Abstract:
A high pressure cutting arrangement is formed by combining a liquid stream, such as water, and a slurry stream, the slurry comprising abrasive particles suspended in a liquid. Energy is supplied to the liquid stream by a first energising means, such as a constant pressure pump. Energy is supplied to the slurry stream by a second energising means, such as by a piston powered by a constant volume pump. The liquid stream and the slurry stream are combined in a cutting tool, in which the supplied energy is converted to kinetic energy to produce a combined liquid and abrasive stream at high velocity.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to cutting (for instance of metals) by jets of liquid including entrained abrasive particles. 
       BACKGROUND TO THE INVENTION 
       [0002]    The use of high velocity water jets containing entrained abrasive particles for cutting purposes has been known since about 1980. Known cutting water jet systems fall into one of two categories: Abrasive water jet (AWJ) systems and Abrasive suspension jet (ASJ) systems. 
         [0003]    AWJ systems typically supply water at extremely high pressure (in the order of 150 to 600 MPa) to a nozzle. A typical AWJ nozzle  10  is shown in  FIG. 1 . The nozzle  10  includes a small orifice  12  (0.2 to 0.4 mm diameter) which leads into a mixing chamber  14 . Water thus flows through the mixing chamber  14  at a high velocity. 
         [0004]    Small grains of abrasive material, typically garnet, are supplied to the chamber, generally by a gravity feed through a hopper  16 . The high water velocity  18  creates a venturi effect, and the abrasive material is drawn into the water jet. 
         [0005]    The water jet then flows through a length of tubing known as a focusing tube  20 . The passage of water and abrasive through the focussing tube acts to accelerate the abrasive particles in the direction of water flow. The focussed water jet  22  then exits through an outlet  24  of the focussing tube. The water jet  22 —or, more accurately, the accelerated abrasive particles—can then be used to cut materials such as metal. 
         [0006]    The energy losses in the nozzle  10  between the orifice  12  and the outlet  24  of the focussing tube  20  can be high. Kinetic energy of the water is lost by the need to accelerate the abrasive material, and also to accelerate air entrained by the venturi. Significant frictional losses occur in the focussing tube  20 , as abrasive particles ‘bounce’ against the walls of the tube. This results in energy loss due to heat generation. As an aside, this phenomenon also results in degradation of the focussing tube, which typically needs replacing after about 40 hours&#39; operation. 
         [0007]    Known AWJ systems are therefore highly inefficient. 
         [0008]    ASJ systems combine two fluid streams, a liquid (generally water) stream and a slurry stream. The slurry contains a suspension of abrasive particles. Both liquid streams are placed under a pressure of about 50 to 100 MPa, and are combined to form a single stream. The combined stream is forced through an orifice, typically in the order of 1.0 to 2.0 mm diameter, to produce a water jet with entrained abrasive particles. 
         [0009]    ASJ systems do not suffer from the same inefficiencies as AWJ systems, as there is no energy loss entailed in combining the two pressurised streams. Nonetheless, known ASJ systems are of limited commercial value. This is partly because ASJ systems operate at significantly lower pressures and jet velocities than AWJ systems, limiting their ability to cut some materials. 
         [0010]    ASJ systems also evidence significant difficulties in operation, primarily due to the presence of a pressurised abrasive slurry, and to the lack of effective means to provide control over its flow characteristics. The parts of the system involved in pumping, transporting and controlling the flow of the abrasive slurry are subject to extremely high wear rates. These wear rates increase as the pressure rises, limiting the pressure at which ASJ systems can safely operate. 
         [0011]    Of possible greater significance are the practical difficulties inherent in starting and stopping a pressurised abrasive flow. When used for machining, for instance, a cutting water jet must be able to frequently start and stop on demand. For an ASJ system, this would require the closing of a valve against the pressurised abrasive flow. Wear rates for a valve used in such a manner are extremely high. It will be appreciated that during closing of a valve the cross-sectional area of flow decreases to zero. This decreasing of flow area causes a corresponding increase in flow velocity during closing of the valve, and therefore increases the local wear at the valve. 
