Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02196941 1999-07-27
TEM File No. 150.2
DUAL CONTAINMENT ASSEMBLY
F~LT~ OF THF TNVENTTON
The present invention relates to tanks for storing liquid substances, and in
particular to dual wall containment tanks for storing liquid substances used
in the
petroleum, industrial, agricultural and petro-chemical industries.
to
BACKGRO TND OF THF TNVENTION
There are a variety of tanks available for storing various substances both
belowground and aboveground. Existing underground tanks suffer from many
drawbacks,
some of which render such tanks unusable as a result of more stringent modern
day
environmental guidelines and requirements. One drawback is that underground
tanks tend
to leak their contents, whether through corrosion or other causes such as
breaks due to
ground movements. Leaks are typically very difFlcult to detect, and, once
detected, it is
virtually impossible to tell how long the leak has been present. Such tanks
not only
require expensive excavation to install, but require further excavation for
removal, repair
or replacement. Underground tanks are usually replaced rather than repaired
due to
impacts and damage during excavation and repair. Reclaiming a contaminated
site is also
very time consuming and expensive. Underground tanks used in the petroleum
industry
further require beans and dykes which claim a large surface area at well
sites, and make
access to the tank more difficult. Removing product from such tanks is
expensive due to
required use of vacuum trucks rather than less expensive pump trucks.
Aboveground tanks, on the other hand, avoid some of these problems. They do
not require excavation, for instance, but may require berms depending on
whether the tank
is single or double walled. Existing aboveground designs have their own
drawbacks,
particularly in extreme cold (or hot) climates. Keeping stored liquid from
freezing and
3o damaging the tank usually requires more than just thermal insulation but a
heating system
of some sort. A disadvantage of prior art systems is that they focus on
heating the liquid
itself, which can be expensive and impractical. Hence, usage of aboveground
tanks in
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CA 02196941 1999-07-27
such extreme climates is not as popular as it should be, even though from an
environmental standpoint, leak detection is much easier and quicker to detect
than in
undergroundtanks.
What is desired therefore is a novel aboveground tank assembly for storing
liquid
substances which overcomes the limitations and problems of these prior art
tanks.
Preferably it should provide a dual containment assembly where a sizeable air
space
between an inner tank for receiving and storing the liquid and outer tank is
provided to
trap any leakage from the inner tank. A means for readily detecting any leak
should be
provided, and a shut-ofl' mechanism should be available to stop liquid flow
and prevent
overflow of the inner tank. Preferably the novel tank should have another air
space
beneath the outer tank communicating with the first air space, and a heater
for heating
these air spaces whereby the stored liquid is prevented from freezing by the
surrounding
air spaces rather than direct heating of the liquid. An air exchange assembly
should
provide for air communication between air spaces, yet prevent any leaked
liquid in the first
air space from entering the second air space. The assembly should be enveloped
and
effectively isolated from the ambient by an insulating layer. The assembly
should further
provide for storage of more than one type of liquid, and for a means of at
least partially
separating solids from the liquid. It should also provide a vent for the inner
tank which
reclaims at least some moisture from vented gases and returns the moisture to
the
2o assembly.
~i 1MMARY OF THF INVENTION
In a preferred aspect the present invention provides an aboveground
containment
assembly for storing liquids comprising:
an inner tank for receiving and storing said liquids;
an outer tank surrounding said inner tank and forming a first air space
therebetween for trapping any of said liquids which might escape said inner
tank;
a support means beneath said outer tank for providing a second air space
between
the outer tank and a ground support surface;
3o an insulating layer for substantially isolating said inner tank, outer tank
and support
means from the ambient;
a vapour exhaust means for said inner tank;
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pipe means for liquid communication between said inner tank and external
liquid
sources for filling and emptying said inner tank;
measurement means for measuring the amount of liquid in said inner tank and
including a shut-off mechanism for stopping the flow of liquid through said
pipe means to
said inner tank upon said liquid reaching a pre-set level therein; and
a detector accessing said first air space for detecting liquid leakage from
said inner
tank.
