Abstract:
A centrifugal pump has impellers for pumping low flow, high viscous materials. The impellers have high exit angles greater than 30 degrees and preferably greater than 50 degrees. The impellers and diffusers have specific geometry that varies with viscosity. The pump has zones of impellers and diffusers with the exit angles and geometry in the zones differing from the other zones. The exit angles decrease and geometry varies in a downstream direction to account for a lower viscosity occurring due to heat being generated in the pump. One design employs small diameter impellers and high rotational speeds.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates in general to electric submersible well pumps. More specifically, this invention relates to submersible well pumps that have an impeller configuration designed for high viscosity fluids and operate at high rotative speeds.  
           [0003]    2. Description of the Prior Art  
           [0004]    Traditionally the use of electric submersible pumps (ESP&#39;s) in low flow viscous crude pumping applications has been limited because of low efficiencies inherent with low capacity centrifugal pumps handling viscous fluids. Low efficiencies result from disk friction losses caused by a layer of viscous fluid adhering to the walls of both rotating and stationary components within the pump impeller and diffuser. Viscous fluids are considered herein to be fluids with a viscosity greater than 500 centipoise.  
           [0005]    Others have made and used ESP&#39;s to pump viscous materials. However, most of these attempts have involved either modifying the material to be pumped or controlling the output of the pump motors with additional equipment to assist in the low flow conditions typical of pumping high viscous materials from wells.  
           [0006]    Others have attempted to pump high viscous materials by simply lowering the viscosity of the material, as opposed to trying to modify the pump or motor to accommodate the high viscous materials. U.S. Pat. Ser. No. 6,006,837 to Breit (hereinafter “Breit Patent”), U.S. Pat. Ser. No. 4,721,436 to Lepert (hereinafter “Lepert Patent”), and U.S. Pat. Ser. No. 4,832,127 to Thomas et al. (hereinafter “Thomas Patent”) are three such examples of this type of invention.  
           [0007]    In the Breit Patent, the viscous fluids that are being pumped are heated in order to lower the viscosity of the fluid being pumped. The Lepert Patent discloses a process for pumping viscous materials by mixing the high viscosity materials with low viscosity materials with the use of a turbine-machine that consists of a turbine and a pump, separating the mixture, and recirculating the low viscosity materials for reuse. The Thomas Patent discloses a process for pumping viscous materials by mixing the high viscosity oil with water to lower the viscosity and then pump the material by conventional methods once the viscosity is suitable for pumping. Each of these references alters the fluid being pumped, without trying to modify the pump or motor to accommodate the fluid being pumped.  
           [0008]    A need exists for an ESP and method of pumping high viscosity materials while maintaining pumping efficiencies, without altering the material being pumped or trying to maintain torque or rpm levels in a pump motor without the use of additional equipment. Ideally, such a system should be capable of being adapted to the specific applications and also be able to be used on existing equipment with minimal modification.  
         SUMMARY OF THE INVENTION  
         [0009]    This invention provides a novel method and apparatus for pumping high viscous fluids from a well by utilizing variations of large impeller vane exit angles and geometry, zones with varying impeller angles and geometry in each zone, smaller diameter impellers, and high rotative speeds for pumping. The impeller vane exit angles are greater than 30 degrees and preferably greater than 50 degrees. The zones have impeller vane exit angles and geometry that vary from zone to zone. In the high rotative speed embodiments, the motor can rotate up to 10,500 rpm, and preferably above 5,000 rpm. When the motor is operated at such a high rotative speed, various impeller diameters can be used, while maintaining the same diameter shaft and diffuser height. The pump diameter can vary, but is limited based upon the fit-up arrangement in the well. Additionally, the present invention can be configured with any of the above traits in a variety of configurations.  
           [0010]    Centrifugal pumps impart energy to the fluid being pumped by accelerating the fluid through the impeller. When the fluid leaves the impeller, the energy it contains is largely kinetic and must be converted to potential energy to be useful as head or pressure. In this invention, energy is imparted to the viscous fluid as rapidly as possible by using impeller vane geometry containing exit angles greater than 30 degrees. The use of large exit angles also minimizes vane length. Vane inlet angles in the range of 0 degrees to 30 degrees are used to minimize impact and angle-of-incidence losses. Diffuser vanes in this invention decelerate and direct the viscous fluid to the next pump stage as rapidly as possible using the same philosophy as used in the impeller, i.e. minimizing vane lengths and rapidly transitioning between the diffuser inlet and exit angles.  
