Patent Publication Number: US-2022220963-A1

Title: Thrust-balancing wet gas compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of U.S. provisional patent application No. 62/725,597 filed Aug. 31, 2018, and entitled “Subsea Compressors with Adjusted Thrust Impellers”, which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     Conventional turbo compressors are typically designed to compress a gas. They are normally composed of many stages (rotating impellers and static diffusers) stacked on a flexible shaft rotating at relative high speed. Critical mechanical elements such as bearings and thrust-balancing devices are often exposed to the process fluid. Any impurities in the process fluid such as solids or liquid may be detrimental to both the thermodynamic and mechanical performance of the compressor. When impurities or liquid are expected to be present in the process stream different types of auxiliary equipment may be utilized to clean or dry the process gas upstream the compressor. 
     Attempts to modify conventional turbo compressors to be so called “liquid tolerant” have sometimes had limited success and only very low liquid volume fractions can be accepted in some cases. However, even in these cases the presence of liquid may cause deterioration in the thermodynamic and mechanical performance. The challenges are even greater when designing a gas compressor for use in a subsea environment. In an attempt to address at least some of these limitations, contra-rotating compressors have been developed that include a first plurality of impellers rotating about a longitudinal axis in a first direction, and a second plurality of impellers interleaved with the first plurality and rotating about the longitudinal axis in a second direction. 
     SUMMARY 
     An embodiment of a contra-rotating compressor for compressing a process fluid comprises a first shaft assembly disposed in a housing and rotatable about a longitudinal axis, the first shaft assembly comprising an outer shaft, and a first plurality of impellers coupled to the outer shaft, wherein the outer shaft comprises a final stage that includes a final impeller of the first plurality of impellers, a second shaft assembly disposed in the housing and rotatable about the longitudinal axis, the second shaft assembly comprising a second plurality of impellers interleaved with the first plurality of impellers, a first pair of annular seals between the final stage and an inner surface of the housing, the pair of annular seals being configured to permit relative rotation between the final stage and the housing, and a third annular seal positioned between the outer surface of the final stage and an inner surface of the second shaft assembly, the third annular seal configured to permit contra-rotation between the final stage and the second shaft assembly. In some embodiments, the final stage comprises an inner cylindrical member, an outer cylindrical member comprising an outlet port, an annular shoulder extending between the inner cylindrical member and the outer cylindrical member, and an annular channel formed between the inner cylindrical member and the outer cylindrical member and terminating the annular shoulder, wherein the final impeller is positioned in the annular channel. In some embodiments, a radially inner end of the final impeller is coupled to the inner cylindrical member of the final stage and a radially outer end of the final impeller is permitted to flex relative to the outer cylindrical member of the final stage. In certain embodiments, the compressor further comprises a pressure balancing circuit configured to be in fluid communication with an inlet flow of the process fluid at an inlet pressure, wherein the pressure balancing circuit comprises a chamber positioned axially between the final stage and the lower shaft assembly. In certain embodiments, the pressure balancing circuit further comprises a first passage extending through the housing, and a second passage extending through the final stage. In some embodiments, the compressor further comprises a pressure balancing circuit configured to be in fluid communication with an inlet flow of the process fluid at an inlet pressure, wherein the pressure balancing circuit comprises a first passage extending through a cylindrical member of the final stage, a second passage extending through the final impeller, and a chamber positioned axially between the final stage and the lower shaft assembly. In some embodiments, the compressor further comprises a pressure balancing circuit configured to be in fluid communication with an inlet flow of the process fluid at an inlet pressure, wherein the pressure balancing circuit comprises a first passage extending through a cylindrical member of the final stage, a second passage extending through the lower shaft assembly, and a chamber positioned axially between the final stage and the lower shaft assembly. In certain embodiments, the compressor further comprises a barrier fluid system that comprises a first barrier fluid seal assembly positioned around the upper shaft assembly and configured to receive a barrier fluid at a first pressure, a second barrier fluid seal assembly positioned around the lower shaft assembly and configured to receive the barrier fluid at the first pressure. 
     An embodiment of a contra-rotating compressor for compressing a process fluid comprises a housing configured to receive an inlet flow of the process fluid at an inlet pressure and output an outlet flow of the process fluid at an outlet pressure, a first shaft assembly disposed in the housing and rotatable about a longitudinal axis, the first shaft assembly comprising an outer shaft, and a first plurality of impellers coupled to the outer shaft, wherein the outer shaft comprises a final stage that includes a final impeller of the first plurality of impellers, a second shaft assembly disposed in the housing and rotatable about the longitudinal axis, the second shaft assembly comprising a second plurality of impellers interleaved with the first plurality of impellers, and a chamber positioned axially between the final stage and the lower shaft assembly, wherein the chamber is configured to be in fluid communication with the inlet flow of the process fluid at the inlet pressure. In some embodiments, the compressor further comprises a pressure balancing circuit that includes the chamber, wherein the pressure balancing circuit comprises a first passage extending through a cylindrical member of the final stage, and a second passage extending through the final impeller. In some embodiments, the compressor further comprises a pressure balancing circuit that includes the chamber, wherein the pressure balancing circuit comprises a first passage extending through a cylindrical member of the final stage, and a second passage extending through the lower shaft assembly. In certain embodiments, the compressor further comprises a first pair of annular seals between the final stage and an inner surface of the housing, the pair of annular seals being configured to permit relative rotation between the final stage and the housing, and a third annular seal positioned between the outer surface of the final stage and an inner surface of the second shaft assembly, the third annular seal configured to permit contra-rotation between the final stage and the second shaft assembly. In certain embodiments, the final stage comprises an inner cylindrical member, an outer cylindrical member comprising an outlet port, an annular shoulder extending between the inner cylindrical member and the outer cylindrical member, and an annular channel formed between the inner cylindrical member and the outer cylindrical member and terminating the annular shoulder, wherein the final impeller is positioned in the annular channel. In certain embodiments, the compressor further comprises a barrier fluid system that comprises a first barrier fluid seal assembly positioned around the upper shaft assembly and configured to receive a barrier fluid at a first pressure, a second barrier fluid seal assembly positioned around the lower shaft assembly and configured to receive the barrier fluid at the first pressure. In certain embodiments, the second plurality of impellers are positioned axially between the first barrier fluid seal assembly and the second barrier fluid seal assembly. 
