Patent Publication Number: US-7713033-B2

Title: Double-acting, duplex pump controlled by two, two position spool valves

Description:
BACKGROUND 
   The present invention relates to pumps and control systems therefore. More particularly, the present invention relates to a double-acting, duplex pump having a power side controlled by two, two position spool valves. 
   Pumps are used in a wide variety of industries to deliver fluids. For instance, the delivery of fluids at high pressure may be accomplished with an intensifier pump. Intensifier pumps are commonly used in a variety of industries where the delivery of fluids at a high pressure is desired. For example, intensifier pumps may be used in conjunction with subterranean operations to deliver cement slurries, stimulation fluids, drilling fluids, or other fluids at the desired pressure. Also, in offshore operations, an intensifier pump may be used to remove the hydrostatic head from seawater that is applied to the downhole fluids. In these instances, the intensifier pump may be located subsurface. 
   Generally, intensifier pumps are reciprocating, fluid-driven apparatuses that comprise one or more large pistons connected to one or more small pistons. Intensifiers are powered by a hydraulic fluid, such as water. In a common intensifier pump, a double-acting, low-pressure, power chamber contains a central piston slidably disposed therein having a high-pressure piston extending from each face. The high pressure pistons extend oppositely from the central piston out of the power chamber into a high-pressure, pumping cylinder. In operation, hydraulic fluid is directed into the low-pressure, power chamber in such a manner to cause the central piston to reciprocate back and forth. The central piston, in turn, drives the high-pressure pistons, which alternately pump an intensified fluid at high pressures to a desired location. As will be understood by those skilled in the art, intensification of the fluid occurs, because the area of the central piston is larger than the area of the high-pressure pistons. 
   When the central piston reaches one end of its stroke, there may be a short delay in the flow of high-pressure fluid. This delay is due to a necessary precompression of the fluid in the high-pressure, pumping cylinder to the operating pressure. Due to this short delay, there may be a pressure dip in the output from the high-pressure, pumping cylinder. This pressure dip is undesirable, inter alia, because it may damage the power system, the pump, the fluid flow lines, and/or the well. To counteract these pressure dips, a double-acting, duplex intensifier pump may be used, wherein two central pistons may be operated in parallel so that one central piston is on its power stroke while the other central piston is changing direction and/or is in a precompression stroke. The operation of the double-acting, duplex intensifier pump must be controlled with precision because the pressure dip may be severe if both central pistons reach the end of their power strokes simultaneously. Therefore, the timing of the power stroke and precompression stroke of the two central pistons should be controlled to provide a substantially constant discharge pressure from the high-pressure, pumping cylinders. One or more control valves may be provided to control the supply of hydraulic fluid to and venting of hydraulic fluid from the low-pressure, power chambers that contain the two central pistons. Conventional control systems may be hydraulically activated. However, timing issues may occur with the hydraulically activated system, thereby disrupting the compression and precompression cycles of the pump. Problems also may be encountered with control systems for pumps other than intensifier pumps. 
   SUMMARY 
   The present invention relates to pumps and control systems therefore. More particularly, the present invention relates to a double-acting, duplex pump having a power side controlled by two, two position spool valves. 
   One embodiment of the present invention provides a fluid control system that includes a source of a pressurized fluid. The fluid control system further includes a first spool valve having at least two positions, wherein the first spool valve is connected in fluid communication with the source of the pressurized fluid. The fluid control system further includes a second spool valve having at least two positions, wherein the second spool valve is connected in fluid communication with the first spool valve and the source of the pressurized fluid. In one aspect, when the first spool valve is in a first position and the second spool valve is in a second position, a first portion of the pressurized fluid is directed from the source of the pressurized fluid to the second spool valve via the first spool valve. In another aspect, when the first spool valve is in the first position and the second spool valve is in a first position, a third portion of the pressurized fluid is directed from the source of the pressurized fluid to the first spool valve via the second spool valve. In another aspect, when the first spool valve is in a second position and the second spool valve is in the first position, a fifth portion of the pressurized fluid is directed from the source of the pressurized fluid to the first spool valve via the second spool valve. In yet another aspect, when the first spool valve is in the second position and the second spool valve is in the second position, a seventh portion of the pressurized fluid is directed from the source of the pressurized fluid to the first spool valve via the second spool valve. 
   Another embodiment of the present invention provides a pump system that includes a first double-acting pump that includes a power chamber. The pump system further includes a fluid control system. The fluid control system includes a first spool valve connected in fluid communication to the power chamber of the first double-acting pump. The fluid control system further includes a second spool valve connected in fluid communication to the first spool valve. The pump system further includes a second double-acting pump that includes a power chamber. The power chamber of the second double-acting pump is connected in fluid communication to the second spool valve. Furthermore, the fluid control system directs delivery of a pressurized fluid to the first double-acting pump and the second double-acting pump. 
   Another embodiment of the present invention provides a method of controlling a pump. The method includes providing a first double-acting pump that includes a power chamber and a piston assembly slidably disposed within the power chamber. The method further includes providing a second double-acting pump that includes a power chamber and a piston assembly slidably disposed within the power chamber. The method further includes positioning a first spool valve in a first position, the first spool valve having at least two positions. The method further includes positioning a second spool valve in a second position, the second spool valve having at least two positions. The method further includes directing a first portion of a pressurized power fluid from a source of the pressurized power fluid to the second spool valve via the first spool valve. And the method further includes directing the first portion of the pressurized power fluid directed to the second spool valve to the power chamber of the second double-acting pump. 
   Another embodiment of the present invention further provides a method of controlling a pump. The method includes providing a double-acting, duplex pump. The double-acting, duplex pump includes a first double-acting pump that includes a power chamber and a piston assembly slidably disposed within the power chamber. The double-acting, duplex pump further includes a second double-acting pump operably connected to the first double-acting pump. The second double-acting pump includes a power chamber and a piston assembly slidably disposed within the power chamber. The method further includes providing a fluid control system. The fluid control system includes a first spool valve having at least two positions, and a second spool valve having at least two positions. And the method further includes directing a pressurized power fluid to the double-acting duplex pump via the fluid control system. 
   The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the certain embodiments that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic diagram of a fluid control system of the present invention with the first spool valve in a first position and the second spool valve in a second position in accordance with an embodiment of the present invention. 
       FIG. 2  is a schematic diagram of a fluid control system of the present invention with the first spool valve in a first position and the second spool valve in a first position in accordance with an embodiment of the present invention. 
       FIG. 3  is a schematic diagram of a fluid control system of the present invention with the first spool valve in a second position and the second spool valve in a first position in accordance with an embodiment of the present invention. 
       FIG. 4  is a schematic diagram of a fluid control system of the present invention with the first spool valve in a second position and the second spool valve in a second position in accordance with an embodiment of the present invention. 
       FIG. 5  is a schematic diagram illustrating the flow control system of the present invention connected to double-acting, duplex pump in accordance with an embodiment of the present invention. 
       FIG. 6  is a schematic diagram of a fluid control system of the present invention connected to the power chambers of a double-acting, duplex pump in accordance with an embodiment of the present invention, wherein the first spool valve is in a first position and the second spool valve is in a second position. 
