EP0358648A1

EP0358648A1 - Abrasive blasting apparatus.

Info

Publication number: EP0358648A1
Application number: EP88902905A
Authority: EP
Inventors: Bruce Nicholson; Ronald Davis
Original assignee: Gagemarch Ltd
Current assignee: Gagemarch Ltd
Priority date: 1987-04-09
Filing date: 1988-04-06
Publication date: 1990-03-21
Anticipated expiration: 2008-04-06
Also published as: DK492889A; DK492889D0; WO1988007915A1; NO885374D0; CA1320643C; AU1544888A; NO169702B; JPH02502982A; IE61329B1; KR890700430A; GB8708499D0; BR8807453A; DE3866355D1; NO169702C; NO885374L; GR880100228A; EP0358648B1; RU2051020C1; IE881032L; DK170697B1

Abstract

Un appareil et un procédé servent à mélanger des milieux conservés séparément, tels ceux utilisés pendant une opération de sablage abrasif humide. L'appareil comprend un réservoir (10) de milieu abrasif/liquide, un réservoir (11) de liquide et une ligne d'alimentation (20) d'un fluide pressurisé. Les réservoirs (10, 11) sont en communication par des tuyaux (22, 23) avec la ligne d'alimentation (20), de sorte que le liquide et le milieu abrasif/liquide soient entraînés par le fluide pressurisé. Les réservoirs communiquent l'un avec l'autre par un orifice (14) de façon à équilibrer la pression hydraulique entre les deux, un dispositif (15, 19) étant également prévu afin de fournir du liquide pressurisé à au moins un des réservoirs (10, 11), d'équilibrer sensiblement le système de pression hydraulique entre les réservoirs (10, 11) et la ligne d'alimentation (20) et de faciliter l'écoulement du milieu abrasif/liquide et du liquide dans le fluide d'entraînement. Le procédé comprend l'entraînement de milieux conservés séparement, tels qu'un milieu abrasif/liquide et un milieu liquide, dans un courant de fluide pressurisé, accompagné de l'application simultanée du fluide pressurisé aux milieux contenus dans les réservoirs (10, 11) afin d'équilibrer le système de pression hydraulique entre les réservoirs eux-mêmes et entre les réservoirs et la ligne d'alimentation, facilitant ainsi leur décharge dans le courant d'entraînement.An apparatus and method is used for mixing separately stored media, such as those used during a wet abrasive blasting operation. The apparatus comprises a reservoir (10) of abrasive / liquid medium, a reservoir (11) of liquid and a supply line (20) of a pressurized fluid. The reservoirs (10, 11) are in communication by pipes (22, 23) with the supply line (20), so that the liquid and the abrasive / liquid medium are entrained by the pressurized fluid. The tanks communicate with each other through an orifice (14) so as to balance the hydraulic pressure between the two, a device (15, 19) being also provided in order to supply pressurized liquid to at least one of the tanks ( 10, 11), to substantially balance the hydraulic pressure system between the reservoirs (10, 11) and the supply line (20) and to facilitate the flow of the abrasive / liquid medium and of the liquid in the fluid training. The method involves entraining separately stored media, such as an abrasive / liquid medium and a liquid medium, in a stream of pressurized fluid, accompanied by the simultaneous application of the pressurized fluid to the media contained in the reservoirs (10, 11 ) in order to balance the hydraulic pressure system between the tanks themselves and between the tanks and the supply line, thus facilitating their discharge into the drive current.

Description

ABRASIVE BLASTING APPARATUS This invention relates to abrasive blasting apparatus especially but not exclusively a wet abrasive blasting apparatus.

In wet abrasive blasting apparatus the abrasive medium is entrained in a pressurised liquid flow or gaseous-entrained liquid flow (hereinafter for convenience simply referred to as "pressurised fluid flow" ) and is directed against the surface or other article to be treated by a controllable nozzle. It is common practice with such an apparatus to add a dry abrasive medium into a gaseous stream and then to add liquid to wet the abrasive medium prior to its egress from the nozzle outlet so that dust generation at the work area is reduced. This liquid is normally delivered into the abrasive and pressurised fluid flow by a separate pump the inclusion of which makes the cost of the blasting apparatus more expensive than is desirable, renders the blasting apparatus more prone to breakdown, and requires a greater technical knowledge of the operator., A further disadvantage is that variations in the entraining fluid pressure require either manual adjustments of the pressure outflow from the pump, or the provision of a pressure balancing control means between the entraining fluid supply and the pump.

