CENTRIFUGAL PUMP

Description

TECHNICAL FIELD

The present invention relates to a centrifugal pump.

BACKGROUND ART

A centrifugal pump includes a motor, a rotary shaft, an impeller, and a housing. The impeller is mounted to the rotary shaft driven by the operation of the motor and rotates together with the rotary shaft, thereby sucking in a pumped liquid from one axial direction side of the rotary shaft and discharging the pumped liquid, for example, outward in the radial direction of the impeller. The impeller includes a plurality of vanes, a front shroud (also referred to as a side plate), and a rear shroud (also referred to as a main plate). In the axial direction, the front shroud covers one direction side of the vanes, and the rear shroud covers the other direction side of the vanes. A portion of the high-pressure pumped liquid discharged from the impeller also flows between the front shroud and the housing and between the rear shroud and the housing, presses the front shroud toward the rear shroud side, and presses the rear shroud toward the front shroud side. The pressure receiving area of the rear shroud is normally larger than the pressure receiving area of the front shroud. Thus, in the axial direction, a force (i.e., an axial thrust force) that pushes the impeller in one direction acts on the impeller.

A multistage centrifugal pump successively discharging a pumped liquid by a plurality of impellers mounted to one rotary shaft for achieving a higher head while improving the suction performance in a centrifugal pump is known (for example, see PTL 1). The axial thrust force described above increases as the number of impellers (i.e., the number of stages) increases. Thus, in the multistage centrifugal pump, the plurality of impellers is divided into a previous-stage group and a subsequent-stage group, and the previous-stage group and the subsequent-stage group are mounted to the rotary shaft in a back-to-back configuration. The impellers in the previous-stage group have the same design as those in the subsequent-stage group except for the direction of rotation, and accordingly, the axial thrust force of the previous-stage group balances that of the subsequent-stage group, and the two axial thrust forces cancel each other out.

CITATION LIST

Patent Literature

[PTL 1] JP2002-21766 A

SUMMARY OF INVENTION

Technical Problem

Normally, a mechanical seal is mounted between the rotary shaft and a partition wall disposed between the previous-stage group and the subsequent-stage group. Thus, the pumped liquid in the subsequent-stage group does not flow into the previous-stage group. However, when the viscosity of the pumped liquid is low, the lubrication performance of the mechanical seal provided by the pumped liquid deteriorates, and thus, a configuration not using a mechanical seal between the partition wall and the rotary shaft may be employed. The pressure of the pumped liquid flowing through the subsequent-stage group is higher than the pressure of the pumped liquid flowing through the previous-stage group. Thus, in such a configuration, the pumped liquid in the subsequent-stage group flows into the previous-stage group through a gap between the partition wall and the rotary shaft; thereby strengthening the axial thrust force of the previous-stage group and weakening the axial thrust force of the subsequent-stage group. As a result, an axial thrust force occurs toward the suction port side of the previous-stage group.

The present invention is directed to reducing the generation of an axial thrust force in a multistage centrifugal pump.

Solution to Problem

A centrifugal pump according to an aspect of the present invention is a centrifugal pump including: a motor; a rotary shaft that rotates by driving of the motor; a first impeller mounted to the rotary shaft and configured to suck in and discharge a pumped liquid; a second impeller mounted to the rotary shaft and configured to suck in and discharge the pumped liquid discharged from the first impeller; a first pump chamber in which the first impeller is accommodated; a second pump chamber that is disposed side by side with the first pump chamber in the axial direction of the rotary shaft and in which the first impeller is accommodated; a central partition wall portion that includes an insertion hole through which the rotary shaft is disposed and that divides the first pump chamber from the second pump chamber; a first partition wall portion that is disposed on a first direction side relative to the first impeller and that defines the first pump chamber together with the central partition wall portion; and a second partition wall portion that is disposed on a second direction side relative to the second impeller and that defines the second pump chamber together with the central partition wall portion, wherein, 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 includes: a first suction port oriented toward the first direction and configured to suck in the pumped liquid from the first direction side; a first shroud facing the central partition wall portion; and a third shroud disposed on the first direction side relative to the first shroud, the second impeller includes: a second suction port oriented toward the second direction and configured to suck in, from the second direction side, the pumped liquid discharged from the first impeller; a second shroud facing the central partition wall portion; and a fourth shroud disposed on the second direction side relative to the second shroud, the third shroud includes a first cylindrical portion forming the first suction port, the fourth shroud includes a second cylindrical portion forming the second suction port, a first space communicating with the insertion hole is defined between the first impeller and the central partition wall portion, a second space communicating with the insertion hole is defined between the second impeller and the central partition wall portion, the second space and the first space communicate with each other through the insertion hole, an outer diameter of the second shroud is larger than an outer diameter of the first shroud, and a second gap between a portion of the second partition wall portion facing the second cylindrical portion and the second cylindrical portion is larger than a first gap between a portion of the first partition wall portion facing the first cylindrical portion and the first cylindrical portion.

Advantageous Effects of Invention

The present invention is capable of reducing the generation of the axial thrust force in the multistage centrifugal pump.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of a centrifugal pump according to the present invention illustrating an embodiment of the centrifugal pump.

FIG. 2 is an enlarged partial sectional view of the centrifugal pump in FIG. 1.

FIG. 3 is a schematic rear elevation view of a central partition wall portion included in the centrifugal pump in FIG. 1.

FIG. 4 is a schematic front elevation view of a second impeller included in the centrifugal pump in FIG. 1.

FIG. 5A is an enlarged partial schematic sectional view of a conventional pump illustrating axial thrust forces in the conventional pump, and FIG. 5B is an enlarged partial schematic sectional view of a conventional pump without a mechanical seal illustrating axial thrust forces in the conventional pump without the mechanical seal.

FIG. 6 is an enlarged partial schematic sectional view of the centrifugal pump illustrating an effect of the second impeller in FIG. 4 on axial thrust forces in the centrifugal pump in FIG. 1.

FIG. 7 is an enlarged partial schematic sectional view of the centrifugal pump illustrating an effect of a second fixed orifice included in the centrifugal pump in FIG. 1 on axial thrust forces in the centrifugal pump in FIG. 1.

FIGS. 8A to 8C each are an enlarged partial schematic sectional view of the centrifugal pump illustrating an effect of a convex portion included in the second impeller in FIG. 4 on axial thrust forces in the centrifugal pump in FIG. 1, in which FIG. 8A illustrates a state in which a first axial thrust force and a second axial thrust force are balanced, FIG. 8B illustrates a state in which the first axial thrust force is greater than the second axial thrust force, and FIG. 8C illustrates a state in which the first axial thrust force is less than the second axial thrust force.

FIG. 9 is an enlarged partial schematic sectional view of a centrifugal pump according to a first modification example.

FIG. 10A is an enlarged partial schematic sectional view of a centrifugal pump according to a second modification example, FIG. 10B is an enlarged partial schematic sectional view of a centrifugal pump according to a third modification example, and FIG. 10C is an enlarged partial schematic sectional view of a centrifugal pump according to a fourth modification example.

FIG. 11 is an enlarged partial schematic sectional view of a centrifugal pump according to a fifth modification example.

DESCRIPTION OF EMBODIMENTS

Embodiments of a centrifugal pump according to the present invention will now be described. In the following description, the drawings are referred to as appropriate. In the drawings, the same members and components are indicated with the same reference signs, and repetitive description thereof will be omitted. Further, the dimensional ratios of the components may be exaggerated for convenience of description and are not limited to the ratios illustrated in the drawings.

Centrifugal Pump

Configuration of Centrifugal Pump

FIG. 1 is a schematic sectional view a centrifugal pump according to the present invention illustrating an embodiment of the centrifugal pump. In the figure, a portion of a housing 2 to be described later is illustrated as a schematic sectional view, and the remaining portion is illustrated in a simplified manner. The figure schematically illustrates a longitudinal section of the centrifugal pump 1 taken along the vertical direction so as to pass through the axis of a rotary shaft 4 to be described later (the same applies to FIG. 2 and FIGS. 5 to 11).

The centrifugal pump 1 sucks in and discharges (i.e., delivers) a pumped liquid. The centrifugal pump 1 includes the housing 2, a motor 3, the rotary shaft 4, bearings 51 and 52, a first impeller 6, and a second impeller 7. That is, the centrifugal pump 1 is a two-stage centrifugal pump including two impellers (i.e., the first impeller 6 and the second impeller 7) and is an example of the centrifugal pump according to the present invention.

The “pumped liquid” is a liquid that is handled (i.e., delivered) by the centrifugal pump 1. The pumped liquid according to the present embodiment is a low-viscosity liquid and is, for example, a cryogenic liquefied gas such as liquefied natural gas or liquid hydrogen.

In the following description, the “forward direction” refers to a direction in which the first impeller 6 and the second impeller 7 are positioned relative to the motor 3, and the “rearward direction” refers to a direction in which the motor 3 is positioned relative to the first impeller 6 and the second impeller 7. The “axial direction” refers to a direction along the central axis of the rotary shaft 4 (i.e., the forward and rearward direction), the “radial direction” refers to a direction along the radius of the rotary shaft 4, and the “circumferential direction” refers to a direction along the circumference of the rotary shaft 4. The “upstream side” refers to the upstream side of a flow of the pumped liquid inside the housing 2, and the “downstream side” refers to the downstream side of the flow of the pumped liquid inside the housing 2. The forward direction is an example of a first direction according to the present invention, and the rearward direction is an example of a second direction according to the present invention.

The housing 2 accommodates the motor 3, the rotary shaft 4, the bearings 51 and 52, the first impeller 6, and the second impeller 7. The housing 2 includes a first partition wall portion 20, a central partition wall portion 21, a second partition wall portion 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 sectional view of the centrifugal pump 1. The figure illustrates a section of the upper half of the centrifugal pump 1, centered around the first impeller 6 and the second impeller 7. In the following description, FIG. 1 is also referred to as appropriate.

