In general terms, the interface is formed by complementary surfaces, i.e. mutually matching surfaces. In the following description of exemplary embodiments, the mutually matching surfaces include a cylindrical surface region or surface portion, co-axial with the ring assembly, and a planar surface portion, orthogonal to the axis of the ring assembly. This is however not the only possible configuration of the interface between first and second ring component.

[0039] In general terms, in embodiments disclosed herein the interface is formed by matching surfaces, which can be adapted to couple the first ring component and the second ring component to one another by interference-fitting, and more specifically by shrink-fitting.

[0040] With continuing reference to Fig.l, an embodiment of the most upstream shroud assembly 20, comprising a ring assembly (ring) 18 and a shroud mounted thereon is shown in Fig.2.

[0041] In this embodiment, the most upstream shroud assembly 20 comprises a ring component 18A and a ring component 18B which are coaxial to one another and constrained to one another to form the ring 18. Each ring component 18 A, 18B can be monolithic, i.e. can be formed by a single piece extending 360° around the axis of symmetry of the shroud assembly 20, which coincides with the rotation axis A-A of the rotor 11.

[0042] The first shroud 19 comprises a plurality of shroud segments 19A arranged generally continuously around the ring 18, and around the rotation axis A-A, and constrained to the ring 18 in a manner to be described.

[0043] The ring component 18 A and the ring component 18B form a seat extending circumferentially around an axis of symmetry of the shroud assembly 20, which coincides with the rotation axis A-A of the expander when the shroud assembly 20 is mounted in the casing 5. In the embodiment of Fig.2 the seat comprises two circular slots 51 and 53. Each circular slot 51, 53 is formed between the ring component 18A and the ring component 18B. The circular slot 51 is open in an aft direction fA and the circular slot 53 is open in a forward direction fF.

[0044] Each shroud segment 19A comprises a respective aft-side hook 55, which is oriented in the forward direction fF and engages the circular slot 51. Each shroud segment 19A further comprises a forward-side hook 57, which is oriented in the aft direction fA and engages the circular slot 53.

[0045] In the embodiment of Fig.2, the ring component 18B has a radially inner surface 18C and a radially outer surface 18D. The ring component 18B has a radially inner surface 18E and a radially outer surface 18F. In the embodiment of Fig.2, the surfaces 18C, 18D, 18E and 18F are cylindrical, to facilitate mounting of the two rings components 18 A, 18B one into the other.

[0046] The ring component 18A and the ring component 18B are concentric to one another, and the radially outer surface 18D of the ring component 18B is in shrink-fit engagement with the radially inner surface 18E of the ring component 18 A. Similarly, the hooks 55 and 57, or at least one thereof, are locked by shrink-fitting in the respective circular slots 51 and 53.

[0047] As a matter of fact, the two ring components 18 A, 18B are therefore coupled to one another by interference-fitting, and specifically by shrink-fitting, along an interface which Is formed by mutually matching surface 18D of ring portion 18B and surface 18E of ring portion 18 A. Both matching surfaces 18D and 18E are surfaces of revolution, i.e. generated by the revolution of a generatrix around the axis A-A of the ring assembly 18. The generatrix of the surface 18D is a straight line, i.e. a rectilinear segment. The generatrix of surface 18E in this embodiment is formed by three rectilinear segments, forming a recessed surface of revolution, wherein the surface of revolution 18D is housed.

[0048] To assemble the components 18 A, 18B and 19A of the shroud assembly 20 of Fig.2 the following method can be used. As a first step, the two ring components 18 A, 18B are brought at different temperatures, such that the diameter of the inner surface 18E becomes larger than the outer diameter of the outer surface 18D. This can be achieved by either heating the outer ring component 18 A, or by cooling the inner ring component 18B, or by both heating the outer ring component 18 A and cooling the inner ring component 18B. In either way, a clearance is obtained between the inner ring component 18B and the outer ring component. The shroud segments 19A are mounted on the inner ring component 18B. This can easily be done if the circular grooves or slots 51, 53 are sufficiently deep, compared to the length of the teeth 55, 57. A relative heat expansion of the shroud segments 19A with respect to the ring component 18B can facilitate the assembling operation.

