EP2653727A1

EP2653727A1 - Multistage pump

Info

Publication number: EP2653727A1
Application number: EP11848993.9A
Authority: EP
Inventors: Keisuke Nagaoka; Yasushi Hashimoto; Makoto Noguchi; Daisuke Nakano
Original assignee: Kubota Corp
Current assignee: Kubota Corp
Priority date: 2010-12-14
Filing date: 2011-12-13
Publication date: 2013-10-23
Anticipated expiration: 2031-12-13
Also published as: WO2012081562A1; CN103261697A; JP5889622B2; CN103261697B; EP2653727A4; EP2653727B1; JP2012140931A

Abstract

Provided is a multistage pump in which a fluid raised in pressure by an impeller at a last stage is guided to a discharge opening through a diffuser 21c at the last stage. A discharge flow path 22 for guiding the fluid from the last-stage diffuser 21c to the discharge opening is formed in a pump casing. The discharge flow path 22 is formed in a direction in which the discharge flow path swirls about the shaft center of a rotating shaft. The cross-sectional area of the discharge flow path 22 in a plane including the shaft center of the rotating shaft increases from the upstream side toward the downstream side of a flow toward the discharge opening in a shaft center direction A and an inward radial direction B of the rotating shaft.

Description

Technical Field

The present invention relates to a multistage pump such as a single-barreled sectional-type multistage pump.

Background Art

Conventionally, as shown in FIG. 12, a multistage pump 71 includes a pump casing 72 composed of a plurality of components including a suction opening 73 and a discharge opening 74. The pump casing 72 contains a plurality of impellers 76a to 76c rotated with a rotating shaft 75. Diffusers 77 are provided outside of the outlets of the impellers 76a to 76c.
When the impellers 76a to 76c rotate with the rotating shaft 75, a fluid such as water flows from the suction opening 73 through a suction flow path 78 and is then guided to and raised in pressure by the first-stage impeller 76a. Next, the fluid such as water is guided from the diffuser 77 to the subsequent-stage impeller 76b through an intermediate flow path 79 and is further raised in pressure by the subsequent-stage impeller 76b. After the fluid such as water is sequentially raised in pressure, the fluid is raised in pressure by the last-stage impeller 76c and is guided from the last-stage diffuser 77 to the discharge opening 74 through a discharge flow path 80.
At this point, as shown in FIG. 13, the fluid swirls through the discharge flow path 80 in a swirl direction 82 which is the same as the rotation direction of the impellers 76a to 76c, and is discharged from the discharge opening 74.
The discharge flow path 80 is torus-shaped so as to surround the shaft center of the rotating shaft 75 and is partly communicated with the discharge opening 74 via a communication flow path 81 formed in the outward radial direction of the discharge flow path 80. The cross-sectional area of the discharge flow path 80 in a plane including a shaft center 75a of the rotating shaft 75 is constant almost over the periphery other than the communication flow path 81.
Patent Literature 1 listed below describes a multistage pump having a configuration in which a fluid fed to a diffuser from a last-stage impeller is guided to a discharge opening through a discharge flow path.

Citation List

Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2005-330878

Summary of Invention

Technical Problem

In the above-described conventional form, the fluid almost uniformly swirls from the last-stage diffuser 77 along the outer periphery in the swirl direction 82 and then flows in the shaft direction. The fluid swirls through the discharge flow path 80 in the swirl direction 82 while increasing in flow rate and is discharged from the discharge opening 74 through the communication flow path 81.
The discharge flow path 80 is shaped like a concentric annulus such that the cross-sectional area of the discharge flow path 80 is constant in the plane including the shaft center 75a of the rotating shaft 75. A portion of the periphery of the discharge flow path 80 is outwardly communicated with the communication flow path 81. In the above-described discharge flow path 80, a region communicated with the communication flow path 81 in the swirl direction 82 is set as a downstream region 84 while a region opposed to the downstream region 84 is set as an upstream region 83. In the upstream region 83, the cross-sectional area of the discharge flow path 80 is extremely large in the plane including the shaft center 75a, for a small flow rate of the fluid into the discharge flow path 80. Thus, the fluid flows through the upstream region 83 of the discharge flow path 80 at a significantly reduced velocity, readily causing stagnation (a dead water region) in the upstream region 83. Hence, the substantial uniform flow of the fluid from the last-stage diffuser 77 into the discharge flow path 80 is disturbed in the peripheral direction, disadvantageously resulting in a reduction in pump efficiency.
An object of the present invention is to provide a multistage pump which is capable of reducing the size of a pump casing and preventing a reduction in pump efficiency.

