Abstract:
To obtain a pressure tank that achieves a high manufacturing efficiency and does not hamper storage of an open/close portion such as a vacuum valve in a pressure tank. A pressure tank of the present invention includes: a tank body having at least one penetrating slit-shaped mortise and having a space formed inside the tank body; a reinforcing member having a tenon portion formed at an end thereof so as to be directed in one direction, and having an electric field relaxation portion on a side opposite to the tenon portion, the reinforcing member being attached to an inner wall surface of the tank body with the tenon portion inserted into the mortise; and a welding portion sealing and fixing the mortise and the tenon portion with no gap therebetween, the welding portion being formed by melting an end of the tenon portion from outside of the tank body.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a pressure tank, a gas insulated switchgear using the same, and a pressure tank manufacturing method. 
       BACKGROUND ART 
       [0002]    A gas insulated switchgear has a structure in which an open/close portion composed of a vacuum valve and the like is fixed in a pressure tank filled with a gas that is excellent in insulation property and arc-distinguishing capability. In general, this pressure tank is filled with the gas at a pressure of about 0.03 to 0.6 MPa, and as compared to a conventional air insulated switchgear, excellent insulation property and excellent arc-distinguishing capability are obtained and significant downsizing is achieved. In addition, since the open/close portion is sealed in the pressure tank, the open/close portion is not subject to contamination, and thus high reliability and safety are obtained. 
         [0003]    In this gas insulated switchgear, it is necessary to stably retain the gas in the pressure tank and prevent the pressure tank from being broken due to stress concentration on a corner portion of the pressure tank by the internal pressure of the filling gas. In order to adapt to such a purpose, the pressure tank is configured such that wall surface materials formed from metal plates are combined, connection parts of the wall surface materials are connected by welding, and further, a rod-like member or a bent plate material is welded to the inner wall surface of the pressure tank, for reinforcement (for example, Patent Document 1 and Patent Document 2). 
         [0004]    In general, as a structure for enhancing the strength of a container such as a tank, there is known a structure in which a plate-shaped reinforcing member is formed between the opposed ones of the wall surface materials composing the tank and thus a wall is formed in the tank (for example, Patent Document 3). For joining between the plate-shaped reinforcing member and the wall surface materials, protruding tenon portions are formed on two opposed sides of the plate-shaped reinforcing member, and the opposed wall surface materials have slit-like mortises into which the tenon portions are inserted and which penetrate through the wall surface materials. From inside of the tank, the tenon portions formed on the two sides of the reinforcing member are inserted and fitted into the mortises of the two opposed wall surface materials, and the tenon portions and the mortises are welded from outside of the tank, whereby the plate-shaped reinforcing member can be fixed between the opposed wall surface materials in the tank. 
       CITATION LIST 
     Patent Document 
       [0005]    Patent Document 1: Japanese Laid-Open Patent Publication No. 5-6825 
         [0006]    Patent Document 2: Japanese Laid-Open Patent Publication No. 6-231097 
         [0007]    Patent Document 3: Japanese Translation of PCT International Application Publication No. 2009-535600 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0008]    However, in the case where the wall surface materials of the tank body forming the pressure tank are reinforced by the rod-like member or the like from inside of the pressure tank, a space for performing the work is narrow, and the work such as welding is performed in the limited space, thus causing a problem that the work efficiency is deteriorated. Further, spatter (weld spatter) occurs around the part where the welding has been performed. Therefore, it is necessary to clean the spatter, and since the structure members are distorted due to heat by welding, it is also necessary to perform work for removing the distortion. 
         [0009]    In addition, in the case where the plate-shaped reinforcing member is formed between the opposed wall surface materials, an area for mounting the open/close portion and the like in the tank body is reduced, thus causing a problem that it is difficult to configure the gas insulated switchgear in a small size. 
         [0010]    The present invention has been made to solve the above problems, and an object of the present invention is to obtain a pressure tank, a gas insulated switchgear using the same, and a pressure tank manufacturing method, which enable easy manufacturing and enable downsizing. 
