Patent Publication Number: US-2011073203-A1

Title: Flow channel switching device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2009-228638, filed Sep. 30, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     An embodiment described herein relates generally to a flow channel switching device for switching flow channels for water or oil etc. 
     BACKGROUND 
     Jpn. Pat. Appln. KOKAI Publication No. 9-52025 (hereinafter referred to as “Patent Document 1”), for example, discloses a reverse osmotic concentration apparatus that incorporates a switch valve as a flow channel switching device for switching flow channels for a high-pressure fluid. The reverse osmotic concentration apparatus is used as, for example, an apparatus for permitting seawater to pass through a reverse osmotic film and thereby be desalted. 
     The switch valve disclosed in Patent Document 1 is switched by a to-be-concentrated liquid pressurized by a double-acting pump. The to-be-concentrated liquid is, for example, seawater. The pressurized to-be-concentrated liquid is sent to a reverse osmotic film tank through the switched valve. At this time, an impermeant liquid of a relatively high pressure, which does not pass through the reverse osmotic film, is returned to the head-side cylinder chamber of the double-acting pump via the switch valve, where it is used as pressurizing energy for the to-be-concentrated liquid. 
     When the piston of the double-acting pump reaches the terminal wall of the cap-side cylinder chamber, then, the pump performs backward operation to retract the piston. As a result, a new to-be-concentrated liquid is fed from a to-be-concentrated liquid containing tank to the cap-side cylinder chamber, and the impermeant liquid used for pressurization is discharged from the head-side cylinder chamber via the switch valve. 
     The switch valve has a spool that is slidable along the inner peripheral wall of the valve chamber. The peripheral surface of the spool functions as a valve body for blocking flow channels when the spool is moving. Further, a circumferential groove for permitting the flow channels to communicate with each other is formed in the periphery of the spool. Namely, the spool of the switch valve is moved in accordance with changes in the pressure of a to-be-concentrated liquid pressurized by the double-acting pump, thereby opening and closing the channel of the to-be-concentrated liquid. 
     However, in the switch valve disclosed in Patent Document 1, since the flow channels are opened and closed by sliding the spool along the inner peripheral wall of the valve chamber, it is difficult to completely close the flow channels if a fluid of an extremely high pressure is handled as in a seawater desalting plant. Namely, if this switch valve is used in the seawater desalting plant, leakage of seawater may well occur. To avoid this, in the switch valve of Patent Document 1, an O-ring is provided on the periphery of the spool for preventing the leakage of seawater. 
     Further, in the switch valve of Patent Document 1, supply and discharge of the to-be-concentrated liquid are performed when the spool is reciprocated once, with the result that the liquid is intermittently fed to the reverse osmotic film tank. This reduces the operation rate of the device. To compensate for this disadvantage, if a pair of devices is connected to a single reverse osmotic film tank and is operated alternately, the entire system will be enlarged and its equipment cost will inevitably increase. 
     In addition, in the switch valve of Patent Document 1, since a plurality of flow channels are simultaneously opened and closed by moving the spool, the flow channels cannot be opened or closed at different times. Accordingly, the switching time of each flow channel cannot be finely adjusted in accordance with, for example, the pressure difference between the flows of the to-be-contracted liquid in the channels. Therefore, the flow channels cannot be smoothly switched. 
     BRIEF SUMMARY 
     It is an object of the invention to provide a flow channel switching device capable of smoothly switching flow channels for high-pressure fluid. 
     To attain the object, a flow channel switching device according to an embodiment comprises: an inlet port through which a high-pressure fluid is introduced; a high-pressure chamber which receives the high-pressure fluid introduced through the inlet port; a first hole and a second hole formed in walls of the high-pressure chamber; a first feed port which feeds the high-pressure fluid discharged from the high-pressure chamber through the first hole; a second feed port which feeds the high-pressure fluid discharged from the high-pressure chamber through the second hole; a first valve body and a second valve body which independently open and close the first and second holes, respectively; and a first actuator and a second actuator which independently drive the first and second valve bodies, respectively, and alternately feed the high-pressure fluid through the first and second feed holes, respectively. 
