Abstract:
A check valve system and method control the opening and closure of a check valve that supplies product fluid to an intensifier pump based on the position of a piston within the intensifier pump. Position sensing allows anticipation of different events along the path traveled by the piston, such as the start and end of advance, retract, and precompression cycles. The system and method operate to selectively open and close associated check valves based on the sensed position to carefully control the delivery of fluid to each intensifier pump. Active control of the check valves based on piston position allows more precise timing of fluid delivery in relation to the piston cycles. Anticipation of the onset of piston advance and retraction cycles can improve valve response time, providing more uniform fluid pressure for a continuous, steady, high pressure flow of fluid with minimal pressure fluctuation.

Description:
TECHNICAL FIELD 
     The present invention relates to valves and, more particularly, to check valve systems for use with intensifier pumps. 
     BACKGROUND INFORMATION 
     Hydraulic intensifier pumps are widely used in applications requiring the delivery of a high pressure jet of fluid. An intensifier pump includes a pump cylinder, a hydraulic working piston, a product intensifier piston, an inlet for the hydraulic working fluid, an inlet for the product fluid to be pressurized, and an outlet for the pressurized fluid. In operation, lower pressure hydraulic fluid is applied to the comparatively large working piston. The working piston, in turn, drives the smaller intensifier piston. The ratio of the hydraulic and product piston areas is the intensification ratio. The hydraulic pressure is multiplied by the intensification ratio to produce an increase in pressure. 
     The fluid to be intensified typically is delivered to the intensifier via an inlet check valve from a low pressure fluid supply pump. The fluid supply pump generally is able to generate sufficient pressure to overcome the tension of an internal poppet spring within the check valve, opening the check valve when the intensifier is in the retraction cycle and allowing product fluid to be delivered to the intensifier cylinder. When the piston begins its advance cycle to expel the pressurized fluid, the higher pressure of the intensified product fluid overcomes the lower supply pressure, closing the inlet check valve and thereby preventing backflow of the intensified fluid into the low pressure supply side of the pump. Many intensifier systems incorporate two or more single acting, single ended intensifier pumps, or two double intensifier pumps, that advance and retract on an alternating basis to provide a substantially continuous fluid jet. When one product intensifier piston retracts, the other advances. The relative timing of the advance and retraction cycles is carefully controlled to provide a substantially constant fluid pressure. Nevertheless, intensifier systems incorporating multiple single or double-acting intensifier pumps typically exhibit minor pressure fluctuations. 
     For industrial applications requiring precise fluid delivery, pressure fluctuation can be highly undesirable. For example, in processing of dispersions, emulsions, liposomes, and the like, the total amount of work, or energy, being applied is a function of both the mechanical power, or shear, and the time the product is in the shear zone. Further, in order to effectively process dispersions, the energy level must be sufficiently high and uniform to disperse agglomerate structure. A gradient of energy levels being applied to a dispersion, a result of processes having pulsation, will result in some of the product being subjected to insufficient processing. Continued processing of the product, under conditions where pulsations exist, cannot compensate for the gradient of energy levels that is less than the energy level required. Other applications that suffer from pulsation include the processing and pumping of coating solutions to a coating process such as a dual layer coating die. 
     SUMMARY 
     The present invention is directed to a high pressure check valve system useful with an intensifier pump. The check valve system is particularly useful in an intensifier pump system designed to be pulsation free, or “pipless.” The check valve system includes a controller that controls the check valve based on the position of a piston within the intensifier pump barrel. The present invention also is directed to an intensifier pump system incorporating such a check valve system, as well as a method for controlling a check valve and an intensifier pump system based on the position of a piston within the intensifier pump barrel. 
     A system and method, in accordance with the present invention, preferably senses a continuous position of one or more intensifier pistons during operation. The term “continuous position,” as used herein, means the position of a hydraulic working piston or product intensifier piston at one of several points along the path traveled by the piston, in contrast to sensing merely a single termination or proximity point, e.g., at the end of a cycle. Continuous position sensing allows anticipation of different events along the path traveled by the piston, such as the start or end of a cycle. In some embodiments, however, use of a proximity sensor may be acceptable. 
