Abstract:
The present invention reduces the amount of gas which leaks and flows from a high pressure side to a low pressure side of the axis and maintains good sealing performance even at high differential pressures. The invention discloses an axis sealing mechanism comprising casings which are supported inside of the stable portion, a plurality of planar plates, which are mutually spaced around the axis, each of which is fixed to the casings, positioned so that an inner end of each planar plate makes an acute angle with a peripheral surface of the axis and contacts the peripheral surface of the axis over a predetermined length in an axial direction, a high pressure side plate which is arranged at a high pressure side and a low pressure side plate which is arranged at a low pressure side so as to hold the planar plates therebetween and a flexible plate which is arranged between the planar plates and the high pressure side plate, and is flexible in an axial direction. The invention also discloses a turbine which provides the above sealing mechanism.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an axis seal mechanism which is suitable for an axis of large turbo machines such as gas turbines, steam turbines, compressors, pumps, etc. The invention also relates to a turbine which generates motive power by converting the thermal energy of a fluid to kinetic rotational energy, and an axis seal mechanism therefor. 
     2. Prior Art 
     Generally, an axis seal mechanism is arranged around the axis of a gas turbine or a steam turbine in order to reduce the amount of gas leaking from the high pressure side of the axis to the low pressure side of the axis. FIG. 12 shows a leaf type seal  1  as one example of a conventional axis seal mechanism. 
     The leaf type seal  1  consists of planar plates  3  each of which has a predetermined width and which are arranged along the axis of the rotation axis  2 . 
     The outer end of each planar plate  3  is welded to a casing  5  by a brazing part  4 . The inner end of each planar plate  3  contacts the peripheral surface of the axis  2  with a predetermined pressure. As shown in FIGS. 12 and 13, the angle between each planar plate  3 , the inner end of which contacts the axis  2 , and the peripheral surface of the axis  2 , which has a direction of rotation indicated by arrow d, is an acute angle. 
     The planar plates  3  are thus fixed to the casing and seal the peripheral surface of the axis  2  so as to separate the area around the axis  2  into a high pressure area and a low pressure area. 
     The casing  5  consists of a first baffle  7  at the high pressure side of the planar plates  3  and a second baffle  8  at the low pressure side of the planar plates  3 , which are arranged in order to hold the planar plates between them and guide the action of the high pressure. 
     In the leaf seal  1  thus constructed, the inner end of each planar plate  3  floats away from the peripheral surface of the axis  2  due to the dynamic pressure caused by the rotation of the axis  2  so that the inner ends of the planar plates  3  do not contact the peripheral surface of the axis  2 , which also prevents wear. 
     However, the above leaf seal  1  has the following problem. 
     The dynamic pressure caused by the axis  2  makes the planar plates  3  float away from the peripheral surface of the axis  2  so as to prevent wear and heating due to friction between the planar plates  3  and the axis  2 . The first baffle  7  and the second baffle  8  are arranged so that the space between the first baffle  7  of the high pressure side and each of the planar plates  3  is the same as the space between the second baffle  8  at the low pressure side and each of the planar plates  3 ; however each planar plate  3  is subject to pressure from the high pressure side to the low pressure side which deforms it towards the radial center of the axis  2 , which makes it difficult to maintain the floating state without contact between the inner end of the planar plate  3  and the axis  2 . 
     In order to overcome the above problems, a mechanism wherein a flexible side leaf is arranged between each planar plate  3  and the first baffle  7  has been proposal. The outer periphery of the side leaf is welded to the first baffle  7  by spot welding. 
     The side leaf of the above leaf seal, when pressure is applied from the high pressure side, bends in the axial direction of the axis  2  and touches the side edge of the planar plates  3  and the space between the planar plates  3  and the first baffle  7  becomes smaller than the space between the planar plates  3  and the second baffle  8 . Therefore gas flowing through the space between the axis  2  and the first baffle  7  flows from the inner ends of the planar plates  3  to the outer ends of the planar plates  3  and the planar plates  3  will float on the flowing gas. 
     However the above leaf seal with the side leaf has the following problem. 
