Patent Publication Number: US-10766009-B2

Title: Slurry injection system and method for operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 16/199,675 filed on Nov. 26, 2018, which is a continuation-in-part of application number Ser. No. 15/927,345, filed on Mar. 21, 2018 which is a continuation-in-part of application Ser. No. 15/888,133, filed Feb. 5, 2018, which is a non-provisional application of provisional application 62/457,447, filed Feb. 10, 2017, the disclosure of which is incorporated by reference herein. This application also relates to U.S. application Ser. No. 15/888,140 filed Feb. 5, 2018 and U.S. application Ser. No. 15/888,154 filed Feb. 5, 2018. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a slurry injection system, and, more specifically, to a method and system for pressurizing concentrated slurry for use in a continuous injection process. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Pumping of process fluids are used in many industries Process fluids may be pumped with various types of pumps such as centrifugal, positive displacement or use of a pressurized drive fluid acting upon the process fluid. A slurry is one type of process fluid used in a process. Slurries are typically abrasive in nature. Slurry pumps are used in many industries to provide the slurry into the process. Sand injection for hydraulic fracturing (fracking), high pressure coal slurry pipelines, mining, mineral processing, aggregate processing, and power generation all use slurry pumps. All of these industries are extremely cost competitive. A slurry pump must be reliable and durable to reduce the amount of down time for the various processes. 
     Hydraulic fracturing of gas and oil bearing formations requires high pressures typically up to 15,000 psi (103421 kPa) with flow rates up to 500 gallons per minute (1892 liters per minute). The total flow rate using multiple pumps may exceed 5,000 gallons per minute (18927 liters per minute). 
     Slurry pumps are subject to severe wear because of the abrasive nature of the slurry. Typically, slurry pumps display poor reliability, and therefore must be repaired or replaced often. This increases the overall process costs. It is desirable to reduce the overall process costs and increase the reliability of a slurry pump. 
     Other components of the hydraulic fracking system also have weaknesses due to the abrasive slurries travelling therethrough. Check valves, pipes, pipe joints and fittings can suffer rapid erosion and “wire drawing” caused by high velocity fluid. Further, pressure vessels may be used and if a large number of penetrations in the tanks are used, those places are also subject to cracking failure from stress concentrations and metal fatigue. 
     SUMMARY 
     The present disclosure is directed to an elongated tank that is used in a system injecting abrasive slurries into a very high-pressure process stream with minimal wear on the system components. The system provides high reliability due to the reduced amount of wear. 
     In one aspect of the disclosure, an elongated tank for a slurry injection system has a side wall disposed in a vertical direction and top wall. The tank further comprises an end cap coupled to the side wall comprising a slurry injection channel and defining a bottom surface of the tank. The bottom side is angled downward from the side wall toward the slurring injection channel. 
     In another aspect of the disclosure, a system for injecting slurry from a slurry source into a slurry injection site comprises a high pressure pump coupled to a water source and a first elongated tank comprising a first end having a first volume and a second end having a second volume. The first volume is separated from the second volume. A first pipe has a first end external to the first elongated tank. The first pipe extends to the first elongated tank so that a second end of the first pipe communicates clear fluid to the first volume. A second elongated tank comprises a first end having a third volume and a second end having a fourth volume, said third volume separated from the fourth volume. A second pipe has a first end external to the second elongated tank. The second pipe extends to the second elongated tank so that a second end of the second pipe communicates clear fluid to the second elongated tank and ends within the third volume. A plurality of slurry valves is fluidically coupled to the first elongated tank and the second elongated tank. The plurality of slurry valves having a first state, a second state and an intermediate state between the first state and the second state. In the first state, the plurality of slurry valves communicates high pressure slurry from the second volume to the slurry injection site and communicates low pressure slurry to the fourth volume. In the second state, the plurality of slurry valves communicates low pressure slurry to the second volume and high pressure slurry from the fourth volume to the slurry injection site. In the intermediate state the plurality of slurry valves communicates high pressure slurry simultaneously from the first elongated tank and the second elongated tank to the slurry injection site. A first clear fluid valve, in the first state, communicates high pressure clear fluid from the high pressure pump to the first volume and, in a second state, communicates high pressure clear fluid to the third volume. A second clear fluid valve, in the first state communicates low pressure clear fluid from the third volume and in the second state communicates low pressure clear fluid from the first volume. A pulsation damper disposed between the high pressure pump and the first clear fluid valve reducing a pressure reduction when the first clear fluid valve changes between the first state and the second state. 
     In another aspect of the disclosure, a slurry injection system comprises a low pressure clear fluid manifold, a high pressure clear fluid manifold, and a plurality of clear fluid pumps receiving low pressure clear fluid from the low pressure clear fluid manifold, pressurizing the low pressure clear fluid into high pressure clear fluid and communicating the high pressure clear fluid into the high pressure clear fluid manifold. A blender unit has low pressure slurry therein. The blender unit comprises an outlet communicating the low pressure slurry therethrough. A sensor system is coupled to the outlet and generates a flow rate signal of the low pressure slurry and a density signal corresponding to a density of the low pressure slurry. A mixer is in fluid communication with the high pressure clear fluid manifold and a slurry pressurizer. The slurry pressurizer is in fluid communication with the high pressure clear fluid manifold through a bypass pump, the mixer, the blender unit and the low pressure clear fluid manifold. The slurry pressurizer forms high pressure slurry by pressurizing the low pressure slurry from the blender unit using high pressure clear fluid from the high pressure clear fluid manifold and the bypass pump. The slurry pressurizer communicates high pressure slurry to the mixer and communicates low pressure fluid to the low pressure clear fluid manifold. The mixer mixes the high pressure slurry and high pressure clear fluid from the high pressure clear fluid manifold to form a mixture that is communicated to a slurry injection site. A controller, coupled to the bypass pump and the sensor system, controls an operation of the bypass pump in response to the flow rate signal and the density signal to control a density of high pressure slurry. 
     In yet another aspect of the disclosure, a method of operating a slurry injection system that has a low pressure clear fluid manifold, a high pressure clear fluid manifold, a plurality of clear fluid pumps, a blender unit having an outlet, a mixer, a bypass pump and a slurry pressurizer comprise receiving low pressure clear fluid from the low pressure clear fluid manifold at the plurality of clear fluid pumps; pressurizing the low pressure clear fluid into high pressure clear fluid at the plurality of clear fluid pumps; communicating the high pressure clear fluid into the high pressure clear fluid manifold from the plurality of clear fluid pumps; communicating high pressure clear fluid from the high pressure clear fluid manifold to the mixer; communicating the high pressure clear fluid from the high pressure clear fluid manifold to the slurry pressurizer using a bypass pump; pressurizing, by the slurry pressurizer, low pressure slurry from the blender unit using high pressure clear fluid from the high pressure clear fluid manifold to form high pressure slurry; communicating low pressure fluid from the slurry pressurizer to the low pressure clear fluid manifold; blending, at a mixer, the high pressure slurry with high pressure clear fluid to form a slurry mixture; and communicating the mixture to a slurry injection site. In one aspect of the disclosure, an elongated tank for a slurry injection system has a side wall disposed in a vertical direction and top wall. The tank further comprises an end cap coupled to the side wall comprising a slurry injection channel and defining a bottom surface of the tank. The bottom side is angled downward from the side wall toward the slurring injection channel. 
     In another aspect of the disclosure, a system for injecting slurry from a slurry source into a slurry injection site comprises a high pressure pump coupled to a water source and a first elongated tank comprising a first end having a first volume and a second end having a second volume. The first volume is separated from the second volume. A first pipe has a first end external to the first elongated tank. The first pipe extends to the first elongated tank so that a second end of the first pipe communicates clear fluid to the first volume. A second elongated tank comprises a first end having a third volume and a second end having a fourth volume, said third volume separated from the fourth volume. A second pipe has a first end external to the second elongated tank. The second pipe extends to the second elongated tank so that a second end of the second pipe communicates clear fluid to the second elongated tank and ends within the third volume. A plurality of slurry valves is fluidically coupled to the first elongated tank and the second elongated tank. The plurality of slurry valves having a first state, a second state and an intermediate state between the first state and the second state. In the first state, the plurality of slurry valves communicates high pressure slurry from the second volume to the slurry injection site and communicates low pressure slurry to the fourth volume. In the second state, the plurality of slurry valves communicates low pressure slurry to the second volume and high pressure slurry from the fourth volume to the slurry injection site. In the intermediate state the plurality of slurry valves communicates high pressure slurry simultaneously from the first elongated tank and the second elongated tank to the slurry injection site. A first clear fluid valve, in the first state, communicates high pressure clear fluid from the high pressure pump to the first volume and, in a second state, communicates high pressure clear fluid to the third volume. A second clear fluid valve, in the first state communicates low pressure clear fluid from the third volume and in the second state communicates low pressure clear fluid from the first volume. A pulsation damper disposed between the high pressure pump and the first clear fluid valve reducing a pressure reduction when the first clear fluid valve changes between the first state and the second state. 
     In one aspect of the disclosure, an elongated tank for a slurry injection system has a side wall disposed in a vertical direction and top wall. The tank further comprises an end cap coupled to the side wall comprising a slurry injection channel and defining a bottom surface of the tank. The bottom side is angled downward from the side wall toward the slurring injection channel. 
     In another aspect of the disclosure, a system for injecting slurry from a slurry source into a slurry injection site comprises a high pressure pump coupled to a water source and a first elongated tank comprising a first end having a first volume and a second end having a second volume. The first volume is separated from the second volume. A first pipe has a first end external to the first elongated tank. The first pipe extends to the first elongated tank so that a second end of the first pipe communicates clear fluid to the first volume. A second elongated tank comprises a first end having a third volume and a second end having a fourth volume, said third volume separated from the fourth volume. A second pipe has a first end external to the second elongated tank. The second pipe extends to the second elongated tank so that a second end of the second pipe communicates clear fluid to the second elongated tank and ends within the third volume. A plurality of slurry valves is fluidically coupled to the first elongated tank and the second elongated tank. The plurality of slurry valves having a first state, a second state and an intermediate state between the first state and the second state. In the first state, the plurality of slurry valves communicates high pressure slurry from the second volume to the slurry injection site and communicates low pressure slurry to the fourth volume. In the second state, the plurality of slurry valves communicates low pressure slurry to the second volume and high pressure slurry from the fourth volume to the slurry injection site. In the intermediate state the plurality of slurry valves communicates high pressure slurry simultaneously from the first elongated tank and the second elongated tank to the slurry injection site. A first clear fluid valve, in the first state, communicates high pressure clear fluid from the high pressure pump to the first volume and, in a second state, communicates high pressure clear fluid to the third volume. A second clear fluid valve, in the first state communicates low pressure clear fluid from the third volume and in the second state communicates low pressure clear fluid from the first volume. A pulsation damper disposed between the high pressure pump and the first clear fluid valve reducing a pressure reduction when the first clear fluid valve changes between the first state and the second state. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1A  is a schematic view of a slurry injection system according to a first example of the present disclosure. 
         FIG. 1B  is a schematic view of a slurry injection system according to a second example of the present disclosure. 
         FIG. 1C  is a side view of a horizontally disposed tank for use in the slurry injection system. 
         FIG. 1D  is a side view of an alternative configuration of a tank disposed horizontally. 
         FIG. 1E  is a cross-sectional view of the tank of  FIG. 1D . 
         FIG. 1F  is an alternative cross-sectional view of the tank of  FIG. 1D . 
         FIG. 1G  is a schematic view of a slurry injection system according to a third example of the present disclosure. 
         FIG. 1H  is a cross-sectional view of a piston formed according to the example of  FIG. 1G . 
         FIG. 1I  is a schematic of a first example of a one tank system with high pressure clear fluid depressurization. 
         FIG. 1J  is a schematic of a second example of a one tank system without high pressure clear fluid depressurization. 
         FIG. 1K  is a schematic of an alternate two tank slurry injection system. 
         FIG. 2A  is a cross-section of an exemplary three-way valve in a first position. 
         FIG. 2B  is a cross-section of the exemplary three-way valve in a second position. 
         FIG. 2C  is a cross-sectional view of the three way valve in an intermediate position. 
         FIG. 3A  is a second example of an exemplary three-way valve in a first position. 
         FIG. 3B  is a second example of the exemplary three-way valve in a second position. 
         FIG. 3C  is a cross-section of a first example of a two-way switch. 
         FIG. 3D  is a cross-section of a second example of two two-way switches having two housings and a common actuator in a first position. 
         FIG. 3E  is a cross-section of the second example of the two two-way switches of  FIG. 3D  in a second position. 
         FIG. 3F  is a cross-section of a third example of a two-way switch. 
         FIG. 3G  is a cross-section of a fourth example of two two-way switches having two housings with a common actuator in a first position. 
         FIG. 3H  is a cross-section of a fifth example of the two-way switches of  FIG. 3G  in a second position. 
         FIG. 3I  is a cross-sectional view of an alternate two-way switch in a closed position having a balance disk. 