     The position of the product intensifier piston may be sensed directly. Alternatively, the position of the hydraulic working position may be sensed as an indication of the position of the product intensifier piston. In other words, the position of the hydraulic working piston will provide an indirect indication of the position of the product intensifier piston. The system and method operate to selectively open and close associated inlet check valves based on the sensed position to carefully control the delivery of product fluid to each intensifier pump. Active control of the check valves based on continuous piston position allows more precise timing of fluid delivery in relation to advance, retraction, and preload stages of the piston cycle. Anticipation of the onset of piston advance and retraction cycles can improve valve response time, providing an actively controlled “smart” valve. Valve operation can be made more efficient, and can be tuned according to the characteristics of the valve and the product fluid. 
     With this check valve system and method, the operation of an intensifier pump can provide more uniform fluid pressure. For example, check valves associated with multiple single acting and double acting intensifier pumps can be coordinated to provide a continuous, steady, high pressure flow of product fluid with minimal pressure fluctuation. In addition, the check valves can be actively controlled with an actuator to provide increased initial closing force, increased seating pressures, and increased opening and closing speeds. Also, in some embodiments, actuation speed can be dynamically controlled by controlling the characteristics of the valve actuator. The result is a check valve having an accelerated response time, allowing precise synchronization with the intensifier piston. 
     With improved response time, the inlet check valve can be opened more quickly to increase the amount of fluid pumped to the intensifier cylinder during the retract cycle. In addition, the check valve can be closed more quickly, minimizing valve leakage upon initiation of the advance cycle of the intensifier piston. The inlet check valve can be particularly useful for applications involving the delivery of pigmented dispersions having higher viscosity levels or particulate structures. Active control based on continuous piston position permits the system to compensate for changes in the characteristics of the product being processed through the inlet check valves. 
     Knowledge of the continuous position of the product intensifier piston enables anticipation of an event such as, for example, the end of the advance cycle or the start of the retract cycle. This anticipation advantage allows check valve actuation to be finetuned according to intensifier pump operation. Also, negative effects on valve hysteresis resulting from product fluid characteristics such as high viscosities and particulate structures can be compensated by tuning check valve actuation. With relatively large opening and closing forces and active actuation, the valve system is able to function positively when encountering high viscosity dispersions having a wide particle size distribution, and need not be subject to a fixed spring bias response. 
     In one embodiment, the present invention provides a system for controlling the flow of fluid to an intensifier pump, the system comprising a check valve housing defining an inlet for communication with a fluid supply, an outlet for communication with the intensifier pump, and a fluid flow channel extending between the inlet and the outlet, a valve poppet that is movable within the fluid flow channel to open and close the flow channel, thereby controlling the flow of fluid to the intensifier pump, an actuator that moves the valve poppet within the fluid flow channel, a position sensor that senses a position of a piston within the intensifier pump, and a controller that controls the actuator to move the valve poppet based on the sensed position of the piston within the intensifier pump. 
     In another embodiment, the present invention provides an intensifier pump system comprising a first intensifier pump having a first piston, a first fluid inlet, and a first fluid outlet, a second intensifier pump having a second piston, a second fluid inlet, and a second fluid outlet, wherein the first and second outlets feed a common fluid flow line, a first check valve that controls the flow of fluid into the first fluid inlet, a second check valve that controls the flow of fluid into the second fluid inlet, a first position sensor that senses a position of the first piston within the first intensifier pump, a second position sensor that senses a position of the second piston within the second intensifier pump, and a controller that controls the first and second check valves based on the sensed positions of the first and second pistons. 
     In a further embodiment, the present invention provides a system for controlling the flow of fluid to an intensifier pump, the system comprising a check valve defining an inlet for communication with a fluid supply, an outlet for communication with the intensifier pump, and a fluid flow channel extending between the inlet and the outlet, a position sensor that senses a position of a piston within the intensifier pump, and a controller that opens and closes the check valve based on the sensed position of the piston within the intensifier pump. 
