Abstract:
A valve for controlling a flow of abrasive particles suspended in a pressurized carrier fluid has at least two apertured valve seats ( 74, 75 ) in face to face contact. One of the valve seats ( 74, 75 ) may be slid between a first position in which the apertures of each valve seat ( 74, 75 ) are aligned to allow fluid flow and a second position in which the aperture is one valve seat ( 74, 75 ) is blocked by the face of another ( 75, 74 ) to stop flow through the valve. The valve seats ( 74, 75 ) each have an outer layer of material with a hardness on the Mohs scale of at least 9, such as diamond. The valve is suitable for use in a fluid jet machining apparatus, particularly apparatus charged with a suspension of abrasive particles such as garnet in water.

Description:
CLAIM OF PRIORITY  
       [0001]    This application is a continuation of International Patent Application No. PCT/GB02/01835 filed on Apr. 25, 2002, published in English on Nov. 7, 2002 as WO 02/087827, which claims priority to application No. GB0110134.4 filed in Great Britain on Apr. 25, 2001, the specification of each of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the production and control of a flow of abrasive particles suspended in a pressurised carrier fluid for cutting materials such as metals, ceramics, polymers and composite materials. More particularly; but not exclusively, it relates to the production of a flow of particles of an abrasive, such as garnet, in an aqueous carrier fluid. The apparatus described is particularly suitable for operation at water pressures above 300 bar, feeding a suspension of fine abrasive particles in water to a cutting nozzle to produce a micro jet less than 100 μm (microns) in diameter.  
         BACKGROUND OF THE INVENTION  
         [0003]    New micro-machining techniques axe required to meet the growing demand for miniaturized products and processes. Abrasive waterjets have the potential to develop into an important micro-machining technique, but before this can happen new technologies are needed to generate and to control the flow of pressurised water flows carrying abrasive particles.  
           [0004]    Micro-abrasive waterjets are formed by passing a pressurised suspension of abrasive particles in a fluid, generally water, through a ceramic or diamond cutting nozzle.  
           [0005]    Abrasive suspensions can be provided pre-mixed, at the concentration required at the cutting nozzle, or alternatively abrasive particles can be metered from a bed of abrasive into a flow of fluid to a cutting nozzle.  
           [0006]    Pre-mixed suspensions are normally formed by mixing abrasive particles and a suspending additive with water. A cartridge is filled with the suspension and is loaded into an abrasive storage vessel that forms part of the apparatus, or the suspension is caused to flow into an abrasive storage vessel. A pressurised source of water is then used to displace the abrasive suspension out of the abrasive storage vessel to a cutting nozzle. If sub micron abrasive particles or a viscous fluid is used, then a suspending additive may not be necessary. An abrasive storage vessel with a volume of one quarter of a liter contains sufficient suspension to cut for an hour with a 15 μm diameter nozzle operating with a water pressure of 700 bar.  
           [0007]    When a micro-abrasive waterjet is to be fed with abrasive particles metered from an abrasive bed, the abrasive is first mixed with the fluid, usually but not necessarily water, and if needed, a Theological modifying additive. A cartridge is filled with the mixture and is loaded into an abrasive storage vessel or the mixture is caused to flow into an abrasive storage vessel. To carry out cutting about 10 percent or so of the flow from a pressurised source of fluid is diverted to the top of the abrasive storage vessel. The fluid flow into the abrasive storage vessel displaces a mixture of abrasive and fluid out of the outlet of the vessel, which mixes into the remaining 90 percent or so of the fluid that is flowing directly to the nozzle. A quarter liter abrasive storage vessel, containing a mixture with 70 percent abrasive by weight, can provide a suspension at a concentration of 10 percent abrasive to a 50 μm diameter nozzle for about one hour when cutting operations are carried out at 700 bar water pressure.  
           [0008]    Cutting technologies using abrasive suspensions have been used in oil and gas well drilling and maintenance operations. Sand particles and/or particles of other materials are suspended in a water-based mud using bentonite and/or water soluble polymers, or in water using water-soluble polymers, and are pumped down a well to one or more relatively large cutting nozzles. More recently, U.S. Pat. No. 5,184,434 has described the use of similar water-soluble polymers in the generation of suspension abrasive waterjets for precision machining. For cutting operations with pre-mixed suspensions, an additive such as xanthan gum with shear-thinning characteristics is desired, so that it may hold abrasive particles in suspension when the suspension is not flowing, but not impede flow when cutting operations are in progress.  
