GB2409992A - Temperature controlled particulate blasting - Google Patents

Abstract

The blasting apparatus includes a pressure vessel (1, figure 1), a conduit 11 connected to a pressurised fluid source and the pressure vessel, a delivery line (14, figure 1) and a pump 13. The pressure vessel contains a particulate blasting medium and a liquid maintained at 4-15{C, preferably by a heat exchanger. Preferably the pressurised fluid is air and the liquid in the pressure vessel is water. Preferably the liquid comes from an insulated tank and the pressure vessel is surrounded by an insulating jacket.

Description

IMPROVEMENTS TO PARTICULATE BLASTING

The present invention relates to a method and apparatus for blasting a surface and in particular to dustless particulate blasting.

In dustless particulate blasting, a particulate, typically an abrasive such as olivine sand, is entrained in a pressurized liquid flow or gaseous-entrained liquid flow and is directed against the surface to be treated by a controllable nozzle. It is the intention of such apparatus to coat each particle of the particulate with the liquid so that each particle thereof is weighted by the liquid and falls safely to the ground after striking the surface to be blasted, generally obviating the requirement for the operator of the apparatus to wear breathing apparatus. The weighted particulate also increases the efficiency of the blasting operation.

Typically, the liquid used in blasting apparatus is water or a water based blasting solution, such as a rust inhibiting solution but it will be appreciated that any suitable liquid could be used. Hence, whenever the term "water" is referred to herein and in the claims it should be understood that the term includes all suitable blasting liquids. Similarly, the pressurized gaseous streams used in blasting operations are typically pressurized air but again the term "air" when referred to herein and in the claims is to be understood as including any gas suitable for use in abrasive blasting operations.

In WO 00/34011 is described a method of abrasive blasting wherein a particulate solid having a particle size from 150 to 250 am is distributed in a fluid carrier to form a spray mixture that is impacted against a surface to be cleaned using a pressurized jet. Further described is the use - 2 - of water as the fluid carrier and it is indicated that the temperature of the water may be the ambient temperature.

However, it is suggested that heating the water to a temperature of between 25 to 40 C reduces water consumption.

The present applicant has carried out extensive tests using water at various temperatures and has found that, in fact, the temperature of the water being used is heated does have a direct effect on the efficiency of the blasting process. In contradiction of the aforementioned patent specification, if the water temperature is within the stated range, it has been found, in particular, that the cohesiveness of the blasting jet is significantly reduced.

This results in an increased scatter of the blasted particulate that not only prolongs the time taken to blast clean a predetermined surface but also tends to causes the particulate within the jet to become "pulsed" resulting in an uneven cleaning effect.

The object of the present invention is therefore to provide a method and apparatus for blast cleaning which improves the efficiency of the blasting process in situations wherein the ambient temperature is likely to be around 20 C or higher and thus have a deleterious heating effect on the water used in the blasting process.

According to a first aspect of the present invention there is provided a method of blasting a surface comprising the steps of containing a particulate blasting medium and a liquid within a pressure vessel; proving an outflow from the vessel into a conduit) providing a controlled flow of pressurized fluid along the conduit) - 3 entraining the contents of the vessel for dispersal into the flow of pressurized fluid for blasting through a delivery line connected to the conduit; and pumping liquid from a liquid source into the vessel behind its contents in terms of its contents' outflow from the vessel into the conduit; and characterized in that the temperature of the liquid within the vessel is retained within the range 4 to 15 C inclusive.

Surprisingly, it has been found that if the liquid used within the blasting process is retained at a temperature within the aforesaid range that the efficiency of the blasting is significantly improved over that when the temperature is of the order of 20 C or higher.

Advantageously, the temperature of the liquid within the vessel is retained within the range 4 to 8 C inclusive.