       FIG. 7  is a schematic diagram of a fluid control system of the present invention connected to the power chambers of a double-acting, duplex pump in accordance with an embodiment of the present invention, wherein the first spool valve is in a first position and the second spool valve is in a first position. 
       FIG. 8  is a schematic diagram of a fluid control system of the present invention connected to the power chambers of a double-acting, duplex pump in accordance with an embodiment of the present invention, wherein the first spool valve is in a second position and the second spool valve is in a first position. 
       FIG. 9  is a schematic diagram of a fluid control system of the present invention connected to the power chambers of a double-acting, duplex pump in accordance with an embodiment of the present invention, wherein the first spool valve is in a second position and the second spool valve is in a second position. 
   

   While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DESCRIPTION 
   The present invention relates to pumps and control systems therefore. More particularly, the present invention relates to a double-acting, duplex pump having a power side controlled by two, two position spool valves. 
   Referring now to  FIG. 1  through  FIG. 4 , a fluid control system of the present invention is shown generally by reference numeral  100 . The fluid control system includes first spool valve  102  and second spool valve  104 . First spool valve  102  and second spool valve  104  are both two-position spool valves, wherein each spool valve has a first position and a second position. First spool valve  102  is shown in its first position in  FIG. 1  and  FIG. 2  and in its second position in  FIG. 3  and  FIG. 4 . Second spool valve  104  is shown in its first position in  FIG. 2  and  FIG. 3  and in its second position in  FIG. 1  and  FIG. 4 . Shifting of the spool valves between their first position and their second position may be accomplished by a variety of shifting mechanisms, a first shifting mechanism  580  and a second shifting mechanism  582  is shown connected to first spool valve  102  and second spool valve  104 , respectively, in  FIG. 5 . For example, the spool valves may be shifted mechanically, hydraulically, electrically, or by any other suitable shifting mechanism. Where shifting is accomplished mechanically, no outside power source is needed to operate fluid control system  100 . Those of ordinary skill in the art should be able to select and implement the appropriate mechanism to shift the spool valves between their two positions. 
   Referring again to  FIG. 1  through  FIG. 4 , first spool valve  102  includes sleeve/body  108  and central spool  106  slidably disposed within sleeve/body  108 . Sleeve/body  108  includes two fluid return ports  110 ,  112  that can be connected to a power fluid return T. Sleeve/body  108  includes two fluid exhaust inlet ports  114 ,  116  that can be connected to a source of an exhaust fluid, e.g., the discharge from the power chamber of an intensifier pump. Sleeve/body  108  includes four fluid supply ports  118 ,  120 ,  122 ,  124  that can be connected to the inlet to the power chamber of an intensifier pump. Sleeve/body  108  includes four load ports  126 ,  128 ,  130 ,  132  adapted to be connected to a load P, e.g., a source of a pressurized fluid. The power fluid return T should be at a lower pressure than the load P, e.g., the source of the pressurized fluid. Sleeve/body  108  includes four connector ports  134 ,  136 ,  138 ,  140  that can be connected to second spool valve  104 . 
   Central spool  106  optionally includes a plurality of seals  142 , for example, O-rings, for providing sealing engagement between central spool  106  and sleeve/body  108 . In some embodiments, a close clearance between central spool  106  and sleeve/body  108  may provide for sealing. Central spool  106  has a first grooved portion  144  for connecting fluid return port  110  and fluid exhaust inlet  114  in fluid communication with each other. Central spool  106  has a second grooved portion  146  for connecting fluid supply port  118  and connector port  140  in fluid communication with each other. Central spool  106  has a third grooved portion  148  for connecting fluid supply port  120  and load port  132  in fluid communication with each other. Central spool  106  has a fourth grooved portion  150  for connecting load port  130  and connector port  138  in fluid communication with each other. Central spool  106  has a fifth grooved portion  152  for connecting fluid supply port  122  and load port  128  in fluid communication with each other. Central spool  106  has a sixth grooved portion  154  for connecting load port  126  and connector port  136  in fluid communication with each other. Central spool  106  has a seventh grooved portion  156  for connecting fluid supply port  124  and connector port  134  in fluid communication with each other. Central spool  106  has an eighth grooved portion  158  for connecting fluid return port  112  and fluid exhaust inlet  116  in fluid communication with each other. 
   As illustrated by  FIG. 1  and  FIG. 2 , in the first position of first spool valve  102 , fluid supply port  118  and connector port  140  are connected in fluid communication, fluid supply port  120  and load port  132  are connected in fluid communication, load port  126  and connector port  136  are connected in fluid communication, and fluid exhaust inlet  116  and fluid return port  112  are connected in fluid communication. 
   As illustrated by  FIG. 3  and  FIG. 4 , in the second position of first spool valve  102 , fluid return port  110  and fluid exhaust inlet  114  are connected in fluid communication, load port  130  and connector port  138  are connected in fluid communication, and fluid supply port  124  and connector port  134  are connected in fluid communication. 
   Second spool valve  104  includes sleeve/body  160  and central spool  162  slidably disposed within sleeve/body  160 . Sleeve/body  160  includes two fluid return ports  164 ,  166  that can be connected to a power fluid return T. Sleeve/body  160  includes two fluid exhaust inlets  168 ,  170  that can be connected to the discharge from the power chamber of an intensifier pump. Sleeve/body  160  includes four fluid supply ports  172 ,  174 ,  176 ,  178  that can be connected to the inlet to the power chamber of an intensifier pump. Sleeve/body  160  includes four load ports  180 ,  182 ,  184 ,  186  adapted to be connected to a load P, e.g., a source of pressurized fluid. Sleeve/body  160  includes four connector ports  188 ,  190 ,  192 ,  194  that can be connected to first spool valve  104 . 
   Central spool  162  optionally includes a plurality of seals  196 , for example, O-rings, for providing sealing engagement between central spool  162  and sleeve/body  160 . In some embodiments, a close clearance between central spool  162  and sleeve/body  160  may provide for sealing. Central spool  162  has a first grooved portion  198  for connecting fluid return port  164  and fluid exhaust inlet  168  in fluid communication with each other. Central spool  162  has a second grooved portion  200  for connecting fluid supply port  172  and connector port  194  in fluid communication with each other. Central spool  162  has a third grooved portion  202  for connecting connector port  192  and load port  186  in fluid communication with each other. Central spool  186  has a fourth grooved portion  204  for connecting load port  184  and fluid supply port  174  in fluid communication with each other. Central spool  162  has a fifth grooved portion  206  for connecting connector port  190  and load port  182  in fluid communication with each other. Central spool  162  has a sixth grooved portion  208  for connecting load port  180  and fluid supply port  176  in fluid communication with each other. Central spool  162  has a seventh grooved portion  210  for connecting fluid supply port  178  and connector port  188  in fluid communication with each other. Central spool  162  has an eighth grooved portion  212  for connecting fluid return port  166  and fluid exhaust inlet  170  in fluid communication with each other. 
   As illustrated by  FIG. 2  and  FIG. 3 , in the first position of second spool valve  104 , fluid supply port  172  and connector port  194  are connected in fluid communication, load port  184  and fluid supply port  174  are connected in fluid communication, load port  182  and connector port  190  are connected in fluid communication, and fluid return port  166  and fluid exhaust inlet  170  are connected in fluid communication. 
