Abstract:
Disclosed is a water jet peening method that includes the steps of: preparing a water jet peening apparatus having a supporting member, a first divider plate, a nozzle support body, and a second divider plate; inserting the water jet peening apparatus into a piping in which a structure or electronic device is mounted that is susceptible to damage by a jet of water discharged from a jet nozzle or by shock waves; disposing either the first divider plate or the second divider plate between the jet nozzle and the structure or electronic device; filling water into an internal area formed in the piping between the first divider plate and the second divider plate; and subjecting the inner surface of the piping to water jet peening by allowing the jet nozzle to discharge a jet of water into the water in the internal area.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method and apparatus for water jet peening, and more particularly to a method and apparatus for water jet peening that is preferably applicable to a nuclear power plant. 
         [0003]    2. Description of the Related Art 
         [0004]    It is known that a residual stress mitigation method is available to give compressive residual stress to the surface of a structural member of a nuclear power plant or the like by subjecting such a structural member by applying water jet peening. 
         [0005]    A water jet peening method described in JP-2010-276491-A subjects the outer surface of a bottom mounted instrument piping to water jet peening by disposing a jet nozzle, which discharges a jet of high-pressure water, at the bottom of a reactor pressure vessel and rotating the jet nozzle, which is discharging a jet of high-pressure water, around the bottom mounted instrument piping. In this manner, compressive residual stress is given to the outer surface of the bottom mounted instrument piping. 
         [0006]    A water jet peening method for giving compressive residual stress to the inner surface of piping is described in JP-2008-14447-A. This water jet peening method inserts a working device in piping with a jet nozzle into a piping, moves the working device within the piping, and allows the jet nozzle to discharge a jet of high-pressure water toward the inner surface of a welding area of the piping. Hence, compressive residual stress is given to the inner surface of the welding area. 
         [0007]    A water jet peening method to be applied in a piping is described in JP-2002-200528-A. In order to subject the inner surface of a welding area formed inner side of a piping to water jet peening, a jet nozzle is inserted into the piping from one end of the piping, and the one end of the piping is sealed with a tube plug while the other end of the piping is sealed with a tube plug or with a valve. Water is then filled into a sealed area in the piping. Eventually, the inner surface of the welding area formed on the piping is subjected to water jet peening by allowing the jet nozzle in the water to discharge a jet of high-pressure water. When the jet of high-pressure water is discharged, air existing in the sealed area is expelled outside through a pipe connected to the tube plug. 
       SUMMARY OF THE INVENTION 
       [0008]    The method described in JP-2002-200528-A forms a water area in the piping targeted for water jet peening and subjects the inner surface of the piping in contact with the water area to water jet peening. However, no tube plug is inserted into the piping in this method. Therefore, it is difficult to subject an arbitrary area in the piping to water jet peening. Further, the water jet peening method described in JP-2002-200528-A is not designed to prevent damage to a structure or electronic device that may exist in the piping and become damaged by shock waves formed by high-pressure water or cavitations. 
         [0009]    The present invention has been made in view of the above-described circumstances to provide a water jet peening method and apparatus for making it possible to prevent damage to a structure or electronic device that is mounted on a piping and is susceptible to damage by a jet of water or by shock waves. 
         [0010]    According to one aspect of the present invention, there is provided a water jet peening method including the steps of: preparing a water jet peening apparatus having a supporting member, a first divider plate mounted on one end of the supporting member, a nozzle support body formed by disposing a jet nozzle around the supporting member, and a second divider plate mounted on the supporting member, the jet nozzle being disposed between the first divider plate and the second divider plate; inserting the water jet peening apparatus into a piping in which a structure or electronic device that is susceptible to damage by shock waves is mounted; disposing either the first divider plate or the second divider plate between the jet nozzle and the structure or electronic device; filling water into an internal area formed in the piping between the first divider plate and the second divider plate; and subjecting the inner surface of the piping to water jet peening by allowing the jet nozzle to discharge a jet of water into the water in the internal area. 
