EXPANDER CONTAINING INNER RINGS SUPPORTING FIXED BLADES

Abstract

The expander (1) comprises an outer casing (3) and an inner casing (5) housed in the outer casing. A plurality of annular arrays of stationary blades (17) are housed in the inner casing. A rotor (11) is housed in the inner casing for rotation therein. The rotor comprises a rotation axis and a plurality of annular arrays of rotor blades (15) surrounding the rotation axis. Each annular array of rotor blades is arranged downstream of a respective one of said annular arrays of stationary blades and forms a respective expander stage therewith. The stationary blades (17) of each annular array of stationary blades are mounted on one respective ring (18) housed in the inner casing. Each ring is in axially oriented pressure contact with two adjacent rings or with one adjacent ring (18) and the inner casing (5). A cooling fluid gap (61) is formed between the rings and the inner casing.

Description

AN EXPANDER INCLUDING INNER RINGS SUPPORTING STATIONARYBLADESDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns power-generating turbomachines. Embodiments disclosed herein concern supercritical carbon dioxide expanders, or more generally expanders operating at high pressure ratios.BACKGROUND ART

[0002] Fossil fuels are a major source of chemical energy used for the generation of mechanical power. Fossil fuels are mixed with air and combusted to generate a combustion gas at high pressure and temperature, which expands in a turbine or an expander. The expander or the turbine converts combustion gas enthalpy into mechanical power available at the output shaft of the expander or turbine and used to drive a load, such as a compressor or compressor train, or to rotate an electric generator and convert mechanical power into electric power.

[0003] 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.

[0004] To reduce the environmental impact of power generation through combustion of fossil fuels, the option of post combustion capture of carbon dioxide has been investigated. Carbon dioxide capture facilities have been developed, to process flue gas, i.e. combustion 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 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.

[0005] In recent years, oxy-combustion cycles, also known as oxy-fuel cycles, havebeen 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 bums in a combustor of an expander, thus producing a pressurized combustion gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0006] The combustion gas is expanded in the expander to generate mechanical power, which can eventually be converted into electric power by an electric generator driven into rotation by the expander. The exhausted combustion 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 combustion gas. A main part of the low-temperature combustion 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 combustion gas is removed and carbon dioxide contained therein is captured.

[0007] 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.

[0008] Oxy -fuel cycles, such as those described above, are particularly interesting in terms of efficiency, reduction of noxious emissions, and more efficient carbon capture. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by a high pressure drop in each stage of the expander, for instance a pressure drop comprised between 20 bar and 60 bar. The high pressure drop across the expander stages leads to high reaction forces on the components which support the stationary blades and other stationary components of the hot gas flow path in the expander. The high reaction forces may result in deformation of the supporting structure, whereto the stationary blades are connected.

[0009] Oxy-fuel expanders are characterized by high temperatures of the expanding combustion gas. Recycled compressed and cooled combustion gas, mainly orexclusively consisting of carbon dioxide, is suggested as a cooling and / or purging medium for the stationary blades and shrouds of the expander. The temperature difference between the cooling fluid and the combustion gas in the flow path of the expander, in combination with the high density of the compressed and recycled combustion gas, is critical for the integrity of the expander components which are in thermal contact with the expanding combustion gas.

[0010] According to one aspect, the present disclosure is aimed at providing a novel structure which overcomes or alleviates the drawbacks mentioned above.SUMMARY

[0011] Disclosed herein is a novel structure for an expander, which overcomes or alleviates the drawbacks mentioned above. In particular, a novel structure is proposed herein, which avoids or reduces elastic deformation of structures supporting hot gas path components, such as in particular stationary blades and shrouds. This is achieved by mechanical decoupling between the stationary blades on the one side and the casing surrounding the rotor on the other. In embodiments disclosed herein, the structure further provides thermal decoupling between a gap, containing a pressurized fluid, which can operate as cooling or purging fluid, and the flow path of the expanding combustion gas.

[0012] As will become clear form the following detailed description of embodiments, the novel structure is beneficial in controlling cooling of the expander components, specifically those facing the expansion flow path. Moreover, the amount of recycling gas used for cooling or purging purposes can be minimized, with beneficial effects in terms of overall efficiency of the thermal cycle. The structure is also useful in controlling clearances between stationary and rotary components of the expander.

