Abstract:
A selflubricating bearing for use submerged in cryogenic fluids. The self-lubricating bearing comprises hardened races, hardened rolling elements and polymeric retainers or cages. More particularly, the rolling elements maybe hardened balls, and the polymeric cages may include PEEK. In particular a bearing for self lubricated use in seal-less, magnetic drive pump for pumping fluids at very low temperaturesand cryogenic temperatures below about −100 degrees centigrade such as liquid nitrogen and temperatures below about −150 degrees centigrade such as liquefied natural gas (LNG). An environment for submerged use of such a bering may be a magnetically driven cryogentic pump with a housing having an intake and exhaust with a back plate mounted therein in which a shaft is journaled in self-lubricating bearings having hardened stainless steel or ceramic balls, stainless steel races and polymeric retainers or cages for retaining the balls for rolling contact in the races. An impeller is mounted on the first end and a first magnet is mounted on the second end of the shaft.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Liquefied natural gas (LNG) and other very low temperature fluids are of increasing commercial importance. There is, therefore, a need for increased facility in handling, storing, and transporting such liquids. For example, LNG is being increasingly utilized as an alternative fuel source for internal combustion engines. Governmental regulations require that LNG be transported at pressures of about 30 psi, but to decrease the amount of LNG that is evaporated or otherwise lost from a stationary storage tank, it is common to store the LNG at pressures of 150 psi. When “bottled” for use as the fuel for an internal combustion engine, it is common to pressurize the LNG to pressures as high as 220 psi. Of course each increase in pressure requires that the LNG be pumped into either a storage tank or into a fuel tank at the next higher pressure such that successful use of LNG as an alternative fuel depends, in effect, upon reliable, safe and energy efficient pumping of high volumes of such fluids at very low temperatures.  
           [0002]    Bearings used submerged in very low temperature fluids often operate without a source of lubrication flowing with the fluid. Oil and grease or other normal lubricants will not function at the very low temperatures. Graphite and other solid friction reducing materials wash away in the fluid and can unacceptably contaminate the fluid.  
           [0003]    Pumps presently used for pumping low temperature fluids all suffer from a variety of disadvantages and limitations which limit their life, require frequent maintenance, and otherwise decrease their utility. This is particularly true when the temperature of the fluid must be very low. For instance, pumps that are currently available for pumping LNG, but wear out quickly and need frequent maintenance and particularly require frequent replacement of the seals. Heretofore known seal-less pumps have not provided a satisfactory solution to this problems. For instance, magnetic drive pumps known in the fluid pump art, are not reliable for use at very low temperatures. Bonding material utilized on the magnets at low temperature nevertheless fails at very low temperature. In the case of LNG, severe problems result from the almost complete lack of lubrication that is provided by the LNG passing through the pump. As a result bearings wear rapidly and need frequent replacement. Sometimes rapid bearing wear leads to catastrophic pump failure.  
           [0004]    It is, therefore, an object of the present invention to provide a pump for use at very low temperatures which is not limited by the disadvantages of known pumps. More specifically, it is an object of the present invention to provide an improved bearing for a seal-less magnetic drive pump for use in pumping at very low temperatures.  
           [0005]    Another object of the present invention is to provide a bearing useful at very low temperatures used herein to mean temperatures lower than about −100° C. Yet another object of the present invention is to provide such a low temperature bearing which is self lubricated when used in a pump for pumping, high volumes of commonly utilized fluids as LNG and liquid nitrogen.  
           [0006]    It is a further object to provide a bearing constructed for self lubrication when it is submerged in cryogenic fluid such as LNG and liquid nitrogen.  
