US3748865A

US3748865A - Storage tank for liquefied gas having bottom insulation gas shielding

Info

Publication number: US3748865A
Application number: US00131716A
Authority: US
Inventors: R Laverman; G Agrawal
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1971-04-06
Filing date: 1971-04-06
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

An enclosed liquefied gas storage tank with a bottom, side wall and roof having a chamber positioned in insulation beneath the tank bottom and extending over the area of the tank bottom, a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough.

Description

United States Patent 11 1 Laverman et al.

STORAGE TANK FOR LIQUEFIED GAS HAVING BOTTOM INSULATION GAS SHIELDING Inventors: Royce Jay Laverman, South Holland; Govind M. Agrawal, Plainfield, both of Ill.

Chicago Bridge & Iron Company, Oak Brook, Ill.

Filed: Apr. 6, 1971 Appl. N0; 131,716

Assignee:

US. Cl 62/50, 220/9 B, 220/9 LG Int. Cl. Fl7c 7/02 Field of Search 62/45, 50, 51, 55;

61/05; 220/9 LG, 9 B

References Cited UNITED STATES PATENTS 5/1955 Morrison 62/50 July 31, 1973 3,196,622 7/1965 Smith et al 62/45 3,246,479 4/1966 Kelley 62/45 2,804,657 9/1957 Munters 220/9 B Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. Capossela Attorney-Merriam, Marshall, Shapiro & Klose [57] ABSTRACT An enclosed liquefied gas storage tank with a bottom, side wall and roof having a chamber positioned in insulation beneath the tank bottom and extending over the area of the tank bottom, a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough.

1 Claim, 9 Drawing Figures PAIENIEU JUL 3 1 I975 sum 1 BF 4 INVENTORS OYCE JAY LAVERMAN GOV/ND M 46/? WAL ATTORNEYS PAIENTED 3.748.865

SHEET 2 0F 4 FIG. 3

, [e INVENTORS ROYCE JAY LAVERMAN W 15 GOV/N0 M AG W4L ATTORNEYS m n mum ms 748 5 sum-3 ur 4 F e. s a

6 63V 4 A B 64 c INVENTORS ROYCE JAYLAl/E/PMA/V GOV/ND M. AGRAWAIL ATTORNEYS PAIENTED 1 3. T48 865 SHEET 4 0F 4 F I 8 K I I z 3 54 A C INVENTORS ROYCE JAY LAVERMA/V GOV/ND M AGRAWAL ATTORNEYS STORAGE TANK FOR LIQUEFIED GAS HAVING BOTTOM INSULATION GAS SIIIELDING This invention relates to storage tanks for liquefied gases. More particularly, this invention concerns improvements in insulated liquefied gas storage tanks characterized by low heat leak and which are economical to construct.

A number of gases can be stored in a liquefied condition. Among these gases are oxygen, hydrogen, natural gas, methane, ethane, ethylene and nitrogen. It is impractical to store some of these gases in a liquid form at about ambient temperature because of the high pressure this would require to prevent them from vaporizing, and it is impossible to store other gases in a liquid form at ambient temperature since any amount of pressure will not result in a liquid phase being fonned. The construction of tanks of adequate strength to store those gases that can be liquefied at about ambient temperature generally would be excessively expensive because of the exceedingly heavy metal walls required to withstand the pressure. These liquefied gases are accordingly usually stored at slightly above atmospheric pressure and at a temperature which will maintain the gas in a liquid form at such pressure. Construction of a tank which need only withstand a pressure slightly above atmospheric pressure, plus the hydraulic pressure of the liquid, is much less expensive and more easily fabricated than large heavy-walled tanks designed to withstand high pressures.

To minimize heat leak into a tank containing a liquefied gas, it is common to employ appropriate insulation. Even though the tank is insulated, there is considerable heat leak into the liquefied gas. The heat leak causes vaporization of liquid in the tank and, to prevent buildup of pressure, the boil-off vapor is removed and used or it is re-liquefied and returned to the tank.

