Patent Publication Number: US-2003224704-A1

Title: Rotary media valve

Description:
FIELD OF THE INVENTION  
       [0001] The present invention is concerned in general, with improvements to blast cleaning apparatus. In particular, the invention is directed to improvements to media valves and a metering and dispensing system used to control the amount of abrasive media directed into a compressed gas stream.  
       BACKGROUND OF THE INVENTION  
       [0002] Blast cleaning equipment of the prior art has generally employed a compressed gas stream that directs the blast media to the target. For more specific blast cleaning, such as confined to cabinet structures, centrifugal wheels have been used to propel blast media towards a target substrate. Standard sand blasting equipment of the prior art pressurized air type consists of a pressure vessel or atmospheric supply pot to hold particles of a blasting medium such as sand, a source of compressed air connected to a conveying hose and a means of metering the blasting medium from the supply pot to the conveying hose. The sand/compressed air mixture is transported via the conveying hose to a nozzle where the sand particles are accelerated and directed toward a work piece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 10-20 pounds per minute. About 0.5 to 1.0 pound of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 0.5 to 1.0.  
       [0003] When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, or to avoid surface alteration of even hard materials such as stainless steel, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate, can be used in place of sand in conventional sand blasting equipment. The media flow rate used for the less aggressive abrasives is substantially less than that used for sand, and has been determined to be from about 0.5 to 10.0 pounds per minute, using similar equipment. The lower flow rates require a much lower media to air ratio, in the range of about 0.05 to 0.5.  
       [0004] However, difficulties are encountered in maintaining continuous media flow at these low flow rates when conventional sand blasting equipment is employed. The fine particles of an abrasive media such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, the bicarbonate media particles tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining substantially uniform flow of bicarbonate material to the blast nozzle has been difficult to achieve. Problems with non-uniform flow of the blast media lead to erratic performance, which in turn results in increased cleaning time and even damage of somewhat delicate surfaces.  
       [0005] Differential pressure systems were developed to address problems such as non-uniform flow of blast media and erratic performance when blast cleaning with the less aggressive media such as sodium bicarbonate. U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose modification of conventional blasting apparatus by providing a separate source of line air to the supply vessel through a pressure regulator to provide greater pressure in the supply vessel than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply vessel and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply vessel to the conveying hose, and ultimately to the nozzle and finally the work piece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having appropriate area, yields acceptable media flow rates in a controlled manner and provides efficient blast cleaning using softer media.  
       [0006] A media metering and dispensing valve which meters and dispenses the abrasive from the supply pot through the metering orifice and to the conveying hose carrying the compressed air stream became a component of the differential pressure systems. The media valve typically operates automatically whenever the compressed air is applied to the blast hose to begin the abrasive blasting operation. A typical media valve for use in the aforementioned differential pressure systems is disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402. This valve is characterized as a Thompson valve and is described in detail in U.S. Pat. No. 3,476,440. The Thompson valve includes a metering valve stem which blocks the output of a discharge tube disposed between the supply pot and an airflow tube, which is secured to and carries the compressed air to the conveying hose. When the blast nozzle is activated, the valve stem is lifted from the valve seat of the Thompson valve and allows a controlled amount of media to flow through the outlet of the discharge tube into the airflow tube. The valve as disclosed in U.S. Pat. No. 3,476,440 has been improved by placing the valve stem within a control sleeve which contains a plurality of orifices having different sizes, one of which can be placed in communication with the outlet of the discharge tube and the air flow tube. When the valve stem is seated within the valve body and closed, the orifice in the control sleeve is blocked such that media cannot flow from the discharge tube through the orifice in the media control sleeve and then into the air flow tube. Upon operation of the blast nozzle, the valve stem is pulled away from the orifice to allow the media flow from the pot to the discharge tube, through the orifice and into the air flow tube.  
       [0007] The plurality of orifices provides another means of controlling the amount of media flowing from the supply pot to the compressed air stream and into the blast nozzle apparatus. Unfortunately, to change the orifice which is in alignment with the media discharge tube and the air flow tube or to clean out a plugged orifice in the Thompson valve, it was required that the valve body holding the stem be taken apart, the valve stem taken out, rotated, placed back in the slot and the valve body then restructured. Obviously, such disassembly and reassembly is cumbersome and does not allow for efficient blast cleaning on the job site.  
