Note: Descriptions are shown in the official language in which they were submitted.
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SCREEN CLOTH FOR VIBRATING OR STATIONARY SCREENS
The present invention relates generally to stationary or vibrating screening
devices. In particular, an impact screen cloth useful in stationary and/or
vibrating
screens for screening oversize objects in a material is provided comprising a
plurality of longitudinal wear bars positioned on the impact screen cloth in
the
direction of travel of the material to be screened.
BACKGROUND OF THE INVENTION
Vibrating and/or stationary screens are used in coal dressing, metallurgy,
mine, power station, and the like. They are primarily used for the
classification of
bulk materials such as coal, minerals, coke, etc. Vibrating and/or stationary
screens are also used in the oil sand industry, in particular, in oil sand
slurry
preparation plants.
Oil sand, such as is mined in the Fort McMurray region of Alberta,
generally comprises water-wet sand grains held together by a matrix of viscous
bitumen. It lends itself to liberation of the sand grains from the bitumen by
mixing
or slurrying the oil sand in water, allowing the bitumen to move to the
aqueous
phase.
As-mined or pre-crushed oil sand is generally mixed with warm or hot
water to yield an oil sand slurry.
The slurry is then conditioned in a
hydrotransport pipeline and subsequently introduced into a large, open-topped,
conical-bottomed, cylindrical vessel commonly termed a primary separation
vessel (PSV) where the more buoyant aerated bitumen rises to the surface and
forms a bitumen froth layer.
It may be desirable to remove the larger aggregates present in oil sand
slurry prior to pipelining in order to avoid blockage or damage of downstream
equipment, e.g., pump component wear. Thus, vibrating screens may be used at
various points during slurry preparation to reject larger lumps of oil sand,
rocks
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and other aggregates, which are large enough to block or damage downstream
equipment, prior to pipeline conditioning. Screens may also be used to further
screen oil sand tailings slurry prior to treating/disposing same.
However, oil sand slurry is extremely heavy and abrasive due to the large
amount of sand, gravel and crushed rock contained therein. Further, primary
vibrating screens are generally vibrating with an acceleration of
approximately 4-
5 g, so that all oil sand slurried material passes over and through the screen
cloths of the vibrating screen. This results in the rapid spalling and
eventual
wearing through of the screen cloths of the vibrating screen ("hole-
throughs"),
which can lead to production interruption and an unplanned maintenance event.
Thus, it is desirable to have an improved screen cloth that can withstand
the abrasiveness of oil sand slurry.
SUMMARY OF THE INVENTION
It was discovered that screen cloths of vibrating and/or stationary
screening devices used to screen frozen lumps, rocks and the like from oil
sand
slurry were wearing through much quicker than desired due to the abrasive
nature of the slurry. In particular, it was discovered that the first few
rows,
referred to herein as the impact rows, of screen cloths of vibrating screening
devices were suffering from localized wear phenomena resulting from the slurry
flow distribution.
In one aspect of the present invention, an impact screen cloth for use in a
screening device for screening out oversize objects in a material flowing in a
direction is provided, comprising:
a metal plate having a perimeter and comprising a plurality of openings
therethrough and forming a grid having longitudinal ligaments substantially
parallel to the direction of the material flow and transverse ligaments
substantially
perpendicular to the direction of the material flow; and
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a number of longitudinal wear bars positioned on an equal number of
longitudinal ligaments to cover a substantial portion of each longitudinal
ligament
without covering the openings.
In one embodiment, the height of the longitudinal wear bar is proportional
to size of the openings to elevate the oversize objects that are larger than
the
size of the openings and thus prevent the oversize objects such as lumps and
rocks from contacting the transverse ligaments. In one embodiment, impact
screen cloths are used in vibrating screening devices and the height of the
longitudinal wear bars are generally about 60% of the width of the openings or
greater. In one embodiment, impact screen cloths are used in stationary or
fixed
screening devices and the height of the longitudinal bars are generally about
40% of the width of the openings or greater.
In one embodiment, the impact screen cloth further comprises a number
of transverse wear bars positioned between the number of longitudinal wear
bars. In another embodiment, the longitudinal wear bars are fabricated from
mild
steel with high wear material and are welded to the longitudinal ligaments of
the
perforated metal plate.
