WO1994020263A1

WO1994020263A1 - Blast-cleaning system

Info

Publication number: WO1994020263A1
Application number: PCT/SE1994/000165
Authority: WO
Inventors: Jan Lindquist; Gösta WIBERG
Original assignee: Wp-System Aktiebolag
Priority date: 1993-03-03
Filing date: 1994-03-01
Publication date: 1994-09-15
Also published as: AU6224194A

Abstract

A blast-cleaning system includes an air compressor, a pressure vessel which pressurizes a blasting material and mixes it with compressed air, and a blasting unit connected to an outlet of the pressure vessel for blasting the interior surface of a structure to be cleaned. A conduit arrangement in the blasting apparatus is divided into two branches downstream from a main air valve. One branch opens into an upper portion of the pressure vessel to pressurize the blasting material; the other branch is connected to a mixing valve for mixing the blasting material with compressed air. Valves arranged in the conduit branches are individually set and maintain a constant pressure difference between the compressed air used for pressurization and the compressed air used for mixing, independent of the level of the operating pressure of the main compressed air from the main air valve. The pressure for pressurizing the blasting material is set higher than the mixing pressure. The blasting unit is provided with rotating, preferably belt-driven blast arms with nozzle outlets through which the surface is blasted at an angle. The blasting unit may be supported within the structure to be cleaned either with adjustable wheels or by a brush arrangement. A vacuum and filtering device may also be connected to the duct so that the pressure vessel, blasting unit, duct and filtering device form a substantially closed system.

Description

BLAST-CLEANING SYSTEM

5 BACKGROUND OF THE INVENTION

Field of the Invention i This invention relates to an apparatus for blast-cleaning, in particular, in a system for blast- cleaning the interior walls of ducts, conduits and other 10 vessels.

Description of the Related Art

There is a common need to clean the inside walls of vessels, ducts, and other similar structures. One common 15 way of doing this is abrasive blasting. In these systems, one or more streams of particles such as sand, powdered metal, etc., are sprayed with high pressure against the interior surfaces of the structures to be blast-cleaned. The abrasive particles are then cleaned out of the 20 interior of the structures and are either discarded or are recycled into a compression system.

Examples of known devices used for abrasive cleaning are described in the following publications:

UK Patent Application GB 2 160 130 A (Applicant: NIS 25 Engineering Ltd.), published 18 December 1985;

UK Patent Application GB 2 119 298 A (Applicant: Brent Chemicals International PLC) , published 16 November 1983;

German published Patent Application DE 34 08 828 Al 30 (Ristow) , published 12 September 1985;

Swiss Patent No. CH 656 330 A5 (Darani) , 30 June 1986; and

U.S. Patent No. 3,074,098 (Downing), 22 January 1963.

Blasting apparatuses such as those described in these i 35 publications typically use compressed air with an operating pressure on the order of 5-7 bar. This pressure is also present within the pressure vessel that is used to propel the abrasive blasting material and also in the valves used for mixing the blasting material with the propellant air. One problem with such conventional systems is that it is often not possible to work with such high pressure, which may damage the objects to be cleaned. An example of such a pressure-sensitive application would be the removal of paint or some other coating from the inner surface of a thin-walled aluminum duct.

One conventional way of dealing with this problem is to use strong solvents at lower pressures. The drawback of such a method is that although the pressure-sensitive structure itself may not be damaged by the solvent, the solvent itself may be damaging to the environment and may also be difficult to handle safely. Furthermore, if one lowers the blasting pressure when working with pressure-sensitive objects below the typical working pressures (5-7 bar) , for example, to as low as 0.5 bar, the pressure will often not be enough to pressurize and force the blasting material out of the pressure vessel to the mixing valve. Moreover, the lower operating pressure must be compensated for by including significantly more of the blasting material in order to achieve satisfactory results.

The object of this invention is therefore to provide a blast-cleaning system with an effective operational pressure range (typically from 0.5 - 9 bar) greater than conventional blasting systems.

Yet another object of the invention is to provide a blast-cleaning system and operation that allow the air for pressurizing the blasting material within the pressure vessel of the apparatus to be higher than the pressure used for mixing the material with the propellant air.

