Patent Publication Number: US-7909564-B2

Title: Gas turbine and gas turbine cooling method

Description:
This application is a continuation application of U.S. application Ser. No. 11/174,555, filed Jul. 6, 2005, now U.S. Pat. No. 7,507,069, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas turbine and a gas turbine cooling method. 
     2. Description of the Related Art 
     In a gas turbine, air is compressed by a compressor and fuel is added to the compressed air to produce an air-fuel mixture. The air-fuel mixture is burnt and resulting high-temperature, high-pressure combustion gases are used to drive the turbine. Thermal efficiency of an overall gas turbine plant can be increased by combining it with another plant, such as a steam turbine. Meanwhile, in a recent gas turbine, a pressure ratio of the combustion gases has been increased with intent to increase the thermal efficiency by using the gas turbine alone. For that reason, the differential pressure across each turbine blade provided in a gas path in a turbine section has been increased in comparison with that in the past. This gives rise to the necessity of reducing the amount of sealing air leaked through gaps between adjacent parts. In order to prevent the combustion gases from flowing into the inside of a turbine rotor, for example, the sealing air supplied to a wheel space on the upstream side must be prevented from leaking to a wheel space on the downstream side through a gap between the turbine rotor as a rotating member and a nozzle vane as a stationary member. To that end, a diaphragm is engaged with a lower portion of the nozzle vane. 
     For the purpose of holding air tightness of a cavity defined by the nozzle vane and the diaphragm, JP-B-62-37204 discloses a structure in which prestress is applied to a foot end of the diaphragm (i.e., a diaphragm hook) such that the diaphragm hook comes into pressure contact with a nozzle vane hook. 
     SUMMARY OF THE INVENTION 
     However, when prestress is applied to the diaphragm hook as disclosed in JP-B-62-37204, this may cause a deterioration of materials. More specifically, temperatures of gas turbine components change from the normal room temperature to a level of 400-500° C. depending on an operating state, and such a large temperature change raises a possibility that the diaphragm hook may be subjected to an excessive load. From the viewpoint of avoiding the possibility, it is desired that no prestress be applied to the diaphragm hook. On the other hand, if the contact between the diaphragm hook and the nozzle vane hook is insufficient, there arise a possibility that most of the sealing air in the cavity may leak to the wheel space on the downstream side where the pressure is relatively low. 
     An object of the present invention is to suppress a reduction in the thermal efficiency of a gas turbine attributable to a leak of the sealing air, which is supplied to the wheel space on the upstream side, from there toward the wheel space on the downstream side. 
     To achieve the above object, according to the present invention, a plurality of engagement portions between a sealing unit and a nozzle vane are provided successively from the upstream side toward the downstream side in a direction of flow of combustion gases, and downstream one of the plurality of engagement portions has a contact interface formed in a direction across a turbine rotary shaft. 
     With the present invention, a reduction in the thermal efficiency of the gas turbine can be suppressed which is attributable to a leak of the sealing air supplied to a wheel space on the upstream side from there toward a wheel space on the downstream side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a nozzle vane and a diaphragm; 
         FIG. 2  is a sectional view of a principal part of a gas turbine according to one embodiment, which is equipped with the nozzle vane and the diaphragm; 
         FIG. 3  is a sectional view taken along the line A-A in  FIG. 1 ; 
         FIG. 4  is a sectional view taken along the line B-B in  FIG. 1 ; 
         FIG. 5  is a perspective view showing engagement between a nozzle vane hook and a diaphragm hook in  FIG. 1 ; 
         FIG. 6  is a perspective view showing a modification of the engagement between the nozzle vane hook and the diaphragm hook; 
         FIG. 7  is a perspective view showing another modification of the engagement between the nozzle vane hook and the diaphragm hook; 
         FIG. 8  is a sectional view taken along the line C-C in  FIG. 1 ; 
         FIG. 9  is a sectional view showing a modification of the diaphragm hook; and 
         FIG. 10  is an enlarged view of the diaphragm hook. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Thermal efficiency of an overall gas turbine plant can be increased by combining it with another plant, such as a steam turbine. In a recent gas turbine, however, a pressure ratio of combustion gases has been increased with intent to increase the thermal efficiency by using the gas turbine alone. In that gas turbine, the differential pressure across each turbine blade in a gas path, i.e., in a gas channel inside the turbine, has been increased in comparison with that in the past. Accordingly, if gaps between adjacent parts remain the same as in the past, the amount of the sealing air flowing through the gaps between adjacent parts is increased to reduce the thermal efficiency of the gas turbine, whereby the advantage resulting from increasing the pressure ratio of the combustion gases is lessened. In other words, to increase the thermal efficiency of the gas turbine having a larger pressure ratio of the combustion gases, it is desired to eliminate or minimize the wasteful leak of the sealing air through the gaps between adjacent parts. 
