Patent Publication Number: US-9903388-B2

Title: Centrifugal pump

Description:
FIELD OF THE INVENTION 
     The present invention relates to a centrifugal pump configured to force a fluid to flow along a volute internal flow channel upon rotation of an impeller disposed in a volute case. 
     BACKGROUND OF THE INVENTION 
     Centrifugal pumps generally include a volute provided inside a casing, and an impeller provided on a rotating shaft projecting into the volute. The impeller has a hub mounted on the rotating shaft and a plurality of vanes provided on the hub. The impeller is disposed inside the volute. When the centrifugal pump is to be driven, a self-priming operation is performed to create a negative pressure inside the volute to thereby introduce a fluid, such as water, into the volute by the effect of the negative pressure. 
     By thus introducing the fluid via the self-priming operation, a steady operation becomes possible. In the steady operation, the fluid is first sucked into the volute, then guided outside the volute (i.e., inside the casing) by the effect of a centrifugal force of the impeller, and finally discharged from the casing to the outside. A typical example of such centrifugal pumps is disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 03-267596. 
     When the centrifugal pump disclosed in JP 03-267596 A performs a self-priming operation, a priming fluid is introduced in the volute and the impeller is rotated by driving the rotating shaft. Upon rotation of the impeller, the priming fluid introduced in the volute is guided by the impeller to flow along the volute to thereby force gas (air) in the volute to be discharged outside the volute. As a consequence of this operation, a negative pressure is created inside the volute and, by the effect of the negative pressure, the fluid is sucked into the volute. A steady operation of the centrifugal pump is now ready to be performed. 
     In a method known as a means for properly discharging gas in the volute to the outside of the volute, the prime fluid in the volute is stirred by utilizing vanes of the impeller. By thus stirring the prime fluid, the gas (in the form of air bubbles) contained in the prime fluid is separated from the prime fluid and discharged from the volute to the outside of the volute. 
     However, in order to stir the prime fluid in the volute by using the vanes of the impeller, distal ends of the vanes should be formed into a shape which is suitable for stirring the prime fluid. The shape of the distal ends of the vanes greatly contributes to the stirring of the prime fluid. Under these circumstances, the degree of freedom in designing the shape of the vanes&#39; distal ends is considerably low, and sufficient elaborately measures cannot be taken to form the distal ends of the vanes into a shape which is suitable for performing a steady operation. 
     It is therefore an object of the present invention to provide a centrifugal pump which is capable of stirring a prime fluid and allows vanes to have distal ends formed into a shape suitable for a steady operation. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a centrifugal pump comprising: a volute case having a volute internal flow channel formed therein; an impeller rotatably disposed in the volute case for forcing a fluid to flow along the volute internal flow channel; and at least one flow-channel recessed portion opening to the volute internal flow channel and formed so as to be recessed in a direction substantially orthogonal to a direction of flow of the fluid. 
     With this arrangement, since the flow-channel recess portion opens to the volute internal flow channel and is formed so as to be recessed in the direction orthogonal to the direction of flow of the fluid, at a time of performing a self-priming operation, gas (air) in the volute case is mixed (or entrained) with the prime fluid in a state of air bubbles. By virtue of the air bubbles contained in the prime fluid, the viscosity and density of the prime fluid are reduced so that the prime fluid can be easily introduced in an internal space of the flow-channel recessed portion. 
     The prime fluid introduced in the flow-channel recessed portion creates a vortex flow within the internal space of the flow-channel recessed portion and the prime fluid is eventually stirred in the internal space of the flow-channel recessed portion. By thus stirring the prime fluid, generation of the air bubbles is promoted, which will insure proper separation of the air bubbles from the prime fluid. The gas (in the form of air bubble thus separated from the prime fluid) can be properly discharged from the volute case to the outside and, hence, the self-priming performance can be achieved with enhanced efficiency. 
     Furthermore, because the flow-channel recessed portion is formed to open to the volute internal flow channel and the prime fluid is stirred within the internal space of the flow-channel recessed portion, it is not necessary to stir the prime fluid by distal ends of vanes of the impeller. The vanes are therefore allowed to have distal ends formed into a shape which is suitable for a steady operation. Since the flow-channel recessed portion is closed at a bottom thereof and hence has a blind-hole shape, the flow-channel recess portion is kept in the state of being filled with the fluid during the steady operation. This arrangement hinders further entry of the fluid into the internal space of the flow-channel recessed portion and allows the fluid to be smoothly guided along the volute internal flow channel without entering the flow-channel recessed portion. By virtue of the vanes having distal ends formed into a shape suitable for the steady operation and by the fluid allowed to smoothly flow during the steady operation, a desired pumping efficiency during the steady operation can be obtained. 
     Preferably, the flow-channel recessed portion has an opening which faces the volute internal flow channel, and a peripheral edge defining the opening, the peripheral edge having a straight section substantially orthogonal to the direction of flow of the fluid. By virtue of the straight section of the opening&#39;s peripheral edge, the opening is allowed to have a sufficiently large width as measured in a direction orthogonal to the direction of flow of the fluid. By thus providing the sufficiently large opening width, the prime fluid can be appropriately guided from the opening into the internal space of the flow-channel recessed portion, and the thus guided prime fluid is able to appropriately create a vortex flow within the internal space of the flow-channel recessed portion. The vortex flow effectively promotes generation of air bubbles from the prime fluid, which will lead to appropriate separation of the air bubbles from the prime fluid. By thus separating the air bubbles from the prime fluid, the gas in the volute internal flow channel can be appropriately discharged to the outside. The self-priming performance can thus be achieved with enhanced efficiency 
     The straight section of the opening&#39;s peripheral edge may be provided on both an upstream side and a downstream side as viewed from the direction of flow of the fluid, or alternately, on only one of the upstream side and the downstream side. In the case where the straight section is provided on both the upstream side and the downstream side of the opening&#39;s peripheral edge, it is possible to provide an opening width which is large enough to secure smooth entry of the prime fluid from the opening into the internal space of the flow-channel recessed portion and enhanced generation of a vortex flow within the internal space of the flow-channel recess portion. 
     In the case where the straight section is provided on one of the upstream side and the downstream side of the opening&#39;s peripheral edge, it is preferable to provide the straight section on the upstream side for the purpose of achieving proper guiding of the prime fluid from the opening into the internal space of the flow-channel recessed portion. More specifically, if the peripheral edge of the opening is formed into a curved shape, the curved peripheral edge section will fail to introduce the prime fluid into the internal space of the flow-channel recessed portion uniformly over the entire width thereof. More specifically, a part of the prime fluid tends to first enter the internal space of the flow-channel recessed portion and this prime-fluid part is restrained from flowing into the internal space of the flow-channel recessed portion due to, for example, the viscosity of that part of the prime fluid which tends to later enter the internal space of the flow-channel recess portion. It is therefore difficult to properly introduce the prime fluid from the opening into the internal space of the flow-channel recessed portion. 
