Patent Publication Number: US-2018030992-A1

Title: Centrifugal compressor and geared centrifugal compressor

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal compressor, and particularly to a centrifugal impeller of a geared centrifugal compressor. 
     Priority is claimed on Japanese Patent Application No.  2015 - 026469 , filed Feb. 13, 2015, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     In general, a centrifugal compressor includes an impeller which is provided in a rotation shaft and a casing which covers the impeller from the outside and defines a flow passage between the casing and the impeller. As the impeller rotates, an external fluid is suctioned through the flow path into the casing by the rotation of the impeller, and the fluid in a high-pressure state is discharged from an outlet of the casing by pressure being applied to the fluid while the fluid flows in the flow passage. 
     As an example of such a technology, a centrifugal compressor disclosed as below in Patent Literature 1 is known. In the centrifugal compressor disclosed in Patent Literature 1, a so-called closed impeller is employed. That is, this device includes a rotation shaft which rotates about an axis, a closed impeller (impeller) which includes a disk attached to the rotation shaft, a plurality of blades arranged on one surface of the disk, and a shroud provided at edges of the plurality of blades at one side in an axial direction, and a casing which covers the closed impeller from the outside. With the above-described configuration, a low-pressure fluid which flows from one side in the axial direction is compressed in accordance with the rotation of the impeller and is led from an outer discharge portion in a radial direction to the outside in the form of a high-pressure fluid. 
     Incidentally, in the above-described device, a labyrinth seal is generally provided in a space between the casing and the shroud and a space between the casing and the disk in order to prevent a reverse flow of the high-pressure fluid. In the device according to Patent Literature 1, a mouse labyrinth is provided between the shroud and the casing in the vicinity of an inlet of the closed impeller. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] 
     Japanese Unexamined Patent Application, First Publication No. H04-203565 
     However, in the configuration disclosed in Patent Literature 1, there is also a possibility that the high-pressure fluid may flow reversely in a space from the discharge portion to the sealing portion (a space between the casing and the shroud). In such a case, a pressure which is generated by the high-pressure fluid is applied to both sides of the closed impeller in the axial direction. Accordingly, a compressing force, which is generated by the shroud and the disk, is applied to the blade. When a device is operated under atmospheric pressure or a device having a relatively low pressure difference is used, such a compressing force is small enough to be ignored. However, in a device having a high pressure difference, since the compressing force inevitably increases, there is a possibility that a durability of the blade may be influenced. 
     As a means for handling such a compressing force, for example, a method of increasing a thickness of the blade or increasing the number of the blades is considered. However, when such a technology is used, while a structural strength of the impeller is compensated, pressure loss of the fluid flowing in the impeller increases. That is, there is a possibility that compression efficiency of the centrifugal compressor is degraded. 
     SUMMARY OF INVENTION 
     One or more embodiments of the present invention provide a centrifugal compressor, in particular a geared centrifugal compressor, with sufficient durability and compression efficiency. 
     According to a first aspect of the present invention, a centrifugal compressor includes: a rotation shaft which extends along an axis; an impeller which includes a disk fixed to the rotation shaft and protruding outward in a radial direction, a plurality of blades provided in a surface facing one side in an axial direction of the disk to be separated from each other in a circumferential direction, and a cover covering the blades from the one side in the axial direction; and a casing which includes a facing inner peripheral surface facing an outer peripheral surface of the cover in the impeller and covers the impeller from an outside. The facing inner peripheral surface includes a cylindrical inner peripheral surface which extends along the axis and an enlarged inner peripheral surface which is connected to another side of the cylindrical inner peripheral surface in the axial direction and extends outward in the radial direction toward the other side. The centrifugal compressor further includes a sealing portion which seals a gap between the enlarged inner peripheral surface and the outer peripheral surface of the cover. 
     According to one or more embodiments, since the sealing portion seals a gap between the enlarged inner peripheral surface and the outer peripheral surface of the cover, it is possible to decrease a possibility that a high-pressure fluid reaches the cylindrical inner peripheral surface beyond the enlarged inner peripheral surface even when the high-pressure fluid flows reversely inside the casing. That is, it is possible to decrease an area of a region on which a pressure of the high-pressure fluid acts in the outer peripheral surface of the cover. Accordingly, it is possible to maintain desired compression efficiency without increasing a structural strength of the impeller. 
     According to a second aspect of the present invention, in the centrifugal compressor according to the first aspect, the sealing portion may include a plurality of sealing fins which are provided in the enlarged inner peripheral surface and extend in a direction from the enlarged inner peripheral surface toward the cover in the impeller. 
     According to one or more embodiments, a reverse flow of the high-pressure fluid between the enlarged inner peripheral surface and the outer peripheral surface of the cover can be suppressed by the plurality of sealing fins. 
