Note: Descriptions are shown in the official language in which they were submitted.
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2549-003-27
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TITL~ OF THE INVENTION
IMPROVED DOUBLE WALLED UNDERGROUND STORAGE TANK
BACKGROUND OF THE INVENTION
Field of the Invention
This invention pertains to tanks or containers
designed to contain liquids, underground, for
dispensing. Specifically, a double walled tank, for
the containment and dispensing of petroleum products,
is addressed.
Background of the Prior Art
By far the dominant method of storing and
dispensing liquids, particular petroleum products,
from, e.g., service stations, is through the use of
underground storage tanks. Conventionally, tanks, of
generally cylindrical design, are prepared and
installed in a pit, subsequently covered, said tanks
having been provided with fittings, piping, valves and
the like, such that gasoline or similar fuel products
can be introduced to the interior of the tank, and then
dispensed, under control. Initially, these tanks were
prepared from steel. Steel continues to be a popular
material from which to construct the storage tanks.
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However, as discussed in U.S. Patents 3,335,904
and 3,700,512 the problems associated with underground
storage tanks, particularly posed by corrosion
problems, and the crushing pressures applied,
particularly if the hole should become filled with
water, require for many situations the use of a
superior material, such as fiber reinforced plastic, a
particular example of which includes chopped fiberglass
reinforced isophthalic resin. Note, in particular, the
discussion at column 1, lines 64-72 and column 2, line
60 - column 3, line 8 of U.S. Patent 3,335,904 and
particularly the discussion at column 2, lines 4-65 of
U.S. Patent 3,700,512.
In this respect, it should be noted that
underground storage tanks, and factors addressed in
their design, production, and maintenance, should be
distinguished from containers intended for above-ground
storage, such as for the storage and/ox transportation
of cryogenic materials. While both types of containers
must have sufficient strength to contain the liquid in
the interior, these cryogenic containers are generally
constructed without regard to, or need to safeguard
against, pressures and corrosive agents applied from
the exterior, as in the case with undergrGund storage
tanks. U.S. Patents 3,895,152 and 3,317,074 are
exemplary references discussing cryogenic containers.
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However, the use of fiber reinforced plastic
materials for the construction of underground storage
tanks--has-not tatally eliminated~the problem of
environmental contamination sr pollution. In
particular, the tanks of the prior art, although
exhibiting superior resistance to corrosive attack,
nonetheless remain potentially subject to cracking
and/or holing, particularly during handling, as well as
after installa~ion. ;For-éxample, the rough treatment a
tank may receive during transportation and manipulation
prior to installation may result in undetected
punctures of the tank. Similarly, when in operation, a
shift in the bed may result in fracturing or holing of
the tank, resulting in the spillage and resulting
hazards addressed in U.S. Patent 3,700,512.
Accordingly, many locations and authorities have
instituted heightened standards for reliability for the
construction and design of underground storage tanks,
such as California. Even in regions where such
stringent regulations are not enforced, avoidance of
environmental pollution remains a concern of the
highest priority.
One widely adopted response to this problem has
been the provision of dual-walled storage tanks. An
example of a double walled storage tank prepared from
steel materials can be found in U.S. Patent
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1,886,074. However, as noted above, steel rusts.
Double walled underground storage tanks have also been
prepared out of fiber reinforced resin systems, such as
that disclosed in U.S. Patent 4,676,093. As described
in both these references, the provision of two walls,
alone, is insufficient. The use of a double walled
storage tank is adopted to allow the installation of an
alarm system, which will detect a leak in either the
inner tank, the outer tank, or both. One such alarm
system consists of detectors sensitive to the presence
of fluid in the space between the inner and the outer
tank. This is generally referred to as a "dry" alarm
system. An alternative, or "wet" system, fills the
space between the inner and the outer tank with a
liquid, such as ethylene glycol, water or brine. When
either the inner or the outer tank is punctured or
otherwise develops a leak, at least a portion of the
liquid contained between the two tanks will flow
through the leak, the reduction in the volume of water
held between the two walls being detected, by a variety
of responsive means. Such systems again are described
in U.S. Patent 4,676,093, as well as U.S. Patent
4,672,366.