     Since in the embodiment, the first and second actuators are controlled to independently operate so as to alternately open and close the first and second holes of the high-pressure chamber using the first and second valve bodies, pressure loss of the high-pressure fluid and water hammer phenomenon can be prevented. As a result, flow channels for the high-pressure fluid can be switched smoothly. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a schematic block diagram illustrating a seawater desalting plant according to an embodiment; 
         FIG. 2  is a schematic block diagram illustrating the internal structure of a power recovery device incorporated in the seawater desalting plant of  FIG. 1 ; 
         FIG. 3  is a sectional view illustrating a five-port switch valve incorporated in the power recovery device of  FIG. 2 ; 
         FIG. 4  is a sectional view illustrating a state into which the state of the five-port switch valve shown in  FIG. 3  is switched; and 
         FIG. 5  is a schematic block diagram useful in explaining the operation of the power recovery device performed when the five-port switch valve is switched to the state shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment will be described in detail with reference to the accompanying drawings. 
     A five-port switch valve  70  according to the embodiment comprises a port  70   c  through which a high-pressure fluid is introduced, a chamber  71   c  which receives the fluid introduced through the port  70   c , holes  75   b  and  75   c  formed in walls of the chamber  71   c , a port  70   b  through which the high-pressure fluid flowing from the chamber  71   c  into the adjacent chamber  71   c  via the hole  75   b  is fed, a port  70   d  through which the high-pressure fluid flowing from the chamber  71   c  into the adjacent chamber  71   d  via the hole  75   c  is fed, two valve bodies  76  which independently open and close the two holes  75   b  and  75   c , and two actuators  72  and  73  which alternately feed the high-pressure fluid via the ports  70   b  and  70   d.    
       FIG. 1  is a schematic block diagram illustrating a seawater desalting plant  100  for converting seawater into plain water. As shown, in the seawater desalting plant  100 , the drawn seawater is subjected to a chemical treatment in a pre-process system  10 , and then fed to a safeguard filter  20  by a feed pump Pu 1  (Q 1 ). Part of the seawater passing through the safeguard filter  20  is fed to a high-pressure pump Pu 2  (Q 2 ), while the other part of the seawater is fed to a power recovery unit  30  (Q 5 ). Assume here that pressure P 3  of the seawater fed from the safeguard filter  20  is about 0.2 MPa. 
     The high-pressure pump Pu 2  boosts the pressure of the seawater fed from the safeguard filter  20 , and feeds the same to a high-pressure reverse osmotic (RO) film  40 . Pressure P 4  of the seawater boosted by the high-pressure RO film  40  is set to an appropriate value. The appropriate value depends upon the type of the high-pressure RO film  40 . In this embodiment, pressure P 4  is set to 6.0 MPa. 
     The high-pressure RO film  40  filters the seawater boosted and fed by the high-pressure pump Pu 2 . If the recovery rate of the high-pressure RO film  40  is 40%, the high-pressure RO film  40  produces 40% by volume of plain water and 60% by volume of highly concentrated brine. At this time, the pressure of the plain water passing through the high-pressure RO film  40  decreases to about 0.2 MPa (=P 3 ), while pressure P 6  of the highly concentrated brine is about 5.8 MPa. 
     The plain water filtered and discharged by the high-pressure RO film  40  is fed to a low-pressure pump Pu 4  (Q plain water), and the highly concentrated brine that did not pass through the high-pressure RO film  40  is fed to the power recovery unit  30  with its pressure unchanged (Q brine). 
