CENTRIFUGAL PUMP

Technical Field The present invention relates to a centrifugal pump, and more particularly to a centrifugal pump having at least one impeller fixed to a main shaft and at least one casing which houses the impeller and is formed through press working (press forming) of sheet metal such as a steel plate.

Background Art As a kind of a centrifugal pump, there has been known a multistage pump which comprises a plurality of impellers fixed to a main shaft coupled to a prime mover such as a motor and a plurality of stacked interstage casings for housing the respective impellers. FIG. 9 is a vertical cross-sectional view showing an example of a conventional vertical-type multistage pump disclosed in Japanese laid-open patent publication No. 2003-214383. As shown in FIG. 9,.. the....conventional . vertical-type multistage pump comprises a plurality of vertically stacked interstage casings 101 which are formed through press working of a steel plate, an upper casing 102 provided on the uppermost interstage casing 101, a lower casing 103 provided under the lowermost interstage casing 101, and an outer casing 104 which houses the vertically stacked interstage casings 101 and the upper casing 102 and is connected to the lower casing 103. A casing cover 105 is provided at upper open ends of the upper casing 102 and the outer casing 104 to seal the upper casing 102 and the outer casing 104 hermetically. A plurality of impellers 111 are fixed to a main shaft 110, and each of the impellers 111 is housed in each of the interstage casings 101. A mechanical seal 112 is mounted on the upper portion of the main shaft 110 to perform a shaft seal of the portion where the main shaft 110 passes through the casing cover 105. A motor base 106 for supporting a motor (not shown) is provided on the casing cover 105. Next, operation of the multistage pump shown in FIG. 9 will be described briefly. When the multistage pump is in operation, liquid drawn in from the suction port 103a of the lower casing 103 is introduced into the first-stage impeller 111 through the lower casing 103, and is pressurized by the first-stage impeller 111 rotated by the main shaft 110. The pressurized liquid passes through passages defined by the return blades 134, and is then introduced into the next- stage impeller 111. In this manner, the liquid is pressurized by each of the impellers 111 in each of the interstage casings 101, and the pressure head of the liquid is recovered while the liquid passes through the passages defined by the return blades 134. Thereafter, . the liquid passes through the upper casing 102 and then the annular passage 155 defined between the outer casing 104 and the multistage interstage casings 101, and is then discharged from the discharge port 103b of the lower casing 103 to the outside of the pump. In the conventional multistage pump shown in FIG. 9, the upper open ends of the upper casing 102 and the outer casing 104 are hermetically sealed by the casing cover 105 which is formed through press working of a stainless steel plate. However, since high fluid pressure created by the multistage impellers is applied to the casing cover 105, the casing cover 105 is required to have relatively high rigidity. Thus, it is necessary for the casing cover 105 to be manufactured through press working of a stainless steel plate thicker than the multistage interstage casing 101, and hence the press working of the casing cover 105 is difficult and material cost of the casing cover 105 is high. Further, the mechanical seal 112 is provided at the portion where the main shaft 110 passes through the casing cover 105. When the mechanical seal 112 is replaced with a new one, the mechanical seal 112 is required to be replaced in such a state that the multistage impellers 111 and the multistage interstage casings 101 are removed from the pump. Thus, replacement work of the mechanical seal 112 is complicated and troublesome. Furthermore, the interstage casing 101 is substantially in the form of a cylindrical receptacle, and the bottom portion 101a of the interstage casing 101 is formed into a flat shape. Therefore, the bottom portion 101a of the interstage casing 101 is liable to be deformed in an axial direction (downward direction) of the interstage-casing .101. . ... ._

Disclosure of Invention The present invention has been made in view of the above drawbacks. It is therefore of an object of the present invention to provide a centrifugal pump which can not only enhance rigidity of a member having a function of a casing cover for hermetically sealing upper open ends of an upper casing and an outer casing but also reduce material cost of such a member, can facilitate replacement of a mechanical seal to improve maintenance efficiency, and can increase pressure resistance of a bottom portion of an interstage casing. In order to achieve the above object, according to a first aspect of the present invention, there is provided a centrifugal pump comprising: at least one impeller fixed to a main shaft coupled to a prime mover; at least one casing provided so as to correspond to the impeller; a casing cover configured to seal an open end of the casing hermetically; and a shaft seal device provided at a portion where the main shaft passes through the casing cover; wherein the casing cover comprises a thick member made of castings, and a thin corrosion-resistant lining member which is tightly fixed to the thick member. According to the present invention, since the casing cover for covering the open end of the pump casing tightly comprises a thick member made of castings, and a thin corrosion-resistant lining member made of a pressed component and adhering to an outer surface of the thick member, rigidity and corrosion resistance of the casing cover can be ensured, and press working of the thin lining member can be easily performed. Further, since the lining member comprises - -a . thin-wall member, material cost... of the lining member can be reduced. In a preferred aspect of the present invention, the lining member may comprise a press-formed member. In a preferred aspect of the present invention, the lining member may be tightly fixed to said thick member by press fitting. In a preferred aspect of the present invention, the thick member may have a hollow cylindrical portion which allows the main shaft to pass therethrough, and the lining member may have an annular portion which is fitted over the hollow cylindrical portion. In a preferred aspect of the present invention, an end portion of the annular portion of the lining member may be expanded radially outwardly so that said lining member is not detached from said thick member. In a preferred aspect of the present invention, the end portion of the annular portion of the lining member may be expanded radially outwardly in a tapered shape so that the end portion of the annular portion of the lining member serves to guide a gasket of the shaft seal device when the gasket is inserted into the annular portion of the lining member. In a preferred aspect of the present invention, the thick member of the casing cover may have a communication hole through which an atmospheric side and a liquid side communicate with each other, a corrosion-resistant pipe may be fitted into the communication hole, and the pipe and the corrosion-resistant lining member may be joined by welding. In a preferred aspect of the present invention, the welding may be laser welding. In a preferred aspect of the present invention, a centrifugal pump may further comprise an outer barrel for preventing.-.press-ure..Liquid generated by .operation-..of-..the pump from leaking to the outside of the pump, the outer barrel having at least one formed portion for mounting an 0-ring at one end or both ends of the outer barrel. In a preferred aspect of the present invention, an end portion of the formed portion of the outer barrel may be expanded radially outwardly to prevent the 0-ring from falling off. In a preferred aspect of the present invention, a tapered portion expanded radially outwardly may be formed on a member to which the outer barrel is attached so that insertion of the 0-ring held by said outer barrel is facilitated. In order to achieve the above object, according to a second aspect of the present invention, there is provided a centrifugal pump comprising: at least one impeller fixed to a main shaft coupled to a prime mover; at least one casing provided so as to correspond to the impeller; the centrifugal pump comprising a multistage pump; the casing including a plurality of stacked interstage casings which house a plurality of impellers, respectively; the interstage casing comprising a first cylindrical portion, a first bottom portion which is held in contact with an axial end face of an adjacent interstage casing, a second cylindrical portion extending axially from the first bottom portion, and a second bottom portion extending radially inward from the second cylindrical portion; and the second bottom portion of the interstage casing being formed into a curved shape of a hemisphere or substantially a hemisphere. According to the present invention, a plurality of stacked interstage casings which house a plurality of impellers are substantially in the form of a cylindrical receptacle- having a-bottom portion, and the- bottom. portion. of each interstage casing is formed into a curved shape of a hemisphere or substantially a hemisphere, and hence the interstage casing has a high pressure resistance. Therefore, the optimum interstage casing configured to accommodate high pressure liquid can be constructed. In a preferred aspect of the present invention, a suction plate may be attached to the second bottom portion of the interstage casing so that an outer circumferential end face of the suction plate is held in contact with an inner surface of a first cylindrical portion of the adjacent interstage casing; and the suction plate, the • second cylindrical portion, the first bottom portion, and the inner surface of the first cylindrical portion of the adjacent interstage casing may form a space for accommodating an 0-ring. In a preferred aspect of the present invention, return guide vanes may be interposed between the suction plate and a side plate. In a preferred aspect of the present invention, a baffle may be provided in the interstage casing so as to cover at least one formed portion located at a corner between the first cylindrical portion and the second bottom portion. In a preferred aspect of the present invention, the baffle may have a plurality of notches at a radially inner portion thereof. In a preferred aspect of the present invention, the maximum diameter of the notch may be smaller than the outer diameter of the impeller. In order to achieve the above object, according to a third aspect of the present invention, there is provided a centrifugal pump comprising: at least one impeller fixed to - a main, shaft- coupled-to a prime mover; at., least . one—.casing-. provided so as to correspond to the impeller; a casing cover configured to seal an open end of the casing hermetically; and a mechanical seal provided at a portion where the main shaft passes through the casing cover; wherein the mechanical seal comprises a sleeve which holds a rotating seal ring and is fixed to the main shaft, and a holder configured to hold a stationary seal ring, the holder comprising a cylindrical member which holds a seal member for sealing liquid between the casing cover and the cylindrical member, and a disk member; and wherein the cylindrical member and the disk member are joined together to form the holder. According to the present invention, because the mechanical seal comprises a cartridge type mechanical seal, only by removing the motor fixed to the motor base and the coupling, the cartridge type mechanical