         SUMMARY OF THE INVENTION  
         [0007]    These objects, and the advantages, of the present invention are met by providing bearing that is useful in a magnetic drive pump for use in pumping fluids at very low temperature, below about −100° C. and more particularly for pumping cryogenic fluids at temperatures below about −150° C. The pump has a back plate with a rotatable shaft journaled therein. The rotatable shaft is journaled in one or more self-lubricating bearings comprising hardened rolling elements and races, such as balls and ball races sized for close tolerance rotation at very low temperatures and polymeric ball retainers or cages providing lubricity to the retained rolling elements at very low temperatures. While stainless steel rolling elements such as stainless steel balls will work, an improved combination provides ceramic rolling elements, particularly ceramic balls, retained with a polymeric cage for rolling in stainless steel races. An impeller of the pump is mounted to the first end of the shaft, and a casing is mounted to the second end of the shaft and a first magnet is contained within the pump casing mounted to the rotatable shaft. The pump casing is comprised of a material having a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the material comprising the magnet. The back plate is mounted within a housing having openings formed therein for intake of a fluid to be pumped at low pressure and an exhaust for output of the high pressure fluid and a second magnet is positioned in close proximity exterior to the housing for rotation therearound, the second magnet being adapted for mounting to the drive shaft of a motor or other drive source for rotating the second magnet around the housing, thereby rotating the first magnet within the housing to pump the fluid.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The foregoing objects, advantages, and features, as well as other objects and advantages, will become more apparent with reference to the description, claims and drawings below, in which like numerals represent like elements and in which:  
         [0009]    [0009]FIG. 1 is a longitudinal sectional view through a preferred embodiment of a pump constructed in accordance with certain teachings of the present invention;  
         [0010]    [0010]FIG. 2 is a sectional view similar to FIG. 1 of an alternative embodiment of the apparatus of the present invention;  
         [0011]    [0011]FIG. 3 is an enlarged side plan view of a ball bearing for use in a magnetic drive pump for pumping low temperature fluid according to one aspect of the present invention; and  
         [0012]    [0012]FIG. 4 is a section view of the bearing of FIG. 3 taken along section line  4 - 4  of a bearing having stainless steel balls, stainless steel rings and polymeric cage; and  
         [0013]    [0013]FIG. 5 is a section view of an enlarged bearing having steel races, ceramic balls and a polymeric cage according to one embodiment of the invention; and  
         [0014]    [0014]FIG. 6 is a section view of the bearing of FIG. 5 taken among section line  5 - 5 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading the present disclosure, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.  
         [0016]    A preferred embodiment of a pump constructed in accordance with the present invention will now be described with reference to FIG. 1 of the drawings. That pump, indicated generally at reference numeral  10 , is comprised of a back plate  12  having a rotatable shaft  14  journaled in a ball bearing  16  therein. An impeller  18  is mounted to the first end of shaft  14  by a screw  20 , a key  22  positioned in the key slots (not numbered) on the rotatable shaft  14  and impeller  18  preventing relative rotation therebetween. A first magnet  24  is mounted to the second end of rotatable shaft  14  by a screw  26  and jam nut  28 , the key  30  and key slots (not numbered) formed in the second end of the rotatable shaft and the magnet  24  preventing relative rotation in the same manner as the key  22  prevents relative rotation between the rotatable shaft  14  and impeller  18 .  
         [0017]    As noted above, one of the objects of the present invention is to provide a seal-less pump which is self-lubricating so as to decrease the need for maintenance of the pump. For that purpose, the ball bearing  16  is comprised of one or more ball races  32  having balls  34  positioned therein with a ball retainer or a cage  37 . The ball races  32  are comprised of an outer race  35  and an inner race  33 . The balls  34  and the ball races  32  being comprised of a hardened material that is both hard and durable at very low temperatures, for example, heat treated  440  stainless steel could be used. In the preferred embodiment, the polymeric material of the cage  37  more specifically comprises a polymer having self-lubricating and “shared lubricating” properties. A polymeric material such as polyether ether ketone sometimes known as PEEK available from ICI America (New York, N.Y.) has been found to work for this purpose. The polymer is believed to act to lubricate itself and to lubricate other parts of the bearing through a property termed herein as “shared lubricating”. The polymer must also avoid becoming unduly brittle at low temperature, it must also have sufficient durability and strength characteristics at low temperatures to function as a retainer or cage for the balls. Applicants have found these properties in the particular polymer, PEEK, and have found such polymer to be useful and particularly advantageous for purposes of this invention. It is theorized by applicants that the self lubrication and shared lubrication property resides in and results from minute wear particles or fine “dust” from the polymer such as PEEK, that roll, slide or otherwise “lubricate” both between the balls and the retainer for self-lubrication and also between the balls and the ball races for “shared lubrication”. It is theorized that “dust” worn off the polymer surfaces of the cage  37  coats and becomes imbedded in micro pores of the hardened surfaces of the balls and races or otherwise attaches onto the surface of the stainless steel balls and on the stainless steel races. It may be recognize from this description that the combination of other polymers and hardened metal balls and races that will operate without brittle fracture below about −100° C. and that have similar self-lubricating and/or “shared lubricating” properties may likewise be suitable for this use without departing from the spirit of this invention.  
         [0018]    The construction of the bearing according to the present invention can be more fully understood with reference to FIG. 3, which is a side plan view of such a bearing as describe above, together with FIG. 4, which is a section view of the bearing of FIG. 3 taken along section line  4 - 4 . The outer race  35  has a close tolerance grove as does the inner race  33 . The balls  34  are held in rolling contact with the races at the low temperature of intended operation with close tolerance maintained by forming the races and the balls of the same material that is both hard and durable at the very low temperature of the fluid to be pumped. It has been found that  440  stainless steel is suitable for this purpose. The balls  34  are spaced and separated by cage  37 , that may be formed of two circular shaped halves  37 ′ and  37 ″ with correspondingly positioned concave hemispherical segments  39 ′ and  39 ″ formed therein for defining spherical arc shaped cage chambers  39  for retaining the balls  34 . The halves  37 ′ and  37 ″ are held together with fasteners  41 , such as rivets, projecting through aligned holes and rigidly secured between the halves.  
