U.S. patent number 3,670,517 [Application Number 04/732,009] was granted by the patent office on 1972-06-20 for apparatus for cooling and filling liquefied gas transport and storage tanks and improvements in said tanks.
This patent grant is currently assigned to John J. McMullen. Invention is credited to Ernst A. Nonnecke.
United States Patent |
3,670,517 |
Nonnecke |
June 20, 1972 |
APPARATUS FOR COOLING AND FILLING LIQUEFIED GAS TRANSPORT AND
STORAGE TANKS AND IMPROVEMENTS IN SAID TANKS
Abstract
A transport tank for carrying liquefied gas at about ambient
pressure including free-standing inner and outer tank walls formed
of material for withstanding low temperatures so that the outer
tank acts as a secondary system in the event the inner tank fails
and a plurality of interconnecting members coupling the inner tank
wall to the outer tank wall so as to limit the relative movement
therebetween but provide 2.degree. of freedom within this limit.
Stress members secured to the inner and outer tank walls are
provided to assure more uniform thermal tank growth in the vertical
direction.
Inventors: |
Nonnecke; Ernst A. (Hamburg,
DT) |
Assignee: |
McMullen; John J. (Montclair,
NJ)
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Family
ID: |
27211794 |
Appl.
No.: |
04/732,009 |
Filed: |
May 15, 1968 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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440081 |
Mar 16, 1965 |
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Current U.S.
Class: |
62/53.2;
220/560.05; 114/74A; 220/901 |
Current CPC
Class: |
F17C
3/025 (20130101); B63B 25/16 (20130101); F17C
6/00 (20130101); F17C 3/10 (20130101); F17C
2223/0161 (20130101); F17C 2270/0105 (20130101); Y10S
220/901 (20130101); F17C 2221/035 (20130101) |
Current International
Class: |
F17C
6/00 (20060101); F17C 3/02 (20060101); F17C
3/00 (20060101); F17C 3/10 (20060101); B63B
25/16 (20060101); B63B 25/00 (20060101); F17c
007/00 () |
Field of
Search: |
;62/45,55 ;220/15
;114/74 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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627,267 |
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Sep 1961 |
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CA |
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1,263,560 |
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May 1961 |
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FR |
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1,354,617 |
|
Jan 1964 |
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FR |
|
Primary Examiner: Perlin; Meyer
Parent Case Text
CROSS REFERENCE TO ANOTHER APPLICATION
This application is a Streamlined Continuation of application, Ser.
No. 440,081, filed Mar. 16, 1965.
Claims
I claim:
1. In combination, a tanker having a cargo hold of predetermined
shape and an insulated container for liquefied gases maintained at
atmospheric pressure and cryogenic temperature mounted within said
hold, said container including:
a. a primary tank substantially similar in shape to said hold,
b. a larger secondary tank substantially similar in shape to said
hold and surrounding said primary tank,
c. thermally conductive rigid structural means for maintaining said
primary tank in fixed, spaced relation within said secondary tank
and for reducing the thermal gradient between said primary and
secondary tanks when said liquefied gases are initially loaded into
said container and during transportation of said gases in said
tanker,
d. said tanks providing primary and secondary liquid-tight barriers
for retaining cryogenic liquefied gas cargo at atmospheric
pressure,
e. foam insulation means applied externally and secured about said
secondary tank in an enveloping relation therewith, said insulation
means constituting substantially the sole insulation means of said
container,
f. a longitudinal bulkhead within said container dividing said
container in substantially liquid-isolated port and starboard
portions,
g. a transverse bulkhead dividing each of said port and starboard
portions into fore and aft portions, and
h. separate submerged pump means located in each of said port and
starboard portions for pumping liquefied gases from said
container.
2. The combination of claim 1 including cooperative key and keyway
means between the exterior of said container and the interior of
said cargo hold to permit relative movement therebetween due to
thermally induced changes in the dimensions of said container.
3. In combination, a tanker having a cargo hold of predetermined
shape and a container for liquefied gases maintained at atmospheric
pressure and cryogenic temperature mounted within said hold, said
container including, an internal longitudinal bulkhead dividing
said container in substantially liquid isolated port and starboard
portions, a transverse bulkhead dividing each of said port and
starboard portions into fore and aft portions, a plurality of
apertured webs extending from side to side of said container, a
primary tank substantially similar in shape to said hold and
secured to the periphery of said webs and periphery of each said
bulkhead, a plurality of stiffening structural members secured to
the exterior of said primary tank for rigidifying said tank, and a
larger secondary tank substantially similar in shape to said
primary tank secured to said structural members in spaced relation
to said primary tank, said structural members in combination with
each said bulkhead and said webs providing a common structural cage
for said primary and secondary tanks, and insulation means secured
to the outer surface of said secondary tank.