         [0012]    In a typical industrial CNC environment, cutting apparatus can be required to start and stop extremely frequently. This translates to frequent opening and closing of valves against pressurised abrasive flow, and rapid wear and deterioration of these valves. As a result, the use of ASJ systems for CNC machining is known to be inherently impractical. 
         [0013]    ASJ systems have found use in on-site environments, such as oil-and-gas installations and sub-sea cutting, where the cutting required is largely continuous. ASJ systems have not been commercially used in industrial CNC machining. 
         [0014]      FIGS. 2   a  and  2   b  show schematic representations of known ASJ systems. In a basic single stream system  30 , as shown in  FIG. 2   a , a high pressure water pump  32  propels a floating piston  34 . The piston  34  pressurises an abrasive slurry  36  and pumps it into a cutting nozzle  38 . 
         [0015]    A simple dual-stream system  40  is shown in  FIG. 2   b . Water from the pump  32  is divided into two streams, one of which is used to pressurise and pump a slurry  36  by means of a floating piston  34  in a similar manner to the single stream system  30 . The other stream, a dedicated water stream  35 , is combined with a pressurised slurry stream  37  at a junction prior to the cutting nozzle  38 . 
         [0016]    Both of these systems suffer from the problems outlined above, and result in very high valve wear rates. Other problems include an inconsistent cutting rate due to extreme wear in the tubes and nozzle. 
         [0017]    An alternative arrangement is proposed in U.S. Pat. No. 4,707,952 to Krasnoff. A schematic arrangement of the Krasnoff system  50  is shown in  FIG. 3   a . The Krasnoff system is similar to the dual-stream system  40 , with the difference being that mixing of the water stream  35  and slurry stream  37  takes place in a mixing chamber  52  within the cutting nozzle  38 . 
         [0018]    A more detailed view of the mixing chamber  52  of Krasnoff is shown in  FIG. 3   b . The nozzle  38  provides a two-stage acceleration. Firstly, the water stream  35  and the slurry stream  37  are accelerated through independent nozzles leading into the mixing chamber  52 . Then the combined water and abrasive stream is accelerated through the final outlet  54 . 
         [0019]    The Krasnoff system is arranged to operate at a pressure of about 16 MPa, significantly lower than other ASJ systems. As such, the impact of the slurry stream  37 , whilst still damaging to valves, results in reduced valve wear rates than in higher pressure systems. The corollary is, of course, that the power output of the Krasnoff system is even lower than other ASJ systems, and thus its commercial applications are small. The applicant is not aware that the Krasnoff system has ever been commercially applied. 
         [0020]    The present invention seeks to provide a system for creating a high pressure water jet with entrained abrasive particles which overcomes, at least in part, some of the above mentioned disadvantages of above AWJ and ASJ systems. 
       SUMMARY OF THE INVENTION 
       [0021]    In essence, the present invention proposes a method which combines many of the advantages of AWJ and ASJ systems whilst reducing some of the disadvantages of each system. 
         [0022]    In accordance with a first aspect of the present invention there is provided a high pressure cutting arrangement comprising a liquid stream and a slurry stream, the slurry comprising abrasive particles suspended in a liquid, energy being supplied to the liquid stream by a first energising means and energy being supplied to the slurry stream by a second energising means, each of the first and the second energising means being selectively operable, wherein the liquid stream and the slurry stream are combined in a cutting tool, at least a portion of the supplied energy being converted to kinetic energy in the cutting tool to produce a combined liquid and abrasive stream at high velocity. The use of separate energising means allows control over stream flows in the system. 
         [0023]    Preferably the energy supplied by the first energising means is provided by a pump, most preferably a constant pressure pump, which pressurises the liquid stream. Similarly, the energy supplied by the second energising means is preferably provided by a pump, most preferably a constant flow pump. This arrangement allows the velocity and volume rate of the combined stream to be regulated by control of the pressure of the constant pressure pump, whilst the flow rate of abrasive material can be independently set by controlling the flow rate of the constant flow pump. Adjustment of the system power, or the fluid:abrasive ratio, can thus be readily achieved. In an alternative arrangement, a single pump may provide energy to both the first and the second energising means. 