[0013] In embodiments disclosed herein, the expander comprises an outer casing and an inner casing housed in the outer casing. A plurality of annular arrays, or annular rows, of stationary blades are housed in the inner casing. The expander further includes a rotor, housed in the inner casing for rotation therein around a rotation axis. The rotor comprises a plurality of annular arrays, or annular rows, of rotor blades surrounding the rotation axis, each annular array of rotor blades being arranged downstream of a respective one of said annular arrays of stationary blades and forming a respectiveexpansion stage therewith. The stationary blades of each annular array of stationary blades are mounted on at least one ring housed in the inner casing. Each ring is in pressure contact with two adjacent rings or with one adjacent ring and the inner casing. A gap containing pressurized fluid, such as cooling or purging fluid, can be formed between the rings and the inner casing. As will become clear from the following description, as understood herein, “pressure contact” is a contact between surfaces belonging to two machine components (e.g. two adjacent rings, or a ring and the inner casing), the contact generating a reaction force at the surface of contact.

[0014] The expander may include further stationary blades, which are supported by different structures or rings, for instance, rings which are differently mounted in the inner casing.

[0015] The blades of each annular array of stationary blades which is mounted on said rings can be mounted on a single ring or on two or more rings. Conversely, one ring can support a single annular array of stationary blades, or more than one such arrays.

[0016] Further rings can be provided in the inner casing, which do not support stationary blades. For instance, a ring supporting a shroud surrounding an annular array of rotor blades can be positioned between two subsequent rings supporting the stationary blades of two adjacent expansion stages. Alternatively, or in combination, one or more rings may support (fully or in part) an annular array of stationary blades and a shroud of an adjacent annular array of rotor blades.

[0017] In some embodiments, at least some of said rings support each a shroud surrounding the annular array of rotor blades arranged downstream of the annular array of stationary blades mounted on the ring.

[0018] In some embodiments, the rings are configured to transfer to the inner casing only axial reaction forces (if the friction forces are neglected) generated by the fluid which flows through the respective stationary blades of one or more stages. The term “axial” as used herein means parallel to the rotation axis of the rotor.

[0019] Further embodiments and features of the expander according to the present disclosure are described below and outlined in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Reference is now made briefly to the accompanying drawings, in which: Fig.l is a sectional view of an expander according to a first embodiment of the present disclosure, taken along a plane containing the rotation axis;Figs. 2A and 2B are enlargements of Fig.l;Fig.3 is an enlarged sectional view of a portion of an expander according to a further embodiment of the present disclosure, taken along a plane containing the rotation axis;Fig.4 is an enlarged sectional view of a modified embodiment; and Fig.5 is a sectional view of a further modified embodiment.DETAILED DESCRIPTION

[0021] A sectional view of an expander 1 according to the present disclosure is shown in Fig.l. 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.

[0022] 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.

[0023] A combustor, such as a can combustor including a plurality of combustion chambers 7 is positioned at the forward side of the expander, upstream of the first annular array of stationary blades or vanes. An annular chamber 6 is positioned between the outer casing 3 and the inner casing 5.

[0024] As used herein “forward” and “aft” are referred to the direction of flow of the process gas, i.e. the combustion 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.

[0025] The expander 1 further comprises a rotor 11 housed in the inner casing 5 andadapted to rotate around the rotation axis A-A. The rotor 11 comprises a rotor shaft 13 and a plurality of annular arrays, i.e. annular rows, or sets of rotor blades. In the exemplary embodiment of Fig.1, the rotor 11 comprises eight annular arrays of rotor blades. Each array of rotor blades extends around the rotation axis A-A of the rotor 11. The rotor blades are labeled 15.j, where j indicates the position of the array in the forward-to-aft direction. Specifically, the rotor blades of the first annular array are labeled 15.1, the rotor blades of the last annular array are labeled 15.8; the blades of the jtharray are labeled 15.j. As used herein, reference number 15 refers to rotor blades of a generic annular array of rotor blades.

[0026] An annular array, i.e. annular row, or set of stationary blades is positioned upstream of each annular array of rotor blades 15.j. The blades of the annular arrays of stationary blades are labeled 17.j. More specifically, the blades of the most upstream annular array of stationary blades are labeled 17.1, the blades of the most downstream array of stationary blades are labeled 17.8. In general, the blades of the jthannular array of stationary blades are labeled 17.j. As used herein reference number 17 refers to stationary blades of a generic array of stationary blades. As used herein “upstream” and “downstream” refer to the direction of flow of the combustion gas flowing through the expander 1.

[0027] Each annular array of stationary blades 17.j and respective annular array of rotor blades 15.j form together a stage of the expander 1.

[0028] The stationary blades 17 are housed in the inner casing 5. As will be described in more detail below, the stationary blades 17 of one, some or each annular array of stationary blades are not mounted directly on the inner casing 5, but rather mounted on at least one respective ring, or on a pair of adjacent rings. In the embodiment of Fig.1, all stationary blades 17 of each annular array or set of stationary blades are supported by a respective single ring. The rings are labeled 18. More specifically, each ring is labeled 18.j (wherein j= 1-7). As used herein, reference number 18 refers to a generic ring. Each annular array of stationary blades 17.j (wherein j=l -7) is mounted on a respective ring 18.j. The most downstream annular array of stationary blades 17.8 is mounted on a ring 18.8 which forms part of an aft portion 5B of the inner casing 5. The aft portion 5B of the inner casing is coupled to a main body 5A of the inner casing 5. The main body 5 A can in turn be formed by a plurality of casing portions. Eachcasing 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.