     According to a third aspect of the present invention, in the centrifugal compressor according to the first aspect, the sealing portion may include a plurality of sealing fins which are provided in the enlarged inner peripheral surface to extend in a direction from the enlarged inner peripheral surface toward an inside in the radial direction of the axis and to be arranged in the axial direction, a protrusion portion which protrudes from the outer peripheral surface and includes a sealing surface extending parallel to the axis may be provided at a position corresponding to the sealing portion in the outer peripheral surface of the cover, and the sealing surface may face the plurality of sealing fins. 
     According to one or more embodiments, the plurality of sealing fins arranged in the axial direction and the sealing surface facing the sealing fins can suppress the reverse flow of the high-pressure fluid between the outer peripheral surface of the cover and the enlarged inner peripheral surface. In addition, it is possible to maintain a separation dimension between the sealing fin and the sealing surface even when a displacement of the rotation shaft in the axial direction occurs. 
     According to a fourth aspect of the present invention, in the centrifugal compressor according to the first aspect, the sealing portion may include a plurality of impeller side fins which are provided in a facing cover surface facing the enlarged inner peripheral surface in the cover and extend in a direction from the facing cover surface toward the enlarged inner peripheral surface. 
     According to one or more embodiments, the reverse flow of the high-pressure fluid between the facing cover surface and the enlarged inner peripheral surface can be sufficiently suppressed by the plurality of impeller side fins. 
     According to a fifth aspect of the present invention, the centrifugal compressor according to the fourth aspect may further include an abradable portion which is provided in the enlarged inner peripheral surface and is formed of a material abradable when brought into contact with front ends of the impeller side fins. 
     According to one or more embodiments, even when the front end of the impeller side fin is brought into contact with the enlarged inner peripheral surface due to a displacement caused by vibration with rotation of the impeller, a processing tolerance of the impeller, or a movement of the impeller in the axial direction, since the abradable portion having machinability is capable of being cut, it is possible to decrease a possibility of causing abrasion of the impeller side fin. Accordingly, it is possible to maintain a sealing property of the impeller side fin. 
     According to a sixth aspect of the present invention, in the centrifugal compressor according to any one of the first to fifth aspects, the sealing portion may be disposed in an area of 90% or less of a diameter dimension of the disk with reference to the axis. 
     According to one or more embodiments, when the sealing portion is provided in an area of 90% or less of the diameter dimension of the disk with reference to the axis, a space between the outer peripheral surface of the cover and the facing inner peripheral surface is divided into an outer radial area and an inner radial area with the sealing portion interposed therebetween. The high-pressure fluid flows in the outer radial area and a low-pressure fluid flows in the inner radial area. Accordingly, it is possible to optimize a pressure distribution applied to the outer peripheral surface of the cover by the high-pressure fluid. 
     According to a seventh aspect of the present invention, the centrifugal compressor according to any one of the first to sixth aspects may further include a back surface sealing portion which seals a gap between a surface of the disk facing the other side in the axial direction and the casing. 
     According to one or more embodiments, since the back surface sealing portion is provided, it is possible to decrease a possibility that the high-pressure fluid flows reversely through a gap between a surface of the disk facing the other side in the axial direction and the casing. Accordingly, it is possible to further decrease a compressing force applied to the impeller from both sides in the axial direction and also to decrease a thrust force applied to the rotation shaft in the axial direction. 
     According to an eighth aspect of the present invention, a geared centrifugal compressor may include: the centrifugal compressor according to any one of the first to seventh aspects; and a speed increasing transmission portion which includes a rotation driving shaft rotationally driven by an external driving source and an accommodation portion accommodating the rotation driving shaft and communicating with an inside of the casing and transmits a rotation of the rotation driving shaft to the rotation shaft of the centrifugal compressor. 
     According to one or more embodiments, it is possible to obtain the geared centrifugal compressor including the centrifugal compressor in which the possibility of the reverse flow of the high-pressure fluid inside the casing is decreased. Particularly, it is possible to sufficiently suppress a reverse flow of the high-pressure fluid directed from the inside of the casing of the centrifugal compressor toward the intake port of the impeller or the accommodation portion of the speed increasing transmission portion, and to reduce the compressing force from both sides of the impeller. 
     According to a ninth aspect of the present invention, in the eighth aspect, the geared centrifugal compressor may further include another centrifugal compressor which is disposed at an opposite side of the rotation shaft of the centrifugal compressor with the speed increasing transmission portion interposed therebetween and is plane symmetric with the centrifugal compressor with reference to a reference plane orthogonal to the axis. 
     According to one or more embodiments, the pair of centrifugal compressors having a plane symmetric shape with the reference plane interposed therebetween rotate. For this reason, even when the thrust force is generated in one centrifugal compressor toward one side of the rotation shaft in the axial direction, the same thrust force is generated in the other centrifugal compressor toward the other side in the axial direction. Accordingly, since these two thrust forces cancel each other, the thrust force of the rotation shaft in the axial direction can be reduced. 
     According to one or more embodiments of the centrifugal compressor and the geared centrifugal compressor, sufficient durability and compression efficiency can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a geared centrifugal compressor according to one or more embodiments of the present invention. 
