WO1990015694A1

WO1990015694A1 - Feeding an abrasive mixture

Info

Publication number: WO1990015694A1
Application number: PCT/GB1990/000927
Authority: WO
Inventors: David Henry Saunders; Edward John Bloomfield
Original assignee: BHR Group Ltd
Current assignee: BHR Group Ltd
Priority date: 1989-06-15
Filing date: 1990-06-15
Publication date: 1990-12-27
Anticipated expiration: 1991-12-15
Also published as: GB8913728D0; AU5825590A; ZA904647B; GB2232620A

Abstract

When an abrasive mixture is supplied through conduits (191) containing valves (309), considerable wear is experienced by the valve when it is operated with abrasive material within it. The present invention aims to reduce wear on such valves by providing means (193, 308) to flush the valve before operation so that the abrasive material content is reduced preferably to zero. This avoids abrasive particles becoming caught between working parts of the valve and causing undue wear. When the conduit leads to a nozzle (222) from which the abrasive mixture is jetted for cutting purposes, the flushing of the valve will cause a reduction in the concentration of abrasive mixture at the nozzle in the normal circumstances. In order to avoid this reduction of concentration, the outlet of the valve can be fed to the nozzle through an abrasive mixture reservoir (310) which will smooth out variations in the abrasive mixture concentration and so maintain the efficiency of the jet from the cutting nozzle.

Description

FEEDING AN ABRASIVE MIXTURE

This invention relates to an abrasive mixture, which comprises abrasive material carried in a carrier liquid. A jet of a pressurised abrasive mixture is useful for cutting or otherwise machining workpieces. It combines the high performance of a gas torch without the disadvantage of heating the workpiece to very high temperatures. The supply of the abrasive mixture does however have the problem that its abrasive nature causes high wear of the conduits and nozzles and these problems have been addressed in earlier patent publications, for example O87/02290 andW086/00224, The present invention is also concerned with the problem of wear, particularly in valves in the conduit carrying the abrasive mixture. Abrasive particles cause high wear in a valve when it is operated, which will soon impair its sealing properties and even its operation. In abrasive mixture supply apparatus, there is provided pressurising means, a passage to supply abrasive mixture to the pressurising means, another passage to supply the pressurised abrasive mixture from the pressurising means to a nozzle. In this conventional arrangement, the pressurising means causes a jet of abrasive mixture at high pressure to be directed from the nozzle to the workpiece to cut or otherwise machine it. It is often desired to control the jet by means of a valve, so that a valve may be provided in a said passage and according to the invention there is provided with means for flushing the valve before operation. This meεtns may comprise a supply of plain carrier liquid and means to force the supply of plain carrier liquid through the valve as the first step in a valve operation process, the second step being to operate the valve when the concentration of abrasive particles in the valve has been reduced below a predetermined value. In one arrangement, two valves may be provided in series, the passage for carrying the abrasive mixture joining a conduit connecting the two valves so that the abrasive mixture passes through the downstream valve only; the flushing means is connected to the upstream side of the two valves.

The pressurising me»ans may comprise a pressure vessel as described in WO87/0229C. Alternatively, it may comprise a pump. Although the flushed valve arrangement of the present invention is presently seen as being more useful in the high pressure part of the apparatus, that is downstream of the pressurising means, it would be possible for the flushed valve to be located upstream of the pressurising means, that is in the low pressure side of the apparatus.

The supply of additional carrier liquid has the purpose of reducing the concentration of abrasive material in the valve, and it also has the effect of reducing the concentration of abrasive material in the conduit downstream of the valve and eventually at the nozzle. If the nozzle is still being used at this time for treating a workpiece, its performance will be affected and an optional feature of the present invention is to provide an averaging device downstream of the flushed valve which will smooth out variations in the concentration of the abrasive mixture at the nozzle. The averaging device may comprise an abrasive mixing reservoir through which the supply of abrasive material passes downstream of the valve on its way to the nozzle.