       [0008] The present inventor has authored or co-authored several patents directed to novel and improved media control valves, which are particularly useful in differential pressure metering systems for dispensing less abrasive blast media. These improved valves and metering systems are disclosed in U.S. Pat. Nos. 5,407,379; 5,421,767; and 5,542,873. Such media valves offer added control with respect to metering the flow of the blast media because they include a control sleeve, which contains a plurality of orifices with different diameters to allow enhanced control of the amount of media dispensed from the supply vessel to the compressed air flow and adjustment of the orifices without disassembly of the valve. The operator can readily adjust the control sleeve so that one orifice is aligned to communicate with the discharge of the media from the supply vessel and the air flow tube to dispense the media into the compressed air flow tube and subsequently into the compressed air stream. Metering of the abrasive media is accomplished by adjustment of the control sleeve, which can be rotated while in place in the valve body to align a different orifice with the media discharge passage in communication with the supply vessel and the compressed air flow tube. Alternative embodiments are provided to index the control sleeve such that an orifice is properly aligned upon rotation of the control sleeve. In one embodiment, the index means comprises a ball spring plunger placed in the valve body and exerted against the control sleeve and a series of detents spaced in the sleeve and aligned with each orifice so as to properly align the orifice with the media discharge passage from the supply vessel and the air flow tube when the ball spring plunger fits within a detent in the sleeve. The control sleeve which contains the valve stem can be easily removed from the valve body in one piece for cleaning and replaced and locked in place in the valve body by means of a lock pin without disassembling the body of the valve. In a second embodiment, the index means comprises a plurality of grooves, which are placed on the face of the bore which receives the control sleeve and which mate with a plurality of teeth on the control sleeve. The teeth are aligned with the orifices. To change orifices, the control sleeve is lifted to disengage the teeth from the grooves and rotated until the teeth and grooves are again aligned and the sleeve then dropped in place in the valve body. The media control valve also includes a manually adjustable multi-port valve placed within the media discharge passage and which can close off the discharge passage from the supply vessel, and allow compressed air to back clean the valve and direct debris out a clean-out port in the valve body.  
       [0009] The prior art systems described above were particularly useful in systems applying a differential pressure across the media-dispensing orifice. While these systems allowed precise control of softer abrasive media dispensing and blast cleaning, such systems are complex and expensive to use and maintain.  
       [0010] Additional prior art disclose various shot blasting devices to propel large volumes of abrasives at high velocities to remove rust, paint and like substances from the substrate surface. For example, centrifugal blasting apparatus comprise a blast wheel with a plurality of throwing blades. In this type of blasting system, the abrasive is fed through the wheel and accelerated by the wheel and discharged toward the substrate surface to be cleaned. U.S. Pat. No. 4,922,664 describes an impeller blasting system that employs a separate liquid and abrasive supply means, where the liquid comes into contact with the abrasive within a chamber at an acute angle and propels the abrasive through an outlet bore to strike a surface to be cleaned. Cleaning a surface by shot blasting and chemical treatment of a cleaned surface normally is effected by separate steps, thereby requiring a variety of relatively complex and expensive process equipment arranged in sequence and involving a time delay between cleaning and treatment steps during which the cleaned surface may oxidize or otherwise becomes contaminated. Abrasive materials such as steel shot is heavier than the liquid in a shot/liquid slurry, the shot nonnally is discharged separate from the liquid off the blades of a centrifugal impelling apparatus, thus impinging the shot on a surface in a different area than the area of impingement of the liquid.  
       [0011] Another centrifugal blasting apparatus described by U.S. Pat. No. 6,126,525, comprises a blast wheel including an axis, a plurality of throwing blades and a central space. In this design the abrasive is fed axially through the center of the wheel and is accelerated by the wheel and discharged radially. The discharged abrasive exits radially at the circumference of the rotor where it immediately and directly impacts the substrate to be cleaned. The centrifugal blasting system has an impeller or throwing wheel that is fed abrasive or slurry axially through the center of the wheel and accelerates the abrasive by sliding it along a vane in the wheel to apply centrifugal force and impart kinetic energy.  
