Patent Publication Number: US-11649926-B2

Title: Pressure vessel

Description:
This application is a division of U.S. patent application Ser. No. 16/564,374, filed Sep. 9, 2019, which claims priority from Japanese Patent Application No. 2018-183429 filed on Sep. 28, 2018, the disclosures of which, including the specification, drawings and abstract, are incorporated herein by reference in their entireties. 
    
    
     INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2018-183429 filed on Sep. 28, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a pressure vessel that includes a liner made of a resin and configured to store a pressurized fluid, and a reinforcing layer made of a fiber-reinforced resin provided around an outer peripheral surface of the liner. 
     2. Description of Related Art 
     Conventional pressure vessels include a pressure vessel including a liner made of a resin and configured to store a pressurized fluid, such as a pressurized gas, and a reinforcing layer with which the liner is reinforced from outside the liner (refer to, for example, Japanese Unexamined Patent Application Publication No. 2015-017641 (JP 2015-017641 A)). The reinforcing layer is formed by winding a strip-shaped fiber-reinforced resin around an outer peripheral surface of the liner by filament winding (FW). 
     SUMMARY 
     However, when the pressure vessel enters a low-temperature and low-pressure state, the liner made of a resin having a higher linear expansion coefficient than that of the reinforcing layer contracts in its radial direction and axial direction, and thus a gap may be formed between the liner and the reinforcing layer. As a result, the liner and the reinforcing layer may be displaced relative to each other in the axial direction of the liner. 
     The present disclosure provides a pressure vessel having a configuration that reduces relative displacement between a liner and a reinforcing layer in the axial direction of the liner. 
     An aspect of the present disclosure relates to a pressure vessel including a liner and a reinforcing layer. The liner is made of a resin. The liner is configured to store a pressurized fluid. The reinforcing layer is made of a fiber-reinforced resin. The fiber-reinforced resin is provided around an outer peripheral surface of the liner. The liner includes a body portion having a tubular shape and a pair of side-end portions each having a domical shape. One of the side-end portions extends continuously from one of two ends of the body portion, and the other one of the side-end portions extends continuously from the other one of the two ends of the body portion. The liner includes a restriction portion provided at a center of the liner in an axial direction of the body portion. The restriction portion is configured to restrict displacement of the reinforcing layer in the axial direction. 
     According to the foregoing aspect, even when the liner is peeled off from the reinforcing layer due to contraction of the liner in the radial direction and the axial direction, the restriction portion provided at the center of the liner enables the reinforcing layer to be kept held by the liner. As a result, it is possible to reduce relative displacement between the reinforcing layer and the liner in the axial direction of the liner. 
     According to the foregoing aspect, the restriction portion is provided at the center of the liner, and thus the position at which the reinforcing layer is held by the liner is at the center of the liner. Therefore, an axial force with which each of the domical side-end portions of the liner is peeled off from the reinforcing layer when the liner contracts in the axial direction can be made lower than that in a case where a restriction portion is provided at a position offset from the center of the liner (e.g., at a position close to one end of the liner). 
     In a case where the caps are provided at the respective end portions of the liner, during contraction of the liner in the axial direction, the caps are kept held by the reinforcing layer and thus a contraction force of the liner acts on the caps in the directions in which the caps are detached from the liner. However, according to the foregoing aspect, the restriction portion is provided at the center of the body portion of the liner, and therefore, such a contraction force is reduced, and thus the force acting on the caps due to contraction of the liner is reduced. Note that, the center of the liner in the axial direction of the body portion signifies a portion including the center of a region between both ends of the liner in the axial direction. 
     As long as the restriction portion is provided at the center of the liner, the restriction portion provided on the outer peripheral surface of the body portion may be in the form of a spot or may be composed of two or more restriction portions that are separated from each other. In the foregoing aspect, the restriction portion may extend along the circumference of the outer peripheral surface of the body portion. With this configuration, the restriction portion provided at the center of the liner enables the reinforcing layer to be stably held by the liner. The expression “the restriction portion extends along the circumference of the outer peripheral surface of the body portion” in the foregoing aspect signifies that an annular (endless) restriction portion is provided on the outer peripheral surface of the body portion of the liner. 
     In the foregoing aspect, the restriction portion may be a recessed portion provided in the outer peripheral surface of the body portion, and a part of the fiber-reinforced resin may be disposed in the recessed portion. With this configuration, the liner and the reinforcing layer are engaged with each other by a part of the fiber-reinforced resin, which is disposed in the recessed portion, and thus it is possible to reduce displacement of the reinforcing layer with respect to the liner in the axial direction of the liner. 
