Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: FLOATING COVER FOR LARGE LIQUID RESERVOIR
FIELD OF THE INVENTION
This invention pertains to flexible floating covers for covering large
liquid reservoirs, and particularly it relates to a floating cover which is
anchored to the perimeter walls of a reservoir and which rises and falls with
the liquid level inside the reservoir.
BACKGROUND OF THE INVENTION
Floating covers are mounted over settling ponds and clarifiers to
contain and collect fermentation gases of mill effluent for example. Floating
covers are also mounted over water reservoirs to prevent contamination of
potable water from acid rain, pollen, leaves, dust, insects, bird droppings,
the
effect of sunlight and from the activities of other animals.
The installation of a floating cover over a large liquid reservoir
represents certain difficulties in that the cover is exposed to the elements
and
to the movement of the liquid under the cover. For example, a slight
accumulation of rain over a cover creates puddles and mounds which catch
the wind and promote waves along the cover and into the liquid under the
cover. The movement of liquid under the cover causes tangential stresses and
constant movement in the cover. These stresses could cause fatigue, localized
elongation and rupture of the cover. The formation of mounds and puddles
on a floating cover is amplified where the cover is installed over a reservoir
that could have gases coming out of the liquid inside the reservoir.
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Examples of floating covers of the prior art are described in the
following documents:
US Patent 3,313,443, issued on April 11, 1967 to H. S. Dial et al.;
US Patent 3,683,428, issued on August 15, 1972 to L. Morris;
US Patent 3,980,199, issued on September 14, 1976 to W. B. Kays;
US Patent 4,139,117, issued on February 13, 1979 to H. S. Dial;
US Patent 4,181,986, issued on January 8, 1980 to H. E. Aine;
US Patent 4,192,025, issued on March 11, 1980 to C. A. Hinsperger;
US Patent 4,438,863, issued on March 27, 1984 to J. V. Wilson et al.;
US Patent 4,503,988, issued on March 12, 1985 to D. H. Gerber;
US Patent 4,603,790, issued on August 5, 1986 to D. H. Gerber;
US Patent 4,672,691, issued on June 16, 1987 to De Garie et al.;
US Patent 5,505,848, issued on April 9, 1996 to Landine et al.;
US Patent 5,587,080, issued on December 24, 1996 to Landine et al.
A common method in the prior art for supporting a floating cover over
a reservoir consists of bonding float blocks to the underside of the
impermeable membrane, or positioning float blocks inside pockets formed in
the membrane. This method has had limited success in the past because the
float blocks and pockets cause obstructions which catch the liquid movement
under the cover and apply tearing stresses along the surface of the cover.
Where the cover is installed over a clarifier or a pond and scum tends to form
at the surface of the pond, the wind-induced movement in the liquid of the
pond and associated scum movement under the cover generate forces that can
rip a float block or a pocket away, and tear the strongest impermeable
membrane.
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Therefore, it is believed that there is a need in the industry for a better
flexible floating cover which is adapted to minimize the formation of puddles
and mounts thereon and which is less susceptible of generating destructive
stresses from wind-induced liquid movement under the cover.
SUMMARY OF THE INVENTION
In the present invention, there is provided a floating cover for liquid
reservoir wherein the structure of the cover is particularly flexible to
follow
the movement of the liquid inside the reservoir without generating excessive
tangential stress in the water-impermeable membrane of the cover. Specific
segments of the cover become quickly submersed during a rainstorm to keep
the cover membrane taut and to limit the formation of randomly spaced
puddles that can deform the cover and create stresses in the cover membrane.
In a first aspect of the present invention there is provided a liquid
reservoir having a floating cover mounted thereon. The floating cover has a
floating grid anchored to the perimeter walls of the reservoir. The floating
grid floats over the liquid inside the reservoir. The floating grid comprises
a keel member and an array of buoyant beams affixed to the keel member and
extending away from the keel member. A water-impermeable membrane is
affixed to the perimeter wall and is loosely laid over the floating grid.