           [0009]    Oil well pumping equipment is large and robust and is capable of pumping abrasive suspensions. However, existing pumps for abrasive waterjet apparatus cannot handle abrasive suspensions in a satisfactory manner. An example of an apparatus that avoids pumping abrasive suspensions to generate abrasive waterjets is described in U.S. Pat. No. 5,184,434. It has valve arrangements to fill abrasive suspension storage vessels at low pressure and to discharge them at high pressure. The valves for such apparatus are required to open and close reliably with abrasive suspensions. However, valve technologies have not yet been available to build reliable valves for such apparatus.  
           [0010]    International Patent Application WO 99/14015 (PCT/GB98/02627) describes apparatus suitable for producing micro abrasive waterjets.  
           [0011]    The pressure differential imposed across settled beds of abrasive particles, with mean particle diameters greater than about 100 μm, causes water to percolate through the bed. Therefore, the mixture flowing out of the bed has a higher water content than is present in the bulk of the bed. Abrasive waterjets operating with settled beds of abrasive particles have relied on this water percolation for the bed to form and to aid in the flow of abrasive particles out of the beds. However, water percolation practically ceases with abrasive particle sizes needed for micro abrasive waterjets and this affects not only how beds can be formed, but also the time dependent Theological properties of abrasive beds and the structure of the beds during operation of abrasive waterjet apparatus.  
           [0012]    Abrasive water mixtures of up to 70 percent by weight abrasive particles are used to form beds in apparatus to generate micro abrasive waterjets. Such mixtures exhibit complex, time dependent properties, such as thixotropy and hindered settling of particles. A bed may retain for several hours the characteristics of a freshly prepared mixture or may not reach a near fully settled state for many days. As particle sizes are reduced to micron and sub micron sizes, abrasive particle/water mixtures can begin to take on the properties of colloidal suspensions.  
           [0013]    Polymer additives that are used to increase the viscosity of water are known to reduce the viscosity of mixtures with high ratios of abrasive particles to water. The additives affect the electrical charges of the particles and the interstitial water to allow easier movement between particles. Additives such as hydroxyethyl cellulose are known to prevent de-watering of abrasive particle/water mixtures by impeding the loss of water from abrasive beds.  
           [0014]    Additives can be added to abrasive/water mixtures to provide benefits in operating abrasive waterjet apparatus. These benefits include:  
           [0015]    a) Decreasing or increasing mixtures viscosity depending on the abrasive, water and additive concentrations, and on particle and additive properties;  
           [0016]    b) Minimizing the-watering of the base of abrasive beds during cutting operations;  
           [0017]    c) Aiding in the diffusion into abrasive beds of water entering the base of beds during pressurization of abrasive waterjet cutting apparatus and during abrasive on/off operations. This prevents the formation of vertical weakness in beds through which water can flow from the top to the bottom of a bed when only part of the bed has been discharged;  
           [0018]    d) Maintaining desirable mixture characteristics for extended periods of time, particularly when abrasive is provided in cartridges that need a long shelf life; and e) Reducing the tendency for blockages to form in passages when conditions exist for abrasive particles to settle out, such as when the apparatus is not used fox an extended period of time and during upset conditions that cause high abrasive concentrations in passage.  
           [0019]    As there is a need to produce abrasive waterjets with a wide range of particle diameters and to control the jet formation and cutting operations it would be beneficial to provide apparatus which can operate with freshly prepared abrasive water mixtures, with abrasive/water mixtures that contain rheological modifying agents, and which can feed cutting nozzles from both suspensions and beds of abrasive particles.  
           [0020]    Water compressibility is a major factor in the design and control of an abrasive waterjet apparatus. The compressed water volume in the abrasive storage vessel can be the equivalent of over 10 seconds of water flow through the apparatus. Precise control of cutting demands that this is vented away from the nozzle, usually by depressurizing the apparatus. When an abrasive waterjet apparatus is depressurized, this compressed water is violently expelled from the abrasive storage vessel through conduits to a vent valve. If the expelled water contains abrasive particles, the sealing capabilities of the valve seats of conventional valves can be destroyed in a single venting operation. There is thus a need for a valve that can handle highly abrasive flows.  
           [0021]    To depressurize and depressurize an abrasive waterjet apparatus between the end of one cut and the start of a new cut may takes several seconds which represents lost machining time. It would be desirable to provide a valve in the flow passage to the cutting nozzle in order to stop the discharge from the nozzle without having to depressurize the apparatus. With a valve in the connection to the nozzle it is not necessary to cycle the pressure in the apparatus from a high to a low pressure in order to stop flow from the nozzle. This has beneficial effects in reducing fatigue loads on apparatus, improving pump and component reliability and reducing energy use.  
