Review of art known to the applicant (s).

The removal of old or failed paint, corrosion products or other coatings, contamination or deposits from surfaces by propelling an abrasive material or a water jet against such layers is well known and usually referred to, respectively, as grit (abrasive) blasting or water (jetting) blasting.

Abrasive materials, both expendable or re-usable, and with varying characteristics, are widely in use for surface preparation tasks such as the need to remove rust and old paint from structural steel, concrete or stonework prior to application of a new protective or decorative coating. An equally important function of the abrasive is to generate a roughened surface profile or anchor pattern necessary for the solid adhesion of the new coating system.

Abrasive blasting is normally achieved by loading a pressure vessel with the desired abrasive and charging the vessel with compressed air to a pressure between 7 and 10 bar (c. 100-150 psi). The abrasive is then released through a regulating valve into the blast hose, with a flow rate of blast air through the hose of typically 10 cubic metres per minute (350 cfm). The mixture of blast air and abrasive then passes through an acceleration nozzle or blast nozzle, as it is more commonly known. The blast nozzle accelerates the grit as it passes through the throat or venturi of the nozzle thus achieving an effective grit blasting tool. The exiting high speed grit then erodes the blast surface on contact to an appropriate blast standard, for example, SA 2, SA 2.5 or SA 3 and surface profile standard, for example 100 microns.

Traditional abrasive methods, particularly those using expendable abrasive, have the disadvantage that they generate considerable dust pollution which restricts the work of others where large scale repainting is being undertaken, for example, on a ship in dry-dock; an offshore oil platform; a land-based petrochemical installation or a steel railway or road bridge.

As well as the interference of dust to the worksite and the neighbourhood, the cost of removal and disposal of used abrasive has risen sharply over the past ten years because of more stringent waste regulations. In many advanced and environmentally-responsible countries, the cost of disposal of used abrasives currently can well equal or exceed the purchase cost of the abrasive in the first place.

More recently, high pressure water jets and ultra high pressure water jets<BR> have become commonly used as an alternative to abrasive or grit blasting.

Water jets at pressures in the range of 1,000 bar (c. 15,000 psi) to 3,800 bar (c. 55,100 psi) can achieve velocities as high as 700 m/sec and have the ability to remove, without the use of abrasive, the most tightly-adherent coatings including almost all of the invisible chloride or chemical contamination. Residual chlorides on abrasive blast-cleaned surfaces are a significant cause of the reoccurrence of substrate corrosion. Water blasting does not cause any dust. Water blasting is also more effective than abrasive blasting at removing flexible coatings such as rubber, plastics or bitumen.

The disadvantage of blasting only with water is that, in general, the average work rate is significantly lower than abrasive blasting and the water jet cannot create the desired anchor pattern for the new coating, instead high pressure water blasting relies on there being a pre-existing anchor pattern under the old coatings (created by abrasive blasting probably when the structure was first built) which it can satisfactorily expose again.

Abrasive blasting with greatly reduced dust can be achieved by either introducing a water jet upstream from the blast nozzle or by premixing the abrasive with water in the blast pot (pressure vessel) and then charging the blast pot with air or water. This method is often referred to as'slurry blasting'.

Fitting an array of high or ultra high water jets down stream of the blast nozzle to wet the abrasive and reduce the dust is referred to as'wet blasting'.

Wet blasting was further developed by the inventors of y-jet, produced by an EC company which currently trades under the name of Mühlhan. The Muhlhan system fires an ultra high pressure water stream at a predetermined angle into the core of the blast media downstream of the blast (acceleration) nozzle in so doing wetting the abrasive. The disadvantage to this method is that a coherent stream of grit and air cannot be maintained. A secondary acceleration (blast) nozzle is introduced to restore this coherent grit (abrasive) stream.

The technology of mixing ultra high pressure water (1,750 bar (c. 25,000 psi) and above) with abrasive is a widely used practice. The UHP water is used to accelerate the grit through a focussing tube. This exiting stream of water air and grit will erode any material placed in its way.

Summary of Invention In its broadest aspect the invention provides an apparatus for slurry blasting wherein the sole or main pressure vessel or blast pot in the apparatus is pressurised using a liquid.

In another aspect the invention provides apparatus for slurry blasting wherein the abrasive is mixed with a liquid in the pressure vessel before it forms an abrasive stream. <BR> <BR> <P>In a further preferred aspect the invention provides apparatus further<BR> comprising an independently controllable pressurised liquid source.

The apparatus permits the delivery of a second source of water from an auxiliary liquid source at a pressure of up to 3,800 bar at varying volumes into the abrasive stream upstream from the blast nozzle.

