US3425234A

US3425234A - Tanks for liquefied gases

Info

Publication number: US3425234A
Application number: US611979A
Authority: US
Inventors: Georges Jean Henri Trepaud
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-01-28
Filing date: 1967-01-26
Publication date: 1969-02-04
Anticipated expiration: 1986-02-04
Also published as: FR92225E; FR1507160A; DE1551625A1; GB1177231A; NL6701460A

Abstract

1,177,231. Liquefied gas containers. G. J. H. TREPAUD. 27 Jan., 1967 [28 Jan., 1966; 10 June, 1966; 12 Jan., 1967], No. 4071/67. Heading F4P. In the event of an undue external temperature rise, liquefied gas e.g. butane stored under pressure at or below a level 16 in an inner tank 1, Fig. 1, is caused to rise up the annular space 7 between tank 1 and a surrounding tank 5 thereby cooling the latter and the so-vaporized butane is freed of entrained liquid particles by passage through a conventional separator 13, 14, 15 and is then passed through a duct 12 containing a bursting disc 23 to an open-air burner system (not shown). The bottom portions of tanks 1, 5 are connected by ducts 8, 8<SP>1</SP> having normally closed valves 10, 10<SP>1</SP> which open at a predetermined temperature to allow access of liquid butane to annular space 7 and also through apertures 20, 21 in hollow tank-supporting legs 2. Liquid particles separated by an annular baffle 14 and a filter mesh 15 are returned through a funnel 13 and a vertical duct 3 to the lower portion of the outer tank 5. The latter is coated externally by an asbestos layer 6. In a modification the upper and lower portions of inner tank 1, Fig. 4, are open to the outer tank 5 and liquid forced up space 7 is vaporized,. freed from liquids by baffles 45, 13, 14 and is relieved through valves 70, 70<SP>1</SP> to the open-air burner. In a further modification, when the upper and lower portions of the inner tank 1, Fig. 5,. are open to the tank 5, the inner tank is formed of separate segments 1a, 1b, Fig. 7, secured along their edges to spaced rectangular sectional ducts 46, 46<SP>1</SP> which extend in vertical meridian planes and are welded to the outer tank 5. The duets are open at their lower ends 47, 47<SP>1</SP> and are connected at their upper ends 48, 48<SP>1</SP> to atmosphere through a fuzible disc 50, Fig. 6 and a next duct 49 and also to the inner tank through a non-return valve 52.

Description

Feb. 4, 1969 G, J. H. TREPAUD TANKS FOR LIQUEFIED GASES sheen; l' om.

Filed Jan. 26, 1967 Feb. 4., 1969 G. J. H. T REPAUD TANKS PoR LIQUEFIED GASES Fuse aan. 26. 1967 Sh'eet g of 4 Feb 4i 1969 G. J. H. TREPAUD 3,425,234

TANKS vFOR LIQUEFIE'D GASES Filed Jan. 26, 1967 Sheet 3 of 4 Feb. 4, 1969 Sheet Filed Jan. 26, 1967 United States Patent O 3,425,234 TANKS FOR LIQUEFIED GASES Georges Jean Henri Trepaud, 44 Rue la Boetie, Paris, France Filed Jan. 26, 1967, Ser. No. 611,979 Claims priority, application France, Jan. 28, 1966, 47,651; June 10, 1966, 64,962; Jan. 12, 1967, 90,845

U.S. Cl. 62-45 25 Claims Int. Cl. F17c 1/12, 7/00 ABSTRACT F THE DISCLOSURE A tank for a liquefied gas, of the type comprising an inner and an outer wall enclosing a narrow, annular space, Means are provided to connect to each other the respective lower ends of this annular space .and of the inside of the inner wall, at least when the temperature outside of thetank increases dangerously; thereby the liquefied gas rising upwardly through the annular space is at least partly vaporized by absorbing the heat transmitted from the outside of the tank. Means are further provided, near to the upper end of the annular space, to separate the vaporized gas from the still liquefied gas, to exhaust the vaporized gas in the atmosphere, and to return the still liquefied gas down to the lower end of the annular space.

Background of the invention This invention relates t0 a tank for liquefied gas, notably for low-molecular weight hydrocarbons, which incorporates ia safety device for protecting the tank against the risks of explosion in case of abnormal increase of the external or surrounding temperature.