         [0011]    Either the first divider plate or the second divider plate is disposed between the jet nozzle and the structure or electronic device that is mounted on the piping that is susceptible to damage by shock waves. Water is filled into the internal area formed in the piping between the first divider plate and the second divider plate. The inner surface of the piping is subjected to water jet peening by allowing the jet nozzle to discharge a jet of water. Consequently, the water jet peening is performed for the inner surface of the piping by shock waves that are generated upon the collapse of cavitations in the jet of water discharged from the jet nozzle into the water existing in the internal area. Further, the jet of water discharged or the shock waves are blocked by the divider plate disposed between the jet nozzle and the structure or electronic device. This makes it possible to prevent the structure or electronic device from being damaged by the jet of water discharged or by the shock waves. 
         [0012]    According to the present invention, a structure or electronic device that is mounted on a piping and is susceptible to damage by a jet of discharged water or by shock waves can be prevented from being damaged when the inside of the piping is subjected to water jet peening. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: 
           [0014]      FIG. 1  is a diagram illustrating a water jet peening method that is applied inside of a piping in accordance with a first embodiment of the present invention; and 
           [0015]      FIG. 2  is a diagram illustrating the water jet peening method that is applied inside of the piping in accordance with a second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Embodiments of the present invention will now be described. 
       First Embodiment 
       [0017]    A water jet peening method according to a first embodiment of the present invention will be described below with reference to  FIG. 1 . 
         [0018]    First of all, a water jet peening apparatus to which the water jet peening method according to the present embodiment is applied will be described. The water jet peening apparatus  6  includes a jet nozzle  20 , a nozzle head  21 , a supporting member  22  shaped like a round bar, divider plates  23 A,  23 B, a high-pressure water supply device  38 , water supply device  39 , a water discharge device  44 , a sealing device  51 , and a rotation body  56 . 
         [0019]    The supporting member  22  is surrounded by the ring-shaped divider plate  23 A. A coupling member  17  couples the divider plate  23 A to one end of the supporting member  22 . A gap between the divider plate  23 A and the supporting member  22  is sealed with a seal member  19  attached to the inner circumference of the divider plate  23 A. The supporting member  22  is penetrated through a cylindrical rotation body  56 . The inner surface of the rotation body  56  is in contact with the outer surface of the supporting member  22 . The rotation body  56  can rotate along the outer surface of the supporting member  22 . A gap between the supporting member  22  and the cylindrical rotation body  56  is sealed with seal members  32 ,  35  attached to the supporting member  22 . The rotation body  56  is surrounded by the cylindrical nozzle head  21 . The inner surface of the nozzle head  21  is in contact with the outer surface of the rotation body  56 . A groove  27  extending in the axial direction of the supporting member  22  is formed in the inner surface of the nozzle head  21 . A key  33  mounted on the outer surface of the rotation body  56  is inserted into the groove  27 . The length of the groove  27  in the axial direction of the supporting member  22  is longer than the length of the key  33  in the axial direction. Hence, the nozzle head  21  can move in the axial direction of the supporting member  22  along the rotation body  56 . A gap between the rotation body  56  and the nozzle head  21  is sealed with a seal member  34  attached to the nozzle head  21 . The nozzle head  21  is surrounded by the ring-shaped divider plate  23 B. The inner surface of the divider plate  23 B is in contact with the outer surface of the nozzle head  21 . A gap between the nozzle head  21  and the divider plate  23 B is sealed with a seal member  36  attached to the divider plate  23 B. The jet nozzle  20  is mounted on a leading end of the nozzle head  21  and inclined with respect to the shaft center of the supporting member  22 . A plurality of combining members  47  attached to the outer surface of the supporting member  22  are used to fasten the divider plate  23 B to an end of the supporting member  22 , which is opposite the end of the supporting member  22  on which the divider plate  23 A is mounted. The combining members  47  are spaced at predetermined intervals and disposed in the circumferential direction of the supporting member  22 . 