       [0012] U.S. Pat. No. 5,975,985 describes a centrifugal blast wheel, wherein a controllably positioned valve is disposed between the vessel and the centrifugal blast wheel to establish a rate at which the abrasive material passes from the vessel to the centrifugal blast wheel, which projects the abrasive material toward the surface.  
       [0013] U.S. Pat. No. 4,907,379 discloses discharge heads, which propel abrasives at a surface to be cleaned. The patent describes a one piece throwing wheel comprising a single side or back plate and angularly spaced throwing blades for propelling shot at a surface to be cleaned.  
       [0014] U.S. Pat. No. 5,637,029 describes an abrasive throwing wheel having a vane impeller rotatably mounted within the casing for receiving abrasive slurry at a relatively low velocity and discharging said slurry at a relatively high velocity. The slurry consists of a liquid carrier and abrasive material consisting of aluminum or steel shot and glass beads.  
       [0015] U.S. Pat. No. 5,879,223 describes a throwing wheel housing for propelling particulate media into the treatment chamber, including a media storage vessel for storing a supply of particulate media and a metering valve for dispensing a controlled flow of particulates from the vessel to the supply conduit. The throwing wheel housing includes a vane impeller and an enclosure having at least a portion shaped in the form of a volute so that the impeller and the enclosure cooperate to act as a suction blower. The throwing wheel housing thereby evacuates particulate media from the supply conduit to obtain an increased flow of particulate media through the throwing wheel housing imparting kinetic energy to the expelled particulate media. The metering valve is only generally described, but appears to be of an impeller design in which rotary vanes within a housing direct media from the media supply vessel to the supply conduit.  
       [0016] U.S. Pat. No. 2,092,201 describes an abrading device comprising, in combination, a fan, a housing surrounding the fan, a nozzle connected in communication with the housing and forming an outlet there from, and means for supplying abrasive to the interior of the nozzle. The supplying means comprises a continuously sealed pocketed feed means. The abrasive feed means of the abrading device comprises a continuously sealed, pocketed feed device made up from a cylindrical housing having an upper abrasive inlet funnel and a lower discharge conduit through the wall of the nozzle and provided with a pocketed rotor, having sufficient number of pockets to maintain the device continuously sealed against any substantial passage of air upwardly there through. The pockets are defined by a series of vanes, which ride against the inner surface of the housing and form a seal therewith. A number of shorter vanes may be interposed between the vanes so as to distribute the abrasive and more evenly provide for a substantially continuous abrasive flow through the feed mechanism. Such vanes would act like an impeller and impart a kinetic energy to the particulate. The abrasive may be supplied to the funnel from the feed hopper in the bottom of which may be placed a selected one of a plurality of plates having varying sized central openings whereby the feed may be regulated according to the type of abrasive used and the velocity of the air stream.  
       [0017] While problems associated with differential pressure blasting systems as described above include the complexity and cost of the overall system, these systems provided precise control over the dispensing of relatively soft abrasives such as sodium bicarbonate into a compressed air stream. While open atmospheric pressure vessels for storing of blast media would greatly reduce the overall cost and complexity of the blasting apparatus, there is still a need to accurately and efficiently control the amount of media, which is directed into the pressurized air stream. It is an objective of the present invention to provide improvements in the metering and dispensing devices used to meter and dispense a blast media into a pressurized air stream, in particular, when the blast media is fed from an open or atmospheric vessel. Specifically, the present invention allows for significantly higher media particle speeds and increased productivity, more efficient media to air ratios for various abrasives, improved precision of media flow rates, ease of operation, produces less dust, has a reduced cost of production, and is easily automated for critical applications.  
