JP7576643B2 - Centrifugal Pump - Google Patents

Description

The present invention relates to a centrifugal pump.

The centrifugal pump comprises a motor, a rotating shaft, an impeller, and a housing. The impeller is attached to the rotating shaft that rotates by the operation of the motor, and rotates together with the rotation of the rotating shaft to suck in the handled liquid from one side in the axial direction of the rotating shaft and discharge the handled liquid, for example, radially outward from the impeller. The impeller comprises a plurality of blades, a front shroud (side plate), and a rear shroud (main plate). In the axial direction, the front shroud covers one side of the blades, and the rear shroud covers the other side of the blades. A portion of the high-pressure handled liquid discharged from the impeller also flows between the front shroud and the housing, and between the rear shroud and the housing, and presses the front shroud toward the rear shroud side, thereby pressing the rear shroud toward the front shroud side. Usually, the pressure-receiving area of the rear shroud is larger than the pressure-receiving area of the front shroud. As a result, a force (axial thrust force) acts on the impeller in one direction.

In order to improve the suction performance of centrifugal pumps and increase the head, multi-stage centrifugal pumps are known that sequentially discharge the handled liquid using multiple impellers attached to one rotating shaft (see, for example, Patent Document 1). The axial thrust force described above increases with the number of impellers (number of stages). For this reason, in multi-stage centrifugal pumps, multiple impellers are divided into a front-stage group and a rear-stage group, and the front-stage group is attached to the rotating shaft back-to-back with the rear-stage group. The design of the impellers in the front-stage group is the same as that of the rear-stage group, except for the direction of rotation, so the axial thrust force of the front-stage group balances with the axial thrust force of the rear-stage group, and both axial thrust forces are canceled out.

JP 2002-21766 A

Normally, a mechanical seal is installed between the partition wall and the rotating shaft, which is located between the front group and the rear group. Therefore, the handled fluid in the rear group does not flow into the front group. However, if the viscosity of the handled fluid is low, the lubricating performance of the mechanical seal by the handled fluid decreases, so a configuration that does not use a mechanical seal between the partition wall and the rotating shaft can be adopted. The pressure of the handled fluid flowing through the rear group is higher than the pressure of the handled fluid flowing through the front group. Therefore, in this configuration, the handled fluid in the rear group flows into the front group through the gap between the partition wall and the rotating shaft, and the axial thrust force of the front group is strengthened and the axial thrust force of the rear group is weakened by this flow. As a result, an axial thrust force toward the suction port side of the front group is generated.

The present invention aims to suppress the generation of axial thrust force in a multi-stage centrifugal pump.

A centrifugal pump according to one embodiment of the present invention includes a motor, a rotating shaft rotated by the drive of the motor, a first impeller attached to the rotating shaft and sucking and discharging a pumped liquid, a second impeller attached to the rotating shaft and sucking and discharging the pumped liquid discharged from the first impeller, a first pump chamber in which the first impeller is housed, a second pump chamber arranged alongside the first pump chamber in the axial direction of the rotating shaft and in which the second impeller is housed, and an insertion hole through which the rotating shaft is inserted, the first pump chamber and the second pump chamber being separated by a central partition. a first partition wall portion disposed on a first direction side with respect to the first impeller and defining the first pump chamber together with the central partition wall portion; and a second partition wall portion disposed on a second direction side with respect to the second impeller and defining the second pump chamber together with the central partition wall portion, in which, in the axial direction, a direction in which the first impeller is disposed with respect to the second impeller is the first direction, and a direction opposite to the first direction is the second direction, the first impeller is oriented in the first direction, and has a first suction port that draws in the handled fluid from the first direction side, and the central partition wall portion the second impeller is oriented in the second direction and has a second suction port that draws in the handled fluid discharged from the first impeller from the second direction side , a second shroud facing the central partition wall portion, and a fourth shroud disposed on the second direction side with respect to the second shroud , the third shroud having a first cylindrical portion constituting the first suction port, the fourth shroud having a second cylindrical portion constituting the second suction port, a first space communicating with the insertion hole is formed between the impeller and the central partition portion, and a second space communicating with the insertion hole is formed between the second impeller and the central partition portion, the second space and the first space communicate with each other via the insertion hole, the outer diameter of the second shroud is larger than the outer diameter of the first shroud, and a second gap between a portion of the second partition portion facing the second cylindrical portion and the second cylindrical portion is larger than a first gap between a portion of the first partition portion facing the first cylindrical portion and the first cylindrical portion .

According to the present invention, it is possible to suppress the generation of axial thrust force in a multi-stage centrifugal pump.

1 is a schematic cross-sectional view showing an embodiment of a centrifugal pump according to the present invention. FIG. 2 is a partially enlarged cross-sectional view of the centrifugal pump of FIG. 1 . FIG. 2 is a schematic rear view of a central partition portion provided in the centrifugal pump of FIG. 1 . FIG. 2 is a schematic front view of a second impeller included in the centrifugal pump of FIG. 1 . FIG. 2A is a schematic partially enlarged cross-sectional view illustrating axial thrust force in a conventional pump, and FIG. 2B is a schematic partially enlarged cross-sectional view illustrating axial thrust force in a conventional pump not equipped with a mechanical seal. 5 is a schematic partially enlarged cross-sectional view illustrating the effect of the second impeller of FIG. 4 on an axial thrust force in the centrifugal pump of FIG. 1 . FIG. 2 is a schematic partially enlarged cross-sectional view illustrating an effect of a second fixed orifice provided in the centrifugal pump of FIG. 1 on an axial thrust force in the centrifugal pump of FIG. 1 . FIG. 5A and 5B are schematic partially enlarged cross-sectional views for explaining the influence of the convex portion of the second impeller of FIG. 4 on the axial thrust force in the centrifugal pump of FIG. 1, in which (a) shows a state in which the first axial thrust force and the second axial thrust force are balanced, (b) shows a state in which the first axial thrust force is greater than the second axial thrust force, and (c) shows a state in which the first axial thrust force is smaller than the second axial thrust force. FIG. 2 is a schematic partially enlarged cross-sectional view of a centrifugal pump according to a first modified example. 1A is a schematic partial enlarged cross-sectional view of a centrifugal pump according to a second modified example, FIG. 1B is a schematic partial enlarged cross-sectional view of a centrifugal pump according to a third modified example, and FIG. 1C is a schematic partial enlarged cross-sectional view of a centrifugal pump according to a fourth modified example. FIG. 13 is a schematic partially enlarged cross-sectional view showing a centrifugal pump according to a fifth modified example.

An embodiment of a centrifugal pump according to the present invention will be described below. In the following description, the various drawings will be referred to as appropriate. In each drawing, the same members and elements are given the same reference numerals, and duplicated descriptions will be omitted. In addition, the dimensional ratios of each element may be exaggerated for the convenience of explanation, and are not limited to the ratios shown in each drawing.

●Centrifugal pump●
Configuration of Centrifugal Pump FIG. 1 is a schematic cross-sectional view showing an embodiment of a centrifugal pump according to the present invention.
In this figure, a part of the housing 2 described later is shown as a schematic cross-sectional view, and other parts are shown in a simplified manner. This figure shows a schematic cross-section of the centrifugal pump 1 cut in the vertical direction along the axial direction of a rotating shaft 4 described later, passing through the axial center of the rotating shaft 4 (the same applies to Figures 2 and 5 to 11).

The centrifugal pump 1 sucks in and discharges (sends) the handled liquid. The centrifugal pump 1 comprises a housing 2, a motor 3, a rotating shaft 4, bearings 51, 52, a first impeller 6, and a second impeller 7. In other words, the centrifugal pump 1 is a two-stage centrifugal pump equipped with two impellers (the first impeller 6 and the second impeller 7), and is an example of a centrifugal pump according to the present invention.

The "handled liquid" is the liquid that is handled (pumped) by the centrifugal pump 1. In this embodiment, the handled liquid is a low-viscosity liquid, for example, a cryogenic liquefied gas (e.g., liquefied natural gas, liquid hydrogen, etc.).

In the following description, the "forward direction" refers to the direction in which the first impeller 6 and the second impeller 7 are located relative to the motor 3, and the "rearward direction" refers to the direction in which the motor 3 is located relative to the first impeller 6 and the second impeller 7. The "axial direction" refers to the direction along the axial centerline of the rotating shaft 4 (front-to-back direction), the "radial direction" refers to the radial direction of the rotating shaft 4, and the "circumferential direction" refers to the circumferential direction of the rotating shaft 4. The "upstream side" refers to the upstream side in the flow of the handled fluid within the housing 2, and the "downstream side" refers to the downstream side in the flow of the handled fluid within the housing 2. The forward direction is an example of the first direction in the present invention, and the rearward direction is an example of the second direction in the present invention.

The housing 2 houses the motor 3, the rotating shaft 4, the bearings 51, 52, the first impeller 6, and the second impeller 7. The housing 2 includes a first partition 20, a central partition 21, a second partition 22, a first pump chamber 23, a second pump chamber 24, a connecting flow path 25, a discharge flow path 26, a suction pipe 27, a discharge pipe 28, and a motor chamber 29.

FIG. 2 is an enlarged partial cross-sectional view of the centrifugal pump 1.
The figure shows a cross section of the upper half of the centrifugal pump 1, centered on the first impeller 6 and the second impeller 7. In the following description, FIG.

The first partition 20, together with the central partition 21, is a partition that divides the first pump chamber 23. The first partition 20 is disposed forward of the first impeller 6, at the front end of the housing 2. The first partition 20 has an inner surface 20a and a first through hole 20b.

The inner surface 20a is a curved surface that is concave toward the front in a generally truncated cone shape to conform to the shape of the first front shroud 63 of the first impeller 6, which will be described later.

In the axial direction, the first through hole 20b is a through hole that penetrates the center of the first partition wall portion 20 in the axial direction in a two-stage cylindrical shape. The first through hole 20b is connected to the first pump chamber 23 and the suction pipe 27. The first through hole 20b has a small diameter portion 20c, a large diameter portion 20d, and a step portion 20e. The inner diameter of the small diameter portion 20c is smaller than the inner diameter of the large diameter portion 20d. In the axial direction, the small diameter portion 20c is disposed in front of the large diameter portion 20d and adjacent to the large diameter portion 20d. The step portion 20e is disposed between the small diameter portion 20c and the large diameter portion 20d, and is a surface that is continuous with each of the small diameter portion 20c and the large diameter portion 20d. In the axial direction, the shape of the step portion 20e is ring-shaped. The small diameter portion 20c functions as a flow path that introduces the handled fluid to the first impeller 6.