     In the foregoing aspect, the restriction portion may be a projection portion provided on the outer peripheral surface of the body portion, and the projection portion may be covered with the fiber-reinforced resin. With this configuration, the liner and the reinforcing layer are engaged with each other by the fiber-reinforced resin with which the projection portion is covered, and thus it is possible to reduce displacement of the reinforcing layer with respect to the liner in the axial direction. 
     The projection portion, which is an example of the restriction portion, may be formed as a part of the liner, for example, by scraping away a part of the outer peripheral surface of the liner, as long as the projection portion is provided at the center of the liner. In the foregoing aspect, the projection portion may be a fixed portion provided around the outer peripheral surface of the body portion so as to project from the outer peripheral surface of the body portion, and fixed to the outer peripheral surface. With this configuration, it is possible to easily provide a fixed portion having a desired shape and a desired size at the center of the liner without the need to scrape away a part of the liner. 
     The fixed portion is not particularly limited as long as it acts as the restriction portion that is a projection portion. In the foregoing configuration, the fixed portion may be a bundle of fibers impregnated with a resin, and the resin of the fixed portion may have higher adhesiveness for the outer peripheral surface of the liner than adhesiveness of the resin included in the fiber-reinforced resin. With this configuration, even when the fiber-reinforced resin is peeled off from the outer peripheral surface of the liner during contraction of the liner, it is possible to more reliably prevent the fixed portion from being peeled off from the outer peripheral surface of the liner. 
     According to the foregoing aspect of the present disclosure, it is possible to reduce relative displacement between the liner and the reinforcing layer in the axial direction of the liner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG.  1    is a schematic sectional view of a pressure vessel according to a first embodiment of the present disclosure; 
         FIG.  2    is an enlarged sectional view of a portion A of the pressure vessel illustrated in  FIG.  1   ; 
         FIG.  3    is a schematic perspective view of a liner of the pressure vessel illustrated in  FIG.  1   ; 
         FIG.  4    is a schematic perspective view for describing a method of manufacturing the pressure vessel illustrated in  FIG.  1   ; 
         FIG.  5 A  is a schematic perspective view illustrating a modification example of the liner of the pressure vessel illustrated in  FIG.  1   ; 
         FIG.  5 B  is an enlarged sectional view of a main portion in a modification example of the pressure vessel illustrated in  FIG.  2   ; 
         FIG.  6    is a schematic perspective view of a liner of a pressure vessel according to a second embodiment of the present disclosure; 
         FIG.  7    is an enlarged sectional view of a main portion of the pressure vessel including the liner illustrated in  FIG.  6   ; 
         FIG.  8 A  is a schematic perspective view illustrating a modification example of the liner of the pressure vessel illustrated in  FIG.  6   ; 
         FIG.  8 B  is an enlarged sectional view of a main portion in a modification example of the pressure vessel illustrated in  FIG.  6   ; 
         FIG.  9    is a schematic perspective view illustrating another modification example of the liner of the pressure vessel illustrated in  FIG.  6   ; and 
         FIG.  10    is an enlarged sectional view of a main portion of the pressure vessel including the liner illustrated in  FIG.  9   . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, several example embodiments of the present disclosure will be described with reference to  FIG.  1    to  FIG.  10   . 
     First Embodiment 
     Configuration of Pressure Vessel  1   
       FIG.  1    is a schematic sectional view of a pressure vessel  1  according to a first embodiment of the present disclosure.  FIG.  2    is an enlarged sectional view of a portion A of the pressure vessel  1  illustrated in  FIG.  1   .  FIG.  3    is a schematic perspective view of a liner  3  of the pressure vessel  1  illustrated in  FIG.  1   .  FIG.  4    is a schematic perspective view for describing a method of manufacturing the pressure vessel  1  illustrated in  FIG.  1   . 
     As illustrated in  FIG.  1   , the pressure vessel  1  is a fuel tank configured to store fuel gas, such as hydrogen gas, to be supplied to a fuel cell in an in-vehicle fuel cell system. The pressure vessel  1  may be used for any purposes other than electricity generation by the fuel cell system. Examples of the substance to be stored in the pressure vessel  1  may include not only hydrogen gas serving as fuel gas, but also various kinds of compressed gas, such as compressed natural gas (CNG), various kinds of liquefied gas, such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG), and various other pressurized substances. 