However, the need to maintain a space between the
inner and outer tanks of a double walled storage tank
implies that two separate, strong tanks be constructed,
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and connected. In U.S. Patent 4,676,093, as well as
many other re~erences, such as U.S. Patent 4,739,659,
this-is-achieved by the provision of ribs which extend
between the first and second tank, providing contact
therebetween, as well as circumferential reinforcement
and deflection resistance. However, even in the design
in U.S. Patent 4,739,659, a certain amount of space
between the inner and the outer wall must be left open,
for the-alarm or leak-detection system. This patent
includes a "gas pervious" material between the inner
and outer walls, the outer wall being discontinuous,
tied into the side of each rib. The gas pervious
material is not load-transferring, and strenqth is
again provided by securing the outer tank wall to the
ribs. Since both tanks have to be resistant to leakage
or penetration, this implies that both tanks must have,
independent, sufficient strength to resist puncture,
fracture and compression, as they cannot share
strength, no load being transmitted across the space
between the two tanks, save for the limited case of the
ribs. While the presence of the ribs may be employed
to reduce deflection therebetween, spacing of the ribs
does not permit a load or impact at a point between the
ribs applied to, e.g., the outer tank, to be
transmitted to, or shared with, the inner tank.
Similarly, loads or impact supplied to the inner tank,
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cannot be shared by the outer tank. This results in
independently strengthened inner and outer shells, and
a significant consumption of material and man hours.
Accordingly, it is an object of this invention to
provide a double walled tank, capable of being equipped
with either a dry or wet alarm system, which comprises
a inner and outer shell, between which load may be
transmitted and shared, so as to reduce the need to
provide two entirely independent tanks, and reduce the
material, and man hours, involved in their
construction.
SUMMARY OF THE INVENTION
The above objects, as well as others that will be
discussed in detail, below, is achieved by providing
two independent rigid shells, as in prior art tanks,
but filling the space therebetween entirely with a load
transmitting, resin impregnable, water-passing
substance. The substance that fills the space between
the two tanks must be capable of transmitting load
across the distance between the two tanks. In order
for the load-transmitting material to transfer load in
the direction normal to a first shell and to the
opposed face, said ma~erial can be fixed in position by
adherence to the shells, when possible, or by close
contact with the outer surface of the inner shell and
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the inner surface of the outer shell. Since,
conventionally, these tanks are made of fiber-
reinforced~resin ~aterials,-an ideal method of securing
the load-transmitting material to the walls of the
tanks is to apply resin thereto, and cure it up against
the tank. Ideally, therefore, the material should be
capable of taking up resin, and curing against the
tank. To be capable of being equipped with a dry alarm
systemj th~ resin-infiltrated-load-transmitting
material should be capable of passing petroleum
liquids. That is, within a relatively short period of
time, regardless of the point of the leak on the
interior tank, the petroleum products stored therein
should be capable of passing along the space between
the two tanks, through the load-transmitting material,
to the bottom of the tank, under the influence of
gravity, to contact the dry alarm system situated
thereat. In order to equip the tank with a wet alarm
system, the load-transmitting material must be capable
of water passage therethrough, i.e., in the ideal
state, a continuous liquid, preferably aqueous, phase
must be present in the load-transmitting material,
without loss of load-transmitting characteristics.
One exemplary material for the load-transmitting
filler is an non-woven composite felt or system,
produced by Ozite Corporation, of Illinois, under the
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name Compozitex~. This material is conventionally sold
as a structural element, for use, e.g., in the
preparation of insulation and the like. It is selected
for use in this invention because of its combination of
load-transmitting capabilities, its easy impregnation,
adherence and curing with resin, and its transparency
to water, or stored petroleum products. In this
regard, such products should be porous. Compozitex~ is
apparently made sufficiently porous by needling.
Other, similar materials, generally manufactured
for use in building insulation and the like, are
available from other manufactures. $his invention is
not limited to any specific load-transmitting material.
A double walled tank, meeting the most stringent
current safety regulations, prepared with an
intermediate layer of Compozitex~ can be prepared
reducing the amount of materials in the outer shell by
half.