       SUMMARY OF THE INVENTION  
       [0018] The present invention is directed to media delivery devices designed to meter abrasive media from a supply vessel into a pressurized gas stream upstream of a nozzle for blast cleaning. The blast media is metered from the supply vessel into the pressurized gas stream, upstream of the nozzle, by means of a rotor, which includes a plurality of depressions placed on the outer circumferential surface thereof. As the rotor revolves, the depressions on the outer surface align with a discharge opening and the blast media flows by gravity from the supply vessel into the circumferential depressions of the rotor. As the rotor spins, the media is then directed to an outlet discharge, transporting media from the supply vessel into the pressurized gas stream. The rotor of the rotary media valve of the present invention contains depressions formed on the outer surface that can be of any shape.  
       [0019] The rotor preferably rotates in a smooth bore of a valve housing and is configured to fit within the valve housing to close tolerances so as to create a metal-to-metal seal between the outer rotor surface and the inner surface of the smooth bore of the valve housing. Accordingly, the preferred rotary media valve does not require side seals as is necessary in the “vane style rotors” of the prior art used to dispense media. The rotor diameter and number of depressions are calculated to maximize circumferential seal area and maintain a precise and proper media to compressed gas mixture at target rotor rpm. The rotor length is calculated so that the lateral seals (smooth outer surfaces), between each of the depressions and between the depressions and the ends of the rotor, are sized to preferably eliminate the need for side seals on the rotor. The presence of multiple small depressions on the outer surface of the rotor minimizes the slugging effect of large vanes dumping abrasive into the blast stream, which would disrupt the ideal media to compressed gas mixture.  
       [0020] Control of the rotor speed may be accomplished by means of a variable speed or indexing servomotor, which in effect sets the media flow rate into the pressurized gas stream. A small vent can be provided and positioned adjacent to the rotor in the valve housing to relieve any air pressure in the rotor depressions before the depressions are rotated and aligned under the supply vessel discharge opening. Scalability of the blast cleaning operation is achieved by sizing the depression volume and number of depressions as well as motor speed to suit the requirements of a given application.  
       [0021] The present invention is also directed to a blasting apparatus and method of using the rotary media valve described above. The invention is particularly useful for blasting with softer media such as sodium bicarbonate and the like. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022]FIG. 1 is a schematic illustration of a blasting system incorporating the metering and dispensing valve of the invention.  
     [0023]FIG. 2 is a cross-section of the rotor at the centerline of the depressions on the rotor.  
     [0024]FIG. 3 is a cross-section of the valve housing identifying the compressed gas, media, and vent flow locations.  
     [0025]FIG. 4 is a side view of the rotor.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0026] The invention can best be described as a blast cleaning device where media is metered from the supply vessel (an open atmospheric hopper) into the pressurized gas stream, upstream of the blast nozzle, by means of a novel media valve that employs an airlock type rotor. Round, slot or other shaped depressions (pockets) are arranged on the circumferential surface of the rotor to transport media from the supply vessel to the pressurized gas stream as the rotor spins within a valve housing. Controlling the rotor speed by means of a variable speed or indexing servomotor controls the media flow rate from the supply vessel into the pressurized gas stream. The rotor is sized to fit in an airtight manner within the valve housing. A small vent in the valve housing is positioned to relieve any pressure in the depressions before the depressions are rotated and aligned with the discharge opening of the supply vessel.  
     [0027] Conventional atmospheric hopper blasting systems siphon media from the hopper by aspiration and mix the media with compressed blast gasses as the compressed blast gas is accelerated at the blast nozzle. The present invention is an open atmospheric supply vessel blasting system wherein media from the supply vessel is mixed with the pressurized gas stream prior to acceleration at the nozzle resulting in significantly higher media particle speeds and increased productivity.  