A number of differently shaped tanks are used for storing liquefied gases. The most common tank is one which has a flat bottom, vertical wall, and a roof which is generally conical or domed. The base of the tank is usually circular and the tank wall is generally cylindrical. Other cryogenic tanks are spherical and ellipsoidal. Regardless of the shape of the tank, it usually comprises at least a single metal shell which is insulated to reduce heat leak.

Studies of heat leak into liquefied gas storage tanks have established that one of the major factors is the heat leak through the tank bottom. Heat which passes through the tank bottom is transferred to the liquid, which it heats, thereby inducing boil-off of vapor. In addition to heat leak through the tank bottom, there is substantial heat leak through the tank wall and roof. This heat leak also induces undesirable boil-off of the liquefied gas. Boiloff of liquid is undesirable since it reduces the amount of liquid in storage. Accordingly, means for reducing heat leak into the tank would provide for a better, more useful, liquefied gas storage tank.

It has been found according to the present invention that heat leak through the bottom of a liquefied gas storage tank to the liquid in the storage tank can be reduced by utilizing the refrigeration capacity, or sensible heat capacity, of boil-off gas or vapor to absorb heat leaking through the bottom of the storage tank plus, if advisable, through the tank roof and tank wall as well. Utilization of the sensible heat capacity of boil-off gas is made possible by improvements in liquefied gas storage tanks provided by this invention. Thus, the invention broadly provides in an enclosed liquefied gas storage tank having a bottom, side wall and roof, the improvement comprising a chamber positioned in insulation beneath the tank bottom and extending over substantially the entire area of the tank bottom, a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough. Boil-off vapor is removed from the vapor space above the liquid by means of the vapor supply conduit and caused to flow through the chamber where it absorbs a substantial part of the heat leaking through the bottom. The vapor is removed from the chamber by means of the vapor removal conduit. The removed vapor can be reliquefied and returned to the tank or the vapor can be sent to a gas distribution line.

The invention will now be described further in conjunction with the attached drawings in which:

FIG. 1 is a vertical sectional view through a cylindrical liquefied gas storage tank provided with a chamber in the insulation beneath the tank bottom through which boil-off vapor can be circulated;

FIG. 2 is a horizontal sectional view along the line 2-2 of FIG. 1;

FIG. 3 is a vertical sectional view through a tank like the one of FIG. 1 but illustrates another chamber construction below the tank bottom;

FIG. 4 is a horizontal sectional view, partially broken away, along the line 33 of FIG. 3;

FIG. 4a is a horizontal sectional view of a tank showing another version of a chamber construction;

FIG. 5 is a vertical sectional view through a tank having a suspended insulated ceiling and a stepped chamber below the tank bottom;

FIG. 6 is a vertical sectional view through a tank having a domed roof, a chamber as shown in FIG. 5, and a vertical heat shield along the tank wall;

FIG. 7 is a vertical sectional view through a tank having a suspended insulated ceiling, a stepped chamber below the tank bottom, a heat shield along the tank wall and a top on the heat shield; and

FIG. 8 is a vertical sectional view through a tank having a stepped chamber below the tank bottom and a multiple layer heat shield suspended from the roof.

So far as is practical, the same or similar elements which appear in the various view of the drawings shall be identified by the same number.

The liquefied gas storage tank 10 shown in FIGS. I and 2 is mounted on foundation II. The tank has an outer shell 12 and an inner shell 13, both advisably made from suitable metal plate. Outer shell [2 has a flat circular metal bottom 14, vertical cylindrical wall 15 joined at its lower edge to bottom 14, and a domed roof 16 supported by wall 15. Inner shell 13 has a flat circular bottom 17 and vertical cylindrical wall 18 joined at its lower edge to bottom 17 and domed roof l9. Suitable insulation 20 is placed in the space between the bottoms, walls and roofs of inner shell 13 and outer shell 12 to thereby insulate the tank. Conduit or pipe 21 is used to feed liquefied gas into the tank and to withdraw it. Vent 22 in the roof of the tank is used to withdraw vapor from the tank to thereby permit regulation of the internal pressure in the tank. The tank as so far described is of conventional well-known construction and is designed to store a liquefied gas, such as liquefied natural gas, at approximately minus 259F. and at about psia pressure.