In another aspect the invention provides a containment assembly for liquid
storage
comprising:
an outer tank located aboveground;
an upper inner tank for primary liquid storage located within said outer tank
so as
to form a first air space therebetween for containing any egress of liquid
from said upper
tank;
a sub-floor assembly beneath said outer tank for elevating and supporting a
base of
said outer tank aboveground and to form a second air space beneath said outer
tank base;
an external insulating layer for substantially isolating said upper inner
tank, outer
tank and sub-floor assembly from the ambient;
a vapour and pressure exhaust means for said upper inner tank;
inlet and outlet pipe means for filling and emptying said upper inner tank;
measurement means for measuring the amount of liquid in said upper inner tank
and including a shut-off mechanism for interrupting the flow of liquid through
said pipe
means upon said liquid reaching a pre-set level within said upper inner tank;
and
a detector accessing said first air space for detecting liquid therein.
DE RTPTTON OF DRAWIN
Embodiments of the invention will now be described, by way of example only,
with
reference to the accompanying drawings, wherein:
Figure 1 is a perspective view on the exterior of a containment assembly
according
to a preferred embodiment of the present invention;
Figure 2 is an elevation view in cross-section of the containment assembly of
fig. l;
Figure 3 is a sectional plan view in cross-section of the containment assembly
of
fig. l;
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Figure 4 is an elevation view similar to fig. 2 showing a fire tube heating
arrangement according to a second embodiment of the present invention;
Figure S is a sectional plan view of the fig. 4 embodiment;
Figure 6 is an elevation view similar to fig. 2 showing a liquid scrubber type
vent
according to a third embodiment of the present invention;
Figure 7 is a sectional plan view of the fig.6 embodiment;
Figure 8 is an elevation view in cross-section of the vent of fig.6;
Figure 9 is a plan view of the vent of fig.8;
Figure 10 is an elevation view similar to fig. 2 showing a second lower inner
tank
according to a fourth embodiment of the present invention; and,
Figure 11 is a sectional plan view of the fig.8 embodiment.
DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is first made to figures l, 2 and 3 which show a preferred
embodiment
of a containment assembly 10 for storing a liquid substance (not shown) above
grade or
ground level 12. The liquid substance may be any one of a number of liquids
capable of
fluid flow which is encountered in the petroleum industry (such as methanol,
lube oil,
diesel fuel, produced water, etc.), the agricultural industry (such as liquid
fertilizers,
pesticides, etc.), the petro-chemical industry (such as chemicals,
contaminated wastes,
etc.), as well as industrial and recreational sites (such as storage of
potable water and
sewage at camp sites, etc.). The liquid may often contain suspended solids of
some sort.
For illustrative purposes and ease of reference, produced water is used herein
as the fluid
substance. "Produced water" is in essence gas-tainted water which is separated
from
natural gas that is pumped out of gas wells. Such separation is typically done
on site at
the well site. Since the produced water normally contains tons, it must be
safely stored
apart from the gas in a container near the well head for later removal and
treatment.
In the present invention, the produced water is transferred from the well head
to
the containment assembly through one or more first inlet pipes 14. A shut-off
mechanism
connected to an emergency shut-down ("ESD") valve 18 is provided to cut off
flow of the
produced water into the assembly 10. The ESD valve 18 may be located elsewhere
along
the inlet pipes 14, such as outside the shed 60. The shut-off mechanism for
triggering the
ESD valve 18 is described in greater detail later. The produced water passes
from the first
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CA 02196941 1999-07-27
inlet pipe 14 through a second inlet pipe 20 and empties into an inner tank 30
which
receives and stores or holds the produced water. The inner tank 30 has a
substantially
planar circular base plate 32 joined with an upstanding continuous
cylindrically-shaped
side wall 34. The base plate 32 and side wall 34 may be of a unitary
construction, or may
be of a mufti-piece construction attached or bonded in a fluid tight manner.
