Abstract:
It is intended to provide a rotary machine provided with an atmosphere relief mechanism by which damage of a rupture disc when attaching the disc can be prevented and which can withstand use over a long period of time. The rotary machine is provided with a casing and an atmosphere relief mechanism  20  for closing an opening  12  for atmospheric release formed in the casing and relieving pressure of inner fluid to atmosphere when a pressure in the casing rises. The atmosphere relief mechanism  20  includes: a rupture disc  24  configured to rupture when the pressure in the casing reaches a predetermined pressure; a pair of annular holding parts  22, 27  arranged to hold an outer edge of the rupture disc  24  from both sides; a plurality of fastening members  29  for fastening the pair of annular holding parts  22, 27  to each other so as to hold the rupture disc  24  between the pair of annular holding parts  22, 27;  and an annular spacer part  25  provided along an outer circumference of the rupture disc  24.

Description:
TECHNICAL FIELD 
       [0001]    A present disclosure relates to a rotary machine provided with an atmosphere relief mechanism for preventing abnormal pressure rise in an interior space of a casing and also relates to a mounting method of the atmosphere relief mechanism. 
       BACKGROUND ART 
       [0002]    A rotary machine such as a steam turbine and a gas turbine includes a casing for housing a rotor so that working fluid (inner fluid) is hermetically sealed in an interior space of the casing. During normal operation of the rotary machine, normally there is pressure difference between the interior space of the casing and an atmospheric side. For instance, some low-pressure casings of the steam turbine are configured so that a rotor being rotated upon receiving steam is covered by an inner casing and the rotor and the inner casing are covered by an outer casing. In this type of configuration, the casing forms an outer shell of the outer casing and the interior space of the casing functions as an exhaust chamber. The exhaust chamber is provided to introduce the steam (exhaust gas) having driven the rotor to a condenser and is maintained at negative pressure during normal operation of the steam turbine. 
         [0003]    However, if the steam leaks from a piping or there is pump failure in a negative-pressure system arranged from the outer casing to the condenser, etc., the pressure in the interior space of the outer casing can increase abnormally. If this pressure exceeds the atmospheric pressure, this causes abnormality in operation of the steam turbine and the condenser due to temperature rise of the steam and change in the pressure direction from external pressure to internal pressure, thereby making it inevitable to stop the operation. To avoid this, an atmosphere relief mechanism is provided to discharge the steam in the outer casing to the atmosphere when the exhaust pressure exceeds the atmospheric pressure. 
         [0004]    Generally, the atmosphere relief mechanism used in the rotary machine is provided with a rupture disc configured to rupture when the pressure in the interior space of the casing exceeds a predetermined pressure. The rupture disc is configured to rupture when the pressure rises abnormally. Once the rupture disc ruptures, the interior space of the casing communicates with the atmosphere side to relieve the pressure of the interior space. Normally, the rupture disc is fixed by a holder to an opening for relieving to the atmosphere, which is formed in the casing. During a normal operation of the rotary machine, the opening is closed by the rupture disc to isolate the interior space from the atmosphere side. 
         [0005]    As a related technique, disclosed in Patent Literature 1 is a mounting device for a floating disc for atmosphere relief provided in the steam turbine. This mounting device is configured to place the atmosphere relief disc made of a lead sheet at the opening of the casing and then fasten outer edge of the atmosphere relief disc to the casing by a holding disc and a bolt. Inside the atmosphere relief disc, the floating disc is provided so that, when the pressure inside the casing increases, the floating disc moves toward the atmosphere side and then causes shear failure of the lead sheet at the outer edge of the floating disc. 
         [0006]    Further, in Patent Literature 2, as a rupture disc used in an exhaust casing of the steam turbine, metal plates made of stainless steel, lead, nickel aluminum, etc. are described. 