As a result, control of the apparatus with particular reference to the abrasive medium/liquid outflow is difficult, in the first instance, with consequent variation in the efficiency of the apparatus, and, in the second instance, the cost of the apparatus is further increased by the need to provide the pressure balancing control means which, in any case, does not provide instantaneous adjustment of the pump upon variations in entraining fluid pressure occurring, again with adverse effects on the efficiency of the apparatus. It is an object of the present invention to provide an apparatus for wet abrasive blasting which obviates or mitigates the aforesaid disadvantages of cost, varying control and efficiency. The apparatus according to this invention may be used dry, or the apparatus may, more generally, be employed for purposes other than abrasive blasting. More detailed references to these alternative uses will be made later . Generally, therefore, apparatus according to the present invention comprises a plurality of receptacles for containing media to be mixed and a source of pressurised fluid adapted to be placed in communication with the receptacles to entrain the media out of the receptacles for delivery out of a pipe, duct or similar connected to the source and to the receptacles, characterised in that a means is provided for directing pressurised fluid from the source into at least one of the receptacles behind the media in terms of media outflow from the receptacle and in that a communication is provided between the receptacles to .provide a substantially balanced fluid pressure system between the receptacles.

Also according to the present invention there is provided a method of mixing a plurality of separately- contained media, by entraining the media from their containers into a flow of pressurised fluid and characterised in that the pressurised fluid is simultaneously applied to the media in their containers to assist egress of the media into the entraining flow and in that a communication exists between the container-"-, substantially to balance the fluid pressure system between the containers.

More specifically there is provided an apparatus for wet abrasive blasting comprising <_ receptacle for a medium, a receptacle for liquid and delivery line along which an entraining pressurised fluid is adapted to be fed and with which the receptacles are adapted to communicate, characterised in that a means adapted Lo deliver said pressurised fluid into at least one of said receptacles is provided and that a communication exists between the receptacles substantially to balance the fluid pressure system between the receptacles themselves and the receptacles and the delivery pipe to facilitate outflow of liquid/abrasive medium and the liquid into the entraining fluid flowing along the delivery line.

Preferably the receptacles are coaxial and concentric .

Preferably an inner receptacle contains the liquid/abrasive medium, while an outer and surrounding receptacle contains the liquid.

Preferably, the communication between each receptacle and the delivery pipe is valve-controlled.

Preferably, the pressurised fluid is delivered to the interior of the inner receptacle and a communication is provided between the inner receptacle and the outer receptacle to provide the balanced fluid pressure system,

Preferably, at least one of the receptacles is provided with a fast pressurised fluid exhaust, which is normally closed.

Preferably, the liquid is water with or without additives e.g. rust inhibitors. Preferably the pressurised fluid is air.

Preferably, within the inner receptacle water is fed in first followed by abrasive, such, ^' for example, as sand to ensure effective mixing and to prevent clogging at the receptacle outlet.

The present invention, therefore, provides a pumpless wet abrasive blasting apparatus and as the entraining pressurised fluid is also applied directly to the liquid/abrasive medium and the separate liquid thert- is instantaneous and equal adjustment to the entraining pressurised fluid and direct pressurised fluid upon any pressure variations occurring at the pressurised fluid source . An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawin , in which:-

Figure 1 is a part sectional elevation of an apparatus for wet abrasive blasting according to the present invention; and

Figure 2 is a corresponding plan view. The apparatus comprises two co-axial concentric receptacles or chambers 10 and 11. The inner chamber 10 serves to contain the abrasive medium which is for example sand, which sand may be wet sand. With the present invention it is not necessar^** as is customary with wet abrasive blasting apparatus using sand as the abrasive medium to employ dry sand which is often delivered to the apparatus bagged. As a consequence the present invention provides a wet abrasive blasting apparatus in which it is possible to re-use wet sand. The inner chamber 10 in use also, of course, contains water and it is to be noted that the water is always introduced first into the inner chamber 10 followed by the sand (or other solid abrasive medium) to ensure that there is effective mixing of the sand and water and no clogging at the outlet 12 of the inner chamber. The introduction of the sand into the water in the inner chamber 10 causes the sand to sink through the water thus getting wet immediately. The introduction of pressurised air into the inner chamber- 10, as hereafter described, causes a swirling action in the inner chamber 10 to complete the mixing. The outlet 12 is disposed at the bottom of th-- chamber 10 and the inlet 13 of the inner chamber 10 is provided at the top of same. The outer chamber 11, in use, contains additional water for addition to the sand/water mixture egressing from the inner chamber 10 as will be described later.