         FIG. 2  is an enlarged view of a main part of a centrifugal compressor according to a first embodiment of the present invention. 
         FIG. 3  is an enlarged view of a sealing portion according to the first embodiment of the present invention. 
         FIG. 4  is an enlarged view of a main part of a centrifugal compressor according to a second embodiment of the present invention. 
         FIG. 5  is an enlarged view of a main part of a centrifugal compressor according to a third embodiment of the present invention. 
         FIG. 6  is an enlarged view of a main part showing a modified example of the centrifugal compressor according to the third embodiment of the present invention. 
         FIG. 7  is an enlarged view of a main part of a centrifugal compressor according to a fourth embodiment of the present invention. 
         FIG. 8  is an enlarged view of a main part of a centrifugal compressor according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a first embodiment of the present invention will be described. 
     As shown in  FIG. 1 , a geared centrifugal compressor  100  according to the embodiment includes a speed increasing transmission portion  200  which includes a rotation driving shaft  102  driven by an external driving source and a pair of centrifugal compressors  1  which are disposed at both sides of the speed increasing transmission portion  200  to interpose the speed increasing transmission portion  200  therebetween. That is, the geared centrifugal compressor  100  is configured as a single-shaft two-stage compressor. Accordingly, a fluid which is compressed by one centrifugal compressor  1  (the first-stage centrifugal compressor  1 ) continuously flows into the other centrifugal compressor  1  (the second-stage centrifugal compressor  1 ). While the fluid flows through the second-stage centrifugal compressor, the fluid is further compressed to become a high-pressure fluid. 
     Further, in the description below, the single-shaft two-stage geared centrifugal compressor  100  will be exemplified. However, the embodiment of the geared centrifugal compressor  100  is not limited thereto, and a compressor having more compression stages and shafts may be employed. 
     More specifically, the geared centrifugal compressor  100  has a configuration in which the pair of centrifugal compressors  1  interposing the speed increasing transmission portion  200  are driven by the same rotation shaft  2 . Further, the pair of centrifugal compressors  1  are formed to be approximately plane symmetric with reference to a reference plane CP, which is an imaginary plane orthogonal to an axis O, of the rotation shaft  2 . In other words, one centrifugal compressor  1  is mirror-symmetric to the other centrifugal compressor  1 . 
     However, dimensions of the parts of the pair of centrifugal compressors  1  may be different. 
     The speed increasing transmission portion  200  includes the rotation driving shaft  102 , which includes a large-diameter gear  103  and is rotationally driven by an external driving source, and an accommodation portion  104 , which accommodates a part of the rotation driving shaft  102  and the rotation shaft  2 . The large-diameter gear  103  of the rotation driving shaft  102  is a disk-shaped gear extending in a plane orthogonal to an axis O of the rotation driving shaft  102 . 
     When a high output and a high torque need to be achieved, a helical gear is suitably used as such a gear. In the large-diameter gear  103 , a pitch of teeth or the like is appropriately set to be meshed with a pinion gear  3  provided in the rotation shaft  2  of the centrifugal compressor  1 . 
     Further, a diameter dimension of the pinion gear  3  is set to be smaller than that of the large-diameter gear  103 . Thus, the number of rotations of the rotation shaft  2  including the pinion gear  3  is larger than the number of rotations of the rotation driving shaft  102  including the large-diameter gear  103 . 
     A bearing device  4  for rotatably supporting the rotation driving shaft  102  and the rotation shaft  2  is provided inside the accommodation portion  104  forming an outer shell of the speed increasing transmission portion  200 . A device for supplying lubricating oil to the bearing device  4  may be separately provided. 
     With the above-described configuration, rotation of the rotation driving shaft  102  is transmitted to the rotation shaft  2  of the centrifugal compressor  1  through the large-diameter gear  103  and the pinion gear  3 . Accordingly, the pair of centrifugal compressors  1  are operated. 
     Next, a configuration of the centrifugal compressor  1  according to the embodiment will be described with reference to  FIG. 2 . Further, as described above, since both of the pair of centrifugal compressors  1  in the geared centrifugal compressor  100  of the embodiment have the same configuration except that both of the compressors are plane symmetric, only one centrifugal compressor  1  will be described in the following description. 
       FIG. 2  is an enlarged view of a main part of the centrifugal compressor  1 . As shown in the same drawing, the centrifugal compressor  1  includes the rotation shaft  2  which extends along the axis O, an impeller  5  which is provided in the rotation shaft  2 , and a casing  6  which covers the impeller  5  from the outside. 
     The rotation shaft  2  is a rotation body which is formed in a columnar shape about the axis O and is rotated about the axis O by a rotational force transmitted from the speed increasing transmission portion  200 . 