       Solution to the Problems 
       [0011]    A pressure tank of the present invention includes: a tank body having at least one penetrating mortise and having a space formed inside the tank body; a reinforcing member having a tenon portion formed at an end thereof so as to be directed in one direction, and having an electric field relaxation portion on a side opposite to the tenon portion, the reinforcing member being attached inside the tank body with the tenon portion inserted into the mortise; and a welding portion sealing and fixing the mortise and the tenon portion with no gap therebetween, the welding portion being formed by melting an end of the tenon portion from outside of a wall surface material. 
       Effect of the Invention 
       [0012]    In the pressure tank of the present invention, the tenon portion, of the reinforcing member, protruding in one direction is inserted into the mortise formed in the wall surface material of the tank body forming the pressure tank, and is fixed by being welded from outside of the tank body. Thus, it becomes possible to obtain a pressure tank which enables easy manufacturing and enables downsizing of a switchgear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic view showing the configuration of a gas insulated switchgear according to embodiment 1 of the present invention. 
           [0014]      FIGS. 2A to 2D  are projection views of a pressure tank of the gas insulated switchgear according to embodiment 1 of the present invention. 
           [0015]      FIGS. 3A to 3C  are projection views of a reinforcing member according to embodiment 1 of the present invention. 
           [0016]      FIG. 4  is an A-A sectional view of the pressure tank shown in  FIG. 2  in the present invention. 
           [0017]      FIGS. 5A and 5B  are B-B sectional views of the pressure tank shown in  FIG. 2  in the present invention. 
           [0018]      FIGS. 6A and 6B  are views illustrating a welding method according to embodiment 1 of the present invention. 
           [0019]      FIGS. 7A to 7C  are projection views of a reinforcing member of a pressure tank according to embodiment 2 of the present invention. 
           [0020]      FIGS. 8A to 8C  are projection views of a reinforcing member of a pressure tank according to embodiment 3 of the present invention. 
           [0021]      FIGS. 9A to 9D  are views showing welding parts in  FIG. 8 . 
           [0022]      FIGS. 10A to 10C  are projection views of a reinforcing member of a pressure tank according to embodiment 4 of the present invention. 
           [0023]      FIGS. 11A to 11D  are views showing welding parts of a reinforcing member of a pressure tank according to embodiment 5 of the present invention. 
           [0024]      FIGS. 12A to 12D  are views showing welding parts of a reinforcing member of a pressure tank according to embodiment 6 of the present invention. 
           [0025]      FIGS. 13A to 13D  are views showing welding parts of a reinforcing member of a pressure tank according to embodiment 7 of the present invention. 
           [0026]      FIG. 14  is a perspective view showing a pressure tank according to embodiment 8 of the present invention. 
           [0027]      FIGS. 15A to 15C  are views showing the pressure tank in  FIG. 14 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    In the descriptions of the embodiments and the drawings, parts denoted by the same reference characters indicate identical or corresponding parts. 
       Embodiment 1 
       [0029]    &lt;Structure of Gas Insulated Switchgear&gt; 
         [0030]      FIG. 1  schematically shows the configuration of a gas insulated switchgear, and a part indicated by a broken line shows an open/close portion stored inside the pressure tank  4 . The gas insulated switchgear includes an insulation rod  6 , a vacuum valve  7 , a fixed terminal  8 , and an insulation holder  9 , and the open/close portion stored in the pressure tank  4  is connected, to an operation spring  2   a  and a spring operation device  1  for causing the open/close portion to operate, via a contact pressure spring  2   b  for applying a contact force to an electrode  7   a  in the vacuum valve  7 . The open/close portion composed of the vacuum valve  7  and the like is separated from the spring operation device  1  via a flange  3 , and the flange  3  is fixed as a lid of the pressure tank  4 . 
         [0031]    In  FIG. 1 , as a basic structure, a tank body  5  is formed which is obtained by welding a plurality of wall surface materials to each other to fix and seal them and which has a rectangular-parallelepiped box shape. Rod-like reinforcing members  10  are provided to the inner surface of the tank body  5 , whereby the pressure tank  4  is formed. That is, the tank body  5  is a container formed by surrounding the periphery by a plurality of wall surface materials, welding them to each other to fix and seal them, and the reinforcing members  10  and the like are attached to the tank body  5 , to enhance the strength thereof, thereby forming the pressure tank  4  which is one of components constituting the gas insulated switchgear. 