4. The combination of claim 3 including cooperative key and keyway
means between the exterior of said container and the interior of
said cargo hold to permit relative movement therebetween due to
thermally induced changes in the dimensions of said container.
5. A tank arranged in a vessel for transporting at extremely low
temperatures a liquefied gas such as methane and the like, said
tank comprising an inner wall and an outer wall spaced from and
enclosing said inner wall and defining a wall space therebetween,
said inner and outer walls being corrugated and having undulations
arranged so that the undulations projecting towards the wall space
are aligned and the undulations projecting away from the wall space
are aligned, a plurality of girders connected to said inner wall
and each having an inner edge spaced inward from said inner wall,
each said girder extending through said inner wall and having a
terminating edge located in said wall space, a stiffening plate
associated with each of said girders and being arranged parallel to
its respective girder, each said stiffening plate extending through
the outer wall of said tank and having an outer edge terminating
near the outer wall, each said girder and its associated stiffening
plate being arranged perpendicular to the longitudinal axes of the
corrugations of said inner and outer walls, said stiffening plate
having an inner edge located in the wall space, a part of said
stiffening plate contacting and overlapping a part of its
respective girder, and mechanical means securing in rigid
engagement the overlapping part of said stiffening member and its
girder, whereby rigid support of said inner and outer tank walls is
afforded by said girder and stiffening member, and cracks
developing in one of said walls are prevented from travelling to
the other of said walls, and wherein said tank has four vertical
sides, each corrugation of said inner and outer walls of each side
extending vertically and wherein at least one of said girders is
arranged in a horizontal plane, the overlapping parts of said
girder and stiffening plate being located between each pair of
corrugations which project toward the wall space, and the parts of
said girder and stiffening plate between the pairs of corrugations
that project away from the wall space being cut to define an
enlarged opening, and said tank further comprising stress members
mounted between said inner and outer walls of the vertical sides of
said tank near the bottom thereof, each stress member comprising a
first and second overlapping, vertically extending plate, said
first plate being integrally mounted on and perpendicular to one
corrugation of said inner wall which projects toward the wall
space, and said second plate being integrally mounted on and
perpendicular to the corrugation of said outer wall which is
aligned with said one corrugation, and mechanical means securing
said first and second plates in rigid engagement.
6. A tank as set forth in claim 5 wherein said corrugations to
which said stress member is mounted project toward the wall
space.
7. A tank as set forth in claim 5 wherein said mechanical means
comprises bolts and said openings are of sufficient size to enable
passage of a man between the undulations that project away from
each other.
8. A transport tank for carrying liquefied gas at about ambient
pressure, said tank having an outer wall and a spaced inner wall,
said walls comprising material capable of withstanding the low
cargo temperatures so that said outer wall acts as a secondary or
back-up barrier in the event of failure of the inner wall, an
interconnecting member is provided between the outer and inner
walls to limit the relative movement therebetween, said
interconnecting member comprising a pair of base members aligned
generally perpendicular to their planes, one base member being
mounted on the outside of the inner wall and the other being
mounted on the inside of the outer wall, an interconnecting arm
means having each end rotatably retained by one of said base
members for transmitting tension and compression forces and
affording within limits two degrees of freedom between said pair of
base members, a girder located inside the inner wall and welded
thereto to reinforce the same, the plane of said girder being
aligned with said interconnecting member, and a pipe arranged
coaxial with said interconnecting member having one end mounted on
the inside of said inner wall and its other end being connected to
said girder for transmitting force loads to and from the aligned
interconnecting member.
9. A transport tank in combination with a ship for storing
liquefied gas at about ambient pressure, said tank having a free
standing outer wall and a spaced free standing inner wall to form a
free space therebetween, said walls being formed of material
capable of withstanding the low cargo temperature, said tank
arranged within the ship's hull and thermal insulation being
disposed between the tank and ship surrounding structure, said tank
comprising stress members to prevent relative movement and unequal
local thermal growth between the outer and inner wall due to
thermal expansion and contraction, each such stress member being
located within the space between said outer and inner wall and
having a pair of mounting plates one of which is secured to the
inside of the outer wall and the other of which is secured to the
outside of the inner wall, said stress member further comprising a
web mounted perpendicularly between and connected to said mounting
plates, said web extending vertically of the tank.