     By contrast, the straight peripheral edge section provided on the upstream side allows entry of the prime fluid from the straight peripheral edge section into the internal space of the flow-channel recessed portion uniformly over the width thereof. The prime fluid can thus be introduced from the opening into the internal space of the flow-channel recessed portion in an appropriate manner, and the introduced prime fluid can properly generate a vortex flow within the internal space of the flow-channel recessed portion. 
     Preferably, the flow-channel recessed portion has a part formed to protrude from the volute internal flow channel in a radial outward direction of the impeller. With this arrangement, the flow-channel recessed portion includes a first part (hereinafter referred to as “an inner flow-channel recess part”) corresponding in position to the volute internal flow channel, and a second part (hereinafter referred to as “an outer flow-channel recessed part”) arranged to protrude from the volute internal flow channel in the radial outward direction of the impeller. The prime fluid, as it flows along the volute internal flow channel, is subjected to a centrifugal force. The prime fluid introduced in an internal space of the inner flow-channel recessed part is subsequently introduced in the form of a vortex flow into an internal space of the outer flow-channel recessed part by the effect of the centrifugal force. Generation of the vortex flow by the prime fluid can be promoted, which will further promote generation of air bubbles. 
     In one preferred form of the invention, the number of the flow-channel recessed portion is plural, and the respective parts of the plural flow-channel recessed portions, which are arranged to protrude from the volute internal flow channel in the radial outward direction of the impeller, are set to be successively smaller along the direction of flow of the fluid. This arrangement ensures that an endmost one of the protruding parts which is located adjacent to a trailing end of the volute case is made sufficiently large, and another endmost protruding part located adjacent to a leading end of the volute case is made sufficiently small. The sufficiently large protruding part effectively promotes generation of a vortex flow by the prime fluid (i.e., stirring of the prime fluid) which will promote generation of air bubbles from the prime fluid. 
     On the other hand, the sufficiently small protruding part located adjacent to the leading end of the volute case is able to appropriately suppress generation of the vortex flow by the prime fluid (i.e., stirring of the prime fluid). The prime fluid guided to a leading end of the volute internal flow channel is smoothly discharged from the leading end of the volute internal flow channel. This will provide a high prime-fluid pumping performance. By virtue of a combination of the enhanced generation of air bubbles on the trailing end side of the volute internal flow channel and the suppressed generation of the vortex flow on the trailing end side of the volute internal flow channel, the gas in the volute internal flow channel can be appropriately discharged to the outside and a further improvement in the self-priming performance can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a centrifugal pump unit according to a preferred embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the centrifugal pump unit shown in a disassembled state; 
         FIG. 3  is a cross-sectional view of a volute case and an impeller of the centrifugal pump unit as there are in a disassembled state; 
         FIG. 4  is an enlarged view of a part indicated by an elongated circle  4  shown in  FIG. 1 ; 
         FIG. 5  is a view in the direction of arrow  5  shown in  FIG. 2 ; 
         FIG. 6  is a plan view of the volute case disassembled from a volute support wall; 
         FIG. 7  is a perspective view showing the volute case and a first flow-channel recessed portion shown in  FIG. 5 ; 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 5 ; 
         FIG. 9  is a perspective view showing the volute case and a fourth flow-channel recessed portion shown in  FIG. 5 ; 
         FIGS. 10A and 10B  are views illustrative of the manner in which air inside a volute internal flow channel of the centrifugal pump unit is entrained in a prime fluid in a state of air bubbles; 
         FIGS. 11A and 11B  are views illustrative of the manner in which generation of a vortex flow by the prime fluid is promoted at the first to fourth flow-channel recessed portions; 
         FIGS. 12A and 12B  are views illustrative of the manner in which the prime fluid and air bubbles are discharged from a volute discharge port of a volute body; 
         FIGS. 13A and 13B  are views illustrative of the manner in which a self-priming operation of the centrifugal pump unit is completed; 
         FIGS. 14A and 14B  are views illustrative of the manner in which a fluid is guided into the volute internal flow channel and caused to flow in a rotating direction of the impeller; 
         FIGS. 15A and 15B  are views illustrative of the manner in which the fluid is discharged from the volute discharge port of the volute body; and 
         FIG. 16  is a view illustrative of the manner in which the fluid discharged into a case internal passage is discharged to the outside of the centrifugal pump unit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A certain preferred structural embodiment of the present invention will be described in greater details below, by way of example only, with reference to the accompanying sheets of drawings. 
     As shown in  FIG. 1 , a centrifugal pump unit  10  generally comprises a base (not shown) supporting the centrifugal pump unit  10 , an engine  12  including a cylinder block  13  mounted on the base, and a centrifugal pump  20  provided on the cylinder block  13  of the engine  12 . 
     The engine  12  includes the cylinder block  13  mounted on the base, and a crankshaft (output shaft)  14  rotatably supported inside the cylinder block  13 . The centrifugal pump  20  has a case member  27  (especially, a partition member  38  of the case member  27 ) mounted to the cylinder block  13  by first to fourth bolts  22 - 25  (the second and third bolts  23 ,  24  being shown in  FIG. 5 ). A sealing washer  26  is disposed between each of the bolts  22 - 25  and the partition member  38  (especially, a volute support wall  61  of the partition member  38 ). 
     The crankshaft  14  has an end portion  14   a  projecting outwardly from the cylinder block  13 , and the end portion  14   a  is connected at its distal end  14   b  with an impeller  28  of the centrifugal pump  20 . With this arrangement, when the engine  12  is driven to rotate the crankshaft  14 , the impeller  28  is rotated by the crankshaft  14 . The case member  27  and the impeller  28  of the centrifugal pump  20  are sealed by mechanical seals  16 ,  17 . 
     As shown in  FIGS. 2 and 3 , the centrifugal pump  20  includes the case member  27  bolted to the cylinder block  13  by the first to fourth bolts  22 - 25 , the impeller  28  disposed inside the case member  27  and connected to the distal end  14   b  of the crankshaft  14 , and a volute case  29  covering the impeller  28 . The centrifugal pump  20  further includes a suction nozzle  32  connected with a suction port or inlet  31  ( FIG. 1 ) of the case member  27 , an opening and closing valve  33  having an upper end  33   a  gripped between the case member  27  and the suction nozzle  32 , and a discharge nozzle  35  connected with a discharge port or outlet  34  of the case member  27 . 
     The case member  27  includes a casing body  37  accommodating therewithin the impeller  28  and the volute case  29 , and the partition member  38  closing an open end  39  of the casing body  37 . The open end  39  of the casing body  37  is closed by the partition member  38 , and the volute case  29  is provided on the partition member  38 . With this arrangement, the casing body  37 , the partition member  38  and the volute case  29  jointly define therebetween an internal flow channel  41 . The internal flow channel  41  has an annular shape formed between the case member  27  and the volute case  29  within the case member  27 . 