There is at least one hole 14 providing a communication between the outer chamber 11 and the inner chamber 10, which hole 14 is disposed at the upper end of both chambers 10, 11.

The inlet 13 to the inner chamber 10 is controlled by an air lock valve, for example a mushroom-type valve, 15, which valve Iδ is closed on application thereto of pressurised air.

The apparatus has as its operational or motive fluid pressurised air which passes from a pressurised air source, such for example as a pressurised air mains (not shown) to a delivery pipe system generally indicated at

16.

Air under pressure is delivered from a source (not shown) , possibly an air compressor to the pipe system 16 at. the main inlet. 17 controlled by a valve 18. Pressurised air is delivered to the valve 15 by a pipe 19, which air then flows from the inner chamber 10 into the outer chamber 11 via the hole 14.

Pressurised air is also delivered by a pipe 20, valve controlled as indicated at 21 to an abrasive blasting hose and nozzle (not. shown) coupled to the pipe 20 by a bayonet joint connection as indicated at 38. The nozzle of the abrasive blasting hose is provided with a deadman ' s handle so that the abrasive blasting operation is controlled by the operator (release of the handle closing the nozzle and discontinuing the abrasive blasting operation) .

Preferably the blasting nozzle is of the lance type and nor. a conventional venturi nozzle as it has been found that abrasive tends to lodge in and clog a venturi nozzle during use of the apparatus . The lance provides an outflow for the pressurised fluid which has a smaller bore than that of the delivery pipe 20 effectively to concentrate the pressurised fluid flow therethrough so that it can be appropriately directed by the operator. It has been found that the lance operates effectively w*hen the ratio of the diameters of the pipe 20 and of the lance is in the range 1.5 : 1 to 10 : 1 inclusive with a lance that is at least 100 mm in length. Typically, the diameter of the bore of the lance is of the order of 15 mm for a 45 mm diameter pipe 20 for delivery of pressurised air at 10 cubic metres per minute at 100 psi. The length of the lance is usually of the order of 450 mm to enable the operator to distance himself from the blasting and grip the lance effectively.

The chambers 10 and 11 communicate with the pipe 20 via pipes 22 and 23 respectively each of which is respectively controlled by a valve 24, 25.

Thus the pipes 22, 23 provide communication between the bottom of the chamber 10 and 11 with the pipe 20.

Adjacent the top of the outer chamber 11 there is provided a fast exhaust pipe 26 controlled by a valve 27.

Water is supplied to the inner and outer chambers from for example a mains water supply (not shown) by a pipe 28 incorporating a non-return valve 29 and an open/close valve 30. The water is delivered directly into the chamber 11 by the pipe 28 to w-hich is connected a pipe 31 for delivering water into the inner chamber 10, this pipe being valve controlled as indicated at 32.

A water level control or overflow pipe 33 communicates with the inner chamber 10, this pipe 33 also being valve controlled as indicated at 34.

The various valves are indicated as being manually controlled but it is clearly to be understood that these valves can be subject to automatic control in any convenient manner. To render the apparatus ready for use the valves 18, 21, 24, 25, 27, 30, 32 and 34 are^" all closed. Valves 30 and 32 are then opened to allow water to flow into the inner and outer chambers 10, 11. A given quantity of abrasive medium for example sand (wet or dry) is then fed through the inlet 13 which is open owing to no air pressure being supplied to the valve 15 to close same. Water continues to be fed into the chambers 10 and 11, and, when the required total volume of sand and water is delivered into the inner chamber 10, water will. flow out of the pipe 33. At this stage, the valve 32 is closed followed by the valve 34. When sufficient water has been fed into the chamber 11, the valve 30 is also closed.

The valve 17 is then opened causing the valve 15 to be moved to the closed position and pressurised air to be introduced into the inner chamber and, through the communication hole 14, into the outer chamber 11.

