Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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STORAGE TANK OVERFLOW CONTAINMENT APPARATUS
FIELD OF THE INVENTION
The present invention relates in general to storage tanks for liquids such as
crude
oil, and in particular relates to apparatus for containing overflow of liquid
through vents
of such tanks.
BACKGROUND OF THE INVENTION
Liquid storage tanks are used in many industrial applications. In the field of
oil
exploration and production, cylindrical field tanks, typically with shallow
conical roofs,
are commonly used as central collectors for crude oil flowing from one or more
producing wells. The tanks are usually set on a layer of sand or gravel fill
over an
impermeable membrane. These tanks require venting to prevent pressure or
vacuum
build-up as the tanks are being filled or emptied, or due to fluctuations in
ambient
temperature and atmospheric pressure. This venting is typically provided in
the form of a
vent pipe extending through the tank roof, usually near the tank wall.
Overflow through storage tank roof vents can be a significant problem,
particularly for oil storage tanks. Occasionally, such storage tanks are
inadvertently
overfilled, due to causes such as human error. A more common problem, however,
is the
phenomenon of "foam-over" that occurs when warm crude oil undergoes a drop in
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temperature, particularly when the ambient humidity is high. Crude oil in
field storage
tanks is typically kept heated by circulating a warm fluid through heating
tubes immersed
in the oil. For various practical reasons, it is common practice to transfer
oil between
field tanks, using tanker trucks that load oil from one tank and pump it into
a tank at a
different location. The oil cools down during transport, with its drop in
temperature
depending on how long the oil is in transit, and also depending on the ambient
outside
temperature (which is a particularly significant factor during in very cold
winter
conditions regularly experienced in Canada and other northern countries). When
the
tanker reaches its destination and pumps the cooler oil into a tank containing
warm oil,
large volumes of foam develop and can rise many feet above the liquid surface
in the
tank. This oil foam commonly builds up to the extent that it spews out of the
tank roof
vents, even when the liquid surface is well below the tank roof.
Foaming can occur even when no oil is being pumped into the tank. A
significant
drop in temperature, particularly when accompanied by high relative humidity,
can be
sufficient to cause a substantial "head" of foam to develop above the surface
of oil stored
in an oil storage tank.
When a storage tank overflows through a roof vent, whether due to overfilling,
foam-over, or some other cause, the liquid or foam (as the case may be)
typically spills
onto the tank roofs, down the tank walls, and onto the ground surface, and may
even
splash over the wall-to-roof junction and outwardly away from the tank. After
such an
overflow, considerable clean-up and/or remediation work is usually required.
First, the
roof and wall surfaces need to be cleaned. Second, even if the gravel fill and
impermeable membrane are in good condition, and thus effective to prevent the
spillage
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from contaminating the subsoil in the vicinity, the affected portion of the
fill usually
needs to be removed and replaced.
These clean-up operations are time-consuming and expensive, in terms of both
labour and equipment requirements. Field tanks are often 30 to 40 feet or more
in height,
S so cleaning the roof and upper wall areas entails the use of man-lifts or
other access
equipment. Such equipment generally is not readily available at the tank site,
and
therefore needs to be transported to the site, adding significantly to the
total cost and
inconvenience of the clean-up. It is also costly to remove and replace
contaminated fill
material around the tanks, not to mention off site treatment and/or disposal
of the fill.
The prior art discloses devices and apparatus directed to the general problem
of
containing overflow from liquid storage tanks. U.S. Patent No. 5,042,519
(Kerlin)
discloses a fuel tank vent having a vent valve. A cylindrical housing is
attached to the
vent of a storage tank and contains a check ball and a float ball. The check
ball and float
ball are in contact with each other, and the check ball is movable to open and
seal a vent
aperture at the top of the housing. During normal operation, vapour from the
tank can
pass around the float ball and the check ball and out the vent aperture. If
the tank is upset
or overfilled, the fuel causes the float ball to move toward the vent aperture
causing the
check ball to do the same. If the overflow reaches a sufficient level, the
check ball seals
off the vent aperture, thus preventing spillage of the fuel.
