Patent Publication Number: US-6666666-B1

Title: Multi-chamber positive displacement fluid device

Description:
FIELD OF THE INVENTION 
     This invention relates to positive displacement fluid devices such as fluid-driven motors and pumps which are operable for pumping high temperature, and contaminated fluids. More particularly, such a fluid device is a circumferential piston pump or motor configured for multi-chambered use in stacked and multi-stage operation. 
     BACKGROUND OF THE INVENTION 
     Conventional methods and apparatus for bringing well fluids to the surface involve various pump systems of different designs and methods of operation. Restrictions on existing pump systems sometimes include dimensional constraints, the ability to handle high temperature and the need to pump contaminated fluids, e.g. high sand content particularly at high temperature. Conventional pumps are limited by their use at high temperature and with contaminant sensitive polymers. 
     Further, pumps having rotating components must have some form of bearing to separate the moving from the stationary components. It is a constant challenge to maintain bearing integrity in high temperature or contaminated environments. Such environments include those typical in the recovery of high temperature hydrocarbons from Steam Assisted Gravity Drainage (SAGD) wells in the heavy oil and bitumen recovery of northern Alberta, Canada. 
     In downhole operations, such as conventional oil recovery operations, progressive cavity (PCP) pumps have been applied to great effectiveness. However, as the well becomes deeper, as the temperature increases, and as the level of contamination increases, the elastomers used begin to fail resulting in pump failure and more frequent and expensive turnovers. 
     As an alternative, one may consider positive displacement pumps which are applied in food and other fluid industries. Among this class of pumps are circumferential piston pumps which have been known since at least 1935 in U.S. Pat. No. 2,096,490 to Hanson and still in production today by Waukesha Delavan, Wis. (Universal II Series) and Tuthill of Alsip, Ill. (HD Series). Conventional circumferential piston pumps utilize opposing, contra-rotating rotors having pistons which are alternately swept through a common chamber. Timing gears coordinate the rotor rotation. Traditionally used in surface applications, significant effort has been applied in order to seal the rotation of the rotors and the resulting pumps to date have been typically used in single stage applications. The rotors are each fitted on a shaft rotatably supported on bearings, either cantilevered or being fit with bearings at each end. The bearings are lubricated and separated from the process fluids by seals (commonly known as external bearings). 
     The usual approach for increasing the volume and fluid flow rate from such positive displacement pumps has been to increase the pump&#39;s dimensions. However, in the restricted space of a wellbore, such dimensional scale-up of pumps is not suitable for providing either the necessary pressure or the flows in the wellbore. 
     In some applications, such as hot, contaminated downhole wellbore operations, there is an objective to increase either the volumetric flow rate or to increase the output pressure beyond that which can conventionally be provided using a conventional circumferential piston pump. Conventional pump technology has not fulfilled these objectives. The design challenges are further increased where the fluid is hot and contaminated, further affecting the challenge of sealing the rotors of such pumps. In particular, in the high pressure, high temperature contaminated environment of oil well downhole operations, there is little opportunity to provide an optimum environment for the bearings. 
     The above problems and challenges are equally applicable to the reverse operation in which fluid is forced through such devices so as to drive a shaft and act as a motor. 
     Accordingly, there is a need for a fluid device which can operate in high temperature, contaminated fluids and which can be further adapted to operate in high volume and pressure operations, even in such restricted spaces as a wellbore. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved positive displacement fluid device, such as a pump, having one or more pump sections, the pump sections being adapted for axial stacking which enables high volume, high pressure transport of high temperature production fluid which can contain a substantial degree of contamination. The novel pumping system overcomes the high temperature limitation as well as being associated with a high tolerance to pump contaminated fluids over a wide viscosity range. The capability to pump high temperature, contaminated fluids is achieved using a circumferential piston pump utilizing a novel sealing arrangement. Further, pump sections are stacked in parallel to achieve required flow rates. The parallel stacked pump sections are in turn stacked in series to meet required discharge head or pressure. Configured as a pump, the fluid device is driven by a drive shaft for pumping fluid. Configured as a motor, fluid is forced through the sections for turning and driving the shaft. Herein, the specification concentrates on a description of the fluid device as a pump although the principle and inventive concepts apply equally to a motor configuration. 
     In a preferred pumping configuration, the invention is a multi-chamber positive displacement fluid device or pump comprising two or more stacked positive displacement pump sections, each pump section having a rotor chamber for pumping fluid from an intake adapted for communication or connection with a fluid source to a discharge manifold and through a fluid discharge adapted for communication or connection to a fluid destination. Each rotor chamber contains rotors driven by common timed drive and idler shafts extending axially through each stacked rotor chamber. Each of the stacked sections has a common discharge manifold which contributes its incremental flow to the common discharge manifold. The sections can be stacked in any combination of parallel or series arrangements, each of which utilizes a drive shaft which extends co-axially through the stack of sections. 