     An embodiment of a method for compressing a process fluid comprises (a) flowing an inlet flow of the process fluid into a housing at an inlet pressure, (b) rotating a first shaft assembly disposed in the housing and comprising a first plurality of impellers about a longitudinal axis in a first rotational direction, (c) rotating a second shaft assembly disposed in the housing and comprising a second plurality of impellers interleaved with the first plurality of impellers about the longitudinal axis in a second rotational direction opposite the first rotational direction, and (d) applying an axially directed pressure force to each end of the lower shaft assembly with the process fluid at the inlet pressure. In some embodiments, (d) comprises (d1) communicating the process fluid at the inlet pressure to a chamber positioned axially between the upper shaft assembly and the lower shaft assembly. In some embodiments, (d) comprises (d2) communicating the process fluid at the inlet pressure through a passage extending through at least one of the first plurality of impellers. In certain embodiments, (d) comprises (d2) communicating the process fluid at the inlet pressure through a passage extending through the lower shaft assembly. In certain embodiments, the method further comprises (e) flowing an outlet flow of the process fluid from the housing at an outlet pressure, (f) leaking a portion of the outlet flow into the chamber, and (g) recirculating the leaked portion of the outlet flow to the inlet flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of exemplary embodiments, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic view of an embodiment of a contra-rotating compressor assembly in accordance with principles disclosed herein; 
         FIGS. 2, 3  are side cross-sectional views of the compressor assembly of  FIG. 1 ; 
         FIG. 4  is a zoomed-in, side cross-sectional view of the compressor assembly of  FIG. 1 ; 
         FIG. 5  is a perspective cross-sectional view of an embodiment of an inner housing of the compressor assembly of  FIG. 1  in accordance with principles disclosed herein; 
         FIG. 6  is a side cross-sectional view of the inner housing of  FIG. 5 ; 
         FIG. 7  is a perspective cross-sectional view of an embodiment of a final stage of the compressor assembly of  FIG. 1  in accordance with principles disclosed herein; 
         FIGS. 8, 9  are side cross-sectional views of the final stage of  FIG. 7 ; 
         FIG. 10  is a side cross-sectional view of another embodiment of a contra-rotating compressor assembly in accordance with principles disclosed herein; 
         FIG. 11  is a side cross-sectional view of another embodiment of a contra-rotating compressor assembly in accordance with principles disclosed herein; and 
         FIG. 12  is a flowchart depicting an embodiment of a method for compressing a process fluid in accordance with principles disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings and description that follow, like parts are typically marked throughout the specification and drawings with the same reference numerals. The drawing figures are not necessarily to scale. Certain features of the disclosed embodiments may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present disclosure is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. 
     Unless otherwise specified, in the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
     Referring to  FIG. 1 , an embodiment of a contra-rotating axial turbo compressor assembly  100  is shown. In the embodiment of  FIG. 1 , compressor assembly  100  is configured for processing multiphase, gas-liquid and wet gasses in a subsea environment. Compressor assembly  100  has a central or longitudinal axis  105  and generally includes a first or upper motor  102 , a second or lower motor  110 , a generally cylindrical compressor outer housing  120  positioned between motors  102 ,  110 , a first or upper shaft assembly  200  rotatably disposed in outer housing  120 , and a second or lower shaft assembly  300  also rotatably disposed in outer housing  120 . Shaft assemblies  200 ,  300  of compressor assembly  100  extend concentrically through outer housing  120 . Upper shaft assembly  200  is rotatably coupled with upper motor  102  such that upper motor  102  may transmit torque and rotate upper shaft assembly  200  within outer housing  120  while lower shaft assembly  300  is rotatably coupled with lower motor  110  such that lower motor  110  may transmit torque and rotate lower shaft assembly  300  within outer housing  120 . Although in this embodiment upper shaft assembly  200  is rotatably coupled with upper motor  102  and lower shaft assembly  300  is rotatably coupled with lower motor  110 , in other embodiments upper shaft assembly  200  may be rotatably coupled with lower motor  110  and lower shaft assembly  300  may be rotatably coupled with upper motor  102 . 
     In this embodiment, outer housing  120  of compressor assembly has a first or upper end  120 A, a second or lower end  120 B opposite upper end  120 A, and a central passage  122  extending between ends  120 A,  120 B. Additionally, outer housing  120  includes a first or inlet port  124  extending radially between central passage  122  and an exterior of outer housing  120 , and a second or outlet port  126  extending radially between central passage  122  and the exterior of outer housing  120 . In this embodiment, compressor assembly  100  includes a generally cylindrical compressor inner housing  130  positioned in the central passage  122  of outer housing  120 . Inner housing  130  includes a plurality of circumferentially spaced fluid inlets  132  proximal a lower end of inner housing  130  and a plurality of circumferentially spaced fluid outlets  134  proximal an upper end of inner housing  130 . Upper shaft assembly  200  and lower shaft assembly  300  of compressor assembly  100  each extend through a central passage of inner housing  130 . Upper shaft assembly  200  includes a plurality of blades or impellers  202  mounted and arranged on an interior thereof while lower shaft assembly  300  includes a corresponding plurality of blades or impellers  302  mounted on an exterior thereof and interleaved with impellers  202  of upper shaft assembly  200 . In this embodiment, the interleaved impellers  202 ,  302  of shaft assemblies  200 ,  300 , respectively, are arranged so as to intermesh through alternating stages or rows of impellers, with each two adjacent rows of impellers rotating in opposite directions. Thus, each row of impellers  202 ,  302  forms a separate stage of compressor assembly  100 . Instead of relying on guide vanes or diffusers between the successive adjacent stages, the process fluid discharged from a stage rotating in one direction immediately enters into the stage rotating in the opposite direction and so on through a number of successive contra rotating stages of compressor assembly  100 . 