U.S. Patent No. 5,346,093 (De Benedittis et al.) discloses an apparatus for
containing overflow through the roof vent of a volatile fuel storage tank due
to
temperature-induced expansion of the fuel in the tank, such as in the
particular case
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where the tank is overfilled in the sense that there is insufficient or no
space at all above
the fuel to accommodate expansion. To address this problem, the De Benedittis
apparatus surrounds the tank vent with a collar that forms an overflow chamber
with an
internal weir. The overflow chamber will retain any expansion-induced overflow
that
does not rise above the weir. If the overflow liquid rises above the weir, it
flows into a
drainage channel into a dike formed around the base of the tank. The overflow
chamber
has a cover member with a downwardly curved splash plate disposed beneath it,
with a
vent space between the cover member and splash plate to allow for venting from
both the
tank and the overflow chamber. The splash plate is configured such that any
fuel that
happens to spurt rapidly upward through the vent will be deflected downward
into the
overflow chamber, rather than spilling out through the vent space of the
apparatus.
Although these prior art apparatus and devices provide benefits in certain
situations, they do not fully address the problems discussed previously. The
Kerlin
apparatus is directed primarily to preventing liquid from exiting a tank in
various
circumstances, and does not address the problem of removing excess liquid in
the event
of overfilling. The De Benedittis apparatus provides for removal of liquid
overflowing
through the tank vent. However, since it is directed only to containment of
comparatively
small overflow amounts due to fuel expansion, it does not provide means for
preventing
overflow and spillage from a tank that is being inadvertently and perhaps
steadily
overfilled beyond its volumetric capacity. If a tank fitted with the De
Benedittis
apparatus were exposed to overfill conditions of that sort, and if the flow
into the
overflow chamber were to exceed the flow capacity of the drainage channel, the
overflow
chamber would become flooded and liquid would escape through the vent space
between
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the cover member and the splash plate, leading to clean-up and remediation
problems of
the type previously described.
For the foregoing reasons, there is a need for improved means for minimizing
or
preventing spillage due to overflow of liquid through tank vents, so as to
reduce or
S eliminate the clean-up efforts and expenses that typically result from such
overflow,
particularly for tanks containing crude oil or other potentially hazardous or
dangerous
liquids. The present invention is directed to these needs.
BRIEF DESCRIPTION OF THE INVENTION
In general terms, the present invention is a spill containment apparatus that
may
be mounted to the vent pipe of a liquid storage tank. The apparatus comprises
a canister
which has an overflow outlet, a canister vent opening, and a check valve
assembly
associated with the canister vent opening. The apparatus also has a tank vent
opening
adaptable for substantially liquid-tight mounting over the tank vent pipe. The
overflow
outlet may be disposed at a distance above the base of the canister, so as to
form a
reservoir in the lower section of the canister, for receiving and containing
liquid
overflowing from the tank through the tank vent. If the volume of overflowed
liquid does
not exceed the reservoir capacity, the liquid in the reservoir drains back
into the tank
when the tank is drawn down or emptied. If the volume of overflowed liquid
exceeds the
reservoir capacity, the apparatus diverts the excess liquid through a conduit
to an
overflow collection receptacle, typically located at grade level near the base
of the tank.
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In either event, the overflowing liquid never contacts or contaminates the
exterior
surfaces of the tank roof and walls, or the fill or natural soil in the
vicinity of the tank.
The provision for a reservoir within the canister, as described above, is not
essential to the invention. In preferred embodiments, the overflow outlet is
located as
S close to the base of the canister as possible in order to minimize the
overall height of the
apparatus, thus facilitating access to the canister vent. It is commonly
desired to add
chemicals such as defoaming agents into oil storage tanks, and this is
commonly done by
pouring the chemicals into the tank vent, which is usually accessible to a
worker standing
on a ladder running up the sidewall of the tank. When the apparatus of the
present
invention is fitted to the tank vent, chemicals can poured in through the
canister vent, but
it is preferable to keep the height to the top of the canister vent low enough
that it can be
conveniently reached from the ladder, without having to climb onto the tank
roof.
