Patent Publication Number: US-8979480-B2

Title: Steam turbine

Description:
TECHNICAL FIELD 
     The present invention relates to a steam turbine, and particularly, to a steam turbine using high-temperature steam having a temperature ranging from approximately 650 to 750° C. 
     BACKGROUND ART 
     A steam turbine using primary steam having a temperature of approximately 600° C. is in practical use from the viewpoint of improvement in turbine efficiency. To further improve the turbine efficiency, studies on increasing the temperature of the primary steam to a value ranging from approximately 650 to 750° C. have been conducted and developments according to the studies have been performed. 
     In such a steam turbine, since the primary steam is of high temperature, it is necessary to use a heat-resistant alloy as in the case of a gas turbine. However, no heat-resistant alloy can be used, for example, because such a heat-resistant alloy is expensive and makes it difficult to manufacture a large component. In such case, the strength of the material of the turbine is insufficient and it is necessary to cool the components of the turbine. 
     Japanese Patent Laid-Open Publication No. 11-200801 (Patent Document 1) discloses a cooling mechanism used with rotor discs integrated with a rotor and studded with blades. The cooling mechanism cools the vicinity of blade studded portions of the rotor discs, in particular, rotor discs in the second state and the following stages. In the cooling mechanism, a cooling fluid is directly supplied into cooling spaces formed by side surfaces of the rotor discs and internal side surfaces of vanes through cooling path holes formed in the rotor. 
     However, it is not easy to readily form the cooling path holes, which are provided to cool the vicinity of the blade studded portions of the rotor discs as described in Patent Document 1, in the rotor inside the rotor discs, and it is also not always preferred to form the cooling path holes from the viewpoint of ensuring the strength of the rotor. 
     Further, in turbine stages that require cooling, such as the rotor discs, the cooling steam that contributed to the cooling in the upstream side turbine stages and then cools the cooling steam increased in temperature in the downstream side turbine stages, which may cause a case of insufficient cooling. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been made in view of the circumstances described above, and an object of the present invention is to provide a steam turbine including a cooling structure capable of ensuring strength of a rotor, rotor discs, and other components of the turbine to maintain integrity thereof even when high-temperature steam is used. 
     Another object of the present invention is to provide a steam turbine in which turbine components in downstream side turbine stages disposed in a range in which cooling is required can be effectively cooled. 
     A steam turbine of the present invention provided for achieving the above objects includes: 
     a rotor; 
     a rotor disc integrated with the rotor; 
     a plurality of blades with which the rotor disc is studded along a circumferential direction of the rotor; 
     a casing that covers the rotor; 
     a plurality of vanes attached to the casing along the circumferential direction of the rotor in positions adjacent to the blades and on an upstream side in an axial direction of the rotor; and 
     an internal diaphragm disposed on rotor-side surfaces of the vanes in the axial direction of the rotor in such a way that the internal diaphragm faces the rotor disc, wherein 
     the vanes and the blades adjacent to each other in the axial direction of the rotor form a turbine stage, 
     in at least one of the turbine stages, a rotor-side cooling path is formed through the rotor disc in the axial direction of the rotor and a diaphragm-side cooling path is formed through the internal diaphragm in the axial direction of the rotor, and 
     a cooling medium flowing through the rotor-side cooling path diverts into the diaphragm-side cooling path and a labyrinth flow path provided between the internal diaphragm and the rotor. 
     In the steam turbine described above, a plurality of turbine stages, each of which has the diaphragm-side cooling path which passes through the internal diaphragm in the axial direction of the rotor and through which the cooling medium flows, are formed, and among the plurality of turbine stages, each of which has the diaphragm-side cooling paths formed therein, the diaphragm-side cooling path is formed in parallel to the axis of the rotor in upstream-side turbine stages, and an outlet of the diaphragm-side cooling path is positioned closer to the rotor than an inlet of the diaphragm-side cooling path in downstream-side turbine stages. 
     According to the present invention, since the cooling medium can cool the rotor, the rotor discs, the internal diaphragms, and other components in a wide range of turbine stages from an upstream side to a downstream side, the strength of each of the turbine components, such as the rotor, can be ensured, and hence, the integrity of each of the turbine components can be maintained even when high-temperature steam is used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view showing a part of a steam turbine according to a first embodiment of the present invention. 
         FIG. 2  is a partial cross-sectional view showing a part of a steam turbine according to a second embodiment of the present invention. 
         FIG. 3  shows variations of a diaphragm-side cooling path in an internal diaphragm shown in  FIG. 2 , and  FIGS. 3(A) to 3(F)  are cross-sectional views showing first to sixth variations. 
         FIG. 4  is a partial cross-sectional view showing a part of a steam turbine according to a third embodiment of the present invention. 
         FIG. 5  is a partial cross-sectional view showing a part of a steam turbine according to a fourth embodiment of the present invention. 
         FIG. 6  shows graphs representing a relationship among the temperature of a cooling medium (cooling steam), the temperature of primary steam, and a target temperature of blade studded portions of a rotor disc. 
         FIG. 7  is a partial cross-sectional view showing a part of a steam turbine according to a fifth embodiment of the present invention. 