The first partition wall portion 20 is a partition wall that defines the first pump chamber 23 together with the central partition wall portion 21. The first partition wall portion 20 is located in the forward direction with respect to the first impeller 6 and is disposed at the front end portion of the housing 2. The first partition wall portion 20 includes an inner surface 20a and a first through hole 20b.

The inner surface 20a is a curved surface recessed forward in a substantially frustoconical shape, so as to follow the shape of a first front shroud 63 of the first impeller 6 to be described later.

When viewed in the axial direction, the first through hole 20b is a through hole that penetrates the central portion of the first partition wall portion 20 in a two-step cylindrical shape along the axial direction. The first through hole 20b communicates with both the first pump chamber 23 and the suction pipe 27. The first through hole 20b includes 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 forward of and adjacent to the large diameter portion 20d. The step portion 20e is a surface that is disposed between the small diameter portion 20c and the large diameter portion 20d and is continuous with both the small diameter portion 20c and the large diameter portion 20d. When viewed in the axial direction, the step portion 20e has a ring shape. The small diameter portion 20c functions as a flow path for introducing the pumped liquid to the first impeller 6.

The central partition wall portion 21 is a partition wall that defines the first pump chamber 23 together with the first partition wall portion 20 and defines the second pump chamber 24 together with the second partition wall portion 22. The central partition wall portion 21 is disposed in the rearward direction with respect to the first partition wall portion 20. The central partition wall portion 21 includes a front surface 21a, a rear surface 21b, a central through hole 21c, and eight recessed portions 21d. The front surface 21a is a surface oriented in the forward direction, and the rear surface 21b is a surface oriented in the rearward direction.

When viewed in the axial direction, the central through hole 21c is a cylindrical through hole that penetrates the central portion of the central partition wall portion 21 along the axial direction. The rotary shaft 4 is disposed through the central through hole 21c. The central through hole 21c is an example of an insertion hole according to the present invention.

FIG. 3 is a schematic rear elevation view of the central partition wall portion 21. The figure schematically illustrates the central partition wall portion 21 as viewed from the rear. The figure also illustrates the rotary shaft 4 and a first hub portion 64 to be described later. Further, the figure also virtually illustrates the second impeller 7 and a convex portion 72c by chain double-dashed lines for convenience of description. In the following description, FIG. 2 is also referred to as appropriate.

The recessed portion 21d reduces the swirl component of the pumped liquid flowing through a second rear-side space S21 to be described later. A portion of the rear surface 21b of the central partition wall portion 21 forms the recessed portion 21d by being recessed in a rectangular shape toward the forward direction. In the circumferential direction, the eight recessed portions 21d are disposed on the rear surface 21b at equiangular intervals (e.g., 45 degrees in the present embodiment). When viewed in the axial direction, the recessed portion 21d has a rectangular shape with the longitudinal direction of the recessed portion 21d extending along the radial direction. When viewed in the axial direction, the inner edge (i.e., the edge on the inner side in the radial direction) of the recessed portion 21d is disposed at the same position as the outer edge of the convex portion 72c of the second impeller 7 to be described later. When viewed in the axial direction, the outer edge (i.e., the edge on the outer side in the radial direction) of the recessed portion 21d is disposed at substantially the same position as an intermediate portion between the inner and outer edges of the second impeller 7.

The following description mainly refers back to FIGS. 1 and 2. The second partition wall portion 22 is a partition wall that defines the second pump chamber 24 together with the central partition wall portion 21. The second partition wall portion 22 is located in the rearward direction with respect to the second impeller 7 and disposed in the rearward direction with respect to the central partition wall portion 21. The second partition wall portion 22 includes an inner surface 22a and a second through hole 22b.

The inner surface 22a is a curved surface recessed rearward in a substantially frustoconical shape, so as to follow the shape of a second front shroud 73 of the second impeller 7 to be described later.

When viewed in the axial direction, the second through hole 22b is a through hole that penetrates the central portion of the second partition wall portion 22 in a three-step cylindrical shape along the axial direction. The second through hole 22b communicates with both the second pump chamber 24 and the connecting flow path 25. The second through hole 22b includes 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 is smaller than the inner diameter of the large diameter portion 22d. In the axial direction, the insertion portion 22f is disposed rearward of and adjacent to the small diameter portion 22c, and the large diameter portion 22d is disposed forward of and adjacent to the small diameter portion 22c. The first step portion 22e is a surface that is disposed between the small diameter portion 22c and the large diameter portion 22d and is continuous with both the small diameter portion 22c and the large diameter portion 22d. The second step portion 22g is a surface that is disposed between the insertion portion 22f and the small diameter portion 22c and is continuous with both the insertion portion 22f and the small diameter portion 22c. When viewed in the axial direction, the first step portion 22e and the second step portion 22g each have a ring shape. The rotary shaft 4 is disposed through the second through hole 22b, and a cylindrical space that functions as a flow path for introducing the pumped liquid to the second impeller 7 is defined between the small diameter portion 22c and the rotary shaft 4.

The first pump chamber 23 accommodates the first impeller 6. The second pump chamber 24 accommodates the second impeller 7. In the axial direction, the first pump chamber 23 is disposed forward of the second pump chamber 24 and is disposed side by side with the second pump chamber 24 in the axial direction, with the central partition wall portion 21 interposed therebetween. That is, the first impeller 6 is disposed forward of the second impeller 7 in the axial direction.

The connecting flow path 25 is a flow path for guiding the pumped liquid discharged by the first impeller 6 to a space inside the small diameter portion 22c. The connecting flow path 25 is configured, for example, by a portion of the housing 2 and communicates with both the first pump chamber 23 and the space inside the small diameter portion 22c. In FIGS. 1 and 2, the connecting flow path 25 is indicated in a simplified manner by a thick solid arrow.

The discharge flow path 26 is a flow path for guiding the pumped liquid discharged by the second impeller 7 to the discharge pipe 28. The discharge flow path 26 is configured, for example, by a portion of the housing 2 and communicates with both the second pump chamber 24 and the discharge pipe 28. In FIGS. 1 and 2, the discharge flow path 26 is indicated in a simplified manner by a dashed arrow.

The front end portion of the housing 2 (i.e., the first partition wall portion 20) extends cylindrically in the forward direction in such a way as to be coaxial with the rotary shaft 4 and forms the suction pipe 27 that sucks (i.e., introduces) the pumped liquid into the first pump chamber 23. Further, a portion of the housing 2 positioned outside the second impeller 7 in the radial direction extends in the tangential direction of the second impeller 7 (i.e., upward) and forms the discharge pipe 28 that discharges the pumped liquid from the second pump chamber 24 (i.e., the discharge flow path 26).

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

The motor 3 is a known motor including a rotor (unillustrated) mounted to the rotary shaft 4 and a stator (unillustrated) that rotates the rotor (unillustrated). The rotary shaft 4 rotates by driving (i.e., rotation) of the motor 3 and transmits the rotation power to the first impeller 6 and the second impeller 7. The rotary shaft 4 has a solid cylindrical shape. The rotary shaft 4 is mounted to the motor 3, and a front portion 4a of the rotary shaft 4 protrudes into the first pump chamber 23 and the second pump chamber 24.

The bearing 51 is disposed forward of the motor 3 and rotatably supports the rotary shaft 4. The bearing 52 is disposed rearward of the motor 3 and rotatably supports the rotary shaft 4. For example, the bearings 51 and 52 are rolling bearings.

The first impeller 6 sucks in and discharges the pumped liquid. The first impeller 6 is mounted to the front portion 4a of the rotary shaft 4 and is accommodated in the first pump chamber 23. That is, the first impeller 6 is disposed between the first partition wall portion 20 and the central partition wall portion 21. The first impeller 6 is a so-called closed-type impeller. The first impeller 6 includes a plurality of first vanes 61, a first rear shroud 62, the first front shroud 63, the first hub portion 64, a first suction port 65, and a first discharge port 66.

The first vanes 61 rotate in the circumferential direction about the rotary shaft 4 as the center of rotation and guide the pumped liquid sucked in through the first suction port 65 to the first discharge port 66. When viewed in the axial direction, each of the plurality of first vanes 61 extends radially from the center side of the first impeller 6 toward the outer edge side and is curved in a spiral shape. The first vanes 61 are disposed between the first rear shroud 62 and the first front shroud 63.

The first rear shroud 62 is a plate (a so-called main plate) that covers the rear of the first vanes 61. The shape of the first rear shroud 62 is a ring-shaped plate. When viewed in the axial direction, the central portion of the first rear shroud 62 protrudes forward in a substantially frustoconical shape. The first rear shroud 62 faces the central partition wall portion 21. The first rear shroud 62 includes an attachment hole 62a and a rear surface 62b. The first rear shroud 62 is an example of the first shroud according to the present invention.

The attachment hole 62a is a through hole through which the front portion 4a of the rotary shaft 4 is disposed. When viewed in the axial direction, the attachment hole 62a is disposed in the central portion of the first rear shroud 62 and extends cylindrically through the central portion along the axial direction.

The rear surface 62b is a surface that is oriented in the rearward direction and faces the front surface 21a of the central partition wall portion 21. A ring-shaped plate-like space (hereinafter referred to as a “first rear-side space S11”) is defined between the rear surface 62b and the front surface 21a. The first rear-side space S11 is an example of the first space according to the present invention.

The first front shroud 63 is a plate (a so-called side plate) that covers the front of the first vanes 61. The shape of the first front shroud 63 is a substantially ring-shaped plate in which the inner edge portion is more convex forward than the outer edge portion. 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 includes a front surface 63a and a cylindrical portion 63b. The first front shroud 63 is an example of the third shroud according to the present invention.