         [0032]    &lt;Structure of Pressure Tank&gt; 
         [0033]      FIGS. 2A to 2D  are projection views of the pressure tank  4 . Specifically,  FIG. 2A  is a front view as seen from a direction in which the open/close portion composed of the vacuum valve  7  and the like is inserted into the tank and the tank is covered by the flange  3 , and  11  denotes a wall surface material on the tank front-surface side.  FIG. 2B  is a side view of the tank, and  12  denotes a wall surface material on the tank side-surface side.  FIG. 2C  is a back view as seen from the back which is opposite to the tank front surface, and  13  denotes a wall surface material on the tank back-surface side.  FIG. 2D  is a plan view as seen from the top, and  14  denotes a wall surface material on the tank top-surface side. Although described in detail later, the wall surface material  12  on the tank side-surface side and the wall surface material  13  on the tank back-surface side each have a plurality of slit-shaped mortises  15  for attaching, inside the tank body  5 , the reinforcing members  10  which are bent so as to have a U-shaped cross section in the present embodiment. It is noted that the reinforcing members  10  shown in  FIG. 2A  are the reinforcing members  10  attached, inside the tank body  5 , to the wall surface material  13  on the tank back-surface side. 
         [0034]    It is noted that the wall surface materials of the tank and the reinforcing members therefor are made from iron and stainless steel or aluminum and copper. 
         [0035]    &lt;Structure of Reinforcing Member&gt; 
         [0036]      FIGS. 3A to 3C  are projection views showing the configuration of a reinforcing member  16  bent so as to have a U-shaped cross section in the present embodiment.  FIG. 3A  shows the sectional shape of the reinforcing member  16 .  FIG. 3B  shows the side shape of the reinforcing member  16 .  FIG. 3C  shows the bottom shape of the reinforcing member  16  as seen from a direction in which tenon portions  17  extend. As described above, the reinforcing members  16  are used to be attached to the wall surface materials  12 ,  13  of the tank body  5  from inside, but  FIGS. 3A to 3C  show the shape of the reinforcing member  16  alone, which has not been attached to the wall surface material  12 ,  13  yet. 
         [0037]    As shown in  FIGS. 3A to 3C , the reinforcing member  16  in the present embodiment has a structure bent so as to have a U-shaped cross section. At the lower part in  FIG. 3B , the tenon portions  17  are formed which are fitted to the slit-shaped mortises  15  formed in the wall surface material, thereby fixing the reinforcing member  16  to the wall surface material  12 ,  13 . As shown in  FIG. 3A , the tenon portions  17  are formed at the lower ends of both bent portions at the right surface and the left surface of the U-shaped cross section. In order that the tenon portions  17  formed at the lower ends of both bent portions at the right surface and the left surface are all inserted and fitted to the mortises  15  formed in the same wall surface material  12 ,  13 , the tenon portions  17  at the right surface and the tenon portions  17  at the left surface of the reinforcing member  16  are formed so as to be directed in the same direction (downward direction in  FIG. 3A ,  FIG. 3B ). 
         [0038]    &lt;Attachment of Reinforcing Members&gt; 
         [0039]      FIG. 4  is a plan sectional view as seen from an A-A direction in  FIG. 2B , when the reinforcing members  16  are attached in the vertical direction inside the tank body  5  shown in  FIGS. 2A to 2D .  FIGS. 5A and 5B  are sectional views as seen from a B-B direction in the tank body  5  shown in  FIG. 2C . 
         [0040]    As shown in  FIG. 4 , the reinforcing members  16  are attached to the wall surface materials  12 ,  13  of the side surfaces and the back surface inside the pressure tank  4 . As described above, the tenon portions  17  of the reinforcing member  16  are formed, so as to be directed in the same direction, at the ends of bent portions on both sides of the reinforcing member  16  bent so as to have a U-shaped cross section. The tenon portions  17  formed at the right surface and the left surface of one reinforcing member  16  are all attached by being inserted and fitted to the plurality of mortises  15  formed in the same wall surface material  12 ,  13 . 