     The impeller  5  is an impeller which is provided at an intermediate position in a direction of the axis O of the rotation shaft  2 . More specifically, the impeller  5  includes a substantially disk-shaped disk  7  which protrudes outward in a radial direction from an outer peripheral surface of the rotation shaft  2 , a plurality of blades  8  which are provided at one surface of the disk  7  in a direction of the axis O, and a cover  9  which covers the plurality of blades  8  from one side in the direction of the axis O. 
     The disk  7  includes a disk support portion  71  which is fitted to a fitting groove  21  formed in the outer peripheral surface of the rotation shaft  2  and an annular disk body  72  which extends in a plate shape outward in a radial direction from the disk support portion  71 . 
     The disk support portion  71  gradually increases in diameter from the inside to the outside in the radial direction from the one side toward the other side in the direction of the axis O. The disk body  72  protrudes outward in the radial direction from the outer peripheral surface at the other side in the direction of the axis O of the disk support portion  71 . That is, the disk body  72  is formed in an annular plate shape. 
     A connection portion  73  between the disk support portion  71  and the disk body  72  is formed in a smooth curved surface shape. One surface in the direction of the axis O of the disk  7  with such a configuration is formed as a disk front surface  7 A. Meanwhile, a surface opposite to the disk front surface  7 A is smoothly formed as a disk back surface  7 B. The disk back surface  7 B extends to a surface substantially orthogonal to the axis O. 
     Each blade  8  is a blade member that has a thin plate shape and extends from the disk front surface  7 A. Although not shown in detail, when viewed from the direction of the axis O, the blade  8  is curved toward one side as it goes from the inside toward the outer side in a radial direction of the disk  7 . 
     Further, a height dimension of the blade  8 , that is, a protrusion dimension from the disk front surface  7 A, gradually decreases from the disk support portion  71  toward the disk body  72 . In other words, an edge of the blade  8  facing the one side in the direction of the axis O, that is, an edge opposite to the disk  7 , is curved so as to substantially correspond to the curved shapes of the disk support portion  71  and the disk body  72 . 
     A plurality of the blades  8  with such a configuration are radially arranged on the disk front surface  7 A about the axis O outward in the radial direction. That is, a circumferential gap is formed between a pair of adjacent blades  8 . 
     Further, each edge of the plurality of blades  8  (the edge opposite to the disk  7 ) is provided with the cover  9  which is provided in an entire extending dimension. In other words, the plurality of blades  8  are covered by the cover  9  from the one side in the direction of the axis O. As described above, since the edge of the blade  8  is curved to correspond to the shape of the disk front surface  7 A, the cover  9  is generally formed in a funnel shape. 
     Further, a separation dimension between the inner peripheral surface of the cover  9  and the disk front surface  7 A gradually decreases from the inside toward the outside in the radial direction when viewed from the radial direction of the axis O. In one or more embodiments, the cover  9  is integrally formed by one member. An outer peripheral surface of the cover  9 , that is, a surface facing the one side in the direction of the axis O, is formed as a facing cover surface  9 A. 
     In the impeller  5  with such a configuration, an impeller flow passage  5 F, which is surrounded by an inner peripheral surface of the cover  9  and the disk front surface  7 A, is defined. Both sides of the impeller flow passage  5 F in a circumferential direction are divided by a pair of adjacent blades  8 . Since one side of the impeller flow passage  5 F in the direction of the axis O is opened toward the one side in the direction of the axis O, an impeller intake port  51  is formed. Meanwhile, since an end opposite to the impeller intake port  51  in the impeller flow passage  5 F is also opened in this way, an impeller discharge port  52  is formed. 
     The casing  6  forms a part of an outer shell of the centrifugal compressor  1  and covers the impeller  5  from the outside by allowing the inner peripheral surface to face the impeller. The casing  6  is provided with an intake flow passage  80  (an intake pipe  80 ) which communicates with the outside to intake air as a working fluid. As shown in  FIG. 1 , the intake pipe  80  is a cone-shaped member that gradually decreases in diameter from the one side toward the other side in the direction of the axis O. Air which enters through the intake flow passage  80  is led to the impeller flow passage  5 F through the impeller intake port  51  inside the casing  6 . 
     Further, a surface in the inner peripheral surface of the casing  6  facing the facing cover surface  9 A in the impeller  5  with a gap therebetween is formed as a facing inner peripheral surface  6 A. A surface which is positioned at the side opposite to the facing inner peripheral surface  6 A with the impeller  5  interposed therebetween in the direction of the axis O is formed as a second facing inner peripheral surface  6 B facing the disk back surface  7 B with a gap interposed therebetween. 
     An area which is surrounded by the facing inner peripheral surface  6 A and the second facing inner peripheral surface  6 B is provided with a diffuser  6 E which is opened toward the outside from an outer end thereof in the radial direction. The diffuser  6 E communicates with an exhaust flow passage  90  (an exhaust flow passage  90 ). As shown in  FIG. 1 , the exhaust flow passage  90  is a pipe body which extends in a spiral shape while surrounding the intake pipe  80  from the outer peripheral side. High-pressure air is supplied to an external device (not shown) through the exhaust flow passage  90  and is used for various purposes. 