         [0024]    In a preferred embodiment, the constant flow pump energises a floating piston, which in turn pressurizes the slurry stream. In this embodiment a valve may be provided between the pump and the floating piston, such that the flow of liquid and therefore energy from the constant flow pump to the floating piston can be instantly prevented. Conveniently, this valve may also act to prevent back flow of liquid from the floating piston. In this way pressure and flow in the slurry stream can be allowed to vary whilst maintaining constant pressure in the liquid stream. The valve may simply act to divert the constant liquid flow away from the floating piston, for instance by returning the liquid to a reservoir of the pump. 
         [0025]    In its preferred form the cutting tool allows the streams to combine in such a way that the pressure of the slurry stream is governed primarily by the pressure of the liquid stream. The cutting tool includes a combining chamber into which the liquid stream, when energised, is provided at a constant pressure; and the slurry stream, when energised, is provided at a constant rate. The pressure at an entry region of the combining chamber is thus set by the pressure of the liquid stream. The point of entry of the slurry stream into the combining chamber is exposed to this pressure, in such a way that the slurry stream is prevented from entering the combining chamber unless the pressure in the slurry stream is marginally higher than the pressure at the combining chamber entry point. The action of the constant volume pump builds the pressure in the slurry stream until it reaches this point. A first equilibrium condition is then achieved where slurry is provided at a constant flow rate, and at the required pressure, into the combining chamber. Under these conditions the constant volume pump effectively acts as a constant displacement delivery pump. 
         [0026]    When the second energising means ceases providing energy to the slurry stream, for instance by closing of the valve between pump and piston in the preferred embodiment, the pressure of the liquid stream in the combining chamber continues to act on the slurry stream. Slurry from the slurry stream continues to enter the combining chamber until such time as the pressure in the slurry stream drops marginally below the pressure in the combining chamber. At this point, the flow of slurry ceases but the pressure in the slurry stream is maintained. This enables a valve in the slurry stream to be closed against a static, albeit pressurised, abrasive stream. The valve is subject to a considerably reduced wear rate in comparison to one closing against a flowing abrasive stream. Closure of this valve ensures that in the only flow to the cutting head is water. Subsequent closure of a valve in the water stream will prevent all flow of liquid through the cutting head. 
         [0027]    Preferably the liquid stream, and hence the slurry stream, operate at a pressure of about 300 MPa. 
         [0028]    It will be appreciated that the ceasing of energy supply from the second energising means results in an almost instantaneous ceasing of slurry, due to the small pressure difference in the slurry between a flowing state and a static state. Similarly, when the second energising means is activated, the required flow of slurry into the combining chamber is achieved almost instantaneously. 
         [0029]    Preferably, the cutting tool includes a combining chamber, the combining chamber having an entry region arranged to receive the liquid stream and the slurry stream, wherein the pressure in the entry region is determined by the pressure in the liquid stream, and the pressure in the entry region acts on the pressure in the slurry stream to regulate the pressure in the slurry stream. 
         [0030]    Preferably the slurry stream and the liquid stream are arranged to enter a nozzle, the nozzle being elongate and the slurry stream and the liquid stream being oriented in the elongate direction. This reduces energy loss involved in changing flow direction, particularly of the slurry. 
         [0031]    In a preferred arrangement the nozzle has a central axis, with the slurry stream being oriented along the central axis and the liquid stream being provided in an anulus about the slurry stream. Such an arrangement provides an efficient means of exposing the slurry stream to the pressure of the liquid stream, and also reduces the propensity for the sides of the nozzle to wear. 
         [0032]    Preferably the nozzle is an accelerating nozzle, with an outlet smaller in diameter than the entry region. This allows the pressure within the streams to be converted to a high velocity output stream. 
         [0033]    The effect is further enhanced by making an outlet smaller in diameter than a diameter of the slurry stream on entry into the nozzle. 
         [0034]    Preferably the nozzle has a constant diameter focussing portion at an outer end thereof, and a conical accelerating portion of reducing diameter between the entry region and the focussing portion. This allows the output stream to achieve both a desired velocity and direction. 