The mouth of
the second inlet pipe 20 is advantageously located slightly above the bottom
of the inner
tank so that some produced water always remains at the bottom of the inner
tank, even
after draining through outlet pipe 24. The residual produced water slows down
the
pressurized produced water exiting the second inlet pipe 20, thus absorbing
its energy to
1o avoid damaging the inner tank by liquid impact.
The inner tank 30 sits within and is surrounded by a larger outer unit or tank
40 for
trapping or containing any produced water which might escape from the inner
tank 30.
The outer tank 40 has an open top and a substantially planar circular floor 42
sealingly
joined to an upstanding continuous outer shell or side wall 44 which is
coextensive with
the inner side wall 34. A stiffening ring 45 may be provided at or near the
top of the outer
shell 44. The base plate 32 sits on and is supported by the floor 42, while
the outer shell
44 is radially spaced from the inner side wall 34 generally uniformly about
the
containment assembly to form a first air space or void space 46. The air space
46 should
be sized to avoid overflow of produced water out of the outer tank should the
inner tank
2o develop a serious leak. It is accepted practice that the size of air space
46 should be at
least about 110% of the volume of the inner tank 30. In the preferred
embodiment the air
space 46 does not extend between the base 32 and the floor 42, although such a
void may
be created if desired. Should produced water enter and accumulate in the air
space 46, a
number of exit valves 47 are located at the bottom of the air space just above
the floor 42
for purging produced water out of the outer tank.
A base portion or sub-floor 48 beneath the outer tank 40 elevates the tank
floor 42
above grade 12 and creates a second air space 50. The sub-floor consists of a
plate 49 of
suitable material such as metal, which may be placed on level soil or gravel,
as desired.
The sub-floor has numerous individual steel supports 52, allowing for heat
transfer
throughout the sub-floor 48. The outer shell 44 of the outer tank 40 extends
only as far as
the top of the sub-floor, and so a separate circumferential shell 54 envelopes
or encloses
the second air space 50, forming an outer perimeter thereabout. The sub-floor
shell 54 is
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2196~~i
not formed integrally with the outer shell 44 to allow the outer tank to be
lifted or
removed from the sub-floor without having to cut the shell. The outer
perimeter 54 is
pierced by one or more holes 58 adjacent a utility shed 60 which houses, inter
alia, the
inlet and outlet pipes 14, 20 & 24. A heater 62 is located by the hole 58 for
introducing
heat into sub-floor 48 adjacent to the hole which then permeates the entire
second air
space 50. The heater's exhaust vent is indicated at 63.
An important feature of this containment assembly is the provision of a number
of
hollow standpipes 64 circumferentially spaced about the outer tank in the
first air space
46. Each standpipe 64 in the preferred embodiment is a vertically oriented
hollow metal
pipe having a bottom end 66 in air communication with the second air space 50
and a top
end 68 in air communication with the first air space 46. The standpipe 64 is
supported on
the floor 42 and is joined thereto in a fluid tight matter to prevent leakage
from the first to
the second air space 46, 50. An open top end 68 of the standpipe 64 is located
at about
the same height as the top edge 36 of outer side wall 34 to prevent ingress of
produced
water into the standpipe, and consequently into the sub-floor 48, should the
produced
water leak from the inner tank 30 into the first air space 46. The standpipes
64 allow the
heated air in the lower air space 50 to migrate upwardly and into the upper
air space 46,
thus heating the upper air space. The heating is achieved by convection,
namely currents
of warm air entering the upper air space from the standpipes, and by radiant
transfer of
heat from the pipe wall to the inner tank sidewall 34. Four standpipes are
shown for the
preferred embodiment, although it will be appreciated that the number may vary
depending
on such factors as the volume of airspace to be heated. The standpipes are
preferably
evenly spaced circumferentially, but other spacings may be more suitable
depending on
local conditions (for instance more standpipes may be provided on the side of
the
containment assembly from which prevailing cold winter winds are encountered).
