Abstract:
A pumping apparatus comprises a rotatable shaft coupled to a motor. At least one control structure is mounted on the shaft, and at least one impeller is carried by the shaft. The impeller is adapted to rotate independently of the shaft so as to pump a fluid. The apparatus also comprises a magnetic coupling between the control structure and the impeller. Further, the impeller is adapted to translate axially along the shaft in response to a change in downstream pressure to alter the strength of the magnetic coupling.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a submersible pump for pumping liquid fuel. More particularly, the invention relates to a submersible pump utilizing a magnetic clutch activated impeller for controlling pressure during periods of low or no flow. 
     BACKGROUND OF THE INVENTION 
     Gasoline service stations normally have underground storage tanks (USTs) from which fuel is pumped to dispensers. A typical installation makes use of a unitized motor and pump (UMP) in the storage tank which operates using one or more impellers to pump gasoline or another liquid fuel to a distribution head located above the tank. The flow path for the fuel includes a vertical column pipe which extends from the pump to the distribution head. From the distribution head, the fuel is supplied to one or more dispensers, each of which may have multiple fueling positions. The fuel is then delivered to a customer&#39;s vehicle tank via a hose and nozzle at each fueling position. 
     Governmental regulations typically limit the flow rate of fuel at each nozzle, for example to 10 gallons per minute. Because service station owners have an interest in servicing customers as quickly as possible, they desire a fuel flow rate approaching this maximum. 
     Submersible pumps are often configured to operate the impeller(s) at a constant RPM even if fuel is not being dispensed. However, operating the pump at a fixed speed may create undesirable high pressures during low or no flow conditions. In particular, when the pump is on and all nozzles are open, pressure in the pump is relatively low. This causes low flow rates at each nozzle. Thus, in some installations two or more UMPs may be manifolded together to achieve higher flow rates when a large number of nozzles are simultaneously open. In any case, when the pump is on and less fuel is being dispensed (i.e., one or more nozzles is closed), the pressure in the pump will increase. This pressure during a stopped flow condition (i.e., all nozzles are closed) may be high enough to damage components of the fuel dispensing system. 
     One prior art solution involves using a variable speed drive (VSD) to control the speed of the impellers in a low or no flow condition. These systems employ some method of feedback to determine when to reduce the impeller speed. For example, a VSD may be provided with a pressure transducer or it may monitor the current delivered to the pump motor. However, the VSD and its associated feedback devices are complex and expensive. 
       FIG. 1  illustrates the above-described operating characteristics of a standard constant speed UMP, a manifolded constant speed UMP, and a UMP using a VSD as the number of nozzles in operation increases. 
     Another potential solution involves using a bypass valve to divert fuel back to the UST during low flow conditions, thereby limiting the pressure. However, this may interfere with existing devices required for leak detection. Specifically, environmental regulations require that USTs be monitored for leaks, and typically a liquid level float is provided for this purpose. The liquid level float is adapted to detect small changes in liquid level to identify potential leaks. Because diverting fuel back into the tank causes liquid surface disturbances, this solution could interfere with the float&#39;s operation. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses disadvantages of prior art constructions and methods. According to one embodiment, the present invention provides a pumping apparatus comprising a pump housing disposed in a storage tank and in fluid communication with fluid piping. The pumping apparatus comprises a rotatable shaft and at least one magnetic clutch assembly coupled with the shaft and configured to pump fluid from the storage tank through the pump housing to the fluid piping. The at least one magnetic clutch assembly comprises an impeller, a control structure, and a magnetic coupling between the impeller and the control structure. The magnetic coupling is defined by a conductive portion and a plurality of magnets proximate the conductive portion such that rotation of the control structure causes rotation of the impeller. The impeller is configured to travel along the shaft relative to the control structure in response to a change in downstream pressure. 