     The casing body  37  includes the open end  39  closed by the partition member  38 , a substantially disc-shaped suction-side end wall  43  opposed to the partition wall  38 , the suction port  31  formed in the suction-side end wall  43 , a suction passage  44  connected to the suction port  31 , a tubular peripheral wall  45  formed along an outer peripheral edge  43   a  of the suction-side end wall  43 , and the discharge port  34  formed at an upper part  45   a  of the peripheral wall  45 . 
     Referring back to  FIG. 1 , the suction port  31  is formed in the suction-side end wall  43  and the suction passage  44  is connected to the suction port  31 . The suction passage  44  is also connected to a suction port or inlet  86  of the volute case  29 . The suction port  86  will be hereinafter referred to as “volute suction port”. The discharge port  34  of the case member  27  is provided at the upper part  45   a  of the peripheral wall  45 , and the discharge nozzle  35  is connected to the discharge port  34 . A fluid supply port  47  is formed in an upper part  35   a  of the discharge nozzle  35 , and the fluid supply port  47  is closed by a supply plug  48 . The fluid supply port  47  is disposed above the volute case  29 . 
     As shown in  FIGS. 4 and 5 , the partition member  38  has a support hole  51  formed through a thickness of the partition member  38  in concentric relation to the crankshaft  14 , first to fourth cylinder attachment portions  52 - 55  (the second and third cylinder attachment portions  53 ,  54  being shown in  FIG. 5 ) disposed at equal circumferential intervals on a circle concentric with the support hole  51 , first to fourth flow-channel recessed portions  56 - 59  formed at positions corresponding to positions of the first to fourth cylinder attachment portions  52 - 54 , and the volute support wall  61  supporting the volute case  29 . 
     The mechanical seal  16  is concentrically supported in the support hole  51  of the partition member  38 . The end portion  14   a  of the crankshaft  14  projects through the mechanical seal  16  into an internal space  63  of the volute case  29 . The mechanical seal  16  is in contact with the mechanical seal  17  of the impeller  28  so that a seal is mechanically provided between the mechanical seal  16  and the mechanical seal  17 . 
     As shown in  FIG. 6 , the first to fourth cylinder attachment portions  52 - 55  are provided equidistantly on the circle concentric to the support hole  51  in the order from a trailing end  88   a  toward a leading end  88   b  of the volute case  29  (especially, a volute body  88  of the volute case  29 ). The first to fourth cylinder attachment portions  52 - 55  have first to fourth through-holes  66 - 69  opening at the first to fourth flow-channel recessed portions  56 - 59 , respectively. The first to fourth flow-channel recessed portions  56 - 59  have first to fourth openings  71 - 74 , respectively, that open at an inner surface of the volute support wall  61  and are located at positions corresponding to the respective positions of the first to fourth cylinder attachment portions  52 - 55 . 
     Referring back to  FIG. 5 , the first opening  71  has a first inner opening part  71   a  facing a volute internal flow channel  84  described later, and a first outer opening part  71   b  located on a radial outer side of the volute internal flow channel  84 . Similarly, the second opening  72  has a second inner opening part  72   a  facing the volute internal flow channel  84 , and a second outer opening part  72   b  located on a radial outer side of the volute internal flow channel  84 . The third opening  73  has a third inner opening part  73   a  facing the volute internal flow channel  84 , and a third outer opening part  73   b  located on a radial outer side of the volute internal flow channel  84 . Similarly, the fourth opening  74  has a four inner opening part  74   a  facing the volute internal flow channel  84 , and a fourth outer opening part  74   b  located on a radial outer side of the volute internal flow channel  84 . The first to fourth flow-channel recessed portions  56 - 59  and the first to fourth openings  71 - 74  will be described in greater detail below. 
     As shown in  FIGS. 4 and 6 , the first bolt  22  is inserted from the first flow-channel recessed portion  56  into the first through-hole  66  in the first cylinder attachment portion  52 , and the first bolt  22  is threadedly engaged with a first attachment screw  76  formed in the cylinder block  13 . Similarly, the second bolt  23  is inserted from the second flow-channel recessed portion  57  into the second through-hole  67  in the second cylinder attachment portion  53 , and the second bolt  23  is threadedly engaged with a second attachment screw  77  formed in the cylinder block  13 . 
     Furthermore, the third bolt  24  is inserted from the third flow-channel recessed portion  58  into the third through-hole  68  in the third cylinder attachment portion  54 , and the third bolt  24  is threadedly engaged with a third attachment screw  78  formed in the cylinder block  13 . Similarly, the fourth bolt  25  is inserted from the fourth flow-channel recessed portion  59  into the fourth through-hole  69  in the fourth cylinder attachment portion  55 , and the fourth bolt  25  is threadedly engaged with a fourth attachment screw  79  formed in the cylinder block  13 . The first to fourth cylinder attachment portions  52 - 55  are attached to the cylinder block  13  by the first to fourth bolts  22 - 25 . In this state, the distal end  14   b  of the crankshaft  14  is arranged to project into the internal space  63  of the volute case  29  and the impeller  28  is attached to the distal end  14   b  of the crankshaft  14 . 
     Respective heads of the first to fourth bolts  22 - 25  are received inside the first to fourth flow-channel recessed portions  56 - 59 , respectively. The first to fourth bolts  22 - 25  can thus be set at positions corresponding to the position of the volute internal flow channel  84  without affecting the pump performance. This arrangement will lead to an improved degree of freedom in fastening the centrifugal pump  20  to the engine  12 . Furthermore, the first to fourth flow-channel recessed portions  56 - 59  are formed by utilizing parts of the case member  27  which are bolted by the first to fourth bolts  22 - 25 . The first to fourth flow-channel recessed portions  56 - 59  can thus be formed without requiring separate parts, which will cause compactification of the centrifugal pump  20 . 
     The impeller  28  is disposed in the internal space  63  of the volute case  29 . The impeller  28  includes a hub  81  mounted on the distal end  14   b  of the crankshaft  14 , and a plurality of vanes  82  provided on the hub  81 . The hub  81  is formed into a circular disc and an outer periphery  81   a  of the hub  81  is formed into a circular-arc shape. The hub  81  has a rear surface  81   b  (which faces the engine  12 ) on which the mechanical seal  17  is provided. The vanes  82  are provided on a front surface  81   c  of the hub  81 . The impeller  28  is received in the internal space  63  of the volute case  29  and covered by the volute case  29 . 
     The volute case  29  is supported on the volute support wall  38  by a pair of support pins  62 . The volute case  29  is a casing which is disposed inside the case member  27  and configured to accommodate the impeller  28 . The volute case  29  and the volute support wall  61  cooperate with each other to ensure that the volute internal flow channel  84  is formed in the internal space  63  of the volute case  29 . More specifically, the volute body  88  of the volute case  29  and a wall part (hereinafter referred to as “volute wall part”)  61   a  of the volute support wall  61  which is opposed to the volute body  88  together form the volute internal flow channel  84  into a hollow shape. The volute wall part  61   a  is formed spirally in confrontation with the volute body  88 . 