   As illustrated by  FIG. 1  and  FIG. 4 , in the second position of second spool valve  104 , fluid return port  164  and fluid exhaust inlet  168  are connected in fluid communication, connector port  192  and load port  186  are connected in fluid communication, load port  180  and fluid supply port  176  are connected in fluid communication, and connector port  188  and fluid supply port  178  are connected in fluid communication. 
   First spool valve  102  and second spool valve  104  are shown connected by four fluid flow lines. These four fluid flow lines are used to direct power fluid between the two spool valves. Connector port  134  of first spool valve  102  and connector port  192  of second spool valve  104  are connected via fluid flow line  214 . Connector port  136  of first spool valve  102  and connector port  188  of second spool valve  104  are connected via fluid flow line  216 . Connector port  138  of first spool valve  102  and connector port  194  of second spool valve  104  are connected via fluid flow line  218 . Connector port  140  of first spool valve  102  and connector port  190  of second spool valve  104  are connected via fluid flow line  220 . In some embodiments, first spool valve  102  and second spool valve  104  are separate units, wherein the fluid flow lines that connect them to each other would be pipes are hoses that extend between the associated spool valves. In another embodiment, first spool valve  102  and second spool valve  104  may be connected by a manifold. In these embodiments, the fluid flow lines would be passages in the manifold extending between the two spool valves. 
   As those of ordinary skill in the art will appreciate, because first spool valve  102  and second spool valve  104  are two-position spool valves, fluid control system  100  has four positions. As illustrated by  FIG. 1 , first spool valve  102  may be in its first position when second spool valve  104  is in its second position. In this position, load port  126  of first spool valve  102  is in fluid communication with fluid supply port  178  of second spool valve  104  via connector port  136  of first spool valve  102 , fluid flow line  216 , and connector port  188  of second spool valve  104 . In operation, when first spool valve  102  is in the first position and second spool valve  104  is in the second position, a first portion of the pressurized fluid is directed from load L (e.g., a source of a pressurized fluid) to second spool valve  104  via first spool valve  102 . A second portion of the pressurized fluid is also directed from load L, to a power chamber or cylinder of a first pump, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 6 , via first spool valve  102 . Additionally, a first exhaust fluid is directed from a source of a first exhaust fluid, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 6 , to power fluid return T via first spool valve  102 . Furthermore, a second exhaust fluid is directed from a source of the second exhaust fluid, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 6 , to power fluid return T via second spool valve  104 . Double-acting pumps  502  and  504  and power chambers or cylinders  508  and  536  will be described in more detail below. 
   As illustrated by  FIG. 2 , first spool valve  102  may be in its first position when second spool valve  104  is in its first position. In this position, load port  182  of second spool valve  104  is in fluid communication with fluid supply port  118  of first spool valve  102  via connector port  190  of second spool valve  104 , fluid flow line  220 , and connector port  140  of first spool valve  102 . In operation, when first spool valve  102  is in the first position and second spool valve  104  is in the first position, a third portion of the pressurized fluid is directed from load L (e.g., a source of a pressurized fluid) to first spool valve  102  via second spool valve  104 . A fourth portion of the pressurized fluid is also directed from load L, to a power chamber or cylinder of a second pump, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 7 , via first second valve  102 . Additionally, a second exhaust fluid is directed from a source of the second exhaust fluid, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 7 , to power fluid return T via second spool valve  104 . Furthermore, a first exhaust fluid is directed from a source of the first exhaust fluid, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 7 , to power fluid return T via first spool valve  102 . Double-acting pumps  502  and  504  and power chambers or cylinders  508  and  536  will be described in more detail below. 
   As illustrated by  FIG. 3 , first spool valve  102  may be in its second position when second spool valve  104  is in its first position. In this position, load port  130  of first spool valve  102  is in fluid communication with fluid supply port  172  of second spool valve  104  via connector port  138  of first spool valve  102 , fluid flow line  218 , and connector port  194  of second spool valve  104 . In operation, when first spool valve  102  is in the second position and second spool valve  104  is in the first position, a fifth portion of the pressurized fluid is directed from load L (e.g., a source of a pressurized fluid) to second spool valve  104  via first spool valve  102 . A sixth portion of the pressurized fluid is also directed from load L, to a power chamber or cylinder of a first pump, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 8 , via first spool valve  102 . Additionally, a first exhaust fluid is directed from a source of a first exhaust fluid, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 8 , to power fluid return T via first spool valve  102 . Furthermore, a second exhaust fluid is directed from a source of the second exhaust fluid, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 8 , to power fluid return T via second spool valve  104 . Double-acting pumps  502  and  504  and power chambers or cylinders  508  and  536  will be described in more detail below. 
   As illustrated by  FIG. 4 , first spool valve  102  may be in its second position when second spool valve  104  is in its second position. In this position, load port  186  of second spool valve  104  is in fluid communication with fluid supply port  124  of first spool valve  102  via connector port  192  of second spool valve  104 , fluid flow line  214 , and connector port  134  of first spool valve  102 . In operation, when first spool valve  102  is in the second position and second spool valve  104  is in the second position, a seventh portion of the pressurized fluid is directed from load L (e.g., a source of a pressurized fluid) to first spool valve  102  via second spool valve  104 . An eighth portion of the pressurized fluid is also directed from load L, to a power chamber or cylinder of a second pump, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 9 , via first second valve  102 . Additionally, a second exhaust fluid is directed from a source of the second exhaust fluid, such as power chamber or cylinder  536  of second double-acting pump  504  as shown on  FIG. 9 , to power fluid return T via second spool valve  104 . Furthermore, a first exhaust fluid is directed from a source of the first exhaust fluid, such as power chamber or cylinder  508  of first double-acting pump  502  as shown on  FIG. 9 , to power fluid return T via first spool valve  102 . Double-acting pumps  502  and  504  and power chambers or cylinders  508  and  536  will be described in more detail below. 
   Referring now to  FIG. 5  is shown fluid control system  100  that directs the power fluid from a load P, e.g., a source of pressurized power fluid (not shown), to double-acting, duplex pump  500 . Double-acting, duplex pump  500  includes first double-acting pump  502  and second double-acting pump  504 . Fluid control system  100  allows one end of first double-acting pump  502  to be on its power stroke while one end of second double-acting pump  502  is on its precompression stroke. Furthermore, fluid control system  100  allows one end of second double-acting pump  504  to be on its power stroke while one end of first double-acting pump  502  is on its precompression stroke. Any of a variety of suitable pumps may be used as first double-acting pump  502  and second double-acting pump  504 . For example, the first double-acting pump  502  and second double-action pump  504 , as shown, in  FIG. 5  are intensifier pumps. 