Hence, according to a second aspect of the present invention there is provided a blasting apparatus comprising a pressure vessel adapted to contain a particulate blasting medium and a liquid and including an inlet and a bottom outlet; a conduit along which an entraining pressurized gas can be fed from a pressurizing gas source and with which the interior of the vessel communicates via an opening in the conduit connected to the outlet, the flow of pressurized fluid through the conduit entraining the contents of the vessel for dispersal into the fluid flow for blasting through a delivery line connected to the conduit; a pressurizing means to pump liquid from a liquid source into the vessel behind its contents in terms of its contents' outflow from the vessel through the bottom outlet; valve means to control the flow of pressurized gas along the conduit; and characterized in that a means is provided whereby the temperature of the liquid within the vessel can be retained within the range 4 to 15 C inclusive. - 4

Preferably, the liquid temperature retaining means comprises an insulating jacket located around the pressure vessel. In this way the temperature of a cool liquid supplied to the vessel can be retained at a lower level than the ambient temperature.

Alternatively or in addition, a heat exchanging means is provided for cooling the temperature of the liquid from the liquid source to a temperature within the 4 to 15 C temperature range prior to its entry into the pressure vessel.

Preferably also, the liquid source comprises a storage tank. In this case, a cooling means is preferably also provided for retaining the temperature of the liquid within the storage tank within the range 4 to 15 C inclusive.

Advantageously, the tank is insulated.

20Preferably also, the temperature of the liquid within the vessel is retained within the range 4 to 8 C inclusive.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a perspective side elevation of a blasting apparatus according to the second aspect of the present invention; Fig. 2 is a schematic view of the interior of a control box located at one side of the apparatus shown in Fig. 1 and a liquid supply for use in one embodiment of the invention; Fig. 3 is a circuit diagram showing pressurized air flow through the apparatus; and 5 Fig. 4 is a diagram showing the cleaning of separate areas in a trial conducted using the blasting apparatus as shown in Figs. 1 to 3.

An apparatus according to the invention comprises a pressure vessel 1 to one side of which is attached a control box 2. Preferably, both the vessel 1 and the control box 2 are mounted on framework forming a wheeled trolley 3 with a handle to enable the apparatus to be wheeled close to an object to be blasted.

The pressure vessel 1 is adapted to contain a blasting mixture of a particulate material and a liquid, which is typically water or a waterbased blasting solution as aforesaid. The vessel 1 is supplied with water either directly from a mains water supply or from a separate supply tank 4, as shown in Fig. 2. At the top of the vessel 1 is a blasting medium inlet 5 that is located centrally at the base of a detachable sieve 6. The vessel also has a bottom outlet 7, which is disposed at a central portion of the bottom of the vessel 1. The inlet 5 can be closed by a seal 8, which is located beneath the sieve 6, in order that the vessel 1 can be internally pressurized. The outlet 7 has no obturating means and communicates directly with the interior of a pipe 9, which therefore effectively forms an extension of the interior of the vessel 1. The pipe 9 communicates the interior of the vessel 1 with a valve block 10 connected into and thereby forming a part of a main air flow conduit 11 through the control box 2, as is further described below. The block 10 comprises a housing for a plunger valve 12 which is used to close the end of the pipe 9 and thereby isolate the interior of the vessel 1 from the pressurized air flow through the conduit 11 which is used to entrain the blasting mixture contained within the vessel 1. The shape of the - 6 - plunger is such that air can flow around it through the conduit when it is both open and closed.

Within the control box 2 is located a pump 13 for pumping liquid from the liquid supply into the upper portion of the vessel 1 via a side inlet (not shown) located between the pump 13 and the vessel 1. Approximately halfway up the vessel 1 but below the level of the side inlet is a side outlet 14, which can be closed by a valve 15 and which comprises an overflow for the contents of the vessel 1.

