Note: Descriptions are shown in the official language in which they were submitted.
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STORAGE TANK WITH INVENTORY REDUCTION
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to floating-roof storage tanks
used to store
liquid natural resources such as cnide oil, gasoline, or the like. The tanks
may have an open top
or have a fixed roof that provides essentially a weather shield for the
floating roof.
[0002] Floating-roof storage tanks are particularly useful in the oil and gas
industry. Oil
refineries and storage ter-minals use floating roof tanks to store liquid
hydrocarbon products that
have a relatively high vapor pressure, such as gasoline, napthas, and crude
oil. The roof on such
tanks floats on the surface of the stored liquid, minimizing the vapor space
and thus limiting
undesired vaporization of the liquid.
[0003] An ordinary 50-foot tall, 150-foot diameter tank can hold approximately
148,000
barrels of liquid. However, some of the effective capacity of a traditional
floating-roof tank is
wasted in what is called a "heel."
[00041 Floating-roof tanks are commonly filled and emptied in cycles. As
liquid is removed
from the tank, the liquid level in the tank descends. The roof, floating on
the surface of the
liquid, also descends, maintaining a low-vaporization condition. However, tank
appurtenances
such as a mixer, interior piping, or nozzles within the tank generally prevent
the roof of the tank
from descending all the way to the floor of the tank. Continuing to empty the
tank after the roof
has descended to its lowermost operating position would create a vapor space
between the roof
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and the surface of the liquid, significantly increasing vaporization of the
liquid and creating a
potentially combustible atmosphere. Consequently, emptying operations are
generally stopped
when the roof reaches its lowermost operating position, and a significant
volume of liquid
remains in the tank. This volume of liquid is the heel.
[00051 In an ordinary 150-foot diameter tank, the lowermost operating position
of the roof
may be four or five feet above the floor of the tank, resulting in a heel of
10,000-15,000 barrels
of liquid. Because that volume of the liquid is not emptied during normal
operations, that heel
represents a significant loss in effective working volume of the tank, and
also represents a
significant inventory of product not used in normal operations.
[0006] There have been efforts to address this problem. Some builders have
sloped the floor
of the tank. However, sloping the floor generally eliminated only 10-30% of
the heel. U.S.
patent no. 4,957,214 proposed mounting a volume-occupying container on the
bottom of the
floating roof or placing a layer of ballast or relatively-dense liquid in a
dam on the floor of the
tank. While this dam arrangement could provide significant heel reduction
(eliminating more
than 50% of the heel), it made maintenance of the tank more difficult. For
example, in order to
check for leaks beneath the ballast, the ballast would have to be removed.
BRIEF SUMMARY
[0007] The applicant has found a solution that can provide significantly more
heel reduction
than a sloped floor and can be significantly easier to maintain than the kind
of dain arrangement
suggested in the '214 patent.
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[0008] Like prior known devices, the new storage tank has cylindrical walls
and a roof that
floats on the surface of liquid stored in the tank. Unlike prior known tanks,
the new tank has an
articulated floor with an annular outer portion, a raised inner portion, and
an intermediate
transition portion that is narrower in width than the raised inner portion.
Liquid-tight seams (for
example, welded seams) connect the intermediate transition portion of the
floor to the outer
portion and to the inner portion.
[0009] Generally, the raised inner portion of the floor will be at least 6"
and no more than 60"
above the outer portion of the floor. The raised inner portion will generally
be made primarily of
steel that is at least 1/8 of an inch thick and no more than'/4 of an inch
thick. In some
embodiments of the invention, the raised inner portion of the floor may
support the roof in a low
position. The inner portion of the floor may also be sloped toward the
intermediate transition
portion of the floor.
[0010] The inclusion of the new transition portion of the floor helps to
maximize the heel
reduction. In floating roof tanks, the roof may have an outer, annular section
that has a bottom
surface with depending pipe elements. In the new arrangement, these elements
may be
positioned so that they fit around the outside of the intermediate transition
portion of the floor.
[0011] Because the raised inner portion and the intermediate transition
portion of the floor are
sealed together, liquid stored in the tank does not enter the space beneath
the raised inner portion
of the floor. Consequently, when the tank is filled, the raised inner portion
and the intennediate
transition portion of the floor may be subjected to significant differential
pressures. To handle
these pressures, the volume under the raised portion of the floor may be
provided with a solid fill
(such as a structural granular fill or concrete), with a metal support
structure, or, if the outer
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portion of the floor is part of a continuous floor that extends beneath the
raised portion of the
floor, with a liquid fill.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may be better understood by referring to the accompanying
drawings, in
which:
[0013] FIGS. 1 a and 1 b are schematic side views of two floating roof tanks
that utilize the
new arrangement. The tank in fig. 1 a is an open-top tank, and the tank in
fig. 1 B is a fixed-roof
tank.
[0014] FIG. 2 is an enlarged, fragmentary view of the lower outer portions of
the tank seen in
fig. 1 a.
[00151 FIG. 3 is a top plan view of the tank, with the floating roof removed.
DETAILED DESCRIPTION
[0016] The tank 10 seen in fig. I a has a cylindrical wall 12, a floating roof
14, and a floor 16.
The tank can be built on any conventional foundation, such as a foundation
ringwall 18 (fig. 2),
slab, berm, etc. The tank seen in fig. lb is similar, but also has a fixed
roof 19.