     The pressure vessel  1  includes the liner  3 , caps  2 A,  2 B, and a reinforcing layer  4 . The liner  3  is made of a resin and configured to store a pressurized fluid. The caps  2 A,  2 B are made of a metal. The cap  2 A is attached to one end portion of the liner  3 , and the cap  2 B is attached to the other end portion of the liner  3 . The reinforcing layer  4  is made of a fiber-reinforced resin. The reinforcing layer  4  is provided such that an outer peripheral surface of the liner  3  is covered with the fiber-reinforced resin and the outer peripheral surface of each of the caps  2 A,  2 B is partially covered with the fiber-reinforced resin. 
     The liner  3  has a storage space  34  for storing fuel gas. The storage space  34  is defined by a resin having a gas barrier property against the fuel gas. Examples of the resin material of the liner  3  include thermoplastic resins, such as polyethylene, polypropylene, nylon polyamide, and an ethylene vinyl alcohol copolymer. 
     The liner  3  includes a body portion  31  having a tubular shape and a pair of side-end portions  32 A,  32 B each having a domical shape. The side-end portion  32 A extends continuously from one end of the body portion  31 , and the side-end portion  32 B extends continuously from the other end of the body portion  31 . The storage space  34  for storing gas is provided inside the liner  3 . Specifically, the body portion  31  is a cylindrical portion having a predetermined length and extending along a central axis X of the pressure vessel  1  illustrated in  FIG.  1   . The side-end portion  32 A has such an outer contour that the side-end portion  32 A hemispherically bulges out from the one end of the body portion  31 . Similarly, the side-end portion  32 B has such an outer contour that the side-end portion  32 B hemispherically bulges out from the other end of the body portion  31 . An opening is provided in a top portion of each of the side-end portions  32 A,  32 B. The cap  2 A made of a metal is fitted in the opening of the side-end portion  32 A. Similarly, the cap  2 B made of a metal is fitted in the opening of the side-end portion  32 B. 
     The cap  2 A is made of a metal, such as aluminum or an aluminum alloy. The cap  2 A includes a tubular portion  21 A that is a main body, and a flange portion  22 A provided on the outer periphery of the tubular portion  21 A. The fiber-reinforced resin of the reinforcing layer  4  is wound around the flange portion  22 A of the cap  2 A and the liner  3 , with the flange portion  22 A pressed against the outer surface of the side-end portion  32 A. A valve (not illustrated) is attached to the cap  2 A. With the valve, fuel gas is discharged from the pressure vessel  1  or is allowed to flow into the pressure vessel  1 . 
     Like the cap  2 A, the cap  2 B is made of a metal, such as aluminum or an aluminum alloy, and includes a tubular portion  21 B that is a main body, and a flange portion  22 B provided on the outer periphery of the tubular portion  21 B. The fiber-reinforced resin of the reinforcing layer  4  is wound around the flange portion  22 B of the cap  2 B and the liner  3 , with the flange portion  22 B pressed against the outer surface of the side-end portion  32 B. The inside of the tubular portion  21 B of the cap  2 B is sealed, and thus the storage space  34  is hermetically sealed by the cap  2 B. 
     The reinforcing layer  4  is made of a fiber-reinforced resin (referred also to as “fiber-reinforced plastic (FRP)”) including reinforcing fibers (filaments) aligned in one direction and impregnated with a matrix resin. In the present embodiment, the fiber-reinforced resin is wound by hoop winding and helical winding such that multiple layers are formed on the outer peripheral surface of the liner  3  and an exposed portion of the outer peripheral surface of each of the caps  2 A,  2 B. 
     Examples of the reinforcing fibers include glass fibers, carbon fibers, aramid fibers, alumina fibers, boron fibers, steel fibers, poly-p-phenylenebenzobisoxazole (PBO) fibers, natural fibers, and high-strength polyethylene fibers. In the present embodiment, a carbon fiber-reinforced resin (referred also to as “carbon fiber-reinforced plastic (CFRP)”) including carbon fibers serving as reinforcing fibers is wound around the outer peripheral surface of the liner  3 , and a glass fiber-reinforced resin (referred also as “glass fiber-reinforced plastic (GFRP)”) including glass fibers serving as reinforcing fibers is further wound around the outer peripheral surface of the liner  3 . 