seal can be mounted on or detached from the main shaft from the location above the main shaft. Therefore, it is not necessary to remove the multistage interstage casings, the multistage impellers, and the like in order to replace the mechanical seal. According to the present invention, because the holder which holds the stationary assembly (including the stationary seal ring) of the mechanical seal comprises the cylindrical member and the disk member which are joined together, the cylindrical member can be formed by working a pipe slightly, and the disk member can be easily formed by blanking or the like. Therefore, the holder having a flange can be manufactured by minimum working processes, and thus manufacturing cost can be greatly reduced. In a preferred aspect of the present invention, the disk member -may .compri.s.e a flat plate made of. sheet.metal. In a preferred aspect of the present invention, the cylindrical member and the disk member may be welded by lasers. In a preferred aspect of the present invention, a gap which is longer than the overall length of the mechanical seal may be formed between the main shaft of the pump and a main shaft of the prime mover, and the mechanical seal may be removed through the gap by removing a coupling which couples the main shaft of the pump and the main shaft of the prime mover. In order to achieve the above object, according to a fourth aspect of the present invention, there is provided a centrifugal pump comprising: at least one impeller fixed to a main shaft coupled to a prime mover; at least one casing provided so as to correspond to the impeller; a casing cover configured to seal an open end of the casing hermetically; a mechanical seal provided at a portion where the main shaft passes through the casing cover; wherein the mechanical seal comprises a sleeve which holds a rotating seal ring and is fixed to the main shaft, and a holder configured to hold a stationary seal ring, the holder comprising a cylindrical member which holds a seal member for sealing liquid between the casing cover and the cylindrical member, and a disk member; and wherein a positioning member is attached to the main shaft, and the sleeve of the mechanical seal is held into contact with the positioning member, thereby positioning the mechanical seal. According to the present invention, when the mechanical seal is mounted on the main shaft of the pump, the end portion of the mechanical seal is brought into contact, with ..the..-positioning member fixed to the., main shaft. Therefore, the mechanical seal can be disposed at an exact position with respect to the main shaft. Thus, the stationary seal ring and the rotating seal ring are held at optimum positions. In a preferred aspect of the present invention, the holder may have a step-like portion for holding the stationary seal ring. In order to achieve the above object, according to a fifth aspect of the present invention, there is provided a centrifugal pump comprising: at least one impeller fixed to a main shaft coupled to a prime mover, the main shaft having a plurality of grooves in an outer circumferential surface thereof for allowing the impeller to be fitted over and allowing the impeller to be prevented from being rotated with respect to the main shaft; at least one casing provided so as to correspond to the impeller; a positioning member for positioning the impeller with respect to an axial direction of the main shaft; and a retainer having a circular disk portion which is brought into contact with an end surface of the positioning member, and a cylindrical portion extending from the circular disk portion so as to enclose an outer peripheral portion of the positioning member. According to the present invention, a positioning member for positioning the impeller with respect to an axial direction of the main shaft is mounted on the main shaft of the pump, and there is provided a retainer which has a circular disk portion which is brought into contact with an end surface of the positioning member, and a cylindrical portion extending from the circular disk portion so as to enclose an outer peripheral portion of the positioning member. Because the positioning member is tightly . fitted.-in., the—main shaft in such a . state that. the positioning member is enclosed by the retainer, even if a fastening force for fixing the impellers is applied to the positioning member, the positioning member is prevented from being deformed. In a preferred aspect of the present invention, the retainer may have an inner diameter shape which is fitted over the groove shape of the main shaft. In a preferred aspect of the present invention, the cylindrical portion of the retainer may be brought into contact with the outer peripheral portion of the positioning member or may be located in close proximity to the outer peripheral portion of the positioning member.

Brief Description of Drawings FIG. 1 is a vertical cross-sectional view of a vertical-type multistage pump according to an embodiment of the present invention; FIG. 2 is an enlarged cross-sectional view of adjacent interstage casings shown in FIG. 1; FIG. 3 is a view showing the interior of the interstage casing, as viewed from an arrow III of FIG. 2; FIG. 4 is an enlarged fragmentary cross-sectional view of the multistage pump shown in FIG. 1; FIG. 5 is a vertical cross-sectional view showing a first modified example of the vertical-type multistage pump shown in FIG. 1; FIG. 6 is a vertical cross-sectional view showing a second modified example of the vertical-type multistage pump shown in FIG. 1; FIG. 7 is an exploded perspective view of a main shaft, a split ring and a ring presser; .FIG. .8. -is. .a. perspective view partly .in ...section showing the state in which the main shaft, the split ring and the ring presser are assembled; and FIG. 9 is a vertical cross-sectional view showing an example of a conventional multistage pump.