         [0020]    A stopper structure  62 , which comes into contact with one end face of the rotation body  56 , is attached to the outer surface of the supporting member  22 . A stopper structure  63 , which comes into contact with the other end face of the rotation body  56 , is attached to the outer surface of the supporting member  22 . The stopper structures  62 ,  63  are not mounted on the rotation body  56 . 
         [0021]    The divider plate  23 A forms a scoop (air gathering area)  9  on its surface facing the divider plate  23 B. An air emission route  16 , which is open in the scoop  9 , is formed in the divider plate  23 B. An air emission route  14  is formed in the supporting member  22  and extended from one end of the supporting member  22 , on which the divider plate  23 A is mounted, to the other end of the supporting member  22 . The air emission route  14  is connected to the air emission route  16 . An air emission hose  43  having a pressure adjustment valve  28 A is connected to the air emission route  14 . 
         [0022]    A high-pressure water route  11  is connected to the jet nozzle  20  is formed in the nozzle head  21 . The high-pressure water supply device  38  includes a high-pressure pump  26 , a water supply pipe  40 , and a high-pressure hose  41 . The water supply pipe  40 , which is connected to a water source  25 , is connected to the high-pressure pump  26 . The high-pressure hose  41 , which is connected to the high-pressure water route  11 , is connected to the high-pressure pump  26 . 
         [0023]    The water supply device  39  includes a check valve  29  and a water supply pipe  45 . The water supply pipe  45 , which is provided with the check valve  29 , is connected to a water supply route  12 , which is formed through the divider plate  23 B. The water supply pipe  45  is connected to the water source  25  and provided with a pump (not shown). The water discharge device  44  includes a pressure adjustment valve  28 B and a water discharge pipe  46 . The water discharge pipe  46 , which has the pressure adjustment valve  28 B, is connected to a water discharge route  18 , which is formed through the divider plate  23 B. 
         [0024]    The sealing device  51  includes ring-shaped hollow seal members  24 A,  24 B made of rubber or other elastic material, a compressor  50 , and air supply pipes  52 ,  53 . A ring-shaped internal space is formed in the hollow seal members  24 A,  24 B. The hollow seal member  24 A is attached to the outer circumference of the divider plate  23 A to surround the divider plate  23 A. An air supply route  8  connected to the internal space of the hollow seal member  24 A is formed in the divider plate  23 A and extended to the inner surface of the divider plate  23 A. The air supply route  8  is connected to an air supply route  7  formed in the supporting member  22 . The air supply route  7  is connected to the air supply pipe  53 , which is mounted on an end of the supporting member  22 . The hollow seal member  24 B is mounted on the outer circumference of the divider plate  23 B to surround the divider plate  23 B. An air supply route  10  connected to the internal space of the hollow seal member  24 B is formed in the divider plate  23 B and extended to the inner surface of the divider plate  23 B. The air supply route  10  is in communication with the air supply pipe  52 , which is mounted on a lateral surface of the divider plate  23 B. The air supply pipes  52 ,  53  are connected to the compressor  50 . An exhaust pipe  58  provided with an on-off valve  59  is connected to the air supply pipe  52 . An exhaust pipe  60  provided with an on-off valve  61  is connected to the air supply pipe  53 . 
         [0025]    A motor  54  is disposed at an end of the supporting member  22  that is opposite the other end of the supporting member  22  to which the divider plate  23 A is attached, and mounted on the outer surface of the supporting member  22 . A gear  55  is coupled to the rotation shaft of the motor  54  through a down speed mechanism (not shown). The gear  55  is in mesh with a gear (not shown) disposed on the outer surface of one end of the rotation body  56 . The rotation body  56  is surrounded by the gear disposed on the outer surface of the rotation body  56 . The rotation body  56 , the motor  54 , and the gear  55  form a rotation device for rotating the nozzle head  21 . 
