Patent Publication Number: US-8113765-B2

Title: Water lubricated line shaft bearing and lubrication system for a geothermal pump

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of geothermal liquid supply systems. More particularly, the invention relates to a water lubricated line shaft bearing and lubrication system for a geothermal production pump. 
     BACKGROUND OF THE INVENTION 
     Downhole geothermal production pumps are adapted to lift geothermal fluid from within a well or column to the ground surface. The geothermal fluid is pumped at a high temperature and pressure, e.g. a temperature in the order of 500° F. and a pressure in the order of 300 psi which is greater than its flash point, in order to ensure continual geothermal liquid flow throughout the geothermal system and thus also prevent scale precipitation. 
     Due to the high temperature and pressure of geothermal fluid, considerable pump bearing wear is noticeable. Petroleum oil is generally used as a lubricant, to prevent excessive wear to a bearing mounted on the main pump shaft. However, the drive shaft and bearings of geothermal production pumps are prone for failure as a result of the intrusion of the high-pressure geothermal fluid into the line through which the lubricant is delivered. Bearing failure is also caused by the precipitation of scale thereon. 
     U.S. Pat. No. 4,276,002 discloses a submerged turbopump unit for pumping hot geothermal liquids from deep wells to the earth&#39;s surface. The wear of the bearings associated with the turbopump is minimized by supplying lubricating liquid, e.g. hot water, thereto which is taken from an intermediate stage of a centrifugal pump at the surface which supplies motive liquid to the turbine. 
     However, no prior art water-lubricated bearings are known to the applicant for the long drive shaft (hereinafter referred to as a “line shaft”) extending from a surface mounted motor to the pump submersed in the water column. U.S. Pat. No. 4,276,002 describes an improved turbopump unit for pumping hot geothermal liquids from deep wells. However, there are many technical challenges of applying geothermal water to line shafts even when taking the teachings of U.S. Pat. No. 4,276,002 into consideration. These challenges include mechanically sealing the shaft at the surface, maintaining pressure above saturation in a low pressure system and, in addition, dealing with the corrosiveness of geothermal fluid to line shaft bearings. For example, it is recited in U.S. Pat. No. 4,276,002 that bled geothermal water needs to be cooled and filtered. 
     It is an object of the present invention to provide a geothermal production pump bearing which is unaffected by the intrusion of geothermal fluid into the lubrication line. 
     It is an additional object of the present invention to provide a reliable water lubricated line shaft bearing. 
     It is an additional object of the present invention to provide a water lubricated geothermal line shaft bearing which is unaffected by the precipitation of scale thereon during the flow of lubrication water. 
     It is yet an additional object of the present invention to provide a water lubricated geothermal line shaft bearing which has sufficient strength to withstand high compressive loads imposed by the rotating line shaft. 
     It is yet a further object of the present invention to provide a lubrication system that ensures sufficient lubrication of the line shaft bearing. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY OF THE INVENTION 
     The present invention provides a water lubricated line shaft bearing assembly, comprising an outer annular steel shell and an inner layer made of low friction material attached to said outer shell, said inner layer having a non-uniform thickness which is formed with wall portions of increased thickness defining a plurality of shaft engaging portions and with wall portions of reduced thickness defining a plurality of lubricant passages. 
     The shaft engaging portions are capable of being journalled on a line shaft adapted to drive a downhole geothermal production pump and the steel shell is engageable with an inner wall of a lubrication tube vertically extending through a water column through which pumped geothermal fluid is delivered, lubrication water bled from the pumped geothermal fluid being used to supply lubrication water through the lubricant passages. 
     The steel shell has sufficient compressive strength to withstand the stress imposed by the high rotational speed of the line shaft of the geothermal production pump. The low friction material of the inner liner allows solid debris present or entrained in the lubrication water to slide over the inner line. Solid debris is prevented from accumulating due to the presence of the passages through which the lubrication water flows. 
     In one aspect, the shaft engaging portions are arcs having a common center which trace a complete circle, and preferably have an equal circumferential length. 
     In another aspect, each lubricant passage is a slot formed within the inner layer being defined by a first wall extending from one end of a first shaft engaging portion to an adjacent wall portion of reduced thickness, a second wall extending from one end of a second shaft engaging portion to the adjacent wall portion of reduced thickness, and a third arc shaped wall extending from the first wall to the second wall, the third wall coinciding with the adjacent wall portion of reduced thickness. 
     In a further aspect, the first and second walls are preferably mutually parallel planar walls. 