The apparatus provides the foregoing benefits without impeding the function of
the tank vent during normal operating conditions. The canister vent allows
vapours to
pass from the tank through the canister to atmosphere during non-overflow
conditions. If
overflow liquid in the canister rises above a certain level, a check valve
closes off the
canister vent, thus preventing spillage through the canister vent, and
ensuring that the
overflow passes out of the canister through the outlet only.
Accordingly, in one embodiment the invention is an overflow containment
apparatus for use with a liquid storage tank having a tank vent, said
apparatus
comprising:
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(a) a canister having a top section, a base section, and sidewall means
interconnecting the top section and base section, wherein:
a.l the top section has a canister vent opening;
a.2 the base section has a tank vent opening; and
a.3 the canister has an overflow outlet;
(b) canister mounting means, for mounting the canister to the tank, with the
tank vent in substantially liquid-tight engagement with the tank vent
opening;
(c) a check valve assembly comprising:
c.l float retainer means disposed inside the canister below the canister
vent opening, said float retainer means having openings through
which liquid can flow; and
c.2 a float retained and vertically movable within the float retainer
means in response to fluctuations in liquid level within the
canister, said float being adapted to form a substantially liquid-
tight seal against the canister vent opening when urged upwardly
thereagainst;
wherein the float retainer means is configured such that the float will be
substantially centrally aligned with the canister vent opening when raised
theretoward;
such that:
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(d) liquid entering the canister through the storage tank vent, and rising
above
the bottom of the overflow outlet, will flow out of the canister through the
overflow outlet; and
(e) liquid rising above the overflow outlet and approaching the top section of
S the canister will raise the float and urge it into substantially liquid-
tight
engagement against the canister vent opening.
The canister may be of any suitable geometric configuration. For instance, the
canister may be of rectilinear, box-like construction, with a plurality of
substantially
planar sidewalk. Alternatively, the canister may be of substantially
cylindrical
configuration, with curvilinear sidewalk.
In the preferred embodiment of the invention, the overflow outlet comprises an
open pipe or other suitable conduit extending laterally through the canister
sidewall
means. Preferably the cross-sectional area of the overflow outlet is larger
than that of the
roof vent, in order to prevent back pressure that might impeded flow out of
the canister.
Oil storage tanks commonly have a second roof vent (typically near the roof
peak), in
addition to a first roof vent (typically near the tank wall) to which the
present invention is
most conveniently and preferably mounted. If flow out of the canister fitted
to the first
roof vent is impeded due to pressure build-up, oil foam can readily rise and
spew out the
peak vent. This potential problem could of course be addressed by fitting the
second or
upper roof vent with the apparatus of the present invention. However, if the
overflow
outlet is sufficiently larger than the (first) roof vent to which it is
mounted, oil foam will
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not rise high enough to reach the second roof vent, so a single canister in
accordance with
the present invention will usually be sufficient to prevent foam over onto the
tank roof
and walls. For example, it has been found through field observation that an
overflow
outlet with a diameter of about 10 inches (250 mm) will generally be
sufficient to prevent
undesirable pressure build-up in canisters mounted to tank roof vents having a
diameter
of about 8.5 inches (215 mm).
Also in the preferred embodiment, the overflow outlet is disposed at a
selected
distance above the canister base, such that the portion of the canister
between the canister
base and the bottom of the overflow outlet defines a reservoir, and such that
liquid
entering the canister from the storage tank through the tank vent will firstly
accumulate in
the reservoir section of the canister, and will flow out through the overflow
outlet when
the liquid level rises above the bottom of the overflow outlet (i.e., above
the invert
elevation of the outlet).
The float must be light enough to float upon the liquid stored in the tank,
and
preferably is made from a material having a specific gravity substantially
less than that of
the liquid, so that a substantial portion of the float will project above the
liquid surface
upon which it floats. In applications where foam-over is a concern, such as in
an oil
storage tank, the float must be light enough to float on the foam so that it
will seal against
the canister vent opening so as to substantially prevent passage of foam
thereinto. For
example, the float may be a hollow float with a thin plastic skin, such that
the effective
density of the float substantially less than the effective density of oil
foam.
