Abstract:
A sealing mechanism includes a pair of space forming rings which face each other and are configured for surrounding a shaft body, an outer bellows-joint having one end connected to an outer periphery of one of the space forming rings and another end connected to an outer periphery of the other of the space forming rings, and an inner bellows-joint having one end connected to an inner periphery of one of the space forming rings and another end connected to an inner periphery of the other of the space forming rings. Each of the inner bellows-joint and the outer bellows-joint is a corrugated plate. The space forming rings space the inner bellows-joint and the outer bellows-joint apart from each other such that the inner bellows-joint and the outer bellows-joint do not come in contact with each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sealing mechanism that is suitably applied to a rotor shaft system of the steam turbine and the like; thereby, the natural frequencies regarding the sealing mechanism can be easily determined; and, the vibration resonance between the mechanism and the rotor shaft system can be easily prevented. The present invention also relates to a steam turbine provided with the sealing mechanism. 
     2. Background of the Invention 
     A steam turbine is provided with an outer turbine casing, and an inner turbine casing inside the outer casing; in the center part of the inner casing, a rotor is rotation freely installed as apart of the rotor shaft. A plurality of rotor blades (a cascade of rotor blades) is fixed to the predetermined positions (locations of the cascades) along the longitudinal direction of the rotor shaft; thereby, the positions are arranged so that the distance between a position and the adjacent position is set in advance. On the other hand, a plurality of stator blades (a cascade of stator blades) is fixed to a stator blade circular-cylinder fixed to the inner casing so that a cascade of stator blades is placed between a cascade of rotor blades and the adjacent cascade of rotor blades. Thus, a cascade rotor blades and a cascade of stator blades are placed in turn in a multi-stage arrangement, so that the cascades of rotor blades and the cascades of stator blades form a steam flow passage; a steam inlet port is provided at the inner casing as well as the outer casing; the steam entering the steam inlet port streams through the steam flow passage so as to rotate the rotor and drive a generator coupled with the rotor shaft. 
     The rotor is rotation-freely supported by a bearing housing that is placed outside of the outer turbine casing regarding the steam turbine; the bearing housing is placed outside of the outer casing and fixed on a foundation made of concrete or the like. A part of the rotor shaft part between the bearing housing and the outer casing is covered with a gland (shaft seal part) provided so as to prevent steam from leaking outside; the gland is supported by the bearing housing via a connecting member; in usual practices, a part of low pressure steam that has been used for rotating the rotor and is discharged from the inside of the inner casing is guided to the gland. 
     Further, in order not to relieve the steam outside from the space around the rotor between the outer turbine casing and the gland, at least one bellows joint of a cylindrical corrugated shape is provided between the outer casing and the gland, so as to surround the rotor shaft; thereby, the bellows joint is able to absorb the relative displacement between the outer casing and the gland, the relative displacement being caused by thermal expansion or working pressure. Incidentally, to make sure, it is noted that the corrugation shape can be seen in a cross-section passing through the axis of the cylindrical shape, but not in a cross-section perpendicular to the axis. 
     JP 2009-235971 (herein referred to as “Patent Reference”) discloses a sealing mechanism configured with the bellows-joint arranged between the outer turbine casing and the gland. The sealing mechanism disclosed by Patent Reference is hereby explained with reference to  FIGS. 4 and 5  attached in this specification. In the sealing mechanism  100 A as shown in  FIG. 4 , a bellows-joint  102  (comprising an inner bellows-joint and an outer bellows-joint) of a double layer structure form a cylindrical corrugated shape so as to surround a space around a rotor shaft (not shown in  FIGS. 4 and 5 ). The bellows-joint  102  are provided so as to connect a ring-shaped end flange  112  to a ring-shaped end flange  114 ; thereby, the ring-shaped end flange  112  is connected to an outer turbine casing (not shown in  FIGS. 4 and 5 ), while the ring-shaped end flange  114  is connected to a gland (not shown in  FIGS. 4 and 5 ); the bellows joint  102  gas-tightly seals the space around the rotor shaft between the outer casing and the gland prevent steam from leaking outside. 