The central partition 21 is a partition that, together with the first partition 20, defines the first pump chamber 23, and, together with the second partition 22, defines the second pump chamber 24. The central partition 21 is disposed behind the first partition 20. The central partition 21 has a front surface 21a, a rear surface 21b, a central through hole 21c, and eight recesses 21d. The front surface 21a is a surface that faces forward, and the rear surface 21b is a surface that faces backward.

When viewed in the axial direction, the central through hole 21c is a cylindrical through hole that penetrates the center of the central partition wall portion 21 along the axial direction. The rotating shaft 4 is inserted into the central through hole 21c. The central through hole 21c is an example of an insertion hole in the present invention.

FIG. 3 is a schematic rear view of the central partition portion 21. As shown in FIG.
This figure shows the central partition 21 as viewed from the rear. The figure also shows the rotating shaft 4 and a first hub portion 64, which will be described later. For ease of explanation, the figure also shows the second impeller 7 and the protruding portion 72c imaginarily by two-dot chain lines. In the following explanation, FIG. 2 will also be referred to as appropriate.

The recess 21d suppresses the swirling component of the handled fluid flowing through the second rear space S21 described later. A part of the rear surface 21b of the central partition wall portion 21 is recessed forward in a rectangular shape to form the recess 21d. In the circumferential direction, the eight recesses 21d are arranged on the rear surface 21b at equal angular intervals (45 degrees in this embodiment). In the axial direction, the shape of the recess 21d is a rectangle whose longitudinal direction is along the radial direction. In the axial direction, the inner edge portion (the edge portion on the inner side in the radial direction) of the recess 21d is arranged at the same position as the outer edge portion of the protrusion 72c of the second impeller 7 described later. In the axial direction, the outer edge portion (the edge portion on the outer side in the radial direction) of the recess 21d is arranged at approximately the same position as the intermediate portion between the inner edge portion and the outer edge portion of the second impeller 7.

The drawings to which primary reference will be made in this description are turned back to FIGS.
The second partition portion 22, together with the central partition portion 21, is a partition that defines a second pump chamber 24. The second partition portion 22 is disposed rearward of the second impeller 7 and rearward of the central partition portion 21. The second partition portion 22 has an inner surface 22a and a second through hole 22b.

The inner surface 22a is a curved surface that is concave toward the rear in a generally truncated cone shape to conform to the shape of the second front shroud 73 of the second impeller 7, which will be described later.

In the axial view, the second through hole 22b is a cylindrical through hole that penetrates the center of the second partition wall portion 22 along the axial direction. The second through hole 22b is connected to the second pump chamber 24 and the connecting flow path 25. The second through hole 22b has a small diameter portion 22c, a large diameter portion 22d, a first step portion 22e, an insertion portion 22f, and a second step portion 22g. The inner diameter of the small diameter portion 22c is larger than the inner diameter of the insertion portion 22f and smaller than the inner diameter of the large diameter portion 22d. In the axial direction, the insertion portion 22f is disposed adjacent to the small diameter portion 22c rearward of the small diameter portion 22c, and the large diameter portion 22d is disposed adjacent to the small diameter portion 22c forward of the small diameter portion 22c. The first step 22e is disposed between the small diameter portion 22c and the large diameter portion 22d, and is a surface that continues to the small diameter portion 22c and the large diameter portion 22d. The second step 22g is disposed between the insertion portion 22f and the small diameter portion 22c, and is a surface that continues to the insertion portion 22f and the small diameter portion 22c. When viewed in the axial direction, the first step 22e and the second step 22g are ring-shaped. The rotating shaft 4 is inserted into the second through hole 22b, and a cylindrical space that functions as a flow path for introducing the handled fluid to the second impeller 7 is formed between the small diameter portion 22c and the rotating shaft 4.

The first pump chamber 23 houses the first impeller 6. The second pump chamber 24 houses the second impeller 7. In the axial direction, the first pump chamber 23 is arranged in front of the second pump chamber 24 and aligned with the second pump chamber 24 in the axial direction with the central partition wall 21 in between. In other words, the first impeller 6 is arranged in front of the second impeller 7 in the axial direction.

The connecting flow passage 25 is a flow passage that guides the pumped fluid discharged by the first impeller 6 to the space within the small diameter portion 22c. The connecting flow passage 25 is, for example, formed by a part of the housing 2, and is connected to the first pump chamber 23 and the space within the small diameter portion 22c. In Figures 1 and 2, the connecting flow passage 25 is simplified and indicated by a thick solid arrow.

The discharge flow passage 26 is a flow passage that guides the handled fluid discharged by the second impeller 7 to the discharge pipe 28. The discharge flow passage 26 is formed, for example, by a part of the housing 2, and is connected to the second pump chamber 24 and the discharge pipe 28. In Figures 1 and 2, the discharge flow passage 26 is simplified and indicated by a dashed arrow.

The front end of the housing 2 (first partition wall portion 20) extends cylindrically forward so as to be coaxial with the rotating shaft 4, forming a suction pipe 27 that draws (introduces) the pumped liquid into the first pump chamber 23. In addition, a portion of the housing 2 that is located radially outward of the second impeller 7 extends in the tangential direction (upward) of the second impeller 7, forming a discharge pipe 28 that discharges the pumped liquid from the second pump chamber 24 (discharge flow passage 26).

The rear half of the housing 2 defines the motor chamber 29 that houses the motor 3 and bearings 51 and 52.

The motor 3 is a known motor that includes a rotor (not shown) attached to the rotating shaft 4, and a stator (not shown) that rotates the rotor (not shown). The rotating shaft 4 rotates when driven (rotated) by the motor 3, and transmits rotational power to the first impeller 6 and the second impeller 7. The rotating shaft 4 is cylindrical in shape. The rotating shaft 4 is attached to the motor 3, and the front part 4a of the rotating shaft 4 protrudes into the first pump chamber 23 and the second pump chamber 24.

Bearing 51 is disposed in front of motor 3 and rotatably supports rotating shaft 4. Bearing 52 is disposed in rear of motor 3 and rotatably supports rotating shaft 4. Bearings 51 and 52 are, for example, rolling bearings.

The first impeller 6 sucks in and discharges the handled liquid. The first impeller 6 is attached to the front part 4a of the rotating shaft 4 and is housed in the first pump chamber 23. That is, the first impeller 6 is disposed between the first partition part 20 and the central partition part 21. The first impeller 6 is a so-called closed type impeller. The first impeller 6 includes a plurality of first blades 61, a first rear shroud 62, a first front shroud 63, a first hub part 64, a first suction port 65, and a first discharge port 66.

The first blades 61 rotate in the circumferential direction around the rotating shaft 4 as the center of rotation, and guide the handled fluid sucked in from the first suction port 65 to the first discharge port 66. When viewed in the axial direction, each of the multiple first blades 61 extends radially from the center side to the outer edge side of the first impeller 6 and is curved in a spiral shape. The first blades 61 are disposed between the first rear shroud 62 and the first front shroud 63.

The first rear shroud 62 is a plate (so-called main plate) that covers the rear of the first blade 61. The shape of the first rear shroud 62 is a ring plate. When viewed in the axial direction, the center of the first rear shroud 62 protrudes forward in a generally truncated cone shape. The first rear shroud 62 faces the central partition portion 21. The first rear shroud 62 has an attachment hole 62a and a rear surface 62b. The first rear shroud 62 is an example of a first shroud in the present invention.

The mounting hole 62a is a through hole through which the front part 4a of the rotating shaft 4 is inserted. When viewed in the axial direction, the mounting hole 62a is located in the center of the first rear shroud 62 and penetrates the center in the axial direction in a cylindrical shape.

The rear surface 62b faces rearward and faces the front surface 21a of the central partition portion 21. A ring-shaped space (hereinafter referred to as the "first rear space S11") is formed between the rear surface 62b and the front surface 21a. The first rear space S11 is an example of the first space in the present invention.

The first front shroud 63 is a plate (so-called side plate) that covers the front of the first blade 61. The shape of the first front shroud 63 is a generally ring-shaped plate with the inner edge being more convex toward the front than the outer edge. The outer diameter of the first front shroud 63 is slightly smaller than the outer diameter of the first rear shroud 62. The first front shroud 63 is disposed forward of the first rear shroud 62. The first front shroud 63 has a front surface 63a and a cylindrical portion 63b. The first front shroud 63 is an example of a third shroud in the present invention.

The front surface 63a faces forward and faces the inner surface 20a of the first partition wall portion 20. Between the front surface 63a and the inner surface 20a, a space having a substantially ring plate shape (hereinafter referred to as the "first front space S12") is formed.

The inner edge of the first front shroud 63 extends cylindrically forward so as to be coaxial with the rotating shaft 4, forming a cylindrical portion 63b that functions as the first suction port 65. In other words, the first front shroud 63 has a cylindrical portion 63b that functions as the first suction port 65. In the first impeller 6, the cylindrical portion 63b is oriented forward and is disposed within the large diameter portion 20d of the first partition portion 20. The cylindrical portion 63b is an example of the first cylindrical portion in the present invention. In the radial direction, the large diameter portion 20d faces the cylindrical portion 63b. A cylindrical gap (hereinafter referred to as the "first cylindrical gap S13") is formed between the large diameter portion 20d and the cylindrical portion 63b. In the radial direction, the length of the first cylindrical gap S13 (the interval between the large diameter portion 20d and the cylindrical portion 63b) is such that the cylindrical portion 63b does not come into contact with the large diameter portion 20d when the centrifugal pump 1 is operating normally, and is set to be equivalent to the same length set for the impeller of a general centrifugal pump. In the axial direction, the step portion 20e faces the cylindrical portion 63b. A ring-shaped gap (hereinafter referred to as the "first ring gap S14") is formed between the step portion 20e and the cylindrical portion 63b. In the axial direction, the length of the first ring gap S14 (the interval between the step portion 20e and the cylindrical portion 63b) is such that the cylindrical portion 63b does not come into contact with the step portion 20e when the centrifugal pump 1 is operating normally, and is set to be equivalent to the same length set for the impeller of a general centrifugal pump. The first cylindrical gap S13 and the first ring gap S14 constitute a first fixed orifice, which will be described later. The large diameter portion 20d and the step portion 20e are an example of a portion that faces the first cylindrical portion in the present invention.