         [0026]    A transfer device  48  is mounted on the outer surface of one end of the rotation body  56 . Although not shown, the transfer device  48  includes a cylinder barrel, a piston disposed in the cylinder barrel, and a piston rod coupled to the piston. The piston rod is coupled to one end of the nozzle head  21 . An air supply hose  49  connected to the compressor  50  is connected to the cylinder barrel of the transfer device  48 . 
         [0027]    A water jet peening method according to the present embodiment, which is exercised by using the water jet peening apparatus  6 , will now be described in detail. 
         [0028]    A target to be subjected to water jet peening is, for example, a piping  1  connected to a vessel  37 . More specifically, the inner surface of a welding area of the piping  1  is subjected to water jet peening. The piping  1  is extended in a vertical direction. There is a gaseous atmosphere in the vessel  37  and in the piping  1 . When the supporting member  22  is pressed into the piping  1 , the water jet peening apparatus  6  is inserted into the piping  1  from the inside of the vessel  37 . In this instance, the divider plates  23 A,  23 B attached to the supporting member  22  are inserted into the piping  1 . As no pressurized air is introduced into the internal spaces of the hollow seal members  24 A,  24 B, the hollow seal members  24 A,  24 B are easily deformed and positioned apart from the inner surface of the piping  1 . Therefore, the divider plates  23 A,  23 B can be easily inserted into the piping  1  and moved within the piping  1 . When the leading divider plate  23 A travels through a residual stress improvement area  2 , which is on the inner surface of the piping  1 , until the residual stress improvement area  2  is positioned between the divider plate  23 A and the divider plate  23 B, the water jet peening apparatus  6  stops its travel in the piping  1 . 
         [0029]    The on-off valves  59 ,  61  are closed. The compressor  50  is driven so that resulting compressed air is discharged from the compressor  50  to the air supply pipes  52 ,  53 . The compressed air discharged to the air supply pipe  52  is supplied to the internal space of the hollow seal member  24 B through the air supply route  10 . The supplied compressed air causes the hollow seal member  24 B to expand so that the outer surface of the hollow seal member  24 B comes into contact with the inner surface of the piping  1  involving the entire circumferential surface of the divider plate  23 B. A gap between the inner surface of the piping  1  and the outer surface of the divider plate  23 B is sealed with the expanded hollow seal member  24 B. The compressed air discharged to the air supply pipe  53  is supplied to the internal space of the hollow seal member  24 A through the air supply routes  7 ,  8 . The supplied compressed air causes the hollow seal member  24 A to expand so that the outer surface of the hollow seal member  24 A comes into contact with the inner surface of the piping  1  involving the entire circumferential surface of the divider plate  23 A. A gap between the inner surface of the piping  1  and the outer surface of the divider plate  23 A is sealed with the expanded hollow seal member  24 A. 
         [0030]    Consequently, an external area  3 , which exists outside the divider plates  23 A,  23 B, and an internal area  4 , which is isolated from external area  3 , are formed in the piping  1  between the divider plate  23 A and the divider plate  23 B. The residual stress improvement area  2  of the piping  1  faces the internal area  4 . The jet nozzle  20  mounted on the nozzle head  21  is located in the internal area  4 . 
         [0031]    The pressure adjustment valve  28 B is closed, whereas the pressure adjustment valve  28 A is open. A pump (not shown) disposed in the water supply pipe  45  is driven so that the water in the water source  25  is pressurized, passed through the water supply pipe  45  with the check valve  29  and through the water supply route  12 , and supplied into the internal area  4 . Air existing in the internal area  4  is pushed upward by the water supplied into the internal area  4 , passed through the air emission route  16  and through the air emission route  14 , forwarded to the air emission hose  43 , and discharged outside. As the water is continuously supplied from the water source  25  to the internal area  4 , the level of the water in the internal area  4  rises so that the internal area  4  is filled with the water before long. In this state, the pump disposed in the water supply pipe  45  stops to shut off the water supply to the internal area  4 . 