Best Mode for Carrying Out the Invention A centrifugal pump according to embodiments of the present invention will be described in detail with reference to the accompanying drawings . As an example of a centrifugal pump according to the present invention, a vertical-type multistage pump which comprises a plurality of vertically stacked interstage casings formed through press working of a steel plate and a plurality of impellers housed in the respective interstage casings will be described. In FIGS. 1 through 8, like or corresponding parts are denoted by like or corresponding reference numerals throughout views, and repetitive description is eliminated. FIG. 1 is a vertical cross-sectional view of a vertical-type multistage pump according to an embodiment of the present invention. A vertical-type multistage pump according to the present embodiment comprises a plurality of vertically stacked interstage casings 1 which are formed through press working of a steel plate, an upper casing 2 provided on the uppermost interstage casing 1, a lower casing 3 provided under the lowermost interstage casing 1, and an outer casing 4 which houses the stacked interstage casings 1 and the upper casing 2 and is connected to the lower casing 3. A pump base 5 is provided under the lower casing 3, and a motor base 6 for supporting a motor (not shown) is provided above the outer casing 4 and the upper casing 2. The. pump base .5..and the motor base 6 are coupled to each other by a plurality of casing bolts 8 and a plurality of nuts 9. Specifically, the lower ends of the casing bolts 8 are screwed into the pump base 5 and the nuts 9 are screwed over the respective upper ends of the casing bolts 8, whereby the multistage interstage casings 1, the upper casing 2 and the lower casing 3 are held between the pump base 5 and the motor base 6. Further, the upper and lower ends of the outer casing 4 are fitted into the lower casing 3 and the motor base 6, respectively, so that the outer casing 4 is supported between the lower casing 3 and the motor base 6. O-rings 7 and 7 are provided at the fitting portion of the outer casing 4 and the lower casing 3 and the fitting portion of the outer casing 4 and the motor base 6. The outer casing 4 constitutes an outer barrel for preventing pressure water (pressurized liquid) generated by operation of the pump from leaking to the outside of the pump. The outer casing 4 constituting the outer barrel have both end portions which are pressed into formed portions 4f, 4f for mounting the 0-rings 7, 7 for sealing. The formed portions 4f, 4f have ends which are expanded radially outwardly to prevent the O-rings 7, 7 from dropping therefrom. A tapered portion expanded radially outwardly is formed on an end portion of a member (the lower casing 3 or a lining member 40 (described later) ) for receiving the outer casing 4 so that the 0-ring 7 held by the formed portion 4f can be easily inserted into the member (the lower casing 3 or the lining member 40) when the outer casing 4 is attached to the lower casing 3 and the motor base 6. A plurality of impellers 11 are fixed to a main shaft 10, and each of the impellers 11 is housed in each of the interstage casings--1 The main., shaft 10 comprises a-.spline shaft, and each of the impellers 11 has a plurality of grooves fitted over the spline shaft at the inner peripheral portion thereof. A mechanical seal 12 is mounted on the upper portion of the main shaft 10 to perform a shaft seal of the portion where the main shaft 10 passes through the pump casing. The upper end of the main shaft 10 is coupled to a main shaft 15 of a motor (not shown) through a coupling 14. A suction nozzle 17 and a discharge nozzle 18 are fixed to the lower casing 3, and a suction flange 19 and a discharge flange 20 are fixed to the forward ends of the suction nozzle 17 and the discharge nozzle 18, respectively. FIG. 2 is an enlarged cross-sectional view showing a portion of the structure including the interstage casings 1 shown in FIG. 1. In the present embodiment, each of the interstage casings 1 is substantially in the form of a cylindrical receptacle. Specifically, as shown in FIG. 2, each of the interstage casings 1 includes a first cylindrical portion 21, a protruding portion 22 protruding radially outward from the first cylindrical portion 21, a first bottom portion 23 extending radially inward from the protruding portion 22, a second cylindrical portion 24 extending axially from the first bottom portion 23, and a second bottom portion 25 extending radially inward from the second cylindrical portion 24. The interstage casings 1 are made of stainless steel plates by press forming. The protruding portion 22 has a maximum outside diameter larger than the outside diameter of the first cylindrical portion 21. The second cylindrical portion 24 has an outside diameter slightly smaller than the inside diameter of the first cylindrical portion 21. The first bottom portion..23 is arranged so as -to. be perpendicular to the first cylindrical portion 21. The first cylindrical portion 21 has an axial end face 21a which is held in contact with a first bottom portion 23 of an adjacent interstage casing 1 substantially over the entire surface of the axial end face 21a. As long as the end face 21a of the first cylindrical portion 21 can maintain a contacting area with