         [0041]    In the present embodiment, as shown in  FIG. 4 , two legs  16   a ,  16   b  forming the U shape of each U-shaped reinforcing member  16  are attached so as to be directed toward the wall surface material  12 ,  13 , and the U-shaped bent portion of the reinforcing member  16  is directed toward the open/close portion which is a high-voltage charge portion. As a result, the U-shaped bent portion of the U-shaped reinforcing member  16  provides, owing to its round shape, an electric field relaxing effect with respect to the open/close portion which is a high-voltage charge portion, thereby improving withstand voltage performance between the open/close portion and the inner surface (including the U-shaped reinforcing members  16 ) of the pressure tank  4 . 
         [0042]    In the pressure tank  4 , the upper end of the U-shaped reinforcing member  16  is located slightly lower than the upper end of the pressure tank  4 , and the lower end of the U-shaped reinforcing member  16  is located slightly higher than the lower end of the pressure tank  4 . As a result, in the pressure tank, it is necessary to improve the withstand voltage performance between the open/close portion which is a high-voltage charge portion and the inner side of the pressure tank  4  which is a ground portion, but since the U-shaped reinforcing member  16  is attached with its U-shaped bent portion directed toward the open/close portion, the electric field between the open/close portion and the inner surface portion (including the U-shaped reinforcing members  16 ) of the pressure tank  4  which is a ground portion can be relaxed, whereby the withstand voltage performance can be improved. As a result, it becomes possible to downsize the pressure tank  4 . In addition, since the upper end and the lower end of the U-shaped reinforcing member  16  are located away from the upper end and the lower end of the pressure tank  4  by a predetermined distance, the U-shaped reinforcing member  16  forms, in the pressure tank  4 , a ventilation duct between the upper end and the lower end of the pressure tank  4 . Therefore, although the temperature in the sealed pressure tank  4  greatly increases due to current application in the open/close portion, since the U-shaped reinforcing member  16  is attached along the up-down direction while having close contact with the wall surface material  12 ,  13 , the sealed gas flows inside the U-shaped reinforcing member  16  by a convection phenomenon toward the upper side of the pressure tank  4 . At this time, the gas flows in contact with the wall surface material  12 ,  13  so that heat in the pressure tank  4  can be efficiently dissipated to outside of the pressure tank  4 . 
         [0043]    In  FIG. 5A , the tenon portion  17  is inserted into the mortise  15  but has not been fixed by welding or the like yet. In  FIG. 5B , an end portion, of the tenon portion  17 , that protrudes from the wall surface material  13  is melted and welded so that the tenon portion  17  is fixed. As shown in detail in  FIG. 5A , the mortise  15  is formed to be slightly larger than the tenon portion  17  so that a slight gap is formed between the mortise  15  and the tenon portion  17 . In addition, the tenon portion  17  is formed to be slightly higher than the thickness of the wall surface material  13  in which the mortise  15  is formed. That is, as shown in  FIG. 5A , at a stage before welding, when the tenon portion  17  is fitted to the mortise  15 , a gap is formed around the tenon portion  17 , and the tenon portion  17  slightly protrudes outward from the outer surface of the wall surface material. 
         [0044]    The protrusion height is set so that the height of the tenon portion  17  becomes substantially the same height as the outer surface of the wall surface material  13  when the tenon portion  17  is melted by welding and the melted part thereof fills the gap between the mortise  15  and the tenon portion. 
         [0045]    Next, the welding method will be described with reference to  FIGS. 6A and 6B .  FIG. 6A  is a view of the mortise  15  as seen from the right in  FIG. 5B , i.e., C direction.  FIG. 6B  is a sectional view as seen from a D-D direction in  FIG. 6A . In  FIG. 6A , the tenon portion  17  is inserted to inside of the mortise  15  with a predetermined gap therebetween.  FIG. 6B  shows the method for welding the engagement part between the mortise  15  and the tenon portion  17 . In the present embodiment, an example using TIG welding is shown.  30  denotes a welding torch for TIG welding,  31  denotes a tungsten electrode of the welding torch  30 ,  32  denotes an inert gas such as argon or helium jetted from the welding torch  30  toward the welding portion. In order to melt the tenon portion  17  uniformly in the mortise  15 , the welding torch is operated in an elliptic shape as indicated by an operation route  33  in  FIG. 6A . In this way, from the state of a tenon portion  17   a  before welding, the end of the tenon portion  17  is melted uniformly in the slit-shaped mortise  15 , to fill the gap in the mortise  15 . 
