Patent Publication Number: US-2016237841-A1

Title: Expansion turbine

Description:
FIELD OF THE INVENTION 
     The present invention has as object an expansion turbine, also simply termed expander, for producing electrical and/or mechanical energy. Preferably but not exclusively, the present invention refers to the expansion turbines used in the apparatuses for producing energy by means of organic Rankine cycle (ORC). In such per se known apparatuses, working fluids are used of organic type in place of the conventional water/steam system, since an organic fluid is able to more efficiently convert heat sources at relatively low temperatures, generally between 100° C. and 300° C., but also at higher temperatures. The ORC conversion apparatuses are therefore finding wider applications in various sectors, for example in the geothermal field, in industrial energy recovery, in plans for producing energy from biomasses and from concentrated solar energy, in re-gasifiers, etc.. 
     BACKGROUND OF THE INVENTION 
     Different types of expanders/turbines are known for converting thermal energy into electrical and/or mechanical energy by means of an organic Rankine cycle (ORC). Among the most well-known types, axial expanders and radial expanders are recalled, with one or more stages in series. 
     In general, a turbine of known type is composed of a fixed body, termed stator, and of a movable part, termed rotor. The rotor part of the turbine is constituted by a shaft to which one or more expansion stages of the working fluid are fixed. The assembly of the rotor part and the stator part is carried out by means of a mechanical group that ensures nearly zero relative movements of the rotor and stator parts except for the rotation around the rotation axis coinciding with the shaft of the machine. 
     The public document WO 2012/093299, on behalf of the same Applicant, illustrates an expansion turbine that comprises: a case having an inlet and an outlet for a working fluid; a stator installed in the case; a rotor installed in the case and rotatable around a respective rotation axis; a short pipe constrained to the case; a mechanical group installed in the short pipe. The mechanical group comprises a bushing and a shaft rotatably installed in the bushing. The shaft is removably connected to the rotor and the entire mechanical group, including the shaft, can be extracted in a block from the short pipe from the side opposite the rotor. The rotor is movable along the axial direction between a first configuration, in which the mechanical group is installed in the short pipe and the rotor is spaced from the short pipe, in a manner so as to be able to rotate under the action of the working fluid, and a second configuration, in which the mechanical group is extracted from the short pipe and the rotor is abutted against the short pipe at a static seal. The turbine also comprises a tie rod which allows tightening the rotor against the shaft and rendering them integral. The shaft has an axial passage inside of which the tie rod is housed. The tie rod has a head that projects from the first end of the shaft and is connected to the rotor. The tie rod is inserted in a central hole of the rotor and the head has a spherical surface that lies against a spherical surface of the rotor directed on the opposite side with respect to the mechanical group. The tie rod has a terminal end opposite the head and projecting from the second end of the shaft. A nut screwed on the terminal end tightens the rotor against the shaft, in a manner such that the rotor and the shaft integrally rotate. 
     The public document WO 2012/004821 illustrates a fluid seal device for rotary machines having a rotor part including at least one disc borne by a rotary shaft that is rotatable with respect to a stator part in the presence of fluid. The rotary shaft is provided with at least one seal system defining a first setting at a first pressure. The stator part has a wall in front of the disc defining therewith a second setting that contains a fluid at a second pressure. The device comprises a movable ring placed between the wall of the stator part and the disc of the rotor part and provided with at least one front seal gasket directed towards said disc. The movable ring is axially movable between an inactive position, in which the front seal gasket is far from said disc, and an active position, in which said front seal gasket abuts against said disc. The movable ring is moved into the active position when the rotary machine is stopped in order to prevent a passage of fluid from one of said settings to the other setting. 
     SUMMARY 
     In such context, the Applicant has observed that the turbine described and illustrated in the abovementioned document WO 2012/093299 can be improved in a manner so as to further facilitate the access to its parts and allow an even easier and more effective maintenance without having to remove the working fluid from the turbine itself. 
     In particular, the Applicant has perceived the need to be able to access the front face of the rotor at the rotation axis, where the means are present that constrain the shaft to the rotor itself, without having to empty the circuit within which the working fluid flows. 
     The Applicant has found that the above-indicated objectives can be reached by arranging, between the rotor and the stator, in a radially inner position with respect to the rotor and stator blades, a static seal which can be brought in abutment, when the turbine is stopped, both against the stator and against the rotor in a manner so as to seal the volume that is radially outside said static seal and delimited between the stator and the rotor. 
