Floating cover for the interior of a storage tank

An internal floating cover for covering and sealing the surface of a liquid stored in a tank having an external roof, has a flexible thin-walled membrane which floats on the surface of the liquid. The membrane is provided with at least one float and a seal around its perimeter edge or rim extending near and along the wall of the storage tank. The dimensions of the membrane are oversized relative to the corresponding liquid surface dimension of the tank and/or the membrane is made with a certain elasticity so that the membrane may bulge in a hood- or dome-shaped out of the plane of its contact with the inner tank surface. The movable connection between the membrane rim or edge includes clamping strips or contact elements on the floats or on a perimeter seal. The bulging of the membrane may occur upwardly under the effect of gas pressure or may occur downwardly under the effect of reduced pressure during draining of the liquid. At least one pressure relief valve is arranged in the membrane. At least one continuous ring-shaped perimeter edge or rim element is attached to the perimeter edge of the membrane in a gas-tight manner, near and along the storage tank wall. The edge or rim element is, at least partly, immersed in the liquid.

FIELD OF THE INVENTION 
The invention relates to an internal floating cover for covering and 
sealing the surface of a liquid stored in a tank which has a roof or 
external cover. Such an internal floating cover includes a flexible 
thin-walled membrane which floats on top of the liquid. The cover further 
includes at least one float, and a seal arranged around the perimeter edge 
of the membrane to contact the vertical walls of the storage tank. 
DESCRIPTION OF THE PRIOR ART 
Internal floating tank covers of the general type described above are known 
in the art. For example, German Patent Publication 632,472 discloses an 
internal floating cover for volatile liquids. The known cover includes a 
ring-shaped float and a plurality of spheres afloat on the liquid to form 
a seal between the float proper and the storage tank wall. The membrane of 
the known internal floating cover should be made of aluminum foil, 
cellular fabric material, thin yet strong or robust metal, celluloid, 
liquid-impermeable cloth or the like, whereby it may also further be 
reinforced or strengthened by a wire mesh. 
Modern internal floating covers of the described type must meet a great 
variety of technical and other requirements. Thus, such covers are not 
only to be produceable at a reasonable price, but also are to be made of 
an extremely thin, lightweight, yet diffusion-impermeable membrane 
material. Nevertheless, the membrane must be able to securely cover and 
seal large planar surfaces of possibly chemically agressive liquids. For 
reasons of environmetal protection, it is further to be assured that gases 
or vapors are not allowed to escape to the atmosphere as free emissions, 
or at least that such emissions are held to an absolute minimum. 
Improvements over the prior art are to be made in these respects. 
OBJECTS OF THE INVENTION 
In view of the foregoing it is the aim of the invention to achieve the 
following objects singly or in combination: 
to construct an improved internal floating cover of the general type 
described above which will securely and reliably withstand not only the 
mechanical and hydraulic loads applied to it by the medium being stored, 
but also the loads of any arising gas pressures; 
to achieve an improved sealing effect in such an internal floating tank 
cover to minimize or eliminate the evaporation of volatile liquids and 
therewith the emission of gases or vapors; 
to achieve a best possible utilization of as much of the entire volume of 
the storage tank for storing the liquid; 
to achieve a simple construction of a floating cover for covering large 
surfaces of possibly chemically agressive liquids, with a thin-walled and 
light, yet diffusion-impermeable membrane, with an acceptably low cost and 
technical effort; 
to construct the cover membrane of several different layers if need be, 
whereby each layer provides optimum physical or chemical characteristics; 
and 
to trap and seal-in any vapors or gaseous emissions from a body of liquid 
covered by the present floating cover. 