[0017] The wall 12 of the tank can vary from 10 feet to 90 feet in height, and
can be made of
any suitable material. In many cases, the wall can be made of 3/16" to I 3/4"-
thick steel plates.
100181 The floating roof 14 is positioned within the tank wall 12, and floats
on the surface of
liquid stored in the tank 10. The illustrated floating roof is a pontoon roof,
and the invention can
also be used with other conventional floating roofs, including pan roofs. The
roof can be made
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of a wide range of materials, including steel, aluminum, composite material,
or other non-
metallic material.
[0019] The illustrated roof 14 has an outer, annular section 20 that has a
bottom surface 22.
Elements such as a drain or a vent may hang below this illustrated annular
section. The drain or
vent can be used to drain liquid off the top of the floating roof.
[0020] As best seen in fig. 2, the floor 16 is articulated, having an outer
portion 30, an
intennediate transition portion 32, and a raised inner portion 34 that are
joined by liquid-tight
seams 38. The various portions of the floor can be made of conventional
materials, such as steel
plates or structural members.
[00211 The outer portion 30 of the illustrated floor 16 can be part of a
continuous floor that
extends beneath the raised inner portion 34 of the floor. Alternatively, the
outer portion of the
floor can terminate at or just inside the lower edge of the intennediate
transition portion 32 of the
floor, as seen in fig. 2 and explained in more detail below. When desired, the
outer portion of
the floor can incorporate a double-bottom configuration or an underbottom
liner.
[0022] The illustrated raised inner portion 34 of the floor 16 is made of'/4"
steel plate and is
positioned about 36" above the outer portion 30 of the floor, creating an
inventory-reduction
compartment 40 beneath the raised inner portion. Because the liquid-tight
floor keeps stored
liquid out of the inventory-reduction compartment, niaintenance is simplified.
[0023] The raised inner portion 34 of the floor 16 can support the roof 14 in
the low position
seen in fig. 1. In fixed-roof tanks, such as the one seen in fig. lb, the
raised inner portion can
also serve as a base for a column 41 used to support the fixed roof. The
illustrated inner portion
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of the floor slopes outwardly toward the outer portion 30 of the floor, rising
approximately 1"
every 10 feet of run. Other configurations can be used. For example, the inner
portion of the
floor can sometimes be as thin as 1/8 of an inch thick or as thick as 3/4 of
an inch thick. It can
sometimes be positioned as little as 6" or as much as 5 feet above the outer
portion of the floor.
It can sometimes be flat or can sometimes slope as much as 6 inches for every
10 feet of run.
[0024] The intermediate transition portion 32 of the floor 16 connects the
outer portion 30 and
the inner portion 34 of the floor, forming the lateral boundary of the
inventory-reduction
compartment 40. The illustrated transition portion of the floor is vertical,
and is spaced
relatively close to the wall 12 of the tank. Preferably, the transition
portion of the floor may be
within 10 feet of the tank wall, but far enough from the tank wall to enable
pipe elements, etc.,
that may hang down from the annular section 20 of the roof 14 to fit around
the transition portion
32. Although other arrangements could be used, this vertical arrangement of
the transition
portion of the floor optimizes the heel-reducing volume of the inventory-
reduction compartment.
[0025] The heel is the volume of the tank below the low operating level of the
roof. The heel
reduction provided by the invention can be measured by comparing the heel of
the product to the
heel that would exist if the inventory-reduction compartment 40 were omitted,
as in a
conventional tank. Above the horizontal plane 50 that extends through the
raised inner portion
of the floor 20, the volume of the illustrated tank is the same as a
conventional tank. Below that
plane, however, the volumes differ. While a conventional tank would have a
heel that
encompasses the entire volume below the plane 50, a tank utilizing the
inventory reduction
chamber has a reduced heel volume 52 that is equal to the difference between
the corresponding
volume of a conventional tank and the volume of the inventory reduction
chamber. Preferably,
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the volume of the heel reduction compartment is at least twice the volume of
the reduced heel
volume. This provides significant advantages over previously- known designs.
[0026] The transition portion 32 of the illustrated floor 16 takes the form of
a step wall that is
made of steel plate or a stnictural member that is welded to a step baseplate
42 on the outer
portion 30 of the floor. When the outer portion of the floor is part of a
continuous floor, this step
baseplate may not be necessary.
[0027] Although the articulated floor 16 offers significant advantages in
simplified
maintenance and heel reduction, the arrangement can present a special design
challenge.
Because the volume beneath the raised inner portion 34 of the floor is not in
fluid
communication with the liquid stored in the tank, the intermediate transition
portion 32 of the
floor and the raised inner portion 34 of the floor become subject to
differential hydraulic
pressures when the tank is filled. Thus, special support for the inventory-
reduction compartment
40 may be needed.
[0028] The support can be provided in a variety of ways. Structural granular
fill or concrete
can be provided beneath the inner portion 34 of the floor 16, as seen in the
figures. A metal
support structure including, for example, conventional structural members such
as I-bearns, can
also be used to support the floor. Alternatively, if the outer portion 30 of
the floor is part of a
continuous floor that seals the bottom of the inventory-reduction compartment
40, the inventory-
reduction compartment can be filled with liquid, such as water, to provide
support.
[0029] This description of various embodiments of the invention has been
provided for
illustrative purposes. Revisions or modifications may be apparent to those of
ordinary skill in
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the art without departing from the invention. The full scope of the invention
is set forth in the
following claims.
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