     Further, the facing inner peripheral surface  6 A includes a cylindrical inner peripheral surface  61 A which is formed in a cylindrical shape and substantially extends along the axis O, and an enlarged inner peripheral surface  62 A which is connected to the other end of the cylindrical inner peripheral surface  61 A in the direction of the axis O and similarly extends outward in the radial direction toward the other side. 
     In addition, the second facing inner peripheral surface  6 B in the inner peripheral surface of the casing  6  extends in a plane shape substantially parallel to the outer peripheral surface of the rotation shaft  2 . A radial gap is formed between the second facing inner peripheral surface  6 B and the outer peripheral surface of the rotation shaft  2 . The gap communicates with the inside of the accommodation portion  104  in the speed increasing transmission portion  200  through a shaft sealing portion  2 S. 
     The shaft sealing portion  2 S is a sealing member that is provided in an area directly facing the rotation shaft  2  in the inner peripheral surface of the casing  6 . The shaft sealing portion  2 S is provided to reduce leakage of air toward the accommodation portion  104  of the speed increasing transmission portion  200 . 
     A sealing portion  10  is provided between the casing  6  and the impeller  5  which are formed as described above. More specifically, the sealing portion  10  according to the embodiment is provided on the enlarged inner peripheral surface  62 A in the facing inner peripheral surface  6 A of the casing  6 . Particularly, in one or more embodiments, a separation dimension D 2  in the radial direction from the axis O to the sealing portion  10  may be 90% or less of a diameter dimension D 1  of the disk as indicated by a dimension line of  FIG. 2  upon providing the sealing portion  10  on the enlarged inner peripheral surface  62 A. 
     Next, a detailed configuration of the sealing portion  10  will be described on the basis of  FIG. 3 . As shown in  FIG. 3 , the sealing portion  10  includes a base portion  11  which is formed in a substantially plate shape and extends along the enlarged inner peripheral surface  62 A inside the casing  6 , and a plurality of sealing fins  12  which extend from the base portion  11  toward the cover  9  of the impeller  5 . 
     Since the sealing fin  12  is formed to be gradually tapered from a side which contacts with the base portion  11  toward a front end thereof, the sealing fin is formed in a wedge shape in a cross-sectional view. Further, in the embodiment, the plurality of sealing fins  12  are arranged on the surface of the base portion  11  at intervals. However, the shape of the sealing fin  12  is not limited thereto and, for example, the sealing fins  12  may be arranged in close contact with each other without spacing. 
     The front end of the sealing fin  12  with such a configuration forms a slight gap with respect to the facing cover surface  9 A of the facing impeller  5 . Since the gap is formed, a possibility of the rotating impeller  5  contacting the sealing fin  12  decreases, and thus sufficient sealing performance can be exhibited. 
     Operations of the centrifugal compressor  1  and the geared centrifugal compressor  100  with the above-described configuration will be described. 
     First, the rotation driving shaft  102  of the speed increasing transmission portion  200  is rotationally driven by an external driving source. As such a driving source, for example, an electric motor or a steam turbine is appropriately selected according to designs and specifications. That is, when an output shaft of the electric motor or the steam turbine is connected to the rotation driving shaft  102 , rotation thereof can be transmitted to the rotation driving shaft  102 . 
     As the rotation driving shaft  102  rotates, the large-diameter gear  103  provided on the rotation driving shaft  102  also rotates. The large-diameter gear  103  meshes with the pinion gear  3  provided on the rotation shaft  2  of the centrifugal compressor  1 . Accordingly, the rotation of the rotation driving shaft  102  is transmitted to the rotation shaft  2  of the centrifugal compressor  1 , and the rotation shaft  2  starts to rotate in a direction opposite to a rotation direction of the rotation driving shaft  102 . 
     Since the rotation shaft  2  rotates, the pair of centrifugal compressors  1  which are provided adjacent to the speed increasing transmission portion  200  are driven. First, the impeller  5  rotates inside the casing  6  in accordance with the rotation of the rotation shaft  2 . As described above, the impeller intake port  51  which intakes air as a working fluid is provided at one side of the impeller  5  in the direction of the axis O. In accordance with an increase in the number of rotations of the impeller  5 , air is taken into the impeller flow passage  5 F through the impeller intake port  51 . 
     The air which is taken into the impeller flow passage  5 F receives torque while flowing toward the impeller discharge port  52  inside the impeller flow passage  5 F due to the rotation of the impeller  5  and is compressed by the impeller flow passage  5 F to become high-pressure air. The high-pressure air passes through the impeller discharge port  52  of the impeller flow passage  5 F and flows toward the diffuser  6 E. The high-pressure air which flows into the diffuser  6 E is led to the outside through the exhaust flow passage  90  provided in the casing  6 . 