     More specifically, according to a first aspect, the present invention regards an expansion turbine, comprising: 
     a case; 
     a stator integrated or installed in the case and having stator blades; 
     a rotor mounted in the case, having rotor blades and facing the stator, wherein the rotor is rotatable around a respective rotation axis; 
     wherein the stator and the rotor delimit between them an expansion volume, for a working fluid, provided with said rotor and stator blades, 
     a short pipe integrated or installed on the case; 
     a shaft rotatably mounted in the short pipe and rotating around the rotation axis; 
     wherein the shaft is removably connected to the rotor and extractable from the short pipe from the side opposite said rotor; 
     characterized in that it comprises: 
     devices for removably connecting the rotor to the shaft having a handling portion placed at a front face of the rotor opposite the short pipe and facing at an space for access to said handling portion; 
     at least one front static seal interposed between the rotor and the stator and placed at a radially inner position with respect to the expansion volume; wherein the front static seal and the rotor are movable with respect to each other between a first position, in which the front static seal clears a passage in fluid communication with the expansion volume and with the space for access to said handling portion, and a second position, in which said front static seal seals said passage and sealingly isolates the expansion volume from the space for access to said handling portion. 
     By “handling portion” it is intended the element on which an operator must act in order to actuate (assemble/disassemble) the removable connection devices. 
     By “space for access to the handling portion” it is intended a space through which an operator can access the handling portion from the outside. The space for access to the handling portion is therefore in communication or it can be placed in communication with outside the case. In other words, the space for access to the handling portion is open towards or can be open towards the case exterior. 
     The front static seal in the second position (sealed passage) allows the operator to access the handling portion and to disassemble the shaft (by extracting it from the short pipe on the side opposite the rotor) while the working fluid remains confined in the expansion volume and cannot exit through the access space. 
     Preferably, such turbine is of radial centrifugal type and is part of plants for the cogeneration of energy of Rankine cycle type, which have closed circuit (such that the working fluid remains in the circuit even during maintenance) and use organic fluids with high molecular weight. The entrance of the fluid into the expansion volume occurs at a radially inner portion of the turbine, the fluid is moved by expanding towards the outside, away from the rotation axis, and exits at a radially outer portion of the turbine. 
     Preferably, the turbine comprises a rotor, or rotor disc, provided with a plurality of rotor blades arranged in series of concentric rings on a respective front face of the rotor. Preferably, the turbine comprises a stator provided with a plurality of stator blades arranged in series of concentric rings on a respective face of the stator. The faces with the blades are placed in front of each other in order to delimit the expansion volume for the working fluid. The series of stator blades are alternated with the series of rotor blades along a radial direction. 
     Preferably, such turbine is a radial centrifugal turbine with final axial stage(s). 
     Preferably, the turbine comprises at least one further stage of axial type arranged in radially outer position to define a radial/axial turbine. Preferably, said at least one stage of axial type is arranged at a radially peripheral edge of the rotor disc. 
     Preferably, said at least one stage of axial type comprises a plurality of rotor blades which are radially extended from the radially peripheral edge of the rotor disc. Preferably, the rotor blades of said at least one stage of axial type are mounted on the radially peripheral edge of the rotor disc. 
     The Applicant has perceived that the radial centrifugal turbine or centrifugal turbine with final axial stages is unexpectedly suitable for the use with this fluid type since:
         the expansions in the ORC cycles are characterized by low enthalpy jumps and the radial centrifugal turbine is adapted for applications with low enthalpy jumps since it obtains, given the same peripheral velocity and degree of reaction, lower work levels with respect to the axial and/or radial centripetal machines ;   the expansions in the ORC cycles are characterized by low rotation velocities and by low peripheral velocities of the rotor, due to the low enthalpy jumps that characterize the abovementioned cycles, moderate temperatures or in any case non-high temperatures as in for example gas turbines, and the radial centrifugal turbine is well suited to situations with low mechanical and thermal stresses;   since the Rankine cycles in general and the ORC in particular are characterized by high volumetric expansion ratios, the radial centrifugal turbine optimizes the height of the blades of the machine, and in particular of the first stage, due to the fact that the diameter of the wheel increases in the direction of the flow; therefore, the total and non-restricted introduction is almost always possible;   given that the structural form of the radial centrifugal turbine allows obtaining multiple expansion stages on a single disc, it is possible to reduce the losses via secondary flows and expansion leaks in radial centrifugal configuration, making it unnecessary to twist the blades on the final stage of the expansion, in this manner simplifying the structure of the machine.       

     In a second aspect in accordance with the first aspect, the front static seal is of the type with variable geometry and is mobile with respect to the stator and to the case between the first and the second position. 