It is known that liquefied gases, notably hydrocarbons that are gaseous under ordinary temperature land pressure conditions, are frequently stored in the liquid state and under pressure in closed and sealed tanks in which the liquefied gas remains in equilibrium, at room temperature, with its satunating vapour, during a relatively long storage period preceding its industrial or domestic use.

Recent accidents, as murderous as spectacular, proved that a tank of this type can burst when submitted to the intense heat released by a nearby fire of which the probability is all the less negligible as these tanks are generally assembled at a relatively high density rate on the storage sites of oil refineries.

T-he tank according to this invention is of the type broadly set forth hereinabove but it is protected against any risk of explosion, even in case of violent fire, and characterised in that it comprises a double wall providing an annular chamber in which, in case of abnormal increase in the external or surrounding temperature, notably in case of fire, liquefied gas rises from the tank and absorbs by evaporating, at least one fraction of the heat transferred through the external wall, thus avoiding the bursting of the tank, and also in that known separating means are provided lat the upper end of said annular chamber for separating the thus evaporated gas from the still liquefied gas, together with a system of open-air burners for venting the vaporized and separated gas.

In case of increase in the external temperature the liquefied gas contained in the tank according to the present invention will thus rise from the bottom to the top of the annular chamber formed between the double wall of the tank, and the fiux of external heat received by the tank and transferred through its external wall will cause the liquefied gas rising in said annular chamber to boil; as a continuous circulation -of liquefied gas is maintained between the two ends of the annular chamber the vaporized fractions of said gas tare vented t0 the atmosphere through said external or open-air burners, preferably at ice a relatively great distance from the seat of the fire, so as to be burned thereat without supplying fresh fuel to said fire. In the following disclosure it will be proved that instead of rising the temperature and pressure prevailing 1n the tank according to this invention will subsequently decrease gradually and that in any case the whole of the tank content can be evacuated gradually in the manner explained hereinabove, until the external fire has been put under control or has ceased, without allowing any risk of explosion to appear at any time.

In a first form of embodiment of the t'ank according to this invention the thickness of the inner wall is adapted to the maximum permissible internal pressure of the liquefied gas contained therein, and the outer wall is considerably thinner, the annular chamber formed between these two walls, which is normally closed, being filled with an inert gas such as nitrogen under a relatively low overpressure, said tank further comprising means for causing the lower end of said annular chamber to communicate with the tank bottom when the external temperature and/or the internal pressure exceed a dangerous value, and also a return line for establishing in this case a continuous circulation of boiling liquefied gas from the lower end to the upper end of said annular chamber.

When a fire breaks out in the vicinity of a tank of known type simply equipped with a safety valve, the liquefied gas stored therein begins to boil tumultuously and unless the level of liquefied gas stored in this tank is extremely low the inlets of the safety valves are flooded with liquefied gas which will vaporize only if allowed to expand as it emerges therefrom, that is, outside the tank, in the open-air burner system communicating with the atmosphere. The heat transmitted from the outside through the wall of the known tank will thus partake completely in the temperature increment yand therefore in the pressure increment of the liquefied gas contained in the tank; under these conditions the safety valves provided therein are inadequate for preventing the explosion of this known type of tank, which is inevitable when the internal pressure exceeds the test or rated pressure. Moreover, these safety valves are subjected to a veritable bombardment by liquid masses thrown in all directions within the tank of known type, said safety valves being therefore more or less damaged by this bombardment, so that if they open it is most likely that they will be unable to be subsequently reclosed in a fluid-tight manner, even if the inner pressure in the known tank eventually drops, for example if the fire has been put out; as a result, the safety valves permit the escape of the whole of the gas contained in the tank of known type from the very moment an increase, even lof temporary nature, of the internal pressure has caused these valves to open. Under these conditions it is clear that even an otherwise small fire is sufficient to cause the loss of the considerable mass of gas contained in a tank of the known type.

Another form of embodiment of the tank according to this invention is free of the inconveniences mentioned hereinabove although it comprises safety valves opening into the surrounding atmosphere as in the known tank type.

This second form of embodiment of this present invention is characterised in that the external wall of the tank has a thickness consistent with the -maximum permissible internal pressure, that the internal wall is considerably thinner and has its upper and lower ends widely open t0 cause said annular chamber of the double-,wall tank structure to communicate with the inner space of the tank, and that safety valves gaged to open at predetermined internal pressures are interposed between the aforesaid separator means and the open-air burner system.