         [0032]    The preparation for water jet peening for the inner surface of the piping  1  is now completed. 
         [0033]    The high-pressure pump  26  is driven to, pressurize the water in the water source  25  and discharge the resulting high-pressure water. The high-pressure water discharged from the high-pressure pump  26  is passed through the high-pressure hose  41  and through the high-pressure water route  11  and supplied to the jet nozzle  20 . The high-pressure water is then discharged in the form of a jet of high-pressure water from the jet nozzle  20  toward the residual stress improvement area  2 , which exists on the inner surface of the piping  1 . Cavitations included in the discharged jet of water collapse to generate shock waves. The generated shock waves are applied to the residual stress improvement area  2 . The shock waves generate compressive residual stress by improving tensile residual stress, which exists in the residual stress improvement area  2 . 
         [0034]    While discharging a jet of high-pressure water, the jet nozzle  20  rotates in a circumferential direction along the inner surface of the piping  1 . The aforementioned rotation device rotates the jet nozzle  20 . When the jet nozzle  20  is to be rotated, the motor  54  is driven to transmit the rotation of the motor  54  to the gear  55 , which then rotates the rotation body  56 . As the key  33  mounted on the rotation body  56  is inserted into the groove  27  formed in the inner surface of the nozzle head  21 , the key  33  transmits the rotation of the rotation body  56  to the nozzle head  21 . This causes the nozzle head  21  to rotate around the supporting member  22  together with the rotation body  56 . Hence, while discharging a jet of water, the jet nozzle  20  attached to the nozzle head  21  rotates in the circumferential direction of the piping  1  to perform water jet peening in the circumferential direction of the residual stress improvement area  2 . The transfer device  48  attached to the outer surface of the rotation body  56  rotates together with the nozzle head  21 . After the jet nozzle  20  makes one revolution around the supporting member  22 , a control device (not shown) exercises control to rotate the motor  54  in opposite direction. As the control device causes the motor  54  to repeatedly rotate in normal direction and in opposite direction, the jet nozzle  20  rotates around the supporting member  22  alternately in normal direction and opposite direction as indicated by an arrow  30 . This prevents the high-pressure hose  41  connected to the rotating nozzle head  21  and the air supply hose  49  connected to the transfer device  48  from being wrung off. 
         [0035]    While the shock waves are applied to the inner surface of the piping  1  in the residual stress improvement area  2  after a jet of water is discharged from the jet nozzle  20 , the cavitations that are discharged, included in the jet of water, and left uncollapsed are raised through the water in the internal area  4  and gathered by the scoop  9  formed on the divider plate  23 A. The gathered cavitations are then discharged from the scoop  9  to the air emission route  16 . Further, the cavitations are discharged to the air emission hose  43  through the air emission route  14 . This makes it possible to prevent the internal area  4  from being filled with the cavitations. 
         [0036]    While the jet of water is being discharged from the jet nozzle  20 , the pressure adjustment valve  28 B is open so that the water in the internal area  4  is discharged to the water discharge pipe  46  through the water discharge route  18 . The degree of opening of the pressure adjustment valve  28 B is adjusted so that the water discharged from the jet nozzle  20  to the internal area  4  entirely flows to the water discharge pipe  46  through the water discharge route  18 . As the water in the internal area  4  is discharged outside the internal area  4  through the water discharge route  18 , it is possible to prevent the pressure in the internal area  4  from being excessively increased by the jet of water discharged from the jet nozzle  20 . 