     The volute case  29  includes the volute suction port  86  communicating with the suction passage  44  of the case body  37 , a circular disc-shaped volute wall  87  extending radially outward from the volute suction port  86 , the volute body  88  formed into a spiral shape around the volute wall  87 , a pair of pin-insertion holes  89  fitted with the pair of support pins  62 , respectively, and first to fourth projecting parts  91 - 94  that cover corresponding ones of the outer opening parts  71   b - 74   b  of the first to fourth openings  71 - 74 . The volute wall  97  is circular-disc-shape so as to be opposed with the vanes  82  of the impeller  28 . The volute body  88  is provided along an outer periphery of the volute wall  87  so that the volute body  88  is disposed on a radial outer side of an outer periphery of the impeller  28 . 
     The volute body  88  is generally J-shaped in cross section. In the front view, the volute body  88  includes the trailing end  88   a  provided on a right side thereof, the leading end  88   b  provided adjacently below the trailing end  88   a , an inner peripheral wall  88   c  extending arcuately from the trailing end  88   a  to the leading end  88   b  along the outer periphery  87   a  of the volute wall  87 , and an outer peripheral wall  88   d  extending spirally from the trailing end  88   a  to the leading end  88   b  on a radial outer side of the inner peripheral wall  88   c.    
     The inner peripheral wall  88   c  is formed into a circular arc which is concentric with a center  96  of the impeller  28  (i.e., the axis of the crankshaft  14 ). The outer peripheral wall  88   d  is formed into a spiral shape which is gradually separated from the inner peripheral wall as it goes in a counterclockwise direction from the trailing end  88   a  to the leading end  88   b.    
     More specifically, the volute body  88  is formed into a counterclockwise spiral shape from the trailing end  88   a  to the leading end  88   b  such that a volute width W 1  increases gradually in a direction from the trailing end  88   a  to the leading end  88   b . The volute body  88  has a volute discharge port  95  formed at the leading end  88   b . With this arrangement, the fluid (including the prime fluid) introduced in the internal space  63  of the volute case  29  is discharged from the volute discharge port  95  to the outside (i.e., the case internal flow channel  41 ). 
     In the self-priming operation, the prime fluid introduced in the volute internal flow channel  84  is discharged from the volute discharge port  95  into the case internal flow channel  41  together with a gas (an air bubble) in the volute internal flow channel  84 . In the steady operation, the fluid introduced from the volute suction port  86  into the volute internal flow channel  84  is discharged from the volute discharge port  95  into the case internal flow channel  41 . 
     As shown in  FIGS. 5 and 6 , the first to fourth projecting parts  91 - 94  projects radially outward from the outer peripheral wall  88   d  of the volute body  88 . The first projecting part  91  covers and closes the first outer opening part  71   b  of the first opening  71 . Similarly, the second projecting part  92  covers and closes the second outer opening part  72   b  of the second opening  72 . Further, the third projecting part  93  covers and closes the third outer opening part  73   b  of the third opening  73 . Similarly, the fourth projecting part  94  covers and closes the fourth outer opening part  74   b  of the fourth opening  74 . 
     As shown in  FIG. 1 , the upper end  33   a  of the opening and closing valve  33  is gripped between the case member  27  and the suction nozzle  32 . With the upper end  33   a  being gripped between the case member  27  and the suction nozzle  32 , the opening and closing valve  33  is pivotally supported to undergo pivotal movement about the upper end  33  between a closed position in which the suction nozzle  32  is closed by the valve  33  and an opened position in which the suction nozzle  32  is opened by the valve  33 . 
     The discharge nozzle  35  is disposed above the volute case  29 . The discharge nozzle  35  is provided with the fluid supply port  47  such that the fluid supply port  47  is located above the volute case  29 . The fluid supply port  47  is closed by the supply plug  48 . The fluid supply port  47  is opened when the supply plug  48  is removed from the fluid supply port  47 . While the fluid supply port  47  is in the opened state, the prime fluid is supplied from the fluid supply port  47  into the case internal flow channel  41 . The prime fluid is such a fluid which can exhibit a priming action when the centrifugal pump performs self-priming operation. 
     Next, the first to fourth flow-channel recessed portions  56 - 59  and the first to fourth openings  71 - 74  that are shown in  FIG. 6  will be described below in greater detail. As shown in  FIGS. 7 and 8 , the first flow-channel recessed portion  56  is formed in such a manner as to correspond to the first cylinder attachment portion  52  of the partition member  38 . The first flow-channel recessed portion  56  is formed such that the first inner opening part  71   a  of the first opening  71  opens to the volute internal flow channel  84 , and is recessed toward the cylinder block  13  in a direction (of arrow C) substantially orthogonal to a direction of flow (direction of arrow B) of the fluid including the prime fluid. 
     More specifically, the first flow-channel recessed portion  56  has a bottom  98  forming a seat for the head of the first bolt  22 , a first peripheral wall  99  extending from the bottom  98  to the volute support wall  61 , and the first opening  71  at which an internal space  101  of the first flow-channel recessed portion  56  opens to the volute internal flow channel  84 . The bottom  98  is formed to have an outline or contour which, in a plan view, is slightly smaller than a first peripheral edge  103  of the first opening  71 . The first flow-channel recessed portion  56  is in the form of a blind hole which is closed at the bottom  98  and opens at the first opening  71 . The first bolt  22  is inserted through the through-hole  66  of the first cylinder attachment portion  52  and threadedly engaged with the first attachment screw  76  in the cylinder block until the head of the first bolt  22  is seated on the bottom  98  of the first flow-channel recessed portion  56 . 
     The first peripheral wall  99  includes an upstream peripheral wall portion  99   a  located on an upstream side with respect to the direction of flow of the fluid (indicated by arrow B), a downstream peripheral wall portion  99   b  disposed downstream of the upstream peripheral wall portion  99   a , an inner peripheral wall portion  99   c  connecting an outer end of the upstream peripheral wall portion  99   a  and an outer end of the downstream peripheral wall portion  99   b , and an outer peripheral wall portion  99   d  connecting an inner end of the upstream peripheral wall portion  99   a  and an inner end of the downstream peripheral wall portion  99   b.    
     The first opening  71  has an outline or contour formed by the first peripheral edge  103 . The first peripheral edge  103  is formed into a portal arch shape at a corner edge formed between the first peripheral wall  99  and the volute support wall  61 . The first peripheral edge  103  includes an upstream straight section  103   a  located on an upstream side with respect to the direction of flow of the fluid (indicated by arrow B), a downstream straight section  103   b  disposed downstream of the upstream straight section  103   a , an inner connecting section  103   e  connecting an inner end  103   c  of the upstream straight section  103   a  and an inner end  103   d  of the downstream straight section  103   b , and an outer connecting section  103   f  connecting an outer end of the upstream straight section  103   a  and an outer end of the downstream straight section  103   b.    