         [0007]    In the case of using the lead plate as the atmosphere relief disc as described in Patent Literature 1 and Patent Literature 2, lead has low elastic coefficient and thus the lead plate is deformed even by a pressure change that does not reach the pressure at which the lead plate ruptures, and repeating of this deformation can cause a bolt hole provided in the lead plate for fastening to stretch. Thus, sealability of the atmosphere relief mechanism declines and the air may enter the interior space of the casing which is maintained at negative pressure during the normal operation. In order to prevent this, the lead plate must be replaced frequently. Moreover, the material of the atmosphere relief plate, lead, is known to be harmful to humans and thus there is a trend to restrict use of this material. 
         [0008]    In view of this, an atmosphere relief disc having stacked layers was proposed. The atmosphere relief disc is formed by stacking thin plates made of material such as SUS and fluorine resin. 
         [0009]    For instance, described in Patent Literature 3 is to arrange a rupture disc in an exhaust gas duct for leading exhaust gas from a gas turbine to a boiler furnace. This rupture disc is manufactured by stacking thin plates made of material such as graphite, SUS 316 and Teflon® and is supported between a pair of flange portions formed in the duct. 
       CITATION LIST 
     Patent Literature 
       [0010]    [PTL 1] 
         [0011]    Japanese unexamined utility model application publication No. 62-81771 
         [0012]    [PTL 2] 
         [0013]    Japanese unexamined utility model application publication No. 2-126001 
         [0014]    [PTL 3] 
         [0015]    Japanese unexamined patent application publication No. 8-226308 
       SUMMARY 
     Technical Problem 
       [0016]    The rupture disc like the one described in Patent Literature 3 is formed by stacking thin plates made of material such as SUS and Teflon and thus, it is not preferable to form a bolt hole in the rupture disc from the perspective of maintaining the sealability around the bolt hole. Therefore, as described in Patent Literature 3, normally the rupture disc is not formed with a bolt hole and instead, held between flanges or the like to be fixed. 
         [0017]    However, in the case where this rupture disc is used for the casing of the rotary machine, if the rupture disc is damaged unintentionally, the rupture disc may not be able to fully satisfy its function to communicate the interior space of the casing with the atmosphere by rupturing at a set pressure (a rupture pressure). Particularly in the case where a slit is formed in the rupture disc so that the rupture disc ruptures at the set pressure, the slit may even tear by repeatedly starting and stopping the rotary machine. 
         [0018]    In view of the above issues, it is an object of the present invention to provide a rotary machine provided with an atmosphere relief mechanism capable of preventing unintended damage of a rupture disc as well as a method for installing the atmosphere relief mechanism of the rotary machine. 
       Solution to Problem 
       [0019]    Inventors of the present invention have studied mechanism of how unintended damage of the rupture disc occurs and found out that mounting structure of the rupture disc to the rotary machine was a cause of the unintended damage of the rupture disc. 
         [0020]      FIG. 7  is a drawing showing an example of a mounting structure of the rupture disc to the rotary machine. The atmosphere relief mechanism illustrated in  FIG. 7  is formed by an attachment seat  51  provided on the casing of the rotary machine, a sheet gasket  52 , a grid panel  53 , a sheet gasket  54 , a rupture disc  55 , a sheet gasket  56 , a holding panel  57 , and a cover  58  arranged in this order. Further, these parts are fastened together by inserting bolts  59  into bolt holes formed in the cover  58 , the holding panel  57 , the grid panel  53 , the sheet gasket  54  and the attachment sheet  51  and hence the rupture disc  55  is held between the holding panel  57  and the grid panel  53 . 