     [0028] This invention is directed to a positive displacement dry powder feed system to transport soft abrasive from a low-pressure environment to a high-pressure environment. A rotary media valve is employed as the dry powder-metering device to direct a blast media from a supply vessel into a pressurized gas stream. The rotary media valve provides precise control of the blast media flow from the supply vessel into the pressurized gas stream to maintain the most efficient media to compressed gas ratio for a given media, nozzle size, and blasting pressure and to insure desired media supply rates into the pressurized gas stream. For any given operating condition, the rotary media valve can introduce the proper amount of media into the compressed gas stream. Too much media (rich mixture) will introduce too many media particles into the compressed gas stream and overload the available compressed gas energy. In a rich mixture, the blast media particles fall out of suspension in the blast hose and cause slugging at the nozzle, resulting in low particle speeds with minimal kinetic energy, thus reducing productivity of each particle and wasting media. Alternatively, too little media (lean mixture) will not introduce the maximum possible amount of media particles into the compressed gas stream, thus wasting compressed gas energy and reducing productivity. Due to the precise metering of media, the rotary media valve of this invention allows the blast system containing same to produce less dust and achieve greater productivity than conventional blast cleaning systems.  
     [0029] The media valve of the present invention is particularly useful for softer abrasives such as salts, including sodium bicarbonate and the like softer abrasives, such as disclosed in the inventor&#39;s previously mentioned patents. Soft abrasive blast media such as sodium bicarbonate, sodium sesquicarbonate, trona, potassium bicarbonate, ammonium bicarbonate, sodium chloride, sodium sulfate and other water soluble salts are meant to be included herein. The very tight clearances (0.0003″ to 0.0009″) between the rotor and the valve housing necessary to create a metal-to-metal dry seal, are designed in particular for dry soft abrasives. Hard abrasives may destroy the dry seals. No side seals are required in the rotary media valve of the present invention.  
     [0030] The ideal media to compressed gas mixture for sodium bicarbonate and air can be described by the following different parameters: 1) between 0.01 and 0.03 pounds of media per cubic foot of air, with a target of about 0.02; 2) between 0.10 and 0.4 pounds of media per pound of air, with a target of about 0.25 (dry air weighs 0.0807 lb/ft 3 ); 3) between 0.0001 and 0.0005 cubic feet of media per cubic foot of air, with a target of 0.00035. These ratios can be broadened by approximately 25% for applications where maximum productivity is not necessarily the goal or for other application specific reasons. These ratios may differ with use of other gasses. The present invention may employ other pressurized gasses such as nitrogen, or any other commercially available inert gas blend.  
     [0031] The basic purpose of any media valve, or media introduction device, is to deliver the most efficient amount of media into the blast stream for the design operating condition to cause blasting at maximum productivity. Most sophisticated blasting apparatus use a pressure vessel containing media under some form of differential pressure control above a variable size orifice to control media flow into the blast stream. This form of control is reasonably accurate but is subject to several significant variables as described below. First, the orifice size and the amount of differential pressure affect flow characteristics through an orifice. The particle size and shape also affect the flow characteristics through an orifice. Additionally, product characteristics such as product formulation (addition of flow aids), product bulk density and the product humidity absorbed from hot wet compressed gas in the pressure vessel affect flow characteristics through an orifice. Flow characteristics through an orifice are also affected by mechanical aids such as vibration. The media head pressure (amount of media in the pressure vessel) also affects flow characteristics through an orifice. Fluidity, or how freely differential compressed gas permeates through the media to facilitate flow, affects flow characteristics through an orifice. The maintenance of differential pressure in the pressure vessel and the angle of repose of the pressure vessel bottom cone also affect flow characteristics through an orifice.  
     [0032] As described above, the rotary media valve is fed from an open atmospheric supply vessel and then discharges the dry soft abrasives radially at the circumference of the rotor into a compressed gas stream, typically 20 to 125 psi, upstream of the blast nozzle. The rotary media valve provides substantially no impeller motion to the media as the media is directed from the supply vessel into the compressed sir stream. An important advantage of the rotary media of the present invention is that it comprises a positive displacement metering device, meaning that assuming the media transfer efficiency is good or at least predictable, the rotary media valve can meter a near exact volume of media per the operating condition (compressed gas usage) to maintain the ideal media to compressed gas ratio. The rotary media valve essentially acts as a mechanical volumetric pump, which pumps media of a known density at a known rpm. The rotary media valve introduces the abrasive into the compressed gas stream, which then flows into a mix chamber allowing acceleration of media particles upstream of a typical blast nozzle. The blast nozzle adds kinetic energy to the media and directs the media to the target surface as is well known in the art. The rate of abrasive metered into a compressed gas stream, in the present invention, is controlled by varying the valve rotor speed (rpm). As a result, the valve rotor speed can be adjusted to hold a particular mixture across an infinite range of operating conditions and media types. Changing media type (density) or operating condition (nozzle size and/or blast pressure) requires a simple calculation to determine proper rpm to maintain the ideal media to compressed gas mixture.  