     Here, as described above, while the high-pressure air exists in the vicinity of the impeller discharge port  52  and the diffuser  6 E, uncompressed air flows in the gap formed by the facing inner peripheral surface  6 A of the casing  6  and the outer peripheral surface (the facing cover surface  9 A) of the cover  9 . Thus, there is a possibility in which the high-pressure air flows into the gap due to a pressure difference. 
     However, as described above, in the centrifugal compressor  1  according to the embodiment, since the facing inner peripheral surface  6 A is provided with the plurality of sealing fins  12  serving as the sealing portion  10 , the flow of the high-pressure air can be sealed. That is, it is possible to decrease a possibility that the high-pressure air flows reversely toward a low-pressure area. 
     Particularly, in the embodiment, the sealing portion  10  is provided on the enlarged inner peripheral surface  62 A of the facing inner peripheral surface  6 A. Accordingly, it is possible to decrease a possibility that the high-pressure air flows over the sealing portion  10  to flow into the cylindrical inner peripheral surface  61 A. 
     Here, when the sealing portion  10  is provided on the cylindrical inner peripheral surface  61 A differently from the configuration of the embodiment, the high-pressure air reaches an area including the cylindrical inner peripheral surface  61 A. In this case, pressure which is generated by the high-pressure air is exhibited in the impeller  5  through the outer peripheral surface of the cover  9 . Further, since the high-pressure air can flow into the gap between the disk back surface  7 B and the second facing inner peripheral surface  6 B of the casing  6 , a compressing force F 1  which is generated by the high-pressure air is applied to both sides of the impeller  5  in the direction of the axis O. 
     However, in the centrifugal compressor  1  according to the embodiment, as described above, the sealing portion  10  is provided on the facing inner peripheral surface  6 A, that is, the enlarged inner peripheral surface  62 A. Accordingly, it is possible to decrease a size of an area into which the high-pressure air can be flown in an entire area of the gap. That is, it is possible to decrease an area of a region that receives a pressure generated by the high-pressure air in the outer peripheral surface of the cover  9 . Accordingly, it is possible to maintain desired compression efficiency without increasing the structural strength of the impeller  5 . 
     Meanwhile, for example, when a thickness of the blade  8  is increased or the number of the blades  8  per impeller  5  is increases in order to improve the structural strength of the impeller  5 , pressure loss of the working fluid flowing inside the impeller  5  increases. That is, the compression efficiency of the centrifugal compressor  1  is degraded. However, in the centrifugal compressor  1  according to the embodiment, it is possible to obtain the above-described operational effect while maintaining the structural strength of the impeller  5 . Thus, it is possible to sufficiently ensure the compression efficiency of the centrifugal compressor  1 . 
     Further, as described above, when the sealing portion  10  is provided in an area of 90% or less of the diameter dimension D 1  of the disk with reference to the axis O, a space between the outer peripheral surface of the cover  9  and the facing inner peripheral surface  6 A is divided into an outer radial area and an inner radial area with the sealing portion  10  interposed therebetween. The high-pressure fluid flows in the outer radial area and the low-pressure fluid flows in the inner radial area. Accordingly, it is possible to optimize a pressure distribution applied to the outer peripheral surface of the cover  9  by the high-pressure fluid. 
     Incidentally, when the sealing portion  10  is provided on the enlarged inner peripheral surface  62 A as described above, while a pressure applied to the outer peripheral surface of the cover  9  decreases, a high pressure between the second facing inner peripheral surface  6 B and the disk back surface  7 B is maintained. Due to the pressure difference, a thrust force F 2  is generated in the rotation shaft  2  and the impeller  5  in a direction from the other side toward the one side in the direction of the axis O (that is, the direction from the disk  7  toward the cover  9  along the axis O). 
     However, as described above, the geared centrifugal compressor  100  according to the embodiment includes the pair of centrifugal compressors  1  which are plane symmetric and the speed increasing transmission portion  200  (the reference plane CP) is interposed therebetween. Thus, when the same sealing portion  10  is provided in the pair of centrifugal compressors  1 , thrust forces F 2  in the centrifugal compressors  1  are exerted along the axis O in a separation direction. Accordingly, since the thrust forces F 2  cancel each other, a thrust load applied to the bearing device  4  can be suppressed. 
     The first embodiment of the present invention has been described with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the embodiment are not particularly limited to the scope of the present invention unless otherwise specified, and various modifications can be made thereto. 
     For example, in the first embodiment, the geared centrifugal compressor  100  having a so-called single-shaft two-stage configuration has been exemplified. However, the shape of the geared centrifugal compressor  100  is not limited thereto and more shafts and more stages may be provided, such as two shafts and four stages, in accordance with designs or specifications. In any configuration, it is possible to obtain the same operational effect as the description in the above-described embodiment in the centrifugal compressor  1  in each stage. 
     Further, in the above-described embodiment, an example in which the plurality of sealing fins  12  are used as the sealing portion  10  has been described. However, the shape of the sealing portion  10  is not limited thereto and any configuration may be employed as long as a fluid in the flow passage is sealed. 