     In general, a nozzle vane in each of second and subsequent stages of the turbine includes a diaphragm disposed between the nozzle vane and a rotor disk as a rotating member on the inner peripheral side. Then, a sealing structure is disposed in a gap between the diaphragm as a stationary member and the rotor disk as the rotating member, to thereby prevent the combustion gases from bypassing through the gap. In this connection, the sealing air is supplied from the nozzle vane side to a cavity inside the diaphragm serving as a sealing means. The sealing air is discharged from the cavity inside the diaphragm to wheel spaces on the upstream and downstream sides. In embodiments described below, it is assumed that the side into which the combustion gases flow from a combustor is the upstream side, and the side from which the combustion gases are discharged after flowing through the turbine (i.e., the gas path outlet side) is the downstream side. If positive sealing is not provided in engagement portions between the diaphragm and the nozzle vane, the sealing air inside the diaphragm leaks to the wheel space on the downstream side through the engagement portion on the downstream side. One reason is that because the pressure of a wheel space atmosphere is higher on the upstream side, the supply pressure of the sealing air must be set higher than the pressure of the wheel space atmosphere on the upstream side. Another reason is that because the differential pressure caused between the wheel spaces on the upstream and downstream sides is large, most of the sealing air leaks to the wheel space on the downstream side unless any sealing means is provided in the downstream-side engagement portion between the nozzle vane and the diaphragm. Such a leak of the sealing air is problematic in that the flow rate of the sealing air supplied to the upstream side becomes insufficient and the amount of the sealing air must be increased correspondingly in the whole of the gas turbine, thus resulting in a reduction in the thermal efficiency of the gas turbine. For the reasons mentioned above, positive sealing is required in the engagement portions between the nozzle vane and the diaphragm. 
     First Embodiment 
     The structure of the gas turbine will be described with reference to  FIG. 2 .  FIG. 2  shows a section of a principal part (blade stage section) of the gas turbine according to a first embodiment. An arrow  20  in  FIG. 2  indicates the direction of flow of combustion gases. Numeral  1  denotes a first stage nozzle vane,  3  denotes a second stage nozzle vane,  2  denotes a first stage rotor blade, and  4  denotes a second stage rotor blade. Also, numeral  5  denotes a diaphragm,  6  denotes a distance piece,  7  denotes a first stage rotor disk,  8  denotes a disk spacer, and  9  denotes a second stage rotor disk. 
     The first stage rotor blade  2  is fixed to the rotor disk  7 , and the second stage rotor blade  4  is fixed to the rotor disk  9 . The distance piece  6 , the rotor disk  7 , the disk spacer  8 , and the rotor disk  9  are integrally fixed by a stub shaft  10  to form a turbine rotor as a rotating member. The turbine rotor is fixed coaxially with not only a rotary shaft of a compressor, but also a rotary shaft of a load, e.g., a generator. 
     The gas turbine comprises a compressor for compressing atmospheric air to produce compressed air, a combustor for mixing the compressed air produced by the compressor with fuel and burning an air-fuel mixture, and a turbine rotated by combustion gases exiting the combustor. Further, the nozzle vanes and the rotor blades are disposed in a channel for the combustion gases flowing downstream inside the turbine. High-temperature and high-pressure combustion gases  20  exiting the combustor are converted to a flow with swirling energy by the first stage nozzle vane  1  and the second stage nozzle vane  3 , thereby rotating the first stage rotor disk  2  and the second stage rotor disk  4 . A generator is rotated with rotational energy of both the rotor disks to produce electricity. A part of the rotational energy is used to drive the compressor. Because the combustion gas temperature in the gas turbine is generally not lower than the allowable temperature of the blade (vane) material, the blades (vanes) subjected to the high-temperature combustion gases must be cooled. 