There
is also provided an array of weight lines anchored to the perimeter walls and
loosely laid over the impermeable membrane. Each of the weight lines is laid
at about halfway between an adjacent pair of the buoyant beams.
The primary advantage of this structure is that the floating grid, the
impermeable membrane and the array ofweight lines constitute three separate
layers that are loosely laid over each other. These three separate layers are
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therefore free to slide upon each other and flex to follow the movement of the
liquid inside the reservoir without generating any destructive tangential
stress
in the impermeable membrane.
In another feature of the present invention, the buoyant beams and the
weight lines are flexible longitudinally whereby a relative movement of the
impermeable membrane between the floating grid and the array of weight
lines does not generate any point of concentrated shear stresses in the
impermeable membrane.
In still another feature of the present invention, the impermeable
membrane has segments that are quickly submersed under the liquid level
during a rainstorm. The submersed segments extend along the keel member
and along the weight lines. These submersed segments are advantageous for
keeping the impermeable membrane in a taut condition during a rainstorm,
and for reducing the formation of puddles and mounds thereon.
In accordance with yet another feature of the present invention, the
impermeable membrane has a series of drain holes therein. The drain holes
are located in a central one third portion of the width of the impermeable
membrane. Due to the location of these drain holes, the submersed segments
remain present on the impermeable membrane for extended period of time
following a rainstorm. Furthermore, the drain holes in the cover of the
present invention represent a distinct advantage over the traditional use of
hazardous electrical sump pumps.
Other advantages and novel features of the present invention will
become apparent from the following detailed description.
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BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention is illustrated in the
accompanying drawings, in which like numerals denote like parts throughout
the several views, and in which:
FIG. 1 is a partial perspective view of a floating cover according to a
preferred embodiment of the present invention installed over a liquid
reservoir;
FIG. 2 is a partial cross-section view of the floating cover;
FIG. 3 is a partial top view of the floating grid supporting the impermeable
membrane;
FIG. 4 is a top view of a rectangular reservoir having the floating cover
according to the preferred embodiment mounted thereon;
FIG. 5 illustrates a cross-section of one of the drain holes through the
impermeable membrane of the floating cover.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
While this invention is susceptible of embodiments in many different
forms, there is shown in the drawings and will be described in details herein
one specific embodiment of the present invention, with the understanding that
the present disclosure is to be considered as an example of the principles of
the invention and is not intended to limit the invention to the embodiment
illustrated and described.
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A preferred embodiment ofthe flexible floating cover according to the
present invention is partly illustrated in FIGS. 1 and 2. The floating grid
supporting the impermeable membrane of the floating cover is partly
illustrated in FIG. 3. These partial drawings are provided herein for clarity.
These drawings are believed to be sufficient for illustrating the concept and
principles of the present invention. Numerous other structural details or
variations may be included in a complete cover installation. However, these
additional structural details and variations are known to those skilled in the
art. The floating cover according to the present invention is an improvement
to the floating cover described in US Patent 4,672,691 of which the first
named inventor is also the inventor of the present invention.
The floating cover according to the preferred embodiment comprises
a flexible water-impermeable membrane 20 which is anchored to the
perimeter wall 22 of a reservoir. A flat bar 24 and a series of anchor bolts
26
are used for clamping the impermeable membrane 20 to the top edge of the
wall 22. The impermeable membrane 20 has sufficient surface to cover the
reservoir at its lowest operating level. The flexible impermeable membrane
is preferably made of a stretch-resistant nylon-based pliable sheet material.
The impermeable membrane 20 is loosely supported at the surface of
20 the reservoir by a floating grid 30. The floating grid is also anchored to
the
perimeter wall 22 by means of tie cables 32 and connector plates 34 mounted
to the anchor bolts 26. The floating grid 30 is made of a series of buoyant
beams 36 attached to and extending from a keel member 38 set along the
centre of the floating grid 30. Each buoyant beam 36 has an inside end
attached to the keel member 38. The outside end of each buoyant beam 36
is retained to the perimeter wall by a tie cable 32.