     During operation of compressor assembly  100 , upper shaft assembly  200  and lower shaft assembly  300  contra-rotate about central axis  105  by motors  102 ,  110 , respectively. An inlet fluid flow (indicated by arrow  125 ) of process fluid at an inlet fluid pressure flows into the central passage  122  of outer housing  120  via inlet port  124 . The process fluid flow then flows through the fluid inlets  132  of inner housing  130  and is urged in an upwards direction (indicated by arrows  127 ) by the contra-rotation of shaft assemblies  200 ,  300 . Particularly, upper motor  102  rotates upper shaft assembly  200  in a first rotational direction about central axis  105 . The rotation of upper shaft assembly  200  in the first rotational direction causes impellers  202  to exert a force on the process fluid in upwards direction  127 , which is primarily parallel to central axis  105 . Additionally, lower motor  110  rotates lower shaft assembly  300  in a second rotational direction, opposite the first rotational direction, about central axis  105 . The rotation of lower shaft assembly causes impellers  302  to exert a force on the process fluid in the same upwards direction  127  as the force imparted on the process fluid by impellers  202  of upper shaft assembly  200 . As the process fluid flows upward it is pressurized by the action of the contra-rotating impellers  202 ,  302  of shaft assemblies  200 ,  300 , respectively, until exiting inner housing  130  via fluid outlets  134 . From fluid outlets  134 , the process fluid flow exits the central passage  122  of outer housing  120  via outlet port  126  as an outlet fluid flow (indicated by arrow  129 ) at an outlet fluid pressure that is greater than the inlet pressure. 
     Referring to  FIGS. 1-6 , cross-sectional views of the outer housing  120 , inner housing  130 , and shaft assemblies  200 ,  300  of compressor assembly are shown in greater detail in  FIGS. 2-4  (the side cross-sectional view of  FIG. 3  is rotated approximately 90° from the side cross-sectional view shown in  FIG. 2 ), and the inner housing  130  of compressor assembly  100  is shown in greater detail in  FIGS. 5, 6 . As shown particularly in  FIGS. 5, 6 , inner housing  130  of compressor assembly  100  includes a central bore or passage  136  defined by a generally cylindrical inner surface  138 , and a generally cylindrical outer surface  140 . The previously described fluid inlets  132  and fluid outlets  134  of inner housing  130  each extend radially between inner surface  138  and outer surface  140 . In the embodiment of  FIGS. 1-6 , the inner surface  138  of inner housing  130  includes an annular shoulder  142 , where a plurality of circumferentially spaced pressure balancing passages  144  extend between shoulder  142  and the outer surface  140  of inner housing  130 . Pressure balancing passages  144  of inner housing  130  are in fluid communication with a plurality of circumferentially spaced pressure balancing passages  128  (shown in  FIG. 4 ) formed in outer housing  120 . Each pressure balancing passage  128  of outer housing  120  extends to an exterior of compressor assembly  100 . As will be discussed further herein, in this embodiment, a pressure balancing conduit  148  (shown in  FIG. 1 ) provides fluid communication between pressure balancing passages  144 ,  128  of housings  130 ,  120 , respectively, and the inlet fluid flow  125 . 
     In this embodiment, upper shaft assembly  200  of compressor assembly  100  generally includes a cylindrical inner shaft  204  coupled to an annular outer shaft  210 . Outer shaft  210  includes a generally cylindrical drum  212  having an inner surface on which impellers  202  of upper shaft assembly  200  are arranged, and an upper or final stage  220  coupled to an upper end of drum  212 . Inner shaft  204  extends from an upper end coupled to upper motor  102  to a lower end coupled to the final stage  220  of outer shaft  210  at an annular interface  203  formed therebetween. In this embodiment, the lower end of inner shaft  204  is coupled to the final stage  220  of outer shaft  210  (e.g., via welding, fasteners, etc.); however, in other embodiments, inner shaft  204  and outer shaft  210  of upper shaft assembly  200  may comprise a single, monolithically formed member. 
     Referring to  FIGS. 1-9 , final stage  220  of the outer shaft  210  of upper shaft assembly  200  is shown in greater detail in  FIGS. 7-9 . In the embodiment of  FIGS. 1-9 , final stage  220  has a first or upper end  220 A, a second or lower end  220 B opposite upper end  220 A, an inner cylindrical member  222  extending from upper end  220 A, and an outer cylindrical member  240  extending from lower end  220 B. 
     Inner cylindrical member  222  of final stage  220  includes a generally cylindrical inner surface  224  and a generally cylindrical outer surface  226 . The inner surface  224  of inner cylindrical member  222  includes an annular shoulder  228  that includes a plurality of circumferentially spaced pressure balancing passages  230 . Particularly, each pressure balancing passage  230  extend from shoulder  228  to an opening  231  spaced from shoulder  228  and formed in the inner surface  224  of inner cylindrical member  222 . As will be discussed further herein, pressure balancing passages  230  of inner cylindrical member  222  are in fluid communication with the pressure balancing passages  144 ,  128  of housings  130 ,  120 , respectively. 
     The outer surface  226  of inner cylindrical member  222  includes an annular shoulder or bridge  232  that connects inner cylindrical member  222  with an upper end of the outer cylindrical member  240  of final stage  220 . Bridge  232  encloses the cylindrical members  222 ,  240  of final stage  220 , and thus, final stage  220  comprises an enclosed final stage  220 . In this embodiment, the outer surface  226  of inner cylindrical member  222  includes a connector  234  for coupling with a final stage impeller  236  (shown in  FIG. 4 ) of outer shaft  210  which, in the interest of clarity, is hidden in  FIGS. 5-7 . Although in this embodiment final stage impeller  236  is coupled to inner cylindrical member  222  via connector  234 , in other embodiments, final stage impeller  236  may be formed monolithically with inner cylindrical member  222 . 
