Abstract:
A stackable multi-stage diffuser with anti-rotation lugs utilizes cast lugs and complementary recesses in the diffuser walls. When stacked in a housing, the lugs of lower diffusers nest inside the recesses of upper diffusers. The diffuser sections interlock and are mechanically locked in the housing to prevent rotation of the diffusers. Each diffuser is sealed directly to adjacent diffusers with o-rings, but do not contact or seal to the housing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates in general to downhole electrical submersible pumps (ESP) and, in particular, to an improved system, method, and apparatus for a stackable multi-stage diffuser with anti-rotation lugs for an ESP. 
         [0003]    2. Description of the Related Art 
         [0004]    Multi-stage diffuser pumps are typically assembled in a stack of diffuser components that are nested together. The diffuser stack slides inside a housing along an axis of the housing. The diffuser stack is subjected to compression by a bearing that is threaded into the housing until it makes contact with the top of the diffuser stack. The bearing is tightened to a calculated length, thus compressing the stack. This design prevents the diffusers from spinning inside the housing during operation due to the impellers that rotate inside them. The stack typically has grooves with O-rings that are located on the outer diameters (OD) of the diffusers to seal directly against the inner diameter (ID) of the housing itself. The O-rings are axially spaced apart in approximately one-foot increments throughout the axial length of the stack to contain any leakage between the diffuser faces and the housing. Although the O-rings are closely spaced apart, they are not used at every stage since the assembly and/or disassembly from the housing would become very difficult. In addition, the O-rings on the OD of the diffusers must slide across the internal threads of the housing when installing the stack into the housing, which can potentially cut and damage the O-rings. Thus, an improved diffuser stack design that overcomes these limitations would be desirable. 
       SUMMARY OF THE INVENTION 
       [0005]    One embodiment of a system, method, and apparatus for a stackable multi-stage diffuser with anti-rotation lugs for an ESP is disclosed. Each stage of the diffuser assembly is provided with one or more “cast-in” lugs and complementary recesses in the diffuser walls. The lug features may comprise cast elements that do not require additional milling operations. The machine turning operations required for other portions of the design are virtually identical to those of other designs and thus add negligible cost to the part. 
         [0006]    When stacked in assembly order, the lugs of one diffuser nest inside the recesses of an adjacent diffuser. The diffuser sections are thus interlocked with each other and mechanically locked in place to prevent rotation relative to the housing and each other as the impellers rotate inside the diffusers. The diffuser stack rests on the base of the assembly and the stages are nested and sealed to each other as described herein. 
         [0007]    In one embodiment, each stage of the stack has an o-ring to provide sealing between the stages without burdening assembly, and there is only one direct seal between the stack and the housing. This design is simpler than conventional designs since the entire stack will easily slide into the housing without interference. Also, by utilizing an o-ring between every stage, the head performance per stage may be increased without losses due to leakage. 
         [0008]    Since there is no compression required in this stack due to the cast-in lugs for anti-rotation, the stack is held in place by other simpler devices, such as a retaining ring on the ID of the housing. Replacement of the conventional threaded top compression bearing allows for more stages per housing to be assembled, thus increasing value. 
         [0009]    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 
         [0010]    So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, 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 which form a part of this specification. It is to be noted, however, that 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. 
           [0011]      FIG. 1  is a perspective view of a centrifugal pump disposed in a fluid in a well, constructed in accordance with the invention; 
           [0012]      FIG. 2  is a sectional side view of one embodiment of a diffuser stack of the pump of  FIG. 1  and is constructed in accordance with the invention; 
           [0013]      FIG. 3  is a lower isometric view of one embodiment of a single diffuser of the diffuser stack of  FIG. 2  and is constructed in accordance with the invention; and 
           [0014]      FIG. 4  is an upper isometric view of the diffuser of  FIG. 3  and is constructed in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to the drawings,  FIG. 1  generally depicts a well  10  with an electrical submersible pump (ESP) assembly  11  installed within. The pump assembly  11  comprises a centrifugal pump  12  that has a seal section  14  attached to it and an electric motor  16  submerged in a well fluid  18 . The motor  16  has a shaft (not shown) that connects to the seal section shaft and is connected to the centrifugal pump  12 . The pump assembly  11  and well fluid  18  are located within a casing  19 , which is part of the well  10 . Pump  12  connects to tubing  25  that conveys the well fluid  18  to a storage tank (not shown). 
         [0016]    Referring to  FIG. 2 , the centrifugal pump  12  has an axis  22  and a tubular housing  27  that protects many of the components of pump  12 . Pump  12  contains a shaft  29  (shown in phantom) that extends axially through the pump. One or more diffusers  21  (e.g., two shown, and hundreds may be used) are co-axially positioned within housing  27  and have an inner portion with a bore  31  through which shaft  29  extends. Each diffuser  21  contains a plurality of passages  32  that extend through the diffuser  21 . Each passage  32  is defined by vanes  23  ( FIG. 3 ) that extend helically outward from a central area. Diffuser  21  is a radial flow type, with passages  32  extending in a substantially radial plane. The invention also is applicable to mixed flow types of diffusers. 