           [0022]    Without a shut off valve before the cutting nozzle, abrasive is discharged through the cutting nozzle in a poorly controller manner during pressurization of abrasive waterjet apparatus. Poor control over abrasive flow has adverse effects on the way jets penetrate into work pieces and in particular can cause local widening of the cut width and cause jets to deviate.  
           [0023]    In order to extend the capabilities of abrasive waterjet cutting apparatus to carry out percussion drilling, milling and marking requires cutting jets to be turned on and off many times per second. An effective way of achieving rapid on/off capabilities is to have an on/off valve in the connection to the cutting nozzle or for the cutting nozzle to be an integral part of an on/off valve.  
           [0024]    Being able to start and stop the flow to an abrasive waterjet cutting nozzle by opening and closing a valve simplifies the control system for an abrasive waterjet apparatus and reduces the incidence of nozzle blockages.  
           [0025]    Also, in the apparatus described in International Patent Application WO 99/14015 and in this application, there is described a means of replenishing the abrasive storage vessel with abrasive mixture from another vessel. This requires valves that operate reliably on abrasive/water mixtures.  
           [0026]    As described above there are many reasons why the operation of abrasive waterjet apparatus would benefit from valves that could operate reliably on abrasive/water mixtures. However, suitable valves have not heretofore been known.  
           [0027]    There are two basic types of mechanical valve mechanisms, both of which involve a port or aperture in a member, referred to as a seat, and a valve element. In one type of valve the element moves along the axis of the seat and in the other the element, or a second seat, moves transversely to the seat.  
           [0028]    Valves that involve elements that move along the axis of a seat are not suitable for use with fluids containing highly abrasive particles because of the brittle mature of the ultra hard materials needed to resist erosion. Substantial forces have to be applied to achieve a seal between an axially moving valve element and a seat. When brittle materials are forced together to stop the flow through a valve, point contacts occur that create local high contact forces and these forces cause fracture of brittle materials.  
           [0029]    Thus, valves for highly erosive conditions need a mechanism involving a valve element moving more or less at right angles to a seat in such a way that abrasive particles cannot get between contacting surfaces. Ball valves and rotary disc type valves, with spring loaded elements to stop abrasive particles getting between contacting surfaces, have been developed for systems that operate with fluids that contain highly erosive particles.  
           [0030]    However, such valves have limitations as regards apparatus to generate micro abrasive waterjets because:  
           [0031]    a) Valve elements and seats cannot be easily fabricated from ultra hard materials to withstand wear if the valves are to be closed or opened under the high pressures in abrasive waterjet cutting apparatus;  
           [0032]    b) The small size of the valve elements needed for micro abrasive water] et apparatus makes it impractical to provide robust drive mechanisms that penetrate through pressure containments to actuate valve elements;  
           [0033]    c) Sealing of valve element drive mechanisms, where they do pass through the pressure containment, is very difficult in the presence of the fine abrasive particles used in micro abrasive waterjet cutting; and  
           [0034]    d) The valves have flow passages that contain spaces where abrasive particles can accumulate and subsequently be released, when the sudden release of accumulated abrasive can cause cutting nozzles on abrasive waterjet apparatus to block.  
           [0035]    It is therefore another object of this invention to provide two mating valve seats that slide relative to one another so that apertures in the seat can be aligned for flow to pass through the valve. Flow may be stopped by sliding the seats relative one to the other until the apertures no longer provide a flow path.  
           [0036]    Although the valves will operate in the presence of abrasive suspensions, it is desirable that the amount of abrasive present during opening and closing of such valves is minimised. A means of momentarily stopping abrasive flow, in order that valves in the connection to the cutting nozzle may be operated in the presence of water alone, is described in International Patent Application WO 99/14015, and is incorporated into certain of the embodiments of the present invention.  
           [0037]    Plunger pumps are conventionally used to power abrasive waterjet apparatus. Such pumps suffer from delivery pressure ripple. Pressure ripple can be minimised by synchronizing the motion of a plurality of pump plungers, as described in International Patent Application WO 99/14015, but some pressure ripple will always remain. Abrasive waterjet apparatus can function satisfactorily in cutting mode with a significant level of pressure ripple but problems arise when the abrasive flow out of an abrasive storage vessel is turned off by stopping the water flow into the top of the vessel. Water compressibility causes the abrasive storage vessel to act as a fluid accumulator, so a drop in pump delivery pressure, or an increase in pressure losses due to operating a valve to turn the abrasive off, causes abrasive to continue to flow out of the abrasive storage vessel.  
