AXIAL THRUST REDUCING DEVICE FOR PUMPS

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 con­ currently 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 sur­ face 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: THRUST LOAD REDUCING DEVICE BACKGROUND OF THE INVENTION This invention relates to thrust load reducing devices, and more particularly it is concerned with a thrust load reducing device for a pump unit driven by a wet motor. Pump units of the type driven by wet motors of the prior art each 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 in the prior art, as disclosed in U.S. Patent No. 3,947,153, for example. 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. SUMMARY OF THE INVENTION 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 invention provides, 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 on its suction side and a rear surface of the thrust disc located on its discharge side, respectively, a thrust load reducing device comprising a first fine 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 fine gap constituting a passage for a fluid discharged by the thrust disc and returning thereto in return flow. Additional and other objects, features and advantages of the invention will become apparent from the description set forth hereinafter when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE 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. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the invention will be described by referring to the accompanying drawings. Fig. 1 shows a pump unit having incorporated therein one embodiment of the invention. The pump unit comprises 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 impeller for 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 9B. 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 lOA and lOB 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. One embodiment of the invention located in the vicinity of the thrust disc 7 will be described in detail by referring to Figs. 1 and 2. A first fine gap defining member 12 cooperating with an outer peripheral surface of the thrust disc 7 for defining a first fine 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 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 fine gap G2. The first and second fine 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 fine 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 function 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 lOB 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 swerving 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 fine gap G1 defined between the first fine 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 fine gap G2 defined between the second fine 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 fine gaps G1 and G2. The pressure P3 can be controlled as desired by arbitrarily setting the dimensions of the first and second fine 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 lOB 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 P1, it is possible to produce a downwardly directed thrust W2 at the thrust dise 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 W1 produced by the main impeller 1 acts, so that it is possible to reduce the thrust load W1 borne by the thrust bearing lOB. 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 A in the axial direction to be performed stably, thereby causing the motor shaft 4 to vibrate. Also, 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 fine 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 fine 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 fine gap G is provided. Thus, the first fine gap G1 is essential in the present invention to achieve the object of reducing the thrust load , WI. When the second fine 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 fine gap G1 to be increased. By this arrangement, it is possible to increase the size of the first fine gap G1 shown in Fig. 3 as compared with the first fine gap G1 shown in Fig. 2 when the first fine gap G1 shown in Fig. 3 offers the same resistance to the flow of a fluid therethrough as the first fine 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 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 fine 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 axially extending second fine gap h 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 fine gap G21 and a second radial fine 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 fine 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 fine 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 fine 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 fine gap 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 fine 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. From the foregoing description, it will be appreciated that 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 1 of the pump unit can be greatly increased.

Claims: WHAT IS CLAIMED IS: 1. 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,1OB) 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, a thrust load reducing device comprising: : a first fine gap (G1) 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. 2. A thrust load reducing device as claimed in claim 1, further comprising a second fine gap (G2) 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 fine gap (G2), and a pressure control chamber (14) defined by the first gap (G1) defining member (12), the second gap (G2) defining member (13), the front surface of the thrust disc (7) on the suction side and the inner peripheral surface of the casing (3). 3. A thrust load reducing device as claimed in claim 2, wherein said first fine gap (G1) defining member (12) has a large axial length to thereby increase the resistance offered by the first fine gap (G1) to the flow of a fluid therethrough. 4. A thrust load reducing device as claimed in claim 3, wherein said second fine gap (G2) 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 therebetween a fine axial gap (G2). 5. A thrust load reducing device as claimed in claim 3, wherein said second fine 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 a fine gap (G2, G21, G22) therebetween. 6. A thrust load reducing device as claimed in claim 3, wherein said second fine 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 a fine gap (G23) therebetween by enclosing an end face and inner and outer peripheral surfaces of the first-mentioned cylindrical body (17). 7. A thrust load reducing device as claimed in any one of claims 1 to 6, wherein said fine gap defining members (12; 13, 15, 16; 17, 18, 19) are each formed with a labyrinth at an inner surface facing the fine gap (G1; G2; G23). 8. A thrust load reducing device as claimed in any one of claims 1 to 6, wherein said fine gap defining members (12; 13; 15, 16; 17, 18, 19) are each formed with a spiral groove at an inner surface facing the fine gap (G1; G2; G23).

Inventors: HAZAKI, EIICHI, INOUE, HIROSHI, NAKADAIRA, SHIRO, OKADA, AKIHISA, YOSHIOKA, MASAHIRO
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: F04D29
IPC codes: F04D29, F04D7, F04D15