10. The combination as set forth in claim 9, wherein said web
comprises two overlapping plates connected by mechanical means,
such as bolts or the like, one of said plates being mounted to one
of said mounting plates and the other of said plates mounted to the
opposite mounting plate.
11. The combination as set forth in claim 10, wherein a plurality
of stress members are provided near the bottom of the upwardly
extending walls of said tank and wherein the vertical dimension of
said web is greater than the horizontal dimension thereof.
12. A transport tank in combination with a ship for transporting a
liquefied gas at about ambient pressure, said tank having a free
standing outer wall and a spaced free standing inner wall to form a
free space therebetween, thermally insulated foundations spaced
from one another and located in the hold of said ship supporting
said tank, and a primary structural tank framework comprising
bottom girders connected to said inner and outer tank walls along
the bottoms thereof to provide primary support for said tanks, said
bottom girders being vertically aligned with said foundations to
transmit the primary force loads directly to said foundations.
Description
The present invention provides an apparatus and method for cooling
a transport or storage container to a predetermined low temperature
and filling the container with a liquid having a low evaporation
point, such as liquefied methane and the like. In a specific
embodiment of the present invention, there is disclosed an
apparatus and method of the type described used in conjunction with
transport tanks on a liquefied methane carrier-ship. The present
invention is also drawn to improvements in the structure of the
transport tanks of the type described.
For many years, some gases, specifically petroleum gases such as
propane and butane, have been transported and stored at ambient
temperatures but under appropriate pressure to avoid evaporation.
However, during recent years increasing attention has been given to
the transportation and storage of liquefied petroleum gases at
ambient pressure but at a temperature below its evaporation point.
This interest has been brought about by the desire to transport and
store liquefied methane, since, as is well known in the art, this
gas cannot be economically carried under enormous pressures
required to obtain ambient temperature. Therefore, the state of the
art has turned toward the development of low temperature containers
for use in handling liquefied gases such as methane and the like at
preferably ambient pressure.
It is generally accepted that one of the most important commercial
problems of carrying liquefied gas below its evaporation
temperature is that great time and expense is necessary in cooling
insulated double-wall containers of large capacity down to and
below the evaporation temperature of the liquefied gas. In the case
of methane, this temperature is in the range of -162.degree. C. or
-260.degree. F. At the present time, it requires nearly a week to
reduce the temperature of an insulated container of the type
described to a temperature, for example, of -162.degree. C. which,
as is readily understood, is commercially expensive from the
standpoint of relative inactivity of machinery, men, vessels and
the like.
The primary object of the present invention is to provide a method
and apparatus for reducing the temperature of the tanks of the type
described in a much more efficient, faster and economic manner.
It is another object of the present invention to provide a method
and apparatus for filling the tanks of the type described with
liquefied gas having a low evaporation temperature in a shorter
time than heretofore realized and to increase the safety by
reducing the possibility of explosion during introduction of the
liquefied gas.
It is yet another object of the present invention to provide
improvements in the double-walled transport tank structure of the
type described which is incorporated in carrier-ships, the
particular structure of the tank being such as to be more
structurally sound and safer than tanks heretofore used for this
purpose.
Other and further objects of the present invention will become
apparent with the following detailed description when taken in view
of the attached drawings in which:
FIG. 1 illustrates a liquefied gas carrier-ship having a number of
cargo tanks therein.
FIG. 2 is a vertical section taken along line 2--2 of FIG. 1.
FIG. 3 is a horizontal section taken along line 3--3 of FIG. 1.
FIG. 4 is an exploded horizontal section of the corner structure of
one of the double-walled tanks of the present invention showing one
embodiment of the double-wall stiffeners.
FIG. 5 is a vertical section taken along line 5--5 of FIG. 4.
FIG. 6 is a vertical section of the bottom corner of one of the
tanks comprising the present invention.
FIG. 7 is a horizontal section taken along line 7--7 of FIG. 6.
FIG. 8 is a horizontal section of a second embodiment of wall
stiffeners.
FIG. 9 is a side elevation taken along line 9--9 of FIG. 8.
FIG. 10 is a vertical section taken along line 10--10 of FIG.