     According to a further embodiment, the present invention provides a pumping apparatus comprising a rotatable shaft coupled to a motor. At least one control structure is mounted on the shaft, and at least one impeller is carried by the shaft. The impeller is adapted to rotate independently of the shaft so as to pump a fluid. The apparatus also comprises a magnetic coupling between the control structure and the impeller. Further, the impeller is adapted to translate axially along the shaft in response to a change in downstream pressure to alter the strength of the magnetic coupling. 
     According to a further embodiment, the present invention provides a method for pumping fluid from a storage tank to fluid piping. The method comprises providing a pump housing having an inlet in fluid communication with the fluid and an outlet in fluid communication with the fluid piping. The method also comprises providing a rotatable shaft located inside the housing, the shaft being in operative communication with a motor. The method also comprises coupling an impeller with the shaft, the coupling allowing the impeller to rotate independently of and translate axially along the shaft. Further, the method comprises mounting a control structure on the shaft such that the control structure rotates with the shaft and magnetically coupling the control structure with the impeller. Finally, the method comprises rotating the control structure to draw the fluid into the inlet using the impeller and altering the strength of the magnetic coupling between the control structure and the impeller in response to a change in downstream pressure. 
     Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
         FIG. 1  is an exemplary graph illustrating the relationship between total dynamic head and flow rate of liquid fuel for a standard constant speed UMP, a manifolded constant speed UMP, and a UMP using a variable-speed drive (VSD). 
         FIG. 2  is a diagrammatic representation of a liquid fuel delivery system in accordance with one embodiment of the present invention. 
         FIG. 3  is a diagrammatic partial cross section of a unitized motor and pump (UMP) having a magnetic clutch activated impeller in accordance with one embodiment of the present invention. 
         FIG. 4  is an exemplary graph illustrating the relationship between pressure and flow rate of liquid fuel for a standard constant speed UMP and a UMP having the magnetic clutch activated impeller of  FIG. 3 . 
         FIG. 5  is an enlarged partial cross section of a magnetic coupling between the impeller and the control disc of a UMP constructed in accordance with an alternative embodiment of the present invention. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of any appended claims and their equivalents. 
     The present invention provides a submersible pump having at least one impeller driven via a magnetic clutch. The magnetic clutch comprises a magnetic coupling which imparts a torque to the impeller that varies as a function of the pressure difference across the impeller. 
       FIG. 2  illustrates a fuel delivery system in a service station environment according to one embodiment of the present invention. A fuel dispenser  10  delivers fuel  12  from an underground storage tank (UST)  14  to a vehicle. Fuel dispenser  10  has a dispenser housing  16  that typically contains an electronic control system  18  and a display  20 . Various fuel handling components, such as valves and meters, are also located inside of housing  16 . These fuel handling components allow fuel  12  to be received from underground piping and delivered through a hose and nozzle to a vehicle, as is well understood. 
     As noted above, fuel  12  is stored in UST  14 . Those of skill in the art will appreciate that there may be a plurality of USTs  14  in a service station environment if more than one type or grade of fuel  12  is to be delivered by fuel dispensers  10 . In this case, UST  14  is a double-walled tank having an inner vessel  22  that holds the fuel  12  surrounded by an outer casing  24 . Any leaked fuel  12  from a leak in inner vessel  22  will be captured in an interstitial space  26  that is formed between inner vessel  22  and outer casing  24 . More information on underground storage tanks in service station environments can be found in U.S. Pat. No. 6,116,815, incorporated herein by reference in its entirety for all purposes. 
     A unitized motor and pump (UMP)  30  is provided to draw fuel  12  from UST  14  and deliver it to fuel dispenser(s)  10 . One example of a prior art UMP is the RED JACKET® submersible turbine pump, manufactured by Veeder-Root Co. of Simsbury, Conn. Another example of a prior art UMP is disclosed in U.S. Pat. No. 6,126,409, incorporated herein by reference in its entirety for all purposes. UMP  30  is modified from the prior art to utilize a magnetic clutch for controlling downstream pressure as will be described herein. 
