Abstract:
A method of limiting core deflection in the manufacture of relatively long sections of progressing cavity pump production tubing wherein a core is held within the bore of a length of tubular casing to provide a form for the molding of a stator element. The core is centered and stabilized within the tubular casing at selected intervals prior to the injection of elastomeric material by one or more core centering bearings. The bearings are effective to prevent core deflection during manufacture and, rather than being removed after curing of the stator, are left in the casing to function as rotor bearings for the progressing cavity pump. The bearings, themselves encapsulated by elastomeric material during the manufacturing process, help maintain the rotor in optimal alignment within the production tubing thereby benefitting pump performance and longevity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to production tubing for Moineau-type progressing cavity pumps, and, more specifically, to minimizing core deflection in the manufacture of stators for relatively long sections of production tubing and providing rotor bearing surfaces at periodic intervals in the manufactured production tubing. 
     2. Background 
     A progressing cavity pump is a positive displacement pump particulary adaptable for pumping viscous, abrasive or corrosive liquids. Rene J. L. Moineau is credited with conceiving the progressing cavity pump in 1932. Such pumps are occasionally referred to as single screw pumps. This name arises since the rotor of a typical progressing cavity pump is a single helix which rolls eccentrically in a stator having a cavity forming a double helix. This single helix rotor/double helix stator combination creates pockets which are moved (progressed) linearly from an inlet end to a discharge end of the pump as the rotor is turned. 
     Applications for progressing cavity pumps fall into two general categories, that is, metering or liquid transfer. Progressing cavity pumps function exceptionally well for metering purposes since they deliver a highly reliable predetermined quantity of liquid for each revolution of the pump rotor. By accurately governing the rate of revolution of the pump rotor, the quantity of liquid delivered by a progressing cavity pump can be accurately repeated. For this reason, progressing cavity pumps are frequently employed in chemical processing systems wherein accurate proportional blending or mixing of liquid components is required. 
     The other basic application for progressing cavity pumps is for liquid transfer using either a constant speed or a variable speed drive. These pumps adapt well to many speciality applications, such as handling abrasive, viscous and two-phase fluids. Progressing cavity pumps can be employed for pumping fluids having a viscosity less than one centipoise. 
     The progressing cavity pump rotor is generally configured with a single screw thread of streamlined design without sharp edges functioning inside the cavity of the stator, the interior wall of which defines the elongated double helix. The rotor and stator are positioned within a length of production tubing (the pump barrel), the rotor being driven by a drive shaft connected to a motor. While the stator can be formed of metal, the most common method of manufacturing progressing cavity pumps is to make the stator of an elastomeric material. The combination of a metallic rotor and an elastomeric stator functions advantageously to provide a pump having great capacity to pump abrasive fluids and to maintain a predetermined discharge pressure. 
     Typically, the maximum pressure that a progressing cavity pump can deliver is directly related to the length of the rotor and stator, and, accordingly, in some applications, such as downhole submersible pumps for the oil field, the rotor and stator can be relatively long compared to their diameters. 
     Conventionally, the manufacture of production tubing involves vertically supporting a length of tubular casing on an injection plate and fixing a core centrally within the casing coincident to the longitudinal axis of the casing. Elastomeric material is then injected under very high pressure into the casing, the inside surface of which has been treated with a bonding material. The elastomeric material flows upward around the core to form the stator after curing. 
     A significant problem in the manufacture of relatively long stators is that the core tends to deflect within the tubular casing during the injecting step. The core is prone to move to one side or the other in response to the very high pressure of the injected elastomeric material. Moreover, if a horizontal injection process is used, the core may deflect or bow due to its own weight. Core deflection results in crooked stators and larger tolerances which, in turn, detrimentally affect pump performance. 
     Prior attempts to correct the problem of core deflection include drilling and tapping a hole in the casing and inserting set screws against the core to maintain its position during the molding process or using a sliding “pig” or disk, essentially a ring, to ride on top of the column of injected elastomeric material to assist the core in maintaining a central alignment. These methods, however, have not achieved great success in preventing core deflection and have significant drawbacks, such as tooling damage caused by the set screws, wobbling or sticking of the sliding ring during the injection process, and the need to recover the ring after manufacture. For these reasons, there remains a need for efficiently and reliably centering the core within a long length of casing during stator formation. 