Valve 21 is now opened allowing pressurised air to flow along the pipe towards the hose and nozzle (not shown) .^' Valves 24 and 25 are then opened to the desired degree to allow the water/sand mixture from the chamber 10 a d additional water from the chamber 11 to be moved into, and be entrained by, the airstream for delivery along the hose out of the nozzle whence it is directed against the surface or other article to be abrasive blasted .

The sand/water mixture and the additional water are subjected to the same air pressure in the upper part of the chambers 10 and 11 as that which is used to entrain them out of their respective chambers.

Thus, any variation in the air pressure at source is automatically and instantaneously applied to both the entraining air and the internal air contained within the chambers 10 and 11. At any stage, the valve 24 can be closed, and water only from the chamber 11 be fed into the airstream, for the purposes of washing down the work area.

Alternatively, the valves 24 and 25 can be closed leaving air travelling through the hose and nozzle, for blowing or drying purposes.

During operation, the air feed pressure can be varied up or down in order to achieve different rates of working or different types or levels of finish on the work area. A pressure gauge may, in these circumstances, be included in the air inlet line 17 in order that the operator can more accurately judge the working pressure.

The normal sequence of operations for shutting off the apparatus is first of all to close the valve 24 to prevent any further emission of abrasive. When all of the abrasive in the hose has cleared, the valve 25 may be closed to prevent any further water going down the line. After all the water has cleared from the hose, the valves 21 and 18 are closed. The valve 27 may then be opened to allow fast reduction of the air pressure in vessels 10 and 11, which in turn allows air lock valve 15.to open.

If, for any reason during normal operation, an emergency stop is required, this can be activated manually, or triggered automatically (by release of the deadman's handle, for example) , by opening the valve 27, preferably with a simultaneous closure of the valve 18.

Means is provided to effect fast filling of the chamber 10 with substances which do not flow quickly on their own (e.f. wet sand or slurry). Such means consists of a water jet 35 situated inside and in close proximity to the wall of the filling hopper 36 at the top of the apparatus. The jet 35 is circumferentially-directed that Is swirls water round the hopper area, when required, and enables the entry of the material through the open inlet 13 and into the chamber 10. A valve 37 controls the flow of water to the jet 35. It is to be noted that additives can be included in either the water in the chamber 10, or alternatively, in the water in the chamber 11, such an additive being, for example, a rust inhibitor if the surface or article being treated is formed of metal, or an anti-freeze material for low temperature working. Alternatively, the additive may be introduced in powder or granular form, mixed with the abrasive. Because the total volume of material in both chambers is known at commencement of the operational sequence, pre-measured doses of additive can be included in either chamber to give an accurate dilution of the additi e .

The wet abrasive blasting apparatus described above can be used dry and in this case the dry abrasive or other abrasive medium is simply contained within the chamber 10, the water supply system being closed off and the sand or other abrasive medium being entrained along pipe 20 through the pipe 22 via the open valve 24, the sand being subjected to the internal air pressure equal to the entraining air pressure as described above.

The apparatus can also be used for ice blasting and in this case the inner chamber will contain ice particles and water arid the outer chamber 11 will contain water. It is to be noted that the ice particles formed from water '-.ill be prevented from coagulating (freezing together) by introducing suitable additives into the water prior to freezing. Suitable additives would be for example poly- phosphates .

The above described apparatus can, as aforesaid, be used either wet or dry as described and it can be used for washing down using water- only from chamber 10 or ever: from chambers 10 and 11.

The apparatus provides faster cleaning than known wet abrasive blasting apparatus (all other operational factors being equal) . It can be up to 20% faster or even more in certain circumstances. The reason for- this is that each particle of abrasive, having being soaked under pressure, is enclosed in a film of water so that its effective weight is increased as it leaves the blast nozzle. Thus its momentum is greater and it does more work when it strikes the workpiece surface.

The apparatus is not subject, to "choking" as are known apparatus which, on occasion, become blocked at the abrasive outlet of the pressure vessel. In practice, with the known apparatus, these blockages are usually relieved by turning off the main air supply (e.g. closing valve 215 momentarily .

The apparatus is relatively cheap to manufacture compared _with known apparatus.

There is less wastage of abrasive on site since with the apparatus according to the present invention all spent abrasive can be re-used in the wet condition i.e. it is not necessary to dry same.