         FIG. 8  is a partial cross-sectional view showing a part of a steam turbine according to a sixth embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     The best mode for carrying out the present invention will be described below with reference to the drawings. However, it is to be noted that the present invention is not limited to the following embodiments. Further, in the following description, it should be understood that the terms “upper”, “lower”, “right”, “left”, and other terms concerning direction are used herein only in the context of illustration or actual installation. 
     [A] First Embodiment (FIG.  1 ) 
       FIG. 1  is a partial cross-sectional view showing a part of a steam turbine according to a first embodiment of the present invention. In a steam turbine  10  shown in  FIG. 1 , high-temperature primary steam  11  having a temperature ranging from approximately 650 to 750° C. is guided via vanes (stationary blades)  12  to blades (moving blades)  13  to rotate a rotor  14  to which the blades  13  are studded so that a generator, not shown, connected to the rotor  14  is rotated. The use of such high-temperature primary steam  11  can improve turbine efficiency. 
     A plurality of blades  13  are studded to the outer peripheral portion of each rotor disc  15 , which is integrated to the rotor  14 , along the circumferential direction of the rotor  14 . 
     The rotor  14  is covered with a casing  16 , to which the a plurality of vanes  12  are attached via an external diaphragm  17  along the circumferential direction of the rotor  14  in positions adjacent to the blades  13  and on the upstream side in the axial direction of the rotor  14 . An internal diaphragm  18  is disposed on the vanes  12  in the axial direction of the rotor  14  in such a way that the internal diaphragm  18  faces the rotor discs  15  of the rotor  14 . The plural vanes  12 , supported by the external diaphragm  17  and the internal diaphragm  18 , guide the primary steam  11  to the blades  13 . 
     The vanes  12  and the blades  13  are alternately arranged in the axial direction of the rotor  14 , and a set of adjacent vanes  12  and blades  13  forms a turbine stage. The turbine stages are numbered as follows: a first stage, a second stage, a third stage, and so on in the direction in which the primary steam  11  flows from the upstream side to the downstream side. A space in which the vanes  12  and the blades  13  are alternately arranged in the axial direction of the rotor  14  forms a steam path  19  through which the primary steam  11  flows. 
     In the thus configured steam turbine  10 , a cooling structure  20  is provided in at least one of the turbine stages to cool the components of the turbine, particularly, the rotor  14  and the rotor disc  15  and internal diaphragm  18 , to ensure the strength of each of the components. The cooling structure  20  in the steam turbine includes a diaphragm-side cooling path  21  and a rotor-side cooling path  22 . 
     The rotor-side cooling path  22  is formed in a rotor disc  15 , which is integrated with the rotor  14 , in the vicinity of a portion  15 A studded with a blade  13 . The rotor-side cooling path  22  extends linearly in parallel to the axis of the rotor  14  through the rotor disc  15  in the axial direction of the rotor  14 . The rotor-side cooling path  22  is actually formed of a plurality of rotor-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . On the other hand, the diaphragm-side cooling path  21  is formed so as to extend linearly in parallel to the axis of the rotor  14  through the internal diaphragm  18  in the axial direction of the rotor  14 . The diaphragm-side cooling path  21  is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . 
     A labyrinth section  23 , which forms a labyrinth flow path  24 , is provided between the internal diaphragm  18  and the rotor  14 . The labyrinth section  23  includes labyrinth teeth  25  protruding from the internal diaphragm  18  and labyrinth pieces  26  protruding from the rotor  14  in a manner that the labyrinth teeth  25  and the labyrinth pieces  26  are alternately arranged along the axial direction of the rotor  14 . The labyrinth section  23  basically seals the gap between the internal diaphragm  18  and the rotor  14  to prevent the primary steam  11  flowing through the steam path  19  from leaking through the gap. The labyrinth flow path  24  is formed by the inner circumferential surface of the internal diaphragm  18  and the outer circumferential surface of the rotor  14  and partitioned by the labyrinth teeth  25  and the labyrinth pieces  26 . 
     A cooling medium  27 , such as cooling steam having a temperature lower than that of the primary steam  11 , flows through the rotor-side cooling paths  22 , the diaphragm-side cooling paths  21 , and the labyrinth flow path  24 . That is, the cooling medium  27  introduced into the rotor-side cooling paths  22  in an upstream rotor disc  15  and passing through the rotor-side cooling paths  22  diverts into the diaphragm-side cooling paths  21  in the downstream internal diaphragm  18  and the labyrinth flow path  24 . The diverted flows of the cooling medium  27  then merge, and the merged cooling medium  27  flows through the rotor-side cooling paths  22  in the same downstream rotor disc  15 , as indicated by the arrows A. 
     The provision of the diaphragm-side cooling paths  21  prevents or substantially prevents the cooling medium  27  having flowed through the rotor-side cooling paths  22  in the upstream rotor disc  15  from flowing into the steam path  19  but allows the cooling medium  27  to flow toward the downstream stage. When the cooling medium  27  having flowed out of the rotor-side cooling paths  22  in the upstream rotor disc  15  flows through the labyrinth flow path  24 , and the cooling medium  27  having flowed through the labyrinth flow path  24  flows into the rotor-side cooling paths  22  in the downstream rotor disc  15 , the upstream and downstream rotor discs  15  and the internal diaphragm  18  (the rotor discs  15 , in particular) are cooled. 