     By “static seal with variable geometry” it is intended an element that can assume at least two “geometric shapes/configurations”: a contracted one, in which it occupies a lower volume in order to remain spaced from a surface with which it must make a seal, and an expanded one, in which it occupies a greater volume in order to touch said surface. 
     In a third aspect in accordance with at least one of the preceding aspects, the turbine comprises a rear static seal defined between the short pipe and/or a rear wall and the rotor; wherein the rear static seal and the rotor are movable with respect to each other between a first position, wherein the rear static seal and the rotor are mutually spaced and a second position, wherein the rotor and said rear static seal abut against each other. The rear static seal allows isolating the housing inside the short pipe and extracting the shaft without the working fluid exiting through the space cleared by the shaft itself. 
     In a fourth aspect in accordance with the preceding aspect, the rear static seal is of the type with variable geometry and is mobile with respect to the stator and to the case between the first and the second position. 
     In a fifth aspect in accordance with one of the preceding aspects, the rotor is movable along an axial direction between a first configuration, in which it is spaced by a rear wall of the case in a manner so as to be able to rotate under the action of the working fluid, and a second configuration, in which it is sealingly abutted against the rear wall. 
     Preferably, the turbine comprises blocking devices adapted to block the rotor against the rear wall, and to prevent the axial translation and rotation thereof while the machine is stopped, for example for repair and/or maintenance operations. 
     In a sixth aspect in accordance with the preceding aspect, the turbine comprises a rear static seal defined between the short pipe and/or the rear wall and the rotor such that the rotor, in the second configuration, abuts against the short pipe and/or the rear wall at such rear static seal. 
     In a seventh aspect, the present invention regards a method for disassembling an expansion turbine in accordance with one or more of the preceding aspects, comprising:
         moving the rotor back with respect to the short pipe until said rotor is brought against said short pipe at the rear static seal;   blocking the rotor against the short pipe;   bringing the front static seal with variable geometry in the second position;   extracting the shaft while the rotor is blocked against said short pipe.       

     In an eighth aspect according to one or more of the aspects from the first to the fourth, the rotor is movable along an axial direction between a first configuration, corresponding to the first position, in which it is spaced from the front static seal in a manner so as to be able to rotate under the action of the working fluid, and a second configuration corresponding to the second position, in which it is abutted against said front static seal. 
     Preferably, the turbine comprises blocking devices adapted to block the rotor against the front static seal and to prevent the axial translation and rotation thereof while the machine is stopped, for example for repair and/or maintenance operations. 
     In a ninth aspect, the present invention regards a method for disassembling an expansion turbine in accordance with one or more of the preceding aspects from the first to the fourth and/or with the eighth aspect, comprising:
         advancing the rotor with respect to the short pipe until said rotor is brought against the front static seal;   blocking the rotor against the front static seal;   bringing the rear static seal with variable geometry in the second position;   extracting the shaft while the rotor is blocked.       

     In a tenth aspect according to one or more of the aspects from the first to the fourth, the rotor is axially fixed and the turbine comprises blocking devices adapted to prevent the rotation thereof while the machine is stopped, for example for repair and/or maintenance operations. 
     In an eleventh aspect, the present invention regards a method for disassembling an expansion turbine in accordance with one or more of the preceding aspects from the first to the fourth and/or with the tenth aspect, comprising:
         blocking the rotation of the rotor;   bringing the front static seal with variable geometry in the second position;   bringing the rear static seal with variable geometry in the second position;   extracting the shaft while the rotor is blocked.       

     In a twelfth aspect in accordance with one or more of the preceding aspects, the shaft is rotatably installed, e.g. by means of bearings, in a bushing mounted in the short pipe. The bushing cannot rotate with respect to the short pipe. The bushing and the shaft form part of a mechanical group that is completely extracted from the short pipe. In other words, the shaft can be axially extracted from the short pipe together with the bushing and with possible bearings. 
     In a thirteenth aspect in accordance with one or more of the preceding aspects, the front static seal has a substantially circular extension coaxial with the rotation axis and is arranged all around the handling portion. 
     In a fourteenth aspect in accordance with one or more of the preceding aspects, the case has a front wall and a rear wall, wherein the stator is obtained in or situated at the front wall; wherein the rotor is interposed between the front wall and the rear wall. 
     In a fifteenth aspect in accordance with one of the preceding aspects, the removable connection devices comprise at least one screw screwed in a front seat of the shaft. 
     In a sixteenth aspect in accordance with the preceding aspect, the handling portion is a head of the screw (e.g. with hexagonal notch or torx). 