SUMMARY OF THE INVENTION 
The above objects have been achieved in a floating tank cover according to 
the invention, wherein the floating cover includes a membrane having 
dimensions which are oversized relative to the tank surface dimensions 
and/or having a certain elasticity, so that the membrane is deformable in 
a hood-shape or bubble-shape out of the plane in which it is attached to 
the float or perimeter seal. The bubble-shaped deformation can occur 
vertically upwardly when the membrane traps gases or vapors released from 
the stored liquid, or vertically downwardly as the tank is being emptied 
to allow removal of essentially all of the stored liquid. Furthermore, the 
vertical bubble-shaped deformation may either occur across one horizontal 
direction, thereby forming one or more essentially cylindrical shaped 
bubbles, or across both horizontal directions, thereby forming an 
essentially hemispherical-shaped bubble. At least one pressure relief 
valve is arranged to vent excessive pressure from under the membrane. At 
least one continuous ring-shaped perimeter edge element is connected in a 
gas-tight manner to the membrane near the storage tank wall. The perimeter 
edge element is at least partially immersed in the stored liquid. 
The membrane of the present internal floating cover has a separating layer 
comprising one or several plies, whereby the length and width or the 
diameter of the membrane is considerably over-dimensioned relative to the 
corresponding dimensions of the liquid surface of the liquid stored in the 
tank in order to permit the above mentioned bubble formation or bulging in 
an upward or downward direction. In the event that gases or vapors are 
developed by and released from the stored liquid, the membrane billows or 
bulges upwardly over the liquid surface due to the developed gas pressure. 
At that time, the gases or vapors cannot escape to the atmosphere. 
Instead, the gases or vapors are held or retained by the membrane. Only 
after the gas pressure exceeds a predetermined value, a safety valve in 
the form of a pressure release valve arranged in the membrane opens, 
whereby excess gas pressure is vented off so that the membrane is not 
damaged. Until the opening pressure of the safety valve is reached, the 
membrane can be completely lifted away and bulge or billow above the 
surface of the liquid, whereby the membrane floats on a gas bubble. In 
order to prevent any leakage losses or escape of gas before the safety 
valve opens, the membrane itself, or the perimeter edge element which is 
connected in a gas-tight manner to the membrane, is immersed in the stored 
liquid around the membrane edge near the storage tank wall, whereby a sure 
sealing is achieved. 
When the tank is being emptied, the membrane may bulge downwardly to 
eventually rest on the surface of the storage tank floor or any other 
equipment, such as pumping equipment, located on the tank floor. This 
downward bulging is also only possible because the membrane is 
over-dimensioned relative to the corresponding storage tank dimension. The 
downward bulging of the membrane during emptying of the tank achieves an 
optimum utilization of the entire volume of the storage tank. That is to 
say, the storage tank may be essentially completely emptied without taking 
any special emptying measures and without losing any volume to the 
structure of the floating cover. 
The material of the membrane is respectively matched to or selected with 
regard to the properties of the stored liquid which is to be covered and 
the gases and vapors which are to be sealed-in, as well as with regard to 
the mechanical requirements. In order to achieve various optimum 
mechanical and chemical properties, the membrane may be made of a single 
ply or layer, or it may include several layers having various properties. 
In order to economically produce a large area membrane, it may be 
assembled of several separate sheets or webs which are attached to each 
other along their respective edges by seam welding or heat fusing, or by 
providing mechanical clamping elements.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
As shown in FIG. 1, an internal floating cover 1 covers and seals the 
surface 2 of a liquid 16 stored in a container or storage tank 4 having an 
external cover or roof 3 which is typically slightly arched or vaulted and 
usually has at least one ventilating stack 8 for venting the tank, or the 
space between the floating cover 1 and the roof 3, to the atmosphere. A 
tank wall 7 of the storage tank 4 defines the cross-sectional shape of the 
tank which may, for example, be circular. The tank 4 is closed at the 
bottom by a floor 15 which may include any necessary pumping equipment or 
outlets. The internal floating cover 1 at least comprises a float 5, a 
flexible thin-wall membrane 6 which usually floats on the surface 2 of the 
liquid 16, and a seal 11 which contacts the tank wall 7 around the 
perimeter edge of the membrane 6. 