     For instance, a steam turbine installed in a nuclear power plant is provided the double layer type bellows-joint (comprising an inner bellows-joint and an outer bellows-joint), from a safety point of view. 
     The bellows-joint  102  comprises an inner bellows-joint  102   a  and an outer bellows-joint  102   b , both the bellows-joints  102   a  and  102   b  having a flat (cylindrical) part  104 . The inner bellows-joint and the outer bellows-joint are formed or manufactured from two cylindrical tubes; the space inside the two superposed tubes is pressurized from the inside toward the outside so that the outer tube and the inner tube are together pressed onto a forging template. On the outer periphery surface of the flat part regarding the outer bellows-joint, a rib  106  of a ring shape is fastened; on the outer periphery surface of the rib  106 , a plurality of holes is provided so that a plurality of weights  108  is inserted therein in order that the rib  106  and the weights  108  configure a vibration control means. Arranging the number of the weights  108  as well as arranging the mass of each weight enables the natural frequencies of the bellows-joint  102  to be adjusted; thus, the resonance between the rotor shaft system vibration and the bellows vibration can be evaded. 
     Patent Reference discloses another configuration example regarding the bellows-joint as shown in  FIG. 5 ; whereby, a sealing mechanism  100 B is provided with a bellows-joint  102  comprising an inner bellows-joint  102   a  and an outer bellows-joint  102   b , both the bellows  102   a  and  102   b  not having a flat (cylindrical) part, and a bellows-joint  116  being arranged outside of the bellows-joint  102 . In addition, both the left and right ends of the bellows-joint  116  are connected an end flange  112  or  114  via a supporting member  118 . In this disclosure (Patent Reference), providing the bellows-joint  116  enables the natural frequencies of the sealing mechanism  100 B to be adjusted. 
     REFERENCES 
     Patent References 
     Patent Reference: JP 2009-235971 
     SUMMARY OF THE INVENTION 
     The natural frequencies of the bellows-joint provided at the gland depends on the stiffness or the flexibility of the bellows-part; in the sealing mechanisms  100 A and  100 B disclosed by Patent Reference, the gap between the inner bellows-joint  102   a  and the outer bellows-joint  102   b  is so narrow that the two bellows-joints stay in closer touch with each other. Thus, based on the degree of the contact as to the joints at the time when the joints are manufactured, the natural frequencies of the bellows-joints in the double layer structure varies structure by structure. Accordingly, it is difficult to set the natural frequencies as a desirable property. As a result, it becomes difficult to establish the natural frequencies of the sealing mechanism so as to evade the resonance between the sealing mechanism and the steam turbine rotor system. 
     Hence, in conventional practices, vibration tests to confirm the natural frequencies are required for each individual turbine. In some cases, it becomes necessary to re-arrange the natural frequencies or re-manufacture the sealing mechanism (or the bellows-joints); thus, the process of installing the sealing mechanism needs surplus time and labor. 
     In order to overcome the above-described difficulties, in the sealing mechanism  100 A of Patent Reference, the rib  106  and the weight  108  are provided; namely, the rib  106  and the weight  108  form a vibration adjusting means for adjusting the natural frequencies of the sealing mechanism  100 A. Providing this vibration adjusting means needs surplus time and labor. 
     Further, in the sealing mechanism  100 B, the third bellows-joint  116  for adjusting the natural frequencies of the sealing mechanism  100 B is provided outside of the bellows-joint  102  of a double layer structure; in this case, however, it becomes necessary that the integrated part of the bellows-joint  116  and the supporting members  118  be divided into two pieces (semi-cylinder pieces or tunnel-shaped pieces) so that the two pieces cover the bellows-joint  102  from outside; this configuration also requires surplus time and labor. 
     In view of the above-described difficulties in the conventional technology, the present invention aims at providing a sealing mechanism free from surplus time and labor in manufacturing or providing the sealing mechanism; thereby, the natural frequencies regarding the sealing mechanism surrounding a to-be-sealed shaft body are easily handled. More concretely, the present invention aims at providing a sealing mechanism including bellows-joints as well as providing a steam turbine having the sealing mechanism. 