     If the sections are stacked in parallel, the volumetric flow rate is incrementally increased. 
     If the sections are stacked in series, the discharge pressure capability is incremented. For a series arrangement, the discharge of one section or stack of sections is fluidly connected to the inlet of a successive stacked section through a crossover section. Sections stacked in series with a cross-over form a pumping stage for incrementally increasing the pressure at the fluid destination. 
     Applied as a motor for a given flow rate of fluid, sections stacked in parallel result in a greater torque at the drive shaft and sections stacked in series result in a greater rotation speed. 
     In a multi-section pump, the invention comprises: two or more axially stacked pump sections, each section having a rotor chamber and associated rotors for pumping fluid from an inlet to a discharge manifold and a drive which extends axially through each rotor chamber for rotating the rotors and pumping fluid. Each section comprises a pump housing for housing the rotor chamber and rotors which are sandwiched between end plates and seals. 
     In a multi-stage pump, the invention comprises: a suction stage have having one or more axially stacked suction pump sections, each section having a rotor chamber and associated rotors for pumping fluid from an inlet to a discharge manifold; and at least one pressure stage, each stage having one or more stacked pressure pump sections, each pressure pump section having a rotor chamber for pumping fluid from a suction manifold to a discharge manifold; a crossover section for fluidly connecting the discharge manifold of the suction stage to the suction manifold of the pressure stage; and a drive which extends axially through each rotor chamber for rotating the rotors and pumping fluid. 
     More preferably, the drive comprises a drive shaft or a plurality of co-axially connected drive shafts extending axially and rotatably to the rotor chamber of each section for rotating one of the rotors; an idler shaft or idler shafts extending rotatably to each rotor chamber for rotating the other rotor; and timing means between the drive shaft and idler shaft for contra-rotating the rotors. 
     The entire stack of sections and crossovers between stages can be fit into the bore of a tubular barrel, compressed sealably together and retained therein, the barrel forming a pump having a fluid intake or inlet ports to a suction stage and having a fluid discharge from a pressure stage. 
     Such a pump has great versatility in its designed flow capacity and lift, all of which can be assembled into a small diameter package and which is driven through a single drive shaft connection; ideal for downhole operations or other space restrictive areas. Configured as a motor, the fluid device demonstrates similar same space and performance advantages in meeting desired output torque and rotational speed characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a - 1   e  are schematic views of the sequential operating principles of a circumferential piston pump; 
     FIG. 2 is an exploded perspective view of a multi-stage circumferential piston pump according to one embodiment of the invention; 
     FIG. 3 is a perspective view of an alternate suction stage according to another embodiment, in which the inlets ports for all pump sections draw from a common suction manifold; 
     FIG. 4 is an exploded perspective view of a pump section configured as a fluid suction section; 
     FIG. 5 is an exploded perspective view of a four parallel pressure pump fluid suction sections of FIG. 5, detailing main drive shaft and idler shaft sections; 
     FIG. 6 is an exploded perspective view of a pump section configured as a pressure pump lift section; 
     FIG. 7 is an exploded perspective view of four parallel pressure pump lift sections of FIG. 6, detailing main drive shaft and idler shaft sections; 
     FIG. 8 is an exploded perspective view of a center timing gear assembly; 
     FIGS. 9 a - 9   d  are various views of a fluid cross-over unit. More particularly, FIG. 9 a  is a perspective view with internal passageway depicted in hidden lines, FIG. 9 b  is top view of FIG. 9 a , FIG. 9 c  is a cross-sectional view of FIG. 9 b  along lines A—A, and FIG. 9 c  is a cross-sectional view of FIG. 9 b  along lines B—B; 
     FIG. 10 is an exploded perspective view of a top bearing assembly; 
     FIG. 11 is an exploded perspective view of a complete pump assembly with outer retaining barrel omitted; and 
     FIGS. 12 a - 12   c  are test results depicting the efficiency, power and torque curves for a five section portion of a pump constructed according to the embodiment of FIG. 2 when pumping water at standard conditions; 
     FIGS. 13 a - 13   c  are test results according to FIGS. 12 a - 12   c , also depicting the efficiency, power and torque curves when pumping SAE30 oil at 70° C.; and 
     FIGS. 14 a - 14   c  are test results according to FIGS. 12 a - 12   c , also depicting the efficiency, power and torque curves when pumping SAE30 oil at 190° C. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The principles of positive displacement pumps are hereby adapted and modified for operation in environments known to be challenging to current pumping technologies. Positive displacement pumps include rotary-actuated gear pumps and circumferential piston pumps. When fluid operated in reverse, a positive displacement device can be used as a motor. Unless the context is specifically otherwise, the description herein applies equally to operation as a pump or as a motor. 