The front surface 63a is a surface that is oriented in the forward direction and that faces the inner surface 20a of the first partition wall portion 20. A substantially ring-shaped plat-like space (hereinafter referred to as a “first front-side space S12”) is defined between the front surface 63a and the inner surface 20a.

The inner edge portion of the first front shroud 63 extends cylindrically in the forward direction in such a way as to be coaxial with the rotary shaft 4 and forms the cylindrical portion 63b that functions as a first suction port 65. In other words, the first front shroud 63 includes the cylindrical portion 63b that functions as the first suction port 65. In the first impeller 6, the cylindrical portion 63b is oriented in the forward direction and is disposed inside the large diameter portion 20d of the first partition wall portion 20. The cylindrical portion 63b is an example of the first cylindrical portion according to the present invention. In the radial direction, the large diameter portion 20d faces the cylindrical portion 63b. A cylindrical gap (hereinafter referred to as a “first cylindrical gap S13”) is defined between the large diameter portion 20d and the cylindrical portion 63b. In the radial direction, the length of the first cylindrical gap S13 (i.e., the interval between the large diameter portion 20d and the cylindrical portion 63b) is set to a length that prevents the cylindrical portion 63b from coming into contact with the large diameter portion 20d during normal operation of the centrifugal pump 1 and is equivalent to the length commonly set for an impeller of a general centrifugal pump. In the axial direction, the step portion 20e faces the cylindrical portion 63b. A ring-shaped plate-like gap (hereinafter referred to as a “first ring gap S14”) is defined between the step portion 20e and the cylindrical portion 63b. In the axial direction, the length of the first ring gap S14 (i.e., the interval between the step portion 20e and the cylindrical portion 63b) is set to a length that prevents the cylindrical portion 63b from coming into contact with the step portion 20e during normal operation of the centrifugal pump 1 and is equivalent to the length set for an impeller of a general centrifugal pump. The first cylindrical gap S13 and the first ring gap S14 constitutes a first fixed orifice to be described later. The large diameter portion 20d and the step portion 20e are examples of the portion that faces the first cylindrical portion according to the present invention.

The first front-side space S12 communicates with both a space located radially outward of the first impeller 6 and the first cylindrical gap S13. The first ring gap S14 communicates with both the first cylindrical gap S13 and a space inside the small diameter portion 20c.

The inner edge portion of the first rear shroud 62 extends rearward in a cylindrical shape in such a way as to be coaxial with the rotary shaft 4, thereby forming the first hub portion 64. The front portion 4a of the rotary shaft 4 is disposed through the first hub portion 64, and the first hub portion 64 is fixed to the front portion 4a of the rotary shaft 4, thereby mounting the first impeller 6 to the front portion 4a. In this state, the first suction port 65 is oriented in the forward direction. The first hub portion 64 covers a portion of the front portion 4a of the rotary shaft 4 and is disposed inside the central through hole 21c.

The outer edge portion of the first vane 61, the outer edge portion of the first rear shroud 62, and the outer edge portion of the first front shroud 63 form the first discharge port 66 from which the pumped liquid flowing through a flow path inside the first impeller 6 is discharged.

The second impeller 7 sucks in and discharges the pumped liquid discharged from the first impeller 6. The second impeller 7 is mounted to the front portion 4a of the rotary shaft 4 and is accommodated in the second pump chamber 24. That is, the second impeller 7 is disposed between the central partition wall portion 21 and the second partition wall portion 22. The second impeller 7 is a so-called closed-type impeller. The second impeller 7 includes a plurality of second vanes 71, a second rear shroud 72, a second front shroud 73, a second hub portion 74, a second suction port 75, and a second discharge port 76.

The specific speed set for a second pump unit P2 configured with the second impeller 7 and the second pump chamber 24 is lower than that set for a first pump unit P1 configured with the first impeller 6 and the first pump chamber 23. According to this configuration, higher suction performance (i.e., a higher discharged flow rate) is achieved by the first pump unit P1, and high head is achieved by the second pump unit P2, in the centrifugal pump 1. Further, the outer diameter (the diameter) of an impeller generally decreases as the specific speed increases. Thus, the outer diameter (the diameter) of the second impeller 7 is larger than the outer diameter (the diameter) of the first impeller 6. The values of the specific speeds are appropriately set based on, for example, the design of the centrifugal pump 1 (e.g., discharged flow rate, head).

The second vanes 71 rotate in the circumferential direction about the rotary shaft 4 as the center of rotation and guide the pumped liquid sucked in through the second suction port 75 to the second discharge port 76. When viewed in the axial direction, each of the plurality of second vanes 71 extends radially from the center side of the second impeller 7 toward the outer edge side and is curved in a spiral shape. The second vanes 71 are disposed between the second rear shroud 72 and the second front shroud 73.

The second rear shroud 72 is a plate (a so-called main plate) that covers the front of the second vanes 71 (i.e., the rear side of the second impeller 7). The shape of the second rear shroud 72 is a ring-shaped plate. When viewed in the axial direction, the central portion of the second rear shroud 72 protrudes rearward in a frustoconical shape. The second rear shroud 72 faces the central partition wall portion 21. The second rear shroud 72 includes an attachment hole 72a, a front surface 72b, and the convex portion 72c. The second rear shroud 72 is an example of the second shroud according to the present invention.

The outer diameter (i.e., the diameter) of the second rear shroud 72 is larger than the outer diameter (i.e., the diameter) of the first rear shroud 62. The outer diameter of the first rear shroud 62 and the outer diameter of the second rear shroud 72 are set, for example, in such a way that the first axial thrust force and the second axial thrust force that are to be described later are approximately balanced when the first impeller 6 and the second impeller 7 (i.e., the centrifugal pump 1) discharge the pumped liquid at the point of maximum efficiency.

The attachment hole 72a is a through hole through which the front portion 4a of the rotary shaft 4 is disposed. When viewed in the axial direction, the attachment hole 72a is disposed in the central portion of the second rear shroud 72 and extends cylindrically through the central portion along the axial direction.

The front surface 72b is a surface that is oriented in the forward direction and that faces the rear surface 21b of the central partition wall portion 21. A ring-shaped plate-like space (hereinafter referred to as the “second rear-side space S21”) is defined between the front surface 72b and the rear surface 21b. The second rear-side space S21 is an example of the second space according to the present invention.

FIG. 4 is a schematic front elevation view of the second impeller 7. The figure schematically illustrates a state of the second impeller 7 as viewed from the forward direction. In the following description, FIG. 2 is also referred to as appropriate.

The convex portion 72c functions as a variable orifice to be described later. A portion of the front surface 72b of the second rear shroud 72 protrudes forward in a ring plate shape in such a way as to be coaxial with the rotary shaft 4, thereby forming the convex portion 72c. That is, when viewed in the axial direction, the convex portion 72c has a ring plate shape that is concentric with the rotary shaft 4. In the radial direction, the convex portion 72c is disposed close to the inner side (i.e., close to the inner edge portion) of the second rear shroud 72. The front surface 72d of the convex portion 72c has a planar shape that is parallel to the rear surface 21b of the central partition wall portion 21. In the axial direction, the interval between the convex portion 72c and the rear surface 21b of the central partition wall portion 21 is narrower than the interval between the front surface 72b excluding the convex portion 72c and the rear surface 21b of the central partition wall portion 21. The convex portion 72c is an example of the protruding portion (i.e., a variable orifice) according to the present invention.

The following description mainly refers back to FIGS. 1 and 2. The second front shroud 73 is a plate (a so-called side plate) that covers the rear of the second vanes 71 (i.e., the front side of the second impeller 7). The shape of the second front shroud 73 is a substantially ring-shaped plate in which the inner edge portion is more convex rearward than the outer edge portion. 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 rearward 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 the fourth shroud according to the present invention.

The rear surface 73a is a surface that is oriented in the rearward direction and faces the inner surface 22a of the second partition wall portion 22. A substantially ring-shaped plat-like space (hereinafter referred to as a “second front-side space S22”) is defined between the rear surface 73a and the inner surface 22a.

The inner edge portion of the second front shroud 73 extends cylindrically in the rearward direction in such a way as to be coaxial with the rotary shaft 4 and forms the cylindrical portion 73b that functions as the second suction port 75. In other words, the second front shroud 73 includes the cylindrical portion 73b that functions as the second suction port 75. In the second impeller 7, the cylindrical portion 73b faces rearward and is disposed inside the large diameter portion 22d of the second partition wall portion 22. The cylindrical portion 73b is an example of the second cylindrical portion according to the present invention. In the radial direction, the cylindrical portion 73b faces the large diameter portion 22d. A cylindrical gap (hereinafter referred to as a “second cylindrical gap S23”) is defined between the large diameter portion 22d and the cylindrical portion 73b. In the radial direction, the length of the second cylindrical gap S23 (i.e., the interval between the large diameter portion 22d and the cylindrical portion 73b) is set to a length that prevents the cylindrical portion 73b from coming into contact with the large diameter portion 22d during normal operation of the centrifugal pump 1 and is longer than the length set for an 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 plate-like gap (hereinafter referred to as a “second ring gap S24”) is defined between the first step portion 22e and the cylindrical portion 73b. In the axial direction, the length of the second ring gap S24 (i.e., the length between the first step portion 22e and the cylindrical portion 73b) is set to a length that prevents the cylindrical portion 73b from coming into contact with the first step portion 22e during normal operation of the centrifugal pump 1 and is longer than the length set for an 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-side space S22 communicates with both a space located radially outward of the second impeller 7 and the second cylindrical gap S23. The second ring gap S24 communicates with both the second front-side space S22 and the space inside the small diameter portion 22c.