Solution to Problem

In order to attain the object, a first invention is a multistage pump including a pump casing which is provided with a suction opening and a discharge opening and contains a plurality of impellers rotated with a rotating shaft, the multistage pump guiding a fluid raised in pressure by the impeller at a last stage to the discharge opening through a pressure recovery part at the last stage, characterized in that a discharge flow path for guiding the fluid from the pressure recovery part at the last stage to the discharge opening is formed in the pump casing, the discharge flow path is formed in a direction in which the discharge flow path swirls about the shaft center of the rotating shaft, and the cross-sectional area of the discharge flow path in a plane including the shaft center of the rotating shaft increases in the shaft center direction and the inward radial direction of the rotating shaft from the upstream side toward the downstream side of a flow toward the discharge opening.
with this configuration, the fluid sucked into the pump casing through the suction opening is sequentially raised in pressure by the plurality of rotating impellers. The fluid raised in pressure by the last-stage impeller flows through the discharge flow path after the last-stage pressure recovery part and is then discharged from the discharge opening.
At this point, the fluid flows from the last-stage recovery part into the discharge flow path while swirling and gradually increasing in flow rate from the upstream side toward the downstream side and is discharged from the discharge opening. Meanwhile, the cross-sectional area of the discharge flow path increases from the upstream side toward the downstream side of the flow toward the discharge opening.
Thus, the flow rate on the upstream side of the discharge flow path is smaller than that on the downstream side. Accordingly, the cross-sectional area of the discharge flow path on the upstream side is smaller than that on the downstream side. This configuration prevents the flow velocity from being significantly reduced on the upstream side of the discharge flow path, thereby preventing the occurrence of stagnation (a water stop region) on the upstream side of the discharge flow path. Thus, when the fluid flows through the discharge flow path from the upstream side toward the downstream side and reaches the discharge opening, the flow of the fluid can be prevented from being largely disturbed on the upstream side of the discharge flow path. Hence, the fluid flows through the discharge flow path from the upstream side toward the downstream side almost uniformly across the cross section of the flow path, thereby reducing an energy loss caused by the disturbance of the flow and preventing a reduction in pump efficiency.
Further, the cross-sectional area of the discharge flow path increases in the shaft center direction and the inward radial direction of the rotating shaft. Thus, the discharge flow path does not expand outwardly in the radial direction, making it possible to reduce the size of the pump casing in the radial direction.
A second invention is the multistage pump, characterized in that the discharge opening side of the discharge flow path is set as the downstream side and the opposite side of the flow direction is set as the upstream side, the dimension of the cross section of the discharge flow path in the plane including the shaft center of the rotating shaft increases in the shaft center direction at a larger rate in a region on the upstream side of the discharge flow path than in a region on the downstream side adjacent to the region on the upstream side, and the dimension of the cross section of the discharge flow path increases in the inward radial direction at a smaller rate in the region on the upstream side of the discharge flow path than in the region on the downstream side adjacent to the region on the upstream side.
With this configuration, when the fluid flows through the discharge flow path from the upstream side toward the downstream side and reaches the discharge opening, the region on the upstream side of the discharge flow path expands more gently in the inward radial direction than the region on the downstream side adjacent to the region on the upstream side. This prevents the flow of the fluid in the inward radial direction from being separated, disturbed, or stagnated in the region on the upstream side of the discharge flow path, further preventing a reduction in pump efficiency.
A third invention is the multistage pump, characterized in that in the upstream region of the discharge flow path, the dimension of the cross section of the discharge flow path increases at a larger rate in the shaft center direction than in the inward radial direction.
This configuration can further prevent the flow in the inward radial direction from being separated or disturbed in the region on the upstream side of the discharge flow path, further preventing a reduction in pump efficiency.
A fourth invention is the multistage pump, characterized in that the cross-sectional area of the discharge flow path in the plane including the shaft center of the rotating shaft linearly increases from the upstream side toward the downstream side at a predetermined rate.
With this configuration, the cross-sectional area of the discharge flow path does not rapidly increase or decrease in a discontinuous manner, thereby reducing an energy loss of the fluid caused by a change of the cross-sectional area and further preventing a reduction in pump efficiency.
A fifth invention is the multistage pump, characterized in that the pump casing is partitioned into a suction casing with the suction opening, a discharge casing with the discharge opening, and an intermediate casing interposed between the suction casing and the discharge casing, a fixing member is provided to fasten the casings in the shaft center direction of the rotating shaft, the discharge flow path is formed in the discharge casing, the suction casing includes a suction flow path for guiding the fluid from the suction opening to the inlet of the impeller at a first stage contained in the intermediate casing, the intermediate casing includes an intermediate flow path for guiding the fluid from the outlet of the impeller to the inlet of the impeller at a subsequent stage, the outside diameter of the discharge flow path and the outside diameter of the intermediate flow path are virtually equal, and the fixing member fixes the suction casing and the discharge casing outside of the outside diameters.
This configuration prevents the discharge casing from becoming extremely larger than the intermediate casing in the radial direction in the assembly of the pump casing, thereby reducing the size of the pump casing in the radial direction.