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Each buoyant beam 36 has a bag-like shape and is filled with chunks
of foam or similar buoyant material. The envelope of each buoyant beam 36
is preferably made ofa different material than the material ofthe impermeable
membrane 20. The envelope of each buoyant beam 36 is preferably made of
a stretch-resistant polyethylene pliable sheet material. The difference in
material between the impermeable membrane 20 and each buoyant beam 36
ensures that the two materials do not fuse together in use, when exposed to
excessive heat from the sun's rays.
Because of the polyethylene sheet material, each buoyant beam 36 is
somewhat flexible lengthwise and widthwise to follow to a certain degree the
wave movements of the liquid under the cover. Moreover, it has been found
that the coefficient of friction between the buoyant beams 36 and the
impermeable membrane 20 in the presence of a water is very low, whereby
a relative movement of the impermeable membrane 20 over one of the
buoyant beams 36 does not apply any significant tangential stress in the
impermeable membrane 20. It is believed that the flexibility of the buoyant
beams 36 is also a contributing factor for providing a low stress contact
between the impermeable membrane 20 and the buoyant beams 36.
The central keel member 38 is made of several plies of the same
material as the buoyant beams 36, that is a stretch-resistant polyethylene
pliable sheet material. The keel member 38 is thereby relatively flexible and
has a surface which offers a low coefficient of friction against the surface
of
the impermeable membrane 20. The keel member 38 comprises a trough 40
and opposite horizontal flaps 42, 44 extending from the trough. The buoyant
beams 36 are attached to the flaps 42, 44 and extend substantially at right
angle with the keel member 38. A series of perforations 45 through the
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bottom segment of the trough 40, evacuate the liquid that may be trapped
inside the trough between the bottom segment of the trough and the central
portion of the impermeable membrane 20.
The height 'A' of the trough 40 constitutes spare surface for
accommodating the widthwise extension and contraction of the floating grid
30 and of the impermeable membrane 20, as the liquid level changes inside
the reservoir. When the liquid level rises in the reservoir to its upper level
as
shown in FIG. 2, the central portion of the impermeable membrane 20
accumulates in the central trough 40.
Optional transverse cables 46 may also be used between the outside
ends of the buoyant beams 36 to retain the buoyant beams in a parallel
orientation with each other. The use of transverse cables 46 is advantageous
for stabilizing a floating grid 30 over a larger reservoir. A grommet 48 on
the
outside end of each buoyant beam 36 is used to retain cables 32 and 46.
The floating cover according to the preferred embodiment also
comprises an array of lateral weight lines 50 laid over the impermeable
membrane 20, each being laid at about halfway between an adjacent pair the
buoyant beams 36. The lateral weight lines 50 are linked to a central weight
line 52 which is laid inside the trough 40 of the keel member 38. The lateral
weight lines 50 and the central weight line 52 are made of a plurality of pipe
sections filed with sand or concrete for example. The pipe sections in the
lateral weight lines 50 and the central weight line 52 are linked to each
other
by rope 54 or light cable, such that each weight line is longitudinally
flexible
to follow the movement ofthe membrane with any wave action in the covered
liquid. The outside end of each lateral weight line 50 is anchored to the
perimeter wall 22 of the reservoir by means of an anchor cable 56 attached to
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an anchor tab 58 mounted to one an anchor bolt 26 above the membrane
clamping flat bar 24.
Because the weight lines 50, 52 and the buoyant beams 36 are
relatively flexible longitudinally, their movement relative to the impermeable
membrane 20 do not apply significant concentrated shear stress in the
impermeable membrane.
The function of the central weight line 54 is to cause the flexible
trough 40 to sink below the level of liquid inside the reservoir and to
entrain
the central portion of the impermeable membrane 20 inside the trough 40.
