Cutting head and cutting nozzle for a liquid/abrasive jet cutting arrangement

1. A cutting tool for a high pressure cutting arrangement, the cutting arrangement comprising a liquid stream and a slurry stream, the slurry comprising abrasive particles suspended in a fluid, the cutting tool including a combining and mixing chamber, the combining and mixing chamber having an entry region arranged to receive the liquid stream and the slurry stream, the slurry stream and the liquid stream being arranged to enter a nozzle, wherein the nozzle is an accelerating nozzle with an outlet smaller in diameter than the entry region, and wherein the outlet is smaller in diameter than a diameter of the slurry stream on entry into the nozzle, and 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.

2. A cutting tool as claimed in claim 1 , wherein the cutting tool includes a slurry valve which can selectively permit or prevent the flow of slurry into the combining and mixing chamber, and a fluid valve which can selectively permit or prevent the flow of liquid into the combining and mixing chamber.

3. A cutting tool as claimed in claim 1 , wherein the nozzle is elongate and the slurry stream and the liquid stream being oriented in the elongate direction.

4. A cutting tool as claimed in claim 3 , wherein the nozzle has a central axis, with the slurry stream being oriented along the central axis and the liquid stream being provided in an annulus about the slurry stream.

5. A nozzle for a high pressure cutting arrangement, the cutting arrangement comprising a liquid stream and a slurry stream, the slurry comprising abrasive particles suspended in a fluid, the nozzle including a combining and mixing chamber, the combining and mixing chamber having an entry region arranged to receive the liquid stream and the slurry stream, wherein the nozzle is an accelerating nozzle with an outlet smaller in diameter than the entry region, and wherein the outlet is smaller in diameter than a diameter of the slurry stream on entry into the nozzle, and 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.

6. A nozzle as claimed in claim 5 , wherein the nozzle is elongate, and the slurry stream and the liquid stream are oriented in the elongate direction.

7. A nozzle as claimed in claim 5 , wherein 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.

8. A nozzle as claimed in claim 5 , wherein 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.

9. A nozzle as claimed in claim 8 , wherein the cone angle is less than 27°.

10. A nozzle as claimed in claim 9 , wherein the cone angle is about 13.5°.

11. A nozzle as claimed in claim 8 , wherein the focussing portion of the nozzle has a length:diameter ratio greater than 5:1.

12. A nozzle as claimed in claim 11 , wherein the focussing portion of the nozzle has a length:diameter ratio greater than 10:1.

13. A nozzle as claimed in claim 8 , wherein the focussing portion of the nozzle has a length:diameter ratio less than about 30:1.

14. A nozzle as claimed in claim 8 , wherein the nozzle is a two-part nozzle, with the combining and mixing chamber defining a first part and the focussing portion contained within a second part.

15. A nozzle as claimed in claim 14 , wherein the second part is a focussing nozzle, and includes an accelerating portion with cone angle greater than or equal to that of the combining and mixing chamber.

16. A nozzle as claimed in claim 15 , wherein the focussing portion has a diameter equal to or slightly smaller than the smallest diameter of the accelerating portion.

17. A nozzle as claimed in claim 15 , wherein the accelerating portion of the focussing nozzle is constructed from an abrasive resistant material such as diamond.

18. A nozzle as claimed in claim 15 , wherein the focussing nozzle has an outlet including an exit chamfer having a cone angle of about 45°.

19. A nozzle for a high pressure cutting arrangement, the cutting arrangement comprising a liquid stream and a slurry stream, the slurry comprising abrasive particles suspended in a fluid, the nozzle including a combining and mixing chamber, the combining and mixing chamber having an entry region arranged to receive the liquid stream and the slurry stream, wherein the nozzle is an accelerating nozzle with an outlet smaller in diameter than the entry region, and wherein the outlet is smaller in diameter than a diameter of the slurry stream at entry to the combining and mixing chamber, and 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.

20. A nozzle as claimed in claim 5 , wherein there is no additional accelerating nozzle along the slurry stream.

21. A cutting tool as claimed in claim 1 , wherein pressure in the entry region acts on the pressure in the slurry stream in such a way that the slurry stream is prevented from entering the combining chamber unless the pressure in the slurry is marginally higher than the pressure at the combining chamber entry point.