8.
FIGS. 11, 12, 15, 17, and 18 are schematic diagrams illustrating
the apparatus and method of the present invention.
FIGS. 13, 14, 16 and 19 are graphs showing pertinent parameters at
various times during the loading and cooling of the liquefied
methane tanks.
Referring to the drawings in detail, FIG. 1 illustrates a methane
carrier-vessel generally indicated as 10 having four cargo tanks 12
spaced throughout the longitudinal axis of the ship. Each tank 12
extends from the bottom to the top of the hull and has a capacity
of 10,000 cubic meters.
As can be seen in FIGS. 2 and 3, the hull 14 of vessel 10 acts as a
housing for tank 12 and said tank 12 is supported by the outer
foundations 16 and a center foundation 18 fitted between the tank
and hull bottoms. Due to the anticipated low temperatures,
insulating material 20, such as balsa wood, expanded plastic,
polyurethane, batted mineral wool or the like, coats the walls,
bottom and top of the hull 14.
Tank 12 comprises an outer corrugated wall 22 and an inner
corrugated wall 24 having undulations such that those undulations
facing toward each other are aligned and those undulations facing
away from each other are aligned. A suitable number of keys (not
shown) are mounted between the insulation and the outer wall 22 to
enable vertical relative movement therebetween due to thermal
expansion and to maintain wall alignment. Inner and outer walls 22
and 24 are spaced from each other for a purpose to be described
hereinbelow, and this space will hereinafter be referred to as the
wall space. Outer wall 22 is also spaced from the insulation 20,
and this space will be hereinafter referred to as the insulation
space. Longitudinal bulkhead 26 and transverse bulkhead 28 divide
the inner tank into four tank sections, each section having
approximately a 10,000 cubic meter volume. Bulkheads 26 and 28 have
a number of stiffeners (not shown) arranged thereon in the
conventional manner.
In order to add additional stiffening to the walls of the tank and
also prevent relative movement between walls 22 and 24, a suitable
number of girders 30 are horizontally mounted at spaced vertical
locations around the sides of the tank. As better seen in FIGS. 4
through 7 and in accordance with one principle of the present
invention, horizontal stiffeners are welded in the wall space to
the outer wall 22 and inner wall 24. The stiffeners comprise two
plates or sections 32 and 34 which face and overlap each other (see
FIG. 5) and are held fast by bolts 36. Plates 32 and 34 are shaped
to overlap only in the regions where the undulations of walls 22
and 24 are closest; see FIG. 4. Where the undulations are farthest
from each other, plates 32 and 34 define an opening 38 of such size
to function as a manhole or crawl space so that personnel or
instruments can move unimpeded within the wall space for the
purpose of conducting safety checks, such as gas leak checks and
the like. Moreover, openings 38 enable a free circulation of inert
gas more fully described below.
During operation, tank outer wall 22 must be at the same low
temperature as inner wall 24 and the cargo being carried in the
event inner wall 24 develops a crack or leak and the liquefied
methane runs into the wall space; a low-temperature outer wall 22
prevents the liquefied methane from vaporizing, and thus, it
reduces the chance of explosion. The overlapping arrangement of
stiffeners 32 and 34, therefore, serves as safety structure because
this arrangement prevents cracks which may develop in inner wall 24
from traveling through the stiffener to the outer wall 22. And in a
like manner, cracks developing in outer wall 22, with the structure
of the present invention, cannot be transmitted to the inner wall
24. As can be readily seen in FIG. 5, cracks developing in any of
the two walls are confined to the respective stiffener associated
therewith and are not transmitted through the other stiffener
section. For this reason, all connections between walls 22 and 24
are bolted or riveted.
Referring to FIGS. 6 and 7, the stiffener section 34 is integral
with the girder 30 extending toward the inside of tank 12. Spaced
at suitable horizontal locations and preferably near the bottom of
the wall space are I-beam stress members 40 having web sections in
a vertical plane and legs 42 and 44 welded to the outer wall 22 and
inner wall 24, respectively. The web section of each I-beam 40 is
made up of two overlapping plates 46 and 48 secured by rivets or
bolts 50 for the same reason as described above for the stiffener
sections 32 and 34. I-beam member 40 not only prevents relative
vertical and horizontal movement between the inner and outer walls
22 and 24, but it also supplies additional vertical support for
inner wall 24. I-beam member 40 is preferably mounted between the
walls at locations where the undulations of outer and inner walls
face each other and the distance therebetween is the smallest.