     The outer cylindrical member  240  of final stage  220  includes a plurality of circumferentially spaced radial ports  244  proximal an upper end of outer cylindrical member  240  and an annular interface  246  configured to couple to an upper end of the drum  212  of outer shaft  210 . Given that final stage  220  comprises an enclosed final stage  220 , outer cylindrical member  240  includes ports  244  for directing the outlet fluid flow  129  towards the fluid outlets  134  of inner cylinder  130 . In this embodiment, an annular channel  248  is formed between inner cylindrical member  222  and outer cylindrical member  240  of final stage  220 . During operation of compressor assembly  100 , the outlet fluid flow  129  shown in  FIG. 1  passes through annular channel  248  and flows through radial ports  244  of final stage  220  prior to flowing through fluid outlets  134  of inner housing  130  and exiting compressor assembly  100  via outlet port  126  of outer housing  120 . 
     As shown particularly in  FIG. 4 , compressor assembly  100  includes a first or upper thrust bearing  400  which is positioned in the central passage  136  of inner housing  130  and engages a cylindrical outer surface of the inner shaft  204  of upper shaft assembly  200  to absorb axially directed thrust loads applied to upper shaft assembly  200 . Additionally, compressor assembly  100  includes a first or upper radial bearing  402  which engages the outer surface of inner shaft  204  proximal upper thrust bearing  400  to support relative rotation between inner shaft  204  and inner housing  130 . In this embodiment, a plurality of barrier fluid passages extend through inner shaft  202  of upper shaft assembly  200  to a lower end thereof. As will be described further herein, the barrier fluid passages of inner shaft  204  are in fluid communication with a barrier fluid system  405  of compressor assembly  100  configured to supply a pressurized barrier fluid (via, e.g., a barrier fluid pump and an associated controller) that is distinct from the process fluid to components of compressor assembly  100 , including a first or upper barrier fluid seal assembly  410  positioned radially between inner shaft  204  and the inner cylindrical member  222  of the final stage  220  of outer shaft  210 . Upper barrier fluid seal assembly  410  assists in ensuring fluid disposed in pressure balancing passages  128 ,  144 , and  230  (collectively comprising a pressure balancing circuit  150  of compressor assembly  100 ) is isolated from other portions of compressor assembly  100 . 
     As shown particularly in  FIGS. 2-4 , in this embodiment, the lower shaft assembly  300  of compressor assembly  100  generally includes a generally cylindrical inner shaft  310  and an annular outer shaft or drum  340  disposed about and coupled to an outer surface of inner shaft  310 . Drum  340  includes an outer surface on which impellers  302  of lower shaft assembly  300  are arranged. Although in this embodiment lower shaft assembly  300  comprises a distinct inner shaft  310  and drum  340 , in other embodiments, inner shaft  310  and drum  340  may comprise a single, monolithically formed member. 
     In this embodiment, an upper end of the inner shaft  310  of lower shaft assembly  300  includes a plurality of barrier fluid passages which are in fluid communication with the barrier fluid passages of upper shaft assembly  200 . An annular contra-rotating bearing is positioned between the inner shaft  310  of lower shaft assembly  300  and the inner shaft  204  of upper shaft assembly  200  to permit contra-rotation therebetween. The barrier fluid passages of lower shaft assembly  300  are configured to supply barrier fluid of barrier fluid system  405  to a second or intermediate barrier fluid seal assembly  412  comprising a contra-rotating seal configured to seal the annular interface formed between inner shaft  310  of the lower shaft assembly  300  and the inner cylindrical member  222  of the final stage  220  of upper shaft assembly  200 . Like the upper barrier fluid seal assembly  410  of barrier fluid system  405 , intermediate barrier fluid seal assembly  412  assists in ensuring fluid disposed in pressure balancing circuit  150  is isolated from other portions of compressor assembly  100 . 
     Compressor assembly  100  additionally includes a second or lower thrust bearing  403  positioned in the central passage  122  of outer housing  120  that engages a cylindrical outer surface of the inner shaft  310  of lower shaft assembly  300  to absorb axially directed thrust loads applied to lower shaft assembly  300 . In this embodiment, compressor assembly  100  further includes a second or intermediate radial bearing  404  and a third or lower radial bearing  406 , each positioned in the central passage  136  of inner housing  130 . Intermediate radial bearing  404  engages the outer surface of the outer shaft  210  of upper shaft assembly  200  proximal a lower end thereof to support relative rotation between outer shaft  210  and inner housing  130 . Lower radial bearing  406  engages an outer surface of the inner shaft  310  of lower shaft assembly  300  to support relative rotation between inner shaft  310  and inner housing  130 . 
     In this embodiment, the barrier fluid system  405  of compressor assembly  100  includes a third or intermediate barrier fluid seal assembly  414  configured to seal the annular interface formed between the inner shaft  310  of lower shaft assembly  300  and the outer shaft  210  of upper shaft assembly  200 . Intermediate barrier fluid seal assembly  414  comprises a contra-rotating seal and is supplied with pressurized barrier fluid via the barrier fluid passages formed in inner shaft  310 . Barrier fluid system  405  further includes a fourth or lower barrier fluid seal assembly  416  is configured to seal the annual interface formed between a lower end of the outer shaft  210  of upper shaft assembly  200  and the inner surface  138  of inner housing  130 . Lower barrier fluid seal assembly  416  is supplied with barrier fluid from barrier fluid system  405  via passages formed in the inner housing  130  (not shown in  FIGS. 2-4 ). 
     As shown particularly in  FIG. 4 , compressor assembly  100  includes an annular first or upper rotating seal assembly  430  positioned between the inner surface  138  of inner housing  130  and final stage  220 . Particularly, upper rotating seal assembly  430  sealingly engages the outer surface  226  of the inner cylindrical member  222  of final stage  220  while permitting relative rotation between final stage  220  and inner cylinder  130 . Compressor assembly  100  additionally includes an annular second or lower rotating seal assembly  434  positioned between the inner surface  138  of inner housing  130  and final stage  220 . Lower rotating seal assembly  434  sealingly engages the outer surface  242  of the outer cylindrical member  240  of final stage  220  while permitting relative rotation between final stage  220  and inner cylinder  130 . Further, in this embodiment, compressor assembly  100  includes an annular contra-rotating seal assembly  438  positioned radially between the final stage  220  of upper shaft assembly  200  and lower shaft assembly  300 . Particularly, contra-rotating seal assembly  438  sealingly engages the outer surface  226  of the inner cylindrical member  222  of final stage  220  and a generally cylindrical inner surface  342  of the drum  340  of lower shaft assembly  300 . 