As these separator means are disposed at the upper end of the annular chamber of the double-wall structure of the tank according to this invention, before or u-pstream of the safety valves, the gaseous mass escaping through these valves will actually carry along but microscopic droplets of liquid fuel which are not capable of submerging or damaging said safety valves, or preventing the yfiuid-tight reclosing thereof, as observed in known tank types, in the absence of said separator means, by the mixture of gas and liquid escaping tumultuously from the upper end of said annular chamber.

Nearly the whole of the heat transmitted from the outside through the tank wall is thus absorbed by the evaporation of the liquefied gas in the annular chamber, so that the temperature and therefore the pressure of the liquefied gas still contained in the inner wall are kept at substantially constant values, thus precluding any risk of explosion.

This increased Safety is due to the fact that the safety or exhaust fvalves equipping this second form of embodiment of the tank according to this invention are fed only with nearly dry vaporized gas, when they are open, as a consequence of the point action of the annular vaporization chamber and of said separator means, as contrasted with the safety valves of known tanks which are practically fiooded with boiling liquefied gas.

According to an advantageous modification of the above disclosed second form of embodiment lof the tank of this invention, a plurality of groups of safety valves are provided, the respective valves of each group being gaged to open at stepped internal pressure valves and having therefore likewise stepped cross-sectional passage areas, whereby, for instance the valves of only one group which lhave a relatively small cross-sectional passage area will open in case of intense sun radiation, the valves of all the groups opening in case of fire.

In the case of the second form of embodiment of the tank according to this invention, when, in case of fire, the partial vaporization of the liquefied gas contained in the tank has strongly reduced the level therein (for example to less than 3 feet in the case of a spherical tank of a diameter approximating 40 feet) only practically dry and even overheated vaporized gas circulates in the upper portion of the annular chamber of the double-wall structure, this vaporized gas being therefore 11nable to absorb the heat transmitted from the external fire through the external 'wall of the tank. In other words, the temperature of the external vvall of the tank may rise, notably towards the upper end of the annular chamber, as the level of liquefied gas decreases in lche tank, to a value whereat the strength of the material constituting said external wall (as a rule steel) is not sufficient to withstand the corresponding internal pressure. This may be the cause of a partial breaking down of the outer wall of the tank and give rise to serious risks in case a certain mass of liquefied gas is still present in the bottom of said tank, the sudden vaporization of this residual gas being most likely under these conditions to produce a violent explosion.

'Ilhis drawback may be avoided -by resorting to the two improvements set forth hereinafter and applicable to the second form of embodiment of the tank according to this invention, independently of, or possibly in combination with, each other:

According to the first improvement a set of substantially vertical ducts or tubes are disposed against the inner face of the outer wall, said ducts or tubes being disposed at spaced intervals along the horizontal section or circumference of the annular chamber, the lower end of each d-uct leading into the tank, near the lower end of said annular chamber, and the upper end of eaoh duct opens into an atmosphere exhaust member connected for example to the open-air burner system, the opening of this member being controlled by the temperature attained by the outer wall in case of fire, when the partial vaporization of the liquefied gas contained in the tank has caused the level therein to drop to such a degree that only vaporized and overheated gas circulates in the -upper portion of the annular chamber.

With this first improvement when the temperature of the outer wall has reached a critical value for the material constituting this wall, in case of fire, the upper end of each duct is caused to communicate through the relevant exhaust member with the atmosphere, whereby the small mass of gas still in the liquid state which remains in the bottom of the tank is rapidly forced by the pressure prevailing within the tank into the lower ends of said ducts where the liquefied gas rises and is finally vented t0 the atmosphere for example through said open-air burner system; under these conditions the tank is drained completely and any risks of explosion as a consequence of the breaking of the outer wall are thus definitely preeluded, since only gas is contained therein.

According to a second improvement applicable in case the tank were supported above the ground by legs or the like, its double-wall structure comprises a cylindrical downward skirt-like extension of smaller transverse dimensions, said skirt-like extension being likewise double-walled so that when the tank proper has been nearly exhausted by a fire having vaporized the liquefied gas therein, a small volume of gas still in the liquid state remains in this hollow extension. As this small residual mass would lvery rapidly evaporate if the external fire continued, this second improvement will enable the tank to be drained out completely, thus definitely avoiding any risks of explosion caused by a partial breaking of the lupper portion of its outer wall.