     The cooling structure of the second stage rotor disk  3  will be described below.  FIG. 1  is a sectional view of the second stage nozzle vane  3  and the diaphragm  5  in an axial direction. A cavity  11  is defined by the second stage nozzle vane  3  and the diaphragm  5 , and air for sealing off wheel spaces  14   a ,  14   b  is supplied to the cavity  11  through a coolant channel provided in the second stage nozzle vane  3 . In this embodiment, air is used as a coolant. The wheel space  14   a  is a gap which is formed by the diaphragm  5  and a shank portion  12  connecting the first stage rotor blade  2  and the rotor disk  7 , and which is positioned upstream of the diaphragm  5 . The wheel space  14   b  is a gap which is formed by the diaphragm  5  and a shank portion  13  connecting the second stage rotor blade  4  and the rotor disk  9 , and which is positioned downstream of the diaphragm  5 . The cavity  11  and the wheel space  14   a  are communicated with each other through a hole  90  formed in the diaphragm  5 . Similarly, the cavity  11  and the wheel space  14   b  are communicated with each other through a hole  91  formed in the diaphragm  5 . Further, the second stage nozzle vane  3  is fixed to an outer casing  93  constituting the turbine, and the diaphragm  5  is engaged with the second stage nozzle vane  3  at plural points. On the other hand, the disk spacer  8  rotates as a rotating member. Then, the diaphragm  5  and the disk spacer  8  provide a sealing structure between them. With that sealing structure, the wheel spaces  14   a  and  14   b  are prevented from spatially communicating with each other and can be formed as independent spaces. Additionally, a coolant  94  is supplied to the cavity  11  through a coolant channel  92  formed in the second stage nozzle vane  3 , followed by flowing into the wheel space  14   a  upstream of the diaphragm  5  and the wheel space  14   b  downstream of the diaphragm  5  through the holes  90 ,  91 , respectively. The coolant  94  is released as sealing air  15   a ,  15   b  into the gas path to prevent the combustion gases  20  from flowing into the interior side from an inner peripheral wall surface of the gas path. 
     When the sealing structure provided by the diaphragm  5  and the disk spacer  8  is formed as a honeycomb seal, the sealing ability is very high. It is therefore desired that the coolant  94  introduced to the cavity  11  be supplied to both the wheel space  14   a  upstream of the diaphragm  5  and the wheel space  14   b  downstream of the diaphragm  5 . On the other hand, when the sealing structure provided by the diaphragm  5  and the disk spacer  8  is formed as a labyrinth seal, the sealing ability is somewhat smaller than that of the honeycomb seal. Taking into account a flow of the coolant  94  directing from the wheel space  14   a  toward the wheel space  14   b  via the labyrinth seal, therefore, the coolant  94  introduced to the cavity  11  may be supplied to only the wheel space  14   a  upstream of the diaphragm  5 . By supplying the coolant  94  from the cavity  11  to only the wheel space  14   a  upstream of the diaphragm  5 , the hole  91  formed in the diaphragm  5  can be dispensed with, thus resulting in an improvement in manufacturability of the diaphragm  5 . 
     If the high-temperature combustion gases  20  flow into the wheel spaces  14   a ,  14   b  and the atmosphere temperatures in the wheel spaces rise correspondingly, the shank portions  12 ,  13  or the diaphragm  5  is thermally damaged by the combustion gases  20 . Further, excessive thermal loads are imposed on the rotor disks  7 ,  9  and the disk spacer  8 . This raises a possibility that thermal stresses increased with the excessive thermal loads may shorten life spans of individual members, and abnormal thermal deformations of the members may cause a trouble in turbine rotation, thus resulting in a difficulty in continuing normal operation of the gas turbine. In order to continue the normal operation of the gas turbine, therefore, it is desired that the sealing air be positively supplied to the wheel spaces  14   a ,  14   b.    