     In a seventeenth aspect in accordance with one or more of the preceding aspects, the turbine comprises a self-centering joint, preferably with toothed coupling (e.g. of Hirth type), connecting the shaft to the rotor. 
     In an eighteenth aspect in accordance with the preceding aspect, a portion of such joint of the shaft is obtained around a front seat of said shaft in which the removable connection devices of the rotor are engaged with the shaft itself. 
     In a nineteenth aspect according to one or more of the preceding aspects, the turbine comprises a substantially tubular wall coaxial with the rotation axis and delimiting the access space. Such wall allows the operator to safely access the handling portion since it prevents any possible contact with the active elements of the turbine (e.g. with the blades). Preferably, the front static seal is placed on a first end edge of the substantially tubular wall directed towards the rotor. Preferably, said passage is delimited between the first end edge of the substantially tubular wall directed towards the rotor and the rotor itself. Preferably, the substantially tubular wall forms part of the stator or in any case is integral with the stator. Preferably, the substantially tubular wall bears, in radially outer position with respect to the access passage, the first series of radially inner stator blades. In a twentieth aspect in accordance with the preceding aspect, an inlet duct for the working fluid in fluid communication with the expansion volume is obtained in a radially outer position with respect to said substantially tubular wall. The inlet duct is defined by a passage with annular section which surrounds the substantially tubular wall. Preferably, a cylindrical body, part of the case, is arranged at the substantially tubular wall and is radially spaced therefrom in a manner so as to delimit the abovementioned inlet duct. 
     In a twenty-first aspect according to one or more of the preceding aspects, the turbine comprises a removable cover adapted to sealingly close said access space. 
     In a twenty-second aspect according to the preceding aspect, the cover sealingly abuts against a second end edge of the substantially tubular wall, axially opposite the first. The seal closure serves to prevent the exit of the working fluid during the operation of the turbine, when the front static seal is in the first position, in which it clears the passage delimited between the rotor and the first end edge of the substantially tubular wall. 
     In a twenty-third aspect according to one or more of the preceding aspects, the shaft, apart from the possible front seat for the removable connection devices, is solid, i.e. it has no axial through duct. In this manner, the shaft is ensured a sufficient twisting resistance necessary for preventing excessive twisting deformations due to the high torques with low rotation velocities typical of the operation of the turbines in the apparatuses with Rankine cycle. Hence the present invention allows disassembling the shaft even in cases with high torque in which the solution with hollow shaft described in WO 2012/093299 is hard to achieve. 
     In a twenty-fourth aspect according to one or more of the preceding aspects, the front static seal is mounted on the stator and, when it is in the second position, abuts and pushes against the rotor. 
     In a twenty-fifth aspect according to one or more of the aspects from the first to the twenty-third, the front static seal is mounted on the rotor and, when it is in the second position, abuts and pushes against the stator. 
     In a twenty-sixth aspect according to one or more of the preceding aspects, the front static seal is axially movable between the first and the second position. The front static seal comes to frontally abut against the rotor or against the stator (depends on where it is mounted). 
     In a twenty-seventh aspect according to one or more of the aspects from the first to the twenty-fifth, the front static seal is radially movable between the first and the second position. 
     In a twenty-eighth aspect according to the preceding aspect, the turbine comprises an appendage with axial extension integral with the rotor, wherein the front static seal is mounted on the stator and acts radially on said appendage. The appendage is cylindrical and preferably coaxial with the rotation axis. The appendage is preferably extended cantilevered from a front face of the rotor. 
     In a twenty-ninth aspect according to the twenty-eighth aspect, the turbine comprises an appendage with axial extension integral with the stator, wherein the front static seal is mounted on the rotor and acts radially on said appendage. In a thirtieth aspect according to one of the aspects from the twenty-seventh to the twenty-ninth, the front static seal has variable geometry and is radially contracted in order to adhere against said appendage. 
     In a thirty-first aspect according to one of the aspects from the twenty-seventh to the twenty-ninth, the front static seal has variable geometry and is radially expanded in order to adhere against said appendage. 
     In a thirty-second aspect according to one or more of the preceding aspects, the front static seal has variable geometry and is an inflatable gasket, i.e. with advancement via pressurization. The first position of the front static seal with variable geometry is obtained by deflating the inflatable gasket. The second position of the front static seal with variable geometry is obtained by inflating the inflatable gasket. 
     In a thirty-third aspect according to one or more of the preceding aspects, the front static seal has variable geometry and is defined by a movable tray preferably moved by a piston, for example with hydraulic actuation. 