In order to assist in the egress of the blasting mixture out of the vessel 1 through the bottom outlet 7 at the same rate as water is pumped into the upper portion of the vessel 1, the wall of the lower portion of the vessel 1 is shallowly dished. In addition, two jets of the pumping liquid are injected into the outlet 7 via hoses 16 at the commencement of a blasting operating to clear any particulate material which may have settled out of the liquid and which may otherwise block the outlet 7 and generally to assist in flow of the blasting mixture out of the outlet 7 and through the pipe 9. The jets are tapped off from the main liquid output supplied by the pump 13.

The apparatus is provided with pressurized air as its operational or motive fluid from a pressurized air source, such as an air compressor (not shown) via a control circuit, which is shown in detail in Fig. 4. Air under pressure is delivered from the source to a main air inlet 17 of the conduit 11 located at one side of the control box 2 and thence to an inlet port of a butterfly control valve 18 operated by an actuator 19. The valve 18 thus controls the flow of air through the conduit 11.

The conduit 11 ends in the valve block 10 and when the valve 18 is open, the pressurized air from the source can - 7 flow through the conduit 11 and the block 10 and thence through an outlet port 20 into a delivery line (not shown) connected thereto. The delivery line typically comprises a flexible hose with a blasting nozzle (not shown) at its end.

The flow of pressurized air in which the blasting mixture has been entrained along the delivery line and out through the nozzle is controlled by an operative using a deadman's handle 21. The handle 21 is preferably connected to the blasting nozzle for the sake of convenience but could be located elsewhere such as attached to the operative's body. The use of the deadman's handle 21 to control the blasting jet enables the blasting operation to be conducted in a fail- safe manner.

The deadman's handle 21 is separately supplied with its own small supply of pressurized air via an air line 22 that is tapped off from the conduit 11 immediately downstream of the inlet 17 and upstream of the control valve 18. It is operationally linked to the control valve 18 and the plunger valve 12 via an air filter 23, which removes stray particulate from the returning air, and a spool valve 24, both of which are located within the control box 2. When the operator wishes to commence a blasting operation, he squeezes a spring-loaded trigger (not shown) in the handle 21 so that pressurized air is permitted to flow through the handle 21 to the spool valve 24, which is thereby operated to permit air to flow through an air line 25 to the actuator 19 of the control valve 18 and an actuator 26 of the plunger valve 12 to cause both valves 12 and 18 to open. However if, for any reason, the operator relaxes his grip on the trigger of the handle 21, the air flow to the spool valve 24 is cut-off.

The spool valve 24 then operates, as described below, to cause air flow through a second air line 27 to the actuators 19 and 26 to cause the control valve 18 and the plunger valve 12 to close, thus cutting off the air flow to the delivery line and halting the blasting operation. - 8

The air line 22 also supplies pressurized air to power the pump 13 via an air pressure regulator 28 and an emergency stop arrangement 29. The regulator 28 controls the air supply to the pump 13, preferably so that the pump 13 is regulated with a 1/3 pressure reduction. Hence, a typical 4 bar air pressure supply will produce a 12 bar water pressure.

The emergency stop arrangement 29 includes a button 30 provided on top of the control box 2 which can be struck in an emergency in order that the supply of pressurized air to the pump 13 can be cut off by venting it to the atmosphere.

Operation of the pump 13 is thereby immediately halted. The arrangement 29 is also interposed between the air filter 23 and the spool valve 24 and on operation acts to cut-off the supply of air returning from the deadman's handle 21 to the spool valve 24. This has the same effect as release of the deadman's handle 21 by the operative and causes the valves 12 and 18 to close thereby shutting off the air supply to the delivery line to halt the blasting operation.

Pressurized air is also supplied from the air line 22 to an air reservoir 31 by way of a non-return valve (not shown).