A further object of the invention is to provide a substantially closed blast-cleaning system in order to reduce the amount of blasting material required, and in order to minimize the impact on the surrounding environment of the blast-cleaning procedure itself. Summary of the Invention

According to the invention, a blast-cleaning system includes a source of compressed air, a pressure vessel that pressurizes a blasting material and mixes it with compressed air, and a blasting unit connected by a flexible hose to an outlet of the pressure vessel for blasting the interior surface of the structure to be cleaned.

A conduit system in the blasting apparatus is divided into two branches downstream from a main air valve. One branch opens into an upper portion of a pressure vessel to pressurize a blasting material using compressed air; the other branch is connected to a mixing valve for mixing the blasting material and the compressed air. Pressure- regulating valves are arranged in the conduit branches and are individually regulated to set and maintain a constant pressure difference between the compressed air used for pressurization and the compressed air used for mixing, independent of the level of the operating pressure of the main compressed air from the main air valve. A desirable, non-pulsating output of the blasting material to the mixing valve and onward to a blasting nozzle is thereby achieved.

According to another aspect of the invention, the pressure of the compressed air used for pressurizing the blasting medium in the pressure vessel is set higher than the pressure of the compressed air used for mixing with the blasting material. In this way, considerably more of the blasting material is supplied to the mixing valve at lower operating pressures for the main compressed air.

According to yet another aspect of the invention, a blasting unit that operates within the conduit or duct to be blast-cleaned is provided with a rotating blasting nozzle, whereby the multi-branched nozzle is belt-driven and blasts the interior surface of the conduit at an angle. The blasting unit may be supported within the conduit to be cleaned either with adjustable wheels or by a brush arrangement.

According to still another aspect of the invention, a vacuum and filtering device is connected to the duct that is being blast-cleaned. The pressure vessel, blasting unit, duct and filtering device thereby form a substantially closed system, which not only allows for recycling of the blasting material but also avoids releasing the blasting material and potentially hazardous particles from the interior of the duct to the surrounding atmosphere.

Brief Description of the Drawings

Fig. 1 shows the main components of the blast-cleaning system according to the invention in place for cleaning the inside of a duct;

Fig. 2 is a side view of a pressure vessel and pressure regulation system for mixing and propelling the blasting material according to the invention; Fig. 3 is a rear view of the pressure vessel and mixing system;

Fig. 4 is an end view of a movable blasting unit within the duct;

Fig. 5 is a side view of the blasting unit shown in Fig. 4;

Fig. 6 illustrates a preferred wheel configuration for the blasting unit shown in Fig. 4;

Fig. 7 is an end view of an alternative brush-support arrangement for the blasting unit; and Fig. 8 is a side view of a laterally movable brush- support arrangement for the blasting unit.

Detailed Description

Fig. 1 shows the main components of the blast-cleaning system according to the invention. These components include a blasting apparatus 10, which is powered by a conventional air compressor 12. Blasting material, which may be of any conventional type, is forced under the pressure of the compressed air from the compressor 12 through a eeder hose 14, which passes through an opening 16 in the duct, conduit, or other structure 18 that one wishes to clean and on to a movable blasting unit or "blaster" 20.

The blaster 20 is pulled forward within the duct 18 by means of a wire 22, whose other end is attached to a reel/winch apparatus 24, which may be electrically or mechanically operated either automatically or under operator control. Blasting material within the duct 22 and the particles blasted from the interior of the duct are collected through a return conduit 26, which is attached and seals over a forward opening (ahead of the blasting unit in the direction of motion of the blasting unit) in the structure 18. The used blasting material and other particles are then fed to a collector 28 which acts as a "vacuum cleaner" to suck and filter out the used blasting material and collected particles. Figs. 2 and 3 show the blasting apparatus 10 in greater detail. The blasting apparatus according to the invention includes as its main component a pressure vessel 30 and pressure fixtures 32 for compressed air. The pressure vessel 30 is preferably made of any conventional material such as steel and includes a mainly cylindrical central portion 34, a curved top portion 36 and a conical lower portion 38.

The pressure vessel 30 of the blasting apparatus 10 is preferably mounted on a supporting structure such as the illustrated tripod structure, which includes wheels 42 so that the blasting apparatus can be moved easily to the work site. Other known supporting structures, with some other number of wheels (including zero) may also be used.