         FIG. 3J  is a cross-sectional view of the two-way switch of  FIG. 3I  in an open position. 
         FIG. 4A  is a first example of a table for the various valve settings used during operation of the example of  FIG. 1A . 
         FIG. 4B  is a second example of a table for the various valve settings used during the operation of the example illustrated in  FIG. 1B . 
         FIG. 4C  is a third example of a table for the various valve settings corresponding to  FIG. 1E . 
         FIG. 4D  is a fourth example of a table for the various valve settings corresponding to  FIG. 1K . 
         FIG. 4E  is a plot of cylinder pressure versus time during the operation of  FIG. 1K . 
         FIG. 4F  is a plot of high pressure clear water flow of clear fluid versus time during the operation of  FIG. 1K . 
         FIG. 4G  is a plot of low pressure clear water flow of clear fluid versus time during the operation of  FIG. 1K . 
         FIG. 5A  is a flowchart for a first example of a method for operating the system of  FIGS. 1A and 1B . 
         FIG. 5B  is a flowchart for a second example of a method for operating the system of  FIG. 1K . 
         FIG. 6A  is a flowchart of a method for switching states between a first tank and a second tank injection slurry of  FIGS. 1A and 1B  B. 
         FIG. 6B  is a flowchart of a method for switching states between a first tank and a second tank injection slurry of  FIG. 1K . 
         FIG. 7A  is a timing chart of an injection system for a multiple unit slurry injection system with a dwell time. 
         FIG. 7B  is a timing chart for a single tank slurry injection system. 
         FIG. 8A  is a schematic of a slurry injection system disposed on trailers. 
         FIG. 8B  is an enlarged cross-sectional view of the static mixer of  FIG. 8A . 
         FIG. 8C  is a schematic of a second example of a slurry injection system having a bypass pump rather than valves for redirecting the fluid to the slurry pressurizer. 
         FIG. 8D  is a schematic of a third example of a slurry injection system having a slurry pressurizer bypass valve. 
         FIG. 8E  is a schematic view of a fourth example of a slurry injection system having a slurry pressurizer bypass valve disposed close to the input to the trailer. 
         FIG. 9A  is a flowchart of a method for operating the system of  FIG. 8A . 
         FIG. 9B  is a flowchart of a method for operating the system of  FIG. 8C . 
         FIG. 9C  is a flowchart of a method for operating the system illustrated in  FIG. 8D . 
         FIG. 9D  is a flowchart of a method for operating when an emergency condition is detected. 
         FIG. 9E  is a flowchart of a method for operating the system when a failure is detected. 
         FIG. 10A  is a top view of a slurry injection module. 
         FIG. 10B  is a side view of two slurry injection modules of  FIG. 10A . 
         FIG. 10C  is a top view of the slurry injection system of  FIG. 10B . 
         FIG. 10D  is a side view of the baseplates of the system of  FIG. 10C  joined together. 
         FIG. 11A  is a schematic view of the two tank slurry injection system of  FIG. 1K  with a modified tank design. 
         FIG. 11B  is an enlarged portion of the endcap of one of the tanks of the system of  FIG. 11A . 
         FIG. 11C  is a top cutaway view of the system of  FIGS. 11A and 11B . 
         FIG. 11D  is an enlarged portion of an alternative example of an endcap of one of the tanks suitable for use in the system of  FIG. 11A . 
         FIG. 11E  is a first alternate example of the end wall of a tank. 
         FIG. 11F  is a second alternate example of the end wall of a tank. 
         FIG. 11G  is a third alternate example of the end wall of a tank. 
         FIG. 11H  is a fourth alternate example of the end wall of a tank. 
         FIG. 11I  is a fourth alternate example of the end wall of a tank. 
         FIG. 11J  is a fourth alternate example of the end wall of a tank 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. The use of the words “low” and “high” are used relative to the pressures suitable for use in fracking. “Low” pressure is suitable for movement of fluids into or out of pipes. “High” pressure is on the order suitable for fracking which is typically thousands of pounds per square inch. 
     A slurry injection system  10  is illustrated for injecting slurry into a high pressure slurry injection site  12 . The injection system  10  may be used alone or in a multi-unit injection system such as the injection system unit  10 A illustrated in fluid communication with the injection site  12 . The injection systems  10 ,  10 A may be operated using a common controller  20  such as a programmable logic controller (PLC). The controller  20  may be used to control the plurality of valves within the injection system  10  and the injection system  10 A based on feedback from sensors such as flow rate sensors  22 ,  24 . The flow rate sensors  22 ,  24  generate a first flow rate signal and a second flow rate signal. 
     The injection system  10  is used for injecting slurry from a slurry source such as a slurry tank  30  using a slurry circulation pump  32 . The slurry, under low pressure from the circulation pump  32 , is communicated to a first tank  40 A and a second tank  40 B through a low pressure slurry inlet pipe  34 . As set forth below, the low pressure slurry is communicated to one tank at a time. 
     The first tank  40 A and the second tank  40 B may be cylindrical or elongated in shape and disposed in a vertical or any angle above horizontal manner as is illustrated. As will also be described below, it may be possible that the tanks may be positioned in a horizontal position. The tanks  40 A,  40 B have respective vertical longitudinal axes  42 A,  42 B, respectively. The tanks  40 A,  40 B each have a respective end cap  44 A,  44 B. The tanks  40 A,  40 B have respective first ends  46 A,  46 B and second ends  48 A,  48 B. The first end  46 A of the first tank  40 A has a first volume  50  of clear fluid and the second end  48 A of the first tank  40 A has a second volume  52  of slurry. The second tank  40 B has a third volume  54  of clear fluid at the second end  46 B and a fourth volume  56  of slurry at the second end  48 B. The volumes  50 ,  52 ,  54  and  56  vary during the process. 
     The tanks  40 A,  40 B each include a longitudinally extending pipe  60 A,  60 B. The pipes  60 A,  60 B may be coaxial with the longitudinal axes at  42 A,  42 B. Each pipe  60 A,  60 B may extend from outside of the respective end caps  44 A,  44 B through an opening  62 A,  62 B. The pipes  60 A,  60 B extend to the first ends  46 A,  46 B through the second ends  48 A,  46 B of the tanks  40 A,  40 B. 
     Flow distribution plates  64 A,  64 B may be disposed at the ends of the pipes  60 A,  60 B toward the first ends  46 A,  46 B of the tanks  40 A,  40 B. The flow distribution plates  64 A,  64 B distribute incoming clear fluid across the diameter of the tanks to minimize the mixing of clear fluid with the slurry. The flow distribution plates  64 A and  64 B helps to minimize mixing in systems using high density slurry and low density slurry. 
     Each tank  40 A,  40 B is separated by a separation region  68 A,  68 B. While the region may be a defined area, in a hydraulic fracturing configuration, clear fluid may be separated from slurry naturally due to the less dense nature of the clear fluid. Should some mixing occur, the concentration of the slurry may be compensated for this. In this example, the clear fluid is disposed at the first ends  46 A,  46 B of the tanks  40 A,  40 B. The slurry is disposed at the second ends  48 A,  48 B. The clear fluid may be water, water mixed with chemicals or slurry additives such as ethylene glycol or other types of hydraulic fluid. “Clear”, in this manner, refers to fluid that does not contain a significant amount of the particles of the slurry. 
     The slurry may contain various types or sizes of sand particles such as small quartz particles. The slurry may also include other types of chemicals to improve the lubrication and movement of the hydraulic fracturing particles therein. 
     The end caps  44 A,  44 B are affixed to the tanks  40 A,  40 B and may include conical portions  70 A,  70 B, respectively. The conical portions  70 A,  70 B may have a larger diameter toward the second volumes  52 ,  56  and taper to a smaller diameter at the bottom or outer end of the end caps  44 A,  44 B (longitudinally away from the second volumes  52 ,  56 ). The end caps  44 A,  44 B may also include cylindrical portions  72 A,  72 B that are coupled to a plurality of slurry valves. The plurality of slurry valves may include outlet valves  80 A,  80 B and inlet valves  82 A,  82 B. Valves  80 A,  80 B are used for communicating slurry under high pressure from the tanks  40 A,  40 B, respectively. In operation, the valves  80 A,  80 B may alternately communicate slurry from the second ends  48 A,  48 B of the respective tanks  40 A,  40 B. 
     Inlet valves  82 A,  82 B communicate fluid from the slurry inlet pipe  34  into the respective tanks  40 A,  40 B. The inlet valves  82 A,  82 B may operate alternately so that each of the valves  82 A,  82 B does not operate at the same time. 
     The valves  80 A through  82 B may be check valves that operate in the manner described below. That is, in general, one tank is receiving high pressure clear fluid to force high pressure slurry from the tank while the other tank is receiving low pressure slurry and removing low pressure clear fluid therefrom. 
     The injection site  12  has an injection manifold  94  that is in communication with a pipe  96  that extends from the check valve  80 A and a pipe  98  that extends from the check valve  80 B. 
     A plurality of clear fluid valves are used for communicating clear fluid to and from each of the tanks  40 A,  40 B and are in fluid communication with a high-pressure clear fluid pump  90  and a clear fluid tank  92 . 
     The clear fluid tank  92  supplies clear fluid to the high-pressure pump  90  through pipe  100  and the flow rate sensor  24 . A pipe  102  supplies pressurized clear fluid to a three-way valve  110 . The three-way valve  110  has an inlet port  110 A, a first outlet port  110 B and an outlet port  110 C. A pipe  112  fluidically communicates fluid from the outlet port  110 C to the injection manifold  94  (or pipe  96  which leads to the injection manifold  94 ). A pipe  114  communicates high pressure clear fluid from the outlet port  110 B to a second three-way valve  120 . The valves  110  and  120  may be referred to as high pressure valves. A bypass valve  124  may also communicate fluid from the high pressure pump  90  to the pipe  114  through pipe  126 . The valve  124  may be a two-way valve used to controllably pressurize port  110 B and pipe  114  during changing states. The operation of valve  124  will be described in further detail below. The three-way valve  110  communicates fluid from the inlet port  110 A to either the outlet port  110 B or the outlet port  110 C under control of the controller  20 . 
     The three-way valve  120  selectively communicates high pressure clear fluid from the inlet port  120 A to either the first outlet port  120 B or the second outlet port  120 C. Outlet port  120 B is in fluid communication with the pipe  60 A through pipe  128 . Outlet port  120 C is in fluid communication with the pipe  60 B through pipe  130 . Valve  120  is under control of the controller  20 . 
     A three-way valve  140  is used for selectively communicating low pressure clear fluid from the tanks  40 A and  40 B under control of the controller  20 . In particular, valve  140  has an inlet port  140 A in fluid communication with pipe  60 A through pipe  142 . The valve  140  also has an inlet port  140 B in fluid communication with pipe  60 B through pipe  144 . An outlet port  140 C of valve  140  is in fluid communication with the clear fluid tank  92 . The fluid from the outlet port  140 C is in fluid communication with the flow rate sensor  22  and a two-way flow control valve  146 . Both the valves  140  and  146  are controlled by the controller  20 . The amount of fluid communicated to the tank  92  may be controlled by selectively controlling the amount of fluid flowing through the valve  146 . The fluid communicated through the valves  140  and  146  is under low pressure as will be described in further detail below. The closed inlet port  140 A or  140 B of valve  140  is under high pressure due to the high pressure slurry operation. A pipe  148  communicates fluid from the valve  146  to the tank  92 . 
     A three-way valve  150  is also in communication with the tanks  40 A and  40 B. In particular, the valve  150  includes an inlet port  150 A in fluid communication with the pipe  60 A through pipe  152 . Inlet port  150 B is in fluid communication with the pipe  60 B through pipe  154 . Outlet port  150 C of valve  150  is in fluid communication with the pipe  148  through pipe  156 . The valve  150 , as will be described in more detail below, is used for equalization or reduction of the pressure within the tanks  40 A,  40 B. The valve  150  is used for communicating clear fluid from the first volume  50  and the third volume  54 . The valve  150  selectively communicates high pressure clear fluid from either the inlet port  150 A or port  150 B to the outlet port  150 C which ultimately communicates clear fluid through the pipe  156  to the tank  92 . The pipe  156  may be directly input into the tank  92  or fluidically coupled to pipe  14 B. The valve  150  is used to lower the pressure within the highly pressurized tanks prior to when the state of the valves  110 ,  120  is changed. Ultimately, the use of the valve  150  helps reduce the overall pressure and thus the effort to switch valves  110  and  120  is lower and potential for valve wear and erosion is reduced. 
     Pipes  60 A and  60 B may also be in fluid communication with a respective valve  160 A,  160 B. The valves  160 A and  160 B are at the lowest point of the respective pipes  60 A,  60 B and are used to purge air from the volume within the respective  40 A,  40 B. 