     Because the side leaf of the above leaf seal is fixed to the first baffle  7 , a bending force acts on the periphery of the side leaf as it bends towards the low pressure area. Because the periphery of the side leaf is welded to the first baffle by spot welding and the strength of the spot welding is relatively low, the side leaf may separate from the first baffle  7  due to the temporary bending force applied to the periphery of the side leaf, and the performance for sealing may become insufficient. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems and seeks to reduce leakage of gas from the high pressure side to the low pressure side. The present invention also seeks to obtain an axis seal mechanism and a turbine which can maintain good sealing performance even at high differential pressures. 
     The axis seal mechanism of the present invention comprises a mechanism which prevents a flow of fluid along an axis through a cylindrical space between a stationary portion and the axis, and comprises casings which are supported inside of the stationary portion, a plurality of planar plates, which are mutually spaced in the circumferential direction of the axis, each of which is fixed to the casings and positioned so that an inner end of each planar plate makes an acute angle with a peripheral surface of the axis and contacts the peripheral surface of the axis over a predetermined length in an axial direction, a high pressure side plate which is arranged at a high pressure side of the planar plates and a low pressure side plate which is arranged at a low pressure side of the planar plates so as to hold the planar plates therebetween and a flexible plate which is arranged between the planar plates and the high pressure side plate and is flexible in the axial direction, wherein the flexible plate is fixed to each of the planar plates. 
     In the above axis seal mechanism, in a sectional view taken along a virtual plane perpendicular to the planar plate, the surface of each planar plate which faces the axis is designated as the bottom face, and the other surface of each planar plate is designated as the upper surface. In this sectional view taken along the virtual plane, the gas pressure which acts on the bottom surface of the planar plate is higher than the gas pressure which acts on the upper surface of the planar plate, therefore, the inner end of each planar plate floats and does not touch the axis. 
     Specifically, because a gas to which pressure is applied from a high pressure side tends to flow towards the low pressure side, and because the flexible plate is arranged between each planar plate and the high pressure side plate, and the space between each planar plate and the high pressure side plate is smaller than the space between each planar plate and the low pressure side plate, gas will flow from the space between the high pressure side plate and each planar plate in a diagonal direction along the upper and the bottom surfaces of the planar plates, and a low pressure area will extend to the outer end of each planar plate. The distribution of the gas pressure which acts on the upper and the bottom surfaces of each planar plate, when viewed in a section taken perpendicular to the width of each planar plate, forms a triangular shape, with the gas pressure tending to become lower from the inner end of each planar plate to the outer end of each planar plate. The distribution of the gas pressure on the upper surface is almost the same as the pattern of gas pressure on the bottom surface. However, each planar plate is inclined with respect to the peripheral surface of the axis, therefore, the pattern of gas pressure on the upper surface shifts with respect to the pattern of gas pressure on the bottom surface, and a gas pressure differential appears between one surface of the planar plate and the other surface of the planar plate at a given point on the surface. 
     The gas pressure Fb which acts on the bottom surface of the planar plate is higher than the gas pressure Fa which acts to the upper surface of the planar plate, therefore there is a force which makes each planar plate float away from the axis. Because the inner end of the planar plate is chamfered, there is no surface to receive the gas pressure near the inner end of the bottom surface of the planar plate, and only the upper surface of the planar plate receives the gas pressure near the inner end of the planar plate. However, the force which thus acts on the upper surface of the planar plate is cancelled by the pressure Fc due to the gas which flows between the inner end of the planar plate and the peripheral surface of the axis. Thus, the total force due to the gas pressure is (Fb+Fc)&gt;Fa, and therefore each planar plate will bend so as to float away from the peripheral surface of the axis. 
     Because the flexible plate is fixed to the planar plates, the displacement in the flexible plate along the axis decreases compared to the case that the flexible plate is fixed to the high pressure plate, and a deformation in the outer periphery of the flexible plate decreases, so that the flexible plate does not become separated from the planar plates. As a result, the axis seal mechanism can maintain its sealing performance at a high differential pressure. 
     In the axis seal mechanism the outer peripheral surface of the flexible plate may be welded to each of said planar plates. 
     Because the flexible plate is strongly fixed to the planar plates, the flexible plate is prevented from separating from the planar plate as a result of a bending force due to a differential pressure between the high pressure side and the low pressure side. 
     In the axis seal mechanism the flexible plate may be inserted into a notched portion which is formed in each of said planar plates. 
     Because the flexible plate is fixed to the planar plates by insertion into the notched portion, it is not necessary to heat the planar plate and the flexible plate for assembly. This prevents the flexible plate and the planar plates thus assembled from being subject to heat deformation or damage, and degradation of the sealing performance is prevented. 