     The low-pressure pump Pu 4  re-pressurizes the plain water discharged from the high-pressure RO film  40 , and feeds the resultant water to a low-pressure RO film  50 . The plain water filtered by the low-pressure RO film  50  and having, for example, contained boron eliminated is fed to a clean water reservoir  60 . The plain water fed to the clean water reservoir  60  is subjected to a chemical treatment, and then supplied as clean water to users via a supply pump Pu 5 . 
     On the other hand, substantially plain water, which passed through the high-pressure RO film  40  but did not pass through the low-pressure RO film  50 , is returned to the pre-process system  10  and re-fed to the seawater desalting plant  100 . 
     The power recover unit  30  utilizes the pressure of the brine, fed from the high-pressure RO film  40 , to further boost the pressure of the seawater fed via the safeguard filter  20 , as will be described later. The seawater boosted by the power recover unit  30  is fed to a boosting pump Pu 3  (Q brine), where it is further boosted to a desired pressure (=P 4 ). 
     The seawater adjusted to the desired pressure by and fed from the boosting pump Pu 3  is fed to the high-pressure RO film  40  (Qin), together with the seawater (Q 2 ) boosted by the high-pressure pump Pu 2 . 
     On the other hand, the brine obtained after being fed to the power recovery unit  30  and utilized to boost the pressure of the seawater is discharged from the power recovery unit  30  with its pressure reduced to substantially the atmospheric pressure. 
       FIG. 2  schematically shows the internal structure of the power recovery unit  30 . 
     As shown, the power recovery unit  30  comprises a five-port switch valve  70 , pressure converters  31  and  32 , a seawater supply unit  34 , and a controller  36 . The five-port switch valve  70  functions as the fluid switching unit of the invention. The controller  36  monitors the operation states of the two pressure converters  31  and  32  to thereby control the switching of the five-port switch valve  70 . 
     As shown in  FIG. 3 , the five-port switch valve  70  comprises a cylinder with five ports  70   a ,  70   b ,  70   c ,  70   d  and  70   e  formed through the peripheral wall of the valve, and two actuators  72  and  73  secured to the opposite ends of the cylinder  71 . 
     The upper actuator  72  functions as a first actuator, while the lower actuator  73  functions as a second actuator. The invention is not limited to use the actuators  72  and  73 , but may use other driving mechanisms. 
     The five ports  70   a ,  70   b ,  70   c ,  70   d  and  70   e  may be formed in portions other than those shown in  FIG. 3 . The central portion  70   c  functions as an inlet port, the port  70   b  functions as a first feed port, and the port  70   d  functions as a second feed port. 
     The cylinder  71  has five cylindrical chambers  71   a ,  71   b ,  71   c ,  71   d  and  71   e , which communicate with the five ports  70   a ,  70   b ,  70   c ,  70   d  and  70   e . Four disk-shaped partitions  74   a ,  74   b ,  74   c  and  74   d  are provided between respective pairs of adjacent chambers. 
     The central chamber  71   c  functions as a high-pressure chamber, and the chambers  71   b  and  71   d  adjacent to the central chamber  71   c  function as first and second chambers, and the opposite end chambers  71   a  and  71   e  function as third and fourth chambers. 
     Circular holes  75   b  and  75   c  to be opened and closed by respective valve bodies  76 , described later, are formed in the central portions of the central partitions  74   b  and  74   c , respectively. Similarly, circular holes  75   a  and  75   d  that are opened and closed by the respective valve bodies  76 , and insert therein piston rods  72   b  and  73   b , respectively, are formed in the central portions of the two outer partitions  74   a  and  74   d , respectively. 
     The hole  75   b  of the partition  74   b  functions as a first hole, the hole  75   c  of the partition  74   c  functions as a second hole, the hole  75   a  of the partition  74   a  functions as a third hole, and the hole  75   d  of the partition  74   d  functions as a fourth hole. 