Reference will now be made to the accompanying drawings illustrating diagrammatically by way of example various forms of embodiment of the tank according to this invention. nI the drawings:

FIGUR-E 1 is a vertical section, taken along a diametral plane, of a first form of embodiment of the spherical tank according to this invention;

FIGURE 2 is a fragmentary section showing on a larger scale of a detail, the section being taken upon the line II II of FIGURE l;

FIGURE 3 is another fragmentary section showing 0n a larger scale detail of a leg, according to a modified embodiment of the tank structure illustrated in FIGURE 1; v FIGURE 4 is another vertical section taken along a diametral plane of a second form of embodiment of the spherical tank according to this invention;

FIGURES 5 and 8 show respectively in vertical section two improvements concerning the tank illustrated in FIGURE 4;

FIGURE 6 shows on a larger scale the detail A of FIGURE 5, and

FIGURE 7 is a fragmentary section taken upon the line VII- VII of FIGURE 5.

The spherical tank illustrated diagrammatically in FIGURE 1 is intended more particularly for the aboveground storage of liquefied hydrocarbons, such as propane, ethane, butane, etc. It comprises essentially a closed and fluid-tight external wall 5, consisting of a hallow steel sphere for example of a diameter of 40 feet, its wall thickness, for example in the case of liquid propane storlage, being of the order of 13716 to 1%6". This sphere 1 is supported by sturdy tubular metal legs, of which a relatively great number, for example eight, are disposed along its outer periphery, although only two legs are visible in the figure and designated by the reference numerals 2. According to this invention, the sphere 1 has mounted therein, coaxially to its vertical diameter, a return tubular duct 3 of a diameter ranging for example from 20 to 24", for a purpose to be explained presently; the two ends of this duct are welded lin a fluid-tight manner to the edges of holes 3' and 3" formed in the sphere 1 at the end of the vertical diameter thereof, so that no direct communication is provided between the inner space of said sphere 1 and that of said duct 3; a bellow 4 is inserted in the return duct 3 to absorb the stress developed by the thermal expansion between said duct 3 and sphere 1. On the other hand, this sphere 1 is surrounded completely by an external wall 5, also closed and fluid-tight but considerably thinner than the wall of sphere 1; this outer wall may consist for example of steel sheets about 0.2" thick; in the form of embodiment illustrated in FIGURE 1 it consists of two semispherical cups S and 5" of a diameter slightly greater than that of said sphere 1 and having their registering edges assembled in a fluid-tight manner, for example by welding, to a relatively shallow cylindrical belt member 5"' located in the vicinity of the equatorial plane of the inner sphere 1. The outer wall assembly 5 is also supported by the legs 2 of the tank structure to which the lower semi-spherical cup 5" is fastened also in a fluid-tight manner, notably by welding. The outer face of wall 5 is covered completely with a layer 6 of asbestos about l" or 1% thick, applied preferably by spraying and covering preferably also the outer surface of each leg 2. Between the sphere 1 constituting the internal wall and the external Iwall S an annular space 7 is formed which will hereinafter be referred to as the annular chamber; the radial dimension of this annular chamber increases from a minimum value approximating 2" at the bottom of the sphere 1 to a maximum value approximati-ng 6" at the top thereof, this radial dimension being approximately V4" in the equatorial plane of the sphere (this radial dimension having been considerably exagigerated in FIGURE l in order to make it more apparent); therefore, the annular charriber 7 has at all points a relatively reduced value in proportion to its diameter which approximates that of said sphere 1, and its cross-sectional dimension increases gradually from the lower end to the upper end of said chamber, as a consequence of its variation in diameter.

In the form of embodiment illustrated in FIGURE 1 a relatively great number of drain pipes y8, 8 (for example up to six or eight) are disposed preferably at spaced intervals about the vertical axis of the sphere 1, so as to permit the communication between the base of this sphere and the cylindrical bottom 9 of the external wall 5. rEach drain pipe y8, 8 extends in a fluid-tight manner through the external wall 5 and the section thereof located externally of this wall 5 comprises a

valve

10 or 10 the opening of which is controlled by a thermometric pickup inconporated in the valve, this pickup consisting for example of a simple fusible eleme-nt so dimensioned that it will melt ywhen the surrounding temperature exceeds the preselected dangerous value, thus causing the automatic opening of the

corresponding valve

10 or 10.