[0029] The rings are centered and locked against rotation with respect to the inner casing by centering and anti-rotation features, known per se to those skilled in the art and not shown.

[0030] Each annular array of rotor blades 15.j is surrounded by a respective shroud. The shrouds are labeled 19.1, ... 19.j, 19.8. In the embodiment of Fig.1, the shrouds 19.2 to 19.7 are supported each by the respective ring 18, on which the stationary blades positioned immediately upstream of the shroud are mounted. Thus, shroud 19.j is mounted on ring 18.j that supports the annular array of stationary blades 17.j, wherein j =2 to 7.

[0031] In the embodiment of Fig.1, the first shroud 19.1 that surrounds the first, i.e., the most upstream annular array of rotor blades 15.1, is supported by an auxiliary ring 18.0, which does not support any stationary blade, and which is positioned between the first ring 18.1 and the second ring 18.2.

[0032] The rings 18 form a structure which supports the stationary blades 15 and the shrouds 17 and which separates the flow path of the hot expanding gas from the inner casing 5. As will be explained in greater detail below, the rings 18 are structured such that they transfer to the inner casing 5 only axial reaction forces generated by the hot gas which expands along the flow path, if the friction forces are neglected. The supporting structure thus designed, which prevents transfer of radial reaction forces between the rings and the casing allows a differential thermal expansion of the rings on the one side and the casing on the other, said differential thermal expansion being determined by a thermal gradient.

[0033] Moreover, the rings 18 decouple the hot gas flow path from the inner casing 5 and form with the latter a fluid gap 61 for a pressurized fluid, specifically a cooling or purging fluid between the rings 18 and the inner casing 5. The pressurized fluid gap 61 will be described in greater detail below. Calibrated flow passages can be provided in the structure formed by the rings, such that a controlled amount of cooling or purging gas can flow from the cooling or purging fluid gap towards the hot gas flow path. Purging or cooling gas can further leak along the contact surfaces between mutuallyabutting rings, or between ring and casing.

[0034] The shape of the rings 18.j and co-action between abutting rings 18, as well as between the rings and the inner casing 5 are best shown in the enlargements of Figs. 2A and 2B and are described here below.

[0035] As will be apparent from the following description, in embodiments disclosed herein, rings whereto the stationary blades or vanes are constrained, are arranged in the casing such that, in use, each ring abuts against at least one downstream ring, or against an upstream and a downstream ring, or against an upstream ring and the inner casing. The most upstream row of stationary blades or vanes is constrained to a peripheral ring which abuts against the inner casing and / or against a downstream ring and are further constrained to a stationary inner structure.

[0036] In the following description, reference will be made to reaction forces generated on the surfaces of contact between abutting the rings 18 and between rings 18 and inner casing 5. The reaction forces to which reference is made herein are mainly generated by the process gas flowing in the hot flow path of the expander and by pressure acting on the rings which support the shrouds and the stationary blades or vanes. Not taken into account are friction forces between mutually contacting components. These forces are generated by expansion of the combustion gas and by the pressure differential across the respective stage. Friction forces can be generated for instance by differential thermal expansions of components in mutual contact with one another during operation transients of the expander 1.

[0037] As shown in Fig.2A, the first ring 18.1 supports the stationary blades 17.1 of the first annular array of stationary blades, i.e., the most upstream set of stationary blades, which are arranged at the outlet of the combustor chambers 7. The stationary blades 17.1 are constrained to the first ring 18.1 and to an inner stationary structure 16.

[0038] The first ring 18.1 comprises a forward surface 31, which can be in pressure contact with the inner casing 5. In other embodiments, the forward surface 31 can be clear of the inner casing 5. In some embodiments, the forward contact surface 31 is a flat surface orthogonal to the rotation axis A-A of the expander 1. Through the forward contact surface, the first ring 18.1 can transmit to the inner casing 5 only axial reaction forces, i.e., forces oriented parallel to the rotation axis A-A. As mentioned, frictionforces generated by mutual displacements between the ring 18.1 and the inner casing 5, due to differential thermal expansion, for instance, are not taken into consideration.

[0039] The first ring 18.1 further comprises an aft contact surface 33 which is in pressure contact with the downstream auxiliary ring 18.0. The aft contact surface 33 can be a flat surface orthogonal to the rotation axis A-A of the expander and therefore adapted to transmit only axial reaction forces.