     As mentioned above, the proportions of the cooling medium  27  having flowed out of the rotor-side cooling paths  22  and diverting into the diaphragm-side cooling paths  21  and the labyrinth flow path  24  are determined based on pressure loss in the diaphragm-side cooling paths  21  and pressure loss in the labyrinth flow path  24 , that is, by controlling the pressure loss in the diaphragm-side cooling paths  21  and the pressure loss in the labyrinth flow path  24 . The pressure loss in the diaphragm-side cooling paths  21  depends on the number of diaphragm-side cooling paths  21  formed in the internal diaphragm  18 , the cross-sectional area of each of the diaphragm-side cooling paths  21 , and other factors. The pressure loss in the labyrinth flow path  24  depends on the number of labyrinth teeth  25 , the dimension “t” from the labyrinth teeth  25  to the outer circumferential surface of the rotor  14 , and other factors. 
     The present embodiment therefore provides the following advantageous effects (1) and (2). 
     (1) The cooling medium  27  having flowed through the rotor-side cooling paths  22  in an upstream-side rotor disc  15  diverts into the diaphragm-side cooling paths  21  in the downstream-side internal diaphragm  18  and the labyrinth flow path  24  provided between the internal diaphragm  18  and the rotor  14 , and the cooling medium  27  is therefore not allowed to flow into the steam path  19 , through which the primary steam  11  flows, or the flow rate of the cooling medium  27  flowing into the steam path  19  can be reduced, and the cooling medium  27  can instead be guided through the diaphragm-side cooling paths  21  into the rotor-side cooling path  22  in the downstream-side rotor disc  15 . As a result, the cooling medium  27  can cool the rotor discs  15  integrated with the rotor  14 , the internal diaphragms  18 , and other components in a wide range of turbine stages from the upstream-side to the downstream-side, and accordingly, the strength of each of the components of the turbine (rotor  14  and the rotor discs  15 , in particular) can be ensured, and hence, the integrity of each of the turbine components can be maintained even when the primary steam  11  used in the turbine has a high temperature ranging from approximately 650 to 750° C. 
     (2) Since the cooling medium  27  flows through the rotor-side cooling paths  22  formed in the rotor discs  15  integrated with the rotor  14  and the diaphragm-side cooling paths  21  formed in the internal diaphragms  18  that support the vanes  12 , the cooling paths can be more readily manufactured than in a case of being formed in the rotor  14 , and the strength of the rotor  14  will not decrease. 
     [B] Second Embodiment (FIG.  2  and FIG.  3 ) 
       FIG. 2  is a partial cross-sectional view showing a part of a steam turbine according to a second embodiment of the present invention.  FIG. 3  shows variations of the diaphragm-side cooling paths in each internal diaphragm shown in  FIG. 2 , in which  FIGS. 3(A) to 3(F)  are cross-sectional views showing first to sixth variations. In the second embodiment, like reference numerals are added to portions or members corresponding or similar to those in the first embodiment described above, and descriptions thereof portions will be simplified or omitted herein. 
     A steam turbine cooling structure  30  according to the second embodiment differs from that in the first embodiment in terms of the shape of a diaphragm-side cooling path  31  formed in each internal diaphragm  18 . The shape of the diaphragm-side cooling path  31  is determined by a portion that particularly requires cooling, pressure loss in the labyrinth flow path  24 , and other factors. 
     That is, the diaphragm-side cooling path  31  is formed in the internal diaphragm  18  so as to be inclined to the axis of the rotor  14  from the side at which the rotor  14  is present toward the vanes  12  and extends linearly through the internal diaphragm  18  substantially in the axial direction of the rotor  14 . The diaphragm-side cooling path  31  is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . The cooling medium  27  having flowed out of the rotor-side cooling paths  22  in an upstream-side rotor disc  15  diverts in positions closer to the rotor  14  than in the first embodiment into the diaphragm-side cooling paths  31  in the downstream-side internal diaphragm  18  and the labyrinth flow path  24  between the internal diaphragm  18  and the rotor  14 . The diverted flows of the cooling medium  27  flow through the diaphragm-side cooling paths  31  and the labyrinth flow path  24  and then merge, and the merged cooling medium  27  flows through the rotor-side cooling paths  22  in the same downstream-side rotor disc  15 , as indicated by arrows B. 
     According to the structure or configuration described above, since the cooling medium  27  having flowed out of the rotor-side cooling paths  22  in the upstream rotor disc  15  diverts in positions close to the rotor  14 , a downstream-side areas α of the upstream-side rotor disc  15  will be particularly cooled. 
     A diaphragm-side cooling path  32  according to the first variation shown in  FIG. 3(A)  is formed in each internal diaphragm  18  so as to be inclined to the axis of the rotor  14  from the side at which the vanes  12  are present toward the rotor  14  (see  FIG. 2 ) and extends linearly through the internal diaphragm  18  substantially in the axial direction of the rotor  14 . The diaphragm-side cooling path  32  is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . The cooling medium  27  having flowed out of the rotor-side cooling paths  22  in an upstream-side rotor disc  15  diverts into the diaphragm-side cooling paths  32  in the downstream-side internal diaphragm  18  and the labyrinth flow path  24  between the internal diaphragm  18  and the rotor  14 . The diverted flows of the cooling medium  27  flow out of the diaphragm-side cooling paths  32  and the labyrinth flow path  24  and merge in positions close to the rotor  14 , and the merged cooling medium  27  flows into the rotor-side cooling paths  22  in the same downstream-side rotor disc  15 . 