In the example embodiment shown in FIG. 1 and in the detail of FIG. 2, the 
float 5 comprises a frame 9 with several floating buoyant bodies 10 which 
are interconnected to form a ring-shaped polygon. Carrier sections 9a, 
which are each straight sections and have a U-shaped cross-section, are 
attached, for example by means of straps 9b, to the likewise straight 
drum-shaped floating bodies 10. Preferably, the ends of the bodies 10 are 
spaced a slight distance from each other. The carrier sections 9a abut one 
another at their facing ends 9c extending at an angle relative to each 
other. The carrier sections may be attached to each other at the 
respective ends 9c. The carrier sections 9a further comprise clamping 
rails 9d for attaching the membrane 6. 
A seal 11 is provided along the storage tank wall 7. The seal 11 includes a 
sealing skirt 12a which bridges an annular gap 12 between the frame 9 and 
the tank wall 7, as well as a sealing element 13 which contacts the tank 
wall 7 as is shown in the detail view of FIG. 2. 
Struts or feet 14 are provided on the bottom of the frame 9 to rest on the 
tank floor 15 when a minimum liquid level is reached as shown by dashed 
lines near the bottom of FIG. 1. The dimensions of the membrane 6 are 
oversized relative to the corresponding dimensions of the storage tank 4, 
and/or the membrane 6 has a certain elasticity sufficient so that the 
membrane 6 may bulge or arch out of the plane 17 of its attachment, for 
example, by means of clamping strips 9d to the floats 5, or rather to the 
frame 9, or alternatively, to a contact element 18 of the seal 11 along 
the tank wall 7. The membrane 6 may bulge or billow upwardly as shown at 
the top of FIG. 1, or may bulge or curve downwardly as shown at the bottom 
of FIG. 1, under different filling states of the tank 4. 
At least one pressure relief valve 19 which is normally closed is arranged 
in the membrane 6. The valve 19 is activated, for example, when a 
sufficient volume 20 of gases is trapped between the membrane 6 and the 
surface 2 of the liquid 16 to develop an excessive pressure. The valve 19 
is normally closed, so that normally an increased gas pressure caused by 
evaporation into the space between the membrane 6 and the liquid surface 
lifts the membrane 6 away from the surface 2 of the liquid 16, whereby the 
membrane 6 takes on an upwardly vaulted shape, depending on its dimensions 
and elasticity as shown at the top of FIG. 1. When the float 5, due to its 
feet 14, comes to rest above the tank floor 15 when the tank is empty, the 
membrane 6 bulges downwardly as shown at the bottom of FIG. 1. This up or 
down vaulted or bulging shape is advantageously predetermined so that the 
membrane 6 may still bulge upwardly, according to the dashed lines shown 
at the top of FIG. 1, without contacting the roof 3 of the storage tank 4 
even when the tank is filled to the maximum permitted liquid level. The 
membrane 6 then bulges up with a hood shape or a hemispherical cap shape 
under the roof 3 before the pressure relief valve 19 is activated. The 
valve 19 is activated before any excessive pressure leads to damage of the 
membrane 6. Thus, the valve 19 prevents an excessive expansion of the 
membrane 6. 
Advantageously, at least one pressure relief valve 19 is arranged at the 
center of the membrane 6. The valve 19 is lifted by the membrane 6 out of 
the liquid 16. The activation or opening pressure for the pressure relief 
valve 19 is, for example, advantageously on the order of the pressure 
exerted by a 1 to 2 mm column of water. 
As shown in FIG. 2, a pressure relief valve 19 comprises a ringshaped float 
19f having a coaxially or centrally arranged throughhole 19g. The float 
19f preferably has a structural height equal to the maximum intended 
bulging height of the membrane 6. A droplet separator 19h is arranged in 
the through-hole 19g. A valve opening 19j at the end of the through-hole 
19g above the drop separator 19h is closed by a valve ball 19i which is 
held in a cage 19k of the pressure relief valve 19. When a predetermined 
gas pressure is exceeded under the membrane 6, the valve ball 19i is 
lifted from the valve opening, 19j and excess gas escapes through the 
pressure relief valve 19 into a space 20' above the membrane 6 until the 
gas pressure is reduced sufficiently to allow the valve ball 19i to again 
close the valve opening 19j. This is achieved, for example, in that the 
membrane 6 lifts the valve 19 with its float 19f from the liquid 16 when 
the bulging of the membrane 6 exceeds a maximum allowable bulge. Then, 
excess gas can escape through the through-hole 19g into the space 20', 
until the bulge is diminished so that float 19f again contacts the liquid 
16. 