           [0038]    There is thus a requirement for an apparatus and a method of operation thereof which may control or eliminate the adverse effects of such pressure variations. In the apparatus described, the pump delivery pressure is increased in a controlled manner when the abrasive off valve is operated. The pressure increase is greater than the sum of pressure variations caused by the pump and the pressure drop caused by operating the abrasive off valve, thereby ensuring that abrasive flow out of the abrasive storage vessel is stopped when the abrasive off valve is operated.  
           [0039]    According to a first aspect of the present invention, there is provided a valve adapted to control a flow of abrasive particles suspended in a pressurised carrier fluid, comprising at least two apertured valve seat means each having a contact face in contact with a corresponding opposing contact face of another of said at least two apertured valve seat means and being translationally slideable in contact therewith and with respect thereto between a first position in which the apertures of each valve seat means are aligned so that fluid may pass through said apertures, and a second position wherein the aperture in one valve seat means is blocked by the contact face of another to stop flow through the valve, wherein the valve seat means each comprise an outer layer of material with a hardness, as measured on the Mohs scale, of at least 9.  
           [0040]    Preferably there are provided two valve seat means, one being translationally slideable in contact with the other and with respect thereto.  
           [0041]    Alternatively, there are provided three valve seat means, a median one of which being translationally slideable in contact with the outer ones and with respect thereto.  
           [0042]    Advantageously, each of the valve seat means comprises diamond.  
           [0043]    At least some of the valve seat means may comprise a composite diamond/ceramic material.  
           [0044]    In this case, a median one of the valve seat means may comprise two layers of such composite material, with their ceramic faces brazed or otherwise joined together.  
           [0045]    The valve may be provided with means to urge said valve seat means together.  
           [0046]    The valve may comprise spring means adapted to urge the valve seat means one towards the other.  
           [0047]    Additionally or alternatively, the means to urge the valve seat means towards one another may comprise the pressure of the carrier fluid exerted on one of the valve seat means.  
           [0048]    In this case, the flow of abrasive particles and carrier fluid may pass to a seat means through a tube adapted to allow sliding movement of the seat means and to transmit thereto a force urging the seat means together.  
           [0049]    The tube should withstand any buckling force.  
           [0050]    The valve may be adapted to operate at a pressure of at least 1000 bar (100 MPa).  
           [0051]    The abrasive particles may have a hardness of at least 6 Mohs.  
           [0052]    The valve may be provided with slide means, to which one of the valve seat means is mounted, adapted to be moveable translationally by external actuating means, thereby causing said one valve seat means to move between said first and said second positions.  
           [0053]    Advantageously, said external actuating means are pneumatic actuating means.  
           [0054]    Optionally, said slide means may be configured to act as a piston means within a double-ended cylinder means provided with inlet means at each end for compressed actuating air.  
           [0055]    Turning means may be provided to rotate at least one of said valve seat means and/or its slide means in relation to the other.  
           [0056]    The valve may have a single inlet means leading to the aperture in one valve seat means and a single outlet means leading from the aperture in the other valve seat means, the valve containing as a result no dead spaces where abrasive particles may accumulate.  
           [0057]    One or each valve seat means may have a contact face grooved to allow replenishment of a lubricating molecular water layer between the contact faces.  
           [0058]    Additionally or alternatively, one or each valve seat means may comprise porous polycrystalline diamond so that a flow of water may penetrate the or each valve seat means sufficient to lubricate the contact surface between the valve seat means.  
           [0059]    Advantageously, there is provided a container assembly adapted to contain supply of abrasive particles for use in an abrasive fluid jet machining apparatus, said assembly comprising a container for said abrasive particles closeable sealably by means of a cap, said cap comprising an inlet means connected to a riser tube within said body, each of such restricted bore as substantially to prevent liquid flow therethrough, except under an imposed pressure differential, and an outlet means, the bore of which comprises such a restriction as substantially to prevent flow therethrough, except under an imposed pressure differential.  
           [0060]    Hence, the inlet means and outlet means are adapted to resist liquid flow out of the container assembly in the absence of sealing means.  
           [0061]    The cap may comprise a substantially circular end face and said outlet means is disposed substantially centrally thereof.  
           [0062]    Advantageously, said inlet means is disposed substantially flush to an end face of said cap.  
           [0063]    The riser tube may extend from an inner face of said cap to a point adjacent but not in contact with a remote end of the container.  
           [0064]    The container may contain a supply of abrasive particles suspended in a carrier fluid.  
           [0065]    Alternatively, the container may contain a supply of abrasive particles immersed in a carrier fluid to form a bed of abrasive particles, adapted initially to occupy approximately 90% of the body of the container.  
           [0066]    Preferably, an upper end of said riser tube is disposed above said bed when the container assembly is oriented with the cap at a lower end thereof.  
           [0067]    The preferred carrier fluid is water.  