     A check valve  170  may also be in communication between the injection manifold  94  and the pump  90 . In the illustration, check valve  170  is fluidly coupled between the pipe  102  and the injection manifold  94 . The check valve  170  is used for directing the flow from the high pressure pump  90  to the injection manifold during upset conditions such as when the fluid paths, the high pressure valves or pipes associated therewith become blocked or equipment, such as the valves, fail. The check valve  170  may also include a spring  172 . The spring  172  keeps check valve  170  closed until the upstream pressure (at the pipe  102 ) exceeds the downstream pressure (in the pipe  96 ) by a certain amount. 
     The valves  110  and  120  may be referred to as high pressure clear fluid valves. Valve  124  may also be included as a high pressure clear fluid valve. Valves  140  and  146  may be referred to as low pressure clear fluid valves. The valve  150  sees a combination of high pressure at the inlet ports  150 A,  150 B and low pressure at the outlet port  150 C. The valve  124  is used for pressurizing the pipe  114  at a certain rate with high pressure clear fluid as will be described in more detail below. Collectively, the valves  110 ,  120 ,  124 ,  140  and  146  may be referred to as clear fluid valves. The plurality of clear fluid valves communicates both high pressure and low pressure fluid to and from the tanks  40 A and  40 B. 
     The valves  110 ,  120  and  140  are capable of handling extreme high pressures such as 15,000 psi (103421 kPa) at flow rates at hundreds of gallons per minute. A sealing force over 50,000 pounds (344737 kPa) may be provided against the valve seat due to an extremely high differential when present. However, the valve  150  may be used to lower the overall pressure during switching. The purpose of valve  110  is to isolate the system by diverting the high-pressure clear fluid flow to the slurry pipe  96  through pipe  112 . After system isolation, valve  150  can bleed of residual high pressure thereby placing components of the system at relatively low pressure. The operation of the valves will be described in more detail below. Although the valves are described as “three-way valves” and “two-way valves” and “check valves”, other types of valves may be substituted therein. The three-way valves may be implemented in a plurality of two-way valves. Of course, other types of valves may be substituted from the valves. Check valves may, for example, be two-way valves controlled by the controller  20 . 
     The valves  110 ,  120 ,  124 ,  140 ,  146  and  150  may be controlled by the controller  20  through the use of electrical signals therefrom. Other valves such as  160 A and  160 B, although not illustrated in communication with the controller  20 , may also be electrically controlled thereby. In addition to electrically, valves  110 ,  120 ,  124 ,  140 ,  146 ,  150 ,  160 A and  160 B may also operate hydraulically or pneumatically 
     Referring now to  FIG. 1B , an injection system  10 ′ is illustrated and labeled identically to that of  FIG. 1A . The system  10 ′ illustrated in  FIG. 1B  operates identically to that set forth in  FIG. 1A  and may also be part of a multi-unit system. The difference between the systems  10  and  10 ′ is the three-way valve  150  has been replaced by a pair of two-way valves  210 A and  210 B for pressure reduction. The two-way valves  210 A,  210 B are used for communicating high pressure clear fluid to the clear fluid tank  92 . The pipe  152  receives clear fluid from the pipe  60 A which in turn is communicated through pipe  156  to the clear fluid tank  92 . The valve  210 B receives clear fluid from the pipe  60 B through pipe  154 . The valve  210 B communicates clear fluid through the pipe  156  to the clear tank  92 . For simplicity in overall maintenance and the like, the two-way valves  210 A and  210  B may be identical to that of valve  124 . That way, maintenance is made easier due to the commonality of parts. The valves  210 A and  210 B all operate at a high differential and thus may need to be serviced more than the other valves in the system which operate with low pressure differentials between the inputs and the outputs. 
     The valves  210 A and  210 B are in electrical communication with the controller  20 . That is, the controller  20  may control the opening and closing of the valves  210 A,  210 B. As will be described in more detail below, the valves may be operated so that either  210 A or  210 B are open but not both. 
     Referring now to  FIG. 1C , the tank  40 A is disposed horizontally. The same reference numerals set forth in  FIGS. 1A and 1B  are used in  FIG. 1C . Because of the higher density of slurry within the tank  40 A, the denser slurry will settle toward the bottom of the tank  40 A. This results in the clear fluid therein pushing a greater amount of fluid near the top relative to the bottom of the tank in the horizontal direction. As is illustrated, the interface  68 A′ is generally at an angle where the top portion is closer to the end cap  44 A. 
     Referring now to  FIG. 1D , an alternative example of the horizontal tank  40 A is set forth. In this example, a plurality of partitions  180  are used to define a plurality of horizontal channels  182 . The partitions  180  help reduce the amount of departure of the interface  68 A″ from vertical. In this example, the partitions  180  are formed with a plurality of types that extend in the longitudinal direction. The pipes in this example are cylindrical in shape and have gaps  184  therebetween. The gaps  184  also define horizontal channels within the tank  40 A. 
     In  FIG. 1D , the end cap  44 A′ is modified to have an enlarged conical portion  70 A′. The conical portion  70 A′ extends in a horizontal direction so as to be in fluid communication with the horizontal channels  182 . That is, the diameter of the conical portion  70 A′ adjacent to the channels  182  has been increased. Thus, the conical portion  70 A′ redirects slurry both to and from the check valves  82 A and  82 B. 
     Referring now to  FIG. 1E , a cross-sectional view of one example of the tank  40 A is set forth. As can be seen, gaps  184  are disposed between the partitions  180  so that horizontal channels  182  are formed. The partitions  180  are cylindrical in shape and may be formed by pipes. The pipe  60 A provides clear fluid to the distributor plate  64 A and distributes the clear fluid through the channels  182  and gaps  184  formed by the partitions  180 . 
     Referring now to  FIG. 1F , a plurality of partitions are illustrated having radially extending walls  190  that extend from the pipe  60 A to the inner wall of the tank  40 A. The walls  190  divide the tank into pie-shaped sectors  192 . The pie-shaped sectors  192  may be further divided by a concentric wall  194 . The concentric wall  194  shares a center point with the tank  40 A and the pipe  60 A. The walls  190 ,  194  act as a partition to reduce the amount of displacement of the slurry in an angular manner as illustrated in  FIG. 1D . 
     Referring now to  FIGS. 1G and 1H , a slurry injection system  10 ″ has each of the tanks  40 A and  40 B with a physical divider between the clear fluid between the first volume  50  and the second volume  52  and between the third volume  54  and the fourth volume  56  rather than the regions  68 A,  68 B described above. The tank  40 A may be disposed horizontally ( FIGS. 1A  and B), vertically ( FIGS. 1C-F ) or at angles therebetween. The physical divider may be a piston  220 A that has a first opening  222  for receiving the pipe  60 A. A piston  220 B configured in the same manner may be used in tank  40 B around pipe  60 B. The piston  220 A is loosely fit around the pipe so that the piston  220 A can freely travel along the pipe  60 A as the pressure of the clear fluid changes. By providing the piston  220 A, a more complete separation of the clear fluid and the slurry is provided with less chance of mixing with the slurry. With very high fill and discharge rates, some turbulence may occur and the slurry may mix with the fluid in the configuration of  FIGS. 1A and 1B . Further, the tanks  40 A,  40 B may be oriented in a horizontal position. Providing pistons  220 A,  220 B also enables the use of the tanks in a horizontal to prevent the mixing of the different fluids within the tank. In tank  40 A, the piston  220 A is shown in a first position and dotted in a second position. Thus, the piston  220 A may move in a longitudinal direction parallel to the longitudinal axis  42 A of the tank. 
     When the piston  220 A reaches the end cap  44 A, a flap  230  disposed within an opening  232  may open by rotating at the hinge  234 . The flap  230  may be a spring-loaded flap and when a sufficient amount of pressure differential is formed between the two sides of the piston  220 A, the flap  230  may provide some clear fluid through the opening  232 . The hinge  234  may instead be spring-loaded to provide a resistance from the flap  230  opening until the piston  220  reaches the end cap  44 A of the tank  40 A. The dotted flap illustrates the open position of the flap  230 . It may be desirable to allow a small amount of clear fluid from volume  50  to travel through the conical portion  70 A into the cylindrical portion  72 A and into opening  62 A of the end cap  44 A so that the clear fluid flows just past check valve  80 A and into the pipe  96 . The conical shape minimizes flow turbulence of the slurry out of the tanks  40 A,  40 B. This allows the check valve  80 A to close within clear fluid and thus in a cleaner environment. Tank  40 B may also be operated in a similar manner in that the clear fluid may transmit through the piston  220 B so that the check valve  80 B closes in a cleaner environment. The force needed to open the flap  230  by overcoming the spring force may be relatively small. That is, only a few pounds per square inch may be sufficient to open the flap to allow the fluid to flush the check valves  80 A or  80 B. 
     Referring now to  FIG. 1J , the same reference numerals are used to indicate the same components as  FIGS. 1A-1D . In this example, only a single tank  40 A is provided with the attached components in the slurry injection  10 ′″. In this example, the tank  40 A may be configured in the same manner as set forth in  FIG. 1A or 1G  in that the separation region  68 A or the piston  220 A (of  FIGS. 1G and 1H ) may be set forth between the first volume  50  and the second volume  52 . 
     One difference between  FIGS. 1A-1H  and  FIG. 1G  is the lack of a slurry tank  30  and pump  32 . In this example, the check valve  82 A is in fluid communication with a low pressure slurry manifold  240 . The low pressure slurry manifold  240  may be a common source shared between multiple tanks in a multiple tank type system. Of course, a slurry tank  30  and pump  32  may be in communication with the low pressure slurry manifold  240 . Another difference between  FIG. 1E  and  FIGS. 1A-1H  is the lack of a high pressure pump  90  and a low pressure fluid tank  92 . A low pressure clear fluid manifold  242  is used to receive the low pressure fluid from the port  140 A′. A high pressure clear fluid manifold  244  is in communication with the valve  110 . Although the pump  90  is not illustrated in this example, a high pressure clear fluid pump may be used somewhere in the system for generating the high pressure within the high pressure clear fluid manifold  244 . 
     In this example, the flow rate sensor  24 ′ has been modified to be positioned downstream of the valve  110  rather than between the clear fluid tank and the pump as illustrated above. The flow rate sensor has been labeled  24 ′ to indicate its change of position. However, the flow rate sensor  24 ′ generates a flow signal corresponding to the amount of flow into the pipe  60 A and thus into the tank  40 A. 
     The valves  120  and  140  illustrated above have been changed from three-way valves to two-way valves and are indicated as valve  120 ′ and  140 ′. Port  120 B′ is in fluid communication with the pipe  60 A. Likewise, port  140 B′ is also in fluid communication with the pipe  60 A. Port  120 A′ is in fluid communication with the flow rate sensor  24 ′, pipe  114  and valve  124  and port  1106  of valve  110 . 
     The slurry injection system  10   iv  may be referred to as an asynchronous system. In the previous figures, the fill rate of given tank is no faster than the discharge rate of the second tank. However, by increasing the fill rate of the slurry, the slurry fill duration can be substantially reduced and the capacity rate to discharge high pressure slurry for each tank is increased. 
     In operation, the three-way valve  110  communicated fluid from port  110 A through  1106 . The high pressure fluid enters pipe  114  and flow rate sensor  24 ′. The valve  120 ′ communicates fluid from port  120 A′ to  120 B′ and into the pipe  60 A. It is presumed that the tank  40 A was previously filled with low pressure slurry. The high pressure clear fluid forces high pressure slurry through check valve  80 A and to the injection site  12  through the injection manifold  94 . As in a similar manner to that set forth above, when the flow rate sensor  24 ′ indicates the volume of clear fluid has flushed the slurry from the tank  40 A and, if desired, passed check valve  80 A, the three-way valve  110  is commanded to divert fluid from port  110 A to outlet port  110 C. Valve  210 A is open which depressurizes the pipe  60 A and the tank  40 A by communicating fluid to the low pressure clear fluid manifold  242 . Valve  120 ′ is closed and valve  140 ′ is open under low pressure. Thereafter the valve  210 A is closed. Low pressure slurry from the low pressure slurry manifold  240  opens the check valve  82 A to allow low pressure slurry to enter the tank  40 A and expel clear fluid through the pipe  60 A. The clear fluid leaves the tank  40 A and pipe  60 A through the open valve  140 ′. The opening and closing of the valves is under the control of the controller  20 . The flow rate sensor  22  is used to indirectly determine the amount of slurry that has entered the tank  40 A. When the desired amount of slurry as determined by monitoring the flow of clear fluid out of the tank is reached, valve  140 ′ is closed and valve  120 ′ is open. The valve  124  is open which directs high pressure clear fluid from the high pressure clear fluid manifold  244  to be communicated to the pipe  114  through the pipe  126 . After some pressure is built up in pipe  114 , valve  110  communicates high pressure clear fluid from port  110 A to port  1106  and valve  124  is closed. High pressure clear fluid then enters the pipe  60 A through valve  120 ′ and check valve  82 A is closed and check valve  80 A is open. 
