EP0121053B1

EP0121053B1 - Axial thrust reducing device for pumps

Info

Publication number: EP0121053B1
Application number: EP84101200A
Authority: EP
Inventors: Masahiro Yoshioka; Hiroshi Inoue; Eiichi Hazaki; Shiro Nakadaira; Akihisa Okada
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1983-03-04
Filing date: 1984-02-06
Publication date: 1987-05-20
Also published as: ATE27341T1; DE3463824D1; JPS59160093A; JPS626119B2; EP0121053A1

Abstract

A thrust load reducing device of a pump unit including a pump section (P) having a main impeller (1), a motor section (M) for driving the impeller (1), a thrust disc (7) serving concurrently as an auxiliary impeller located at one end of a motor shaft (4) in the motor section (M) for supplying motor cooling water flowing in circulation to the motor section (M), and thrust bearings (10A, 10B) mounted on an inner surface of the casing (3) in positions in which they are juxtaposed against a front surface of the thrust disc (7) located on its suction side and a rear surface of the thrust disc (7) located on its discharge side, respectively. The thrust load reducing device includes a first fine gap defining member (12) located on the inner surface of the casing (3) and cooperating with an outer peripheral surface of the thrust disc (7) to define therebetween a first fine gap (G1) constituting a passage for a fluid discharged by the thrust disc (7) and returning thereto in return flow.

Description

This invention relates to a thrust load reducing device in a pump unit comprising a pump section having a main impeller, a motor section for driving the main impeller, a thrust disc serving concurrently as an auxiliary impeller located at one end of a motor shaft in the motor section for supplying motor cooling water flowing in circulation to the motor section, and thrust bearings mounted on an inner surface of a casing in positions in which they are juxtaposed against a front surface of the thrust disc located on its suction side and a rear surface of the thrust disc located on its discharge side, respectively.
Such a pump unit of the type driven by a wet motor as disclosed in the US-A-3 947 153 comprises a main impeller and guide vanes mounted in a pump chamber, and a motor mounted in a motor chamber. In this type of pump unit, a hydrodynamic thrust load directed axially of the pump is produced by the main impeller. All the thrust load is borne by thrust bearings located in the motor chamber. The reason why thrust load bearing means of this construction is used is because the pump chamber lacks enough space to mount the thrust bearings in the vicinity of the main impeller. The thrust bearings of this type inevitably become large in size and high in cost because they have to bear all the thrust load produced by the main impeller, as described hereinabove. Such thrust bearings are unable to have a long service life because they operate under severe load conditions.
Similar arrangements for cooling the pump motor and lubricating the bearing means are described in the GB-A-518 428 and GB-A-1 351 826.
This invention has as its object the provision of a thrust load reducing device capable of effectively lessening a thrust load applied to the thrust bearings.
To accomplish the aforesaid object, the thrust load reducing device of the generic kind comprises a first small gap defining member located on the inner surface of the casing and cooperating with an outer peripheral surface of the thrust disc to define therebetween a first small gap constituting a passage for a fluid discharged by the thrust disc and returning thereto in return flow and a second small gap defining member secured to the inner surface of the casing and cooperating with an inner peripheral surface of the thrust disc on the suction side to define therebetween a second small gap, and a pressure control chamber defined by the first small gap defining member, the second small gap defining member, the front surface of the thrust disc on the suction side and the inner peripheral surface of the casing.
The present invention enables a thrust load applied to the thrust bearings provided to the thrust disc serving concurrently as an auxiliary impeller to be reduced. This enables a compact size to be obtained in a thrust bearing and allows the service life of the thrust bearings to be prolonged. As a result, the reliability of the pump unit can be greatly increased.
Preferably the first small gap defining member has a large axial length to thereby increase the resistance offered by the first small gap to the flow of a fluid therethrough.
Conveniently the second small gap defining member is constituted by a cylindrical member connected to the casing and juxtaposed against the inner peripheral surface of the thrust disc on its suction side to define axially therebetween the second small gap.
Advantageously the second small gap defining member is constituted by a cylindrical body connected to the thrust disc on its suction side, and another cylindrical body connected to the casing and juxtaposed against the first-mentioned cylindrical body to define the second small gap therebetween.
In another embodiment the second small gap defining member is constituted by a cylindrical body connected to the thrust disc on its suction side, and two other cylindrical bodies connected to the casing and cooperating with the first-mentioned cylindrical body to define the second small gap therebetween by enclosing an end face and inner and outer peripheral surfaces of the first-mentioned cylindrical body.
The small gap defining members can each be formed with a labyrinth or a spiral groove at an inner surface facing the small gap.
Embodiments of the invention are further explained by means of drawings.