     Comparing the atmosphere pressures in the second stage nozzle vane  3 , the pressure in the wheel space  14   a  on the upstream side is higher than the pressure in the wheel space  14   b  on the downstream side. Although such a pressure difference changes depending on various conditions, it is usually about twice. Accordingly, when the sealing air is supplied to the wheel space  14   a , the pressure in the cavity  11  is preferably set higher than the pressure in the wheel space  14   a . A plurality of engagement portions between the second stage nozzle vane  3  and the diaphragm  5  are provided successively from the upstream side toward the downstream side in the direction of flow of the combustion gases, and the cavity  11  is defined by an inner surface of the diaphragm  5  and a lower surface of the second stage nozzle vane  3 . In this embodiment, the engagement portions between the second stage nozzle vane  3  and the diaphragm  5  are provided two, i.e., one on each of the upstream side and the downstream side. If air tightness of the cavity  11  is not held, the sealing air leaks to the downstream side where the pressure is relatively low, and the sealing air cannot be supplied to the upstream side in sufficient amount. In the gas turbine having a larger pressure ratio of the combustion gases, there is a tendency that the differential pressure between the upstream side and the downstream side of the nozzle vane increases. For that reason, if air tightness of the cavity  11  is not ensured, the amount of the sealing air leaking through the engagement portion on the downstream side is increased. If the amount of the sealing air supplied to the cavity  11  is increased to ensure a sufficient amount of the sealing air on the upstream side without reducing the amount of the sealing air leaking through the engagement portion on the downstream side, the amount of the sealing air leaking to the downstream side is increased in proportion to the increased amount of the sealing air supplied. To ensure a sufficient amount of the sealing air on the upstream side in such a manner, the sealing air must be supplied in a larger amount. Such an increase in the amount of the sealing air supplied lessens the effect of increasing the thermal efficiency of the gas turbine having a larger pressure ratio of the combustion gases. 
     With intent to avoid the above-mentioned drawback, this embodiment includes a plurality of engagement portions between respective hooks of the second stage nozzle vane  3  and the diaphragm  5  both constituting the cavity  11 . In this embodiment, those engagement portions are provided two, i.e., one on each of the upstream side and the downstream side. In the upstream one of the two engagement portions, a sealing interface  60  is formed by a nozzle vane hook  30  and a diaphragm hook  31  in the circumferential direction of a circle about a turbine rotary shaft. Then, the nozzle vane hook  30  and the diaphragm hook  31  are mated with each other at the sealing interface  60 . At this time, to ensure positive contact for sealing-off on the downstream side, the nozzle vane hook  30  and the diaphragm hook  31  forming the engagement portion on the upstream side are arranged such that gaps  97  and  98  are left as clearances in the axial direction to hold the two hooks from not contacting with each other in the axial direction. 
     In the engagement portion on the downstream side, a nozzle vane hook  33  is inserted in a diaphragm hook  32  formed substantially in a U-shape. A set pin  50  is inserted to extend through the diaphragm hook  32  and the nozzle vane hook  33  to hold them in a fixed positional relationship, whereby motions of the diaphragm  5  are restrained. Additionally, a proper gap  52  is left between the set pin  50  and an inner periphery of a pin bore  51  formed in the nozzle vane hook  33 . In other words, the pin bore  51  formed in the nozzle vane hook  33  has a larger diameter than the set pin  50 . Usually, the position and dimension of the set pin  50  are decided in consideration of design errors so that the positional relationship between the nozzle vane hook  33  and the diaphragm hook  32  is accurately held fixed even during the operation of the gas turbine. However, if no gap  52  is left between the set pin  50  and the inner periphery of the pin bore  51  formed in the nozzle vane hook  33 , the set pin  50  is not adaptable to thermal deformations of the nozzle vane hook  33  and the diaphragm hook  32 , and excessive thermal stresses are generated around the pin bore  51 . The thermal deformations of the nozzle vane hook  33  and the diaphragm hook  32  can be absorbed by setting the diameter of the pin bore  51  formed in the nozzle vane hook  33  larger than that of the set pin  50  and leaving the gap  52  in such a size as being able to accommodate those thermal deformations. Further, a sealing interface  61 , i.e., a contact interface, between the nozzle vane hook  33  and the diaphragm hook  32  is formed in a direction across the turbine rotary shaft. A recessed step portion  35  is formed in a part of the diaphragm hook  32  at a position nearer to the outer peripheral side than the sealing interface, and a recessed step portion  36  is formed in a part of the nozzle vane hook  33  at a position nearer to the inner peripheral side than the sealing interface. Each of those recessed step portions has a level difference defined by both the contact surface and a plane shifted from the contact surface in the axial direction of the turbine rotary shaft. 
       FIG. 3  shows a cross-section of the nozzle vane hook  33  taken along the line A-A in  FIG. 1 .  FIG. 4  shows a cross-section of the diaphragm hook  32  taken along the line B-B in  FIG. 1 . As shown in  FIG. 3 , a boundary  38  of the recessed step portion  36  is formed to extend substantially linearly. As shown in  FIG. 4 , a boundary  37  of the recessed step portion  35  is also formed to extend substantially linearly. Since the recessed step portions  35 ,  36  of the diaphragm hook  32  and the nozzle vane hook  33  have the substantially linear boundaries  37 ,  38 , those members can be machined more easily than the case of the boundaries being curved. Note that there is no problem even if the boundaries  37 ,  38  are not exactly linear due to machining errors. 