Chamber 23, shown in FIGS. 1 and 2, is placed in the insulation below bottom 17. The chamber has central

horizontal header pipes

24 and 240 which extend outwardly from juncture with vapor supply conduit 25. The upper end 26 of conduit is located in the vapor space above the design liquid level storage capacity-of the tank to receive boil-off vapor and feed it to chamber 23. Projecting, such as perpendicular, from both sides of header pipe 24 is a series of vapor distributor pipes 27 which join with collector pipe 28. Vapor removal pipe 29 joins collector pipe 28 and extends outside of the tank to remove vapor therefrom. Similarly, header pipe 24a has a series of vapor distributor pipes 27a extending from both sides and they in turn join with collector pipe 28a from which vapor is removed by vapor removal pipe 29a. The described series of pipes comprising the chamber 23 are calculated to be of sufficient size to permit adequate flow of boil-off vapor beneath the tank bottom to effect substantial absorption by the vapor of heat leaking through the bottom. Suitable pump means, not shown, can be placed in communication with exit pipes 29 and 29a to induce or enhance vapor flow through the chamber by reducing the pressure in the chamber.

Chamber 23 extends over substantially the entire area of the tank bottom to provide a substantially uniform heat absorption means. Supplying the boil-off vapor to the central area of the chamber by conduit 25, and removing the vapor from the periphery of the chamber, facilitates uniform heat absorption.

FIGS. 3 and 4 illustrate a liquefied gas storage tank essentially identical to the tank of FIGS. 1 and 2 but with a differently constructed chamber. Therefore, those elements in FIGS. 3 and 4 common to the tank of FIGS. 1 and 2 will carry the same identifying number and will not be described again.

The chamber 30 shown in FIGS. 3 and 4 has the general shape of a hollow flat sided disc placed horizontally. The chamber has a top sheet 31, a bottom sheet 32 and a peripheral wall 33 which together enclose and form the chamber. To strengthen the chamber and to aid in vapor distribution throughout the chamber, two semicircular

corrugated sheet sections

34 and 34a are placed in the chamber with their

straight edges

35 and 35a spaced apart from one another to provide a header corridor 36 through which vapor can flow to feed each vapor path defined by the corrugations. Baffles 37 are placed at the ends of corridor 36 to aid in vapor distribution. As the vapor leaves the corrugated paths it enters peripheral corridors 38 and 380 between the circular edges of

sheet sections

34 and 34a and peripheral wall 33 and then is conducted to vapor removal conduit 39. If desirable, more than one conduit 39 can be used to remove vapor to effect even flow of vapor through the chamber. The lower end of vapor supply conduit 25 advisably has an enlarged portion 40 which communicates with corridor 36.

The chamber structure of FIG. 4a is similar to that shown in FIGS. 3 and 4 except that no corrugated sheet sections are installed in the chamber. Instead, a series of radial walls 41 are placed in the chamber and they extend from the enlarged lower section 40 of vapor supply conduit 25 outwardly to near the chamber peripheral wall 33. If desired, the space between the ends of adjoining walls 41 can be provided with baffles with appropriately sized openings to aid in effecting uniform vapor distribution throughout the chamber space.