[0040] In other embodiments, the first ring 18.1 can be in pressure contact with the auxiliary ring 18.0 only, and have no contact at 31 with the casing 5. Alternatively, the first ring 18.1 can be in pressure contact at 31 with the inner casing 5 and have no contact with the auxiliary ring 18.0.

[0041] In any event, the resulting force generated at the areas of pressure contact is oriented axially and no radial forces are transferred from the first ring 18.1 to the inner casing 5, except those generated by friction.

[0042] The aft contact surface 33 of the first ring 18.1 corresponds to a forward contact surface of the auxiliary ring 18.0. The forward contact surface of auxiliary ring 18.0 is labeled with the same reference number 33. The auxiliary ring 18.0 further includes an aft contact surface 35, wherewith the auxiliary ring 18.0 is in pressure contact with the second ring 18.2. Reaction forces applied to the auxiliary ring 18.0 at surfaces 33 and 35 balance the thrust generated on the shroud 19.1 by the pressure difference across the expansion stage.

[0043] In the embodiment of Figs. 1 and 2A, 2B, the second ring 18.2 supports the stationary components of the second expansion stage, namely the stationary blades 17.2 of the second annular array of stationary blades, and the shroud 19.2, which surrounds the rotor blades 15.2 of the second annular array of rotor blades. The second ring 18.2 includes a forward contact surface, which corresponds to the aft contact surface 35 of the auxiliary ring 18.0 and is referred to with the same reference number 35. The second ring 18.2 further comprises an aft contact surface 37, whereat the second ring 18.2 is in pressure contact with the third ring 18.3.

[0044] In some embodiments, one or both the forward contact surface 35 and the aft contact surface 37 of the second ring 18.2 with the upstream neighboring ring 18.0 andwith the downstream neighboring ring 18.3. are flat and orthogonal to the rotation axis A-A of the expander 1, such that the reaction forces generated at said surfaces is oriented axially, i.e., parallel to the rotation axis A-A.

[0045] In some embodiments, as shown in Fig.2A, the second ring 18.2 is designed such that the forward contact surface 35 thereof is at a distance from the rotation axis A-A which is greater than the distance of the aft contact surface 37, such that reaction forces applied to the contact surfaces 35, 37 of the second ring 18.2 generate a torque in the clockwise direction (in the drawing), which balances the counter-clockwise torque applied to the second ring 18.2 by the stationary blades 17.2 of the second expansion stage, said counter-clockwise torque being generated by the pressure difference across the row of stationary blades 17.2 and by the force applied by the expanding combustion gas which flows through the row of stationary blades 17.2. This results in a better balancing of the reaction forces on the second ring 18.2. Additionally, this design limits the elastic deformation of the second ring with the aim of avoiding or reducing variations of the clearance between statoric and rotoric components during operation of the machine.

[0046] The next, third ring 18.3 supports the stationary components of the third expansion stage, namely the blades 17.3 of the third annular array of stationary blades, and the shroud 19.3 which surrounds the rotor blades 15.3 of the third annular array of rotor blades. The third ring 18.3 comprises a forward contact surface, which corresponds to aft contact surface 37 of the second ring 18.2 and is labeled with the same reference number. An aft contact surface of the third ring 18.3, whereat the third ring 18.3 is in pressure contact with the fourth ring 18.4, is shown at 39. In embodiments disclosed herein, the aft contact surface 39 of the third ring 18.3 is flat and orthogonal to the rotation axis A-A, such as to transmit only axial forces to the neighboring, downstream ring 18.4.

[0047] A reduction of the reaction forces applied to the third ring 18.3 is achieved by positioning the aft contact surface 39 at a distance from the rotation axis A-A which is as close as possible to the distance of the forward contact surface 37 to said axis. In the embodiment shown, the aft and forward contact surfaces 39, 37 of the third ring 18.3 are substantially at the same radial distance from the rotation axis A-A.

[0048] The fourth ring 18.4 supports the stationary components of the fourth expansion stage of the expander 1, namely the stationary blades 17.4 of the fourth annular array of stationary blades and the shroud 19.4 surrounding the rotor blades 15.4 of the fourth annular array of rotor blades.

[0049] The fourth ring 18.4 has a forward contact surface which is in pressure contact with the inner casing 5. More specifically, the fourth ring 18.4 features a radial projection 18.41, forming the contact surface 41 between the fourth ring 18.4 and the inner casing 5. The contact surface 41 is flat and orthogonal to the rotation axis A- A, such that the reaction force between the fourth ring 18.4 and the inner casing 5 is oriented axially, i.e. parallel to the rotation axis A-A, and no radial reaction forces are transmitted by the fourth ring 18.4 to the inner casing 5.

