Patent Publication Number: US-6908006-B2

Title: High-pressure tank and method for fabricating the same

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to improvements of a high-pressure tank into which high-pressure gas is charged and improvements of a method for fabricating the same. 
     (2) Description of the Related Art 
     High-pressure tanks into which a gas, such as natural gas or hydrogen gas, is charged and stored at high pressure, are generally subjected to winding which is a technique for wrapping carbon fibers or the like around the tank body for reinforcement. A cylindrical gas discharge section and a continuous dome section of the tank, in particular, are likely to concentrate stress and therefore must be reinforced firmly. However, the vicinity of the boundary between the dome section and the cylindrical gas discharge section is difficult to subject to winding and thus difficult to reinforce. 
     Therefore, the dome section and the cylindrical gas discharge section are generally increased in thickness as compared with a cylindrical middle section of the tank by necking a predetermined region of an elongated hollow cylindrical blank continuing from its opening end by means of spinning. 
     In this connection, the applicant filed a patent application on a technique for further thickening the dome section and the cylindrical gas discharge section by flow forming the predetermined region of the elongated hollow cylindrical blank continuing from its opening end into a larger thickness than the other region and then necking the thickened predetermined region through spinning, and has already obtained a patent for the technique (see, for example, patent document 1) 
     [Patent Document 1] 
     Japanese Patent Publication No. 3251216 (page 3 and FIG. 1) 
     For the first-mentioned technique, however, there is a limit to how much the thickness of the dome section and the cylindrical gas discharge section can be increased because of the use of an elongated hollow cylindrical blank having a uniform thickness as a whole. Therefore, this technique is difficult to apply particularly to high-pressure tanks for hydrogen gas into which the gas is charged at a pressure of between 35 MPa and 75 MPa. 
     To cope with this, it can be considered to ensure the thickness of the dome section and the cylindrical gas discharge section by increasing the thickness of the entire middle section of the elongated hollow cylindrical blank. In this case, however, the entire high-pressure tank thus fabricated will also be thickened and thereby increased in weight, which hinders weight reduction. In addition, since the tank uses the thick elongated hollow cylindrical blank, its material cost will be high. 
     On the other hand, for the second-mentioned patent document 1, since the predetermined region of the elongated hollow cylindrical blank continuing from the opening end is increased in thickness as compared with the other region, the dome section and the cylindrical gas discharge section can have a larger thickness as compared with the first-mentioned technique and the entire tank weight can be reduced by thinning the middle section. This patent document 1 technique, however, requires the elongated hollow cylindrical blank to undergo two process steps of flow forming and spinning, which takes much labor. In addition, in order to ensure a sufficient thickness of a dome section and a cylindrical gas discharge section in a high-pressure tank for hydrogen gas, it is necessary to increase the amount of reduction of the blank. This may invite the complete closure of the cylindrical gas discharge section and thereby necessitate subsequent boring of a gas outlet in a later process step, which requires extra labor. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing points and therefore its object is to provide easily and inexpensively a high-pressure tank capable of withstanding high pressures of 35 to 75 MPa while keeping its lightness in weight. 
     To attain the above object, the present invention is characterized in that a reinforcing member is fitted integrally on the exterior of the tank between the cylindrical gas discharge section and the dome section, and takes the following solutions. 
     The invention claimed in claims  1  to  3  is concerned with a high-pressure tank. The invention claimed in claim  1  is characterized by comprising: a metallic tank body in which a cylindrical gas discharge section is protruded integrally from one end of a cylindrical middle section through a dome section and an annular fitting recess is formed circumferentially in the exterior of the tank body in the vicinity of the boundary between the dome section and the cylindrical gas discharge section; and a cylindrical metallic reinforcing collar having an annular boss and engaged integrally on the exterior of the tank body between the cylindrical gas discharge section and the dome section with the boss fitted in the fitting recess of the dome section through fitting of the reinforcing collar onto the cylindrical gas discharge section of the tank body. 
     With the above structure, in the invention of claim  1 , the cylindrical gas discharge section and the neighboring dome section, likely to concentrate stress, can be increased in effective thickness by the thickness of the reinforcing collar, thereby obtaining sufficient strength in these sections. This provides a high-pressure tank strong enough to withstand high pressures of 35 to 75 MPa. Further, since the reinforcing collar is fitted not on the entire tank body but only on its part: the dome section and the cylindrical gas discharge section, this does not so much increase the weight of the entire high-pressure tank and maintains light weight, and provides facilitated fabrication and reduced cost. 
     The invention claimed in claim  2  is characterized in that in the invention of claim  1 , the reinforcing collar is made of steel alloy or titanium alloy. 
     With the above structure, in the invention of claim  2 , the dome section and the cylindrical gas discharge section, likely to concentrate stress, can be further increased in strength thanks to the mechanical properties of steel alloy or titanium alloy which is a material for the reinforcing collar. 
     The invention claimed in claim  3  is characterized in that in the invention of claim  1 , the tank body is made of aluminum alloy. 