   Generally, first double-acting pump  502  includes housing  506  that defines power chamber or cylinder  508  and a pair of pumping cylinders  510 ,  512  on opposite sides of power chamber or cylinder  508 . As shown in  FIG. 5 , each of the pair of pumping cylinders  510 ,  512  is of smaller diameter than power chamber or cylinder  508 . However, each of the pair of pumping cylinders  510 ,  512  need not be of smaller diameter than power chamber or cylinder  508 . As those of ordinary skill in the art will appreciate, housing  506  may be a single or multi-piece housing. For example, the pair of pumping cylinders  510 ,  512  may each be in a separate housing from power chamber or cylinder  508 . Housing  506  further includes a pair of power fluid intake ports  514 ,  516  connected to power chamber or cylinder  508  and a pair of power fluid exhaust ports  518 ,  520  connected to power chamber or cylinder  508 . Slidably disposed within power chamber or cylinder  508  is piston assembly  522  that includes a central piston  524  and a pair of high-pressure pistons  526 ,  528  of smaller diameter than central piston  524  that extend oppositely from central piston  524  into the pair of pumping cylinders  510 ,  512 . The ratio of the diameter of central piston  524  to the diameter of the pair of high-pressure pistons  526 ,  528  may be varied, inter alia, to modify the compression ratio of the first double-acting pump  502 . Furthermore, piston assembly  522  may be made of multiple pieces, inter alia, to permit replacement of the pair of high-pressure pistons  526 ,  528  independently to the replacement of central piston  524 . At the opposite ends of housing  506  are a pair of valve assemblies  530 ,  532  that permit the inflow of the low-pressure fluid to be pumped into each pumping cylinder  510 ,  512  on the intake stroke, and the outflow of high-pressure fluid from each pumping cylinder  510 ,  512  on the power stroke. In some embodiments, the pair of valve assemblies  530 ,  532  may comprise a check or a ball valve. The low-pressure fluid may be any of a variety of fluids, including water (e.g., seawater) and subterranean treatment fluids (e.g., drilling fluids, workover fluids, and completion fluids). Other fluids may be pumped by first double-acting pump  502  as desired by one of ordinary skill in the art. Furthermore, additional components, such as a sealing means between housing  506  and the pair of high-pressure pistons  526 ,  528 , may be included in first double-acting pump  502  as desired. 
   Generally, second double-acting pump  504  includes housing  534  that defines power chamber or cylinder  536  and a pair of pumping cylinders  538 ,  540  on opposite sides of power chamber or cylinder  536 . As shown in  FIG. 5 , each of the pair of pumping cylinders  538 ,  540  is of smaller diameter than power chamber or cylinder  536 . However, each of the pair of pumping cylinders  538 ,  540  need not be of smaller diameter than power chamber or cylinder  536 . As those of ordinary skill in the art will appreciate, housing  534  may be a single or multi-piece housing. For example, the pair of pumping cylinders  538 ,  540  may each be in a separate housing from power chamber or cylinder  536 . Housing  534  further includes a pair of power fluid intake ports  542 ,  544  connected to power chamber or cylinder  536  and a pair of power fluid and a pair of power fluid exhaust ports  546 ,  548  connected to power chamber or cylinder  536 . Slidably disposed within power chamber or cylinder  536  is piston assembly  550  that includes a central piston  552  and a pair of high-pressure pistons  554 ,  556  of smaller diameter than central piston  552  that extend oppositely from central piston  552  into the pair of pumping cylinders  538 ,  540 . The ratio of the diameter of central piston  552  to the diameter of the pair of high-pressure pistons  554 ,  556  may be varied, inter alia, to modify the compression ratio of second double-acting pump  504 . Furthermore, piston assembly  550  may be made of multiple pieces, inter alia, to permit replacement of the pair of high-pressure pistons  554 ,  556  independently to the replacement of central piston  552 . At the opposite ends of housing  534  are a pair of valve assemblies  558 ,  560  that permit the inflow of the low-pressure fluid to be pumped into each pumping cylinder  538 ,  540  on the intake stroke, and the outflow of high-pressure fluid from each of pumping cylinder  538 ,  540  on the power stroke. In some embodiments, the pair of valve assemblies  558 ,  560  may comprise a check or a ball valve. The low-pressure fluid may be any of a variety of fluids, including water (e.g., seawater) and subterranean treatment fluids (e.g., drilling fluids, workover fluids, and completion fluids). Other fluids may be pumped by second double-acting pump  504  as desired by one of ordinary skill in the art. Furthermore, additional components, such as a sealing means between housing  534  and the pair of high-pressure pistons  554 ,  556 , may be included in second double-acting pump  504  as desired. 
   First double-acting pump  502  is connected to first spool valve  102  of fluid control system  100  by a variety of fluid flow lines. These fluid flow lines are used to direct power fluid to first double-acting pump  502  or to direct power fluid expelled from first double-acting pump  502  to first spool valve  102 . Fluid exhaust inlet  114  of first spool valve  102  and power fluid exhaust port  518  of first double-acting pump  502  are connected via power fluid exhaust line  562 . Fluid supply port  118  of first spool valve  102  and power fluid intake port  514  of first double-acting pump  502  are connected via power fluid intake line  564 . Furthermore, fluid intake line  564  connects power fluid intake port  514  of first double-acting pump  502  to fluid supply port  120  of first spool valve  102  by its branch  565 . Fluid exhaust inlet  116  of first spool valve  102  and power fluid exhaust port  520  of first double-acting pump  502  are connected via power fluid exhaust line  566 . Fluid supply port  124  of first spool valve  102  and power fluid intake port  516  of first double-acting pump  502  are connected via power fluid intake line  568 . Furthermore, power fluid intake line  568  connects power fluid intake port  516  of first double-acting pump  502  to fluid supply port  122  of first spool valve  102  by its branch  569 . In some embodiments, the plurality of fluid flow lines that connect first spool valve  102  and first double-acting pump  502  may be pipes or hoses that extend between them. In another embodiment, first spool valve  102  and first double-acting pump  502  may be connected by a manifold, wherein the fluid flow lines that connect them would be passages in the manifold extending between first spool valve  102  and first double-acting pump  502 . 
   Second double-acting pump  504  is connected to second spool valve  104  of fluid control system  100  by a variety of fluid flow lines. These fluid flow lines are used to direct power fluid to second double-acting pump  504  or to direct power fluid expelled from second double-acting pump  504  to second spool valve  104 . Fluid exhaust inlet  168  of second spool valve  104  and power fluid exhaust port  546  of second double-acting pump  504  are connected via power fluid exhaust line  570 . Fluid supply port  172  of second spool valve  104  and power fluid intake port  542  of second double-acting pump  504  are connected via power fluid intake line  572 . Furthermore, power intake line  572  connects power fluid intake port  542  of second double-acting pump  504  to fluid supply port  174  of second spool valve  104  by its branch  573 . Fluid exhaust inlet  170  of second spool valve  104  and power fluid exhaust port  520  of second double-acting pump  504  are connected via power fluid exhaust line  574 . Fluid supply port  178  of second spool valve  104  and power fluid intake port  544  of second double-acting pump  504  are connected via power fluid intake line  576 . Furthermore, power intake line  576  connects power fluid intake port  544  of second double-acting pump  504  to fluid supply port  176  of second spool valve  104  by its branch  577 . In some embodiments, the plurality of fluid flow lines that connect second spool valve  104  and second double-acting pump  504  may be pipes or hoses that extend between them. In another embodiment, second spool valve  104  and second double-acting pump  504  may be connected by a manifold, where the fluid flow lines that connect them would be passages in the manifold extending between first spool valve  102  and first double-acting pump  502 . 