Similar I-beam members 52 are spaced at suitable horizontal
positions between the inner and outer bottom of the tank in the
same manner as I-beam member 40. As further described below, the
temperature difference between tank top and bottom should
preferably not exceed 25.degree. C. so that great stresses from
thermal expansion do not appear in walls 22 and 24. If this maximum
temperature differential is exceeded, more stress members 40 are
needed to prevent structural failure.
FIGS. 8, 9 and 10 illustrate an alternative arrangement for
stiffening the inner and outer walls; horizontal girders 30 are
welded on the inside of wall 24 as described above. A flexible
stiffening member 100 has a base plate 102 welded to the narrow
space undulation 24'. Welded on the base plate 102 is a disk 104
having a depression 106 therein. A cup-shaped retain 108 is also
welded to plate 102 coaxially with disk 104. A connecting arm 110
having a ball seat at each end bitted within each depression 106
functions to maintain the wall distance. The member 110 also has
enlarged ball-shaped ends which cooperate with the inner surface of
retainers 108. Pipe 112 is preferably coaxial with assembly 100 and
welded to the wall 24 and girder 30.
This flexible stiffener operates to enable slight relative movement
(for example, 2 or 3 millimeters) between walls 22 and 24 resulting
from, for example, thermal expansion and contraction. Moreover,
cracks appearing in one wall will not be transmitted to the other.
Fabrication of the tank is also enhanced by this embodiment because
the assembly 100 is welded as a unit to one wall; next, the other
wall is positioned and the other end of the assembly is welded to
the other wall.
The stress members 52 located between the bottoms of the
double-walled tank need not necessarily be of the flexible assembly
100 because it is anticipated the bottom of the tank will be
uniform in temperature distribution unlike the vertical walls 22
and 24 of the tank 12.
At the bottom corners of the tank, there is also provided vertical
girders 56 having one end welded to a horizontal girder 30 and its
other end acting as a stiffener 34 within the wall space.
Elbow-shaped plates 58 and 60 are welded to the horizontal and
vertical girders 30 and 56, respectively, to further reinforce and
increase the rigidity of the entire tank structure. Flared inserts
or shoes 62 are welded between girder 30 and the inward facing
undulations of inner wall 24 to increase the base area and spread
the supporting forces more uniformly over girder 30.
Each tank section of tank 12 is fitted with one electro-driven
submerged pump located on the tank bottom with a capacity of
approximately 350 cubic meters per hour. Pumps of this type are
well known in the art, and it should operate satisfactorily down to
a level of 125 millimeters above the pump section inlet. An
equalization gate-valve (not shown) is fitted very closely to the
bottom of transverse bulkhead 28 between two tank sections so that
the pump of one section can be used as standby pump for the other
section. The wall space is provided with one emergency pump to
empty that space in the event it becomes necessary, and this pump
should have a capacity of approximately 45 cubic meters per
hour.
Each tank section is fitted with a filling line, a discharge line,
a gas suction line, and an inert gas line, safety valves, vacuum
valves (none of which are shown) and any other necessary
connections now commonly found on tanks of this type. The safety
valves comprise two escape systems, one starboard and one port, and
the vacuum valves serve to protect the tanks against undue
underpressures. These vacuum valves are connected with a methane
pressure system which is held under low overpressure.
Referring now to FIG. 11, the method and apparatus for cooling and
filling tanks of the aforementioned type will now be described.
It is well known that liquefied methane should not be poured into a
tank which is at ambient temperature, and, for safety reasons, the
tank must be cooled down to at least -140.degree. C. before
introduction of liquefied methane cargo begins.
Before beginning the cooling of the tanks of the type described,
the tanks must be purged with an inert for safety reasons. For this
purpose, a quantity of nitrogen is generated by a
nitrogen-generating plant located on shore or on the vessel, and
the gas is stored under pressure in large tanks.
Any suitable apparatus can be used to purge the spaces and tank
with inert gas. For example, pipes can be installed running through
the insulation space to the bottom thereof and communicate with the
wall space at the bottom through the outer gas-tight wall 22.
Openings at the bottom of the portion of the pipe within the
insulation space enable introduction of gas therein. A separate
pipe from the inert gas source communicates with the inner tank.
Appropriate collecting manifolds are mounted at the top of the
insulation and wall spaces and deliver the inert to a blower.
