Patent Publication Number: US-11021960-B2

Title: Set of turbines and a turbine train comprising at least one such set

Description:
FIELD OF THE INVENTION 
     The object of the present invention is a set of turbines and a turbine train comprising at least one such set. 
     The present invention falls within the scope of power plants for generating electrical energy and is intended for expansion systems configured to convert the energy of an expanding working fluid into mechanical energy, making one or more turbine rotors rotate, and then into electrical energy. 
     The present invention particularly refers to systems constituted by a plurality of turbines that are mechanically connected in series with their shafts aligned (turbine train). 
     BACKGROUND OF THE INVENTION 
     There are several known public documents that illustrate solutions with two or more turbines, the shafts of which are connected to each other so as to realize an array of aligned turbines. 
     For example, document WO2013/007463 illustrates a power generation system provided with a train of turbines with the rotors thereof set in serial arrangement and operatively coupled to each other and to a generator. 
     Document U.S. Pat. No. 2,174,806 illustrates a pair of turbines (a high-pressure turbine and a low-pressure turbine). The rotors of the turbines are aligned and connected by a coupling. 
     Document FR928577 illustrates two steam turbines that are coupled by means of a flexible shaft. 
     Document U.S. Pat. No. 5,780,932 illustrates an electricity generating unit comprising a gas turbine, a steam turbine having a plurality of modules, and an electricity generator, the above being mounted in succession along a common axis with the respective shafts being connected by rigid couplings. 
     Document GB492081 illustrates a power system comprising a high-pressure gas turbine and a low-pressure gas turbine connected in series by means of a coupling that rigidly connects the respective shafts. 
     Document EP0852660 illustrates a system with a first gas turbine, a generator, a low-pressure steam turbine, a high-pressure steam turbine and a second gas turbine, in a sequence. 
     Document U.S. Pat. No. 5,737,912 illustrates a power station comprising a gas turbine group and a steam turbine coupled by means of a coupling to the gas turbine group. 
     Document U.S. Pat. No. 5,042,247 illustrates a system comprising a steam turbine made up of two sections that are joined to each other by a rigid coupling. 
     Document GB929323 illustrates a flexible coupling for rotating parts, particularly for rotary shafts of turbine-generator sets. 
     Also known is patent US2012/177494 (also published as EP2479377), which illustrates the rotor of a steam turbine having a high-temperature section with a respective rotor wheel and a low-temperature section with a respective rotor wheel, both being made of different materials. The two rotor wheels are directly interconnected to each other by means of a coupling that prevents relative rotation of the two sections, such as: a circumferential array of axially-extending bolts or a reduced diameter portion housed in a blind bore and bolts oriented radially or a threaded stud screwed into a threaded, blind bore or a grooved stud housed in a grooved seat. 
     Lastly, document WO2012/156520 illustrates a system comprising a gas turbine having a shaft and auxiliary equipment, such as a compressor, an oil pump and mechanical or electrical accessories. The auxiliary equipment is provided with an overhung shaft configured to be connected to the turbine shaft. A flexible joint is located between the overhung shaft and the turbine shaft. 
     SUMMARY 
     In this field, the Applicant has observed that several aspects of the solutions of the prior art relating to turbine trains can be improved. 
     In particular, the Applicant has observed that the turbine trains of the prior art are complex and costly, for the individual turbines making up the train must, in any case, be designed “ad hoc” in order to make it possible to interconnect the shafts. Each individual turbine must be devised for extraction of the rotor shaft from the respective case so as to enable connection of both ends of the shaft to the adjacent turbines or to a turbine and to a generator. It is therefore also necessary to arrange two seals on the shaft to prevent leakage of the working fluid. 
     Moreover, the rigid connection of the turbines of the prior art implies that the turbines to be combined must be designed together, taking into account the characteristics of both so as to prevent the rotordynamics of one from negatively influencing the other. This involves a great number of hours of engineering to design a set for each new application. 
     The Applicant has thus perceived the need to propose a new set of two turbines. 
     In particular, the Applicant has noted that there is a need to:
         obtain a simple solution for connecting two turbines in series so as to realize a simple, reliable and relatively low-cost set;   obtain a set of turbines that makes it possible to realize turbine trains made up of one or more of such sets, possibly also to be combined with other prior-art turbines.       

     The Applicant has found that the objectives listed above and others as well can be achieved by a set of turbines comprising two turbines of the overhung type and connected on the free side of the respective rotors by means of an elastic joint. 
     More specifically, according to an independent aspect, the present invention concerns a set of turbines, which comprises: 
     a first turbine comprising: a first case, a first shaft supported in the first case, first support elements radially interposed between the first shaft and the first case and configured to enable free rotation of said first shaft with respect to the first case about a first main axis, and a first rotor provided with first rotor blades and joined to a distal end of the first shaft, wherein said first rotor is supported in an overhung manner with respect to the support elements, wherein said first rotor has a first front face facing the opposite side with respect to the first support elements; a second turbine comprising: a second case, a second shaft supported in the second case, second support elements radially interposed between the second shaft and the second case and configured to enable free rotation of said second shaft with respect to the second case about a second main axis, and a second rotor provided with second rotor blades and joined to a distal end of the second shaft, wherein said second rotor is supported in an overhung manner with respect to the second support elements, wherein said second rotor has a second front face facing the opposite side with respect to the second support elements. 
