Abstract:
Durable pumps for abrasives are provided. An example centrifugal pump stage for subsurface operation has a thrust washer located inside the circumference of an outboard clearance seal between an impeller shroud and the diffuser. Relocation of the thrust washer allows the clearance seal to protect the thrust washer from abrasives. A centrifugal action of an outboard seal lip effects a separation of the particles from the fluid nearest the gap of the outboard seal and drives the particles away from the gap of the outboard seal. When an abrasive particle in the leakage flow does clear the outboard seal, the particle is readily flushed across the thrust washer by the leakage flow to the central fluid inlet to prevent wear of the thrust washer. In an implementation, the inside diameter of the thrust washer is approximately flush with the bore of a central fluid inlet of the pump stage.

Description:
RELATED APPLICATIONS 
       [0001]    This continuation patent application claims the benefit of priority to U.S. patent application Ser. No. 13/174,343 to Eslinger, filed Jun. 30, 2011 and entitled “Durable Pump for Abrasives,” which is incorporated herein by reference in its entirety, which in turn claims priority to U.S. Provisional Patent Application No. 61/365,695, filed Jul. 19, 2010, and entitled, “Centrifugal Pump with Increased Abrasion Resistance,” and to U.S. Provisional Patent No. 61/360,431, filed Jun. 30, 2010, and entitled, “Device and Means to Reduce Downthrust in a Multistage Centrifugal Pump,” both of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Oilfields sometimes use electric submersible pumps staged in series to pump downhole fluids. A number of centrifugal pump stages can be stacked together along their axial direction for ganged lift in a subsurface environment. Such subsurface multistage pumps are frequently employed to move fluids consisting of liquid hydrocarbon mixtures that may have some mixed and suspended earth solids. The fluid may also contain gaseous components and water. Particles and chunks of rock and sand are usually present to some degree. Such heterogeneous “liquid sandpaper” may result in cavitation and abrasion issues for pumps, especially if the solids cause deposits to build up against some surfaces of the pump or if the fluid itself has a slurry-like consistency. The viscosity and other flow characteristics of a particular liquid mixture may result in high velocity flow of the abrasive fluid around certain pump parts. Impellers used in downhole centrifugal pumps experience significant abrasion of the downthrust washers (hereinafter, “thrust washers”) when pumping fluids containing abrasives. Thus, the art of pump design aims to minimize abrasion and prolong the life of the pump. The particular composition and behavioral characteristics of the abrasive fluid to be pumped often allow particular pumps to be custom-designed and optimized for particular types of unrefined fluids. 
       SUMMARY 
       [0003]    Durable pumps for abrasives are provided. An example centrifugal pump stage for subsurface operation has a thrust washer located inside the circumference of an outboard clearance seal betwe en an impeller shroud and the diffuser. Relocation of the thrust washer allows the clearance seal to protect the thrust washer from abrasives. A centrifugal action of an outboard seal lip effects a separation of the particles from the fluid nearest the gap of the outboard seal and drives the particles away from the gap of the outboard seal. When an abrasive particle in the leakage flow does clear the outboard seal, the particle is readily flushed across the thrust washer by the leakage flow to the central fluid inlet to prevent wear of the thrust washer. In an implementation, the inside diameter of the thrust washer is approximately flush with the bore of a central fluid inlet of the pump stage. This summary section is not intended to give a full description of durable pumps for abrasives. A detailed description with example implementations follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a diagram of an example stage of a multistage subsurface pump for pumping fluids containing abrasives. 
           [0005]      FIG. 2  is a diagram of example inboard thrust washers for a subsurface pump for abrasives. 
           [0006]      FIG. 3  is a diagram of example reduction in diameter of an inboard thrust washer. 
           [0007]      FIG. 4  is a diagram of example reduction in diameter of a thrust washer to balance pressure areas to reduce friction. 
           [0008]      FIG. 5  is a diagram of an example reduction in thrust washer diameter to decrease friction and reduce power loss. 
           [0009]      FIG. 6  is a diagram of an example reduction in thrust washer diameter to decrease moment arm of a braking torque to reduce power loss. 
           [0010]      FIG. 7  is a flow diagram of an example method of making an abrasion-resistant subsurface pump. 
           [0011]      FIG. 8  is a flow diagram of an example method of increasing the durability and efficiency of an abrasion-resistant pump. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Overview 