     In an added embodiment, the present invention provides a method for controlling the flow of fluid from a fluid supply to an intensifier pump via a check valve, the method comprising sensing a position of a piston within the intensifier pump, and controlling the check salve to selectively open and close based on the sensed position of the piston within the intensifier pump. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagram of a high pressure check valve system; 
     FIG. 2 a  is a is a conceptual diagram of an intensifier pump system incorporating a check valve system as shown in FIG. 1 and a linear position transmitter (LPT) arrangement for piston position sensing; 
     FIG. 2 b  is a conceptual diagram of another intensifier pump system incorporating a check valve system as shown in FIG. 1 and a linear variable displacement transducer (LVDT) for piston position sensing; 
     FIG. 3 is a graph illustrating operation of an intensifier pump in a system as shown in FIGS. 2 a  and  2   b;    
     FIG. 4 is graph illustrating operation of complementary intensifier pumps in a system as shown in FIGS. 2 a  and  2   b;    
     FIG. 5 is a graph illustrating operation of a check valve system as shown in FIG. 1; 
     FIG. 6 is a graph illustrating operation of check valve systems as shown in FIG. 1 in conjunction with complementary intensifier pumps as shown in FIGS. 2 a  and  2   b;  and 
     FIG. 7 is a flow diagram illustrating operation of a check valve system as shown in FIG.  1 . 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     FIG. 1 is a diagram of a high pressure check valve system  10  in accordance with an embodiment of the present invention. Valve system  10  may be particularly useful in the delivery of continuous, steady, high pressure flow of pigmented dispersions via an intensifier pump, where avoidance of significant pressure fluctuation is desirable. An example application is the delivery of coating compositions for manufacture of magnetic data storage media. In such an application, an intensifier pump may be used to deliver pigmented dispersions having abrasive materials with particles that range from submicron sizes to sizes that exceed those captured by a 60 mesh screen, at throughputs exceeding 2 gpm, and for periods of time exceeding 100 hours of operation. Typical fluid pressure may range from 0 psi to 40,000 psi, or greater, during each intensifier cycle. 
     As shown in FIG. 1, check valve system  10  includes a check valve  11  with a housing that includes a valve body  12 , a valve seat nut  14 , and a valve adapter  16 . Valve adapter  16  defines an inlet  18  for communication with a product fluid supply. Valve body  12  defines an outlet  20  for communication with an intensifier pump or other fluid destination. Valve body  12 , valve seat nut  14 , and valve adapter  16  together define a fluid flow channel  22  that extends between inlet  18  and outlet  20 . Check valve  11  further includes a valve poppet  24  that is movable within fluid flow channel  22  to open and close the flow channel, thereby controlling the flow of fluid from inlet  18  to outlet  20 . The structure of valve body  12 , including poppet  24 , may conform substantially to that of a valve disclosed in U.S. Pat. No. 5,482,077 to Serafin. Valve  11  need not incorporate a spring bias, however, for activation of poppet  24 . 
     An actuator  26  moves valve poppet  24  within fluid flow channel  22 . Actuator  26  may take the form of a shaft-like member having one end  28  that is coupled to an inlet side of poppet  24 . The opposite end  30  of actuator  26  is coupled to a piston  32  that is mounted in an air cylinder  34 . In operation, air cylinder  34  is controlled to selectively move actuator  26  up and down within flow channel  22 . Air cylinder  34  can be coupled to a pneumatic supply via one or more valves. One or more pneumatic solenoids associated with air cylinder  34  are actuated to open and close the valves, and thereby selectively actuate the actuator  26 . Piston  32  retracts and extends relative to air cylinder  34  to drive actuator  26 . In turn, actuator  26  moves poppet  24  up and down, sealing and unsealing the poppet against a valve seat o-ring  36 , to thereby open and close valve  11 . With actuator  26 , valve  11  does not require a spring to bias poppet  24  in a desired position. Instead, air cylinder  34  and piston  32  actively control the position of poppet  24 . 