Examples of the invention will now be described v/ith reference to the accompanying drawings in which:-

Figure 1 is a schematic diagram of apparatus described in WO/8702290, Figure 2 is a schematic diagram of a modification of the apparatus of Figure 1 according to the present invention,

Figure 3 is a detail of the mixing reservoir,

Figure 4 is a schematic diagram of a flushed valve arrangement on the upstream side of the pressurising device, and

Figure 5 is a schematic diagram of supply apparatus incorporating a pump as pressurising means.

In the apparatus of Figure 1, abrasive material is fed, either in dry or in slurry form, into a hopper 205 filled with water extending to a maximum depth controlled by an overflow 207. Material from the base of the hopper can be drawn upwards through a passageway 192 to the pressure vessel 201, the passageway 192 including vertical tube 206 leading to a trap 208 through a valve 211, the location of the valve being such that the volume of the conduit below the valve is greater than the volume of the conduit above the valve and below the trap by such a factor that when abrasive material of the operative concentration in the carrier fluid is present in the conduit above and below the valve and the flow stops, the abrasive material in the conduit will settle over a time to a maximum level which is below the level of the valve. This can be achieved by making the lower portion of the tube 206 of larger cross section than the portion above the valve. The valve in the rest state will then be in clear carrier fluid and the valve can operate without drawing abrasive material into its working parts. The minimum value of the factor depends on the concentration of abrasive material in the carrier liquid, but the apparatus can be designed with a factor suitable for most working concentrations. Pressure vessel 201 has two co-axial conduits at its upper end and a trap type outlet at its lower end. The inner co-axial conduit 225 is connected through trap 208 and valve 211 to the tube 206. A high pressure water pump 209 feeds water in two branches; one branch leads through a variable flow restrictor 217, a flowmeter 216, a non-return valve 220 provided with a strainer 227 at the entry to the vessel 201. A junction between the valve 213 and the outer co-axial conduit 226 leads through a valve 221 to a suction pump 210 which feeds water into the top of the hopper 205- The pump 210 is capable of handling an inlet suction of 63cm Hg and low concentration slurries, since some fine abrasive material will be passed by the strainer 227. A suitable pump is a pneumatic powered diaphram pump. The other branch 193 from the junction at the outlet of the pump 209 feeds through a non-return valve 219 to a junction from which one branch is connected through a valve 212 to the outlet conduit 191 of the pressure vessel 201 and the other branch is connected to a discharge nozzle 222. The non-return valves 219 and 220 are chosen so that sufficient pressure differential is created to pass a required flow through the pressure vessel 201, the remaining outlet of the pump by-passing the pressure vessel 201 through the valve 219. Relief valves 218 are provided for safety.

At the start of operations, the empty pressure vessel 201 is filled with water from the pump 209_* After valve 213 has been closed and 221 opened, the suction pump 210 is energized to circulate water from conduit 226 at the top of the pressure vessel 201 through the valve 221 into the hopper 205 and from the base of the hopper- through the tube 206 back to conduit 225 of the vessel 201. Grit is supplied to the hopper and settles to the bottom. The pressure difference generated within the tube 206 and the locally increased liquid velocity fluidises abrasive material at the inlet to the tube 206 and a slurry of the water and the par iculate material contained in the hopper 205 is drawn into the pressure vessel 201 where the arrangement of components and the rate of flow are chosen so that the abrasive material settles out from the slurry while the water continues its circulation through conduit 226 to the pump 210. Eventually, the settled material will 5 reach the level of a strainer 227 at the entrance to the outer co-axial conduit 226 at the top of the container, stopping the flow when the strainer mesh becomes blocked. The abrasive material is chosen to be in a restricted band of particle sizes, so that there are plenty of voids in the material in the vessel 201 allowing 10 liquid to flow therethrough. The presence of a significant proportion of fines in such material reduces the flow of liquid through the settled material and furthermore such fines are not efficient when the abrasive material is entrained in a jet of carrier fluid and used for cutting purposes.