The upper portion of the external Wall 5 of the tank opens into the base of a cylindrical dome 11 having a vertical axis coincident with the vertical axis of the splhere 1, this dome consisting for example and likewise of relatively thin sheet metal and being assembled in a fluidtight manner, for` example by welding, to the top portion of said wall v5. From the top of this dome 11 emerges a duct 12 leading to one or a plurality of open-air burners (not shown in FIGURE l).

This open-air burner syste-m is located preferably as remotely as possible (horizontally and/ or vertically) from the tank. `On the other hand, the dome 11 contains known means acting as gas-liquid separators and in the example illustrated in FIGURE 1 these means consist of a funnelshaped member 13 coaxial with the sphere 1, the lower tubular portion of this funnel opening into the upper orifice 3 of the ret-urn tube 3'; of an annular trough-like member 14 secured to the inner side wall of dome 11 so as to guide the liquid dripping along this wall towards the upper collector of said funnel 13, and, finally, of a filtering mesh structure 1S covering the complete cross-sectional area of the cylindrical dome 11, in the upper portion thereof.

Conventional Valves are provided in the known fashion to permit the picking up of liquefied gas from the lower 6 portion of the spherical container 1 through the external wall 5, but they are not shown in FIGURE l.

The asbestos layer 6 may also be covered in turn with a thin external shell of polished metal such as aluminium.

When the tank according to this invention is put into service, the air contained in the annular chamber 7 is forced out and replaced by an inert gas such as nitrogen, retained therein by a slight overpressure (for example 1A p.s.i.) 'by a gaged exhaust member of known type such as a breaking disk 23 mounted notably at the inlet of tube 12.

When the surrounding temperature in the vicinity of lthe tank exceeds a preselected critical Value considered as dangerous for various reasons, such as the breaking out of a fire in component elements of the liquefied gas storage plant Iwhich are located in the vicinity of the tank, the melting of the fusible elements equipping the

valves

10, 10 etc. causes the nearly instantaneous opening of said valves, whereby the liquefied gas enclosed in the sp'here 1 for example up to the level 16 begins to flow through the pipes l8, 8' etc., at the bottom of the sphere 1, into the bottom 9 of the external wall 5 so as to gradually fill same and rise in the annular chamber 7 where the pressure increases until it breaks the disk 23. Since this annular chamber 7 now communicates with the external atmosphere through the pipe 12 and the open-air burner system to which this pipe is connected, the liquefied gas, of which the pressure in the sphere 1 was that of its saturating vapour, undergoes a considerable pressure reduction; at the same time, the intense heat flux having caused the melting of the fusible elements associated with the

valves

10, 10 etc. is transmitted to the liquefied gas, in spite of the asbestos layer `6 covering same, to the external wall 5. As in the evaporator of a refrigeration system, the liquefied and expanded gas rising in the annular chamber 7 begins to boil (for example at about 22 F. (-30 C.) in the case of propane, with due consideration for the pressure losses in the annular chamber), so as to absorb at least one fraction of the flux of external heat transmitted thereto. The liquid and gas mixture attaining lthe upper end of the annular chamber 7 is separated by the above-described means 13, 14, 15 mounted within the dome 11, the gaseous fraction being sucked up through the duct 12 and directed t0 the open-air burner system where it is burned off at a distance from the tank sufficient to prevent this fraction from adding extra fuel to the fire to which the tank is exposed, the liquid fraction being returned on the other hand by the funnel 13 and return duct 3 to the bottom 9 of the external wall 5, Thus the return duct 3` will produce a continuous circulation of boiling liquefied gas from the lower end to the upper end of the annular chamber 7; the boiling liquefied gas circulating therein constitutes a kind of heat screen about the sphere 1, thus protecting the liquefied gas still contained therein against the intense heat flux from the outside; the complete tank structure may be so dimensioned that this protection be perfect or, in other words, that even in case of very intense external heat ux such as caused by a very violent fire in close proximity of the tank the temperature and therefor the pressure of the liquefied gas still present in the sphere 1, instead of gradually increasing as in a known tank, will not only remain constant but even decrease below the relatively high lbut not dangerous values which they had attained just before the protection measure were started. In this case the tank operates as a real opencycle refrigeration machine fed from the liquefied-gas tank and taking heat both from the surrounding atmosphere and from the container in which liquefied gas is stored, since the annular chamber 7 is in heat-transfer contact with both media. As long as the fiux of external heat is continued the level 16 of liquefied gas in the spihere 1 decreases and a corresponding quantity of gas is burned off in the open-air burner system without allowing in any case the pressure in the sphere 1 to exceed the rated or test pressure lvalue which may then not exceed the value contemplated and prescribed by official regulations in force. When the external lire is extinguished the vaporization of liquefied gas, if any is still present in the sphere 1, will firstly decrease and then cease. The tank may possibly still contain a considerable fraction of the initial liquefied gas under conditions in which a tank of known type, provided with only conventional safety valves, would be completely empty. -If on the other hand .the duration of the external tire were such that the tank will eventually be fully exhausted, the protection provided 4by the boiling of the liquefied gas will cease at the same time as the risk of explosion created by the presence of this liquefied gas in the closed container 1.