[0049] Once the shroud segments 19A are mounted around the ring component 18B on the inner side thereof, the two ring components 18 A, 18B can be brought in the coaxial and concentric position, as shown in Fig.2. The two ring components 18 A, 18B are then brought at the same temperature again. The inner diameter of surface 18E and the outer diameter of surface 18D are such that, when they are at the same temperature, the two ring components 18 A, 18B are shrink-fit to one another, i.e. constrained to one another by interference. The radial dimension of at least one of the teeth 55, 57 of each shroud segment 1A, and the radial dimension of the circular slots or grooves 51, 53 are such that the shroud segments 19A are locked to the ring 18 by shrink-fit.

[0050] With continuing reference to Figs 1 and 2, a further embodiment of the most- upstream shroud assembly 20 is illustrated in Fig.3.

[0051] In the embodiment of Fig.3, the ring component 18A and the ring component 18B form a seat extending circumferentially around an axis of symmetry of the shroud assembly 20, which coincides with the rotation axis A-A of the expander when the shroud assembly 20 is mounted in the casing 5. In the embodiment of Fig.3 the seat comprises two circular slots 51 and 53. The circular slot 51 is formed between the ring component 18A and the ring component 18B. The circular slot 53 is formed as groove in the ring component 18B, and specifically on the forward surface thereof, i.e. the surface of the ring component 18B which faces in the forward direction fF. The circular slot 51 is open in an aft direction fA and the circular slot 53 is open in a forward direction fF.

[0052] Each shroud segment 19A comprises a respective aft-side hook 55, which is oriented in the forward direction fF and engages the circular slot 51. Each shroud segment 19A further comprises a forward-side hook 57, which is oriented in the aft direction fA and engages the circular slot 53.

[0053] In the embodiment of Fig.3, the ring component 18B has a radially inner surface 18C and a radially outer surface 18D. The ring component 18B has a radially inner cylindrical surface 18E and a radially outer surface 18F. The ring component 18A and the ring component 18B are coaxial to one another but shifted one with respect to the other in the axial direction (parallel to rotation axis A-A). The radially outer cylindrical surface 18D of the ring component 18B is in shrink-fit engagement with the radially inner cylindrical surface 18E of the ring component 18 A. Similarly, the hooks 55 and 57 are locked by shrink-fitting in the respective circular slots 51 and 53 and specifically at least hook 55 is shrink-fitted in the circular slot 51 facing in the aft direction fA.

[0054] The ring component 18B further comprises a flat, i.e. planar surface 18G, which is in contact with a corresponding flat or planar surface 18H of the ring component 18 A. The flat surface 18G faces in the aft direction fA and the flat surface 18H faces in the forward direction fF.

[0055] Thus, in the embodiment of Fig.3 the two ring components 18 A, 18B which form the ring assembly 18 are coupled to one another along matching surfaces in the form of ruled surfaces of revolution, which are formed as follows. The first surface is a surface of revolution forming part of ring component 18A and being generated by a generatrix comprising: a rectilinear segment orthogonal to the axis of the ring assembly 18 and which generates the flat surface 18H; a rectilinear segment parallel to the axis of the ring assembly 18 and which generates the cylindrical surface 18D. These two surfaces of revolution match with the planar or flat surface of revolution 18G, belonging to ring portion 18B and generated by a rectilinear segment orthogonal to the axis of the ring assembly 18, and with the cylindrical surface of revolution 18D, belonging to ring portion 18B and generated by a rectilinear segment parallel to the axis of the ring assembly 18.