In one embodiment, wear materials for the longitudinal wear bars can be
ceramics, chromium carbide, tungsten carbide or sintered tungsten carbide.
These materials can be thermally (e.g., vulcanizing, welding, brazing, etc.),
chemically (e.g., epoxy) or mechanically (e.g. bolted, dovetailed, etc.)
attached to
the wear bars.
In yet another embodiment, the perforated metal plate is made from
structural steel (e.g., high tensile steel, stainless steel, carbon steel,
etc.) and is
overlayed with multi-pass layers of chromium carbide, tungsten carbide (PTA or
Technoginia products) or cast wear products (e.g., ceramic, Kencast or
sintered
tungsten carbide tile) to increase its thickness. Thus, the increase in
structural
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competence by the addition of longitudinal wear bars minimizes the spalling of
the overlay on the metal plate.
In another aspect of the present invention, a screen for use in a vibrating
or stationary screening device, the screen having a feed end and a discharge
end, for screening oversize objects in a material is provided, comprising:
a number of screening rows positioned end to end between the feed end
and the discharge end, each row having at least one screen cloth and each
screen cloth comprising a metal plate having a perimeter and having a
plurality of
openings therethrough to form a grid having longitudinal ligaments
substantially
parallel to the direction of the material flow and transverse ligaments
substantially
perpendicular to the direction of the material flow;
whereby the at least one screen cloth of at least one screening row further
comprises a number of longitudinal wear bars positioned on an equal number of
longitudinal ligaments to cover a substantial portion of each longitudinal
ligament
without substantially interfering with the openings.
In one embodiment, the at least one screening row is the screening row
closest to the feed end. Thus, the primary impact zone is reinforced by
providing
additional sacrificial material in the form of longitudinal wear bars.
In one embodiment, the screen for use in the vibrating or stationary
screening device comprises at least two screening rows, each screening row
having at least one screen cloth, wherein the at least one screen cloth of the
screening row closest to the feed end has a greater number of longitudinal
wear
bars than the at least one screen cloth of the next screening row.
Other features will become apparent from the following detailed
description. It should be understood, however, that the detailed description
and
the specific embodiments, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various changes and
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modifications within the spirit and scope of the invention will become
apparent to
those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like reference numerals indicate similar
parts throughout the several views, several aspects of the present invention
are
illustrated by way of example, and not by way of limitation, in detail in the
following figures. It is understood that the drawings provided herein are for
illustration purposes only and are not necessarily drawn to scale.
Fig. la is a top view of a screen cloth of the prior art.
Fig. lb is a cross-section of a screen cloth of the prior art.
Fig. lc is an enlargement of a portion of the cross-sectional view of Fig.
lb.
Fig. Id is a perspective view of another screen cloth of the prior art.
Fig. le is the longitudinal section of the screen cloth of the prior art shown
in Fig. Id.
Fig. 2a is a top view of an embodiment of an impact screen cloth of the
present invention.
Fig. 2b is a cross-section of the impact screen cloth embodiment shown in
Fig. 2a.
Fig. 2c is an enlargement of a portion of the cross-sectional view of Fig.
2b.
Fig. 2d is a perspective view of the impact screen cloth shown in Fig. 2a.
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Fig. 2e is the longitudinal section of the impact screen cloth shown in Fig.
2d.
Fig. 3 is a top view of a screen useful in a vibrating screening device
comprising a plurality of impact screen cloths of the present invention.
Fig. 4 is a perspective view of the impact screen cloth shown in Fig. 2a in
operation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The detailed description set forth below in connection with the appended
drawings is intended as a description of various embodiments of the present
invention and is not intended to represent the only embodiments contemplated
by the inventor. The detailed description includes specific details for the
purpose
of providing a comprehensive understanding of the present invention. However,
it will be apparent to those skilled in the art that the present invention may
be
practiced without these specific details.
Turning first to Figs. 1 a, lb and 1 c (Prior Art), screen cloth 10 (partial)
is
made from a perforated plate 14 having a side edge 18 (the opposite side edge
is not shown), a bottom edge 26 and a top edge 20. Perforated plate 14 can be
made of a number of different materials, preferably, structural steel.
