Patent Publication Number: US-7909090-B2

Title: System, method and apparatus for scale resistant radial bearing for downhole rotating tool components and assemblies

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to radial bearings and, in particular, to a system, method and apparatus for scale resistant radial bearing designs for electrical submersible pump components and assemblies. 
     2. Description of the Related Art 
     In many downhole pumping systems, such as rotating equipment like electrical submersible pumps (ESP), gas separators and intakes, the problem of scale build up is observed in the clearances of radial bearings. Scale may include any kind of surface deposit that might tend to develop due to environmental exposure during operation of the equipment. One problem is that the formation of scale impedes the axial movement or stroke of the shaft (i.e., the rotating assembly stack) relative to the stationary support housing. This problem can become critical even when the amount of scale build up is very thin (e.g. on the order of 0.001 inches or more). 
     Referring to  FIG. 1 , a conventional radial bearing  11  typically comprise stacked sleeves  13  (one shown) on the shaft  15  where all of the sleeves are formed at the same diameter and engage the bushing  17 . For example, some radial bearing bushings and sleeves have a total diameter difference or clearance of about 0.003 to 0.015 inches between their inner and outer diameters, respectively. Scale deposits develop in the clearance on the outer surface of the sleeve that protrudes axially beyond the bushing inner surface. Upon any shaft axial stroke, the scale build up is forced into the tight clearance  19  between the bushing  17  and sleeve  13 . As the scale build up is drawn into the clearance, a tremendous frictional drag is introduced in the radial bearing. 
     A compounding issue for radial bearings is the presence of a chamfer  21  on the face edges of the bushings  17  and sleeves  13 . As the shaft  15  is axially stroked, the chamfers  21  on the leading edges act like a funnel or cam to force more scale into the bearing clearance  19 . The additional friction due to these issues can cause numerous common failure modes. For example, the bearing and/or sleeve can overheat, the bearing can fail due to loss of lubrication and overheating, and the sleeve can seize inside the bushing. 
     In addition, the scale can limit the life or prevent reuse of the pump, gas separator or intake due to limited axial shaft stroke or seized shaft. Moreover, the pump can lock up and prevent the motor from starting, and extreme heating can cause motor failure. Furthermore, extreme frictional drag can cause shearing of the key alignment feature that is located under the sleeve, and then continued operation may result in extreme wear and weaken or destroy the shaft. Thus, an improved design that overcomes the limitations and problems associated with prior art designs would be desirable. 
     SUMMARY OF THE INVENTION 
     Embodiments of a system, method, and apparatus for reducing scale build up in radial bearing designs for electrical submersible pump (ESP) components and assemblies are disclosed. The invention is well suited for use in downhole rotating equipment such as pumps, gas separators and intakes. For example, scale resistant and abrasive resistant (AR) sleeves and AR bushings (such as PTFE-impregnated, tungsten carbide designs, etc.) may be used in place of conventional materials. 
     In another embodiment, the axial lengths of the sleeves are kept within the axial length of the bushings, or vice versa, no matter the axial stroke of one component relative to the other. In addition, sharp corners may be formed on the sleeve or bushing axial faces (i.e., at their respective interfacing diameters). As the shaft moves axially, the sharp corner on one component scrapes off the scale on the other component. This design discards the scale rather than force it into the clearance between the sleeve and bushing. 
     In still another embodiment, smaller diameter, scale resistant spacer sleeves (i.e., on both axial ends of the sleeve) may be used so that scale build up on the spacer sleeves is farther away from the bushing inner diameter and cannot cause a scale-related problem. This design also gives any scale that is scraped away the opportunity to fall away from the bearing. Additional running clearance (e.g., 0.001 inches) between the sleeve and bushing may be added to provide extra lubrication flow and cooling of the components. This element also may be needed for some applications due to the sharp corners on the sleeves or bushings. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention are attained and can be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings. However, the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic sectional side view of a conventional radial bearing installation; 
         FIG. 2  is a schematic sectional side view of one embodiment of a radial bearing installation constructed in accordance with the invention; 
         FIG. 3  is a schematic sectional side view of another embodiment of a radial bearing installation constructed in accordance with the invention; 
         FIG. 4  is a schematic sectional side view of still another embodiment of a radial bearing installation constructed in accordance with the invention; 
         FIG. 5  is a schematic side view of one embodiment of a downhole rotating tool constructed in accordance with the invention; and 
         FIG. 6  is an enlarged side view of one embodiment of a “sharp edge” for one or more of the radial bearing installations disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 2-6 , embodiments of a system, method and apparatus for reducing scale build up in radial bearings for downhole tools are disclosed. The invention is well suited for downhole rotating equipment, such as electrical submersible pump (ESP) assembly components (e.g., pumps, gas separators, intakes, etc.). 
