Abstract:
The present invention provides an apparatus for maintaining the operation of a geothermal production pump which comprises one or more impellers and a vertical line shaft for driving said one or more impellers, comprising a liquid buffer for isolating a discharge column through which pumped geothermal fluid including non-condensable gases flows from a lubrication column through which oil for lubricating one or more bearings of the line shaft flows, the liquid buffer being interposed between the discharge column and an outlet of the lubrication column to prevent infiltration of the non-condensable gases into the lubrication column.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of downhole pumps. More particularly, the invention relates to apparatus for maintaining the operation of a geothermal production pump. 
       BACKGROUND 
       [0002]    A geothermal production pump is used to extract geothermal fluid from a deep well. A typical downhole geothermal production pump is a vertical turbine pump which has three main parts: the head assembly which comprises the motor for driving the pump, the shaft and column assembly, and the pump bowl assembly. The lower pump bowl assembly comprises the stationary bowl and the impeller which rotates within the bowl for enhancing the flow of the fluid to be extracted. The shaft and column assembly provides the connection between the head assembly and the pump bowl assembly. The line shaft transmits torque from the pump motor to the impellers and rotates internally to the column. The extracted fluid is transported within the discharge column to the surface. 
         [0003]    Due to the high temperature and pressure of the geothermal fluid, a bearing mounted on the line shaft is subjected to considerable wear. To prevent excessive wear to the bearing mounted on the line shaft, the line shaft rotates within a lubrication column through which oil flows. However, the line shaft and bearings are prone to damage as a result of the intrusion of the high-pressure geothermal fluid into the lubrication column. In particular, non-condensable gases (NCG&#39;s) present in the geothermal fluid, especially when the geothermal fluid contains a large amount of NCG&#39;s, are liable to infiltrate into the lubrication column, causing sluggish oil flow and even damage to the pump as a result of repeated cycles of bubble formation and cavitation. 
         [0004]    It is an object of the present invention to provide an apparatus for maintaining operation of a geothermal production pump when extracting geothermal fluid having a relatively high NCG content. 
         [0005]    Other objects and advantages of the invention will become apparent as the description proceeds. 
       SUMMARY 
       [0006]    The present invention provides an apparatus for maintaining the operation of a geothermal production pump which comprises one or more impellers and a vertical line shaft for driving said one or more impellers, comprising a liquid buffer for isolating a discharge column through which pumped geothermal fluid including non-condensable gases flows from a lubrication column through which oil for lubricating one or more bearings of the line shaft flows, said liquid buffer being interposed between said discharge column and an outlet of said lubrication column to prevent infiltration of the non-condensable gases into the lubrication column. 
         [0007]    In one aspect, the liquid buffer is defined by a cup structure which comprises an upper oil receiving section facing the lubrication column outlet, and a lower securing element related to said receiving section for defining an interior of said receiving section and for securing said cup structure to the line shaft, spent oil from the lubrication column being introducible into, and overflowable from, said oil receiving section. The spent oil that overflows from the receiving section is entrainable by the pumped geothermal fluid. 
         [0008]    The present invention is also directed to a geothermal production pump, comprising a structure and bowl arrangement in which the cup structure and a throttle bearing, lubricated by oil flowing through the lubrication column, are housed, said bowl arrangement being in fluid communication with, and being located above, a further bowl arrangement in which an uppermost impeller of said geothermal production pump is housed. 
         [0009]    In one aspect, the flow rate of the pumped geothermal fluid through the bowl arrangement and through the further bowl in which the uppermost impeller is housed is substantially equal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings: 
           [0011]      FIG. 1  is a vertical cross sectional view of a portion of a production well and pump; 
           [0012]      FIG. 2  is a vertical cross sectional view of a portion of a production pump, according to one embodiment of the present invention; 
           [0013]      FIG. 3  is a cross section view of the pump of  FIG. 1 , along line A-A; and 
           [0014]      FIG. 4  is a vertical cross sectional view of a portion of the pump of  FIG. 1 , showing the cup structure in a raised position when the line shaft is subjected to thermal expansion. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Geothermal fluid extracted by a production pump  1  flows upwardly within a discharge column to ground level, for use in power production or any other suitable use (see  FIG. 1 ). Discharge column  25  surrounds lubrication column  2  of geothermal production pump  1  within which long vertical line shaft  5  rotates transmitting torque generated by motor  4  to the production pump impellers  8 . Motor  4  is supported by landing head  28 , which is positioned in overlying relation to, and connected to discharge column  25 . The geothermal fluid delivered upwardly by production pump  1  flows through the annulus of discharge column  25  and of landing head  28 , and then exits via discharge pipe  3  connected to a fitting of landing head  28 . The lubrication column  2  provides lubrication oil for bearings that support line shaft  5 , and terminates after lubricating the bearing, which maintains the radial position of the line shaft  5 . 
         [0016]    With respect to prior art pumps, lubricating oil can exit the bottom of lubrication column and is disposed of via channels  14  shown in  FIG. 2  to bypass ports  16  (see  FIG. 2 ), allowing the oil to be discharged into the annulus of the production well casing. As the pumped geothermal fluid which includes brine and NCG&#39;s flows adjacent the lubrication column outlet, it has been found that some NCG&#39;s infiltrate the lubrication column. The infiltrated NCG&#39;s consequently flow upwardly within the lubrication column and restrict oil flow, resulting in sluggish and non-uniform oil flow that reduces its lubricating capability. At times, the infiltrated oil cavitates and wear to the line shaft bearings is accelerated. 