     In a thirty-fourth aspect according to one or more of the aspects from the third to the thirty-second, the rear static seal has variable geometry and is an inflatable gasket, i.e. with advancement via pressurization. The first position of the rear static seal with variable geometry is obtained by deflating the inflatable gasket. The second position of the rear static seal with variable geometry is obtained by inflating the inflatable gasket. 
     In a thirty-fifth aspect according to one or more of the preceding aspects, the rear static seal has variable geometry and is defined by a movable tray preferably moved by a piston, e.g. with hydraulic actuation. 
     In a thirty-sixth aspect according to the thirty-second and/or the thirty-fourth aspect, the turbine has at least one channel in fluid communication with the inflatable gasket and with a pressurized gas source, wherein said source can be activated on command (e.g. by means of a valve). 
     In a thirty-seventh aspect according to the thirty-second and/or the thirty-fourth and/or the thirty-sixth aspect, the inflatable gasket/gaskets contains/contain an inert gas, e.g. nitrogen. This solution is especially preferred if the working fluid is inflammable. 
     In a thirty-eighth aspect according to one or more of the aspects from among the thirty-second and/or the thirty-fourth and/or the thirty-sixth and/or the thirty-seventh, the inflatable gasket/gaskets is/are made of elastomeric material preferably reinforced with cords. Preferably, the material of the gasket is selected as a function of the type of working fluid so that it is not altered when interacting with said working fluid. 
     In a thirty-ninth aspect in accordance with the seventh or the ninth or the eleventh aspect, the method comprises, after having brought the front static seal and/or the rear static seal in the second position and before extracting the shaft:
         removing the cover;   acting on the handling portion and removing the connection devices.       

     In a fortieth aspect in accordance with one or more of the preceding aspects, the present invention regards a method for disassembling an expansion turbine in accordance with one or more of the preceding aspects, comprising:
         blocking the rotation of the rotor;   bringing the front static seal and the rotor to the respective second position, by moving the front static seal, if this is of the type with variable geometry, towards the rotor or the rotor towards the front static seal;   bringing the rear static seal and the rotor to the respective second position, by moving the rear static seal, if this is of the type with variable geometry, towards the rotor or the rotor towards the rear static seal;   removing the cover;   acting on the handling portion and removing the connection devices;   extracting the shaft while the rotor is blocked.       

     Further characteristics and advantages will be clearer from the following detailed description of a preferred but not exclusive embodiment of an expansion turbine in accordance with the present invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Such description will be set forth hereinbelow with reference to the enclosed drawings, provided only as a non-limiting example, in which: 
         FIG. 1  illustrates, in section view, an expansion turbine according to the present invention; 
         FIG. 2  illustrates a portion of the turbine of  FIG. 1  in a first operative configuration; 
         FIG. 3  illustrates the portion of  FIG. 2  in a second operative configuration; 
         FIG. 4  illustrates a different embodiment of the portion pursuant to  FIGS. 2 and 3  in the first operative configuration; 
         FIG. 5  illustrates the different embodiment of  FIG. 4  in the second operative configuration; 
         FIG. 6  illustrates a different embodiment of the seal pursuant to  FIGS. 2 and 3 ; 
         FIG. 7  illustrates a different portion of the turbine of  FIG. 1  according to a variant of the present invention; and 
         FIG. 8  illustrates the portion of  FIG. 7  with a different embodiment of the seal pursuant to  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the abovementioned figures, reference number  1  indicates overall an expansion turbine in accordance with the present invention. 
     The illustrated expansion turbine  1  is of radial centrifugal (outflow) type used in the apparatuses for the generation of mechanical and/or electrical energy by means of organic Rankine cycle (ORC). The turbine  1  can have at least one further stage of axial type (not illustrated) arranged in radially outer position to define a radial/axial turbine. 
     The turbine  1  comprises a rotor  2 , or rotor disc, provided with a plurality of rotor blades  2   a  arranged in series of concentric rings on a respective front face of the rotor  2 . The turbine  1  comprises a stator  3  provided with a plurality of stator blades  3   a  arranged in series of concentric rings on a respective face of the stator  2 . The faces with the blades  2   a,    3   a  are placed in front of each other to delimit an expansion volume “V” for the working fluid. The series of stator blades  3   a  are alternated with the series of rotor blades  2 a along a radial direction. In a non-illustrated embodiment variant, the turbine  1  has a series of fixed blades situated around the rotor disc and adapted to deflect the radial flow exiting peripherally into an axial flow that subsequently moves into one or more axial turbine stages. 