The first front space S12 is connected to the space located radially outward of the first impeller 6 and to the first cylindrical gap S13. The first ring gap S14 is connected to the first cylindrical gap S13 and the space within the small diameter portion 20c.

The inner edge of the first rear shroud 62 extends cylindrically rearward so as to be coaxial with the rotating shaft 4, forming a first hub portion 64. The front portion 4a of the rotating shaft 4 is inserted into the first hub portion 64, and the first impeller 6 is attached to the front portion 4a of the rotating shaft 4 by fixing the first hub portion 64 to the front portion 4a. At this time, the first suction port 65 faces forward. The first hub portion 64 covers a portion of the front portion 4a of the rotating shaft 4, and is disposed within the central through hole 21c.

The outer edge of the first blade 61, the outer edge of the first rear shroud 62, and the outer edge of the first front shroud 63 form a first discharge port 66 through which the handled fluid flowing through the flow passage in the first impeller 6 is discharged.

The second impeller 7 sucks in and discharges the handled fluid discharged from the first impeller 6. The second impeller 7 is attached to the front part 4a of the rotating shaft 4 and is housed in the second pump chamber 24. That is, the second impeller 7 is disposed between the central partition part 21 and the second partition part 22. The second impeller 7 is a so-called closed type impeller. The second impeller 7 has a plurality of second blades 71, a second rear shroud 72, a second front shroud 73, a second hub part 74, a second suction port 75, and a second discharge port 76.

The specific speed set for the second pump section P2, which is composed of the second impeller 7 and the second pump chamber 24, is smaller than the specific speed set for the first pump section P1, which is composed of the first impeller 6 and the first pump chamber 23. With this configuration, in the centrifugal pump 1, the first pump section P1 provides high suction performance (discharge flow rate), and the second pump section P2 provides high head. In addition, the outer diameter of the impeller generally becomes smaller as the specific speed increases. Therefore, the outer diameter of the second impeller 7 is larger than the outer diameter of the first impeller 6. The values of these specific speeds are appropriately set based on, for example, the design of the centrifugal pump 1 (e.g., discharge flow rate, head, etc.).

The second blades 71 rotate in the circumferential direction around the rotating shaft 4 as the center of rotation, and guide the handled fluid sucked in from the second suction port 75 to the second discharge port 76. When viewed in the axial direction, each of the multiple second blades 71 extends radially from the center side to the outer edge side of the second impeller 7 and is curved in a spiral shape. The second blades 71 are disposed between the second rear shroud 72 and the second front shroud 73.

The second rear shroud 72 is a plate (so-called main plate) that covers the front of the second blade 71 (toward the rear side of the second impeller 7). The shape of the second rear shroud 72 is a ring plate. When viewed in the axial direction, the center of the second rear shroud 72 protrudes rearward in a truncated cone shape. The second rear shroud 72 faces the central partition wall portion 21. The second rear shroud 72 has an attachment hole 72a, a front surface 72b, and a protrusion 72c. The second rear shroud 72 is an example of a second shroud in the present invention.

The outer diameter (diameter) of the second rear shroud 72 is larger than the outer diameter (diameter) of the first rear shroud 62. The outer diameters of the first rear shroud 62 and the second rear shroud 72 are set, for example, so that when the first impeller 6 and the second impeller 7 (i.e., the centrifugal pump 1) are discharging the handled liquid (at the most efficient point), the first axial thrust force and the second axial thrust force described below are roughly balanced.

The mounting hole 72a is a through hole through which the front portion 4a of the rotating shaft 4 is inserted. When viewed in the axial direction, the mounting hole 72a is located in the center of the second rear shroud 72 and penetrates the center in the axial direction in a cylindrical shape.

The front surface 72b faces forward and faces the rear surface 21b of the central partition 21. A ring-shaped gap (hereinafter referred to as the "second rear space S21") is formed between the front surface 72b and the rear surface 21b. The second rear space S21 is an example of the second space in the present invention.

FIG. 4 is a schematic front view of the second impeller 7. As shown in FIG.
2. This figure shows a schematic view of the second impeller 7 as seen from the front. In the following description, FIG.

The protrusion 72c functions as a variable orifice, which will be described later. A part of the front surface 72b of the second rear shroud 72 protrudes forward in a ring-shaped plate shape so as to be coaxial with the rotating shaft 4, forming the protrusion 72c. That is, in the axial view, the protrusion 72c is a ring-shaped plate shape concentric with the rotating shaft 4. In the radial direction, the protrusion 72c is disposed toward the inside (near the inner edge) of the second rear shroud 72. The front surface 72d of the protrusion 72c is a flat surface parallel to the rear surface 21b of the central partition wall 21. In the axial direction, the distance between the protrusion 72c and the rear surface 21b of the central partition wall 21 is narrower than the distance between the front surface 72b excluding the protrusion 72c and the rear surface 21b of the central partition wall 21. The protrusion 72c is an example of a protrusion (variable orifice) in the present invention.

The drawings to which primary reference will be made in this description are turned back to FIGS.
The second front shroud 73 is a plate (so-called side plate) that covers the rear of the second blade 71 (the direction of the front side of the second impeller 7). The shape of the second front shroud 73 is a substantially ring-shaped plate with an inner edge that is more convex toward the rear than the outer edge. The outer diameter of the second front shroud 73 is slightly smaller than the outer diameter of the second rear shroud 72. The second front shroud 73 is disposed on the rear side of the second rear shroud 72. The second front shroud 73 includes a rear surface 73a and a cylindrical portion 73b. The second front shroud 73 is an example of a fourth shroud in the present invention.

The rear surface 73a faces rearward and faces the inner surface 22a of the second partition wall portion 22. A substantially ring-shaped space (hereinafter referred to as the "second front space S22") is formed between the rear surface 73a and the inner surface 22a.

The inner edge of the second front shroud 73 extends cylindrically toward the rear so as to be coaxial with the rotating shaft 4, forming a cylindrical portion 73b that functions as the second suction port 75. In other words, the second front shroud 73 has a cylindrical portion 73b that functions as the second suction port 75. In the second impeller 7, the cylindrical portion 73b is directed toward the rear and is disposed within the large diameter portion 22d of the second partition wall portion 22. The cylindrical portion 73b is an example of a second cylindrical portion in the present invention. In the radial direction, the cylindrical portion 73b faces the large diameter portion 22d. A cylindrical gap (hereinafter referred to as the "second cylindrical gap S23") is formed between the large diameter portion 22d and the cylindrical portion 73b. In the radial direction, the length of the second cylindrical gap S23 (the distance between the large diameter portion 22d and the cylindrical portion 73b) is set to be such that the cylindrical portion 73b does not come into contact with the large diameter portion 22d when the centrifugal pump 1 is operating normally, and is longer than the same length set for the impeller of a general centrifugal pump. That is, in the radial direction, the length of the second cylindrical gap S23 is longer than the length of the first cylindrical gap S13. In the axial direction, the cylindrical portion 73b faces the first step portion 22e. A ring-shaped gap (hereinafter referred to as the "second ring gap S24") is formed between the first step portion 22e and the cylindrical portion 73b. In the axial direction, the length of the second ring gap S24 (the length between the first step portion 22e and the cylindrical portion 73b) is set to be such that the cylindrical portion 73b does not come into contact with the first step portion 22e when the centrifugal pump 1 is operating normally, and is longer than the same length set for the impeller of a general centrifugal pump. That is, in the axial direction, the length of the second ring gap S24 is longer than the length of the first ring gap S14.

The second front space S22 is connected to the space radially outward of the second impeller 7 and the second cylindrical gap S23. The second ring gap S24 is connected to the second front space S22 and the space within the small diameter portion 22c.

The inner edge of the second rear shroud 72 extends cylindrically forward so as to be coaxial with the rotating shaft 4, forming the second hub portion 74. The front portion 4a of the rotating shaft 4 is inserted into the second hub portion 74, and the second impeller 7 is attached to the front portion 4a of the rotating shaft 4 by fixing the second hub portion 74 to the front portion 4a. At this time, the second suction port 75 is directed rearward. The second hub portion 74 covers a part of the front portion 4a of the rotating shaft 4 and is disposed in the central through hole 21c. The second hub portion 74 abuts against the first hub portion 64, and a cylindrical space (hereinafter referred to as the "cylindrical space S3") is formed between the central through hole 21c and the first hub portion 64 and the second hub portion 74. The cylindrical space S3 is in communication with the first rear side space S11 and the second rear side space S21. That is, the first rear side space S11 and the second rear side space S21 are in communication with each other via the cylindrical space S3 (central through hole 21c).

In this way, the first impeller 6 and the second impeller 7 are attached to one rotating shaft 4 back-to-back with the central partition 21 in between, forming a two-stage centrifugal pump. The first pump chamber 23 and the second pump chamber 24 are partitioned by the central partition 21 and communicate with each other via the cylindrical space S3. In other words, no shaft sealing structure such as a mechanical seal is provided between the first pump chamber 23 and the second pump chamber 24.

Operation of the Centrifugal Pump Next, a description will be given of the operation of the centrifugal pump 1. In the following description, reference will be made to Figures 1 and 2 as appropriate.

When the centrifugal pump 1 starts to operate, the motor 3 is driven and the rotating shaft 4 rotates. The first impeller 6 sucks the handled fluid introduced into the small diameter portion 20c from the front and discharges it into the first pump chamber 23. The handled fluid discharged into the first pump chamber 23 is sent to the space in the small diameter portion 22c through the connecting flow path 25. At this time, a part of the handled fluid discharged into the first pump chamber 23 flows into the first back side space S11 and flows into the second pump chamber 24 (second back side space S21) through the cylindrical space S3. In addition, another part of the handled fluid discharged into the first pump chamber 23 flows (returns) into the space in the small diameter portion 20c through the first front side space S12, the first cylindrical gap S13, and the first ring gap S14. This flow of the handled fluid returning has a negative effect on the suction performance of the first impeller 6. Therefore, the flow is mainly restricted by the first cylindrical gap S13 and the first ring gap S14. In other words, the first cylindrical gap S13 and the first ring gap S14 function as a restriction (first fixed orifice) for the flow. The first fixed orifice (first cylindrical gap S13, first ring gap S14) is an example of the first gap in the present invention.