Above the curved top portion 36, the blasting apparatus 10 preferably includes a supply hopper 44 of a conventional type for holding and feeding into the pressure vessel 30 any conventional blasting material such as sand, metallic powders, ground glass, etc. The amount of blasting material fed into the pressure vessel 30 from the supply hopper 44 is preferably controlled by a conventional valve arrangement. Blasting material is preferably fed into the pressure vessel 30 when the vessel is decompressed (see below) .

An outlet opening 46 near the bottom or tip 48 of the conical lower portion 38 of the pressure vessel 30 is connected via an outlet conduit 50 to a first inlet 52 of a mixing valve 54. The blasting material passes from the blasting apparatus 10 via the mixing valve 54 onward to the downstream elements of the blast-cleaning system (described below) .

The pressure fixtures 32 for compressed air include a regulated main air valve 56, which may be adjusted for setting the operating pressure of the air supplied by the compressor between a minimum and a maximum pressure. The available range of operating pressures depends on the pressure of the compressor 12 chosen for the particular job. A preferred pressure range for the main compressed air will be from approximately 0.1 to approximately 9.0 bar and preferably from 0.5 to 9.0 bar. The compressor 12 is connected to the main air valve 56 via a conventional inlet fitting 58. The main air valve 56 has an outlet 60, to which a conduit arrangement 62 is connected. The conduit arrangement 62, which is described in greater detail below, may be made of conventional straight, curved, or angled pipes or tubes that are able to withstand the pressures within the system.

The conduit arrangement 62 divides into a first branch 64 and a second branch 66 near and downstream from the main air valve 56. The first conduit branch 64 opens preferably into the central portion 34 of the pressure vessel 30. More specifically, the first branch 64 continues through the exterior wall of the central portion 34 and opens into a region directly under the supply hopper 44. Compressed air from the main air valve 56 is conveyed through the first branch 64 to the upper region of the pressure vessel 30 in order to pressurize the blasting material from above. The second branch 66 of the conduit arrangement 62 is connected to a second inlet 68 of the mixing valve 54. Compressed air that enters the system through the second branch 66 is there mixed with the blasting material which, via the outlet opening 46, the outlet conduit 50, and the first inlet 52, is fed from the pressure vessel 30 to the mixing valve 54. In the mixing valve 54, the compressed air and blasting material are mixed in a known, regulated manner by means of a knob 70. The mixture is then fed out via an outlet 72 and a flexible hose 74 or other conduit to blasting nozzles of the movable blaster 20 (see below) .

In each of the branches 64 and 66 there is a pressure-regulating valve 76 and 78, respectively. Each valve 76, 78 is individually adjustable by means of knobs

80 and 82, respectively. According to the invention, the operator turns the knobs (controls the setting of the valves 76, 78) so as to set a desired differential pressure between the compressed air used for pressurizing the blasting material via the first branch 64 and the compressed air used for mixing with the blasting material that is forced out via the second branch 66.

The pressure-regulating valves 76 and 78 are also arranged so that rises and falls in the pressure in either one are matched by corresponding rises and falls in pressure in the other in order to maintain the set constant differential pressure between the two valves 76, 78. This constant pressure differential may be set by the operator by turning the knobs 80, 82 and may be set independent of the operating pressure of the main compressed air from the main air valve 56. In this way, it is not necessary to adjust the settings of the pressure-regulating valves 76, 78 every time the operating pressure is changed; furthermore, a desired, non-pulsating output stream of blasting material to the mixing valve 54 and onward to the blasting nozzle (see below) of the blaster 20 is achieved.

As an example, in order to achieve a mild blasting effect on sensitive or in some way fragile structures (such as very thin-walled aluminum ducts) , the pressure of the air used to pressurize the blasting material in the pressure vessel 30 may be set higher, for example 5 bar, than the pressure of the air used to mix with the exiting blasting material, which can be as low as 0.1 - 0.5 bar. As another example, if one wishes to have a pressure differential of 0.5 bar, one could set the pressurization pressure at 1.0-1.2 bar while the mixing pressure is at the lower level of 0.5-0.7 bar. In this way, much more blasting material can be forced out to the actual blasting site at the blaster 20 with a lower operating pressure for the main supply of compressed air than is possible with conventional blasting arrangements. Furthermore, the differential pressure can be set regardless of the absolute pressure of the main supply.

