Abstract:
A target fluid discharged from a pump is partially diverted, by a lead-in duct, into a housing chamber including a motor therein. A top bearing supporting a motor shaft is placed in the lead-in duct, and lubricated by the target fluid flowing through the lead-in duct. A top seal is installed downstream from the top bearing in the lead-in duct. The top seal has a slight gap from the motor shaft to form a throttle structure which increases a flow resistance. The throttle structure maintains a pressure in an area near the top bearing, to thereby suppress presence of gases.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2003-338540 filed on Sep. 29, 2003. The content of the application is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a submersible pump integrally configured by a pump and a motor to drive the pump, and submerged in a reserved target fluid, for use in pumping up the target fluid.  
         [0004]     2. Description of the Related Art  
         [0005]     To pump up liquefied natural gas (LNG), liquefied petroleum gas, or the like, there has been utilized a submersible pump in which a pump, such as a centrifugal pump, and a motor for driving the pump are integrated into one unit. Such a submersible pump is used in a state that it is entirely immersed in the target fluid to be pumped by the pump. The submersible pump is configured so as to allow entry of the target fluid into regions around a stator and a rotor of the motor and around bearings for shafts of the motor and the pump in order to utilize the target fluid for cooling, lubrication, or the like.  
         [0006]     In one example pump, a target fluid discharged by a pump is conveyed to a bearing located at an upper end of a shaft to lubricate the bearing. Such a pump is described in Japanese Utility Model Application No. Hei 5-26949 (Japanese Utility Model Laid-Open Publication No. Hei 6-80896).  
         [0007]     However, in such submersible pump, when the target fluid contains a easily vaporizable constituent, it is common for the bearing to be insufficiently lubricated due to vaporization of the constituent in the vicinity of the bearing, leading to malfunction of the pump.  
       SUMMARY OF THE INVENTION  
       [0008]     This invention provides a submersible pump in which a bearing is advantageously protected against insufficient lubrication.  
         [0009]     A submersible pump according to the present invention comprises a lead-in duct through which a target fluid discharged from a pump is partially directed to flow through a bearing supporting a shaft of a motor almost at an upper end of the shaft and then conveyed into a housing in which the motor is installed, and a throttle structure installed downstream from the bearing in the lead-in duct.  
         [0010]     Further, in the submersible pump, an entrance of a reflux channel for returning, to a pump unit, the target fluid existing in the housing in which the motor is installed is disposed at a place situated higher than the bearing located near the upper end of the motor shaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  schematically shows a configuration of a submersible pump according to an embodiment;  
         [0012]      FIG. 2  shows a detail of the structure near an upper end of a motor shaft in the submersible pump; and  
         [0013]      FIG. 3  shows another detailed structure near the upper end of the motor shaft in the submersible pump. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     Referring now to drawings, an embodiment of this invention will be described below.  FIG. 1  schematically shows a configuration of a submersible pump  10  according to this embodiment. The submersible pump  10  has a structure in which a pump unit  12  including a pump for boosting a pressure of a target fluid to be pumped up and a motor unit  14  for driving the pump are contained in a pot  16 . An opening at the top of the pot  16  is covered by a head plate  18 . The target fluid is introduced from an intake pipe  20  into the pot  16 , while gases are removed through a degassing pipe  22  to enable the pot  16  to fill with the target fluid. The submersible pump  10  should be placed in the same vertical position as illustrated in  FIG. 1  in actual use. In other words, the submersible pump  10  is placed so that the pump unit  12  will be located below the motor unit  14 .  
         [0015]     The pump unit  12  includes a multistage centrifugal pump in which a plurality of centrifugal pumps  26  are arranged on a common pump shaft  24 . A lower part of the multistage centrifugal pump, more specifically, a suction port of the centrifugal pump  26  on the first stage is attached to a suction pipe  28  which includes, in the inside thereof, a inducer  30  mounted to the pump shaft  24  and actuated with the pump shaft  24 .  
         [0016]     The motor unit  14  located above the pump unit  12  includes an electric motor  32  for driving the multiphase centrifugal pump and a housing cylinder  34  placed so as to enclose the motor  32 . The motor  32  comprises a motor shaft  36  shared as the pump shaft  24 , a rotor  38  fixed to the motor shaft  36 , and a stator  40  fixed on the housing cylinder  34 . The bottom of the housing cylinder  34  is joined to an attachment plate  42 , and the pump unit  12  is also joined to the attachment plate  42  to be integral with the motor unit  14 . The top of the housing cylinder  34 , on the other hand, is connected to a discharge manifold  44 . Thus, the housing cylinder  34 , the attachment plate  42 , and the discharge manifold  44  constitute a housing in which the motor  32  is installed.  