15 Grit is discharged from the pressure vessel 201 by applying water under pressure from the pump 209 through the valve 213 to the outer co-axial conduit 226, the valves 211 and 221 being closed and the valve 212 open. This flow of water in reverse to the previous flow clears grit from the strainer 227 and water passes

20 through the settled material to the base of the pressure vessel where the local flow pattern adjacent a trap 204 at the outlet of the vessel 201 fluidises the material which passes through the trap 204 and the valve 212 to the nozzle 222. The discharge of the pressure vessel 201 can be stopped at any time by closing the valve

25213, so that all the water from the pump 209 is then diverted through the non-return valve 219. Closure of valve 212 must be delayed after the flow of abrasive mixture has stopped, to allow abrasive material to settle into the conduit below the valve, leaving clear carrier fluid in the valve at the time it is closed

30 to avoid damage. The apparatus of Figure 2 adds some components to those of Figure 1. There is a pressure control device in the form of a bleed conduit 311 leading from the junction of the valves 213 and 221 back to the hopper- 205 which provides a continuous bleed from the outlet 226. There is an accumulator 301 in the outlet from pump 209, together with a dump valve 302, a pressure relief valve 303, a flowmeter 304 and a pressure gauge 305. A valve 306 and a flow limiter 307 are located between the flow restrictor 217 (which in Figure 2 with respect to Figure 1 is reversed in position with the flowmeter 216) and the non-return valve 220. The flow limiter 307 prevents excessive surges in the flow of fluid when the appropriate valve 213 is opened which might cause a consequent pressure drop elsewhere, for example in the pressure vessel being discharged. This valve 306 and others such as 213 and 221 are illustrated with rectangles alongside them, the rectangles denoting pneumatic control devices. Such devices are connected to a central controller (not shown) to control the operation of the system. The single valve 212 in the bottom outlet of the pressure vessel 201 in Figure 1 is replaced by valves 308 and 309 one on either side of the junction with the conduit 193, -and these valves opeerate in place of the settling action associated with the valve 212 of Figure 1. A grit mixer 310 is locat»sd downstream of the valve 309 and upstream of the nozzle 222 in order to smooth variations in the concentration of the abrasive mixture reaching the nozzle as a result of the flushing of valve 309-

The flushing action operates as follows: when the abrasive mixture is being supplied under pressure from one vessel 201 to the nozzle 222, the valve 309 is open and the corresponding valve 308 is closed. When it is desired to close off the conduit 191 from the nozzle 222 by closing the valve 309, the valve 213 is closed to relieve the applied pressure of the carrier liquid and the valve 308 is opened to transfer the carrier liquid flow from the upper passage 194 to the lower conduit 193• The flow of carrier liquid from the valve 308 through the valve 309 flushes the valve 309 and reduces the concentration to a satisfactory low value. The valve 309 can then safely be operated without abrasive particles causing undue wear of the moving parts.

The grit mixer is shown in Figure 3 and contains a pressure envelope 41 with a pair of coaxial passages for entry and exit of the abrasive material. Abrasive mixture enters through the inner passage 42 and leaves it at exit slots 52. The inner passage continues above the slots 5 to an air bleed passage 53_* An annular exit passage 51 is located at the base of the envelope 41, leading to the nozzle 222. A reserve supply of abrasive mixture is contained in the envelope 41 around the central passages and as the flow of mixture reaches the outlet 51, the variations in concentration of the mixture are smoothed out.

In the apparatus of Figure 2, there are two vertical tubes 206 each leading through respective valves 211 and traps 208 to respective pressure vessels 201. Similarly the feed water from the flowmeter 216 divides into two branches, one for each pressure vessel 201, each branch having its own flow limiter 307, non-return valve 220, valve 213, valve 221, and relief valve 218, with a connection from the junction of the valves 213 and 220 to the outlet 226 of the respective vessel 201. There is a branch of the conduit downstream of the non-return valve 219 for each vessel 201, each branch having its own valves 308 and 309, the branches joining up again upstream of the grit mixer 310.