       FIG. 5  shows the downstream-side engagement portion between the diaphragm hook  32  and the nozzle vane hook  33  which are formed as described above. The provision of the recessed step portions  35 ,  36  allows the sealing interface  61  to have any suitable width in practice. If the width of the sealing interface  61  is too narrow, the sealing interface is not adaptable for a shift of the mating between the diaphragm and the nozzle vane. Conversely, if it is too wide, the surface pressure is reduced. For those reasons, the width of the sealing interface  61  is preferably in the range of 3-7 mm. Note that, in  FIG. 5 , the sealing interface  61  having a band-like shape is indicated by a hatched area. 
     A description is made of the action of the engagement portion between the diaphragm hook  32  and the nozzle vane hook  33  in this embodiment during the operation of the gas turbine. Referring to  FIG. 10 , due to the differential pressure between the upstream side and the downstream side, an action force  70  acts on the diaphragm  5  toward the downstream side. As a force opposing the action force  70 , a reaction force  72  is generated to act on the sealing interface  61 . Because the action force  70  and the reaction force  72  are not in a coaxial relation, there occurs a moment  77  acting on the diaphragm  5 . At this time, the diaphragm  5  is going to rotate in the direction of the moment  77  with the upstream-side engagement portion serving as a fulcrum. However, since a downstream-side end  65  of the diaphragm hook  32  contacts with an inner-peripheral end wall  66  of the second stage nozzle vane  3  and is restrained from moving unintentionally, a diaphragm sealing surface and a nozzle vane sealing surface are held in parallel relation. Then, action forces  71 ,  73  are generated to act on the diaphragm hook  31  and the downstream-side end  65  of the diaphragm hook  32 , respectively. In the upstream-side engagement portion, therefore, the nozzle vane hook  30  and the diaphragm hook  31  are further fastened together by the action force  71 . Accordingly, the surface pressure at the upstream-side sealing surfaces is increased and the sealing effect is enhanced. The upstream-side sealing surfaces are contacted with each other in the circumferential direction of a circle about the turbine rotary shaft.  FIG. 8  shows the sealing surfaces as a sectional view taken along the line C-C in  FIG. 1 . As shown in  FIG. 8 , the thermal deformations of the nozzle vane hook  30  and the diaphragm hook  31  change the radii of curvatures of their sealing surfaces contacting with each other, thereby generating a small gap  96  between both the hooks. However, the differential pressure across the upstream-side engagement portion, i.e., the differential pressure between the cavity  11  and the wheel space  14   a , is relatively small, and the surface pressure at the upstream-side sealing surfaces is increased by the action force  71 . As a result, the leak amount of the sealing air can be reduced to a negligible level. 
     The upstream-side engagement portion is of a structure in which the diaphragm hook  31  is latched by the nozzle vane hook  30 . Thus, because the diaphragm hook  31  and the nozzle vane hook  30  are in a relatively movable state, a leak of the sealing air through both the upstream-side engagement portion and the downstream-side engagement portion can be reduced by effectively utilizing the above-mentioned moment  77 . As a result, a reduction in the thermal efficiency of the gas turbine can be suppressed which is attributable to the leak of the sealing air supplied to the wheel space on the upstream side from there toward the wheel space on the downstream side. 
     On the other hand, in the downstream-side engagement portion, the diaphragm hook  32  receives the reaction force  72  from the nozzle vane hook  33  such that both the hooks are pressed against each other, and a large force of the magnitude almost equal to that of the action force  70  acts on the sealing interface  61 . At this time, since the sealing interface  61 , i.e., the contact interface formed in the downstream-side engagement portion, is formed to extend in the direction across the turbine rotary shaft, a large force of the magnitude almost equal to that of the action force  70  acts on the entire sealing interface  61 . Preferably, the sealing interface  61  is substantially perpendicular to the turbine rotary shaft. Also, since the sealing interface  61  as the contact interface is a flat plane, a plane deviation is small even when both the hooks are thermally deformed. Further, since the surface pressure is increased with the sealing interface  61  having a band-like shape, no gap is generated at the sealing interface  61  and positive sealing can be realized even when subjected to a large differential pressure. Stated another way, since the upstream-side sealing interface of the downstream-side engagement portion does not provide contact in the circumferential direction of a circle about the turbine rotary shaft, but forms the contact interface extending in the direction across the turbine rotary shaft, it is possible to provide a reliable sealing structure between the nozzle vane and the diaphragm, which causes no performance reduction due to the leak of the sealing air. 