     The upstream straight section  103   a  is formed by a corner edge formed between the upstream peripheral wall portion  99   a  and the volute support wall  61 . The upstream straight section  103   a  and the upstream peripheral wall portion  99   a  extend linearly in a direction substantially orthogonal to the direction of flow of the fluid (i.e., in a direction toward the outside of the volute body  88 ) within a range H 1 . Furthermore, the inner end  103   c  of the upstream straight section  103   a  and the inner end of the upstream peripheral wall portion  99   a  are located adjacent to the outer periphery  81   a  of the hub  81 . 
     The downstream straight section  103   b  is formed by a corner edge formed between the downstream peripheral wall portion  99   b  and the volute support wall  61 . The downstream straight section  103   b  and the downstream peripheral wall portion  99   b  extend linearly in the direction (of arrow C) orthogonal to the direction of flow of the fluid within the range H 1 , in the same manner as the upstream straight section  103   a  and the upstream peripheral wall portion  99   a . The inner end  103   d  of the downstream straight section  103   d  and the inner end of the downstream peripheral wall portion  99   b  are located adjacent to the outer periphery  81   a  of the hub  81 . 
     The distance between the upstream straight section  103   a  and the downstream straight section  103   b  is an opening width W 2  of the first opening  71 . The upstream straight section  103   a  and the downstream straight section  103   b  extend parallel to each other almost throughout the range H 1  and, hence, the opening width W 2  of the first opening  71  is constant almost throughout the range H 1 . 
     The inner connecting section  103   e  is formed by a corner edge formed between the inner peripheral wall portion  99   c  and the volute support wall  61 . The inner connecting section  103   e  is connected with the inner end  103   c  of the upstream straight section  103   a  and the inner end  103   d  of the downstream straight section  103   b . The inner connection section  103   e  is disposed at a position located radially outward of, and adjacent to, the outer periphery  81   a  of the hub  81 . The inner connecting section  103   e  is formed into a circular arc shape which is concaved toward a center of the first opening  71  along the outer periphery  81   a  of the bub  81 . Likewise the inner connecting section  103   e , the inner peripheral wall portion  99   c  is formed into a circular-arc shape recessed toward a central axis of the opening  71  along the outer periphery  81   a  of the hub  81 . 
     The outer connecting section  103   f  is formed by a corner edge formed between the outer peripheral wall portion  99   d  and the volute support wall  61 . The outer connecting portion  103   f  is connected to the outer end of the upstream straight section  103   a  and the outer end of the downstream straight section  103   b . Further, the outer connecting section  103   f  is formed into a curved shape which makes the first opening  71  to swell in a radial outward direction. Likewise the outer connecting section  103   f , the outer peripheral wall portion  99   d  is formed into a curved shape swelling radially outward. 
     The first inner opening part  71   a  of the first opening  71  is arranged to face the volute internal flow channel  84 . That part  56   a  of the first flow-channel recessed portion  56  which includes the first inner opening part  71   a  is disposed adjacent to the volute internal flow channel  84 . The part  56   a  including the first inner opening part  71   a  will be hereinafter referred to as “first flow-channel inner recessed part  56   a ”. Since the first inner opening part  71   a  is arranged to face the volute internal flow channel  84 , the first inner flow-channel recessed part  56   a  communicates with the volute internal flow channel  84  via the first inner opening part  71   a.    
     The first outer opening part  71   b  of the first opening  71  is located on the radial outer side of the volute internal flow channel  81  and closed by the first projecting part  91 . That part  56   b  of the first flow-channel recessed portion  56  which includes the first outer opening part  71   b  is arranged to protrude from the volute internal flow channel  84  in a radial outward direction of the impeller  28 . The part  56   b  including the first outer opening part  71   b  will be hereinafter referred to as “first outer flow-channel recessed part  56   b ”. The first inner flow-channel recessed part  56   a  and the first outer flow-channel recessed part  56   b  communicate with each other and jointly form the first flow-channel recess portion  56  such that the first flow-channel recessed portion  56  is communicated with the volute internal flow channel  84  via the first inner opening part  71   a.    
     As shown in  FIGS. 5 and 6 , the volute body  88  is formed spirally such that the outer peripheral wall  88   d  of the volute body  88  gradually separates outwardly from the inner peripheral wall  88   c  as it goes from the trailing end  88   a  toward the leading end  88   b . The volute body  88  is formed such that the volute width W 1  increases gradually in a direction from the trailing end  88   a  toward the leading end  88   b  of the volute body  88 . Furthermore, the first and fourth flow-channel recessed portions  56 - 59  are arranged to locate on the same circumference of a circle, and the respective inner connecting sections  103   e ,  105 ,  106 ,  107  of the first to fourth flow-channel recessed portions  56 - 59  are spaced by a fixed distance L 1  from the inner peripheral wall  88   c  of the volute body  88 . 
     The outer peripheral wall  88   d  of the volute body  88  is outwardly offset to a greater extent from the first to fourth inner connecting sections  103   e ,  105 ,  106 ,  107  as it goes from the trailing end  88   a  to the leading end  88   b  of the volute body  88 . The first flow-channel recessed portion  56  is located adjacent to the trailing end  88  of the volute body  88  so that the volute width W 1  is controlled to have a small value at the first flow-channel recessed portion  56 . With this arrangement, as shown in  FIG. 7 , the first outer opening part  71   b  is allowed to have a large area S 1  and the first outer flow-channel recessed part  56   b  is also allowed to have a large capacity. Furthermore, since the inner end  103   c  of the upstream straight section  103   a  and the inner end of the upstream peripheral wall portion  99   a  are located adjacent to the outer periphery  81   a  of the hub  81 , the opening width W 2  of the first inner opening part  71   a  can be secured in a wide range H 2  which is substantially equal to the entire width of the volute internal flow channel  84 . 
     By thus securing the opening width W 2  in the wide range H 2 , it is possible to appropriately introduce the prime fluid from the first inner opening part  71   a  into the internal space  101  of the first flow-channel recessed portion  56 . Furthermore, the thus introduced prime fluid is able to appropriately generate a vortex flow within the internal space  101  of the first flow-channel recessed portion  56 . The prime fluid in the volute internal flow channel  84  contains gas in the form of air bubbles. The prime fluid mixed with air bubbles has reduced viscosity and density so that the prime fluid can be easily introduced into the first inner flow-channel recessed part  56   a.    
     The vortex flow generated within the internal space  101  of the first flow-channel recessed portion  56  promotes generation of air bubbles from the prime fluid and the air bubbles can be appropriately separated from the prime fluid. By thus separating the air bubbles from the prime fluid, the gas can be appropriately discharged from the volute internal flow channel  84  to the outside. The self-priming performance of the centrifugal pump can thus be improved. 