         [0013]    This disclosure describes durable pumps for abrasives. The described pumps provide higher wear and longer life than conventional designs, especially when pumping subsurface fluids containing some solids that tend to be abrasive when pumped, or when pumping slurries.  FIG. 1  shows a cross-section of a centrifugal pump stage  100  of a multistage submersible pump stack  102 . The multistage submersible pump stack  102  includes a number of the centrifugal pump stages  100  stacked together along their axial direction for ganged lift to generate axial fluid flow  104  in a subsurface environment.  FIG. 2  shows example inboard thrust washers.  FIGS. 3-6  show reduction of the diameter of example thrust washers, and associated benefits.  FIGS. 7-8  show example methods of increasing the durability of pumps for abrasive fluids. 
       Example System Environment 
       [0014]    Electric submersible pumps for abrasive fluids usually have at least one surface that is an impeller housing, or “shroud,” i.e., a solid part of the impeller assembly extending radially outward from a more central hub to strengthen and attach the impeller blades on one side, and also serving to screen or shield the impeller blades, at least in part, from the fluid on the other side of the shroud, since the shroud is solid. Impeller blades are typically attached to the shroud, and the shroud is typically attached to a hub that receives the rotational drive power of the pump, or, the shroud is an extension of the hub itself. Such a shroud may “underlie” the bottom sides of the impeller blades, or two shrouds may enclose both the top and bottom sides of the impeller blades in a “closed-impeller” or “enclosed” design in which only the radial ends of the impeller blades are open, as opposed to open-style impeller blades that are exposed to the fluid being pumped on all sides of the blades. Open-style impellers (without a shroud) are more susceptible to abrasive wear than a shrouded impeller, because high velocity fluid on the impeller blades is in close proximity to the casing walls (“diffuser” or housing), creating rotating vortices that accelerate wear when abrasives are present in the fluid. 
         [0015]    Centrifugal pumps for moving fluids that may have abrasive properties typically incorporate a single shroud, located on the bottom of the impeller, or an enclosed design with both top and bottom shrouds. In an abrasive setting, the shroud(s) also provide additional structural support and reinforcement to protect against blade collapse or deformation. Such enclosed or semi-open impeller designs are well suited for handling solids in applications where the blades might encounter high impact loads from rocks and solids. A semi-open impeller also has an ability to pass solids in a manner similar to that of an open impeller type. With a single shroud the semi-open impeller is also relatively easy to manufacture. 
         [0016]    High axial thrust is the primary drawback of semi-open and enclosed impeller designs: the rotating impeller creates a net fluid flow  104  along the axial direction but also creates large reaction forces, which thrust the shrouded impeller back in the opposite direction of the axial fluid flow. On a semi-open impeller, the entire backside surface of the shroud is subject to the full impeller discharge pressure. The front side of the shroud is at suction pressure at the eye of the impeller, where the fluid is inlet, and increases along the impeller radius due to centrifugal action. 
         [0017]    The differential between the pressure profiles along the two sides of the shroud creates the axial thrust imbalance, referred to herein as downthrust. The downthrust can be countered with a thrust washer, which radially supports the backside of the shroud. There is also an efficiency loss due to disk friction caused by the impeller shroud turning in close proximity to the stationary casing wall. The downthrust forces are resisted by thrust washers on each stage for floater style pumps. Impellers of the mixed-flow type usually have balance rings which assist to keep these forces within acceptable limits. However, radial-flow impellers do not have such balance rings due to the need to minimize the stage axial length. The height of the wear rings, thrust washers, or other balance rings in the axial direction is of primary concern because this height directly affects the overall height of each pump stage, which is critical in many multistage pump designs. Therefore, radial impellers tend to have high thrust loads which lead to high mechanical friction power losses and a high thrust washer wear rate. 
         [0018]    In centrifugal pumps, a portion of the fluid exiting from the rotating impeller characteristically leaks back to the pump suction by traveling through the gap between the impeller shroud and the casing. A semi-open impeller typically has wear rings or a front seal to control this leakage. In some pump stage designs, the outer edges of the thrust washer may perform this leakage-control role. Thus, the thrust washer may also aim to provide a fluid seal. 
         [0019]    The thrust washers control recirculation through flow restriction, and may also be used in conjunction with impeller balance holes to control the axial thrust. However, the flow restriction created by tight clearances between rotating and stationary thrust washer faces causes very high local fluid velocities and thus a high wear rate. Conventional thrust washers, because they are subject to this high flow velocity, have a short life span in an abrasive environment, even when hard materials and treated surfaces are used. 