         [0046]    Thus, the end portion of the tenon portion  17  or the wall surface of the mortise  15  is melted by TIG welding, whereby the gap between the mortise  15  and the tenon portion  17  can be filled in a sealed state by the melted material and the U-shaped reinforcing member  16  can be firmly fixed to the wall surface material  13 . It is noted that, if the volume of a part, of the tenon portion  17 , that protrudes from an end surface of the wall surface material  13  is designed to be the same as the volume of the melted part (i.e., welding portion  18 ) in the gap between the mortise  15  and the tenon portion  17 , a recess of the mortise  15  formed in the wall surface material  13  disappears after welding, and thus the mortise  15  part can be made flush with the other surface of the wall surface material  13 . Thus, by making the mortise  15  part flush with the other surface of the wall surface material  13 , an effect of improving the strength of the joining part is obtained. In addition, an effect of preventing deterioration in the appearance of the outer surface of the pressure tank  4  is obtained. 
         [0047]    In TIG welding, only a welding target (in the present embodiment, the wall surfaces of the tenon portion  17  and the mortise  15 ) is melted by an arc from the tungsten electrode  31 , and therefore, since a welding material is not supplied from outside, the melting amount in welding can be easily grasped. Therefore, the gap of the mortise  15  and the height of the tenon portion  17  can be easily designed, whereby it becomes possible to easily make the welding finished surface of the mortise  15  part flush with the surface of the wall surface material  12 ,  13 , as compared to the other welding methods. 
         [0048]    As described above, it is preferable that the height of the tenon portion  17  is greater than the thickness of the wall surface material  12 ,  13  as described above, and it is preferable that it is possible to, by melting, fill the gap between the mortise  15  and the tenon portion  17  and make the surface of the pressure tank  4  flat. In addition, the magnitude relationship between the tenon portion  17  and the mortise  15  needs to be a magnitude relationship that allows the tenon portion  17  to be inserted and fitted into the mortise  15  and thereafter, by melting the end of the tenon portion  17 , allows the gap between the mortise  15  and the tenon portion  17  to be filled to form the welding portion  18  so that they are fixed. 
         [0049]    In the present embodiment, the height of the tenon portion  17  is greater than the thickness of the wall surface material by 0.3 mm, but is not limited thereto. For example, by making the height of the tenon portion  17  greater by 0.05 mm to 0.5 mm, the same effect as in the present embodiment can be obtained. In addition, even in the case of making the height of the tenon portion  17  greater by 0.5 mm to 1 mm, the same effect can be obtained though the working time is slightly prolonged. 
         [0050]    In the present embodiment, the dimensions of the mortise  15  in the longitudinal direction and the lateral direction are both greater than those of the tenon portion  17  by 0.3 mm, but are not limited thereto. For example, even in the case of making the dimensions of the mortise  15  in the longitudinal direction and the lateral direction greater by 0.05 mm to 0.5 mm, the same effect as in the present embodiment can be obtained. Even in the case of making these dimensions of the mortise  15  greater by 0.5 mm to 1 mm, the same effect can be obtained though the working time is slightly prolonged. 
         [0051]    In the present embodiment, the reinforcing members  16  bent so as to have a U-shaped cross section are fixed to the inner sides of the wall surface materials  12 ,  13  of the tank body  5 , whereby the strength of the pressure tank  4  can be greatly enhanced. Since each reinforcing member  16  is bent so as to have a U-shaped cross section and has the tenon portions  17  directed in the same direction, the strength of the pressure tank  4  can be enhanced without limiting the size and the configuration of the open/close portion stored in the tank body  5  as in the case where, for example, the tenon portions  17  are provided at two opposed sides at both ends of a reinforcing member having a plate-shaped cross section and the reinforcing member is fixed so as to stride between the two opposed wall surface materials. 
         [0052]    Further, in the present embodiment, the mortises  15  are formed in the wall surface materials  12 ,  13  of the tank body  5  forming the pressure tank  4 , and the tenon portions  17  formed on each reinforcing member  16  are inserted and fitted thereto from inside of the tank body  5 , whereby the welding work can be performed from outside of the tank body  5 . Thus, it becomes unnecessary to clean spatter after welding, in the pressure tank  4 , and the welding work itself is also facilitated. 