Fig. 1 is a vertical sectional view of one example of the pump unit incorporating therein one embodiment of the thrust load reducing device in conformity with the invention;
Fig. 2 is a vertical sectional front view, on an enlarged scale, of the embodiment of the thrust load reducing device incorporated in the motor section of the pump unit shown in Fig. 1;
Fig. 3 is a vertical sectional front view of another embodiment of the thrust load reducing device in conformity with the invention;
Fig. 4 is a vertical sectional front view of still another embodiment of the thrust load reducing device in conformity with the invention; and
Fig. 5 is a vertical sectional front view of a further embodiment of the thrust load reducing device in conformity with the invention.

Fig. 1 shows a pump unit comprising a pump section P, and a motor section M for driving the pump section P. The pump section P comprises a main impeller 1 and guide vanes 2, and is located in a fluid passage L defined in a casing C for delivering a fluid in the direction of an arrow or downwardly in the plane of Fig. 1. The motor section M which constitutes a wet motor is secured to the casing C as a motor casing 3 is joined to the casing C. The motor section M comprises a motor shaft 4, a rotor 5 located on the motor shaft 4, and a stator 6 supported on the motor casing 3 and juxtaposed against the rotor 5. The motor shaft 4 has its one end attached to a thrust disc 7 serving concurrently as an auxiliary impellerfor circulating motor cooling water and is connected to the main impeller 1 of the pump section P at the other end thereof. The thrust disc 7 is formed with a multiplicity of ducts 8 for performing a pumping action. The motor shaft 4 is journalled by radial bearings 9A and 98. Thrust bearings 10A and 10B are located in positions in which they are juxtaposed against a front surface of the thrust disc 7 or auxiliary impeller located on its suction side and a rear surface thereof located on its discharge side, respectively. The thrust bearings 10A and 10B and thrust disc 7 perform the function of bearing a hydrodynamic thrust load applied by the main impeller 1 and acting upwardly in the plane of Fig. 1. Cooling water pressurized by the thrust disc 7 or auxiliary impeller cools the stator 6 of the motor section M and flows through a heat exchanger 11 located outside the pump unit before returning to the suction side of the thrust disc 7 serving as an auxiliary impeller.
Referring to Figs. 1 and 2 a first small gap defining member 12 cooperating with an outer peripheral surface of the thrust disc 7 for defining a first small gap G₁ serving as a passage for a back flow of the water discharged by the auxiliary impeller and flowing to its suction side is located on an inner surface of the motor casing 3. A second small gap defining member 13 extends from the inner surface of the pump casing 3 along an inner peripheral surface of the thrust disc 7 to define therebetween a second small gap G₂. The first and second small gap defining members 12 and 13 which are each in the form of a cylinder define a pressure control chamber 14 between the front surface of the thrust disc 7 on its suction side and the inner surface of the pump casing 3. The pressure control chamber 14 has its pressure set at a predetermined level by a drop. in pressure caused by a loss of pressure by the fluid due to the presence of the first and second small gaps G₁ and G₂. The pressure in the pressure control chamber 14 acts on the front surface of the thrust disc 7 on its suction side and, combined with the pressure of a fluid acting on the rear surface of the thrust disc 7 on its discharge side, performs the functions of reducing the thrust load produced by the main impeller 1.
Operation of the embodiment shown in Figs. 1 and 2 will now be described. Actuation of the motor section M causes the main impeller 1 and the thrust disc 7 serving as an auxiliary impeller to rotate, so that the main impeller 1 delivers a fluid in the passage L downwardly in the plane of Fig. 1 as indicated by the arrow, while the thrust disc 7 functions as an auxiliary impeller to draw cooling water through the suction side as shown in Fig. 2 and discharge same after pressurizing same by the pumping action performed through the ducts 8. The major portion of the discharged cooling water flows through the thrust bearing 10B on the rear surface of the thrust disc 7 to cool same and then along an outer periphery of the motor shaft 4, from which it flows further upwardly to cool the stator 6 of the motor section M, before flowing into the heat exchanger 11 located outside the pump unit as shown in Fig. 1. The cooling water that has performed cooling is cooled by heat exchange performed in the heat exchanger 11 and returns to the suction side of the thrust disc 7 serving as an auxiliary impeller.
A portion of the cooling water discharged by the thrust disc 7 serving as an auxiliary impeller flows through the first small gap G₁ defined between the first small gap defining member 12 and the outer peripheral surface of the thrust disc 7 into the pressure control chamber 14 to cool the thrust bearing 10A therein, before flowing through the second small gap G₂ defined between the second small gap defining member 13 and the inner peripheral surface of the thrust disc 7 and returning in return flow to the suction side of the auxiliary impeller or an inlet to the ducts 8 formed therein. A pressure P₃ in the pressure control chamber 14 is intermediate between a suction pressure P₁ of the auxiliary impeller and a discharge pressure P₂ thereof due to a loss of pressure in the first and second small gaps G₁ and G₂. The pressure P₃ can be controlled as desired by arbitrarily setting the dimensions of the first and second small gaps G₁ and G₂ or their widths and lengths.
Meanwhile, the main impeller 1 usually produces an upwardly directed thrust load W₁ as shown in Fig. 2. The load W₁ which is borne by the thrust bearing 10B can be reduced by varying the pressure P₃ in the pressure control chamber 14. More specifically, the pressure P₃ in the pressure control chamber 14 acts on the front surface of the thrust disc 7 and the discharge pressure P₂ of the thrust disc 7 acts on the rear surface of the thrust disc 7. Thus, by reducing the pressure P₃ in the pressure control chamber 14 to a level below the discharge pressure P₂ and close to the suction pressure P_i, it is possible to produce a downwardly directed thrust W₂ at the thrust disc 7 by the relative pressures acting on the front surface and the rear surface of the thrust disc 7. The thrust W₂ acts in a direction opposite to the direction in which the thrust load W, produced by the main impeller 1 acts, so that it is possible to reduce the thrust load W₁ borne by the thrust bearing 10B. This is conductive to prevention of damage which might otherwise be caused to the thrust bearing 10B, and the service life of the thrust bearing 10B can be prolonged.