     The sealing engagement between final stage  220  and the drum  340  of lower shaft assembly  300  provided by contra-rotating seal assembly  438  forms an annular pressure balancing chamber  238  that is in fluid communication with pressure balancing passages  230  of first stage  220 , and thus comprise a portion of the pressure balancing circuit  150  described above. Particularly, pressure balancing chamber  238  extends radially between the inner surface  342  of the drum  340  of lower shaft assembly  300  and an outer surface of the inner shaft  310  of lower shaft assembly  300 . 
     As described above, pressure balancing circuit  150  of compressor assembly  100  is in fluid communication with the inlet fluid flow  125  via pressure balancing conduit  148 , and thus, fluid pressure within pressure balancing chamber  238 , as well as the pressure balancing passages (e.g., passages  128 ,  144 ,  230 ) of pressure balancing circuit  150  is substantially equal to the inlet pressure of the inlet fluid flow  125 , the inlet fluid pressure of inlet fluid flow  125  entering outer housing  120  being substantially less than an outlet fluid pressure of the outlet fluid flow  129  exiting outer housing  120 . The inlet fluid pressure disposed in pressure balancing circuit  150  provides a thrust load against portions of lower shaft assembly  300  in a second or downwards direction (generally opposite the axial upwards travel of fluid flow  127 ). As shown particularly in  FIG. 4 , the portion of lower shaft assembly  300  exposed to the inlet fluid pressure of pressure balancing circuit  150  comprises a circular, inner axially-projected surface area  350  defined by a diameter  352  that is equal to a diameter of contra-rotating seal assembly  438 . During operation of compressor assembly  100 , a portion of the outlet fluid flow  129  exiting final stage  220  may bleed or leak across contra-rotating seal  438  and into the pressure balancing chamber  238  of pressure balancing circuit  150 . 
     In this embodiment, pressure balancing circuit  150  is configured to recirculate any outlet fluid bled into pressure balancing chamber  238  into the inlet fluid flow  125  via pressure balancing conduit  148 , thereby providing an outlet for the high pressure outlet fluid. Given that inlet fluid pressure applies a pressure force against lower shaft assembly  300  in the upwards direction  127 , the downwards pressure force applied to the inner axially-projected surface area  350  of lower shaft assembly  300  by the inlet fluid pressure does not produce a net thrust load on lower shaft assembly  300 . In other words, the downwards thrust load applied to the inner axially-projected surface area  350  of lower shaft assembly  300  is balanced by the upwards thrust load applied to a corresponding inner axially-projected surface area located near a lower end of the lower shaft assembly  300 . In this manner, lower shaft assembly  300  of compressor assembly  100  comprises a thrust-balanced lower shaft assembly  300 . 
     Particularly, without the sealing engagement provided by contra-rotating seal assembly  348 , the inner axially-projected surface area  350  of lower shaft assembly  300  would be exposed to the outlet fluid pressure of the outlet fluid flow  129  exiting the final stage  220  of upper shaft assembly  200 , and thus, the thrust loads imparted to lower shaft assembly  300  would be increased. Therefore, by exposing inner axially-projected surface area  350  of lower shaft assembly  300  to the inlet fluid pressure rather than the greater outlet fluid pressure, contra-rotating seal assembly  438  reduces the total thrust load imparted to lower shaft assembly  300  in the downwards direction by the action of the contra-rotating impellers  202 ,  302  of shaft assemblies  200 ,  300 , respectively. By reducing the amount of thrust load imparted to lower shaft assembly  300 , the differential pressure between the outlet fluid flow  129  and inlet fluid flow  125  achieved by compressor assembly  100  may be increased without jeopardizing the structural integrity of lower shaft assembly  300 . Thus, contra-rotating seal  438  is configured to maximize the achievable differential pressure between fluid flows  129 ,  125 , thereby increasing the efficiency of compressor assembly  100 . 
     In this embodiment, while the downwards thrust load applied to lower shaft assembly  300  is reduced by the action of pressure balancing circuit  150  as described above, a reduced net axially directed thrust load in the downwards direction is applied to lower shaft assembly  300  to lower shaft assembly  300  to prevent lower shaft assembly  300  from floating or chattering within outer housing  120  during the operation of compressor assembly  100 . Particularly, the net downwards thrust load applied to lower shaft assembly  300  corresponds to an annular outer axially-projected surface area of lower shaft assembly  300  defined by an outer radius  356  extending between contra-rotating seal assembly  438  and an outer cylindrical surface of the drum  340  of lower shaft assembly  300 . Thus, the amount of downwards thrust load imparted to lower shaft assembly  300  may be tailored as desired by adjusting the size of outer radius  356 . 
     In this embodiment, compressor assembly  100  is also configured for increasing the maximum differential pressure between fluid flows  125 ,  129  safely achievable by compressor assembly  100  by distributing thrust loads across the final stage  220  of upper shaft assembly  200 . Particularly, torque and thrust loads applied to final stage impeller  236  may be transferred to the inner cylindrical member  222  of final stage  220  via the connection formed therebetween via connector  234 . The loads transferred from final stage impeller  236  to inner cylindrical member  222  of final stage  220  may be distributed to outer cylindrical member  240  via the annular bridge  232  coupling outer cylindrical member  240  of final stage  220  with inner cylindrical member  222 . In this manner, thrust loads applied to final stage  220  may be shared or distributed between cylindrical members  222 ,  240 , thereby increasing the amount of thrust loads that may be safely applied to final stage  220  without damaging final stage  220 . 
