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RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application No. 61/676,774, filed on Jul. 27, 2012, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to non-explosive mining techniques for mining operations. 
     2. Description of the Related Art 
     Non-explosive mining techniques offer an alternative to the increasing costs associated with explosive excavation. Explosive excavation is a cyclic process requiring several steps: blast holes are drilled into a rock face, explosive charges are loaded into the blast holes, the surrounding area is evacuated, the explosives are detonated, and the area is ventilated and cleared. Explosive excavation incurs significant costs associated with security and environmental damage, such as the generation of toxic gases. 
     Mechanized non-explosive mining may be carried out with fewer personnel and reduce the security and environmental costs of high explosives. This approach also increases processing efficiency by allowing selective mining of the ore veins. Mechanical impact hammers can be used to excavate hard rock, but the process is slow; the hammers and support equipment are very heavy and the impact tools wear out quickly. 
     Another example of mechanized non-explosive mining is an impact piston water cannon, in which compressed air drives a heavy piston that impacts and pushes a quantity, or slug, of water. The water slug impacts the rock face to cause erosion and excavation. While impact piston devices have been shown to generate high pressures, their use in commercial excavation work has been limited due to the significant wear on the pistons and cylinders of the devices. Further, the mechanical system that must be maneuvered at the rock face is prohibitively bulky. 
     As an alternative to an impact piston cannon, a compressed water cannon designed for hard rock mining is described in “A Hydraulic Pulse Generator for Non-Explosive Excavation,” by Kolle, J. J., in  Mining Engineering , July 1997, pg. 64-72, which is herein incorporated by reference in its entirety. The compressed water cannon comprises a heavy pressure vessel charged to very high pressures (100-400 MPa, or 14,500-60,000 psi). At these pressures, the water is substantially compressed and stores a considerable amount of energy. After charging, the water is discharged through a fast-opening valve, which causes the resulting pulse of water to impact the rock face. Discharge of a 100 to 400 MPa pulse onto the face of hard rock will have little or no effect in rock fragmentation. To perform rock fragmentation, the compressed water cannon nozzle must be inserted and discharged into a pre-drilled blast hole. Discharge of the pulse into the blast hole generates tensile stresses in the rock and allows effective excavation. The productivity and flexibility of this approach, called bench blasting, is limited because drilling is the most time-consuming aspect of the operation. 
     As reported by Mauer, W. C. in  Advanced Drilling Techniques , pg. 302-348, Petroleum Publishing Inc., 1980, hyper-pressure pulses that are over 1 GPa, or 145,000 psi, have been shown to efficiently excavate hard rock by cratering, eliminating the need for a pre-drilled blast hole. Accordingly, it would be desirable to enable a compressed water cannon to be employed without the need for a pre-drilled blast hole. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the problems above are addressed with a hyper-pressure water cannon. The hyper-pressure water cannon, or pulse excavator, is able to discharge fluid pulses at extremely high velocities to fracture a rock face in excavation applications. A compressed water cannon can be used to generate hyper-pressure pulses by discharging the pulse into a straight nozzle section which leads to a convergent tapered nozzle. The water cannon design is relatively compact, and the pulse generator can readily be maneuvered to cover the face of an excavation as part of a mobile mining system. As an alternative, the pulse could be generated by a propellant gun. 
     Hyper-pressure pulse excavation, or cratering, is an application of the water cannon that eliminates the need for drilling a blast hole. The high-velocity water pulse is discharged into a combination straight and tapered nozzle that can amplify the peak pulse pressure by a factor of 10 or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  illustrates a cross-sectional schematic view of a complete hyper-pressure pulse excavator  100  including an electrical trigger, vent valve assembly  150 , pressure vessel  110 , and two-part nozzle assembly ( 120  and  132 ); 
         FIGS. 1B-1E  illustrate the hyper-pressure pulse excavator  100  in various stages of preparing to fire a water pulse; 
         FIGS. 2A-2C  illustrate exemplary measurements for various sizes of the hyper-pressure pulse excavator  100 ; 
         FIGS. 3A-3C  show nozzle inlet pulse measurement charts based on a 230 MPa discharge from the exemplary embodiment shown in  FIG. 2A ; 
         FIG. 4  illustrates the process of unsteady flow acceleration of a water pulse through straight and tapered nozzle sections; 
         FIG. 5A-5C  illustrate the hyper-pressure outlet pulse measurement charts; and 
         FIG. 5D  shows a chart displaying an exemplary exponentially convergent tapered nozzle profile. 
         FIG. 5E  shows a chart displaying the internal pressure profiles inside an exponentially tapered nozzle at three locations of the fluid pulse. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