A first function of the lateral weight lines 50 is to cause lateral
depressions on the membrane surface around several drain holes 60 through
the impermeable membrane 20. These drain holes 60 are located between the
buoyant beams 36, and in the central portion 'B' of the cover, as shown in
FIG. 4. This central portion 'B' represents about one third of the width 'W'
of the cover. The ropes 54 of the weight line above a drain hole 60 are
preferably attached to the drain hole to retain the weight line to that drain
hole. When rainwater is considered a contaminant relative to the content of
the reservoir, the drain holes 60 are connected to each other and to one or
more drain pipes 62 which are routed outside the reservoir.
Referring again to FIG. 4, it will be better understood that the flexible
trough 40 accumulates a spare surface of the impermeable membrane 20 for
accommodating extension and contraction of the impermeable membrane 20
across the width 'W' ofthe reservoir, when the liquid level changes inside the
reservoir. Similarly, a second function of the lateral weight lines 50 is to
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cooperate with the buoyant beams 36 and form peaks and valleys across the
length 'L' of the reservoir to accumulate a spare surface of the impermeable
membrane 20 along the length 'L' of the reservoir, to accommodate for the
lengthwise extension and contraction ofthe impermeable membrane 20 as the
liquid level change inside the reservoir. The lengths of the anchor cables 56,
of the tie cables 32 and of the intermediate ropes 54, and the height 'A' of
the
trough 40 are selected to allow unrestricted vertical movement of the
impermeable membrane 20 over the expected level variations of the liquid
inside the reservoir.
FIG. 4 also illustrates a typical accumulation of rainwater over the
cover. As mentioned before, the buoyant beams 36 cause transverse ridges in
the impermeable membrane 20, and the lateral weight lines 50 causes
depressions in the membrane between the ridges. Rainwater accumulates
inside the trough 40 and in the depressions much like according to the
illustrated contour line 64. As rainwater is evacuated through the drain holes
60 the size of the puddles recedes toward the central trough 40 until the
water
line is within the central region of the cover such as illustrated by label
66.
The rainwater remaining inside and along the central trough 40 is slowly
evacuated by evaporation.
The advantages of this installation is that rainwater has a stabilizing
effect on the cover during a rainstorm by submersing the central segment 68
of the membrane and a series of rib-like lateral segments 70. Because of
these submersed segments 68, 70, the impermeable membrane 20 is kept taut
and the surface of the membrane exposed to uplifting wind forces is greatly
reduced. Because of the position of the drain holes 60 in the central one
third
portion of the impermeable membrane 20, the submersed segments 68, 70
are still present when the rainwater recedes to a low level 66 beyond the
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holes 60. The stabilizing effect is therefore maintained during and after a
rainstorm.
Additionally, the floating grid 30, the impermeable membrane 20 and
the array of weight lines 50, 52 are free to move relative to each other.
Therefore, any liquid movement under the membrane 20 is less susceptible
of applying excessive tangential stress in the membrane.
The ridges created by the buoyant beams 36 still offer gas passages
under the membrane 20 whereby any off gas generated by the content of the
reservoir can be evacuated along the buoyant beams 36 and toward the
perimeter wall 22 of the reservoir, such as illustrated by arrows 72.
Referring now to FIG. 5, there is illustrated therein the structural
arrangement of a preferred drain hole 60. Each drain hole 60 has an inverted
Y-shaped fitting 80, the legs 82 of which retain segments of the drain hose
62. The fitting 80 has a flange 84 which is bolted to a pair of washers 86,
one
on each side of the impermeable membrane 20. The washers 86 have an
inside diameter 'D' which is larger than the maximum width 'C' of the fitting
80, across the legs 82. The fitting 80 is removable from the washers 86 by
removing a series of bolts 88. The fitting 80 and the drain hose segments 62
are retrievable from under the impermeable membrane 20 through the
opening 'D', for inspection, repair or replacement of the drainage system,
without removing the cover.
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While one embodiment of the present invention has been illustrated in
the accompanying drawings and described hereinabove, it will be appreciated
by those skilled in the art that various modifications, alternate
constructions
and equivalents may be employed without departing from the true spirit and
scope of the invention. Therefore, the above description and the illustrations
should not be construed as limiting the scope of the invention which is
defined by the appended claims.
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