     In addition, even when the plurality of sealing fins  12  are provided, the sealing fins  12  may be formed to have different shapes and dimensions. Specifically, dimensions of the sealing fin  12  may be formed to increase or decrease from one side toward the other side in the plurality of sealing fins  12 . 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 4 . Further, the same reference numerals will be given to the same components and members as those of the first embodiment, and detailed descriptions thereof will be omitted. 
     As shown in  FIG. 4 , in the centrifugal compressor  1  according to the embodiment, a shape of the sealing portion  10  is different from that of the first embodiment. That is, the sealing portion  10  of the embodiment is formed by a plurality of impeller side fins  121  which are provided on an outer peripheral surface of the cover  9  of the impeller  5 . Each of the impeller side fins  121  is formed in a thin plate shape extending toward the facing inner peripheral surface  6 A (the enlarged inner peripheral surface  62 A) separated from the outer peripheral surface of the cover  9  with a gap interposed therebetween. Here, in the example of  FIG. 4 , the impeller side fin  121  is formed without interposing the base portion  11  described in the first embodiment. Further, desirably, the impeller side fin  121  is integrally formed in the cover  9 . 
     Similarly to the first embodiment, it is desirable to set installation positions of the impeller side fins  121  so that the separation dimension D 2  from the axis O to the impeller side fin  121  in the radial direction is 90% or less of the diameter dimension D 1  of the disk. 
     Even in the above-described configuration, reverse flow of a high-pressure fluid between the enlarged inner peripheral surface  62 A and the outer peripheral surface of the cover  9  can be sufficiently suppressed by the plurality of impeller side fins  121 . Accordingly, it is possible to ensure sufficient compression efficiency without specially compensating the structural strength of the centrifugal compressor  1 . 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 5 . Further, the same reference numerals will be given to the same components and members as those of the first embodiment, and detailed descriptions thereof will be omitted. 
     As shown in  FIG. 5 , in the centrifugal compressor  1  according to the embodiment, a back surface sealing portion  101  is provided between the second facing inner peripheral surface  6 B of an inner peripheral surface of the casing  6  and the disk back surface  7 B in addition to the sealing portion  10  of the first embodiment. Similarly to the configuration shown in  FIG. 3 , the back surface sealing portion  101  includes the base portion  11  which is provided in the second facing inner peripheral surface  6 B and the plurality of sealing fins  12  which are provided in the base portion  11 . 
     As described above, there is a possibility that the high-pressure air flows reversely from the vicinity of the diffuser  6 E into the gap formed between the second facing inner peripheral surface  6 B and the disk back surface  7 B. Particularly, in the geared centrifugal compressor  100 , the accommodation portion  104  of the speed increasing transmission portion  200  communicates with the inside of the casing  6 . In this case, a possibility that the high-pressure air flows reversely toward the accommodation portion  104  having a relatively low pressure further increases. 
     However, when the back surface sealing portion  101  is provided like in the embodiment, it is possible to seal the flow of the high-pressure air which flows reversely. Accordingly, it is possible to reduce a pressure applied from the disk back surface  7 B to the impeller  5  by the high-pressure air. 
     Thus, since it is possible to obtain the same operational effect as those of the above-described embodiments while maintaining the structural strength of the impeller  5 , it is possible to sufficiently ensure the compression efficiency of the compressor. 
     Additionally, in the embodiment, as the back surface sealing portion, a configuration which includes the base portion  11  provided in the second facing inner peripheral surface  6 B and the plurality of sealing fins  12  provided in the base portion  11  has been exemplified. However, the shape of the back surface sealing portion  101  is not limited thereto. For example, as shown in  FIG. 6 , the plurality of sealing fins  12  may be integrally provided on the disk back surface  7 B of the impeller  5  to form the back surface sealing portion  101 . 
     According to such a configuration, the reverse flow of the high-pressure fluid can be sufficiently suppressed by the plurality of sealing fins  12 . Accordingly, it is possible to ensure sufficient compression efficiency without specially compensating the structural strength of the centrifugal compressor  1 . 
     Further, when the fins are integrally formed in the impeller  5  (the disk back surface  7 B) as described above, it is possible to reduce the number of steps and costs necessary for manufacturing the impeller  5  and the casing  6 . 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described with reference to  FIG. 7 . Further, the same reference numerals will be given to the same components and members as those of the first embodiment, and detailed descriptions thereof will be omitted. 
     As shown in  FIG. 7 , in the centrifugal compressor  1  according to the embodiment, a shape of the sealing portion  10  is different from those of the above-described embodiments. That is, in the centrifugal compressor  1  according to the embodiment, an abradable portion  70  is provided on the facing inner peripheral surface  6 A facing the impeller side fin  121  in addition to the sealing portion  10  of the second embodiment, in other words, on the plurality of impeller side fins  121  provided on the outer peripheral surface of the cover  9 . 