   First spool valve  102  is connected in fluid communication to load P, e.g., a source of pressurized power fluid, such as a pump (not shown) or a pressurized reservoir (not shown), by a plurality of fluid lines (not shown) that are connected to the four load ports  126 ,  128 ,  130 ,  132  of first spool valve  102 . In some embodiments, the source of the pressurized power fluid may be a high pressure pump. First spool valve  102  is connected to a power fluid return T, such as a tank (not shown) or sump (not shown), by a plurality of fluid lines (not shown) that are connected to the two fluid return ports  110 ,  112  of first spool valve  102 . Second spool valve  104  is connected in fluid communication to a load P, e.g., a source of pressurized power fluid, such as a pump (not shown) or a pressurized reservoir (not shown), by a plurality of fluid lines (not shown) that are connected to the four load ports  180 ,  182 ,  184 ,  186  of second spool valve  104 . In some embodiments, the source of the pressurized power fluid may be a high pressure pump. Second spool valve  104  is connected to a power fluid return T, such as a tank (not shown) or sump (not shown), by a plurality of fluid lines (not shown) that are connected to the two fluid return ports  164 ,  166  of second spool valve  104 . As those of ordinary skill in the art will appreciate, the power fluid return T should be a lower pressure than the load P. Generally, the load P will be the same for first spool valve  102  and second spool valve  104 . Likewise, the power fluid return T will also be the same for first spool valve  102  and second spool valve  104 . Even further, for example, the power fluid return T and load P may be located remote to the fluid control system  100 , for example, where fluid control system  100  is located subsurface, or may be located proximate to fluid control system  100 . The power fluid may be any of a variety of power fluids suitable for driving a pump, including, but not limited to, hydraulic fluids and water. 
   Furthermore, first shifting mechanism  580  for shifting first spool valve  102  between a first position and a second position is shown connected to first spool valve  102  and first double-acting pump  502 . Second shifting mechanism  582  for shifting second spool valve  104  between a first position and a second position is shown connected to second spool valve  104  and second double-acting pump  504 . First shifting mechanism  580  and second shifting mechanism  582  may be any suitable shifting mechanism for shifting the spool valves between their two positions. For example, the shifting mechanisms may operate mechanically, hydraulically, electrically, or by any other suitable method of operation. Those of ordinary skill in the art will be able to select and implement the appropriate shifting mechanism to shift the spool valves between their two positions. 
   Operation of fluid control system  100  to control double-acting, duplex pump  500  will be described in more detail by  FIG. 6  through  FIG. 9 . Pumping cylinders  510 ,  512  of first double-acting pump  502 , valve assemblies  530 ,  532  of first double-acting pump  502 , pumping cylinders  538 ,  540  of second double-acting pump  504 , valve assemblies  558 ,  560  of second double-acting pump  504 , first shifting mechanism  580 , and second shifting mechanism  582  are omitted from  FIG. 6  through  FIG. 9  because the present invention does not lie in their details.  FIG. 6  illustrates fluid control system  100  with first spool valve  102  in its first position and second spool valve  104  in its second position.  FIG. 7  illustrates fluid control system  100  with first spool valve  102  in its first position and second spool valve  104  in its first position.  FIG. 8  illustrates fluid control system  100  with first spool valve  102  in its second position and second spool valve  104  in its first position.  FIG. 9  illustrates fluid control system  100  with first spool valve  102  in its second position and second spool valve  104  in its second position. 
   Referring now to  FIG. 6 , first spool valve  102  is shown in its first position and second spool valve  104  is shown in its second position. In this position, fluid control system  100  directs the flow of power fluid so that piston assembly  550  of second double-acting pump  504  is on a power stroke and piston assembly  522  of first double-acting pump  502  is on a precompression stroke. 
   Power fluid exhaust port  520  of first double-acting pump  502  is connected to a power fluid return T via power fluid exhaust line  566 , fluid exhaust inlet  116  of first spool valve  102 , and fluid return port  112  of first spool valve  102 . Power fluid intake port  514  of first double-acting pump  502  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via power fluid intake line  564 , branch  565  of power fluid intake line  564 , fluid supply port  120  of first spool valve  102 , and load port  132  of first spool valve  102 . Branch  565  of power fluid intake line  564  includes flow restriction  602 , e.g., an orifice, in the fluid flow path therethrough. Flow restriction  602  restricts the flow of power fluid through branch  565 . In some embodiments, flow restriction  602  may be a variable orifice. 
   Power fluid exhaust port  546  of second double-acting pump  504  is connected to a power fluid return T via power fluid exhaust line  570 , fluid exhaust inlet  168  of second spool valve  104 , and fluid return port  164  of second spool valve  104 . Power chamber or cylinder  536  of second double-acting pump  504  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid, via fluid control system  100 . Power fluid intake port  544  of second double-acting pump  504  is connected in fluid communication with fluid supply port  178  of second spool valve  104  via power fluid intake line  576 . Load port  126  of first spool valve  102  is in fluid communication with fluid supply port  178  of second spool valve  104  via connector port  136  of first spool valve  102 , fluid flow line  216 , and connector port  188  of second spool valve. Accordingly, power fluid intake port  544  of second double-acting pump  504  is connected in fluid communication to load P, e.g., a source of pressurized power fluid (not shown), via load port  126  of first spool valve  102 , connector port  136  of first spool valve  102 , fluid flow line  216 , connector port  188  of second spool valve  104 , fluid supply port  178  of second spool valve  104 , and fluid intake line  576 . Power fluid intake port  544  of second double-acting pump  504  is also connected in fluid communication with a load P, e.g., a source of power fluid supply (not shown), via power fluid intake line  576 , branch  577  of power fluid intake line  576 , fluid supply port  176  of second spool valve  104 , and load port  180  of second spool valve  104 . However, branch  577  of power fluid intake line  576  includes a flow restriction  604 , e.g., an orifice, in the fluid flow path therethrough. Flow restriction  604  restricts the flow of power fluid through branch  577 . In some embodiments, flow restriction  604  may be a variable orifice. 
   In operation, as illustrated in  FIG. 6 , a first portion of the pressurized power fluid is directed to power chamber or cylinder  536  of second double-acting pump  504  via fluid control system  100 . The first portion of the pressurized power fluid from the load P (e.g., the source of the pressurized power fluid) is directed to second spool valve  104  via first spool valve  102 . The first portion of the pressurized power fluid directed to second spool valve  104  is directed to the power chamber or cylinder  536  of second double-acting pump  504 . More particularly, the first portion of the pressurized power fluid flows into first spool valve  102  via load port  126  and then into second spool valve  104  via connector port  136  of first spool valve  102 , fluid flow line  216 , and connector port  188  of second spool valve  102 . The first portion of the pressurized power fluid then flows into power chamber or cylinder  536  on one side of central piston  552  via fluid supply port  178  of second spool valve  104 , power fluid intake line  576 , and power fluid intake port  544 . The first portion of the pressurized power fluid directed to power chamber or cylinder  536  applies pressure on central piston  552  and moves piston assembly  550  to the right so that piston assembly  550  of second double-acting pump  504  is on a power stroke. As piston assembly  550  is moved to the right, power fluid is discharged from power chamber or cylinder  536  of second double-acting pump  504  on the other side of central piston  552  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to second spool valve  104  via power fluid exhaust port  546 , power fluid exhaust line  570 , and fluid exhaust inlet  168  of second spool valve  104 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  164  of second spool valve  104 . Those of ordinary skill in the art will appreciate, that as piston assembly  550  is moved to the right high-pressure fluid is discharged from second double-acting pump  504  via valve assembly  560  (shown on  FIG. 5 ) and the low-pressure fluid to be pumped enters second double-acting discharge pump  504  via valve assembly  558  (shown on  FIG. 5 ). At a predetermined point in the power stroke of piston assembly  550  of second double-acting pump  504 , e.g., when piston assembly  550  reaches the end of its stroke, second shifting mechanism  582  (shown on  FIG. 5 ) shifts second spool valve  104  to its first position. As previously discussed, this may be accomplished mechanically, hydraulically, electrically, or by any other suitable mechanism. 