     [0033] A blast cleaning system containing the rotary media valve of this invention can be described by reference to FIG. 1. Referring to FIG. 1, a blast cleaning system is provided comprising a supply vessel  2  (an open atmospheric hopper) containing a supply of blast media  3 , a source of pressurized gasses  4 , a blast nozzle  18 , and a rotary media valve  1  that comprises a rotor  10  contained within a valve housing  13  in an airtight manner. Cylindrical, slot or other shaped depressions  20  are arranged around the circumference of rotor  10  to transport media  3  from the supply vessel  2  to the pressurized gas stream, upstream of blast nozzle  18 , as the rotor spins. Controlling the rotor speed by means of a variable speed or indexing servomotor  16  (FIG. 4) via respective motor and rotor shafts  44  and  46 , controls the media flow rate from supply vessel  2  into the pressurized gas stream. The compressed gas stream is provided by compressed gas source  4 , valve housing entry port  5 , or alternative entry port  9 , airflow line  6 , and outlet  7 , mixing chamber  15  and conveying hose  17  which directs the mixture of media and compressed gas to blast nozzle  18 . A small vent  14  contained in valve housing  13  is positioned to relieve any pressure in the depressions before the depressions  20  are rotated under the supply vessel  2 .  
     [0034] The compressed gas, such as air, nitrogen, or any other commercially available inert gas blend, preferably at 20 to 125 psi, is directed from source  4  typically through a filter  34 , a dryer  36 , a regulator  38 , and a pressure gauge  40 , to control the gas flow properties. An on/off switch  42  connected to a 12 V DC or AC power supply initiates gas flow only or both compressed gas and media flow. Once on/off switch  42  is on for both compressed gas and media flow, compressed gas is directed through the compressed gas entry port  5  or  9  of housing  13 . The compressed gas is then directed to gas flow line  6  in housing  13  wherein the media  3  is discharged from rotor  10  and outlet discharge  19 . The mixture of media  3  and the compressed gas in the gas flow line  6  then proceeds through outlet  7  of housing  13  and into mix chamber  15  to provide a uniform mix of the media particles within the compressed gas stream.  
     [0035]FIGS. 1, 3, and  4  illustrate the novel rotary media valve of the invention. Referring to FIGS. 1 and 3, it can be seen that valve housing  13  includes an gas passage below rotor  10  which gas passage is comprised of an entry port  5  communicating with a compressed gas source  4 , an gas flow line  6  and an outlet  7 , which communicates with mixing chamber  15 . An alternate gas flow inlet  9  is provided to direct compressed gas up through outlet discharge  19  to assist the gravity feed of media  3  from the pockets  20  of rotor  10  into the compressed gas stream. The upward motion of incoming gasses from entry port  9  toward discharge  19  and pockets  20  is designed to assist in scowering media out of the pockets and improve media transfer efficiency. An additional focusing jet (not shown) can be placed into port  9  to further focus the incoming gas very close to the discharge  19  to increase this action. For directing blast media flow, valve housing  13  contains a conical discharge opening  26  above rotor  10 , which communicates with the supply vessel  2  which can be situated, juxtaposed on discharge opening  26  in any manner which will maintain supply vessel  2  in place. The media  3  passes by gravity from the supply vessel  2  to the conical discharge opening  26  in the valve housing  13  and then through media outlet  8  into the pockets or depressions  20  of rotor  10 .  