     The axis seal mechanism may also comprise a mechanism which prevents a flow of fluid along the axis through the cylindrical space between a static portion and the axis, comprising casings which are supported inside the static portion, a plurality of planar plates which are mutually spaced in the peripheral direction of said axis, the outer end of each being fixed to the casings, the inner end of each slidably contacting the peripheral surface of said axis at an acute angle over a predetermined length along said axis, a low pressure side plate, which is arranged in a low pressure side of the axis, and a high pressure side plate, which is arranged in a high pressure side of the axis, which hold the planar plates therebetween, a flexible plate which is arranged between the planar plates and said high pressure side plate and is flexible in an axial direction of said axis, wherein the flexible plate comprises a projecting portion which is held between the casing and the planar plates. 
     Because the flexible plate can be arranged between each of the planar plates and the high pressure side plate by providing the projecting portion of the flexible plate between the casings and the planar plates, when the outer ends of the planar plates are fixed to the casings, deformation and damage due to heating and forces applied during construction, and degradation of the sealing performance of the axis seal mechanism, are prevented. 
     In the axis seal mechanism, it is possible for the flexible plate to contact a side edge of each of the planar plates. 
     Because the flexible plate contacts the side edges of each of the planar plates and is supported by each planar plate, the force which acts on the outer periphery of the flexible plate becomes smaller. The flexible plate thus supported is prevented from separating from the planar plates and the sealing performance is maintained even at high pressures. 
     A turbine which converts thermal energy into kinetic rotational energy by blowing a high pressure and high temperature fluid flowing onto an active blade in a casing may comprise a turbine which consists of any one of the above axis seal mechanisms. 
     Because the turbine comprises the axis seal mechanism which reduces the amount of gas leakage even at high differential pressures, it is possible to minimize power loss due to leakage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the first embodiment of the present invention. 
     FIG. 2 is an oblique perspective view of the leaf seal (axis seal mechanism) of the first embodiment. 
     FIG. 3 is a sectional view, taken along the axis, of the leaf seal of the first embodiment. 
     FIG. 4 is another sectional view, taken along the axis, of the leaf seal of the first embodiment. 
     FIG. 5 is a sectional view taken along the line B—B of the leaf seal of the first embodiment. 
     FIG. 6 is a sectional view, taken along the axis, of the leaf seal of the second embodiment. 
     FIG. 7 is a sectional view, taken along the axis, of the leaf seal of the third embodiment. 
     FIG. 8 is a sectional view, taken along the axis, of the leaf seal of the fourth embodiment. 
     FIG. 9 is an oblique perspective view of a flexible plate of the fourth embodiment. 
     FIG. 10 is a sectional view, taken along the axis, of the leaf seal of the fifth embodiment. 
     FIG. 11 is an oblique perspective view of a flexible plate of the fifth embodiment. 
     FIG. 12 is an oblique perspective view of a conventional axis seal mechanism. 
     FIG. 13 is a cross sectional view of the conventional axis seal mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, embodiments of an axis seal mechanism and turbo machinery which includes the axis seal mechanism are described. However the invention is not particularly limited thereto. The invention is explained by reference to an embodiment of a gas turbine, however the invention is not limited to gas turbines and can be applied to other kinds of turbines. 
     The first embodiment will be explained with reference to FIGS. 1 to  7 . 
     FIG. 1 shows a schematic view of a gas turbine. In FIG. 1, numeral  20  indicates a compressor, numeral  21  indicates a combustor and numeral  22  indicates a turbine. The compressor  20  intakes air and compresses the intaken air. Similarly to a conventional turbine, the compressor  21  is connected to the turbine  22  by an axis  23  and is driven by a part of a driving force which is generated by the turbine  22 . The combustor  21  combusts the fuel which is mixed with the air which is compressed by the compressor  21 . The turbine is driven by the expansion of the combustion gas (fluid) which originates in the combustor  21 . Specifically, the expanding combustion gas drives dynamic blades  23   e  which are fixed to the axis  23  so as to convert the thermal energy of the combustion gas to kinetic energy to rotate the axis  23 . 