     The actuator  72  comprises a piston cylinder  72   a  secured to an end wall of the cylinder  71 , and a piston rod  72   b  inserted through the hole  75   a  of the partition  74   a . Similarly, the actuator  73  comprises a piston cylinder  73   a  secured to the other end wall of the cylinder  71 , and a piston rod  73   b  inserted through the hole  75   d  of the partition  74   d . A valve body  76  is provided as the distal end of the piston rod  72   b  for closing the holes  75   a  and  75   b  of the two partitions  74   a  and  74   b . Similarly, another valve body  76  is provided as the distal end of the piston rod  73   b  for closing the holes  75   c  and  75   d  of the two partitions  74   c  and  74   d.    
     The valve body  76  of the actuator  72  functions as a first valve body, and is positioned in the chamber  71   b . The valve body  76  of the actuator  73  functions as a second valve body, and is positioned in the chamber  71   d . Further, the respective pistons  77  movable within the piston cylinders  72   a  and  73   a  are provided as the proximal ends of the piston rods  72   b  and  73   b.    
     The actuators  72  and  73  are connected to respective pumps (not shown). Namely, air is alternately supplied into two pressure chambers  78  and  79  defined in the piston cylinders  72   a  and  73   a , respectively, thereby operating the pistons  77  to axially move the valve bodies  76  as the distal ends of the piston rods  72   b  and  73   b . Not only air pressure but also hydraulic pressure may be used to drive the actuators  72  and  73 . 
     When the actuator  72  is driven, the hole  75   a  formed in the partition  74   a  between the chambers  71   a  and  71   b  of the cylinder  71 , and the hole  75   b  formed in the partition  74   b  between the chambers  71   b  and  71   c  of the cylinder  71  are alternately opened and closed by the corresponding valve body  76 . Further, when the other the actuator  73  is driven, the hole  75   c  formed in the partition  74   c  between the chambers  71   c  and  71   d  of the cylinder  71 , and the hole  75   d  formed in the partition  74   d  between the chambers  71   c  and  71   d  of the cylinder  71  are alternately opened and closed by the corresponding valve body  76 . 
     In the embodiment, the two valve bodies  76  are independently openable and closable. The valve bodies  76  function as grove valves that axially move with respect to the holes  75   a ,  75   b ,  75   c  and  75   d  of the partitions  74   a ,  74   b ,  74   c  and  74   d  to open and close the holes. This valve structure is suitable for the seawater desalting plant  100  that handles high-pressure fluid. 
     Brine is introduced from the high-pressure RO film  40  into the port  70   c  communicating with the central chamber  71   c  of the cylinder  71 . Further, the brine introduced into the central chamber  71   c  through the port  70   c  is alternately fed into the pressure converters  31  and  32  through the ports  70   b  and  70   d  communicating with the two chambers  71   b  and  71   d  adjacent to the central chamber  71   c . The ports  70   a  and  70   e  communicating with the opposite end chambers  71   b  and  71   d  are joined together downstream of the switch valve  70 , and are used to discharge the respective flows of brine fed from the pressure converters  31  and  32  with their pressures reduced. 
     Returning to  FIG. 2 , the pressure converters  31  and  32  comprise cylinders  31   a  and  32   a , and pistons  31   b  and  32   b  for defining axial chambers in the cylinders  31   a  and  32   a , respectively. The piston  31   b  axially moves to offset the pressure difference between the two pressure chambers  31   c  and  31   d  defined on the opposite sides of the piston  31   b . Similarly, the piston  32   b  axially moves to offset the pressure difference between the two pressure chambers  32   c  and  32   d  defined on the opposite sides of the piston  32   b . The pressure chambers  31   c  and  32   c  are connected to the ports  70   b  and  70   d  of the five-port switch valve  70 , respectively. The other pressure chambers  31   d  and  32   d  are connected to a seawater feed unit  34 , described later. Namely, brine is fed into the pressure chambers  31   c  and  32   c , while seawater is fed into the other pressure chambers  31   d  and  32   d . In other words, the seawater and brine are prevented from mixing in the pressure converters  31  and  32 . 