     In the present embodiment, a thermosetting resin is used as the matrix resin. Examples of the thermosetting resin include an epoxy resin, modified epoxy resins typified by a vinyl ester resin, a phenol resins, a melamine resins, a urea resin, an unsaturated polyester resin, an alkyd resin, a polyurethane resin, and a thermosetting polyimide resin. 
     In the present embodiment, a thermosetting resin is used as the matrix resin. However, a resin with which a bundle of fibers is impregnated may be a thermoplastic resin as long as the thermoplastic resin can be heated to a softening point (glass transition point) or higher. Examples of the thermoplastic resin include a polyester resin, a polypropylene resin, a nylon resin (e.g., a 6-nylon resin or a 6,6-nylon resin), a polyamide resin, a polycarbonate resin, an acrylic resin, and an Acrylonitrile Butadiene Styrene (ABS) resin. 
     In the present embodiment, a restriction portion is provided at the center of the liner  3  in a direction along the central axis X (the axial direction) of the body portion  31 . The restriction portion has such a shape as to restrict displacement of the reinforcing layer  4  in the axial direction. As illustrated in  FIG.  2   , an example of the restriction portion is a recessed portion  33 A provided in the outer peripheral surface of the body portion  31 , and a part of the fiber-reinforced resin of the reinforcing layer  4  is disposed in the recessed portion  33 A. In the present embodiment, the fiber-reinforced resin may be disposed in the recessed portion  33 A in any manner, as long as the fiber-reinforced resin of the reinforcing layer  4  does not come out of the recessed portion  33 A (that is, as long as the fiber-reinforced resin is kept engaged with recessed portion  33 A) even when the liner  3  contracts in the radial direction. The maximum depth of the recessed portion  33 A is desirably set to such a depth that the fiber-reinforced resin of the reinforcing layer  4  does not come out of the recessed portion  33 A of the liner  3  when the liner  3  contracts in the radial direction. It is desirable to set the maximum depth of the recessed portion  33 A to a depth greater than the maximum amount by which the liner  3  contracts in the radial direction under a given usage environment of the pressure vessel  1 . 
     In the present embodiment, the recessed portion  33 A is an annular groove extending along the circumference of the outer peripheral surface of the body portion  31 . The recessed portion  33 A is a concave portion recessed with respect to the outer peripheral surface of the liner  3 , in a section taken along the axial direction of the liner  3 . Specifically, a wall surface that defines the recessed portion  33 A (a side wall surface that defines the groove) is a curved wall surface. With this configuration, it is possible to reduce the stress to be concentrated on the recessed portion  33 A of the liner  3  when the reinforcing layer  4  is engaged with the recessed portion  33 A. The width of the recessed portion  33 A (groove width) is not limited to any particular width, and may be, for example, within a range of 5% to 15% of the total length of the liner  3 . 
     The recessed portion  33 A can be formed, for example, by scraping away a portion of a conventional liner. However, the recessed portion  33 A may be formed in any method. For example, the recessed portion  33 A may be formed by deforming a wall portion of the body portion  31  of the liner  3  with the thickness of the liner  3  kept substantially constant. 
     In the present embodiment, the center of the liner  3  where the recessed portion  33 A, which is an example of the restriction portion, is provided signifies a portion including a center line C located at the middle of a region between both ends of the liner  3 . The recessed portion  33 A may be any portion including the center line C. Therefore, where the length of the liner  3  in its axial direction is denoted by L, the recessed portion  33 A is provided so as to include a position that is at a distance of L/2 from each end of the liner  3  in the axial direction. 
     In the pressure vessel  1  configured as described above, for example, when the fuel gas is discharged from the pressure vessel  1 , the fuel gas adiabatically expands in the storage space  34 . This may cause a decrease in the internal pressure of the storage space  34 , leading to a decrease in the temperature of the fuel gas in the storage space  34 . In this case, the liner  3  may contract in its radial direction and axial direction. Even when the liner  3  is peeled off from the reinforcing layer  4  due to contraction of the liner  3 , the recessed portion  33 A (an example of the restriction portion) provided at the center of the liner  3  enables the reinforcing layer  4  to be kept held by the liner  3 . Specifically, the liner  3  and the reinforcing layer  4  are engaged with each other by a part of the fiber-reinforced resin of the reinforcing layer  4 , which is disposed in the recessed portion  33 A. As a result, it is possible to reduce displacement of the reinforcing layer  4  with respect to the liner  3  in the axial direction. 