Another arrangement provides two pipes each extending through the
insulation and wall space, respectively, with openings at the
bottoms thereof.
It should be understood that any suitable arrangement can be used
as long as the inner and outer walls 22 and 24 are maintained gas
tight.
Before filling the insulation and wall space with inert gas, the
air within these spaces is circulated two or three times over the
dehydration units to reduce the moisture content thereof. This
procedure prevents condensation from forming during later
operations.
After dehydration, the inert gas -- in this example, nitrogen -- is
delivered at zero degrees centigrade from the storage tanks to the
inner tank, insulation space from top to bottom and up the wall
space of the cargo tank to a blower and out an exhaust until the
spaces and tank are purged. At this time, the nitrogen gas is at
0.degree. C, and within two or three volume exchanges, the tank,
wall space, and insulation space are uniformly cooled to about
0.degree. C. This step takes approximately 5 hours for one tank
having a capacity of 10,000 cubic meters. After purging, the
exhaust is closed, and the nitrogen recirculated to the heat
exchanger.
After purging is completed, the cooling of the tanks begins. The
heat exchanger is first fed from a source of liquefied gas (in this
example, methane) wherein the nitrogen assumes a lower temperature
than during inerting mentioned above. Again, the nitrogen gas is
circulated by a blower to the tank insulation space and between the
wall space so that the inner and outer walls of the tank 22 and 24,
respectively, are cooled in a uniform manner. At the same time, the
methane exhausting from the heat exchanger, although now a vapor,
has a temperature much below the ambient, and this vaporized
methane is fed through pipes and subsequently released directly
into the tank to cool the interior thereof. The rising methane gas
within the tank is collected and fed to heater where it is heated
to approximately 15.degree. C. and then supplied to a gas turbine
or a fuel storage bin. If this collected methane gas is not needed
for fuel, it is recirculated to a cooling unit on shore where it is
again converted to liquefied gas and fed to the main source tank.
To speed the lowering of the tank temperature, a small amount of
liquefied gas, such as methane, is also sprayed during this time
directly in the tank.
The blower and the heat exchanger capacities are preferably set in
such a way that at the beginning of the cooling procedure the
difference in temperature between the tank top and tank bottom does
not exceed a maximum value of 25.degree. C. for safety reasons and
the aforementioned structural reasons.
After a cooling period of approximately 65 hours, the insulation
space and wall space and tank temperature is roughly -130.degree.
C. Once the tank temperature reaches -130.degree. C., it is
anticipated the rate of lowering the tank temperature could be
speeded up in any number of ways.
One method of reducing the temperature of the tank and the
insulation and wall spaces even further is to feed liquid nitrogen,
which has an evaporation temperature of approximately -190.degree.
C. to the heat exchanger in place of the liquefied methane. The
circulation of the nitrogen gas continues for approximately another
15-hour period after which the tank bottom reaches a temperature of
-140.degree. C. With the temperature of the insulation space, the
wall space, the tank and the tank sidewalls at approximately
-140.degree. C., the liquefied methane is then fed through the fill
line directly into the storage tanks.
Another way to reduce the tank temperature from -130.degree. C. is
to change from a heat exchanger having 100 square meter surface
area to another heat exchanger having a considerably greater
surface area.
The method of the present invention enables four 10,000 cubic
metric tanks to be cooled in approximately 80 hours total time from
the initial dehydration step to the final filling of the tank with
liquefied methane. It is important to note that the insulation
space, the wall space, the inner and outer walls 22 and 24, and the
inner tank are cooled at the same uniform rate due to the passing
of the cooled nitrogen gas between the insulation space and the
wall space of the tank.
After the tank has been completely filled with liquefied methane,
its low temperature is maintained by evaporation; during boil off,
the gas is collected and can be fed as fuel to appropriate gas
storage areas of the vessel. It has been found that a cargo of
10,000 tons yields about 30 to 40 tons of boil off per day, which
is a sufficient quantity to efficiently propel the vessel.
According to the present invention, the pumps located in the bottom
of the tanks deliver a small amount of liquefied methane to the
heat exchanger as the nitrogen is circulated within the insulation
and wall spaces in order to keep the circulating nitrogen at about
-145.degree. C.