     Another problem regularly encountered in long lengths of production tubing involves the misalignment of the rotor within the stator cavity. Preferably there should be an equal interference between the rotor and stator around the entire circumference of the rotor to ensure optimum pump performance. Should the rotor wobble or “chunk out” to one side of the stator cavity the pump loses the beneficial equal interference and performance is degraded. If areas of significant interference are created by the misalignment the torques required to operate the pump increase to the detriment of the system. 
     SUMMARY OF THE INVENTION 
     We have developed a method of manufacture in which the core is centered and stabilized within the tubular casing at selected intervals prior to the injection of elastomeric material by one or more core centering bearings. The bearings are effective to prevent core deflection during manufacture and, rather than being removed after curing of the stator, are left in the casing to function as rotor bearings for the progressing cavity pump. The bearings, themselves encapsulated by elastomeric material during the manufacturing process, help maintain the rotor in optimal alignment within the production tubing thereby benefitting pump performance and longevity. 
     More fully described, the inventive method is implemented in the manufacture of relatively long sections of progressing cavity pump production tubing wherein a core is held within the bore of a length of tubular casing to provide a form for the molding of a stator element. One or more cylindrical bearings are permanently mounted within the tubular casing, preferably prior to the insertion of the core within the casing. Each of the bearings has an outer diameter marginally smaller than the inner diameter of the casing so as to be concentrically received therein. The inner diameter of the bearing is marginally greater than the outer diameter of the core (which corresponds to the outer diameter of a complementary rotor). A flow of elastomeric material is injected into the casing to form the stator element. The flow encapsulates each bearing with elastomeric material so that a layer of elastomeric material covers substantially all of the innermost surface of the bearing. 
     To restrain the bearings within the casing during the injecting step, it is preferred that one or more holes be drilled and tapped into the casing at the desired location in a plane transverse to the longitudinal axis of the casing to receive set screws. Preferably, each bearing possesses an outer circumferential groove to receive and seat the set screws. Damage to the outer surface of the core is thereby avoided. 
     Because the outer and inner diameters of the bearings are marginally smaller than the inner diameter of the casing and outer diameter of the core, respectively, two small spaces are created once the bearings have been located and restrained within the casing and the core has been inserted. A first space is defined between the outermost surface of each bearing and the inner surface of the casing while a second space is defined between the innermost surface of each bearing and the outer surface of the core. As elastomeric material is injected into the casing, the elastomeric material flows through the first and second spaces whereby upon completion of the injecting step, the bearing is substantially encapsulated by elastomeric material. 
     In another preferred aspect of the invention, each bearing possesses an inner circumferential chamfer. The chamfered end of the bearing is directed to first receive the flow of elastomeric material so as to decrease the impedance of the flow and provide a holding surface for the elastomeric material. To maintain a smaller bearing surface against the rotor of the progressing cavity pump, and to thereby decrease working torques, it is particularly preferred that each end of the bearing possess an inner circumferential chamfer. In the most preferred embodiment, one end of the bearing has a chamfer of greater length than the other, and the end having the chamfer of greater length is directed to first receive the flow of elastomeric material. 
     The bearings, being encapsulated in the elastomeric material and restrained by set screws, are not removed from the casing after manufacture, but are left in the casing and thereafter function as rotor bearings to provide additional support to the progressing cavity pump rotor during use. Thus, the invention also provides inventive production tubing for a progressing cavity pump. 
    
    
     A better understanding of the present invention, its several aspects, and its objects and advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a manufacturing assembly employing the present invention in the making of a stator in a length of casing. 
     FIG. 2 is a top view of the preferred core centering/rotor bearing. 
     FIG. 3 is a cross section taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a cross section of a length of production tubing illustrating the use of the preferred core centering/rotor bearing. 