[0050] The fourth ring 18.4 also comprises a further forward contact surface in pressure contact with the upstream, third ring 18.3. Said further forward contact surface coincides with the aft contact surface 39 of the third ring 18.3 and is labeled with the same reference number.

[0051] The fourth ring 18.4 further comprises an aft contact surface 43, at which the fourth ring 18.4 is in pressure contact with the fifth ring 18.5. The aft contact surface 43 has a convex cylindrical shape coaxial with the rotation axis A-A of the expander 1. At the cylindrical surface forming the aft contact surface 43 of the fourth ring 18.4 and the forward contact surface of the fifth ring 18.5 a radial reaction force, i.e., a force oriented orthogonal to the rotation axis A-A, is generated. The radial reaction force applied by the fifth ring 18.5 to the fourth ring 18.4 is oriented towards the rotation axis A-A. The torque generated by the radial reaction force applied to ring 18.4 at the aft surface 43 thereof (which is oriented clockwise in Fig.2A) balances the counterclockwise oriented torque applied to said ring 18.4 by the expanding combustion gas through the stationary blades 17.4 as well as the counter-clockwise torque generated by the axial reaction forces acting at the forward contact surfaces 41 and 39..

[0052] The fifth ring 18.5, which supports the stationary blades 17.5 and the shroud 19.5 of the fifth expansion stage, features an annular groove or rabbet at the forward end thereof, wherein the aft end of the fourth ring 18.4 engages. The annular groove or rabbet has a concave, i.e. inner cylindrical surface. The inner cylindrical surface ofthe annular groove corresponds to the aft contact surface 43 of the fourth ring and is labeled with the same reference number. The inner cylindrical contact surface 43 therefore represents a forward contact surface of the fifth ring 18.5, in pressure contact with the upstream ring 18.4.

[0053] The inner cylindrical contact surface 43 can be continuous, or can present one or more discontinuities, therein, such as slots, holes, or indentations, for instance. The same is true for the aft end of the fourth ring 18.4, which engages in the groove forming the inner cylindrical contact surface 43. The aft end of the fourth ring can be a continuous cylindrical surface, or can include one or ore discontinuities, such as indentations, slots, or holes.

[0054] The fifth ring 18.5 further comprises an aft contact surface 45, in pressure contact with a contact surface of the inner casing 5. The contact surface 45 is positioned nearer to the rotation axis A-A than the contact surface 43, such that a better balancing of the torque applied to ring 18.5 by the expanding combustion gas can be achieved. In some embodiments, the aft contact surface 45 is flat and orthogonal to the rotation axis A-A, such as to transmit only axial reaction forces to the inner casing 5.

[0055] The next, sixth ring 18.6, which supports the stationary blades 17.6 and the shroud 19.6 of the sixth expansion stage, has an aft contact surface 49 in pressure contact with the downstream, seventh ring 18.7. In some embodiments, the aft contact surface 49 has an outer cylindrical surface portion 49A coaxial to the rotation axis A- A of the expander, and a flat surface portion 49B, orthogonal to the rotation axis A-A. The inner cylindrical contact surface portion 49A surrounds the aft portion of the upstream ring 18.6, i.e. the aft portion of the upstream ring 18.6, which forms the aft contact surface of the ring 18.6, engages in the annular groove at the forward end of ring 18.7.

[0056] An axial reaction force is applied at the flat surface portion 49B and a radial reaction force is applied at the outer cylindrical surface portion 49A.

[0057] The aft end of the sixth ring 18.6, which features the aft contact surface 49, engages in an annular groove or rabbet formed in the forward end of the next, seventh ring 18.7, that supports the stationary blades 17.7 and the shroud 19.7 of the seventh expansion stage. The annular groove of the seventh ring 18.7 features a forwardcontact surface in pressure contact with the aft end of the sixth ring 18.6. The forward contact surface of ring 18.7 has a shape which is complementary to the shape of the the aft contact surface of ring 18.6, i.e. comprises an inner cylindrical surface portion labeled again 49 A, and a flat surface portion labeled again 49B.

[0058] At surface 49 A, 49B, therefore, axial and radial reaction forces are transmitted between the sixth ring 18.6 and the seventh ring 18.7. The latter supports the stationary blades 17.7 and the shroud 19.7 of the seventh expansion stage.

[0059] In addition to the forward contact surface 49A, 49B, the seventh ring 18.7 features an aft contact surface 51, which is in pressure contact with ring 18.8, which forms the aft portion 5B of the inner casing 5. In some embodiments, the aft contact surface 51 is flat and orthogonal to the rotation axis A- A, such as to transmit an axial reaction force to the eight ring 18.8 forming part of the aft portion 5B of the inner casing 5.