the adjacent first bottom portion 23, the first bottom portion 23 may be inclined at a predetermined angle with respect to the first cylindrical portion 21 rather than perpendicular to the first cylindrical portion 21. The second bottom portion 25 of the interstage casing 1 is formed into a curved shape of a hemisphere or substantially a hemisphere. A suction plate 30 is attached to the second bottom portion 25 by welding. A plurality of return guide vanes 34 are interposed between the suction plate 30 and a side plate 32 and attached to the suction plate 30 and the side plate 32 by welding. The suction plate 30 is also formed into a curved shape of a hemisphere or substantially a hemisphere so as to be held in contact with a surface of the second bottom portion 25. The suction plate 30 has an outer circumferential end face 30a which is held in contact with an inner surface of a first cylindrical portion 21 of an adjacent interstage casing 1. The suction plate 30, the second cylindrical portion 24, the first bottom portion 23, and the inner surface of the first cylindrical portion 21 of the adjacent interstage casing 1 form a space for accommodating an O-ring 27. Thus, according to the present invention, the interstage casing 1 has a simple structure which can be manufactured simply through press forming. Accordingly, it is possible to readily form a space for an O-ring disposed between adjacent interstage...casings with high-.accuracy. Thus,, it is possible to maintain stable sealing capability and stable sealing accuracy. Such a space for an O-ring may be changed in shape such that a commercially available O-ring is fitted in the space. Thus, since any commercially available O-ring can be used, manufacturing cost of the pump can be reduced. Further, since the second bottom portion 25 of the interstage casing 1 is formed into a curved shape of a hemisphere or substantially a hemisphere, the interstage casing 1 has a high pressure resistance. A liner ring 28 is mounted on an innermost portion 26 of the second bottom portion 25 of the interstage casing 1 to prevent water having a high pressure from leaking into the suction side of the impeller 11. A baffle 36 is disposed in the interstage casing 1 so as to cover the protruding portion 22, the first bottom portion 23, the second cylindrical portion 24, and an outer peripheral portion of the second bottom portion 25. The baffle 36 is bonded to the second bottom portion 25 of the interstage casing 1 by welding. Even if solid matter such as stones flows into the interstage casing 1, the baffle 36 can protect the protruding portion 22, the first bottom portion 23, the second cylindrical portion 24, and the outer peripheral portion of the second bottom portion 25 from the solid matter. Particularly, the baffle 36 protects a stepped portion formed at a corner of the interstage casing 1. The baffle 36 may be eliminated depending on purposes of the pump. FIG. 3 is a view taken in a direction of an arrow III of FIG. 2. FIG. 3 shows an interior of the interstage casing 1. As shown in FIG. 3, the baffle 36 has a spline structure at a radially inner portion thereof. Specifically, the baffle 36 has a number of notches -(groaves) .36a extending radially outward Accordingly,, the second bottom portion 25 of the interstage casing 1 is exposed at the notches 36a of the baffle 36 and covered with the baffle 36 at portions other than the notches 36a of the baffle 36. Thus, depressions and projections are formed on a surface of the second bottom portion 25 facing the impeller 11 by the notches 36a of the baffle 36. The depth of the depressions and the thickness of the projections correspond to the thickness of the baffle 36. Rotational energy is applied to a fluid in a space between the impeller 11 and the second bottom portion 25 by rotation of the impeller 11. The depressions and projections formed on the surface of the second bottom portion 25 can reduce the rotational energy of the fluid between the impeller 11 and the second bottom portion 25. Accordingly, it is possible to prevent a static pressure of the fluid between the impeller 11 and the second bottom portion 25 from being lowered. Thus, a thrust force applied to the impeller 11 can be reduced. Next, the mechanical seal 12 for performing a shaft seal of the portion where the main shaft 10 passes through the pump casing, and its associated components will be described below with reference to FIGS. 1 and 4. FIG. 4 is an enlarged fragmentary cross-sectional view of the multistage pump shown in FIG. 1. As shown in FIGS. 1 and 4, a lining member 40 which is manufactured through press working of a stainless steel plate adheres to a liquid contact portion of the motor base 6. The motor base 6 comprises castings of gray cast iron, and the motor base 6 and the lining member 40 comprising a pressed component constitute a casing cover for covering an open end of the pump casing tightly. The lining member 40 adheres to the motor base 6 by press fitting. An annular -.portion.