     The seawater feed unit  34  comprises four check valves  34   a ,  34   b ,  34   c  and  34   d  connected in series. The check valves  34   a ,  34   b ,  34   c  and  34   d  open and close independently of each other in accordance with the pressure difference between the opposite ends of each valve. Namely, the four check valves  34   a ,  34   b ,  34   c  and  34   d  feed the seawater from the safeguard filter  20  into the pressure chambers  31   d  and  32   d  of the pressure converters  31  and  32 , and feed the seawater from the pressure chambers  31   d  and  32   d  into the above-mentioned boosting pump Pu 3 . 
     The controller  36  monitors the operation of the two pressure converters  31  and  32  and independently controls the two actuators  72  and  73  of the five-port switch valve  70 . 
     Referring now to  FIGS. 2 to 5 , a description will be given of the operation of the power recovery unit  30  constructed as the above, and the operation of the five-port switch valve  70 . 
     As indicated by the arrows in  FIG. 3 , the brine fed from the high-pressure RO film  40  at relative high pressure P 6  (=5.8 MPa) flows into the central chamber  71   c  via the central port  70   c  of the five-port switch valve  70 . At this time, if the controller  36  switches the two actuators  72  and  73  to the state shown in  FIG. 3 , the brine in the chamber  71   c  flows into the adjacent chamber  71   b  through the hole  75   b  of the partition  74   b , and then into the pressure chamber  31   c  of the pressure converter  31  via the port  70   b.    
     The state in which the controller  36  moves the two valve bodies  76  to the positions shown in  FIG. 3  will be hereinafter referred to as “the first state.” In the first state, the controller  36  sets the actuator  72  so that the corresponding valve body  76  blocks the hole  75   a  of the partition  74   a , and sets the other actuator  73  so that the corresponding valve body  76  blocks the hole  75   c  of the partition  74   c.    
     The brine flowing into the pressure chamber  31   c  of the pressure converter  31  pushes, by the pressure difference between the pressure chambers  31   c  and  31   d , the piston  31   b  in the direction indicated by the arrow in  FIG. 2  to thereby push the seawater filled in the pressure chamber  31   d  into the seawater supply unit  34 . 
     At this time, pressure P 8  of the seawater pushed out of the pressure chamber  31   d  is slightly reduced to about 5.75 MPa by the friction of the piston  31   b.    
     Pressure P 3  of the seawater fed from the safeguard filter  20  into the seawater supply unit  34  is about 0.2 MPa as mentioned above. Accordingly, when the five-port switch valve  70  is in the first state, the check valve  34   a  is closed by the difference between the pressures P 3  and P 8 . 
     Further, at this time, since pressure P 11  between the check valves  34   b  and  34   c  is maintained at substantially the same pressure as pressure P 8  as described later, the check valve  34   b  is opened. Furthermore, since pressure P 13  between the check valves  34   c  and  34   d  is close to the atmospheric pressure as described later, the check valve  34   c  is closed by the difference between pressures P 11  and P 13 . 
     As a result, the brine pushed out of the pressure chamber  31   d  by a pressure of about 5.75 MPa is fed to the boosting pump Pu 3  through the check valve  34   b . The boosting pump Pu 3  slightly boosts the pressure of the brine from the pressure chamber  31   d  (5.75 MPa→6.0 MPa) and feeds the resultant brine to the high-pressure RO film  40 . Namely, the power recovery unit  30  converts, using the pressure converter  31 , the energy of the brine discharged from the high-pressure RO film  40  into the energy for boosting seawater. 
     Referring back to  FIG. 3 , in the above-mentioned first state, the valve body  76  of the other actuator  73  blocks the hold  75   c  of the partition  74   c . In other words, in the first state, the hole  75   d  of the partition  75   d  is open to thereby connect the chambers  71   d  and  71   e . As described above, in the first state, the chamber  71   e  is set at substantially the same pressure as the atmospheric pressure via the port  70   e , and hence the chamber  71   d  is also set at substantially the same pressure as the atmospheric pressure. Further, the pressure chamber  32   c  of the pressure converter  32 , which communicates with the chamber  71   d  via the port  70   d , is open to the atmosphere. 