In the foregoing description, the thrust load W₁ has been assumed to be higher than the thrust W₂. However, the thrust W₂ might become higher than the thrust load W₁ depending on the conditions under which the main impeller 1 operates. In this case, the load acting on the thrust bearing 10B would become zero or too small to allow positioning of the motor shaft 4 in the axial direction to be performed stably, thereby causing the motor shaft 4 to vibrate. An excessively high load would be applied to the thrust bearing 10A which is designed to have an ability to bear a load substantially of the same level as the weight of a rotary member from the point of view of reducing a mechanical loss, thereby causing damage to the thrust bearing 10A. The damage caused to the thrust bearing 10A can be avoided by adjusting the pressure P₃ in the pressure control chamber 14 by varying the dimensions of the first and. second small gaps G₁ and G₂.
As can be clearly understood in the foregoing description, the pressure P₃ in the pressure control chamber 14 would become equal to the discharge pressure P₂ of the thrust disc 7 if the first small gap defining member 12 did not exist in Fig. 2, so that no thrust tending to reduce the thrust load W1 produced by the main impeller 1 would not be produced at the thrust disc 7. Therefore, it would be impossible to reduce the thrust load W1 unless the first small gap G₁ is provided. Thus, the first fine gap G₁ is essential for reducing the thrust load W₁.
When the second small gap defining member 13 were not provided in Fig. 2, the pressure P₃ acting on the front surface of the thrust disc 7 or the suction side thereof would become equal to the suction pressure P₁ of the thrust disc 7. This would make the pressure P₃ lower than the discharge pressure P₂ acting on the rear surface of the thrust disc 7. Thus, a thrust load tending to reduce the thrust load W₁ produced by the main impeller 1 which, like the thrust W₂ shown in Fig. 2, would act in a direction opposite to the direction in which the thrust load W₁ acts, would be produced. Thus, it would be possible to reduce the thrust load W₁ produced by the main impeller 1 by such thrust load reversed in direction. However, since the pressure P₃ is uniquely decided by the relation between it and the suction pressure P₁, the thrust load reversed in direction would be constant in value and such value would not be adjustable.
Fig. 3 shows another embodiment, in which the thrust disc 7 serving as an auxiliary impeller is formed with a multiplicity of oblique ducts 8A which are directed obliquely upwardly in going from the center of the thrust disc 7 toward its outer periphery, to enable the length of the first small gap G₁ to be increased. By this arrangement, it is possible to increase the size of the first small gap G₁ shown in Fig. 3 as compared with the first small gap G₁ shown in Fig. 2 when the first small gap G₁ shown in Fig. 3 offers the same resistance to the flow of a fluid therethrough as the first small gap G₁ shown in Fig. 2. As a result, when the pump shaft 4 vibrates, it is possible to prevent the outer peripheral surface of the thrust disc 7 and the first small gap defining member 12 from being brought into contact with each other while allowing the thrust load W₁ to be reduced.
Fig. 4 shows still another embodiment in which the second small gap G₂ is modified. As shown, a first cylindrical body 15 extends downwardly from a lower end of the thrust disc 7 in such a manner that inner peripheral surfaces of the first cylindrical body 15 and the thrust disc 7 form a straight line, and a second cylindrical body 16 located outwardly of the first cylindrical body 15 is secured to the motor casing 3. By this arrangement, the axiatly extending second small gap G₂ is defined between an outer peripheral surface of the first cylindrical body 15 and an inner peripheral surface of the second cylindrical member 16 and, at the same time, a first radial small gap G₂₁ and a second radial small gap G₂₂ are defined between an end face of the first cylindrical body 15 and an inner peripheral surface of the motor casing 3 and between an end face of the second cylindrical body 16 and the lower end of the thrust disc 7, respectively. The result of this is that this embodiment can achieve the same effects as the embodiment shown in Fig. 3 because it is possible to alter the resistance offered to the flow of the fluid through the small gaps. At the same time, the pumping efficiency of the auxiliary impeller is improved because leaks of the fluid in the pressure control chamber 14 to the suction side of the thrust disc 7 can be minimized by the centrifugal pumping action performed by the end portions of the cylindrical bodies 15 and 16.
Fig. 5 shows a further embodiment in which the second small gap G₂ is further modified. In this embodiment, a first cylindrical body 17 extends downwardly from a lower end of the thrust disc 7 in such a manner that inner peripheral surfaces of the first cylindrical body 17 and the thrust disc 7 form a straight line, and a second cylindrical body 18 and a third cylindrical body 19 are secured to the motor casing 3 to define a small gap G₂₃ between inner and outer peripheral surfaces and an end face of the first cylindrical body 17 and inner peripheral surfaces of the second and third cylindrical bodies 18 and 19. By this arrangement, the length of the small gap G₂₃ can be increased and a thrust load of reverse direction produced by the thrust disc 7 can be controlled. Moreover, an increase in the size of the gap prevents the cylindrical body 17 from coming into contact with the cylindrical bodies 18 and 19, and the pumping efficiency of the auxiliary impeller can be improved.
In all the embodiments shown and described hereinabove, all the small gaps have been described as being constant in size. However, a labyrinth or a spiral groove may be provided to one of the surfaces of the members defining the gaps.