-40a . is .formed at the inner -peripheral portion of the lining member 40, and covers a hollow cylindrical portion 44h of the mechanical seal holder 44. The end portion of the annular portion 40a of the lining member 40 is expanded radially outwardly so that the lining member 40 does not fall out from the motor base 6. Further, the outer peripheral portion 40b of the lining member 40 is expanded radially outwardly in a tapered shape so that the O-ring 7 can be easily inserted into the annular portion 40a. A cartridge type mechanical seal 12 is mounted on the upper part of the main shaft 10 to perform a shaft seal of the portion (hollow cylindrical portion 6h) where the main shaft 10 passes through the casing cover (composed of the motor base 6 and the lining member 40) . According to the multistage pump shown in FIG. 1, since the casing cover for covering the open end of the pump casing tightly comprises the motor base 6 comprising a thick member made of castings, and the lining member 40 comprising a thin member made of a pressed component and adhering to the outer surface of the motor base 6, rigidity and corrosion resistance of the casing cover can be ensured, and press working of the lining member 40 can be easily performed. Further, since the lining member 40 comprises a thin-wall member, material cost of the stainless steel plate can be reduced. Further, as shown in FIG. 1, the motor base 6 has a vertical hole 6b and a lateral hole 6c for priming and air- bleeder. A pipe 51 made of stainless steel is fitted into the vertical hole 6b, and a cylindrical receptacle-like member 52 made of stainless steel is fitted into the lateral hole 6c. A plug 53 is screwed into the open end of the cylindrical receptacle-like member 52. With the above arrangement shown in FIGS. 1 and 4, in order-to -form the hole for priming and .air-bleeder in the motor base 6 made of gray cast iron, the pipe 51 and the cylindrical receptacle-like member 52 which are made of stainless steel are provided in the liquid contact portion. Therefore, even if the motor base 6 is made of gray cast iron having a poor corrosion resistance, the liquid contact portion in the motor base 6 is entirely lined with a material made of stainless steel, thus ensuring corrosion resistance of the liquid contact portion. The contacting portion of the lining member 40 and the pipe 51 and the contacting portion of the pipe 51 and the cylindrical receptacle-like member 52 are welded by lasers to ensure water-tightness. As shown in FIGS. 1 and 4, the cartridge type mechanical seal 12 comprises a cartridge sleeve 41 which is mounted on the main shaft 10 and rotates together with the main shaft 10, a rotating assembly 42 including a rotating seal ring which is mounted on the cartridge sleeve 41 and rotates together with the cartridge sleeve 41, a stationary assembly 43 including a stationary seal ring which is brought into sliding contact with the rotating assembly 42, and a holder 44 which holds the stationary assembly 43 and is fixed to the motor base 6. The holder 44 comprises a cylindrical member 44a which holds the stationary assembly 43 including the stationary seal ring and has an 0-ring 45 thereon, and a circular plate 44b welded to the cylindrical member 44a by lasers. The cylindrical member 44a has a step portion 44s for holding the stationary assembly 43 including the stationary seal ring. The holder 44 is fixed to the motor base 6 by fastening the circular plate 44b to the motor base 6 with a plurality of bolts 46. On the other hand, a sleeve presser 47 is provided on -the .upper-.end of the cartridge sleeve - 41,.__and..a plurality of setscrews 48 are screwed into the sleeve presser 47. The cartridge sleeve 41 has through-holes 41h at the locations corresponding to the setscrews 48. By fastening the setscrews 48 against the main shaft 10, the sleeve presser 47 rotates together with the main shaft 10 to cause the cartridge sleeve 41 to rotate together with the main shaft 10. The rotating seal ring is pressed against the stationary seal ring by a compression spring (not shown) . Further, an adjustment sleeve 49 is provided between the rotating assembly 42 including the rotating seal ring and the cartridge sleeve 41. With the above arrangement, in a case where the cartridge type mechanical seal 12 is removed from the main shaft 10, the coupling 14 is removed from the main shaft 10 and the main shaft 15, and then the motor (not shown) is removed from the motor base 6. In this state, there is nothing left above the cartridge type mechanical seal 12 except for the upper end portion of the main shaft 10. Next, the setscrews 48 are loosened, the sleeve presser 47 becomes in a free state with respect to the main shaft 10, and then the bolts 46 are removed from the motor base 6. Thereafter, the holder 44 is lifted using a driver or the like. Thus, the inner peripheral portion of the circular plate 44b of the holder 44 is brought into close contact with the end surface of the sleeve presser 47. As a result, the holder 44, the sleeve presser 47, and the cartridge sleeve 41 are integrally lifted. Then, the holder 44 is further lifted, and thus the cartridge type mechanical seal 12 in its entirety is removed upwardly from the main shaft 10. When the cartridge type mechanical seal 12 is mounted on the main shaft 10, the cartridge sleeve 41 which holds the rotating assembly 42 including the rotating seal ring, the holder 44 which holds the stationary assembly 43 including the stationary seal ring, and the sleeve presser 47 mounted on the upper end of the cartridge sleeve 41 are integrated in advance, and in this state, the integrated assembly including