     In one embodiment a circumferential piston pump is applied to overcome the pumping challenges identified by the applicant. The principles of circumferential piston pumps are well known and are summarized briefly herein for reference. 
     Generally, and using illustrations of a circumferential piston pump as an example (FIGS. 1 a - 1   e ), a positive displacement pump comprises at least a rotor chamber  10 , rotors  11  fitted into the rotor chamber, a fluid inlet  6  and a fluid discharge  7 . In a single stage implementation, the inlet  6  is connected to a fluid source and its discharge  7  is connected to a fluid destination. In the case of an elementary gear pump, two rotors  11  such as meshing gears are rotated in the rotor chamber  10 . The rotors  11 , 11  are contra-rotated for effective fluid flow—either being driven by the fluid as is the case for a motor, or driving the fluid as a pump. 
     Specifically for a circumferential piston pump, two contra-rotating piston rotors  11 , 11  are rotated in the rotor chamber  10  about cylindrical machined bosses  12 . Annular piston bores  14  are formed between the rotor chamber  10  and the bosses  12 . Each rotor  11 , 11  has one or more arcuate pistons  15  which travel in circular paths in their respective annular piston bores  14 . The piston bores  14 , 14  meet at a common point of intersection C in the center of the rotor chamber  10 . The center of rotation of each rotor is spaced outside of the major diameter (sometime known as external) of the opposing rotors. The point of intersection C of the piston bores  14 , 14  is connected at one side to the pump&#39;s inlet  6  and at an opposing side to the pump&#39;s outlet  7 . Each piston  15  alternates passing through the point of intersection C. Each piston  15  has a trailing edge and a leading edge. As the trailing edge of a rotor&#39;s piston  15  leaves the point of intersection C, the volume of its piston bore is steadily yet temporarily increased, causing a suction and a resulting inflow of fluid from the inlet  6  or suction side. This is the suction portion of the cycle of each rotor  11 . The leading edge of the same piston  15  then seals the piston bore  14  which traps the fluid drawn from the inlet  6  and positively displaces it to the outlet  7  or discharge side. While one rotor&#39;s piston  15  is displacing fluid out of its piston bore  14 , the other rotor&#39;s piston  15  is drawing fluid into its piston bore  14 . The suction inlet  6  and discharge outlet  7  are constantly isolated, despite the common point of intersection C, due to the continual presence of one rotor  11  or the other rotor  11  sealing between its respective piston bore  14  and against the opposing rotor&#39;s cylindrical boss  12 . 
     In example sequential steps of operation, starting at FIG. 1 a , an Open-to-Inlet (OTI) volume is defined in a rightmost rotor bore  14  by the rotor chamber  10  and by the departing the rightmost rotor piston  15 . The rightmost rotor piston  15  fluid seals the OTI volume at the point of intersection C where the piston meets and seals against the opposing rotor&#39;s cylindrical boss  12 . Comparing FIG. 1 a  and  1   e , the OTI volume alternates between the piston bores  14 , 14  as the pistons  15 , 15  alternately enter or leave the point of intersection C. Normally, neither the rotors  11 , 11  nor the pistons  15 , 15  contact each other and only close tolerance fluid seals exist between the rotor  11  and the opposing rotor&#39;s boss  12 . As the rightmost rotor bore  14  forms the OTI volume (FIG. 1 a ), an Open-to-Outlet (OTO) volume is defined in the leftmost rotor bore  14  by the rotor chamber  10  and the surfaces of rotor pistons  15  between their fluid seal contacts with the opposing boss  12  where they leave the point of intersection C. Observing the rightmost piston  15 , FIGS. 1 a  and  1   b  illustrate the OTI suction portion of the cycle, while FIG. 1 c  illustrates the trapping of the fluid and its positive displacement towards the OTO volume. FIGS. 1 d  and  1   e  illustrate the continuous discharge of the trapped fluid to the outlet  7 . As is shown in FIG. 1 c , the OTI suction cycle for the leftmost rotor  11  begins when the rightmost rotor  11  is completed its OTI cycle. 
     In the conventional mode of operations, radial surfaces and axial-end surfaces of the rotor pistons  15  run in close-clearance contact with the walls of the rotor chamber  10 , and due to the reality of manufacturing tolerances, load-bearing contact may occasionally occur in these zones. Annular apertures defined by the running clearances therebetween determine the amount of fluid leakage from the outlet  7  to the inlet  6 , being from the OTO volume to the OTI volume, for a given pressure difference and a given effective viscosity. For each rotor chamber  10 , each rotor  11 , 11  alternately supports the driving torque. 