The inner edge portion of the second rear shroud 72 extends forward in a cylindrical shape in such a way as to be coaxial with the rotary shaft 4, thereby forming the second hub portion 74. The front portion 4a of the rotary shaft 4 is disposed through the second hub portion 74, and the second hub portion 74 is fixed to the front portion 4a of the rotary shaft 4, thereby mounting the second impeller 7 to the front portion 4a. In this state, the second suction port 75 faces rearward. The second hub portion 74 covers a portion of the front portion 4a of the rotary shaft 4 and is disposed inside the central through hole 21c. The second hub portion 74 abuts the first hub portion 64, and a cylindrical space (hereinafter referred to as a “cylindrical space S3”) is defined between the central through hole 21c and the first and second hub portions 64 and 74. The cylindrical space S3 communicates with both 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 communicate with each other through the cylindrical space S3 (i.e., the central through hole 21c).

In this way, the first impeller 6 and the second impeller 7 are mounted to one rotary shaft 4 in a back-to-back state in such a way as to sandwich the central partition wall portion 21, thereby forming a two-stage centrifugal pump. The first pump chamber 23 and the second pump chamber 24 are divided by the central partition wall portion 21 and communicate with each other through the cylindrical space S3. That is, no shaft seal structure based on a mechanical seal or the like is provided between the first pump chamber 23 and the second pump chamber 24.

Operation of Centrifugal Pump

Next, the operation of the centrifugal pump 1 will be described. In the following description, FIGS. 1 and 2 are referred to as appropriate.

When the centrifugal pump 1 starts operating, the motor 3 drives the rotary shaft 4, causing the rotary shaft 4 to rotate. The first impeller 6 sucks in the pumped liquid introduced to the small diameter portion 20c from the front and discharges the pumped liquid into the first pump chamber 23. The pumped liquid discharged into the first pump chamber 23 is delivered to the space inside the small diameter portion 22c through the connecting flow path 25. At this time, a portion of the pumped liquid discharged into the first pump chamber 23 flows into the first rear-side space S11 and flows into the second pump chamber 24 (i.e., the second rear-side space S21) through the cylindrical space S3. Further, another portion of the pumped liquid discharged into the first pump chamber 23 flows into (i.e., returns to) the space inside the small diameter portion 20c through the first front-side space S12, the first cylindrical gap S13, and the first ring gap S14. The returning flow of the pumped liquid adversely affects the suction performance of the first impeller 6. Thus, the returning flow is primarily restricted by the first cylindrical gap S13 and the first ring gap S14. That is, the first cylindrical gap S13 and the first ring gap S14 function as restrictors (i.e., a first fixed orifice) for the returning flow. The first fixed orifice (i.e., the first cylindrical gap S13 and the first ring gap S14) is an example of the first gap according to the present invention.

The second impeller 7 sucks in the pumped liquid delivered to the space inside the small diameter portion 22c from the rear and discharges the pumped liquid into the second pump chamber 24. The pumped liquid discharged into the second pump chamber 24 is discharged to discharge piping (unillustrated) through the discharge flow path 26 and the discharge pipe 28. At this time, a portion of the pumped liquid discharged into the second pump chamber 24 flows into the second rear-side space S21. Herein, the pressure of the pumped liquid inside the second pump chamber 24 is higher than the pressure of the pumped liquid inside the first pump chamber 23. Thus, the pumped liquid flowing into the second rear-side space S21 flows into the first pump chamber 23 (i.e., the first rear-side space S11) through the cylindrical space S3 in such a way as to push back the pumped liquid that has flowed in from the first pump chamber 23. In this way, a flow of the pumped liquid returning to the first pump chamber 23 from the second pump chamber 24 (hereinafter referred to as a “return flow”) occurs in the centrifugal pump 1. The second rear-side space S21 and the cylindrical space S3 constitute a flow path (hereinafter referred to as a “return flow path RL”) through which a portion of the pumped liquid inside the second pump chamber 24 flows to the first rear-side space S11 (i.e., the first pump chamber 23). Further, another portion of the pumped liquid discharged into the second pump chamber 24 flows into (i.e., returns to) the space inside the small diameter portion 22c through the second front-side space S22, the second cylindrical gap S23, and the second ring gap S24. The returning flow of the pumped liquid may adversely affect the suction performance of the second impeller 7. Thus, the returning flow is primarily 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 restrictors (i.e., a second fixed orifice) for the returning flow. The second fixed orifice is larger than the first fixed orifice. Accordingly, the degree of restriction on the flow rate by the second fixed orifice is less than the degree of restriction on the flow rate by the first fixed orifice. In other words, the flow rate of the pumped liquid that flows through the second fixed orifice is higher than the flow rate of the pumped liquid that flows through the first fixed orifice. The second fixed orifice (i.e., the second cylindrical gap S23 and the second ring gap S24) is an example of the second gap according to the present invention.

As described later, in the centrifugal pump 1 in which the pumped liquid flows in this way, an axial thrust force different from that in a conventional two-stage centrifugal pump (hereinafter referred to as a “conventional pump”) acts. It is assumed herein that the configuration of the “conventional pump” is in common with the configuration of the centrifugal pump 1 except that the outer diameter of the first impeller (i.e., the outer diameter of the first rear shroud) is equal to the outer diameter of the second impeller (i.e., the outer diameter of the second rear shroud) and the cylindrical space is shaft-sealed by a mechanical seal. That is, the conventional pump is a two-stage centrifugal pump in which the first impeller and the second impeller are mounted to one rotary shaft in a back-to-back state. In order to distinguish between the centrifugal pump 1 and the conventional pump, components in the conventional pump corresponding to those in the centrifugal pump 1 are denoted by the same reference signs with the suffix sign “z” in the following description.

The “axial thrust force” is a force acting on the rotary shaft 4 in such a way as to move the rotary shaft 4, the first impeller 6, and the second impeller 7 in the axial direction. The axial thrust force includes a first axial thrust force that acts in such a way as to move the rotary shaft 4 forward and a second axial thrust force that acts in such a way as to move the rotary shaft 4 rearward. The axial thrust force is mainly generated by the pressure balance between the front and rear sides of the first impeller 6 and the second impeller 7 in the centrifugal pump 1.

Axial Thrust Forces in Conventional Pump

Herein, prior to description of axial thrust forces in the centrifugal pump 1, axial thrust forces in a conventional pump 1z and the conventional pump 1z not including a mechanical seal Mz will be described below.

FIG. 5A is an enlarged partial schematic sectional view of the conventional pump 1z illustrating axial thrust forces in the conventional pump 1z, and FIG. 5B is an enlarged partial schematic sectional view of the conventional pump 1z without the mechanical seal Mz illustrating axial thrust forces in the conventional pump 1z without the mechanical seal Mz. In the figures, only a central partition wall portion 21z, a rotary shaft 4z, a first impeller 6z, a second impeller 7z, and the mechanical seal Mz are illustrated in a simplified manner for convenience of description. A return flow is indicated by an outlined arrow in FIG. 5B.

In the following description, a pressure “P_f3” indicates the pressure applied to a first front shroud 63z of the first impeller 6z by the pumped liquid flowing through a first front-side space S12z, a pressure “P_r3” indicates the pressure applied to a first rear shroud 62z of the first impeller 6z by the pumped liquid flowing through a first rear-side space S11z, a pressure “P_f4” indicates the pressure applied to a second front shroud 73z of the second impeller 7z by the pumped liquid flowing through a second front-side space S22z, and a pressure “P_r4” indicates the pressure applied to a second rear shroud 72z of the second impeller 7z by the pumped liquid flowing through a second rear-side space S21z. A force “F_f3” based on the pressure “P_f3” acts rearward on the first impeller 6z, and a force “F_r3” based on the pressure “P_f3” acts forward on the first impeller 6z. Since the pressure “P_f3” is lower than the pressure “P_r3,” a force (i.e., a 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” based on the pressure “P_f4” acts forward on the second impeller 7z, and a force “F_r4” based on the pressure “P_r4” acts rearward on the second impeller 7z. Since the pressure “P_f4” is lower than the pressure “P_r4”, a force (i.e., a second axial thrust force) equivalent to the difference between the two forces (F_r4−F_f4) acts rearward on the second impeller 7z. In FIGS. 5A and 5B, the length of a thin arrow illustrates the magnitude of each of the pressures “P_f3”, “P_r3”, “P_f4”, and “P_r4”. The length of a thick arrow illustrates the magnitude of each of the force “F_f3”, “F_r3”, “F_f4”, and “F_r4”.

As illustrated in FIG. 5A, the configuration of the first impeller 6z is in common with the configuration of the second impeller 7z except for the direction of rotation in the conventional pump 1z. Specifically, 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. The mechanical seal Mz is mounted to a cylindrical space S3z, and a first pump chamber 23z does not communicate with a second pump chamber 24z. The pressure receiving area (i.e., the liquid contact area) of the first front shroud 63z is smaller than the pressure receiving area of the first rear shroud 62z and is equal to the pressure receiving area of the second front shroud 73z. Further, the pressure receiving area of the first rear shroud 62z is equal to 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 is greater than the force “F_f3” acting on the first front shroud 63z and is equal to the force “F_r4” acting on the second rear shroud 72z. The force “F_f3” acting on the first front shroud 63z is equal to the force “F_f4” acting on the second front shroud 73z. Accordingly, the force (i.e., the first axial thrust force) equivalent to the difference between the two forces (F_r3−F_f3) acts forward on the first impeller 6z. In contrast, the force (i.e., the second axial thrust force) equivalent to the difference between the two forces (F_r4−F_f4) 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 (i.e., the axial thrust forces are balanced). Note that in practice, the first axial thrust force acts on the rotary shaft 4z due to the pressure difference between the pumped liquid inside the first pump chamber 23z and the pumped liquid inside the second pump chamber 24z. However, the first axial thrust force is normally absorbed by a bearing. Thus, a problematic axial thrust force does not act on the rotary shaft 4z in the conventional pump 1z.