         [0021]    According to knowledge of the inventors, the main cause of the unintended damage of the rupture disc  55  is the gap  60  formed on an outer circumference of the rupture disc  55 . This gap  60  is formed by holding the outer circumferential edge of the rupture disc  55  between the holding panel  57  and the grid panel  53 , based on the design idea that it is no longer necessary to form fastening holes in the rupture disc  55  and also that it is made easy to attain sealability of the rupture disc  55 . When the holding panel  57  and the grid panel  53  are fastened together by the bolts  58  with the gap  60  existing therebetween, there is no surface on the outer circumferential side of the bolts  58  to receive load of the holding panel  57 . Thus, the outer circumferential edge of the holding panel  57  deforms slightly toward the grid panel  53 . As a result, a center part of the holding panel  57  bulges out toward the atmosphere side and accordingly the rupture disc  55  warps (a center part of the rupture disc  55  bulges out toward the atmosphere side). Therefore, the rupture disc  55  becomes damaged before the pressure in the casing reaches the set pressure (a rupture pressure). Particularly, in the case where a slit is formed in the rupture disc  55 , the slit is subjected to high stress due to warping of the rupture disc  55  caused by presence of the gap  60 . This may even cause tearing of the slit when attaching the rupture disc  55 . 
         [0022]    Therefore, a rotary machine according to the present invention comprises
       a casing; and   an atmosphere relief mechanism for closing an opening for atmosphere relief formed in the casing and relieving pressure of inner fluid to atmosphere when a pressure in the casing rises.       
 
         [0025]    And the atmosphere relief mechanism comprises:
       a rupture disc configured to rupture when the pressure in the casing reaches a predetermined pressure;   a pair of annular holding parts arranged to hold an outer edge of the rupture disc from both sides;   a plurality of fastening members for fastening the pair of annular holding parts to each other so as to hold the rupture disc between the pair of annular holding parts; and   an annular spacer part provided along an outer circumference of the rupture disc.       
 
         [0030]    According to the above rotary machine, the rupture disc held by the pair of annular holding parts is arranged inside the area where the fastening members are arranged and the annular spacer part is formed along the outer circumference of the rupture disc. As a result, the force applied by the fastening force of the fastening members to the casing side from the annular holding part located on the atmosphere side can be received almost evenly by the rupture disc and the annular spacer part. Therefore, it is possible to prevent deformation of the annular holding part located on the atmosphere side during fastening and also to prevent deformation of the rupture disc. As a result, unintended damage of the rupture disc can be prevented. 
         [0031]    In the above rotary machine, a liquid gasket may be provided between the rupture disc and a first holding part of the pair of annular holding parts that is disposed on a side of an interior space of the casing. 
         [0032]    In the case of providing the gasket between the rupture disc and the first holding part of the pair of annular holding parts, e.g. a grid panel, a gap corresponding to the thickness of the gasket is formed between the rupture disc and the first holding part. Therefore, if the pressure in the casing changes repeatedly in association with start and stop of the rotary machine, the rupture disc is repeatedly displaced by the amount corresponding to the thickness of the gasket (the amount of the gap between the rupture disc and the first holding part). This can enhance unintended damage of the rupture disc. 
         [0033]    In view of this, a liquid gasket which is significantly thinner than a regular sheet gasket is provided as the gasket to be provided between the rupture disc and the first holding part. As a result, it is possible to suppress repeated displacement of the rupture disc in association with start and stop of the rotary machine while maintaining high sealability between the rupture disc and the first holding part. This makes it possible to prevent unintended damage of the rupture disc more effectively. 
         [0034]    In the above rotary machine, the annular spacer part may be a plate-like liner formed separately from the pair of annular holding parts. 
         [0035]    As the annular spacer part is constituted of the plate-like liner formed separately from the pair of annular holding parts, it is made easier to adjust the thickness of the liner corresponding to the thickness of the rupture disc. As a result, it is possible to prevent unintended damage of the rupture disc effectively. 
         [0036]    Further, the liner may have elastic coefficient approximately the same as or higher than the rupture disc. 
         [0037]    In the case where the liner has elastic coefficient lower than the rupture disc, the liner deforms more than the rupture disc when fastening by the fastening members and this may result in slight deformation of the second holding part. In view of this, the liner is configured to have elastic coefficient approximately the same as or higher than the rupture disc so as to firmly support the outer circumferential edge of the second holding part. As a result, it is possible to prevent deformation of the second holding part. 