         [0017]    An impeller  20  (only one shown) is typically located within each diffuser  21  to form a “stage” or sub-assembly. Impeller  20  also includes a bore  33  that extends the length of impeller  20  for rotation relative to diffuser  21  and is engaged with shaft  29 . Impeller  20  also contains passages  34  that correspond to the openings in the diffuser  21 . Washers (not shown) are placed between the upper and lower portions between the impeller  20  and diffuser  21 . 
         [0018]    Impellers  20  rotate with shaft  29  to increase the velocity of the fluid  18  ( FIG. 1 ) being pumped as the fluid is discharged radially outward through passages  34 . The fluid flows inward through passages  32  of diffuser  21  and returns to the intake of the next stage impeller  20 , which increases the pressure of fluid  18  flowing therethrough. Increasing the number of stages by adding more impellers  20  and diffusers  21  can increase the pressure of the fluid. 
         [0019]    Referring again to  FIGS. 2-4 , one embodiment of diffuser assembly comprises the housing  27  having one or more of the diffusers  21  coaxially mounted therein. The stack of diffusers  21  rests on the bottom of the assembly in a conventional manner. As will be described herein, the diffusers  21  are mounted within the housing  27  to prevent relative rotation therebetween. In one embodiment, a radial clearance in a range of 0.003 to 0.005 inches separates outer diameters of the diffusers  21  and an inner diameter of the housing  27 . Thus, the individual stages in the stack do not make contact with or seal to the housing  27 . 
         [0020]    Each diffuser  21  comprises a cylindrical body having at least one lug  41  (e.g., three) extending therefrom in an axial direction. A complementary recess  43  is formed in the cylindrical body extending in the axial direction opposite the lug  41 . In the embodiment shown, the lugs  41  and recesses  43  are generally rectangular in profile, but arcuate in shape to match the curvature of the cylindrical diffuser  21 . The lugs  41  and recesses  43  may be symmetrically arrayed castellations that engage and nest in equal numbers as shown. 
         [0021]    The complementary recesses  43  receive and nest respective lugs  41  on axially adjacent diffusers  21  to form an interlocked stack that prevents relative rotation between the diffusers  21  and the housing  27 . However, in one embodiment, the recesses  43  and lugs  41  do not make axial contact with each other (see small axial gaps therebetween illustrated in  FIG. 2 ), but only prevent rotation via contact between lateral shoulders  61 ,  63  (compare  FIGS. 3 and 4 ) on lugs  41  and in recesses  43 , respectively. In addition, the radial outermost shoulders  65 ,  67  also do not make axial contact with each other as shown by the small axial gaps therebetween in  FIG. 2 . 
         [0022]    In addition, a sealing member, such as an o-ring  45  ( FIG. 2 ), is located on an exterior surface of each diffuser  21 . In the embodiment shown, O-rings  45  seat in a circumferential recess  49  ( FIG. 3 ) located below (i.e., axially spaced apart from) and radially inward of the recesses  43 . The O-rings  45  seal against a radially recessed inner surface  47  (best shown in  FIG. 4 ) formed on axially adjacent diffusers  21 . Each diffuser  21  also has a pair of axially opposed upper and lower shoulders  51 ,  53  upon which the assembled diffusers make axial contact and seat. In the embodiment shown, shoulder  51  is formed adjacent the radially recessed inner surface  47  axially opposite lugs  41 , while shoulder  53  is located on an exterior of diffuser  21 , both axially and radially positioned between vanes  23  and recess  49 . 
         [0023]    In one embodiment, an upper end of the stack of diffusers  21  are mechanically locked to the housing with a retaining ring  71  that is mounted in an inner circumferential recess  73  formed in the housing  27 . A retaining tube  75  is mounted to and extends from the retaining ring  71  and is coupled to the diffusers  21  with, e.g., a housing adapter  77  that mechanically engages an adjacent one of the diffusers  21 , such that the housing adapter  77  vertically supports the stack in the housing  27 . In the embodiment shown, the retaining ring  71  and the retaining tube  75  are located above the stack. The housing adapter  77  is sealed to the adjacent diffuser with an o-ring  45  (described above) that seals against surface  47 . The housing adapter  77  also has a housing seal member  79  for sealing with the housing  27 . In one alternate embodiment, a groove may be milled on the ODs of the diffusers to accept complementary keystock in order to hold the stages together. 
         [0024]    In certain applications such as steam assisted gravity drain (SAGD), the heat of the well may cause the housing to stretch which would cause previous designs to lose compression. This loss of compression for stages is such that the rate of thermal expansion for the stage material is different than that of the housing which, through thermal cycling, eventually causes the stages to compression set. Any subsequent thermal cycling where the stage expansion cannot keep up with the housing expansion will lose compression on the stage stack. However, with the invention, any housing stretch is compensated for and the lugs still maintain their locked position between stages. Also, with the previously required compression bearing eliminated from the invention, the housing is not subjected to any undue stress from the torquing of the bearing. 
         [0025]    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.