In an alternative embodiment as shown schematically in FIG. 2a, a pressure 
relief valve 19a comprises at least one hose-or pipe shaped element 19b 
which is immersed vertically in the liquid 16. The pipe 19b has one open 
end 19c located at a sufficient safety distance h above the surface 2 of 
the liquid 16. That is to say, the end 19c of the pipe element 19b opens a 
sufficient distance h above the membrane 6 to prevent any liquid 16 from 
being expelled through the opening 19c. At a spacing H, see FIG. 2a, below 
the surface 2 of the liquid 16, the pipe element 19b comprises at least 
one opening 19d. A weight 19e, or the like, assures that the pressure 
relief valve 19a or specifically, the pipe element 19b assumes a vertical 
hanging position in the normal operating state. When the membrane 6 is 
lifted by increasing gas pressure out of the position in which it is 
resting on the surface 2 of the liquid 16 as shown in FIG. 2a, into the 
bulging position shown by dashed lines at the top of FIG. 1, spacing H is 
reduced until it becomes zero and the opening 19d emerges above the 
surface 2 of the liquid 16. 
Prior to the just mentioned rise of the valve pipe element 19b the liquid 
16 effectively closed the opening 19d and the gas pressure or tank 
pressure was not sufficient to force liquid 16 into the pipe element 19b. 
However, after the opening 19d has been lifted above the surface 2 of the 
liquid 16, excess gas 20 is free to escape through the opening 19d into 
the pipe element 19b and finally exit from the open end 19c into the space 
20'. 
Thus, either pressure relief valve 19 or 19a is hydraulically activated or 
controlled. That is to say, it operates on the basis of its position 
relative to the surface 2 of the liquid 16. The hose or pipe element 19b 
penetrates essentially perpendicularly through the plane of the membrane 6 
and is attached and sealed in a liquid and gas pressure-tight manner to 
the membrane 6. Such an attachment and sealing is also true for the 
components of the pressure relief valve 19. 
The floating cover 1 according to the invention further includes at least 
one continuous ring-shaped perimeter edge element attached in a gas-tight 
manner to the perimeter edge of the membrane 6. The perimeter edge element 
extends close to and along the tank wall 7 and is at least partially 
immersed in the liquid 16. This perimeter edge element assures that the 
membrane 6 can form a billowed or bulged hood for confining a gas bubble 
without leaking or billowing upwards along the perimeter edge of the 
membrane 6. This perimeter edge element may be formed by the seal 11, or 
by the float 5, or by an immersed seal part 13 of the seal 11. The 
immersed or submerged seal member 13 may basically also be a part of the 
membrane 6 if the membrane 6 is made of a highly chemically resistive and 
heat-fusible material. In this manner the attachment by means of clamping 
strips 9d may be avoided if the membrane 6 also directly forms the seal 
skirt 12a as well as the immersed seal element 13. Usually, however, the 
membrane 6 is attached to the frame 9 in a gas-tight manner together with 
the separate seal skirt 12a of the seal 11. A surfacially heat-fused 
membrane 6, for example formed of separate membrane strips or webs joined 
together by heat-fusing as described below, can be extended to reach 
completely to the contact element 18 of the seal 11 only in the case of 
chemically highly resistive and heat-fusible or weldable membrane 
materials with resultant chemically resistive heat-fused seams. 
Finally, the seal 11 includes several spring elements 21 which may, for 
example, be U-shaped and which connect the seal 11 with the floats 5 or 
the frame 9, and which additionally press the seal 11 outwardly for 
sealing against the tank wall 7. 
