EP0952352A2

EP0952352A2 - Thrust-balancing device

Info

Publication number: EP0952352A2
Application number: EP99107770A
Authority: EP
Inventors: Yasushi c/oNikkiso Co. Ltd. Kubota
Original assignee: Nikkiso Co Ltd
Current assignee: Nikkiso Co Ltd
Priority date: 1998-04-20
Filing date: 1999-04-19
Publication date: 1999-10-27
Anticipated expiration: 2019-04-19
Also published as: EP0952352B1; KR19990083231A; EP0952352A3; TW406166B; DE69922729T2; KR100295011B1; CN1120937C; CN1232928A; DE69922729D1

Abstract

A flow channel and pressure equalizing sections introduce a fluid having substantially no angular momentum into a thrust balance chamber of a thrust balance device. The introduction of this fluid reduces the angular momentum of the fluid in the thrust balance chamber, facilitating the discharge of fluid out of the thrust balance chamber through a variable orifice. The thrust balance chamber exerts a variable pressure onto a rear surface of an impeller of a centrifugal pump. This pressure prevents significant displacement of the impeller during pump operation. The result is a centrifugal pump having good thrust balance properties regardless of flow rate and impeller speed.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a thrust balance device. More specifically, the present invention relates to a thrust balance device significantly improving thrust balance in a device such as a canned motor pump.
Conventional canned motor pumps include an impeller mounted on a rotating shaft. In the type of canned motor pump, a fluid is sucked through a suction opening i.e. inlet which opens axially. Centrifugal force from the impeller causes discharge of the sucked fluid from radial discharging openings i.e. outlet. Since the suction opening is oriented toward an end of the rotation shaft, a force is applied on the impeller in the direction of thrust. Thus, in primitive canned motor pumps, the impeller is pushed toward an inner wall of the chamber holding the impeller. This pushing force interferes with the rotation of the impeller. As a result, almost all recent canned motor pumps are equipped with a thrust balance mechanism.
Suction of the fluid generates a pressure in the direction of thrust. A thrust balance mechanism prevents obstruction of the rotation of the impeller caused by the pressure of the sucked fluid. Generally, a thrust balance mechanism includes:

(1) a fixed orifice formed between an outer surface of a ring-shaped cylinder formed on a rear surface of an impeller having a balance hole and a cylindrical inner perimeter surface of a cavity disposed on a casing by inserting the ring-shaped cylinder into the cavity to form a gap between the outer perimeter surface of the cylinder and the cylindrical inner perimeter surface of the cavity;
(2) a thrust balance chamber which is formed from the following: a bottom surface of the cylinder; an inner perimeter surface of the cylinder; a surface of a first projection projected from the casing toward an inner space of the cylinder, the surface thereof facing and being separated from the bottom surface of the cylinder by a prescribed gap; and an outer perimeter surface of a ring-shaped second projection, surrounding the rotating axis and projecting further than the first projection; and
(3) a variable orifice which is formed from the rear surface of the impeller and an end surface of the second projection facing it.