     The abradable portion  70  is a material having good machinability obtained by being compacted and molded, for example, a fine aluminum powder. That is, the abradable portion  70  has a property such that the abradable portion is easily cut when brought into contact with other members and does not influence abrasion of the contacted member. 
     According to such a configuration, since the abradable portion  70  having machinability is cut even when a front end of the impeller side fin  121  is brought into contact with the enlarged inner peripheral surface  62 A due to a displacement of the impeller  5  caused by vibration with a rotation, a processing tolerance of the impeller  5 , and a movement of the impeller  5  in the direction of the axis O, a possibility of causing abrasion of the impeller side fin  121  can be reduced. Accordingly, it is possible to maintain a sealing property due to the impeller side fin  121 . 
     Additionally, in the embodiment, an example using a material, which is compacted fine aluminum powder, as the abradable portion  70  has been described. However, the material and the shape of the abradable portion  70  are not limited thereto and any material may be used as long as satisfactory machinability is ensured. That is, the shape is not uniquely limited as long as the shape exhibits hardness lower than a material of the impeller  5  (the cover  9 ) and has machinability. 
     Fifth Embodiment 
     Next, a fifth embodiment of the present invention will be described with reference to  FIG. 8 . Further, the same reference numerals will be given to the same components and members as those of the first embodiment, and detailed descriptions thereof will be omitted. 
     As shown in  FIG. 8 , in the centrifugal compressor  1  according to the embodiment, an extension direction of the sealing fin  12  in the sealing portion  10  is mainly different from those of the above-described embodiments. That is, in the embodiment, the sealing portion  10  is provided inside an accommodation groove  62 B and all sealing fins  12  extend inward in the radial direction of the axis O. 
     Further, in the embodiment, a protrusion portion  9 B is formed at a position corresponding to the sealing portion  10  on the facing cover surface  9 A of the impeller  5 . The protrusion portion  9 B protrudes from the facing cover surface  9 A substantially toward one side in the direction of the axis O as shown in  FIG. 8 . The protrusion portion  9 B has a substantially triangular shape which extends in a normal direction of a bottom side corresponding to a part of the facing cover surface  9 A. 
     Particularly, a sealing surface  9 C which is a surface facing the outside in the radial direction of the axis O among two surfaces forming a cross-section is formed to be substantially parallel to an arrangement direction of the sealing fins  12  in the sealing portion  10 . In other words, the sealing surface  9 C forms a plane which is substantially parallel to the axis O. Further, a part of the protrusion portion  9 B including the sealing surface  9 C extends to the inside of the accommodation groove  62 B. Accordingly, front ends of the plurality of sealing fins  12  face the sealing surface  9 C to seal a gap between the facing cover surface  9 A and the facing inner peripheral surface  6 A. 
     According to the above-described configuration, since the sealing fins  12  of the sealing portion  10  are arranged in the direction of the axis O, it is possible to maintain a separation dimension between the front end of the sealing fin  12  and the sealing surface  9 C facing the front end even when the rotation shaft  2  is displaced in the direction of the axis O. Thus, it is possible to seal a gap between the facing inner peripheral surface  6 A and the facing cover surface  9 A regardless of an operation state of the centrifugal compressor  1 . 
     INDUSTRIAL APPLICABILITY 
     According one or more embodiments of the above-described centrifugal compressor and the geared centrifugal compressor, sufficient durability and compression efficiency can be realized. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
       1  Centrifugal compressor 
       2  Rotation shaft 
       2 S Shaft sealing portion 
       3  Pinion gear 
       4  Bearing device 
       5  Impeller 
       6  Casing 
       7  Disk 
       8  Blade 
       9  Cover 
       10  Sealing portion 
       11  Base portion 
       12  Sealing fin 
       21  Fitting groove 
       51  Impeller intake port 
       52  Impeller discharge port 
       5 F Impeller flow passage 
       6 A Facing inner peripheral surface 
       6 B Second facing inner peripheral surface 
       6 E Diffuser 
       7 A Disk front surface 
       7 B Disk back surface 
       9 A Facing cover surface 
       9 B Protrusion portion 
       9 C Sealing surface 
       61 A Cylindrical inner peripheral surface 
       62 A Enlarged inner peripheral surface 
       62 B Accommodation groove 
       70  Abradable portion 
       71  Disk support portion 
       72  Disk body 
       73  Connection portion 
       80  Intake flow passage 
       80  Intake pipe 
       90  Exhaust flow passage 
       100  Geared centrifugal compressor 
       101  Speed increasing transmission portion 
       200  Back surface sealing portion 
       102  Rotation driving shaft 
       103  Large-diameter gear 
       104  Accommodation portion 
       121  Impeller side fin 
     CP Reference plane 
     D 1  Diameter dimension of disk 
     D 2  Separation dimension of radial direction 
     F 1  Compressing force 
     F 2  Thrust force 
     O Axis