     FIG. 5 is a cross section showing a long length of production tubing possessing several core centering/rotor bearings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein in like reference numerals designate identical or corresponding parts throughout the several views, and specifically to FIG. 1 which exemplifies a manufacturing assembly employing the present invention, a tubular casing  10  is vertically supported upon an injection fixture  12  by a pipe retention mechanism  14 . The injection fixture  12  has a passageway  16  for delivering elastomeric material from a source (not shown) into the bore  18  of the casing  10 . Vertically supported coaxially within the casing  10  is a core  20 . The outer surface  22  of the core functions as a mold to define the desired double helix configuration of the interior wall of the stator cavity. The core  20  is supported in the manufacturing assembly by bolts  24 ,  26 . At the top end of the assembly the casing  10  is closed off by a topcap  28  held rigidly in place by a bolt  24  and nut  30  combination. A perforated bottom plate  29  supports the casing  10  upon the injection fixture  12 . 
     Within the bore  18  of the casing  10 , and at one or more desired locations along the extent of the casing  10  as may be necessary due to the length of the casing  10  and core  20 , there is provided in accordance with the present invention one or more core centering/rotor bearings  32 . Referring now to the remainder of the drawing figures, the bearing  32  is cylindrical or tubular in shape having a bore  34  therethrough. It is preferably machined from brass, although other metals, Teflon or hard plastics may be utilized alternatively. As best shown in the cross section of FIG. 3, the bearing  32  has a first end  36 , a second in  38 , an outermost surface  40 , and an innermost surface (or rotor bearing surface)  42 . The outer diameter of the bearing  32  is marginally smaller than the inner diameter of the casing  10  so as to be concentrically received therein as illustrated in the figures. As shown best in FIG.  1  and FIG. 4, the inner diameter of the bearing  32  is marginally greater than the outer diameter of the core  20  at its widest point. This allows for the easy placement of the bearing  32 , and subsequently the core  20 , within the casing and enables use of the invention with existing tooling. 
     The bearing  32  is preferably mounted within the casing  10  by drilling and tapping one or more, preferably three, holes  44  in the casing  10  at the desired location, the holes  44  being spaced around the casing  10  in a plane transverse to the longitudinal axis  46  of the casing  10 , and inserting one or more, preferably three, set screws  48  through the holes  44  to restrain the bearing  32  where desired. The bearing  32  preferably possesses an outer circumferential groove  50  to receive and seat the set screws  48  in order to better restrain the bearing  32  against the tremendous pressures encountered during the injection of the elastomeric material. 
     The tubular casing  10  is prepared for the injection of elastomeric material by sandblasting or glass-bead-blasting the inner surface  52  of the casing  10  until a white metal finish is obtained. The surface  52  is then cleaned with a degreasing composition and coated with a bonding agent. At predetermined locations, depending upon the length of the particular casing and the anticipated deflection to be encountered in the core  20 , one or more sets of holes  44  are drilled and tapped to receive the set screws  48 . 
     Each bearing  32  is prepared in a fashion similar to the inside surface  52  of the casing  10 . The bearing  32  is first lightly sandblasted or glass-bead-blasted. It is then cleaned with a degreasing solution, coated with a bonding agent and located within the casing  10  at the desired location. The bearing  32  may be positioned with the bore  18  of the casing  10  with a reach-rod or other centering device that allows for the proper placement of the bearing  32  within the bore  18  of the casing  10  such that alignment is achieved between the holes  44  and the circumferential groove  50 . 
     Once the bearings  32  have been properly located and restrained with the set screws  48 , the core  20  is inserted coaxially within the casing and secured by bolts  24 ,  26 , topcap  28  and bottom plate  29 . At this point the assembly (comprising the casing  10 , the mounted bearing(s)  32 , the mounted core  20  and the topcap  28  and bottom plate  29 ) is preheated, as is well known in the art, to facilitate the injection of molten elastomeric material. The temperature to which the assembly is preheated is dependant upon the type of elastomeric compounds to be utilized in forming the stator element and is within the skill of the art. After being preheated, the assembly is affixed to the injection fixture  12 , such as by the pipe retention mechanism  14 . 
