Patent Publication Number: US-7214042-B2

Title: Progressing cavity pump with dual material stator

Description:
The present invention is directed to a progressing cavity pump, and more particularly, to a progressing cavity pump having a stator made of more than one material. 
   BACKGROUND 
   Progressing cavity pumps typically have a single threaded screw, termed a rotor, located inside a stator having a double threaded cavity located therein. The rotor and the stator are shaped to create cavities along the length of the pump. As the rotor is rotated within the stator, the cavities progress from an inlet end of the pump to an outlet or discharge end. Thus, rotation of the rotor inside the stator pumps material located in the pump from the inlet end to the outlet end. 
   Stators may be formed of or coated with an elastomeric material to ensure a strong seal between the stator and rotor. The rotor and elastomeric stator form a compressive fit therebetween which allows the progressing cavity pump to self-prime, suction lift fluids (i.e. pump against gravity) and pump against a pressure (i.e., pump against a back pressure). However, stators lined with elastomeric material may have a performance disadvantage, especially when pumping moderate-to-high viscosity fluids due to pressure limitations of the elastomeric materials and frictional forces between the rotor and the stator. 
   Stators that are not lined with an elastomeric material (also known as “rigid stators”) may be formed of relatively rigid materials such as steel. Progressing cavity pumps having rigid stators may have a gap or clearance between the rotor and the stator. The clearances between the rotor and stator reduce friction and allow for more efficient pumping of moderate viscosity fluids (i.e. having a viscosity of between about 3000 centipoise and about 20,000 centipoise) and high viscosity fluids (i.e. having a viscosity of greater than about 20,000 centipoise). In particular, when pumping moderate-to-high viscosity fluids, the viscous fluids fill the gaps or clearances between the rotor and stator to allow efficient pumping operations. However, the gap between the rotor and the stator may prevent the pump from being self-priming, can limit its ability to suction lift fluids, and may limit its volumetric efficiency, especially when pumping relatively low viscosity fluids (i.e. having a viscosity of less than about 300 centipoise, or less than about 100 centipoise, or between about 0.5 centipoise and about 100 centipoise). 
   Accordingly, there is a need for a progressing cavity pump, and in particular, a stator for use with a progressing cavity pump which can be self-priming, and can create sufficient suction and can pump against high pressure, while providing efficient pumping operations. 
   SUMMARY 
   The present invention is a progressing cavity pump, and in particular, a stator which can be used with a progressing cavity pump which can form a seal with the rotor yet provides high pumping efficiencies for moderate-to-high viscosity fluids. In particular, in one embodiment, the present invention is a hybrid stator having a relatively soft or elastomeric portion at one axial end and a rigid portion at the other axial end. The soft stator portion may be located at the inlet end of the stator to provide the desirable suction characteristics. The rigid stator portion may be located at the outlet end which allows high pumping pressures to be developed. 
   In particular, in one embodiment the invention is a progressing cavity pump including a rotor and a stator having an inlet and an outlet. The rotor is rotationally disposed inside of the stator such that rotation of the rotor causes fluid in the pump to be pumped from the inlet toward the outlet in a downstream direction. The stator has an inner surface having a first portion made of a first material and a second portion made of a second material, the second portion being located in the downstream direction relative to the first portion. 
   In another embodiment, the invention is a progressing cavity pump including a rotor and a stator having an inlet and an outlet. The rotor is rotationally disposed inside of the stator such that rotation of the rotor causes fluid in the pump to be pumped from the inlet toward the outlet in a downstream direction. The stator has an inner surface having a first portion having a material property and a second portion having a material property that differs from the material property of the first portion. 
   Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front perspective view of one embodiment of the pump of the present invention, with portions of the pump cut away for illustrative purposes; 
       FIG. 2  is a side view of one embodiment of the stator of the present invention; 
       FIG. 3  is a side cross section of the stator of  FIG. 2  with a rotor received therein; and 
       FIG. 4  is an exploded view of the stator of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , the progressing cavity pump  10  of the present invention may include a generally cylindrical stator tube  12  having a stator  14  located therein. The stator  14  has an opening or internal bore  16  extending generally longitudinally therethrough in the form of a double lead helical nut to provide an internally threaded stator  14 . The pump  10  includes an externally threaded rotor  18  in the form of a single lead helical screw rotationally received inside stator  14 . The rotor  18  may include a single external helical lobe  20 , with the pitch of the lobe  20  being twice the pitch of the internal helical grooves. 
   The rotor  18  fits within the stator bore  16  to provide a series of helical seal lines  22  where the rotor  18  and stator  14  contact each other or come in close proximity to each other. In particular, the external helical lobe  20  of the rotor  18  and the internal helical grooves of the stator  14  define the plurality of cavities  24  therebetween. The stator  14  has an inner surface  36  which the rotor  18  contacts or nearly contacts to create the cavities  24 . The seal lines  22  define or seal off defined cavities  24  bounded by the rotor  18  and stator  14  surfaces. 