     In particular, because the recessed portion  33 A, which is an example of the restriction portion, is provided at the center of the liner  3 , the position at which the reinforcing layer  4  is held by the liner  3  is at the center of the liner  3 . Therefore, an axial force with which each of the side-end portions  32 A,  32 B of the liner  3  is peeled off from the reinforcing layer  4  when the liner  3  contracts in the axial direction can be made lower than that in a case where a restriction portion is provided at a position offset from the center of the liner  3  (e.g., at a position close to one end of the liner  3 ). 
     In the present embodiment, the caps  2 A,  2 B are brought into contact with the respective end portions of the liner  3  from outside of the liner  3 , as described above. Therefore, when the liner  3  contracts in the axial direction, the caps  2 A,  2 B around which the fiber-reinforced resin is provided are kept held by the reinforcing layer  4 . Thus, a contraction force of the liner  3  acts on the caps  2 A,  2 B in the directions in which the caps  2 A,  2 B are detached from the liner  3 . However, the recessed portion  33 A, which is an example of the restriction portion, is provided at the center of the body portion  31  of the liner  3 , and therefore, such a contraction force is reduced, and thus the force acting on the caps  2 A,  2 B due to contraction of the liner  3  is reduced. 
     In particular, because the recessed portion  33 A extends along the circumference of the outer peripheral surface of the body portion  31 , the recessed portion  33 A provided at the center of the liner  3  enables the reinforcing layer  4  to be stably held by the liner  3 . As long as a part of the fiber-reinforced resin of the reinforcing layer  4  can be disposed in the recessed portion  33 A and the reinforcing layer  4  can be stably held by the liner  3 , the recessed portion  33 A may be composed of two or more recessed portions that are separated from each other, that are extending in the circumferential direction of the liner  3 , and that are disposed at the center of the liner  3 , for example, as illustrated in  FIG.  5 A . 
     As long as a sufficient strength of the liner  3  is maintained when the reinforcing layer  4  is engaged with the recessed portion  33 A, instead of the recessed portion  33 A illustrated in  FIG.  2   , a recessed portion  33 B defined by a bottom surface  33   a  and side wall surfaces  33   b  rising from the bottom surface  33   a  may be provided, as illustrated in  FIG.  5 B . 
     Method of Manufacturing Pressure Vessel  1   
     A method of manufacturing the pressure vessel  1  will be described below. First, the liner  3  as illustrated in  FIG.  3    is produced. Specifically, a pre-molded tubular member corresponding to the body portion  31  and domical members corresponding to the side-end portions  32 A,  32 B are joined together. Next, the caps  2 A,  2 B are fixed to the respective end portions of the liner  3  with, for example, an adhesive. Next, the recessed portion  33 A illustrated in  FIG.  3    is formed at the center of the liner  3  by scraping away a portion of the body portion  31 . 
     In the present embodiment, the recessed portion  33 A is formed at the center of the liner  3  after the caps  2 A,  2 B are fixed to the respective end portions of the liner  3  with, for example, an adhesive. Alternatively, the recessed portion  33 A may be formed in advance in a tubular member corresponding to the body portion  31 . 
     A roughening treatment, such as a blasting treatment or an etching treatment, may be performed on the surface of the recessed portion  33 A of the liner  3 . The surface roughness of the recessed portion  33 A is greater than the surface roughness of the other portion of the outer peripheral surface of the liner  3  than the recessed portion  33 A. Thus, the reinforcing layer  4  is easily kept held in the recessed portion  33 A due to an anchoring effect. 
     Next, a fiber-reinforced resin  41  (prepreg) is prepared. The fiber-reinforced resin  41  includes continuous reinforcing fibers aligned in one direction and impregnated with an uncured thermosetting resin. The fiber-reinforced resin  41  that has been prepared is in the form of a strip or a string. The fiber-reinforced resin  41  is wound, in layers, around the outer peripheral surface of the liner  3  to which the caps  2 A,  2 B have been attached, while a predetermined winding tension is applied to the fiber-reinforced resin  41 . By filament winding (FW), the reinforcing layer  4  is formed in the following manner. First, the fiber-reinforced resin  41  is moved in the axial direction while the liner  3  is rotated, so that the fiber-reinforced resin  41  is wound around the outer peripheral surface of the liner  3  by hoop winding while overlaps of the fiber-reinforced resin  41  are created (see, for example,  FIG.  4   ), and then the fiber-reinforced resin  41  is wound around the liner  3  by, for example, helical winding. At this time, the fiber-reinforced resin enters the recessed portion  33 A provided in the outer peripheral surface of the liner  3 . Then, the reinforcing layer  4  that has been formed is heated to cure the thermosetting resin that has not been cured. In this way, the pressure vessel  1  is obtained. 