[0060] Thus, the only portion of the inner casing 5 which is loaded with a radial force is the aft portion 5B, whereto reaction forces are applied, adapted to balance the torque generated by expansion of the combustion gas in the last stage, i.e. on stationary blades 17.8 and shroud 19.8, while the remaining sections of the inner casing 5 are only loaded with axial reaction forces transmitted by the structure formed by the rings 18.1- 18.7.

[0061] As shown in Figs. 2A and 2B, the structure formed by the stacked rings 18 defines a cooling or purging fluid gap 61 between the surface of the rings 18 facing radially outwardly, and the inner surface of the inner casing 5. The cooling or purging fluid gap 61 is adapted to receive pressurized cooling or purging fluid, for instance cool recycled gas. The cooling or purging fluid can be used to cool the stationary blades 17 or purge empty volumes surrounding the stationary blades, for instance. Pressurized cooling or purging fluid can leak in a controlled manner from the cooling or purging fluid gap 61 toward the stationary blades 17 and / or towards the shrouds 19. Control leak of the cooling or purging fluid can be obtained along the contact surfaces at which the rings are in mutual pressure contact with one another and / or thought ducts formed in the rings. By way of non-limiting example one such duct is shown at 62 in Fig.2B.

[0062] Thus, the set of rings 18 provide mechanical decoupling between the inner casing 5 and the stationary components (stationary blades 17 and shrouds 19). This mechanical decoupling allows thermal expansion of the inner rings during operation of the expander. At the same time, the set of rings 18 provide a thermal and fluid decoupling between the cooling or purging gap 61 and the stationary components 17, 19. The latter are at a high temperature determined by the temperature of the expanding combustion gas which flows in the flow path of the expander. Controlled leak of cooling or purging fluid from the gap 61 towards the stationary blades 17 and the shrouds 19 prevents thermally induced damages to the stationary blades 17 and shrouds 19, which would be caused by uncontrolled or excessive flow of cooling or purging fluid. The controlled leak cools the stationary components of the expander.

[0063] Controlling and minimizing the flowrate of the cooling or purging fluid is particularly beneficial when the cooling or purging fluid has high density, as in the case where chilled, pressurized and recycled carbon dioxide is used as cooling or purging medium, since the high density and high Reynolds number of the cooled and compressed recycling carbon dioxide implies a high coefficient of thermal transmission by convection against the hot stationary components (blades 17 and shrouds 19) of the expander.

[0064] Additionally, a controlled and minimized leak of cooling or purging fluid is also beneficial in terms of overall efficiency of the expander, since less power is lost through compressed and recycled combustion gas; a higher flowrate of recycled combustion gas is available for power generation purposes.

[0065] While in the embodiment described so far, each annular array of stationary blades is carried by only one ring, in other embodiments, an array of stationary blades can be carried by two adjacent rings, and / or the same ring can support two or more annular arrays of stationary blades.

[0066] Fig.3 illustrates an enlarged sectional view of the most upstream stages of an expander, wherein the stationary blades 17.2 are mounted on two adjacent rings, labeled 18A and 18B. Similarly, the stationary blades 17.3 are mounted on two adjacent rings 18B, 18C. In this embodiment, the ring 18A supports also the shroud 19.1 and the ring 18B supports the shroud 19.2.

[0067] Referring now to the enlargement of Fig.4, one embodiment of devices for mechanically attaching stationary blades 17 to rings 18 will be described. With general reference to any stationary blade 17 starting from the second expansion stage, each stationary blade or stationary vane 17 comprises an outer platform 71 and can further include an inner platform. The outer platform 71 features the coupling members which connect the stationary blade 17 to the respective supporting ring 18, or to the casing directly.

[0068] The outer platform 71 comprises a radially outer surface 71.1 facing the ring 18 (or the casing). The outer platform 71 further comprises a radially inner surface 71.2 facing the inner platform 73 and the rotation axis A-A of the rotor 11.

[0069] The outer platform 71 further comprises a forward edge 71.3 and an aft edge 71.4, as well as a mechanical coupling feature adapted to mechanically attach the outer platform 71 to a supporting structure, which in Fig. 4 is featured by the supporting ring 18.

[0070] Each outer platform 71 can be integrally formed with a single stationary blade 17, i.e. each stationary blade 17 may have its own outer platform 71. This, however, is not mandatory. In some embodiments, or for one or more expansion stages of the expander, the stationary blades 17 can be configured as arcuate segments, wherein each segment comprises two or more stationary blades 17, integrally formed with a single outer platform 71. The stationary blades integrally formed with a common outer platform 71 are further provided with a common inner platform 73.