           [0011]    Inherent in the operation of centrifugal pumps, the energy dissipated as a result of frictional losses is absorbed as heat by the viscous crude oil, resulting in a temperature rise as the oil passes through the pump. The temperature rise in turn lowers the crude oil viscosity. The temperature rise can be significant in an ESP because of the length and number of stages contained in a typical ESP application. The present invention seeks to take advantage of the decreasing viscosity by assembling the pump in zones or modules with the impeller and diffuser geometry in each zone or module optimized for the viscosity and/or NPSH (net positive suction head) conditions of the viscous crude oil passing through that zone. Geometry refers to the configuration of the vanes with respect to the exit angles and number of vanes.  
           [0012]    Flow rate varies directly with rotative speed and head or pressure varies with the square of rotative speed in centrifugal pumps. Reducing the impeller diameter minimizes disk friction but reduces the head and flow of the pump. When higher rotative speeds are coupled with vane geometry optimized for viscous pumping, performance per stage is restored and efficiency is further increased by reducing the amount of time in which the impeller and/or diffuser are in contact with the viscous fluids relative to the flow rate of the pump. As a practical limit, rotative speeds will be limited to 10,500 rpm, which corresponds to the speed of a two-pole electric motor operating at a frequency of 180 Hz. The present invention seeks to minimize disk friction by shortening the distance that the viscous fluid must travel as it moves through the pump. At the same time, clearances between rotating and stationary components are optimized to minimize the effect of boundary layer losses on non-pumping surfaces.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of the invention&#39;s scope as it may admit to other equally effective embodiments.  
         [0014]    [0014]FIG. 1 is a perspective view of a centrifugal pump disposed in a viscous fluid within a well, constructed in accordance with this invention.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of two stages in the centrifugal pump of FIG. 1.  
         [0016]    [0016]FIG. 3 is a cross-sectional view of an impeller of the centrifugal pump of FIG. 1.  
         [0017]    [0017]FIG. 4 is a sectional view of an impeller taken along the line  4 - 4  of FIG. 3 with 5 vanes, equally spaced.  
         [0018]    [0018]FIG. 5 is a cross-sectional view of a diffuser of the centrifugal pump of FIG. 1.  
         [0019]    [0019]FIG. 6 is a sectional view of a diffuser showing nine diffuser vanes, equally spaced, taken along the line  7 - 7  of FIG. 5.  
         [0020]    [0020]FIG. 7 is a sectional view of an impeller similar to the impeller of FIG. 4, but with a 50° exit angle.  
         [0021]    [0021]FIG. 8 is a sectional view of an impeller similar to the impeller of FIG. 4, but with a 60° exit angle.  
         [0022]    [0022]FIG. 9 is a sectional view of an impeller similar to the impeller of FIG. 4, but with a 70° exit angle.  
         [0023]    [0023]FIG. 10 is a partial cross-sectional view of two stages in a pump constructed in accordance with the invention, but with a shortened impeller diameter and higher rotating shaft speed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Referring to the drawings, FIG. 1 generally depicts a well  10  with a submersible pump 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 shaft of motor  16  connects to the seal section shaft  15  (not shown) 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 is needed to convey the well fluid  18  to a storage tank (not shown).  
         [0025]    Referring to FIG. 2, centrifugal pump  12  has a housing  27  (not shown in FIG. 2) that protects many of the pump  12  components. Pump  12  contains a shaft  29  that extends longitudinally through the pump  12 . Diffusers  21  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. 6) that extend helically outward from a central area. Diffuser  21  is a radial flow type, with passages  32  extending in a radial plane.  
         [0026]    An impeller  20  is placed within each diffuser  21 . 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 . Passages  34  are defined by vanes  22  (FIG. 4). Washers are placed between the upper and lower portions between the impeller  20  and diffuser  21 .  
         [0027]    Impellers  20  rotate with shaft  29 , which increases the velocity of the fluid  18  being pumped as the fluid  18  is discharged radially outward through passages  34 . The fluid  18  flows inward through passages  32  of diffuser  21  and returns to the intake of the next stage impeller  20 , which increases the fluid  18  pressure. Increasing the number of stages by adding more impellers  20  and diffusers  21  can increase the pressure of the fluid  18 .  
         [0028]    As shown in FIGS. 4, 7,  8  and  9 , the number of and exit angle b 2  of the impeller vanes  22  and diffuser vanes  23  can vary. The exit angle b 2  is measured from a line tangent to the circular periphery of impeller  20  to a line extending straight from vane  22 . FIG. 4 is a cross-sectional view of impeller  20 , which has five equally spaced impeller vanes  22  and with an exit angle b 2  of 55 degrees. Passages  34  increase greatly in width and their flow area from the central areas to the periphery. FIGS. 7 through 9 show impellers with five equally spaced vanes with a discharge angle of b 2 , 50, 60, and 70 degrees respectively. The inlet angles b 1  are in the range from 20 to 30 degrees for each impeller  20  of FIGS.  4  and FIGS. 7 through 9. As the vane exit angle b 2  increases, the vanes  22  become straighter and thus shorter. The length L from impeller  20  of FIG. 4 is longer than the length of the vanes  22  of the other FIGS. A shorter vane  22  increases pressure head but, generally speaking, creates more turbulence losses. A shorter vane also reduces the effects of boundary layer.  
