U.S. patent application number 10/155032 was filed with the patent office on 2003-12-04 for rotary media valve.
Invention is credited to Shank, James.
Application Number | 20030224704 10/155032 |
Document ID | / |
Family ID | 29582135 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030224704 |
Kind Code |
A1 |
Shank, James |
December 4, 2003 |
Rotary media valve
Abstract
A blast cleaning apparatus containing a rotary media valve with
a rotor, which contains depressions on the surface thereof,
delivers blast media from a supply vessel into a pressurized gas
stream upstream of a nozzle for blast cleaning. As the rotor
revolves, the depressions align with the supply vessel where the
particulate blast media cascades into the depressions. The media is
transported from the supply vessel to the pressurized gas stream as
the rotor spins.
Inventors: |
Shank, James; (Princeton,
NJ) |
Correspondence
Address: |
Stuart D. Frenkel
Law Office of Stuart D. Frenkel, P.C.
3975 University Drive, Suite 330
Fairfax
VA
22030
US
|
Family ID: |
29582135 |
Appl. No.: |
10/155032 |
Filed: |
May 28, 2002 |
Current U.S.
Class: |
451/38 |
Current CPC
Class: |
B24C 1/086 20130101;
B24C 7/0092 20130101 |
Class at
Publication: |
451/38 |
International
Class: |
B24C 001/00 |
Claims
What is claimed:
1. An apparatus for blast cleaning, comprising: a vessel means for
containing a quantity of particulate abrasive blasting medium; a
source of pressurized gas; a gas conveying line being in fluid
communication with said source of pressurized gas; a rotary media
valve for feeding said blasting medium from said vessel to a
location of said gas conveying line to form a mixture of blasting
medium and pressurized gas; a blast nozzle at the end of said
conveying line and downstream of said location.
2. The apparatus for blast cleaning of claim 1 wherein said vessel
comprises an open vessel for containing abrasive blasting
media.
3. The apparatus for blast cleaning of claim 1 wherein said
pressurized gas comprises air, nitrogen, inert gas, or blends
thereof.
4. The apparatus for blast cleaning of claim 1 including a mix
chamber positioned between said location and said blast nozzle to
form a more uniform mix of said blasting media in said pressurized
gas stream.
5. The apparatus for blast cleaning of claim 1 wherein said rotary
media valve comprises a rotor with at least one depression on the
surface thereof capable of delivering media from said vessel into
said gas conveying line.
6. The apparatus for blast cleaning of claim 4 wherein said rotor
has a plurality of said depressions arranged in a spaced
configuration around substantially the entire circumference of the
rotor.
7. The apparatus for blast cleaning of claim 5 wherein said
depression is a slot.
8. The apparatus for blast cleaning of claim 5 wherein said
depression is round.
9. The apparatus for blast cleaning of claim 5 wherein said
depression depth ranges from 0.05 to 1.0 inch.
10. The apparatus for blast cleaning of claim 5 wherein said rotor
is contained within a bore of a valve housing.
11. The apparatus for blast cleaning of claim 10 wherein said rotor
and said bore of said valve housing are sized to create a
metal-to-metal dry seal.
12. The apparatus for blast cleaning of claim 11 wherein said rotor
and said bore have a clearance space there between of 0.0001" to
0.002".
13. The apparatus for blast cleaning of claim 5 wherein rotation
speed of said rotor is controlled by means of a variable speed or
indexing servomotor communicating with said rotor.
14. The apparatus for blasting of claim 1 wherein said blast nozzle
has an orifice diameter of up to 5/8".
15. The apparatus for blasting of claim 1 wherein said blast nozzle
has an orifice diameter of at least 0.01".
16. The apparatus for blasting of claim 5 wherein said rotor is
contained within a housing, said housing containing a vent
communicating with said rotor and positioned to relieve any
pressure in said at least one depression before said at least one
depression on said rotor is rotated to said supply vessel.
17. The apparatus of claim 5 wherein said rotor is placed below
said vessel means and above said gas conveying line.
18. The apparatus for blasting of claim 1 wherein said rotary media
valve feeds said blast medium to said gas conveying line without
impeller action.
19. The apparatus for blast cleaning of claim 1 wherein said rotary
media valve acts as a mechanical volumetric pump.
20. A method for blast cleaning, comprising: providing a quantity
of blasting medium within a vessel; feeding said blasting medium
from said vessel, through a rotary media valve into pressurized gas
to form a blast steam; discharging said blast stream through a
nozzle placed downstream of where said blasting medium is fed into
said source of pressurized gas.
20. The method for blast cleaning of claim 20 wherein the blasting
medium comprises sodium bicarbonate, potassium bicarbonate,
ammonium bicarbonate, sodium sesquicarbonate, sodium chloride,
sodium sulfate or mixture thereof.
21. The method for blast cleaning of claim 20 wherein the
pressurized gas is between about 20 to 125 psig.
22. The method for blast cleaning of claim 21 wherein said blasting
medium is fed from said blast nozzle at a flow rate of from about
0.1 to 10 pounds per minute.
23. The method for blast cleaning of claim 23 wherein said blasting
medium to pressurized gas weight ratio is about 0.1 to 0.4.
24. The method for blast cleaning of claim 20 wherein said rotary
media valve comprises a rotor with at least one depression on the
surface thereof capable of delivering media from said vessel into
said source of pressurized gas, rotating said rotor such that said
media is transported from said supply vessel to said at least one
depression and subsequently from said at least one depression into
said pressurized gas.
25. The method for blast cleaning of claim 24 wherein said rotor
delivers said blasting medium via a plurality of said depressions
arranged in a spaced configuration around substantially the entire
circumference of the rotor.
26. The method for blast cleaning of claim 25 comprising
controlling the RPM of said rotor by means of a variable speed
motor or servo motor.
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'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.sup.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 5/8" typical commercial size, and most
preferably between {fraction (1/32)}" and 1/8". 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.
* * * * *