An adjustable valve 84 for starting and stopping the blasting operation is provided in the second branch 66 between the pressure-regulating valve 78 and the mixing valve 54; the valve 84 is preferably adjusted by the operator. Simultaneously with the operation of the adjustable valve 84, the mixing valve 54 is opened and closed so that none of the blasting material exits the pressure vessel 30 and reaches the mixing valve during breaks in operation. In this way, the mixing valve 54, the hose 74 and the blasting nozzle (described below) are relieved of pressure while the pressurization of the blasting material within the pressure vessel 30 is still maintained. When the blasting operation is then re-started, it will not be necessary to wait for re-pressurization of the blasting material within the pressure vessel 30. A vibrator 86 is preferably mounted on the exterior of the conical lower portion 38 of the pressure vessel in order to improve the feeding of the blasting material to the mixing valve 54. The vibrator 86 may be of a conventional type such as those that contain an eccentrically mounted rotating momentum wheel. As is well known, the vibrator 86 causes the blasting material within the lower-conical portion 38 to "flow" or be fluidized so that it runs down toward the outlet opening 46 of the pressure vessel 30.

Outside of the "region of fluidization," the vibrator 86 could cause compaction of the blasting material. In order to counteract such compaction, an arrangement including an inlet to the lower, conical portion 38 may be provided for pulsating application of compressed air from below and into the blasting material. In Fig. 3, such an otherwise conventional arrangement is indicated as an inlet end of a pressurization line 87 for providing the pulsating, compressed air and/or driving the vibrator. To fill the pressure vessel 30 with blasting material via the supply hopper 44, the pressure vessel 30 must be decompressed. This may be done in a conventional manner by releasing the compressed air used for pressurizing the blasting material out into the atmosphere via an outlet valve 88 and a sound muffler 90, which may be combined with a filter for preventing escape of the blasting medium from the pressure vessel 30.

Figs. 4 and 5 are end and side views, respectively, of a preferred embodiment of the blaster 20 in operating position within the structure 18 that is to be blast- cleaned. The blaster 20 includes a central, hollow hub 100, through which compressed air and blasting material is distributed from the hose 14, via a generally cylindrical pressure chamber 101, to a plurality of tubular blast arms 104 that radiate outward from the central hub 100 and rotate about a central axis 106. A blast nozzle 108 is provided at the end of each arm 104. As Fig. 4 shows, the arms 104 preferably extend radially outward from the central axis 106, which also defines a longitudinal direction for the blaster. A radial direction is then defined as the direction of a line segment perpendicular to the central axis and with its end on the central axis; a tangential direction is then also defined as the direction of a vector that is perpendicular to both the central axis and the radial direction. The blast nozzles 108 are preferably angled so that the mixture of compressed air and blasting material is applied to the inner surface of the duct 18 at an angle, so that the direction of blast has radial, tangential and longitudinal components. Although the angle may be varied depending on the application, the preferable ranges of angle of the blast nozzles 108 are from 20 to 40 degrees to the surface pointing forward and from 30 to 50 degrees. By angling the blast nozzles in this manner (preferably all in the same direction, either clockwise or counter-clockwise) , experience has indicated that the effectiveness of blast-cleaning using the invention is improved, especially when removing from the inner surface of the structure 18 materials such as paint that tend to "flake". The blaster 20 preferably includes front and back mounting plates 109, 110, respectively, which preferably extend mainly parallel to the central axis 106. The mounting plates 109, 110 are joined by longitudinally extending struts 112. The struts are preferably dimensioned (using known techniques) and made of a material such as polyurethane so that they are rigid enough to support and join the mounting plates 109, 110 securely yet flexible enough tc allow the plates 109, 110 to flex relative to one another. In this way, the blaster according to the invention is better able to negotiate bends, angles and curves within the structure while still maintaining efficient contact with the structure's inner surface to provide effective blasting.

Wheels 114 are mounted on the front and back mounting plates 109, 110 via pivoting arms 116. The arms 116 are in turn joined with the front and back mounting plates 109, 110 by means of pivots or journals 118 so that the wheels 114 can independently move radially inward and outward. Springs 120 or other conventional tensioning members connect the pivoting arms 116 with the respective mounting plates 109, 110, optionally via lugs 122. The arms 116 and thereby the wheels 114 are thus biased radially outward so that the wheels 114 press against and roll along the inner surface of the structure 18.