Air flow into the reservoir 31 from the air line 22 occurs until the pressure in the reservoir 31, as indicated by a pressure gauge 32, balances that within the air line 22. The air reservoir 31 is provided as a safety feature of the apparatus and is linked to the spool valve 24 by an air line 33, the pressure in which is controlled so that normally the spool valve is operated to permit air flow through the line keep the valves 12 and 18 open. However, if there is a failure in the supply of pressurized air from the source, for example if the supply hose is cut or the air compressor fails, the supply of air to the spool valve 24 from the deadman's handle 21 will fail. The connection of the air reservoir 31 to the spool valve 24 is set up such that if - 9 this occurs, the spool valve operates to cause pressurized air to flow from the reservoir 31 through the valve 24 down the line 27 to cause the main valve 18 and the plunger valve 12 to close. It will be appreciated that this also occurs when the deadman's handle 21 is released by the operative or the emergency stop arrangement 29 is activated. However, in both the latter cases, the reservoir 31 is immediately replenished with air from the line 22, whereas in the first case when there is an air supply failure, the reservoir 31 ensures that there is sufficient pressurized air available to place the apparatus in a safe shut-down mode. It will be appreciated that in these circumstances, when the apparatus is re-started the valves 12 and 18 can only be opened by operation of the deadman's handle 21 acting on the spool valve 24 after the reservoir 31 has been replenished.

In use, the apparatus must be set up so that the pump 13 is supplied with a continuous supply of blasting liquid and the air inlet 17 is connected to a source of pressurized air.

Prior to commencing a blasting operation, the vessel 1 must be filled with a blasting medium. The valve 15 is opened and the vessel 1 is filled with water from either a mains source or from the tank 4 through the top opening 5 until the water starts to overflow from the vessel 1 through the side outlet 14. It will be appreciated that the vessel 1 is now approximately half-full of water. The dry abrasive to be used in the blasting operation is then poured into the vessel 1 through the sieve 6, which prevents any clumps of abrasive or large foreign bodies from entering the vessel 1. Typically, around 75 kilos of abrasive are poured into the vessel 1.

This will displace an equal volume of water from the vessel through the side outlet 14. The valve 15 is then closed and the vessel 1 is topped up with water 1 so that all air is expelled therefrom. The valve seal 8 is then used to close the top inlet 5. -

The air supply to the inlet 17 can then be switched on and the water pump 13 set to pump up to 1 lithe of water per minute into the vessel 1 from either the mains supply or the tank 4, as determined by a dosing valve (not shown) attached thereto, to control the rate of supply of blasting medium from the vessel 1 to the supply line as desired. Typically between one quarter and one half a litre of water per minute is required. Water is also injected into the outlet 7 via hoses 16 to clear any particulate material which may have settled out and be blocking the conduit 9 Once the air supply has been switched on, the air reservoir 31 fills and the deadman's handle 21 is supplied with a pressurized air flow via the air line 22. The operative can then commence a blasting operation when desired by squeezing the trigger of the handle 21. As described above, this permits air to flow to the spool valve 24, which operates to permit air to flow through the air line 25 to the actuator 19 of the control valve 18 and the actuator 26 of the plunger valve 12 and to cause both valves 12 and 18 to be opened.

Air is now permitted to blast through the conduit 11 and the block 10 over the top of the open valve opening 37.

As a result, the air entrains the liquid-coated particulate blasting medium which is supplied to the block 10 under pressure along the conduit 9 from the vessel 1 so that the blasting jet issuing from the nozzle at the end of the delivery line is substantially dry and contains an even mix of pressurized air and blasting medium. In addition, the shallow dished shape of the lower portion of the vessel 1 assists in ensuring that as much liquid-coated particulate medium as is possible can flow through the pipe 9 under pressure from the water being pumped into the vessel 1 by the pump 13. As each particle of the particulate blasting medium is coated with a film of liquid, this has a beneficial effect on the blasting process as the momentum of each coated particle blasted from the nozzle is thereby increased and results in a more efficient cleaning action.

The operative can continue blasting using the deadman's handle 21 to control the flow of air and blasting media along the delivery line until the supply of particulate medium in the vessel 1 has been used. When it is desired to stop lO blasting, the operative releases the handle 21, which cuts off its supply of air to the spool valve 24 which then operates to permit air from the reservoir 31 to flow down the line 27 to cause the main valve 18 and the plunger valve 12 to close. Blasting is then stopped.