       FIG. 1A  illustrates a schematic of an exemplary hyper-pressure pulse excavator  100 , shown after firing a water pulse. The pulse excavator  100  includes a pressure vessel  110  and a two-part nozzle assembly, which includes a straight nozzle section  120  and a tapered nozzle section  132  within a nozzle housing  130 . The pressure vessel  110  includes a supply tube  112 , a poppet sleeve  114 , a sleeve port  116 , and a poppet  118 . When poppet  118  is closed, it sits against poppet seat  119  at the end of pressure vessel  110 . When the poppet  118  is opened, or pushed away from the poppet seat  119 , the poppet  118  and poppet seat  119  together act as a dump valve, and pressurized fluid in the pressure vessel  110  is discharged into the straight nozzle section  120 . The junction of the pressure vessel  110  and the straight nozzle section  120  includes an opening connected to an air compressor  126  and a second opening connected to a metering pump  122  and a gel supply  124 . The electrical subsystem of the pulse excavator  100  includes a push button switch  170 , arm light  172 , arm switch  174 , relay switch  176 , and the solenoid valve  180  (including battery power for the solenoid). 
     Fluids within the hyper-pressure pulse excavator  100  build to extremely high pressures and must be discharged very quickly to effectively crater rock. Additionally, an excavating tool such as the pulse excavator  100  should not be so unwieldy and large as to prevent moving the tool around the rock face. Off-the-shelf valve systems offering suitable performance in both size and speed for such operation are typically not available. Instead, as shown in  FIG. 1A , a series, or system, of cascading valves leading to the pressure vessel  110  can be used. Each subsequent stage the handles progressively larger volumes and pressures, and the final stage opens the poppet  118  in the pressure vessel  110 . While  FIG. 1A  shows an exemplary series of cascading valves, different types and arrangements of valves may be used to operate the poppet  118  in the pressure vessel  110 . 
     The series of cascading valves includes the solenoid valve  180 , the hydraulic pump return valve  146 , the pressurized water supply valve  184 , and the vent valve assembly  150 . In operation, the accumulator  140 , return tank  142 , and hydraulic pump  148 , and isolator piston  144  serve to maintain a pressure on the vent valve assembly  150  until the solenoid valve  180  can open. In the discharged state after firing, the hydraulic pump return valve  146  is open, resulting in water pressure from pressurized water supply  182  moving the isolator piston  144  to its upper position. The hydraulic pump  148  is also shown with a return tank  142  and an accumulator  140 . Additionally, the pressurized water supply valve  184  is open, and the solenoid valve  180  to the tank  178  is closed and unarmed. Additional details of the valve operation can be seen in U.S. Pat. No. 5,000,516 to Kolle, entitled “Apparatus for rapidly generating pressure pulses for demolition of rock having reduced pressure head loss and component wear,” issued Mar. 19, 1991, which is incorporated herein in its entirety. 
     In a preferred embodiment of the invention, the pulse excavator  100  further includes a vent valve assembly  150 . The vent valve assembly  150  includes a vent valve housing  158  with vent valve vents  160 . Although the pressurized water supply valve  184  is open, the vent valve piston  156  in the vent valve housing  158  is not pressurized to a sufficient level to tightly hold the poppet  154  against its seat  152 . The vent valve assembly  150  is connected to the supply tube  112  of the pressure vessel  110 . An ultra-high pressure pump  162  with a water inlet  164  is also coupled to the vent valve assembly. 
       FIG. 1B  shows the system ready to fire a water, or water-based, pulse. The pressurized water supply valve  184  is closed. The hydraulic pump return valve  146  of the hydraulic pump  148  is closed, and the hydraulic pump  148  has been actuated, pressurizing the top of the isolator piston  144  with oil, water, or another fluid. The other side of the isolator piston  144  contains water. When the top of the isolator piston  144  is pressurized, the left side of the vent valve piston  156  is pressurized, causing the vent valve piston  156  to push against and hold the vent valve poppet  154  against the vent valve poppet seat  152 . The ultra-high pressure pump  162  is then actuated and used to charge the pressure vessel  110  through the supply tube  112  into the cavity between the poppet sleeve  114  and poppet  118  within the pressure vessel  110 . This pressurization pushes the poppet  118  against its seat  119  at the outlet of the pressure vessel  110 , closing the fluid path to the straight nozzle section  120 . With the poppet  118  seated against the straight nozzle section  120 , the sleeve port  116  is exposed, allowing water to flow into the pressure vessel  110  through the supply tube  112 . As more water is pumped into the pressure vessel  110 , the pressure within the pressure vessel  110  builds, typically to 100 to 400 MPa. 
     In parallel, the air compressor  126  may supply compressed air to the straight nozzle section  120 . This helps to empty the straight nozzle section  120  and tapered nozzle section  132  of any residual water (for example, from the previous water pulse firing). In one embodiment, a small volume of a gelled fluid  125  such as agar, polyacrylamide, or bentonite gel may be metered using the metering pump  122  from into the straight nozzle section  120  immediately below the poppet seat  119 . This precharges the straight nozzle section  120  with the gelled fluid  125 , allowing the gelled fluid  125  to be on the leading edge of the fluid pulse when the pulse excavator  100  fires. This gelled fluid may also be weighted with a substance such as salt to increase its density. The arm switch  174  electrical circuit is then armed, the air valve of the air compressor  126  is closed, and the system  100  is ready to fire. 