   The rotor  18  is rotationally coupled to a drive shaft  30  by a pair of gear joints  32 ,  34  and by a connecting rod  36 . The drive shaft  30  is rotationally coupled to a motor (not shown). Thus, when the motor rotates the drive shaft  30 , the rotor  18  is rotated about its central axis and thus eccentrically rotates within the stator  14 . As the rotor  18  turns within the stator  14 , the cavities  24  progress from an inlet or suction end  40  of the rotor/stator pair to an outlet or discharge end  42  of the rotor/stator pair. The pump  10  includes a suction chamber  44  in fluid communication with the inlet end  40  into which fluids to be pumped may be introduced. During a single 360° revolution of the rotor  18 , one set of cavities  24  is opened or created at the inlet end  40  at exactly the same rate that a second set of cavities  24  is closing or terminating at the outlet end  42  which results in a predictable, pulsationless flow of pumped fluid. 
   The pitch length of the stator  14  may be twice that of the rotor  18 , and the present embodiment illustrates a rotor/stator assembly combination known as 1:2 profile elements, which means the rotor  18  has a single lead and the stator  14  has two leads. However, the present invention can also be used with any of a variety of rotor/stator configurations, including more complex progressing cavity pumps such as 9:10 designs where the rotor has nine leads and the stator has ten leads. In general, nearly any combination of leads may be used so long as the stator  14  has one more lead than the rotor  18 . U.S. Pat. Nos. 2,512,764, 2,612,845, and 6,120,267, the contents of which are hereby incorporated by reference, provide additional information on the operation and construction of progressing cavity pumps. 
   In the embodiment shown in  FIG. 1 , the stator  14  includes a first or upstream portion  50  made of a first material and a second or downstream portion  52  made of a second material. The first  50  and second  52  portions may abut against each other at a transition location  54 , and are shaped and aligned such that the internal bore  16  transitions smoothly from the first portion  50  to the second portion  52  while maintaining a smooth and continuous helical nut shape. Thus, the first portion  50  extends from the inlet end  40  to the transition location  54 , and the second portion extends from the transition location  54  to the outlet end  42 . If desired, an O-ring (not shown) may be included in a groove (not shown) at the mating surfaces of the transition location  54  to seal the surfaces at the transition location  54 . 
   The first portion  50  may be located at or adjacent to the inlet end  40  of the stator  16  and is made of a relatively soft material, such as elastomeric materials, elastomers, nitrile rubber, natural rubber, synthetic rubber, fluoroelastomer rubber, urethane, ethylene-propylene-diene monomer (“EPDM”) rubber, polyolefin resins, perfluoroelastomer, hydrogenated nitrites and hydrogenated nitrile rubbers, polyurethane, epichlorohydrin polymers, thermoplastic polymers, polytetrafluoroethylene (“PTFE”), polychloroprene (such as Neoprene), synthetic elastomers such as HYPALON® polyolefin resins and synthetic elastomers sold by E. I. du Pont de Nemours and Company located in Wilmington Del., synthetic rubber such as KALREZ® synthetic rubber sold by E. I. du Pont de Nemours and Company, tetrafluoroethylene/propylene copolymer such as AFLAS® tetrafluoroethylene/propylene copolymer sold by Asahi Glass Co., Ltd. of Tokyo, Japan, acid-olefin interpolymers such as CHEMROZ® acid-olefin interpolymers sold by Chemfax, Incorporated of Gulfport Miss., and various other materials. The elastomeric material may have a hardness of between about 35 Shore A and about 85 Shore A, or less than about 35 Shore A, or less than about 85 Shore A, or more than about 85 Shore A 
   Rather than being made entirely of a soft or elastomeric portion (as is the portion of the stator shown in  FIG. 1 ), the first portion  50  may be made of a relatively rigid material, such as steel, with the relatively soft coating on its inner surface  36 . For example,  FIG. 3  illustrates the first portion  50  including a rigid (steel) inner core  51  with an elastomeric coating  53  located thereon. The helical groove of the stator portion  50  and/or the lobe  20  of the rotor  18  may be shaped and sized to form a compressive fit therebetween to allow the progressing cavity pump  10  to self-prime, suction, lift fluids and pump against a pressure (i.e., pump fluids against a back pressure). 
   The second portion  52  of the stator  14  may be located at or adjacent to the outlet end of the stator  42  and is made of a relatively rigid material, such as steel, carbon steel, tool steel, TEFLON® fluorinated hydrocarbons and polymers sold by E.I. duPont de Nemours and Company, A2 tool steel, 17-4 PH stainless steel, crucible steel, 4150 steel, 4140 steel or 1018 steel, or other suitable materials which can be cast or machined. The rigid stator material  52  may have a hardness of between about 25 Rockwell C or about 60 Rockwell C, or greater than about 25 Rockwell C or greater than about 60 Rockwell C. In this case, the helical groove of the second portion  52  and/or the lobe  20  of the rotor  18  may be sized somewhat differently from the first portion of the stator  50 . In particular, the stator portion  52  and rotor  18  may have a gap or clearance therebetween, which provides high pumping efficiencies, especially for high viscosity fluids. The gap between the rotor  18  and stator portion  52  is relatively small, and thus is not shown in the attached drawings. 