           [0068]    In this case, the bed of abrasive particles additionally comprises a water-retention aid.  
           [0069]    Advantageously, said abrasive particles comprise particles of garnet, olivine or aluminum oxide.  
           [0070]    Optionally, said abrasive particles may have a mean particle diameter of between 10% and 50% of the diameter of the nozzle. The mean particle diameter may be less than 10 μm.  
           [0071]    According to a second aspect of the present invention, there is provided an apparatus for machining a workpiece, comprising pressurizing means, a storage vessel for a supply of abrasive particles, a nozzle, and a valve as described above adjacently upstream of the nozzle, adapted to interrupt flow through the nozzle.  
           [0072]    The pressurizing means may further comprise means momentarily to increase the pressure at a point between the nozzle and the storage vessel prior to actuation of the valve to interrupt flow through the nozzle.  
           [0073]    The pressure at said point may be raised to a level exceeding that present in the storage vessel.  
           [0074]    The apparatus may include valve means openable to cause an increased proportion of the fluid to flow from the pressurizing means directly to the point.  
           [0075]    The apparatus may comprise means to control the pressurizing means to vary the delivery pressure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0076]    Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which:  
         [0077]    FIGS.  1  to  3  show alternative flow circuits for abrasive waterjet apparatus;  
         [0078]    [0078]FIG. 4 shows a cross-section of a shut-off valve;  
         [0079]    [0079]FIG. 5 shows a cross-section of an abrasive cartridge assembly; and  
         [0080]    FIGS.  6  to  10  show cross-sections of alternative embodiments of a shut-off valve. 
     
    
     DETAILED DESCRIPTION  
       [0081]    Referring now to the drawings, and to FIG. 1 in particular, a flow circuit is shown similar to that disclosed in International Patent Application WO 99/14015, with the addition of a buffer volume  24 , a non return valve  26  and a shut-off valve  27 . Buffer volume  24  is not necessary if valve  21  is not damaged by abrasive laden flows.  
         [0082]    Pressurised water from a pump  25  enters the apparatus through conduit  1 . When valve  5  is open, a major proportion of the water passes through conduit  4  and valve  5  and thence, via conduit  7 , to a junction  6 , where it recombines with a small proportion of the water flow which has passed through conduit  2  and a first restrictor  3 . Of the total flow from the pump  25 , about ninety percent flows from junction  6 , through a second restrictor  10  and conduit  11 , which is provided with a non-return valve  26 , to junction  14 , bypassing an abrasive storage vessel  19 . The remaining ten percent or so of the water flows through the buffer volume  24  and conduit  9  to the storage vessel  19 , where it displaces abrasive particles and water out of the bottom of the storage vessel  19  through conduit  1  S, an abrasive flow restrictor  17  and conduit  20  to junction  14 . At junction  14 , the flow from the storage vessel  19  joins the ninety percent or so  
         [0083]    of the flow that bypassed the storage vessel  19 . From junction  14  the water and abrasive particles pass through conduit  15 , which is provided with a shut-off valve  27 , to a cutting nozzle  16 , where the pressure energy of the fluid is converted to velocity energy to form an abrasive fluid j et  23 . The percentage of water that flows to the top of the abrasive storage vessel  19  depends mainly on the cross-sectional areas of the restrictors  10  and  17  and conditions within the abrasive bed in the abrasive storage vessel  19 .  
         [0084]    When valve  5  is closed all the flow from conduit  1  passes through the first restrictor  3  across junction  6  and into the second restrictor  10 . The combination of the first restrictor  3  and the second restrictor  10  forms a jet pump. This results in the static pressure in conduit  11  being higher than in conduit  7 , causing flow to reverse in the abrasive storage vessel  19  as fluid from conduit  7  is entrained into the jet from the first restrictor  3  at junction  6 . Abrasive flow to the nozzle  16  is therefore stopped by the closing of valve  5  and turned on by opening valve  5 , while clean water flow continues.  
         [0085]    When required, the whole apparatus may be depressurized by opening the vent valve  21 .  
         [0086]    The buffer volume  24  prevents abrasive particles carried out of the abrasive storage vessel  19  during depressurization of the apparatus from reaching the vent valve  21 . The clean water flow to the top of the abrasive storage vessel  19  during. pressurization of the vessel and during normal cutting operations flushes abrasive particles back from the buffer volume  24  into the abrasive storage vessel  19 .  
         [0087]    The non-return valve  26  provided in conduit I 1  prevents abrasive particles from the base of storage vessel  19  from reaching the vent valve  21  during depressurization of the apparatus.  