     This ends a review of the more conventional aspects of the circumferential piston pump, the principles of which are common with positive displacement pumps generally and with the present invention. Such conventional pumps utilize a pump body or housing having a single inlet  6  and an outlet  7 . The typical means for increasing a pump&#39;s volume (OTI,OTO) and fluid flow rate has been to increase the pump&#39;s dimensions. However, in the restricted space of a wellbore, such dimensional scale-up of pumps is not suitable for providing either the necessary pressure or the flows in the wellbore. 
     Therefore, with reference to FIGS. 2 for the overall arrangement and FIGS. 4 and 5 for details, and turning to a first embodiment of the present invention, a novel pump  20  comprises two or more positive displacement chambers  10 , 10  . . . , stacked axially one chamber  10  atop another chamber  10 . Each chamber  10  is provided with its respective rotors  11 , 11 , bosses  12 , 12  and an end plate  13  for forming a section  21 . In stacking the sections  21  and thus stacking the chambers  10 , 10  . . . , the respective rotors  11 , 11  of each discrete chamber  10  are aligned along the same axes and can thereby be driven through a common drive shaft and idler shaft. 
     Two or more stacked sections  21  having their outlets  7  conjoined into a common discharge are stacked to form a pump stage  22 . A pump  20  can merely have a single stage  22  of one or more parallel stacked sections  21 . Practically however, for increased head or discharge pressure, a pump  20  preferably comprises two or more stages; a suction stage  22   s  (FIGS. 4 and 5) and at least one pressure stage  22   p  (FIGS.  6  and  7 ). 
     Each stage  22 , whether suction or pressure  22   s , 22   p , comprises one or more pump sections  21  arranged or stacked axially in parallel for obtaining the desired capacity or fluid flow rates. Stages  22  can also be stacked axially in series  22   s , 22   p , 22   p , . . . for obtaining the desired discharge pressure from the ultimate outlet from the pump  20 . 
     As shown in FIG. 2, a complete pump  20  consists of pump sections  21  combined in multiples in a stack  23  and preferably having two or more stages  22  operating in series  22   s , 22   p , 22   p.    
     The stack  23  of pump sections  21  and drive components (described later) are sandwiched together for fluid tight connections therebetween. While other means such as threading section  21  to section  21  together or joining by fasteners could be employed, one convenient means for assembling a multiplicity pump sections  21  and their associated drive components is to fit the stack  23  into an outer cylindrical retaining barrel  24 . The length of the outer retaining barrel  24  is complementary to the overall height of the stack  23  so that when installed into the outer retaining barrel, end retaining nuts  25  are secured into each end of the outer retaining barrel  24  for engaging the stack ends  26  and retaining them together. 
     While each section  21  may actually be identical, the section&#39;s location in the stack can define its role as either a suction or a pressure section  21   s , 21   p . A suction section  21   s , multiple sections  21   s , 21   s  . . . , or a suction stage  22   s  is located adjacent to and in fluid communication with a fluid source and draws the design flow rate of fluid into the pump  20 . As shown in FIG. 2, such a suction stage  22   s , can draw fluid independently into each section  21   s , 21   s  . . . through a plurality of corresponding inlets  6 , 6  . . . in the sections  21  and corresponding inlet ports  27  in the outer retaining barrel  24 . Alternately, as shown in FIG. 3, the fluid can be drawn through a combined suction intake  34 . 
     With reference to FIG. 4, a section  21   s  configured for suction is illustrated. Each section  21  comprises a pump body or pump housing  30  forming at least two chambers: a pumping or rotor chamber  10  and a discharge chamber  31 . For ease of manufacture and assembly, the rotor chamber  10  of each section  21  is sandwiched and sealed between end plates  13 . A pair of bosses  12  extends from one side of the end plate  13  and project into the rotor chamber  10 . The end plate  13  blocks one side of the rotor chamber  10 , shown in this configuration as a top end plate for one pump section while also forming a bottom end plate for the next adjacent pump section. At an extreme bottom end of a stack of pump sections, a termination plate  32  without bosses is provided. 