     Further to the dynamic blade  23   e , the turbine  22  consists of static blades  24   a  which are fixed to a casing  24 . The dynamic blades  23   e  and the static blades  24   a  are positioned one after the other. The dynamic blades  23   e  drive the axis  23  by receiving the pressure of the combustion gas which flows along the axis  23 . The axis  23  transmits the rotational power at an axial end portion of the axis  23 . A leaf seal  25  is arranged between the static blades  24   a  and axis  23  as an axis seal mechanism which prevents the leaking of the combustion gas which flows from a high pressure side of the axis  23  to a low pressure side of the axis  23  via circular cavities between the static blades  24   a  and the axis  23 . 
     As shown in FIG. 2, the leaf seal  25  consists of a pair of casings  26 , which are supported by the inner face of the static blade  24   a , and a plurality of planar plates  28  which are aligned separated by a mutual space  27  around the axis  23 . 
     One of the pair of casings  26  has a high pressure side plate  29 , which guides the direction of the gas, at a high pressure side of the planar plate  28  and the other one of the pair of casings  26  has a low pressure side plate  30 , which also guides the direction of the gas, at a low pressure side of the planar plate  28 . 
     The outer end  28   a  of each planar plate  28  is fixed to the inner faces of the pair of casings  26  and the inner end  28   b  of each planar plate  28  touches a peripheral surface  23   a  of the axis  23  so as to slide along the peripheral surface  23   a . The angle between the planar plate  28  and the peripheral surface  23   a  is acute. Each planar plate  28  is rigid in directions within its plane, but is flexible in a direction which perpendicular to the plane. In other words the planar plate  28  is not readily bent in a direction parallel to the axis but can be easily bent in a direction tangential to the axis. A flexible plate  31  which has flexibility in a direction perpendicular to the flexibility of the planar plates  28  is arranged between the planar plates  28  and the high pressure side plate  29 . 
     FIG. 3 is a cross sectional view taken along the arrow A of leaf seal  25 . As shown in FIG. 3, the cross sections of the combined pair of casings  26  and each planar plate  28  have a T shape. 
     The outer end of the flexible plate  31  is rigidly fixed to a bottom end of a head portion H of the planar plate  28  having a T shape. The flexible plate  31  touches the side edge  33  of each planar plates  28 . The flexible plate  31  can be bent by pressure from the high pressure side, and touches the side edges  33  of the planar plates  28  and is supported by the side edges  33 . 
     The space  34  at the high pressure side, between the high pressure side plate  29  and the flexible plate  31 , is narrower than the space  35  at the low pressure side, between the low pressure side plate  30  and the planar plates  28 . 
     As shown in FIGS. 4 and 5, because the space  34  at the high pressure side is narrower than the space  35  at the low pressure side, the gas g which is subjected pressure from the high pressure area to the low pressure area will broadly flow along the upper surface  36  and the bottom surface  37  of each planar plate  28 , and the area of low pressure will expand to the outer end  28   a . In other words, the distribution patterns  40   a  of the gas pressure on the upper surface  36  and the bottom surface  37  of the planar plates  28  have a triangular shape in which the gas pressure is highest at the corner r 1  which is located at the inner end  28   b  of the planar plate  28  and faces the high pressure side plate  29  and the gas pressure becomes lower at the diagonally oppsosite corner r 2 . 
     Specifically, the gas g from the high pressure area passes between the inner ends  28   b  of the planar plates  28  and the peripheral surface  23   a  of the axis  23  and flows along the upper surface  36  and the bottom surface  37  of the thin plats  28  to the low pressure area. Therefore a stream of the gas which flows into a space between the high pressure side plate  29  and the peripheral surface  23   a  of the axis  23  expands in the radial direction from the corner r 1  to the corner r 2 , and the low pressure area will expand forwards the outer end  28   a . As shown in FIG. 5, the pressure pattern  40   b  of gas which is perpendicular to the upper surface  36  of the planar plate  28  and the pressure pattern  40   c  of gas which is perpendicular to the bottom surface  37  of the planar plate  28  have a triangular shape in which the pressure is higher near the inner end of the planar plate  28  and also is lower near the outer end of the planar plate  28 . 
     The pressure pattern  40   b  on the upper surface  36  is nearly the same as the pressure pattern  40   c  on the bottom surface  37 . However, the planar plates  28  are inclined with respect to the peripheral surface  23   a  of the axis  23  so as to make at an acute angle, therefore the pressure patterns  40   b  and  40   c  are shifted with respect to each other by a distance s 1 . At a point P between the outer end  28   a  and the inner end  28   b , the pressure Fb applied to the bottom surface  37  is higher than the pressure Fa applied to the upper surface  36 , therefore a force which bends the planar plates  28  is exerted so as to make the planar plates  28  float away from the peripheral surface  23   a  of the axis  23 . 