Claims

1. A thrust load reducing device in a pump unit comprising a pump section (P) having a main impeller (1), a motor section (M) for driving the main impeller (1), a thrust disc (7) serving concurrently as an auxiliary impeller located at one end of a motor shaft (4) in the motor section (M) for supplying motor cooling water flowing in circulation to the motor section (M), and thrust bearings (10A, 10B) mounted on an inner surface of a casing (3) in positions in which they are juxtaposed against a front surface of the thrust disc (7) located on its suction side and a rear surface of the thrust disc (7) located on its discharge side, respectively, characterized by a first small gap defining member (12) located on the inner surface of the casing (3) and cooperating with an outer peripheral surface of the thrust disc (7) to define therebetween a first small gap (G_i) constituting a passage for a fluid discharged by the thrust disc (7) and returning thereto in return flow and a second small gap defining member (13) secured to the inner surface of the casing (3) and cooperating with an inner peripheral surface of the thrust disc (7) on the suction side to define therebetween a second small gap (G₂), and a pressure control chamber (14) defined by the first small gap defining member (12), the second small gap defining member (13), the front surface of the thrust disc (7) on the suction side and the inner peripheral surface of the casing (3).

2. A thrust load reducing device as claimed in claim 1, wherein said first small gap defining member (12) has a substantially large axial length as compared with the size of the first small gap (G_i) to thereby increase the resistance offered by the first small gap (G_i) to the flow of a fluid therethrough.

3. A thrust load reducing device as claimed in claim 1 or 2, wherein said second small gap defining member (13) is constituted by a cylindrical member connected to the casing (3) and juxtaposed against the inner peripheral surface of the thrust disc (7) on its suction side to define axially therebetween the second small gap (G₂).

4. A thrust load reducing device as claimed in claim 1 or 2, wherein said second small gap defining member is constituted by a cylindrical body (15) connected to the thrust disc (7) on its suction side, and another cylindrical body (16) connected to the casing (3) and juxtaposed against the first mentioned cylindrical body (15) to define the second small gap (G₂₂, G₂, G₂₁) therebetween.

5. A thrust load reducing device as claimed in claim 1 or 2, wherein said second small gap defining member is constituted by a cylindrical body (17) connected to the thrust disc (7) on its suction side, and two other cylindrical bodies (18, 19) connected to the casing (3) and cooperating with the first-mentioned cylindrical body (17) to define the second small gap (G₂₃) therebetween by enclosing an end face and inner and outer peripheral surfaces of the first-mentioned cylindrical body (17).

6. A thrust load reducing device as claimed in any one of claims 1 to 5, wherein said small gap defining members (12; 13, 15, 16; 17, 18, 19) are each formed with a labyrinth at an inner surface facing the small gap (G₁; G₂; G₂₁, G₂₂, G₂₃)-

7. A thrust load reducing device as claimed in any one of claims 1 to 5, therein said small gap defining members (12; 13; 15, 16; 17, 18, 19) are each formed with a spiral groove at an inner surface facing the small gap (G₁; G₂, G₂₁, G₂₂; G₂₃).