     With further reference to FIG. 1, when check valve  11  is used to control product fluid delivery to an intensifier pump, a position sensor  38  preferably senses the continuous position of a piston within the intensifier pump. Monitoring of continuous piston position allows anticipation of the onset of piston advance and retraction cycles, improving response time of valve  11 . Based on the sensed position of the piston, a controller  40  controls actuator  26  to move valve poppet  24 . In particular, controller  40  controls air cylinder  34  to move piston  32  and thereby open and close valve  11 . In this manner, the operation of check valve  11  is actively controlled. The delivery of fluid to the intensifier pump can be controlled on a closed-loop basis in synchronization with the pumping cycle of the pump. As a result, check valve  11  can provide precise control of fluid delivery to the intensifier pump. In some embodiments, use of a proximity sensor may be acceptable. 
     A check valve  11  as shown in FIG. 1 provides a number of advantages. As a first example, active control and actuation of valve  11  via air cylinder  34  can provide the valve with increased initial closing force. Initial seating pressures of 400 to 700 psi at o-ring  36  can be readily achieved. To facilitate increased seating pressures, the area ratio between air cylinder  34  and o-ring  36  can be increased. Second, active control of valve  11  can increase the opening and closing speeds of the valve, relative to passive, spring-loaded valves. Third, actuation speed can be dynamically controlled by remotely adjusting the volume of air delivered to air cylinder  34 . Fourth, actuation speed can be further increased by selection of the pneumatic solenoid used to deliver air to air cylinder  34 . Specifically, a pneumatic solenoid with an increased actuation speed will likewise increase the actuation speed of air cylinder  34  and valve  11 . 
     FIG. 2 a  is a conceptual diagram of an intensifier pump system  42  incorporating a pair of high pressure check valve systems  10  as shown in FIG. 1. A check valve system  10  may be used in a system incorporating a single product intensifier piston. Multiple check valves and intensifier pistons can be coordinated, however, to provide substantially continuous high pressure flow in duplex or multiplex intensifier systems. With reference to FIG. 2 a,  system  42  includes a first intensifier  44  having a hydraulic cylinder  45  with a hydraulic working section  46  and a product intensifier barrel  48 . Intensifier barrel  48  has a significantly smaller diameter than that of working section  46 , promoting increased fluid pressure within the intensifier barrel. Working fluid delivered via an inlet  50  drives a working piston  52  along working section  46 . Working piston  52 , in turn, drives product intensifier piston  54  along intensifier barrel  48 . Intensifier barrel  48  receives product fluid via an inlet  55  and a check valve system  10   a.  Intensifier piston  54  expels product fluid from an outlet  56  and through a check valve  58  for delivery to a product outflow line  60 . 
     As further shown in FIG. 2 a,  system  42  includes a second intensifier  62  that conforms substantially to first intensifier  44 . In particular, second intensifier  62  has an intensifier cylinder  63  that includes a hydraulic working section  64  and product intensifier barrel  66 . Intensifiers  44 ,  62  further include retraction intensifiers  51 ,  61 , respectively. Working fluid delivered via an inlet  68  drives a hydraulic working piston  70  along working section  64 . Working piston  70  drives intensifier piston  72  along intensifier barrel  66  and within intensifier barrel  66 . Intensifier piston  72  expels fluid from an outlet  74  and through a check valve  76  for delivery to product outflow line  60 . Intensifier barrel  66  receives product fluid via an inlet  77  and check valve system  10   b . The advance and retract cycles of intensifiers  44 ,  62  are controlled by the delivery of hydraulic working fluid to hydraulic working barrels  46 ,  64 , respectively. Coordinated control of duplex intensifiers is well known in the art. 
     The operation of intensifiers  44 ,  62  is offset such that one intensifier advances under the force of hydraulic working fluid to deliver product fluid to outflow line  60  while the other retracts to fill with hydraulic working fluid and product fluid. Thus, intensifiers  44 ,  62  work in tandem to provide a substantially continuous flow of product fluid to product outflow line  60 . Check valve systems  10   a,    10   b  ensure the delivery of product fluid to intensifier barrels  48 ,  66 , respectively, in manner that promotes a substantially continuous flow of product fluid in product outflow line  60  and minimizes pressure fluctuations. As described with reference to FIG. 1, each check valve system  10   a,    10   b  includes, respectively, a check valve  11   a,    11   b  an air cylinder  34   a,    34   b,  a position sensor  38   a,    38   b,  and a controller  40   a,    40   b.    