     UMP  30  includes a distribution head  32  that incorporates power and control electronics. The distribution head  32  is typically placed inside a sump  34 . Electronics in the distribution head  32  may be communicatively coupled to a tank monitor  36 , site controller  38 , or other control system via a communication line  40 . An example of a tank monitor  36  is the TLS-450 manufactured by the Veeder-Root Co. An example of a site controller  38  is PASSPORT® point-of-sale system manufactured by Gilbarco Inc. of Greensboro, N.C. 
     Distribution head  32  is fluidly connected to a column pipe  42  which provides fluid communication to fuel  12  inside of UST  14 . Column pipe  42  is surrounded by a riser pipe  44  which is mounted (using a mount  46 ) to the top of the UST  14 . In particular, the column pipe  42  extends down into the UST  14  and is terminated with a boom  48 . Boom  48  is coupled to a pump housing  50  that contains a motor and at least one impeller. The inlet  52  of pump housing  50  is located near the bottom of UST  14  as shown. 
     In operation, impeller(s) inside the housing  50  rotate to draw fuel  12  into the housing inlet  52  and thus into the boom  48 . The fuel  12  is pushed through column pipe  42  and delivered to the main fuel piping conduit  54 . In this embodiment, main fuel piping conduit  54  is a double-walled piping having an interstitial space  56  formed by outer wall  58  to capture any leaked fuel. Finally, main fuel piping conduit  54  is coupled to the fuel dispensers  10  in the service station whereby fuel  12  is delivered to a vehicle. 
       FIG. 3  illustrates the lower portion of pump housing  50  showing a magnetic clutch activated impeller in accordance with one embodiment of the present invention. Those of skill in the art will appreciate that UMP  30  may comprise more than one magnetic clutch driven impeller, as needed or desired. For simplicity of explanation, however, only one magnetic clutch driven impeller is discussed below. Further, although the below discussion contemplates a magnetic clutch of the eddy-current type, those of skill in the art will appreciate that other types of magnetic clutches may be used. 
     In one embodiment, housing  50  contains a magnetic clutch assembly  60  and a pressure control assembly  62  positioned axially along pump shaft  64 . A motor located downstream of pressure control assembly  62  in housing  50  is in electrical communication with distribution head  32  and is operative to rotate pump shaft  64 . 
     In this embodiment, magnetic clutch assembly  60  comprises an impeller control disc  66  which rotates with shaft  64 , a spring  68 , and an impeller  70  which is magnetically coupled with control disc  66  as described in more detail below. Control disc  66  may be configured as a wheel-like structure formed of steel or other suitable material. This structure defines a plurality of radial ribs which allow liquid fuel to pass through disc  66 . Impeller control disc  66  is preferably keyed to shaft  64  to prevent relative rotation therebetween. Set screws  72  or the like may be used as necessary or desired to maintain disc  66  in position. Importantly, control disc  66  is also provided with a circumferential array of closely-spaced permanent magnets  74 . The magnets  74  are arranged such that their poles alternate between North and South. 
     Spring  68  is fixed between a flange  76  of impeller  70  and thrust bearing  78 . Spring  68  may preferably be a helical compression spring that is preloaded by an amount sufficient to resist further compression until a predefined pressure in UMP  30  is reached. Because impeller  70  typically has a lower rotational speed than control disc  66 , thrust bearing  78  is provided to facilitate relative rotational motion between spring  68  (which rotates with impeller  70 ) and control disc  66 . Thrust bearing  78  also supports axial compression of spring  68  as the pressure in UMP  30  increases. 
