Note: Descriptions are shown in the official language in which they were submitted.
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TITLE. Method and system for reducing and preventing emissions from liquid
storage
tanks.
The invention relates to a system for use interiorly in a storage tank, said
storage tank
being suitable for storing a liquid load, the system comprising: at least one
flexible
liquid holder for containing the liquid load.
Large storage tanks for liquid products, like petroleum and chemical products,
usually are of the type "fixed" of "floating" roof tanks.
The "fixed roof" storage tanks have a fixed interior volume. Said storage
tanks
may be located upon solid base ground like in a storage yard or may be located
upon
a floating device like a ship, which is arranged for transportation of
liquids; or like a
tanker; or like a so-called FPSO (Floating Production, Storage and Offloading)
provided
with storage compartments. Generally the tank volume is partially filled with
a liquid
product, the remaining part of the volume being filled with ambient air. This
ambient
air will saturate with vapour resulting from the liquid, until the vapour
pressure,
corresponding to the temperature in the tank, is reached. Furthermore
evaporation is
determined by the so-called breathing of the tank, whereby saturated vapour is
discharged out of the tank and clean air from outside is drawn in into the
tank. While
filling this type of storage tanks containing liquid products, the air present
in the tank,
comprising vapour from the liquid, will be vented from the tank and emitted to
the
ambient air, unless suctioned in and recycled.
The "floating roof" storage tanks have a variable volume. The roof of the tank
floats
upon the liquid contained in the tank, so that usually, as a result, no free
space is
present within this type of tank. When the liquid is pumped out of the tank,
the roof
will move down until it reaches a mechanically determined minimum height;
usually
about 1,5 meter above the base floor of the tank, whereby the roof is
supported upon
legs mounted onto the underside the roof. When the tank is further drained,
the free
space between the roof, supported on its legs, and the liquid is filled with
ambient air.
This air will also get saturated with vapour resulting from the liquid until
the vapour
pressure, corresponding to the temperature in the tank, is reached. While the
tank is
refilled with the liquid product, the air, saturated with vapour, will be
vented to
atmosphere outside of the tank through the so-called "rim seals". This vapour-
air
mixture generally has a higher specific weight than ambient air and remains
therefore
usually within the cylindrically shaped upright wall, which e.g. may lead to
so-called
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"rim fires": fires originating from lightning strokes on the tank, thereby
causing
ignition of said vapour-air mixture.
Besides vapour-phase emissions also emissions of the liquid products from both
type
of tanks may occur. Generally these emissions are caused by small scale
leakages like
e.g. not thoroughly closed flanged connections or leaking valves. The liquid
losses
caused by small leakages generally disappear into the base ground nearby the
tank.
Large losses will occur as a result of the failure of the wall of the tank.
Each tank, is
therefore surrounded by a so-called "bundwall". This is a dike around the
tank,
whereby the height of the dike and the interior surface are such dimensioned
in that
the holding capacity for the amount of liquid corresponds to the maximum tank
volume. Finally, subsequent to pumping the major part of the liquid out of the
tank,
the remaining liquids are lost during cleaning of the tank.
From DE 10 2004 009398 and US 3,537,608 is it is known to store liquids into
compartments of flexible liquid holders, so that cleaning of the storage tank
is not
necessary when changing the stored liquid and so that vapour formation in the
free
space is reduced. However, within these flexible liquid holders a gas or
vapour phase
is formed, for example by evaporation of liquid or gas expelled from the
liquid, like
with partially degassed crude oil extracted from bored wells. Due to this
effect the
flexible liquid holder is further inflated by the gas or the vapour phase, so
that after
draining the liquid holder by removal of the liquid phase, a substantial
amount of gas
or vapour is retained within the flexible holder which may contminate the next
liquid
charge or may lead to undesired emissions and/or loss of product or to costs
for
cleaning.
Moreover the presence of (large) amounts of inflammable vapours and gasses is
undesirable because of the danger of fire or explosion.
From Dutch Patent 1030882 in the name of the applicant, an inflatable element
is
known (also indicated as airbag) for use interiorly in a holder, the holder
being
suitable for storing and/or transporting of a liquid load, in particular for a
holder of a
transport or storage device like a tank lorry, a tank container or a storage
tank. The
inflatable element may be adjusted to the free space above the liquid in the
holder, so
that both an anti-slosh device and a reduction of the vapour space is
realized.