     The rotor  2  is situated in a case that comprises a front wall  4   a,  which defines in part the stator  3 , and which also comprises a rear wall  4   b  placed on the opposite side of the rotor  2  with respect to the stator  3 . The rear wall  4   b  is jointed to a short pipe  5  integral with the case. 
     Inside the short pipe  5 , a mechanical group  6  is mounted in a reversible manner, which bears a shaft  7  that is extended along a rotation axis “X-X” and is free to rotate around said axis “X-X” together with the rotor  2 , to which it is connected at a first end  7 ′ thereof. More in detail, the mechanical group  6  comprises a bushing  8  coaxial with the short pipe  5 . The bushing  8  and/or the short pipe  5  also have grooves and/or keys, not illustrated, which prevent the relative rotation between bushing  8  and short pipe  5  around the rotation axis “X-X”. 
     In the bushing  8 , ball bearings  9  are mounted which support the shaft  7  in a manner such that it is free to rotate around the rotation axis “X-X” within the bushing  8  itself. 
     In the illustrated non-limiting embodiment, the mechanical group  6  comprises two bearings  9  side-by-side and situated in an axially intermediate position of the bushing  8  and a further bearing  9  positioned at one end of the bushing  8  close to a second end  7 ″ of the shaft  7 . 
     The mechanical group  5  also comprises a seal device  10  arranged between the bushing  8  and the shaft  7  at the first end  7 ′ of the shaft  7  and an oil seal  11  arranged between the bushing  8  and the shaft  7  at the second end  7 ″ of the shaft  7 . 
     On the first end  7 ′ of the shaft  7 , a toothing, not illustrated, is obtained which, when the turbine  1  is mounted and ready to work, is engaged with a toothing, not illustrated, obtained on the rotor  2 . The two toothings define a self-centering joint of Hirth or equivalent type. 
     The shaft  7  has a threaded front seat  12  obtained on the first end  7 ′ and extended along the rotation axis “X-X”. Such seat  12  is extended for a limited section inside the shaft  7 . 
     The rotor  2  has a central through hole  13  and a screw  14  is removably engaged in the central through hole  13  and in the threaded front seat  12  in order to integrally constrain the rotor  2  to the shaft  7 . The screw  14  has a head  15  that abuts against an abutment surface  16  delimited in the central through hole  13 . The screw  14 , once tightened, draws the rotor  2  against the shaft  7  and maintains the toothings of the self-centering joint engaged, preventing any mutual axial and rotational movement between the rotor  2  and the shaft  7 . The screw  14 , the central through hole  13  and the threaded front seat  12  define devices for removably connecting rotor  2  to the shaft  7 . The head  15  of the screw  14 , which has for example a cavity  17  for engaging a suitable tool, defines a handling portion for said removable connection devices  14 ,  13 ,  12 . 
     On the face of the short pipe  5  and/or of the rear wall  4   b  directed towards the rotor  2 , an element  18  (illustrated in a schematic manner) of a rear static seal is obtained which faces a corresponding element (not illustrated) of said rear static seal present on the rotor  2 . In the illustrated schematic representation, such element  18  is an annular relief and the rear static seal is defined by the abutment of such annular relief  18  against a rear surface  19  of the rotor  2 , as will be further detailed below. During the normal operation of the turbine  1 , such annular relief  18  remains spaced from the rear surface  19  of the rotor  2 . 
     The case comprises a substantially tubular wall  20  which is placed in front of the rotor  2  in a radially inner central position with respect to the expansion volume “V”. The substantially tubular wall  20  is coaxial with the shaft  2  and internally delimits an access space  21  facing the head  15  of the screw  14 . Said access space  21  is defined by the through duct defined by the substantially tubular wall  20 . The substantially tubular wall  20  is integral with the remaining parts of the case and with the stator  3 . A first axial end of the substantially tubular wall  20  has a first end edge provided with a first surface  22  directed towards the rotor  2  and spaced therefrom by an interspace. A second axial end of the substantially tubular wall  20 , opposite the first axial end, has a second end edge provided with a second surface  23 . The first axial end also bears a kind of flange  24 . 
     A cylindrical body  25  is arranged around the substantially tubular wall  20 , is coaxial with the shaft  2  and is integral with the walls  4 a,  4 b of the case and with the stator  3 . Between the cylindrical body  25  and the substantially tubular wall  20 , an inlet duct  26  is delimited for the working fluid in the expansion volume “V”. The inlet duct  26  has an annular shape, in a section orthogonal to the rotation axis “X-X”. A first axial end of the inlet duct  26  close to the rotor  2  is in fluid communication with the expansion volume “V”. In particular, said first axial end of the inlet duct  26  is delimited by the cylindrical body  25  and by the flange  24 . The flange  24  has a curved surface  27  which determines the passage of the working fluid from an axial direction (into the inlet duct) to a radial direction (into the expansion volume “V”). The flange  24  also bears the first series, radially more internal, of stator blades  3   a  and therefore forms part of said stator  3 . 