         [0029]    [0029]FIG. 6 depicts a cross-sectional view of diffuser  21 , which has nine equally spaced vanes  23  taken along the line  6 - 6  of FIG. 5. The entrance and exit angles of vanes  23  are selected to minimize losses due to the angle of incidence and will depend on which impeller exit angle b 2  is chosen. Each diffuser passage  32  increases in flow area from the periphery inward. As the shaft rotates impellers  20 , fluid flows radially outward through passages  34 . The velocity increases, then the energy is largely kinetic. The fluid turns upward and flows into diffuser passages  32 . The velocity slows as the fluid flows radially inward, converting energy to potential energy. Diffuser vanes  23  decelerate and direct the viscous fluid to the next pump stage as rapidly as possible by minimizing the vane lengths and rapidly transitioning between the diffuser inlet and exit angles. Clearances between rotating and stationary pump components are also optimized to minimize the effect of boundary layer losses on non-pumping surfaces.  
         [0030]    The centrifugal pump  12  can have a plurality of zones in order to take advantage of the viscosity change of the well fluid  18  as the fluid  18  is heated by the pumping process. Referring to FIG. 1, three zones  36 ,  38 , and  40  are illustrated. Each zone comprises a plurality of impellers  20  and diffusers  21 . Preferably all of the impellers  20  within a zone  36 ,  38 , and  40  will have the same impeller vane  23  discharge angle B 2 . Frictional losses cause a temperature rise across each stage that varies with viscosity. Consequently, the well fluid is more viscous in zone  36  than in zone  38 , which in turn is more viscous than in zone  40 . Consequently, the exit angle b 2  in impellers  20  of zone  36  is higher than in zone  38 . Similarly, the exit angle b 2  in impellers  20  of zone  38  is higher than zone  40 . For example, zone  36  could be designed for greater than 500 centipoise viscosity, zone  38  for 300-500 centipoise, and zone  40  for 100-300 centipoise. There could be more than three zones and the stages in the zones do not have to be equal in number.  
         [0031]    The method of pumping the viscous well fluid  18  with a submersible pump assembly  11  can also be accomplished by rotating the pump  12  at a higher speed than normally used with viscous fluids. High speed is defined as a speed greater than 3,500 rpm and may be as high as about 10,500 rpm with the preferred speed being above 5,000 rpm. The use of the high speed reduces the required diameter of the impellers, so a small impeller diameter  20 , for example less than 2.75 inches, can be used in the high speed embodiments of this invention, as shown in FIG. 10. The impeller diameter Id can be shortened in this embodiment, while the shaft diameter Sd and the diffuser height Dh remain the same as in the lower speed embodiments of FIGS.  1 - 9 . Any size diameter  20  can be used, but the size can be limited due to the pump fit-up arrangement in the well. As a result, the ratio of shaft diameter Sd to impeller diameter Id is at least 0.30 and preferable 0.33 and the ratio of diffuser height Dh to impeller diameter Id is at least 0.70 and preferably 0.72. These ratios can be utilized in all embodiments of the invention that operate at a high pumping speed. In the embodiments of FIGS.  1 - 9 , the ratio of shaft diameter Sd to impeller diameter Id is a prior art dimension of 0.28 and the ratio of diffuser height Dh to impeller diameter Id is a prior art dimension of 0.57.  
         [0032]    The impellers  20  of FIG. 10 have the same high exit angles as in the other embodiments, preferably greater than 30 degrees. Although the rotational speed is much higher than in the embodiments of FIGS.  1 - 9 , the tip velocities are approximately the same because of the shorter radius. The typical prior art speed is 3,500 rpm. Reducing the impeller  20  diameter reduces disk friction but reduces the head and flow of the pump. Increasing the rotative speed increases head and flow. The higher rotative speed and high exit angle geometry are efficient for viscous fluids because of the reduced amount of time in which the impeller and/or diffuser are in contact with the viscous fluids relative to the flow rate of the pump.  
         [0033]    The invention has significant advantages. The high exit angles increase pump efficiency for viscous fluids by shortening the lengths of the flow paths through the impellers. The multiple zones, each with impellers having different exit angles, allows optimizing as heat reduces the viscosity of the well fluid flowing through the pump. Higher rotative speeds and smaller diameter impellers also increases efficiency for viscous fluids.  
         [0034]    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.