     On the other hand, the seawater fed from the safeguard filter  20  to the seawater supply unit  34  under pressure P 3  is further fed into the pressure chamber  32   d  of the pressure converter  32  via the check valve  34   d . At this time, the pressure in the pressure chamber  32   d  becomes slightly higher than that in the pressure chamber  32   c , whereby the piston  32   b  is moved in the direction indicated by the arrow of  FIG. 2 . As a result, the pressure in the pressure chamber  32   d  also becomes close to the atmospheric pressure. At the same time, the brine filled in the pressure chamber  32   c  is pushed by the piston  32   b  to the five-port switch valve  70 . 
     At this time, the check valve  34   d  is opened by the difference between pressure P 3  and the pressure in the pressure chamber  32   d , thereby permitting the seawater to flow therethrough. The check valves  34   a  and  34   c  are closed by the pressure of the brine as mentioned above. Accordingly, the seawater fed from the safeguard filter  20  flows into the pressure chamber  32   d  through the check valve  34   d.    
     As described above, in the first state in which the two actuators  72  and  73  of the five-port switch valve  70  assume the positions shown in  FIG. 3 , the controller  36  monitors the positions of the pistons  31   b  and  32   b  of the pressure converters  31  and  32 , and independently controls the switching of the actuators  72  and  73  when each of the pistons  31   b  and  32   b  reaches an end. 
     Basically, when the piston  31   b  of the pressure converter  31  is pressed by the brine and the volume of the chamber  31   d  becomes substantially zero, the controller  36  switches the actuator  72  to the position shown in  FIG. 4 . As a result, the hole  75   a  of the partition  74   a  is opened, the hole  75   b  of the partition  74   b  is closed by the valve body  76 , thereby connecting the chambers  71   a  and  71   b.    
     Similarly, when the piston  32   b  of the other pressure converter  32  is pressed by the seawater and the volume of the chamber  32   d  becomes substantially zero, the controller  36  switches the actuator  73  to the position shown in  FIG. 4 . As a result, the hole  75   c  of the partition  74   c  is opened, the hole  75   d  of the partition  74   d  is closed by the valve body  76 , thereby connecting the chambers  71   c  and  71   d . The state in which the actuators  72  and  73  are set at the positions shown in  FIG. 4  will be hereinafter referred to as “the second state.” 
     However, since the five-port switch valve  70  of the embodiment can independently control the operations of the two actuators  72  and  73 , it is not always necessary to simultaneously switch the positions of the actuators  72  and  73 . Further, since there is a difference between the pressures applied to the pistons  31   b  and  32   b  of the two pressure converters  31  and  32 , the two pistons  31   b  and  32   b  may reach their respective ends at different times even if the positions of the two actuators  72  and  73  are simultaneously switched. 
     For instance, if the two actuators  72  and  73  are completely simultaneously switched from the positions shown in  FIG. 3  to those shown in  FIG. 4 , a state, in which the two valve bodies  76  block none of the check valves  75   a ,  75   b ,  75   c  and  75   d , will occur for a slight period immediately after the two valve bodies  76  start to move. In this case, the pressure in the central chamber  71   c  is reduced, which is regarded as a pressure loss. 
     To avoid such a disadvantage as the above, a method could be employed, in which, firstly, only the actuator  72  is switched from the position shown in  FIG. 3  to thereby block the hold  75   b  of the one partition  74   b  defining the central chamber  71   c , and then the other actuator  73  is switched to block the hold  75   c  of the other partition  74   c  defining the central chamber  71   c.    