         [0038]    In the above rotary machine, the annular spacer part may be a stepped portion formed by an outer rim of one of the pair of annular holding parts protruding toward the other of the pair of annular holding parts. 
         [0039]    As the annular spacer part is constituted of the stepped portion, it is possible to prevent unintended damage of the rupture disc. Further, the annular spacer part (the stepped portion) can be formed integrally with the one of the pair of annular holding parts and thus, the number of parts can be reduced. 
         [0040]    The present invention provides a method for attaching to a rotary machine an atmosphere relief mechanism which comprises a rupture disc configured to close an opening formed in a casing of the rotary machine and to rupture when a pressure in the casing reaches a predetermined pressure, and a pair of annular holding parts for holding an outer edge of the rupture disc from both sides. The method comprises the steps of:
       placing the rupture disc and a liner on a first holding part of the pair of annular holding parts that is disposed on a side of an interior space of the casing;   placing on the rupture disc and the liner, a second holding part of the pair of annular holding parts that is disposed on an atmosphere side; and   fastening the pair of annular holding parts to each other by a plurality of fastening members,   wherein in the step of placing the rupture disc and the liner, the liner is placed along an outer circumference of the rupture disc.       
 
         [0045]    According to the above method for attaching the atmosphere relief mechanism, the liner is arranged along the outer circumference of the rupture disc and thus, a force applied by a fastening force of the fastening members to the casing side from the second holding part can be received almost evenly by the rupture disc and the liner. Therefore, it is possible to prevent deformation of the second holding part when fastening by the fastening member and also to prevent deformation of the rupture disc. As a result, unintended damage of the rupture disc can be prevented. 
       Advantageous Effects 
       [0046]    According to the present invention, it is possible, by providing the annular spacer part (e.g. the liner, the stepped portion or the like) along the outer circumference of the rupture disc, to suppress warping of the rupture disc caused by deformation of the annular holding part disposed on the atmosphere side during fastening by means of the fastening members and also to prevent unintended damage of the rupture disc due to repeated operation of the rotary machine. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0047]    [ FIG. 1 ]  FIG. 1  is a cross-sectional view of a structural example of a steam turbine according to an embodiment of the present invention. 
           [0048]    [ FIG. 2 ]  FIG. 2  is a cross-sectional view of a structural example of an atmosphere relief mechanism according to the embodiment of the present invention. 
           [0049]    [ FIG. 3 ]  FIG. 3  is an enlarged view of portion B of the atmosphere relief mechanism of  FIG. 2 . 
           [0050]    [ FIG. 4 ]  FIG. 4  is a plain view of members constituting respective layers of the atmosphere relief mechanism which are illustrated in the order of the layers. 
           [0051]    [ FIG. 5 ]  FIG. 5  is a flow chart of an example of a mounting method of the atmosphere relief mechanism according to the embodiment of the present invention. 
           [0052]    [ FIG. 6 ]  FIG. 6  is a cross-sectional view of a modified example of the atmosphere relief mechanism according to the embodiment of the present invention. 
           [0053]    [ FIG. 7 ]  FIG. 7  is an explanatory drawing of a cause of rupture of a rupture disc of the atmosphere relief mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]    An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, relative positions and the like of components in this embodiment shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
         [0055]    As one example of a rotary machine according to this embodiment of the present invention, a steam turbine provided with an atmosphere relief mechanism is described. However, the rotary machine according to the embodiment of the present invention is not limited to the steam turbine. For instance, it includes other types of rotary machine provided with the atmosphere relief mechanism such as a gas turbine and a compressor. 
         [0056]      FIG. 1  is a cross-sectional view of a structural example of the steam turbine according to the embodiment of the present invention. 