Therefore there is a need for a system whereby continuously and reliably, at
low
costs, with a considerable reduced risk of fire or explosion, with a minimum
chance on
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failure and without any effect upon the quality of the stored liquid, the
storage tank
may be operated in such a way that vapour or gas formation in the liquid
holders,
located inside the storage tank, is avoided.
The object of the invention is therefore to provide a system whereby vapour
and gas formation by liquids present in the tank, vapour and gas emissions to
the
environment and also uncontrollably and undesirably expelling of liquids from
the tank
during filling and emptying is prevented, and whereby the risk of fire and/or
an
explosion is reduced and a dike or 'bundwall' is superfluous for retaining the
liquid
product or liquid load in case of failure of the storage tank wall or of
failure of the
holder of the liquid products.
This object of the invention is achieved by a system according to the
preamble; the
system further comprises: at least one inflatable element, suitable for
containing an
inflating gas.
By combining the liquid load in the liquid holder with an inflatable element
the gas and
vapour formation is reduced, formation of a gas or vapour phase in the liquid
holder is
prevented as a result of inflating the inflatable element by means of the
inflating gas.
In a preferred embodiment the inflatable element is provided with an accordion
wall or otherwise extendable or unfoldable wall or walls, which unfold or fold
up
respectively by supplying or discharging inflating gas.
These measures allow for the volume of inflatable element to be varied easily
without
the need of applying a high pressure within the inflatable element.
Preferably the flexible liquid holder comprises flexible walls and/or is
provided
with an accordion wall or otherwise extendable or unfoldable wall or walls,
which
unfold or fold up respectively by supplying or discharging the liquid load.
These measures enable that the volume of the liquid holder may be varied
easily
without the need of bringing the liquid load to a high pressure.
Particularly the inflatable element and/or the liquid holder are configured
out of
two or more elements or are provided with two or more compartments.
As a result thereof large elements or holders for large storage tanks may be
manufactured easily and at low costs.
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The invention also provides for an inflatable support element provided
interiorly
within the liquid holder, which support element, in its inflated use position,
being
suitable as support for the roof of the storage tank or as support for the
inflatable
element.
These inflatable support elements, provided inside the liquid holder, occupy a
portion
of the liquid holder so that the roof or an inflatable element located upon
the liquid
holder, may be supported and inflow of ambient air is prevented or strongly
reduced.
In an alternative embodiment the liquid holder, positioned within the storage
tank, on its upper side is not provided with a wall, so that as a result the
inflatable
element rests upon the liquid surface of the liquid load.
In this embodiment of the liquid holder, being open at the upper side and
whereby the
upper side is covered by the inflatable element, the free space above the
liquid - and
therewith the formation of vapour and gas - is reduced by the inflatable
element.
In particular means are provided for adapting the volume of the inflatable
element and/or the support element when the volume of the flexible liquid
holder
changes.
Preferably the means are selected from: vacuum en overpressure means; a
pressure or level controlled air system with air pump or compressor or other
overpressure air system; safety, warning or registration system for liquid
present
between the flexible liquid holder and the wall of the storage tank.
Therewith the formation of vapour and gas in the liquid holder is decreased
and
vapour emissions and uncontrollable and undesirable expelling of liquids from
the
storage tank, during filling and draining, is prevented.
Particularly the roof of the storage tank is a floating roof, and that the
inflatable
element, at its roof side, is provided with a shape corresponding to the
interior shape
of the roof.
Preferably the inflatable element, with the therein contained inflating gas,
is
mounted upon the upper side of the flexible liquid holder such that in the use
position
within the storage tank essentially the inflatable element abuts against het
roof of the
holder and the flexible liquid holder essentially abuts against the base of
the storage
tank.
The inflatable elements closely adjacently link up against the roof in order
to prevent
vapour and gas formation, while the liquid holder closely abuts against the
base
preventing emission and leakage of liquid.
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In an alternative embodiment the inflatable element and the flexible liquid
holder each are provided with connection means, such as inflow en outflow
means,
with which inflating gas and liquid load respectively, without contacting the
storage
5 tank or the tank wall, are supplied in or out of the storage tank.
In particular the inflatable element only occupies a portion of the volume
difference between the volume of the storage tank and the volume of the liquid
holder.
By means of the connection means the liquid holder and the inflatable element
may be
filled and drained independently with respect to each other.
As a result, if required, the system may be operated in such a way, that the
liquid
holder and the inflatable element together will not completely fill up the
entire volume
of the storage tank.