     In the embodiment of FIG. 2 a,  each position sensor  38   a,    38   b  takes the form of a linear position transducer (LPT) that provides a continuous, accurate position of product pistons  54 ,  72  during the entire length of the piston cycle, allowing anticipation of the start or end of a particular cycle. Each LPT  38   a,    38   b,  as is well known, may include a rod that is physically coupled to a working piston  52 ,  70  or a product piston  54 ,  72 , respectively. Movement of the rod in response to movement of the respective piston is transduced by a potentiometer associated with LPT  38   a,    38   b  to indicate the position of product piston  54 ,  72 , respectively. Each LPT  38   a,    38   b  transmits a signal providing a voltage, current, or frequency that indicates the position to controllers  40   a,    40   b,  respectively. In some applications, the signal transmitted by LPT  38   a,    38   b  can be digitally encoded. 
     As an alternative, the position sensors can be realized by linear variable displacement transducers (LVDT). FIG. 2 b  illustrates the use of LVDT&#39;s  39   a,    39   b  in a system as shown in FIG. 2 a.  An LVDT requires no physical connection to pistons  52 ,  70  or  54 ,  72 . Instead, as is well known, the LVDT operates to sense position electromagnetically by reference to piston  52 ,  70  or  54 ,  72  or a component carried by the respective piston. In particular, the LVDT may include a core mounted on or within hydraulic piston  46 ,  64  and a coil mounted about the piston. Like the LPT, the LVDT produces a signal that varies with linear displacement of the respective piston. The signal can be digitally encoded, if desired. LPT and LVDT sensors are described herein for purposes of example and not limitation. Accordingly, other position sensors can be used to ascertain piston position. With either an LPT or LVDT, the sensed position provides an indication, directly or indirectly, of the continuous position of product pistons  54 ,  72 , thereby allowing synchronization of check valves  11   a,    11   b  with the product pistons to deliver fluid to intensifier barrels  48 ,  66 . 
     Also, such sensors may sense the position of either hydraulic working pistons  52 ,  70  or product intensifier pistons  54 ,  72 . Working pistons  52 ,  70  move together with intensifier pistons  54 ,  72 , respectively. Hence, the position of a working piston  52 ,  70  is indicative of the product intensifier piston  54 ,  72 , respectively. For an LPT, it may be most convenient to provide a physical connection to product pistons  54 ,  72 . With an LVDT, however, electromagnetic interaction with working pistons  52 ,  70  or product pistons  54 ,  72  can be readily achieved. In either case, the sensed position provides an indication, directly or indirectly, of the continuous position of product pistons  54 ,  72 , allowing synchronization of the check valves  11   a,    11   b  with the product pistons to deliver product fluid to intensifier barrels  48 ,  66 . 
     Controllers  40   a,    40   b  drive air cylinders  34   a,    34   b,  respectively, to actuate check valves  11   a,    11   b,  and control delivery of product fluid to intensifier barrels  48 ,  66 . Each controller  40   a,    40   b  may take the form of a programmable processor, microcontroller, or ASIC arranged to control check valves  11   a,    11   b.  If embodied as a processor, each controller  40   a,    40   b  may reside on a general purpose computer with a single- or multi-chip microprocessor such as a Pentium® processor, a Pentium Pro® processor, an 8051 processor, a MIPS processor, a Power PC® processor, or an Alpha® processor. Alternatively, the processor may take the form of any conventional special purpose microprocessor. As a further alternative, controller  40   a,    40   b  can be realized by discrete circuitry that processes position signals generated by position sensors  38   a,    38   b,  or  39   a,    39   b,  to generate control signals that drive air cylinders  34   a,    34   b  to open and close check valves  11   a,    11   b.  Thus, in contrast to microprocessor embodiments, controllers  40   a,    40   b  could be realized by simple circuitry embodiments that compare the position signals to reference levels. 