         [0088]    Vessel  19  can be replenished with abrasive using cartridges as described in International Patent Application WO 99/14015, the specification of which is incorporated by reference, or through conduit  140  and valve  141 .  
         [0089]    The shut-off valve  27 , located in conduit  15  upstream of the nozzle  16 , is used to stop flow from the nozzle  16 . Before closing the shut-off valve  27 , valve  5  is closed. After a short delay for the resulting clean water flow to clear abrasive from conduit  15 , valve  27  is then closed.  
         [0090]    If the pressure drop across the restrictor  3  is too high and/or if the delivery pressure from the pump  25  is decreasing, for instance due to pressure ripple, abrasive flow out of the abrasive storage vessel  19  may not immediately stop on closing valve  5 . The pump  25  will usually be provided with two or more plungers powered by pneumatics, hydraulics or linear electric actuators. These methods of actuation allow the pump pressure to be rapidly varied. By increasing the delivery pressure from the pump  25  in a controlled manner when valve  5  is closed, flow out of the abrasive storage vessel  19  to junction  14  can be stopped controllably. Valve  27  can then be closed, or nozzle  16  can be moved rapidly from the end of a completed cut to the start of a new cut with only water discharging from the nozzle. In the new cutting position, cutting is restarted by opening valve  27  (if it has been closed), opening valve  5  and reducing the water pressure from the pump  25  to the normal cutting pressure. During this brief period of decaying water pressure, the abrasive concentration at the nozzle  16  is higher than the steady state cutting concentration. This higher abrasive concentration is beneficial in enabling a jet to make an initial penetration into the material being cut.  
         [0091]    The operation of the flow circuit shown in FIG. 1, using a jet pump arrangement, begins to break down as nozzle diameters are reduced to the point where laminar flow occurs in parts of the circuit. It is then more appropriate to use the flow circuit of FIG. 2 which shows the circuit for a basic abrasive waterjet apparatus. A limitation of the circuit shown is its inability to stop abrasive discharge controllably. Any drop in delivery pressure from the pump  25  with valve  5  closed causes flow out of the bottom of the abrasive storage vessel  19 . This flow has a high concentration of abrasive, which can settle out and block conduit  15  and nozzle  16 . Hence it is preferred to use the circuit with a control strategy that increases the pump delivery pressure when valve  5  is closed. With increasing pump delivery pressure water flows back up conduit  18  into the base of the abrasive storage vessel  19 , stopping the flow of abrasive to the nozzle  16 . The nozzle  16  can then be moved rapidly from the end of a completed cut to the start of a new cut with only water discharging, or if a shutoff valve  27  is fitted, the shut-off valve  27  can be safely closed with only water passing therethrough.  
         [0092]    [0092]FIG. 3 shows a flow circuit for operating an apparatus in which the abrasive storage vessel  19  contains a suspension of abrasive particles at the same abrasive/water weight ratio as is required at the nozzle  16 . In the circuit shown in FIG. 3, the non return valve  29  is spring-loaded to give a pressure drop greater than the pressure ripple from pump  25 . When valve  28  is open all the water entering conduit  1  flows to period of decaying water pressure, the abrasive concentration at the nozzle  16  is higher than the steady state cutting concentration. This higher abrasive concentration is beneficial in enabling a jet to make an initial penetration into the material being cut.  
         [0093]    The operation of the flow circuit shown in FIG. 1, using a jet pump arrangement, begins to break down as nozzle diameters are reduced to the point where laminar flow occurs in parts of the circuit. It is then more appropriate to use the flow circuit of FIG. 2 which shows the circuit for a basic abrasive waterjet apparatus. A limitation of the circuit shown is its inability to stop abrasive discharge controllably. Any drop in delivery pressure from the pump  25  with valve  5  closed causes flow out of the bottom of the abrasive storage vessel  19 . This flow has a high concentration of abrasive, which can settle out and block conduit  15  and nozzle  16 . Hence it is preferred to use the circuit with a control strategy that increases the pump delivery pressure when valve  5  is closed. With increasing pump delivery pressure water flows back up conduit  18  into the base of the abrasive storage vessel  19 , stopping the flow of abrasive to the nozzle  16 . The nozzle  16  can then be moved rapidly from the end of a completed cut to the start of a new cut with only water discharging, or if a shutoff valve  27  is fitted, the shut-off valve  27  can be safely closed with only water passing therethrough.  