The tank 50 shown in FIG. 5 has an external shell comprising a flat metal bottom 51, a cylindrical wall 52 and a domed metal, noninsulated roof 53, and an internal shell comprising a flat metal bottom 54 and a cylindrical wall 55. Insulation 60 is placed between outer bottom 51 and inner bottom 54, and between outer wall 52 and inner wall 55. An insulated ceiling 56 is suspended by rods 57 from roof 53. Vent pipe 58 extends through ceiling 56 and roof 53 and can be used to remove vapor from the tank as desired. Pipe 61 is used to supply liquid to the tank and to remove it. Centrally positioned in the tank is vapor supply conduit 25 having its upper end 26 located above the design liquid level capacity of the tank. The lower end of conduit 25 communicates with vapor chamber 59 located in the insulation 60 beneath inner bottom 54. Chamber 59 has a top 62 and a bottom 63 which are supported in spaced apart position by blocks 64. Top 62 and bottom 63 can be made of flat sheet material such as wood or glass fiber reinforced plastic material. The hollow central section A of the chamber is cylindrical and the hol low sections B and C extending outwardly in series therefrom are each ring-like and of increasing size. Section B is a step lower than section A and section C is a step lower than section B. Section B is in vapor communication with both sections A and C so that vapor can flow from the central section of the chamber to the lower most peripheral section. Chamber 59 has an outer solid peripheral wall 65, and an inner peripheral wall 66 with holes therein so that vapor can flow around the corridor 67 between walls and 66 and out pipe 68.

With reference to FIG. 6, the basic tank structure shown therein is the same as in FIG. 1 so that the same elements in FIG. 6 are given the same number as in FIG. 1. Similarly, the chamber 59 in FIG. 6 is the same as in FIG. 5 and so its elements haveaccordingly been identified with the same numbers used in FIG. 5. FIG. 6 however illustrates a novel heat shield 70 positioned in the tank in spaced away relationship from internal wall 18. Brackets 71 are used to support the shield in position. The lower edge 72 of shield 70 is positioned above inner bottom 17 so that when liquefied gas is supplied by pipe 21, or is removed thereby, both liquid and vapor can flow to and from both sides of shield 70. As a result, the surface of the liquefied gas in the tank is always the same on both sides of the shield. The top edge 73 of the shield is spaced below inner roof 19. The resulting clearance provides a passage by which vapor flowing upwardly in corridor space 74, defined by shield 70 and wall 18, can travel to the vapor space in the tank.

As heat is conducted through the portion of the tank wall insulation below the liquid surface, the liquefied gas is heated, resulting in the production of boil-off vapor which is released from the surface of the liquid between shield 70 and inner wall 18. This vapor is then heated by the heat which is conducted through the tank wall above the liquid level in the tank. The vapor flowing upwardly in the corridor absorbs a large amount of this heat and, to the extent absorbed, the heat is prevented from radiating thru the vapor space of the tank above the liquid level to the surface of the liquid.

Shield 70 can be made of any suitable material. It can be made of a conventional boardlike material, with or without insulating properties. Although not essential to achieve some advantage, it is advisable to provide the surfaces of the shield facing the tank interior with a bright shiny heat reflecting surface. Such a surface can be provided by a painted coating or by a thin metal layer or foil, such as aluminum foil. A thin metal layer advisably is supported by an appropriate strength supplying backing.

The tank structure and vapor chamber of FIG. 7 is essentially like that shown in FIG. 5. Each figure therefore has the same identifying number for identical elements pertaining to such structure and chamber. The tank shown in FIG. 7 furthermore has a heat shield 80 essentially like heat shield 70 shown in FIG. 6. Shield 80 is supported by brackets 81 from the inside wall 55. Extending inwardly from the upper edge of shield 80 is top shield 82 of sheet material. Top 82 has an opening 83 by which vapor can pass from below top 82 to above it and thus out vent 58. Of course, if desired, opening 83 can be eliminated and the vapor from below top 82 can be fed down conduit 25.

Top 82 functions as a continuation of side heat shield 80. As vapor flows upwardly from corridor 74 its further flow is through corridor 84 defined by top 82 and the bottom of ceiling 56. As the vapor flows through corridor 84 it absorbs heat conducted through the tank roof and suspended ceiling 56. By absorbing much of such heat, the amount of heat which passes into vapor space 85 is reduced. Less heat thus enters the tank to produce boil-off vapor.