     However, if the state, in which the corresponding valve body  76  blocks the hole  75   b  of the partition  74   b  and the other valve body  76  blocks the hole  75   c  of the partition  74   c , is prolonged, the pressure in the thus-sealed central chamber  71   c  increases, whereby a water hammer phenomenon may well occur in which when the valve body  76  blocking the hole  75   c  is opened as shown in  FIG. 4 , the brine pressurized in the chamber  71   c  will rapidly flow. 
     Furthermore, in the first state shown in  FIG. 3 , since the two communicating chambers  71   b  and  71   c  are kept under high pressure, it is easy to open the valve body  76  blocking the hole  75   c , whereas a relatively high torque is necessary to move the other valve body  76  to open the hole  75   a  of the partition  74   a . Namely, even if the controller  36  switches the two actuators  72  and  73  at desired times, there may occur a light difference between the movements of the two valve bodies  76 . 
     Therefore, in the embodiment, in consideration of the difference between the movements of the two valve bodies  76 , the operation times of the two actuators  72  and  73  are set so that almost simultaneously when the corresponding valve body  76  blocks the hole  75   b  of the partition  74   b , the other valve body  76  opens the hole  75   c  of the partition  74   c . This structure can minimize the above-mentioned pressure loss, prevent the water hammer phenomenon, and realize smooth switching of the five-port switch valve  70 . 
     Referring again to  FIG. 4 , after the controller  36  switches the five-port switch valve  70  to the second state, the high-pressure brine filling the central chamber  71   c  flows into the chamber  71   d  through the hole  75   c  of the partition  74   c , and then into the pressure chamber  32   c  of the other pressure converter  32  through the port  70   d.    
     In the state (i.e., the first state) assumed before the controller  36  switches the five-port switch valve  70  to the second state, the two pressure chambers  32   c  and  32   d  of the pressure converter  32  are under low pressures substantially equal to the atmospheric pressure. Accordingly, if high-pressure brine flows into the pressure chamber  32   c  of the pressure converter  32 , the piston  32   b  is pushed in the direction indicated by the arrow in  FIG. 5  as a result of the pressure difference between the chambers. 
     Consequently, the pressure of the seawater filled in the pressure chamber  33   d  is increased and the thus pressurized seawater is pushed into the seawater supply unit  34 . At this time, pressure P 13  of the seawater pushed from the pressure chamber  32   d  is slightly reduced to about 5.75 MPa by the friction of the piston  32   b.    
     On the other hand, pressure P 3  of the seawater fed from the safeguard filter  20  to the seawater supply unit  34  is about 0.2 MPa as described previously. Accordingly, in the second state in which the five-port switch valve  70  is switched as shown in  FIG. 4 , the check valve  34   d  is closed by the difference between pressure P 1  and pressure P 13 . 
     Further, at this time, since pressure P 11  assumed immediately before switching to the second state is substantially maintained at high, pressures P 13  and P 11  are substantially the same pressure, and hence the check valve  34   c  will be opened. Further, since pressure P 8  is set to a value substantially equal to the atmospheric pressure as will be described later, the check valve  34   b  is closed by the difference between pressures P 11  and P 8 . 
     Thus, the brine pushed from the pressure chamber  32   d  under about 5.75 MPa is fed into the booster pump Pu 3  through the check valve  34   c . The booster pump Pu 3  slightly boosts the pressure of the brine fed from the pressure chamber  32   d  (5.75 MPa→6.0 MPa), and feeds the resultant brine to the high-pressure RO film  40 . Namely, the power recovery unit  30  converts, using the pressure converter  32 , the energy of the brine discharged from the high-pressure RO film  40  into the energy for boosting seawater. 
     In contrast, in the second state shown in  FIG. 4 , the valve body  76  of the actuator  72  blocks the hole  75   b  of the partition  74   b  and opens the hole  75   a  of the partition  74   a , thereby connecting the two chambers  71   a  and  71   b . Since in this state, the port  70   a  is open to the atmosphere, the two communicating chambers  71   a  and  71   b  are also open to the atmosphere. Similarly, the pressure chamber  31   c  of the pressure converter  31  is open to the atmosphere through the port  70   b.    