     In this case, since the cooling medium  27  having flowed out of the diaphragm-side cooling paths  32  in the downstream internal diaphragm  18  and the cooling medium  27  having flowed out of the labyrinth flow path  24  merge in positions close to the rotor  14 , and the merged cooling medium  27  flows into the rotor-side cooling paths  22  in the same downstream-side rotor disc  15 , upstream-side areas β ( FIG. 2 ) of the downstream-stage rotor disc  15  can particularly be cooled. 
     On the other hand, a diaphragm-side cooling path  33  according to the second variation shown in  FIG. 3(B)  is formed in each internal diaphragm  18  so as to be inclined to the axis of the rotor  14  from the side at which the rotor  14  (see  FIG. 2 ) is present toward the vanes  12 , extends linearly to a point somewhere in the middle of the internal diaphragm  18 , and further extends in parallel to the axis of the rotor  14  through the internal diaphragm  18  in the axial direction of the rotor  14 . The diaphragm-side cooling path  33  is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . The cooling medium  27  flows substantially in the same manner as in the case of the diaphragm-side cooling path  31  shown in  FIG. 2 , and the downstream-side area α ( FIG. 2 ) of the upstream-side rotor disc  15  can particularly be cooled. Further, by guiding the cooling medium  27  flowing through the diaphragm-side cooling paths  33  to positions closer the rotor  14  than in  FIG. 2 , desired areas of the downstream rotor disc  15  will be suitably cooled and the cooling medium  27  will be prevented from flowing into the steam path  19 . 
     A diaphragm-side cooling path  34  according to the third variation shown in  FIG. 3(C)  is formed in each internal diaphragm  18  so as to be inclined to the axis of the rotor  14  from the side at which vanes  12  are present toward the rotor  14  (see  FIG. 2 ), extends linearly to a point somewhere in the middle of the internal diaphragm  18 , and further extends in parallel to the axis of the rotor  14  through the internal diaphragm  18  in the axial direction of the rotor  14 . The diaphragm-side cooling path  34  is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14 . The cooling medium  27  flows substantially in the same manner as in the case of the diaphragm-side cooling path  32  shown in  FIG. 3(A) , but the positions where the cooling medium  27  having flowed out of the diaphragm-side cooling paths  34  merges with the cooling medium  27  having flowed out of the labyrinth flow path  24  can be set in desired positions closer to the blades  13  than the upstream-side areas β. 
     Diaphragm-side cooling paths  35 ,  36 , and  37  represented by the fourth, fifth, and sixth variations respectively shown in  FIGS. 3(D) ,  3 (E), and  3 (F) are formed in each internal diaphragm  18  and have the same shapes as those of the diaphragm-side cooling path  21  ( FIG. 1 ), the diaphragm-side cooling path  31  ( FIG. 2 ), and the diaphragm-side cooling path  32  ( FIG. 3(A) ) except that each of the diaphragm-side cooling paths  35 ,  36  and  37  is actually formed of a plurality of diaphragm-side cooling paths disposed in parallel to the radial direction of the rotor  14  and the cross-sectional area thereof is smaller. Each of the plurality of diaphragm-side cooling paths  35 ,  36  and  37  is further formed of a plurality of diaphragm-side cooling paths disposed at predetermined intervals in the circumferential direction of the rotor  14 . 
     In the fourth, fifth and sixth variations, each of the plurality of diaphragm-side cooling paths  35 ,  36  and  37 , has a smaller cross-sectional area, resulting in greater pressure loss produces therein. The fourth, fifth and sixth variations are therefore used in a case where the labyrinth flow path  24  between each internal diaphragm  18  and the rotor  14  produces large pressure loss and can divert the cooling medium  27  having flowed out of the rotor-side cooling paths  22  (see  FIG. 2 ) in an upstream-side rotor disc  15  in a satisfactory manner into the diaphragm-side cooling paths  35 ,  36 , or  37  and the labyrinth flow path  24 . The fourth, fifth and sixth variations, of course, function in ways similar to those in the first embodiment ( FIG. 1 ), the second embodiment ( FIG. 2 ), and the first variation (FIG.  3 (A)), respectively. 
     The steam turbine cooling structure  30  according to the second embodiment, including the first to sixth variations thereof described above, also achieves or provides advantageous effects similar to the advantageous effects (1) and (2) provided in the first embodiment described hereinbefore. 
     [C] Third Embodiment (FIG.  4 ) 
       FIG. 4  is a partial cross-sectional view showing a part of a steam turbine according to a third embodiment of the present invention. In the third embodiment, like reference numerals are added to portions or members corresponding or similar to those in the first embodiment, and descriptions of these portions will be simplified or omitted herein. 
     A steam turbine cooling structure  40  according to the present embodiment differs from the first embodiment described above in that a movable fin  41  that is moved by the cooling medium  27  in the axial direction of the rotor  14  is disposed in each internal diaphragm  18  in this fourth embodiment. 
     That is, a bifurcated diaphragm-side cooling path  42  is formed in the internal diaphragm  18 . The bifurcated diaphragm-side cooling path  42  is a combination of the diaphragm-side cooling path  21  according to the first embodiment ( FIG. 1 ) and the diaphragm-side cooling path  32  according to the first variation of the second embodiment ( FIG. 3(A) ). The movable fin  41  is arranged on the downstream-side of the diaphragm-side cooling path  42  to a portion thereof corresponding to the diaphragm-side cooling path  21  with the movable fin  41  urged by a spring  43  or any other suitable urging member. 