     Second Embodiment 
     Hereinafter, a pressure vessel according to a second embodiment of the present disclosure and a method of manufacturing the pressure vessel will be described with reference to  FIG.  6    to  FIG.  8 B . A pressure vessel  1  according to the second embodiment is different from the pressure vessel  1  according to the first embodiment, in the shape of a restriction portion provided on the liner  3 . Therefore, the same portions as those in the first embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted. 
     As illustrated in  FIG.  6    and  FIG.  7   , in the present embodiment, a restriction portion is provided at the center of the liner  3  in the axial direction of the body portion  31 . The restriction portion has such a shape as to restrict displacement of the reinforcing layer  4  in the axial direction. As illustrated in  FIG.  6   , an example of the restriction portion is a projection portion  34 A provided on the outer peripheral surface of the body portion  31 , and the projection portion  34 A is covered with the fiber-reinforced resin of the reinforcing layer  4 . In the present embodiment, the center of the liner  3  where the projection portion  34 A, which is an example of the restriction portion, is provided signifies a portion including a center line located at the middle of a region between both ends of the liner  3 , as in the first embodiment. The projection portion  34 A may be any portion including the center line. It is desirable to set the maximum height of the projection portion  34 A to a height greater than the maximum amount by which the liner  3  contracts in the radial direction. 
     In the present embodiment, the projection portion  34 A is an annular ridge extending along the circumference of the outer peripheral surface of the body portion  31 . The width of the projection portion  34 A (ridge width) is not limited to any particular width, and may be, for example, within a range of 5% to 15% of the total length of the liner  3 . The projection portion  34 A is a convex portion that bulges with respect to the outer peripheral surface of the liner  3 , in a section taken along the axial direction of the liner  3 . Specifically, a wall surface that defines the projection portion  34 A is a curved wall surface. With this configuration, it is possible to reduce the stress to be concentrated on the projection portion  34 A of the liner  3  when the reinforcing layer  4  is engaged with the projection portion  34 A. 
     According to the present embodiment, even when the liner  3  is peeled off from the reinforcing layer  4  due to contraction of the liner  3 , the projection portion  34 A provided at the center of the liner  3  enables the reinforcing layer  4  to be kept held by the liner  3 , as in the first embodiment. Specifically, the liner  3  and the reinforcing layer  4  are engaged with each other by the fiber-reinforced resin of the reinforcing layer  4  with which the recessed portion  33 A is covered. As a result, it is possible to reduce displacement of the reinforcing layer  4  with respect to the liner  3  in the axial direction. Further, it is possible to reduce an axial force that acts when each of the side-end portions  32 A,  32 B of the liner  3  is peeled off from the reinforcing layer  4  during contraction of the liner  3  in the axial direction. Further, it is possible to reduce forces acting on the caps  2 A,  2 B due to contraction of the liner  3 . 
     In particular, because the projection portion  34 A extends along the circumference of the outer peripheral surface of the body portion  31 , the projection portion  34 A provided at the center of the liner  3  enables the reinforcing layer  4  to be stably held by the liner  3 . As long as the projection portion  34 A is covered with the fiber-reinforced resin of the reinforcing layer  4  and thus the reinforcing layer  4  can be stably held by the liner  3 , the projection portion  34 A may be composed of two or more projection portions that are separated from each other, that are extending in the circumferential direction of the liner  3 , and that are disposed at the center of the liner  3 , for example, as illustrated in FIG. 
       8 A. 
     As long as a sufficient strength of the reinforcing layer  4  is maintained when the reinforcing layer  4  is engaged with the projection portion  34 A, instead of the projection portion  34 A illustrated in  FIG.  7   , a projection portion  34 B having a rectangular section may be provided, as illustrated in  FIG.  8 B . 