     Impeller  70  is preferably a radial flow impeller comprising a plurality of vanes supported by a shroud as is well understood. In this embodiment, shaft  64  is provided with a sleeve  80 , to which the inner race of radial ball bearing  82  is slidably affixed. Impeller  70  is received over shaft  64  such that the outer race of bearing  82  is affixed to central bore  84  of impeller  70 . Bearing  82  thus facilitates relative rotation between shaft  64  and impeller  70 . Sleeve  80  is preferably formed of a low friction material, such as polytetrafluoroethylene (PTFE), to allow bearing  82  and impeller  70  to translate along the axis of shaft  64  to the extent of sleeve  80 . Thus, impeller  70  may both rotate at a speed independent of shaft  64  and move axially along shaft  64  when the pressure in UMP  30  overcomes the preload in spring  68 . 
     A cylinder  86 , preferably formed of steel, is affixed to the peripheral edge of impeller  70 . Cylinder  86  preferably has a length L such that when spring  68  is in its initial, preloaded state, distal end  88  of cylinder  86  overlaps peripheral magnets  74  on control disc  66 . In one embodiment, cylinder  86  is provided with a ring  90  of highly conductive material (e.g., aluminum or copper) affixed to its interior peripheral edge and flush with distal end  88 . There is preferably a small gap G between magnets  74  and ring  90  sufficient to allow control disc  66  and impeller  70  to rotate freely. 
     Those of skill in the art will appreciate that as shaft  64  and control disc  66  rotate, the array of magnets  74  on control disc  66  apply a time-varying magnetic field to the conductive ring  90  to induce eddy currents therein. The eddy currents in the conductive ring  90  create an electromagnetic force (EMF) that acts to oppose the field applied by control disc  66 . The interaction of these fields causes the ring  90  (and thus the cylinder  86  and impeller  70 ) to rotate with the control disc (and shaft  64 ). However, because relative motion is required to produce the time-varying magnetic field, the rotational speed of the impeller will be less than the rotational speed of shaft  64  and control disc  66 . This difference in speed is referred to as “slip.” 
     The torque that the input shaft  64  imparts to the impeller  70  is directly proportional to the flux density of the magnetic field applied by magnets  74  to conductive ring  90  and to the EMF induced in ring  90 . The flux density through conductive ring  90  will depend on the amount of surface area perpendicular to the magnetic field induced by the magnets  74 . Thus, for example, the strength of the coupling will increase as the overlap increases between the conductive ring  90  on the cylinder  86  and the peripheral array of magnets  74  on the control disc  66 . (In addition, the strength of the coupling will increase as the gap G between the conductive ring  90  on the cylinder  86  and the magnetic array  74  on control disc  66  decreases. Thus, those of skill in the art can select a suitable value for dimension G based on various factors.) 
     Further, the EMF induced in ring  90  is related to the time rate of change of magnetic flux. Generally, as the difference in rotational speed between control disc  66  and cylinder  86  increases, the rate of change of flux increases. Thus, the torque transferred from shaft  64  is proportional to the slip. For example, to compensate for an increase in load torque on the impeller  70  (e.g., when pressure increases), the slip will also increase (i.e., the rotational speed of impeller  70 , cylinder  86 , and ring  90  will decrease relative to the rotational speed of shaft  64  and control disc  66 , which remains constant). Where control disc  66  is rotating at a much higher speed than cylinder  86 , there is a high rate of change of magnetic flux, which generates a large EMF and hence the larger torque required. Notably, because the rotational speed of impeller  70  has decreased, the rate of pressure increase across UMP  30  will also decrease. 
     The pressure control assembly  62  comprises a support disc  92 , a diffuser  94 , and a plurality of spring-loaded guide pins  96 . Support disc  92  is preferably formed of steel and defines a plurality of apertures  98  to allow liquid fuel to flow therethrough. Support disc  92  is fixed to housing  50  via suitable mounting hardware  100  and thus does not rotate with or translate along shaft  64 . Shaft  64  penetrates a central bore  102  of support disc  92  and may be provided with a bushing sleeve  104 . In any case, the diameter of central bore  102  is large enough for shaft  64  to rotate freely without contacting support disc  92 . In alternative embodiments, a bearing may be provided in central bore  102 . 