     [0036] The novel rotary media valve  1  of the invention comprises a bore  28  within the valve housing  13 , which contains rotor  10 . Bore  28  is sized to create a metal-to-metal dry seal between the inner surface  25  of bore  28  of valve housing  13  and outer circumferential surface  21  of rotor  10 . Tight clearances of 0.0003″ to 0.0009″ between surface  21  of the rotor  10  and the inner surface  25  of bore  28  provide the metal-to-metal seal. The rotary media valve  1  of this invention is particularly useful for transportation of dry soft abrasives. The use of hard abrasives such as sand may destroy the metal-to-metal seals formed between rotor  10  and valve housing  13 . No side seals are required in the rotary media valve of the present invention.  
     [0037] Referring to FIG. 4, it can be seen that a large area of outer surface  21  of rotor  10  exists between depressions  20  and the respective ends  30  and  32  of rotor  10  in addition to surface area between the individual depressions  20 . This large surface area is provided to create a sufficient metal-to-metal seal between rotor  10  and housing  13  to prevent air leakage that would cause consequent media  3  migration onto the seal areas that could cause binding between surface  21  of the rotor  10  and the inner surface  25  of bore  28 .  
     [0038] Rotor  10  of media valve  1  is structured to direct a precise amount of media  3  into the compressed gas stream at outlet discharge  19 . Referring to FIGS. 1, 2 and  4 , rotor  10  contains at least one, preferably a plurality of depressions  20  which are spaced on outer surface  21 . The depressions  20  receive and transport media  3  as the rotor  10  spins and aligns with outlet  8 . Upon further rotation of rotor  10 , the media  3  is deposited at outlet discharge  19  and into the pressurized gas stream flowing through gas flow line  6 . The depressions  20  of any shape or number are organized circumferentially around the rotor  10  and are preferably separated equidistantly on surface  21  by land width  22 . A rotor vent  14  is located within the housing  13 , and serves to relieve any air pressure in the depressions  20  via vent line  24  before the depressions  20  are again rotated under the conical discharge opening  26  to receive media  3  from the supply vessel  2 . The rotor depressions  20  may be of a round or elongated slot depression design. The exact shape is not critical so long as the depressions  20  have sufficient surface area and depth to receive and transport media  3 .  
     [0039] The flow rate of media  3  from supply vessel  2  to gas flow line  6  is affected by a number of variables that must be considered when fashioning the rotor. The size of the outlet  8  which permits media  3  to flow from the supply vessel  2  to the rotor depressions  20 , as well as the size, volume, depth and number of the depressions  20 , will affect the flow rate of media from the supply vessel  2  to the gas flow line  6 . In a preferred embodiment, depression  20  depth is between 0.03″ and 1.0″. Media transfer efficiency is dependent upon how completely the depressions  20  in rotor  10  fill with media  3  and how completely the depressions  20  empty the media  3  into outlet discharge  19  and the compressed gas stream in gas flow line  6 . The speed at which the rotor  10  spins, as well as the depression depth and other significant depression geometry variations can be selected to achieve the desired media transfer from supply  2  to the pressurized gas stream. Optimizing the media flow rate can be achieved by sizing the outlet  8  of the conical discharge opening  26 , which directs media  3  from the supply vessel  2 , relative to the size of depressions  20  of the rotor  10 . If the diameter of outlet  8  is significantly less than the diameter of the individual depressions  20  of rotor  10 , this may create a void in depressions  20  preventing the efficient gravity fill of the depressions  20  and resulting in reduced media transfer efficiency. Additionally, an outlet  8  with a diameter that is significantly greater than the diameter of depression  20  of rotor  10  will require a depression of increased depth for equivalent depression volume transfer and subsequent increase in overall diameter of the rotor  10  to provide an equivalent media flow rate. Optimally, outlet  8  will have a diameter that is equal to the diameter of the depression  20  of the rotor  10 . In a preferred embodiment, outlet  8  is aligned axially with depression  20  and is of a round or slot shape having a circumferential width allowing exposure to at least one depression  20  and at most five depressions simultaneously.  