     In order to achieve the objectives, the present invention provides a sealing mechanism comprising a bellows-joint surrounding a to-be-sealed shaft body and connected to a pair of support members arranged around the to-be-sealed shaft body on both ends thereof, the sealing mechanism hermetically sealing an area around the to-be-sealed shaft body while absorbing relative displacement between the pair of support members, wherein the bellows-joint includes an outer bellows-joint and an inner bellows-joint nested in the outer bellows-joint so that the inner and outer bellows-joints are arranged concentrically with respect to the to-be-sealed shaft body, each of the outer and inner bellows-joints is manufactured by corrugating a metal plate, and the outer and inner bellows-joints are arranged so as not to come in contact with each other. 
     As described above, in the sealing mechanism according to the present invention, the outer and inner bellows-joints are arranged so as not to come in contact with each other. Thus, it is not necessary to take the degree of the contact into consideration. Therefore, the natural frequencies of the sealing mechanism can be easily computed, and designing or handling the natural frequencies regarding the sealing mechanism becomes easy and simple. Accordingly, surplus time and labor in adjusting natural frequencies can be dispensed with. Further, vibration tests to confirm the natural frequencies can be also dispensed with; in addition, the expenditure for performing the vibration tests can be remarkably reduced; moreover, surplus labor and expenditure for re-manufacturing the bellows-joint can be dispensed with. 
     The sealing mechanism disclosed by Patent Reference is provided with the special means for adjusting the natural frequencies or vibration properties; on the contrary, according to the present invention, it is unnecessary to provide such a special adjusting means; further, the sealing performance of the present invention is not inferior to that of the Patent Reference. 
     Further, in the sealing mechanism disclosed by Patent Reference as depicted in  FIG. 5 , the outer bellows-joint  116  that is placed outside of the bellows-joint  102  has to be divided into at least two pieces (e.g. two semi-cylinder pieces or two tunnel-shaped pieces) so that the outer bellows-joint  116  surrounds the bellows-joint  102 . On the contrary, according to the present invention, the outer bellows-joint and the inner bellows-joint are arranged so that the inner bellows-joint is placed inside of the outer bellows-joint in a nesting condition; namely, it is unnecessary to divide the outer bellows-joint into two or more pieces. Thus, the sealing mechanism according to the present invention is far easily manufactured than that according to Patent Reference. 
     Preferably, the above-described sealing mechanism further comprises a pair of space forming rings respectively arranged on end surfaces of the support members facing with each other so as to surround the to-be-sealed shaft body, the outer bellows-joint being connected to outer periphery of each of the pair of space forming rings, the inner bellows-joint being connected to inner periphery of each of the pair of space forming rings. 
     In this way, the outer and inner bellows-joints are secured to the support members via the space forming rings. Therefore, it is easy to secure the outer and inner bellows-joints to the support members so that the outer and inner bellows-joints have a gap therebetween. 
     Preferably, the above-described sealing mechanism is applied to a gland of a steam turbine. Specifically, the to-be-sealed shaft body may be a rotor shaft of a steam turbine including an outer turbine casing and a gland, and the pair of support members may be the turbine casing and the gland; and the outer and inner bellows-joints are hermetically provided between the outer turbine casing and the gland so as to hermetically seal an area around the rotor shaft. 
     In this way, the natural frequencies of the sealing mechanism can be adjusted in a simple manner without surplus expenditure in time and labor; and, the resonance between the sealing mechanism and the rotor shaft system can be easily avoided. 
     Preferably, at least one of plate thickness, the number of corrugations, pitch of the corrugations and material of the outer and inner bellows-joints is selected so that natural frequency requirements and environment conditions are satisfied. 
     In this way, the plate thickness, the number of the corrugations, the pitch of the corrugations or the material is selected; thus, the natural frequencies can be further easily determined so as to meet the natural frequencies requirements; and, the enhanced strength or durability of the sealing mechanism can be achieved in response to the environment conditions regarding the bellows-joints. 