         [0019]    In a further discovery it has been found that a bearing for use submerged in a cryogenic fluid can also uniquely be constructed to be self lubricating, where the ceramic balls as depicted in FIGS. 5 and 6.  
         [0020]    [0020]FIG. 5 is a side view of a bearing with ceramic balls, stainless steel races and a polymeric cage. FIG. 6 is a cross sectional view taken along section line  6 - 6  of FIG. 5. The outer race  350  has a close tolerance grove as does the inner race  330 . The balls  340  are held in rolling contact with the races at the low temperature of intended operation with close tolerance maintained by forming the races of stainless steel and the balls of ceramic material having compatible thermal expansion characteristics and a ceramic material that is both hard and durable at the very low temperature of the fluid to be pumped. The balls  340  are spaced and separated by a cage  370 , that may be formed comprising a polymeric material that provides lubrication to the bearing. The construction of the cage  370  may, for example, comprise two generally circular shaped halves  370 ′ and  370 ″ with correspondingly positioned concave hemispherical segments  390 ′ and  390 ″ formed therein for defining spherical arc shaped cage chambers  390  for retaining the balls  340 . The halves  370 ′ and  370 ″ are held together with fasteners  410 , such as rivets, projecting through aligned holes and rigidly secured between the halves. In particular it has been found that cages made from or coated with PEEK will provide self lubrication between the ceramic balls and cage and shared lubrication between the balls and the races.  
         [0021]    It has been theorized that the minute poracity of the ceramic balls uniquely facilitate the lubrication provided by the minute particles or fine “dust” worn from the cage during use. Whether the porocity facilitates the continuous wearing of the cage to provide the fine dust, provides sites in the pores for the particles to lodge, or both is not yet known. However, in theory both modalities can be present to unique advantage. The thermal shrinkage characteristics of ceramic balls is not a problem where the composition of the ceramic balls and the relative sizes between the balls the races and the cage are properly adjusted to minimize the effect of the respective thermal coefficients of expansion for each of the components.  
         [0022]    Also as noted above, a problem arises with the use of magnetic drive pumps as a result of the use of the pump at very cold temperatures in that the material comprising the magnet is unable to withstand the cold temperatures. In more detail, it is the material which bonds (or “pots”) the magnetic material  36  comprising the magnet  24  to the carrier  38  which fails at cold temperatures rather than the magnetic material  36  itself. One such material is sold as part of the magnet and carrier assemblies commercially available under the brand name CHEMREX by Ugimag, Inc. (Valparaiso, Ind.). To overcome that limitation of prior art magnetic drive pumps, the magnet  24  of pump  10  is provided with a casing  40  carried on rotatable shaft  14  which is trapped between the jam washer  28  and the spacer  41  which traps the ball races  32  against the shoulder  42  formed on shaft  14  and which encases the magnetic material  36 . In the preferred embodiment, the casing  40  is comprised of a metallic or other material having a coefficient of thermal expansion which is greater than that of the material  36  comprising the magnet  24  so that, as temperature decreases, the material comprising casing  40  contracts at a rate faster than the rate of contraction of the material  36  comprising the magnet  24  so that the material  36  is held tightly in place on rotatable shaft  14 . The magnitude or rate of thermal contraction corresponds to the magnitude of the coefficient of thermal expansion. Thus, for a casing that fits tightly on the E magnet at room temperature (i. e., both the magnet and the casing are expanded) the casing will contract faster for each degree that the temperature drops when exposed to cryogenic fluid and the casing will therefore fit tighter on the magnet when operating to pump very low temperature fluids.  
         [0023]    Back plate  12  is mounted within a housing  44  having openings formed therein for intake and exhaust  46  and  48 , respectively, of the fluid to be pumped through pump  10 . In the preferred embodiment shown in FIG. 1, the back plate  12  is provided with a flange  50  which is confined between front and back halves  44 ′ and  44 ″ of housing  44  by a pumping of screws  52  (only one of which is seen in the view shown in FIG. 1), shoulders (not numbered) being provided for appropriately sized gaskets  54  for scaling the two halves  44 ′ and  44 ″ to the flange  50 . The front interior half  44 ′ of housing  44  forms the volute of pump  10 .  
         [0024]    A second magnet  56  is positioned in close proximity to the housing  44 - for rotation therearound and is adapted, as with a concave coupler  57  and a corresponding cylindrical shape, for mounting to a drive shaft  58  of a motor  60 . When the motor  60  is operated, the first magnet  24  within the housing  44  is rotated under the magnetic influence of second magnet  56  transferred through housing  44 , to drive the rotatable shaft  14  the impeller  18 .  