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In the preferred embodiment of the invention, the canister vent opening is
circular
and the float is substantially spherical. The diameter of the float is
sufficiently greater
than the diameter of the canister vent opening such that the float will
readily seal against
the canister vent opening when pushed up against it by buoyancy forces from
overflow
liquid in the canister, but will not be susceptible to becoming jammed or
stuck in the
canister vent opening when the overflow liquid recedes.
The float retainer means preferably will be a cage-like enclosure suspended
beneath the top section of the canister. Alternatively, the float retainer
means may be a
largely solid-walled enclosure with openings or perforations to permit inflow
of liquid
from the canister. The float retainer means is preferably of generally
cylindrical
configuration and disposed substantially concentrically with the canister vent
opening,
such that the float will tend to be centered on the canister vent opening when
raised
theretoward. Where a spherical float is used, the lower portion of the float
retainer is
preferably rounded to conveniently accommodate the shape of the float.
The apparatus may also include a secondary vent pipe extending upward from the
canister vent opening. In alternative embodiments, the secondary vent pipe may
extend
below the top section of the canister, in which case the bottom of the
secondary vent pipe
will effectively serve as the canister vent opening against which the float
will seal when
the liquid level rises high enough within the canister.
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The canister mounting means may include a threaded fitting, whereby a threaded
connection can be made between the canister and the tank vent pipe.
Alternatively, the
connection between the canister and the tank vent pipe may be made with a slip-
on or
press-fit type of fitting, using suitable gaskets or other appropriate sealing
materials to
provide a substantially liquid-tight connection.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described with reference to the
accompanying figures, in which numerical references denote like parts, and in
which:
FIGURE 1 is a cross-sectional side view of an overflow containment
apparatus in accordance with one embodiment of the present invention,
shown with the canister of the invention partially filled with overflow
liquid from a storage tank vent, and with the liquid being discharged from
the canister through the canister outlet.
FIGURE 2 is a cross-sectional side view of the apparatus of Figure l,
with the canister flooded above the level of the canister outlet, and with
the float engaging the canister vent opening.
FIGURE 3 is a partial elevation of a liquid storage tank with the overflow
containment apparatus of the present invention mounted to the tank vent
pipe, with a conduit fitted to the canister outlet for conveying liquid from
the canister to a collection receptacle.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Refernng to Figures 1 and 2, the spill containment apparatus of the present
invention (generally indicated by reference numeral 10) comprises a canister
20 having a
top section 22, a base section 24, and sidewall means 26 interconnecting the
top section
22 and the base section 24. The top section 22 has a canister vent opening 30,
which in
the preferred embodiment will be circular. Also in the preferred embodiment,
the
apparatus 10 includes a secondary vent pipe 32 associated with the canister
vent opening
30.
As conceptually illustrated in Figures 2 and 3, the base section 24 has a tank
vent
opening 40 with associated for mounting the canister 20 to the vent pipe of a
liquid
storage tank in substantially liquid-tight fashion. The canister mounting
means 42 may
include a threaded, welded, or friction-fit connection means, or any other
known type of
connection for making liquid-tight, non-pressurized connections.
An overflow outlet 50 is provided in association with the sidewall means 26,
for
discharging liquids that accumulate in the canister 20. A conduit 52 may be
connected to
the overflow outlet 50 to convey liquids from inside the canister 20 to a
collection
receptacle (not shown).
A check valve assembly 60 is disposed inside the canister 20 beneath the top
section 22 and generally centered under the canister vent opening 40. The
check valve
assembly 60 comprises float retainer means 62 which retainingly encloses a
float 64. The
float retainer means 62 is may be of any suitable construction that allows for
free flow of
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fluids into and out of the space inside the float retainer means 62. In the
preferred
embodiment, the float retainer means 62 is provided in the form of a cage-like
structure.
Alternatively, the float retainer means 62 may be fashioned with a screen-type
material,
or it may be in the form of a largely solid-walled enclosure having multiple
openings to
allow passage of fluids.