In the above thrust balance mechanism, the centrifugal force from the rotation of the impeller causes fluid to be discharged radially. A portion of the fluid discharged in the centrifugal direction flows into the thrust balance chamber via the fixed orifice. The fluid which enters the thrust balance chamber flows out from the thrust balance chamber through the variable orifice. The fluid exiting the thrust balance chamber passes through the balance hole and combines with the fluid to be discharged.
If the pressure of the sucked and discharged fluid increases, a pressure in the direction of thrust is applied to the impeller. This pressure causes the back surface of the impeller to approach the casing surface facing it. However, pressure from the fluid also increases the flow rate, resulting in higher fluid pressure within the thrust balance chamber. The increase in fluid pressure in the thrust balance chamber causes a pressure to be applied on the rear surface of the impeller to push the impeller away from the casing surface facing it. This pressure is sometimes referred to as independent pressure. Fluid pressure within the thrust chamber causes the impeller to move against the pressure from the fluid being sucked and discharged.
The gap in the variable orifice increases when the impeller is displaced away from the casing surface facing its rear surface, i.e., when the impeller is shifted so that it moves away from the casing surface facing the rear surface of the impeller. This movement causes high-pressure fluid to flow rapidly from the variable orifice. As a result, fluid pressure within the thrust balance chamber drops. The pressure in the thrust direction applied to the impeller from the fluid being sucked and discharged becomes greater than the fluid pressure within the thrust balance chamber. The pressure in the thrust direction causes the impeller to shift toward the casing surface facing the rear surface of the impeller.
As described above, in order to balance the pressure within the thrust balance chamber and the pressure from the fluid being sucked and discharged, the impeller changes its position according to the gap in the fixed orifice, the gap in the variable orifice, as well as the volume of the thrust balance chamber. The change of position of the impeller maintains balance for the rotating axis along the thrust direction.
However, with a thrust balance chamber in conventional thrust balance devices, the rear surface of the impeller is a rotating surface, while the casing surface facing the impeller is a fixed surface. Thus, fluid flowing into the thrust balance chamber receives an angular momentum energy from the impeller rotation. Additionally, fluid flowing into the thrust balance chamber rotates together with the impeller. As a result, the fluid rotating in the thrust balance chamber with the impeller generates a very high flow-path resistance.
The flow-path resistance of the fluid interposed between the rotating surface and the fixed surface is proportional to the square of the peripheral speed of the fluid rotating with the rotating surface. Thus, in a high-speed pump in which its impeller rotates at a very high speed, the flow-path resistance of the fluid in the thrust balance chamber thereof is high. Further, in a large pump wherein a large amount of fluid exist at the gap between the fixed and rotating surfaces thereof, even if it is not a high speed pump, and consequently, the peripheral speed of the rotating fluid is high, the flow-path resistance of the fluid in the thrust balance chamber thereof is also high. Such a high flow-path resistance prevents the thrust balance of the pump from being maintained appropriately.
In order to overcome this problem, bypass structures known as pressure-equalizing holes or pressure-decreasing holes have been conventionally formed in the fixed surface of the thrust balance chamber. However, these pressure-equalizing holes have been unable to lower the flow-path resistance and maintain thrust balance. While forming this kind of bypass may be able to increase the independent pressure, this kind of bypass cannot significantly reduce the angular momentum of the fluid inside the thrust chamber.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thrust balance device which overcomes the foregoing problems.
It is another object of the present invention to provide a thrust balance device which has superior thrust balance properties.
It is a further object of the present invention to provide a thrust balance device which has good thrust balance properties regardless of pump discharge rate.
It is still a further object of the present invention to provide a thrust balance device which has good thrust balance properties regardless of impeller rotation speed.
Briefly stated, the present invention provides a flow channel and pressure equalizing sections introducing a fluid having substantially no angular momentum into a thrust balance chamber of a thrust balance device. The introduction of this fluid reduces the angular momentum of the fluid in the thrust balance chamber, facilitating the discharge of fluid out of the thrust balance chamber through the variable orifice. The thrust balance chamber exerts a variable pressure onto a rear surface of an impeller of a centrifugal pump. This pressure prevents significant displacement of the impeller during pump operation. Therefore, the present invention provides a centrifugal pump with a good thrust balance property regardless of flow rate and impeller speed of the centrifugal pump.
According to an embodiment of the present invention, there is provided a thrust balance device in a centrifugal pump comprising a fixed orifice permitting flow of a portion of a fluid passing through the centrifugal pump into a thrust balance chamber of the thrust balance device, the thrust balance chamber facing the rear surface of the impeller in the centrifugal pump, a variable orifice permitting a variable flow of the portion from the thrust balance chamber depending on a balance between a fluid pressure in the thrust balance chamber and the pressure from the fluid being pumped, and means for introducing a fluid having substantially no angular momentum into the thrust balance chamber, whereby the introduced fluid facilitates the flow of the portion to flow out through the variable orifice.
According to another embodiment of the present invention, there is provided a thrust control device for controlling an axial position of an impeller of a centrifugal pump, comprising an impeller having a first surface exposed to a pressure of a fluid being pumped, a thrust balance chamber adjacent a second surface of the impeller, at least one balance hole communicating between the first surface and the second surface, a projection facing the second surface of the impeller, a fixed orifice permitting a controlled leakage of the fluid from an outlet of the centrifugal pump into the thrust balance chamber, a variable orifice adjusting flow rate of fluid flowing out from the thrust balance chamber when the impeller is displaced axially at a predetermined distance in the direction toward said projection, whereby the controlled leakage is enabled to increase a fluid pressure in the thrust balance chamber, and thereby to resist axial displacement of the impeller in said direction, and at least one stationary flow channel conveying a portion of the fluid with substantially reduced angular momentum to the thrust balance chamber.
According to still another embodiment of the present invention, there is provided a device for feeding fluid having substantially no angular velocity to a thrust balance chamber of a centrifugal pump comprising a radially arranged opening and a ring-shaped groove opening at the thrust balance chamber. The ring-shaped groove connects the inner end of the opening with the thrust balance chamber. The fluid is accepted at the outer end of the opening and conducted to its inner end, then fed into the thrust balance chamber through the ring-shaped groove.
In the above embodiment, the device for feeding fluid having substantially no angular velocity to the thrust balance chamber can have a plurality of the radially arranged openings.
Additionally, both of the opening area of the ring-shaped groove and the sum total of the cross-sectional area of the radially arranged opening(s) are preferably larger than the opening area of the balance hole of the impeller.
The present invention achieves these objects by providing a thrust balance device that includes the following elements:

(1) A fixed orifice forming a gap between an outer perimeter surface of a cylinder formed on a rear surface of an impeller in a centrifugal pump and a cylindrical inner perimeter surface of a cavity disposed in a casing thereof, wherein the impeller has at least one balance hole and the cylinder is inserted into the cavity;
(2) A thrust balance chamber which is formed from the following: a base surface of the cylinder; an inner perimeter surface of the cylinder; a surface of a first projection projected from the casing toward an inner space of the cylinder, the surface being separated from the rear surface by a prescribed gap; and an outer perimeter surface of a ring-shaped second projection, surrounding the rotating axis, projecting further than the first projection;
(3) A variable orifice which is formed between the rear surface of the impeller and an end surface of the second projection facing it;
(4) A ring-shaped groove, surrounding the rotating axis, which is formed on the first projection; and
(5) A pressure-equalizing section which leads to the ring-shaped groove and the cavity.

The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a longitudinal section of a thrust balance device of one embodiment of the present invention.
Fig. 2 is a longitudinal section of another embodiment of a thrust balance device of the present invention.
Fig. 3 is a longitudinal section of a thrust balance device of yet another embodiment of the present invention.
Fig.4 is a longitudinal section of still another embodiment of a thrust balance device of the present invention.
Fig.5 is a graph indicating the variation of the remaining thrust value to various discharge amount for canned motor pump having the thrust balance device shown in Fig.1 and for canned motor pump that is the same as the above canned motor pump except for not having the above thrust balance device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig.1, a centrifugal pump 1 equipped with a thrust balance device according to the present invention includes an impeller 6 mounted on a rotating axis 5. Impeller 6 is positioned in a pump chamber 4 formed by a casing 2 and a liner disk 3.
A suction opening of centrifugal pump 1 (not shown) is formed at an axial orientation relative to impeller 6. A cylindrical guide path 7, co-axial with rotating axis 5, extends from the suction opening to pump chamber 4.
Impeller 6 includes a base 8 which has a circular shape when seen from an axial direction. Impeller 6 rotates together with rotating axis 5, discharging fluid introduced through guide path 7 in a centrifugal direction. Thus, in centrifugal pump 1, a discharging opening (not shown) is formed centrifugally in the relationship to impeller 6.
A cylinder 9 projects from a rear surface, i.e. the surface facing liner disk 3, of base 8, which is a section of impeller 6. Cylinder 9 projects toward liner disk 3. Furthermore, a balance hole 10 extends from the rear surface of base 8 to the front surface thereof toward guide path 7.
A cavity 11 is formed on a surface of liner disk 3 facing base 8. Cavity 11 has a cylindrical inner perimeter surface which has an inner diameter slightly larger than a diameter of cylinder 9. When cylinder 9 is inserted into cavity 11, a slight gap is formed between an outer perimeter surface of cylinder 9 and an inner perimeter surface of cavity 11. This gap serves as a fixed orifice 12.
A disc-shaped first projection 13, on cavity 11 inward from cylinder 9, projects toward a rear surface of impeller 6. A second projection 14, having a ring-shaped end surface, is located on cavity 11 at a position inward from first projection 13, toward rotating axis 5. Second projection 14 projects closer to the rear surface of impeller 6 than first projection 13. An end surface of first projection 13 facing a bottom surface of cylinder 9 has a ring shape. When cylinder 9 is inserted into cavity 11, a prescribed gap is formed between an outer perimeter surface of first projection 13 and an inner perimeter surface of cylinder 9. This gap is much larger than the gap formed by fixed orifice 12. The ring-shaped end surface of second projection 14 has a ring-like shape when seen from an axial direction.
A thrust balance chamber 15 is formed as a space created between the ring-shaped end surface of first projection 13 (this surface is also a fixed surface) and a bottom surface of cylinder 9 (this surface is the rear surface of impeller 6 and is also a rotating surface).