the cartridge sleeve 41, the holder 44 and the sleeve presser 47 is fitted over the main shaft 10 from the upper end of the main shaft 10. Thereafter, the bolts 46 are screwed into the pump base 6, and the setscrews 48 are tightened against the main shaft 10. Since the annular portion 40a of the lining member 40 has an end portion which is expanded radially outwardly, when the mechanical seal 12 is mounted, the O-ring 45 held by the holder 44 can be easily inserted into the annular portion 40a of the lining member 40. As shown in FIG. 1, a double-split ring 91 (described later on) is attached to the main shaft 10, and when the cartridge type mechanical seal 12 is mounted on the main shaft 10, the lower end portion of the cartridge type mechanical seal 12 is brought into close contact with the double-split ring 91, thereby positioning the cartridge type mechanical seal 12. According to the cartridge type mechanical seal having the above structure, only by removing the coupling 14 and the motor (not shown) fixed to the motor base 6, the cartridge type mechanical seal 12 can be mounted on or detached from the main shaft 10 from the location above the main shaft 10. Therefore, it is not necessary to remove the multistage interstage casings 1, the impellers 11, and the like in order to replace the mechanical seal. Further, according to the cartridge type mechanical seal, because the holder 44 which holds the stationary assembly (including the stationary seal ring) of the mechanical seal comprises the cylindrical member 44a and the -circular .plate .44b which are joined., together by laser welding, the cylindrical member 44a can be formed by working a pipe slightly, and the circular plate 44b can be easily formed by blanking or the like. Therefore, the holder 44 having a flange can be manufactured by minimum working processes, and thus manufacturing cost can be greatly reduced. Next, operation of the multistage pump shown in FIGS. 1 through 4 will be described briefly. When the multistage pump is in operation, liquid drawn in from the suction nozzle 17 is introduced into the lower casing 3, and is pressurized by the first-stage impeller 11 in the lowermost interstage casing 1. The liquid discharged from the first-stage impeller 11 passes through passages defined by the return blades 34 provided between the suction plate 30 and the side plate 32, and is then introduced into the next-stage impeller 11. In this manner, the liquid is pressurized by each of the impellers 11 in each of the interstage casings 1, and the pressure head of the liquid is recovered while the liquid passes through the passages defined by the return blades 34 and the like. Finally, the liquid is pressurized by the final- stage impeller 11 located at the uppermost stage, and the pressure head of the liquid is recovered after passing through the final-stage passages defined by the return blades 34 and the like. Thereafter, the liquid flows into the upper casing 2. A plurality of discharge holes 2a are formed in the upper casing 2. Thus, the liquid in the upper casing 2 passes through the discharge holes 2a and then the annular passage 55 defined between the outer casing 4 and the multistage interstage casings 1, and is then discharged from the discharge nozzle 18 to the outside. Although the vertical-type multistage pump has been,...descr-ibed -in the above embodiments, this. pump, can also be used as a horizontal type multistage pump. FIG. 5 is a vertical cross-sectional view showing a first modified example of the vertical-type multistage pump shown in FIG. 1. In the multistage pump shown in FIG. 5, the cartridge type mechanical seal 12 can be removed from the main shaft 10 without detaching the motor (not shown) from the motor base 6. Further, when the cartridge type mechanical seal 12 is mounted on the main shaft 10 of the pump, the motor is not required to be detached from the motor base 6. A gap Li which is longer than the overall length of the cartridge type mechanical seal 12 is formed between the main shaft 10 of the pump and the main shaft 15 of the motor. Therefore, the cartridge type mechanical seal 12 can be removed in such a state that the motor is mounted on the motor base 6. Specifically, the double-split coupling 14 is removed from the main shaft 10 and the main shaft 15, and then the cartridge type mechanical seal 12 in its entirety is lifted to the space between the main shaft 10 of the pump and the main shaft 15 of the motor, whereby the cartridge type mechanical seal 12 can be removed from the main shaft 10. Rectangular windows 6w are formed in the side wall of the motor base 6, and hence the cartridge type mechanical seal 12 which has been lifted is taken out laterally through one of the windows 6w. Conversely, when the cartridge type mechanical seal 12 is mounted on the main shaft 10, the cartridge type mechanical seal 12 is introduced into the motor base 6 through the window 6w, and then the cartridge type mechanical seal 12 is mounted on the main shaft 10 through the space between the main shaft .10- .and-the._main .shaft 15. _ .. According to the coupling structure shown in FIG. 5, the motor which is a heavy object is not required to be moved, and the mechanical seal can be inspected or replaced only by removing the coupling. FIG. 6 is a vertical cross-sectional view showing a second