     In an additonal aspect, pairs of passages are diametrically opposite to each other and are arranged such that a first planar wall portion of a passage is collinear with the second planar wall portion of a diametrically opposite passage. 
     The low friction material is selected from the group of Teflon and glass blended with Teflon. 
     Lubrication water is preferably bled from the pumped geothermal fluid by means of a lubrication system operable to ensure that the inner layer of the bearings is continuously moist. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are described below by way of example and with reference to the accompanying drawings wherein: 
         FIG. 1  is a cross sectional view of a line shaft water-lubricated bearing assembly, according to one embodiment of the invention; 
         FIG. 2  is a front view of the bearing assembly of  FIG. 1 ; 
         FIG. 3  is a schematic vertical cross section of a portion of a water column and of a lubrication tube extending vertically within the water column of a geothermal production well, illustrating a water-lubricated bearing mounted on a line shaft and engaged with the lubrication tube; 
         FIG. 4  is a schematic vertical cross section of upper and lower portions of water column of a geothermal production well, illustrating a submerged pump and a line through which lubrication water is bled from discharged geothermal fluid; and 
         FIG. 5  is a cross sectional view of the bearing of  FIG. 1  which is journalled on a line shaft. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a cross sectional view of a line shaft water-lubricated bearing assembly of a downhole geothermal production pump, according to one embodiment of the present invention. The line shaft bearing assembly designated by numeral  10  has a novel partial arc configuration by which the bearing can be journalled to the line shaft, yet permits the passage of solid debris entrained in the lubrication water that is bled from the pumped geothermal fluid. 
     As shown in  FIG. 1 , bearing assembly  10  comprises an outer annular steel shell  5 , e.g. made of carbon steel, e.g. standard boiler steel and an inner layer  15  made of low friction material, e.g. Teflon®, glass blended with Teflon® to provide thermal stability. Preferably, less than about 10% glass is used in the glass blended with Teflon® option. Inner layer  15  is attached to shell  5  by means of pins  8 A-D radially extending from outer surface  11  of inner layer  15 , which are received in complementary recessed portions formed in the inner surface of shell  5 , so that the radial clearance between shell and inner liner  15  is e.g. about 0.025 in. Inner layer  15  has a non-uniform thickness which defines shaft engaging portions  16 A-D and lubricant passages  18 A-D. That is, inner layer  15  is formed from two types of wall portions: wall portions  12 A-D of increased thickness from outer periphery  11  of inner layer  15  to shaft engaging portions  16 A-D, respectively, and wall portions  19 A-D of reduced thickness from outer periphery  11  of inner layer  15  to lubricant passages  18 A-D, respectively. 
     Shaft engaging portions  16 A-D are arcs of preferably an equal circumferential length having a common center and which trace a complete circle, to allow the line shaft to be received thereby. Lubricant passages  18 A-D are slots formed within inner layer  15 , and are preferably arranged, as shown in the illustrated arrangement, such that two passages are diametrically opposite to each other and that two adjacent passages are equally angularly spaced. Each of the four passages  18 A-D has a corresponding first planar wall portion  23 A-D extending from the circumferential end of one adjacent shaft engaging portion, a second planar wall portion  25 A-D extending from the circumferential end of the other adjacent shaft engaging portion, and an arc shaped recessed wall portion  27 A-D extending from the first to second wall portion. Preferably, the first planar wall portion is collinear with the second planar wall portion of the diametrically opposite passage. With respect to an illustrative, exemplary bearing assembly, the outer diameter of the steel shell is 2.875 in., the inner diameter of the steel shell is 2.500 in., the distance between diametrically opposite recessed wall portions is 2.300 in., the distance between diametrically opposite shaft engaging portions is 1.9625 in., and the distance between first and second wall portions is 1.00 in. 
     It will be appreciated that inner layer  15  may be configured differently, such as with any other number and circumferential length of shaft engaging portions. 
       FIG. 2  illustrates a front view of bearing assembly  10 . The outer surface of shell  5  is formed with threads  9  which are engageable with threads formed within the inner wall of a lubrication tube. 