     The valve  170  is configured in a similar manner to that described above relative to the spring  172 . That is, the valve  170  may also include a spring  172  in a similar manner to that set forth above. Valve  170  opens when a sufficient force is between the high pressure clear fluid manifold  244  and the pipe  96  or the injection site  12 . Again, the valve  170  is open when damage to the valve or other components of the system is present or flow conditions have been upset. The valve  170  should be open when the upstream pressure is about 100 pounds per square inch higher than the downstream pressure. This ensures that the valve will not open during normal operation. 
     Referring now to  FIG. 1J , the same components set forth in  FIG. 1E  are labeled in the same manner. In this example, the slurry injection system 10 IV  has valves  120 ′ and  140 ′ that are presumed to be robust enough to be switched under high pressure. Thus, there is no pressure relief using valves  110  and  124  as in the previous examples. In this example, the flow rate sensor  24 ′ measures the amount of high pressure clear fluid that is communicated to the pipe  60 A through the valve  120 ′. Valve  140 ′ is closed. Clear fluid displaces slurry through the check valve  80 A. When the amount of slurry through the check valve  80 A has reached clear fluid as determined by the flow rate sensor  24 ′ and thus the volume of fluid, the valve operation is reversed in that valve  120 ′ is closed while valve  140 ′ is open. This allows the tank  40 A to be depressurized and low pressure slurry is then communicated to the tank to restart the process. When the amount of clear fluid that leaves the tank corresponds to a desired amount of slurry being input to check valve  80 A, the valves  120 ′ and  140 ′ are reversed in operation. 
     Referring now to  FIG. 1K , a similar example to that set forth in  FIG. 1B  is set forth. A slurry injection system  10 ″ has a generally simpler layout. In this example, valves  210 A,  210 B and piping  152 ,  154  and  156  have been removed. Likewise, the valve  124 , piping  126 , and valve  110  have also been removed. Further, the three-way valve  140  has also been removed and replaced by two-way valves  246 A and  246 B. A first pipe  248 A fluidically connects valve  246 A and pipe  60 A. A pipe  248 B connects valves  246 A,  246 B and the flow meter  22  which ultimately is in fluid communication with the tanks  92 . A pipe  246 C couples the valve  246 B to pipe  60 B. The two-way valves  246 A,  246 B are used as a return path for low pressure clear fluid being returned to the tank  92  through the flow meter  22 , valve  146  and pipe  148 . 
     The 3-way valve  120  connects the high pressure pump to either tank  40 A or  40 B. The moment that the 3-way valve  120  begins to open, fluid begins to flow from the high pressure tank  40 A/B to the low pressure tank  40 B/A. In a fraction of a second, both tanks  40 A/B would be pressurized to a value roughly equal to 50% of the pressure of the pressurized tank before the 3-way valve  120  is fully opened. This is highly desirable because the high pressure pump  90  does not experience as great of a pressure drop during the valve switching process. 
     By adding a pulsation dampener  245 , the residual brief drop in pressure before the high pressure pump  90  can fully pressurize the lower pressure tank  40 A/B is reduced. The pulsation dampener  145  is disposed fluidically between the high pressure pump  90  and the three way valve  120 . In  FIG. 1K  the pulsation dampener  245  may be disposed in pipe  120  or pipe  126 . 
     In operation, tank  40 A is pressurized to 10,000 psi and 3-way valve  120  begins to open, there will be an immediate flow to tank  40 B which was at low pressure. This may be referred to as an intermediate state. Both tank  40 A and  40 B will quickly reach an equilibrium pressure of about 5,000 psi. HP pump  90  experiences the drop in discharge pressure until pressure in tank  40 B builds up to 10,000 psi. At that time check valve  80 B would open thus allowing slurry flow to be established. Pulsation dampener  245  would reduce the severity and length of this pressure reduction to a negligible level. 
     In this example, the pump  90  is in communication with the three-way valve  120  and, in particular, port  120 C through pipe  126 . As will be described below, the valve  120  may also have an intermediate state between the first state and the second state. That is, the three-way valve  120  typically communicates fluid to either pipe  60 A or  60 B. However, in the intermediate position, the valve  120  may communicate fluid to both pipes  60 A and  60 B for a predetermined time period through ports  120 A and  120 B, respectively. A configuration of the valve  120  is set forth in further detail below in  FIG. 2C . 
     Referring now to  FIGS. 2A and 2B , a valve  250  suitable for use as valve  110  or  120  in  FIGS. 1A-1C  is set forth. The valve  250  has a housing  252  which may be cylindrical in shape. The housing  252  includes an inlet or central port  254 , a first outlet port  256  and a second outlet port  258 . A pair of valve seats  260 ,  262  extends from the interior cavity  264  of the housing  252 . The valve seat  260  is disposed between the inlet port  254  and the outlet port  258 . The valve seat  262  is disposed between the inlet port  254  and the outlet port  256 . 
     An actuator  270  has a rod or spindle  272  which has a first valve disk  274  and a second valve disk  276  fixedly coupled thereto. The valve disk  274  is disposed between the inlet port  254  and the valve seat  262 . The valve disk  276  is disposed between the inlet port  254  and the valve seat  260 . Packing  280  may be disposed between the spindle  272  and the housing  252  to facilitate longitudinal movement of the spindle  272  and the valve disks  274  and  276  as in the direction illustrated by the arrows  282  and to prevent leakage of fluid from cavity  264 . In  FIG. 2A , the valve disks  274 ,  276  and spindle  272  are moved in a longitudinally outward direction away from the actuator  270  so that the fluid flows between the inlet port  254  and the outlet port  258  as illustrated by the path  284 . In  FIG. 2B , the spindle is moved in the rightmost position toward the actuator  270  so that fluid travels from the inlet port  254  to the outlet port  256 . 
     The actuator  270  may be various types of actuators such as an electrical actuator or a hydraulic actuator. In this example, an electric actuator has been used. The actuator  270  is sized to move the disks  274 ,  276  so that high pressure flow between the inlet port and one of the outlet ports is provided (once resumed during the state switching process). Further, the path corresponding to  FIG. 1A  is a high pressure clear fluid path of clear fluid. The other port which is closed is at a low pressure. The low pressure port corresponds to tank  40 A or  40 B depending on the state of operation. The high pressure input to the housing “pushes” the closed valve disk against the corresponding valve seat to ensure a very high closing force to prevent leakage. Because the high pressures are relieved during the switching process, the actuator is sized to overcome very little force (a little more than the packing places on the spindle  272 ). 
     Referring now to  FIG. 2C , the valve  250  illustrated above is shown in an intermediate position in which fluid is communicated between both the inlet port  254  and the first outlet port  256  and the second outlet port  258 . That is, fluid is communicated from the first inlet port  254  simultaneously to the first outlet port  256  and the second outlet port  258 . The amount of fluid or time in the intermediate state is governed by the pressures involved as well as the distance D between the valve disk  274  and the valve disk  276 . The distance D and spindle velocity correspond to the duration of the intermediate state. 
     Referring now to  FIGS. 3A and 3B , a valve  250 ′ is illustrated having the same reference numerals as  FIGS. 2A and 2B  except for the changed components. The valve  250 ′ illustrated in  FIGS. 3A and 3B  is suitable for use as valve  140  illustrated in  FIGS. 1A-1C . In this example, the valve seats  260  and  262  have been changed to valve seats  260 ′ and  262 ′. In this example, the valve seats  260 ′,  262 ′ are moved closer to the central port  290 , which is an outlet port. The valve disks  274  and  276  are moved outboard of the valve seats  260 ′ and  262 ′. In this example, the valve  250 ′ has one outlet port  290  and two inlet ports  292 ,  294 . One of the inlet ports is at low pressure and one is at high pressure during operation. The open port is at low pressure. The high pressure on the valve disk at high pressure forces it in communication with the associated valve seat. For example, in  FIG. 3A , valve disk  274  is forced against valve seat  262 ′. In  FIG. 3B , valve disk  276  is forced against valve seat  260 ′. In  FIG. 3A , fluid path  296  communicates fluid from the second tank  40 B to the outlet port  290  through fluid path  296 . In  FIG. 3B , fluid is communicated from tank  40 A to outlet port  290  through fluid path  298 . In each of the cases of  FIGS. 2A-3B , a gap between the valve disks and the valve seats allows fluid to pass therethrough. 
     Referring now to  FIG. 3C , a two-way valve  310  may be used to replace the three-way valves illustrated in  FIGS. 1A-1F . That is, more than one two-way valve may be used to replace the three-way valves illustrated in the example set forth above. For a two-tank operation, two two-way valves may be used to replace a three-way valve. The valve  310  may also be used in a one tank solution as valves  120 ′ or  140 ′. The valve  310  includes a housing  312  that has an inlet port  314  and an outlet port  316 . The housing  312  includes packing  320  through which an actuator rod  322  extends therethrough. The actuator rod  322  includes a valve disk  324  which is moved by an actuator  326  coupled to the rod  322 . The valve disk  324  may be displaced against the valve seat  327 . When the valve disk  324  contacts the valve seat  327 , the valve  310  is sealed and thus no flow from the inlet port  314  to the outlet port  316  takes place. In this example, when each of the three-way valves are replaced with two-way valves, each valve may have an independent actuator  326  to allow independent control. Thus, each two-way valve may have greater freedom in valve timing. 
     Referring now to  FIGS. 3D and 3E , a pair of two-way valves  310  are illustrated coupled to a common actuator  326 . The common components of each valve are primed. In  FIG. 3D , the right valve  310 ′ is open while the left valve  310  is closed. In  FIG. 3E , the left valve  310  is open and the right valve  310 ′ is closed. By providing the exact same functionality as a three-way valve, the examples illustrated in  FIGS. 3D and 3E  may have some manufacturing advantages in resisting very high pressure operations due to the small size of each valve component. 
     Referring now to  FIG. 3F , a two-way valve  330  suitable for draining low pressure fluid from a tank is set forth. In this example, a port  332  is an inlet port within the housing  334 . The inlet port  332  communicates fluid to a drain port  336 . The housing  334  has packing  340  that receives the actuator spindle  342  coupled to the actuator  344 . The spindle  342  moves the valve disk  350  toward or away from the valve seat  352 . In  FIG. 3F , the valve seat  352  is spaced apart from the valve disk  350  and thus fluid flows between the inlet port  332  and the outlet port  336 . As mentioned above relative to  FIG. 3C , the actuator  344  may be provided for each valve so that when two two-way valves replace a three-way valve, independent timing and control may be performed by the actuators for each valve. 
     Referring now to  FIGS. 3G and 3H , two valves  330 ,  330 ′ may be in communication with a common actuator  344 . In  FIG. 3G , the left valve  330  is closed and the right valve  330 ′ is open. The common components of valve  330  are primed in valve  330 ′. In  FIG. 3H , the left valve is open and the right valve is closed. It should be noted that the valve disk  350  is located between the valve port  332  and the valve seat  352 . When closed, the high pressure forces the valve disk  350  against the valve seat  352 . 
     Referring now to  FIGS. 3I and 3J , a two-way valve  330 ′ similar to that illustrated in  FIG. 3F  is set forth. The common components to those set forth in  FIG. 3F  are labelled the same. The valve  330 ′ is suitable for use as the two-way valves of  FIG. 1K . In this example, the actuator spindle  342  has both a valve disk  350  and a balance disk  360 . The balance disk  360  defines a chamber  362  between the balance disk  360  and the housing  334 . The chamber  362  may also be partially formed by packing  364 . The packing  364  may be an annular layer disposed on a portion of the inner surface of an inner wall of the housing  334 . The chamber  362  is disposed on the actuator  344  side of the housing  334 . The packing  364 , a sealing surface between the balance disk  360  and the housing  334 . 
     The force pushing the valve disk  350  against the valve seat  352  may exceed 50,000 pounds in various applications. The chamber  362  is exposed to the same pressure as inlet  332 . A passage  370  that, in this example, is within the housing  334  communicates fluid from the inlet  332  to the chamber  362 . By balancing the force upon the disk  350  by the pressure in the chamber  362 , the actuator rod  342  essentially only has to overcome the friction force of the packing  364  and seals  376 ,  378  in the housing  334  to open the valve. 
     The valve disk  350  may have a diameter  366 . The balance disk  360  has a diameter  368 . By changing the relative diameters of the balance disk  360  and the valve disk  350 , the net force to open or close the valve may be changed. The diameter may be changed to allow the valve  330 ′ to fail open or fail closed should the actuator  344  malfunction. That is, if the diameter  368  of the balance disk  360  were substantially larger than the diameter  366  of the valve disk  350 , the valve  330 ′ would automatically open if the actuator  344  were to fail. That is, the actuator  344  would need to exert a force toward itself to the keep the valve disk  350  against the valve seat  352 . The diameter  366  and  368  may be referred to as a sealing diameter. 
     The first seal  376  may be disposed adjacent to the actuator spindle  342  and adjacent to the chamber  362 . The second seal  378  may also be disposed adjacent to the actuator spindle  342  closer to the actuator  344 . The seal  378  may also be disposed within the housing  334 . A drain line  380  may be disposed between the first seal  376  and the second seal  378 . The drain  380  provides a path of high pressure fluid out of the housing and away from personnel should the seal  376  fail. Seal  378  prevents high pressure fluid from escaping toward the actuator  344 . 