     A removable cover  28  sealingly closes the access space  21  by abutting against the second surface  23  of the substantially tubular wall  20 . Such cover  28  is fixed, for example with non-illustrated removable screws, to the substantially tubular wall  20 . An annular flange  28   a  instead stably closes a second axial end of the inlet duct  26 , opposite the first. The cylindrical body  25  also has a radial opening  29  for the introduction of the working fluid coming from the circuit placed upstream of the turbine  1 . 
     A front static seal with variable geometry  30  is positioned on the first surface  22  of the substantially tubular wall  20  and is directed towards the rotor  2 . In the illustrated embodiment, the front static seal with variable geometry  30  is an inflatable gasket attached to the first surface  22  and has an annular shape (not visible in the figures) which is extended all around the access space  21  that is opened towards the rotor  2  and towards the head  15  of the screw  13 . The inflatable gasket  30  is connected by means of one or more channels  31  obtained in the substantially tubular wall  20  to a pressurized air/gas source  32  (e.g. nitrogen). The inflatable gasket  30  can be inflated and deflated on command. The inflatable gasket  30  is made of elastomeric material reinforced with cords and has a base portion  33 a stably housed in a seat made in the first surface  22  and an active portion  33 b intended, when inflated, to be expanded, to come into contact and press against a surface  34  of the rotor  2  in a manner so as to ensure the seal for preventing the passage of the working fluid ( FIGS. 2 and 3 ). 
     In a non-illustrated embodiment variant, the inflatable gasket  30  is mounted on the rotor  2  and the active portion  33   b  is intended, when inflated, to be expanded, to come into contact and to press against the first surface  22 . 
     In a different embodiment, schematically illustrated in  FIGS. 4 and 5 , the inflatable gasket  30  acts radially. The rotor  2  comprises an appendage  35  defined by a cylindrical body coaxial with the rotation axis and with the shaft  7  that is extended cantilevered from the rotor  2  itself and enters partially inside the access space  21 . The base portion  33   a  of the inflatable gasket  30  is housed in a seat made in a radially inner surface  36  of the substantially tubular wall  20  and the active portion  33   b  faces towards a radially outer surface  37  of the appendage  35 . The active portion  33   b  is intended, when inflated, to be expanded, to come into contact and to press against the radially outer surface  37  of the appendage  35  in a manner so as to ensure the seal for preventing the passage of the working fluid. In an embodiment variant, not illustrated, of the different embodiment the inflatable gasket  30  is mounted on the appendage  35  and the active portion  33   b  is intended, when inflated, to be expanded, to come into contact and to press against the radially inner surface  36  of the substantially tubular wall  20 . 
     When the turbine  1  is correctly mounted for working ( FIG. 1 ), the bushing  8  is inserted and locked in the short pipe  5  by means of screws  38  (of which only the axes are illustrated) inserted in a flange  39  of the bushing  8  and in the short pipe  5  itself. The shaft  7  is integral with the rotor  2  (by means of the screw  14  and of the toothed joint) and the rotor  2  is spaced both from the wall  4  (and from the annular element  18 ) and from the inflatable gasket  30 . The shaft  7  and the rotor  2  are free to rotate but are axially fixed. The inflatable gasket  30  is deflated and clears a passage  40  (front static seal with variable geometry  30  in the first position,  FIGS. 2 and 4 ) between the first axial end of the substantially tubular wall  20  and the rotor  2  through which the working fluid can pass. The removable cover  28  sealingly closes access space  21 . 
     During operation, the working fluid flows through the radial opening  29  into the inlet duct  26 , flows axially towards the expansion volume “V” and is expanded herein, making the turbine  1  rotate. While the turbine  1  rotates, the working fluid also fills the access space  21  so that it is free to flow through the passage  40 . 
     In a different embodiment of the front static seal  30 , in place of an inflatable gasket, a movable tray gasket is employed which has a fixed gasket  41  positioned on a tray  42 ; the movable tray gasket is moved by one or more hydraulic pistons  43  supplied by means of one or more channels  31 . 