The apparatus can be readily used in high humidity areas since contamination by moisture will not adversely affect i .

The apparatus is spark free and static free. It is pumpless as will be clear from the above and due to the lack of pump there is obviously no need to provide a pump driving generator. The apparatus has no mechanical working parts and due to the simple nature of the apparatus all parts can easily be changed and renewed in a short time.

The apparatus Is safe to use and as a result can be used by unskilled labour. The apparatus, when used in a wet-blasting process, produces far less air-borne duκ^τ than comparable known apparatus, and therefore far* less fall-out at any given distance from h" work ar^ . This makes it environ¬ mentally more acceptable. Finally, the apparatus can be used with a wide range of easily ^"obtainable abrasives wet and dry and in use it is considered that there will be a lower usage of abrasive than with other wet abrasive blasting systems (in the region of 20% less) and the apparatus involves less clearing up on site than conventional wet abrasive blasting apparatus.

The apparatus according to the present invention is extremely versatile and can be used, inter alia , as follows:

1. Wet or dry abrasive cleaning of stone, brick, terracotta, steel, iron, and all other hard or semi- hard surfaces.

2. Stripping of paints and other coatings from these surfaces, either in one operation or layer by layer.

3. Washing off of these surfaces.

4. Removal of contaminants from surfaces, e.g. radio¬ active contamination or biological contamination (possibly using chemical additives in the water to improve the efficiency of the treatment) ; removal of oils and greases from surfaces (possibly using hot ater, or solvents in place of water, to speed removal ) .

5. Removal of graffiti from stone and other surfaces. 6. Texturing of surfaces either for aesthetic- appearance or as a preparation for over coating or bonding of the surface; the exposing of grain on timber surfaces; the 'frosting' of glass to make it opaque; the roughening or matting of plastic and all other surfaces.

7, Engraving of glass, stone, plastic, and other surfaces, using suitable masks where appropriate.

8. Mixing of materials, dilution of mixes, and spray application of materials, the mixing of substances in one or more forms (such as solids in powder, granular- or fibrous form, or semi-solid materials, or liquids) w th themselves or with each other, and the spray or 'blast' application of the resultant mixes. For example, the mixing and application of renders, slurries, coatings, paints, adhesives, de- icing mixes (e.g. mixing salt with sand or gravel and spraying onto roads liable to freezing, or de- icing mixes onto aircraft), the mixing of plant seeds into culture media and the spray application of such; the mixing and spraying of fertilisers and weedkillers onto agricultural land; the mixing of or coating of textile fibres with bonding agents and their spray application in, for example, bonded fabric production; the impregnation of surfaces with another substance; the variegated colouring of surfaces such as floors, walls, tiles, by using different coloured materials in the different chambers or of different densities on one chamber. Modifications may be made to the above described apparatus . For example a porous wall may be provided between the chambers 10, 11 in order to allow for the movement of, say, a liquid from chamber 11 into chamber 10. For example, when ice blasting, ice and water would be put into chamber 10, and water into chamber 11. If the water in chamber 10 is reduced too much during the blasting operation, then water can percolate from chamber 11 to chamber 10 and so prevent clogging of the ice in chamber 10.

In another embodiment of the invention, an outer chamber is used for the abrasive/liquid medium and an inner chamber for the liquid medium. Here, the inner chamber can be of a small volume with respect to the outer chamber and a porous wall can be used to divide the chambers so that the liquid flows from the outer chamber into the inner chamber to charge it with liquid prior to and during the blasting to obviate the need for separa e pipework and other filling means. Some of the abrasive may also pass into the inner chamber through the wall but this does not inhibit the mixing and blasting process. However, this is preferably prevented by making the pore 5 size in the wall such that passage of the abrasive therethrough is prevented. Effectively, in this embodiment the inner chamber is reduced to a porous pipe located in the outer chamber and communicating therewith above the. level of the media contained therein, through _]_o which pipe liquid can be drawn off from the liquid/ abrasive medium and entrained separately in the pressurised fluid from the abrasive/liquid medium.

The apparatus may comprise more than two chambers provided all the chambers are in communication so that τ_5 there is in the chambers air pressure equal to the entraining fluid pressure.

The chambers may be located one within the other and be coaxial and concentric as in the above described apparatus or they may be disposed in side-by-side

2o relationship either parallel one with another or in juxtaposed coaxial relationship.