[0056] To assemble the components 18 A, 18B and 19A of the shroud assembly 20 of Fig.3 the following method can be used. As a first step, the two ring components 18 A, 18B are brought at different temperatures, such that the diameter of the inner cylindrical surface 18E becomes larger than the outer diameter of the outer cylindrical surface 18D. This can be achieved by either heating the outer ring component 18 A, or by cooling the ring component 18B, or by both heating the outer ring component 18A and cooling the ring component 18B. In either way, a clearance is obtained between the ring component 18B and the ring component and specifically between the two cylindrical surfaces 18D and 18E thereof. The shroud segments 19A are mounted on the ring component 18B. This can easily be done if the circular grooves or slots 51, 53 are sufficiently deep, compared to the length of the teeth 55, 57. A relative heat expansion of the shroud segments 19A with respect to the ring component 18B can facilitate the assembling operation.

[0057] Once the shroud segments 19A are mounted around the ring component 18B on the inner side thereof, the two ring components 18 A, 18B can be brought in the coaxial position, as shown in Fig.3. The two ring components 18 A, 18B are then brought back at the same temperature. The inner diameter of the cylindrical surface 18E and the outer diameter of cylindrical surface 18D are such that, when the two ring components are at the same temperature, they are shrink-fit to one another. I.e., the two ring components 18A, 18B are constrained to one another by interference. The radial dimension of at least one of the teeth 55, 57 of the shroud segments 19A and the radial dimensions of the circular slots or grooves 51, 53 are such that the shroud segments 19A are locked to the ring 18 by shrink-fit. Preferably both teeth 55, 57 of the circumferentially arranged shroud segments 19A are locked by shrink-fitting in the respective circular slots 51, 53.

[0058] The remaining rings 18, starting from the third stage, to the seventh stage of the expander 1, are designed substantially in the same way and only the shroud assembly of the third expansion stage will be described in detail, reference being made to Figs. 4 to 9. [0059] While in Figs 1 to 3 the seat, where the shroud segments 19A are retained and locked, comprises two circular slots oriented in opposite (aft and forward) directions, and the teeth of the shroud segments are faced towards one another, in the embodiment of Figs. 4 to 9 the shroud segments are each provided with two teeth facing in opposite (i.e. aft and forward) directions, which engage circular slots or grooves which are oriented one towards the other.

[0060] More specifically, the shroud 19 of the embodiment of Figs. 4 to 9 comprises again a ring component 18A and a ring component 18B, which form a seat extending circumferentially around an axis of symmetry of the shroud assembly 20. In the embodiment of Figs.4 to 9 the seat comprises two circular slots again labeled 51 and 53. The circular slot 51 is formed between the ring component 18A and the ring component 18B. The circular slot 53 is formed as an annular groove in the ring component 18B, and specifically on the forward surface thereof, i.e. the surface of the ring component 18B, which faces in the forward direction fF. The circular slot 51 is open in an aft direction fA and the circular slot 53 is open in a forward direction fF.

[0061] Each shroud segment 19A comprises a respective aft-side hook 55, which is oriented in the aft direction fF and engages the circular slot 51. Each shroud segment 19A further comprises a forward-side hook 57, which is oriented in the forward direction fA and engages the circular slot 53.

[0062] In the embodiment of Figs. 4 to 9, the ring component 18A and the ring component 18B are coaxial to one another, but shifted one with respect to the other in the axial direction (parallel to rotation axis A- A), the ring component 18B extending in the forward direction beyond a forward surface of the ring component 18 A.

[0063] The ring component 18B has a radially inner cylindrical surface 18C and a radially outer cylindrical surface 18D. The ring component 18B has a radially inner cylindrical surface 18E and a radially outer surface 18F. The radially outer cylindrical surface 18D of the ring component 18B is in shrink-fit engagement with the radially inner cylindrical surface 18E of the ring component 18A. Similarly, the hooks 55 and 57 are locked by shrink-fitting in the respective circular slots 51 and 53 and specifically at least hook 55 is shrink-fitted in the circular slot 51 facing in the aft direction fA.

[0064] The ring component 18B further comprises a flat, i.e. planar surface 18G, which is in contact with a corresponding flat or planar surface 18H of the ring component 18 A. The flat surface 18G faces in the aft direction fA and the flat surface 18H faces in the forward direction fF.