         [0017]    In the geothermal production pump of the present invention, infiltration of the NCG&#39;s to the lubrication column is prevented by providing a cup structure attached to the line shaft. Consequently, the lubrication oil exiting the bottom of the lubrication column accumulates in the cup interior and induces an upward oil flow to counteract the effect of NCG&#39;s infiltration. 
         [0018]      FIG. 2  illustrates a vertical cross sectional view of a portion of a production pump generally designated  10 , according to one embodiment of the present invention. The pumped geothermal fluid F flows upwardly within discharge column  35 , which is annular in shape and surrounds line shaft  15 , and is then diverted to the ground surface by an elbow or any other flow directing device (not shown). 
         [0019]    Vertically disposed line shaft  15  of pump  10 , which transmits torque from the pump motor of the head assembly, is engaged with first stage impeller  18  and second stage impeller  19 , or any other number of impeller stages, and causes the same to rotate within the bowl assembly, which includes a suction bell (not shown) located at the bottom of first bowl  33 , a first bowl  33  in which first stage impeller  18  is housed, a second bowl  34  in which second stage impeller  18  is housed, and spacer bowl  36  located above second stage bowl  34 . During rotation of impellers  18  and  19 , the momentum of the fluid to be extracted from the well in which pump  10  is disposed is increased, causing the fluid to rise through the suction bell. A diffuser  27  located above each impeller converts the tangential flow of increased pressure diverging from impellers  18  and  19  to an axial flow of fluid F rising within spacer bowl  36 . 
         [0020]    Within spacer bowl  36  is housed cup structure  20 . Cup structure  20  comprises cylindrical wall  22  which is substantially coaxial with line shaft  15 , and securing element  21  located below wall  22  for securing cup structure  20  to line shaft  15 , preferably by a heat shrink fit. Securing element  21  may gradually taper as shown from the bottom of cylindrical wall  22  to the outer surface of line shaft  15 . Cup structure  20  therefore rotates together with line shaft  15 . 
         [0021]    Throttle bearing  9  is advantageously able to be longer than the journal bearing of prior art production pumps, for increased support to line shaft  15  in the radial direction, due to the added volume afforded by spacer bowl  36 . Throttle bearing  9 , which prevents lateral movement of line shaft  15 , is supported by a plurality of elongated retainers  13  radially extending from tube  11  and connected to lubrication column  12  in the vicinity of throttle bearing  9  to the casing of discharge column  35 . 
         [0022]    Lubrication oil pumped by a pump located at the head assembly flows downward within lubrication column  12  surrounding line shaft  15 . After lubricating throttle bearing  9 , the lubrication oil is discharged into the interior  26  of cup structure  20  between cylindrical wall  22  to the outer surface of line shaft  15 . The discharged lubrication oil accumulates within interior  26  and eventually overflows, flowing upwardly over cup structure rim  23 , which is located above the bottom surface of throttle bearing  9 . 
         [0023]    The mass of oil disposed in cup structure  20  serves as a liquid buffer between lubrication column  12  and spacer bowl  36 . The pressure of the pumped geothermal fluid flowing upwardly within the discharge of spacer bowl  36  in this case is not high enough to dissolve all the NCG&#39;s that comprise the geothermal resource. In contrast to prior art production pumps which suffer from sluggish oil flow due to the infiltration of the liberated NCG&#39;s through the bottom of the lubrication column, the NCG&#39;s in the pump of the present invention are not directly exposed to the bottom of the lubrication column, but rather to the top of the oil mass overflowing rim  23 . The oil near rim  23  resists the mass transport of NCG&#39;s through the cup structure interior  26  to such a degree that the pressure of the NCG&#39;s at the bottom of lubrication column  12  is no greater than, and is generally less than, the pressure of the oil flowing through the lubrication column. Accordingly, the infiltration of the NCG&#39;s into lubrication column  12  will be negligible or just about nonexistent. 
         [0024]    The overflowing oil is entrained into the pumped high pressure geothermal fluid F and carried with the pumped geothermal fluid. Since the bowl assembly is provided with blocked bypass ports  17 , or with a casing made without any bypass ports, the discharged oil is not induced outwardly from the bowl assembly into the well annulus, as has been practiced heretofore. 
         [0025]    As shown in  FIG. 3 , the downward oil flow that overflows from the cup structure is limited by the small clearance  38  between throttle bearing  9  and line shaft  15 . The flow rate of overflowing oil that is entrained in the pumped geothermal fluid may be as little as about 1-10 ppm. 
         [0026]    As shown in  FIG. 4 , line shaft  15  is subject to thermal expansion when the hot geothermal fluid F flows through discharge column  35  and elongation due to the downward thrust. As a result of the expansion, cup structure  20  rises with respect to throttle bearing  9  and the volume of the cup structure interior becomes reduced. 
         [0027]    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 exceeding the scope of the claims.