     The first front face faces the second front face and the first main axis is substantially aligned with the second main axis. 
     The set of turbines further comprises a connection element connected to the first front face and to the second front face to transmit rotation from the first shaft to the second shaft or vice versa. 
     Said connection element comprises at least one elastic joint configured to minimize the rotordynamic influence of the first turbine and the second turbine on each other. 
     The first and/or the second rotor comprise(s) a rotor disc bearing the respective rotor blades on the front face and/or on a respective, radially peripheral portion. 
     Each one of the two turbines is an “overhung” turbine. The term “overhung turbine” is used to indicate that the rotor or rotor disc is located axially to the side of all the support elements, such as bearings, of the respective shaft. In other words, the rotor or rotor disc of each turbine has a rear face, opposite the front face, from which the respective shaft extends and said rear face faces the elements supporting the shaft in the respective case. The overhung turbine does not have other support elements located in front of the front face. 
     The elastic, or flexible, joint is a device used to connect two shafts/rotors together with the aim of transmitting torque even when the two shafts are slightly misaligned. The elastic, or flexible, joint also enables damping of torsional vibrations. The connection element rotates together with the first and the second rotor and it is capable of absorbing possible misalignments of the respective main axes. 
     The terms “slightly misaligned” or “substantially aligned” indicate that the axes are inclined by a few degrees with respect to each other (e.g. by about 0.2° to about) 3° and/or that said axes are radially offset with an offset of a few millimetres (e.g. by about 2 mm to about 10 mm). 
     In the present description and in the appended claims, the adjective “axial” is used to define a direction directed parallel to an axis of rotation of the turbine. The adjective “radial” is used to define a direction directed in the same manner as the radii extending perpendicularly from the axis of rotation. The adjective “circumferential” is understood as referring to directions tangent to circumferences coaxial with the axis of rotation. 
     The Applicant has verified that the invention makes it possible to connect the two turbines very easily and thus create a relatively simple, reliable and low-cost module. 
     The Applicant has verified that the elastic joint connecting the two turbines makes it possible to decouple the rotordynamics of the individual turbines, which will thus have little impact on each other. 
     The Applicant has verified that as the two overhung turbines are connected at the front faces of the respective rotor discs, that is, at the free side of the rotor, it is possible to arrange a single seal on each shaft to isolate the process from the external environment. 
     The Applicant has also verified that the overhung structure of the two turbines, which face each other, also makes it possible to make use of the space between the two rotor discs to house other elements of the set (such as the conduits for the working fluid which connect the two turbines) so as to make the set very compact and easily installable. 
     The Applicant has also verified that the set according to the invention makes it possible to configure the turbine train very easily based on the requirements of the specific project, combining a number of sets and/or adding other turbines even of a known type (radial, axial, radial/axial overhung or not overhung) to said one or more sets. 
     Moreover, the Applicant has also verified that the set/module of the present invention is suited to a connection in series of a number of turbines on a single generator. 
     In a second aspect, the present invention also concerns a turbine train comprising at least one set of turbines in accordance with the first aspect and/or with the following aspects. 
     In one aspect, said at least one elastic joint is elastic in flexure. Preferably, said flexure is such as to permit an inclination of the rigid parts which said joint connects of at least 0.2°, preferably of up to about 3°. 
     In one aspect, said at least one elastic joint is elastic in traction/compression. Preferably, this traction/compression is such as to enable the rigid parts, which said joint connects, to move about +/−5 mm away from/towards each other. 
     In one aspect, said connection element enables relative movements of the first and the second rotor and said movements consist of the following for example: translational movement along three axes that are perpendicular to each other and/or rotation about a plurality of axes differing from the first and the second main axis. 
     In one aspect, the rotation about the first and/or the second main axis (torsion) is prevented so as to enable proper transmission of the torque. 
     In one aspect, said at least one elastic joint is of the type having flexible discs (or lamellae). 
     In one aspect, the connection element is located at the first and second main axis. 
     In one aspect, the connection element is connected to the respective centres of the first rotor and the second rotor. 
     In one aspect, the connection element comprises at least one drive shaft. In one aspect, said at least one drive shaft is substantially aligned with the first shaft and with the second shaft. 
     In one aspect, the connection element comprises two elastic joints, each one being located at one end of the drive shaft. The elastic joints are located at the opposite ends of the drive shaft and at the centres of the first rotor and the second rotor. 
     In one aspect, the set comprises a casing connected to the first case and to the second case. 
     In one aspect, the connection element in contained in said casing. 
     In other words, with the first and the second case, the casing forms a single, sealed container that contains the rotors, the shafts and also the connection element. The casing also contains the working fluid that passes through the first rotor and/or the second rotor. Preferably, the connection element is immersed in the working fluid. 
     In one aspect, the casing delimits at least one conduit for a working fluid passing through the first turbine and/or the second turbine. 