     With reference to FIG. 5, four suction pump sections  21   s  are shown with the discharges  31  of each of the pump housings  30  and end plates  13  being aligned for forming a discharge manifold  31   m  for contiguous fluid passage therethrough. Inlets  6  are shown extending from the rotor chamber and through the pump housing  30 . The pump housing, may or may not have a suction chamber  33  which mirrors the discharge chamber  31 . In this embodiment, a suction chamber  33  would be a mere artifact of the implementation of pump housings which are interchangeable for either suction or pressure section use. As shown in FIG. 2, the assembled suction stage  22   s  draws fluid from a fluid source outside the pump  20 , typically from a wellbore. Fluid enters the suction stage through a series of inlet ports  27  formed in the outer retaining barrel  24 . The inlet ports  27  align with corresponding inlets  6  in each of the suction stages  21   s ; typically one inlet port  27  per suction pump section  21 . While this arrangement does require some accuracy in matching inlet ports  27  and pump section inlets  6 , use of individual inlet ports  27  does minimize fluid restriction and ensures a substantially equal supply of fluid to each pump section  21 . Each suction pump section  21  transports substantially an equal amount of fluid from the inlet  6  and delivers it to the common discharge manifold  31   m  which is located 180 degrees opposite to the suction manifold  33   m . The discharge manifold  31   m  runs along the full axial length of each pump stage  22   s , 22   p  . . . , through both the pump housings  10  and the end plates  13  for accumulating and delivering the discharge fluid to the next pump stage  22 . 
     In the alternate embodiment shown in FIG. 3, the multiple-stacked chambers of the suction stage  22   s  can all draw from suction intake  34 . The suction sections  21   s , 21   s  . . . have their inlets  6  extending only from the rotor chamber  10  to a suction chamber  33  as part of an overall common suction manifold  33   m . This simplifies the pump assembly and avoids the need to accurately align individual section inlets  6  with the inlet ports  27  in the outer retaining barrel  24 . Accordingly, a common or combined suction intake  34  is formed at the initial suction section  21   s  or the suction stage  22   s . The intake  34  is formed in the termination plate  32 . In this embodiment, the suction manifold  33   m  is required to pass the entire design fluid flow rate, and thus the pressure drop therethrough must be considered in the design such as increasing the manifold  33   m  cross-section accordingly. The suction manifold  33   m  may have sufficient cross-sectional areas to supply fluid to all of the multi-chambers  10  in the stage  22  without starving the latter sections  21  of fluid flow. The suction manifold  33   m  for all pump sections  21  may be increased in size. The inlets  6  for each section  21  are all joined through the common suction manifold  33   m . In the alternate embodiment in FIG. 3, it is clear that the pressure and suction sections  21   p , 21   s  may be identical for simplification and economy of manufacture. 
     Turning to FIG. 6, a pressure pump section  21   p  is shown herein as differing from an independent Inlet operating suction pump section  21   s  by the absence of an inlet  6  extending through the pump housing  30  which forms a suction manifold  33   m . As shown individually In FIG.  6  and stacked in FIG. 7, the pressure pump sections  21   p  correspond in all other respects to the suction pump sections  21   s  set forth in FIGS. 4 and 5 except that the suction chamber  33  now forms the inlet to each section  21 . The suction chamber  33  Is isolated from the outer retaining barrel  24  and is enclosed wholly within the pump housing  30 . A pressure stage  22   p  is typically configured to accept fluid from the suction stage&#39;s common discharge, process the fluid through the one or more sections  21   p  in parallel and also discharge the fluid through a common discharge  31  or manifold  31   m.    
     The end plates  13  are also fitted with suction and discharge chambers  33 , 31  which are complementary to the pump housing&#39;s suction and discharge chambers  33 , 31  for forming respective suction and discharge manifolds  33   m , 31   m  extending continuously along the pump  20  for contiguous fluid communication between stacked pump stages  22   s , 22   p , 22   p  . . . As noted above, end plates  13  throughout a suction stage  22   s  may or may not include a suction chamber  33  as the suction section&#39;s pump housing  30  may be absent such a chamber, being fitted only with an inlet  6 . 
     With reference to FIG. 7, four pressure pump sections  21   p , 21   p  . . . are shown with each of the respective suction and discharge manifolds  33   m , 31   m  of the pump housings  30  and end plates  13  being aligned for contiguous fluid passage therethrough. 
     Rotors  11  and their pistons  15  are mounted rotatably over the bosses  12  for rotation in the rotor chamber  10 . Single lobed rotors  11  are shown although double lobed or other rotor arrangements are possible. In U.S. Pat. No. 2,642,808 to Thomas, the entirety of which is incorporated herein by reference, double-lobed rotors are implemented. Further, the circumferential piston  15  can extend axially from the rotor  11  to overhang the boss  12 , as illustrated herein, or can be cantilevered, as taught by Thomas. 