         [0094]    [0094]FIG. 3 shows a flow circuit for operating an apparatus in which the abrasive storage vessel  19  contains a suspension of abrasive particles at the same abrasive water weight ratio as is required at the nozzle  16 . In the circuit shown in FIG. 3, the non return valve  29  is spring-loaded to give a pressure drop greater than the pressure ripple from pump  25 . When valve  28  is open all the water entering conduit  1  flows to the nozzle  16 . When valve  28  is closed the spring-loaded non-return valve  26  opens and fluid flows to the top of the abrasive storage vessel. Opening valve  28  causes valve  26  to close and the pressure to rise at junction  14 , and this stops or reverses the flow out of abrasive storage vessel  19 .  
         [0095]    When vessel  19  is replenished with abrasive suspensions through conduit  140  and valve  141 , the vessel may be provided with a floating piston to separate water entering through conduit  9  from mixing with the abrasive suspension in the vessel  19 .  
         [0096]    Abrasive concentrations in the abrasive storage vessel  19  can be varied from about seventy percent by weight of abrasive in water, down to less than ten percent. In the embodiment shown in FIG. 4 two pneumatic cylinders  72 , mounted to the valve body  70 , carry the slide  71 , which is provided with a plurality of seals  73 , and effectively forms the piston for both of the pneumatic cylinders  72 . Application of compressed air at ports  83  and  84  thus opens and closes the valve. Movement of the slide  71  could also be produced by other forms of actuation. The movement range of the slide  71  is limited by stops  82  provided on the body  70  and on the slide  71 . The inlet connection  80  may be offset laterally from the outlet connection  81  by half the permitted movement range of the slide  71 . The tendency for buckling of tube  78  may thereby be minimised.  
         [0097]    [0097]FIG. 6 shows a form of the valve installed at the base of an abrasive storage vessel  19 , such as is shown in FIG. 5, which shows a cartridge assembly that can be installed in the abrasive storage vessels  19  of FIGS.  1  to  3 . The cartridge assembly is formed by a cartridge  41 , a cap  42 , the abrasive flow restrictor  17 , a riser tube  44  and two seals  45  and  46 , and is installed in an abrasive storage vessel consisting of a pressure vessel made up of a barrel  51  and a base  50 . As shown, the nozzle assembly made up of nozzle  16 , extension  55 , tube  15  and seal  54  is mounted in base  50 .  
         [0098]    Pressurised water entering through conduit  9  flows through passageways in the base  50  to an inlet plenum  52 , formed between the base  50  and the cap  42 , that is sealed by seals  45  and  46 . The water enters a passage in the cap  42  that communicates with the riser tube  44 , and discharges from the riser tube  44  into a water-filled volume  49  above a bed of abrasive  47 . The flow of water into the cartridge assembly causes abrasive and water to flow out through the abrasive flow restrictor  17  into an outlet plenum  53 , where they mix with water entering the outlet plenum  53  via conduit  11  and passageways in the base  50 . The combined flow passes through conduit  15  to the cutting nozzle  16 . During cutting operations, with an abrasive bed containing about seventy percent concentration by weight of abrasive, the water flow in conduit  9  is about ten percent of the water flow in conduit  11 .  
         [0099]    The bore of the abrasive flow restrictor  17  in the cap  42  is sized, in combination with restrictor  10  in the circuits feeding water to conduits  9  and  11 , to regulate the water flows in order to achieve a particular abrasive concentration at the cutting nozzle.  
         [0100]    The abrasive flow restrictor  17  in the cap  42  and the long narrow bore of the riser tube  44  both inhibit abrasive and water flow out of, and air flow into, the cartridge assembly while it is being fitted and removed from the base  50 . This arrangement avoids the need for seals in the flow connections between the passageways in the base  50  and the cartridge cap  42 , as are required in the abrasive storage vessel arrangements disclosed in International Patent Application WO 99/14015, the specification of which is incorporated herein by reference.  
         [0101]    The arrangement shown allows one central physical connection to be used in place of the two physical connections used in the arrangement disclosed in the above International Patent Application. With only central physical connection cartridge assemblies are far easier to fit into the base  50  and no misalignment of connections is possible. The removals of cartridges from the base  20  can be aided by applying compressed air through conduit  56  once the barrel  51  is undone. Plug  57  in the barrel  51  provides a small annular gap between the plug and barrel, through which air can pass when the barrel is slid over the cartridge. The annular gap between plug  57  and  75  and their carriers  76  can be rotated in small increments from time to time so that erosive wear is evenly spread.  