     Further, while a radially inner end of the final stage impeller  236  is connected to the inner cylindrical member  222  of final stage  220 , in this embodiment, a radially outer end of final stage impeller  236  is not connected to outer cylindrical member  240 . Thus, final stage impeller  236  is permitted to flex relative outer cylindrical member  240  and final stage impeller  236 , which has a relatively thin cross-sectional area relative cylindrical members  222 ,  240 , is substantially isolated from thrust loads applied to the outer cylindrical member  240  of final stage  220 . In this manner, final stage impeller  236  may be at least partially isolated from torque, centrifugal loads, and thrust loads, protecting final stage impeller  236  from damage during the operation of compressor assembly  100 . Although in this embodiment final stage impeller  236  is not connected to outer cylindrical member  240  of final stage  220 , in other embodiments, final stage impeller  236  may be connected with both inner cylindrical member  222  and outer cylindrical member  240  of final stage  220 . For instance, in certain embodiments, cylindrical members  222 ,  240  and final stage impeller  236  may comprise a single, monolithically formed member. 
     Beyond reducing the thrust load applied to lower shaft assembly  300 , by exposing inner axially-projected surface area  350  of lower shaft assembly  300  to the inlet fluid pressure, pressure balancing conduit  150  of compressor assembly  100  is configured to simply the configuration barrier fluid system  405 , thereby reducing the size, weight, cost, and/or complexity of compressor assembly  100 . Particularly, barrier fluid system  405  is configured to supply barrier fluid to each barrier fluid seal assembly  410 ,  412 ,  414 , and  416  at a pressure that is slightly higher than the fluid pressure to which each barrier fluid seal assembly  410 ,  412 ,  414 , and  416  is exposed such that any leakage across barrier fluid seal assemblies  410 ,  412 ,  414 , and/or  416  comprises barrier fluid leaking into the process fluid flow (i.e., fluid flows  125 ,  127 , and  129 ) rather than process fluid leaking into barrier fluid system  405 . 
     In this embodiment, intermediate barrier fluid seal assembly  414  and lower barrier fluid seal assembly  416 , each positioned near a lower end of inner housing  130  where the inlet fluid flow  125  enters fluid inlets  132  of inner housing  130 , are each exposed to the inlet fluid pressure. Additionally, due to the supply of inlet fluid pressure via pressure balancing circuit  150  and the sealing engagement provided by contra-rotating seal assembly  438 , upper barrier fluid seal assembly  410  and intermediate barrier fluid seal assembly  412  are also each exposed to the inlet fluid pressure. Thus, each of the barrier fluid seal assemblies  410 ,  412 ,  414 , and  416  of barrier fluid system  405  are exposed to the inlet fluid pressure. Given that barrier fluid seal assemblies  410 ,  412 ,  414 , and  416  are each exposed to substantially the same fluid pressure, the barrier fluid supplied to each of barrier fluid seal assemblies  410 ,  412 ,  414 , and  416  may be disposed at a single pressure that is slightly greater than the inlet fluid pressure of compressor assembly  100 . Therefore, instead of needing to supply barrier fluid at varying pressures (requiring multiple barrier fluid pumps, controllers, etc.), barrier fluid system  405  need only supply a barrier fluid at a single pressure for each of the barrier fluid seal assemblies  410 ,  412 ,  414 , and  416 , thereby simplifying the configuration of the barrier fluid system  405  of compressor assembly  100 . 
     Although the embodiment shown in  FIGS. 1-9  includes an upper shaft assembly  200  comprising an enclosed final stage  220 , other embodiments of compressor assemblies including a thrust-balanced lower shaft assembly may employ an open final stage. For example, referring to  FIG. 10 , an embodiment of a compressor assembly  500  is shown including an upper shaft assembly  530  having an open final stage  540 . Compressor assembly  500  of  FIG. 10  includes features in common with the compressor assembly  100  shown in  FIGS. 1-9 , and shared features are labeled similarly. Particularly, in the embodiment of  FIG. 10 , compressor assembly  500  has a central or longitudinal axis  505  and generally includes an outer housing  502 , an inner housing  510  received in a central passage of outer housing  502 , first or upper shaft assembly  530 , and a second or lower shaft assembly  300 ′ similar in configuration as the lower shaft assembly  300  of compressor assembly  100  and configured to contra-rotate relative upper shaft assembly  530  of compressor assembly  500 . 
     The upper shaft assembly  530  of compressor assembly  500  generally includes inner shaft  204  coupled to an annular outer shaft  532 . Outer shaft  532  of upper shaft assembly  530  includes drum  212 , and upper or final stage  540  coupled to the upper end of drum  212 . In this embodiment, final stage  540  of outer shaft  532  includes an inner cylindrical member  542  extending from an upper end of final stage  540 , and an outer cylindrical member  550  extending from a lower end of final stage  540 . Unlike the final stage  220  of the compressor assembly  100 , final stage  540  of compressor assembly  500  does not include an annular shoulder or bridge connecting the inner cylindrical member  542  with outer cylindrical member  550 . Instead, process fluid exits compressor assembly  500  as an outlet fluid flow (indicated by arrows  507  in  FIG. 10 ) via the annular opening formed between an upper end of outer cylindrical member  550  and a generally cylindrical outer surface  544  of the inner cylindrical member  542  of final stage  540 . Outlet fluid flow  507  exits compressor assembly  500  via a plurality of circumferentially spaced fluid outlets  512  formed in inner housing  510 , and an outlet port (not shown in  FIG. 10 ) formed in outer housing  502 . 
     In this embodiment, the inner cylindrical member  542  of final stage  540  includes an annular shoulder  546  having a plurality of circumferentially spaced pressure balancing passages  548  formed therein, each pressure balancing passage  548  extending to a lower end of inner cylindrical member  542 . Final stage  540  additionally includes a final stage impeller  554  extending between inner cylindrical member  542  and outer cylindrical member  550 . In this embodiment, final stage impeller  554  is formed monolithically with cylindrical members  542 ,  550 ; however, in other embodiments, final stage impeller  554  may be separately coupled with cylindrical members  542 ,  550 . A pressure balancing passage  556  extends through the final stage impeller  554 . Pressure balancing passage  556  is in fluid communication with both the pressure balancing passage  548  of inner cylindrical member  540  of final stage  540  and an annular pressure balancing passage  511  formed radially between an outer surface of the drum  212  of upper shaft assembly  530  and a generally cylindrical inner surface  514  of inner housing  510 . 