         [0025]    In a preferred embodiment, a frequency inverter  61  is uniquely used with the pump to not as with normal use of a frequency inverter which is to vary the speed of the motor from its standard maximum rated speed to slower speeds, but to increase the speed of the pump from the maximum standard of 3600 rpm to about 7200 rpm. This advantageously accomplishes the increased speed desired for low temperature fluid pumping simply and without the use of pulleys and a V-belt as known in the art. This increase in the speed of the motor facilitates the increase in the pressure of the fluid from pumping because head pressure is proportional to the square of the impeller speed. In the embodiment shown in FIG. 1, a jacket  62  is bolted between the back half  44 ″ of housing  44  and the motor  60  for enclosing the second magnet  56 . Jacket  62  is provided with an inlet and outlet  64  and  66 , respectively, for purging of fluids therethrough to eliminate around housing  44  any fluids such as liquid water, water vapor, or other constituents of ambient air that will freeze at the very low temperatures resulting at housing  44  due to very low temperature fluid being pumped therein. The purging with a dry and contaminant free fluid advantageously prevents the formation of water ice (or other frozen constituents of ambient air) around the housing  44  and between the magnets  56  and housing  44 . Alternatively, as shown in hidden lines in FIG. 1, opening  66  may be closed, as with cap  67 , and a desiccant material  65  may be enclosed within jacket  62 , as through opening  64 , to absorb water so that no moisture is allowed to condense on the moving parts inside the jacket  62 , which could freeze up the motor  60 . In one preferred embodiment, the desiccant  65  may be made periodically replaceable, as by treading it into opening  64 , to maintain functional drying throughout the life of the pump.  
         [0026]    As a preventative measure to reduce the formation of areas of temperature variation and mechanical size variation caused by thermal expansion, the fluid being pumped through pump  10  is circulated within the housing  44 . This interior circulation is accomplished by provision of a passage  68  in back plate  12  having a plug  70  positioned in a well  72  formed therein, the plug  70  having an orifice  74  therethrough. The orifice  74  and passage  68  allow high pressure fluid to pass from the volute formed inside the front half of housing  44 ′ through the back plate  12  to the back half of housing  44 ″ until sufficient back pressure builds behind back plate  12  to cause the fluid to return to the intake  46  of housing  44 ′. Return to the intake  46  is through the hollow shaft  14  and along the outside of the shaft  14  through the ball bearings  16  into the chamber  76  which connects through the hole  78  formed near the base of the blades  80  comprising impeller  18  (e.g., in the lower pressure portion of the volute).  
         [0027]    Referring to FIG. 2, a second alternative embodiment of the pump of the present invention is indicated generally at reference numeral  182 . In this second embodiment, all the component parts thereof are similar to those of the embodiment shown in FIG. 1 and are numbered with the same reference numeral preceded with a “1,” e.g., impeller  18  in FIG. 1 corresponds generally to impeller  118  in FIG. 2. Pump  182  is particularly intended for use with those fluids such as liquid nitrogen which can be vented to the atmosphere and circulates the fluid being pumped internally in the same manner as does pump  10  in FIG. 1, but also provides a check valve  184  through which a portion of the pumped fluid is passed to the atmosphere through a vent tube  185  from the back half  144 ″ of housing  144 . Where the pumped fluid is known to be free of water moisture and other materials that might freeze at the low temperature of the fluid to being pumped, the vented fluid can also alternatively be captured as a purging fluid by a line  186  (shown in shadow lines to indicate that it is optional) and, which line  186  is constructed to act as a vaporizer to assure that the vented fluid is converted into warm purge gas, circulated into the inlet  164  and through the jacket  162  for the purpose of purging the jacketed volume  163  of any moisture capable of freezing. If the fluid being pumped is LNG, the pumped fluid vented through vent  185  can also be routed through a line  187  to a stack (not shown) for burning or otherwise for safe disposal or recycling of it.  
         [0028]    Although described in terms of the above-illustrated preferred embodiments, those skilled in the art who have the benefit of this disclosure will recognize that many changes can be made to the component parts of the illustrated embodiments which do not change the manner in which these parts function to achieve their intended advantageous results. For instance, in the event that a potting material is found for bonding the magnetic material  36  of first magnet  24  at very low temperatures, it is not necessary that the magnetic material  36  be encased in the casing  40  for them to function for their intended purpose. Further, depending upon the working environment of the pump  62  and other factors known in the art, it may not be necessary to circulate moisture free purging fluid through the jacket  62  and certainly the purging fluid can be circulated through the jacket  62  in different routes. These and all other such changes are intended to fall within the spirit of the present invention as defined by the following claims.