     The function of the casing is not only that of protecting the connection element, but also that of delimiting conduits for the working fluid. This also makes it possible to arrange just one seal for each one of the two shafts between the working fluid and the external environment. Moreover, no seals are needed on the connection element because as specified above, the connection element can be immersed in the working fluid passing through/remaining in the above-mentioned casing. 
     In one aspect, said at least one conduit sets the first rotor and the second rotor in fluid communication with each other. Carried out in this manner, the realization of the conduits for communication between the turbines makes it possible to utilize the available space and to make the set compact. 
     In one aspect, the first and the second turbine are connected in parallel as concerns the flow of the working fluid. 
     In one aspect, the first and the second turbine are connected in series as concerns the flow of the working fluid. 
     In other words, from the process/energetic point of view, the expansion of the two turbines can be in parallel or in series. An in-parallel connection makes it possible to “dispose of” more mass flow and eliminate the volume flow rate limit at discharge. The in-series connection makes it possible to increase the specific enthalpy and thus also to achieve high volumetric ratios between the inlet and outlet. Furthermore, an in-series connection allows for a second intermediate inlet for a second lower-pressure supply (admission) of the main inlet or steam extraction (tapping). 
     In one aspect, said at least one conduit is an inlet conduit for admitting the working fluid into the first and/or second turbine. In one aspect, said at least one conduit is an outlet conduit for letting the working fluid flow out from the first and/or second turbine. 
     In one aspect, said at least one conduit is an inlet conduit for admitting the working fluid into the first rotor and the second rotor. In one aspect, said at least one conduit is an inlet conduit for admitting the working fluid into the first rotor and an outlet conduit for letting it flow out from the second rotor or vice versa. 
     In one aspect, the casing has an inlet mouth for admitting the working fluid into the set of turbines. In one aspect, the casing has an intermediate mouth for admitting the working fluid into the set of turbines. In one aspect, the casing has an outlet mouth for letting the working fluid flow out from the set of turbines. In one aspect, the casing has an intermediate mouth for tapping the working fluid from the set of turbines. 
     In one aspect, the casing comprises at least one tubular or substantially tubular body extending between the first case and the second case and it is substantially coaxial with the connection element. 
     In one aspect, the casing comprises a single tubular or substantially tubular body extending between the first case and the second case and it is substantially coaxial with the connection element. 
     In one aspect, said at least one conduit is delimited between the single tubular or substantially tubular body and the connection element. 
     In one aspect, the casing comprises a radially internal tubular body and a radially external tubular body. 
     In one aspect, said at least one conduit is delimited between the radially internal tubular body and the radially external tubular body. 
     In one aspect, the radially internal tubular body surrounds the connection element. 
     In one aspect, the casing incorporates or bears a plurality of first stator blades of the first turbine and/or a plurality of second stator blades of the second turbine. 
     In one aspect, the first turbine and/or the second turbine comprise(s) a sleeve housed or that can be housed in a seat in the respective case and containing the respective support elements and the respective shaft. In one aspect, said sleeve is extractable from the respective seat on the opposite side with respect to the respective rotor and together with the support elements and the respective shaft. 
     In one aspect, a proximal end of the first and/or the second shaft and opposite the distal end protrudes from the respective first/second case. 
     In one aspect, the proximal end comprises connection devices configured to enable joining preferably to another set of turbines or to another turbine or to a generator. 
     In one aspect, the first turbine and/or the second turbine are of the type selected from the group comprising: radial turbines (centrifugal and centripetal), axial turbines, and radial/axial turbines. 
     In one aspect, the first rotor and/or the second rotor are of the radial type and comprise(s) a plurality of first/second rotor blades arranged on the respective first/second front face according to at least one annular array concentric with the respective first/second main axis. In one aspect, the first/second rotor blades are arranged according to a number of concentric annular arrays on the respective first/second front face. Said first/second rotor blades extend axially from the respective front face. 
     In one aspect, the first rotor and/or the second rotor are of the axial type and comprise(s) a plurality of first/second rotor blades arranged on a radially peripheral portion of the respective rotor according to at least one circumferential array. In one aspect, the first/second rotor blades are arranged according to a number of circumferential arrays that are axially spaced on the radially peripheral portion of the respective rotor. Said first/second rotor blades extend radially from the respective radially peripheral portion. 
     In one aspect, the first rotor and/or the second rotor are of the radial/axial type and comprise(s) a plurality of first/second radial rotor blades arranged on the respective first/second front face according to at least one annular array concentric with the respective first/second main axis, and a plurality of first/second axial rotor blades arranged on a radially peripheral portion of the respective rotor according to at least one circumferential array. 
     In one aspect, the turbine train comprises at least two sets. 
     In one aspect, said two sets are connected one to the other at the proximal ends of the respective first or second shaft. In one aspect, a generator is interposed between the two sets. In one aspect, at least one preferably elastic joint is interposed between the two sets. 
     The present invention also concerns a power plant for generating electrical energy, for example a power station comprising at least one set according to the present invention and/or a turbine train according to the present invention. 
     Further characteristics and advantages will become more apparent from the detailed description of preferred, but not exclusive, embodiments of a set of turbines according to the present invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       This description is provided herein below with reference to the attached drawings, which are provided solely for purpose of providing approximate and thus non-limiting examples, and of which: 
         FIG. 1  schematically illustrates a set of turbines according to the present invention. 