         [0035]    The cone angle of the accelerating portion should not exceed 27°. Preferably, the cone angle should be about 13.5°. This provides a good balance between efficient acceleration and maintaining non-turbulent flow. 
         [0036]    Preferably, the focussing portion of the nozzle should have a length:diameter ratio greater than 5:1, preferably about 10:1. It is also preferred that the length:diameter ratio is less than about 30:1. 
         [0037]    The nozzle may be a compound nozzle, with the accelerating portion formed from a material harder than that of the focussing portion. 
         [0038]    The focussing portion may have a diameter equal to or slightly smaller than the smallest diameter of the accelerating region, to guard against the introduction of turbulence. 
         [0039]    The outlet may include an exit chamfer having a cone angle of about 45°. Such an angle is sufficient to ensure flow separation at the outlet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    It will be convenient to further describe the invention with reference to the accompanying drawings which illustrate preferred embodiments of the high pressure cutting arrangement of the present invention. Other embodiments are possible, and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. In the drawings: 
           [0041]      FIG. 1  is a schematic cross sectional view of a cutting tool of an AWJ system of the prior art; 
           [0042]      FIG. 2   a  is a schematic view of a single fluid ASJ system of the prior art; 
           [0043]      FIG. 2   b  is a schematic view of a dual fluid ASJ system of the prior art; 
           [0044]      FIG. 3   a  is a schematic view of a dual fluid ASJ system of the prior art where fluids are injected into a cutting nozzle; 
           [0045]      FIG. 3   b  is a cross sectional view of the prior art cutting nozzle of  FIG. 3   a;    
           [0046]      FIG. 4  is a schematic view of the high pressure cutting arrangement of the present invention; 
           [0047]      FIG. 5  is a cutting tool from within the cutting arrangement of  FIG. 4 ; 
           [0048]      FIG. 6  is a cross sectional view of a portion of the cutting tool of  FIG. 5 , including a nozzle; 
           [0049]      FIG. 7  is a cross sectional view of a focussing nozzle within the cutting tool of  FIG. 5 ; 
           [0050]      FIG. 8  is a cross sectional view of an alternative embodiment of a focussing nozzle for use within the cutting tool of  FIG. 5 ; and 
           [0051]      FIG. 9  is an alternative embodiment of a cutting tool for use within the cutting arrangement of  FIG. 4 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0052]      FIG. 4  shows a schematic arrangement of a high pressure cutting system  100 . The cutting system  100  has a cutting tool  110 , to which is attached two input lines: a fluid or water flow stream  112  and a slurry flow stream  114 . Each of the water flow stream  112  and the slurry flow stream  114  are supplied to the cutting tool  110  under pressure. 
         [0053]    Pressure is applied to the water flow stream  112  by a first energising means, being a constant pressure pump  116 . In this embodiment, the constant pressure pump  116  is an intensifier type pump. The constant pressure pump  116  ensures that pressure in the water flow stream  112  is maintained at a constant, desired pressure. The desired pressure may be altered by control of the constant pressure pump  116 . A typical available pressure range may be 150 MPa to 600 MPa. In typical operation, water pressure of about 300 MPa will provide a useful result. 
         [0054]    Pressure is applied to the slurry flow stream  114  by a second energising means. The second energising means comprises a floating piston  118  which is powered by a constant flow water pump  120 . In this embodiment, the constant flow water pump  120  is a multiplex pump. The floating piston  118  pushes a suspension of abrasive particles in water along the slurry flow stream  114 , at a high density and low flow rate. The flow rate of the slurry stream  114  is governed by the flow rate of water  122  being pumped by the constant flow water pump  120 . The desired flow rate of slurry may be altered by control of the constant flow pump  120 . A typical flow rate of slurry is about one litre per minute. 
         [0055]    The second energising means includes a valve  124  located along the water flow  122  between the constant flow pump  120  and the floating piston  118 . Closure of the valve  124  redirects the water flow  122  away from the floating piston  118 , and back to the constant flow pump  120 . Closure of the valve  124  thus immediately ceases the supply of pressure to slurry stream  114 . The valve  124  also prevents the backflow of water from the floating piston  118  to the constant flow pump  120 , and thus hydraulically locks the floating piston  118 , thereby also preventing the backflow of slurry from the sluny stream  114 . 