     The related art disclosed in JP-B-62-37204 employs a structure in which prestress is applied to the diaphragm hook, and accompanies with a possibility of causing a deterioration of diaphragm materials. Also, because the gas turbine is operated under a wide variety of temperature conditions, there is a possibility of affecting durability of the diaphragm in all the operating states of the gas turbine. In contrast, this embodiment has the structure in which the diaphragm hook  31  is latched by the nozzle vane hook  30  and no prestress is applied to the diaphragm hook  31 . Accordingly, durability of the diaphragm can be maintained in all the operating states of the gas turbine. 
     As shown in  FIGS. 3 to 5 , the sealing surface boundaries  37 ,  38  defined by the recessed step portions  35 ,  36  are formed substantially linearly. Therefore, even when the parallelism between the sealing surface of the diaphragm hook and the sealing surface of the nozzle vane hook in the downstream-side engagement portion is deviated in a small range due to, e.g., thermal deformations of those hooks during the gas turbine operation, such a deviation can be accommodated. For example, when the nozzle vane hook  33  is rotated relative to the diaphragm hook  32  in the direction of an arrow  80 , a sealing edge of a linear-contact sealing portion  63  is maintained tight so as to suppress the generation of a gap. Also, when the nozzle vane hook  33  is rotated relative to the diaphragm hook  32  in the direction of an arrow  81 , a sealing edge of a linear-contact sealing portion  64  is maintained tight so as to suppress the generation of a gap. With such a sealing manner, even in the case of operating the gas turbine having a larger pressure ratio of the combustion gases, it is possible to reduce the amount of the sealing air unintentionally leaked from the cavity  11  through the downstream-side engagement portion. Then, the sealing air can be positively supplied from the cavity  11  to both the wheel spaces  14   a  and  14   b . Further, the amount of the sealing air used in total can be reduced to the least necessary amount, and therefore a reduction in the thermal efficiency of the gas turbine can be suppressed. Note that, since the provision of at least one of the recessed step portions  35 ,  36  is enough to form the contact interface extending in the direction across the turbine rotary shaft, similar advantages to the above-mentioned ones can also be obtained with only one of the recessed step portions  35 ,  36 . 
     In this embodiment, unlike the related art, any additional member, e.g., a packing, is not provided on each of the diaphragm hook and the nozzle vane hook. The members of the downstream-side engagement portion, i.e., a set of the nozzle vane hook and its contact portion contacting with the diaphragm hook and a set of the diaphragm hook and its contact portion contacting with the nozzle vane hook, are each formed as an integral part. This structure contributes to avoiding damage of the members and improving reliability in operation. Furthermore, this embodiment can be realized with a simpler structure and easier machining because of using no complicated means, such as a spring and packing. 
     Moreover, as shown in  FIG. 1 , an upper surface of the diaphragm hook  32  formed substantially in a U-shape and a lower surface of an intermediate portion  96 , to which the nozzle vane hook  33  is fixed, are held in surface contact with each other in the circumferential direction of a circle about the turbine rotary shaft. With that surface contact, even when a moment acts on the diaphragm  5 , it is possible to restrict a displacement of the diaphragm  5  relative to the second stage nozzle vane  3 . If the displacement of the diaphragm  5  relative to the second stage nozzle vane  3  can be restricted, the engagement at the most-downstream end between the diaphragm hook  32  and the nozzle vane hook  33  (i.e., the intermediate portion  96 ) is not essential in this embodiment. In other words, the construction of this embodiment may be modified, by way of example, as shown in  FIG. 9  without problems. In any case, the displacement of the diaphragm  5  can be restricted by contacting the diaphragm  5  and the second stage nozzle vane  3  with each other at a position closer to the downstream-side engagement portion to such an extent that the displacement of the diaphragm  5  relative to the second stage nozzle vane  3  can be restricted. Such contact minimizes the displacement of the diaphragm  5  relative to the second stage nozzle vane  3 . That contact is also effective in facilitating mutual positioning of the nozzle vane hook  33  and the diaphragm hook  32  when they are assembled together in a turbine assembly process. 