         FIG. 2  schematically illustrates a turbine train comprising a number of sets according to the present invention. 
         FIG. 3  illustrates the set appearing in  FIG. 1  with a first type of fluid coupling between the turbines. 
         FIG. 4  illustrates the set appearing in  FIG. 1  with a different type of fluid coupling between the turbines. 
         FIG. 5  illustrates an example of a connection element between the turbines making up the set according to the invention. 
         FIG. 6  illustrates an example of an elastic joint that is part of the connection element of  FIG. 5 . 
         FIG. 7  illustrates a first embodiment of the set appearing in  FIG. 1 . 
         FIG. 8  illustrates a second embodiment of the set appearing in  FIG. 1 . 
         FIG. 9  illustrates a third embodiment of the set appearing in  FIG. 1 . 
         FIG. 10  illustrates a fourth embodiment of the set appearing in  FIG. 1 . 
         FIG. 11  illustrates a fifth embodiment of the set appearing in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the figures cited, a set of turbines in accordance with the present invention is indicated in its entirety by the reference number  1 . The set  1  comprises a first turbine  2  and a second turbine  3 , each one being of the overhung type and they are connected to each other by a connection element  4 . 
     As shown schematically in  FIGS. 1, 2, 3 and 4 , the first turbine  2  comprises a first rotor  5  joined to a distal end  6  of a first shaft  7 . The first shaft  7  is supported in a first case  8  (not visible in the schematic  FIGS. 1, 2, 3 and 4 , but represented in detail in  FIGS. 6-10 ) by two first support elements  9  (bearings) that are radially interposed between the first shaft  7  and the first case  8  and configured to enable free rotation of said first shaft  7  together with the first rotor  5 , with respect to the first case  8 , about a first main axis “X 1 ”. A proximal end  10  of the first rotor  7  is provided with a connection flange  11  for the transmission of power. The connection flange  11  defines or is part of a connection device configured to enable joining, for example, to another set of turbines  1  or to another turbine of a known type or to a generator. A mechanical seal  12  surrounds the first shaft  7  in proximity to the first rotor  5 . 
     Likewise, the second turbine  3  comprises a second rotor  13  joined to a distal end  14  of a second shaft  15 . The second shaft  15  is supported in a second case  16  (not visible in the schematic  FIGS. 1, 2, 3 and 4 , but represented in detail in  FIGS. 6-10 ) by two second support elements  17  (bearings) that are radially interposed between the second shaft  15  and the second case  16  and configured to enable free rotation of said second shaft  15  together with the second rotor  13 , with respect to the second case  16 , about a second main axis “X 2 ”. A proximal end  18  of the second rotor  13  is provided with a connection flange  19  for the transmission of power. The connection flange  19  defines or is part of a connection device configured to enable joining, for example, to another set of turbines  1  or to another turbine of a known type or to a generator. A mechanical seal  20  surrounds the second shaft  15  in proximity to the second rotor  3 . 
     The first and the second rotor  5 ,  13  are supported in an overhung manner with respect to the first and second support elements  9 ,  17  so that the respective first and second front face  21 ,  22  of the rotors  5 ,  13  remain free from support elements. Said first front face  21  and said second front face  22  face each other and the first and second shaft  7 ,  15  are substantially aligned, that is, the first and the second main axis “X 1 ”, “X 2 ” substantially coincide. The connection element  4  connects the first front face  21  to the second front face  22  and, in the illustrated embodiments, it comprises a drive shaft  23 . A third main axis “X 3 ” of the drive shaft  23  is substantially aligned with the first shaft  7  and the second shaft  15 . The opposite ends of the drive shaft  23  are connected to the centres of the first rotor  5  and the second rotor  13  by means of respective elastic joints  24 . 
     Each elastic joint  24  may also be known in itself. 
     In the embodiment illustrated in  FIGS. 5 and 6 , the elastic joint  24  is of the type having a flexible disc or discs. The illustrated elastic joint  24  comprises a flange  25  that is connected rigidly, for example by means of bolts, to the respective first or second rotor  5 ,  13 . The flange  25  bears a tubular body  26  terminating with a first annular edge  27 . A second annular edge  28  is rigidly connected to or is part of each end of the drive shaft  23 . One or more flexible discs  29  substantially annular in shape are located between the first annular edge  27  and the second annular edge  28 . The surface of the flexible discs  27  is substantially perpendicular to the main axes “X 1 ”, “X 2 ” and to the axis of the drive shaft  23 . The first annular edge  27  is connected to the flexible disc(s)  29  by means of first bolts and the second annular edge  28  is connected to the same flexible discs  29  by means of second bolts  31 . The first bolts  30  are circumferentially alternated with the second bolts  31 . 
     The flexible discs  29  of each one of the two elastic joints  24  permit limited relative movements between the drive shaft  23  and the respective flange  25 . For example, these relative movements consist of the following: translational movement along three axes that are perpendicular to each other and/or rotation about a plurality of axes differing from the first, the second and third main axis “X 1 ”, “X 2 ”, “X 3 ”. The rotation about the first and/or the second and/or the third main axis “X 1 ”, “X 2 ”, “X 3 ” (torsion) is instead preferably prevented so as to enable proper transmission of the torque. 