         [0020]    The flow restriction at the thrust washer also causes solids to dam up at this location. Conventionally, as shown in the top of  FIG. 2 , an outboard thrust washer  202  is located radially outward from an impeller-to-diffuser shroud clearance seal  204  or other seal. Such a clearance seal  204  is typically a finely machined, close-fitting, close-running, metal-to-metal interface between the impeller shroud  206  and the diffuser (casing walls)  208  of the pump. The conventional wisdom of this arrangement is to support the impeller  210  against reactive forces from axial fluid flow  104 , supporting the impeller  210  around a ring that has a substantial diameter under the impeller at some median radius of the impeller shroud  206 . But a shortcoming of the outboard thrust washer  202  arrangement is that abrasive particles carried by fluid leakage from the impeller tip tend to accumulate at deposit location  212  in  FIG. 2 . This build-up of abrasive particles is due to the fact that the thrust washer axial clearance is larger than the radial clearance of front seal action and therefore the front seal acts as a particle dam. Accumulated abrasive particles rapidly wear the outboard thrust washer  202 . 
         [0021]    The thrust created by the impeller  210  in each stage of a submersible pump can be problematic in a variety of submersible pump types, including pumps with mixed flow stages and pumps with radial flow stages. In some floater style designs, for example, a significant portion of power loss in the pump is due to thrust friction occurring at an outer thrust washer due to relatively high friction-induced torque at this radially outlying position. If the outer thrust washer is removed from the floater style stage, however, the lack of any seal functionality increases leakage loss. 
       Example Pump and Impeller Design 
       [0022]    As shown in the bottom part of  FIG. 2 , in one implementation of a pump stage  100 , an example pump impeller  214  has an inboard thrust washer (pad, ring)  216  that is relocated inboard in relation to a seal  204  that defines a boundary of a fluid chamber of the diffuser (i.e., the stationary housing around the impeller). The relocation of the inboard thrust washer  216  “behind” the seal  204  protects the inboard thrust washer  216  from abrasive fluids being pumped and thus, from conventional abrasion and wear. The term “inboard,” as used herein, means “radially inward, toward, or closer to the axial center of rotation of the pump,” while “outboard” means “radially outward, away from, or further from the axial center of rotation of the pump.” 
         [0023]    The aforementioned seal  204  may be a wear ring, or may be a finely machined, close-running interface between a rotating part of the impeller  214 , usually an impeller shroud  218 , and the stationary diffuser housing: i.e., an impeller shroud-to-diffuser clearance seal  204 . With regard to abrasive fluid, since the protecting seal  204  is upstream from the inboard thrust washer  216  (with respect to fluid trying to return from the impeller  214  to the pump inlet  220 ) the amount of abrasive particles reaching the protected inboard thrust washer  216  is greatly reduced or eliminated. In conventional designs, an outboard thrust washer  202  may be in direct contact or even fully immersed in the fluid being pumped. The thrust washer  216  thus relocated and protected counteracts and supports against reactionary downthrust forces generated by the pumping impeller while providing higher wear and longer life than in conventional pumps used for pumping abrasive fluids in a multistage, subsurface environment. 
         [0024]    In the same or another implementation, as shown in  FIG. 3 , the diameter (size, or “ring-size”) of a seal or a thrust washer at the bottom (i.e., back) of the impeller is strategically reduced in order to expose more surface area of the bottom impeller shroud to the pressured fluid being pumped. In some designs a seal, wear ring, or close-fitting interface between moving impeller and stationary diffuser forms the extent of the fluid space under the impeller, while in other designs the thrust washer  202  itself plays this role. The thrust washer  202  will be used as an example for the sake of description below, since it plays the additional role of a “wear ring” type seal in some pumps. 
         [0025]    As shown in the top part of  FIG. 4 , a conventional thrust washer  202  defines the extent of a fluid chamber  402  at a bottom impeller shroud  206 . The top of the impeller  210  has a fluid chamber  404  that exposes a greater amount of surface area at the top of the impeller  210  to pressured fluid, resulting in a pressure imbalance area  406 , which thrusts the impeller  210  down into the thrust washer  202 , where friction results in power loss. Downthrust forces tend to be high because pressure acting on the impeller bottom shroud surface  206  is sealed at the outside diameter  408  of the thrust washer  202 , while pressure forces acting on the impeller top shroud surface  410  are sealed at the diffuser hub inside diameter  412 . 
         [0026]    In the bottom part of  FIG. 4 , reducing the diameter of the conventional thrust washer  202  to a smaller diameter thrust washer  216  when designing and manufacturing a pump, increases the extent of the bottom fluid chamber  414  and increases the amount of surface area of the bottom impeller shroud  416  that is exposed to the pressured fluid underneath. Referring to  FIG. 4 , reducing the diameter of the thrust washer  216  effectively reduces the pressure imbalance area  418 , as given in Equation (1): 
         [0000]      Δ A =(π/4)( d   1   2   −d   2   2 )  (1)
 