The above-described form of embodiment of the tank according to this invention is susceptible of many modifications and variations as will readily occur to anybody conversant with the art, but it is obvious that these modiiications and variations would not constitute a departure from the basic principles of the invention. Thus, more particularly, the shape and dimensions of the closed tank or container are immaterial, and besides this invention is also advantageously applicable to existing tanks, irrespective of their shape, for example to cylindrical tanks; this invention is also of considerable interest both in the case of relatively small tanks, for example in domestic installations, and in the case of average-capacity or large-capacity tanks of industrial plants, notably in storage zones of oil refinery plants. Of course, the dimensions of the annular chamber, notably its width, must be adapted to the dimensions and therefore to the volumetric capacity of the tank surrounded thereby, in orderA to provide the necessary efficiency and protection characteristics. In all cases the external wall may consist of a relatively thin material since as a rule there is no difference in pressure between its two faces. As contrasted with the structure illustrated in FIGURE l, the valves provided for connecting the bottom of the closed container to the corresponding end of the annular chamber may be secured directly in or on the wall of the lower portion of said closed container, instead of being located externally of the outer wall of the assembly, so as open directly into the annular chamber, for example above the bottom thereof; of course, in this case the thermometric pickups controlling the opening of said valves must still be located externally of the tank so as to be responsive to the temperature of the surrounding medium; save for this requirement, these thermometric pick-up devices lend themselves to a great number of different forms of embodiment, many of which are well known to those conversant with the art and are adaptable to the present invention; besides, it may be advantageous to associate with the various valves thermometric pickups of different designs, although their operation is controlled by the same critical temperature; some of these pickups may comprise for example fusible elements, others may comprise bimetallic strips, etc., in order to increase the probability of opening at least one of the valves aforesaid which are relied upon for starting the protection measures. If the valves are secured in or to the surface of the inner wall of the container, the opening of at least some of these valves may also advantageously be controlled by the pressure prevailing in the inner space of the container, by means of at least one manometric pickup device disposed in said inner space and possibly incorporated in the valve; in this case the last-named valve may be in the form of a relief valve opening into the annular chamber, notably a breaking-disk valve. The dome 11 and the separator means 13, 14, 15 mounted therein are also adapted to be constructed in many different manners as to their shapes, dimensions and forms of embodiment which are also well known to those conversant with vaporization techniques. Finally, the asbestos layer provided on the outer surface of the external wall of the tank according to this invention is optional.

Experience having taught that the damages caused by the bursting of a liquefied-gas tank carried by legs are further increased by the breaking of at least some of these legs and by the fall of the tank, it is advantageous, in this case, to provide, according to another feature characterising this invention, means for deriving into inner ducts formed in these legs one fraction of the boiling liquefied gas circulating in the annular chamber in case of abnormal increase in the external temperature; in the tank construction illustrated in FIGURE l, of which one leg is shown in vertical section in FIGURE 3, these means consist in providing inside the tubular -legs 2 a cylindrical tube 18 having a vertical axis and its upper end supported for example by the wall of the inner sphere 1, at least one aperture 19 being provided between its lower end and the base of leg 2; thus, the liquefied gas rising in the lannular chamber 7 penetrates into each tube 18 and then rises in the annular space formed between this tube 18 and the tubular leg 2, to re-enter the annular chamber 7 through the lateral ports shown at 21. This circulation of boiling liquefied gas (at about 22 F. or 30 C. in the case of propane) will cool the leg 2 and concurrently with the external asbestos layer 6 protect this leg against the heat flux. Of course, this cooling arrangement may be modified in many different manners as will readily occur to those skilled in the art.