The second impeller 7 sucks the handled fluid sent to the space in the small diameter portion 22c from the rear and discharges it into the second pump chamber 24. The handled fluid discharged into the second pump chamber 24 is discharged to the discharge piping (not shown) through the discharge flow path 26 and the discharge pipe 28. At this time, a part of the handled fluid discharged into the second pump chamber 24 flows into the second back side space S21. Here, the pressure of the handled fluid in the second pump chamber 24 is greater than the pressure of the handled fluid in the first pump chamber 23. Therefore, the handled fluid that has flowed into the second back side space S21 flows into the first pump chamber 23 (first back side space S11) through the cylindrical space S3, so as to push back the handled fluid that has flowed in from the first pump chamber 23. In this way, in the centrifugal pump 1, a flow of the handled fluid returning from the second pump chamber 24 to the first pump chamber 23 (hereinafter referred to as the "return flow") is generated. The second back side space S21 and the cylindrical space S3 form a flow path (hereinafter referred to as the "return flow path RL") through which a part of the handled fluid in the second pump chamber 24 flows into the first back side space S11 (first pump chamber 23). Another part of the handled fluid discharged into the second pump chamber 24 flows (returns) into the space in the small diameter portion 22c through the second front side space S22, the second cylindrical gap S23, and the second ring gap S24. This return flow of the handled fluid may adversely affect the suction performance of the second impeller 7. Therefore, the flow is mainly restricted by the second cylindrical gap S23 and the second ring gap S24. That is, the second cylindrical gap S23 and the second ring gap S24 function as a restriction (second fixed orifice) for the flow. The second fixed orifice is larger than the first fixed orifice. Therefore, the amount of flow restriction by the second fixed orifice is less than the amount of flow restriction by the first fixed orifice. In other words, the flow rate of the handled fluid flowing through the second fixed orifice is greater than the flow rate of the handled fluid flowing through the first fixed orifice. The second fixed orifice (second cylinder gap S23, second ring gap S24) is an example of the second gap in the present invention.

As described below, in the centrifugal pump 1 through which the handled liquid flows in this manner, an axial thrust force different from that of a conventional two-stage centrifugal pump (hereinafter referred to as the "conventional pump"). In this specification, the configuration of the "conventional pump" is assumed to be the same as that of the centrifugal pump 1, except that the outer diameter of the first impeller (outer diameter of the first rear shroud) and the outer diameter of the second impeller (outer diameter of the second rear shroud) are the same, and the cylindrical space is sealed with a mechanical seal. In other words, the conventional pump is a two-stage centrifugal pump in which the first impeller and the second impeller are attached to one rotating shaft back-to-back. In order to distinguish between the centrifugal pump 1 and the conventional pump, in the following description, the configuration indicating the conventional pump is designated by the same reference numeral as the centrifugal pump 1, with the letter "z" added.

The "axial thrust force" is a force acting on the rotating shaft 4 (first impeller 6, second impeller 7) to move the rotating shaft 4 in the axial direction. The axial thrust force includes a first axial thrust force acting to move the rotating shaft 4 forward, and a second axial thrust force acting to move the rotating shaft 4 backward. The axial thrust force is mainly generated by the pressure balance between the front side and the back side of the first impeller 6 and the second impeller 7 of the centrifugal pump 1.

Axial thrust force in conventional pump Before describing the axial thrust force in the centrifugal pump 1, the axial thrust force in a conventional pump 1z and a conventional pump 1z not equipped with a mechanical seal Mz will be described below.

FIG. 5(a) is a schematic partially enlarged cross-sectional view illustrating the axial thrust force in a conventional pump 1z, and (b) is a schematic partially enlarged cross-sectional view illustrating the axial thrust force in a conventional pump 1z that does not have a mechanical seal Mz.
For ease of explanation, only the central partition wall 21z, the rotary shaft 4z, the first impeller 6z, the second impeller 7z, and the mechanical seal Mz are shown in the simplified form in Fig. 2. Fig. 2(b) shows the return flow with a hollow arrow.

In the following description, pressure " _Pf3 " indicates the pressure that the first front shroud 63z of the first impeller 6z receives from the handled fluid flowing in the first front side space S12z, pressure " _Pr3 " indicates the pressure that the first rear shroud 62z of the first impeller 6z receives from the handled fluid flowing in the first rear side space S11z, pressure " _Pf4 " indicates the pressure that the second front shroud 73z of the second impeller 7z receives from the handled fluid flowing in the second front side space S22z, and pressure " _Pr4 " indicates the pressure that the second rear shroud 72z of the second impeller 7z receives from the handled fluid flowing in the second rear side space S21z. A force " _Ff3 " based on the pressure " _Pf3 " acts rearward on the first impeller 6z, and a force " _Fr3 " based on the pressure " _Pr3 " acts forward. Since the pressure "P _f3 " is smaller than the pressure "P _r3 ", a force (first axial thrust force) equivalent to the difference between the two forces (F _r3 -F _f3 ) acts forward on the first impeller 6z. Similarly, a force "F _f4 " due to the pressure "P _f4 " acts forward on the second impeller 7z, and a force "F _r4 " due to the pressure "P _r4 " acts backward. Since the pressure "P _f4 " is smaller than the pressure "P _r4 ", a force (second axial thrust force) equivalent to the difference between the two forces (F _r4 -F _f4 ) acts backward on the second impeller 7z. In FIG. 5, the thin arrows show the magnitudes of the pressures "P _f3 ", "P _r3 ", "P _f4 ", and "P _r4 " by the lengths of the arrows. The thick arrows indicate the magnitude of the forces "F _f3 ", "F _r3 ", "F _f4 ", and "F _r4 " by way of example, using the lengths of the arrows.

As shown in Fig. 5(a), in the conventional pump 1z, the configuration of the first impeller 6z is common to the configuration of the second impeller 7z, except for the direction of rotation. That is, as described above, the outer diameter of the first rear shroud 62z of the first impeller 6z is the same as the outer diameter of the second rear shroud 72z of the second impeller 7z. A mechanical seal Mz is attached to the cylindrical space S3z, and the first pump chamber 23z does not communicate with the second pump chamber 24z. The pressure-receiving area (liquid-contacting area) of the first front shroud 63z is smaller than the pressure-receiving area of the first rear shroud 62z and is the same as the pressure-receiving area of the second front shroud 73z. In addition, the pressure-receiving area of the first rear shroud 62z is the same as the pressure-receiving area of the second rear shroud 72z. In this configuration, the pressure "P _f3 " is equal to the pressure "P _f4 ", and the pressure "P _r3 " is equal to the pressure "P _r4 ". As a result, the force "F _r3 " acting on the first rear shroud 62z becomes larger than the force "F _f3 " acting on the first front shroud 63z, and becomes equal to the force "F _r4 " acting on the second rear shroud 72z. The force "F _f3 " acting on the first front shroud 63z becomes equal to the force "F _f4 " acting on the second front shroud 73z. Therefore, a force (first axial thrust force) equivalent to the difference between the two forces (F _r3 -F _f3 )) acts forward on the first impeller 6z. Meanwhile, a force (second axial thrust force) equivalent to the difference between the two forces ( _Fr4 - _Ff4 ) acts rearward on the second impeller 7z. As a result, the first axial thrust force and the second axial thrust force cancel each other out (the axial thrust forces are balanced). In reality, the first axial thrust force acts on the rotating shaft 4 due to the pressure difference between the handled fluid in the first pump chamber 23z and the handled fluid in the second pump chamber 24z. However, this first axial thrust force is usually absorbed by the bearing. Therefore, in the conventional pump 1z, no problematic axial thrust force acts on the rotating shaft 4z.

Next, as shown in FIG. 5B, in the conventional pump 1z not provided with the mechanical seal Mz, the first pump chamber 23z communicates with the second pump chamber 24z through the cylindrical space S3z. In this configuration, since the pressure of the handled fluid in the second pump chamber 24z is higher than the pressure of the handled fluid in the first pump chamber 23z, a flow (return flow) occurs in which the handled fluid in the second pump chamber 24z flows into the first pump chamber 23z through the cylindrical space S3z. At this time, in the second rear side space S21z, a flow is generated from the outer edge side to the inner edge side of the second rear shroud 72z while rotating in the rotation direction of the second impeller 7z. Therefore, as shown by the dashed line and the solid line in FIG. 5B, the pressure " _Pr4 " of the handled fluid flowing in the second rear side space S21z becomes low. On the other hand, in the first rear side space S11z, a flow is generated from the inner edge side to the outer edge side of the first rear side shroud 62z while rotating in the rotation direction of the first impeller 6z. Therefore, as shown by the dashed line and the solid line in Fig. 5(b), the pressure "P _r3 " of the handled fluid flowing in the first rear side space S11z becomes high. Therefore, the force "F _r3 " acting on the first rear side shroud 62z becomes large, and the force "F _r4 " acting on the second rear side shroud 72z becomes small. As a result, the first axial thrust force acting on the rotating shaft 4z (first impeller 6z, second impeller 7z) becomes larger than the second axial thrust force, and the first axial thrust force acts on the rotating shaft 4z. In this manner, when the first pump chamber 23z communicates with the second pump chamber 24z via the cylindrical space S3z, the balance of the axial thrust forces in the conventional pump 1z is lost, and an axial thrust force (first axial thrust force) acts on the rotating shaft 4z.

These axial thrust forces increase as the rotation speed of the motor 3 increases. Therefore, for example, when a handling liquid that requires high-speed rotation (e.g., a handling liquid with low viscosity) is pumped, the axial thrust force cannot be absorbed by the bearings, and a mechanism to counter the axial thrust force (e.g., a balance piston, a balance sheet, etc.) is required. In this case, not only does the conventional pump 1z become larger overall, but the rotating shaft 4z becomes longer. As a result, it becomes difficult to handle high-speed rotation. In addition, when the handling liquid is used to lubricate the bearings, even a slight axial thrust force generates surface pressure on the sliding surface of the bearing, especially if the viscosity of the handling liquid is low. As a result, the temperature of the sliding surface rises and the life of the bearing is reduced. Therefore, a structure that generates as little axial thrust force as possible (ideally, a structure in which the axial thrust force becomes "0") is required.