         [0056]    The cutting tool  110  includes a substantially cylindrical body portion  126  having a substantially cylindrical nozzle  128  extending from an outer end thereof. An inner end of the body portion  126  is connected to two injectors: an axial slurry injector  130  and an annular water injector  132 . The injectors are arranged such that the water stream and the slurry stream both enter the body portion  126  in an axial direction, with the water stream being annularly positioned around the slurry stream. The water injector  132  includes flow straighteners to substantially remove turbulence from the water flow before entry into the body porion  126 . In the embodiment of the drawings, water flow enters the water injector  132  in a radial direction and is then redirected axially. The flow straighteners, being a plurality of small tubes, assist in removing the turbulence created by this redirection. 
         [0057]    The cutting tool  110  includes a slurry valve  131  located upstream of the slurry injector  130 , and a water valve  133  located upstream of the water injector  132 . The slurry valve  131  and the water valve  133  are each independently operable, and can be open or shut to permit or prevent flow. 
         [0058]    An axial connection  135  between the sluny valve  131  and the slurry injector  130  is of variable length. 
         [0059]    The nozzle  128  can be best seen in  FIG. 6 . The nozzle includes a combining chamber  134  and a focussing region  136 . The combining chamber includes an entry region  138 . The combining chamber  134  is also a conical accelerating chamber, with a cone angle of about 13.5°. 
         [0060]    The focussing region  136  is a constant-diameter portion of the nozzle immediately adjacent a nozzle outlet  140 . The focussing region has a length:diameter ratio of at least 5:1, and preferably greater than 10:1. 
         [0061]    The entry region  138  is arranged to receive slurry flow through an axially inlet tube  142  of substantially constant diameter. The entry region is also arranged to receive water through an axially aligned annulus  144  about the inlet tube  142 . The annulus  144  has an outer diameter about three to four times the diameter of the inlet tube  142 . The annulus  144  joins the inner wall of the combining chamber  134  in a continuous fashion, thus reducing any propensity for the introduction of turbulence into the water flow. 
         [0062]    The position of the entry tube  142 , and hence the entry region  138 , is variable. The position can be varied by adjustment of the axial connection  135 . The axial positioning of the entry region  138  allow for the water flowing through the annulus  144  to be accelerated to a desired velocity before it enters the entry region  138 . This allows for the calibration of the flows of water and slurry, and may allow an operator to adjust for wear or loss of power. 
         [0063]    In the embodiment of the drawings the focussing region  136  is formed within a separate focussing nozzle  146  which is axially connected to the combining chamber  134 . The focussing nozzle  146 , as shown in  FIG. 7 , includes an accelerating region  148  immediately prior to the focussing region  136 . The accelerating region  148  has a cone angle greater than or equal to that of the combining chamber  134 . The accelerating region  148  has a diameter at inlet substantially identical to the diameter at an outlet of the combining chamber  134 . It is considered desirable that the inlet diameter of the accelerating region  148  be not significantly greater than the outlet diameter of the combining chamber  134  in order to reduce any propensity for the introduction of turbulence. 
         [0064]    The focussing nozzle  146  may be formed of a harder, more abrasive resistant material than that of the combining chamber  134 . As such, the respective portions of the nozzle  128  may be designed such that the fluid/abrasive stream is accelerated to a first velocity, for instance 250 m/sec, in the combining chamber, and then accelerated to its final velocity in the accelerating region  148 . The respective velocities can be designed and selected in accordance with the abrasive resistance of the materials used in the two portions. 
         [0065]    In an alternative embodiment, as shown in  FIG. 8 , the focussing nozzle  146  is a compound nozzle, with the accelerating region  148  formed from a particularly hard, abrasive resistant material such as diamond and the focussing region  135  formed from another suitable material such as a ceramic material. In this embodiment the diameter of the focussing region  136  is designed to be equal to or slightly smaller than the minimum (exit) diameter of the accelerating region  148 . 