Advantageous Effects of Invention

As has been discussed, the present invention can prevent a reduction in the pump efficiency of a multistage pump and achieve a size reduction of the multistage pump.

Brief Description of Drawings

[FIG. 1] FIG. 1 is a cross-sectional view of a multistage pump according to a first embodiment of the present invention.
[FIG. 2] FIG. 2 is a cross-sectional view of a discharge casing as viewed along the shaft center direction of a rotating shaft in the multistage pump and shows that the discharge casing is provided with a diffuser according to the first embodiment.
[FIG. 3] FIG. 3 is a cross-sectional view of the discharge casing as viewed along the shaft center direction of the rotating shaft in the multistage pump and shows that the diffuser is removed according to the first embodiment.
[FIG. 4] FIG. 4 is a cross-sectional view showing the cross sections of the discharge casing in FIG. 3.
[FIG. 5] FIG. 5 is a cross-sectional view showing the cross sections of the discharge casing in FIG. 3.
[FIG. 6] FIG. 6 is a cross-sectional view showing the cross sections of the discharge casing in FIG. 3.
[FIG. 7] FIG. 7 is a cross-sectional view showing that the discharge flow path of the multistage pump expands in the shaft center direction and the inward radial direction of the rotating shaft according to the first embodiment.
[FIG. 8] FIG. 8(a) is a graph indicating the dimensions of the discharge flow path along the shaft center direction of the rotating shaft in positions V1 to V16 of FIG. 3; and FIG. 8(b) is a graph indicating the dimensions from the outside to the inside in the radial direction of the cross section of the discharge flow path in the positions V1 to V16 of FIG. 3.
[FIG. 9] FIG. 9 is a cross-sectional view of the discharge casing including a discharge opening in the multistage pump according to the first embodiment.
[FIG. 10] FIG. 10 is a cross-sectional view of a multistage pump according to a second embodiment of the present invention.
[FIG. 11] FIG. 11 is a graph indicating the cross-sectional areas of a discharge flow path in positions V1 to V16 of a multistage pump according to a third embodiment of the present invention.
[FIG. 12] FIG. 12 is a cross-sectional view of a multistage pump according to the related art.
[FIG. 13] FIG. 13 is a cross-sectional view taken along the arrows X-X of FIG. 12.