A variable orifice 16 is formed of a space between the ring-shaped end surface of second projection 14 and a bottom surface of cylinder 9, i.e. the rear surface of base 8.
A ring-shaped groove 17, centered on rotating axis 5, is positioned between first projection 13 and second projection 14. Ring-shaped groove 17 has a space surrounded by an opening facing a ring-shaped end surface of first projection 13, an inward inner perimeter surface which is an outer perimeter surface of a cylinder, and an outward inner perimeter surface which is an inner perimeter surface of a cylinder. The resulting space is a ring-shaped space centered on rotating axis 5. In Fig. 1, which is a longitudinal section of the centrifugal pump 1, a line representing a longitudinal section of an inward inner perimeter surface of ring-shaped groove 17 is parallel with a line representing a longitudinal section of an outward inner perimeter surface of ring-shaped groove 17.
Pressure-equalizing sections 18 are openings extending from an outer perimeter surface of first projection 13 to ring-shaped groove 17. Pressure-equalizing sections 18 are connected with ring-shaped groove 17 and cavity 11, Preferably, twelve pressure-equalizing sections 18 are formed at first projection 13. Each of pressure-equalizing sections 18 has a circular cross-sections cut along a plane perpendicular to the axis thereof. In other words, pressure-equalizing sections 18 have cylindrical inner spaces.
The following is a description of how centrifugal pump 1 operates together with the thrust balance device of the present invention.
Rotating axis 5 rotates together with impeller 6. Fluid introduced from the suction opening flows through guide path 7 into pump chamber 4. Since impeller 6 is rotating inside pump chamber 4, the fluid is discharged through a discharging opening by centrifugal force. This is the standard operation of centrifugal pump 1.
A portion of the fluid in pump chamber 4 flows through fixed orifice 12, into thrust balance chamber 15. The fluid passes through variable orifice 16 and balance hole 10 to return to a front side of impeller 6.
If the discharge pressure on impeller 6 increases, impeller 6 is displaced toward liner disk 3 by this increase in the discharge pressure. This pressure change causes the width of the opening of variable orifice 16 to be decreased, lowering the flow through variable orifice 16. While the width of the gap at variable orifice 16 decreases, the width of the gap at fixed orifice 12 remains unchanged. Thus, fluid continues to flow into thrust balance chamber 15, increasing the fluid pressure inside thrust balance chamber 15 until the fluid pressure in thrust balance chamber 15 exceeds the discharge pressure.
When the fluid pressure of thrust balance chamber 15 exceeds the discharge pressure, impeller 6 is displaced in the direction where cylinder 9 is pushed out from cavity 11, This displacement of impeller 6 increases the width of the opening in variable orifice 16. As the width of the opening in variable orifice 16 increases, the amount of fluid coming out through variable orifice 16 from balance chamber 15 exceeds the amount of fluid going into balance chamber 15 through fixed orifice 12. Thus, the fluid and the fluid pressure in thrust balance chamber 15 are reduced, displacing impeller 6 toward liner disk 3. When the fluid pressure in thrust balance chamber 15 is in equilibrium with the discharge pressure toward impeller 6, displacement of impeller 6 stops.
Fluid inside thrust balance chamber 15 rotates together with the rotation of impeller 6. Fluid rotating inside thrust balance chamber 15 has an angular momentum and generates flow-path resistance. If this flow-path resistance is high, the flow of fluid in thrust balance chamber 15 through variable orifice 16 is hindered, even when the opening in variable orifice 16 enlarges.
The object of the present invention is to reduce the flow-path resistance caused by the angular momentum of the fluid in thrust balance chamber 15. Ring-shaped groove 17 and pressure-equalizing sections 18 help achieve this goal. Fluid having no angular momentum flows from pressure-equalizing sections 18 into thrust balance chamber 15 via ring-shaped groove 17, mixing with fluid having angular momentum. The addition of a fluid having no angular momentum into thrust balance chamber 15 dramatically reduces the angular momentum of fluid in thrust balance chamber 15. Thus, by reducing the flow-path resistance caused by angular momentum of fluid in thrust balance chamber 15, fluid in thrust balance chamber quickly and smoothly flows out through variable orifice 16.