It will
further be appreciated that the standpipes need not be straight but could be
curved and
bent into different shapes to enhance distribution of heat circumferentially
about the inner
tank. However, such shaping is not preferred due to increased production cost,
and
because good results have been realized with the straight shapes shown.
The inner tank 30 has a circular shaped roof 70 which is supported on and
connected to the top perimeter of sidewall 34. It is important that the
connection be of
sufficient strength so that the roof and inner tank may be removed as one unit
from the
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CA 02196941 1999-07-27
outer tank by lifting the roof using hook 71 and a crane or other hoisting
means. The roof
is slightly bowed or arched upwardly to encourage and direct any precipitation
to travel
ofl= the roof. A vapour exhaust or vent 72 accesses the inner tank at or near
the roof s
highest point, namely the centre in the fig.2 embodiment. Fig. 2 shows a
standard vent
having a 180 degree bend so that the open exterior end faces downwardly to
discourage
ingress of precipitation or other foreign substances. An alternate embodiment
of the vent
is described later.
A roof mounted blow down vent or port 73 of, say, 8 inch diameter pipe
provides
a means of relieving excessive pressure build-up in the inner tank which may
from time to
to time exceed the venting capacity of the vent 72. In the fig. 2 embodiment,
the flanged lid
74 is slideably located on several long, slender rods or wires (not shown)
extending from
the port's periphery so that excessive inner tank gas pressure will lift the
lid for escape to
the ambient, and then the rods will guide the falling lid back into position
atop the port.
Alternately, the lid 74 may be hinged at one side and have a string, spring or
other means
on the other side for returning an open lid to a properly closed position. A
larger manway
may be located on the roof if desired to provide workers access into the inner
tank for
inspection and/or repairs. A ladder 75 to the port 73 should also be provided
outside the
containment assembly.
The outer periphery of the roof 70 forms an overhanging portion 76 extending
2o radially outwardly from the inner tank sidewall 34 over the upper air space
46 and
terminates just above the outer shell 44 of the outer tank. Preferably the
overhang 76 is
not attached to the outer shell, leaving a small gap 78 therebetween. This
arrangement
allows the inner tank to be lifted out of the outer tank without having to
detach the roof
from the outer tank sidewall.
A utility shed 60 located adjacent the outer tank 40 houses the inlet and
outlet
pipes 14, 20, 24, valves 18, 22 and the heater 62 as shown in figs. 2&3. A
pump may be
provided within the shed to allow transfer of produced water out of the inner
tank 30 by
reversing flow through pipe 20 and discharging the water through outlet pipe
24.
Periodically, the produced water must be drained from the inner tank to a
tanker truck via
3o the control valve 22 and the outlet pipe 24. A detector 26 for detecting
leaks of produced
water out of the inner tank is also located in the shed. In the preferred
embodiment the
detector is a sight glass arrangement which pierces the outer shell 44 to
penetrate the
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CA 02196941 1999-07-27
upper air space for visual leak inspection thereof. Any produced water sighted
in the air
space indicates a leak of the inner tank. In alternate embodiments electronic
sensors may
be located in the upper air space which trigger alarms upon detecting produced
water in
the air space. Such sensors may also be adapted to trigger a shut-off
mechanism, such as
the ESD valve 18, to disrupt flow into the inner tank, as is known in the art.
Another important feature of the invention is a thermal insulation layer or
barner
80 which envelopes or surrounds the containment assembly, including the shed
60, as
shown in figures 1, 2 and 4. The insulation covers and adheres to the outer
shell 44 along
its entire height and over the shell 54 right down to grade 12 so as to
include the sub-floor
l0 48. The insulation extends from the outer shell 44 over the gap 78 and onto
the outside
surface of the roof 70, and so effectively encloses the inner tank 30 as well
as both the
upper and lower air spaces 46, 50. A number of small opening may be made in
the
insulation at the gap 78 if increased air circulation in the air spaces is
needed to vent any
moisture due to temperature condensation or the like. The insulation is not
required on
the portion of the outer shell 44 enclosed by the shed 60 since the insulation
extends about
the periphery of the shed. It will be appreciated that the insulation may in
effect envelope
the shed by constructing it with pre-fabricated insulated panels. The port 73
and the vent
72 are not covered with the insulation 80. In the preferred embodiment,
therefore, the
containment assembly is virtually completely thermally isolated from the
ambient, which is
2o very desireable in cold, hot, or other hostile climates.