       FIG. 1C  illustrates the start of the firing sequence. The push button switch  170  is closed or depressed, causing the relay switch  176  to close and the solenoid valve  180  to open. As the solenoid valve  180  opens, the isolator piston  144  moves down at constant pressure. The opening time of the solenoid valve  180  is preferably very short, such as on the order of 100 milliseconds so, but there is a limit to the opening speed of solenoid valves. The isolator piston  144  and accumulator  140  assembly give the solenoid valve  180  time to open fully by maintaining pressure on the vent valve poppet  154  before the isolator piston  144  reaches the end of its travel. As soon as the isolator piston  144  reaches the end of its travel, the left side of the vent valve piston  156  is depressurized, and the ultra-high pressure on the face of the vent valve poppet  154  causes it to open. 
       FIG. 1D  illustrates the continuation of the firing sequence, with the vent valve poppet  154  fully open. This depressurizes the water in the supply tube  112  and the volume of water in the cavity between the poppet  118  and poppet sleeve  114  in the pressure vessel  110 . Because the section area of the poppet  118  is larger than the seal area of the poppet seat at the base of the straight nozzle section  120 , a large force lifts the poppet  114  from its seat. The poppet  118  opens very quickly, acting like a fast-opening dump valve and discharging the compressed water from the body of the pressure vessel  110 . Once the poppet  118  is open, the water contained in the pressure vessel  110  begins accelerating through the straight nozzle section  120 . As mentioned above, if gel has been metered out into the straight nozzle section  120 , the gel slug is also pushed by the accelerating water pulse. The gel slug and water slug are pushed through the straight nozzle section  120  as well as the nozzle housing  130 , as shown in  FIG. 1E . The nozzle housing  130  contains a tapered nozzle section  132 , which tapers from the diameter of the opening of the straight nozzle section  120 . 
     Due to the unsteady flow phenomenon, the gel and water slugs are extruded though the tapered nozzle section  132  at extremely high velocities. The process of unsteady flow acceleration is illustrated in  FIG. 4 . When a fluid pulse moving at uniform velocity, U o , enters a tapered nozzle, the leading edge of the pulse accelerates (U e ), while the trailing edge of the pulse slows (U b ). The velocities can be calculated for a given nozzle profile based on the principles of continuity of momentum and volume. If no gel is used, then the water will be at the leading edge of the pulse. In a preferred embodiment of the invention, the tapered section  132  is exponential. 
     Due to the extreme pressures generated in employing this technique, nozzle wear and fatigue of the cannon body are concern for long-term operation. The tapered nozzle section  132  is preferably fabricated from a hard erosion-resistant material such as hardened steel or carbide. This material may be held by a nozzle housing  130  made of high strength steel. The two part construction of the tapered nozzle allows the use of hard, erosion-resistant materials that may have low tensile strength. Conversely, the tapered nozzle can be fabricated from one part if a sufficiently high strength steel is used. 
       FIGS. 2A-2C  illustrate exemplary dimensional measurements for various sizes of the hyper-pressure pulse excavator  100 . The productivity of hyper-pressure pulse excavation can be expressed in terms of specific energy, which is the ratio of the pulse energy to the volume of rock removed. Increasing the scale of the system increases efficiency substantially, since the specific energy required for breaking is inversely proportional to the rock fragment size. As described above, impact piston cannons provide a means of generating hyper-pressure pulses, but the mechanism for these devices is very bulky and generates large reaction forces. Further, as also described above, their use in commercial excavation work has been limited due to the significant wear on the pistons and cylinders of the devices. The compressed water cannon as described herein can provide the similar pressure levels more efficiently. As described above, the pulse excavator  100  uses the system of cascaded valves to build to sufficient pressure levels. In a smaller embodiment, such as the one seen in  FIG. 2A , alternate valve systems, such as a hand valve or a large solenoid valve, may be used. This may allow the pulse excavator  110  to be operated with a single- or dual-level valve system. For larger embodiments, such as the ones seen in  FIGS. 2B and 2C , single- or dual-level valve systems will likely not provide the performance required for operation. Additionally, the cascaded valve system allows for smaller valves to be used at the various stages, further allowing for the use of smaller batteries to actuate the solenoid valve  180 . 
     The specifications for the exemplary embodiment shown in  FIG. 2A  of the compressed water cannon for use in hyper-pressure pulse excavation are as follows:
         1.8-liter internal volume;   15 kJ @ 240-MPa charge pressure; and   12.7-mm-diameter discharge nozzle.       