As previously explained, should an emergency arise requiring the emergency stop button 30 to be depressed or should the supply of pressurized air fail, then the spool valve 24 is also operated to cause air from the reservoir 31 to flow down the line 27 to cause the main valve 18 and the plunger valve 12 to close and thereby stop the blasting operation.

It will be appreciated that the aforementioned blasting apparatus is a conventional apparatus and can be used within any conventional particulate blasting medium, for example sand; grit-like materials such as garnets, or olivine sands; sodium carbonate; calcium carbonate; calcium magnesium carbonate; calcium oxide; calcium bicarbonate; calcium magnesium carbonate; magnesium oxide) magnesium sulphatei and soda ash as well as small glass beads.

However, by conducting scientifically based trials it has been found that the efficiency of a blasting operation is significantly affected by the temperature of the water within the pressure vessel. In temperate and cold climates, the - 12 water for use in the blasting operation is typically taken from a below-ground mains water supply. As a consequence the temperature of the water is usually between 4 and 8 C.

However, if a blasting operation is carried out on a warm day or in a country where the ambient temperature is normally in excess of 15 C, then the temperature of the water within the vessel 1 rises gradually to the ambient temperature. This has a deleterious effect on the blasting efficiency. Reference is now made to the following trial, which were scrutinized and verified by an independent witness, namely a senior trading standard officer. Trial

With reference to Fig. 4, two areas A and B to blasted were marked out on the doors of a large steel container. Each of these areas was subdivided into two 1 m2 areas to ensure each presented comparative painted surface coatings to be removed during the trial. The trial comprised the blast cleaning of the paintwork off each of these areas using a blasting apparatus as described above wherein all parameters were kept the same, including the same operative, apart from the temperature of the water retained in and supplied to the vessel 1. A blasting process using cold water was applied to areas Al and B1 and a blasting process using heated water was applied to areas A2 and B2.

The blasting media used for all trials was GMA Garnet 30/60 mesh and the air compressor throughout was operated at 9.5 bar. The ambient temperature throughout the trial was never higher than 20 C.

The water temperature within the vessel 1 at the start of the cold blasting process was found to be 4.2 C. The water temperature within the vessel 1 at the start of the heated blasting process was found to be 39. 3 C and at the end of the - 13 process had only dropped to 37.5 C. Each of the cleaning processes was timed by a stopwatch. The results were as follows.

Cleaning Times for Cleaning Times for Cold Water Blasting Heated Water Blasting Panel A1 2 mins 44 sees Panel A2 4 mins 05 sees Panel B1 2 mins 22 sees Panel B2 3 mins 26 sees Total time 5 mins 06 sees Total time 7 mins 31 sees Hence, it can be seen that the use of heated water significantly increases the cleaning time for any given blasting apparatus by almost 50% over the cleaning time when using cold water. The reasons for this are those quoted above, namely that the cohesiveness of the blasting jet was deleteriously affected when heated water was used resulting in an increased scatter of the blasted particulate. This prolonged the time taken to blast clean the areas A2 and B2.

It was also apparent that the particulate density within the jet became uneven and tended to be "pulsed", resulting in an uneven cleaning effect.

Further experiments have also been carried out by the applicant which were not independently scrutinized and it has been established that the efficiency of the apparatus it begins to deteriorate once the temperature of the water within the vessel rises above 15 C regardless of the particulate material being used.

It is a known phenomenon with apparatus as described above that if the temperature of the water in the vessel 1 falls below 4 C that ice pellets begin to form within the blasting jet. Hence, the optimum temperature range for the water in the vessel 1 is between 4 and 15 C inclusive.