     With the above structure, in the invention of claim  3 , the tank body of aluminum alloy is light in weight, which further reduces the tank weight as a whole. 
     The invention claimed in claim  4  is concerned with a method for fabricating a high-pressure tank. The method is characterized by comprising the steps of: plastically deforming a hollow cylindrical blank of metal with rotation to form a tank body in which a cylindrical gas discharge section is protruded integrally from one end of a cylindrical middle section through a dome section, followed by forming an annular fitting recess circumferentially in the exterior of the tank body in the vicinity of the boundary between the dome section and the cylindrical gas discharge section; and then fitting a cylindrical metallic reinforcing collar having an annular boss onto the cylindrical gas discharge section of the tank body to fit the boss in the fitting recess of the dome section and engaging the reinforcing collar integrally on the exterior of the tank body between the cylindrical gas discharge section and the dome section by shrinkage. 
     With the above structure, in the invention of claim  4 , the additional reinforcing collar need only be fitted onto the cylindrical gas discharge section of the tank body. Accordingly, a high-pressure tank having a light weight and capable of withstanding high pressures of 35 to 75 MPa can be fabricated easily without requiring much labor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of a high-pressure tank. 
         FIG. 2  is a longitudinal cross-sectional view of a reinforcing collar. 
         FIG. 3A  is a longitudinal cross-sectional view of a short hollow cylindrical blank provided for flow forming in a high-pressure tank fabrication process. 
         FIG. 3B  is a diagram showing a flow forming step in the high-pressure tank fabrication process. 
         FIG. 3C  is a diagram showing a spinning step in the high-pressure tank fabrication process. 
         FIG. 3D  is a longitudinal cross-sectional view showing a state that, a dome section of a tank body is formed with a fitting recess in the high-pressure tank fabrication process. 
         FIG. 3E  is a longitudinal cross-sectional view showing a state before a reinforcing collar is fitted onto the tank body by shrinkage in the high-pressure tank fabrication process. 
         FIG. 3F  is a longitudinal cross-sectional view showing a state that the reinforcing collar is fitted onto the tank body by shrinkage in the high-pressure tank fabrication process. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  shows a high-pressure tank  1  according to the embodiment of the present invention. The high-pressure tank  1  includes a tank body  2  into which high-pressure gas, such as hydrogen gas, of 35 to 75 MPa is charged. The tank body  2  is formed so that a cylindrical gas discharge section  5  circular in cross section is protruded integrally from one end of a cylindrical middle section  3  circular in cross section through a dome section  4 , the cylindrical gas discharge section  4  is formed with a gas outlet  6  and the cylindrical middle section  3  is integrally formed at the other end with a bottom section  7 . The tank body  2  is provided internally with a hollow part  8  for containing high-pressure gas. 
     The high-pressure tank  1  is made of aluminum alloy such as JIS A 6061 or JIS A 6062, formed by plastically deforming a hollow cylindrical blank and subjected to heat treatment such as T 6  treatment after the forming. The dome section  4 , the cylindrical gas discharge section  5  and the bottom section  7  are formed to have greater thicknesses than the cylindrical middle section  3 . In particular, the dome section  4  is, due to its forming process, gradually increased in thickness from that of the middle section  3  to that of the cylindrical gas discharge section  5  in proceeding from the middle section  3  to the cylindrical gas discharge section  5 , thereby reinforcing the dome section  4  which is likely to concentrate stress. Further, an annular fitting recess  9  is formed circumferentially in the exterior of the tank body in the vicinity of the boundary between the dome section  4  and the cylindrical gas discharge section  5 . 
     As a feature of the present invention, a cylindrical metallic reinforcing collar  10  is engaged integrally onto the exterior of the tank body  2  between the cylindrical gas discharge section  5  and the dome section  4  by shrinkage. The reinforcing collar  10  is, as also shown in  FIG. 2 , composed of a cylindrical part  11  of circular cross section having substantially the same thickness as the cylindrical gas discharge section  5 , and an extension  12  formed integrally at the lower end of the cylindrical part  11  to extend outwardly. The extension  12  is gradually reduced in thickness proceeding to its outer edge, so that the outer edge of the extension  12  merges with the exterior of the dome section  4  smoothly. The back of the extension  12  is formed integrally with an annular boss  13  which protrudes downwardly. The reinforcing collar  10  is formed internally with a fitting hole  14  which passes through the cylindrical part  11  and the extension  12  vertically. The reinforcing collar  10  is made of steel alloy, such as SNCM440, SCM440 or SKD61, or titanium alloy and formed by forging or turning. Materials for the reinforcing collar, however, are not limited to the above but need only be those higher in strength-to-weight ratio than aluminum. Such materials make a great contribution to weight reduction. Further, the cylindrical part  11  of the reinforcing collar  10  is fitted integrally on the cylindrical gas discharge section  5  of the tank body  2  by shrinkage with the cylindrical gas discharge section  5  inserted in the fitting hole  14 , and the extension  12  of the reinforcing collar  10  is bonded integrally to the exterior of the dome section  4  by shrinkage with the boss  13  fitted in the fitting recess  9  of the dome section  4 . 