Axial Thrust Force in Centrifugal Pump Next, the axial thrust force in the centrifugal pump 1 will be described below. In the following description, Figs. 1 to 5 will also be referred to as appropriate.

FIG. 6 is a schematic partially enlarged sectional view illustrating the influence of the second impeller 7 on the axial thrust force in the centrifugal pump 1. As shown in FIG.
In the figure, for the sake of convenience, only the central partition wall 21, the rotating shaft 4, the first impeller 6, and the second impeller 7 are shown in a simplified manner, and the protruding portion 72c is omitted. The white arrows in the figure indicate the return flow. In addition, in the figure, in order to facilitate understanding of the force acting on the second impeller 7, the second impeller 7 is shown such that only the outer diameter of the second rear shroud 72 is larger than the outer diameter of the first rear shroud 62, and the outer diameter of the second front shroud 73 is the same as the outer diameter of the first front shroud 63.

In the following description, pressure " _Pf1 " indicates the pressure that the first front shroud 63 of the first impeller 6 receives from the handled fluid flowing in the first front side space S12, pressure " _Pr1 " indicates the pressure that the first rear shroud 62 of the first impeller 6 receives from the handled fluid flowing in the first rear side space S11, pressure " _Pf2 " indicates the pressure that the second front shroud 73 of the second impeller 7 receives from the handled fluid flowing in the second front side space S22, and pressure " _Pr2 " indicates the pressure that the second rear shroud 72 of the second impeller 7 receives from the handled fluid flowing in the second rear side space S21. A force " _Ff1 " based on the pressure " _Pf1 " acts rearward on the first impeller 6, and a force " _Fr1 " based on the pressure " _Pr1 " acts forward. Since pressure "P _f1 " is smaller than pressure "P _r1 ", a force (first axial thrust force) equivalent to the difference between the two forces (F _r1 -F _f1 ) acts forward on the first impeller 6. Similarly, a force "F _f2 " based on pressure "P _f2 " acts forward on the second impeller 7, and a force "F _r2 " due to pressure "P _r2 " acts backward. A force (second axial thrust force) equivalent to the difference between the two forces (F _r2 -F _f2 ) acts backward on the second impeller 7. In FIG. 6, the thin arrows show the magnitudes of pressures "P _f1 ", "P _r1 ", "P _f2 ", and "P _r2 " by the lengths of the arrows. The thick arrows indicate the magnitudes of the forces "F _f1 ", "F _r1 ", "F _f2 ", and "F _r2 " by the lengths of the arrows.

As described above, the outer diameter of the second rear shroud 72 is larger than the outer diameter of the first rear shroud 62. Therefore, as shown in Fig. 6, the pressure-receiving area of the second rear shroud 72 is larger than the pressure-receiving area of the first rear shroud 62. As a result, the force " _Fr2 " acting on the second impeller 7 becomes greater than the force " _Fr4 " acting on the second impeller 7z of the conventional pump 1z. Therefore, by designing the outer diameter of the second rear shroud 72 (i.e., the outer diameter of the second impeller 7) to be larger than the outer diameter of the first rear shroud 62 (i.e., the outer diameter of the first impeller 6), the second axial thrust force ( _Fr2 - _Ff2 ) acting on the second impeller 7 (rotating shaft 4) becomes larger than the second axial thrust force ( _Fr4 - _Ff4 ) acting on the second impeller 7z (rotating shaft 4z), and the axial thrust force (first axial thrust force) acting on the rotating shaft 4 becomes smaller (suppressed) than the axial thrust force (first axial thrust force) acting on the rotating shaft 4z. As a result, the first axial thrust force ( _Fr1 - _Ff1 ) and the second axial thrust force ( _Fr2 - _Ff2 ) acting on the rotating shaft 4 are (approximately) balanced.

FIG. 7 is a schematic partially enlarged sectional view illustrating the influence of the second fixed orifice (the second cylindrical gap S23 and the second ring gap S24) on the axial thrust force in the centrifugal pump 1.
In the figure, for the convenience of explanation, only the first partition 20, the central partition 21, the second partition 22, the rotating shaft 4, the first impeller 6, and the second impeller 7 are shown in a simplified manner, and the protruding portion 72c is omitted. The white arrows in the figure indicate the return flow. In addition, in the figure, in order to explain only the effect of the second fixed orifice, the second impeller 7 is shown so that the outer diameter of the second impeller 7 is the same as the outer diameter of the first impeller 6.

As described above, in the radial direction, the length of the second cylindrical gap S23 is longer than the length of the first cylindrical gap S13. In the axial direction, the length of the second ring gap S24 is longer than the length of the first ring gap S14. In this configuration, the flow rate of the handled fluid that has flowed into the second front side space S22 and is throttled by the second fixed orifice is smaller than the flow rate of the handled fluid that has flowed into the first front side space S12 and is throttled by the first fixed orifice. That is, the flow rate of the handled fluid flowing through the second fixed orifice (the flow rate of the flow indicated by the dashed arrow in FIG. 7) is greater than the flow rate of the handled fluid flowing through the first fixed orifice. Therefore, the pressure "P _f2 " that the second front shroud 73 receives from the handled fluid is smaller than the same pressure (indicated by the dashed line in FIG. 7) when the flow rate of the handled fluid passing through the second fixed orifice is the same as the flow rate of the handled fluid passing through the first fixed orifice. As a result, the force acting on the second impeller 7 (second axial thrust force) is larger than the force acting when the flow rate of the handled fluid flowing through the second fixed orifice is the same as the flow rate of the handled fluid flowing through the first fixed orifice. Therefore, by designing the flow rate of the handled fluid flowing through the second fixed orifice to be larger than the flow rate of the handled fluid flowing through the first fixed orifice, the second axial thrust force acting on the second impeller 7 (rotating shaft 4) becomes large, and the axial thrust force (first axial thrust force) acting on the rotating shaft 4 becomes small (suppressed). Here, normally, when the flow rate of the handled fluid flowing through the first fixed orifice increases, the suction performance of the first impeller 6 deteriorates, and the efficiency of the centrifugal pump 1 deteriorates. On the other hand, even if the flow rate of the handled fluid flowing through the second fixed orifice increases, the effect on the suction performance of the second impeller 7 is small. Therefore, although this configuration may cause some deterioration in the performance of the centrifugal pump 1, the improvement in efficiency of the centrifugal pump 1 due to the improvement in the axial thrust force acting on the rotating shaft 4 outweighs this deterioration.

Next, as described above, a plurality of recesses 21d are arranged on the rear surface 21b of the central partition wall portion 21. The recesses 21d face the second rear side space S21, and the flow of the handled fluid in the second rear side space S21 is interfered with by the recesses 21d. The flow of the handled fluid in the second rear side space S21 is a swirling flow that swirls in the same rotational direction as the second impeller 7 in response to the rotation of the second impeller 7. That is, the flow of the handled fluid in the second rear side space S21 contains a large amount of swirling components. Of the flow of the handled fluid in the second rear side space S21, a part of the flow close to the central partition wall portion 21 flows into the recesses 21d and is obstructed by the side wall of the recesses 21d. At this time, the swirling component of the flow that flows into the recesses 21d is suppressed, and the flow changes direction to the radially inward side and the axial side. Therefore, the flow also interferes with the flow that does not flow into the recesses 21d, and the swirling component of the interfered flow is also slightly suppressed. As a result, the swirling component of the flow of the handled fluid in the second rear side space S21 is suppressed. When the swirling component of the handled fluid is large, the pressure " _Pr2 " of the handled fluid decreases, so when the swirling component of the handled fluid is suppressed, the pressure " _Pr2 " of the handled fluid increases. In other words, by suppressing the swirling component of the handled fluid in the second rear side space S21, the force acting on the second impeller 7 (second axial thrust force) becomes large, and the axial thrust force acting on the rotating shaft 4 (first axial thrust force) becomes small (suppressed).

In this way, the centrifugal pump 1 has a configuration for suppressing the axial thrust force (first axial thrust force) by the outer diameter of the second rear shroud (hereinafter referred to as the "first suppression configuration"), a configuration for suppressing the axial thrust force (first axial thrust force) by the second fixed orifice (hereinafter referred to as the "second suppression configuration"), and a configuration for suppressing the axial thrust force (first axial thrust force) by the recess 21d (hereinafter referred to as the "third suppression configuration"). In the centrifugal pump 1, the first to third suppression configurations are combined so that the axial thrust forces acting on the rotating shaft 4 are balanced.

Figure 8 is a schematic partially enlarged cross-sectional view illustrating the effect of the protrusion 72c on the axial thrust force in the centrifugal pump 1, where (a) shows a state in which the first axial thrust force and the second axial thrust force are balanced, (b) shows a state in which the first axial thrust force is greater than the second axial thrust force, and (c) shows a state in which the first axial thrust force is smaller than the second axial thrust force. For ease of explanation, only the central partition 21, the rotating shaft 4, and the second impeller 7 are shown in a simplified form in the figure.

As shown in FIG. 8(a), when the first axial thrust force and the second axial thrust force are in balance, the rotating shaft 4 (first impeller 6, second impeller 7) is disposed at a predetermined position in the axial direction. At this time, a gap S4 having a distance of "L1" is formed between the protrusion 72c and the rear surface 21b of the central partition wall portion 21 in the axial direction. The handled fluid in the second rear side space S21 flows while swirling from the outer edge side to the inner edge side of the second impeller 7, and the flow rate is restricted in the gap S4.