     Particular attention is now directed to FIG.  4 . Elastomeric material is injected through the passageway  16  of the injection fixture  12  so as to traverse upward through the bore  18  of the casing  10  to fill the space between the inner surface  52  of the casing  10  and the outer surface  22  of the core  20 . Once injected (and after curing), the elastomeric material forms the stator  54 . After the completion of the injecting step, the assembly is removed from the injection fixture  12  and transferred into an autoclave for curing. Cure time and temperature is, of course, also dependent upon the particular elastomeric compound utilized as well as the diameter and length of the casing  10 . After curing, the assembly is cooled and all tooling is removed to obtain a length of production tubing  56  (FIG.  5 ). As shown in FIG. 5, the ends  58  of the production tubing  56  may be machined such as to threadably receive a coupler  60  or to otherwise be joined to additional sections of production tubing or linkages as is well known in the art to form a relatively long progressing cavity pump. The heads of the set screws  48  are preferably ground down and smoothly welded to the outside of the casing  10 . 
     Referring back to FIG. 4, it is preferred that an outer layer of elastomeric material  62  be provided over most of the bearing surface  42  of the bearing  32 , and it is most preferred that the bearing  32  be completely encapsulated by the elastomeric material. This is achieved during the injecting step when there exists a tolerance between the bearing  32  and the adjacent structure. FIG. 4 shows a space  64 , defined on one side by the outermost surface  40  of the bearing  32  and on the other side by the inner surface  52  of the casing  10 , filled with elastomeric material, and in like manner a similarly filled space  66  defined on one side by the innermost surface  42  of the bearing  32  and on the other side by the outer surface  22  of the core  20 . In the most preferred embodiment the spaces  64 ,  66  are on the order of 3 to 5 thousandths of an inch in width. Covering most of the innermost surface  42  of the bearing  32  with elastomeric material is advantageous insofar as the thin layer of elastomeric material filling the space  66  helps to provide a good interface between the rotor  67  (FIG. 5) and stator  54  and resists abrasions such as might otherwise be caused by suspended solids in the fluid being pumped. Pertaining to the use herein of the terms “substantially” or “most of” as applied to the covering of elastomeric material over the innermost surface  42  of the bearing  32 , it is to be recognized and appreciated that at the point where the innermost surface  42  of the bearing  32  is closest to the core  20 , i.e. adjacent the widest portion of the core  20 , there may be very little or no elastomeric material applied. This is due to the close tolerance between the surface  42  and the core  20 . Moreover, if a thin layer of elastomeric material is applied during the manufacturing process, it may wear so as to expose a small circumferential area of bronze bearing surface. In either instance, the object of the invention to provide a bearing surface for the rotor is no less achieved. 
     Another preferred aspect of the invention is best shown in FIGS. 3 and 4. In this regard it is preferred that one end, i.e. first end  36 , of the bearing  32  possess an inner circumferential chamfer  68 , and, in the most preferred embodiment, that the second end  38  of the bearing  32  possess a second inner circumferential chamfer  70 . Moreover, most preferred is a bearing  32  wherein the first chamfer  68  is of a length l 1  that is greater than the length l 2  of the second chamfer  70  and wherein the bearing  32  is mounted in the casing with the end  36  having the longer first chamfer  68  directed to first receive the flow of elastomeric material, as illustrated in FIG. 4 wherein the arrow  72  designates the direction of flow. 
     Several advantages are obtained by a bearing  32  having the preferred chamfers  68 ,  70 . The chamfers  68 ,  70  decrease the resistance to the flow of elastomeric material during the injecting step and prevent a peeling or flaking back of the material. The chamfers  68 ,  70  also provide a good grabbing and holding surface for the elastomeric material which helps to lock and stabilize the bearing  32  within the stator  54 . The chamfers  68 ,  70  also reduce the dimensions of the bearing surface  42  which keeps operating torques at or below those of production tubing where no bearing is utilized. The chamfers  68 ,  79  further minimize any distortion in the double helix form of the stator  54 . 
     In the foregoing manner, the aforedescribed invention achieves good success in minimizing core deflection in the manufacture of stators for relatively long sections of progressing cavity pump production tubing and provides a beneficial bearing surface for the rotor of the progressing cavity pump. With the invention a desirable interface is established between the rotor and stator and proper alignment is preserved. The maintenance of equal interference between the rotor and stator improves pump performance and longevity. The bearings prevent the rotor from wobbling or chunking out to one side while having no adverse impact on the torques required to operate the pump. 
     While the invention has been described with a certain degree of particularity, it is understood that the invention is not limited to the embodiment(s) set for herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.