GB2077158A - Wet grit blasting - Google Patents

Abstract

Air from a compressor (T) is used both for entraining grit in a line from a regulator valve (E) to a nozzle (N) and for pressurizing in a tank (A) so that a stream of compressed air, pressurized liquid and grit is ejected from the blast nozzle to effect wet grit blasting of structural fabrications. The pressurized liquid passes through a valve (P) to an injector (K) or downstream or even in the nozzle, to join compressed air from the valve (E). The grit is fed by a screw metering unit (W) from a tank (L) pressurized by the air. Pilot valves (H) are controlled by a deadman's valve (X). This controls the outlet (P) and grit tank pressurization. Helmets (R) are supplied by the air via individual valves (Y). A dry grit blasting system is convertible to a wet grit blasting system above. <IMAGE>

Description

SPECIFICATION Wet grit blasting This invention relates to wet grit blasting removing fouling, scale, oxidisation and successive layers of paint from structural fabrications.

Wet grit blasting systems already known generally fall into two categories in one of which no compressed air is used. Water is supplied to the blast nozzle at high pressure velocity by pumps and grit is entrained in the water stream by partial vacuum at the nozzle. Water quantities and pressures in the system are extremeley high, typical figures for a single nozzle being as follows: Water pressure 1000 to 3000 p.s.i.

Water consumption 2 to 5 galls/min.

Grit consumption 1 to 3 Ibs./min.

In the other category compressed air transmits grit to the nozzle at which point water pressurized by pumps is injected. Typical figures for a single nozzle are as follows: Air pressure 80 to 120 p.s.i.

Water pressure 100 to 1 50 p.s.i.

Water consumption 3 to 1 5 pints/min.

Grit consumption 5 to 1 5 Ibs/min.

These systems use valves for controlling the water content of the blast and it is found in practice that fine control over water is difficult to obtain.

The cost of fresh water on offshore sites is high since it either has to be transported to the offshore site or distilled from sea water. It is therefore an object of the present invention to reduce the consumption of fresh water.

In accordance with one aspect of the invention there is provided a method of wet grit blasting wherein compressed air is used both for entraining grit and for pressurising liquid and a stream of compressed air, pressurised liquid and grit is ejected from a blast nozzle.

In accordance-with another aspect of the invention there is provided a wet grit blasting system comprising a source of compressed air, container means for liquid, a line connecting the source of compressed air to the container means whereby to pressurise liquid in the container means, a nozzle, a line connecting the source of compressed air to the nozzle whereby to deliver compressed air to the nozzle, line means connecting the container means to the nozzle for delivery of liquid under pressure to the nozzle, a grit supply unit, a line connecting the source of compressed air to the grit supply unit whereby to pressurise the unit, and a line from the grit supply unit to supply grit under pressure to the nozzle whereby to eject a stream of compressed air, pressurised liquid and grit from the blast nozzle.

In accordance with yet another aspect of the invention there is provided equipment for modifying into a wet grit blasting system a dry grit blasting system comprising a source of compressed air, a grit supply unit and a nozzle from which grit can issue entrained in a jet of compressed air, the equipment comprising container means for liquid, a first connection for the source of compressed air, a line from the connection to the container means whereby to pressurise liquid in the container means, a line from the first connection to a second connection for the nozzle for delivering compressed air to the nozzle, line means from the container for delivery of liquid under pressure to the nozzle, and a line from the first connection to a third connection for the grit unit whereby to pressurise the grit unit.

The invention has control over the amount of dust produced and the dust affected.area, the control being proportional to the amount of water being used. With the invention, sea water can be used for initial blasting, final blasting and flushing being carried out using fresh water.

An example of a system according to the present invention is shown schematically in the accompanying drawing.