         [0037]    Compressed air discharged from the compressor  50  is supplied to a lower chamber (not shown) in the cylinder barrel of the transfer device  48  through the air supply hose  49 . The piston in the cylinder barrel is then pushed upward so that the nozzle head  21 , which is coupled to the piston by way of the piston rod, moves upward in the axial direction of the supporting member  22  (see an arrow  31 ). The nozzle head  21  moves in the axial direction because the key  33  mounted on the rotation body  56  is inserted into the groove  27  longer than the key  33  to let the nozzle head  21  move along the key  33 . Therefore, the jet nozzle  20 , which is discharging a jet of water, can be moved upward without allowing the divider plates  23 A,  23 B to move in the axial direction of the piping  1  while the jet nozzle  20  is rotating around the supporting member  22  within the internal area  4 . As the jet nozzle  20  rotates within the internal area  4  and moves in the axial direction of the supporting member  22 , the residual stress improvement area  2  in the circumferential direction and axial direction of the piping  1  can be entirely subjected to water jet peening. 
         [0038]    To move the divider plates  23 A,  23 B in the axial direction of the piping  1  within the piping  1  to reposition the jet nozzle  20 , it is necessary to discharge the water in the internal area  4  to the outside through the water discharge route  18  and expel the compressed air from the hollow seal members  24 A,  24 B as described later to contract the hollow seal members  24 A,  24 B. Further, after the supporting member  22 , the divider plates  23 A,  23 B, and the like are moved in the axial direction within the piping  1  to reposition the jet nozzle  20 , it is necessary to introduce compressed air into the hollow seal members  24 A,  24 B and fill the internal area  4  with water. Thus, it takes a long period of time to reposition the jet nozzle in the axial direction of the piping  1  by moving the divider plates  23 A,  23 B. In the present embodiment, however, the nozzle head  21  can move in the axial direction of the supporting member  22  as described earlier. Therefore, the jet nozzle  20  can move in the axial direction of the piping  1  within a short period of time. 
         [0039]    After completion of water jet peening for the inner surface of the piping  1 , the on-off valves  59 ,  61  open. The compressed air in the hollow seal member  24 A is then discharged outside through the air supply routes  8 ,  7 , the air supply pipe  53 , and the exhaust pipe  60 . Further, the compressed air in the hollow seal member  24 B is discharged outside through the air supply route  10 , the air supply pipe  52 , and the exhaust pipe  58 . The hollow seal members  24 A,  24 B contract and leave the inner surface of the piping  1 . Subsequently, the water jet peening apparatus  6  is extracted from the piping  1  and removed outside the vessel  37 . 
         [0040]    According to the present embodiment, the sizes of the divider plates  23 A,  23 B, which define the internal area  4 , are smaller than the transverse cross sectional area of the inner surface of the piping  1  to be subjected to water jet peening. Therefore, the water jet peening apparatus  6  can be easily inserted into the piping  1  and subjected to water jet peening at an arbitrary position within the piping  1 . Particularly, as the contractible, circular, hollow seal members  24 A,  24 B are disposed on the outer surfaces of the divider plates  23 A,  23 B, the water jet peening apparatus  6  can be easily inserted into and extracted from the piping  1 . 
         [0041]    When compressed air is introduced into the internal spaces of the hollow seal members  24 A,  24 B to expand the hollow seal members  24 A,  24 B while the water jet peening apparatus  6  is inserted in the piping  1 , it is possible to seal the gap between the inner surface of the piping  1  and the divider plate  23 A and the gap between the inner surface of the piping  1  and the divider plate  23 B. Thus, the internal area  4  can be isolated from the external area  3 . This makes it possible to fill the internal area  4  with water and subject the residual stress improvement area  2  to water jet peening. 