Next, in the conventional pump 1z without the mechanical seal Mz, the first pump chamber 23z communicates with the second pump chamber 24z through the cylindrical space S3z, as illustrated in FIG. 5B. In this configuration, since the pressure of the pumped liquid inside the second pump chamber 24z is higher than the pressure of the pumped liquid inside the first pump chamber 23z, a flow of the pumped liquid inside the second pump chamber 24z flowing into the first pump chamber 23z through the cylindrical space S3z (i.e., a return flow) occurs. At this time, a flow flowing from the outer edge side toward the inner edge side of the second rear shroud 72z while swirling in the direction of rotation of the second impeller 7z is generated in the second rear-side space S21z. Thus, as indicated in FIG. 5B by a broken line and a solid line, the pressure “P_r4” of the pumped liquid that flows inside the second rear-side space S21z decreases. In contrast, a flow flowing from the inner edge side toward the outer edge side of the first rear shroud 62z while swirling in the direction of rotation of the first impeller 6z is generated in the first rear-side space S11z. Thus, as indicated in FIG. 5B by a broken line and a solid line, the pressure “P_r3” of the pumped liquid that flows inside the first rear-side space S11z increases. Accordingly, the force “F_r3” acting on the first rear shroud 62z increases, and the force “F_r4” acting on the second rear shroud 72z decreases. As a result, the first axial thrust force acting on the rotary shaft 4z, the first impeller 6z, and the second impeller 7z becomes greater than the second axial thrust force, and the first axial thrust force acts on the rotary shaft 4z. Thus, when the first pump chamber 23z communicates with the second pump chamber 24z through the cylindrical space S3z, the balance between the axial thrust forces in the conventional pump 1z is lost, and an axial thrust force (i.e., the first axial thrust force) acts on the rotary shaft 4z.

The axial thrust forces increase as the rotation rate of a motor 3 increases. Thus, for example, when a pumped liquid requiring high-speed rotation (for example, a low-viscosity pumped liquid) is delivered, the axial thrust force cannot be absorbed by the bearing, and a mechanism for cancelling the axial thrust force (for example, a balancing piston, or a balancing sheet) is required. In this case, not only the size of the entire conventional pump 1z increases but the length of the rotary shaft 4z also increases. As a result, high-speed rotation becomes more difficult to achieve. Further, when the pumped liquid is used for lubrication of the bearing, particularly in the case of low-viscosity liquid, even a slight axial thrust force causes surface pressure to be generated on the sliding surface of the bearing. As a result, the temperature of the sliding surface rises, and the lifespan of the bearing decreases. Accordingly, a structure that minimizes the generation of an axial thrust force (ideally, a structure in which the axial thrust force becomes “zero”) is required.

Axial Thrust Forces in Centrifugal Pump

Next, axial thrust forces in the centrifugal pump 1 will be described below. In the following description,, FIGS. 1 to 5 are also referred to as appropriate.

FIG. 6 is an enlarged partial schematic sectional view of the centrifugal pump 1 illustrating an effect of the second impeller 7 on axial thrust forces in the centrifugal pump 1. In the figure, only the central partition wall portion 21, the rotary shaft 4, the first impeller 6, and the second impeller 7 are illustrated in a simplified manner, and illustration of the convex portion 72c is omitted, for convenience of description. An outlined arrow in the figure indicates a return flow. Further, the second impeller 7 is illustrated in the figure in such a way 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 equal to the outer diameter of the first front shroud 63 for ease of understanding of forces that act on the second impeller 7.

In the following description, a pressure “P_f1” indicates the pressure applied to the first front shroud 63 of the first impeller 6 by the pumped liquid flowing through the first front-side space S12, a pressure “P_r1” indicates the pressure applied to the first rear shroud 62 of the first impeller 6 by the pumped liquid flowing through the first rear-side space S11, a pressure “P_f2” indicates the pressure applied to the second front shroud 73 of the second impeller 7 by the pumped liquid flowing through the second front-side space S22, and a pressure “P_r2” indicates the pressure applied to the second rear shroud 72 of the second impeller 7 by the pumped liquid flowing through the second rear-side space S21. A force “F_f1” based on the pressure “P_f1” acts rearward on the first impeller 6, and a force “F_r1” based on the pressure “P_r1” acts forward on the first impeller 6. Since the pressure “P_f1” is lower than the pressure “P_r1”, a force (i.e., a 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_r2” based on the pressure “P_f2” acts forward on the second impeller 7, and a force “F_r2” based on the pressure “P_r2” acts rearward on the second impeller 7. A force (i.e., a second axial thrust force) equivalent to the difference between the two forces (F_r2−F_f2) acts rearward on the second impeller 7. In the figure, the lengths of thin arrows illustrate the respective magnitudes of the pressures “P_f1”, “P_r1”, “P_f2”, and “P_r2”. The lengths of thick arrows illustrate the respective magnitudes of the forces “F_f1”, “F_r1”, “F_f2”, and “F_r2”.

As described above, the outer diameter of the second rear shroud 72 is larger than the outer diameter of the first rear shroud 62. Thus, as illustrated 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 “F_r2” acting on the second impeller 7 increases more than the force “F_r4” acting on the second impeller 7z in the conventional pump 1z. Accordingly, by designing the outer diameter of the second rear shroud 72 (that is, the outer diameter of the second impeller 7) to be larger than the outer diameter of the first rear shroud 62 (that is, the outer diameter of the first impeller 6), the second axial thrust force (F_r2−F_r2) acting on the second impeller 7 mounted to the rotary shaft 4 becomes greater than the second axial thrust force (F_r4−F_f4) acting on the second impeller 7z mounted to the rotary shaft 4z, and the axial thrust force (i.e., the first axial thrust force) acting on the rotary shaft 4 becomes less (i.e., is reduced) than the axial thrust force (i.e., the first axial thrust force) acting on the rotary shaft 4z. As a result, the first axial thrust force (F_r1−F_f1) acting on the rotary shaft 4 and the second axial thrust force (F_r2−F_r2) acting on the rotary shaft 4 are (substantially) balanced.

FIG. 7 is an enlarged partial schematic sectional view of the centrifugal pump 1 illustrating an effect of the second fixed orifice (i.e., the second cylindrical gap S23 and the second ring gap S24) on axial thrust forces in the centrifugal pump 1. In the figure, only the first partition wall portion 20, the central partition wall portion 21, the second partition wall portion 22, the rotary shaft 4, the first impeller 6, and the second impeller 7 are illustrated in a simplified manner, and illustration of the convex portion 72c is omitted, for convenience of description. An outlined arrow in the figure indicates a return flow. Further, the second impeller 7 is illustrated in the figure in such a way that the outer diameter of the second impeller 7 is equal to the outer diameter of the first impeller 6, in order to illustrate only the effect of the second fixed orifice.

As described above, the length of the second cylindrical gap S23 is longer than the length of the first cylindrical gap S13 in the radial direction. The length of the second ring gap S24 is longer than the length of the first ring gap S14 in the axial direction. In this configuration, the flow rate of the pumped liquid that flows into the second front-side space S22 and is restricted by the second fixed orifice is lower than the flow rate of the pumped liquid that flows into the first front-side space S12 and is restricted by the first fixed orifice. That is, the flow rate of the pumped liquid that flows through the second fixed orifice (i.e., the flow rate of a flow indicated by a broken arrow in FIG. 7) is greater than the flow rate of the pumped liquid that flows through the first fixed orifice. Thus, the pressure “P_f2” applied to the second front shroud 73 by the pumped liquid becomes lower than the pressure when the flow rate of the pumped liquid passing through the second fixed orifice is equal to the flow rate of the pumped liquid passing through the first fixed orifice (indicated by a broken line in FIG. 7). As a result, the force acting on the second impeller 7 (i.e., the second axial thrust force) becomes greater than the force when the flow rate of the pumped liquid flowing through the second fixed orifice is equal to the flow rate of the pumped liquid flowing through the first fixed orifice. Accordingly, by designing the flow rate of the pumped liquid flowing through the second fixed orifice to be greater than the flow rate of the pumped liquid flowing through the first fixed orifice, the second axial thrust force acting on the second impeller 7 mounted to the rotary shaft 4 increases, and the axial thrust force acting on the rotary shaft 4 (i.e., the first axial thrust force) decreases (i.e., is reduced). Herein, normally, as the flow rate of the pumped liquid 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. In contrast, even when the flow rate of the pumped liquid flowing through the second fixed orifice increases, it has little effect on the suction performance of the second impeller 7. Thus, in this configuration, although a slight deterioration in the performance of the centrifugal pump 1 may occur, the improvement in the efficiency of the centrifugal pump 1 due to the reduction in the axial thrust force acting on the rotary shaft 4 outweighs the deterioration.