     Controllers  40   a,    40   b,  although represented separately in FIGS. 2 a  and  2   b,  can be realized by a single controller that operates in response to position signals from position sensors  38   a,    38   b  to control both check valve  11   a  and check valve  11   b.  In a processor embodiment, program code executed by controllers  40   a,    40   b  is arranged to drive air cylinders  34   a,    34   b  in a coordinated mode such that product fluid is fed to duplex intensifiers  44 ,  62  in an alternating fashion that is synchronized with the advance and retract cycles of pistons  54 ,  72 . By sensing the continuous position of working pistons  52 ,  70  or intensifier pistons  54 ,  72  via position sensors  38   a,    38   b  , controllers  40   a,    40   b  are capable of anticipating advance and retract cycles, and thereby optimizing the opening and closing of check valves  11   a,    11   b  to maximize product fluid volumes on the retract cycle and minimize leakage and backflow on the advance cycle. 
     FIG. 3 is a graph illustrating operation of an intensifier pump in a system as shown in FIGS. 2 a  and  2   b.  The graph of FIG. 3 plots time on the X axis versus position, as indicated by LPT voltage, on the Y axis. With reference to intensifier  62 , intensifier product piston  72  undertakes a retract cycle in which intensifier barrel  66  fills with product fluid. In the retract cycle, the product fluid is pumped via a low pressure supply pump through check valve  11   a  and inlet  77 . At the same time, hydraulic fluid is pumped into retraction intensifier  61 , thereby purging hydraulic cylinder  63  of hydraulic working fluid. Intensifier piston  72  then enters a precompression cycle and a stall stage prior to beginning an advance cycle. During the advance cycle, hydraulic cylinder  64  fills with working fluid, moving hydraulic piston  70  and product piston  72 . In the advance cycle, product piston  54  expels product fluid from intensifier barrel  66 . 
     FIG. 4 is a graph illustrating operation of complementary intensifiers  44 ,  62  operating in a duplex mode in a system as shown in FIGS. 2 a  and  2   b.  As shown in FIG. 4, intensifiers  44 ,  62  operate in an alternating manner such that one intensifier expels product fluid while the other takes in product fluid. Thus, the advance and retract cycles of intensifiers  44 ,  62  temporally overlap. In this manner, intensifiers  44 ,  62  together feed a substantially continuous flow of product fluid to outlet line  60 . The relative timing of intensifiers  44 ,  62  can be controlled by a system that modulates the delivery of working fluid via inlets  50 ,  68 . Such systems are well known in the art. Check valves  11   a,    11   b,  in accordance with the present invention, are controlled in synchronization with the movement of product intensifier pistons  54 ,  72 . 
     With further reference to FIG. 4, each intensifier  44 ,  62  has a cycle that includes the retract cycle, precompression cycle, and advance cycle. During the retract cycle for intensifier  44 , intensifier barrel  48  of intensifier  44  fills with product fluid. The next cycle, occurring at the start of the advance cycle, is the precompression cycle. During the precompression cycle, product fluid within intensifier barrel  48  is pumped, via intensifier product piston  54 , ramping up pressure until the pressure level is almost at the same level as that of the second intensifier  62 . At this point, product intensifier pistons  54 ,  72  are at almost the same pressure level. Consequently, product intensifier piston  54  effectively stops until the second intensifier piston  72  completes its advance cycle. Thus, intensifier piston  54  enters a momentary stall cycle. The final portion of the cycle is the advance cycle, in which the pressure of intensifier piston  54  exceeds that of intensifier piston  72 . Intensifier product piston  54  then expels the product fluid from intensifier barrel  48 . 
     FIG. 5 is a graph illustrating operation of a check valve  11   a  as shown in FIGS. 2 a  and  2   b  relative to the operation of an intensifier  44 . The operation of intensifier  44  is illustrated in terms of an LPT voltage indicating the position of pistons  52 ,  70 . The operation of check valve  11   a  is illustrated in terms of check valve pressure. As shown in FIG. 5, check valve  11   a  is actuated to deliver product fluid to the intensifier barrel  48  based on the continuous position signal provided by position sensor  38   a.  When the LPT signal indicates that the intensifier  44  is starting the retraction cycle, valve  11   a  is opened, as indicated by reference numeral  78 , allowing delivery of product fluid to fill intensifier barrel  48 . When the LPT signal indicates that intensifier  44  is ending the retraction cycle and entering the precompress cycle, valve  11   a  is closed as indicated by reference numeral  80 , terminating delivery of product fluid and preventing backflow of intensified fluid when the intensifier begins the advance cycle. 