In another preferred aspect the invention provides an apparatus wherein the auxiliary liquid source acts on the abrasive stream to increase the velocity of substantially the whole of the abrasive stream downstream from the blast nozzle. The apparatus permits the introduction into the abrasive stream of a water jet which may be a high pressure or ultra high pressure water jet with the objective to increase the mean velocity of the abrasive particle stream from 200m/sec to approximately 600 m/sec before it enters the blast or acceleration nozzle in order to increase the blasting work rate.

In another aspect the invention provides a method of blasting using the apparatus herein described.

The inventor has previously developed a slurry blasting system-Aquablaster - whereby the blast pot (pressure vessel) is pressurised by water rather than air. Mixing the wet abrasive by water jet in the pot and pumping it out into the compressed air blast line (hose) through a control valve at a differential pressure of 2-4 bar (30-60 psi) increases blasting efficiency and lowers abrasive consumption dramatically.

To further increase blasting rates, the inventor has introduced an auxiliary pressurised liquid source to further boost the velocity of the abrasive particle stream.

A control system is provided which permits the abrasive blasting operator to activate, either independently or concurrently, the abrasive stream, second source of water and pressurised liquid source or a shut-down/override device for the immediate termination of all the above functions in the event of an emergency.

Description of Preferred Embodiments.

The invention will now be described by way of example and with reference to accompanying Figure 1, which shows two diagrammatic cross-sectional views of one preferred embodiment of the apparatus according to the invention wherein an auxiliary liquid source is provided.

A pressurised abrasive stream (siurry stream) consisting of grit, air and water travels down the blast hose indicated by the arrow into the Superstripper Nozzle Body, item (1) in figure 1. An auxiliary liquid source is introduced along the centre line of the abrasive stream as well as the centre line of the nozzle body, by first travelling through an aero-foiled nozzle body, (2) and then through a nozzle extension (4) The following design criteria for item (2) and (4) is specified as follows : Item (2) has an aerofoil (double aeroplane wing) design to stabilise the already turbulent inlet abrasive flow and reduce wear on the exposed leading edge of the profile caused by the abrasive particles striking it at 200 m/sec.

An attack angle of the order of 113 degrees to the centre line enhances the aerodynamic properties of the aerofoil. The aero-foiled body clamped into the base of the Superstripper body aligns the nozzle extension with the centre line of the abrasive stream.

The length of the nozzle extension, item (4), varies dependent on; the volume of air supplied via the blast nozzle ; the volume of wet grit forced into the blast hose through the control valve ; the pressure of the auxiliary liquid source; the water flow rate of the auxiliary liquid source.

The front face of the nozzle extension (the point where the water jet exists the UHP nozzle extension) is positioned between 60 and 120 mm from the throat area of the blast nozzle, item (3), (the point where the grit blasting nozzle centre hole is at its minimum). This positioning maximises the acceleration of the abrasive stream without encouraging excessive turbulence. The configuration ensures that the abrasive stream is accelerated from 200 m/sec to around 600 m/sec prior to entering the blast nozzle throat area (venturi).

The invention provides a slurry blasting apparatus that reduces abrasive consumption by approximately 50 % compared with traditional methods. This makes the use of specialised but more expensive abrasives more cost effective. Specialised abrasives are, in particular, harder and non-toxic, generally naturally occurring minerals such as garnet which increase work rate.

The introduction of an auxiliary liquid source into the abrasive stream by means of the Superstripper nozzle described here will accelerate the abrasive stream to approximately 600 m/sec before it enters the venturi of the blast nozzle where it is further accelerated. The design boosts the performance (production) rate of a conventional dry blasting nozzle from, for example, 10 m2 to rates between 17-22 m2/hour depending on the coating being removed.

Dust is eliminated, collection and disposal of spent abrasive is greatly reduced.

Chloride contamination of a steel substrate is greatly reduced in comparison to dry abrasive blasting methods because of the water jet's ability to dossolve salts. The additional process of washing down prepared steel with fresh water to remove salt is therefore eliminated.

Traditional open dry abrasive blasting with expendable abrasives averages about 50 Kgs of abrasive consumption per square metre in, for example, most European ship repair yards and production rates average about 10 square metres per hour on flat surfaces.

Using expendable abrasives in the apparatus described herein results in abrasive consumption falling to between 25-30 Kgs per square metre without significant loss in production rate.

Using harder, recyclable abrasives such as almandite garnet, for example, has the effect of lowering abrasive consumption to between 12-17 Kgs per square metre and increasing production rates from an average of 10 square metres per hour to rates of between 15 and 22 square metres per hour.

Boosting the performance of the Aquablaster with ultra high pressure water increases the abrasive stream velocity and hence the production rate with no corresponding increase in abrasive consumption.