     Referring now to  FIG. 3J , the valve  330 ′ of  FIG. 3I  is illustrated in the closed position in which the chamber  362  is at a minimum volume. 
     Referring now to  FIG. 4A , a state table illustrated by  FIG. 1A  illustrating a transition of the valves from a first state to a second state is set forth. The first state “A” corresponds to the tank  40 A injecting high pressure slurry into the injection site  12  while high pressure clear fluid is an input to the first tank  40 A. At the same time, tank  40 B is receiving low pressure slurry from the slurry tank  30  and expelling clear fluid to the fluid tank  92 . During the first state illustrated as “A” in  FIG. 4A , valve  80 A is open, valve  82 A is closed, valve  82 B is open and valve  80 B is closed. Three-way valve  140  is communicating fluid from port  140 B to port  140 C to the clear fluid tank  92 . Valve  120  is communicating clear fluid from port  120 A to  120 B. Valve  110  is communicating clear fluid from port  110 A to port  110 B which ultimately communicates fluid through valve  120  and into tank  40 A. Valve  124  is closed and valve  146  is partially open. 
     In the table set forth in  FIGS. 4A  (and  4 B), the bolded cells indicate a change in the valve state. Thus, only the bolded cells will be described in the various states A 1 -A 5 . To begin the transition to state A′, multiple valve states are changed in sequence. State A 1  is achieved by changing the state of the three-way valve  110  to communicate fluid from inlet port  110 A to outlet port  110 C. The high pressure clear fluid from the pump  90  is diverted to the injection manifold  94 . The high pressure clear fluid is no longer directed through the valve  120 . 
     In state A 2 , the valve  150  is switched from communicating clear fluid from between port  150 B and port  150 C to communicating clear fluid from between port  150 A and port  150 C. The switching pressure differential of the valve  150  is reduced since the high pressure fluid is no longer being communicated to tank  40 A through valve  120  due to the relief of high pressure clear fluid flowing to the injection manifold  94  through valve  110 . 
     In state A 3 , the depressurization of the tank  40 A results in the check valves  80 A,  82 A and  82 B switching states. That is, valve  80 A is closed, valve  82 A is open, and valve  82 B is closed. Check valve  80 B remains closed for this portion of the state change. The three-way valves  140  and  120  are also changed in state. That is, valve  140  switches to communicate clear fluid from port  140 A to port  140 C. Three-way valve  120  communicates fluid from port  120 A to port  120 C. Notice, the switching of valves  120  and  140  are performed when low pressures are at all the ports. 
     In state A 4 , valve  124  is open which results in the check valve  80 B being open. By opening the valve  124 , pipe  114  and thus the flow through valve  120  is increased. Port  110 B also sees an increased pressure. 
     In state A 5 , valve  124  is closed and valve  110  is switched in state to terminate the diversion of high pressure clear fluid from the pump  90  to the injection manifold  94 . That is, valve  110  switches states so that clear fluid is communicated between inlet port  110 A and outlet port  110 B. The switching is performed while both ports  110 A and  110 B are under a high pressure due to the diversion of high pressure clear fluid through valve  124 . 
     In this example, states A 1 -A 5  are switched 0.20, 0.3, 0.2, 0.25 and 0.20 seconds respectively for a total switching state time of 1.15 seconds. Of course, the timing may be adjusted based on various conditions. 
     In state A′, a steady state of operation is achieved with check valve  80 A closed, check valve  82 A open, check valve  82 B closed, check valve  80 B open, and three-way valve  140  communicating low pressure fluid to the tank  40 A. Valve  120  is communicating high pressure clear fluid to the tank  40 B which results in high pressure slurry being injected into the injection site  12  through the injection manifold  94 . The valve  150  is communicating fluid from tank  40 A while valve  124  is closed. Valve  146  is partially open. As will be described in more detail below, the switching of the valves takes place based upon comparison from the signals from the flow rate sensors  22  and  24 . A comparison of the flow signals from flow rate sensors  22  and  24  are compared. The flow rate sensor signals correspond to the volume of clear fluid entering one tank and leaving the other tank. 
     Referring now to  FIG. 4B , operation of the injection system  10 ′ illustrated by  FIG. 1B  is illustrated. All of the states are the same except for the valve  150  has been replaced by the valves  210 A and  210 B. In steady state A, valve  210 A is closed and valve  210 B is open while the remaining valves are the same as in  FIG. 4A . In state A 1 , valves  210 A and  210 B remain closed and open, respectively. In step A 2 , valve  210 A is open while valve  201 B is closed. This allows the pressure in the first tank  40 A to be depressurized or relieved of pressure. In the remaining states A 3 -A 5 , valve  210 A remains open while valve  210 B is closed. Likewise, in steady state A′, valves  210 A and  210 B are open and closed, respectively. 
     Referring now to  FIG. 4C , a state diagram illustrating the operation or position of the valves of  FIG. 1I  is set forth. In the slurry discharge state, valve  80 A is open, valve  82 A is closed, valve  120 ′ is open, valve  140 ′ is closed, valve  110  is open, valve  124  is closed and valve  140  is closed. Valve  146  is partially closed so that the flow to the low pressure clear fluid source is regulated. The state A illustrated a slurry discharge state in steady state operation. To transition through the process after slurry has been fully discharged as indicated by the output of the flow rate sensors, state A 1  operates in the same manner except for valve  110  has diverted flow to the manifold  94 . The remaining states are the same. In state A 2 , valve  80 A is closed and valve  140  is open. This allows the tank  40 A to be depressurized. 
     In state A 3 , valves  82 A,  120 ′ and  140 ′ are switched states so that valve  82 A is open, valve  120 ′ is closed and valve  140 ′ is open. In state A′, slurry begins to fill the tank. Slurry fills the tank until a predetermined amount of clear fluid is discharged as determined by the flow rate sensor  22 . In state A′- 1 , the slurry discharge cycle is started by changing the states of valves  120 ′ to open and  140 ′ to closed. This stops the slurry fill. In state A′- 2 , the valve  110  is open. The system then continues in this state which corresponds to state A where slurry is discharged. The process then starts over again. 
     Referring now to  FIG. 4D , the operation of the system illustrated in  FIG. 1K  is set forth. In state A, in which tank  40 A is pumping high pressure slurry and tank  40 B is filling with low pressure slurry, check valve  80 A is open, check valve  82 A is closed, check valve  80 B is closed and check valve  82 B is open. Three-way valve  120  is communicating high pressure clear fluid to the tank  40 A. The two-way valve  246 B is returning low pressure clear fluid to the tank  92  and two-way valve  246 A is closed. In state A- 1 , the state of the two-way valve  246 B is changed from open to closed. In state A- 2 , the check valve  80 B is closed and the check valve  82 B is open. The three-way valve  120  communicates fluid to both tank  40 A and tank  40 B during the period of switching states. That is, the three-way valve  120  communicates high pressure clear fluid to the first volume of the first tank  40 A and the third volume of tank  40 B. The other valves remain the same as in state A- 1 . Both tanks  40 A and  40 B are providing high pressure fluid to the injection site  12 . 
     In state A′, three-way valve  120  has completed switching, the cylinders are switched and the check valve  80 A is closed, check valve  82 A is open. Two-way valve  246 A is open and two-way valve  246 B is closed. In this state, the tank  40 B is providing high pressure slurry to the injection site. 
     Referring now to  FIG. 4E , a plot of the tank pressure versus time for the operation set forth in  FIG. 4D  is set forth. “ 120 SW” refers to the three-way valve switching state or intermediate state between a first state and a second state. The “C” or “O” next to the valve name denotes the valve as closed or open. Within the intermediate state, both tanks are at high pressure. The drain valve for the tank about to be pressurized is closed before the three-way valve  120  switches states. That is, at time period  420 , valve  246 B is closed, valve  120  switches states and then valve  246 A is opened. The tank pressure at tank  2  increases while the pressure within tank  1  decreases during time period  420 . At time period  422 , valve  246 A is closed, then valve  120  switches states and valve  246 B is opened thereafter. At time period  422 , the pressure within tank  1  increases and tank  2  decreases. It is noted that at the time that the valve  120  switches states, the intermediate time period or intermediate state is illustrated which allows both tank  40 A and  40 B to communicate high pressure slurry. Time periods  424  and  426  correspond directly to time periods  420  and  422 . 
     Referring now to  FIG. 4F , the high pressure flow into both cylinders is illustrated. At time period  420 , the high pressure flow from tank  1  is transitioning from a high pressure to a low pressure while the high pressure flow into tank  2  is increasing from a low pressure to a high pressure. A crossing takes place during the intermediate in which switch  120  is switching states. As the high pressure fluid into tank  2  is increasing, the high pressure into tank  1  is decreasing. This causes an approximate balance in the output of the high pressure slurry as illustrated by the total flow line  430 . As is illustrated, the total flow  430  is constant throughout the operating of the system. 
     Referring now to  FIG. 4G , the low pressure flow from each cylinder is illustrated. The flow rate is reduced during the change of states in the two-way valves  246 A and  246 B and stops completely when the three-way valve  120  is in the intermediate state. The average flow reduction during the time span may be about 60% of the full flow. If the average flow is about 600 gallons per minute, the total time to switch states for the valves  120  and  246 A and  246 B is 0.9 seconds. The flow reduction is about 6 gallons over 0.9 seconds. The volume of the accumulator  249  of  FIG. 1K  may be about 18 gallons to reduce the flow variation experienced by the pump so that the flow variation is a negligible value. As is illustrated, during the intermediate state, zero low pressure slurry flow takes place during time periods  420 ,  422 ,  424  and  426  during the switching portion of switch  120  or the intermediate state. 
     Referring now to  FIG. 5A , a method of operating the system set forth in  FIGS. 1A-1H  is set forth. In step  510 , the second volume  52  is filled with slurry in tank  40 A and a first volume  50  is filled with clear fluid. In step  512 , a third volume  54  of the second tank is filled with clear fluid which is reduced by filling the fourth volume  56  with slurry from the low pressure slurry tank  30 . 
     In step  514 , the plurality of high pressure valves  110  and  120 , in particular, communicate high pressure clear fluid from the pump  90  and into the pipe  60 A. In response to communicating the high pressure clear fluid, step  516  moves the region  68 A toward the end cap (downward in  FIG. 1A ) and high pressure slurry is communicated through the check valve  80 A into the injection manifold  94  and injection. 
     In step  518 , a fourth volume  56  is fluidically coupled to allow the slurry from the slurry pump  32  and slurry tank  30  to increase the fourth volume. In response to increasing the fourth volume, clear fluid is reduced within the third volume which is displaced through the pipe  60 B and is fluidically communicated through the valves  140  and  146  into the clear fluid tank  92 . In step  522 , the fluid flow rate of fluid from the tank  92  is measured by flow rate sensor  24 . In step  524 , a second fluid flow rate is determined from fluid flowing from the third volume  54  into the tank  92 . That is, the amount of clear fluid from the tank  40 B communicated to the clear fluid tank  92  is measured. In step  526 , the controller  20  compares the first fluid flow rate and the second fluid flow rate. The flow rates correspond to the volumes entering and leaving tank  92 . In step  528 , the drain valve  146  is controlled in response to comparing so that the flow through the valve  146  is increased or decreased based upon the comparison. Ultimately, the amount of fluid flowing from the second tank may be controlled so that the amount of slurry ready to be injected from the second tank  40 B is available when the tank  40 A is depleted of slurry. Preferably, while draining the tank  40 A of slurry, the amount of clear fluid may extend through the end cap and just past the check valve  80 A so that the check valve  80 A closes in a clean fluid environment rather than in a slurry environment in step  530 . It is desirable to have the tank  40 B and thus volume  56  at a process maximum before states A 1 -A 5  of  FIGS. 4A and 4B  are performed. 
     In step  532 , the plurality of clear fluid valves are controlled to cause tank  40 A to depressurize. This takes place in states A 1  and A 2  of  FIGS. 4A and 4B . 
     In step  534 , the valves are changed in state in steps A 3 -A 5  so that the second tank  40 B is pressurized while tank  40 A is depressurized and fills with slurry. The switching process is described below. 
     Referring now to  FIG. 5B , the operation of the system illustrated in  FIG. 1K  is set forth. Steps  540 - 544  are the same as steps  510 - 514  in  FIG. 5A . Thus, the description of the operation of these steps is not set forth. In step  546 , the step is performed in a similar manner to that set forth in step  516 . In step  546 , in response to the high pressure clear fluid in the first volume, the region  68 A or the piston is moved. High pressure slurry is communicated from the first tank to the injection site. Valve  246 A is closed during step  546 . 
     In step  548 , the fourth volume is coupled to a low pressure slurry pump and slurry tank to increase the fourth volume. In step  550 , in response to step  548 , the third volume is reduced and low pressure fluid is displaced through the valve  246 B and through valve  146 . 