     Accordingly, and referring to FIGS.  2  and  4 - 7 , when assembled into a typical pump  20  configuration, a suction stage  22   s  is demonstrated as having fifteen stacked suction pump sections  21   s  and fifteen corresponding inlet ports  27 . All fifteen suction pump sections  21   s  discharge to the common discharge manifold  33   m . The fluid in the suction stage&#39;s discharge manifold  31   m  is directed to a first pressure stage  22   p . The first pressure stage  22   p  is also illustrated as having fifteen stacked pressure pump sections  21   p . All fifteen pressure pump sections  21   p  draw from a common suction manifold  33   m  and discharge to a common discharge manifold  31   m . The fluid in the first pressure stage&#39;s discharge manifold  31   m  is directed to a second pressure stage  22   p . The second pressure stage  22   p  is also illustrated with fifteen pressure pump sections  21   p . All fifteen pressure pump sections  21   p  also draw from a common suction manifold  33   m  and discharge to a common discharge manifold  31   m.    
     Turning to FIGS. 7 and 8, one rotor  11  is driven by one or more drive shafts  40 , 40  . . . which extend through each rotor chamber  10  and which are connected end to end for co-rotation. The opposing rotor  11  is driven by one or more idler shafts  41 , 41  . . . which are also connected end to end for co-rotation. The one or more drive shafts  40  and one or more idler shafts  41  are hereinafter referred to collectively and simplistically as singular drive shaft  40  and idler shaft  41  respectively. 
     As shown in FIGS. 7 and 8, the pump sections  21  are driven using the drive shaft  40 , extending axially through each pump section  21  and connecting driven rotors  11  in each stacked pump stage  22 . The rotors  11  in each pump stage  22  rotate in the same contra-rotating directions as they are driven by one common input main drive shaft  40 . The opposing rotor  11  in each pump section  21  is driven by paired sets of timing gears  50 , connected to the drive shaft  40  and the parallel idler shafts  41 . The plurality of discontinuous, yet co-axial, conjoined idler shafts  41  each being driven through the timing gears  50 . The timing gears  50  have a dual function: to drive the idler shaft  41  and their associated rotors  11 , and to ensure that the rotors&#39; pistons  15  are timed correctly so that they do not contact or clash. 
     A person of skill in the art can design one or more shafts  40 , 40  . . . and  41 , 41  . . . for assembly into a single co-rotating shaft  40  or  41 . As shown in FIG. 7, an individual shaft  40  or  41  may be conjoined at splined connections  42  at its respective and common rotor  11 . For example, the ends of the shafts  40 , 41  can be fitted with an external involute spline  42  which fits cooperatively with an internally splined coupling bushing (or rotor  11  or gear  50 ) to co-axially connect the shaft sections of each of the stacked pump stages  22 . Further, as shown in FIG. 8, the shafts may be conjoined with splined connections at the timing gears  50 . 
     The timing gears  50  are housed in timing assemblies  51 , 51  . . . which are located at regular intervals between multiple stacked pump sections  21 , and thereby provide accurate timing for the piston sections  21 , 21  . . . Typically, a timing assembly  51  is sandwiched between every four of five pump sections  21 . The timing gears  50  are contained in separate timing assemblies  51 , fully integrated in each pump stage  22 . 
     Regardless of the form of connection to a fluid source, the common discharge manifold  31   m  of the suction stage  22   s  delivers pumped fluid to the next successive pump stage  22 , in this case being the first pressure pump stage  22   p . The first pressure stage  22   p  and successive pressure pump stage  22   p  is similar in design and construction to the previous suction pump stage  22   s , excluding the suction inlets  6  and inlet ports  27 . 
     At the discharge of each stage  22 , such as between the suction stage  22   s  and a pressure stage  22   p , the discharge manifold  31   m  is routed to the suction manifold  33   m  of the successive pump stage. In order to maintain common rotational axes for the drive shaft  40  and idler shaft  41 , and to pump the discharge flow to the common suction manifold  33   m  of the successive stage  22   p , the fluid needs to cross-over 180 degrees to flow into the common suction manifold  22   s  of the successive stage  22 . 
     With reference to FIGS. 2 and 9 a - 9   d , a fluid flow cross-over section  60  comprises a cylindrical block forming an end wall  61  for blocking the preceding stage&#39;s suction manifold  33   m  and a fluid inlet  62  for accepting fluid flow from the preceding stage&#39;s discharge manifold  31   m . The fluid from the preceding stage&#39;s discharge manifold  31   m  is routed through a fluid flow passage  63  to a fluid outlet  65 . The fluid outlet  65  is arranged for discharge into the suction manifold  33   m  of the successive stage  22 . As shown in FIGS. 9 b  and  9   d , the cylindrical block is fitted with a bore  66  for forming a through passage for the drive shaft  40 . The idler shafts  41 , being driven by timing assemblies  51  positioned periodically along the pump, are able to terminate either side of the cross-over section  60 . Accordingly, the fluid flow passage  63  is neither obstructed nor interrupted by the drive shaft  40  or idler shafts  41 , 41 . 