     In other words, with respect to a plane in which the flexible discs lie in their flat and undeformed configuration, said discs bend/deform outside of said plane. 
     Therefore, the elastic joint is yielding and elastic in flexure and traction/compression, but not in torsion. The flexure is such as to permit an inclination, for example of 0.2°, of the rigid parts, which said joint connects. This traction/compression is such as to enable the rigid parts, which said joint connects, to move about +/−5 mm away/towards each other. 
     The set of turbines  1  can be used alone connected to one or two generators  32  by means of the connection flange  11  of the first shaft  7  and/or the connection flange  19  of the second shaft  15 . 
       FIG. 2  illustrates a turbine train  33  comprising a plurality of sets  1  of the type described above, each of which thus constituting one module of the train  33 . In the illustrated embodiment, the turbine train  33  comprises a first pair of sets  1  and a second pair of sets  1 . Each set  1  of each pair is connected to the other set  1  by means of an auxiliary connection element  34  interposed between the connection flanges  11 ,  19 . This auxiliary connection element  34  may be similar or identical to the connection element  4  described hereinabove or it may be a rigid joint. The two pairs are connected to opposite shafts of a single generator  32  by means of auxiliary connection elements  34 . 
       FIGS. 3 and 4  illustrate two types of fluidic connection between the first and the second turbine  2 ,  3  of a set  1 . In  FIG. 3 , the first and the second turbine  2 ,  3  are connected in parallel. The working fluid enters through a common inlet and it is subdivided into two streams, each one entering in one of the turbines  2 ,  3 . In  FIG. 4 , the first and the second turbine  2 ,  3  are connected in series. The working fluid passes first through the second turbine  3 , exits from the second turbine and then passes through the first turbine  2 . 
     A first embodiment of the set  1  is illustrated in greater detail in  FIG. 7 . 
       FIG. 7  shows the first case  8  of the first turbine  2  in which the first shaft  7  and the first rotor  5  are housed. In particular, the first shaft  7  is inserted and rotatably supported by means of the first support elements  9  in a first sleeve  35 . The mechanical seal  12  of the first shaft  7  is located at one end of the first sleeve  35 , said end facing the first rotor  5 , and it is radially interposed between said first sleeve  35  and said first shaft  7 . 
     The first sleeve  35  is inserted in a seat  36  afforded in the first case  8  and it is fixed to the first case  8 . The distal end  6  of the first shaft  7  projects out from the first sleeve  35  and from the mechanical seal  12  and projects inside the first case  8 . 
     The proximal end  10  of the first shaft  7  and the connection flange  11  project out from the first sleeve  35  and also from the first case  8 . The first sleeve  35  is extractable from the respective seat  36  on the opposite side with respect to the first rotor  5  and together with the support elements  9  and the first shaft  7  (after having disconnected it from the first rotor  5 ). 
     The first case  8  delimits a first housing space  37  for the first rotor  5  and a first annular discharge space  38  that surrounds the first housing space  37 . A first discharge opening  39  connects the first annular discharge space  38  with the exterior or with a suitable circuit. 
     The first turbine  2  is of a centrifugal radial (outflow) type. The first rotor  5  comprises concentric annular arrays of first rotor blades “P 1 ” arranged on the first front face  21  at a first transit and expansion space for the working fluid. 
     Each of the first rotor blades “P 1 ” extends away from the first front face  21  with the leading edge and trailing edge thereof substantially parallel to the first main axis “X 1 ”. 
       FIG. 7  shows the second case  16  of the second turbine  3  in which the second shaft  15  and the second rotor  13  are housed. The second shaft  15  is inserted and rotatably supported by means of the second support elements  17  in a second sleeve  40 . The mechanical seal  20  of the second shaft  15  is located at one end of the second sleeve  40 , said end facing the second rotor  13 , and it is radially interposed between said second sleeve  40  and said second shaft  15 . 
     The second sleeve  40  is inserted in a seat  41  afforded in the second case  16  and it is fixed to the second case  16 . The distal end  14  of the second shaft  15  projects out from the second sleeve  41  and from the mechanical seal  20  and projects inside the second case  16 . The proximal end  18  of the second shaft  15  and the connection flange  19  project out from the second sleeve  40  and also from the second case  16 . The second sleeve  40  is extractable from the respective seat  41  on the opposite side with respect to the second rotor  13  and together with the support elements  17  and the second shaft  15  (after having disconnected it from the second rotor  13 ). 
     The second case  16  delimits a second housing space  42  for the second rotor  13  and a second annular discharge space  43  that surrounds the second housing space  42 . A second discharge opening  44  connects the second annular discharge space  43  with the exterior or with a suitable circuit. 
     The second turbine  3  is also of the centrifugal radial (outflow) type. The second rotor  13  comprises concentric annular arrays of second rotor blades “P 2 ” arranged on the second front face  22  at a second transit and expansion space for the working fluid. 
     Each of the second rotor blades “P 2 ” extends away from the second front face  22  with the leading edge and trailing edge thereof substantially parallel to the second main axis “X 2 ”. 