   Furthermore, while the piston assembly  550  of second double-acting pump  504  is on a power stroke, piston assembly  522  of first double-acting pump  502  is on a precompression stroke. Accordingly, a second portion of the pressurized power fluid is directed to power chamber or cylinder  508  of first double-acting pump  502  via first spool valve  102 . More particularly, the second portion of the pressurized power fluid flows into first spool valve  102  via load port  132  of first spool valve  102  and then into power chamber or cylinder  508  on one side of central piston  524  via fluid supply port  120  of first spool valve  102 , branch  565  of power fluid intake line  564 , power fluid intake line  564 , and power fluid intake port  514 . The flow of the second portion of the pressurized power fluid is restricted so that piston assembly  522  of first double-acting pump  502  is on a precompression stroke. To restrict the flow of the second portion of the pressurized power fluid into power chamber or cylinder  508  of first double-acting pump  502 , branch  565  of power fluid intake line  564  includes flow restriction  602  in the fluid flow path therethrough. Flow restriction  602  should be designed to control the pressure applied by the power fluid to central piston  524  so that the low-pressure fluid to be pumped in pumping cylinder  510  of first double-acting pump  504  is brought up to the desired operating pressure prior to the compression stroke. During the precompression stroke, the second portion of the pressurized power fluid applies pressure to central piston  524  and may move piston assembly  522  to the left. As piston assembly  522  is moved to the left, power fluid is discharged from power chamber or cylinder  508  of first double-acting pump  502  on the other side of central piston  524  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to first spool valve  102  via power fluid exhaust port  520 , power fluid exhaust line  566 , and fluid exhaust inlet  116  of first spool valve  102 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  112  of first spool valve  102 . 
   Referring now to  FIG. 7 , first spool valve  102  is shown in its first position and second spool valve  104  is shown in its first position. In this position, fluid control system  100  directs the flow of power fluid so that piston assembly  522  of first double-acting pump  502  is on a power stroke, and piston assembly  550  of second end of second double-acting pump  504  is on a precompression stroke. 
   Power fluid exhaust port  548  of second double-acting pump  504  is connected in fluid communication to a power fluid return T via power fluid exhaust line  574 , fluid exhaust inlet  170  of second spool valve  104 , and fluid return port  166  of second spool valve  104 . Power fluid intake port  542  of second double-acting pump  504  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via power fluid intake line  572 , branch  573  of power fluid intake line  572 , fluid supply port  174  of second spool valve  104 , and load port  184  of second spool valve  104 . Branch  573  of power fluid intake line  572  includes flow restriction  702 , e.g., an orifice, in the fluid flow path therethrough. Flow restriction  702  restricts the flow of power fluid through branch  573 . In some embodiments, flow restriction  702  may be a variable orifice. 
   Power fluid exhaust port  520  of first double-acting pump  502  is connected to a power fluid return T via power fluid exhaust line  566 , fluid exhaust inlet  116  of first spool valve  102 , and fluid return port  112  of first spool valve  102 . Power chamber or cylinder  508  of first double-acting pump  502  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid, via fluid control system  100 . Power fluid intake port  514  of first double-acting pump  502  is connected in fluid communication with fluid supply port  118  of first spool valve  102  via power fluid intake line  564 . Load port  182  of second spool valve is in fluid communication with fluid supply port  118  of first spool valve  102  via connector port  190  of second spool valve  104 , fluid flow line  220 , and connector port  140  of first spool valve  102 . Accordingly, power fluid intake port  514  of first double-acting pump  502  is connected in fluid communication to a load P, e.g., a source of pressurized power fluid (not shown), via load port  182  of second spool valve  104 , connector port  190  of second spool valve  104 , fluid flow line  220 , connector port  140  of first spool valve  102 , fluid supply port  118  of first spool valve  102 , and power fluid intake line  564 . Power fluid intake port  514  of first double-acting pump  502  is also connected in fluid communication with a load P, e.g., a source of power fluid supply (not shown), via power fluid intake line  564 , branch  565  of power fluid intake line  564 , fluid supply port  120  of first spool valve  102 , and load port  132  of first spool valve  102 . However, branch  565  of power fluid intake line  564  includes flow restriction  602  that restricts the flow of power fluid through branch  565 . 
   In operation, as illustrated in  FIG. 7 , a third portion of the pressurized power fluid is directed to power chamber or cylinder  508  of first double-acting pump  502  via fluid control system  100 . The third portion of the pressurized power fluid from the load P (e.g., the source of the pressurized power fluid) is directed to first spool valve  102  via second spool valve  104 . The third portion of the pressurized power fluid directed to first spool valve  102  is directed to the power chamber or cylinder  508  of first double-acting pump  502 . More particularly, the third portion of the pressurized power fluid flows into second spool valve  104  via load port  182  and then into first spool valve  102  via connector port  190  of second spool valve  104 , fluid flow line  220 , and connector port  140  of first spool valve  104 . The third portion of the pressurized power fluid then flows into power chamber or cylinder  508  on one side of central piston  524  via fluid supply port  118  of first spool valve  102 , power fluid intake line  564 , and power fluid intake port  514 . The third portion of the pressurized power fluid directed to power chamber or cylinder  508  applies pressure on central piston  524  and moves piston assembly  522  to the left so that piston assembly  522  of first double-acting pump  502  is on a power stroke. As piston assembly  522  is moved to the left, power fluid is discharged from power chamber or cylinder  508  of first double-acting pump  502  on the other side of central piston  524  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to first spool valve  102  via power fluid exhaust port  520 , power fluid exhaust line  566 , and fluid exhaust inlet  116  of first spool valve  102 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  112  of first spool valve  102 . Those of ordinary skill in the art will appreciate, that as piston assembly  522  is moved to the left high-pressure fluid is discharged from first double-acting pump  502  via valve assembly  530  (shown on  FIG. 5 ) of first double-acting pump  502  and the low-pressure fluid to be pumped enters first double-acting pump  502  via valve assembly  532  (shown on  FIG. 5 ) of first double-acting pump  502 . At a predetermined point in the power stroke of piston assembly  522  of first double-acting pump  502 , e.g., when piston assembly  522  reaches the end of its stroke, first shifting mechanism  580  (shown on  FIG. 5 ) shifts first spool valve  102  to its second position. As previously discussed, this may be accomplished mechanically, hydraulically, electrically, or by any other suitable mechanism. 