     One embodiment of the invention is shown in  FIG. 2 . The downhole tool has a housing  31  with an axis  33  and a hole  35  extending through the housing  31  along the axis  33 . A shaft  37  is located in and extends through the hole  35  in the housing  31  along the axis  33 . The shaft  37  is rotatable relative to the housing  31  and has a limited range of axial motion, depending on the application and installation. 
     A radial bearing  41  is installed in the downhole tool for reducing scale build up. The radial bearing  41  is located in the hole  35  of the housing  31  for supporting the shaft  37  relative to the housing  31 . The radial bearing  41  comprises a bushing  43  mounted to the housing  31 , and a sleeve  45  mounted to the shaft  37  for engaging the bushing  43 . The sleeve  45  moves rotationally and axially with the shaft  37  relative to the housing  31  and bushing  43 . A clearance  47  is located between an inner diameter of the bushing  43  and an outer diameter of the sleeve  45 . 
     In the embodiment shown in  FIG. 2 , the sleeve  45  has a short axial length  51  and the bushing  43  has a long axial length  53  that is greater than the short axial length  51 . As such, the axial ends  55  of the short axial length  51  of the sleeve  45  never extend axially beyond the axial ends  57  of the long axial length  53  of the bushing  43  throughout the limited range of axial motion of the shaft  37 .  FIG. 3  depicts an alternate embodiment wherein the sleeve  45  is axially longer than the bushing  43 . Similarly, the axial ends  57  of the bushing  43  never extend axially beyond the axial ends  55  of the sleeve  45  throughout the limited range of axial motion of the shaft  37 . 
     In some embodiments, the bushing  43  and the sleeve  45  are formed from scale resistant and abrasive resistant materials. For example, the bushing  43  and sleeve  45  may be formed from PTFE-impregnated, tungsten carbide. Alternatively, these components may be coated, impregnated or otherwise formed from other types of scale and abrasive resistant materials. 
     In other embodiments, the bushing  43 , the sleeve  45  or both may be provided with a sharp corner(s)  61  (schematically depicted in  FIG. 6 ) on an axial end(s)  57 ,  55 , respectively, thereof. In  FIG. 6 , the features are exaggerated for clarity. Sharp corners  61  may be provided on one or both axial ends of the component to scrape scale off of the other component of the radial bearing (i.e., the bushing scrapes the sleeve, and/or the sleeve scrapes the bushing) at their respective interfacing diameters. 
     This design helps to remove and discard the scale rather than force it into the clearance  47  between the bushing  43  and sleeve  45 . For example, corner  61  may be provided with a maximum radius of 0.005 inches, and employ a face angle  63  of less than 90° as shown (e.g., 85° to 89°). The face angle  63  enhances the scraping action and extends the life of the sharp corner in the event of surface wear. With an angle of less than 90°, the scraping corner is “self sharpening” as surface wear progresses, prolonging the scale-resistance of the design. 
     Referring now to  FIG. 4 , the invention may further comprise smaller diameter, scale resistant spacer sleeves  71  located on and abutting axial ends  55  of the sleeve  45 . The spacer sleeves  71  provide mechanical limits to ensure that the bearing sleeve is located in the correct axial position on the shaft. Retaining rings  73  or other mechanical features also may be used on the shaft to keep the spacer sleeves  71  in the correct axial positions. In some embodiments, the hubs of pump impellers provide and act to keep the spacer sleeves in the correct axial position. Each of these axial stroke limiters, may be employed for the various other embodiments depicted and described herein (e.g.,  FIGS. 1-3 ). 
     Additional running clearance (e.g., 0.001 inches) between the sleeve and bushing also may be added to provide extra lubrication flow and cooling of the components. This element also may be needed for some applications due to the sharp corners on the sleeves or bushings. 
     Referring now to  FIG. 5 , one embodiment of a downhole tool for a well  110  is shown. The downhole tool comprises an electrical submersible pump (ESP) assembly  111  installed within the well  110 . The pump assembly  111  may comprise a centrifugal pump  112  with an intake  113  and an internal gas separator. A seal section  114  is attached to pump  112  and to an electric motor  116  and submerged in a well fluid  118 . The motor  116  has a shaft that connects to the seal section shaft and is connected to the shaft in the centrifugal pump  112 . The pump assembly  111  and well fluid  118  are located within a casing  119 , which is part of the well  110 . Pump  112  connects to tubing  125  that conveys the well fluid  118  to a storage tank (not shown). The radial bearing designs disclosed herein may be employed in the pump, gas separator, intake or still other components that are suitable for downhole applications. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, a chamfer may be formed on corner(s) of the longer component of the bushing or sleeve so as to allow the longer component to be inserted more easily into the bushing bore. However, the components are prevented from sliding under the chamfers under any thermal expansion condition or shaft stroke mechanical limits.