Openings
12 in perforated plate 14 are generally of a consistent size, wherein the size
is
dependent on the size of the oversize that one desires to screen out. In this
embodiment, openings are round. Perforated plate 14 is constructed from a
large piece of steel, which forms the perimeter of the screen cloth, and a
regular
pattern of openings or holes are cut and/or punched (flame, plasma)
therethrough to form the mesh of the screen plate. It can further be seen from
Fig. 1 a that the direction of flow of material to be screened is from top
edge 20 to
bottom edge 26.
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. .
Fig. lb is a cross-section of screen cloth 10 and Fig. 1 c is an enlargement
of a portion of screen cloth 10 shown in Fig. lb. It can be seen from Fig. lc
that
screen cloth 10 has an overlay 16 such as tungsten carbide. However, it was
õ
found that these prior art screens wore very quickly, despite overlay 16.
Fig. Id is a perspective view of another prior art screen cloth where
additional supporting structures have been added to increase the strength of
the
screen cloth. In this embodiment screen cloth 110 is fabricated from a large
steel
plate 114 having a perimeter comprising sides 118a and 118b, top edge 120 and
bottom edge 126. The direction of flow is from the top edge 120 to the bottom
edge 126. A plurality of substantially square openings 112 are cut or punched
(flame, plasma) on the plate 114 to form a mesh comprising longitudinal
ligaments 144 and transverse ligaments 146.
It is understood that multiple of these screen cloths will be installed in a
vibrating or stationary screening device's main cross members to form the
screening deck (also referred to herein simply as the screen) of the vibrating
or .
stationary screening device. Screen cloths can be attached to the main cross
members by means of bolts inserted through bolt holes 150.
In the embodiment shown in Fig. id, multiple reinforcement gusset plates
140 are strategically welded under and along the entire length of the
longitudinal
ligaments 144 to increase the rigidity of screen cloth 110. This can be seen
more
clearly in Fig. le, which is the longitudinal section of the screen cloth in
id.
Bolting plate 148 provides further support when bolting the screen cloth 110
to
the screening device. All top surfaces of screen cloth 110 can be protected
from
additional wear by either overlay products such as chromium carbide, tungsten
carbide (PTA or Technoginia products) or cast wear products (ceramic, Kencast
or sintered tungsten carbide tile). These products can be welded, bolted
and/or
brazed onto the screen cloth.
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Figs. 2a to 2e illustrate one embodiment of an impact screen cloth of the
present invention. Impact screen cloth 210 comprises perforated plate 214
having a first side edge 218a, a second side edge 218b, a top edge 220
(material
or feed end) and a bottom edge 226 (oversize exit end). Openings 212 in
perforated plate 214 are rectangular in shape and the size of the openings is
dependent on the size of the oversize that one desires to screen out.
Generally,
the openings are of a consistent size as well. As was the case with perforated
plate 114 of Fig. 1d, perforated plate 214 may also be constructed from a
large
piece of steel, which forms the perimeter of the screen cloth 210, and the
openings 212 are cut and/or punched (flame, plasma) therethrough to form the
mesh of screen cloth 210 which also comprises longitudinal ligaments 244 and
transverse ligaments 246.
In the embodiment shown in Fig. 2a, impact screen cloth 210 further
comprises longitudinal wear bars 222, wherein each longitudinal wear bar 222
extends essentially from the top edge 220 to the bottom edge 226 of perforated
plate 214 and covers a substantial portion of the longitudinal ligaments 244.
In
Fig. 2a, the longitudinal wear bars 222 are formed using four separate bar
segments to minimize distortion of the perforated plate 214, however, it is
understood that the longitudinal wear bars can also be a single structure.
Further, longitudinal wear bars 222 are transversely spaced so that they do
not
substantially interfere with the size of the openings 212 and generally a
longitudinal wear bar 222 is positioned directly adjacent both the first side
edge
218a and the second side edge 218b.