A conventional air motor 124 is also preferably mounted on the front mounting plate 109 with a motor shaft 126 that extends mainly parallel to the central axis 106. A secondary compressed air hose 128 or flexible conduit connects the compressor 12 (see Fig. 1) or other pressure source with the air motor 124, whose shaft 128 rotates when compressed air is applied to the motor 124. Rotational motion of the air motor's 124 shaft 126 is transmitted to the central hub 100 via a belt 130, whereby conventional belt drive wheels 131, 133 are mounted on the motor shaft 126 and a shaft 135 of the central hub 100, respectively.

As Fig. 5 shows, the wire 22 used to pull the blaster within the conduit, duct, or other structure 18 is connected to the blaster preferably using a conventional rotating coupling mounted on the central hub 100 along the central axis 106. As compressed air is applied to the air motor 124, the arms 104 rotate so that the whole interior surface of the structure 18 is blast-cleaned as the blaster 20 is pulled forward by the wire 22.

In Figs. 4 and 5, the blaster 20 is shown with six wheels 114 at the front and six wheels 114 at the rear of the blaster 20. The number of wheels may, however, be varied depending on the type of structure one expects the blaster 20 to be used the most in. Furthermore, although it will typically be preferable to arrange the wheels 114 with equal spacing about the central axis 106 for reasons of balance, this is not necessary. For example, if it is anticipated that the blaster will be operating mostly within square or rectangular ducts, it may be preferable to rearrange the wheels, or to make some of the pivoting arms 116 longer than others.

Even in such cases, however, the six-wheel arrangement shown in the figures would be able to adapt to the interior of such a structure since the wheels can move radially against the force of the springs 120. Another advantage of spacing the wheels evenly about the central axis 106 is that the blaster 20 will then more easily be able to follow bends and other changes of direction of the duct 18. Furthermore, the arrangement of the wheels 114 as shown in the figures allows them to hold the central hub 100 near the center of the duct 18. This in turn improves the ability of the blaster 20 to blast the interior surface of the structure 18 evenly.

Fig. 6 is a front view of a preferred configuration of the wheels 114 in an unstressed configuration. As Fig. 6 illustrates, the wheels preferably have a pointed profile (as opposed to having the profile of a smooth torus) . The wheels 114 will then have the minimum contact surface with the interior of the structure 18 as the blaster 20 moves through the structure. This also minimizes the degree to which movement of the blaster 20 will be hindered within the structure 18 because of rivets, screws, and other fittings, fixtures and irregularities.

Fig. 7 is an end view of an alternative support arrangement for the blaster 20. In this alternative support arrangement, the arms 104 still extend radially outward from the central hub 100, but the wheels are replaced by a brush support arrangement 132 in which stiff bristles extend radially outward from the surface of a generally cylindrical mounting member 134. This blaster thus has the general appearance of a fuzzy caterpillar.

As before, the blast nozzles 108 are preferably angled relative to the arms 104. As before, the nozzles 5 108 may also be angled relative to the arms 104 to provide radial, tangential, and/or longitudinal blasting force components. One advantage of the brush support and spacing arrangement 132 is that it is simple and also will tend to help "scrub" particles off of the inner surface of 0 the structure 18.

Fig. 8 shows a brush-support and spacing arrangement which is particularly suitable for blast-cleaning of structures with rectangular cross-sections. In this arrangement, brush elements or bristles 136 extend from a 5 main mounting plate 138, which is installed so as to be essentially parallel to one side of the inner surface of the structure 18. The brush elements 136 extend on all four sides over the length of the blaster 20 (once again resembling a mechanical fuzzy caterpillar) preferably so 0 as to be in contact with all four inner surfaces of the structure 18, but preferably at least with a bottom surface and both side surfaces.

The central hub 100 and blast arms 104 are thereby mounted in a conventional bearing in the mounting member 5 so that the hub 100 and therefore also the arms 104 can move sideways (perpendicular to the longitudinal direction) within the structure 18. As the arms 104 rotate, the arms and hub may then be moved back and forth in the direction of the arrow 140 so as to blast-clean the 0 entire inner surface of rectangular structure 18.