     In step  552 , a first flow rate of clear fluid from the clear fluid tank  92  to the first volume  50  is measured. In step  554 , a second fluid flow rate of fluid from the third volume  54  through the valve  246 B is measured using the flow meter  22 . In step  556 , the flow rate or volume of the clear fluids based on the flow rates is compared. In step  556 , the drain valve  146  is controlled in response to comparing to increase or reduce the clear fluid from the third volume. In step  560 , the optional step of communicating clear fluid past the check valve such as the check valve  80 A is set forth. 
     Referring now to  FIG. 6, 6A , the switching of the valves in the states between state A and state A′ is illustrated in flowchart form. In step  610 , if the desired amount of clear fluid being removed from the second tank  40 B has been reached, the switching process begins. As mentioned above, this corresponds to the flow rate or volume determined by the flow rate. Step  610  uses the comparison of step  526  to make this determination. In step  612 , the process of switching from state A to A′ of  FIGS. 4A and 4B  is set forth. In step  614 , high pressure clear fluid is redirected to the injection manifold through the three-way valve  110 . In step  616 , the first tank  40 A is coupled to the clear fluid tank through valve  150 . Check valve  80 A closes in step  618  when the pressure in the tank  40 A is reduced. The pressure reduction may be to or nearing to ambient pressure. In step  620 , the state of the three-way valves  150  and  120  are also changed as the valves are changed toward state A′. 
     In step  622 , the pipe to the three-way valve  120  is pressurized by opening the valve  124 . In step  624 , and in response to the bypass valve  124  being closed, valve  82 B is closed and check valve  80 B is open. Thereafter, in step  626  the state of the three-way valve  110  is changed to communicate high pressure clear fluid to tank  40 B through the three-way valve  120 . In step  628 , high pressure slurry is discharged from tank  40 B. In step  630 , the check valve  82 A is open to force clear fluid from the tank by displacing the clear fluid with low pressure slurry from the tank  15  and the slurry circulation pump  32 . In step  632 , clear fluid is communicated to the tank  92  through the valve  140  and valve  146 . 
     Referring now to  FIG. 6B , the switching process of  FIG. 1K  is set forth.  FIG. 6B  continues the process of  FIG. 5B . In step  640 , when the programmed amount of clear fluid from the second tank  40 B is removed, the switching process begins in step  642 . In step  644 , valve  246 B is closed. In step  646 , the three-way valve  120  is placed into an intermediate position so both valves  80 A and  80 B supply high pressure slurry to the injection site  12 . In step  648 , the three-way valve  120  continues switching to change state from the first state to the second state through the intermediate state. In step  568 , valve  246 A is open. When the three-way valve  120  completes switching, tank  40 A is no longer communicating high pressure fluid because the high pressure pump is no longer communicating high pressure fluid to the first volume. When valve  246 A opens, the pressure is reduced and the check valve  80 A is opened in step  642 . The three-way valve continues to change state to allow only tank  40 B to begin pumping slurry to the injection site  12 . In step  656 , high pressure slurry is continued to be discharged through the tank  40 B. In step  658 , the check valve  82 A opens to force clear fluid into pipe  60 A. In step  660 , clear fluid is directed from the clear fluid tank  92  from tank  40 A to the two-way valve  246  and valve  146 . 
     Referring now to  FIG. 7A , a timing diagram for a multiple unit system is set forth. Each unit referred to in  FIG. 7A  comprises a pair of tanks. In  FIG. 1 , only two units are illustrated. However, as mentioned above, a plurality of units are set forth. In this example, five units, unit  1 -unit  5 , are controlled having staggered starts of two seconds. That is, unit  1  starts at time  0  while unit  2  starts at 2 seconds, unit  3  at 4 seconds, unit  4  at 6 seconds, and unit  5  at 8 seconds. A small dwell time such as one-half second may be used in between each cycle for each unit to accommodate slightly slower cycle rates or other variations. That is, the nominal cycle illustrated in  FIG. 7A  is 9.5 seconds with a one-half second dwell time therebetween. Unit  1  restarts after the dwell time at 10 seconds, unit  2  at 12 seconds, unit  3  at 14 seconds, unit  4  at 16 seconds and unit  5  at 18 seconds. 
     By staggering the start times and maintaining such during operation, the amount of slurry injected during the process may be maintained at a constant rate. If the units would be in sync in terms of start times (all start at the same time) this may generate stress in the piping, valves and other components. Preferably, the number of units may equal the cycle time divided by the switching time plus the dwell time which is multiplied by an integer. In this case, the cycle time is 10 seconds divided by the 2 second. The results are 5 units to obtain minimal flow variation. However integer multiples 10, 15 or 20 units (or more) may also be used to minimize flow variation. 
     Referring now to  FIG. 7B , a timing chart of a system is set forth using single tank control such as the single tank of  FIGS. 1E and 1F  above. Single tank control with rapid slurry charging is set forth in  FIG. 7B . In this example, each unit has a nominal five-second cycle time with four-second slurry discharge and a one-second slurry fill. In this example, no dwell time is assumed. However, a dwell time may be used in operation. The ratio of the slurry fill time to the slurry discharge time is 1:5. That is, the discharging of slurry takes place 80% of the time and slurry filling takes place 20% of the time. This is increased over the examples including two tanks in which half the time the tank is charging while the other half of the time the tank is discharging (i.e., 50%). The preferred number of tanks equals the cycle time divided by the slurry fill time in the preceding example, this would be five (5) tanks. In the example set forth in  FIG. 7B , based on the aforementioned slurry discharge and fill rates with five (5) tanks, the rate of high pressure slurry output and low pressure slurry input is uniform. In the example set forth in  FIG. 7B , the same discharge and fill times are used but in this case, six (6) tanks are used thus the rate of slurry output of the entire system is not uniform. Each of the single tanks will operate in phase for one second of every five seconds. As shown in  FIG. 7B , the simultaneous slurry filling of units  1  and  6  happens at the time between four and five seconds, nine and ten seconds, fourteen and fifteen seconds and nineteen and twenty seconds. The amount of high pressure clear fluid used for slurry pressurization drops by one unit of 16.7% for one second every five seconds. Since the high pressure pumps are positive displacement pumps running at a constant speed, the excess flow is diverted by the valve  170  illustrated in  FIGS. 1E and 1F . When the tanks are in-phase and thus reducing the flow of high pressure fluid to the tanks, the excess high pressure fluid flow is diverted to the slurry manifold  94  through the check valve  170 . The dilution of the slurry caused by the diversion of flow may be accommodated by making the slurry more concentrated. In the present example, a slurry concentration increase of 3.3% is used to accommodate the extra high pressure clear fluid that is bypassed through check valve  170 . 
     Referring now to  FIG. 8A , a slurry concentrate pressurizer configuration  810  is illustrated. In this example, clear fluid is provided from a clear fluid source such as a reservoir  812  or tank. The reservoir  812  is in fluid communication with a low pressure pump  814 . Clear fluid from the reservoir  812  enters a low pressure clear fluid manifold  816 . The low pressure clear fluid manifold  816  is in communication with a trailer  818 . Because the systems are moved from wellsite to wellsite, mounting the system components to a trailer is suitable. The trailer  818  may also be referred to as a “missile.” The trailer  818  has a portion of the low pressure clear fluid manifold  820  and a portion of a high pressure clear fluid manifold  822  coupled thereto. The low pressure manifold  820  is in fluid communication with a plurality of high pressure clear fluid pumps  830 . The pumps  830  may be referred to as fracking pumps. The plurality of pumps  830  may all be disposed on trailers  832  that may be hooked to a semi for easy transport from fracking site to fracking site. One or more pumps  830  may be disposed on each trailer  832 . Each of the pumps  830  draws low pressure fluid from the low pressure manifold  820  through an inlet pipe  840  and discharges high pressure clear fluid through an outlet pipe  842 . Only one each of pipes  840  and  842  are labeled. The high pressure manifold  822  is in fluid communication with a static mixer  852  through a two-way valve  850 . The static mixer  852  is in communication with a well head  854 . The high pressure clear fluid manifold  822  is in fluid communication with a slurry pressurizer  860  through a valve  862 . A controller  864  is used to control the valves  850 ,  862  so that a portion of the high pressure clear fluid transmits through the valve  850  and a portion of the high pressure clear fluid is communicated to the slurry pressurizer  860 . The controller  864  may be a programmable logic controller (PLC) that acts in response to one of more flow rate sensors  866 A,  866 B or  866 C. Of course, flow rate sensors  866 A,  866 B,  866 C may be disposed at various locations throughout the system  810  and generate flow signals that the controller  864  uses to control the system. 
     The slurry pressurizer  860  receives high pressure clear fluid and generates high pressure slurry through an output pipe  870  which is in fluid communication with the static mixer  852 . That is, the pipe  870  is in fluid communication with a point between the valve  850  and the static mixer  852 . The static mixer  852  forms a mixture of concentrated high pressure slurry from slurry pressurizer  860  and water from the valve  850 . 
     The slurry pressurizer  860  also receives low pressure slurry from a blender unit  872  through a pipe  874 . The blender unit  872  may also receive additive from an additive tank  876  which is in fluid communication with a dosing pump  878 . The dosing pump  878  communicates the fluid from the additive tank  876  into the blender unit  872 . The additive within the additive tank  876  may comprise a gel or other types of additive using in the fracking process. The slurry unit  872  may blend slurry, fluid and additives to form the low pressure slurry. 
     A centrifugal separator  880  receives low pressure fluid from the slurry pressurizer  860  through pipe  881 . The centrifugal separator  880  may separate any residual slurry from within the low pressure discharge and communicate the slurry matter to the blender unit  872  through the pipe  882  for re-use. The separator  880  may also communicate clear fluid to the low pressure manifold through pipe  884 . 
     The blender unit  872  may also receive low pressure clear fluid from the low pressure manifold  816  through a pipe  886 . The low pressure clear fluid may be used to form the slurry. 
     In operation, the slurry pressurizer  860  may be disposed on a trailer  890 . The slurry pressurizer unit  860  may be one or more of the examples set forth in  FIG. 1A-1F . Both single or double tank slurry injection units may be used for the slurry pressurizer  860 . In operation, the controller  864  controls the valve  850 . The valve  850  may be used to create a differential pressure between a pipe  892  and pipe  870 . The differential pressure may be 75 psi or less. The valve  862  may be precisely controlled so that the pressure thereacross is between 1 and 20 psi. Valve  850  may be designed to not fully close. That is, a predetermined amount of flow through the valve  850  so that a predetermined amount of pressure differential is present across the valve  850 . For example, a 100 psi pressure differential may be used. The valve cannot fully close preventing accidental over pressurizing of the pumps and the piping. A suitable valve may be a leaky butterfly valve or a ball valve that is not allowed to physically close due to the geometry therein. 
     Referring now to  FIG. 8B , the static mixer  852  is illustrated in further detail. The static mixer  852  has mixing elements  910  set forth therein for mixing the slurry and clear fluid communicated through the valve  850 . The static mixer  852  blends the clear fluid and the concentrated slurry from the slurry pressurizer  860  that is received through the pipe  870 . 
     In the example set forth in  FIGS. 8A and 8B , the fracking pumps are supplied by a single low pressure clear fluid line. The slurry pressurizer  860  draws low pressure clear fluid and returns low pressure clear fluid back to the clear fluid manifold  816 . These connections minimize the amount of piping in a system. The centrifugal separator  880  separates the slurry particulates from the low pressure clear fluid to negligible amounts so that a minimum amount of particulates are in the clear fluid when entering the high pressure pumps. If various ones of the pumps  830  fail, the system can continue to pressurize slurry with minimal effect on the operation. The trailers  832  containing the pumps  830  may easily be maneuvered to allow additional or replacement pumps to be quickly connected to the trailer  818 . 
     Because of the configuration, all the high pressure slurry mass is provided by the trailer containing the slurry pressurizer  860 . The slurry pressurizer  860  may use vertical cylinders which keep the slurry in drive fluid from excessive mixing. As mentioned above, pistons may also be used within the various tanks to prevent mixing of the fluids therein, particularly if the tanks are disposed at another angle other than vertical. The valves within the slurry pressurizer have a generally low cycle rate of once every five to ten seconds versus six times per second in a typical fracking pump. Valves designed for low velocities and materials that minimize erosion from concentrated slurry may be used. 
     Some slurry processes use 0.5 pounds of sand per gallon of high pressure clear fluid or about 6% concentration by weight. Highly dense slurry may contain five pounds of sand per gallon. Based on the ratio, the slurry pressurizer  860  may only need to handle a flow of approximately 10% of the total high pressure clear fluid flow to achieve a desired slurry concentration downstream of the static mixer  852 . The additive tank  876  may pass the additives through the slurry pressurizer  860  and the capacity of the slurry pressurizer may be reduced. Because the slurry pressurizer  860  may provide a highly concentrated slurry due to the later mixing within the static mixer  852 , the system may be referred to as a slurry concentrate pressurizer. The slurry pressurizer is capable of handling very high slurry concentrations due to low flow velocities and relatively long cycle times which minimize wear of the check valve. Therefore, the fluid capacity hence the size of the equipment can be relatively small. 