         [0042]    The jet nozzle,  20  is disposed between the divider plate  23 A and the divider plate  23 B. Therefore, even when a sensor or other electronic device  5  (or structure) that may be damaged by shock waves generated upon the collapse of cavitations included in a jet of water discharged from the jet nozzle  20  is disposed in the piping  1 , the divider plate  23 A can be positioned between the electronic device  5  and the jet nozzle  20 . Thus, the shock waves generated in the internal area  4  during water jet peening can be blocked by the divider plate  23 A to prevent the shock waves from being applied to the electronic device  5 . In the present embodiment, the jet nozzle  20  is disposed between the divider plate  23 A and the divider plate  23 B. This makes it possible to prevent the electronic device  5  mounted on the piping  1  from being damaged by the shock waves. 
         [0043]      FIG. 1  shows that the electronic device  5  is protruded inward from the inner surface of the piping  1 . However, when the electronic device  5  is mounted on the piping so that a leading end of the electronic device  5  is positioned on the inner surface of the piping  1  without allowing the electronic device  5  to protrude inward from the inner surface of the piping  1 , the water jet peening apparatus  6  can be inserted into the piping  1  to the extent that the water jet peening apparatus  6  is positioned behind the electronic device  5 . When the inner surface of the piping  1  is to be subjected to water jet peening at a place behind the electronic device  5 , the divider plate  23 B is disposed between the jet nozzle  20  and the electronic device  5 . When water jet peening is performed in this state, the divider plate  23 B blocks the shock waves and prevents the electronic device  5  from being damaged by the shock waves. Even when the electronic device  5  is mounted on the piping  1  while it is filled with water, it is possible to prevent the electronic device  5  from being damaged as far as the divider plates  23 A,  23 B are disposed between the jet nozzle  20  and the electronic device  5 . 
         [0044]    In the present embodiment, the stopper structures  62 ,  63  prevent the rotation body  56  from moving in the axial direction of the supporting member  22 . Therefore, even when the transfer device  48  moves the nozzle head  21  in that axial direction, the rotation body  56  does not move in that axial direction. Further, the stopper structure  62  prevents the rotation body  56  from falling. 
         [0045]    As the nozzle head  21  is surrounded by the divider plate  23 B, it is easy to connect the high-pressure hose  41  to the high-pressure water route  11 , which is formed in the rotating nozzle head  21 , and connect the air supply hose  49  to the transfer device  48 , which rotates together with the rotation body  56 . The nozzle head  21  and the transfer device  48  can also be rotated. Further, as the nozzle head  21  is surrounded by the divider plate  23 B, the spacing interval between the divider plate  23 A and the divider plate  23 B can be narrowed to ease the handling of the water jet peening apparatus  6  and improve the installability of the water jet peening apparatus  6  in the piping  1 . 
       Second Embodiment 
       [0046]    The water jet peening method according to a second embodiment of the present invention will now be described with reference to  FIG. 2 . 
         [0047]    In the first embodiment, water jet peening is performed for the inner surface of the piping  1  that is vertically extended. In the second embodiment, however, water jet peening is performed for the inner surface of a piping  1 A that is horizontally extended. 
         [0048]    A water jet peening apparatus  6 A to which the water jet peening method according to the second embodiment is applied is configured so that the scoop (air gathering area)  9  included in the water jet peening apparatus  6  to which the water jet peening method according to the first embodiment is applied is replaced by a scoop  9 A. The other components of the water jet peening apparatus  6 A are the same as those of the water jet peening apparatus  6 . 
         [0049]    The scoop (air gathering area)  9 A is formed on a circumferential surface of the divider plate  23 A in the water jet peening apparatus  6 A that faces the divider plate  23 B. When the water jet peening apparatus  6 A is to be inserted into the piping  1 A that is horizontally extended and connected to the vessel  37 , the scoop  9 A is located at the highest position in the piping  1 A as shown in  FIG. 2 . 
         [0050]    The water jet peening performed by the water jet peening apparatus  6 A with respect to the residual stress improvement area  2 , which is the inner surface of the piping  1 A, is the same as the water jet peening performed by the water jet peening apparatus  6  according to the first embodiment. 
         [0051]    The second embodiment provides the same advantages as the first embodiment.