Technical Field The present invention relates to a centrifugal pump, and more particularly to a centrifugal pump whose components such as an impeller and a casing are manufactured from a sheet metal and a method of manufacturing such a centrifugal pump.

Background Art Generally, in order to efficiently convert velocity energy of a fluid into pressure energy, a centrifugal pump is required to have a mechanism for decreasing a velocity of the fluid discharged from an impeller so as to recover a pressure head. Further, a multistage centrifugal pump having aplurality of impellers disposed in series is required to have a mechanism for leading a fluid from a certain-stage impeller to a next-stage impeller. Thus, in a multistage centrifugal pump, a guide vane is widely used as a mechanism for decreasing a velocity of a fluid and leading the fluid to a next-stage impeller. This guide vane comprises diffuser passages -for decreasing a velocity of a fluid discharged from an impeller, and return passages for leading the fluid, which has passed through the diffuser passages, to the next-stage impeller, as disclosed in the Japanese laid-open utility model publication No. 6-40958. Since the above-mentioned diffuser passages and the returnpassageshave acomplicatedshape, ithasbeencustomary to form the guide vane by using resin or by casting. The resin guide vane and the cast guide vane can have smooth passages thereinforleadingthefluidtoasuctionportofthenext-stage impeller, and hence an excellent pump performance can be obtained. However, the resin guide vane maybe corroded depending oncharacteristicofthe fluid. Therefore, thetypes of fluids which the pump can treat with are limited. Further, if the pump is used for delivering waste water, the resin guide vane is worn by suspended substances such as sands contained in the waste water. On the other hand, in a case of using the cast guide vane, the corrosion and the wear can be prevented from occurring. However, the cast guide vane causes a manufacturing cost to increase. In order to solve such problems, there has beenproposed a centrifugal pump having a diffuser section for decreasing a velocity of a fluid discharged from a rotating impeller, a plurality of return vanes for leading the fluid, which has passed through the diffuser section, toward a discharge side, and a main plate to which the diffuser section and the return vanes are fixed, all of which are manufactured from a sheet metal.

Disclosure of Invention The centrifugal pump having the diffuser section, the return vanes, and the main plate, which are-manufactured from- a sheet metal, are excellent in corrosion resistance and wear resistance, andcanpressurizethe fluidwithahighefficiency. An object of the present invention is to provide a centrifugal pump which has such advantages and can further improve a pump performance, and to provide a method of manufacturing such a centrifugal pump. In order to achieve the above object, according to one aspect of the present invention, there is provided a centrifugal pump for pressurizing a fluid by rotating an impeller, the centrifugalpump comprising: a diffuser section for decreasing a velocity of the fluid discharged from the impeller; a return vane for leading the fluidwhich has passed through the diffuser section to a discharge side; amainplate to which the diffuser section and the return vane are fixed; and a structural member for smoothening a step formed in a passage extending from the diffuser section to the return vane. According to the present invention, because the structural member smoothens the step formed in the passage extending from the diffuser section to the return vane, a resistance against the fluid flowing through the passage can be small and a loss can thus be small. Therefore, a high-efficient centrifugal pump can be achieved. In a preferred aspect of the present invention, the diffuser section, the return vane, the main plate, and the structural member are formed from a sheet metal. In a preferred aspect of the present invention, the structural member is a cover plate formed from a single sheet metal. In a preferred aspect of the present invention, the return vane engages with the cover plate to prevent the cover plate from moving. ■■ According -to the present invention, because all the components such as the diffuser section, the return vane, the main plate, and the structural member are formed from a sheet metal such as stainless steel, a centrifugal pump having an excellent corrosion resistance and an excellent wear resistance can be achieved. According to another aspect of the present invention, there is provided a method of manufacturing a centrifugal pump for pressurizing a fluid by rotating an impeller, the method comprising: forming a diffuser section from a sheet metal, the diffuser section being provided for decreasing a velocity of the fluid discharged from the impeller; forming a returnvane froma sheetmetal, the returnvanebeingprovided for leading the fluid which has passed through the diffuser section to a discharge side; forming a main plate from a sheet metal/ the diffuser section and the return vane being fixed to the main plate; forming a structural member from a sheet metal, the structural member being provided for smoothening a step formed in a passage extending from the diffuser section to the return vane; and assembling the diffuser section, the return vane, the main plate, and the structural member by weldingprocesses, theweldingprocesses beingperformed from the same side. According to the present invention, because all the components are formed from a sheet metal and are assembled by the welding processes which are performed from the same side, the centrifugal pump having an excellent corrosion resistance and an excellent wear resistance can be easily manufactured. In a preferred aspect of the present invention, the structuralmember andthe returnvane are integrallyassembled by a single welding process. According to the present invention, the structural •member and-the return vane can be easily-assembled.