     In contrast, because the inner edge of the planar plate  28  is chamfered so as to make a plane  38  for touching the peripheral surface  23   a , a force due to the gas pressure only acts to the upper surface  36  in the vicinity of the outer end  28   a  of the planar plate  28 . Because the pressure of the gas which flows between the peripheral surface  23   a  of the axis  23  and the inner end  28   b  of the planar plate  28  produces a force Fc which floats the inner end  28   b  of the planar plate  28  away from the peripheral surface  23   a , the force acting on the upper surface  36  is cancelled and there is no force pushing the outer end  28   a  of the planar plate  28  towards the peripheral surface  23   a  of the axis  23 . The forces Fa, Fb and Fc which act on each planar plate  28  can be expressed by the equation (Fb+Fc)&gt;Fa, and therefore the planar plates  28  can be bent so as to float away from the peripheral surface  23   a . The planar plates  28  thus constructed can be maintained in a non-touching state where the outer end  28   a  floats away from the peripheral surface  23   a  due to the pressure differential between the upper surface  36  and the bottom surface  37 . 
     Next, a method to assemble the casing  25  will be explained. 
     (i) Aligning the planar plates  28  which are formed by masking and etching and are separated from each other by a space  27  along the periphery of the axis  23 . 
     (ii) Brazing each outer end  28   a  of each planar plate  28  so as to connect one planar plate  28  to another planar plate  28 . 
     (iii) Welding the bottom end of the head portion H to the periphery of the flexible plate  31 . 
     (iv) Aligning a pair of separated casings  26  above the low pressure side of each planar plate  28  and the flexible plate  31  so as to enclose the planar plates  28  and the flexible plate  31  by the pair of casings  26 . 
     Besides brazing the outer ends  28   c  of the planar plates  28 , it is possible to braze a side portion of the outer end  28   c  of the planar plates  28 . 
     Because the leaf seal  25  thus constructed has the flexible plate  31  which makes the high pressure space  34  between the high pressure side plate  29  and each of the planar plates  28  smaller than the low pressure space  35  between the low pressure side plate  30  and each of the planar plates  28 , a pressure differential between the upper face  36  and the bottom face  37  of each planar plate  37  [Fa&lt;(Fb+Fc)] occurs and as a result each of the inner ends  28   b  of the planar plates  28  can float away from the peripheral surface of the axis  23  so that there is no contact between the planar plates  28  and the peripheral surface  23   a  of the axis  23 . Therefore overheating and wear of the planar plates  28  and axis  23  are prevented. 
     Because the flexible plate  31  is fixed to the flexible planar plates  28  which can easily move in accordance with the deformation of the flexible plate  31 , the flexible plate  31  can be easily bent along the axis  23  and the bending force applied to the periphery of the flexible plate  31  is reduced so that the flexible plate  31  is not displaced with respect to the planar plates  28 , as compared with the case that the flexible plate  31  is blazed to the rigid high pressure side plate  29 . Furthermore, because the flexible plate  31  is rigidly fixed to the planar plates  28  by brazing, the flexible plate is not readily separated from the planar plates  28  by a bending force which is due to the differential pressure between the high pressure area and the low pressure area. 
     The flexible plate  31  is supported by all of the planar plates  28 , by contacting with their side edges  33 , and therefore the force which bends the outer periphery of the flexible plate  31  is reduced and a displacement of the flexible plate  31  is prevented. According to the flexible plate  31  thus constructed, the leaf seal maintains its sealing performance even at high differential pressures. 
     It is not necessary to modify or work on the planar plates  28  in order to attach the flexible plate  31  to the planar plates  28 . 
     A gas turbine which consists of the casing  25  thus constructed can maintain its sealing performance, and therefore has reduced loss of power due to gas leakage. 
     Hereinafter, other embodiments of the present invention will be explained. The embodiments are explained with priority given to specific subjects, and the same reference numerals are used for the members of the following embodiments which are equivalent to those of the first embodiment, and explanations for the equivalent members are omitted. Because the gas turbines of the following embodiments are the same as that of the first embodiment, explanations for the gas turbines are omitted. 