The containment assembly preferably incorporates a means for measuring the
level
or amount of produced water in the inner tank 30. Such means can range from
complex
data acquisition equipment employing remote sensing technology, to simple
mechanical
device deployed on-site. Figs. 1&2 illustrate one such simple measuring
assembly
employing an external gauge board 84 with a slideable indicator 86 operatively
connected
by a line 88 through opening 90 to a float 92 within the inner tank. The
volume of product
inside the inner tank is therefore readily shown on the board 84.
The measuring assembly further cooperates with a shut-off mechanism for
disrupting or stopping the flow of produced water into the inner tank. In the
preferred
3o embodiment a pneumatic shut down switch 96 (indicated in figs.l&2) is
located near the
lower end of the gauge board 84. The location is selected so that as the
produced water in
the inner tank reaches a pre-set maximum volume or level, the indicator 86
reaches and
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CA 02196941 1999-07-27
trips the switch 96. The switch in turn activates the EDS valve 18, thereby
stopping the
flow of produced water through the inlet piping 14, 20. The switch is simply a
precautionary measure since the inner tank capacity will be chosen taking into
account the
anticipated maximum flow or production of produced water and the frequency
with which
the tank is to be emptied. It will be appreciated that the switch 96 may be
any one of a
variety of types, such as electro-mechanical, photo-electric, magnetic, etc.
Figures 4&5 show a second embodiment of the invention in which the heater 62
within the insulating layer is substituted by a fire-tube type heater 98
located outside the
insulating layer 80, although the fire-tube heater 98 may be added in addition
to the heater
l0 62 as a back-up heating system if desired. The fire-tube heater delivers
heat directly into
the inner tank 30 and the produced water within. The heater 98 transmits
heated air
within an inlet conduit 100 which extends into the inner tank 30 through both
the outer
shell 44 and sidewall 34 in a sealingly type manner. The inlet conduit extends
within the
inner tank a sufficient distance for the desired transfer of heat thereinto.
The inlet conduit
~s 100 is then looped or extended back out of the inner tank to vent the
circulated air
through an outlet conduit 102 to the ambient. A joining plate 104 provides
access into the
conduits 100, 102 for cleaning and maintenance from outside the insulating
layer 80, and
likewise another joining plate 106 is provided within the first air space 46
so that the
conduits on either side of the plate 106 may be disengaged so as not to
obstruct removal
20 of the inner tank out of the outer tank, should such operation be required.
In a third embodiment of the invention shown in figs. 6-9, the vent 72 is
substituted with a liquid scrubber 108 to recover liquids from gases exiting
the inner tank
and discharging through the scrubber, and to return these recovered liquids
back into the
inner tank. The scrubber has an inlet portion 110 through which the gases
enter the
25 scrubber. Upon entry, the upward gas flow is first intercepted or
obstructed by a concave
plate 112 supported on pillar-like legs 114 which forces the gas flow around
the sides of
the plate 112. The plate's shape forces any liquid condensed thereon to fall
onto a lower
scrubber surface 116 and return into the inner tank via inlet portion 110. The
exiting gas
next encounters two (or more) baffle members 118a, 118b which are angled
upwardly
3o away from the scrubber wall 117, and are arranged in a vertically stacked
relationship on a
central support as shown to form a snaking passage within the scrubber. As the
rising
gases flow through the passage toward an outlet portion 120 for escape to the
ambient or
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CA 02196941 1999-07-27
other closed system for collection, they contact the baffle surfaces which
facilitate or
promote condensation of liquids thereon. Liquids collected on the baffles are
urged by
gravity to trickle down the baffles and through gaps 119a and 119b to the
bottom of the
scrubber and back into the inner tank. A bolted cover 122 atop the scrubber
provides
s interior access for maintenance and repairs.