         [0053]    In the case of performing welding from inside of the pressure tank  4 , the material melted at a high temperature is rapidly cooled at a part where the welding has been completed, so that distortion occurs in a direction in which the welding work surface is dented. However, in the present embodiment, since welding is performed from outside of the pressure tank  4 , such distortion can be reduced, and the time taken to remove the distortion after welding of the pressure tank  4  can be shortened. 
         [0054]    Such a reinforcement structure using the reinforcing member  16  in the pressure tank  4  allows the number, the size, the attachment interval, and the like of the reinforcing members to be adjusted in accordance with the size of a device placed in the pressure tank  4 , the pressure of a gas to fill the pressure tank  4 , the electric field intensity in the pressure tank  4 , and the like. The mortise  15  is not limited to a rectangle shape, but may be, for example, a hole having a shape such as square, ellipse, or polygon having three or more sides, or a special shape such as L shape or V shape. 
       Embodiment 2 
       [0055]    In the present embodiment, only the shape of the reinforcing member is different as compared to embodiment 1, and the other structures, working conditions, and the like are the same. 
         [0056]      FIGS. 7A to 7C  show projection views of a reinforcing member  19  having a plate-shaped cross section in the present embodiment.  FIG. 7A  shows the sectional shape of the reinforcing member  19 , and  FIG. 7B  and  FIG. 7C  show the side shape and the bottom shape thereof, respectively. The present embodiment is different from embodiment 1 in that, in embodiment 1, the reinforcing member is bent in a U shape and has tenon portions  17  directed in the same direction at both ends of the U shape, whereas in the present embodiment, the tenon portions  17  are formed at one end of the plate-shaped reinforcing member  19 . 
         [0057]    In addition, the plate-shaped reinforcing member  19  has a round shape at corner portions at an end on a side opposite to the tenon portions  17 . Therefore, an electric field relaxing effect is provided with respect to the open/close portion which is a high-voltage charge portion, and the withstand voltage performance between the open/close portion and the inner surface (including the reinforcing members  19 ) of the pressure tank  4  can be improved. 
         [0058]    Also in the present embodiment, by performing welding from outside of the pressure tank  4 , the welding process can be simplified, spatter cleaning after welding becomes easy, and the distortion removing process after welding can be shortened, for example. Thus, the same effects as in embodiment 1 can be obtained. 
       Embodiment 3 
       [0059]    In the present embodiment, only the shape of the reinforcing member is different as compared to embodiments 1 and 2, and the other structures, working conditions, and the like are the same. 
         [0060]      FIGS. 8A to 8C  show projection views of a reinforcing member  20  having substantially a quadrangle rod shape in the present embodiment.  FIG. 8A  shows the sectional shape of the reinforcing member  20 , and  FIG. 8B  and  FIG. 8C  show the side shape and the bottom shape thereof, respectively. The present embodiment is different from embodiment 1 in that, in embodiment 1, the reinforcing member is bent in a U shape and has tenon portions  17  directed in the same direction at both ends of the U shape, whereas in the present embodiment, tenon portions  17  having substantially a quadrangle block shape are formed on one surface of the reinforcing member  20  having substantially a quadrangle rod shape, and corner portions thereof on a side opposite to the tenon portions  17  are rounded to form electric field relaxation portions  20   a . Since the electric field relaxation portions  20   a  are located so as to be directed toward the open/close portion which is a high-voltage charge portion, the withstand voltage performance between the open/close portion and the inner surface of the pressure tank  4  can be improved owing to the electric field relaxing effect by the rounded electric field relaxation portions  20   a.    