     [0040] The distance between the round or slot depressions  20  of the rotor  10  is referred to as the land width  22 , FIG. 4. The size of the land width  22  may affect the formation of the metal-to-metal dry seals between rotor  10  and housing  13 , and, as well, affect the filing and discharging of media  3  into and from depressions  20 . A land width  22  approaching the width of outlet  8  (greater than 75%) increases the center seal area of rotor  10 , but will necessitate an increase in the diameter of rotor  10  to accommodate a sufficient number of depressions on the circumferential rotor surface to efficiently transport media  3  and may limit more than one depression  20  from filling or discharging simultaneously. Large land widths  22  may minimize airborne media  3  trapped in an emptied depression  20 , but may produce “pulsing” as slugs (emptying of individual depression loads) of media into nozzle  18 . Thus, large land widths  22  separating depressions  20  cause depressions  20  to discharge media  3  individually as opposed to smaller land widths  22 , which allow for continuous discharge of media  3  from depressions  20  into the pressurized gas stream. However, land widths  22  which are significantly smaller than the diameter of outlet  8  (less than 50%) minimize the center seal area of rotor  10  and may promote leakage, and pressurizing of the depressions  20  with airborne particulate causing “blow by” through vent  14  and reduce media transfer efficiency. In a preferred embodiment, land widths  22  are sized in a range from a minimum of 0.03″ to a maximum of depression  20  circumferential width, thus allowing a minimum of one depression  20  exposure to the outlet  8  or outlet discharge  19 .  
     [0041] The operation of the blast cleaning system using the rotary media valve  1  of this invention can be described by referring to FIG. 1. The blast cleaning system has a supply vessel  2  at least partially filled with blast media  3 . Supply vessel  2  is not pressurized with compressed gas and can be open to the atmosphere. To operate the system, the switch  42  should be turned to allow the flow of compressed gas from the source of pressurized gas  4  through a filter  34 , a dryer  36 , a regulator  38 , and a pressure gauge  40 , which act to control the gas flow properties. Once the pressurized gas is flowing properly, the switch  42  can be turned to allow both gas and media flow, whereby rotor  10  is rotated within housing  13 .  
     [0042] As the rotor  10  rotates in the direction of arrow  50 , the blast media  3  is fed from the open atmospheric supply vessel  2  into conical discharge opening  26  of the valve housing  13 , through outlet  8  into the depressions  20  of the rotor  10  of the rotary media valve  1 . The speed of rotation of rotor  10  and the width and depth of the depressions  20  regulate the flow of media  3  from supply  2  through discharge outlet  19  into the pressurized gas stream in gas flow line  6 . The empty depressions  20  rotate around to vent  14 , which depressurizes the depressions  20  prior to alignment with the outlet  8 , whereby the depressions  20  are subsequently gravity filled with media  3  from the supply vessel  2 .  
     [0043] The mixture of media  3  and the compressed gas flow line  6  then proceeds through outlet  7 , of housing  13  into mix chamber  15 , which has a larger diameter than gas flow line  6  to provide a uniform mix of the media particles within the compressed gas stream. The media  3  and the pressurized gas proceed through the mix chamber  15 , into the blast hose  17  and to the blast nozzle  18 , where the blast media is accelerated and directed to a surface to be cleaned. Blast nozzle  18  is preferably of a venturi type wherein the mix of media particles and pressurized gas are directed through a venturi orifice before being directed from the outlet of the nozzle. The size (throat diameter) of the nozzle  18  may vary. In a preferred embodiment, compatible nozzle  18  sizes range from 0.10″ micro blaster size to ⅝″ typical commercial size, and most preferably between {fraction (1/32)}″ and ⅛″. The blast hose  17  diameters may vary. In a preferred embodiment, the size of the blast hose  17  is selected to permit the high and low limits for gas speed in the blast hose. The sizes of the nozzle  18  and the blast hose  17  are chosen to maintain media suspension and minimize slugging at the nozzle  18 .  
     [0044] In another embodiment, the rotary media valve of the present invention can be easily and inexpensively automated to control the media to gas ratio automatically through a simple computer employing an algorithm. The algorithm relates media bulk density, media to gas mixture and rotor speed to control a variable speed motor to maintain ideal rotor speed.  
     [0045] In another embodiment, the rotary media valve of the present invention could also be automated to dose media in an automatic, programmed fashion by using a servo or stepper motor to control the motion of the rotor in a non-continuous rotational manner.