     A casing  45  is interposed between the first case  8  and the second case  16  and connects them so as to form, together with said first and second case  8 ,  16 , a single box-like containment body. 
     The casing  45  appearing in  FIG. 7  comprises a radially internal tubular body  46  that surrounds the drive shaft  23  and a radially external tubular body  47  arranged coaxially around the radially internal tubular body  46 . 
     The casing  45  further comprises a first wall  48  that extends radially around a first end of the radially external tubular body  47  and that is connected to the first case  8  and closes a front opening of said first case  8 , and a second wall  49  that extends radially around a second end of the radially external tubular body  46  and that is connected to the second case  16  and closes a front opening of said second case  16 . 
     One face of the first wall  48  inside the first case  8  bears concentric annular arrays of first stator blades “S 1 ” that are radially alternated with the annular sets of first rotor blades “P 1 ”. Likewise, one face of the second wall  49  inside the second case  16  bears concentric annular arrays of second stator blades “S 2 ” that are radially alternated with the annular arrays of second rotor blades “P 2 ”. 
     The radially innermost annular array of first stator blades “S 1 ” and the radially innermost annular array of second stator blades “S 2 ” connect the radially internal tubular body  46  to the radially external tubular body  47 . The radially external tubular body  47  thus supports the radially internal tubular body  46  by means of said radially innermost annular arrays of first and second stator blades “S 1 ”, “S 2 ”. 
     The radially internal tubular body  46  and the radially external tubular body  47  delimit together a conduit  50  having a substantially cylindrical shape with the opposite ends thereof terminating at the radially innermost annular arrays of first and second stator blades “S 1 ”, “S 2 ”. 
     An inlet mouth  51  is defined on the radially external tubular body  47  and it extends perpendicular to a central axis of said radially external tubular body  47  and permits the working fluid to enter the conduit  50 . The inlet mouth  51  is located in an axially middle area of the radially external tubular body  47  so that the incoming fluid divides into two streams: a first stream directed towards the radially innermost first stator blades “S 1 ” and a second stream directed towards the radially innermost second stator blades “S 2 ”. The first stream passes radially through the first rotor  5  of the first turbine  2 , as it expands, thereby determining the rotation thereof; it then enters into the first annular discharge space  38  and flows out from first turbine  2  through the first discharge opening  39 . The second stream passes radially through the second rotor  13  of the second turbine  3 , as it expands, thereby determining the rotation thereof; it then enters into the second annular discharge space  43  and flows out from the second turbine  3  through the second discharge opening  44 . The conduit  50  is therefore an inlet conduit for the working fluid in both turbines  2 ,  3  of the set  1 . As concerns the flow of fluid, the first and the second turbine  2 ,  3  are connected in parallel. 
     A second embodiment of the set  1  is illustrated in  FIG. 8 . 
     The first turbine  2  of the set  1  appearing in  FIG. 8  is identical to the first turbine  2  of the first embodiment described previously (the reference numbers are the same) and therefore it is not be described in further detail herein below. 
     The second turbine  3  of the set  1  appearing in  FIG. 8  differs from the second turbine  3  in the first embodiment described previously in that it is a radial centripetal (inflow) turbine. The second case  16  delimits an annular inlet space  52  that surrounds the second housing space  42 . An inlet opening  53  connects the annular inlet space  52  with a suitable circuit. The working fluid enters into the second turbine  3  through the inlet opening  53  and flows into the annular inlet space  52 . Moving radially towards the first main axis “X 1 ”, and expanding, the working fluid thus passes through the second rotor  13  of the second turbine  3 , thereby determining the rotation thereof, and then flows into the conduit  50  through the radially innermost second stator blades “S 2 ” towards the radially innermost first stator blades “S 1 ” of the first turbine  2 . 
     In this second embodiment, the inlet mouth  51  constitutes an intermediate admission mouth through which an additional stream of working fluid is admitted. 
     This additional stream enters into the conduit  50  and, dragged by the working fluid coming from the second turbine  3  and directed by the internal shape of the inlet mouth  51 , it too flows towards the radially innermost first stator blades “S 1 ” of the first turbine  2 . The working fluid, the sum total of the fluid coming from the second turbine  3  and the fluid coming from the intermediate admission mouth  51 , passes through the first rotor  5  of the first turbine  2 , as it expands, thereby determining the rotation thereof; it then enters into the first annular discharge space  38  and flows out from the first turbine  2  through the first discharge opening  39 . The conduit  50  is thus an inlet conduit for admitting the working fluid into the first rotor  5  and an outlet conduit for letting it flow out from the second rotor  13 . It should be noted that the remaining elements of the set  1  in this second embodiment are the same as those in the first embodiment appearing in  FIG. 7  and they have the same reference numbers. As concerns the flow of fluid, the first and the second turbine  2 ,  3  are connected in series. 
     A third embodiment of the set  1  is illustrated in  FIG. 9 . In this case as well, the elements of the set  1  that are identical to those of the preceding embodiments have the same reference numbers. 
     The third embodiment differs from the preceding embodiments in that both turbines  2 ,  3  are of the axial type. 