A computer was used to simulate the thrust balance in rotating axis 5 for a pump having pressure-equalizing sections 18 and ring-shaped groove 17 versus a pump having only pressure-equalizing sections 18. According to the results of the simulation, the outgoing flow from variable orifice 16 was 290 liters/min. for the pump having only pressure-equalizing sections 18. The flow pressure at the backside of impeller 6 (the pressure inside thrust balance chamber 15) was 2363 N (241 kgf). With the pump having pressure-equalizing sections 18 and ring-shaped groove 17, the outgoing flow from variable orifice 16 was 301 liters/min. and the flow pressure at the back side of impeller 6 was 2157 N (220kgf), thus showing a dramatic reduction in flow path resistance caused by angular momentum of fluid in thrust balance chamber 15. In these calculations the pump specifications were as follows: SUC 125 A, DIS 100 A, 200m³/h × 32m × 2900 rpm, impeller diameter, 190mm.
The groove space of ring-shaped groove 17 formed on first projection 13 can be of any shape, as long as it surrounds rotating axis 5.
Referring to Fig. 2, an alternate embodiment of the present invention is shown. Ring-shaped groove 17 has a groove space surrounded by an opening facing the ring-shaped end surface of first projection 13, an inward inner perimeter surface corresponding to an outer perimeter surface of a cylinder co-axial with rotating axis 5, and an outward inner perimeter surface corresponding to an inner perimeter surface of a cone that is co-axial with rotating axis 5. The longitudinal section of the groove space of ring-shaped groove 17 of this embodiment of the present invention forms a wedge shape as shown in Fig. 2.
Referring to Fig. 3, an alternate embodiment of the present invention is shown. Ring-shaped groove 17 has a groove space surrounded by an opening facing a ring-shaped end surface of first projection 13, an inward inner perimeter surface corresponding to on outer perimeter surface of a cone that is co-axial with rotating axis 5, and an outward inner perimeter surface corresponding to an inner perimeter surface of a cone that is co-axial with rotating axis 5. The longitudinal section of the groove space of ring-shaped groove 17 of this embodiment forms a wedge shape having a configuration opposite of the wedge shape of the embodiment shown in Fig. 2.
Referring to Fig. 4, in yet another embodiment of the present invention, ring-shaped groove 17 has a groove space surrounded by an opening facing a ring-shaped end surface of first projection 13, an inward inner perimeter surface corresponding to an outer perimeter surface of a cone that is co-axial with rotating axis 5, and an outward inner perimeter surface of a cone that is co-axial with rotating axis 5. The longitudinal section of the groove space of ring-shaped groove 17 of this embodiment of the present Invention forms a v-shape.
Regardless of the shape of the groove space formed by ring-shaped groove 17, it is desirable that A, the sum total of the cross-sectional area of each of pressure-equalizing sections 18 cut along the plane perpendicular to the axis thereof or the circular cross-section area of pressure-equalizing sections 18 (A is calculated as ${n × (π/4) ×d}_{1}^{2}$
, where n is the number of pressure-equalizing sections 18 and d₁ is the diameter of the circular cross-section) is equal to or smaller than B, an area of the opening of ring-shaped groove 17 (B is calculated as ${(π/4) × (D}_{2}^{2} {-D}_{3}^{2}$
, where D₂ and D₃ are the outer and inner diameters of the opening of ring-shaped groove 17, respectively), i.e. A≦B.
There are no specific restrictions placed on the number of pressure-equalizing sections 18.
The present invention provides a thrust balance device having superior thrust balance properties, Furthermore, the present invention provides a thrust balance device having good thrust balance properties, regardless of the discharge from the pump. The present invention also provides a thrust balance device having good thrust balance properties, regardless of the speed of rotation of the impeller.
Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.