     A preferable embodiment according to the present invention is a steam turbine comprising an outer turbine casing, an inner turbine casing housed in the outer turbine casing, a rotor shaft housed in the inner turbine casing, a plurality of rotor blades arranged at predetermined intervals in an axial direction of the rotor shaft, a plurality of stator blades provided inside the inner turbine casing, each of the stator blades being arranged between adjacent two of the rotor blades, a bearing arranged outside the outer turbine casing so as to rotation-freely support the rotor shaft, a gland for preventing steam leak provided in a location at which the rotor shaft penetrates the outer turbine casing, and a sealing mechanism according to the outer turbine casing and the gland of the present invention. 
     Thus, according to the above-described embodiment, applying the sealing mechanism to the rotor shaft of the steam turbine can surely prevent steam from leaking outward from the space around the rotor shaft between the outer turbine casing and the gland, and it is possible to avoid the resonance between the sealing mechanism and the rotor shaft. 
     Based on the sealing mechanism according to the present invention, the sealing mechanism comprises a bellows-joint surrounding a to-be-sealed shaft body and connected to a pair of support members arranged around the to-be-sealed shaft body on both ends thereof, the sealing mechanism hermetically sealing an area around the to-be-sealed shaft body while absorbing relative displacement between the pair of support members, wherein the bellows-joint includes an outer bellows-joint and an inner bellows-joint nested in the outer bellows-joint so that the inner and outer bellows-joints are arranged concentrically with respect to the to-be-sealed shaft body, each of the outer and inner bellows-joints is manufactured by corrugating a metal plate, and the outer and inner bellows-joints are arranged so as not to come in contact with each other. 
     Thus, the natural frequencies of the sealing mechanism can be adjusted in a simple manner without surplus expenditure; and, the resonance between the sealing mechanism and the to-be-sealed shaft body can be evaded. Further, the durability of the sealing mechanism according to the present invention is not inferior to that according to the conventional technology using the bellows-joint of a double layer type; thus, the present invention provides the steal mechanism of a long life. 
     Further, based on the steam turbine provided with the sealing mechanism according to the present invention, the steam turbine comprises, an outer turbine casing, an inner turbine casing housed in the outer turbine casing, a rotor shaft housed in the inner turbine casing, a plurality of rotor blades arranged at predetermined intervals in an axial direction of the rotor shaft, a plurality of stator blades provided inside the inner turbine casing, each of the stator blades being arranged between two adjacent rotor blades, a bearing arranged outside the outer turbine casing so as to rotation-freely support the rotor shaft, a gland for preventing steam leak provided in a location at which the rotor shaft penetrates the outer turbine casing, and a sealing mechanism according to any one of claims  1  to  4  which is arranged between the outer turbine casing and the gland. 
     Thus, a steam turbine according to the present invention is provided with the sealing mechanism according to the present invention as described above; therefore, the steam leakage outward from the space around the rotor shaft between the outer turbine casing and the gland can be surely prevented, and it is possible to avoid the resonance between the sealing mechanism and the rotor shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in greater detail with reference to the preferred embodiments of the invention and the accompanying drawings, wherein: 
         FIG. 1  shows a cross-section regarding a rotor and a rotor shaft of a steam turbine and the neighborhood thereof, a sealing mechanism being applied to the steam turbine, according to a first embodiment of the present invention; 
         FIG. 2  shows a cross-section regarding the sealing mechanism according to the first embodiment; 
         FIG. 3  shows a cross-section regarding the sealing mechanism according to the second embodiment; 
         FIG. 4  shows a cross-section regarding the sealing mechanism according to the conventional technology; 
         FIG. 5  shows a cross-section regarding another sealing mechanism according to the conventional technology. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, the present invention will be described in detail with reference to the embodiments shown in the figures. However, the dimensions, materials, shape, the relative placement and so on of a component described in these embodiments shall not be construed as limiting the scope of the invention thereto, unless especially specific mention is made. 
     (First Embodiment) 
     With reference to  FIGS. 1 and 2 , the detail of the first embodiment of the present invention is now explained; thereby, a sealing mechanism according to a first embodiment is applied to a steam turbine. Firstly, based on  FIG. 1 , the configuration on the steam exhaust side of the low-pressure steam turbine  10  is explained. As shown in  FIG. 1 , an outer turbine casing  12  is provided so as to surround a rotor  20 ; in the outer turbine casing  12 , a flow guide  16  forms an exhaust hood  14  through which low-pressure steam ‘s’ is discharged, the steam ‘s’ being the steam having been used for making the rotor rotate; and, the flow guide  16  is fitted to a support member  18 . 