     The movable fin  41  is provided so as not to overlap with a fixed fin  44  provided on the adjacent rotor disc  15  when the movable fin  41  substantially retracts in the internal diaphragm  18  due to the urging force produced by the spring  43 . According to this configuration, the movable fin  41  is prevented from interfering with the fixed fin  44  when the vanes  12 , the external diaphragm  17  and the internal diaphragm  18  are assembled to the casing  16 . 
     When the cooling medium  27  is introduced into the rotor-side cooling paths  22  (see  FIG. 1 ) in an upstream-side rotor disc  15 , the cooling medium  27  having flowed out of the rotor-side cooling paths  22  diverts into the diaphragm-side cooling path  42  in the downstream-side internal diaphragm  18  and the labyrinth flow path  24 . The diverted flows of the cooling medium  27  flow out of the portion of the diaphragm-side cooling path  42  that corresponds to the diaphragm-side cooling path  32  and the labyrinth flow path  24  and merge, and the merged cooling medium  27  flows into the rotor-side cooling path  22  in the same downstream-side rotor disc  15 . In this process, the upstream-side and downstream-side rotor discs  15  (the downstream-side rotor disc  15  in particular) are cooled. 
     At this moment, the cooling medium  27  having flowed into the portion of the diaphragm-side cooling path  42  that corresponds to the diaphragm-side cooling path  21  presses the movable fin  41  in the axial direction of the rotor  14  against the urging force produced by the spring  43 . The movable fin  41  then protrudes toward the adjacent rotor disc  15  and overlaps with the fixed fin  44  thereon as shown in  FIG. 4  to thereby narrow the gap between the movable fin  41  and the fixed fin  44 . 
     The thus configured present embodiment provides not only provides advantageous effects similar to the advantageous effects (1) and (2) attained by the first embodiment described above, but also the following advantageous effect (3). 
     (3) Since each internal diaphragm  18  has the movable fin  41  disposed therein, which can be moved by the cooling medium  27  in the axial direction of the rotor  14  to narrow the gap between the movable fin  41  and the fixed fin  44  on the adjacent rotor disc  15 , the cooling medium  27  will not flow into the steam path  19  and the primary steam  11  in the steam path  19  will not flow into the space between the rotor disc  15  and the internal diaphragm  18  where the cooling medium  27  flows. 
     [D] Fourth Embodiment (FIGS.  5  and  6 ) 
       FIG. 5  is a partial cross-sectional view showing a part of a steam turbine according to a fourth embodiment of the present invention. In the fourth embodiment, like reference numerals are added to portions or members corresponding or similar to those in the first embodiment, and descriptions of these portions will be simplified or omitted herein. 
     A steam turbine cooling structure  50  according to the present embodiment differs from those in the first to third embodiments in that among a plurality of turbine stages disposed along the axial direction of the rotor  14 , a cooling-requiring turbine stage range where the rotor  14 , rotor discs  15 , internal diaphragms  18 , and other turbine components require cooling (for example, the cooling-requiring range including the first to sixth turbine stages) have diaphragm-side cooling paths  51 A,  51 B,  51 C,  51 D, and so on formed in the internal diaphragms  18  and that the shapes of the diaphragm-side cooling paths  51 A to  51 D and so on are different between upstream-side and downstream-side turbine stages in the cooling-requiring range. 
     The diaphragm-side cooling paths  51 A to  51 D and so on are formed through the internal diaphragms  18  in the axial direction of the rotor  14 , and the cooling medium  27 , such as cooling steam, flows through the diaphragm-side cooling paths  51 A to  51 D and so on, as in the cases of the diaphragm-side cooling paths  21  and others according to the first to third embodiments described hereinbefore. Each of the diaphragm-side cooling paths  51 A to  51 D and so on is actually formed of a plurality of diaphragm-side cooling paths formed through the internal diaphragms  18  at predetermined intervals in the circumferential direction of the rotor  14 . 
     The diaphragm-side cooling path  51 A in the internal diaphragm  18  in each upstream-side turbine stage (first and second turbine stages, for example) is formed so as to linearly extend in parallel to the axis of the rotor  14 , as in the case of the diaphragm-side cooling path  21  according to the first embodiment. The diaphragm-side cooling paths  51 B to  51 D and so on in the internal diaphragms  18  in downstream-side turbine stages (third to sixth turbine stages, for example) are formed so as to be inclined to the axis of the rotor  14  from the side at which the vanes  12  are present toward the rotor  14  and linearly extend. As a result, outlets  53  of the diaphragm-side cooling paths  51 B to  51 D and so on are closer to the rotor  14  than inlets  52  thereof in the radial direction of the internal diaphragms  18 . That is, in the present embodiment, the inlets  52  and the outlets  53  of the diaphragm-side cooling paths  51 A in the upstream-side turbine stages are formed in the uniform radial position, whereas the outlets  53  of the diaphragm-side cooling paths  51 B to  51 D and so on in the downstream-side turbine stages are formed in positions radially inside the inlets  52  thereof. 