After the vessel reaches its destination and the cargo is unloaded
down to ballast condition, the remaining liquefied methane is kept
cool and the aforementioned 25.degree. C. temperature differential
is maintained by the circulation of nitrogen through the heat
exchanger with pumped liquefied methane as the medium. The
liquefied methane is delivered to the top of the tank through pipes
after it exhausts from the heat exchanger and is sprayed into the
tank to maintain the top of the tank within the aforementioned
temperature differential. It is commonly known that balast trips
are somewhat dangerous, and the aforementioned recirculation of
nitrogen and methane vapors also acts as a safety device to prevent
explosion and rapid evaporation. It should be understood that any
suitable media can be used in the heat exchanger during cargo and
ballast trips, and any suitable liquefied gas can be stored on
board under pressure or a liquid nitrogen generator can be provided
on board for this purpose.
The following is but one example of the present invention to be
conducted on a 10,000 cubic meter tank (surface area between tank
walls: 1,500 m.sup.2 ; total surface area of nitrogen space is
6,800 m.sup.2).
A nitrogen generator is used to extract nitrogen from the
atmosphere and deliver the same in liquid form to large storage
tanks on shore. Dehydration of the insulation and the insulation
space is effected by circulating three volume exchanges of air in
the insulation and wall spaces of two hydration units. For
inerting, liquid nitrogen is fed from shore to a heat exchanger on
the vessel where it is vaporized and subsequently fed to the
insulation space at about 0.degree. C. The entire tank, insulation
and wall spaces are purged by use of a blower (capacity: 60,000
m.sup.3 /hr.) mounted adjacent the tank drawing from these spaces
and circulating the air out an exhaust. This flushing of tank and
spaces continues for two or three volume exchanges, approximately 5
hours. When all but nitrogen is removed from these spaces, the
exhaust is closed, and the blower then feeds the heat exchanger and
the source of liquid nitrogen is shut off. In this way, the
insulation, wall spaces, blower, heat exchanger and connecting
lines define a closed circuit filled with nitrogen gas in
continuous circulation.
Next, liquefied methane is fed to the heat exchanger from large
storage tanks on shore and used as a sink for the circulating
nitrogen gas. When the methane is first (time zero) introduced to
the heat exchanger, the blower is advanced to circulate the
nitrogen at 40 exchanges per hour. The heat exchanger has an area
of about 100 m.sup.2, and the nitrogen is rapidly cooled. See FIG.
13. The methane exhausting the heat exchanger is in vapor form, and
it is returned to shore to collecting tanks to be reliquefied. The
heat exchanger and blower are set so that the temperature
difference between the top and bottom of the tank is not more than
25.degree. C. See FIG. 14. After a period of about 20 hours, the
condition of the tank is that illustrated by FIG. 15.
Approximately six tons of liquefied methane is sprayed directly
into the tank (see FIG. 16) over a period of 75 hours beginning at
the fifth hour or when the tank wall temperature is about
-20.degree. C. When the tank wall and methane vapor temperatures
are within approximately 10.degree. C. of each other, the input of
liquefied methane should be gradually reduced to zero over a
20-hour period (see FIG. 17, summation of liquefied methane, from
hours 30 to 50).
After 65 hours, the tank and gas temperatures are practically the
same so that the tank temperature will no longer rapidly lower due
to inherent heat absorption thereof. At this time, liquid nitrogen
is fed to the heat exchanger in place of liquefied methane. FIG. 13
illustrates the respective evaporation temperatures of nitrogen and
methane. At the same time, liquefied methane is again fed directly
into the tank at a rapid rate (see FIG. 16) until the desired tank
temperature is reached.
Liquid nitrogen is fed to the heat exchanger for a period of about
15 hours until the nitrogen gas and tank cool to about -140.degree.
C. For the tank to reach this temperature in about 80 hours,
approximately 82 tons of methane and 36 tons of nitrogen are
vaporized. After 80 hours, the distribution of temperatures is that
illustrated by FIG. 18.
On a ballast trip, the nitrogen gas is kept moving at a rate of 20
exchanges per hour whereby the difference between top and bottom
tank temperature is not greater than 25.degree. C. During the
ballast trip, 46 tons of liquefied methane per hour is delivered
through the heat exchanger to maintain the nitrogen at its low
temperature of -141.degree. C., and the methane from the heat
exchanger is sprayed directly back into the tank. See FIG. 12.
Under loaded conditions, the liquefied methane from the heat
exchanger is heated to 15.degree. C. and fed to the boiler as
fuel.
It is readily understood that other and further modifications can
be made without departing from the spirit and scope of the present
invention.
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