Next, as shown in FIG. 8B, when the first axial thrust force is greater than the second axial thrust force, the rotating shaft 4 (first impeller 6, second impeller 7) moves forward in the axial direction, and the gap S4 narrows to "L2". At this time, the amount of the handled fluid throttled by the convex portion 72c increases. Therefore, in the second back side space S21, the pressure " _Pr2 " of the handled fluid upstream (outer side in the radial direction) of the convex portion 72c increases. In addition, the flow rate of the handled fluid flowing through the cylindrical space S3 decreases, and the pressure " _Pr1 " of the handled fluid in the first back side space S11 decreases. As a result, the force " _Fr2 " acting on the second impeller 7 increases, and the force " _Fr1 " acting on the first impeller 6 decreases, and finally, the first axial thrust force and the second axial thrust force are balanced. In this way, the convex portion 72c narrows a portion of the second rear side space S21 (i.e., a portion of the return flow path RL) as the rotating shaft 4 (first impeller 6, second impeller 7) moves forward, thereby balancing the axial thrust force (first axial thrust force, second axial thrust force) acting on the rotating shaft 4.

On the other hand, as shown in FIG. 8(c), when the first axial thrust force is smaller than the second axial thrust force, the rotating shaft 4 (first impeller 6, second impeller 7) moves rearward in the axial direction, and the gap S4 becomes wider and becomes "L3". At this time, the amount of the handled fluid throttled by the convex portion 72c becomes smaller. Therefore, in the second front side space S22, the pressure "P _r2 " of the handled fluid upstream of the convex portion 72c becomes smaller. In addition, the flow rate of the handled fluid flowing through the cylindrical space S3 increases, and the pressure "F _r1 " of the handled fluid in the first back side space S11 increases. As a result, the force "F _r2 " acting on the second impeller 7 decreases, and the force "F _r1 " acting on the first impeller 6 increases, and finally, the first axial thrust force and the second axial thrust force are balanced. In this way, the convex portion 72c widens a portion of the second rear side space S21 (i.e., a portion of the return flow path RL) as the rotating shaft 4 (first impeller 6, second impeller 7) moves rearward, thereby balancing the axial thrust force acting on the rotating shaft 4.

In this way, the protrusion 72c functions as a variable orifice that adjusts the flow rate of the handled fluid in the second front space S22 (and, as a result, the axial thrust force acting on the rotating shaft 4) according to the axial thrust force acting on the rotating shaft 4.

As described above, the second rear side space S21 is a ring-shaped space. Therefore, the cross-sectional area (cross-sectional area of the flow path) of the second rear side space S21 in a cut surface along the circumferential direction becomes smaller as the cut surface approaches the inner edge of the second rear shroud 72. In other words, as the cut surface approaches the inner edge of the second rear shroud 72, the throttling amount of the variable orifice increases with respect to the amount of movement of the convex portion 72c in the axial direction. Therefore, if the convex portion 72c is disposed closer to the inner edge of the second rear shroud 72, the amount of change in the pressure " _Pr2 " of the handled fluid upstream of the convex portion 72c becomes larger even if the amount of movement of the convex portion 72c in the axial direction is small.

In this way, the centrifugal pump 1 is designed so that the force " _Fr2 " acting on the second impeller 7 is increased by the first to third suppression configurations. Therefore, the first to third suppression configurations are designed so that the first axial thrust force and the second axial thrust force acting on the rotating shaft 4 are perfectly balanced, so that even if the handled liquid in the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3 (return flow path RL), no axial thrust force acts on the rotating shaft 4. Here, in this embodiment, the balance between the first axial thrust force and the second axial thrust force acting on the rotating shaft 4 is roughly adjusted (coarsely adjusted) by the first suppression adjustment, and is supplementarily adjusted (finely adjusted) by the second suppression configuration and the third suppression configuration. Moreover, even if an axial thrust force acts on the rotating shaft 4, the coaxial thrust force is suppressed to a slight force by the first to third suppression configurations, so that the axial thrust force is easily and automatically adjusted by the variable orifice.

Summary According to the embodiment described above, the centrifugal pump 1 includes the central partition wall portion 21, the first pump chamber 23, the second pump chamber 24, the rotating shaft 4, the first impeller 6, and the second impeller 7. The central partition wall portion 21 includes a central through hole 21c through which the rotating shaft 4 is inserted. The first pump chamber 23 is partitioned from the second pump chamber 24 by the central partition wall portion 21, and is disposed in front of and alongside the second pump chamber 24. The first suction port 65 of the first impeller 6 faces forward, and the second suction port 75 of the second impeller 7 faces backward. A first rear side space S11 is formed between the first impeller 6 and the central partition wall portion 21, and a second rear side space S21 is formed between the second impeller 7 and the central partition wall portion 21, and the first rear side space S11 communicates with the second rear side space S21 via the cylindrical space S3. The outer diameter of the second rear shroud 72 is larger than the outer diameter of the first rear shroud 62. According to this configuration, the pressure-receiving area of the second rear shroud 72 increases, thereby increasing the force " _Fr2 " acting on the second impeller 7. Therefore, even if the handled fluid in the second pump chamber 24 flows into the first pump chamber 23 via the cylindrical space S3, the axial thrust force acting on the rotating shaft 4 is suppressed. Therefore, by adjusting the outer diameter of the second rear shroud 72 so that the axial thrust force acting on the rotating shaft 4 is balanced, the axial thrust force acting on the rotating shaft 4 can be suppressed to a minimum.

In addition, according to the embodiment described above, the specific speed set in the first pump section P1 consisting of the first impeller 6 and the first pump chamber 23 is greater than the specific speed set in the second pump section P2 consisting of the second impeller 7 and the second pump chamber 24. In general, as the specific speed increases, the opening on the suction side between the impeller blades becomes larger and the outer diameter of the impeller becomes smaller. Therefore, according to this configuration, the suction performance (discharge flow rate) of the first pump section P1 is improved, and the head of the second pump section P2 is improved, thereby improving the performance of the centrifugal pump 1. In addition, the outer diameter of the second rear shroud 72 is inevitably greater than the outer diameter of the first rear shroud 62. As a result, even if the handled liquid in the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3, the axial thrust force acting on the rotating shaft 4 is suppressed.

Further, according to the embodiment described above, the centrifugal pump 1 includes the first partition portion 20 and the second partition portion 22. The first impeller 6 includes the first front shroud 63 and the cylindrical portion 63b functioning as the first suction port 65. The second impeller 7 includes the second front shroud 73 and the cylindrical portion 73b functioning as the second suction port 75. In the first partition portion 20, the large diameter portion 20d and the step portion 20e face the cylindrical portion 63b. In the second partition portion 22, the large diameter portion 22d and the first step portion 22e face the cylindrical portion 73b. The size (length) of the second cylindrical gap S23 and the second ring gap S24 (second fixed orifice) is larger (longer) than the size (length) of the first cylindrical gap S13 and the first ring gap S14 (first fixed orifice). According to this configuration, the pressure " _Pf2 " that the second front shroud 73 receives from the handled fluid is smaller than the pressure " _Pf2 " when the flow rate of the handled fluid passing through the second fixed orifice is the same as the flow rate of the handled fluid passing through the first fixed orifice. As a result, the force acting on the second impeller 7 (second axial thrust force) is larger than the force acting on the second impeller 7 (second axial thrust force) when the flow rate of the handled fluid passing through the second fixed orifice is the same as the flow rate of the handled fluid passing through the first fixed orifice. Therefore, even if the handled fluid in the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3, the axial thrust force acting on the rotating shaft 4 is further suppressed.

Furthermore, according to the embodiment described above, the central partition portion 21 is provided with eight recesses 21d. The recesses 21d face the second rear side space S21 and are configured to suppress the swirling component of the handled fluid flowing through the second rear side space S21. With this configuration, the pressure " _Pr2 " of the handled fluid increases, the second axial thrust force acting on the second impeller 7 (rotating shaft 4) increases, and the axial thrust force (first axial thrust force) acting on the rotating shaft 4 is further suppressed.

Furthermore, according to the embodiment described above, the outer diameter of the first rear shroud 62 and the outer diameter of the second rear shroud 72 are set, for example, so that the first axial thrust force and the second axial thrust force acting on the rotating shaft 4 are balanced when the first impeller 6 and the second impeller 7 are discharging the handled fluid. With this configuration, the axial thrust force acting on the rotating shaft 4 is suppressed to a minimum.

Furthermore, according to the embodiment described above, the second rear shroud 72 of the second impeller 7 has a protruding portion 72c. The protruding portion 72c functions as a variable orifice that narrows a portion of the second rear side space S21 (a portion of the return flow path RL) as the rotating shaft 4 moves forward, and widens the same portion as the rotating shaft 4 moves rearward. With this configuration, even if an axial thrust force acts on the rotating shaft 4, the coaxial thrust force is suppressed to a small force by the first to third suppression configurations, and therefore the axial thrust force is easily and automatically adjusted by the variable orifice.

Furthermore, according to the embodiment described above, the variable orifice is configured by a protrusion 72c that protrudes forward in a ring shape from the second rear shroud 72. With this configuration, the variable orifice can be formed by a simple configuration in which a portion of the second rear shroud 72 protrudes forward.

Furthermore, according to the embodiment described above, the convex portion 72c is disposed radially inward of the second rear shroud 72. The cross-sectional area of the second rear side space S21 in a cut surface along the circumferential direction decreases as the cut surface approaches the inner edge of the second rear shroud 72. Therefore, according to this configuration, even if the amount of movement of the convex portion 72c in the axial direction is small, the amount of change in the pressure " _Pr2 " of the handled fluid upstream of the convex portion 72c increases. In other words, the sensitivity to pressure fluctuations due to the variable orifice is improved.

●Variations●
Next, modified examples of the centrifugal pump 1 will be described below, focusing on the differences from the previously described embodiment (hereinafter referred to as the "first embodiment"). In the following modified examples, for ease of explanation, the same components as in the first embodiment and components having common functions are denoted by the same reference numerals as in the first embodiment. In the following modified examples, reference will be made to Figures 1 to 8 as appropriate.

First Modification FIG. 9 is a schematic partially enlarged cross-sectional view of a centrifugal pump 1A according to a first modification.
In the figure, for the sake of convenience, only the central partition wall 21, the rotary shaft 4, the first impeller 6, and the second impeller 7 are shown in a simplified manner, and the protrusion 72c is omitted. The white arrow in the figure indicates the return flow.