The arrangement of Figure 2 has two systems in parallel leading to a common outlet containing the grit mixer 310 and nozzle 222. With this arrangement, one system can be operating to supply an abrasive mixture under high pressure to the nozzle 222 while the other system is being replenished. This is useful when a continuous output is required from the nozzle 222 whereas the presuurising Svsfcem of the vessel 201 is essentially a batch process, but the flushed valve arrangement of the present invention could be used with a single pressurising system. If this single pressurising means system requires alternate filling and discharge cycles, the supply of pressurised mixture from the nozzle 222 could then only be discontinuous.

The flushed valve arrangement is shown in Figure 2 downstream of the pressure vessel. In Figure 4, it is schematically shown upstream of the pressure vessel, that is in the low pressure side of the apparatus, but the same principles apply.

Similarly, the same principles apply when a pressurising means other than the pressure vessel illustrated in Figure 2 is used. For example, a rotary piston pump, such as that shown schematically in Figure 5 could be used and the flushed valve arrangement could be located upstream or downstream of the pump. The pump has the advantage that it can operate continuously so that it is not then necessary to provide the twin system of Figure 2 in order to provide a substantially continuous jet at the nozzle 222, but on the other hand a pump has moving parts which are more subject to wear from the passage of the abrasive material than the relatively static system of Figure 2.

Figure 6 is a schematic diag',ram of the invention applied to a system with two pressingvessels aranged in series.

It would be possible to have two pressure vessels arranged in series as shown in Figure 6. In this arrangement, two T-arrangements of valves 308 and 309 are arranged at the outlets of respective pressure vessels 201. Carrier liquid in conduit 194 can be applied to the inlet of the top pressure vessel 201 through valve 213, or to the top of the lower pressure vessel 201' through valve 308 or to the nozzle conduit 191 through a second valve 308'.

There are valves 309 below the respective junctions of the valves 308 and 308' with the conduit arrangement 191 leading to the nozzle 222.

Claims

CLAIMS:

1. Aα £.braεive mixture supply apparatus ccmprising pressurising means (61, 201), a nozzle (222), a passage (192) to supply abrasive mixture to the pressurising means, another passage (191) to supply the pressurised abrasive mixture from the pressurising means to the nozzle and a valve (309) in a said passage, characterised by means (193, 308) for flushing the valve (309) before operation.

2. Apparatus as claimed in Claim 1 wherein the abrasive mixture comprises abrasive particles in a carrier liquid and the flushing means comprises means (193, 308) for supplying additional carrier liquid to the valve.

3. Apparatus as claimed in Claim 1 or Claim 2 wherein the flushing means comprises a further valve (308) upstream of said valve (309) and not located in said passage (191), the further valve (308) being openable to flush the first mentioned valve (309).

4« Apparatus as claimed in any one of Claims 1 to 3 comprising an abrasive mixing reservoir (310) in said other passage (191) downstream of said abrasive material flushing means (308, 193).

5- Apparatus as claimed in any one of Claims 1 to 4 comprising two said pressurising means (6l, 201) interconnected in parallel to enable the means to discharge abrasive slurry alternately to provide a continuous flow of abrasive slurry from said nozzle (222), there being a said abrasive material flushing means (193, 3θ8) associated with each pressurising means.

6. Apparatus as claimed in Claim 4 and Claim 5 wherein said valves (308) associated with said flushing means (193,308) are arranged in parallel with their outlets connected to a common mixing reservoir (310).

7- Apparatus as claimed in any one of the preceding Claims wherein the pressurising means comprises a pressure vessel (201).

8. Apparatus as claimed in any one of Claims 1 to 6 wherein the pressurising means comprises a pump (6l).