     In the cooling-requiring turbine stage range, the cooling medium  27  having flowed out of the rotor-side cooling paths  22  in the rotor disc  15  in an adjacent turbine stage diverts into one of the diaphragm-side cooling paths  51 A to  51 D and so on in the turbine stage and the labyrinth flow path  24 . The cooling medium  27  having flowed out of the one of the diaphragm-side cooling paths  51 A to  51 D and so on and the cooling medium  27  having flowed out of the labyrinth flow path  24  merge, and the merged cooling medium  27  flows into the rotor-side cooling paths  22  in the rotor disc  15  in the same turbine stage. According to the configuration or arrangement described above, the cooling medium  27  is prevented or substantially prevented from flowing into the steam path  19 , and the rotor  14 , the rotor discs  15  and the internal diaphragms  18  can be hence cooled. 
     As shown in  FIG. 6 , since the cooling medium  27  (cooling steam, for example) absorbs more heat when it travels downstream through the turbine stages, the temperature of the cooling medium  27  (cooling medium temperature Tc) gradually becomes higher, whereas since the primary steam  11  dissipates more heat when it travels downstream through the turbine stages, the temperature of the primary steam  11  (primary steam temperature Tg) becomes gradually lower. On the other hand, the temperature of a rotor disc  15 , in particular, a target temperature Tm of the blade studded portions  15 A of a rotor disc  15 , is set at a lower value in a more downstream-side turbine stage. The reason for this matter resides in that the height of the blades  13  becomes greater in a more downstream-side turbine stage and the centrifugal force acting thereon increases or the force acting on the blade studded portions  15 A of the rotor disc  15  increases accordingly, and in this case, necessary strength thereof can be ensured only by lowering the target temperature Tm. 
     Further, the temperature of the blade studded portions  15 A of a rotor disc  15  is nearly equal to that of the primary steam  11  unless the portions  15 A are cooled by the cooling medium  27 . In order to lower the temperature of the blade studded portions  15 A of a rotor disc  15  at least to the target temperature Tm, it is necessary to satisfy the following Expression (1):
 
 X 1×( Tg−Tm )≦ X 2×( Tm−Tc )  (1)
 
     In Expression (1), each of the coefficients X 1  and X 2  is a function of the following parameters: the length of a cooling path formed of one of the diaphragm-side cooling paths  51 A to  51 D and so on and the rotor-side cooling path  22  in the same turbine stage, the flow rate of the cooling medium  27 , and other factors. That is, Expression (1) indicates that the amount of heat dissipated from a rotor disc  15  through the cooling medium  27  (cooling steam, for example) needs to be equal to or higher than the amount of heat transferred from the primary steam  11  to the rotor disc  15 . 
     In a cooling-requiring turbine stage range, since the temperature Tc of the cooling medium  27  is much lower than the target temperature Tm of the blade studded portions  15 A of a rotor disc  15  in an upstream-side turbine stage (the turbine stage A and a turbine stage close thereto in  FIG. 6 , for example), the temperature difference (Tm−Tc) becomes large, and hence, the cooling capacity of the steam turbine cooling structure  50  using the cooling medium  27  has extra capacity. The right-hand side value of Expression (1) is therefore greater than the left-hand side value of Expression (1), and Expression (1) is satisfied. In this case, in an upstream-side turbine stage within the cooling-requiring turbine stage range, the rotor  14 , the rotor disc  15 , and the internal diaphragm  18 , particularly the blade studded portions  15 A of the rotor disc  15 , are suitably cooled even if the diaphragm-side cooling path  51 A is formed so as to extend linearly in parallel to the axis of the rotor  14  as shown in  FIG. 5 . 
     In contrast, in a downstream-side turbine stage within the cooling-requiring turbine stage range (the turbine stage C and a turbine stage close thereto shown in  FIG. 6 , for example), since the temperature difference (Tm−Tc) between the target temperature Tm of the blade studded portions  15 A of the rotor disc  15  and the temperature Tc of the cooling medium  27  decreases, the coefficient X 2  needs to be greater in order to achieve a greater value of the right-hand side of Expression (1). To this end, for example, it is conceivable to increase the length of the cooling path formed of one of the diaphragm-side cooling paths  51 B to  51 D and so on and the rotor-side cooling path  22 . 
     To achieve the above object, in the downstream-side turbine stages within the cooling-requiring turbine stage range, the diaphragm-side cooling paths  51 B to  51 D and so on are formed to be inclined to the axis of the rotor  14  and the outlets  53  are formed so as to be positioned closer to the rotor  14  than the inlets  52 , as shown in  FIG. 5 . According to the configuration described above, it becomes possible to increase the length from the outlet  53  of any one of the diaphragm-side cooling paths  51 B to  51 D and so on to the inlet of the rotor-side cooling path  22  in the rotor disc  15  in the same turbine stage. As a result, the length of the cooling path formed of any one of the diaphragm-side cooling paths  51 B to  51 D and so on and the rotor-side cooling path  22  is increased, and the cooling medium  27  flows out of any one of the diaphragm-side cooling paths  51 B to  51 D and so on and impinges on the side surface of the rotor disc  15  in the same turbine stage, and the rotor disc  15  (including the blade studded portions  15 A) is thereby cooled through the side surface. The cooling capacity of the steam turbine cooling structure  50  is thus increased. 