Description of Embodiments

Referring to the accompanying drawings, the following will describe embodiments of the present invention.

(First Embodiment)

First, a first embodiment will be described with reference to FIGS. 1 to 9. As shown in FIG. 1, reference numeral 1 denotes a single-barreled sectional-type multistage pump including a pump casing 2 provided with a suction opening 3 and a discharge opening 4. The pump casing 2 contains a plurality of impellers 6a to 6c rotated with a rotating shaft 5. The pump casing 2 is partitioned into a suction casing 7 having the suction opening 3, a discharge casing 8 having the discharge opening 4, and a plurality of sectional-type intermediate casings 9a and 9b placed between the suction casing 7 and the discharge casing 8.
The casings 7, 8, 9a, and 9b are fastened and fixed by a fixing member 11 along a shaft center direction A of the rotating shaft 5. The fixing member 11 has a plurality of fixing bolts 12 and nuts 13. Each of the fixing bolts 12 is inserted into the suction casing 7 and the discharge casing 8 located at two ends of the fixing bolt 12 along the shaft center direction A. The nuts 13 are screwed at the two ends of the fixing bolt 12 to fix the suction casing 7 and the discharge casing 8.
The rotating shaft 5 is inserted through the pump casing 2 and is sealed by sealing members 15 such as gaskets at shaft seal parts 14. The impellers 6a to 6c are fitted onto the rotating shaft 5 so as to rotate with the rotating shaft 5 and are contained in the intermediate casings 9a and 9b and the discharge casing 8. The impellers 6a to 6c each have an outlet 16 and an inlet 17. The outlet 16 is located outside of the inlet 17 in the radial direction of the rotating shaft 5.
A suction flow path 19 is formed in the suction casing 7 to guide water 18 (an example of a fluid) from the suction opening 3 to the inlet 17 of the impeller 6a at the first stage. The suction flow path 19 is torus-shaped so as to surround the rotating shaft 5. This configuration allows the water 18 to flow into the inlet 17 of the impeller 6a as uniformly as possible.
The intermediate casings 9a and 9b each have an intermediate flow path 20. The intermediate flow paths 20 guide the water 18 from the outlets 16 of the impellers 6a and 6b to the inlets 17 of the impellers 6b and 6c at the subsequent stages. The intermediate flow paths 20 have toric diffusers 21a and 21b formed outside of the outlets 16 of the impellers 6a and 6b.
As shown in FIG. 2, a toric diffuser 21c (an example of a pressure recovery part) at the last stage and a discharge flow path 22 are provided in the discharge casing 8. The diffuser 21c at the last stage is formed outside of the outlet 16 of the impeller 6c at the last stage. The discharge flow path 22 guides the water 18 having passed through the last-stage diffuser 21c to the discharge opening 4 and is formed spirally in a direction in which the discharge flow path 22 swirls about the shaft center 5a of the rotating shaft 5.
As shown in FIGS. 3 to 7, a downstream side 22a of the discharge flow path 22 is close to the discharge opening 4 while an upstream side 22b of the discharge flow path 22 is opposite to a flow direction 23 toward the discharge opening 4. The cross-sectional area of the discharge flow path 22 in a plane including a shaft center 5a of the rotating shaft 5 gradually increases from the upstream side 22b toward the downstream side 22a in the shaft center direction A and an inward radial direction B of the rotating shaft 5. In this case, when, in the shaft center direction A, the side of the suction opening 3 is set as the front and the side of the discharge opening 4 is set as the rear, the cross-sectional area of the flow path increases toward the rear in the shaft center direction A (that is, from the intermediate casing 9b toward the discharge casing 8). The downstream side 22a of the discharge flow path 22 is positioned behind the upstream side 22b without overlapping the upstream side 22b in the shaft center direction A. In FIG. 4(a), the cross-sectional area of the discharge flow path 22 apparently increases in an outward radial direction. This is because a communication path leading to the discharge opening 4 is shown in the drawing.