Preferably, the temperature is kept significantly lower than the upper level of this range and between 4 to 8 C inclusive. - 14

In order to retain the temperature of the water within the vessel 1 at these low temperatures, the vessel 1 preferably comprises an insulating jacket, which can be formed by making the vessel with a double wall, the cavity between the walls being filled with an insulating material.

Such a vessel 1 will keep any water it contains at substantially the same temperature as that when the vessel 1 was filled. Hence, water supplied from a mains supply will be retained at a low temperature even though the ambient temperature may be greater than 15 C.

Alternatively or in addition, the water for the vessel 1 can be supplied from the tank 4 and either cooled prior to its entry into the vessel l by being passed through an suitable heat-exchanger 34 or by being cooled whilst retained within the tank 4 by a refrigerating cooling means 35 linked to the tank 4. In the latter case, the tank 4 should also be insulated to reduce the tendency of the water therein from being heated by ambient air.

By means of the aforesaid arrangements, it can be ensured that a blasting operation can be carried out with the blasting liquid at a temperature wherein the efficiency of the blasting operation is not impaired. -

Claims (12)

1. CT,AIMS _. . _., I. A method of blasting a surface comprising the steps

   of containing a particulate blasting medium and a liquid within a pressure vessel; proving an outflow from the vessel into a conduit) providing a controlled flow of pressurized f]uid along the conduit; entraining the contents of the vessel for lO dispersal into the flow of pressurized fluid for blasting through a delivery line connected to the conduit) and pumping liquid from a liquid source into the vessel behind its contents in terms of its contents' outflow from the vessel into the conduit; characterized in that the temperature of the liquid within the vessel is retained within the range 4 to 15 C inclusive.
2. 2. A method as claimed in Claim 1, characterized in that the temperature of the liquid within the vessel is retained within the range 4 to 8 C inclusive.
3. 3. A blasting apparatus comprising a pressure vessel adapted to contain a partcu]ate blasting medium and a liquid and including an inlet and a bottom outlet; a conduit along which an entraining pressurized gas can be fed from a pressurizing gas source and with which the interior of the vessel communicates via an opening in the conduit connected to the outlet, the flow of pressurized fluid through the conduit entraining the contents of the vessel for dispersal into the fluid flow for blasting through a delivery line connected to the conduit; - 16 a pressurizing means to pump liquid from a liquid source into the vessel behind its contents in terms of its contents' outflow from the vessel through the bottom outlet; valve means to control the flow of pressurized gas along the conduit; characterised in that a means is provided whereby the temperature of the liquid within the vessel can be retained within the range 4 to 15 C inclusive.
4. 4. An apparatus as claimed in Claim 3, characterised in that the liquid temperature retaining means comprises an insulating jacket located around the pressure vessel.
5. 5. An apparatus as claimed in Claim 3 or Claim 4, characterised in that a heat exchanging means is provided for cooling the temperature of the liquid from the liquid source to a temperature within the 4 to 15 C temperature range prior to its entry into the pressure vessel.
6. 6. An apparatus as claimed in any of Claims 3 to 5, characterised in that the liquid source comprises a storage tank.
7. 7. An apparatus as claimed in Claim 6, characterised in that a cooling means is provided for retaining the temperature of the liquid within the storage tank within the range 4 to 15 C inclusive.
8. 8. An apparatus as claimed in Claim 6 or Claim 7, characterised in that the tank is insulated.
9. 9. An apparatus as claimed in any of Claims 3 to 9, characterised in that the temperature of the liquid within the vessel is retained within the range 4 to 8 C inclusive.
10. 10. The use of a blasting apparatus as claimed in Claim 3, characterized in that the temperature of the liquid pumped into the vessel from the liquid source is within the range 4 to 15 C inclusive.
11. 11. A method of blasting a surface substantially as described herein with reference to Figs. 1 to 3 of the accompanying drawings.
12. 12. A blasting apparatus substantially as described herein with reference to Figs. 1 to 3 of the accompanying drawings.