     A downstream turbine stage within a cooling-requiring turbine stage range used herein refers to a turbine stage downstream of a turbine stage (turbine stage B shown in  FIG. 6 , for example) at which the temperature difference (Tm−Tc) between the target temperature Tm of the blade studded portions  15 A of the rotor disc  15  and the temperature Tc of the cooling medium  27  is at least equal to the temperature difference (Tg−Tm) between the target temperature Tm of the blade studded portions  15 A of the rotor disc  15  and the temperature Tg of the primary steam  11 . 
     A turbine stage, at which the temperature difference (Tm−Tc) is equal to the temperature difference (Tg−Tm), may also be configured as a downstream-side turbine stage at which any of the diaphragm-side cooling paths  51 B to  51 D and so on is formed to be inclined to the axis of the rotor  14 . Such downstream-side turbine stages are, for example, the third to sixth turbine stages as described above, and upstream-side turbine stages within the cooling-requiring turbine stage range are those other than the downstream-side turbine stages described above, for example, the first and second turbine stages. 
     Further, the diaphragm-side cooling paths  51 B to  51 D and so on in the downstream-side turbine stages within the cooling-requiring turbine stage range in the present embodiment are formed so that the inclination angles thereof to the axis of the rotor  14  are designed to be greater in further downstream-side turbine stages, and that the outlets  53  thereof are positioned radially closer to the rotor  14  (further inward in the radial direction) in further downstream-side turbine stages, as shown in  FIG. 5 . The reason for this matter is to handle the situation in which the temperature Tc of the cooling medium  27  becomes gradually higher in a further downstream-side turbine stage and the cooling capacity of the cooling medium  27  becomes gradually lower accordingly. In order to lower the temperature of the blade studded portions  15 A of a rotor disc  15  at least to the target temperature Tm thereof in consideration of the fact described above, the length of the cooling path formed of any one of the diaphragm-side cooling paths  51 B to  51 D and so on and the rotor-side cooling path  22  needs to be gradually longer in a further downstream-side turbine. 
     Therefore, the thus configured present embodiment provides not only advantageous effects similar to the advantageous effects (1) and (2) provided in the first embodiment described above but also the following advantageous effects (4) to (6). 
     (4) In the downstream-side turbine stages within a cooling-requiring turbine stage range at which the cooling is required, since the diaphragm-side cooling paths  51 B to  51 D and so on formed in the internal diaphragms  18  are formed so as to position the outlets  53  thereof to be closer to the rotor  14  than the inlets  52  thereof, the length of the cooling path formed of each of the diaphragm-side cooling paths  51 B to  51 D and so on and the rotor-side cooling path  22  provided in the rotor disc  15  in the same turbine stage can be increased. 
     Furthermore, the cooling medium  27  having flowed out of the outlet  53  of each of the diaphragm-side cooling paths  51 B to  51 D and so on impinges on the side surface of the rotor disc  15  in the same turbine stage, and therefore, the rotor disc  15  including the blade studded portions  15 A can be cooled through the side surface. The turbine components in the downstream-side turbine stages within the cooling-requiring turbine stage range, particularly the rotor discs  15  including the blade studded portions  15 A, can be suitably cooled even if the temperature of the cooling medium  27  flowing through the diaphragm-side cooling paths  51 B to  51 D and so on in the downstream-side turbine stages increases. 
     (5) The diaphragm-side cooling path  51 A in an upstream-side turbine stage within the cooling-requiring turbine stage range is formed in parallel to the axis of the rotor  14  and linearly passes through the internal diaphragm  18 . In the upstream-side turbine stage, since the temperature Tc of the cooling medium  27  is sufficiently low, the cooling medium  27  can suitably cool the rotor  14 , the internal diaphragm  18 , and the rotor disc  15  including the blade studded portions  15 A. Furthermore, the diaphragm-side cooling path  51 A, in a state in parallel to the axis of the rotor  14 , can be readily machined through the internal diaphragm  18 , resulting in the reduction in machining cost. 
     (6) The diaphragm-side cooling paths  51 B to  51 D and so on in the downstream-side turbine stages within the cooling-requiring turbine stage range are formed so that the outlets  53  thereof are positioned gradually closer to the rotor  14  in further downstream-side turbine stages. Thus, the temperature Tc of the cooling medium  27  gradually becomes higher in a further downstream-side turbine, and the cooling capacity of the cooling medium decreases, and accordingly, in the configuration described above, the length of the cooling path formed of any one of the diaphragm-side cooling paths  51 B to  51 D and so on and the rotor-side cooling path  22  can be made gradually longer in a further downstream-side turbine. As a result, the temperature of the blade studded portions  15 A of the rotor disc  15  can be efficiently cooled at least to the target temperature Tm thereof. 
     [E] Fifth Embodiment (FIG.  7 ) 
       FIG. 7  is a partial cross-sectional view showing a part of a steam turbine according to a fifth embodiment of the present invention. In the fifth embodiment, like reference numerals are added to portions or members corresponding or similar to those in the first embodiment ( FIG. 1 ) and the fourth embodiment ( FIG. 5 ), and descriptions of these portions will be simplified or omitted herein. 
     A steam turbine cooling structure  60  according to the present embodiment differs from the steam turbine cooling structure  50  according to the fourth embodiment in terms of the inclination angles and the positions of the outlets  53  of diaphragm-side cooling paths  61 B to  61 D and so on formed in the internal diaphragms  18  in the downstream-side turbine stages within a cooling-requiring turbine stage range. 