         [0017]     The target fluid is pressurized in the pump unit  12 . The attachment plate  42  holds a bottom bearing  46  which is a rolling bearing rotatably supporting the motor shaft  36 , and includes a bottom seal  48  for suppressing the flow of pressurized target fluid into the motor unit  14 . Because a small gap is present between bottom seal  48  and the pump shaft  24 , a small amount of the target fluid enters a housing chamber  49 , which is a space formed in the housing of the pump unit  12 . This small amount of target fluid flows through the bottom bearing  46  while lubricating the bottom bearing  46 . The discharge manifold  44  holds a top bearing  50  rotatably supporting the motor shaft  36  in the proximity of an upper end of the motor shaft  36 . Further, in the discharge manifold  44 , there is provided a discharge port  54  communicating with a discharge hole  52  formed in the head plate  18 . The target fluid delivered from the multistage centrifugal pump is conveyed to the discharge port  54  through a discharge channel  56  running around components, such as, for example, the stator  40  of the motor unit  14 , and then pumped to the outside.  
         [0018]     The discharge manifold  44  comprises a lead-in duct  58  for diverting the target fluid from the discharge port  54  into the housing chamber  49 . The top bearing  50  is installed in the lead-in duct  58 , and a top seal  60  is installed downstream from the top bearing  50 . These components associated with the lead-in duct  58  will be described below. An entrance  62   a  to a reflux channel  62  for returning the target fluid contained in the housing chamber  49  to the pump unit  12  is provided in the housing chamber  49 , at a location higher than the top bearing  50 , preferably at a topmost part of the housing chamber  49 . Through the reflux channel  62 , gases accumulated in the housing chamber  49  is collected and conveyed to the pump unit  12  together with the target fluid.  
         [0019]      FIG. 2  shows a detailed configuration of the lead-in duct  58 . The lead-in duct  58  directs the target fluid from the discharge port  54  to the top bearing  50  which is a rolling bearing, preferably a ball bearing. The target fluid flows downstream and passes through gaps between an inner race and a rolling element of the top bearing  50  and between an outer race, an inner race and the rolling element. The target fluid passing through the top bearing  50  functions as a lubricant for the top bearing  50 . The target fluid flowing past the top bearing  50  reaches the top seal  60  which includes a small clearance  64  between the seal and the outside of the motor shaft  36 . Along with the outside perimeter of the motor shaft  36  and the clearance  64 , the top seal  60  constitutes a throttle structure in which a flow path in the lead-in duct  58  is narrowed, wherein flow resistance in the lead-in duct  58  increases. This throttle structure causes a pressure differential to develop across the clearance  64 , with the result that the area around the top bearing  50  is maintained at a pressure close to the pressure raised by the multistage centrifugal pump, while the inside of the housing chamber  49  is maintained at a relatively lower pressure. In addition, a flow rate of the target fluid passing through the lead-in duct  58  is restricted by the throttle structure, which suppresses any decrease in discharge from the submersible pump  10 , in other words, any decrease in flow rate of the target fluid to be pumped.  
         [0020]     By maintaining the area around the top bearing  50  at a high pressure, the target fluid is protected from vaporizing in the area around the bearing, even if a constituent prone to vaporization is present in the target fluid, which in turn ensures sufficiency of the lubrication of the bearing. Accordingly, the dimensions of each component of the throttle structure, such as, for example, clearance and length, should be set a dimension that will ensure the capability of maintaining a pressure which can prevent, or significantly suppress, vaporization of the target fluid in the area around the bearing. The shape of the flow path in the throttle structure may be, for example, crank-shaped rather than straight. Further, labyrinth structure  66  as shown in  FIG. 3  may be formed in the seal  60  so as to alternately dispose narrow and broad gaps.  
         [0021]     Because the gas which occurs in the housing chamber  49  is returned to the reflux channel  62  from the position located higher than the top bearing  50 , it is possible to prevent a situation in which lubrication will be affected by lowering of the fluid level by the gas accumulated in the housing chamber  49  to a level reaching the top bearing  50 .  
         [0022]     The submersible pump  10  according to this embodiment may be preferably used as a pump for handling LNG which contains carbon monoxide. When the LNG contains carbon monoxide, a gas (carbon monoxide) appears at a lower temperature because the boiling point of carbon monoxide is lower than those of other constituents in the LNG, and lower than the boiling point of another LNG which contains no carbon monoxide. As described above, by installing the throttle structure downstream from the top bearing  50 , poor lubrication due to the occurrence of gas in the vicinity of the top bearing  50  can be prevented.  
         [0023]     Although an example three-stage centrifugal pump was described to illustrate a preferred embodiment of the present invention, the number of stages is not so limited, and may be changed as appropriate according to design requirements. Similarly, bearings other than the ball bearing may be used as the top and bottom bearings, and may be selected in consideration of other design requirements.