Next, as described above, a plurality of recessed portions 21d is disposed on the rear surface 21b of the central partition wall portion 21. The recessed portions 21d face the second rear-side space S21, and the flow of the pumped liquid inside the second rear-side space S21 is interfered by the recessed portions 21d. The flow of the pumped liquid inside the second rear-side space S21 becomes a swirl flow rotating in the same direction as the second impeller 7, due to the rotation of the second impeller 7. That is, the flow of the pumped liquid inside the second rear-side space S21 includes a large amount of the swirl component. A portion of the flow close to the central partition wall portion 21 in the flow of the pumped liquid inside the second rear-side space S21 flows into the recessed portion 21d and is blocked by the side wall of the recessed portion 21d. At this time, the swirl component of the flow flowing into the recessed portion 21d is reduced, and the direction of the flow changes toward the radially inner side and the axial direction. Thus, the flow interferes with the flow that does not flow into the recessed portion 21d, and the swirl component of the interfered flow is also slightly suppressed. As a result, the swirl component of the flow of the pumped liquid inside the second rear-side space S21 is reduced. Since the pressure “P_r2” of the pumped liquid decreases as the swirl component in the pumped liquid increases, the pressure “P_r2” of the pumped liquid increases as the swirl component of the pumped liquid is reduced. That is, As the swirl component of the pumped liquid inside the second rear-side space S21 is reduced, the force acting on the second impeller 7 (i.e., the second axial thrust force) increases, and the axial thrust force acting on the rotary shaft 4 (i.e., the first axial thrust force) decreases (i.e., is reduced).

In this way, the centrifugal pump 1 includes a configuration for reducing an axial thrust force (i.e., the first axial thrust force) by the outer diameter of the second rear shroud (hereinafter referred to as a “first reduction configuration”), a configuration for reducing an axial thrust force (i.e., the first axial thrust force) by the second fixed orifice (hereinafter referred to as a “second reduction configuration”), and a configuration for reducing an axial thrust force (i.e., the first axial thrust force) by the recessed portion 21d (hereinafter referred to as a “third reduction configuration”). The first to third reduction configurations are combined in the centrifugal pump 1 in such a way that the axial thrust forces acting on the rotary shaft 4 are balanced.

FIGS. 8A to 8C each are an enlarged partial schematic sectional view of the centrifugal pump 1 illustrating an effect of the convex portion 72c on axial thrust forces in the centrifugal pump 1, in which FIG. 8A illustrates a state in which the first axial thrust force and the second axial thrust force are balanced, FIG. 8B illustrates a state in which the first axial thrust force is greater than the second axial thrust force, and FIG. 8C illustrates a state in which the first axial thrust force is less than the second axial thrust force. In the figure, only the central partition wall portion 21, the rotary shaft 4, and the second impeller 7 are illustrated in a simplified manner for convenience of description.

As illustrated in FIG. 8A, in the state in which the first axial thrust force and the second axial thrust force are balanced, the rotary shaft 4, the first impeller 6, and the second impeller 7 are disposed at a predetermined position in the axial direction. In this state, a gap S4 with a spacing of “L1” is defined between the convex portion 72c and the rear surface 21b of the central partition wall portion 21 in the axial direction. The pumped liquid inside the second rear-side space S21 flows while swirling from the outer edge side of the second impeller 7 toward the inner edge side of the second impeller 7, and the flow rate of the flow is restricted at the gap S4.

Next, as illustrated in FIG. 8B, in the state in which the first axial thrust force is greater than the second axial thrust force, the rotary shaft 4, the first impeller 6, and the second impeller 7 move forward in the axial direction, and the width of the gap S4 is reduced to “L2”. In this state, the degree of restriction of the pumped liquid by the convex portion 72c increases. Thus, the pressure “P_r2” of the pumped liquid on the upstream side of the convex portion 72c (i.e., the outer side in the radial direction) in the second rear-side space S21 increases. Further, the flow rate of the pumped liquid flowing through the cylindrical space S3 decreases, and the pressure “P_r1” of the pumped liquid in the first rear-side space S11 decreases. As a result, the force “F_r2” acting on the second impeller 7 increases, the force “F_r1” acting on the first impeller 6 decreases, and the first axial thrust force and the second axial thrust force are eventually balanced. In this way, as the rotary shaft 4, the first impeller 6, and the second impeller 7 move forward, the convex portion 72c narrows a portion of the second rear-side space S21 (that is, a portion of the return flow path RL) and balances the axial thrust forces acting on the rotary shaft 4 (i.e., the first axial thrust force and the second axial thrust force).

In contrast, as illustrated in FIG. 8C, in the state in which the first axial thrust force is less than the second axial thrust force, the rotary shaft 4, the first impeller 6, and the second impeller 7 move rearward in the axial direction, and the width of the gap S4 increases to “L3”. In this state, the degree of restriction of the pumped liquid by the convex portion 72c decreases. Thus, the pressure “P_r2” of the pumped liquid on the upstream side of the convex portion 72c in the second front-side space S22 decreases. Further, the flow rate of the pumped liquid flowing through the cylindrical space S3 increases, and the pressure “F_r1” of the pumped liquid in the first rear-side space S11 increases. As a result, the force “F_r2” acting on the second impeller 7 decreases, the force “F_r1” acting on the first impeller 6 increases, and the first axial thrust force and the second axial thrust force are eventually balanced. In this way, as the rotary shaft 4, the first impeller 6, and the second impeller 7 move rearward, the convex portion 72c widens a portion of the second rear-side space S21, that is, a portion of the return flow path RL, and balances the axial thrust forces acting on the rotary shaft 4.

In this way, the convex portion 72c functions as a variable orifice that adjusts the flow rate of the pumped liquid inside the second front-side space S22 and, as a result, the axial thrust force acting on the rotary shaft 4, depending on the axial thrust force acting on the rotary shaft 4.

As described above, the second rear-side space S21 is a ring-shaped plate-like space. Thus, the sectional area of the second rear-side space S21 (i.e., the cross-sectional area of the flow path) in a section along the circumferential direction decreases as the section approaches the inner edge portion of the second rear shroud 72. In other words, as the section approaches the inner edge portion of the second rear shroud 72, the degree of restriction of the variable orifice relative to the amount of movement of the convex portion 72c in the axial direction increases. Accordingly, when the convex portion 72c is disposed close to the inner edge of the second rear shroud 72, the amount of change in the pressure “P_r2” of the pumped liquid on the upstream side of the convex portion 72c increases even when the amount of movement of the convex portion 72c in the axial direction is small.

In this way, the centrifugal pump 1 is designed to increase the force “F_r2” acting on the second impeller 7 by means of the first to third reduction configurations. Thus, by designing the first to third reduction configurations to completely balance the first axial thrust force and the second axial thrust force that act on the rotary shaft 4, an axial thrust force does not act on the rotary shaft 4 even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3 (i.e., the return flow path RL). According to the present embodiment, the balance between the first axial thrust force and the second axial thrust force that act on the rotary shaft 4 is roughly adjusted (i.e., coarse-adjusted) by first reduction configuration and is accessorily adjusted (i.e., fine-adjusted) by the second reduction configuration and the third reduction configuration. Further, even when the axial thrust force acts on the rotary shaft 4, the axial thrust force is reduced to a slight force by the first to third reduction configurations, and thus, the axial thrust force is easily and automatically adjusted by the variable orifice.

CONCLUSION

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 rotary shaft 4, the first impeller 6, and the second impeller 7. The central partition wall portion 21 includes the central through hole 21c through which the rotary shaft 4 is disposed. The first pump chamber 23 is divided from the second pump chamber 24 by the central partition wall portion 21 and is disposed forward of and side by side with the second pump chamber 24. The first suction port 65 of the first impeller 6 is oriented in the forward direction and the second suction port 75 of the second impeller 7 is oriented in the rearward direction. The first rear-side space S11 is defined between the first impeller 6 and the central partition wall portion 21, the second rear-side space S21 is defined 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 through 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 force “F_r2” acting on the second impeller 7 increases by increase in the pressure receiving area of the second rear shroud 72. Thus, the axial thrust force acting on the rotary shaft 4 is reduced even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3. Accordingly, the outer diameter of the second rear shroud 72 is adjusted in such a way that the axial thrust forces acting on the rotary shaft 4 are balanced, thereby allowing the axial thrust force acting on the rotary shaft 4 to be reduced to a minimum.

According to the embodiment described above, a specific speed set for the first pump unit P1 configured with the first impeller 6 and the first pump chamber 23 is higher than a specific speed set for the second pump unit P2 configured with the second impeller 7 and the second pump chamber 24. In general, as a specific speed increases, the opening on the suction side between impeller vanes increases, and the outer diameter of the impeller decreases. Thus, according to this configuration, the suction performance (i.e., the discharged flow rate) of the first pump unit P1 is improved, and the head generated by the second pump unit P2 is improved, thereby enhancing the performance of the centrifugal pump 1. Further, the outer diameter of the second rear shroud 72 becomes larger than the outer diameter of the first rear shroud 62, as a matter of course. As a result, the axial thrust force acting on the rotary shaft 4 is reduced even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3.

According to the embodiment described above, the centrifugal pump 1 includes the first partition wall portion 20 and the second partition wall portion 22. The first impeller 6 includes the first front shroud 63 and the cylindrical portion 63b that functions as the first suction port 65. The second impeller 7 includes the second front shroud 73 and the cylindrical portion 73b that functions as the second suction port 75. The large diameter portion 20d and the step portion 20e in the first partition wall portion 20 face the cylindrical portion 63b. The large diameter portion 22d and the first step portion 22e in the second partition wall portion 22 face the cylindrical portion 73b. The sizes (i.e., lengths) of the second cylindrical gap S23 and the second ring gap S24 (i.e., the second fixed orifice) are greater (i.e., longer) than the sizes (i.e., lengths) of the first cylindrical gap S13 and the first ring gap S14 (i.e., the first fixed orifice). According to this configuration, the pressure “P_f2” applied to the second front shroud 73 by the pumped liquid is lower than the pressure “P_f2” when the flow rate of the pumped liquid passing through the second fixed orifice is equal to the flow rate of the pumped liquid passing through the first fixed orifice. As a result, the force acting on the second impeller 7 (i.e., the second axial thrust force) is greater than the force (i.e., the second axial thrust force) when the flow rate of the pumped liquid passing through the second fixed orifice is equal to the flow rate of the pumped liquid passing through the first fixed orifice. Thus, the axial thrust force acting on the rotary shaft 4 is further reduced even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3.