An air compressor T supplies compressed air through a separator F with an automatic drain to an adjustable pressure regulating valve I and separately to a pilot-controlled pressure regulating valve E. Pressure regulated compressed air is fed from the valve I to a tank A pressurised by the compressed air, via a pressure gauge EE with a stop valve. Desirably, the valves I and E enable the air pressure to be adjusted within the range of O to 1 20 p.s.i.

As shown in the broken box line a the pressurised liquid tank A has an air vent and filling valve D. Connected across the tank A, via shut-off valves AA, is a liquid level indicator B. Liquid is fed under pressure from the tank A via a drain valve CC, a flow-regulating valve Z and a flowrate indicator C.

The pressurised water storage and feeding arrangement within the box a may be provided again as shown by the box a', the two arrangements being connected in parallel and the tank A iri arrangement a' being smaller than in arrangement a. It is envisaged that the large tank A may contain sea water or fresh water and the small tank A may- contain a concentrated solution of liquid rust inhibitor. Alternatively the large tank A may contain a desired solution of liquid rust inhibitor in water and the small tank A may contain fresh water for final flushing. The liquid from one or both of the arrangements a and a' is fed, via a pneumatically-operated stop valve P, to an injector K.

On the downstream side of the valve E is a spur connected to a pressure gauge EE provided with a stop valve. The pressure-regulated compressed air is fed from the valve E to operator's helmets R via a common air filter/separator 0 with an automatic drain and individual flowregulating valves Y. Furthermore, the pressure-regulated air is fed from the valve E, via a relief valve G, to a unit H comprising a pilot operated air stop valve and a quick exhaust valve.

In the unit H pressure-regulated compressed air is fed through the pilot-operated air stop valve, via a check valve J and a restricted S to the injector K where it is mixed with liquid from the tank(s) A. The pressure-regulated compressed air from the valve E is also passed to one end of the pilot-operated air stop valve in the unit H tending to close the valve and additionally fed, via a flexible hose to a "dead-man" manually-operated valves where it acts against manual operation to open the valve X to exhaust. The pressure-regula'ted compressed air is fed back, via another flexible hose to the other end of the pilot-operated air stop valve in the unit, one end of the exhaust valve in the unit H and to the pneumatic actuator of the water stop valve P.

Pressure-regulated compressed air is also fed from the pilot-operated air stop valve from a point upstream of the check valve J, via a flexible hose, to a grit hopper unit L where it acts on one end of a grit stop valve DD and also pressurises a tank containing grit. A line inlcluding a flexible hose, returns from the grit tank to the exhaust valve in the unit H which is connected to a silencer U. Compressed air in this line is also fed to the other end of the exhaust valve tending to open the valve. Pressure regulated compressed air is additionally fed from the pilot-operated air stop valve from the same point upstream of the check valve J to an air motor V for driving an archimedean screw grit feed metering unit W.

In the grit hopper unit L grit is supplied via a hopper and the grit stop valve DD into the grit tak. Grit leaves the tank, via a grit valve M, and is supplied to the unit W wherefrom the flow of grit is metered. Air and liquid are fed from the injector, via a flexible hose and a pressure gauge EE with a stop valve and joins the metered flow of grit from the unit W. Air, liquid and grit continue, via a flexible hose, to a blast nozzle N.

In operation, compressed air is fed from the compressor T to the valve I and thence to-the tank(s) A at an adjusted pressure. Compressed air is also fed from the compressor T to the valve E where any excessive pressure fluctuations are evened out by exhausting air from the valve.

The adjustment of air pressure within the desired range of O to 1 20 p.s.i. afforded by the valve E means that the blast pressure of the nozzle N is adjustable for selective removal of coatings from the surface to be blasted. In addition to the helmet R, the pressure regulated compressed air is fed to the unit H. Provided that the "dead-man" valve X is manually depressed so that the valve is closed the pilot-operated valve in the unit H is open, the exhaust valve in the unit H is closed and the valve P is open. This means that compressed air is supplied to the injector K, the valve DD is closed and the tank in the unit L is pressurised, and the motor V is operating, the air pressure in the tank A being slightly higher than that at the injector K because of the orifice S.