[0065] The surfaces 18G, 18E and 18H, 18D are ruled surfaces of revolution, i.e. surfaces of revolution generated by straight lines or segments. More specifically, the ruled surfaces of revolution 18E, 18D are cylindrical surfaces and the surfaces of revolution 18H and 18E are planar surfaces of revolution, as they are generated by a rectilinear segment orthogonal to the axis of the ring assembly 18.

[0066] To mount the components 18 A, 18B and 19A of the shroud assembly 20 of Figs.4 to 9 the following method can be used. As a first step, the two ring components 18 A, 18B are brought at different temperatures, such that the diameter of the inner cylindrical surface 18E becomes larger than the outer diameter of the outer cylindrical surface 18D. This can be achieved by either heating the outer ring component 18 A, or by cooling the inner ring component 18B, or by both heating the outer ring component 18A and cooling the inner ring component 18B. In either way, a clearance is obtained between the inner ring component 18B and the outer ring component. The shroud segments 19A are mounted on the inner ring component 18A by engaging the respective hooks 55 in the circular slot 51.

[0067] Once the shroud segments 19A are engaged with the ring component 18 A, the two ring components 18 A, 18B can be brought in the coaxial position, as shown in Fig.4, forming the circular slot 53 therebetween, with the hooks 57 of the shroud segments 19A engaged therein. The two ring components 18 A, 18B are then brought back at the same temperature. The inner diameter of the cylindrical surface 18E and the outer diameter of cylindrical surface 18D are such that, once brought at the same temperature, the two ring components 18 A, 18B are shrink-fit to one another and connected to one another by interference.

[0068] The radial dimension of at least one of the teeth 55, 57 of the shroud segments 19A and the radial dimensions of the circular slots or grooves 51, 53 are such that the shroud segments 19A are held inside slot of the 18. At least one and preferably both the teeth 55 and 57 of each shroud segment 19A is held in the respective circular slot 51 or 53. [0069] As shown in particular in Fig.4, the ring component 18A and the ring component 18B are in mutual contact at the cylindrical shrink-fitting surfaces 18D and 18E, and further in mutual contact at the annular flat contact surfaces 18H and 18G. These mutually matching surfaces are surfaces of revolution generated by generatrixes formed by straight segments, which are respectively orthogonal and parallel to the axis of the ring assembly 18, similarly to the configuration of Fig. 3.

[0070] While in the previously described figures, the interface between the two ring components 18 A, 18B are mainly formed by continuous surfaces of revolution generated by generatrixes in the form of straight segments, i.e. they are ruled surfaces, this is not mandatory, even if it may be preferred in some embodiments.

[0071] In some embodiments, the matching surfaces of contact can be discontinuous, rather than continuous. Moreover, the generatrixes can be curvilinear, rather than rectilinear, and/or can include a combination of rectilinear and curvilinear portions. Furthermore, the surfaces of revolution of the two ring portions 18 A, 18B do not need to be in intimate contact along the entire development thereof. It can also be envisaged, that the surfaces of revolution be formed by generatrixes including a plurality of rectilinear and/or curvilinear segments in combination.

[0072] Some exemplary embodiments will clarify this aspect.

[0073] In Fig.10 the surfaces 18G, 18H are shaped in the form of opposing toothed surfaces, each comprising teeth in the form of circular projections extending around the axis of the ring assembly 18. Compared with the embodiment of Fig.4, a more extensive surface of shrink-fitting is obtained, since the two ring components 18 A, 18B are coupled by interference not only along surfaces 18D, 18E, but also at each tooth forming the surfaces 18H, 18G.

[0074] Fig.11 shows a sectional view similar to Fig.4 of a further embodiment. In this embodiment, the surfaces 18H and 18D are in fact formed by a single curved surface of revolution, generated by a generatrix which includes a rectilinear segment parallel to the axis of the ring assembly 18 and a rectilinear segment which is inclined with respect to the axis by an angle different than 90°. This later rectilinear segment generates a conical surface of revolution. The two straight segments are connected to one another by a curve. The generatrix is thus comprised of a first rectilinear segment parallel to the axis of the ring assembly 18, a curve and a second rectilinear segment inclined with respect to the axis and not orthogonal thereto. In this embodiment the surfaces 18G and 18E belonging to the ring portion 18A are identical, to the surfaces 18H and 18D, but this is not mandatory, as will be clarified later on.