     Again, the actuation of check valve  11   a  can be actively controlled based on the continuous position of product intensifier piston  54 , which is indicative of the intensifier piston cycle. In particular, the continuous position signal allows anticipation of an event, such as the advance cycle. This allows check valve  11   a  to be closed, for example, prior to the onset of the advance cycle. In this manner, active control of check valve  11   a  enables optimal filling of intensifier barrel  48  with product fluid during the retract cycle, and prevents fluid leakage and backflow during the advance cycle. Active control of check valve  11   a  also can provide enhanced response time and seating pressure. Such advantages make check valve system  10  especially useful with high viscosity dispersions having particulate structures and wide particle size distribution. In particular, check valve system  10  can be tuned to compensate for valve hysteresis resulting from product fluid variations. 
     Notably, an increased response time in opening check valve  11   a  can actually reduce the duration of the precompress cycle. When valve  11   a  is opened earlier in the retract cycle, the valve stays open longer. As a result, intensifier barrel  48  is able to take on a greater volume of product fluid. With a greater volume of product fluid, product intensifier barrel  48  is able to achieve target pressure more quickly in the precompress cycle. This results in a shorter time duration for the precompress cycle and a longer stall cycle. With more time allowed for product fluid to be pumped into product intensifier barrel  48 , a greater volume of product fluid is provided. A full intensifier barrel  48  is able to develop product pressure in less time than an intensifier barrel that is less full. 
     FIG. 6 is a graph illustrating operation of check valves  11   a,    11   b  as shown in FIGS. 2 a  and  2   b  in conjunction with duplex intensifiers  44 ,  62  as shown in FIG.  2 . Like FIG. 5, FIG. 6 illustrates intensifier operation in terms of intensifier piston position and check valve operation in terms of valve pressure. As illustrated by FIG. 6, check valves  11   a,    11   b  operate in an alternating manner, opening and closing in response to the sensed position of the respective working piston  52 ,  70 . Notably, system  42  is scalable such that multiple check valve systems  10  could be employed with multiple intensifiers. For example, check valve systems  10  could be applied to intensifier systems having three, four, or more intensifiers to optimize product fluid volumes and minimize leakage and backflow among the alternating intensifiers. Accordingly, application of check valve system  10  is not limited to intensifier systems having only one or two intensifiers. 
     FIG. 7 is a flow diagram illustrating operation of a check valve  11   a  as shown in FIGS. 2 a  and  2   b.  The flow diagram of FIG. 7 illustrates control of the actuation of check valve  11   a  based on the sensed position of product intensifier piston  54  as an indication of intensifier cycle position. In operation, controller  40   a  continuously samples the LPT signal generated by position sensor  38   a,  as indicated by block  82 , to obtain a continuous indication of the position of product piston  54 . If the LPT signal indicates that product piston  54  entered the precompress cycle and is in a stall condition, as indicated by block  84 , controller  40   a  drives air cylinder  34   a  to close valve  11   a  in anticipation of the advance cycle, as indicated by block  86 . Thus, valve  11   a  terminates delivery of product fluid to intensifier barrel  48  and closes to prevent leakage and backflow. 
     Meanwhile, controller  40   a  continues to sample the LPT signal, as indicated by loop  88  and block  82 . In the event the LPT signal generated by position sensor  38   a  does not indicate the precompress condition, controller  40   a  determines whether the product intensifier piston  54  has reached the end of the advance cycle, as indicated by block  90 . Valve  11   a  remains closed until the end of the advance cycle. When the LPT signal indicates that the product intensifier piston  54  has completed the advance cycle and is about to enter the retraction cycle, controller  40   a  activates air cylinder  34   a  to open valve  11   a,  as indicated by block  92 , and allow product fluid to flow into intensifier barrel  54 . Then, controller  40   a  continues to sample the LPT signal as indicated by loop  94  and block  82 . If the advance cycle is not complete, controller  40   a  continues to sample the LPT signal, as indicated by loop  96  and block  82 . This routine is generally continuous and operates in an alternating manner with valve system  10   b.    
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.