~RIEF DESCRIPTION OF THE DRAWING
The Figure is a representational plan view of an
underground storage tank, bearing external ribs, and a
load-transmitting substance filling the space between
inner and outer shells.
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DETAILED DESCRIPTION OF THE INVENTION
The underground storage tank of this invention is
broadly illustrated in the Figure. It consists of an
outer tank, 10, comprised of chopped fiberglass-
reinforced resin, which has been cured up, of similar
or identical composition to inner tank wall 14. Lying
between, and adhered to both walls, is the load-
transmitting material 12, entirely filling the annular
space between tank wa~ls l0 and-14. Ideally, these
tanks bear external ribs 16, for reinforcement, and to
prevent deflection. The number, and placement, of ribs
16, will vary with tank diameter and length.
Outer shell 10 can be prepared through a female
molding process, such as that disclosed in U.S. Patent
4,363,687, the entire content of which is incorporated
herein, by reference. Briefly, a half mold,
corresponding to the topography of one-half of the
length of tank l, is provided with an internal lining
or release agent, making detachment of the mold from
the finished shell possible. On the internal wall of
the female mold, a mixture of discontinuous fibers,
generally a fiberglass roving or similar material, and
resin, is blown or sprayed onto the internal wall.
Generally, the resin is in the form of a liquid
mixture, also containing a catalyst, for auto-curing of
the mixture. In one preferred embodiment, the female
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mold is caused to rotate about its longitudinal axis,
while the wall material is sprayed from a central
position, which passes along the length of the mold.
Pursuant to the current invention, once this
external shell is formed, a layer which is either pure
resin, or extremely resin rich, is sprayed upon the
interior of the formed wall. This is semi-cured, to a
tacky condition, at which time the load transmitting
material, such as Compozitex~, is applied thereto. As
this material is a workable felt, which can be provided
in long strips, sheets, etc., it is a straightforward
operation simply to place the load-transmitting
material against the tacky wall, and then roll out air
pockets between the felt and the outer wall, with a
conventional roller, similar to those used in painting
and the like. As noted previously, the load-
transmitting material quickly takes up the resin, and
is adhered to the outer tank thereby.
Other load-transmitting materials may be used,
such as composite honeycomb or plastic webbing. A
particularly preferred webbing is available as an
extruded net or webbing having continuous passages
therethrough. One supplier of such material is the
Conwed Corporation of Minneapolis, Minnesota. If the
material is thermoplastic it may be necessary to alter
its surface properties to adhere to the resin of the
tank. Similar load-transmitting materials may be used.
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Once the load-transmitting material is secured
against the outer tank, the molcling process is resumed,
with a first-layer of resin or resin rich material
applied against the internal surface of the load-
transmitting material, to ensure bonding of that phase
to the outer surface of the internal wall 14. The
internal wall 14 is then similarly sprayed up, and the
two halves are joined, as is conventional in the art.
It should be noted that ribs 16 may be prepared
according to conventional processes. If there is no
manway provided, and the two molded tank halves are
mated only from the ou-tside, care must be taken to
ensure the annular material 12 is continuous across the
joint.
It should be noted that adhesion of the load-
transmitting material to the sides of the tank shells
i5 unnecessary to achieve transmission of the applied
load in a direction normal to the surface to which the
load is applied. The configuration of the tank fixes
the material in position. Since the tank, in use, is
generally under compression, load will be transmitted
across the material. Adhesion of the material to the
outer surface of the inner wall and inner surface of
the inner wall does impart shear resistance not
otherwise obtained, and accordingly remains a preferred
embodiment.
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Alternatively, the double wall tank of this
invention can be prepared through a male molding
process, such as that described in U.S. Patent
4,676,093, column 2, line 11 - column 3, line 4. It
should be noted that, unlike that prior patent, there
is no need to provide or maintain holes in ribs, or
specific passageways. In the claimed invention, the
load-transmitting material extends between the rib
surfaces of the external and internal shell, and
accordingly, where a wet alarm system is desired, the
alarm liquid passes through this layer.