     In the first state assumed before switching the five-port switch valve  70  to the second state, the other pressure chamber  31   d  of the pressure converter  31  is kept under high pressure. Accordingly, when the controller  36  switches the five-port switch valve  70  to the second state to cause the pressure in the pressure chamber  31   d  to be substantially equal to the atmospheric pressure, the piston  31   b  is moved in the direction indicated by the arrow shown in  FIG. 5  by the difference between the pressures in the pressure chambers  31   c  and  31   d . As a result, the pressure in the pressure chamber  31   d  is also reduced to a value close to the atmospheric pressure, and pressure P 8  is reduced to a value close to the atmospheric pressure. 
     Namely, at this time, the check valve  34   a  is opened by the difference between pressures P 3  and P 8 , whereby the seawater fed from the safeguard filter  20  flows into the pressure chamber  31   d  of the pressure converter  31  through the check valve  34   a.    
     After that, the controller  36  monitors the positions of the pistons  31   b  and  32   b  of the pressure converters  31  and  32 , and slightly moves, as mentioned above, the switching of the actuators  72  and  73  when each of the pistons  31   b  and  32   b  reaches an end, thereby switching the five-port switch valve  70  to the first state shown in  FIG. 3 . 
     As described above, the controller  36  of the power recovery unit  30  repeats the above-mentioned operations to alternately switch the five-port switch valve  70  between the first and second states, thereby re-feeding seawater to the high-pressure RO film  40  using the pressure of the brine fed from the high-pressure RO film  40  for boosting the pressure of the seawater. In the power recovery unit  30  of the embodiment, since the five-port switch valve  70  is operated in the manner as described above, the flow channels for brine having an extremely high pressure that is about 60 times higher than the atmospheric pressure can be smoothly switched without pressure loss and water hammer phenomenon. 
     In contrast, in the case of driving one actuator having the two valve bodies formed integral as one body, it is necessary to enhance the dimension accuracy of the two valve bodies  76 , and the holes  75   a ,  75   b ,  75   c  and  75   d  of the partitions  74   a ,  74   b ,  74   c  and  74   d , which inevitably increases the manufacturing cost of the five-port switch valve  70 . 
     If the dimension accuracy is degraded, clearances will be formed between the valve bodies  76 , and the holes  75   a ,  75   b ,  75   c  and  75   d , resulting in leakage of brine therethrough, i.e., in pressure loss. In particular, if the two valve bodies  76  are formed integral as one body, a clearance will be formed between one of the valve bodies and a hole, with the other valve body kept in contact with another hole. To avoid this, high accuracy of dimension is required. Further, if the two valve bodies  76  are formed as one body, they cannot be operated independently, with the result that the above-mentioned water hammer phenomenon cannot be avoided. 
     This being so, in an apparatus that handles an extremely high pressure fluid, like the above-described seawater desalting plant  100 , it is advantageous to employ, as in the five-port switch valve  70 , actuators  72  and  73  that can independently drive two valve bodies  76  for opening and closing flow channels for a high-pressure fluid. 
     While a certain embodiment of the invention has been described, the embodiment has been presented by way of example only, and is not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention. 
     For instance, in the above-described embodiment, a description is given of the case where the invention is applied to the five-port switch valve  70  incorporated in the power recovery unit  30  of the seawater desalting plant  100 . However, the invention may be also applied to a valve for switching flow channels for another high-pressure fluid such as oil. 
     In addition, a method utilizing air pressure, water pressure, oil pressure or a solenoid coil is possible as a method of switching the positions of the actuators  72  and  73  of the five-port switch valve  70 . In particular, brine, seawater fed from the feed pump Pu 1 , or brine fed from the high-pressure pump Pu 2 , may be used as a hydraulic source used as a power source for switching the positions of the actuators  72  and  73 .