modified example of the vertical-type multistage pump shown in FIG. 1. In the multistage pump shown in FIG. 6, a housing bracket 80 is fixed to the motor base 6, and a large ball bearing 81 for receiving a thrust load generated in the pump is housed in the housing bracket 80. The main shaft 10 of the pump and the main shaft 15 of the motor are coupled by a first coupling 82 and a second coupling 83. A gap L2 which is longer than the overall length of the cartridge type mechanical seal 12 is formed between the upper end of the main shaft 10 and the lower end of the first coupling 82. Therefore, the cartridge type mechanical seal 12 can be removed in such a state that the motor is mounted on the housing bracket 80. Specifically, the double-split type second coupling 83 is removed from the main shaft 10 and the first coupling 82, and then the cartridge type mechanical seal 12 in its entirety is lifted to the space between the main shaft 10 of the pump and the first coupling 82, whereby the cartridge type mechanical seal 12 can be removed from the main shaft 10. Rectangular windows 6w are formed in the side wall of the motor base 6, and hence the cartridge type mechanical seal 12 which has been lifted is taken out laterally through one of the windows 6w. Conversely, when the cartridge type mechanical seal 12 is mounted on the main shaft 10, the cartridge type mechanical seal 12 is introduced into the motor base 6 through the window 6w, and then the cartridge type mechanical seal 12 is mounted on the main...shaft .10 through the space between, .the.. main shaft 10 and the first coupling 82. According to the coupling structure shown in FIG. 6, the motor which is a heavy object is not required to be moved, and the mechanical seal can be inspected or replaced only by removing the coupling. Further, by providing the housing bracket 80 having the large ball bearing 81 therein, the thrust load generated in the pump can be received by the large ball bearing 81, and hence the general-purpose small motor which is available in low cost on the market can be used. Thus, the range of selection of the motor can be broaden, and the reduction of the cost of the pump including the motor can be achieved. FIGS. 7 and 8 are views showing the structure for fixing the uppermost impeller 11 to the main shaft 10 of the pump. FIG. 7 is an exploded perspective view of the main shaft 10, a double-split ring 91 and a ring presser 92, and FIG. 8 is a perspective view partly in section showing the state in which the main shaft 10, the double- split ring 91 and the ring presser 92 are assembled. As shown in FIG. 7, a circular groove 1Og is formed in the main shaft 10 at the location near the upper end of the spline section 10s. The double-split ring 91 is fitted in the circular groove 1Og. The double-split ring 91 constitutes a positioning member for positioning the final- stage impeller 11 with respect to the main shaft 10 in an axial direction of the main shaft 10. The ring presser 92 shown below the main shaft 10 in FIG. 7 has a circular disk portion 92a which is brought into close contact with the end surface of the double-split ring 91, and a cylindrical portion 92a extending perpendicularly from the outer periphery of the circular disk portion 92a and provided so as to enclose--the outer peripheral portion of the. double- split ring 91. The ring presser 92 constitutes a retainer for retaining the double-split ring 91 mounted on the main shaft 10. The ring presser 92 constituting the retainer has a plurality of spline grooves serving as an inner diameter shape which is fitted over the spline grooves provided in the spline section 10s of the main shaft 10. First, the ring presser 92 is moved upwardly along the spline section 10s, and the ring presser 92 is brought into close contact with the double-split ring 91 mounted on the main shaft 10. At this time, the cylindrical portion 92b of the ring presser 92 is brought into contact with the outer peripheral portion of the double-split ring 91 or is located in close proximity to the outer peripheral portion of the double-split ring 91. Thereafter, the multistage impellers 11 are mounted on the main shaft 10, and finally, a nut 93 is threaded over the lower end of the main shaft 10, thereby fixing the multistage impellers 11 to the main shaft 10. Instead of the double-split ring 91, a C-type snap ring 94 located above the main shaft 10 in FIG. 7 may be used. According to the present embodiment, the multistage impellers 11 are fixed to the main shaft 10 by fastening the nut 93. At this time, the fastening force of the nut 93 is transmitted to the double-split ring 91 through the ring presser 92. However, since the double-split ring 91 is tightly fitted in the circular groove 1Og of the main shaft 10 in such a state that the double-split ring 91 is enclosed by the ring presser 92, the double-split ring 91 is prevented from being deformed. As shown in FIG. 1, the double-split ring 91 serves also as a positioning member for positioning the lower end portion of the cartridge type mechanical seal 12.

Industrial Applicability The present invention is applicable to a centrifugal pump having at least one impeller fixed to a main shaft and at least one casing which houses the impeller and is formed through press working (press forming) of sheet metal such as a steel plate.