At least one vacuum relief valve 22 is located in the frame 9. The vacuum 
relief valve 22 is activated when the struts or feet 14 of the frame 9 or 
floats 5 come to rest on the tank floor 15 during emptying of the tank 4 
when any liquid remaining between the membrane 6 and the tank floor 15 is 
still to be pumped out. In this case, a slight vacuum is formed under the 
membrane 6, whereby the vacuum relief valve 22 opens, so that air streams 
from the space above the membrane 6 into the space between the tank floor 
15 and the membrane 6. Thus, the remaining liquid may be pumped out as the 
membrane 6 progressively comes to rest on the tank floor 15 as is shown at 
the bottom of FIG. 1. As shown in the detail of FIG. 2, the vacuum relief 
valve 22 preferably comprises a vertically arranged valve basket 23 with a 
central pipe 23a and a valve ball 24 which is normally submerged in and 
buoyed by the liquid 16. After the feet 14 have come to rest on the tank 
floor 15 and the level of the liquid 16 continues to drop, the valve ball 
24 drops downwardly along with the liquid level and thereby opens the 
valve opening of the central pipe 23a. In this manner, air enters the 
space under the membrane as described above. 
The membrane 6 comprises at least one layer or ply which is tear and 
puncture resistant as well as impermeable to diffusion and also 
anti-static or resistant to accumulation of static electric charge. In 
view of the possible broad range of physical and chemical properties of 
the respective different liquids 16 to be covered, the requirements to be 
met by the membrane 6 can be widely variable. As the case may be, the 
membrane 6 at least comprises a metal film or a diffusion impermeable 
synthetic film which is provided on at least one side with a protective 
armor layer. 
As shown particularly in FIG. 7, in order to cover and seal volatile 
hydrocarbons, the membrane 6 advantageously includes a special barrier 
film 25 which is durable and impermeable to diffusion by hydrocarbon 
vapors. Polyester, polyamide or polyvinyl alcohol is especially suitable 
as a material for such a barrier film 25. However, these films typically 
do not provide sufficient mechanical strength and are usually 
electro-statically chargeable. Therefore, in addition to a barrier film 25 
made of polyester, polyamide or polyvinyl alcohol, the membrane 6 
comprises a layer of fabric 26 as an anti-static puncture protective layer 
on top of the barrier film 25, and a further anti-static bottom layer 27 
underneath the barrier film 25. Both the fabric layer 26 and the bottom 
layer 27 may comprise a reinforcing fabric or mesh with an anti-static 
thermoplastic coating, for example, a coating of polyethylene. 
As further shown in FIG. 7, the separate layers or plies of the membrane 6, 
that is, the polyethylene coated fabric 26, the barrier film 25, and the 
anti-static bottom layer 27, lie loosely on top of one another. In other 
words, the various layers are not laminated or surfacially attached to one 
another. It is further provided that the length dimensions of the 
diffusion impermeable barrier film 25 are larger than the corresponding 
dimensions of the armor layer or polyethylene coated fabric layer 26. 
Thus, any arising tension forces in the multilayer membrane 6 are taken up 
in the armor fabric 26, with the result that the delicate barrier layer 25 
and its bottom layer coating 27 remain free of undesirable loads or 
stress. 
In another example embodiment of the membrane 6 according to FIG. 8, a 
fabric 26a serving as an armor layer is arranged above a barrier film 25, 
and a further fabric armor layer 26b is arranged below the barrier film 
25. Similarly, as described above, in this embodiment the barrier film 25 
lies relatively loosely between the two fabric layers 26a and 26b and is 
therefore not loaded by any arising forces. In fact, the load free 
condition of the barrier film 25 in this embodiment is independent of 
forces acting vertically upwardly or vertically downwardly. Thus, in this 
embodiment, the barrier film 25 remains unloaded when the membrane 6 is 
billowing upwardly due to a gas pressure developing under the membrane as 
well as when the membrane 6 is bulging downwardly due to draining of the 
liquid from the tank. 