[0075] Fig.12 shows a sectional view along line XII-XII of Fig.11 in a possible modified embodiment. In this embodiment, each surface of revolution is discontinuous and includes indentations 16A, 16B. Other forms of discontinuities can be present in the two matching surfaces of revolution along which the two ring components 18 A, 18B are coupled to one another, provided sufficient shrink-fitting pressure is generated between the surfaces, and sufficient sealing is provided.

[0076] A further embodiment of a ring assembly 18 is shown in the sectional view of Fig. 13. The same reference numbers designate parts which are equivalent or functionally equivalent to those of Fig.4. In the embodiment of Fig.13 the mutually matching surfaces of revolution are generated by generatrixes formed by a first rectilinear segment parallel to the axis of the ring assembly 18 and by a second rectilinear segment which is inclined with respect to said axis, and forms therefore a conical surface of mutual contact between the two ring components 18 A, 18B.

[0077] The exemplary embodiment of Fig.13 also shows that the seat where the shroud segments 19A are retained may have a non-continuous shape and may include indentations 16 A, for instance.

[0078] Finally, as mentioned above, the mutually matching continuous or discontinuous surfaces of revolution along which the first and second ring portions 18 A, 18B are coupled do not need to be identical to one another as shown above in Figs. 4 to 13. For instance, Fig.14 shows that an annular empty volume 22 can be formed between the mutually matching surfaces of revolution, when the two surfaces are not identical to one another.

[0079] In the above-described embodiments, the ring assemblies 18 are configured to support the shroud of the respective stage. In some embodiments, the ring assemblies can be configured to support not only the shroud but also the stationary vanes, or only the stationary vanes.

[0080] For instance, the rings 18 of the shroud assemblies 20 of the second to seventh expansion stage of the expander 1 further comprise each an attachment feature adapted to attach an annular row of the corresponding stationary blades or vanes 17. As best shown in Figs. 1 and 4 in some embodiments the attachment feature is positioned on only one of the ring components 18 A, 18B and specifically on the ring component 18B.

[0081] In some embodiments, the attachment feature includes an annular groove 61 facing in the forward direction, i.e. formed on a forward-facing surface of the ring . Each stationary blade 17 comprises a corresponding tooth 63, which is oriented in the aft direction and engages the corresponding annular groove 61.

[0082] In the area of contact between abutting rings, or between ring and casing, stationary seals (not shown) can be positioned, to prevent, limit or control the amount of cooling or purging gas leaking through the mutually contacting surfaces.

[0083] As shown in the attached figures, the most downstream expansion stages are devoid of rings 18, and the stationary vanes and shrouds are mounted directly on the casing. This makes the arrangement simpler. The simplified arrangement is particularly suitable in those stages where less critical thermodynamic conditions (temperature and pressure) exist, i.e. the most downstream ones.

[0084] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A shroud assembly for a power-generating turbomachine, the shroud assembly comprising a ring and a plurality of shroud segments arranged generally continuously around the ring and retained thereto; wherein the ring comprises a first ring component extending around an axis of symmetry and a second ring component extending around the axis of symmetry; wherein the first ring component and the second ring component are constrained to one another; wherein the first ring component and the second ring component form a seat extending circumferentially around the axis of symmetry; wherein the seat comprises a first circular slot formed between the first ring component and the second ring component and a second circular slot, opposite to the first circular slot; and wherein each shroud segment comprises a first hook engaging the first circular slot and a second hook engaging the second circular slot.

2. The shroud assembly of claim 1, wherein the first ring component is a single piece ring component extending 360° around the axis of symmetry; and wherein the second ring component is a single piece ring component extending 360° around the axis of symmetry.