         [0057]    The steam turbine  1  of  FIG. 1  is configured such that an inner casing  3  is arranged in an outer casing  2 . The steam turbine  1  is a double flow steam turbine in which steam enters a steam inlet flow path  9  located near a center of the casing and then flows in right-hand and left-hand directions. The steam turbine  1  may be a low pressure turbine into which the steam flows after having performed work in a high pressure turbine or an intermediate pressure turbine. 
         [0058]    The steam turbine  1  has a rotor  4  which is surrounded by the outer casing  2  and the inner casing  3 . The rotor  4  is rotatably supported by a rotor bearing outside the outer casing  2  and multiple stages of the blade rows  5  are provided symmetrically along the axial direction O. These blade rows  5  are covered by the inner casing  3 . The inner casing  3  forms extraction rooms  6   a,    6   b,    6   c  that are arranged outside the blade rows  5  in the radial direction. These extraction rooms  6   a,    6   b,    6   c  are configured to extract steam of prescribed pressure from each row of the multiple stages of blade rows  5  and lead it to outside. The rotor  4  and the inner casing  3  are covered by the outer casing  2 . For supplying the steam from the inner casing  3  to the blade rows  5 , a steam supply tube  7  and the steam inlet flow path  9  formed by a partition wall  8  are provided. In an interior space of the casing  10  forming an outer shell of the outer casing  2 , an exhaust chamber  11  where the steam having passed through the multiple stages of the blade rows  5  (exhaust gas) is exhausted. The exhaust chamber  11  communicates with a condenser (not shown) and is maintained vacuum during the normal operation. 
         [0059]    In abnormal situations such as when the steam leaks from a piping or there is pump failure in a negative-pressure system arranged from the exhaust chamber  11  to the condenser, etc., the pressure in the exhaust chamber  11  can increase abnormally. Therefore, an atmosphere relief mechanism  20  is provided in the casing  10  so as to relieve the steam inside the exhaust chamber  11  to the atmosphere at the time of the abnormal pressure rise in the exhaust chamber  11 . The atmosphere relief mechanism  20  is arranged to cover an opening  12  of the casing  10  formed in the casing  10 . For instance, the atmosphere relief mechanism  20  may be provided at four places on a top surface of the casing  10 . 
         [0060]    In reference to  FIG. 2  through  FIG. 4 , a detailed structure of the atmosphere relief mechanism  20  is explained  FIG. 2  is a cross-sectional view of a structural example of the atmosphere relief mechanism according to the embodiment of the present invention.  FIG. 3  is an enlarged view of portion A of the atmosphere relief mechanism of  FIG. 2 .  FIG. 4  is a plain view of members constituting respective layers of the atmosphere relief mechanism that are illustrated in the order of the layers. 
         [0061]    As illustrated in  FIG. 2  and  FIG. 3 , the atmosphere relief mechanism  20  has a layered structure having a plurality of members stacked together. The atmosphere relief mechanism  20  is arranged on a pedestal  13  which is provided on the casing  10  to form an opening  12  for atmosphere relief. The atmosphere relief mechanism  20  may be formed by a gasket  21 , a grid panel  22 , a gasket  23 , a rupture disc  24 , a liner  25 , a gasket  26 , a holding panel  27  and a cover  28  which are arranged in this order from the exhaust chamber  11  side toward the atmosphere. Further, the layers of the atmosphere relief mechanism  20  are fastened to each other by means of fastening members constituted by bolts  29  and fixed to the casing  10 . 
         [0062]    Each layer of the atmosphere relief mechanism is explained in details in reference to  FIG. 4 . 
         [0063]    The pedestal  13  projects from the casing  10  toward the atmosphere side and its projecting portion has a flat annular top surface. The atmosphere relief mechanism  20  is placed on the top surface of the projecting portion of the pedestal  13  (the flat annular surface). In the pedestal  13 , a plurality of bolt holes  13   a  are provided along an outer circumference of the opening  12 . The bolts  29  (see  FIG. 2 ,  FIG. 3 ) are screwed into the bolt holes  13   a.  Further, the pedestal  13  may have a rib  13   b  provided to traverse the opening  12 . 