Liquid is supplied from the tank(s) A to the injector K and the mixture of air and liquid from the injector K is joined with grit from the unit W so that air, liquid and grit issues from the nozzle.

In the event that the "dead-man" valve X ceases to be manually depressed and thus becomes open, the air pressure in the lines to and from the valve X drops so that the pilot-operated valve in the unit H is closed, the exhaust valve in the unit H is opened and the valve P is closed. This means that compressed air ceases to be supplied to the injector K, the tank in the unit L ceases to be pressurised and the motor V ceases to operate. This, in turn, means that liquid ceases to be supplied to the injector K and grit ceases to be supplied by the unit W. The system can also be used, by closing of the grit supply, to subject a surface liquid washing at any desired air pressure and liquid flow rate.

In a. modification, the injector K is omitted and the liquid is instead introduced at the nozzle N so that only compressed air is fed to join the metered flow of grit from the unit W. Thus, air and grit continue, via the flexible hose, to the nozzle N where they are joined by the liquid so that again air, liquid and grit issue from the nozzle N. Alternatively, the liquid may be introduced at any point downstream of the point of introduction of the grit into the line to the nozzle N.

Instead of using the air motor V and the unit W the grit may be deliverd from the tank in the unit L into the line to the nozzle N by means of air pressure and gravity via a ball valve.

In order that the system described and illustrated may cater for a plurality of nozzles N, the one compressor T may deliver compressed air via individual lines to the respective nozzles, the same tank(s) A may deliver water under pressure to the said individual lines and the grit may be supplied to the individual lines from respective grit units L such that the air pressure and liquid content in the lines to the individual nozzles are accurately and individually controllable.

Since many users will already possess dry grit blasting systems comprising an air compressor, a grit unit, nozzles, helmets and sundry lines, it is envisaged to supply not only a complete wet grit blasting system but also equipment such as enclosed in the dot-dash-line block b in the drawing for modifying into a wet grit blasting system a dry grit blasting system.

Claims (8)

1. A method of wet grit blasting wherein compressed air is used both for entraining grit and for pressurising liquid and a stream of air, pressurised liquid and grit is ejected from a blast nozzle.

2. A wet grit blastic system comprising a source of compressed air, container means for liquid, a line connecting the source of compressed air to the container means whereby to pressurise liquid in the container means, a nozzle, a line connecting the source of compressed air to the nozzle whereby to deliver compressed air to the nozzle, line means connecting the container means to the nozzle for delivery of liquid under pressure to the nozzle, a grit supply unit, a line connecting the source of compressed air to the grit supply unit whereby to pressurise the unit, and a line from the grit supply unit to supply grit under pressure to the nozzle whereby to eject a stream of compressed air, pressurised liquid and grit from the blast nozzle.

3. A system according to claim 2 comprising an operator's helmet and a line connecting the source of compressed air to the helmet.

4. Equipment for modifying into a wet grit blasting system, a dry grit blasting system comprising a source of compressed air a grit supply unit and a nozzle from which grit can issue entrained in a jet of compressed air, the equipment comprising container means for liquid, a first connection for the source of compressed air, a line from the connection to the container means whereby to pressurise liquid in the container means, a line from the first connection to a second connection for the nozzle for delivering compressed air to the nozzle line means from the container means for delivery of liquid under pressure to the nozzle, and a line from the first connection to a third connection for the grit unit whereby to pressurise the grit unit.

5. Equipment according to claim 4 comprising a line from the first connection to a fourth connection for an operator's helmet.

6. A method of wet grit blasting substantially as hereinbefore described with reference to the accompanying drawing.

7. A wet grit blasting system substantially as hereinbefore described with reference to the accompanying drawing.

8. Equipment for modifying into a wet grit blasting system a dry grit blasting system, the equipment being substantially as hereinbefore described with reference to the accompanying drawing.