     The first rotor  5  comprises circumferential arrays of first rotor blades “P 1 ” arranged at a radially peripheral portion of the first rotor  5  and at a first transit and expansion space for the working fluid. The first rotor blades “P 1 ” extend in a radial pattern away from the first main axis “X 1 ” with the leading edge and trailing edge thereof substantially perpendicular to the first main axis “X 1 ”. 
     The second rotor  13  comprises circumferential arrays of second rotor blades “P 2 ” arranged at a radially peripheral portion of the second rotor  5  and at a second transit and expansion space for the working fluid. The second rotor blades “P 2 ” extend in a radial pattern away from the second main axis “X 2 ” with the leading edge and trailing edge thereof substantially perpendicular to the second main axis “X 2 .” 
     The casing  45  comprises the radially internal tubular body  46  and the radially external tubular body  47  with the inlet mouth  51  and they delimit the conduit  50 , but it does not have the first and the second wall that extend radially. 
     Unlike the first and the second embodiments, circumferential arrays of first stator blades “S 1 ” are afforded on a radially internal surface of the radially external tubular body  47 . Said first stator blades “S 1 ” radially extend towards the first rotor  5 , that is, towards the first main axis “X 1 ”, and they are radially alternated with the circumferential arrays of first rotor blades “P 1 ”. The circumferential array of first stator blades “S 1 ” bordering on the conduit  50  is connected to and supports the radially internal tubular body  46 . Likewise, circumferential arrays of second stator blades “S 2 ” are afforded on a radially internal surface of the radially external tubular body  47 . Said second stator blades “S 2 ” radially extend towards the second rotor  13 , that is, towards the second main axis “X 2 ”, and they are radially alternated with the circumferential arrays of second rotor blades “P 2 ”. The circumferential array of second stator blades “S 2 ” bordering on the conduit  50  is connected to and supports the radially internal tubular body  46 . 
     The fluid entering through the inlet mouth  51  is divided into two streams: a first stream directed towards the first stator blades “S 1 ” and first rotor blades “P 1 ” and a second stream directed towards the second stator blades “S 2 ” and second rotor blades “P 2 ”. The first stream passes axially through the first rotor  5  of the first turbine  2 , as it expands, thereby determining the rotation thereof; it then enters into the first annular discharge space  38  and flows out from first turbine  2  through the first discharge opening  39 . The second stream passes axially through the second rotor  13  of the second turbine  3 , as it expands, thereby determining the rotation thereof; it then enters into the second annular discharge space  43  and flows out from the second turbine  3  through the second discharge opening  44 . As concerns the flow of fluid, the first and the second turbine  2 ,  3  are connected in parallel. 
     A fourth embodiment of the set  1  is illustrated in  FIG. 10 . In this case as well, the elements of the set  1  that are identical to those of the preceding embodiments have the same reference numbers. 
     This fourth embodiment of the set  1  differs from the third embodiment appearing in  FIG. 9  in that the working fluid passes through first turbine  2  and the second turbine  3  (again of the axial type) in series. Furthermore, the radially internal tubular body  46  is not present and the conduit  50  is delimited by the radially external tubular body  47  alone. 
     The first case  8  delimits an annular inlet space  52  that partly surrounds the first housing space  37 . An inlet opening  53  connects the annular inlet space  52  with a suitable circuit. 
     The working fluid enters into the first turbine  2  through the inlet opening  53  and flows into the annular inlet space  52 . Moving axially and expanding, the working fluid passes through the first rotor and stator blades “P 1 ”, “S 1 ” of the first rotor, thereby determining the rotation thereof; it then flows into the conduit  50  and subsequently, moving axially and expanding, it passes through the second rotor and stator blades “P 2 ”, “S 2 ” of the second rotor  13 , thereby determining the rotation thereof. In this fourth embodiment as well, the inlet mouth  51  constitutes an intermediate admission mouth through which an additional stream of working fluid is admitted. This additional stream enters into the conduit  50  and, dragged by the working fluid coming from the first turbine  2 , it too flows towards the second turbine  3 . The working fluid, the sum total of the fluid coming from the first turbine  2  and the fluid coming from the intermediate admission mouth  51 , passes axially through the second rotor  13  of the second turbine  3 , as it expands, thereby determining the rotation thereof; it then enters into the second annular discharge space  43  and flows out from the second turbine  3  through the second discharge opening  44 . 
     A fifth embodiment of the set  1  is illustrated in  FIG. 11 . In this case as well, the elements of the set  1  that are identical to those of the preceding embodiments have the same reference numbers. 
     This fifth embodiment of the set  1  differs from the preceding embodiments in that the first turbine  2  is of the centrifugal radial type, whereas the second turbine  3  is of the axial type. The first turbine  2  is similar to the first turbine  3  of the first and the second embodiment (centrifugal radial,  FIGS. 7 and 8 ), but it does not have the first annular discharge space  38  or the first discharge opening  39 . The second turbine  3  is substantially identical to the second turbine  3  of the third embodiment described hereinabove ( FIG. 9 ). 
     The casing  45  comprises the radially internal tubular body  46  and the radially external tubular body  47  with the inlet mouth  51  and they delimit the conduit  50 . 