     If the upstream straight section  103   a  is replaced by a curved section, the upstream curved section will fail to introduce the prime fluid into the internal space  101  of the first flow-channel recessed portion uniformly over the entire width of the upstream curved section. More specifically, a part of the prime fluid tends to first flow into the internal space  101  of the first flow-channel recessed portion  56  and this prime fluid part is restrained from flowing into the internal space  101  of the first flow-channel recessed portion  56  due to, for example, the viscosity of that part of the prime fluid which tends to later flow into the internal space  101  of the first flow-channel recessed portion  56 . It is therefore difficult to appropriately introduce the prime fluid from the first inner opening part  71   a  into the internal space  101  of the first flow-channel recessed portion  56 . 
     By contrast, the upstream straight section  103   a  provided on the upstream side is able to secure uniform entry of the prime fluid into the internal space  101  of the first flow-channel recessed portion  56  over the width thereof. The prime fluid can thus be appropriately introduced from the first inner opening part  71   a  into the internal space  101  of the first flow-channel recessed portion  56 . The thus introduced prime fluid can appropriately generate a vortex flow within the internal space  101  of the first flow-channel recessed portion  56 . 
     The first outer flow-channel recessed part  56   b  of the first flow-channel recessed portion  56  is arranged to protrude from the volute internal flow channel  84  in a radial outward direction of the impeller  28 . The prime fluid as it flows along the volute internal flow channel  84  is subjected to a centrifugal force. Under such condition, upon entry from the first inner opening part  71   a  into the first inner flow-channel recessed part  56   a , the introduced prime fluid is guide into the first outer flow-channel recessed part  56   b  in the form of a vortex flow by the effect of the centrifugal force. This will promote generation of the vortex flow by the prime fluid, which will further promote generation of air bubbles from the prime fluid. 
     The first peripheral wall  99  of the first flow-channel recessed portion  56  has the outer peripheral wall portion  99   d  ( FIG. 7 ) formed into a curved shape swelling radially outward. The thus curved outer peripheral wall portion  99   d  is able to further promote generation of the vortex flow by the prime fluid which is introduced into the first flow-channel recessed portion  56 . 
     Referring back to  FIG. 5 , the second flow-channel recessed portion  57  is formed to be separated in a counterclockwise direction at a fixed interval from the first flow-channel recessed portion  56 . Likewise the first flow-channel recessed portion  56 , the second flow-channel recessed portion  57  is arranged such that the opening width W 2  of the second inner opening part  72   a  can be secured in a wide range H 3  which is substantially equal to the entire width of the volute internal flow channel  84 . With this arrangement, the prime fluid can be appropriately introduced from the second inner opening part  72   a  into an internal space of the second flow-channel recessed portion  57  and the thus introduced prime fluid can appropriately generate a vortex flow. 
     The third flow-channel recessed portion  58  is formed to be separated in the counterclockwise direction at the fixed interval from the second flow-channel recessed portion  57 . Likewise the first flow-channel recessed portion  56 , the third flow-channel recessed portion  58  is arranged such that the opening width W 2  of the third inner opening part  73   a  can be secured in a wide range H 4  which is substantially equal to the entire width of the volute internal flow channel  84 . This arrangement ensures that the prime fluid can be appropriately introduced from the third inner opening part  73   a  into an internal space of the third flow-channel recessed portion  58 , and the thus introduced prime fluid can appropriately generate a vortex flow. 
     The fourth flow-channel recessed portion  59  is formed to be separated in the counterclockwise direction at the fixed interval from the third flow-channel recessed portion  58 . Likewise the first flow-channel recessed portion  56 , the fourth flow-channel recessed portion  59  is arranged such that the opening width W 2  of the fourth inner opening part  74   a  can be secured in a wide range H 5  which is substantially equal to the entire width of the volute internal flow channel  84 . With this arrangement, the prime fluid can be appropriately introduced from the fourth inner opening part  74   a  into an internal space of the fourth flow-channel recessed portion  59 , and the thus introduced prime fluid can appropriately generate a vortex flow. The second, third and fourth flow-channel recessed portions  57 ,  58  and  59  have shapes similar to the shape of the first flow-channel recessed portion  56  and a detailed description of these flow-channels  57 - 59  can be omitted. 
     As shown in  FIGS. 5 and 9 , the fourth flow-channel recessed portion  59  is formed to be located adjacent to the leading end  88   b  of the volute body  88 . Since the volute width W 1  of the volute body  88  is formed to increase gradually in a direction from the trailing end  88   a  toward the leading end  88   b  of the volute body  88 , the volute width W 1  at the fourth flow-channel recessed portion  59  is secured to have a large value. With this arrangement, the area S 1  of the fourth outer opening part  74   b  is set to be small and a fourth outer flow-channel recess part  59   b  is also set to be small. 
     Because of the volute width W 1  formed to be increase gradually from the trailing end  88   a  to the leading end  88   b  of the volute body  88 , the area S 1  of the second outer opening part  72   b  of the second flow-channel recessed portion  57  is smaller than the area S 1  of the first outer opening part  71   b  of the first flow-channel recessed portion  56 . That is, a second outer flow-channel recessed part  57   b  of the second flow-channel recessed portion  57  is configured to be smaller than the first outer flow-channel recessed part  56   b  of the first flow-channel recessed portion  56 . 
     Furthermore, the area S 1  of the third outer opening part  73   b  of the third flow-channel recessed portion  58  is smaller than the area S 1  of the second outer opening part  72   b  of the second flow-channel recessed portion  57  and larger than the area S 1  of the fourth outer opening  74   b  of the fourth flow-channel recessed portion  59 . That is, a third outer flow-channel recessed part  58   b  of the third flow-channel recessed portion  58  is configured to be smaller than the second outer flow-channel recessed part  57   b  of the second flow-channel recessed portion  57  and larger than the fourth outer flow-channel recessed part  59   b  of the fourth flow-channel recessed portion  59 . 
     As is apparent from the foregoing, the areas S 1  of the first to fourth outer opening parts  71   b - 74   b  are set to be smaller successively along the direction of flow of the fluid, and the first to fourth outer flow-channel recess parts  56   b - 59   b  are set to be smaller successively along the direction of the fluid. With this arrangement, the first outer flow-channel recessed part  56   b  on the trailing end  88   a  side is secured to be sufficiently large, and the fourth outer flow-channel recessed part  59   b  on the leading end  88   b  side is reduced to be small. By thus providing the sufficiently large first outer flow-channel recessed part  56   b  on the trailing end  88   a  side of the volute body  88 , the prime fluid is introduced into the first outer flow-channel recessed part  56   b  under the effect of a centrifugal force. This will promote generation of a vortex flow by the prime fluid (i.e., stirring of the prime fluid), which in turn promotes generation of air bubbles from the prime fluid. 