The chambers may in fact be separate one from another and there may be two or more chambers provided there is communication between them to ensure common 5 fluid pressure within all the chambers, which pressure is equal to and variable with the entraining fluid pressure.

0

5

Claims

1. An apparatus comprising a plurality of receptacles for containing media to be mixed and a source of pressurised fluid adapted to be placed in communication with the receptacles to entrain the media out of the receptacles for delivery out of a pipe, duct or similar connected to the source, characterised in that a means is provided for directing pressurised fluid from the source into at least one of the receptacles behind the media in terms of media outflow from the receptacle and in that a communication is provided between the receptacles to provide a substantially balanced fluid pressure system between the receptacles .

2. An apparatus for abrasive blasting comprising a receptacle for an abrasive or a liquid/abrasive medium, a receptacle for a liquid and a delivery line along which an entraining pressurised fluid is adapted to be fed and with which the receptacles are adapted to communicate, characterised in that a means adapted to deliver said pressurised fluid into at least one of said receptacles is provided and in that a communication exists between the receptacles substantially to balance the fluid pressure system between the receptacles themselves and the receptacles and the delivery line and to facilitate outflow of the abrasive or liquid/abrasive medium and the liquid into the entraining fluid flowing along the delivery line.

3. An apparatus for abrasive blasting as claimed in Claim 2, characterised in that the receptacles are located one within the othe .

4. An apparatus for abrasive blasting as claimed In Claim 3, characterised in that the receptacles are coaxial and concentric.

5. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 4 , characterised in that one of the receptacles is provided with a fast pressurised fluid exhaust, which is normally closed.

6. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 5 , characterised in that the media inlet in the receptacle for the abrasive or the liquid/ abrasive medium is provided with a valve which is closed by the application thereto of pressurised fluid from within the receptacle.

7. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 6 , characterised in that a means to enable a circumferentially directed jet of liquid is provided to swirl the abrasive to assist entry into its receptacle.

8. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 7 , characterised in that the delivery line terminates in a lance.

9. An apparatus for abrasive blasting as claimed in Claim 8, characterised in that the ratio of the diameters of the delivery line and of the lance is between 1.5:1.-0 and 10:1 inclusive.

10. An apparatus for abrasive blasting as claimed in Claim 8 or Claim 9, characterised in that the lance is at least 100 mm in length.

11. An apparatus for abrasive blasting as claimed in any one of Claims 3 to 10, characterised in that the receptacles are separated by a porous wall.

12. An apparatus for abrasive blasting as claimed in Claim 11, characterised in that the outer receptacle is adapted to contain the liquid/abrasive medium and the inner receptacle is adapted to contain the liquid and is filled by passage of the liquid through the porous wall from the outer receptacle.

13. An apparatus for abrasive blasting as claimed in any one of Claims 3 to 11, characterised in that the inner receptacle is adapted to contain the liquid/ abrasive medium and the outer receptacle is adapted to contain the liquid.

14. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 11 or Claim 13, characterised in that the receptacle for the liquid is filled by means of a pipe incorporating a non-return valve and an open/close valve.

15. An apparatus for abrasive blasting as claimed in any one of Claims 2 to 14, characterised in that the communication between each receptacle and the delivery line is valve-controlled.

16. A method of mixing a plurality of separately- contained media by entraining the media from their containers into a flow of pressurised fluid and characterised in that the pressurised fluid is simultaneously applied to the media in their containers to assist egress of the media into the entraining flow and in that a communication exists between the containers substantially to balance the fluid pressure system between the containers .

17. A method as claimed in Claim 16, characterised in that at least one of the media is a liquid.

18. A method as claimed in Claim 17, characterised in that the temperature of the liquid is above the ambient temperature.

19. A method as claimed in any one of Claims 16 to 18, characterised in that one of the media is water with or without additives and another is an abrasive and water mixture .

20. A method as claimed in Claim 19, characterised in that the abrasive is ice.

21. A method as claimed in Claim 19 or Claim 20, characterised in that the container for the abrasive and water mixture is filled by feeding in water first followed by the abrasive to ensure effective mixing and to prevent the abrasive clogging the outlet of the container.

22. A method as claimed in any one of Claims 16 to 21, characterised in that the pressurised fluid is air.