   Furthermore, while piston assembly  522  of first double-acting pump  502  is on a power stroke, piston assembly  550  of second double-acting pump  504  is on a precompression stroke. Accordingly, a fourth portion of the pressurized power fluid is directed to power chamber or cylinder  536  of second double-acting pump  504  via second spool valve  104 . More particularly, the fourth portion of the pressurized power fluid flows into second spool valve  104  via load port  184  of second spool valve  104  and then into power chamber or cylinder  536  on one side of central piston  552  via fluid supply port  174  of second spool valve  104 , branch  573  of power fluid intake line  572 , power fluid intake line  572 , and power fluid intake port  542 . The flow of the fourth portion of the pressurized power fluid is restricted so that piston assembly  550  of second double-acting pump  504  is on a precompression stroke. To restrict the flow of the fourth portion of the pressurized power fluid into power chamber or cylinder  536  of second double-acting pump  504 , branch  573  of power fluid intake line  572  includes flow restriction  702  in the fluid flow path therethrough. Flow restriction  702  should be designed to control the pressure applied by the power fluid to central piston  552  so that the low-pressure fluid to be pumped in pumping cylinder  538  of second double-acting pump  502  is brought up to the desired operating pressure during the precompression stroke. During the precompression stroke, the fourth portion of the pressurized power fluid applies pressure to central piston  552  and may move piston assembly  550  to the left. As piston assembly  550  is moved to the left, power fluid is discharged from power chamber or cylinder  536  of second double-acting pump  502  on the other side of central piston  552  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to second spool valve  104  via power fluid exhaust port  548 , power fluid exhaust line  574 , and fluid exhaust inlet  170  of second spool valve  104 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  166  of second spool valve  104 . 
   Referring now to  FIG. 8 , first spool valve  102  is shown in its second position and second spool valve  104  is shown in its first position. In this position, fluid control system  100  directs the flow of power fluid so that piston assembly  550  of second double-acting pump  504  is on a power stroke and piston assembly  522  of first double-acting pump  502  is on a precompression stroke. 
   Power fluid exhaust port  518  of first double-acting pump  502  is connected in fluid communication to a power fluid return T via power fluid exhaust line  562 , fluid exhaust inlet  114  of first spool valve  102 , and fluid return port  110  of first spool valve  102 . Power fluid intake port  516  of first double-acting pump  502  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via power fluid intake line  568 , branch  569  of power fluid intake line  568 , fluid supply port  122  of first spool valve  102 , and load port  128  of first spool valve  102 . Branch  569  of power fluid intake line  568  includes flow restriction  802 , e.g., an orifice, in the fluid flow path therethrough. Flow restriction  802  restricts the flow of power fluid through branch  569 . In some embodiments, flow restriction  802  may be a variable orifice. 
   Power fluid exhaust port  548  of second double-acting pump  504  is connected to a power fluid return T via power fluid exhaust line  574 , fluid exhaust inlet  170  of second spool valve  104 , and fluid return port  166  of second spool valve  104 . Power chamber or cylinder  536  of second double-acting pump  504  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid, via fluid control system  100 . Power fluid intake port  542  of second double-acting pump  504  is connected in fluid communication with fluid supply port  172  of second spool valve  104  via power fluid intake line  572 . Load port  130  of first spool valve  102  is in fluid communication with fluid supply port  172  of second spool valve  104  via connector port  138  of first spool valve  102 , fluid flow line  218 , and connector port  194  of second spool valve  104 . Accordingly, power fluid intake port  542  of second double-acting pump  504  is connected in fluid communication to a load P, e.g., a source of pressurized power fluid (not shown), via load port  130  of first spool valve  102 , connector port  138  of first spool valve  102 , fluid flow line  218 , connector port  194  of second spool valve  104 , fluid supply port  172  of second spool valve  104 , and power fluid intake line  572 . Power fluid intake port  542  of second double-acting pump  504  is also connected in fluid communication with a load P, e.g., a source of power fluid supply (not shown), via power fluid intake line  572 , branch  573  of power fluid intake line  572 , fluid supply port  174  of second spool valve  104 , and load port  184  of second spool valve  104 . However, branch  573  of power fluid intake line  572  includes flow restriction  702  that restricts the flow of power fluid through branch  573 . 
   In operation, as illustrated in  FIG. 8 , a fifth portion of the pressurized power fluid is directed to power chamber or cylinder  536  of second double-acting pump  504  via fluid control system  100 . The fifth portion of the pressurized power fluid from the load P (e.g., the source of the pressurized power fluid) is directed to second spool valve  104  via first spool valve  102 . The fifth portion of the pressurized power fluid directed to second spool valve  104  is directed to the power chamber or cylinder  536  of second double-acting pump  504 . More particularly, the fifth portion of the pressurized power fluid flows into first spool valve  102  via load port  130  and then into second spool valve  104  via connector port  138  of first spool valve  102 , fluid flow line  218 , and connector port  194  of second spool valve  102 . The fifth portion of the pressurized power fluid then flows into power chamber or cylinder  536  on one side of central piston  552  via fluid supply port  172  of second spool valve  104 , power fluid intake line  572 , and power fluid intake port  542 . The fifth portion of the pressurized power fluid directed to power chamber or cylinder  536  applies pressure on central piston  552  and moves piston assembly  550  to the left so that the piston assembly  550  of second double-acting pump is on a power stroke. As piston assembly  550  is moved to the left, power fluid is discharged from power chamber or cylinder  536  of double-acting intensifier pump  504  on the other side of central piston  552  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to second spool valve  104  via power fluid exhaust port  548 , power fluid exhaust line  574 , and fluid exhaust inlet  170  of second spool valve  104 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  166  of second spool valve  104 . Those of ordinary skill in the art will appreciate, that as piston assembly  550  is moved to the left high-pressure fluid is discharged from second double-acting pump  504  via valve assembly  558  (shown on  FIG. 5 ) of second double-acting pump  504  and the low-pressure fluid to be pumped enters second double-acting pump  504  valve assembly  560  (shown on  FIG. 5 ) of second double-acting pump  504 . At a predetermined point in the power stroke of piston assembly  550  of second double-acting pump  504 , e.g., when piston assembly  550  reaches the end of its stroke, second shifting assembly  582  (shown on  FIG. 5 ) shifts second spool valve  104  to its second position. As previously discussed, this may be accomplished mechanically, hydraulically, electrically, or by any other suitable mechanism. 
   Furthermore, while piston assembly  550  of second double-acting pump  504  is on a power stroke, piston assembly  522  of first double-acting pump  502  is on a precompression stroke. Accordingly, a sixth portion of the pressurized power fluid is directed to power chamber or cylinder  508  of first double-acting pump  502  via first spool valve  102 . More particularly, the sixth portion of the pressurized power fluid flows into first spool valve  102  via load port  128  of first spool valve  102  and then into power chamber or cylinder  508  on one side of central piston  524  via fluid supply port  122  of first spool valve, branch  569  of power fluid intake line  568 , power fluid intake line  568 , and power fluid intake port  516 . The flow of the sixth portion of the pressurized power fluid is restricted so that piston assembly  522  of first double-acting pump  502  is on a precompression stroke. To restrict the flow of the sixth portion of the pressurized power fluid into power chamber or cylinder  508  of first double-acting pump  502 , branch  569  of power fluid intake line  568  includes flow restriction  802  in the fluid flow path therethrough. Flow restriction  802  should be designed to control the pressure applied by the power fluid to central piston  524  so that the low-pressure fluid to be pumped in pumping cylinder  512  of first double-acting pump  502  is brought up to the desired operating pressure during the precompression stroke. During the precompression stroke, the sixth portion of the pressurized power fluid applies pressure to central piston  524  and piston assembly  522  may be moved to the right. As piston assembly  522  is moved to the right, power fluid is discharged from power chamber or cylinder  508  of first double-acting pump  502  on the other side of central piston  524  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to first spool valve  102  via power fluid exhaust port  518 , power fluid exhaust line  562  and fluid exhaust inlet  114  of first spool valve  102 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  110  of first spool valve  102 . 