         [0064]    The gasket  21  is formed into a circular shape and is used for the purpose of improving the sealability by interposing between the pedestal  13  and the grid panel  22 . As the gasket  21 , a sheet gasket is preferably used. For instance, a flexible gasket, a metal gasket, a semi-metal gasket, or the like are used. In this gasket  21  as well, bolt holes  21   a  are formed corresponding to the bolt holes  13   a  of the pedestal  13 . Further, this gasket  21  may be omitted. 
         [0065]    The grid panel  22  includes an annular holding part  22   a  constituting a first holding part, a grid part  22   b  provided in an opening of the annular holding part  22   a , and a plurality of bolt holes  22   c  formed corresponding to the bolt holes  13   a  of the pedestal  13 . This grid panel  22  is provided so that the rupture disc  24  is prevented by the grid part  22   b  from bending toward the exhaust chamber  11  side when the inside of the exhaust chamber  11  is evacuated. 
         [0066]    The gasket  23  is formed into a circular shape and is provided between the grid panel  22  and the rupture disc  24  and the liner  25  to improve the sealability. Further, the gasket  23  is provided for the purpose of forming a precise flat surface for the rupture disc  24  and the liner  25  to be placed on. The gasket  23  may have the same configuration as the gasket  21 . However, from the perspective of reducing the gap between the grid panel  22  and the holding panel  27 , a liquid gasket which can be formed thinner than the sheet gasket is preferably used. As the liquid gasket, a liquid gasket of silicone type may be used, for instance. Further, the liquid gasket may be provided only on an inner circumferential side of the bolts  29  or on both the inner and outer circumferential sides of the bolts  29 . 
         [0067]    The rupture disc  24  is formed into a circular shape and has a plurality of slits  24   a.  The rupture disc  24  is configured to rupture when the pressure reaches a prescribed rupture pressure. For instance, the rupture disc  24  may be configured by interposing a fluorine contained resin sheet securing the sealability between stainless thin panels formed with slits. In the case of application to the low pressure steam turbine  1  as described in this embodiment, the rupture disc  24  configured to rupture at pressure slightly above the atmospheric pressure may be used. This rupture disc  24  contacts the annular holding part  22   a  of the grid panel  22  via the gasket  23  at an outer circumferential edge on the exhaust chamber  11  side and contacts an outer circumferential edge of the holding panel  27  via the gasket  26  at an outer circumferential edge on the atmosphere side. Further, the rupture disc  24  is held between the grid panel  22  and the holding panel  27  by means of the bolts  29  to be fixed to the casing  10 . 
         [0068]    The bolt holes formed for the bolts  29  are not formed in the rupture disc  24  itself and the bolts  29  are arranged on an outer circumferential side of the rupture disc  24  to avoid the rupture disc  24 . In an area  30  (a fastening member arrangement area) where the bolts  29  are arranged is an annular-shaped area between the rupture disc  24  and the liner  25  which is described next. More specifically, in the fastening member arrangement area  30 , neither the rupture disc  24  nor the liner  25  is provided and a space is formed for the bolts  29  to pass through. Further, the bolt holes of the members of the layers other than the rupture disc  24  and the liner  25  ( 13   a,    21   a,    22   c,    26   a,    27   c ,  28   c ) are respectively provided corresponding to the fastening member arrangement area  30  so that the bolts  29  passing through the fastening member arrangement area  30  can be screwed into these bolt holes. 