         [0061]    Next, the welding structure in the present embodiment will be described with reference to  FIGS. 9A to 9D .  FIG. 9A  and  FIG. 9B  show the mortise  15  and the tenon portion  17  combined with each other before welding.  FIG. 9A  is a side sectional view showing a state in which the tenon portion  17  penetrates through the mortise  15  to be coupled therewith.  FIG. 9B  is a sectional view (E-E sectional view) of the mortise  15  and the tenon portion  17  coupled with each other in  FIG. 9A , as seen from the axial direction of the reinforcing member  20 .  FIG. 9C  and  FIG. 9D  show the mortise  15  and the tenon portion  17  combined with each other after welding, and respectively correspond to  FIG. 9A  and  FIG. 9B .  FIG. 9D  is a view showing an F-F cross section in  FIG. 9C . In  FIG. 9C  and  FIG. 9D, 18  denotes a welding portion formed by the inner wall of the mortise  15  and the tenon portion  17  being melted by welding. 
         [0062]    It is noted that the above-described tenon portions  17  having a quadrangle block shape are not limited to a quadrangle shape, but may have a cylindrical block shape. 
         [0063]    Also in the present embodiment, by performing welding from outside, the welding process can be simplified, spatter cleaning after welding becomes easy, and the distortion removing process after welding can be shortened, for example. Thus, the same effects as in embodiment 1 can be obtained. 
       Embodiment 4 
       [0064]    In the present embodiment, only the shape of the reinforcing member is different as compared to embodiments 1 to 3, and the other structures, working conditions, and the like are the same. 
         [0065]      FIGS. 10A to 10C  show projection views of a rod-shaped reinforcing member  21  having a mountain-shaped cross section in the present embodiment.  FIG. 10A  shows the sectional shape of the reinforcing member  21 , and  FIG. 10B  and  FIG. 10C  show the side shape and the bottom shape thereof, respectively. The present embodiment is different from embodiment 1 in that, in embodiment 1, the reinforcing member is bent in a U shape and has tenon portions  17  directed in the same direction at both ends of the U shape, whereas in the present embodiment, tenon portions  17  having a quadrangle block shape are formed on one bottom surface of the rod-shaped reinforcing member  21  having a mountain-shaped cross section. 
         [0066]    It is noted that the tenon portions  17  having a quadrangle block shape are not limited to a quadrangle shape, but may be a cylindrical block shape. 
         [0067]    Also in the present embodiment, by performing welding from outside, the welding process can be simplified, spatter cleaning after welding becomes easy, and the distortion removing process after welding can be shortened, for example. Thus, the same effects as in embodiment 1 can be obtained. 
       Embodiment 5 
       [0068]    In the present embodiment, the feature of the shape of the welding portion  18  will be described. 
         [0069]    As shown in  FIGS. 11A to 11D , the welding portion in the present embodiment 5 is formed such that the welding portion  18  bulges in a dome shape from the surface of the wall surface material  13 .  FIG. 11A  corresponds to  FIG. 5A  in embodiment 1 and shows the mortise  15  and the tenon portion  17  coupled with each other before welding.  FIG. 11B  is a view showing a G-G cross section in  FIG. 11A .  FIG. 11C  corresponds to  FIG. 5C  in embodiment 1 and shows the mortise  15  and the tenon portion  17  welded with each other after welding.  FIG. 11D  is a view showing an H-H cross section in  FIG. 11C . In the drawings,  18  denotes the welding portion  18  bulging in a dome shape by a height of h from the surface part of the wall surface material  13 . 
         [0070]    Such a shape provides an effect of improving the joining strength between the wall surface material  13  and the U-shaped reinforcing member, though the appearance is slightly deteriorated. 
       Embodiment 6 
       [0071]    In the present embodiment, the feature of the shape of the welding portion  18  different from embodiment 5 will be described. 
         [0072]    As shown in  FIGS. 12A to 12D , the welding portion in the present embodiment 6 is formed such that the welding portion  18  is flush with the surface of the wall surface material  13 .  FIG. 12A  corresponds to  FIG. 5A  in embodiment 1 and shows the mortise  15  and the tenon portion  17  coupled with each other before welding.  FIG. 12B  is a view showing a J-J cross section in  FIG. 12A .  FIG. 12C  corresponds to  FIG. 5C  in embodiment 1 and shows the mortise  15  and the tenon portion  17  welded with each other after welding.  FIG. 12D  is a view showing a K-K cross section in  FIG. 12C . In the drawings,  18  denotes the welding portion  18  formed to be flush with the surface part of the wall surface material  13 . 
         [0073]    Such a shape provides an effect of improving the joining strength between the wall surface material  13  and the U-shaped reinforcing member and achieving an excellent appearance. 