       FIG. 3  illustrates a schematic vertical cross section of a portion of water column  35  of a geothermal production well. Also shown is a portion of a line shaft  31  driven by a surface mounted motor for transmitting torque to pump  55  ( FIG. 4 ), e.g. a multi-stage impeller pump or turbine pump, submersed in water column  35 , through which geothermal fluid having a temperature ranging from about 275° F. (135° C.) to 400° F. (205° C.) is delivered at a flow rate ranging from about e.g. 1000 to 3500 gpm. These flow conditions prevent the flashing and the resultant precipitation of scale within the pumped geothermal fluid. Line shaft  31  extends downwardly from the surface mounted motor substantially through the center of lubrication tube  38 . Bearing assembly  10 , which is shown in front view, is journalled on line shaft  31  and is engaged with the inner wall of lubrication tube  38  by use of threads  9  present on the outer surface of shell  5  of bearing assembly  10  (see  FIG. 2 ). Bearing assembly  10  has a height of about e.g. 4 in. and is journalled on line shaft  31  at a distance ranging from about 4 in. to 6 in., e.g. 5 feet, from an adjacent bearing. Lubrication tube  38  in turn extends substantially through the center of water column  35 . During operation of the geothermal production pump, geothermal fluid  37  is raised to ground level so that it can be used for power production or for any other suitable industrial process, through the annulus of column  35  and lubrication tube  38 . Lubrication water  39  is supplied from the pump discharge and is delivered to the bearings along the length of lubrication tube  38 . 
       FIG. 4  illustrates a schematic vertical cross section of upper and lower portions of water column  35  of a geothermal production well. Surface mounted motor  51  of pump  55  enclosed by casing  52  is supported by casing head flange  56 , which is positioned in overlying relation to, and bolted to a flange  59  of, water column  35 . Water column flange  59  is generally located above ground level GL. Lubrication tube  38 , through which line shaft  31  ( FIG. 3 ) transmits torque generated by motor  51  to pump  55 , extends from throat  53  of casing  52  to the upper end of pump  55 . Annular landing head  57 , which is attached to both throat  53  and casing head flange  56 , is in communication with the pumped geothermal fluid. The geothermal fluid delivered upwardly by pump  55  flows through the annulus of water column  35  and of landing head  57 , and then exits via discharge pipe  65  connected to fitting  63  of landing head  57 . A portion of the discharged geothermal fluid is bled from pipe  65  via line  69  to the inlet of lubrication tube  38  which is located within throat  53  of motor casing  52 . 
       FIG. 5  illustrates a cross section of lubrication tube  38  when line shaft  31  is received by shaft engaging portions  16 A-D ( FIG. 1 ) of bearing assembly inner layer  15 . As shown, the interior of lubrication tube  38  is occupied by shell  5  engaged to the inner face of lubrication tube  38  by use of threads  9  present on the outer surface of shell  5  of bearing assembly  10  (see  FIG. 2  ) and inner layer  15  of the bearing assembly, and by line shaft  31 . Reduced wear of inner liner  15  with respect to metallic bearings is noticeable due to the high lubricity of the low friction material from which inner liner  15  is made. The material from which steel shell  5  is made has sufficient compressive strength to withstand the stress imposed on the low friction material of inner layer  15  by the rotation of line shaft  31  at a rate ranging from about 1750 to 2500 rpm and by the thermal expansion of inner layer  15 . Cavities defined by passages  18 A-D remain between inner layer wall portions of reduced thickness and the outer periphery of line shaft  31 , and the lubrication water bled from discharge pipe  65  via line  69  ( FIG. 4 ) flows through passages  18 A-D. The lubrication water serves to cool inner layer  15 . Lubrication water flows across shaft engaging portions  16 A-D which are in contact with line shaft  31 . The low friction material advantageously allows solid debris present or entrained in the lubrication water to slide over inner layer  15 . The presence of passages  18 A-D permits the flow of debris across the passages and prevents its accumulation. 
     Due to the configuration of line  69  and of the associated flow control devices, which will be described hereinafter, the flow rate of lubrication water within passages  18 A-D can be e.g. about 10 gpm, while the lubrication water has a temperature ranging from about 60° F. (15.5° C.) to 400° F. (205° C.) and a pressure ranging from about 40 to 200 psi. These flow conditions provide lubrication and prevent the flashing and the resultant precipitation of scale within the lubrication water. 
     Even though the low friction material of inner liner  15  advantageously permits solid debris present or entrained in the lubrication water to slide over the inner layer during the flow of lubrication water, it is susceptible to damage if allowed to run dry. To prevent damage to inner liner  15  during a pump startup or unanticipated pump malfunction when the inner liner may be dry, the lubrication system is advantageously provided with control valves which cause the lubrication water to change direction in order to keep inner liner  15  moist. Tolerances on pump throttle bushing have been increased to allow more “leakage” of fluid into the line shaft allowing lubricating fluid flow. No such modification is required in the top down design. 
     While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.