     FIG. 6 shows a cross sectional view of the leaf seal  25  of the second embodiment taken along the axis  23 . A notched portion  41  is formed in each bottom portion of the high pressure side head H of planar plates  28 . The outer periphery of the flexible plate  41  is inserted into the notched portion  41  so as to be fixed thereto. 
     The notched portion  41  is tightened once the outer periphery of the flexible plate  31  has been inserted into the notched portions  41 , so as to tightly fix the flexible plate  31  to each of the planar plates  28 . 
     With the leaf seal  25  of the second embodiment, it is possible to fix the flexible plate  31  to each of the planar plates  28  without heating for welding or brazing. Therefore, heat deformation and damage due to heating of the flexible plate  31  and planar plates  28  are prevented so as not to degrade the sealing performance. 
     The leaf seal  25  of the third embodiment of the present invention will be explained with reference to FIG. 7. A notched portion  42  is formed in the side edge  33  of the leaf seal  25  of the third embodiment. The notched portion  42  is directed parallel to the axis  23  and is engaged with a projecting portion  43  which is formed in the outer end of the flexible plate  31 . 
     It is possible to tightly fix the projecting portion  43  of the flexible plate  31  into the notched portion  42  of each of the planar plates  28  by inserting the projecting portion  43  into the notched portions  42 . 
     Because the opening of the notched portion  43  faces a direction parallel to the axis  23 , the projecting portion  43  of the flexible plate  31  engages with the notched portion  42  so that the load of the flexible plate  32  is supported by the projecting portion  43 . Therefore it is not necessary to clamp the notched portion  42  after inserting the projecting portion  43  of the flexible plate  31  and it is also not necessary to heat the flexible plate  31  for welding or brazing. By the above method for attaching the flexible plate  31  to the planar plates  28 , which can be performed without heating or clamping, deformation and damage of the leaf seal  25  are prevented, thus ensuring its performance. 
     The leaf seal  25  of the fourth embodiment of the present invention will be explained with reference to FIGS. 8 and 9. 
     The thickness of the heads H at the high pressure side of the planar plates  25  is smaller than the thickness width of the heads H at the low pressure side of the planar plates  25 . The flexible plate is formed so as to have projecting portion  44  which projects towards the high pressure side and is held between the bottom end of the high pressure side head H of the planar plates  28  and the casing  26  is formed. 
     The flexible plate  31  is arranged to touch the side edges  33  of each of the planar plates  28  and the pair of casings  26  which are aligned along the axis are combined so as to enclose the planar plates  28  and the flexible plate  31 . Then the projecting portion  44  of the flexible plate  31  is held between the casing  26  and each of the planar plates  28  and the flexible plate  31  is thus fixed. 
     Because the projecting portion  44  of the flexible plate  28  is held between the casing  26  and each of the planar plates  28  so as to be held therein, deformation and damage of the planar plates  28  and the flexible plate  31  are prevented, thus ensuring the sealing performance of the leaf seal  25 . 
     Next, the leaf seal  25  of the fifth embodiment of the present invention will be explained with reference to FIGS. 10 and 11. 
     As shown in FIG. 10, the high pressure side of the head portion of each planar plate  28  is omitted, and therefore each planar plate  28  is L-shaped so that the head portion of each planar plate  28  only projects towards the low pressure side. 
     Projecting portions  45  are formed in the outer periphery of the flexible plate  31 . Each projecting portion  45  is arranged at a predetermined space from the next projecting portion  45  around the peripheral direction of the axis  23 . 
     The leaf seal  25  of the fifth embodiment can be assembled by arranging each portion of the casing  26  to enclose the planar plates  28  and combining the pair of casings  26  so as to hold each planar plate  28  and the flexible plate  31  between the casings  26 . 
     Because the projecting portions  45  are formed in the flexible plate  31  separated from each other by a predetermined spacing around the axis  23 , flexible plate  31  can be easily bent in accordance with the moving of the side edge  33  of each planar plate  28  and the gas g from the high pressure side does not flow into a space between the flexible plate  31  and each planar plate  28 . Therefore, the pattern of the gas pressures  40   b  and  40   c  perpendicular to the upper surface  36  and the bottom surface  37  of each planar plate  28  form a triangular shape in which the pressure is the higher the pressed point is closer to the inner end  28   b  of each planar plate  28  and is also lower the pressed point is closer to the outer end  28   a  of the planar plate  28 .