[0071] In general, the inner platform 71 and the outer platform are shaped such that when the annular row of stationary blades 17 is assembled with the respective ring 18, a plurality of platforms 71 and 73 are circumferentially arranged around the rotation axis A-A and form respective circular bands, framing therebetween a plurality of airfoils, described below.

[0072] Each stationary blade 17 comprises, or consists of, an airfoil 75, which extends from the radially inner surface 71.2 of the outer platform 71 to the radially outer surface 73.1 of the inner platform 73 and comprising a leading edge and a trailing edge. Each airfoil 75 includes a leading edge 75.1 oriented forwardly, and a trailing edge 75.2, oriented aftwardly.

[0073] Similarly to Figs. 1 and 2, the mechanical coupling feature wherewith each stationary blade 17 is coupled to the ring 18 or other supporting structure housed in the casing, comprises a forward hook 77, which projects from the radially outer surface 71.1 of the outer platform 71 and is oriented towards the aft edge 71.4 of the outer platform 71. More specifically, in the embodiment of Fig.4, the forward hook 77 comprises a foot 77.1 at the radially outer surface 71.1 of the outer platform 71, and a projection or tooth 77.2 at the distal end of the forward hook 77, the projection 77.2 being oriented in the aft direction, i.e. towards the aft edge 71.2 of the outer platform 71.

[0074] The forward hook 77 is positioned in an intermediate position between the forward edge 71.3 and the aft edge 71.4 of the outer platform 71. Considering a geometric plane P-P orthogonal to the rotation axis A-A, and equidistant from the forward edge 71.3 and the aft edge 71.4, which divides the outer platform 71 in a forward platform portion 71F and an aft platform portion 71 A, the forward hook 77 is preferably constrained to the outer platform 71 in the forward portion 7 IF thereof. Similarly to the embodiment of Figs. 1 and 2, therefore, the forward hook 77 is arranged on the side of the outer platform 71 nearer to the forward edge 71.1 thereof. In some embodiments, the distal projection 77.2 can project beyond the median plane P-P towards the aft edge 71.4 of the outer platform 71.

[0075] In some embodiments, the outer platform 71 comprises a rib 71.5 which projects radially outwardly from the radially outer surface 71.1 of the outer platform 71 and forms a resting, i.e. an abutting surface adapted to rest against the respective ring 18. The rib 71.5 extends adjacent the aft edge 71.4 of the outer platform 71 and, in the embodiment of Fig.4, the rib 71.5 is in contact with a cylindrical inner surface 18C of the ring 18. The diameter of the inner cylindrical surface 18C is such that each stationary blade 17 can be mounted on the ring 18 with a forward-to-aft insertion movement.

[0076] The cylindrical surface 18C forms a cylindrical resting surface on which the rib 71.5 abuts and whereto radially outwardly oriented reaction forces are transferred from the stationary blade 17 to the ring 18 or directly to the casing 5. If no rings 18 are provided, the cylindrical surface 18C can be formed on the inner surface of the casing 5.

[0077] In some embodiments, the aft platform portion 71 A, which extends from the forward hook 77 to the aft edge 71.4, has a thickness in radial direction, which is smaller thank the thickness in radial direction of the forward platform portion 7 IF. Moreover, in some embodiments, the outer platform can comprise an indentation 71.6 on the radially outer surface. The indentation 71.6 can be positioned between the forward hook 77 and the aft edge 71.4 of the outer platform 71, and more precisely between the foot of the forward hook 77 and the rib 71.5, and can extend approximately parallel to the forward edge 71.3 and the aft edge 71.4 of the outer platform 71. As in the embodiment of Figs. 1 and 2, the indentation 71.6 provides a reduction of the thickness of the aft portion 71 A of the outer platform 71.

[0078] Each forward hook 77 engages a circular front groove 18A formed in an annular surface 18B of the respective ring 18. The annular surface 18B can be a planar, i.e. flat surface, orthogonal to the rotation axis A-A. The circular groove 18A has a forward-facing opening for insertion of the forward hooks 77 of the outer platforms 71, and more specifically the distal projection 77.2 thereof.

[0079] Differently from the embodiment of Figs 1, 2A, 2B, in the embodiment of Fig. 4 the rib 71.5 does not engage a radial groove of the ring 18, but rests on the inner cylindrical surface 18C thereof, which extends in a forward direction till the front planar annular surface 18B where the circular groove 18A is formed. With this arrangement, each stationary blade 17, with its respective outer platform 71, can be mounted on the supporting ring 18 thereof with a simple movement in a forward-to-aft direction (arrow f71 in Fig.4). The stationary blades 17 can therefore be mounted on a monolithic ring, i.e. on a ring which has a continuous extension of 360° around the symmetry axis thereof, i.e. has a monolithic development around 360°. A monolithic ring having no discontinuities in the tangential or circumferential direction is particularly beneficial in terms of ability to resist mechanical stresses, specifically in case of high pressure drops across the respective expansion stage, and furthermore prevents leakages.