     During manufacturing of the pressure vessel  1 , the projection portion  34 A may be formed, for example, by scraping away a portion of a conventional liner before the formation of the reinforcing layer  4 . Alternatively, for example, two tubular members having shapes obtained by splitting the shape of the body portion  31  at the center of the body portion  31  are prepared. Then, when end surfaces of the two tubular members are brought into contact with each other and joined together by thermal fusion bonding, portions of the two tubular members, which include the end surfaces, are deformed into the projection portion  34 A. The projection portion  34 A may be formed during molding of a tubular member corresponding to the body portion  31 . After the liner  3  illustrated in  FIG.  6    is prepared, the reinforcing layer  4  is formed by winding the fiber-reinforced resin  41  around the outer peripheral surface of the liner  3  including the projection portion  34 A, in the same method as the method described with reference to  FIG.  4   . As a result, the pressure vessel can be obtained. In this way, the surface of the projection portion  34 A is covered with the fiber-reinforced resin of the reinforcing layer  4 . 
     In  FIG.  6    to  FIG.  8 B , the projection portion  34 A,  34 B is formed so as to be integral with the liner  3 . Alternatively, for example, as illustrated in  FIG.  9   , the projection portion provided on the liner  3  may be a fixed portion  34 C that is provided around the outer peripheral surface of the body portion  31  so as to project from the outer peripheral surface of the body portion  31  and fixed to the outer peripheral surface of the body portion  31 . The fixed portion  34 C may be made of, for example, a resin such as a thermosetting resin, or metal as long as the fixed portion  34 C functions as a restriction portion. In the present embodiment, the fixed portion  34 C is a bundle of fibers impregnated with a resin. 
     A resin with which a bundle of fibers is impregnated is desirably a resin having higher adhesiveness (higher affinity) for the outer peripheral surface of the liner  3  than the matrix resin included in the fiber-reinforced resin of the reinforcing layer  4 . For example, when the liner  3  is made of a nylon resin (nylon 6, nylon 66, or the like) and the matrix resin is an epoxy resin, an acrylic modified silicone resin or the like may be an example of the resin with which a bundle of fibers of the fixed portion  34 C is impregnated. When the liner is made of a polyethylene resin and the matrix resin is an epoxy resin, cyanoacrylate or the like may be an example of the resin with which a bundle of fibers of the fixed portion  34 C is impregnated. 
     The fixed portion  34 C can be obtained by winding, multiple times, a bundle of fibers impregnated with an uncured resin around the outer peripheral surface of the body portion  31  at the center of the liner having a smooth outer peripheral surface, and then thermally curing the resin. Then, in the same method as the method described with reference to  FIG.  4   , the fiber-reinforced resin  41  is wound around the outer peripheral surface of the liner  3  including the fixed portion  34 C to form the reinforcing layer  4 . As a result, the pressure vessel  1  is obtained. In this way, as illustrated in  FIG.  10   , the surface of the fixed portion  34 C is covered with the fiber-reinforced resin of the reinforcing layer  4 . 
     The resin included in the fixed portion  34 C is a resin having higher adhesiveness for the outer peripheral surface of the liner  3  than the resin included in the fiber-reinforced resin of the reinforcing layer  4 . Thus, even when the fiber-reinforced resin is peeled off from the outer peripheral surface of the liner  3  during contraction of the liner  3 , it is possible to more reliably prevent the fixed portion  34 C from being peeled off from the outer peripheral surface of the liner  3 . 
     Before the fiber-reinforced resin is wound around the outer peripheral surface of the liner  3 , a roughening treatment may be performed on the surface of the projection portion  34 A, the outer surface of the projection portion  34 B or the outer surface of the fixed portion  34 C. In this way, the surface roughness of the projection portion  34 A, the projection portion  34 B or the fixed portion  34 C that has been subjected to the roughening treatment is greater than the surface roughness of the remaining portion of the outer peripheral surface of the liner  3 . Thus, due to an anchoring effect, the reinforcing layer  4  is easily kept held by the projection portion  34 A, the projection portion  34 B or the fixed portion  34 C that has been subjected to the roughening treatment. 
     Some embodiments of the present disclosure have been described in detail above. However, an applicable embodiment of the present disclosure is not limited to the foregoing embodiments, and various design changes may be made to the foregoing embodiments within the scope of the appended claims. 
     For example, the fixed portion according to the second embodiment is made of a bundle of fibers impregnated with a resin. Alternatively, the fixed portion may be made of a resin having a higher linear expansion coefficient than that of the resin material of the liner. In this way, the contraction amount of the fixed portion is greater than the contraction amount of the resin material of the liner, and therefore, the fixed portion is more reliably fixed to the outer peripheral surface of the liner.