In graph (a) of FIG. 8, the abscissa indicates positions V1 to V16 along a peripheral direction D of the discharge flow path 22 in FIG. 3. The positions V1 to V16 are set 22.5° apart from each other along the peripheral direction D. Further, the ordinate in the graph (a) indicates a linear dimension C in the shaft center direction A of the cross section of the discharge flow path in FIG. 9. According to the graph (a), the dimension C in the shaft center direction A of the discharge flow path 22 is set to increase at such a rate that a predetermined upstream region 25 including the start position V1 (start point) of the discharge flow path 22 is larger than a predetermined downstream region 26 adjacent to the region 25. The increase rate corresponds to inclinations α1 and α2 in the graph (a). The inclination α1 in the predetermined upstream region 25 (at substantially 180° from the upstream side) is set larger than the inclination α2 in the predetermined downstream region 26.
In graph (b) of FIG. 8, the abscissa indicates the positions V1 to V16 along the peripheral direction D of the discharge flow path 22 in FIG. 3, while the ordinate indicates a linear dimension F from the outside to the inside in the radial direction of the cross section of the discharge flow path 22 in FIG. 9. According to the graph (b), the dimension F of the discharge flow path 22 is set to increase in the inward radial direction B at such a rate that a predetermined upstream region 27 including the start position V1 (start point) of the discharge flow path 22 is smaller than a first downstream region 28 adjacent to the region 27. The increase rate corresponds to inclinations β1 to β3 in the graph (b). The inclination β1 in the predetermined upstream region 27 including the start position V1 is set smaller than the inclination β2 in the first downstream region 28 adjacent to the region 27. Further, the inclination β3 in a second downstream region 29 on the downmost stream side adjacent to the first downstream region 28 is set smaller than the inclination β1 in the predetermined upstream region 27. This is because the inclination β3 in the second downstream region 29 cannot be set large due to a component, e.g., a balancing disc provided in the inward radial direction B of the discharge flow path 22. In the absence of a component such as a balancing disc, the inclination β3 can be set equal to or larger than the inclination β2.
In the upstream regions of the discharge flow path 22, the increase rate of the dimension C in the shaft center direction A of the discharge flow path 22 (that is, the inclination α1 in the graph (a) of FIG. 8) is larger than that of the dimension F in the inward radial direction B of the discharge flow path 22 (that is, the inclination β1 in the graph (b) of FIG. 8).
As shown in FIG. 9, an outside diameter G1 of the discharge flow path 22 is kept constant from the start point (the upstream start position V1 in FIG. 3) to the flow path communicating with the discharge opening 4 (the downstream position V16 in FIG. 3). As shown in FIG. 1, the outside diameter G1 of the discharge flow path 22 is substantially equal to an outside diameter G2 of the intermediate flow path 20, and an outside diameter G3 of the suction flow path 19 is smaller than the outside diameters G1 and G2. The fixing bolts 12 are located outside of the outside diameters G1 and G2.
The following will describe the action of the above-described configuration.
As shown in FIG. 1, the impellers 6a to 6c are rotated by a rotation of the rotating shaft 5. The water 18 suctioned from the suction opening 3 into the pump casing 2 flows into the inlet 17 of the first-stage impeller 6a through the suction flow path 19 and flows out of the outlet 16 of the first-stage impeller 6a. The water 18 flows through the intermediate flow path 20 after the first-stage diffuser 21a. Then, the water 18 flows into the inlet 17 and out of the outlet 16 of the subsequent-stage impeller 6b. The water 18 then flows through the intermediate flow path 20 after the subsequent-stage diffuser 21b. The water 18 sequentially raised in pressure thus flows into the inlet 17 and out of the outlet 16 of the last-stage impeller 6c. The water 18 then flows into the discharge flow path 22 through the last-stage diffuser 21c and is discharged from the discharge opening 4.
As has been discussed, the water 18 is discharged from the discharge opening 4 after being sequentially raised in pressure by the impellers 6a to 6c. At this point, the water 18 uniformly flows into the discharge flow path 22 through the last-stage diffuser 21c in the peripheral direction. The water 18 swirls from the upstream side 22b toward the downstream side 22a of the discharge flow path 22 while gradually increasing in flow rate, and the water 18 is then discharged from the discharge opening 4. Meanwhile, as shown in FIG. 3, the cross-sectional area of the discharge flow path 22 gradually increases from the upstream side 22b toward the downstream side 22a of the flow toward the discharge opening 4.