     Compressor assembly  500  includes an annular first or upper rotating seal assembly  570  positioned radially between the inner surface  514  of inner housing  510  and the outer surface  544  of the inner cylindrical member  542  (proximal an upper end thereof) of final stage  540 , and is configured to seal the annular interface formed therebetween. Additionally, compressor assembly  500  includes an annular second or lower rotating seal assembly  574  positioned radially between a generally cylindrical outer surface  551  of the outer cylindrical member  550  of final stage  540  and the inner surface  514  of inner housing  510 , and is configured to seal the annular interface formed therebetween. Further, compressor assembly  500  includes an annular contra-rotating seal assembly  578  positioned radially between the outer surface  544  of the inner cylindrical member  542  (proximal a lower end thereof) of final stage  540  and the inner surface  342  of the drum  340  of lower shaft assembly  300 ′. 
     The sealing engagement between final stage  540  and the drum  340  of lower shaft assembly  300 ′ provided by contra-rotating seal assembly  578  forms an annular pressure balancing chamber  560  that is in fluid communication with pressure balancing passages  511 ,  548 , and  556 . Pressure balancing chamber  560  and pressure balancing passages  511 ,  548 , and  556  collectively comprise a pressure balancing circuit  562  of compressor assembly  500 . Pressure balancing passage  511  formed between inner housing  510  and the drum  212  of upper shaft assembly  530  is in fluid communication with the fluid inlets of inner housing  510 , and thus the fluid disposed in pressure balancing circuit  562  is disposed at substantially the inlet fluid pressure of the inlet fluid flow entering inner housing  510 . 
     In the configuration described above, a portion of lower shaft assembly  300 ′ is exposed to the inlet fluid pressure of pressure balancing circuit  562 , the portion comprising a circular, inner axially-projected surface area  580  defined by a diameter  582  that is equal to a diameter of the contra-rotating seal assembly  578  of compressor assembly  500 . Therefore, similar to the operation of the pressure balancing circuit  150  of compressor assembly  100 , the pressure balancing circuit  562  of compressor assembly  500  reduces the net downwards thrust load applied to lower shaft assembly  300 ′ by balancing the downwards thrust applied to the inner axially-projected surface area  580  of lower shaft assembly  300 ′ with a corresponding upwards thrust load applied to lower shaft assembly  300 ′ from the inlet fluid pressure exposed to a lower end of lower shaft assembly  300 ′. 
     However, unlike compressor assembly  100 , compressor assembly  500  thrust-balances lower shaft assembly  300 ′ using a pressure balancing circuit  562  that includes an open final stage  540 . In some applications, it may be preferable to employ an open final stage  540 , which does not require outlet fluid flow  507  to flow through a plurality of circumferentially spaced ports formed in the final stage. Additionally, instead of recirculating entrained outlet fluid flow that has leaked past seals  570 ,  574 , and/or  578  via passages formed in outer housing  502 , outlet fluid flow that has leaked into pressure balancing circuit  562  is recirculated to the fluid inlets of inner housing  510  via pressure balancing passage  511  (indicated by arrows  584 ). 
     Referring to  FIG. 11 , another embodiment of a compressor assembly  600  is shown including an upper shaft assembly  610  having an open final stage  620 . Compressor assembly  600  of  FIG. 11  includes features in common with the compressor assembly  100  shown in  FIGS. 1-9  and the compressor assembly  500  shown in  FIG. 10 , and shared features are labeled similarly. In the embodiment of  FIG. 11 , compressor assembly  600  has a central or longitudinal axis  605  and generally includes outer housing  502 , inner housing  510 , first or upper shaft assembly  610 , and a second or lower shaft assembly  650  configured to contra-rotate relative upper shaft assembly  610  of compressor assembly  600 . 
     The upper shaft assembly  610  of compressor assembly  600  generally includes inner shaft  204  coupled to an annular outer shaft  612 . Outer shaft  612  of upper shaft assembly  610  includes drum  212 , and upper or final stage  620  coupled to the upper end of drum  212 . In this embodiment, final stage  620  of outer shaft  612  includes an inner cylindrical member  622  extending from an upper end of final stage  620 , and an outer cylindrical member  640  extending from a lower end of final stage  620 . Similar to the final stage  540  of the compressor assembly  500  shown in  FIG. 10 , final stage  620  of compressor assembly  600  does not include an annular shoulder or bridge connecting the inner cylindrical member  622  with outer cylindrical member  640 . Thus, process fluid exits compressor assembly  500  as an outlet fluid flow (indicated by arrows  607  in  FIG. 11 ) via the annular opening formed between an upper end of outer cylindrical member  640  and a generally cylindrical outer surface  624  of the inner cylindrical member  622  of final stage  620 . 
     In this embodiment, the inner cylindrical member  622  of final stage  620  includes pressure balancing passages  548  formed therein and a final stage impeller  630  formed monolithically with cylindrical members  620 ,  640 . However, unlike final stage impeller  554  of final stage  540 , the final stage impeller  630  of final stage  620  does not include an internal pressure balancing passage for communicating inlet fluid pressure. Additionally, while final stage impeller  630  is formed monolithically with cylindrical members  622 ,  640 , in other embodiments, final stage impeller  630  may be separately coupled with cylindrical members  622 ,  640 . 
     Lower shaft assembly  650  generally includes a generally cylindrical inner shaft  652  and an annular outer shaft or drum  670  disposed about and coupled to an outer surface of inner shaft  652 . Similar to drum  340  of lower shaft assembly  300  shown in  FIGS. 2-9 , drum  670  of lower shaft assembly  650  includes an outer surface on which impellers  302  are arranged. In this embodiment, the inner shaft  652  of lower shaft assembly  650  includes a plurality of circumferentially spaced pressure balancing passages  654  extending from an upper end thereof, wherein pressure balancing passages  654  are in fluid communication with the fluid inlets (not shown in  FIG. 11 ) of the inner housing  510  of compressor assembly  600 . 