     As shown in  FIG. 1 , the final stage rotator comprises a rotor  20 , a disc  22  and a plurality of rotor blades  24 ; thereby, the disc  22  and the rotor blades  24  are integrated into one body as a part of the rotor  20 . Just on the upstream side of the rotor blades  24 , a plurality of stator blades (not shown) provided on the final stage stator (not shown) that is fixed to the inner casing (that is not shown, however, is inside of the whole outer turbine casing as usual (cf. another Patent Reference JP3831617 etc.)) is arranged. A rotor shaft  26  as an extended part of the rotor  20  is rotation freely supported by a bearing pad (not shown) provided inside of the bearing housing  28  that is placed outside of the outer turbine casing  12 ; and, the bearing housing  28  is fixed on a foundation  30  made of concrete or the like. 
     A gland (shaft seal part)  32  is arranged between the outer turbine casing  12  and the bearing housing  28  so as to prevent steam from leaking through the gap between the rotor shaft  26  and the outer turbine casing  12 . In usual practices, a part of low pressure steam that has been used for rotating the rotor and streams in the exhaust hood  14  is guided to the gland  32  in order to shut the steam leakage through the gap; incidentally, the steam guided to the gland  32  is called sealing steam. A casing  34  that covers the gland  32  is connected to the bearing housing  28  via a connecting member  36 . Inside of the gland  32 , a labyrinth seal  38  is provided so as to surround the rotor shaft  26  and form a labyrinth gap passage to evade steam leakage through the gap. 
     A shield wall  19  is connected to the support member  18  of the outer turbine casing  12 ; in order to close a space around the rotor shaft between the shield wall  19  and the casing  34  of the gland  32 , as well as, in order to prevent steam from escaping outside of the space, a sealing mechanism  40 A is provided according to the first embodiment. The configuration of the sealing mechanism  40 A is hereafter explained with reference to  FIG. 2 . 
     In  FIG. 2 , onto the surface of the shield wall  19  on the side of the gland  32 , an end flange  42  is gas-tightly fixed by means of welding or bolt-fastening; and, onto the surface of the casing  34  on the side of the shield wall  19 , an end flange  44  is gas-tightly fixed by means of welding or bolt-fastening. As shown in  FIG. 2 , the end flanges have a rectangular cross-section. 
     An outer bellows-joint  46  and an inner bellows-joint  48  are inserted between the end flanges  42  and  44  and attached thereto, by means of welding or bolt fastening; each bellows-joint is manufactured by corrugating a metal plate (a to-be-corrugated membrane) formed in a cylindrical shape, the metal plate being made of a single kind of metal material. Each of the left end and the right end of the outer bellows-joint  46  is gas-tightly welded to the outer edge of the end flange  42  or  44 ; each of the left end and the right end of the inner bellows-joint  48  is gas-tightly welded to the inner edge of the end flange  42  or  44 . Further, in order to evade the contact between the outer bellows-joint  46  and the inner bellows-joint  48 , a distance (a gap) C 1  in the radial direction between the innermost radius of the outer bellows-joint  46  and the outermost radius of the inner bellows-joint  48  is provided. For instance, the distance C 1  is not shorter than 0.1 mm so that the both the joints do not come in contact with each other even when the sealing mechanism  40 A is vibrated. 
     According to the first embodiment, an appropriate distance is put between the outer bellows-joint  46  and the inner bellows-joint  48  so that the contact between both the joints can be always evaded; while the conventional manner accompanies the difficulty that the natural frequencies of the sealing mechanism vary case by case, the present invention can overcome the difficulty. Thus, it becomes easy to design the natural frequencies of the sealing mechanism  40 A under the condition that the sealing mechanism  40 A is installed in the steam turbine and the turbine is operated. In other words, the natural frequencies of the sealing mechanism  40 A can be easily established in advance so that the natural frequencies are apart from those of the rotor shaft  26 . Further, different from the sealing mechanism disclosed by Patent Reference, no special arrangement means for arranging the natural frequencies of the sealing mechanism, in this embodiment. Accordingly, this embodiment improves cost effectiveness. 