     Diffuser  94  is mounted on spring-loaded guide pins  96  in close proximity to impeller  70 . Shaft  64 , which may be provided with a bushing sleeve  104 , penetrates a central bore  106  of diffuser  94 . As with bore  102  of support disc  92 , the diameter of bore  106  is sufficiently large to allow shaft  64  to rotate freely in relation to diffuser  94 . Diffuser  94  defines a plurality of stationary guide vanes to direct the liquid fuel axially as it is thrust radially upwards by impeller  70 . Thus, guide pins  96  are preferably slidably mounted in a plurality of bores  108  angularly spaced about diffuser  94  to support diffuser  94  for axial movement while preventing its rotation. In some exemplary embodiments, four guide pins  96  may be provided. Springs  110 , which may be compression springs having a smaller spring constant than spring  68 , are fixed on guide pins  96  between support disc  92  and diffuser  94  to maintain diffuser  94  and impeller  70  in close proximity. 
     Diffuser  94  will thus translate axially along shaft  64  toward control disc  66  when the downstream pressure in UMP  30  increases above the preload of spring  68 . A thrust bearing  112  is provided between diffuser  94  and impeller  70  to receive the axial load of diffuser  94  while allowing relative rotation between impeller  70  and diffuser  94 . Slide bearings  114  may be mounted in bores  108  of support disc  92  to facilitate linear motion of guide pins  96  as springs  110  compress and expand. 
     The configuration of components of magnetic clutch assembly  60  may be altered within the scope of the present invention. For example, the configuration of impeller  70  and control disc  66  may be reversed in some embodiments, such that impeller  70  is disposed upstream of control disc  66 . Further, in some embodiments the magnetic coupling may be proximate impeller  70  rather than control disc  66 ; in such a case, cylinder  86  may be affixed to the periphery of control disc  66  and magnetic array  74  may be provided on impeller  70 . Moreover, the positions of ring  90  and magnetic array  74  may be reversed, such that ring  90  is coupled with control disc  66  and magnetic array  74  is coupled with cylinder  86 . Those of skill in the art will appreciate that additional configurations are contemplated. 
     Referring now to  FIG. 4 , the operation of the magnetic clutch operated impeller of UMP  30  will be described. In particular,  FIG. 4  illustrates an exemplary relationship between pressure and flow rate of liquid fuel in a standard constant speed UMP in comparison with a UMP having the magnetic clutch activated impeller of  FIG. 3 . In a standard UMP, the pressure steadily increases as flow rate decreases, approaching a maximum at very low flow rates. There is concern that the high pressures caused by low and no flow conditions may damage the UMP. 
     In contrast, in a UMP having the magnetic clutch activated impeller of  FIG. 3 , the magnetic clutch assembly  60  and the pressure control assembly  62  cooperate to reduce the rate of increase in pressure across UMP  30  as flow rate decreases and maintain the pressure at a desirably lower level. More specifically, during a high flow condition, such as when all nozzles are open, the pressure across UMP  30  is relatively low. In this case, the load torque on impeller  70  is also relatively low, so the relative velocity (i.e., slip) between the impeller  70  and the control disc  66  will also be low. 
     As noted above, as nozzles begin to close and the flow rate decreases, the pressure across UMP  30  will increase. Because diffuser  94  may translate axially, this pressure increase will force diffuser  94  toward control disc  66 . However, preloaded spring  68  will counteract this force. As a result, impeller  70  and diffuser  94  will be maintained in a fixed location along the axis of shaft  64  until the pressure increases above a predetermined threshold. 
     When the pressure increases such that the force on diffuser  94  and impeller  70  is greater than the preload of spring  68 , spring  68  will begin to compress and the diffuser  94  and impeller  70  will begin to translate axially along shaft  64  toward control disc  66 . As the impeller  70  translates toward control disc  66 , conductive ring  90  will move axially away from magnetic array  74 . This reduces the amount of conductive ring  90  perpendicular to the magnetic field of magnetic array  74  and hence reduces the torque imparted by control disc  66 . 