Figures 10 and 11 show a fourth embodiment of the invention where the sub-
floor
48 and second air space 50 incorporate or house a second or lower inner tank
130. In
figs. 10,11 similar reference numerals designate like features of the
preferred embodiment.
The peripheral sidewall 132 is of a similar diameter to that of the upper
inner tank 30, and
1o a removable metal plate-like lid 134 caps the lower tank 130 to discourage
entry of
foreign matter. The floor 42 of the outer tank 40 sits on and is supported by
lid 134. A
plurality of first supports 136 elevate the lower tank 130 above the ground
plate 49 for
extending the second air space s0 to provide air and heat circulation
thereunder. The first
supports are vertically aligned with corresponding second supports 138
extending through
is the lower tank and engaging the underside of lid 134, thereby transferring
load from the
outer tank 40 to the grade through first supports 136. The underside of the
lid 134 has
clasps (not shown) which engage the second supports to prevent lateral
movement and
keep them vertically alligned. The lower tank 130 may be accessed by removing
the outer
tank 40.
2o In the fig 10/11 embodiment the lower tank 130 is partitioned into two
container
areas 139 and 140 by an upstanding central wall or divider 142 for selective
storage of one
or more liquids. Each container area 139 and 140 has a dedicated set of inlet
and outlet
pipes 143a, 143b and 144a, 144b, respectively, for handling selected liquids.
In this
embodiment the divider 142 also allows selective migration of liquids between
the
2s container areas 139, 140. The divider wall 142 is built lower than the
peripheral sidewall
132, and so as liquid is pumped into, say, the first container area 139, it
may overflow into
the second container area 140 over the divider 142 upon reaching capacity.
Such an
arrangement is particularly useful where the liquid to be stored has a
significant amount of
suspended solids (for example, sewage from a camp site), and it is desired to
at least
30 partially separate the solids from the liquid. In this embodiment, the
solids will have an
opportunity to settle in the first container area 139 while the "clarified"
liquid escapes over
the top of the divider 142 into the second container area 140.
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Figure 11 also illustrates a further alternate version of a heating means
which may
be employed on its own or in addition to the heaters discussed earlier. This
system
employs a continuous pipe 150 located near the bottom of the first air space
40 and
extends circumferentially about the upper inner tank 30. The pipe 150 is
connected to a
source which circulates a heated liquid therethrough to heat the first air
space 46. Such
system is advantageously used where glycol is heated elsewhere on site for use
in another
process. Since the glycol is still warm after use, it may be recirculated
through pipe 150
for efficient use and conservation of energy and reduction of heating costs.
In the preferred embodiment, good results have been achieved using an inner
tank
made of a substantially fiberglass material which is corrosion resistant,
economical and
simple to fabricate, install and replace. A metal outer tank is preferred for
its strength and
puncture resistance. It will be understood that other materials and
combinations thereof
may be employed (eg. both inner and outer tanks made of metal), depending on
local
conditions, economies and material availability. Regardless of material,
however, a
minimum radial distance of at about 1 foot (aprox. 300 mm) should be
maintained
between the outer shell 44 and the inner sidewall 34, to minimize the chances
of having
the inner tank damaged by impact or movement of the outer tank. The 1 foot
spacing also
ensures an upper air space capacity of at least 110% as noted earlier for most
anticipated
tank sizes, except for exceptionally large tank designs where such spacing may
have to be
increased.
Many advantages and benefits of the present invention, as well as its
operation,
may now be better appreciated. First, minimal site disturbance and preparation
is required
as compared to prior art tanks, particularly underground storage tanks. A site
near the
well head must be cleared of obstructions and graded to provide a level ground
surface 12.