In view of the relatively large size of the surface 2 of the liquid 16 to 
be covered by the membrane 6 in storage tanks of a typical size, the 
membrane 6 is preferably assembled of separate sheets or webs 28 which are 
connected to each other along their edges 29, 30. Whenever possible, this 
connection along the edges 29, 30 is carried out by means of welding 
(heat-fusing) or gluing. However, when the material characteristics of the 
films, layers, or fabrics do not allow the seams to be heat-fused, or if 
the resulting bonded or fused seams are not sufficiently chemically 
resistant and liquid and gas-tight, then flexible clamping and holding 
sections are provided along the seams as connection members. When only 
some of the layers of the membrane 6 are sufficiently heat-fusible, 
adjacent webs 28 may be joined along edges 29, 30 by heat-fusing and by 
providing clamping members. 
In the example embodiment shown in FIG. 3, two clamping and holding 
sections 31, 32 each having an L-shaped cross-section are provided as 
connecting elements. The lengthwise edges 29, 30 of adjacent sheets or 
webs 28 are bent or angled upwardly away from the liquid 16 and are 
received between the two-L-shaped clamping sections 31, 32. Bolts 33 
tightly clamp the clamping and holding sections 31 and 32 together, with 
the web edges 29 and 30 clamped in between. Each clamping and holding 
section 31, 32 has one L-shank 34 contacting an edge 29, 30 of the web 28 
and the other L-shank 35 resting on top of the web 28 essentially parallel 
to the plane of the membrane 6. In this clamping arrangement the edges 29, 
30 of the membrane webs 28 are lifted away and out of the liquid 16 so 
that the web edges 29, 30, which may be more susceptible to chemical 
attack, are not directly exposed to the agressive liquid 16. Furthermore, 
the essentially horizontal L-shanks 35 of the clamping and holding 
sections 31, 32 are slightly curved or rounded at their outer edges so 
that the membrane web 28 is not exposed to any sharp edge or corner. 
Whenever the edges 29, 30 of one or another layer of the membrane webs 28 
may be heat-fused together, this may be carried out in addition to the 
connection provided by the clamping sections 31, 32. The layer or layers 
of the webs 28 which are fused together along their edges 29, 30 may, for 
example, form a tube-shaped or hose-shaped protruding bead 36. For 
example, when using a polyethylene coated fabric 26 as an upper cover or 
armor layer, the edges of this coated fabric 26 may be heat-fused together 
while forming a hose-shaped bead 36 as described above. 
FIG. 4 shows another embodiment for connecting the edges of adjacent 
membrane webs 28 by means of a clamping and holding section 37 having an 
essentially U-shaped cross-section with essentially vertical shanks 38 and 
an essentially horizontal connecting web or U-stem 40. Slightly bent or 
bow-shaped supporting flanges 39 project essentially horizontally from the 
lower ends of the shanks 38. The edges 29, 30 of the separate layers of 
the membrane webs 28 overlap in the area of the upper web or stem 40 of 
the U-shaped clamping and holding section 37, and are clamped together by 
means of bolts 41 tightly clamping a clamping rail 40' against the U-stem 
40. In this clamping arrangement with a U-shaped clamping and holding 
section 37, the overlapping edges 29, 30 of the membrane webs 28 are 
lifted out of the liquid in a manner similar to that described above with 
respect to FIG. 3. 
According to FIGS. 5 or 6, floats 42 or 43 are arranged under the membrane 
6 and at least partially lift the membrane 6 out of the liquid 16. 
Advantageously, such floats 42 or 43 are placed primarily near the regions 
of the interconnection seams 44 between adjacent membrane webs 28, so that 
the connection seams 44 are lifted out of the liquid 16 so that they are 
not subjected to the chemically attacking nature of the liquid 16. In 
order to fix or attach a float 43 in the area of a connecting seam 44, 
loop-shaped holding elements 45 may extend from the seam 44, or rather 
from the webs 28 to reach around and hold the float 43. 
FIGS. 7 and 8 have been described in detail above, but it should be noted 
here that they additionally show adjacent membrane webs 28 interconnected 
along their edges at connecting seams 44. 
Although the invention has been described with reference to specific 
example embodiments, it will be appreciated, that it is intended to cover 
all modifications and equivalents within the scope of the appended claims.