     Drum  670  of lower shaft assembly  650  includes an annular shoulder  672  proximal an upper end of drum  670 , where annular shoulder  672  engages the upper end of the inner shaft  652  of lower shaft assembly  650 . In this embodiment, drum  670  includes a plurality of circumferentially spaced pressure balancing passages  674 , each passage  674  extending axially between upper and lower ends of shoulder  672  and in fluid communication with a corresponding pressure balancing passage  654  of inner shaft  650 . Compressor assembly  600  includes rotating seal assemblies  570 ,  574 , and contra-rotating seal assembly  578 , thereby defining an annular pressure balancing chamber  632  formed about the inner shaft  652  of lower shaft assembly  650  and extending axially between a lower end of the inner cylindrical member  622  of final stage  620  and the upper end of the annular shoulder  672  of drum  670 . Pressure balancing chamber  632  is in fluid communication with pressure balancing passages  548  of final stage  620  and the pressure balancing passages  654 ,  674  of the inner shaft  652  and drum  670 , respectively, of lower shaft assembly  670 . Pressure balancing chamber  632  and pressure balancing passages  548 ,  654 , and  674  collectively comprise a pressure balancing circuit  680  of compressor assembly  600 . With pressure balancing passages  654  of the inner shaft  652  of lower shaft assembly  650  in fluid communication with the fluid inlets of the inner housing  510 , fluid disposed in pressure balancing circuit  680  is disposed at substantially the inlet fluid pressure of the inlet fluid flow entering inner housing  510  of compressor assembly  600 . 
     In the configuration described above, a portion of lower shaft assembly  650  is exposed to the inlet fluid pressure of pressure balancing circuit  680 , the portion comprising a circular, inner axially-projected surface area  682  defined by a diameter  684  that is equal to a diameter of the contra-rotating seal assembly  578  of compressor assembly  600 . Therefore, similar to the operation of the pressure balancing circuit  562  of compressor assembly  500 , the pressure balancing circuit  680  of compressor assembly  600  reduces the net downwards thrust load applied to lower shaft assembly  650  by balancing the downwards thrust applied to the inner axially-projected surface area  682  of lower shaft assembly  650  with a corresponding upwards thrust load applied to lower shaft assembly  650  from the inlet fluid pressure exposed to a lower end of lower shaft assembly  650 . Additionally, in this embodiment, outlet fluid flow that has leaked past seals  570 ,  574 , and/or  578  and into pressure balancing circuit is recirculated to the fluid inlets of inner housing  510  via pressure balancing passages  654 ,  674  (indicated by arrows  686 ). 
     Unlike the pressure balancing circuit  562  of compressor assembly  500 , where inlet fluid pressure was communicated to circuit  562  via annular pressure balancing passage  511  formed between inner housing  510  and drum  212 , inlet fluid pressure is communicated to the pressure balancing circuit  680  of compressor assembly  600  via the plurality of pressure balancing passages  654  formed within the inner shaft  652  of lower shaft assembly  650 . In some applications, it may be preferable to communicate inlet fluid pressure via pressure balancing passages  654  of internal shaft  652  in lieu of an annular passage formed between drum  212  and inner housing  510  (e.g., due to spatial constraints or other limitations constraining the design of the compressor assembly). Additionally, given that final stage impeller  630  does not include an internal passage  630 , the cross-sectional area of final stage impeller  630  may be greater than the final stage impeller  554  of compressor assembly  500 , and thus, final sage impeller  630  of compressor assembly  600  may be able to withstand relatively greater torque, centrifugal loads, and thrust loads than final stage impeller  554 . 
     Referring to  FIG. 12 , a flowchart of a method  700  for compressing a process fluid is shown. At block  702  of method  700 , an inlet flow of a process fluid is flowed into a housing at an inlet pressure. In some embodiments, block  702  includes flowing inlet fluid flow  125  into outer housing  120  of compressor assembly  100  at an inlet fluid pressure. In other embodiments, block  702  comprises flowing inlet fluid flow  125  into the outer housing  502  of compressor assemblies  500  and/or  600  at the inlet pressure. At block  704  of method  700 , a first shaft assembly disposed in the housing and comprising a first plurality of impellers about a longitudinal axis in a first rotational direction. In some embodiments, block  704  comprises rotating upper shaft assembly  200 , including impellers  202 , about central axis  105  in a first rotational direction. In other embodiments, block  704  comprises rotating upper shaft assemblies  530 ,  610  about central axes  505 ,  605 , respectively in the first rotational direction. 
     At block  706  of method  700 , a second shaft assembly disposed in the housing and comprising a second plurality of impellers interleaved with the first plurality of impellers is rotated about the longitudinal axis in a second rotational direction opposite the first rotational direction. In some embodiments, block  706  comprises rotating lower shaft assembly  300 , including impellers  302 , about central axis  105  in a second rotational direction. In other embodiments, block  706  comprises rotating lower shaft assemblies  300 ,  650  about central axes  505 ,  605 , respectively in the second rotational direction. At block  708  of method  700 , an axially directed pressure force is applied to each end of the lower shaft assembly with the process fluid at the inlet pressure. In some embodiments, block  708  comprises communicating a portion of the inlet fluid flow  125  at the inlet pressure to a pressure balancing chamber (e.g., pressure balancing chambers  238 ,  560 , and  632 ) positioned axially between an upper shaft assembly (e.g., upper shaft assemblies  200 ,  530 , and  610 ) and a lower shaft assembly (e.g., lower shaft assemblies  300 , and  650 ) via a pressure balancing circuit (e.g., pressure balancing circuits  150 ,  562 , and  680 ) that includes the pressure balancing chamber, thereby applying a pressure force against an upper end of the lower shaft assembly via fluid disposed in the pressure balancing chamber at the inlet pressure. 
     The above discussion is meant to be illustrative of the principles and various embodiments of the present disclosure. While certain embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The embodiments described herein are exemplary only, and are not limiting. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.