       Embodiment 7 
       [0074]    In the present embodiment, the feature of the shape of the welding portion  18  further different from embodiment 5 will be described. 
         [0075]    As shown in  FIGS. 13A to 13D , the welding portion in the present embodiment 7 is formed such that the welding portion  18  is recessed from the surface of the wall surface material  13 .  FIG. 13A  corresponds to  FIG. 5A  in embodiment 1 and shows the mortise  15  and the tenon portion  17  coupled with each other before welding.  FIG. 13B  is a view showing an L-L cross section in  FIG. 13A .  FIG. 13C  corresponds to  FIG. 5C  in embodiment 1 and shows the mortise  15  and the tenon portion  17  welded with each other after welding.  FIG. 13D  is a view showing an M-M cross section in  FIG. 13C . In the drawings,  18  denotes the welding portion  18  recessed in a reverse dome shape by a depth of h from the surface part of the wall surface material  13 . 
         [0076]    Such a shape provides an effect of improving the joining strength between the wall surface material  13  and the U-shaped reinforcing member and enabling a plurality of pressure tanks  4  to be arranged in close contact with each other, though the appearance is slightly deteriorated. 
       Embodiment 8 
       [0077]    In the present embodiment, the reinforcement structure of the pressure tank further advanced from embodiment 1 will be described. 
         [0078]    As shown in  FIG. 14  and  FIGS. 15A to 15C , in the pressure tank in the present embodiment 8, the U-shaped reinforcing members  16  are attached so as to be perpendicular to each other.  FIG. 14  is a perspective view of the pressure tank  4  as seen from the obliquely front side.  FIG. 15A  is a side view of the pressure tank in  FIG. 14 .  FIG. 15B  is a front sectional view as seen from a P-P direction in  FIG. 15A .  FIG. 15C  is a plan sectional view as seen from a Q-Q direction in  FIG. 15A . In the drawings, in the vertical direction, two U-shaped reinforcing members  16  are provided on the front side and three U-shaped reinforcing members  16  are provided on the back side such that they are parallel to each other, and in the horizontal direction, two upper and lower U-shaped reinforcing members  16  are provided, between the front-side two members and the rear-side three members in the vertical direction, so as to be directed in the horizontal direction. 
         [0079]    Such a configuration provides an effect of improving the strength of the wall surface materials  12 ,  13  against bending loads thereon in the horizontal direction and the vertical direction, in the example shown in  FIG. 14  and  FIGS. 15A to 15C . 
         [0080]    An area where the electric field intensity is severe between the open/close portion and the pressure tank inner surface (ground portion) is an area between each vacuum valve  7  and the inner surface of the pressure tank  4 . On the lateral sides of the vacuum valves  7 , the U-shaped reinforcing members  16  in the vertical direction are not provided but the U-shaped reinforcing members  16  in the horizontal direction are provided along the axial direction of the vacuum valves  7 , whereby the electric field intensity can be relaxed between the open/close portion and the pressure tank inner surface (ground portion). As a result, an effect of enabling downsizing of the pressure tank  4  is obtained. 
         [0081]    It is noted that, within the scope of the present invention, each of the above embodiments may be modified or simplified as appropriate. 
       DESCRIPTION OF THE REFERENCE CHARACTERS 
       [0000]    
       
         
           
               1  spring operation device 
               2   a  operation spring 
               2   b  contact pressure spring 
               3  flange 
               4  pressure tank 
               5  tank body 
               6  insulation rod 
               7  vacuum valve 
               7   a  electrode 
               8  fixed terminal 
               9  insulation holder 
               10  reinforcing member 
               11  wall surface material on tank front-surface side 
               12  wall surface material on tank side-surface side 
               13  wall surface material on tank back-surface side 
               14  wall surface material on tank top-surface side 
               15  mortise 
               16  reinforcing member 
               16   a ,  16   b  leg 
               17  tenon portion 
               17   a  tenon portion before welding 
               18  welding portion 
               19  reinforcing member 
               20  reinforcing member 
               20   a  electric field relaxation portion 
               21  reinforcing member 
               30  welding torch 
               31  tungsten electrode 
               32  welding gas 
               33  operation route