         [0102]    [0102]FIG. 7 shows a version of the valve shown in FIG. 4 that is particularly suited to applications in which one of the valve connections is to a low pressure region, such as the vent valve  21  of FIGS.  1  to  3 . The valve exploits the flexible nature of the small diameter, high pressure tubing used to connect components of micro abrasive waterjet apparatus. Flow enters through a flexible inlet tube  86  to seat  75 , and leaves through seat  74  and outlet tube  85 . Seat  75  is mounted to slide  87 . Seat  74  is located in carrier  113 , which is loaded by a spring  77  to hold valve seats  75  and  74  together with a force that is typically 1.5 times the force exerted by the pressure in tube  86  acting on an area equal to the cross sectional area of the aperture in slide  89 . Actuation of the valve follows that for the valve in FIG. 4. The force exerted by the spring  77  may be supplemented by fluid pressure from connection  110  acting on carrier  13  in plenum  114  formed between seals  111  and  112 .  
         [0103]    To avoid spring and fluid pressure loads on seats  74  and  75  causing the seats to tip relative to on another it is desirable to make the seat diameter larger than that required to achieve on and off operations. However, since the friction coefficient of diamond sliding on diamond more than doubles without a molecular film of water or other fluid at the sliding interface, patterns of grooves in the sliding faces of seats  74  and  75  can be used to allow replenishment of the molecular water layer. Porous polycrystalline diamond can also be used for seats  74  and  75  to allow a minute flow of water to escape and in the process lubricate the sliding interface of seats  74  and  75 .  
         [0104]    A form of the valve that is particularly suited to apparatus for feeding cutting nozzles less than about 50 μm diameter is shown in FIG. 8. The valve has a slide  93  separating the seats  74  and  75 . The slide  93  has an aperture that can be moved into alignment with the apertures in seats  74  and  75  or to block off the connection between the apertures in seats  74  and  75 . Spring  77  can provide the total sealing force on the seats  74  and  75  and slide  93 , or part of the sealing force can come from axial loads on tube  78  of FIG. 4, or from fluid loading on carrier  76  as described in relation to FIG. 7.  
         [0105]    The part of the slide  93  that moves relative to the seats  74  and  75  can take the form of a separate double faced seat  89  in the slide  93 . Seat  89  can be rotated periodically along with seats  74  and  75  in order to even out the wear.  
         [0106]    A robust slide for the valve in FIG. 8 can make use of diamond materials produced for diamond tipped tools for high speed machining. For instance the slide can be fabricated from items cut from a lapped disc of polycrystalline diamond, 0.5 mm thick on a 1 mm thick ceramic base. By brazing two pieces of material, ceramic to ceramic, 3 mm thick, diamond faced slides of sufficient strength can be produced. Machining of the composite diamond/ceramic material .and its subsequent brazing and drilling uses techniques developed for diamond tipped tooling.  
         [0107]    An ideal location to stop and start discharge through a nozzle is adjacent to the nozzle. FIG. 9 shows an arrangement of the valve where outlet seat  100  also acts as the nozzle  16 . Fluid and suspended abrasive flows through tube  91  to seat  75  that is held in contact with seat/nozzle  100 . Seat  75  can be slid laterally over seat/nozzle  100  by actuator  103  acting through member  95  and seat carrier  76  to align apertures in seat  75  and seat/nozzle  100  or to misalign the apertures to effect a seal. In the arrangement shown the spring  97  acting on collar  96  attached to tube  91  applies the sealing force between seat  75  and the seat/nozzle  100 . Tube  91  deflects on movement of the seat  75  in a similar manner to tube  78  of FIG. 4.  
         [0108]    As described in relation to the slide in FIG. 8 the seat/nozzle  100  can be a composite construction of diamond on ceramic discs brazed back to back, with the nozzle bore drilled through the outer diamond layer.  
         [0109]    Another arrangement of valve integrated with the nozzle assembly is shown in FIG. 10. Multiple nozzles  105  are drilled in a diamond or diamond/ceramic disc  106  that is rotated by shaft  104  to align or not the nozzle drillings with the aperture in seat  75  that is connected to tube  91 . A spring  77  can provide the total sealing force between seat  75  and the seat/nozzle  100  or part of the sealing force can come from axial loads on tube  91  in a similar manner to the fluid pressure load on tube  78  of FIG. 4.  
         [0110]    As can be seen, the valve apertures are connected to tubes for carrying abrasive suspensions. The mating seats are made of ultra hard materials with a low coefficient of sliding friction, particularly of polycrystalline and carbon vapor deposition diamond, that can withstand highly erosive conditions and can move relatively freely under high loads. The valves have actuating mechanisms that do not pass through the pressure containment and the valve flow passages have no spaces where abrasive particles can accumulate.  
         [0111]    Commercially available industrial diamond materials with highly polished, ultra flat surfaces can be used for the valve seats and components. Thus the valves are compact, economic to manufacture and can be actuated by linear and rotary drives, including advanced actuators based on shape memory alloys and piezoelectric transducers. Versions of the valve can make use of the cutting nozzle as one of the valve seats.