         [0069]    The liner  25  constitutes an annular spacer part and is arranged on an outer circumferential side of the fastening member arrangement area  30  to surround the outer circumference of the rupture disc  24 . Further, the liner  25  has thickness which corresponds to thickness of the rupture disc  24 . More specifically, the liner  25  may be approximately as thick as the rupture disc  24  or slightly thinner than the rupture disc  24  to make it easier to transmit a fastening force by the bolts  29  to the rupture disc  24  side. 
         [0070]    Furthermore, the liner  25  may be made of material having elastic coefficient approximately the same as or higher than the rupture disc  24 , preferably metal material. For instance, in the case that SUS316 is used as the main material of the rupture disc  24 , SS400 can be used for the liner  25 . In this manner, the liner  25  has elastic coefficient approximately the same as or higher than the rupture disc  24  and thus the outer circumferential edge of the holding panel  27  can be securely held toward the casing  11  side by the liner  25 , thereby preventing deformation of the holding panel  27 . 
         [0071]    The gasket  26  is formed into a circular shape and is provided between the rupture disc  24  and the holding panel  27 . The gasket  26  is used for the purpose of improving the sealability between the rupture disc  24  and the holding panel  27 . The gasket  26  has almost the same configuration as the gasket  21  and has a plurality of bolt holes  26   a  at positions corresponding to the bolt holes of the members of other layers. Further, this gasket  26  may be omitted 
         [0072]    The holding panel  27  includes an annular holding part  27   a  constituting a second holding part, a lid part  27   b  for covering an atmosphere-side surface of the annular holding part, and a plurality of bolt holes  27   c  formed at positions corresponding to the bolt holes of the members of other layers. This holding panel  27  is configured to form a space  27   d  by the annular holding part  27   a  and the lid part  27   b  on a side facing the rupture disc  24  (see  FIG. 3 ). When the pressure in the exhaust chamber  11  reaches the prescribed pressure (e.g. during abnormal temperature rise), the rupture disc  24  bulges out in the space  27   d  and hence ruptures. The lid part  27   b  is provided for the purpose of preventing the rupture disc  24  from being broken by a fallen object from outside, or the like. However, the lid part  27   b  may be omitted. 
         [0073]    The holding panel  27  itself may be omitted. In this case, the second holding part is constituted by a flange part  28   a  of the cover  28  described later. The cover  28  includes the flange part  28   a,  a frame  28   b  provided to traverse the opening of the flange part  28   a,  and a plurality of bolt holes  28   c  formed in the flange part  28   a  at positions corresponding to the bolt holes of the members of other layers. 
         [0074]    As illustrated in  FIG. 2  and  FIG. 3 , the bolts  29  constituting the fastening member are screwed into the holes  13   a  of the pedestal  13  through respective bolt holes of the holding panel  27 , the gasket  26 , the grid panel  22  and the gasket  21 . By the fastening force of the bolts  29 , the above-described components are fixed to the casing  10  side and also the rupture disc  24  and the liner  25  are held between the grid panel  22  and the holding panel  27  so as to be fixed to the casing  10  side. 
         [0075]    In this process, the force applied by the fastening force of the bolts from the holding panel  27  to the casing  10  side can be received almost evenly by the rupture disc  24  on the inner circumferential side of the bolts  29  and by the liner  25  on the outer circumferential side of the bolts  29 . Thus, it is possible to prevent deformation of the holding panel  27  and deformation of the rupture disc  24  when being fastened. Therefore, in the atmosphere relief mechanism  20  of this embodiment, it is possible to prevent unintended damage of the rupture disc  24 . 
         [0076]    Next, in reference to  FIG. 5 , a method for attaching the atmosphere relief mechanism  20  is explained  FIG. 5  is a flow chart of an example of a mounting method of the atmosphere relief mechanism according to the embodiment of the present invention. 