Claims

CLAIMS 1. A system for blast-cleaning an interior surface of a structure such as a duct, conduit or vessel by abrasion of a blasting material propelled by compressed air, comprising:

A. a pressure vessel with an upper portion and a lower portion for containing and pressurizing the blasting material and including:

1) an upper material inlet through which the blasting material is fed into an interior of the pressure vessel;

2) a lower material outlet through which the pressurized blasting material exits the pressure vessel;

B. a source of compressed air; C. a common main air valve connecting the source of compressed air and the pressure vessel for setting a main operating pressure of supply air from the source of compressed air;

D. a mixing valve connected to the lower material outlet and to the source of compressed air for mixing the pressurized blasting material with compressed air at a mixing pressure to form pressurized, mixed blasting material, and including:

1) a compressed air intake; 2) a pressurized material intake; and

3) a mixed air/material outlet;

E. a pressure-regulated conduit system including:

1) a first conduit branch leading from the common main air valve and opening into the upper portion of the pressure vessel;

2) a first pressure-regulating valve connected in the first conduit branch for setting a material pressurization pressure for the blasting material within the pressure vessel; 3) a second conduit branch leading from the common main air valve to an air intake of the mixing valve; 4) a second pressure-regulating valve connected in the second conduit branch for setting a mixing pressure within the mixing valve;

F. a blasting unit including a blasting nozzle for applying pressurized mixed blasting material to the surface of the structure to be blast-cleaned;

G. a main blast supply hose connecting the mixed air/material outlet and the blasting unit; whereby an operator may set the pressurization pressure and the mixing pressure independent of the setting of the common main air valve and thereby maintain a constant pressure differential between the pressurization pressure and the mixing pressure independent of the operating pressure; and whereby a pre-determined, non-pulsating feed of the blasting material is applied via the mixing valve to the blasting unit.

2. A system as defined in claim 1, in which the material pressurization pressure is greater than the mixing pressure.

3. A system as defined in claim 1, in which a start/stop valve for starting and stopping the blasting operation is arranged in the second conduit branch between the second pressure-regulating valve and the mixing valve, whereby the blasting operation may be started and stopped with maintained pressurization of the blasting material within the pressure vessel.

4. A system as defined in claim 1, further including a vibrator for fluidizing the blasting material within the pressure vessel adjacent to the lower outlet opening of the pressure vessel.

5. A system as defined in claim 4, further including a source of pulsating compressed air, in which: the pressure vessel has a fluidization inlet near the lower portion; and the source of pulsating compressed air is connected to the fluidization inlet for applying the pulsating compressed air from below to the blasting material; whereby compaction of the blasting material because of the vibrator is counteracted.

6. A system as defined in claim 1, in which the blasting unit includes: a pressure chamber connected to the main blast supply hose; a hollow hub that is rotatably journalled on the blasting unit at an upstream end of the blasting unit and is rotated about a central axis that substantially coincides with a longitudinal axis of the structure; hollow, tubular blasting arms that extend substantially in a radial direction and are inwardly connected to the hollow hub; and a blast nozzle connected at an outer end of each blasting arm; whereby the pressurized, mixed blasting material passes from the pressure chamber to the blasting nozzles via the central hub and the blasting arms.

7. A system as defined in claim 6, further including: an air motor, which has an output shaft, mounted on the blasting unit; a second flexible tube connecting the air motor with the source of compressed air; and a transmission arrangement connecting the output shaft of the air motor with the central hub for rotating the blasting arms.

8. A system as defined in claim 7, in which the blasting arms extend radially outward from the central hub and in which the blast nozzles are angled relative to the blast arms.

9. A system as defined in claim 7, in which the output shaft of the air motor is coupled via a belt drive to the central hub.

10. A system as defined in claim 6, in which: the blasting unit further includes: wheels; a wheel support system for mounting and biasing the wheels against the interior surface of the structure to be blast-cleaned; and each wheel has a substantially pointed profile in order to minimize the contact area between the wheels and the interior surface of the structure.

11. A system as defined in claim 10, in which: the blasting unit includes front and back mounting plates that extend substantially perpendicular to a central axis of the blasting unit; the front and a back mounting plates are joined by lengthwise extending supporting struts; and the supporting struts are flexible; whereby the front and back mounting plates are able to flex relative to one another and the blasting unit is able to follow bends and other changes of direction within the structure while maintaining contact between the wheels and the interior surface of the structure.