   The bore  16  of the first stator portion  50  and the bore  16  of the second stator portion  52  may have the same size and shape, with the exception that the bore  16  of the first stator portion  50  may have an elastomeric coating on its inner surface  32  to provide a sealing surface. However, the size and shape of the bores  16  of the stator portions  50 ,  52  may vary from each other in order to provide optimal pumping performance inside each stator portion  50 ,  52 . For example, as noted above, the bore  16  in the rigid stator portion  52  may be slightly larger than the bore  16  in the flexible stator portion  50 . 
   The hybrid stator  14  of the present invention includes two portions  50 ,  52 , which together allow for self-priming, suctioning, lifting fluids and pumping against a pressure, while also providing high pumping efficiencies for medium-to-high viscosity fluids. Accordingly, the hybrid stator  14  provides the advantages of both soft and rigid stators in a single stator, while limiting the drawbacks of either type of stator. 
     FIG. 1  illustrates a single stator tube  12  receiving two stator portions  50 ,  52  therein.  FIGS. 2–4  illustrate an alternate embodiment of the hybrid stator  14  of the present invention, in which a first stator tube  12   a  receives the first stator portion  50  therein, and a second stator tube  12   b  receives the second stator portion  52  therein. The first and second stator tubes  12   a,    12   b  may be joined together to form the hybrid stator  12 . In this embodiment, providing two separate stator portions  50 ,  52  and tubes  12   a,    12   b  which can be releasably joined together allows each separate stator section  50 ,  52  to be replaced, serviced and/or repaired without having to replace or access the entire hybrid stator. 
   In particular, a stator tube portion  12   a  which includes an elastomeric material on its inner surface will typically wear faster than a rigid stator tube portion  12   b.  Thus, stator tube portion  12   a  may need repair and/or replacement prior to stator tube portion  12   b.  Furthermore, the use of two separate stator portions  50 ,  52  which are joined together allows the stator bores  16  of each stator portion  50 ,  52  to be individually sized and shape to provide optimum pumping performance. 
   Any of a wide variety of methods for joining the two stator tubes  12   a,    12   b  together may be utilized without departing from the scope of the present invention. However, in the illustrated embodiment, each stator tube  12   a,    12   b  includes an annular recess  60 ,  62  located on its outer surface and adjacent to the transition location  54 . The stator  12  may include a pair of retaining rings  64 ,  66 , with each retaining ring received in an associated recess  60 ,  62 . A clamp ring  68 ,  70  is located on either side of an associated retaining ring  64 ,  66 , and a seal ring  72  is located between the seal rings  64 ,  66  and clamp rings  68 ,  70 . 
   Bolts  76  are passed through the retaining rings  68 ,  70  and washers  78  and nuts  80  are located over the ends of the bolts  76  and tightened down to attach the first  12   a  and second  12   b  stator tubes in a sealed manner. The assembled hybrid stator  14  may then be utilized in a progressing cavity pump such as, for example, the pump shown in  FIG. 1 . 
   The hybrid stator  14  illustrated herein includes first  50  and second  52  stator portions of equal axial length. However, the relative lengths of the first  50  and second  52  portions may be adjusted in order to adjust the performance characteristics of the hybrid stator  14  in the desired manner. In other words, the transition location  54  may be located at any point along the length of the stator  14 . Furthermore, if desired, more than two different types of materials may be included in the stator  12 , or more than one material may be used at more than one location inside the stator  12 . 
   Furthermore, the relatively hardness/softness of the stator portions  50 ,  52  is not the only characteristic which may differ between the two stator portions  50 ,  52 . Instead, the first  50  and second  52  stator portions may differ in a wide variety of material properties, including but not limited to lubricity, hardness, temperature resistance (i.e., softening and/or melting point), chemical resistance, crystalline structure, strength, density, elasticity, thermal expansion coefficient, etc. The properties may differ sufficiently such that recognizably different pump characteristics are provided. 
   The rotor  18  can be made of any of a wide variety of materials, including steel or any of the materials listed above for the stator portions  50 ,  52 . In addition, rather than having a hybrid stator made of more than one material, the rotor  18  may be a “hybrid rotor.” In particular, one axial half or portion of the rotor may be made of or coated with a first material  90  (i.e. an elastomeric material) and the second half or portion of the rotor may be coated with or made of a second material (i.e. metal)  92 . A hybrid rotor can be manufactured by making standard rotor using common manufacturing techniques while reducing the size of a portion of the rotor (i.e. the portion which will receive the elastomeric coating). The first and second materials of the outer surface of the rotor may have the differing characteristics outlined above for the differing materials of the stator. In this case the hybrid rotor can provide the same or similar benefits as the hybrid stator discussed above. The hybrid rotor can be used in conjunction with a standard (non-hybrid) stator, or in conjunction with a hybrid rotor.  FIG. 3  illustrates a hybrid rotor having two sections  90 ,  92  used in conjunction with a hybrid stator (although the two sections  90 ,  92  are not shaded differently to show the different types of materials). 
   Having described the invention in detail and by reference to the preferred embodiments, it will be apparent that modifications and variations thereof are possible without departing from the scope of the invention.