3. The shroud assembly of claim 1 or 2, wherein: the first circular slot has an annular aperture facing in a forward direction and the first hook of each shroud segment extends into the first circular slot through said annular aperture in an aft direction; or the first circular slot has an annular aperture facing in an aft direction and the first hook of each shroud segment extends into the first circular slot through said annular aperture in a forward direction.

4. The shroud assembly of any one of the preceding claims, wherein the first ring component and the second ring component are coupled to one another by shrink- fit.

5. The shroud assembly of any one of the preceding claims, wherein the shroud segments are constrained to the first ring component and the second ring component by shrink-fit.

6. The shroud assembly of any one of the preceding claims, wherein the first hook of each shroud segment is positioned at a forward side of the respective shroud segment.

7. The shroud assembly of any one of the preceding claims, wherein the second circular slot is formed by the first ring component and the second ring component.

8. The shroud assembly of any one of the claims 1 to 6, wherein the second circular slot is formed in one of said first ring component and second ring component.

9. The shroud assembly of any one of the preceding claims, wherein the first circular slot and the second circular slot have respective annular apertures facing each other; and wherein the first hook and the second hook of each shroud segment are oriented in opposite directions.

10. The shroud assembly of any one of the preceding claims, wherein the first circular slot and the second circular slot have respective annular apertures opposing each other and the first hook and the second hook of each shroud segment are oriented towards each other.

11. The shroud assembly of any one of the preceding claims, wherein the second hook of each shroud segment is positioned at an aft side of the respective shroud segment.

12. The shroud assembly of any one of the preceding claims, wherein: the first ring component has a radially inner surface and a radially outer surface; the second ring component has a radially inner surface and a radially outer surface; and the first ring component and the second ring component are concentric to one another, the radially outer surface of the second ring component being in shrink-fit engagement with the radially inner surface of the first ring component

13. The shroud assembly of any one of claims 1 to 11, wherein the first ring component and the second ring component are in mutual contact at cylindrical shrink-fitting surfaces and at annular flat contact surfaces.

14. The shroud assembly of any one of the preceding claims, wherein the ring comprises an attachment feature, adapted to attach an annular row of stationary blades to the ring.

15. The shroud assembly of claim 14, wherein the attachment feature is positioned on only one of said first ring component and second ring component.

16. The shroud assembly of claim 14 or 15, wherein the attachment feature comprises an annular groove formed on a forward-facing surface of the ring

17. A power-generating turbomachine, comprising: a casing; a rotor housed in the casing for rotation therein around a rotation axis; at least one annular row of rotor blades extending radially outwardly from the rotor and positioned in an expansion flow path; and at least one stationary shroud assembly stationarily housed in the casing and surrounding the annular row of rotor blades; wherein the shroud assembly is according to any one of the preceding claims,

18. The turbomachine of claim 17, further comprising a plurality of further annular rows of rotor blades; the annular rows of rotor blades being positioned in sequence along said expansion flow path, and each annular row of rotor blades comprising a respective stationary shroud assembly according to any one of claims 1 to 16.

19. The turbomachine of claim 17 or 18, further comprising at least one annular row of stationary blades, positioned upstream of each annular row of rotor blades; and wherein the stationary blades are attached to one of the first ring component and second ring component of the respective shroud assembly.

20. The turbomachine any one of the preceding claims, further comprising a combustor upstream of the expansion flow path.

21. A method for assembling a shroud assembly for a turbomachine, the shroud assembly comprising a plurality of shroud segments coupled to a ring, the ring comprising a first ring component and a second ring component, coupled to one another; the method comprising the following steps: assembling together a plurality of shroud segments and one of a first ring component and a second ring component; bringing the first ring component and the second ring component together, thus forming a circumferential seat, in which the shroud segments are engaged; and coupling to one another said first ring component and said second ring compo- nent with said shroud segments locked in the circumferential seat.

22. The method of claim 21, wherein the step of coupling the first ring component and second ring component comprises the step of shrink-fitting the first ring component and the second ring component to one another thus locking the shroud segments in the circumferential seat by shrink-fit.