Thus, the flow rate on the upstream side 22b of the discharge flow path 22 is smaller than that on the downstream side 22a. In accordance with the smaller flow rate, the cross-sectional area of the discharge flow path 22 on the upstream side 22b is smaller than that on the downstream side 22a, thereby suppressing a significant reduction in flow velocity on the upstream side 22b of the discharge flow path 22. This can prevent the stagnation of the water 18 (the flow of the water 18 is disrupted, namely, a dead water region) from occurring on the upstream side 22b of the discharge flow path 22. Thus, when the water 18 flows through the discharge flow path 22 from the upstream side 22b toward the downstream side 22a and reaches the discharge opening 4, the flow of the water 18 can be prevented from being greatly disturbed on the upstream side 22b of the discharge flow path 22. Further, the water 18 flows through the discharge flow path 22 from the upstream side 22b toward the downstream side 22a almost uniformly across the cross section of the flow path. This can reduce an energy loss caused by the disturbance of a flow, thereby preventing a reduction in pumping efficiency.
As shown in FIG. 7, the cross-sectional area of the spiral-shaped discharge flow path 22 increases from the upstream side 22b toward the downstream side 22a not in the outward radial direction but in the shaft center direction A and the inward radial direction B of the rotating shaft 5. Thus, the discharge flow path 22 does not expand in the outward radial direction, so that the pump casing 2 does not increase in size in the radial direction. Hence, even when the discharge flow path 22 is spiral-shaped to improve the pump efficiency, the pump casing 2 (the discharge casing 8) can be reduced in size in the radial direction.
Further, before the water 18 reaches the discharge opening 4 through the discharge flow path 22, since the flow condition in the upstream region 27 of the discharge flow path 22 is particularly heavily affected by the swirling flow in the outward radial direction, the water 18 is not likely to flow in the inward radial direction of the discharge flow path 22. Meanwhile, as shown in the graph (b) of FIG. 8, the upstream region 27 of the discharge flow path 22 expands in the inward radial direction B more gradually than the first downstream region 28 adjacent to the region 27. This can particularly prevent the flow of the water 18 close to the inside in the radial direction of the discharge flow path 22 from being separated, disturbed, or stagnated in the upstream region 27 of the discharge flow path 22, further preventing a reduction in pump efficiency.
Moreover, since the inclination α1 in the graph (a) of FIG. 8 is larger than the inclination β1 in the graph (b) of FIG. 8, the flow of the water 18 close to the inside in the radial direction of the discharge flow path 22 can be further prevented from being separated or disturbed in the upstream region of the discharge flow path 22, further preventing a reduction in pump efficiency.
As shown in FIG. 1, the pump casing 2 is assembled as follows: the intermediate casings 9a and 9b are interposed between the suction casing 7 and the discharge casing 8; the fixing bolts 12 are inserted into the suction casing 7 and the discharge casing 8; and the nuts 13 are screwed to fix the casings 7, 8, 9a, and 9b. At this point, since the outside diameter G1 of the discharge flow path 22 and the outside diameter G2 of the intermediate flow path 20 are substantially the same in dimension, the discharge casing 8 does not become extremely larger than the intermediate casings 9a and 9b in the radial direction, so that the pump casing 2 can be reduced in size in the radial direction.
The outside diameter G1 of the discharge flow path 22 and the outside diameter G2 of the intermediate flow path 20 are substantially the same in dimension, which includes the cases where the outside diameters G1 and G2 are totally the same and the outside diameters G1 and G2 are slightly different. For example, even in the case where the outside diameter G1 of the discharge flow path 22 is slightly larger than the outside diameter G2 of the intermediate flow path 20, the outside diameters G1 and G2 may be considered to be the same in dimension.