   Referring now to  FIG. 9 , first spool valve  102  is shown in its second position and second spool valve  104  is shown in its second position. In this position, fluid control system  100  directs the flow of power fluid so that piston assembly  522  of first double-acting pump  502  is on a power stroke and piston assembly  550  of second double-acting pump  504  is on a precompression stroke. 
   Power fluid exhaust port  546  of second double-acting pump  504  is connected in fluid communication to a power fluid return T via power fluid exhaust line  570 , fluid exhaust inlet  168  of second spool valve  104 , and fluid return port  164  of second spool valve  104 . Power fluid intake port  544  of second double-acting pump  504  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via power fluid intake line  576 , branch  577  of power fluid intake line  576 , fluid supply port  176  of second spool valve  104 , and load port  180  of second spool valve  104 . Branch  577  of power fluid intake line  568  includes flow restriction  604  that restricts the flow of power fluid through branch  577 . 
   Power fluid exhaust port  518  of first double-acting pump  502  is connected to a power fluid return T via power fluid exhaust line  562 , fluid exhaust inlet  114  of first spool valve  102 , and fluid return port  110  of first spool valve  102 . Power chamber or cylinder  508  of first double-acting pump  502  is connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via fluid control system  100 . Power fluid intake port  516  of first double-acting pump  502  is connected in fluid communication with fluid supply port  124  of first spool valve  102  via power fluid intake line  568 . Load port  186  of second spool valve  104  is in fluid communication with fluid supply port  124  of first spool valve  102  via connector port  192  of second spool valve  104 , fluid flow line  214 , and connector port  134  of first spool valve  102 . Accordingly, power fluid intake port  516  of first double-acting pump  502  is connected in fluid communication to a load P, e.g., a source of pressurized power fluid (not shown), via load port  186  of second spool valve  104 , connector port  192  of second spool valve  104 , fluid flow line  214 , connector port  134  of first spool valve  102 , fluid supply port  124  of first spool valve  102 , and power fluid intake line  568 . Power fluid intake port  516  of first double-acting pump  502  is also connected in fluid communication with a load P, e.g., a source of pressurized power fluid (not shown), via power fluid intake line  568 , branch  569  of power fluid intake line  568 , fluid supply port  122  of first spool valve  102 , and load port  128  of first spool valve  102 . However, branch  569  of power fluid intake line  568  includes flow restriction  802  that restricts the flow of power fluid through branch  565 . 
   In operation, as illustrated in  FIG. 9 , a seventh portion of the pressurized power fluid is directed to power chamber or cylinder  508  of first double-acting pump  502  via fluid control system  100 . The seventh portion of the pressurized power fluid from the load P (e.g., the source of the pressurized power fluid) is directed to first spool valve  102  via second spool valve  104 . The seventh portion of the pressurized power fluid directed to first spool valve  102  is directed to the power chamber or cylinder  508  of first double-acting pump  502 . More particularly, the seventh portion of the pressurized power fluid flows into second spool valve  104  via load port  186  and then into first spool valve  102  via connector port  192  of second spool valve  104 , fluid flow line  214 , and connector port  134  of first spool valve  104 . The seventh portion of the pressurized power fluid then flows into power chamber or cylinder  508  on one side of central piston  524  via fluid supply port  124  of first spool valve  102 , power fluid intake line  568 , and power fluid intake port  516 . The seventh portion of the pressurized power fluid directed to power chamber or cylinder  508  applies pressure on central piston  524  and moves piston assembly  522  to the right so that piston assembly  522  of first double-acting pump  502  is on a power stroke. As piston assembly  522  is moved to the right, power fluid is discharged from power chamber or cylinder  508  of first double-acting pump  502  on the other side of central piston  524  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to first spool valve  102  via power fluid exhaust port  518 , power fluid exhaust line  562 , and fluid exhaust inlet  114  of first spool valve  102 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  110  of first spool valve  102 . Those of ordinary skill in the art will appreciate, that as piston assembly  522  is moved to the right high-pressure fluid is discharged from first double-acting pump  502  via valve assembly  532  (shown on  FIG. 5 ) of first double-acting pump  502  and the low-pressure fluid to be pumped enters first double-acting pump  502  via valve assembly  530  (shown on  FIG. 5 ) of first double-acting pump  502 . At a predetermined point in the power stroke of piston assembly  522  of first double-acting pump  502 , e.g., when piston assembly  522  reaches the end of its stroke, first shifting assembly  580  (shown on  FIG. 5 ) shifts first spool valve  104  to its first position. As previously discussed, this may be accomplished mechanically, hydraulically, electrically, or by any other suitable mechanism. 
   Furthermore, while the piston assembly  522  of first double-acting pump  502  is on a power stroke, piston assembly  550  of second double-acting pump  504  is on a precompression stroke. Accordingly, an eighth portion of the pressurized power fluid is directed to power chamber or cylinder  536  of second double-acting pump  504  via second spool valve  104 . More particularly, the eighth portion of the pressurized power fluid flows into second spool valve  104  via load port  180  of second spool valve  104  and then into power chamber or cylinder  536  on one side of central piston  552  via fluid supply port  176  of second spool valve  104 , branch  577  of power fluid intake line  576 , power fluid intake line  576 , and power fluid intake port  544 . The flow of the eighth portion of the pressurized power fluid is restricted so that piston assembly  550  of second double-acting pump  504  is on a precompression stroke. To restrict the flow of the eight portion of the pressurized power fluid into power chamber or cylinder  536  of second double-acting pump  504 , branch  577  of power fluid intake line  576  includes flow restriction  604  in the fluid flow path therethrough. Flow restriction  604  should be designed to control the pressure applied by the power fluid to central piston  552  so that the low-pressure fluid to be pumped in pumping cylinder  540  of second double-acting pump  504  is brought up to the desired operating pressure during the precompression stroke. During the precompression stroke, the eighth portion of the pressurized power fluid applies pressure to central piston  552  and may move piston assembly  550  to the right. As piston assembly  550  is moved to the right, power fluid is discharged from power chamber or cylinder  536  of second double-acting pump  502  on the other side of central piston  552  and directed to a power fluid return T via fluid control system  100 . More particularly, the discharged power fluid flows to second spool valve  104  via power fluid exhaust port  546 , power fluid exhaust line  570 , and fluid exhaust inlet  168  of second spool valve  104 . Next, the discharged power fluid flows to a power fluid return T via fluid return port  164  of second spool valve  104 . 
   Therefore, the present invention is well-adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. In particular, as those of skill in the art will appreciate, steps from the different methods disclosed herein can be combined in a different manner and order. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.