Brief Description of Drawings FIG.1 is a view showing components constituting a guide vane incorporated in a centrifugal pump according to an embodiment of the present invention; FIG. 2A is a plan view showing the guide vane of the centrifugal pump according to the embodiment of the present invention; FIG. 2B is a vertical cross-sectional view showing the guide vane of the centrifugal pump according to the embodiment of the present invention; FIG. 3 is a schematic view showing the guide vane of the centrifugalpumpaccordingto the embodimentofthepresent invention; FIG.4 is a plan view illustrating the manner of welding andassemblingthe guidevaneofthe centrifugalpumpaccording to the embodiment of the present invention/ FIG. 5 is a vertical cross-sectional view illustrating the manner of welding and assembling the guide vane of the centrifugal pump according to the embodiment of the present invention; FIG. 6 is a view illustrating the manner of welding a diffuser section, a return vane, and a main plate of the centrifugal pump according to the embodiment of the present invention; FIG. 7 is a view illustrating the manner of welding the diffuser section and the main plate; FIG. 8 is a view illustrating the manner of welding the main plate, a cover plate, and the return vane; and FIG.9 is aviewshowinganessentialpart ofamultistage centrifugal pump according to the embodiment of the present invention.

Best Mode for Carrying Out the Invention A centrifugal pump according to an embodiment of the present invention will be described below with reference to the drawings. As shown in FIGS. 1 through 3, a guide vane 10 of the centrifugal pump comprises a diffuser section 11 for forming diffuser passages 15, a plurality of return vanes 14 for forming return passages 16, and a main plate 12 to which the diffuser section 11 and the return vanes 14 are fixed. The diffuser section 11, the return vanes 14, and the main plate 12 are formed from a sheet metal such as stainless steel. The diffuser section 11 is divided into each of the diffuser passages 15, and is fixed to the main plate 12 by welding. The return vanes 14 are divided into each of the return passages 16, and are fixed to the main plate 12 by welding. In the structures of these components, as shown in FIG. 1, a step 17 of the main plate 12 is formed in the fluid passages, andhence an integral coverplate 13 is additionally provided between the return vanes 14 and the main plate 12 so as to smoothen the step 17. As shown in FIG. 1, the cover plate 13 is formed from a single sheet metal. Thus, the guide vane 10 can be easily assembled, and each of the return passages 16 has a smooth flow passage. The cover plate 13 is assembled as shown in FIG. 8. Specifically, the cover plate 13 is placed between • the main plate 12 and the return vane 14, and the main plate 12, the cover plate 13 and the return vane 14 are welded and integrated by penetration welding of laser using a nozzle 20 provided at the side of the main plate 12. In this manner, the main plate 12, the cover plate 13 and the return vane 14 are assembled by a single welding process, i.e., the penetration welding. Further, as shown in FIG. 1, engaging portions 18 and 19 are provided on the return vane 14 and the cover plate 13, respectively, and when welding finishes, as shown in FIG. 3, the engaging portion 19 of the cover plate 13 is pressed by the engaging portion 18 of the return vane 14, so that the outer peripheral portion of the cover plate 13 is prevented from moving. As described above, the guide vane 10 comprises the diffuser section 11 for forming the diffuser passages 15, the return vanes 14 for forming the return passages 16, the mainplate 12 for fixing the diffuser section 11 andthe return vanes 14, and the cover plate 13 interposedbetween the return vanes 14 and the main plate 12. Therefore, as indicated by the arrow A, a liquid discharged from a rotating impeller (described later) changes its course and flows into the return passages 16, and is then led to a discharge side (i.e., a discharge port of the pump or a next-stage impeller) . FIGS. 4 and 5 show the manner in which the guide vane 10 is assembled. As showninFIGS.4 and5, the diffuser section 11, the cover plate 13 and the return vanes 14 are fixed to the main plate 12 by laser welding. The arrow B in FIG. 4 indicates aweldingdirection, and laserwelding (for example, YAG laser welding) is performed from the front side of the sheet surface of FIG. 4. The arrow C shown in FIG. 4 indicates a rotating direction of the impeller. In a welding portion Ll shown in FIG. 4, as shown in FIG. 6, the main plate 12, the return vanes 14 and the diffuser section 11 are welded from the side of the diffuser section 11 by penetration welding of laser using the nozzle 20, so that these three components 11, 12 and 14 are welded simultaneously. Further, in a welding portion L2, as shown in FIG. 7, the main plate 12 and the diffuser section 11 are welded from the side of the diffuser section 11 by penetration welding of laser using the nozzle 20. In a welding portion L3, as-shown-in F-IG.8, the cover plate 13 is -interpo-sedbetween the main plate 12 and the return