     That is, the diaphragm-side cooling paths  61 B to  61 D and so on in the downstream-side turbine stages within the cooling-requiring turbine stage range are designed to have the same inclination angle with respect to the axis of the rotor  14  that is necessary in the most downstream-side turbine stage and the uniform radial position of the outlet  53  that is necessary in the most downstream-side turbine stage. Each of the diaphragm-side cooling paths  61 B to  61 D and so on is actually formed of a plurality of diaphragm-side cooling paths arranged at predetermined intervals in the circumferential direction of the rotor  14  and passing through the internal diaphragm  18  substantially in the axial direction of the rotor  14 . 
     The inclination angle necessary in the most downstream-side turbine stage and the outlet position necessary in the most downstream-side turbine stage are set to provide a cooling path having a length necessary to lower the temperature of the blade studded portions  15 A of the rotor disc  15  in the most downstream-side turbine stage at least to the target temperature Tm thereof in consideration of the temperature Tc of the cooling medium  27  flowing through the most downstream-side turbine stage within the cooling-requiring turbine stage range. 
     Therefore, the thus configured present embodiment provides not only advantageous effects similar to the advantageous effects (1) and (2) provided in the first embodiment described above and advantageous effects similar to the advantageous effects (4) and (5) provided in the fourth embodiment described above but also the following advantageous effect (7). 
     (7) The positions of the outlets  53  of the diaphragm-side cooling paths  61 B to  61 D and so on in the downstream-side turbine stages within the cooling-requiring turbine stage range are designed to be the same outlet position necessary in the most downstream-side turbine stage. The diaphragm-side cooling paths  61 B to  61 D and so on can therefore be readily machined, and hence, the machining cost can be reduced as compared with a case where the positions of the outlets  53  of the diaphragm-side cooling paths are positioned closer to the rotor  14  in the further downstream-side turbine stages. 
     [F] Sixth Embodiment (FIG.  8 ) 
       FIG. 8  is a partial cross-sectional view showing a part of a steam turbine according to a sixth embodiment of the present invention. In the sixth embodiment, reference numerals are added to portions or members corresponding or similar to those in the first embodiment ( FIG. 1 ) and the fourth embodiment ( FIG. 5 ), and descriptions of these portions will be simplified or omitted herein. 
     A steam turbine cooling structure  70  according to the present embodiment differs from the steam turbine cooling structure  50  according to the fourth embodiment in terms of the shape of a diaphragm-side cooling path  71  formed in the internal diaphragm  18  in a downstream-side turbine stage within a cooling-requiring turbine stage range. 
     That is, the diaphragm-side cooling path  71  in the downstream-side turbine stage is formed through the internal diaphragm  18  so as to be inclined to the axis of the rotor  14  from the side at which the vanes  12  are present toward the rotor  14 , extends linearly to a point somewhere in the middle of the internal diaphragm  18 , and further extends in parallel to the axis of the rotor  14  in the axial direction of the rotor  14 . 
     The diaphragm-side cooling path  71  is actually formed of a plurality of diaphragm-side cooling paths passing through the internal diaphragm  18  and arranged at predetermined intervals in the circumferential direction of the rotor  14 . The inlet  52  of the diaphragm-side cooling path  71  is provided at an end of the inclined portion of the diaphragm-side cooling path  71 , and the outlet  53  of the diaphragm-side cooling path  71  is provided at an end of the parallel portion of the diaphragm-side cooling path  71 . That is, in the present embodiment, the diaphragm-side cooling path  71  is characterized in that at least a part thereof has a portion parallel to the axis of the rotor  14 . 
     The outlet  53  of the diaphragm-side cooling path  71  may alternatively be positioned closer to the rotor  14  in a further downstream-side turbine stage as in the fourth embodiment, or may alternatively have the same position necessary in the most downstream-side turbine stage as in the fifth embodiment.  FIG. 8  shows an example of the latter case (same position setting). 
     Therefore, the thus configured present embodiment provides the following advantageous effect (8) in addition to the advantageous effects similar to the advantageous effects (1) and (2) provided in the first embodiment described above, the advantageous effects similar to the advantageous effects (4) to (6) provided in the fourth embodiment described above, and the advantageous effects similar to the advantageous effect (7) provided in the fifth embodiment described above. 
     (8) The diaphragm-side cooling path  71  formed in the internal diaphragm  18  in a downstream-side turbine stage within a cooling-requiring turbine stage range is formed so as to be inclined to the axis of the rotor  14 , extends to a point somewhere in the middle of the internal diaphragm  18 , and further extends in parallel to the axis of the rotor  14 . The inlet  52  is provided at an end of the inclined portion and the outlet  53  is provided at an end of the parallel portion. According to the configuration described above, since the cooling medium  27  flowing through the parallel portion of the diaphragm-side cooling path  71  and flowing out of the outlet  53  thereof impinges on the side surface of the rotor disc  15  in the same turbine stage at a right angle, the cooling medium  27  can efficiently cool the rotor disc  15  (including the blade studded portions  15 A). 
     It is to be noted that the present invention is not limited to the embodiments described above and many other changes and modifications may be made without departing from the scope of the appended claims.