According to the embodiment described above, the central partition wall portion 21 includes eight recessed portions 21d. The recessed portions 21d face the second rear-side space S21 and are configured in such a way as to reduce the swirl component of the pumped liquid flowing through the second rear-side space S21. According to this configuration, the pressure “P_r2” of the pumped liquid increases, the second axial thrust force acting on the second impeller 7 mounting to the rotary shaft 4 increases, and the axial thrust force acting on the rotary shaft 4 (i.e., the first axial thrust force) is further reduced.

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, in such a way that the first axial thrust force and the second axial thrust force that act on the rotary shaft 4 balance each other when the first impeller 6 and the second impeller 7 discharge the pumped liquid. According to this configuration, the axial thrust force acting on the rotary shaft 4 is reduced to a minimum.

According to the embodiment described above, the second rear shroud 72 of the second impeller 7 includes the convex portion 72c. The convex portion 72c functions as the variable orifice narrowing a portion of the second rear-side space S21 (i.e., a portion of the return flow path RL) as the rotary shaft 4 moves forward and widens the portion as the rotary shaft 4 moves rearward. According to this configuration, even when an axial thrust force acts on the rotary shaft 4, the axial thrust force is reduced to a slight force by the first to third reduction configurations, and thus, the axial thrust force is easily and automatically adjusted by the variable orifice.

According to the embodiment described above, the variable orifice is configured by the convex portion 72c that protrudes forward in a ring shape from the second rear shroud 72. According to this configuration, the variable orifice is made formable by a simple configuration in which a portion of the second rear shroud 72 protrudes forward.

According to the embodiment described above, the convex portion 72c is disposed close to the inner side of the second rear shroud 72 in the radial direction. The sectional area of the second rear-side space S21 in a section along the circumferential direction decreases as the section approaches the inner edge portion of the second rear shroud 72. Thus, according to this configuration, the amount of change in the pressure “P_r2” of the pumped liquid on the upstream side of the convex portion 72c increases even when the amount of movement of the convex portion 72c in the axial direction is small. In other words, the sensitivity to pressure variation provided by the variable orifice improves.

Modification Examples

Next, modification examples of the centrifugal pump 1 will be described below focusing on points different from the embodiment described above (hereinafter referred to as a “first embodiment”). In the following modification example, the same members and the members with a common function as in the first embodiment are indicated with the same reference signs as in the first embodiment for convenience of description. In the following modification examples, FIGS. 1 to 8 are referred to as appropriate.

First Modification Example

FIG. 9 is an enlarged partial schematic sectional view of a centrifugal pump 1A according to a first modification example. In the figure, only the central partition wall portion 21, the rotary shaft 4, the first impeller 6, and the second impeller 7 are illustrated in a simplified manner, and illustration of the convex portion 72c is omitted, for convenience of description. An outlined arrow in the figure indicates a return flow.

In the centrifugal pump 1A according to the first modification example, the shape of the second impeller 7 differs from that in the first embodiment. Specifically, the outer diameter of the second rear shroud 72 is larger than the outer diameter of the first rear shroud 62, whereas the outer diameter of the second front shroud 73 of the second impeller 7 is equal to the outer diameter of the first front shroud 63. In this configuration, only the pressure receiving area of the second rear shroud 72 is larger than the pressure receiving area of the first rear shroud 62. Thus, the degree to which the force “F_r2” acting on the second impeller 7 increases in response to the amount of enlargement of the outer diameter of the second rear shroud 72 is greater than that in the first embodiment. Accordingly, in the present modification example, even when the amount of enlargement of the outer diameter of the second rear shroud 72 is smaller than that in the first embodiment, a reduction effect on the axial thrust force, similar to that in the first embodiment, can be obtained. Further, compared with a configuration in which only the outer diameter of the second rear shroud 72 is increased as illustrated in FIG. 6, the size of the second vane 71 is also increased in the circumferential direction in this configuration. As a result, the pressure applied to the pumped liquid by the second impeller 7 in the first modification example is higher than the pressure applied to the pumped liquid by the second impeller 7 illustrated in FIG. 6. Accordingly, the force (i.e., the discharge pressure) of the second impeller 7 in the first modification example is greater than the force of the second impeller 7 illustrated in FIG. 6.

Second to Fourth Modification Examples

FIG. 10A is an enlarged partial schematic sectional view of a centrifugal pump 1B according to a second modification example, FIG. 10B is an enlarged partial schematic sectional view of a centrifugal pump 1C according to a third modification example, and FIG. 10C is an enlarged partial schematic sectional view of a centrifugal pump 1D according to a fourth modification example.

The configuration of the variable orifice in each of the centrifugal pumps 1B to 1D according to the second to fourth modification examples differs from that in the first embodiment. Specifically, the second rear shroud 72 does not include the convex portion 72c, and the central partition wall portion 21 includes a convex portion 21e in the centrifugal pump 1B according to the second modification example. The convex portion 21e is disposed close to the inner edge of the rear surface 21b of the central partition wall portion 21 (i.e., close to the central through hole 21c). The convex portion 21e is an example of the variable orifice according to the present invention. The central partition wall portion 21 includes the convex portion 21e, and the second rear shroud 72 also includes the convex portion 72c in the centrifugal pump 1C according to the third modification example. The convex portion 21e and the convex portion 72c are disposed at 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 according to the first embodiment. The convex portions 21e and 72c are examples of the variable orifice according to the present invention. In the centrifugal pump 1D according to the fourth modification example, the second rear shroud 72 does not include the convex portion 72c, and an enlarged-diameter portion 21f and a step portion 21g are formed by enlarging the diameter of the rear end of the central through hole 21c Further, an annular member R is mounted to the rotary 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 the variable orifice according to the present invention. In these configurations, similarly to the first embodiment, the degree of restriction in a portion of the return flow path RL changes according to movement of the rotary shaft 4 in the forward-rearward direction. Thus, each of these configurations functions as the variable orifice similar to that in the first embodiment.

Fifth Modification Example

FIG. 11 is an enlarged partial schematic sectional view of a centrifugal pump 1E according to a fifth modification example.

The centrifugal pump 1E according to the fifth modification example differs from the first embodiment in the number of impellers. Specifically, the centrifugal pump 1E includes a housing 2E, the motor 3, the rotary shaft 4, the bearings 51 and 52, the first impeller 6, the second impeller 7, a third impeller 8, and a fourth impeller 9. That is, the centrifugal pump 1E is a four-stage centrifugal pump including four impellers (i.e., the first impeller 6 to the fourth impeller 9).

The housing 2E accommodates the motor 3, the rotary shaft 4, the bearings 51 and 52, and the first impeller 6 to the fourth impeller 9. The housing 2E includes the first partition wall portion 20, the central partition wall portion 21, the second partition wall portion 22, the first pump chamber 23, the second pump chamber 24, the connecting flow path 25, the discharge flow path 26, the suction pipe 27, the discharge pipe 28, the motor chamber 29, a front partition wall portion 2a, a rear partition wall portion 2b, a third pump chamber 2c, and a fourth pump chamber 2d. The front partition wall portion 2a is disposed forward of the first partition wall portion 20 and defines, together with the first partition wall portion 20, the third pump chamber 2c accommodating the third impeller 8. The rear partition wall portion 2b is disposed rearward of the second partition wall portion 22 and defines, together with the second partition wall portion 22, the fourth pump chamber 2d accommodating the fourth impeller 9. The third pump chamber 2c is disposed forward of and side by side with the first pump chamber 23, and the fourth pump chamber 2d is disposed rearward of and side by side with the second pump chamber 24. The connecting flow path 25 is a flow path that guides the pumped liquid discharged by the first impeller 6 to the fourth impeller 9. The suction pipe 27 is formed on the front partition wall portion 2a instead of the first partition wall portion 20.

The configuration of the third impeller 8 is in common with the configuration of the first impeller 6. The configuration of the fourth impeller 9 is in common with the configuration 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 equal to each other and are smaller than the outer diameter of the second impeller 7. The third impeller 8 is mounted to the front portion 4a of the rotary shaft 4 and forward of the first impeller 6. The fourth impeller 9 is mounted to the front portion 4a of the rotary shaft 4 and rearward of the second impeller 7. The first impeller 6 and the second impeller 7 are disposed in a back-to-back state in such a way as to sandwich the central partition wall portion 2, similarly to the first embodiment. The third impeller 8 and the first impeller 6 constitute a previous-stage group that sucks in the pumped liquid from the front, and the fourth impeller 9 and the second impeller 7 constitute a subsequent-stage group that sucks in the pumped liquid from the rear. The pumped liquid flows in order of the suction pipe 27, the third impeller 8, the first impeller 6, the connecting flow path 25, the fourth impeller 9, the second impeller 7, the discharge flow path 26, and the discharge pipe 28 in the centrifugal pump 1E. In other words, the second impeller 7 sucks in and discharges, through the connecting flow path 25 and the fourth impeller 9, the pumped liquid discharged from the first impeller 6.

In this configuration, the difference between the pressure of the pumped liquid inside the second pump chamber 24 and the pressure of the pumped liquid inside the first pump chamber 23 is approximately twice the difference in the first embodiment. Thus, the flow rate of the pumped liquid that flowing through the return flow path RL increases compares to that in the first embodiment. Accordingly, the axial thrust force acting on the rotary shaft 4 (i.e., the first axial thrust force) based on the pumped liquid flowing through the return flow path RL becomes greater than the axial thrust force in the first embodiment. Even in such a case, the first to third reduction configurations and the variable orifice according to the present invention are capable of reducing and adjusting the axial thrust force.

Note that, in this modification example, the configuration of the fourth impeller 9 may be the same as the configuration of the second impeller 7.