vane 14, and the main plate 12, the cover plate 13 and the return vane 14 are welded from the side of the diffuser section 11 by penetration welding of laser using the nozzle 20, so that these three components 12, 13 and 14 are welded simultaneously. In this manner, all of the welding processes illustrated in FIGS. 6 through 8 areperformed fromthe same side, i.e., the side of the diffuser section 11. FIG.9 is a view showing an essential part of amultistage centrifugal pump according to the embodiment of the present invention. As shown in FIG. 9, the multistage centrifugal pump 50 comprises a plurality of impellers 51, a casing 52 in which the impellers 51 are housed, and a rotatable main shaft 53 on which the impellers 51 are mounted. The casing 52 is divided into a plurality of interstage casings 52A. 0-rings 54 are provided respectively at connecting portions of the adjacent interstage casings 52A. The impellers 51 are disposed at equal intervals on the main shaft 53, and are integrally rotated with the main shaft 53. The suction ports 51a of the impellers 51 are in the same direction, and the impellers 51 are disposed on the main shaft 53 in series. The main shaft 53 is coupled to a motor (not shown) , and the impellers 51 are rotated by the motor through themain shaft 53. The impeller 51 andthe casing 52 are formed from a sheet metal such as stainless steel. The multistage centrifugal pump 50 has a plurality of guide vanes 56 each having the same structure as the above-mentioned guide vane 10. Each of the guide vanes 56 comprises adiffuser section 57 for formingdiffuserpassages, a plurality of return vanes 58 for forming return passages, and a main plate 59 to which the diffuser section 57 and the return vanes 58 are fixed. Although not shown in FIG. 9, a cover plate-i-s- provided between the return vanes- 58 and the- main plate 59 in the same manner as described above so as to smoothen a step formed therebewteen. Further, each of the guide vanes 56 is fixed to each of the inner circumferential surfaces of the interstage casings 52A, and is disposed in thevicinityoftheouterperipheryandthebackside (discharge side) of each of the impellers 51. A first annular partition wall 60 constituting a part of each of the return passages is fixed to the backside (discharge side) of the return vanes 58. The first partition wall 60 has a first through-hole 60a having a small inner diameter. A second annular partition wall 61 is provided at the discharge side of the first partition wall 60, and a space 62 is defined between the first partition wall 60 and the second partition wall 61. The second partition wall 61 has a second through-hole 61a having an inner diameter substantially equal to the inner diameter of the first through-hole 60a. In this embodiment, the first partition wall 60 and the second partition wall 61 are formed from a sheet metal such as stainless steel. In this embodiment, a portion extending from the interstage casing 52A constitutes the second partition wall 61. With the multistage centrifugal pump having the above structure, when the impellers 51 are rotated by the motor, a liquidis introducedinto the impeller 51 throughthe suction port 51a in the direction of arrow D shown in FIG. 9. The liquid introduced into the impeller 51 is pressurized by the rotating impeller 51, and is discharged from the outer periphery of the impeller 51 toward the guide vane 56. The liquid introduced into the guide vane 56 flows inthe direction of arrow E in the guide vane 56. At this time, the liquid passes throughthediffuser section57 todecrease itsvelocity, andthusvelocityenergyofthe liquidis efficientlyconverted into-pres-sure energy of the liquid. The l-iquid-which has passed through the diffuser section 57 is led by the return vanes 58 to the suction port 51a of the next-stage impeller 51. In this manner, the liquid is pressurized successively by the multistage impellers 51, and the pressure head of the liquid is recovered successively by the multistage diffuser sections 57. Finally, the pressurized liquid is discharged from the discharge port (not shown) of the multistage centrifugal pump. As described above, the liquid is successively pressurized by each of the multistage impellers 51, and the liquids having different pressures are partitioned by the first partition wall 60 and the second partition wall 61 into a high-pressure side and a low-pressure side. According to the multistage centrifugal pump of the present embodiment, in order to prevent the liquid in the casing 52 from leaking from the high-pressure side toward the low-pressure side, a floating-type liner ring 63 is provided. Although a certain preferred embodiment of the present inventionhasbeendescribedindetail, it shouldbeunderstood that various changes and modifications may be made without departing from the scope of the appended claims for patent, and the scope of the technical concept described in the specification and drawings.

Industrial Applicability The present invention is applicable to a centrifugal pump whose components such as an impeller and a casing are manufactured from a sheet metal and a method of manufacturing such a centrifugal pump.