         [0066]    In both embodiments the nozzle  128  is of sufficient length to allow the required velocity of a water/slurry mix to be met, typically up to 600 m/sec. It will be noted that, in the embodiment of the drawings, this requires the diameter of the focussing region  136  to be less than that of the slurry inlet tube  142 . 
         [0067]    The nozzle includes a chamfered exit  150  at the outlet  140 . The cone angle of the chamfer is sufficient to ensure separation of flow at the exit  150 . In the embodiment of the drawings, this angle is 45°. 
         [0068]    In a further alternative embodiment, as shown in  FIG. 9 , the focussing nozzle  146  is contained within an external holder  152 . The chamfered exit  150  in this embodiment is formed within the external holder  152 . 
         [0069]    In use, water is pressurised to the required pressure by the constant pressure pump  116 . It is pumped under this pressure to the cutting tool  110 , through the annular water injector  126 , and then into the annulus  144 . From the annulus it enters the entry region  138 , and establishes a pressure in the entry region  138  close to the pressure at which it was pumped. 
         [0070]    Slurry, energised by the floating piston  118 , is pumped along to the cutting tool  110 , through the slurry injector  130  into the inlet tube  142 . 
         [0071]    It will be appreciated that slurry will only proceed into the entry region  138  when pressure in the inlet tube  142  exceeds the pressure in the entry region  138 . When slurry is flowing, the action of the floating piston  118  (powered by the constant flow pump  120 ) acts to increase pressure in the slurry flow stream until it is sufficiently high to enter the entry region  138  of the combining chamber  134 . It will be appreciated that this is marginally higher than the pressure created in the entry region  138  by the water flow. When this pressure is established in the slurry stream, the action of the pump  120  will result in slurry being continuous supplied to the chamber  134  at a constant rate and pressure. 
         [0072]    Water and slurry will be rapidly advanced and mixed along the chamber  134 . The annular water flow will largely protect the walls of the chamber  134  from the abrasive action of the slurry, at least at the inner part of the nozzle  128 . 
         [0073]    By the time the flow has been accelerated to the focussing nozzle  146 , the water and slurry will be well mixed. At least an entry portion of the focussing nozzle must therefore be constructed from an abrasion-resistant material, such as diamond. 
         [0074]    The flow will exit the focussing nozzle  146  through the outlet  140  at an extremely high velocity, suitable for cutting many metals and other materials. 
         [0075]    When cutting is to be stopped, the valve  124  is activated to immediately cease operation of the floating piston  118 . It will be appreciate that the valve  124  is only acting against water, not abrasive material, and therefore is not subject to extreme wear. 
         [0076]    The ceasing of the floating piston  118  will cause energy to stop being added to the slurry stream  114 . This will result in pressure dropping in the slurry stream  114  and the inlet tube  142 . 
         [0077]    As soon as pressure in the inlet tube  142  drops marginally below the water pressure in the entry region  138 , the water pressure will prevent the flow of slurry into the entry region  138 . It will be appreciated that this occurs virtually instantaneously on activation of the valve  124 . The output jet will change from being a water/slurry jet to being a water only jet. 
         [0078]    At this point the slurry stream  114  will be maintained under high pressure, zero velocity conditions. In these conditions the slurry valve  131  can be closed without subjecting the valve  131  to excessive wear. 
         [0079]    Once the slurry valve  131  has been closed, the water valve  133  can be closed in order to cease the flow of water. This sequence of valve closures can be controlled rapidly, thus providing a convenient means to start and stop cutting at the cutting head  110 . 
         [0080]    When cutting is to be recommenced, the valve control sequence can be implemented in reverse, with water valve  133  being opened first, followed by slurry valve  131 . Subsequent opening of the valve  124  will result in a virtually instantaneous reestablishment of the slurry flow into the combining chamber  134 . 
         [0081]    Control over the cutting properties of the exit flow can be achieved through several measures, including changing the operating pressure of the constant pressure pump  116 , changing the volume supplied by the constant volume pump  120 , and changing the density of the slurry supplied to the system. 
         [0082]    Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.