     Referring now to  FIG. 8C , the schematic of  FIG. 8A  has been modified to remove the valves  850  and  862  and replace the valves with the pump  894 . The pump  894  may be referred to as a bypass or transfer pump in which high pressure clear fluid from the high pressure clear fluid manifold  822  is redirected to the slurry pressurizer  860 . A variable frequency drive  896  is in communication with the controller  864 . The controller  864  controls the variable frequency drive  896  to control the speed of the pump  894  so that a desired amount of high pressure clear fluid is directed to the slurry pressurizer  860 . The controller  864  may provide feedback from the flow meters  866 A,  866 B and  866 C. Further, the controller  864  may receive feedback from the flow meters  22  and  24  in the various stages of the slurry pressurizers. One or more flow signals from the flow meters may be used to control the speed of the transfer pump  894 . The remaining portions of  FIG. 8A  that are illustrated in  FIG. 8C  are not described because the operation is the same. The bypass pump  894  in conjunction with the variable frequency drive  896  develops the necessary boost to achieve the desired flow rate of high pressure slurry from the slurry pressurizer and thus the desired amount of output through the pipe  870  prior to communication with the mixer  852 . Although a variable frequency drive  896  is not necessary, by providing the variable frequency drive  896 , more precise adjustment of pressure boosting may be provided. The pressure provided by the pump  894  may be between about 40 psi and 100 psi depending on the desired slurry flow rate and the viscosity of the slurry. 
     The slurry pressurizer  860  is illustrated having a first stage  898 A and a second stage  898 B. As will be described in more detail below, providing two stages addresses the fact that the amount of proppant in fracking operations may vary widely depending on the type of geological formation and the preferences of the operation. When “slick” water is used, the proppant concentration may be as low as a few percent. Thus, the slurry pressurizer  860  may operate at a proppant concentration of 50% or higher and thus may only need to handle a very small fraction of the total flow. For example, if the final slurry concentration is to be about 5%, the slurry pressurizer unit when operating at 50%, would only need to handle about 10% of the total flow and thus may single stage  898 A may be used. However, some fracking operations may be proppant concentrations of 20%-30%. In such cases, the slurry pressurizer may use two stages such as stages  898 A and  898 B. However, different numbers of stages may also be used. This is described in  FIGS. 7A, 7B . The module configuration of the slurry pressurizer may be desirable, but if the slurry pressurizer handles about 50% concentration, the proppant concentration desired at the well head is 25%, then the slurry pressurizer needs to handle about 50% of the flow. By providing a modularized slurry pressure unit, the costs are minimized and thus the proper amount of stages may be used. If a pressurizer fails, a new stage may be easily input into the system. 
     Referring now to  FIG. 8D ,  FIGS. 8C and 8A  are similar in that a separator  880  is used for separating any potential contamination from the low pressure clear fluid from the slurry pressurizer  860 . The pipe  881  communicates low pressure clear fluid to the separator  880  where most of the fluid is separated. In the example illustrated in  FIG. 8D , a portion of the separated water and the slurry may be communicated through the pipe  882  to the blender unit  872 . Under certain conditions, all of the clear fluid may be communicated from the slurry pressurizer  860  to the blender unit  872 . In such a case, the separator  880  may be eliminated. 
     In  FIG. 8D , a configuration that is suitable for compensating for emergency or failure detections is set forth. 
     A first isolation valve  912 A is disposed within the pipe  892  between the bypass pump  894  and the high pressure clear fluid manifold  822 . A second isolation valve  912 B is disposed within the pipe  870  communicating high pressure slurry to the mixer  852 . 
     A bypass pipe  914  is in fluid communication with the pipe  874  and communicates low pressure slurry to the low pressure clear fluid manifold  816  through the valve  916 . The valve  916  is normally closed and is operated under various conditions such as an emergency or failure condition or a condition when an amount of slurry produced by the slurry pressurizer  860  is insufficient. Insufficient slurry concentration may be developed most likely at the end of the slurry process where process requirements require the maximum slurry density. 
     A densometer  918  is also disposed within the pipe  874 . The densometer  918  may be referred to as a sensor, a plurality of sensors or a sensor system. The densometer  918  generates a signal corresponding to the density of the slurry from the blender unit  872  within the pipe  874 . A flowrate signal may also be generated by the densometer  918 . The flowrate signal corresponds to the flowrate of the low pressure slurry that is communicated from the blender unit  872  to the slurry pressurizer  860 . By using the flowrate signal and density signal, a constant high density slurry with a predetermined rate of slurry injection into the mixer  852  is used to achieve the desired slurry density flowing to the injection site  854 . By changing the pressure from the pump  894 , the rate of flow of slurry injection may be changed. The pressure is used to overcome the flow resistance of the slurry through various check valves and the flow resistance of water through the control valves. A very low flow may be obtained with about 0.5 bars pressure boost with flowrates increasing as the change in pressure increases. By using the techniques, about a five to one flow variation may be achieved. Data from the densometer  918  is communicated to the controller  864  through a data line  921 A. A data line  921 B and  921 C are used to control the valve  916  and the pump  894 , respectively. Using these techniques, the slurry concentration flowing to the injection site  854  may be varied within a few seconds and thus very precise control of the slurry concentration is obtained. As mentioned briefly above, the concentrated slurry injected at the maximum flowrate may be insufficient to achieve a desired overall concentration. The slurry pressurizer bypass valve  916  may be opened in such conditions to allow some low pressure slurry to flow to the clear water manifold  816 . The data line  921 B sends a control signal to open the valve  916 . The high pressure pumps  830  will thus be exposed to slurry but only for brief periods at a significantly lower concentration than otherwise required. The pressure in line  914  is higher than the pressure in the low pressure clear water manifold  816 . Referring now to  FIG. 8E , a modification of the valve  916 ′ is moved closer to the missile  818  and the high pressure pumps  830 . Thus, the slurry, which may change in density to meet rapidly changing process requirements, exiting blender  872 , through pipe  914 ′ and circulating through pipe  922 , is injected with less delay to missile  818  than in  FIG. 8D . In this example, the bypass pipe  914 ′ is extended so that the valve  916 ′ is closer to the missile  818 . A return line  922  is provided so that a continuous amount of fluid may flow in a loop between the bypass pipe  914 ′, the return line  922  and the blender unit  872 . By providing a continuously circulating amount of slurry, when the valve  916 ′ opens, the slurry in the pipe  914 ′ is always at the desired current density thus the correct slurry density will be immediately available to missile  818 . The continuous supply of slurry to the well head is provided through the recirculation line  922  in case the slurry pressurizer stops. A flow meter  924  is disposed within the recirculation line  922 . The flow meter  924  communicates a flow signal corresponding to the flow of fluid within the recirculation line  922 . 
     Although not illustrated in  FIGS. 8D and 8E , flowmeters  866 A,  866 B and  866 C, each coupled to the controller  864  may also be incorporated into the system for monitoring the slurry pressurizer and to allow control of the various components therein. 
     Referring now to  FIG. 9A , a method for operating the system of  FIGS. 8A and 8B  is set forth. In step  930 , clear fluid is received from a tank or reservoir at a low pressure pump. In step  932 , the low pressure clear fluid is communicated to a low pressure manifold. The low pressure is not high enough to operate the slurry pressurizer illustrated in  FIG. 8A . In step  934 , a plurality of clear fluid pumps that are coupled to both a high pressure clear fluid manifold and a low pressure clear fluid manifold increase the pressure of the low pressure clear fluid. In step  936 , the high pressure clear fluid generated at the plurality of clear fluid pumps is communicated to a high pressure clear fluid manifold. 
     In step  938 , a portion of the high pressure clear fluid is communicated from the high pressure clear fluid manifold through a static mixer through a first valve. In step  940 , a portion of the high pressure clear fluid from the high pressure clear fluid manifold is communicated to a slurry pressurizer through a second valve. In step  942 , the flow through the first valve and second valve is adjusted based on a flow rate or a pressure monitored within the system. 
     In step  944 , additives may be added to the low pressure slurry. For example, the additives may be a gel or other types of additives suitable for improving the slurry fracking process. Step  944  is an optional step. Additives may be communicated to a slurry unit from a tank and a dosing pump. 
     In step  946 , low pressure slurry is communicated to the slurry pressurizer. In step  948 , high pressure slurry is communicated to the static mixer from the slurry pressurizer. In step  950 , the low pressure clear fluid from the slurry pressurizer is communicated to a separator. The low pressure clear fluid is the result of the pressure transfer at the slurry pressurizer of high pressure from the high pressure clear fluid to the increase in pressure of the low pressure slurry to high pressure slurry. In step  952 , the slurry residue may be extracted at a separator. The clear fluid may have a small amount of slurry therein. In step  954 , the separated slurry at the extractor is communicated to the slurry unit and is later used for reinjecting to the slurry pressurizer. In step  956 , low pressure clear fluid is communicated to the low pressure clear fluid manifold from the separator. Should the clear fluid have an acceptably low amount of slurry particles therein, the separator may be eliminated from the system. 
     Referring now to  FIG. 9B , the operation of the system illustrated in  FIG. 8C  is described in detail. The operation of  FIG. 9B  is similar to that of  FIG. 9A . Steps  958  through  966  are identical to those set forth as steps  930 - 936  and thus will not be described in greater detail. In step  966 , a portion of the high pressure clear fluid is communicated to the blender unit  872  from the clear fluid manifold. In step  940 , a portion of the high pressure clear fluid is communicated to a slurry pressurizer through the pump  894 . In step  970 , the flow through the pump is adjusted based upon the various flow rates. The flow rates from the flow meters  22  and  24  or the flow meters  866 A- 866 B may be used individually or in combination. Steps  972 - 984  are identical to those set forth as steps  944 - 956  and thus will not be described in further detail. 
     Referring now to  FIG. 9C  a method for operating the system of  FIG. 8B  is set forth. In step  986  a sensor or sensor system such as the densometer is used to generate a density signal and flow signal of the slurry leaving of the blender unit  872 . In step  987  a desired density of the slurry for the injection site is determined at the controller  864 . 
     The speed of the pump  894  is changed to increase or decrease the density of the slurry to well  854  to meet process requirements in step  990 . In step  989  if it is determined that the pump is not at maximum capacity (i.e. slurry pressurizer  860  is not at maximum capacity), the system repeats back to step  986 . When the slurry pressurizer  860  is at capacity (maximum flow rate) in step  989 , step  990  increases the pump speed. After step  988 , when the slurry density is to be decreased, step  990  decreases the pump speed. Then, step  994  opens the slurry pressurizer bypass valve  916 . Slurry is communicated from the blender outlet  875  to the low pressure clear fluid manifold in step  996 . In step  998  slurry is communicated through the high pressure pumps where the slurry is added to the slurry from the slurry pressurizer  860 . The slurry density of the slurry injected at the injection site  854  is changed accordingly in step  999 . 
     Another way to control of slurry density entering well  854  is to vary the slurry density exiting blender  872 . However, the slurry pressurizer, handling a concentrated slurry, allows another option which is to keep the blender slurry concentration constants but vary the rate of injection into mixer  852  leading to well  854 . The later method has a faster response as, with the former method, the relatively large blender tank takes a while to adjust to the desired concentration and then adjusted slurry needs to flow all the way through the array of high pressure pumps  830  and missile  818 . By simply changing the rate of slurry concentrate injection from slurry pressurizer via adjustment of speed of pump  892  allows a change in a matter of seconds to well  854 . Both methods can be mentioned but the preferred method may be to simply vary the speed of pump  892  in many conditions. 
     Referring now to  FIG. 9D , step  902 D determines whether an emergency condition is detected. Step  902 D is repeated when an emergency condition is not detected. When an emergency condition is detected in step  902 D, the pump  894  is shut down in step  904 D. The emergency condition may correspond to a broken pipe within the system. A broken pipe may be detected by one of the flow meters. In step  906 D the fracking operation is continued for one more cycle to depressurize the system. In step  908 D the system is shut down and the blender unit  872  or the pump  873  therein is depowered. 
     In step  910 D, fracking operations continue for one more cycle to depressurize the pressure in the various parts of the system. 
     Referring now to  FIG. 9E , in step  902 E the presence of a failure is determined. When a failure is not detected in step  902 E the system continuously monitors step  902 E to determine if there is a failure in the system. A failure in the system may correspond to a failure of the slurry pressurizer  960 . If a failure is detected in step  902 E, step  904 E turns off the pump  874 . Valve  916  is opened in step  906 E. In step  908 E, the isolation valves  912 A and  912 B are closed. In step  910 E, fracking operations continue for one more cycle to depressurize the pressure in the various parts of the system. 