The float 64 is configured and adapted such that it will form a substantially
liquid-
tight seal with the canister vent opening 40 when urged against the canister
vent opening
40. Accordingly, in the preferred embodiment, where the canister vent opening
40 is
circular, the float 64 is spherical, with a diameter greater than the diameter
of the circular
canister vent opening 40. The float 64 has a specific gravity less than that
of the liquid
stored in the tank to which the canister 20 is mounted. The diameter and
specific gravity
of the float 64 are preferably selected such that as buoyancy forces from
liquid
accumulating inside the canister 20 cause the float 64 to rise inside the
float retainer
means 62, the float 64 will seal against the canister vent opening 40 before
the liquid
level reaches the canister vent opening 40, as illustrated in Figure 2.
The overflow outlet 50 may be located at or near the base of the sidewall
means
26, in which case liquid overflowing from the storage tank into the canister
20 will tend
to flow directly out of the canister 20 through the overflow outlet 50.
However, in
alternative embodiments, as illustrated in Figures 2 and 3, the overflow
outlet 50 is
positioned in the sidewall means 26 at a distance above the base section 24
such that
there will be a lower sidewall section 28 below the overflow outlet 50. In
this
configuration, the portion of the canister 20 below the lower sidewall section
28 defines a
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reservoir 29 in which a certain volume of overflow liquid can accumulate
before liquid
begins flowing out through the overflow outlet 50.
The operation and functioning of the overflow containment apparatus 10 may be
readily understood by reference to Figures 1 and 2 along with Figure 3, which
illustrates
a liquid storage tank 70 with sidewall 72, roof 74, and vent pipe 76 disposed
in roof 74.
The canister 20 is secured to vent pipe 76 by means of canister mounting means
42 in
substantially liquid-tight fashion so that any liquid overflowing from storage
tank 70
through vent pipe 76 will flow into canister 20. Overflow conduit 52 is
connected to
overflow outlet 50 and leads to an overflow collection receptacle.
During normal operating conditions, when the liquid level in the storage tank
70
is below the vent pipe 76, float 64 sits at the bottom of float retainer means
62 and away
from canister vent opening 40. Vapours from storage tank 70 can exit storage
tank 70
through vent pipe 76 into canister 20, and then into float retainer means 62
and out to
atmosphere through canister vent opening 40, as indicated by vapour flow
arrows V.
During tank overflow conditions, liquid from storage tank 70 rises into
canister 20
through vent pipe 76. As long as the overflow liquid is at or below level L-1
(i.e., where
overflow outlet 50 meets lower sidewall section 28), the overflow is retained
in the
reservoir section 29 of canister 20. As the contents of storage tank 70 are
drawn down,
the overflow liquid accumulated in the reservoir 29 will flow back into
storage tank 70 by
gravity. If the overflow liquid rises above level L-1, it will begin to flow
out overflow
outlet 50 through conduit 52 to an overflow collection receptacle.
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As the liquid rises further within canister 20, it begins to flow into the
float
retainer means 62 and exerts buoyancy forces on the float 64. If the liquid
level in the
canister 20 continues to rise, it will urge the float 64 (guided by the float
retainer means
62) into substantially liquid-tight sealing engagement with canister vent
opening 40, as
shown in Figure 3. Overflow liquid will continue to flow out of canister 20
through
overflow outlet 50, and thence to the collection receptacle, but float 64
prevents any
outflow of liquid through canister vent opening 40.
In the foregoing fashion, the present invention prevents liquids overflowing
through storage tank vents from spilling onto the tank roof and walls, and
onto
surrounding grade surfaces. The invention provides these significant
operational benefits
without impeding the venting of vapours from the tank during normal operating
conditions.
It will be readily appreciated by those skilled in the art that various
modifications
of the present invention may be devised without departing from the essential
concept of
1 S the invention, and all such modifications are intended to be included in
the scope of the
claims appended hereto.
In this patent document, the word "comprising" is used in its non-limiting
sense to
mean that items following that word are included, but items not specifically
mentioned
are not excluded. A reference to an element by the indefinite article "a" does
not exclude
the possibility that more than one of the element is present, unless the
context clearly
requires that there be one and only one such element.