Other Embodiments

Note that, according to the embodiment described above, the centrifugal pump 1 includes configurations (i.e., the second and third reduction configurations) other than the first reduction configuration for increasing the force “Fr2” acting on the second impeller 7. Instead, the centrifugal pump 1 need not include a portion or all of the other configurations. Specifically, for example, the size of the second fixed orifice (i.e., the flow rate through the second fixed orifice) may be equal to that of the first fixed orifice. Further, for example, the central partition wall portion 21 need not include the recessed portion 21d. Since a configuration that contributes most significantly to the reduction of an axial thrust force is the first reduction configuration, the axial thrust force acting on the rotary shaft 4 is reduced even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3 as long as the centrifugal pump 1 includes the first reduction configuration. When the centrifugal pump 1 includes only the first reduction configuration among the first to third reduction configurations, the outer diameter of the first rear shroud 62 and the outer diameter of the second rear shroud 72 are set, for example, in such a way 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) discharge the pumped liquid at the maximum efficiency point. Specifically, for example, when the two-stage centrifugal pump operates under the conditions of a discharge flow rate of 30 m³/h, a total head of 65 m, water as the pumped liquid (specific gravity: 1, viscosity: 1 cP), and a rotational speed of 3,000 rpm, and when the outer diameter of the first impeller 6 is 150 mm and that of the second impeller 7 is 158 mm, both the first axial thrust force and the second axial thrust force become 2,100 N. Herein, the flow rate of the return flow is assumed to be 10% (3 m³/h).

Further, in the present invention, the second impeller 7 need not include the convex portion 72c (i.e., the variable orifice). Also in this configuration, the axial thrust force acting on the rotary shaft 4 is reducible to a minimum by the first to third reduction configurations.

In the present invention, the central partition wall portion 21 may include a convex portion instead of the recessed portion 21d. In this case, the convex portion has the same size (i.e., volume) as the recessed portion 21d, for example.

In the present invention, the number of the recessed portions 21d is not limited to that in the present embodiment. Specifically, for example, the number of the recessed portions 21d may be “one”, an odd number equal to or greater than “three”, or an even number excluding “eight”.

In the present invention, the central partition wall portion 21 may include one or more convex portions instead of some of a plurality of recessed portions 21d. Specifically, for example, the central partition wall portion 21 may include four recessed portions 21d and four convex portions disposed between the respective recessed portions 21d.

In the present invention, the position of the recessed portion 21d is not limited to that in the present embodiment. Specifically, for example, the inner edge portion of the recessed portion 21d may be disposed outside the convex portion 72c in the radial direction.

In the present invention, the position of the convex portion 72c is not limited to that in the present embodiment. Specifically, for example, the convex portion 72c may be disposed in the central portion of or at the outer edge portion of the front surface 72b in the radial direction.

In the present invention, a specific speed set for the first pump unit P1 may be equal to a specific speed set for the second pump unit P2 excluding 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.

In the present invention, the number of impellers included in the centrifugal pump 1 is not limited to two, provided that the number is even. Specifically, for example, the number of impellers may be “four” as described in the fifth modification example or may be “six” or greater.

In the present invention, the pumped liquid is not limited to a liquefied gas. Specifically, for example, the pumped liquid may be water.

Aspects of Present Invention

Next, aspects of the present invention conceived from the embodiments described above will be described below with reference to the terms and reference signs described in the embodiments.

A first aspect of the present invention is a centrifugal pump (for example, the centrifugal pump 1, 1A to 1E) including: a motor (for example, the motor 3); a rotary shaft (for example, the rotary shaft 4) configured to rotate by driving of the motor; a first impeller (for example, the first impeller 6) mounted to the rotary shaft and configured to suck in and discharge a pumped liquid; a second impeller (for example, the second impeller 7) mounted to the rotary shaft and configured to suck in and discharge the pumped liquid discharged from the first impeller; a first pump chamber (for example, the first pump chamber 23) in which the first impeller is accommodated; a second pump chamber (for example, the second pump chamber 24) that is disposed side by side with the first pump chamber in an axial direction of the rotary shaft and in which the second impeller is accommodated, and a central partition wall portion (for example, the central partition wall portion 21) that includes an insertion hole (for example, the central through hole 21c) through which the rotary shaft is disposed and that divides the first pump chamber from the second pump chamber, wherein, in the axial direction, a direction in which the first impeller is disposed relative to the second impeller is a first direction (for example, the forward direction), and a direction opposite to the first direction is a second direction (for example, the rearward direction), the first impeller includes: a first suction port (for example, the first suction port 65) oriented toward the first direction and configured to suck in the pumped liquid from the first direction side; and a first shroud (for example, the first rear shroud 62) facing the central partition wall portion, the second impeller includes: a second suction port (for example, the second suction port 75) oriented toward the second direction and configured to suck in, from the second direction side, the pumped liquid discharged from the first impeller; and a second shroud (for example, the second rear shroud 72) facing the central partition wall portion, a first space (for example, the first rear-side space S11) communicating with the insertion hole is defined between the first impeller and the central partition wall portion, a second space (for example, the second rear-side space S21) communicating with the insertion hole is formed between the second impeller and the central partition wall portion, the second space and the first space communicate with each other through 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, the axial thrust force acting on the rotary shaft 4 is reduced even when the pumped liquid inside the second pump chamber 24 flows into the first pump chamber 23 through the cylindrical space S3.

A second aspect of the present invention is the centrifugal pump in the first aspect, in which a specific speed set for a second pump unit (for example, the second pump unit P2) configured by the second impeller and the second pump chamber is lower than a specific speed set for a first pump unit (for example, the first pump unit P1) configured 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 rotary shaft 4 is reduced.

A third aspect of the present invention is the centrifugal pump in the first or second aspect, further including: a first partition wall portion (for example, the first partition wall portion 20) that is disposed on the first direction side relative to the first impeller and that defines the first pump chamber together with the central partition wall portion; and a second partition wall portion (for example, the second partition wall portion 22) that is disposed on the second direction side relative to the second impeller and that defines the second pump chamber together with the central partition wall portion, wherein the first impeller includes a third shroud (for example, the first front shroud 63) disposed on the first direction side relative to the first shroud, the second impeller includes a fourth shroud (for example, the second front shroud 73) disposed on the second direction side relative to the second shroud, the third shroud includes a first cylindrical portion (for example, the cylindrical portion 63b) forming the first suction port, the fourth shroud includes a second cylindrical portion (for example, the cylindrical portion 73b) forming the second suction port, and a second gap (for example, the second cylindrical gap S23 and the second ring gap S24) between a portion (for example, the large diameter portion 22d and the first step portion 22e) of the second partition wall portion facing the second cylindrical portion and the second cylindrical portion is larger than a first gap (for example, the first cylindrical gap S13 and the first ring gap S14) between a portion (for example, the large diameter portion 20d and the step portion 20e) of the first partition wall portion facing the first cylindrical portion and the first cylindrical portion. According to this configuration, the axial thrust force acting on the rotary shaft 4 is further reduced.

A fourth aspect of the present invention is the centrifugal pump in any one of the first to third aspects, in which the central partition wall portion includes at least one convex portion or recessed portion (for example, the recessed portion 21d) facing the second space and configured to reduce a swirl component of the pumped liquid that flows through the second space. According to this configuration, the axial thrust force acting on the rotary shaft 4 is further reduced.

A fifth aspect of the present invention is the centrifugal pump in any one of the first to fourth aspects, in which the outer diameter of the first shroud and the outer diameter of the second shroud are set in such a way that a first axial thrust force acting in such a way as to move the rotary shaft toward the first direction and a second axial thrust force acting in such a way as to move the rotary shaft toward the second direction are balanced when the first impeller and the second impeller discharge the pumped liquid. According to this configuration, the axial thrust force acting on the rotary shaft 4 is reduced to a minimum.

A sixth aspect of the present invention is the centrifugal pump in any one of the first to fifth aspects, in which the second space and the insertion hole constitute a return flow path (for example, the return flow path RL) through which a portion of the pumped liquid discharged by the second impeller flows to the first space, and the centrifugal pump further includes a variable orifice (for example, the convex portion 72c) configured to narrow a portion of the return flow path as the rotary shaft moves toward the first direction and widens the portion as the rotary 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 aspect of the present invention is the centrifugal pump (for example, the centrifugal pump 1, 1A to 1C, 1E) in the sixth aspect, in which the variable orifice is configured by: a protruding portion (for example, the convex portion 72c) that protrudes toward the first direction from the second shroud; and/or a protruding portion (for example, the convex portion 21e) that protrudes toward the second direction from the central partition wall portion. According to this configuration, the variable orifice is made formable by a simple configuration.

An eighth aspect of the present invention is the centrifugal pump in the seventh aspect, in which the protruding portion is ring-shaped along a circumferential direction of the rotary shaft, and is disposed, in a radial direction of the rotary shaft, close to an inner edge portion of the second shroud and/or close to an inner edge portion of the central partition wall portion. According to this configuration, the sensitivity to pressure fluctuations caused by the variable orifice is improved.

REFERENCE SIGNS LIST

• • 1 Centrifugal pump
  • 20 First partition wall portion
  • 20d Large diameter portion (Portion facing first cylindrical portion)
  • 20e Step portion (Portion facing first cylindrical portion)
  • 21 Central partition wall portion
  • 21c Central through hole (Insertion hole)
  • 21d Recessed portion
  • 22 Second partition wall portion
  • 22d Large diameter portion (Portion facing second cylindrical portion)
  • 22e First step portion (Portion facing second cylindrical portion)
  • 3 Motor
  • 4 Rotary 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-side space (First space)
  • S13 First cylindrical gap (First gap)
  • S14 First ring gap (First gap)
  • S21 Second rear-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