     The operating pressure of the pressure vessel  110  alone is limited by practical considerations to 100-400 MPa (14,500-60,000 psi). However, the pressure required to effectively break harder rock requires fluid pulses with stagnation pressures above 2 GPa (300,000 psi). As mentioned above, the straight nozzle section  120  and tapered nozzle section  132  are used to amplify the velocities of fluid pulses to achieve the stagnation pressures required to effectively break rock. The diameter of the straight nozzle section  120  may be equal to the diameter of the discharge valve of the pressure vessel  110 . The diameter of the straight nozzle section  120  is smaller than the diameter of the pressure vessel  110  bore—typically, around 20% to 30% of the bore is preferred, though the range could be 10% to 50%. 
     The length of the straight nozzle section  120  is determined by observing the discharge characteristics of the pressure vessel  110  without the nozzle section attached.  FIG. 3A  shows the observed stagnation pressure from a water pulse discharged from the exemplary embodiment shown in  FIG. 2A  (without the attached nozzle) when the pressure vessel  110  is charged to 230 MPa versus time. Note that the peak stagnation pressure is substantially less than the charge pressure of 230 MPa. Further, the rise time of the pressure release is very fast, on the order of 1-2 ms. The fast rise time is facilitated by the presence of the fast-opening dump valve, such as the poppet valve  118 .  FIG. 3B  shows the velocity of the pulse as a function of pulse length as calculated from the stagnation pressure profile. A uniform-velocity slug of water is needed to generate a hyper-pressure pulse in a tapered nozzle section  132 . In practice, the velocity of water exiting the cannon valve varies continuously, however a pulse of about 0.5 m length with a velocity of over 500 m/s is generated. The kinetic energy of the pulse rises linearly up to around 0.5 m and then increases at a lower rate. The velocity is slow as the valve opens, peaks after the valve is opened, and then drops as the cannon decompresses. A straight nozzle section  120  accumulates the water in the leading edge of the pulse and allows the higher-velocity fluid to catch up, forming a uniform-velocity slug. Once the slug velocity starts to drop, the slug will stretch and break up. 
     Based on a measurement of the discharge pressure of the pressure vessel  110  at 230 MPa, the velocity of the water pulse can be measured against the length of the pulse. To reach efficiencies, pulse velocity and length should be maximized. For the pressure vessel  110  of the exemplary embodiment shown in  FIG. 2A , a pulse length of 0.5 meters was chosen based on the chart shown in  FIG. 3B . The point representing the pulse length of 0.5 meters in  FIG. 3B  was selected as maximizing both pulse velocity and length because the pulse velocity begins to decrease more substantially after the pulse length of 0.5 meters. Accordingly, the length of the straight nozzle section  120  was set at 0.5 meters. The final volume of the straight nozzle section  120  may be preferably between 2-10% of the volume of the pressure vessel  110 . 
     Given a 20 inch long (i.e., roughly 0.5 meter) slug with a diameter of 0.5 inch, the tapered nozzle parameters may be determined. As mentioned above, the tapered nozzle section  132  accelerates the leading edge of the pulse to hyper velocity through unsteady flow dynamics. Given a convergent tapered nozzle  132  with an arbitrary profile, it is possible to calculate the velocity of the slug of water everywhere as the slug is extruded though the taper by solving the equations for continuity of volume and momentum. This may be determined using a numerical simulation of these continuity equations for various nozzle profiles. The internal pressure along the length of the nozzle can also be calculated from the local acceleration. The details of this calculation are described in Glenn, Lewis A. (1974) “On the dynamics of Hypervelocity liquid jet impact on a flat rigid surface,”  Journal of Applied Mathematics and Physics  ( ZAMP ), vol. 25. 
     A numerical analysis indicates that the exemplary compressed water cannon tool from  FIG. 2A  can produce a compressed water pulse that is 300-mm in length, traveling at a velocity of about 520 m/s, as shown  FIG. 5A . The theoretical profile agrees reasonably well with the observed profile shown in  FIG. 3B . The theoretical velocities of the leading and trailing edges (shown as U e  and U b , respectively) of this water slug as it moves through the tapered nozzle are shown in  FIG. 5B . The leading edge accelerates to over 2000 m/s, while the trailing edge decelerates. The peak velocity drops rapidly, to under 1000 m/s after 200 μsec. In this time the leading edge of the pulse will travel 0.4 m (16 in.). The nozzle should be located at a fraction of this distance from the target to maximize effectiveness. The velocity profiles may be calculated by assuming that the water is an incompressible fluid, although water is compressible at such velocities. The peak velocity of the discharged jet may be limited by the speed of sound in water (around 1500 m/s), which may limit the peak velocities to values lower than those shown in  FIG. 5B . The compressed water pulse will convert to a 2-GPa pressure spike in a 150-mm-long convergent tapered nozzle, as shown in  FIG. 5B , with 80% energy conversion above 1 GPa, as shown in  FIG. 5C . 
     An example of the internal pressure profiles inside an exponentially tapered nozzle at three locations of the pulse is provided in  FIG. 5E . The internal pressure builds as the pulse enters the tapered section. The peak pressure occurs at the moment that the pulse reaches the exit of the nozzle. The peak internal pressure is less than 1 GPa (145,000 psi) which is within the capacity of the nozzle materials available. In a preferred embodiment of the invention, the nozzle comprises a carbide inner section that is pressed into a sleeve to provide a preload on the carbide. Those skilled in the art will understand that a composite nozzle of this type provides higher internal pressure capacity than a monobloc nozzle. 
     The cross-sectional area of the tapered nozzle section  132  is denoted as A(x), and it decreases exponentially along the length of the tapered nozzle section  132 , which is denoted as x. The relationship between the length and cross-sectional area of the tapered nozzle section  132  is shown according to the following exponential equation: 
               A   ⁡     (   x   )       =       A   i     ⁢     exp   ⁡     (         -   x     ⁢           ⁢     ln   ⁡     (   R   )           l   t       )               
In this equation, R is the inlet/outlet area ratio; and l t  is the total length of the tapered nozzle section  132 . An example of a nozzle profile is as shown in  FIG. 5D , which is derived from the data in the following Table 1.
 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Length, in. 
                 Diameter, in. 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Straight 
                 20 
                 0.500 
               