         [0077]    The thickness of the liner  25  is adjusted corresponding to the thickness of the rupture disc  24  (S 1 ). More specifically, the thickness of the liner  25  is adjusted to the same thickness or slightly smaller than the thickness of the rupture disc  24 . Then, the gasket  21  and the grid panel  22  are placed on the pedestal  13  provided on the casing  10  (S 2 ). On this grid panel  22 , the liquid gasket is applied as the gasket  23  (S 3 ). On this liquid gasket, the rupture disc  24  and the liner  25  whose thickness has been adjusted are placed (S 4 ). Then, on the rupture disc  24  and the liner  25 , the gasket  26  and the holding panel  27  are placed (S 5 ), and the cover  28  is placed on this holding panel  27  (S 6 ). Further, when stacking these members, positions of respective bolt holes are matched. Finally, the bolts  29  are inserted in the bolt holes and these members are fastened to the casing  10  (S 7 ). 
         [0078]    As described above, according to this embodiment, the annular spacer part (the liner  25 ) having the thickness corresponding to that of the rupture disc  24  is provided outside the fastening member arrangement area  30 . As a result, it is possible to suppress warping of the rupture disc  24  caused in association with deformation of the holding panel  27  during fastening by means of the bolts  29  and thus it is possible to prevent unintended damage of the rupture disc  24 . 
         [0079]    By using the thin liquid gasket as the gasket  23  to be interposed between the rupture disc  24  and the grid panel  22 , it is possible to suppress repeated displacement of the rupture disc  24  in association with start and stop of the steam turbine while maintaining high sealability between the rupture disc  24  and the grid panel  22 . As a result, it is possible to prevent unintended damage of the rupture disc  24  more effectively. 
         [0080]    Further, in the above embodiment, the liner  25  is used as the annular spacer. However, this is not limitative and instead of the liner  25 , a stepped portion  22   d  may be used as the annular spacer as illustrated in  FIG. 6 .  FIG. 6  is a cross-sectional view of a modified example of the atmosphere relief mechanism according to this embodiment of the present invention. 
         [0081]    In the atmosphere relief mechanism  20 ′ according to this modified example, a grid panel  22 ′ includes an annular holding part  22   a ′ constituting the first holding part, a grid part  22   b ′, bolt holes  22   c ′ and a stepped portion  22   d.  The stepped portion  22   d  is formed by an outer part of the annular holding part  22   a ′ which is on the outer circumferential side of the bolt holes  22   c ′ and the outer part protrudes toward the rupture disc  24 . A surface of the stepped portion  22   d  on the rupture disc  24  side forms a flat surface contacting the holding panel  27  via the gasket  26 . Further, the stepped portion  22   d  is formed to have height corresponding to height of the rupture disc  24 . 
         [0082]    As the annular spacer part is constituted of the stepped portion  22   d,  it is possible to suppress warping of the rupture disc  24  caused in association with deformation of the holding panel  27  during fastening by means of the bolts  29  and thus it is possible to prevent unintended damage of the rupture disc  24 . Further, it is possible to provide the annular spacer part (the stepped portion  22   d ) integrally with the grid panel  22 ′ and thus, the number of parts can be reduced. 
         [0083]    Further, the stepped portion  22   d  is formed in the grid plate  22 ′ in the configuration illustrated in  FIG. 6 . However, the stepped portion may be formed in the holding panel  27 . In this case, the gasket  26  is omitted or the liquid gasket is provided as the gasket  26 . 
         [0084]    While the embodiment of the present invention has been described in detail, it is obvious that various modifications and changes may be made without departing from the gist of the invention. 
         [0085]    For instance, in the above embodiment, the atmosphere relief mechanisms  20 ,  20 ′ having the stacked layer structure as illustrated in  FIG. 4  and  FIG. 6  are described. However, the stacked layer structure of the atmosphere relief mechanism is not limited to this as long as it is configured so that the outer circumferential edge of the rupture disc  24  is held from both sides by a pair of annular holding parts and fastened by a fastening member (e.g. the bolts  29 ) and the annular spacer part (the liner  25  and the stepped portion) is provided outside the fastening member arrangement area  30 .