     Referring now to  FIG. 10A , a top view of a single stage  1010  corresponding to one of the stages  698 A or  698 B of  FIG. 8C  is set forth. In this example, each stage  1010  may be disposed on a baseplate  1012 . Each baseplate  1012  may include the first tank  40 A, the second tank  40 B and one or more valves. In this example, which corresponds to  FIG. 1K , the two-way valves  246 A and  246 B are illustrated together with three-way valve  120 . A plurality of pipes is used to interconnect the module  1010  with various other modules and to the injection site and the slurry and clear fluid sources. Pipe  1020  is a low pressure clear fluid pipe. A low pressure slurry pipe  1022  communicates low pressure slurry to the tanks  40 A,  40 B. A high pressure slurry pipe  1024  communicates high pressure slurry to the injection site that has been pressurized by the tanks  40 A and  40 B. A high pressure clear fluid pipe  1026  communicates high pressure clear fluid to the tanks  40 A,  40 B to displace high pressure fluid from the tanks  40 A,  40 B. Each stage operates according to the manner set forth with respect to  FIG. 1K . A first crossover pipe  1030  is fluidically coupled to pipe  1024 . A second crossover pipe  1032  is fluidically coupled to pipe  1022 . 
     As is set forth in  FIG. 10A , the high pressure pipes  1024  and  1026  are located on one side of the base place  1012  for safety purposes. That is, the high pressure pipes  1024  and  1026  are isolated away from the low pressure pipes  1020 ,  1022 . The low pressure pipes typically require more personnel access. 
     Referring now to  FIGS. 10B and 10C , a side view of a first stage  1010  and a second stage  1010 ′ are illustrated. The base plates  1012  and  1012 ′ may be joined together using spacer blocks  1040  and pipe couples  1042 . The blocks  1040  have the length to allow the installation of the pipe couplings  1042  to connect the various pipes of the various modules. 
     Referring now to  FIG. 10D , the base plates  1012  and  1012 ′ are illustrated being coupled together by the spacer block  1040 . However, the systems are typically used in various non-ideal conditions such as in a field or from a vehicle trailer. Consequently, the base plates  1012 ,  1012 ′ may be mounted to an adjustable pad  1050 ,  1050 ′. The pads  1050 ,  1050 ′ may be coupled to an adjustment bolt  1052 ,  1052 ′ that may be turned to change the distance between the pad  1050  and a flange  1054 ,  1054 ′, respectively. Thus, the adjustment bolt  1052  provides vertical adjustment for each of the base plates  1012 ,  1014 . 
     A shoulder bolt  1056 ,  1056 ′ is illustrated in a horizontal direction and thus provides horizontal alignment of the spacer  1040  with the base plates  1012 ,  1056 . The shoulder bolts  1056 ,  1056 ′ may be coupled to flanges  1058 ,  1058 ′ that are mounted to the base plates  1012 ,  1012 ′. A drift pin or alignment pin (not shown) may also be used to achieve horizontal alignment prior to installing the shoulder bolts  1056 ,  1056 ′. 
     The first stage  1010  may include a programmable logic controller or controller and thus each additional module may become slaves of the first module using standard electrical interface plugs and connectors. The programmable logic controller or controller may be programmed to handle the necessary number of modules. Preferably, the controller is located at the low pressure side of the base plate  1012  so that the module may be easily reached without being directly adjacent to the high pressure pipes. 
     Referring now to  FIGS. 11A, 11B and 11C , the system set forth in  FIG. 1K  is illustrated having a modified first tank  40 A′ and a modified second tank  40 B′. During slurry discharge in which water is being emitted to the top of the tank through the flow distribution plate  64 A, the slurry may try to settle. Therefore, the tank may be modified to remove the flat bottom portions illustrated in the examples set forth above. As set forth below, a conical or hemispherical end cap may be used at the bottom of the tank that does not induce diffusion during the slurry filling cycle. As will be described in more detail below an angle of the cone of between twenty and thirty degrees may be used to prevent sand from settling but not causing diffusion of the entering slurry flow during the slurry fill cycle. 
     Tanks  40 A′ and  40 B′ are illustrated as tanks that are vertically disposed. In this example the tanks  40 A′ and  40 B′ are modified. In particular, the endcaps  44 A′ and  44 B′ are modified to reduce the amount of sediment buildup in the bottom of the tank due to gravity. The teaching set forth in  FIGS. 11A-11C  apply equally to the tanks set forth in  FIGS. 1A, 1B, 1G, 1I, and 1J , each of which illustrate vertically disposed tanks. That is, one or both of the tanks  40 A′ and  40 B′ may be incorporated into one of those systems. 
     The length of the tank L 1  may be about seven or eight times the inside diameter W 1  of the tank  40 A′. This allows the tank enough length for the incoming jet of slurry to dissipate before the slurry reaches the end wall  41 . 
     During slurry discharge in which water is being emitted to the top of the tank through the flow distribution plate  64 A, the slurry may try to settle. Therefore, the tank may be modified to remove the flat bottom portions illustrated in the examples set forth above. As set forth below, a conical or hemispherical end cap may be used at the bottom of the tank that does not induce diffusion during the slurry filling cycle. As will be described in more detail below an angle of the cone of between twenty and thirty degrees may be used to prevent sand from settling but not causing diffusion of the entering slurry flow during the slurry fill cycle. 
     In this example, the endcap  44 A′ is modified so that a bottom surface  1120  of the tank is formed between the sidewall  1122  and the slurry injection channel  1110 . That is a first edge  1120 A of surface  1120  is near the sidewall  1122  and a second edge  1120 B of surface  1120  is near the slurry injection channel  1110 . 
     A clear fluid injection pipe  1124  which is in fluid communication with the flow distribution plates  64 A is also illustrated. In the present example the clear fluid injection pipe  1124  corresponds to the longitudinal axis  42 A of the system. Also, the tank  40 A′, the slurry injection channel  1110  and the endcap  44 A′ are all centered along the longitudinal axis  42 A. However, such alignment is not necessary. The top of the endcap  1126  forms a horizontal plane which is perpendicular to the longitudinal axis  42 A of the system and which is illustrated as a horizontal line  1126 . The horizontal line  1126  defines the intersection of the sidewalls  1122  and the endcap  44 A′. 
     The bottom surface  1120  of the tank  40 A′ angles from the sidewall  1122  downward toward the slurry injection channel at an angle A. This forms a conical surface. The angle A is greater than zero degrees from the horizontal plane which is illustrated as a line  1126 . The actual angle A may vary for different systems depending upon the size of the sand particles within the slurry and the type of fluid and chemicals within the tank. The angle A is greater than zero degrees but less than about forty-five degrees. In another example, the angle may be between about twenty degrees and about forty-five degrees. It is believed that an angle A less than twenty degrees may allow slurry particles to settle and thus not be washed into the slurry injection channel  1110  where they can be recirculated. However, as mentioned above the amount of settling may occur due to various factors of the particles in the fluid and the fluid itself. With the particles settling into the slurry injection channel  1110  as new slurry is received through the valve  82 A, pipe  34  and slurry tank  30 , the particles within the slurry injection tank are picked up in new slurry and mixed within the tank  40 A′. An angle of about forty-five degrees may allow the slurry injection jet to be diffused resulting in low mixing turbulence and thus allowing slurry particles to lend toward settling. It is further believed that a range of about thirty-five to about forty degrees may be desirable. In operation, the valve  82 A allows slurry to be injected into the tank  40 A′ using low pressure. A jet of slurry forces the clear fluid toward the top of the tank  40 A′. The bottom surface  1120  and the angle provided therein prevent the slurry particles from building up on the bottom surface of the tank. 
     Referring now to  FIG. 11D , a bottom surface  1130  is hemispherical in shape. That is, the surface  1130  is curved downward from the side wall to the slurry injection channel  1110 . The endcap  44 A″ may have a hemispherical shape with the slurry injection channel  1110  at the bottom of the surface  1130 . The pipe  60 A is disposed within the slurry injection channel  1110  as described above in  FIG. 11B . The hemispherical center is at point C. The tangent to the hemisphere is greater than a predetermined angle such as 20 degrees because the bottom of the circle has been removed at the slurry injection channel  1110 . Of course various other curved shapes may be used as long as a diffuser is not formed. 
     Referring now generally to  FIGS. 11E-J , in the previous examples, the top of the tank is flat and thus no potential for sand accumulation is present. This maximizes the tank volume for a given length. However, it has been found that a conical shape at the first end  46 A of the tank may be suitable for diffusion of the clear water to minimize disturbance of the water/slurry interface  68 A. A very dense slurry may benefit from a flat faced design as is illustrated above in various figures such as  FIGS. 1A-1D, 1G, 1I, 1J, 1K and 11A . The flat upper surface design prevents the clear water jet from deeply penetrating the slurry volume. The flat surface also maximizes the usable tank volume. In the designs set forth below, a modified first end  46 A may be used in low density slurry to minimize the disturbance of the slurry/water interface  68 A caused by the jet of incoming clear water through the pipe  60 A. The above examples largely eliminate diffusion of the incoming slurry to maximize formation of a jet moving upward in the center of the tank caused by the incoming slurry. The jet dissipates toward the top of the slurry volume at the first end of the tank  46 A. At the point when the slurry volume is close to the top of the tank, the jet has entirely dissipated at the interface area  68 A thus allowing a sharply defined interface to form. However, at the point of entry at the bottom of the tank through channel  1110 , the jet is defined and causes a great amount of turbulence which minimizes the settling that may occur keeping the slurry density uniform. The length of the tank L 1  may be about seven or eight times the inside diameter W 1  of the tank  40 A′. This allows the tank enough length for the incoming jet of slurry to dissipate before the slurry reaches the end wall  41 . 
     Referring now to  FIG. 11E , the first end  46 A of a tank  40 A has been modified. An external clear water pipe  1132  is used in place of pipe  60 A and the diffuser  64 A. Of course, in a two tank design both tanks  40 A and  40 B may be configured with the external clear water pipe  1132 . In  FIG. 11E  the pipe  1132  has a diameter D 2 . The opening in a top surface  1142  of the tank  40 A is also D 2 . The example illustrated in  FIG. 11E  is particularly suitable for high density slurry. 
     Referring now to  FIG. 11F , a diffuser piece  1140  is attached within the first end  46 A of the first tank  40 A. The diffuser piece  1140  extends between the distribution plate  64 A and a top surface  1142  of the tank  40 A. In  FIG. 11F  the top surface  1142  is generally planer or perpendicular to the longitudinal direction of the tank. Angle B illustrated in the figure allows the surface  1144  to be conical in shape. Various angles for angle B may be used. In this example an angle B of about fifty degrees to about sixty degrees from the horizontal may be used. A flat horizontal surface  1146  may also be formed at the interface or top portion of the diffuser piece  1140 . The diffuser piece  1140  has the flat surface or horizontal surface  1146  having a particular length. The length/diameter D 1  of the surface  1146  may be about twice the diameter D 2  of the pipe  60 A. Of course, different dimensions for the angle B and the distance D 1  may be used. In fact D 2  and D 1  may be equivalent and thus eliminating the surface  1146 . One example is of this is shown below in  FIG. 11J . 
     Referring now to  FIG. 11G , the diffuser piece  1140 ′ has been modified to fit into the tank  40  with a hemispheric end wall  1142 ′. The shape of the surface  1144  may be similar to that described above with respect to  FIG. 11F , which is conical with a horizontal portion  1146 . Likewise, the angle B may also be similar. 
     Referring now to  FIG. 11H , the end wall  1142  and  1142 ′ illustrated in  FIGS. 11F and 11G  have been modified to form end walls  1150  and  1152 . The end wall  1150  and  1152  may be disposed at the angle B. In a sense, the walls  1150  and  115  may act functionally as a diffuser. 
     Referring now to  FIG. 11I , the end wall of the tank  40 A′ is formed in a similar manner to that of  11 H with the exception of an additional wall  1154 . The wall  1150  and  1152  angle in a similar manner to that set forth above with respect to  FIG. 11E  in that the interior portions of the tank or the interior portions of the walls  1150  and  1152  are disposed at an angle B to form an integrated diffuser piece. The walls  1150  and  1152  are one continuous wall that forms a conical surface. In  FIG. 11H  the third wall  1154  is horizontal and may have a similar width D 1  which is about two times the width of the pipe D 2 , in the present example. 
     Referring now to  FIG. 11J , the tank  40 A the diffuser piece  1140  includes a nozzle  1156 . The nozzle expands from the diameter of the pipe D 1  to the diameter D 3  at the lower surface of the diffuser piece  1140 . The angle B relative to horizontal may also vary as described above. The diameter D 3  may be twice the diameter D 1  to obtain the diffusing effect. The diffuser  1156  lowers the exit velocity of the clear fluid entering the tank  40 A. The configuration of  FIG. 11J  is suitable for low density slurry. 
     The external pipe  1132  illustrated above may be substituted into the embodiments like  FIGS. 1A-1K  with the internal pipe  60 A/ 60 B while removing the flow distribution plates. 
     The embodiments shown in  FIGS. 11F, 11G, 11H, 11I and 11J  are particularly suitable when the unit is handling a low density slurry. The conical shape top end of the tank allows the diffusion of the incoming clear water flow. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.