     As described above, the cylindrical gas discharge section  5  and the neighboring dome section  4  has a larger thickness as compared with the cylindrical middle section  3 . The fitting of the reinforcing collar  10  on these sections can further increase their effective thickness, and in addition increase their strength much more in combination with the mechanical properties of steel alloy or titanium alloy which is a material for the reinforcing collar  10 . Accordingly, there can be provided a high-pressure tank  1  strong enough to withstand high-pressures of 35 to 75 MPa. 
     Further, since the fitting position of the reinforcing collar  10  is not the entire tank body  2  but limited to the dome section  4  and the cylindrical gas discharge section  5  which are likely to concentrate stress, this avoids a large increase in weight of the high-pressure tank  1 , thereby keeping its lightness in weight, and provides facilitated fabrication and reduced cost. Furthermore, since the tank body  2  is made of aluminum alloy and therefore light in weight, the tank weight can be further reduced as a whole. 
     Next, description will be made about an example of fabrication methods for the high-pressure tank  1  having the above-described structure. 
     First, prepared is a short hollow cylindrical blank  21  of aluminum alloy as shown in FIG.  3 A. The short hollow cylindrical blank  21  is formed, for example, by forging a solid cylindrical billet, which is not shown, and has the shape of an end-closed cylinder in which a cylindrical middle section  31  is formed integrally with a bottom section  71 . Alternatively, the shape of an end-closed cylinder may be formed by drawing a disk-shaped material with a die and a punch. 
     Then, the short hollow cylindrical blank  21  is flow formed. The manner of the flow forming is as shown in FIG.  3 B: the short hollow cylindrical blank  21  is fitted on a mandrel  15 , the mandrel  15  is rotated on its axis to rotate the short hollow cylindrical blank  21  as a single unit, and a forming roll  16  is pressed against the outer periphery of the short hollow cylindrical blank  21  to rotate while applying force to the middle section  31  in the axial direction. In this manner, the short hollow cylindrical blank  21  is plastically deformed to form an elongated hollow cylindrical blank  211 . At this stage, the thicknesses of a middle section  311  and a bottom section  711  become equal to those of a middle section  3  and a bottom section  7  of a tank body  2  in a tank as a final product. 
     Subsequently, the elongated hollow cylindrical blank  211  is held by an unshown chucking device, and a predetermined region starting from the opening end is necked by spinning. The manner of the necking in process is as shown in FIG.  3 C: the elongated hollow cylindrical blank  211  is rotated on its axis, and in this state a forming roll  17  is pressed in an inclined position against the predetermined region starting from the opening end of the elongated hollow cylindrical blank  211  to rotate while moving obliquely with respect to the axis of the elongated hollow cylindrical blank  211 . In this manner, the predetermined region starting from the opening end of the elongated hollow cylindrical blank  211  is plastically deformed, resulting in the formation of a tank body  2  in which a cylindrical gas discharge section  5  is protruded integrally from one end of a cylindrical middle section  3  through a dome section  4 . The tank body  2  is formed so that the dome section  4  is gradually increased in thickness proceeding from the cylindrical middle section  3  to the cylindrical gas discharge section  5  through the necking by spinning as described above. 
     Then, the tank body  2  is loaded into a cutting machine, and as shown in  FIG. 3D , an annular fitting recess  9  is formed circumferentially in the exterior of the tank body  2  in the vicinity of the boundary between the dome section  4  and the cylindrical gas discharge section  5 . 
     Thereafter, as shown in  FIG. 3E , prepared is a reinforcing collar  10  of steel alloy or titanium alloy obtained by an additional forging or turning. As described above, the reinforcing collar  10  has a cylindrical part  11  and an extension  12  formed integrally at the lower end of the cylindrical part  11 , the back of the extension  12  is formed integrally with an annular boss  13 , and the reinforcing collar  10  is formed internally with a fitting hole  14  which passes through the cylindrical part  11  and the extension  12  vertically (see FIG.  2 ). The inner diameter of the fitting hole  14  is selected by taking into consideration a fitting allowance for shrinkage relative to the outer diameter of the cylindrical gas discharge section  5 . 
     Then, as shown in  FIG. 3F , the reinforcing collar  10  is fitted onto the cylindrical gas discharge section  5  of the tank body  2  so that the boss  13  of the reinforcing collar  10  is fitted in the fitting recess  9  of the dome section  4 . Subsequently, the reinforcing collar  10  is engaged integrally on the exterior of the tank body  2  between the cylindrical gas discharge section  5  and the dome section  4  by shrinkage. 
     As can be understood from the above, the additionally prepared reinforcing collar  10  need only be fitted onto the cylindrical gas discharge section  5  of the tank body  2 . Accordingly, a high-pressure tank  1  can be fabricated easily without requiring much labor. 
     In the above embodiment, an end-closed cylindrical blank is illustrated as the short hollow cylindrical blank  21  provided for flow forming. However, the short hollow cylindrical blank  21  may be a hollow cylinder both ends of which are open. In this case, predetermined regions respectively starting from both opening ends will be necked by spinning.