     On the other hand, since the fourth outer flow-channel recessed part  59   b  on the leading end  88   b  side of the volute body  88  is reduced to be small, the prime fluid is uneasy to enter the fourth outer flow-channel recessed part  59   b  by the effect of the centrifugal force so that the generation of a vortex flow by the prime fluid (i.e., stirring of the prime fluid) is suppressed. The prime fluid introduced to the leading end  88   b  side of the volute body  88  can be smoothly discharged from the volute discharge port  85  so that excellent prime-fluid discharging performance can be obtained. 
     Generation of the air bubbles from the prime fluid can thus be sufficiently promoted on the trailing end  88   a  side of the volute body  88 , and the excellent prime-fluid discharging performance can be obtained on the leading end  88   b  side of the volute body  88 . As a result, gas can be appropriately discharged from the interior of the volute body  88  (i.e., the volute internal flow channel  84 ) to the outside (i.e., the case internal flow channel  41 ) as indicated by arrow E, and a further improvement in the self-priming performance of the centrifugal pump can be achieved. 
     Furthermore, since the second outer flow-channel recessed part  57   b  of the second flow-channel recessed portion  57  is smaller than the first outer flow-channel recessed part  56   b , generation of a vortex flow (i.e., stirring of the prime fluid) by the second outer flow-channel recessed part  57   b  is properly suppressed as compared to that by the first outer flow-channel recessed part  56   b . Similarly, because the third outer flow-channel recessed part  58   b  of the third flow-channel recessed portion  58  is smaller than the second outer flow-channel recessed part  57   b , generation of a vortex flow (i.e., stirring of the prime fluid) by the third outer flow-channel recessed part  58   b  is properly suppressed as compared to that by the second outer flow-channel recessed part  57   b.    
     By thus providing the first to fourth flow-channel recessed portions  56 - 59  opening to the volute internal flow channel  84  and by stirring the prime fluid within the internal spaces  101  of the first to fourth flow-channel recessed portions  56 - 59 , it is not necessary for stirring the prime fluid by the impeller  28  (especially by distal ends or outer circumferential ends  82   a ) of the vanes  82 . The distal ends  82   a  of the vanes  82  are allowed to be formed into a shape which is suitable for a discharge amount of the fluid during the steady operation. 
     As shown in  FIG. 8 , the first flow-channel recessed portion  56  is in the form of a blind hole which is closed at the bottom  98  and opens at the first opening  71 . During the steady operation, the internal space  101  of the first flow-channel recessed portion  56  is kept in the state of being filled with the fluid, making it difficult for the fluid to flow into the internal space  101  of the first flow-channel recessed portion  56 . The fluid can thus be smoothly guided along the volute internal flow channel  84 . 
     As shown in  FIG. 5 , the second to fourth flow-channel recessed portions  57 - 59  are also in the form of blind holes in the same manner as the first flow-channel recessed portion  56 . Thus, during the steady operation, the internal spaces of the second and fourth flow-channel recessed portions  57 - 59  are kept in the state of being filled with the fluid. This arrangement hinders further entry of the fluid into the internal spaces of the second to fourth flow-channel recessed portions  57 - 59 . The fluid can thus be smoothly guided along the volute internal flow channel  84 . Since the vanes  82  are allowed to have distal ends  82   a  so shaped as to be suitable for the steady operation, and since the fluid can smoothly flow during the steady operation, a desired pumping efficiency during the steady operation can be suitably obtained. 
     Next, a self-priming operation of the centrifugal pump  20  will be described with reference to  FIGS. 10 to 13 . As shown in  FIG. 10A , when the impeller  28  of the centrifugal pump  28  is in a stop state, gas (for example, air) is reserved inside the internal space  63  of the volute case  29 . In this condition, the supply plug  48  is removed from the fluid supply port  47  as indicated by arrow F to thereby open the fluid supply port  47 . While the fluid supply port  47  is in an open state, a prime fluid  112  is supplied from the fluid supply port  47  into the interior (i.e., the case internal flow channel  41 ) of the case member  27  as indicated by arrow G. 
     As shown in  FIG. 10B , the prime fluid  112  supplied to the internal flow channel  41  of the case member  27  is reserved in the volute internal flow channel  84  via the internal flow channel  41 . In this condition, the centrifugal pump  20  is driven by the engine  12  ( FIG. 10A ) to rotate the impeller  28  as indicated by arrow H. Rotation of the impeller  28  causes the prime fluid  112  to flow through the volute internal flow channel  84  as indicated by arrow I whereupon the gas in the volute internal flow channel  84  is entrained with the prime fluid in the form of air bubbles. 
     As shown in  FIG. 11A , due to the gas in the volute internal flow channel  84 , which is now contained in the prime fluid  112  in the form of air bubbles, the viscosity and density of the prime fluid  112  are reduced. As a result, the prime fluid  112  can be easily introduced into the respective inner flow-channel recessed parts  56   a - 59   a  of the first to fourth flow-channel recessed portions  56 - 59  as indicated by arrow J shown in  FIG. 10B . 
     As shown in  FIG. 11B , the first outer flow-channel recessed part  56   b  of the first flow-channel recessed portion  56  is arranged to protrude from the volute internal flow channel  84  in the radial outward direction of the impeller  28 . Furthermore, the prime fluid  112  as it flows along the volute internal flow channel  84  is subjected to a centrifugal force acting in the radial outward direction of the impeller  28 . Under such condition, upon entry from the first inner opening part  71   a  into the first inner flow-channel recessed part  56   a , the prime fluid  112  is guided into the first outer flow-channel recessed part  56   b  in the form of a vortex flow by the effect of the centrifugal force as indicated by arrow K. In the first flow-channel recessed portion  56 , generation of the vortex flow by the prime fluid  112  is promoted and generation of air bubbles from the prime fluid  112  is also promoted. The prime fluid  112 , which has promoted the generation of air bubbles, is then introduced from the first flow-channel recessed portion  56  into the volute internal flow channel  84 . 
     When introduced into each of the second to fourth inner flow-channel recessed parts  57   a - 59   a  shown in  FIG. 10B , the prime fluid  112  will be guided into a corresponding one of the second to fourth outer flow-channel recessed parts  57   b - 59 B in the form of a vortex flow by the effect of a centrifugal force, in the same manner as the prime fluid  112  introduced into the first inner flow-channel recessed part  56   a . In the second to fourth flow-channel recessed portions  57 - 59 , generation of the vortex flow by the prime fluid  112  is promoted and generation of air bubbles from the prime fluid  112  is also promoted. 
     As shown in  FIG. 12A , the prime fluid  112  is introduced into the fourth inner flow-channel recessed part  59   a  of the fourth flow-channel recessed portion  59  as indicated by arrow L. In this instance, since the fourth outer flow-channel recessed part  59   b  of the fourth flow-channel recessed portion  59  is set to be small, the prime fluid  112  introduced in the fourth inner flow-channel recessed part  59   a  is not easily guided into the forth outer flow-channel recessed part  59   b  by the effect of the centrifugal force. As a result, generation of a vortex flow by the prime fluid  112  (i.e., stirring of the prime fluid  112 ) within the fourth flow-channel recessed portion  59  can be appropriately suppressed. The prime fluid  122  which has promoted generation of air bubbles will be introduced from the fourth flow-channel recessed portion  59  into the volute internal flow channel  84  as indicated by arrow M. 
