Known liquid abrasive cutting techniques are carried out by pressurising a liquid to a high pressure, typically 2000 Bar and generating a fine high pressure jet which is able to cut a range of target materials. For cutting soft materials, the liquid jet alone is sufficient to produce a cut. However, for harder more difficult materials an abrasive material is introduced into the liquid jet.

This method of cutting generates very low reactive forces and is essentially a cold cutting, non-spark technique even on the most difficult materials. For this reason, liquid abrasive cutting has been adopted for cutting explosive ammunition and charges as a means for disposal. The cutting application is generally conducted as a precursor to explosive ordnance disposal by burning and is operated remotely. There are also experimental applications for the technique such as use to allow access to weapons, weapon components or live ordnance in a post accident recovery situation.

A disadvantage of known liquid abrasive cutting techniques is the potential for the abrasive material to come into contact with and become embedded within any highly explosive material present, thereby increasing its explosive sensitivity.

Accordingly, the present invention provides a two phase cutting device having a cutting element comprising a cooled pressurised fluid jet with solid abrasive particles entrained therein, wherein the abrasive particles are formed from a material which is fluid at normal ambient conditions but which is solid at the conditions within the fluid jet.

An advantage of the invention over known liquid abrasive cutting systems arises from the fact that the abrasive particles are solid whilst entrained within the cutting jet but become fluid (i. e. either a liquid or a gas) at normal ambient conditions. Therefore,

once the cutting operation is completed any abrasive particles in contact with the explosive material will melt or sublime as the temperature and pressure of the particles returns to normal ambient conditions. Thus, the abrasive particles will not remain embedded in the explosive material and will not be explosive sensitising. The two phase cutting system will effectively leave only innocuous residues or, in some cases, no residues at all on the material being cut.

The equipment for generating such a two phase jet can be similar to conventional systems but with additional mechanisms to generate the solid abrasive particles within the environment of the jet.

In conventional liquid abrasive cutting systems, the most effective cutting is generally achieved if the abrasive particles are suspended in the liquid before the mixture is pressurised through a pumping system. However, a disadvantage of this arrangement is abrasive wear, especially in the pumping system, which results in the requirement for replacement parts after only a few hours of operation.

In the present invention, the solid abrasive particles may be formed post pressurisation and therefore the pumping system or other pressure intensifier is only exposed to fluids, minimising mechanical wear. Thus, a further advantage of the invention over known liquid abrasive cutting systems is the reduction of wear in the mechanical components of the system.

In one embodiment of the invention, a liquid with a low freezing point is chosen as a carrier fluid. A second liquid with a much higher freezing point but which is liquid at ambient conditions is introduced into the carrier fluid to produce the solid abrasive particles within the jet. For example, if a light silicone oil, perhaps at-40°c, is chosen as the carrier fluid, the abrasive solid particles can be generated by introducing water into the carrier fluid. To reduce the cooling requirement associated with the phase transition of the water to ice, the water can be pre- cooled to near freezing point before being fed into the carrier fluid.

There are a number of ways in which the two phase cutting device can be constructed.

In a first device, a reservoir tank of silicone oil is cooled to -40°c using either conventional refrigeration equipment or compressed carbon dioxide released from a cylinder. The cooled silicone oil is then pressurised through a conventional pressure intensification system, with optional further cooling post pressurisation. The cold pressurised oil is fed to the jet nozzle where pre-cooled water is injected to form ice crystals within the liquid jet. The injection of cooled water into the oil stream may be achieved using known systems for the injection of abrasive slurries into high pressure jets by utilising a proportion of the pressurised flow. An advantage of the use of water rather than an abrasive slurry is that injection is easier to control as there are none of the attendant clogging problems associated with suspended particulates. The vessel containing the water in this type of injection system should be rated at the jet operating pressure including adequate safety factors.

In an alternative device, a silicone oil/water emulsion is generated within the reservoir tank. The emulsion may be generated using a high shear rate, high energy mixer, possibly with the addition of suitable emulsifying agents. This emulsion is pressurised though a conventional pressure intensification system and cooled at the jet orifice using a high capacity cooling system, possibly based on liquid nitrogen or carbon dioxide.

The end objective of either of the devices described above is the same. Namely, to produce a cold silicone oil jet at a pressure of up to 2500 bar with ice crystals entrained within it.

The two phase cutting jet can also be generated using a single material in two phases, for example water and ice. In such a system, the temperature of the jet will have to be accurately controlled to ensure that the two phases of the material can co- exist within the cutting jet.

Alternatively, the carrier fluid could be provided by a gas which is cooled sufficiently to solidify the abrasive particle material

to provide solid abrasive particles entrained within the gas. This arrangement would operate upon similar principles to grit-blasting but with the"grit"being provided by ice crystals or some other solid particles which are fluid (either liquid or gas) at ambient conditions.

Certain gases (e. g. carbon dioxide) can exist as liquids at pressures suitable for liquid jet cutting and at temperatures below 0°c. Hence pressurised cold liquid carbon dioxide (or a gas with similar properties) may be used as a carrier fluid and water may be injected at the jet nozzle to produce a liquid carbon dioxide/ice jet.

Conventional liquid abrasive cutting techniques are effective over a wide range of pressures and the operating parameters chosen depend upon the material being cut. The parameters may need to be regulated to optimise the time of cut, waste material production and consumption of operating fluid. Clearly, in the two phase cutting device, the temperature of the pressurised jet must also be such that the two phases can co-exist. Thus, the operating temperature of the jet preferably falls in the range-0°c to-50°c, typically-40°c. Due to the high capital funding of very high pressure systems, the operating pressure of the jet will preferably fall within the range 200 bar to 2500 bar, typically 2000 bar.

Two embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein Fig 1. Shows a schematic of a two phase cutter in which pre-cooled water is injected into the jet carrier fluid at the jet nozzle.

Fig 2. Shows a schematic of a two phase cutter in which water and the jet carrier fluid are mixed to form an emulsion which is pressurised and then cooled at the jet nozzle.

Referring to Figure 1, a reservoir tank 1 is filled with a light silicone oil 2 which is cooled to-40°c. The light silicone oil 2 (which acts as the carrier fluid) is then pressurised using a

pumping system 3 and fed to an outlet nozzle 4. At the outlet nozzle, pre-cooled water is mixed with the silicone oil 2 and forms ice crystals which are entrained within the silicone oil, to form the cutting jet 5.

Referring to Figure 2, in an alternative embodiment of the invention, a reservoir tank 1 is filled with a mixture of light silicone oil and water 2 which is mixed with a mixer 6 to form an emulsion. This emulsion is then pressurised using a pumping system 3 and fed to an outlet nozzle 4. As the emulsion is ejected from the outlet nozzle 4, it is rapidly cooled using liquid nitrogen or carbon dioxide, thereby forming ice crystals within the jet of silicone oil, to form the cutting jet 5.

It will be appreciated that the two-phase jet can be provided by a number of different combinations of materials, provided the carrier fluid is a liquid or gas at the pressure and temperature conditions in the pressurised jet and the abrasive particle material is a solid at the conditions in the pressurised jet and a liquid or gas at ambient conditions.