     The conduit  50  is connected to the second turbine  3  in the same manner as in the third embodiment ( FIG. 9 ). The conduit  50  is connected to the first turbine  3  at the radially outermost annular array of first rotor blades “P 1 ” of the first rotor  5 . 
     The casing  45  further comprises an auxiliary portion  54  located around the connection element  4  in a radially internal position with respect to the radially internal tubular body  46 . The auxiliary portion  54  comprises a radially internal tubular wall  55  and a radially external wall  56  that delimit an annular inlet space  52  and an auxiliary conduit  57 . A pipe  58  is in fluid connection with the annular inlet space  52 , passes through the radially internal tubular body  46  and the radially external tubular body  47 , and exits from the set  1  through an inlet opening  53  configured to be connected to a suitable circuit. 
     The auxiliary conduit  57  extends from the annular inlet space  52  and terminates at the radially innermost annular array of first rotor blades “P 1 ” of the first rotor  5 . The radially internal annular array of first stator blades “S 1 ” of the first turbine  2  is located and supported in this area by the casing  45 . The casing  45  also supports the other arrays of first stator blades “S 1 .” 
     Admitted through the inlet opening  53 , the working fluid flows into the pipe  58  to the inside of the annular inlet space  52 . The working fluid passes axially through the auxiliary conduit  57  and then it is deviated along radial directions and passes radially through the first rotor and stator blades “P 1 ”, “S 1 ” of the first rotor  5  of the first turbine  2 , as it expands, thereby determining the rotation thereof. Subsequently, the working fluid flows into the conduit  50  and then, moving axially and expanding, it passes through the second rotor and stator blades “P 2 ”, “S 2 ” of the second rotor  13 , thereby determining the rotation thereof. 
     In this fifth embodiment as well, the inlet mouth  51  constitutes an intermediate admission mouth through which an additional stream of working fluid is admitted. This additional stream enters into the conduit  50  and, dragged by the working fluid coming from the first turbine  2 , it too flows towards the second turbine  3 . The working fluid, the sum total of the fluid coming from the first turbine  2  and the fluid coming from the intermediate admission mouth  51 , passes axially through the second rotor  13  of the second turbine  3 , as it expands, thereby determining the rotation thereof; it then enters into the second annular discharge space  43  and flows out from the second turbine  3  through the second discharge opening  44 . 
     In all of the embodiments described, the connection element  4  is contained in the casing  45  and it is immersed in the working fluid. The drive shaft  23  is coaxial with the radially external and/or internal tubular body  47 ,  46 . The centre of both the first and the second rotor  5 ,  13  is free of the rotor blades and it is provided with the flange  25  of the respective elastic joint  24 . 
     In other unillustrated embodiments, at least one of the turbines  2 ,  3  of the set  1  may be of the radial/axial type, that is, it may comprise at least one annular array of first/second rotor blades “P 1 ”, “P 2 ” located on the respective front face  21 ,  22  (as in  FIGS. 7, 8 ) and at least one circumferential array of first/second rotor blades “P 1 ”, “P 2 ” arranged at a radially peripheral portion of the first/second rotor  5 ,  13  (as in  FIGS. 9, 10 ). In other unillustrated embodiments, the casing  45  can be configured so as to have an outlet mouth for letting the working fluid flow out from the set  1  of turbines and/or an intermediate mouth for extracting the working fluid from the set  1  of turbines. 
     LIST OF ELEMENTS 
     
         
           1  set of turbines 
           2  first turbine 
           3  second turbine 
           4  connection element 
           5  first rotor 
           6  distal end of first shaft 
           7  first shaft 
           8  first case 
           9  first support elements 
           10  proximal end of first shaft 
           11  connection flange of first shaft 
           12  mechanical seal of first shaft 
           13  second rotor 
           14  distal end of second shaft 
           15  second shaft 
           16  second case 
           17  second support elements 
           18  proximal end of second shaft 
           19  connection flange of second shaft 
           20  mechanical seal of second shaft 
           21  first front face 
           22  second front face 
           23  drive shaft 
           24  elastic joints 
           25  flange 
           26  tubular body 
           27  first annular edge 
           28  second annular edge 
           29  flexible discs 
           30  first bolts 
           31  second bolts 
           32  generator 
           33  turbine train 
           34  auxiliary connection element 
           35  first sleeve 
           36  seat for the first case 
           37  first housing space 
           38  first annular discharge space 
           39  first discharge opening 
           40  second sleeve 
           41  seat for the second case 
           42  second housing space 
           43  second annular discharge space 
           44  second discharge opening 
           45  casing 
           46  radially internal tubular body 
           47  radially external tubular body 
           48  first wall 
           49  second wall 
           50  conduit 
           51  inlet mouth 
           52  annular inlet space 
           53  inlet opening 
           54  auxiliary portion 
           55  radially internal tubular wall 
           56  radially external tubular wall 
           57  auxiliary conduit 
           58  pipe 
         X 1  first main axis 
         X 2  second main axis 
         X 3  third main axis 
         P 1  first rotor blades 
         P 2  second rotor blades 
         S 1  first stator blades 
         S 2  second stator blades