(Second Embodiment)

In a second embodiment, as shown in FIG. 10, an outside diameter G1 of a discharge flow path 22, an outside diameter G2 of an intermediate flow path 20, and an outside diameter G3 of a suction flow path 19 are substantially the same in size.

(Third Embodiment)

The following will describe a third embodiment with reference to the graph of FIG. 11.
In the graph of FIG. 11, the abscissa indicates the positions V1 to V16 along the peripheral direction D of the discharge flow path 22 in FIG. 3, while the ordinate indicates the cross-sectional area of the discharge flow path 22 in the plane including the shaft center 5a of the rotating shaft 5. The graph shows that the cross-sectional area of the discharge flow path 22 linearly and gradually increases from the start position V1 on the upstream side toward the position V16 on the downstream side at a predetermined rate. In other words, the positions V1 to V16 along the peripheral direction D of the discharge flow path 22 are directly proportional to the cross-sectional area of the discharge flow path 22, and the predetermined rate corresponds to an inclination γ in the graph.
Thus, since the cross-sectional area of the discharge flow path 22 linearly and gradually increases from the upstream side toward the downstream side, the cross-sectional area of the discharge flow path 22 does not rapidly increase or decrease in a discontinuous manner. Accordingly, the flow velocity does not vary with a change in the cross-sectional area of the discharge flow path 22, thereby reducing an energy loss of the fluid and further preventing a reduction in pump efficiency.
In the third embodiment, a multistage pump 1 exhibits both the relationship between the positions V1 to V16 and the cross-sectional area of the discharge flow path 22 in the graph of FIG. 11 and the relationships between the positions V1 to V16 and the linear dimensions C and F of the discharge flow path 22 in the graphs of FIGS. 8(a) and 8(b) of the first embodiment. However, the multistage pump 1 may exhibit only the relationship in the graph of FIG. 11 but may not exhibit the relationships in the graphs of FIGS. 8(a) and 8(b).
In the above-described embodiments, the multistage pump 1 includes the three (multiple) impellers 6a to 6c. The number of the impellers is not limited to three. Two or four or more impellers may be provided. Further, the two intermediate casings 9a and 9b are provided but one or three or more intermediate casings may be provided in accordance with the number of the impellers.
In the above-described embodiments, the sectional-type multistage pump 1 includes the pump casing 2 partitioned into the suction casing 7, the discharge casing 8, and the multiple sectional-type intermediate casings 9a and 9b. The multistage pump 1 may be, for example, a horizontally-partitioned multistage pump including the pump casing 2 partitioned into multiple casings along the cross sections of the pump casing 2 parallel to the shaft center 5a of the rotating shaft 5.
In the above-described embodiments, as shown in FIG. 3, the downstream side 22a of the discharge flow path 22 is positioned behind the upstream side 22b without overlapping the upstream side 22b in the shaft center direction A. However, the downstream side 22a may overlap the upstream side 22b.
In the above-described embodiments, the cross-sectional area of the discharge flow path 22 gradually increases toward the downstream side 22a. The discharge flow path 22 may have a region in which the cross-sectional area is kept constant, between the upstream side 22b and the downstream side 22a.
Further, the discharge flow path 22 is formed over a range of substantially 360° but may be formed over a range smaller or larger than 360°.
The rate at which the dimensions C and F of the cross section of the discharge flow path 22 increase in FIG. 8, positions where the increase rate changes, and the number of the changes are not limited to the relationships indicated by the rectilinear graphs. The relationships may be indicated by curvilinear graphs.
In the above-descried embodiments, as shown in the graph (b) of FIG. 8, the inclination β1 is smaller than the inclination β2. Conversely, the inclination β2 may be smaller than the inclination β1. Alternatively, the inclination β2 and the inclination β3 may be zero. In this case, in the first downstream region 28 and the second downstream region 29, the dimension F of the cross section of the discharge flow path 22 is kept at a constant value. The most effective way to prevent a reduction in pump efficiency is to make the inclination β1 smaller than the inclination β2 as in the graph (b) of FIG. 8.
In the above-described embodiments, the multistage pump 1 is illustrated. The same effect as in the multistage pump 1 can be produced in a single-stage pump by increasing the cross-sectional area of the discharge flow path 22 in the shaft center direction A and the inward radial direction B.
In the above-described embodiments, the diffusers 21a to 21c are used as an example of a pressure recovery part. The diffusers 21a to 21c may be vaned or vaneless. Alternatively, as another example of a pressure recovery part, at least one volute may be used.