     Sockets  67  and bearings (not shown) are provided for the termination of a preceding idler shaft and for the termination of a successive idler shaft. Such sockets  67  can be machined into the cross-over section  60  or into specialized end plates (not shown) which can be provided as matter of economics so as to avoid further machining of the cross-over section  60 . 
     As known by those of skill in the art of positive displacement pumps, each rotor  11 , 11  is rotated in close non-contacting tolerance to their respective bosses  12 , 12  and to the rotor chamber  20  and the opposing rotor  11  so as to effect a positive displacement motoring or pumping action. To maintain such operational tolerances, the rotors  11 , 11  are mounted securely to their respective shafts  40 , 41  and the shafts themselves are supported concentrically in the bosses  12 , 12  using bearings  70 . Unlike the conventional wisdom applied to such circumferential piston pumps, the bearings  70  employed herein are not supported external to the rotor chamber in a protected environment. Recognizing the oft times harsh conditions experienced by pumps in hot, or contaminated environments, face-to-face hard bearing surfaces, including tungsten carbide, silicon carbide, and ceramics are provided inside each boss  12 , 12  and on the corresponding locations on the main drive shaft  40  and idler shaft  41 . Best shown in FIGS. 6 and 8, bearings  70  are fit into each boss  12 . Mating bearings  70  are also fit to the shafts  40 , 41  (obscured in FIGS.  6  and  8 —an example shown in FIG.  8 ). Similar complementary bearings  70  are employed in each timing assembly  51 . 
     Best seen in FIGS. 4 and 6, sealing between the individual components of the pump housings  30 , end plates  13 , timing assemblies  51 , and fluid cross-over sections  60  is accomplished using specially molded high temperature O-ring seals  90 . The seals  90  are fitted in corresponding shaped grooves  91  formed in each pump housing  30 , providing full sealing around the perimeter of each chamber  30 , each stacking Interface and each individual lubricant and instrumentation port hole  81 , running through the full length of the pump stage  22 . 
     As discussed earlier, each complete assembled pump stage  22  is mounted inside an outer retaining barrel  24  for supporting the complete assembly. Accordingly, each complete stacked pump stage  22  is free of any internal mechanical fasteners. 
     The outside pump retaining barrel  24  is precision ground and polished on its inside diameter, and provides close tolerance support for each internally mounted section  21 , 21  and stage  22 . The extreme ends  29  of the outer retaining barrel  24  are internally threaded, and each match with the externally threaded retaining nut. The retaining nut  25  can also be provided by a threaded fluid cross-over  60 . Once the retaining nuts  25  are threaded into each end of the outer retaining barrel, they sandwiches the stacked pump sections  21  and stages  23  together, compressing the O-ring seals  90  and thereby providing full internal sealing of the internal pump stage components  21 , 51 , 60 . 
     The assembly is aided by compressing the stack of pump components  21 , 51 , 60  using opposing mandrels. The end retaining nuts  25  are then threaded into each end of the outer retaining barrel to retain the compressed stack in the outer retaining barrel  24 . Depending upon the number of sections  21  for the particular pump configuration, and as an example, for three stages of fifteen sections/stage about 10,000 to 20,000 pounds force is applied. 
     In operation, each stage of a circumferential piston pump produces a characteristic pulsing at each discharge. Accordingly, and in a preferred aspect, such pulsing is minimized by slightly rotationally incrementing each pair of rotors  11 , 11  for each successive section  21 , 21 . One approach is to mount the rotors  11 , 11  on the drive shafts  40  and idler shafts  41  such that the pump OTI/OTO timing for a complete pump stage  22  is incremented, at equal angular intervals throughout the entire 360° shaft circumference, so as to equally divide the pulsing throughout each 360 degrees revolution. The resulting fluid flow has an overall reduced variation in pulsation at the discharge manifold  31   m  and provides continuous low pulsation fluid intake and fluid flow discharge characteristics. For example, for a stage  22  having fifteen pump sections  21 , each rotor  11  of a rotor pair would be incrementally rotated about 24 degrees on the main drive shaft ( 360 / 15 ). The rotors  11  are connected to the drive and idler shafts  40 , 41  by means of splines  42  and shaft keys (not shown). As is the convention in rotating machines, shaft keyways are rounded with radius ends, to reduce stresses on the shafts  40 , 41 . 