Fig. 2b is a cross-section of screen cloth 210 and Fig. 2c is an
enlargement of a portion of impact screen cloth 210 shown in Fig. 2b. It can
be
seen from Figs. 2b and 2c that longitudinal wear bars 222 are raised and the
height of each longitudinal wear bar 222 is proportional to size of the
openings
212 (e.g., in this embodiment, the height of the longitudinal wear bars is
about
315th the width of the openings 212). In addition to the longitudinal wear
bars
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222, it can be seen in Fig. 2c that perforated plate 214 may be coated with
multi-
pass layers of tungsten carbide 216 to increase its thickness as compared to
prior art screen cloth shown in Fig. 1c. Thus, the increase in structural
competence by the addition of longitudinal wear bars minimizes the spalling of
the thicker tungsten carbide layer. Further wear material 224 can be provided
for
the longitudinal wear bars as well, for example, ceramic carbide (e.g.,
tungsten
carbide) or brazed-on high wear material (e.g., chrome white iron material).
As was the case in the embodiment shown in Fig. 1d, and which can be
seen more clearly in longitudinal section 2e and perspective view 2d, multiple
reinforcement gusset plates 240 may be strategically welded under and along
the
entire length of the longitudinal ligaments 244 to increase the rigidity of
screen
cloth 210. In addition, transverse stiffeners 246 also provide additional
support.
Bolting plates 248 aid in the secure fastening of the screen cloth 210 to the
screening device by means of bolts inserted through bolt holes 250.
It is understood that longitudinal wear bars can be fixed to the perforated
plate 214 by any means known in the art, for example, welding the bars
thereon.
In one embodiment, impact screen cloth 210 of the present invention comprises
a frame having first side edge 218a, second side edge 218b, top edge 220 and
bottom edge 226) which supports a plurality of longitudinal ligaments 244 and
a
plurality of transverse ligaments 246 which ligaments intersect to form a mesh
or
grid. The openings 222 formed between the longitudinal ligaments 244 and
transverse ligaments are generally uniform in size. The longitudinal ligaments
extend essentially from the feed end to the oversize exit end and the
transverse
ligaments extend essentially from the first side edge to the second side edge.
The longitudinal ligaments are raised relative to the transverse ligaments as
a
result of the attached longitudinal wear bars 222.
Figure 2e show bolting plates 248 which provides further support when
bolting the screen cloth 210 to the screening device. Optionally, the screen
cloth
210 may further comprise transverse stiffeners 249.
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Fig. 3 is a top view of a portion of a screen 402 useful in a vibrating
screening device. As can be seen in Fig. 3, screen 402 comprises a plurality
of
individual screen cloths of three different kinds, namely, 410a, 410b and
410c.
Screen cloths 410a form the first row of screen cloths (Row 1), screen cloths
410b form the second row of screen cloths (Row 2), and screen cloths 410c form
the third row of screen cloths (Row 3). The feed (e.g., oil sand slurry to be
screened) is initially deposited on Row 1 and then travels the entire length
of the
screen 402 in the direction of flow as shown in Fig. 3. Thus, the screen
cloths
which will receive the greatest wear would be screen cloths 410a. Thus, screen
cloths 410a are reinforced with longitudinal wear bars, 422a, positioned on
each
longitudinal ligament 444a. Screen cloths 410b will have less wear than screen
cloths 410a and even though screen cloths 410b are reinforced with
longitudinal
wear bars 422b, only every other longitudinal ligament 444b is reinforced.
Screen cloths 410c (and subsequent screen cloths) do not have any longitudinal
wear bars.
The use of longitudinal wear bars on the first two rows of screen cloths
increased the life of the screens from about 500 hours to about 2000 hours.
Even more importantly, however, is that such an arrangement of more
reinforcement in Row 1, less in Row 2, and none in subsequent rows, results in
uniform wear across all screen cloths of the screen 402. This optimizes the
run
time of the vibrating screen device so that the operator does not have to shut
the
device down multiple times to change screen cloths.
Fig. 4 is a perspective view of the impact screen cloth 210 shown in Figs.
2a-2e in operation. As it can be seen in Fig. 4, longitudinal wear bars 222
prevent rock 250 from impacting on the screen cloth perforated plate 214.
Further, in this embodiment, the longitudinal wear bars are spaced fairly
close
together, which allows rock 250 to essentially ride on top of the longitudinal
wear
bars 222 in the direction of flow.
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