The tank structure of FIG. 8 is identical to the tank of FIG. 5 except that the suspended insulating ceiling 56 of FIG. 5 has been replaced by a suspended multiple layer heat shield 90. The heat shield 90 comprises five separated

layers

91, 92, 93, 94 and 95, with layer 91 being the bottom layer and 95 the top layer. The layers are spaced apart from adjoining layers by spacers 96 and are closed at their peripheral edges by sides 97. Each layer is more or less horizontally positioned. The heat shield 90 is suspended by bars 57 from roof 53.

Each of the layers 91 to 95 has at least one opening in it which permits boil-off vapor to flow from one side of each layer to the other side of the layer. The opening in each layer is remotely located from the openings in any adjoining layers so that the flow of vapor in the corridors between layers is made to extend, as close as feasible, over the entire space between layers. This is sometimes not possible with a single opening in each layer, so to facilitate maximum vapor flow coverage in the corridor spaces a plurality of holes 98 is generally used in each layer. The openings, however, are strategically placed to obtain serpentine flow of the boil-off vapors as they progress upwardly through the series of vapor corridors. As the vapor leaves the openings in the top layer it flows upwardly and can be removed through vent 100 in the roof of the tank.

Heat shield 90 shown in FIG. 8 is positioned inside of wall 55, of the inner shell, and adjacent its top portion. the shield thus is located to handle effectively heat which leaks through the roof 53. Boil-off vapor from the liquid in the tank, in flowing through the heat shield, absorbs heat and thus prevents such absorbed heat from flowing downwardly into the liquid. In addition, by using layers which have one or more bright surfaces, heat which flows through the roof can be reflected. Each layer can be made from an insulating or noninsulating material or a combination thereof. Thus, the layers can be made of wood, composition board, aluminum, plastic reinforced with fiber glass or polyurethane foam. It is advisable, however, to employ for the layer a material which has or is provided with a bright, shiny heat reflecting surface. It can be a composite layer having an aluminum sheet or foil bonded to polyurethane board. The aluminum sheet has a bright surface and low emmissivity while the polyurethane board has excellent insulating properties. Heat leak is thus reduced in at least two ways when a shield is produced as described. The bright surface provides radiation shielding to reduce heat flow into the liquid and the heat capacity of the boil-off gas flowing through the shield corridors absorbs a substantial amount of the heat which would otherwise pass into the liquid.

The shield 90, shown in FIG. 8, has five separated layers. However, it need not have more than two layers to be useful although such a shield will generally be less efficient.

The foregoing detailed description has been given for illustration purposes and it is not intended that the invention be limited to these embodiments.

What is claimed is:

1. In an enclosed liquefied gas storage tank having a bottom, side wall and roof, the improvement comprismg:

a chamber positioned in insulation beneath the tank bottom and extending over substantially the entire area of the tank bottom,

a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and

a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough,

said chamber having a central hollow cylindrical section with a plurality of hollow ring-like sections of increasing size in series extending outwardly therefrom with the inner most ring spaced below and in communication with the central hollow cylindrical section, and each ring out therefrom is spaced below the next adjacent inner ring, said vapor supply conduit communicating with the ccntral hollow cylindrical section and the vapor removal conduit communicates with the outer-most ring-like section.

Claims

1. In an enclosed liquefied gas storage tank having a bottom, side wall and roof, the improvement comprising: a chamber positioned in insulation beneath the tank bottom and extending over substantially the entire area of the tank bottom, a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough, said chamber having a central hollow cylindrical section with a plurality of hollow ring-like sections of increasing size in series extending outwardly therefrom with the inner most ring spaced below and in c0mmunication with the central hollow cylindrical section, and each ring out therefrom is spaced below the next adjacent inner ring, said vapor supply conduit communicating with the central hollow cylindrical section and the vapor removal conduit communicates with the outer-most ring-like section.