     Further, since the second stage nozzle vane  3  and the diaphragm  5  are engaged with each other in the upstream-side engagement portion and the upper surface of the diaphragm hook  32  and the lower surface of the intermediate portion  96 , to which the nozzle vane hook  33  is fixed, are held in surface contact with each other in the downstream-side engagement portion, a maximum displacement of the diaphragm  5  relative to the second stage nozzle vane  3  is restricted. Therefore, the nozzle vane hook  33  and the diaphragm hook  32  in the downstream-side engagement portion can be avoided from excessively displacing from each other. The contact surface formed in the downstream-side engagement portion to extend in the direction across the turbine rotary shaft is adaptable for a slight displacement between the second stage nozzle vane  3  and the diaphragm  5 , but it accompanies with a possibility that the effect of the contact surface may not be developed when the displacement increases. With this embodiment, however, since the diaphragm and the nozzle vane are mutually supported at two points, i.e., two engagement portions between them on the upstream side and the downstream side, a maximum displacement of the diaphragm relative to the nozzle vane can be restricted. Additionally, when the diaphragm is supported on the nozzle vane at two points through two engagement portions between them on the upstream side and the downstream side, more positive sealing can be realized by forming the downstream-side engagement portion such that the contact surface extends in the direction across the turbine rotary shaft. Preferably, the contact surface is substantially perpendicular to the turbine rotary shaft. 
     While the advantages of this first embodiment have been described in connection with the second stage nozzle vane and the diaphragm, the structure of this first embodiment is not limited to the second stage and is applicable to the nozzle vane and the diaphragm in each stage of the gas turbine including many stages of nozzle vanes and diaphragms. 
     Second Embodiment 
       FIG. 6  shows a second embodiment of the present invention. According to this embodiment, in the downstream-side engagement portion between the second stage nozzle vane  3  and the diaphragm  5 , a slope  39  is formed in the diaphragm hook  32  on the side closer to the outer periphery from the sealing interface. Further, a slope  40  is formed in the nozzle vane hook  33  on the side closer to the inner periphery from the sealing interface. More specifically, each slope  39 ,  40  is formed as a hook wall surface inclined at any desired angle from the direction perpendicular to the turbine rotary shaft. Even with such a structure, a sealing interface  61   b  (indicated by a hatched area in  FIG. 6 ) is formed substantially in a band-like shape, and therefore the amount of the sealing air unintentionally leaking through the downstream-side engagement portion can be reduced. Further, similar advantages can also be obtained with such a modification that a recessed step portion is formed in one of the diaphragm hook and the nozzle vane hook and a slope is formed in the other hook. The shape of each slope is not limited to particular one, and similar advantages can also be obtained with a linear or curved slope so long as the sealing interface is formed substantially in a band-like shape. 
       FIG. 7  shows another example in which the boundaries of the recessed step portions of the diaphragm and the nozzle vane are each formed as an angularly bent line. It is desired that the boundaries of the band-shaped sealing surfaces of the diaphragm and the nozzle vane be as linear as possible. However, when a difficulty arises in forming the boundaries to be linear because of a structure using coupled vanes, the recessed step portions may be modified, as indicated by  35   b ,  36   b , such that their boundaries have angularly bent points  45 ,  46  and an angularly bent sealing interface  61   c  is formed (as indicated by a hatched area in  FIG. 7 ). A sufficient sealing effect is obtained when the parallelism between the sealing surfaces of both the hooks is substantially held, as with the above-described engagement structure of the nozzle vane and the diaphragm. Although the sealing effect is somewhat reduced, a practically advantageous effect is obtained even when the boundary of the sealing interface is formed as a gently curved line or a linear line having a plurality of angularly bent points. 
     Thus, by employing any of the structures for supporting the nozzle vane hook and the diaphragm according to the embodiments described above, the amount of the sealing air unintentionally leaking from the cavity defined by the nozzle vane and the diaphragm can be reduced in the gas turbine having a large pressure ratio of the combustion gases. Further, a high reliable gas turbine can be provided by positively supplying the sealing air to the upstream side while avoiding a possibility that an increase in the thermal efficiency of the gas turbine, which is resulted from setting a larger pressure ratio of the combustion gases, may be reduced with a leak of the sealing air through the diaphragm.