EXAMPLE

1. Example 1

For a canned motor pump (type: HN25E) having a thrust balance device with the structure shown in Fig.1, a difference between the fluid pressure in thrust balance chamber 15 and the discharge pressure at impeller 6, i.e., remaining thrust was measured with varying discharge of the canned motor pump from 10 to 140 m³/hr.
Both of A, the sum total of the areas of the circular cross section of pressure-equalizing sections 18 and B, the area of the opening of ring-shaped groove 17 were larger than the sum total of the area of the openings of balancing holes 10 on base 8. Further, A is less than B. Applying an alternating current of 50 Hz drove the canned motor pump. Results are shown in Fig.5.
As shown in Fig. 5, at a discharge of 10 to 140 m³/hr, the canned motor pump of Example 1 showed almost no remaining thrust, in other word, the fluid pressure in thrust balance chamber 15 balanced well to the discharge pressure on impeller 6.

2. Comparative Example 1

Remaining thrust is measured for a canned motor pump (type: HN25E-F4) having the same structure as that of the canned motor pump of Example 1 except for having neither ring-shaped groove 17 nor pressure-equalizing sections 18. Results are shown in Fig 5.
As shown in Fig. 5, the above canned motor pump showed a maximum remaining thrust of about 70 kgf.

Claims

A thrust balance device in a centrifugal pump comprising:
a fixed orifice permitting flow of a portion of a fluid passing through said centrifugal pump into a thrust balance chamber of said thrust balance device;

said thrust balance chamber facing the rear surface of the impeller of said centrifugal pump; and

a variable orifice permitting a variable flow of a fluid from said thrust balance chamber depending on a balance between a fluid pressure in said thrust balance chamber and a fluid being pumped;

characterized in having means for introducing a fluid having substantially no angular momentum into said thrust balance chamber, whereby the flow-path resistance of the fluid in the thrust barance chamber thereof is reduced.
A thrust balance device according to claim 1, wherein:
said means includes one or more pressure-equalizing sections connected to at least one stationary flow channel;

said at least one stationary flow channel having an opening to permit flow of the fluid in the pressure-equalizing sections into said thrust balance chamber.
A thrust balance device according to claim 2, wherein said stationary flow channel is a ring shaped groove.
A thrust balance device according to claim 3, wherein the outer perimeter surface of said ring shaped groove and inner perimeter surface thereof are parallel.
A thrust balance device according to claim 3, wherein said ring shaped groove has a wedge shape in cross-section.
A thrust balance device according to claim 3, wherein each of said pressure-equalizing sections has a circular cross section:
each of said circular cross sections having a cross-sectional area wherein a sum total of said cross-sectional area of said pressure-equalizing section is smaller than an area of said opening of said ring-shaped groove.
A thrust balance device including:
(1) A fixed orifice forming a gap between an outer perimeter surface of a cylinder formed on a rear surface of an impeller in a centrifugal pump and a cylindrical inner perimeter surface of a cavity disposed in a casing thereof, wherein sad impeller have at least one balance hole and the cylinder is inserted into the cavity;

(2) a thrust balance chamber which is formed from the following: a base surface of the cylinder; an inner perimeter surface of the cylinder; a surface of a first projection projected from the casing toward an inner space of the cylinder, the surface being separated from the rear surface by a prescribed gap; and an outer perimeter surface of a ring-shaped second projection, surrounding the rotating axis and projecting further than the first projection; and

(3) a variable orifice which is formed between the rear surface of the impeller and an end surface of the second projection facing it;
characterized in having:
a ring-shaped groove formed on the first projection and surrounding the rotating axis of the impeller, and

a pressure-equalizing section which leads to the ring-shaped groove and the cavity.
A thrust balance device according to claim 7, wherein the outer perimeter surface of said ring shaped surface and inner perimeter surface thereof are parallel.
A thrust balance device according to claim 7, wherein said ring shaped groove has a wedge shape in cross-section.
A thrust balance device according to one of claims 7 to 9 having a plurality of said pressure-equalizing sections.
A thrust balance device according to claim 10, wherein each of said pressure-equalizing sections has a circular cross section, and a sum total of the area of the circular cross sections of said pressure-equalizing sections is smaller than the area of said opening of said ring-shaped groove.
A thrust balance device according to claim 10 or 11, wherein both of the opening area of said ring-shaped groove and the sum total of the cross-sectional area of said pressure-equalizing sections are larger than the opening area of the balance hole of the impeller.