The invention also relates to a method for supplying and discharging of liquid
load to a
storage tank which is provided with a system according to the invention
comprising
the following steps:
- supplying and discharging of liquid load to the flexible liquid holder such
that
the volume changes of the liquid holder,
- registration of the volume change of the liquid holder,
- supplying and discharging inflating gas into the inflatable element, such
that
the volume change in the liquid holder is compensated.
In particular the method further comprising the following step:
supplying or discharging of inflating gas into of from the inflatable element
and/of the
inflatable support element for displacing the roof of the storage tank.
The invention also provides for an optional addition, which measures,
registers or
reports the possible presence of liquid between the flexible liquid holder and
the tank
wall. This addition may optionally be connected to a safety or warning system
of the
tank terminal, where the storage tank is a part of.
The invention also provides for the possibility to let the upper side of the
inflatable
element function as the roof of the tank, such that this upper side of the
inflatable
element is always retained at the same position, identical to the roof of a
"fixed roof"
tank and in which the remaining part of the inflatable element is adaptable to
a variety
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of shapes so that free air is not available in the tank, or in other words,
the entire
volume of the tank is filled by the liquid and the inflatable element.
The invention takes care for both considerably decreasing the possibility of
the
creation and the consequences of a fire or an explosion and for an alternative
configuration during a catastrophe with two independently functioning flexible
holders
within the storage tank: an inflatable element and a liquid holder, whereby
the current
storage tank outside wall constitutes the outer boundary. As a result it is
possible to
build and operate the storage tanks closer to each other in a safe way thereby
using
less ground for the storage of a certain amount of liquid.
The invention is further explained by means of a drawing and some embodiments
of
the method and system having an inflatable element and the flexible liquid
holder
whereby features and other advantages will come forward.
Fig. 1 shows a perspective view of a storage tank having a "floating roof",
provided
with a system according to the invention,
Fig. 2 shows a perspective view of a storage tank with a "fixed roof" provided
with the
system according to the invention,
Fig. 3 shows a perspective view of an "open" storage tank provided with the
system
according to the invention.
Figure 1 shows a storage tank 2 with a roof 6, which may vertically displace
within the storage tank (floating roof), provided with inflatable support
elements 10
within the liquid holder 1. On the right side of the storage tank 2 is shown
liquid
holder 1 wherein liquid load 3 is contained. The liquid holder 1 may be
provided with a
flexible wall 8 and/of an accordion wall 7 or otherwise extendable or
unfoldable wall so
that the volume of the liquid holder may be easily, without the need of
applying a high
internal pressure, be enlarged or made smaller when the liquid load 3 is
introduced
into or out of the liquid holder, respectively. When the storage tank 2
empties, the
roof 6 will move downwards. In order to prevent that the roof 6 will rest upon
the
ground base 16 of the storage tank 2 and in order to reduce the free space
within the
liquid holder, inflatable support elements 10 are provided within the liquid
holder 1. In
the left part of the storage tank 2 in Fig.1 the liquid holder 1 is omitted so
that the
inflatable support elements 10 are visible as support of the roof 6. The
inflatable
support elements 10 may also be used as support for an inflatable element 11
(see
Fig.2) which is mounted on top of the liquid holder 1.
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Figure 2 shows a storage tank 2, having a roof 6' which cannot be vertically
displaced within the storage tank (fixed roof), provided with an inflatable
element 11
and a liquid holder 1 according to the invention. In this embodiment the
liquid holder
1 may be open on its upper side or constitute a completely enclosed space.
Both the
liquid holder 1 and the inflatable element 11 may be provided with an
accordion wall 7
respectively 12. The inflatable element 11 and/or the liquid holder 1 may each
comprise one space, but may also be formed from two or more elements and/or be
provided with two or more compartments. In order to prevent formation of
vapour or
gas within the storage tank 2 and/or within the liquid holder 1, the
inflatable element
11 and the liquid holder 1 together are exactly filling up the internal volume
of the
storage tank 2. When the liquid load 3 is introduced into the liquid holder or
when gas
or vapour formation occurs in the liquid holder 1, the volume of the liquid
holder will
increase so that as a result the volume of the inflatable element 11 must
decrease and
consequently the amount of inflating gas 13 must be reduced. When liquid load
3 is
removed from the liquid holder 1 or the amount of vapour or gas within the
liquid
holder 1 decreases, the volume of the inflating gas 13 within the inflatable
element 11
consequently must be increased. Therefore for compensation, the system
according to
the invention provides for means by which the volume of the inflatable element
11
may be changed oppositely, when the volume of the flexible liquid holder 1
changes.
These means may be a warning or registration system, whereby for example the
pressure in the liquid holder 1 and/or the inflating element 11 is measured.