               
                   
                 Taper 
                 0 
                 0.500 
               
               
                   
                   
                 2 
                 0.429 
               
               
                   
                   
                 4 
                 0.369 
               
               
                   
                   
                 6 
                 0.316 
               
               
                   
                   
                 8 
                 0.272 
               
               
                   
                   
                 10 
                 0.233 
               
               
                   
                   
                 12 
                 0.200 
               
               
                   
                   
               
             
          
         
       
     
     An exponential tapering is used for the tapered nozzle section  132 , as opposed to a linear tapering, to prevent the tapered section from being blown off from the pressure release during a firing. An external nut may be used to clamp the tapered nozzle section  132  to the straight nozzle section  120 . This nut may be attached with a torque of about 2000 ft-lbf. Based on the configuration of the straight nozzle section  120  and tapered nozzle section  132 , a water cannon may be converted into the hyper-pressure water cannon  100  suitable for use in excavation applications. 
     Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description.

Summary:
A hyper-pressure water cannon, or pulse excavator, is able to discharge fluid pulses at extremely high velocities to fracture a rock face in excavation applications. A compressed water cannon can be used to generate hyper-pressure pulses by discharging the pulse into a straight nozzle section which leads to a convergent tapered nozzle. The hyper-pressure water cannon design is relatively compact, and the pulse generator can readily be maneuvered to cover the face of an excavation as part of a mobile mining system.