In the centrifugal pump 1A according to the first modification, the shape of the second impeller 7 is different from that of the first embodiment. Specifically, the outer diameter of the second rear shroud 72 is larger than that of the first rear shroud 62, but the outer diameter of the second front shroud 73 of the second impeller 7 is the same as that of the first front shroud 63. In this configuration, only the pressure-receiving area of the second rear shroud 72 is larger than that of the first rear shroud 62. Therefore, the degree of increase of the force " _Fr2 " acting on the second impeller 7 relative to the amount of expansion of the outer diameter of the second rear shroud 72 is larger than that in the first embodiment. Therefore, in this modification, even if the amount of expansion of the outer diameter of the second rear shroud 72 is smaller than that in the first embodiment, the same effect of suppressing the axial thrust force as in the first embodiment can be obtained. In addition, in this configuration, the second blades 71 are also larger in the circumferential direction compared to the configuration in which only the outer diameter of the second rear shroud 72 is larger as shown in FIG. 6. As a result, the pressure that the second impeller 7 in the first modified example exerts on the handled fluid is greater than the pressure that the second impeller 7 in the first modified example exerts on the handled fluid shown in Fig. 6. Therefore, the force (discharge pressure) of the second impeller 7 in the first modified example is greater than the force of the second impeller 7 shown in Fig. 6.

●Second to fourth modified examples Figure 10(a) is a schematic partial enlarged cross-sectional view of a centrifugal pump 1B relating to the second modified example, (b) is a schematic partial enlarged cross-sectional view of a centrifugal pump 1C relating to the third modified example, and (c) is a schematic partial enlarged cross-sectional view of a centrifugal pump 1D relating to the fourth modified example.

The centrifugal pumps 1B to 1D according to the second to fourth modified examples differ from the first embodiment in the configuration of the variable orifice. Specifically, in the centrifugal pump 1B according to the second modified example, the second rear shroud 72 does not have a convex portion 72c, and the central partition portion 21 has a convex portion 21e. The convex portion 21e is arranged closer to the inner edge portion (closer to the central through hole 21c) of the rear surface 21b of the central partition portion 21. The convex portion 21e is an example of a variable orifice in the present invention. In the centrifugal pump 1C according to the third modified example, the central partition portion 21 has a convex portion 21e, and the second rear shroud 72 also has a convex portion 72c. The convex portions 21e and 72c are arranged in positions facing each other. In the axial direction, the length of the convex portion 72c is shorter than the length of the convex portion 72c in the first embodiment. The convex portions 21e and 72c are an example of a variable orifice in the present invention. In the centrifugal pump 1D according to the fourth modification, the second rear shroud 72 does not have a protruding portion 72c, and the rear end of the central through hole 21c is expanded to form an expanded diameter portion 21f and a step portion 21g. In addition, an annular member R is attached to the rotating shaft 4. In the axial direction, the annular member R is disposed between the second impeller 7 and the step portion 21g, and faces the step portion 21g. The annular member R is an example of a variable orifice in the present invention. In these configurations, as in the first embodiment, the amount of throttling of a part of the return flow passage RL changes in response to the movement of the rotating shaft 4 in the forward and backward directions. Therefore, these configurations function as a variable orifice similar to that of the first embodiment.

Fifth Modification FIG. 11 is a schematic partially enlarged cross-sectional view showing a centrifugal pump 1E according to a fifth modification.

The centrifugal pump 1E according to the fifth modified example differs from the first embodiment in the number of impellers. Specifically, the centrifugal pump 1E includes a housing 2E, a motor 3, a rotating shaft 4, bearings 51 and 52, a first impeller 6, a second impeller 7, a third impeller 8, and a fourth impeller 9. In other words, the centrifugal pump 1 is a four-stage centrifugal pump equipped with four impellers (first impeller 6 to fourth impeller 9).

The housing 2E houses the motor 3, the rotating shaft 4, the bearings 51, 52, and the first impeller 6 to the fourth impeller 9. The housing 2E includes a first partition 20, a central partition 21, a second partition 22, a first pump chamber 23, a second pump chamber 24, a connecting flow path 25, a discharge flow path 26, a suction pipe 27, a discharge pipe 28, a motor chamber 29, a front partition 2a, a rear partition 2b, a third pump chamber 2c, and a fourth pump chamber 2d. The front partition 2a is disposed in front of the first partition 20, and defines the third pump chamber 2c, which houses the third impeller 8, together with the first partition 20. The rear partition 2b is disposed behind the second partition 22, and defines the fourth pump chamber 2d, which houses the fourth impeller 9, together with the second partition 22. The third pump chamber 2c is arranged in front of and alongside the first pump chamber 23, and the fourth pump chamber 2d is arranged behind and alongside the second pump chamber 24. The connecting passage 25 is a passage that guides the pumped fluid discharged by the first impeller 6 to the fourth impeller 9. The suction pipe 27 is formed in the front partition 2a instead of the first partition 20.

The configuration of the third impeller 8 is the same as that of the first impeller 6. The configuration of the fourth impeller 9 is the same as that of the first impeller 6, except for the direction of rotation. That is, the outer diameters of the first impeller 6, the third impeller 8, and the fourth impeller 9 are the same as each other and are smaller than the outer diameter of the second impeller 7. The third impeller 8 is attached to the front part 4a of the rotating shaft 4 in front of the first impeller 6. The fourth impeller 9 is attached to the front part 4a of the rotating shaft 4 behind the second impeller 7. The first impeller 6 and the second impeller 7 are arranged back to back so as to sandwich the central partition wall 21, as in the first embodiment. The third impeller 8 and the first impeller 6 constitute a front group that draws in the handled fluid from the front, and the fourth impeller 9 and the second impeller 7 constitute a rear group that draws in the handled fluid from the rear. In the centrifugal pump 1E, the handled liquid flows in the following order: suction pipe 27, third impeller 8, first impeller 6, connecting passage 25, fourth impeller 9, second impeller 7, discharge passage 26, and discharge pipe 28. In other words, the second impeller 7 sucks in the handled liquid discharged from the first impeller 6 via the connecting passage 25 and the fourth impeller 9, and discharges it.

In this configuration, the pressure difference between the pressure of the handled fluid in the second pump chamber 24 and the pressure of the handled fluid in the first pump chamber 23 is approximately twice the pressure difference in the first embodiment. Therefore, the flow rate of the handled fluid flowing through the return flow path RL is increased compared to the same flow rate in the first embodiment. Therefore, the axial thrust force (first axial thrust force) acting on the rotating shaft 4 by the handled fluid flowing through the return flow path RL is greater than the coaxial thrust force in the first embodiment. Even in such a case, the first to third suppression configurations and the variable orifice in the present invention can suppress and adjust the axial thrust force.

In this modified example, the configuration of the fourth impeller 9 may be the same as the configuration of the second impeller 7.

Other embodiments In the above-described embodiment, the centrifugal pump 1 includes, in addition to the first suppression configuration, other configurations (second to third suppression configurations) for increasing the force " _Fr2 " acting on the second impeller 7. Alternatively, the centrifugal pump 1 may not include some or all of the other configurations. That is, for example, the size (flow rate) of the second fixed orifice may be the same as the size (flow rate) of the first fixed orifice. Also, for example, the central partition portion 21 may not include the recess 21d. Since the configuration that contributes most to suppressing the axial thrust force is the first suppression configuration, if the centrifugal pump 1 includes the first suppression configuration, the axial thrust force acting on the rotating shaft 4 is suppressed even if the handled fluid in the second pump chamber 24 flows into the first pump chamber 23 via the cylindrical space S3. Here, when the centrifugal pump 1 has only the first suppression configuration among the first to third suppression configurations, the outer diameters of the first rear shroud 62 and the second rear shroud 72 are set, for example, so that the first axial thrust force and the second axial thrust force are balanced when the first impeller 6 and the second impeller 7 (i.e., the centrifugal pump 1) are discharging the handled liquid (at the most efficient point). That is, for example, when the two-stage centrifugal pump operates under the conditions of a discharge rate of 30 m ³ /h, a total head of 65 m, the handled liquid is water (specific gravity: 1, viscosity: 1 cP), and a rotation speed of 3,000 rpm, when the outer diameter of the first impeller 6 is 150 mm and the outer diameter of the second impeller 7 is 158 mm, both the first axial thrust force and the second axial thrust force are 2,100 N. Here, the flow rate of the return flow is assumed to be 10% (3 m ³ /h).

In addition, in the present invention, the second impeller 7 does not need to have a protrusion 72c (variable orifice). Even in this configuration, the axial thrust force acting on the rotating shaft 4 can be minimized by the first to third suppression configurations.

Furthermore, in the present invention, the central partition 21 may have a protrusion instead of the recess 21d. In this case, the protrusion has, for example, the same size (volume) as the recess 21d.

Furthermore, in the present invention, the number of recesses 21d is not limited to the number in this embodiment. That is, for example, the number of recesses 21d may be "1", an odd number equal to or greater than "3", or an even number excluding "8".

Furthermore, in the present invention, the central partition 21 may have protrusions instead of some of the recesses 21d among the plurality of recesses 21d. That is, for example, the central partition 21 may have four recesses 21d and four protrusions arranged between each recess 21d.

Furthermore, in the present invention, the position of the recess 21d is not limited to the position in this embodiment. That is, for example, the inner edge of the recess 21d may be located radially outward from the protrusion 72c.

Furthermore, in the present invention, the position of the protrusion 72c is not limited to the position in this embodiment. That is, for example, in the radial direction, the protrusion 72c may be located in the center or outer edge of the front surface 72b.

Furthermore, in the present invention, the specific speed set for the first pump section P1 may be the same as the specific speed set for the second pump section P2, except for the difference due to the difference between the outer diameter of the first rear shroud 62 and the outer diameter of the second rear shroud 72.

Furthermore, in the present invention, the number of impellers provided in the centrifugal pump 1 may be an even number and is not limited to two. That is, for example, the number of impellers may be "4" as shown in the fifth modified example, or may be "6" or more.

Furthermore, in the present invention, the handled liquid is not limited to liquefied gas. That is, for example, the handled liquid may be water.

●Embodiments of the present invention●
Next, the embodiments of the present invention that can be understood from the above-described embodiments will be described below, using the terms and symbols described in the respective embodiments.