     Further, each of the outer bellows-joint  46  and the inner bellows-joint  48  is gas-tightly connected to the end flanges  42  and  44 ; thus, the sealing effect according to this embodiment is equal to the sealing effect according to the conventional sealing mechanism where the bellows-joint of a double layer type is applied. 
     Further, according to the present invention, the outer bellows-joint  46  is connected to the outer edge of the end flange  42  or  44 ; and, the inner bellows-joint  48  is connected to the inner edge of the end flange  42  or  44 ; thus, both the bellows-joints  46  and  48  are fastened to the shield wall  19  and the casing  34  via either of the end flanges  42  and  44 . Accordingly, the bellows-joints  46  and  48  are easily positioned without mutual contact. 
     As described thus far, the design of the natural frequencies regarding the sealing mechanism  40 A is easily performed with accuracy; thus, the process of re-manufacturing the sealing mechanism  40 A becomes unnecessary; further, the vibration tests to confirm that the resonance between the bellows-joint vibrations and the rotor shaft vibrations is evaded at a time point after the sealing mechanism  40 A and rotor shaft  26  are assembled. 
     (Second Embodiment) 
     In the next place, a sealing mechanism according to a second embodiment of the present invention is now explained with reference to  FIG. 3 . In the first embodiment, as for the corrugation of bellows of both the outer bellows-joint  46  and the inner bellows-joint  48 , the thickness (of the membrane), the number of corrugations, the amplitude of corrugation, the pitch of corrugation and the metal material of the membrane are assumed to be fixed. In a sealing mechanism  40 B according to the second embodiment, it is assumed that the thickness, the corrugation number, the corrugation amplitude, the corrugation pitch and the membrane material may be different between an outer bellows-joint  50  and the inner bellows-joint  48 . Apart from this point, the configuration (including the fitting approaches regarding the bellows-joints) of this second embodiment is the same as that of the first embodiment. This second embodiment is applied to the rotor shaft  26  of the low-pressure steam turbine  10 . 
     In more detail, the amplitude H 1  and the pitch P 1  regarding the outer bellows-joint  50  are determined so that the relations P 2 &lt;P 1  and H 1 &lt;H 2  are hold where P 2  and H 2  are the pitch and the amplitude regarding the outer bellows-joint  48 . Incidentally, as just described, the amplitude regarding the outer bellows-joint  50  is smaller than the amplitude regarding the inner bellows-joint  48 . In addition, a distance (a gap) C 2  in the radial direction between the innermost radius of the outer bellows-joint  50  and the outermost radius of the inner bellows-joint  48  is provided; thereby, the distance C 2  is taken so as to be longer than the distance C 1  in the first embodiment. Further, the stainless steel (JIS SUS310 or SUS410) that has fine characteristics in heat resistance property is used as the membrane material regarding the inner bellows-joint  48  that is exposed to elevated temperatures; and, the chromium steel (JIS SCr) that has fine characteristics in anti-corrosion property is used as the membrane material regarding the outer bellows-joint  50 . 
     As described above, according to the second embodiment, the specifications of the outer bellows-joint  50  are prescribed, differently from those of the inner bellows-joint  48 , in response to the specific requirement or the environment factor regarding each specification item by item; in this way, the design of natural frequencies can be further easily performed; in addition, the life of the sealing mechanism can be prolonged. Naturally, in this second embodiment, the membrane thickness of the outer bellows-joint  50  may be varied from that of the inner bellows-joint  48 . 
     Industrial Applicability 
     According to the present invention, a sealing mechanism can be realized whereby the design regarding the natural frequencies of the sealing mechanism can be easily performed; and, the vibration resonance between the sealing mechanism and the to-be sealed rotor shaft can be surely evaded. The sealing mechanism is suitably applied to a steam turbine so as to seal the area around the gland of the rotor shaft in the steam turbine.