[0080] In the embodiments described so far, each ring 18 is monolithic, or can in some cases be split into two symmetrical semi-rings, each developing around 180°, for instance, to facilitate mounting of the stationary blades. In other embodiments, however, one, some or all rings can be split into two partial rings, i.e. into a first ring component and as second ring component, which are easier to manufacture and whichcan be coupled to one another by interference fitting, preferably by shrink-fitting. The shroud 19 can be mounted between the two partial rings, or ring components. Each shroud 19 can be formed by a plurality of shroud segments, mounted between the two ring components and retained therebetween. Fig.5 illustrates one of the rings 18 of the expander split into two partial rings or ring components 18 A, 18B, which are coupled to one another by shrink-fitting. The two ring components are coaxial and each ring component 18A, 18B is a one-piece, monolithic piece extending around 360°.

[0081] In all embodiments disclosed herein, the rings 18 are axially retained in the casing by pressure contact between adjacent rings and / or between rings and casing, without the need for additional or separate locking or retention devices, such as bolts or the like, providing axial retaining forces. Only anti-rotation features may be added to prevent rotation of the rings in the casing.

[0082] 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. Expander containing: - outer casing; - an inner casing placed in an outer casing; - a plurality of annular arrays of fixed blades located in the inner casing; - a rotor located in an inner casing with the possibility of rotation therein; a rotor containing an axis of rotation and a plurality of annular arrays of rotor blades around the axis of rotation; wherein each annular array of rotor blades is located downstream from the corresponding one of the said annular arrays of fixed blades and forms with it a corresponding stage of the expander; wherein the fixed blades of each of the said annular arrays of fixed blades are mounted on at least one corresponding ring located in the inner casing; wherein each ring is in contact under pressure with at least one adjacent ring or at least with the inner housing; wherein at least the intermediate ring is in contact under pressure at a forward contact surface with the upstream ring and at a rear contact surface with the downstream ring; and the forward contact surface is at a greater distance from the axis of rotation than the rear contact surface; so that in operation the reaction forces applied to the rear contact surface and to the front contact surface create a torque on said intermediate ring in a direction opposite to the torque applied by the fixed blades supported by said ring; and / or wherein: at least one of said rings comprises an annular groove forming a front contact surface which is in contact under pressure with an upstream ring; and the upstream ring has a rear end forming a rear contact surface which is engaged with the annular groove and surrounded by said ring.

2. An expander according to claim 1, wherein the front contact surface formed by the annular groove has a cylindrical shape and the rear contact surface has a cylindrical shape.

3. An expander according to claim 1 or 2, in which a gap is formed between the rings and the inner housing for cooling or blowing liquid under pressure.

4. An expander according to any one of the preceding claims, wherein the fixed blades of at least one annular array of fixed blades are mounted on a single ring.

5. An expander according to any one of the preceding claims, wherein the fixed blades of at least one annular array of fixed blades are mounted on two adjacent rings.

6. An expander according to any of the preceding claims, wherein the rings are configured to transmit to the inner housing only the axial reaction forces created by the flow of fluid through the corresponding fixed blades.

7. An expander according to any one of the preceding claims, in which at least one of said rings supports a casing surrounding an annular array of rotor blades located downstream or upstream of the annular array of stationary blades mounted on said at least one of said rings.

8. An expander according to any one of the preceding claims, wherein at least one of said rings comprises a passage for a cooling or purging fluid under pressure.

9. An expander according to any one of the preceding claims, wherein at least some of said rings comprise a forward contact surface in contact with an upstream ring or with an inner casing, and a rear contact surface in contact with a downstream ring or with an inner casing.

10. The expander of claim 9, wherein the front contact surface, the rear contact surface, or both the front contact surface and the rear contact surface are flat and perpendicular to the axis of rotation.

11. An expander according to any one of the preceding claims, wherein each ring is a single ring that is continuous and extends around a circumference of 360°.

12. An expander according to any of the preceding claims, wherein the inner housing is horizontally divided.

13. An expander according to any of the preceding claims, wherein the outer housing is vertically divided.

14. An expander according to any one of the preceding claims, further comprising a combustion chamber on the front side of the expander upstream of the most upstream annular array of fixed blades.

15. An expander according to any one of the preceding claims, wherein at least one of said rings is formed by a first ring element and a second ring element connected to each other.

16. An expander according to claim 15, wherein the first annular element and the second annular element are connected to each other by means of an interference fit, in particular by means of a shrink fit.