Other
suitable measurement methods are: a level measurement or a weight measurement
of
the liquid load or a pressure measurement of the walls with a resistance
strain gauge
or similar. Furthermore a flow-measurement may be provided at the connection
means 9, 17. After the volume change of the liquid holder 1 is measured and/or
registered, the control part of the system will charge or discharge inflating
gas 13 to
or from the inflatable element 11, such that the volume change within the
liquid
holder will be compensated. The charging or discharging may be executed by
means
of a feedback loop, but may also be executed by means of a calculation of a
certain
amount of required volume. Because of the explosion, safety and vapour
requirements, the control part of the system may be set up in such a way, that
the
volume of the storage tank 2 is continuously completely filled up, but if
required may
also be set up, such that the liquid holder 1 and the inflatable element 11
only occupy
a portion of the volume of the storage tank 2. The system may comprise vacuum
or
overpressure facilities and an air system comprising an air pump or compressor
15.
In a preferred embodiment the inflatable element 11 on its upper or roof side
is
provided with a shape corresponding to the interior shape of the roof. In
Fig.2 both
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the roof 6' and the inflatable element 11 are provided with a conical shape.
As a result
the inflatable element 11 closely links up with the roof 6' so that
deformation of the
inflatable element 11 or creation of free spaces between the inflatable
element 11 and
the roof 6' is prevented. The corresponding shape of a roof 6 (like in Fig.1)
has the
advantageous effect of a uniform distribution of the forces acting upon the
roof, so
that the inflatable element 11 has a small chance on deformation and/or
leakage and
moreover the roof will be uniformly supported over its entire surface and can
be
moved vertically.
The liquid holder 1 in Fig.2 may be open at its upper side, so that the
inflatable
element 11 rests upon or is supported by the liquid surface of the liquid load
3. The
liquid holder rests upon or is supported by the base ground 16 of the holder
2.
Because the inflatable element 11 is in contact with the liquid load 3 the
vapour and
gas formation above the liquid is prevented. Preferably the liquid holder 1 is
a
completely enclosed space so that as a result the liquid load 3 does not come
in
contact with the walls of the storage tank 2 and the walls of the inflatable
element 11
so that contamination of the inflatable element 11 and/or the walls may not
occur.
Preferably the system is provided with a supply system 15, 17 for a gaseous
product (also inflating gas 13), usually ambient air, which is available at
the storage
tank 2 location by means of an air or gaseous supply system, or otherwise by
means
of a facility directly coupled with inflating element 11 like an air or gas
compressor or
pump 15 or a high pressure storage vessel. The supply system consists of a
discharge
system for the gaseous product comprising a direct exhaust facility when
applying
ambient air or comprising a system wherein cleaning facilities or other
possibilities are
provided for reuse or another responsible method for removal of the gaseous
product.
In its basic configuration the system is pressure controlled. When filling the
storage
tank 2 or the holder with the liquid load 3 by means of connection 9 the
pressure
within the inflatable element 11 will rise so that the pressure controlled
system,
comprising the inflatable element 11, will discharge gaseous filling out of
the inflatable
element 11 by means of connection 17 in such a way that the otherwise
resulting
pressure increase is neutralised. During the removal of liquid load 3 from the
storage
tank 2 or from the holder of the liquid load 3, the pressure within the
inflatable
element 11 will decrease so that the pressure controlled system, comprising
the
inflatable system 11, will supply gaseous product into the inflatable element
11 in
such a way that the otherwise resulting pressure drop is counter acted and
neutralized. In case of a gas or vapour formation in the liquid holder 1 the
pressure
within the inflatable element - for the reduction of the formation of this
vapour or gas
formation - is set up at a small overpressure, for example between 0 - 1 bar
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overpressure and preferably the pressure is selected from the range 0 - 0.2
bar
overpressure.
Figure 3 shows a storage tank 2 without roof having an inflatable element 11.
In this embodiment the inflatable element 11 functions as the roof of the
'open'
storage tank 2. With an "open" liquid holder 1 the bottom side 18 of the
inflatable
element rests upon the liquid surface of the liquid load 3. In a preferred
embodiment a
closed liquid holder 1 is provided whereby the bottom side of the inflatable
element
also rests upon the upper side of the liquid holder 1.
The inflatable element of the liquid holder may be manufactured from (a
combination
of) a suitable material like a plastic or metal.
The storage tank may be positioned upon a solid base ground, but may also be
part of
a floating object like a tanker of a floating storage tank.