     Referring to both FIGS. 2 and 10, the drive shaft  40 , running through the full length of the complete pump  20 , is supported at the discharge end of the pump  20  by a thrust/radial bearing assembly  100 . The thrust bearing assembly comprises a bearing housing  101  located on top of the uppermost pump stage  22   p , and forms an integral part of the pump  20  when installed into the outer retaining barrel  24 . The thrust bearing assembly  100  contains double thrust bearings  102 , 102  and double radial bearings  103 , 103  fit with bearing housings  104  to prevent axial and radial driveshaft movement. The bearing assembly  100  is a sealed unit, with high temperature mechanical seals  105 , 105  located at the upper and lower end of the drive shaft bearing assembly  100 . The bearing assembly  100  is filled with high temperature lubricant oil to lubricate the bearings  102 , 103 . The bore of the bearing housing  101  contains the combined stack of bearings  102 , 103  and has an additional lubricant oil reservoir  106  surrounding the bearing assembly  100 . The reservoir  106  can be refreshed or topped up through a lube oil connection (not shown) at the top of the pump  20  adjacent the production line connection  110 . 
     Alignment of the stacked components  21 , 51  is accomplished by hollow alignment dowels  80 , located in integral lubricant/instrumentation galleries  81  running through the full length of the complete pump  20 . Each pump housing  30 , end plate  13 , timing assembly  51  and fluid cross-over section  60  have such galleries  81  into which are fit hollow dowels  81  for alignment as well as for lubricant instrumentation purposes. Each pump section  21  is located and rotationally locked to the adjacent section  21  using the dowels  80 . Further, through the use of hollow dowels  81 , one through four galleries  80  can be formed along the length of the pump  20 . For example, the oil reservoir  106  surrounds the bearing assembly  100  and is also supplied with lubricant externally through one of the galleries  80  running through the full length of the pump  20 . 
     As shown in FIG. 11, assembly of the pump sections  21  comprises first stacking each of two or more pump housings  30  and rotors  11 , 11  between end plates  13 , 13 . The end plate&#39;s bosses  12 , 12  center and locate the rotors  11 , 11  in the pump housing  30 , and also rotatably support the main drive shaft  40  and idler shaft  41  bearings  70 . Pump housings  30  and end plates  13 , 13  are stacked back to back, with timing assemblies  51  at regular intervals, to form one or more stages  22 . As shown in FIG. 2, the entire stack  30 , 13 , 51  . . . is compressed and installed in the outer retainer barrel  24  for form the complete pump  20 . 
     The discharge fluid is delivered from the uppermost pump stage  22   p  via the common discharge manifold  31   m  to a last cross-over section  60 , connecting to the production pipe line  110  for directing the fluid to the fluid destination. In a pump  20  fit to a wellbore, the fluid destination would be the earth&#39;s surface. 
     EXAMPLE 
     Operations for a pump  20  capable of operation in a 9⅝″ wellbore casing include a plurality of  8 ″ diameter pump housings  30  comprises a suction stage  22   s  and two pressure stages  22   p , 22   p . Each pump section  20  has a rotor chamber  10  and rotor  11 , 11  combination having a displacement of 0.833 liters per rotor revolution. Timing gears  50  are provided every five pump sections  21 , or three assemblies  51  per stage  22 . Rotational speed of the pump sections  21  can vary between about zero to over 600 rpm, limited only by mechanical constraints such as the means for driving the drive shaft and depending on the characteristics of the fluid. Operating with drive means such as conventional top drives rotating at 400 rpm, such a pump  20  can produce flow rates of about 1000 liters/minute at 4500 kPa on fluid such as oil having gravity and viscosity equivalent to fluid similar to a SAE30 oil. 
     Having reference to FIGS. 12 a - 12   c , a single stage  22  having five sections  21  of the above pump  20  was manufactured, assembled and operated on water at 30° C. The water had a viscosity of less than about 1 mPa·s. The figures are graphs of pump performance versus fluid discharge flow rates and discharge pressure. FIG. 12 a  demonstrates test results for pump efficiency pumping water at 30° C. FIGS. 12 b  and  12   c  illustrate the pump power and torque. FIGS. 13 a - 13   c  illustrate the same parameters of efficiency, power and torque curves when pumping SAE30 oil at 70° C. and FIGS. 14 a - 14   c  illustrated efficiency, power and torque curves when pumping SAE30 oil at 190° C. 
     With oil at 70° C., the 5 stages produced flow rates in the order of 340-300 l/min at between 350-1400 kPa respectively. Through extrapolation to 15 sections  21  per stage 22, one would expect to get about three times the flow rate or upwards of 1000 liters/min, and when pumped through two additional pressure stages, each having 15 sections for maintaining the flow rates, one could expect discharge pressures of up to about 4200 kPa.