     As shown in  FIG. 12B , by virtue of the appropriately controlled vortex-flow generation in the internal space of the fourth flow-channel recessed portion  59 , the prime fluid  112  and the air bubbles can be smoothly guided to the leading end  88   b  of the volute body  55 . The prime fluid  112  and the air bubbles (i.e., the gas) thus guided to the leading end  88   a  of the volute body  88  can be appropriately discharged from the volute discharge port  95  as indicated by arrow N. 
     As shown in  FIG. 13A , the gas discharged from the volute discharge port  95  ( FIG. 12B ) is then discharged to the outside of the centrifugal pump  20  successively through the case internal flow channel  41 , the discharge port  34  and the discharge nozzle  35  as indicated by arrow O. By thus discharging the gas, a negative pressure is developed within the internal space  63  of the volute case  29 , which will cause the opening and closing valve  33  to open as indicated by arrow P. 
     As shown in  FIG. 13B , opening of the opening and closing valve  33  ensures a suction performance by which a fluid  113  is sucked from the volute suction port  86  into the internal space  63  of the volute case  29  as indicated by arrow Q. By thus achieving the suction performance, the centrifugal pump  20  completes the self-priming operation. 
     Next, the steady operation of the centrifugal pump  20  will be described with reference to  FIGS. 14 to 16 . As shown in  FIG. 14A , after the self-priming operation is completed, the impeller  28  continues to rotate as indicated by arrow H. In this instance, the impeller  28  (especially the distal ends  82   a  of the vanes  82 ) is formed into a shape which is suitable for the steady operation. The internal space  63  of the volute case  29  communicates with the suction nozzle  34  via the volute suction port  86 , the suction passage  44  and the case suction port  31 . 
     With this arrangement, due to a negative pressure created in the internal space  63  of the volute case  29 , the fluid  113  for the steady operation is sucked into the suction nozzle  32 , the case suction port  31 , the suction passage  44  and the volute suction port  86  successively, as indicated by arrow Q. The fluid  113  sucked into the volute suction port  86  is subsequently introduced into the volute internal flow channel  84  of the volute case  29 . 
     As shown in  FIG. 14B , the fluid  113  introduced in the volute internal flow channel  84  is guided by the vanes  82  of the impeller  28 . The fluid  82  while being guided by the vanes  82  flows in a rotating direction of the impeller  28  as indicated by arrow R. 
     As shown in  FIG. 15A , the first flow-channel recessed portion  56  is in the form of a blind hole which is closed at the bottom  98  and opens at the first opening  71 . With this arrangement, the internal space  101  of the first flow-channel recessed portion  56  is kept in the state of being filled with the fluid  113 , hindering further entry of the fluid  113  from the volute internal flow channel  84  into the internal space  101  of the first flow-channel recessed portion  56 . 
     As shown in  FIG. 15B , each of the second to fourth flow-channel recessed portions  57 - 59  is also in the form of a blind hole which is closed at the bottom and opens at a corresponding one of the second to fourth openings  72 - 74  in the same manner as the first flow-channel recessed portion  56 . Thus, the internal spaces of the second and fourth flow-channel recessed portions  57 - 59  are kept in the state of being filled with the fluid  113 . 
     By thus keeping the first to fourth flow-channel recessed portions  56 - 59  in the state of being filled with the fluid  113 , it is possible to hinder entry of the fluid  113  from the volute internal flow channel  84  into the internal spaces of the first to fourth flow-channel recessed portions  56 - 59 . The fluid  113  is therefore allowed to smoothly flow along the volute internal flow channel  84  as indicated by arrow R until it reaches the volute discharge port  95 , and subsequently the fluid  113  is discharged from the volute discharge port  95  to the case internal flow channel  41  as indicated by arrow S. 
     As shown in  FIG. 16 , the fluid  113  discharged into the case internal flow channel  41  is subsequently discharged to the outside of the centrifugal pump  20  successively through the case discharge port  34  and the discharge nozzle  35  as indicated by arrow T. Since the distal ends  82  of the vanes  82  are so shaped as to be suitable for the steady operation, and since the fluid  113  is allowed to smoothly flow through the volute internal flow channel  84 , a desired pumping efficiency can be securely obtained. 
     The centrifugal pump according to the present invention should by no means be limited to the one discussed in the afore-mentioned embodiment, and various changes and modifications are possible. For example, in the illustrated embodiment, the first to fourth peripheral edges  103  (only the first peripheral edge being shown) each have both of the upstream straight section  103   a  and the downstream straight section  103   b , however, only one of the upstream and downstream straight sections  103   a ,  103   b  may be provided. 
     In this case, it is preferable to provide the upstream straight section  103   a  because the prime fluid can be appropriately introduced from the upstream straight section  103   a  to the internal space of each of the first to fourth flow-channel recessed portions  56 - 59  (the internal space  101  of the first flow-channel recessed portion  101  being only shown). This arrangement ensures that a vortex flow of the prime fluid  112  can be appropriately generated within the internal spaces of the first to fourth flow-channel recessed portions  56 - 59 . 
     Although in the illustrated embodiment, the peripheral edges of the first to fourth flow-channel recessed portions  56 - 59  (the first peripheral edge  103  being only shown) is formed into a portal arch shape, other shapes such as a substantially oblong shape, a substantially rectangular shape, etc. can be employed for the peripheral edges of the first to fourth flow channel recessed portions  56 - 59 . 
     Furthermore, in the illustrated embodiment, the first to fourth flow-channel recessed portions  56 - 59  are formed by utilizing that parts of the case member  27  which are bolted by the first to fourth bolts  22 - 25 . According to the invention, the first to fourth flow-channel recessed portions  56 - 59  can be formed without using the bolted parts of the case member  27 . 
     Although in the illustrated embodiment, four flow-channel recessed portions (i.e., the first to fourth flow-channel recessed portions  56 - 59 ) are provided, the number of the flow-channel recessed portions can be properly selected. 
     Furthermore, the shape and configuration of the centrifugal pump unit, the centrifugal pump, the impeller, the volute case, the first to fourth flow-channel recessed portions, the first to fourth outer flow-channel recessed parts, the first to fourth openings, the volute internal flow channel, the volute body, the first peripheral edge, and the upstream and downstream straight sections should by no means be limited to those shown in the illustrated embodiment but can be changed appropriately. 
     The present invention is particularly suitable for an application to a centrifugal pump configured to force a fluid to flow along a volute internal flow channel upon rotation of an impeller disposed in a volute case. 
     Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.