12. A system as defined in claim 6, in which the blasting unit includes a brush support and spacing arrangement with bristle elements extending away from a central mounting member toward and in contact with the interior of the structure to be blast-cleaned.

13. A system as defined in claim 12, in which the central hub and arms are mounted in bearings on the central mounting member to allow lateral movement of the hub and arms relative to the mounting member, whereby the blasting unit is able to apply blasting material to the entire interior surface of rectangular structures.

14. A system as defined in claim 1, in which the structure has a forward opening located ahead of the blasting unit in a direction of motion of the blasting unit, further including: a vacuum source connected to and sealing the forward opening in the structure for removing and filtering out particles blasted from the interior surface of the structure; whereby the source of compressed air, the pressure vessel, the blasting unit, and the vacuum source form a closed system for removal and collection of material blasted from the interior of the structure.

15. A system for blast-cleaning an interior surface of a structure such as a duct, conduit or vessel by abrasion of a blasting material propelled by compressed air, comprising: A. a pressure vessel with an upper portion and a lower portion for containing and pressurizing the blasting material and including:

B. a source of compressed air;

C. a common main air valve connecting the source of compressed air and the pressure vessel for setting a main operating pressure of supply air from the source of compressed air; D. a mixing valve connected to the lower material outlet and to the source of compressed air for mixing the pressurized blasting material with compressed air at a mixing pressure to form pressurized, mixed blasting material, and including:

1) a compressed air intake;

2) a pressurized material intake; and

3) a mixed air/material outlet;

E. a pressure-regulated conduit system including: 1) a first conduit branch leading from the common main air valve and opening into the upper portion of the pressure vessel;

2) a first pressure-regulating valve connected in the first conduit branch for setting a material pressurization pressure for the blasting material within the pressure vessel;

3) a second conduit branch leading from the common main air valve to an air intake of the mixing valve; 4) a second pressure-regulating valve connected in the second conduit branch for setting a mixing pressure within the mixing valve, whereby an operator may set the pressurization pressure and the mixing pressure independent of the setting of the common main air valve and thereby maintain a constant pressure differential between the pressurization pressure and the mixing pressure independent of the operating pressure;

G. a main blast supply hose connecting the mixed air/material outlet and the blasting unit;

H. a vibrator for fluidizing the blasting material within the pressure vessel adjacent to the lower outlet opening of the pressure vessel;

I. a vacuum source connected to and sealing a forward opening in the structure for removing and filtering out particles blasted from the interior surface of the structure; in which:

J. the material pressurization pressure is greater than the mixing pressure;

K. a start/stop valve for starting and stopping the blasting operation is arranged in the second conduit branch between the second pressure-regulating valve and the mixing valve, whereby the blasting operation may be started and stopped with maintained pressurization of the blasting material within the pressure vessel; L. the blasting unit includes:

1) a pressure chamber connected to the main blast supply hose; 2) a hollow hub that is rotatably journalled on the blasting unit at an upstream end of the blasting unit and is rotated about a central axis that substantially coincides with a longitudinal axis of the structure;

3) hollow, tubular blasting arms that extend substantially in a radial direction and are inwardly connected to the hollow hub;

4) a blast nozzle connected at an outer end of each blasting arm, whereby the pressurized, mixed blasting material passes from the pressure chamber to the blasting nozzles via the central hub and the blasting arms;

5) an air motor, which has an output shaft, mounted on the blasting unit;

6) a second flexible tube connecting the air motor with the source of compressed air; and 7) a transmission arrangement connecting the output shaft of the air motor with the central hub for rotating the blasting arms, in which the output shaft of the air motor is coupled via a belt drive to the central hub; 8) wheels, each wheel having a substantially pointed profile in order to minimize the contact area between the wheels and the interior surface of the structure;

9) a wheel support system for mounting and biasing the wheels against the interior surface of the structure to be blast-cleaned;

10) front and back mounting plates that extend substantially perpendicular to a central axis of the blasting unit;

11) flexible, longitudinally extending struts joining the front and a back mounting plates, whereby the front and back mounting plates are able to flex relative to one another and the blasting unit is able to follow bends and other changes of direction within the structure while maintaining contact between the wheels and the interior surface of the structure; whereby the source of compressed air, the pressure vessel, the blasting unit, and the vacuum source form a closed system for removal and collection of material blasted from the interior of the structure.