Claims

A multistage pump comprising a pump casing which is provided with a suction opening and a discharge opening and contains a plurality of impellers rotated with a rotating shaft, the multistage pump guiding a fluid raised in pressure by the impeller at a last stage to the discharge opening through a pressure recovery part at the last stage, characterized in that
a discharge flow path for guiding the fluid from the pressure recovery part at the last stage to the discharge opening is formed in the pump casing,
the discharge flow path is formed in a direction in which the discharge flow path swirls about a shaft center of the rotating shaft, and
a cross-sectional area of the discharge flow path in a plane including the shaft center of the rotating shaft increases in a shaft center direction and an inward radial direction of the rotating shaft from an upstream side toward a downstream side of a flow toward the discharge opening.
The multistage pump according to claim 1, characterized in that
a discharge opening side of the discharge flow path is set as the downstream side and an opposite side of the flow direction is set as the upstream side,
a dimension of a cross section of the discharge flow path in the plane including the shaft center of the rotating shaft increases in the shaft center direction at a larger rate in a region on the upstream side of the discharge flow path than in a region on the downstream side adjacent to the region on the upstream side, and
a dimension of the cross section of the discharge flow path increases in the inward radial direction at a smaller rate in the region on the upstream side of the discharge flow path than in the region on the downstream side adjacent to the region on the upstream side.
The multistage pump according to claim 2, characterized in that
in the upstream region of the discharge flow path, the dimension of the cross section of the discharge flow path increases at a larger rate in the shaft center direction than in the inward radial direction.
The multistage pump according to claim 1, characterized in that
the cross-sectional area of the discharge flow path in the plane including the shaft center of the rotating shaft linearly increases from the upstream side toward the downstream side at a predetermined rate.
The multistage pump according to any one of claims 1 to 4, characterized in that
the pump casing is partitioned into a suction casing with the suction opening, a discharge casing with the discharge opening, and an intermediate casing interposed between the suction casing and the discharge casing,
a fixing member is provided to fasten the casings in the shaft center direction of the rotating shaft,
the discharge flow path is formed in the discharge casing,
the suction casing includes a suction flow path for guiding the fluid from the suction opening to an inlet of the impeller at a first stage contained in the intermediate casing,
the intermediate casing includes an intermediate flow path for guiding the fluid from an outlet of the impeller to an inlet of the impeller at a subsequent stage,
an outside diameter of the discharge flow path and an outside diameter of the intermediate flow path are virtually equal to each other, and
the fixing member fixes the suction casing and the discharge casing outside of the outside diameters.