     Importantly, the reduced rotational speed of impeller  70  caused by the reduction in surface area perpendicular to the magnets  74  substantially reduces the rate at which pressure will increase in UMP  30 . The change in slope of the pressure-flow curve which occurs as the preload of spring  68  is overcome is labeled in  FIG. 4  as a “knee.” Those of skill in the art may select the preload such that the knee occurs at a predetermined pressure level in UMP  30 . As the flow rate continues to decrease beyond the knee, the pressure will be relatively stable because the magnetic clutch will continue to adjust to slight increases in pressure as described above. The slope of the pressure-flow curve beyond the knee is determined by the spring constant K of spring  68 . 
     When customers begin to dispense fuel again and the flow becomes less restricted, the downstream pressure at UMP  30  will decrease. This causes spring  68  to uncompress, the surface area of ring  90  perpendicular to the magnetic field to increase, and control ring  66  to impart more torque to impeller  70 . To compensate, the relative velocity between impeller  70  and control disc  66  will decrease (i.e., the impeller  70  will increase in speed relative to control disc  66 ). The slip will thus decrease until the torque imparted from shaft  64  and control ring  66  is equal to the load torque, when the system will be at equilibrium. 
     According to a further embodiment, the impeller of a UMP may be driven using a synchronous magnetic coupling. More particularly,  FIG. 5  is an enlarged view of a synchronous magnetic coupling, generally indicated at  120 , between a control disc  122  and a cylinder  124 . Control disc  122  and cylinder  124  may preferably be analogous to control disc  66  and cylinder  86  such that rotation of control disc  122  causes rotation of cylinder  124  (and its associated impeller) as described below. 
     In this embodiment, control disc  122  is provided on its circumference with a low energy, ferromagnetic material  126 . Material  126  may preferably be Alnico  5  or a similar material. It is known that such materials can be magnetized to produce permanent magnets. In some embodiments of the present invention, however, material  126  is not magnetized prior to installation on control disc  122 . Additionally, cylinder  124  is provided with an array of high energy magnets  128  affixed to its interior peripheral edge. Magnets  128 , which may preferably be formed of Somarium Cobalt or Neodymium, are preferably arranged such that their poles alternate between North and South. It will be appreciated that the arrangement of material  126  and magnets  128  may be reversed, such that material  126  is provided on cylinder  124  and magnets  128  are provided on control disc  122 . As discussed above, there is preferably a small gap G between magnets  128  and material  126  sufficient to allow control disc  122  and the impeller to rotate freely. 
     In many embodiments, it may be desirable to configure material  126  as a plurality of individual elements respectively opposing each magnet  128  (although in other embodiments material  126  may be provided as a continuous ring). Where material  126  is configured as a plurality of individual elements, each element will magnetize “ad hoc” (i.e., acquire a polarity opposite its corresponding magnet  128 ) such that cylinder  124  will “track,” or rotate with, control disc  122  in a synchronous fashion. Thereby, cylinder  124  (and, thus, the impeller) will not slip at low torques. Higher torques may cause cylinder  124  to slip relative to control disc  122 , although magnetic coupling  120  may still transmit torque to the impeller. In particular, as cylinder  124  slips relative to control disc  122 , elements of material  126  will repolarize at a rapid rate. This repolarization causes a recovery torque as the elements of material  126  attempt to realign with magnets  128 . Thus, this recovery torque operates to limit the amount of slip which occurs. It will be appreciated that the physical characteristics of magnetic coupling  120  may be selected to produce slip at a desired torque level. 
     The coupling between material  126  and magnets  128  will tend to counteract axial movement of the impeller toward control disc  122 , and thus spring  68  may not be necessary in this embodiment. However, because the force of magnetic coupling  120  counteracting axial movement of the impeller is nonlinear, a spring analogous to spring  68  may be disposed between the impeller and control disc  122  to provide linearity. 
     While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the scope and spirit thereof.