The present invention relates to an abrasive blasting apparatus and
in particular to a dustless abrasive blasting apparatus.
In dustless abrasive blasting, an abrasive 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 abrasive with the liquid so that the abrasive 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 abrasive also increases the efficiency of the blasting operation.
Typically, the liquid used in abrasive 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.
Conventionally, a dry abrasive is entrained into a gaseous stream
and liquid is then added to wet the abrasive prior to its egress from the nozzle.
In this way, dust generation at the work area is reduced. Such an arrangement, however,
requires a separate pump to be used to deliver the liquid into the abrasive and
gaseous stream, which makes the cost of the blasting apparatus more expensive than
is desirable and renders the blasting apparatus more prone to breakdown.
In WO 88/07915 is described an apparatus which overcomes the aforementioned
problems by entraining separately contained liquid and an abrasive/liquid mixture
in a flow of pressurized fluid whilst simultaneously applying the pressurized fluid
to the receptacles containing the liquid and the abrasive/liquid medium in order
to balance the fluid pressure between the receptacles themselves and the receptacles
and the delivery line. This has the advantage of requiring only a single pressurized
fluid source and therefore of reducing the cost of the apparatus.
The aforementioned apparatus was further refined in WO 00/51787 by
the injection of a pressurized fluid into the lower portion of the receptacle directly
into the contents, which is thereby agitated. A control means is then provided to
control the pressure within the receptacle to ensure that during blasting the pressure
of the pressurized fluid entering the vessel through the injection means is always
maintained at a higher level than the pressure of the pressurized fluid directed
into the vessel by the pressurizing means. This enabled fine abrasive particles
in addition to conventional abrasive materials such as various sands and grit, to
be used in the apparatus.
However, the aforementioned apparatus all suffer from the disadvantage
that a considerable quantity of water or other blasting liquid is used during a
blasting operation. This makes the operation messy as large quantities of liquid
must be disposed of and the object of the cleaning operation is thoroughly wetted,
which can be disadvantageous.
Other apparatus entrain an abrasive/liquid mixture contained in a
pressure vessel directly in a flow of pressurized air, the vessel being pressurized
by water from a mains supply rather than by pressurized air. Such an arrangement
is described in EP 1034891.
This significantly reduces the quantities of waste water which is
produced. However, this apparatus also suffers from the disadvantage that the entrained
abrasive particles tend not to be evenly dispersed within the pressurized blasting
fluid but clumped together in groups or pockets so that the resulting jet of blasting
fluid is not uniform in nature. This slows down a blast cleaning operation as the
operator must ensure that the blasting jet is played over the surface to be cleaned
for a sufficient length of time to compensate for the unevenness of the jet.
The object of the present invention is to overcome the aforementioned
disadvantages by considerably increasing the quantity of abrasive entrained in the
pressurized fluid. This has the effect of increasing the ratio of abrasive to blasting
liquid content, thus reducing the large quantities of liquid waste which results
from a blasting operation. Also, the blasting jet is made more uniform and smooth
According to 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 bottom
outlet, the flow of pressurized gas through the conduit entraining the contents
of the vessel for dispersal into the gaseous 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 characterised in that the opening connected to the bottom
outlet of the vessel is defined in a lower side of the conduit in order that the
pressurized gas passes over the opening; and in that a venturi is located in the
conduit immediately upstream of the opening connected to the bottom outlet of the
vessel so that the velocity of the pressurized gas is increased as it passes over
the opening thereby to increase the quantity of particulate blasting medium entrained
by the gas.
Preferably, the outlet from the pressure vessel communicates directly
with the delivery line via a pipe which terminates in the opening.
Preferably also, the opening and the venturi respectively comprise
inlet ports into a block which forms a part of the conduit, the block defining an
outlet for the flow of pressurized gas and entrained media in a side directly opposite
Preferably also, the opening is defined by the lowermost side of the
Preferably also, the opening comprises a valve opening which can be
obturated by a valve member to close off the supply of particulate blasting medium
and liquid from the vessel into the conduit. Advantageously, the block houses the
Preferably also, the valve member comprises a plunger around which
pressurized gas in the conduit can flow.
Advantageously, means are provided to inject liquid into the pipe
to clear any blockage therefrom.
Preferably, the wall of the lower portion of the vessel is dished
to encourage flow of the particulate material through the bottom outlet.
Preferably also, the valve means comprises an actuator which is powered
by the pressurizing gas.
Preferably also, the apparatus comprises a gas reservoir capable of
powering the actuator to close the valve means if the supply of pressurized gas
from the source is cut-off.
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 present invention;
- Fig. 2 is a perspective view similar to that of Fig. 1 but from the opposite
side of the apparatus and with components at the top of a pressure vessel of the
apparatus shown in explosion;
- Fig. 3 is a schematic view of the interior of a control box located at one side
of the apparatus;
- Fig. 4 is a circuit diagram showing pressurized air flow through the apparatus;
- Fig. 5 is view to an enlarged scale and in partial cross-section of an air flow
conduit located at the lower portion of the control box.
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 water based blasting
solution as aforesaid. The vessel 1 is provided with a side inspection plate 4,
which during operation of the apparatus is kept shut, and a top blasting medium
inlet 5. The inlet 5 is defined centrally at the base of a detachable sieve 6 located
at the top of the vessel 1. 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 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
a supply such as the mains water 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 block 10 as shown in Fig. 3 and as is described
in greater detail below with reference to Fig. 5. When the valve 18 is open, the
pressurized air from the source can 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
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.
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 25 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 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.
Turning now to the block 10, as previously described this comprises
a housing for the plunger valve 12 which is used to open and to close the pipe 9
and thereby permit or prevent the abrasive blasting medium contained with the vessel
from the entrained into the pressurized air flow passing through the block 10.
With particular reference to Fig. 5, the block 10 defines an air inlet
34 and an air outlet 35 located respectively directly opposite one another on opposite
sides of the block 10. It is through the inlet 34 and the outlet 35 that the pressurized
air can enter and exit from the block 10. The valve 12 is located vertically within
the block with a valve member 36 disposed so that it can open and close a valve
opening 37 defined on the lowermost side of the block 10 over which the air flow
through the block passes. The actuator 26 for the valve 12 is mounted above the
block 10. The valve opening 37 is formed at the end of the pipe 9 and thereby effectively
forms a main outlet from the vessel 1 for the blasting medium as the pipe 9 is connected
directly to the interior of the vessel 1 via the bottom outlet 7.
The inlet 34 is fashioned into a venturi 38, which increases the velocity
of the pressurized air as it passes over the valve opening 37. This in turn significantly
increases the quantity of particulate blasting medium entrained by the air and thereby
increases the ratio of abrasive to blasting liquid content, thus reducing the large
quantities of liquid waste which results from a blasting operation. It has also
been found that the blasting jet issuing from the blasting nozzle is made more uniform
and smooth in nature than conventional apparatus.
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
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 litre of water per minute into the vessel 1, 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. As described above, the venturi
38 at the inlet 34 greatly increases the quantity of abrasive entrained relative
to the liquid 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 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.
The blasting apparatus can be used with any suitable 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 sulphate; and
soda ash as well as small glass beads. The inclusion of the venturi 38 in the valve
block 10 has been found to increase by up to four fold the quantity of such blasting
media entrained by the pressurized air, which greatly increases the efficiency and
efficacy of the blasting operation.