     For disassembling the mechanical group  6  and the shaft  7 , after having stopped the rotor  2 , the screws  38  are first extracted, in a manner so as to remove the axial block between the bushing  8  and the short pipe  5 , and the entire mechanical group  6  (shaft  2 , seal device  10 , bearings  9 , bushing  8 , oil seal  11 ) and the rotor  2  integral therewith (together with the screw  14  and the toothed joint) are moved axially back (to the right in  FIG. 1 ) until the rear surface  19  of the rotor  2  is brought against the annular relief  18  and achieves the rear static seal. This translation moves the surface  34  of the rotor  2  away from the inflatable gasket  30  by slightly increasing the passage  40  (visible in the comparison between  FIGS. 2 and 3 ). At this point, the rotor  2  is blocked against the rear wall  4 b with suitable devices, not shown. 
     Subsequently, the gas is inserted through the channels  31  into the inflatable gasket  30  in a manner so to inflate it until the active portion  33   b  is brought against the surface  34  of the rotor  2 , coming to sealingly close the passage  40  (front static seal with variable geometry  30  in the second position,  FIG. 3 ). While the gasket  30  is inflated, the active portion  33   b  translates in axial sense by following the entire distance that separated it from the rotor  2 , before then being abutted and pushing against the rotor  2  itself. In such second position, the inflatable gasket  30  seals and isolates the access space  21  from the expansion volume “V” and from the rest of the circuit. 
     At this point, possibly after having extracted (with suitable non-illustrated devices) from the access space  21  the limited quantity of working fluid present therein, the removable cover  28  is disassembled, separating it from the second surface  23  of the substantially tubular wall  20 . 
     The operator can then freely access the head  15  of the screw  14 , unscrew it and extract it, releasing the shaft  7  from the rotor  2 . 
     The entire mechanical group  6  (constituted by the bushing  8 , by the shaft  7 , by the bearings  9 , by the seal  10  and by the oil seal  11 ) is extracted by removing it from the short pipe  5  and moving it away from the rotor  2  (from left to right in  FIG. 1 ) while the rotor  2  itself remains in its position. The rear static seal  18 ,  19  prevents the working fluid from flowing inside the short pipe  5 , into the space cleared by the mechanical group  6 . The inflated front static seal with variable geometry  30  prevents the working fluid from flowing inside the handling space  21  lacking cover  28 . 
     After having executed the necessary operations on the mechanical group  6  (for example: testing, maintenance, repair), the latter is reinserted in the short pipe  5  (from right to left in  FIG. 1 ) until the first end  7 ′ of the shaft  7  is brought against the rotor  2  (at the self-centering joint) while the rotor  2  is still blocked against the rear wall  4   b.  The screw  14  is reinserted through the rotor  2  and into the shaft  7  and tightened. The access space  21  is re-closed by means of the removable cover  28 . 
     At this point, the inflatable gasket  30  is deflated, bringing it back into the first position and freeing the passage  40 . 
     The rotor  2  is released from the rear wall  4   b  and the entire mechanical group  6  (shaft  2 , seal device  10 , bearings  9 , bushing  8 , oil seal  11 ) and the rotor  2  integral thereto (together with the screw  14  and the toothed joint) are axially advanced (to the left in  FIG. 1 ) until the flange  39  of the bushing  8  is brought against the short pipe  5 . Finally, the screws  38  are inserted and tightened in their housings in the flange  39  and in the bushing  8 . 
     In the different embodiment illustrated in  FIGS. 4 and 5 , the disassembly/reassembly operations are the same as those described above except for the fact that while the gasket  30  is inflated, the active portion  33   b  is moved in a radial sense, following the entire distance that separated it from the radially outer surface  37  of the appendage  35 , before then being abutted and pushing against said radially outer surface  37 . 
     In a different embodiment, not illustrated in its entirety, the front static seal  30  is fixed while the rear static seal  18  has variable geometry. In  FIGS. 7 and 8 , respective embodiment variants of the rear static seal  18  with variable geometry are represented: one is an inflatable gasket and the other is of tray type. For disassembling the mechanical group  6  and the shaft  7 , the rotor  2  is advanced (rather than moved back) in order to sealingly abut against the front static seal  30  and the rear static seal  18  is moved (inflated or moved) in order to be abutted against the rotor  2 . 
     In a further different embodiment, not illustrated in its entirety, both the front static seal  30  and the rear static seal  18  have variable geometry (like the inflatable gasket of  FIG. 7  or like the tray gasket of  FIG. 8 ). For disassembling the mechanical group  6  and the shaft  7 , the rotor  2  remains axially fixed. The two static seals  30 ,  18  are moved in order to sealingly abut against the two opposite sides of the rotor  2 .