A first embodiment of the present invention includes a motor (e.g., motor 3), a rotating shaft (e.g., rotating shaft 4) rotated by driving the motor, a first impeller (e.g., first impeller 6) attached to the rotating shaft and sucking and discharging a handled fluid, a second impeller (e.g., second impeller 7) attached to the rotating shaft and sucking and discharging the handled fluid discharged from the first impeller, a first pump chamber (e.g., first pump chamber 23) in which the first impeller is accommodated, and a front pump chamber (e.g., first pump chamber 24) in the axial direction of the rotating shaft. The pump pump has a second pump chamber (e.g., second pump chamber 24) arranged side by side with the first pump chamber and accommodating the second impeller, and a central partition wall portion (e.g., central partition wall portion 21) having an insertion hole (e.g., central through hole 21c) through which the rotating shaft is inserted and partitioning the first pump chamber and the second pump chamber, and in the axial direction, a direction in which the first impeller is arranged relative to the second impeller is a first direction (e.g., forward direction), and a direction opposite to the first direction is a second direction (e.g., rearward direction). ), the first impeller is oriented in the first direction and includes a first suction port (e.g., first suction port 65) that draws in the handled fluid from the first direction side, and a first shroud (e.g., first rear shroud 62) that faces the central partition wall portion, and the second impeller is oriented in the second direction and includes a second suction port (e.g., second suction port 75) that draws in the handled fluid discharged from the first impeller from the second direction side, and a second shroud (e.g., second rear shroud 7 2), a first space (e.g., a first rear-side space S11) communicating with the insertion hole is formed between the first impeller and the central partition portion, a second space (e.g., a second rear-side space S21) communicating with the insertion hole is formed between the second impeller and the central partition portion, the second space and the first space communicate with each other via the insertion hole, and an outer diameter of the second shroud is larger than an outer diameter of the first shroud.
According to this configuration, even if the pumped fluid in the second pump chamber 24 flows into the first pump chamber 23 via the cylindrical space S3, the axial thrust force acting on the rotating shaft 4 is suppressed.

A second embodiment of the present invention is a centrifugal pump in the first embodiment, wherein a specific speed set in a second pump section (e.g., second pump section P2) formed by the second impeller and the second pump chamber is smaller than a specific speed set in a first pump section (e.g., first pump section P1) formed by the first impeller and the first pump chamber.
According to this configuration, the performance of the centrifugal pump 1 is improved, and the axial thrust force acting on the rotating shaft 4 is suppressed.

A third embodiment of the present invention is a centrifugal pump according to the first or second embodiment, comprising a first partition portion (e.g., first partition portion 20) disposed on the first direction side with respect to the first impeller and partitioning the first pump chamber together with the central partition portion, and a second partition portion (e.g., second partition portion 22) disposed on the second direction side with respect to the second impeller and partitioning the second pump chamber together with the central partition portion, the first impeller being provided with a third shroud (e.g., a first front shroud 63) disposed on the first direction side with respect to the first shroud, and the second impeller being provided with a fourth shroud (e.g., a the third shroud has a first cylindrical portion (e.g., cylindrical portion 63b) constituting the first suction port, the fourth shroud has a second cylindrical portion (e.g., cylindrical portion 73b) constituting the second suction port, and a second gap (e.g., second cylindrical gap S23 and second ring gap S24) between a portion of the second partition portion facing the second cylindrical portion (e.g., large diameter portion 22d and first step portion 22e) and the second cylindrical portion is larger than a first gap (e.g., first cylindrical gap S13 and first ring gap S14) between a portion of the first partition portion facing the first cylindrical portion (e.g., large diameter portion 20d and step portion 20e) and the first cylindrical portion,
It is.
With this configuration, the axial thrust force acting on the rotating shaft 4 is further suppressed.

A fourth embodiment of the present invention is a centrifugal pump in any one of the first to third embodiments, wherein the central partition portion faces the second space and has at least one convex or concave portion (e.g., concave portion 21d) configured to suppress the swirling component of the handled fluid flowing through the second space.
With this configuration, the axial thrust force acting on the rotating shaft 4 is further suppressed.

A fifth embodiment of the present invention is a centrifugal pump in any one of the first to fourth embodiments, wherein the outer diameter of the first shroud and the outer diameter of the second shroud are set so that a first axial thrust force acting to move the rotating shaft in the first direction and a second axial thrust force acting to move the rotating shaft in the second direction are balanced when the first impeller and the second impeller are discharging the handled fluid.
According to this configuration, the axial thrust force acting on the rotating shaft 4 is suppressed to a minimum.

A sixth embodiment of the present invention is a centrifugal pump in any one of the first to fifth embodiments, wherein the second space and the insertion hole constitute a return flow path (e.g., a return flow path RL) through which a portion of the handled fluid discharged by the second impeller flows into the first space, and the centrifugal pump has a variable orifice (e.g., a convex portion 72c) configured to narrow a portion of the return flow path as the rotating shaft moves toward the first direction and to widen the portion as the rotating shaft moves toward the second direction.
According to this configuration, the axial thrust force is easily and automatically adjusted by the variable orifice.

A seventh embodiment of the present invention is a centrifugal pump (e.g., centrifugal pumps 1, 1B to 1D) in the sixth embodiment, wherein the variable orifice is constituted by a protrusion (e.g., convex portion 72c) protruding from the second shroud in the first direction and/or a protrusion (e.g., convex portion 21e) protruding from the central partition portion in the second direction.
According to this configuration, the variable orifice can be formed with a simple structure.

An eighth embodiment of the present invention is a centrifugal pump in the seventh embodiment, wherein the protrusion is ring-shaped along the circumferential direction of the rotating shaft and is positioned radially toward the inner edge of the second shroud and/or toward the inner edge of the central partition in the radial direction.
This configuration improves the sensitivity to pressure fluctuations caused by the variable orifice.

1 Centrifugal pump 20 First partition wall portion 20d Large diameter portion (portion facing the first cylindrical portion)
20e Step portion (portion facing the first cylindrical portion)
21 Central partition wall portion 21c Central through hole (insertion hole)
21d Recess 22 Second partition wall portion 22d Large diameter portion (portion facing the second cylindrical portion)
22e: First step portion (portion facing the second cylindrical portion)
3 Motor 4 Rotating shaft 6 First impeller 62 First rear shroud (first shroud)
63 First front shroud (third shroud)
63b Cylindrical portion (first cylindrical portion)
65 First suction port 7 Second impeller 72 Second rear shroud (second shroud)
72e Convex portion (variable orifice)
73 Second front shroud (fourth shroud)
73b Cylindrical portion (second cylindrical portion)
75 Second suction port S11 First rear space (first space)
S13 First cylindrical gap (first gap)
S14 First ring gap (first gap)
S21 Second back side space (second space)
S23 Second cylindrical gap (second gap)
S24 Second ring gap (second gap)
S3 Cylindrical space (insertion hole)
RL Return flow path

Claims (7)

A motor;
A rotating shaft that rotates by driving the motor;
a first impeller attached to the rotary shaft for sucking and discharging the pumped fluid;
a second impeller attached to the rotary shaft, which sucks in the handled fluid discharged from the first impeller and discharges it;
a first pump chamber in which the first impeller is accommodated;
a second pump chamber that is arranged alongside the first pump chamber in the axial direction of the rotary shaft and that accommodates the second impeller;
a central partition portion including an insertion hole through which the rotating shaft is inserted, the central partition portion separating the first pump chamber and the second pump chamber;
a first partition portion disposed on a first direction side with respect to the first impeller and defining the first pump chamber together with the central partition portion;
a second partition wall portion disposed on a second direction side with respect to the second impeller and defining the second pump chamber together with the central partition wall portion;
and
In the axial direction, a direction in which the first impeller is disposed relative to the second impeller is the first direction, and a direction opposite to the first direction is the second direction,
The first impeller is
a first suction port oriented in the first direction and configured to suck in the pumped fluid from the first direction;
a first shroud facing the central partition portion;
a third shroud disposed on the first direction side with respect to the first shroud;
With
The second impeller is
a second suction port oriented in the second direction and configured to suck in the handled fluid discharged from the first impeller from the second direction side;
a second shroud facing the central partition portion;
a fourth shroud disposed on the second direction side with respect to the second shroud;
With
The third shroud comprises:
A first cylindrical portion constituting the first suction port;
With
The fourth shroud includes:
A second cylindrical portion constituting the second suction port;
With
A first space communicating with the insertion hole is formed between the first impeller and the central partition wall,
A second space communicating with the insertion hole is formed between the second impeller and the central partition wall,
The second space and the first space communicate with each other via the insertion hole,
The outer diameter of the second shroud is larger than the outer diameter of the first shroud,
a second gap between a portion of the second partition wall facing the second cylindrical portion and the second cylindrical portion is larger than a first gap between a portion of the first partition wall facing the first cylindrical portion and the first cylindrical portion;
Centrifugal pump. a specific speed set in a second pump section formed by the second impeller and the second pump chamber is lower than a specific speed set in a first pump section formed by the first impeller and the first pump chamber;
2. The centrifugal pump of claim 1. The central partition portion has at least one convex portion or concave portion facing the second space and configured to suppress a swirling component of the handled fluid flowing through the second space.
2. The centrifugal pump of claim 1. an outer diameter of the first shroud and an outer diameter of the second shroud are set so that a first axial thrust force acting to move the rotating shaft in the first direction and a second axial thrust force acting to move the rotating shaft in the second direction are balanced when the first impeller and the second impeller discharge the handled fluid.
2. The centrifugal pump of claim 1. the second space and the insertion hole form a return flow passage through which a portion of the handled fluid discharged by the second impeller flows into the first space,
a variable orifice configured to narrow a portion of the return passage as the rotating shaft moves in the first direction and to widen the portion as the rotating shaft moves in the second direction.
5. A centrifugal pump according to any one of claims 1 to 4 . The variable orifice is
A protrusion protruding from the second shroud in the first direction, and/or
a protrusion protruding from the central partition wall in the second direction;
Consists of:
6. The centrifugal pump according to claim 5 . The protruding portion is a ring-shaped portion extending in a circumferential direction of the rotating shaft,
and/or,
In the radial direction, the central partition wall portion is disposed closer to an inner edge portion of the central partition wall portion.
7. The centrifugal pump according to claim 6 .

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