Whether prepared through a female or male mGlding
process, the resulting double wall tank has
significantly greater strength against compression than
prior double wall tanks, in regions between the ribs of
the tank cylinder. Specifically, the outer and inner
shell, by virtue of the load-transmitting material
therebetween, now act together, in resisting applied
force. ~hus, to secure a tank of strength equal to
prior art embodiments where the outer and the inner
tank must necessarily be of independently sufficient
strength and thickness, one can prepare a tank
according to the claimed invention, where the thickness
of at least one of the two shells is reduced to that of
a liner. Alternatively, the thickness of both shells
may be reduced sharply to provide a symmetrical
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system. In any event, the total consumption of
material is significantly reduced over currently
available systems.-- Thus, i:n one embodiment, the
internal shell 14 has a thickness of about 3/16 inch,
the load-transmitting material has a thickness of about
a l/8-inch, and the exterior tank has a thickness of
about 1/4 inch. These dimensions are exemplary only.
It is possible to vary thickness in both directions and
mai-ntai~n a generous~safety factor. As the cost of the
load-transmitting material, and the labor involved in
its application, is sharply lower than that of the
exterior tank 10, this reduction in material
const~tutes a significant reduction in cost of the
tank, both in terms of material used, and labor
required.
The total annular space is sharply reduced in the
invention, over prior art systems. Thus, using
Compozitex~, a total value of perhaps 270-30n gallons
is required to fill the space of a 10,000 gallon tank,
as opposed to 1,000 gallons or more for prior art
systems. Additionally, since the annular space is
continuous, the entire tank can be monitored by one
alarm means, without the need to provide passageways
through the ribs, as envisioned in U.S. Patent
4,739,659. In general, the volume will range from 100-
300 gallons. This is not only more economical, it
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renders the system with a wet alarm more sensitive to
leakage. Moreover, a storage tank of this design can
be shipped from the factory with its annulus filled, a
great advantage over prior systems.
Where a dry alarm system is desired, a leak sensor
such as that described in U.S. Patent 4,672,366 may be
employed. Alter~ative systems may be used. In U.S.
Patent 4,672,366 a sensor cable, comprised of a twisted
pair of conductors separated by insulation materials is
placed at the bottom of the tank. The insulating
materials are specially designed to degrade in the
presence of petroleum products, such as gasoline and
diesel fuels, or other materials to be stored in the
tank such as machine oil, and the like. If the inner
tank develops a leak, as noted above, the stored
petroleum products will pass easily through the load-
transmitting layer, under the influence of gravity, to
the bottom, where the insulation material will be
contacted, and degraded. This causes a change in
resistance in the twisted cable, and eventually, a
short circuit, which sets off an alarm.
Alternatively, a wet alarm system, such as that
described in U.S. Patent 4,676,093, can be employed.
Wet alarm systems are capable of detecting leaks in
either the outer shell 10 or the inner shell 14, or
both. An exemplary system is described in U.S. Patents
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4,676,093 and 1,996,0~4. In such a system, the space
between the inner and outer tank is completely filled
with a freeIy flowIng liquid, such as ethylene glycol,
water or brine, which may be given a distinctive color,
odor, etc. The fill line for this space rests above
the installation, and can be visually or electronically
monitored. ~n operation, the space is filled, and the
column leading to this space is similarly filled to a
prede~ermlned Ievel,-above the tank, whi~n may be
monitored. Should a leak develop in either the inner
or the outer tank, the level in the standpipe will drop
in response thereto, signaling a need for immediate
repairs. It is also possible to use a specific liquid
for the detection material which can cause a sensor in
the interior of the tank to respond, if desired. It
is, of course, critical that the liquid solution of the
wet alarm system be able to freely move in the annular
space, and the load-transmitting material of this
invention allows such movement.
Of course, fittings for filling the interior tank,
vapor recovery, level measuring, a manway, if desired,
etc. are provided on and through the two tanks. Such
fittings are well described in the art, and constitute
no part of this invention, per se.
Obviously, numerous modifications and variations
of the present invention are possible in light of the
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above teachings. It is therefore to be understood that
within the scope of the appended claims, the invention
may be practiced otherwise than as specifically
described-herein.