Pit excavation (particularly difficult in winter), liners and backfilling is
not required, nor
the laying of gravel or other substrate, unless the soil is of particularly
poor quality. The
entire prefabricated containment assembly is then delivered and placed on the
prepared
surface 12. Once the required external connections are made (eg. inlet pipe 14
must be
connected to the supply for produced water, fuel gas tubing (not shown) must
be
connected to a fizel gas supply, and instrumentation (not shown) is connected
to sensing
units), the assembly is ready for use. The average time for preparing the site
and setting
up the assembly has been found to take less than half a day, and as little as
2 to 3 hours,
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well below that for underground storage tanks (which is normally in excess of
a day).
Likewise, removal of an assembly 10 can be completed in about the same time.
Berms and dykes are not required around the assembly 10 because an integral
outer containment unit 40 is provided, which not only saves
installation/reclamation time
and costs, but does not obstruct access to the assembly 10. This also results
in much less
surface area having to be set aside for the assembly 10 (typically about 120
square feet for
100 BBL tanks) as compared to underground tanks with berms (typically about
300
square feet for 100 BBL tanks).
Easy accessibility to the assembly 10 allows an operator to quickly and
conveniently read the level of produced water in the inner tank from the
external gauge
board, without having to even leave his vehicle. As well, the sight glass 26
is readily
reached for rapid confirmation that there is no produced water in the upper
air space 46,
and so no leakage of the inner tank.
Should produced water be detected in the first air space 46 indicating a leak,
the
assembly's design and above-ground location results in easy drainage of the
outer tank by
gravity through drain 47, thus eliminating the need for using more expensive
vacuum
trucks. Gravity may also be employed for the inner tank. In any event, pump
trucks can
be used for such liquid removal rather than the more expensive vacuum trucks
typically
required for belowground tanks. The former trucks also reduce damage to roads
and the
well site. Upon evacuation of the liquid, the inner tank may then be inspected
and, if
necessary, repaired or replaced by removing it from the outer tank 40. To
remove the
inner tank from the outer tank, a circumferential cut in the insulation layer
80 should be
made at the gap 78 and the roof and inner tank lifted with a crane or the
like. Costly
excavation of the site is avoided, as are the chances of damaging the assembly
with
digging equipment. A new tank may then be reinserted into the outer tank, and
the top
portion of the insulation layer 80 is replaced and bound to the insulation
layer below.
The design of the assembly provides the option of moving and reinstalling the
assembly at other sites. Such re-use is difficult, and generally assumed not
probable, for
underground tanks due to the corrosive effects of soil on metal tanks and the
chances of
damaging the tank when excavating.
The peripheral heating system of the preferred embodiment of the present
invention avoids direct contact and heating of the produced water or other
substances
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21y~~~~
stored within the inner tank, but instead heats the created air spaces
exterior to and
around the tank. Likewise, the means of preventing freezing of the produced
water is not
solely restricted to an insulation layer which by itself may not be adequate
in many
temperature extreme climates. The air space and insulation also act to prevent
evaporation loss of liquid in the inner tank.
As noted above, the present invention may be used for storing various types of
liquid substances, such as Tube oil, glycol and many types of chemicals. In
certain
applications, such as storage of methanol and particular fuels, a steel inner
tank is
preferably used rather than a fiberglass one for fire retardation. In
applications where the
substance being stored produces flammable gases which must be sent to flare or
a V.R.U.
(ie. vapour recovery unit), a "closed" system design should be employed,
namely
processing the gases through the scrubber 108 and then a flare stack rather
than venting
directly to the ambient through the type of vent indicated by 72.
Addition of a sub-base beneath the outer tank 40 provides stiffness and
stability to
the assembly for easy and safe lifting, moving and transport of the assembly,
as well as an
air space for insulation purposes.
The above description is intended in an illustrative rather than a restrictive
sense
and variations to the specific configurations described may be apparent to
skilled persons
in adapting the present invention to specific applications. Such variations
are intended to
form part of the present invention insofar as they are within the spirit and
scope of the
claims below. For example, the heater 62 need not be provided for storage of
methanol
and fizels/chemicals having freezing points which exceed the coldest air
temperatures
expected at a particular site.
30
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