         [0000]    where d 1  is the conventional outside diameter of the pressure imbalance area  406  and d 2  is the outside diameter of the reduced pressure imbalance area  418 . Reducing the pressure imbalance area  418  in this manner increases the pressure at the bottom impeller shroud  416  thereby lifting the impeller  420  off the thrust washer  216  to some degree. The lift may not be a physical movement of the impeller  420  off the thrust washer  216 , but may be a reduction in the net downthrust force acting on the impeller  420 , or a reduction in the normal force F n  on the friction surface of the thrust washer  216 , thus sparing the thrust washer  216 . The friction on the surface of the thrust washer  216  may be approximated by the dry friction expression in Equation (2): 
         [0000]        F   f   ≦μF   n   (2)
 
         [0000]    where F f  is the force of friction exerted by each surface on the other, and is parallel to the surface in a direction opposite to the net applied force; μ is the coefficient of friction, which is an empirical property of the materials used to make the thrust washer  216 , and F n  is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface. 
         [0027]    The diameter of the thrust washer  216  (or other seal) can thus be selectively reduced to strategically balance the exposed surface area and pressure at the bottom of the impeller  420  with the exposed surface area and pressure at the top of the impeller  420  to reduce friction and power loss. This balancing of pressures at the top and bottom of the impeller  420  through seal or washer size selection also provides additional benefits. 
         [0028]    As shown in  FIG. 5 , in reducing the diameter of the conventional thrust washer  202 , the reduced diameter of the smaller thrust washer  216  also reduces power loss because of less surface area for friction to occur on the smaller thrust washer  216 . The reduction in surface area for friction to occur is given by Equation (3), using the radii shown in  FIG. 5 : 
         [0000]      Δ A =π[( r   1   2   −r   2   2 )−( r   3   2   −r   4   2 )]  (3)
 
         [0029]    For a reduction in the outside diameter of a conventional thrust washer  202  in which the new outside diameter of the smaller thrust washer  216  still remains larger than the initial inside diameter of the conventional thrust washer  202 , the reduction in surface area for friction to occur may be given by Equation (4): 
         [0000]      Δ A =(π/4)( d   1   2   −d   2   2 )  (4)
 
         [0000]    where d 1  is the outside diameter of the conventional thrust washer  202  and d 2  is the outside diameter of the new, smaller thrust washer  216 . 
         [0030]    Further, as shown in  FIG. 6 , since the radius of the circle or ring defined by the conventional thrust washer  202  is reduced to that of the new smaller thrust washer  216 , the moment arm  602  of the incidental braking force is reduced  604 . The braking force is a high-friction-induced torque acting between the rotating shroud and the thrust washer  216 , or between the stationary diffuser and the thrust washer  216 , depending on set-up, as the thrust washer  216  undesirably acts like elements of a disk brake. The frictional torque is given by Equation (5): 
         [0000]      τ= r×F   (5)
 
         [0000]    where τ is the frictional braking torque, r is the moment arm  602  (or lever arm) and F is the friction force approximated by Equation (2) above. Thus, the reduction in frictional braking torque may be given by Equation (6), using the radii shown in  FIG. 6 : 
         [0000]      Δτ= F ( r   1   −r   2 )  (6)
 
         [0031]    Relocating the seal or thrust washer may also increase efficiency of the pump and reduce wear by placing the thrust washer  216  or other seal where there is less agitation and turbulence in the abrasive fluid and/or where there is improved laminar flow away from closely interacting moving parts. 
       Example Methods 
       [0032]      FIG. 7  is an example method  700  of making an abrasion-resistant subsurface pump. In the flow diagram, the operations are summarized in individual blocks. 
         [0033]    At block  702 , a pump for moving fluids containing abrasives in a subsurface location is made, including an impeller, a casing, and a thrust washer. 
         [0034]    At block  704 , a seal and a thrust washer are placed in relation to each other to resist a flow of the abrasives to the thrust washer. 
         [0035]      FIG. 8  is an example method  800  of increasing the durability and efficiency of an abrasion-resistant pump. In the flow diagram, the operations are summarized in individual blocks. 
         [0036]    At block  802 , an impeller for pumping a fluid is made, including a thrust washer for supporting the impeller and for restricting a flow of the fluid. 
         [0037]    At block  804 , the diameter of the thrust washer is reduced to balance a first pressure at the bottom of the impeller with a second pressure at the top of the impeller, to reduce a friction of the impeller on the thrust washer. 
         [0038]    At block  806 , the diameter of the thrust washer is reduced to reduce a surface area subject to friction and to reduce a moment arm of a braking torque on the impeller, to reduce power loss in the pump. 
       CONCLUSION 
       [0039]    Although exemplary systems and methods have been described in language specific to structural features or techniques, the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claimed systems, methods, and structures.