The spherical tank illustrated diagrammatically in FIG- URE 4, consists essentially of an external spherical steel wall 5 constituting a closed and sealed chamber, the thickness of this wall 5 being consistent with the maximum permissible internal pressure; thus, in the case of a propane tank having a volumetric capacity of 1,000 cubic meters (35,000 cu. ft.) and therefore a diameter of about 40 feet, the thickness of the outer wall 5 must be approximately 1.34 (34 mm.) if the maximum permissible temperature ofthe liquefied gas is 122 F. (50 C.) which, in the case of propane, corresponds to a maximum permissible internal pressure of the order of 230 p.s.i. This spherical wall 5 opens at its lower end into `a kind of cylindrical vat 9 closed by a bottom 35 which may be detachable or not and covered with an external protection layer 6 of asbestos; this tank is supported by a plurality of vertical legs 2 of -which only two are shown in FIGURE 4. Internally of the external wall 5 an inner wall 1 of relatively thin steel sheet is secured by adequate means (not shown), the thickness of this inner wall being preferably of the order of 0.08 for example in the case of a tank having the numerical characteristics mentioned hereinabove. This inner Wall 1 is so shaped and dimensioned'that when it is secured within the outer wall 5 the double-wall structure thus obtained provides therebetween an annular chamber 7 of which the radial width varies preferably from the lower end to the upper end as illustrated in the figure, the annular chamber 7 having for example a minimuni radial width in the equatorial plane of the sphere 5 and a maximum width at the ends. The thin inner wall 1 is connected at its lower end to a neck 1 projecting considerably into the vat 9, and its upper end is wide open as at 1 so that the annular chamber 7 of the double-Wall structure will communicate with the inner space of the tank, that is, the inside of the inner wall 1. Disposed near the upper pole of the spherical external wall 5 are

safety valves

10, 10 inserted in pipe lines extending through the external wall 5 and leading to a common open-air burner system 12. (not shown). In this specific form of embodiment, two groups of safety valves, respectively 10 and 10', are provided and have different characteristics and functions. The single safety valve 10' of the lirst group has a relatively small cross-sectional passage area and is gaged to open for example when the liquefied gas contained in the tank has been heated, for example in case of intense sunning, up to a temperature of about 86 F. (30 C.); in other words, the safety valve 10 is so gaged that it will open when the pressure prevailing in the tank is of the order of p.s.i., which is the vapour tension of propane at 86 F. (30 C). Both valves 10 of the second group aforesaid have on the other hand definitely larger cross-sectional passage areas; they are gaged to open when the gas contained in the tank has been heated, for example as a consequence of a fire in the vicinity of said tank, to a temperature approximating 122 F. (50 C); in other words, the safety valves 10 yare gaged to open at the maximum permissible internal pressure of 230 p.s.i., which is the vapour tension of propane at 50 C. Of course, this second form of embodiment of the 'tank according to this invention may be equipped With any desired number of groups or series of safety valves, of which the respective valves, of which the number is immaterial, are `gaged to open at stepped, predetermined pressure values, their cross-sectional passage areas being also stepped accordin ly.

gAt the upper end of the annular chamber 7, that is, above the upper aperture 1" of the inner wall 1, known separator means such as those illustrated in the lower portion of the dome 11 of FIGURE 1, may be provided; in this case they consist of an annular trough 14 secured to the inner surface of fthe outer Wall 5, and of a funnel 13 underlying the aforesaid annular trough 14 so that its discharge orifice overlies the upper aperture 1 of the inner Wall 1. Beneath the separator means 13 and 14 and above the edge of the upper aperture 1 of said inner wall 1 is an annular baflie or defiector 45 also secured to the internal surface of the external wall 5.

Under normal service conditions the liquefied gas in equilibrium with its vapour at room temperature rises to the same level 16 in the space enclosed by the inner wall 1 and in the annular chamber 7. The heat penetrating into the tank from the outside firstly increases the temperature of the external wall and is subsequently absorbed by the major portion of the liquefied gas contained in the annular chamber 7 below the level 16. As a result, a certain quantity of the liquefied gas contained in the annular chamber 7 is vaporized, thus tending to raise the vapour pressure in the upper portion of said annular chamber 7 to the value corresponding to the thus increased temperature of the liquefied gas contained in its lower portion. As the upper ends of the annular chamber 7 and of the internal wall 1 communicate through the relatively wide aperture 1' and as the gas pressure in said inner wall space 1 is constant, the gas vaporized in the annular chamber 7 rises towards the top of this chamber and carries along a certain amount of liquid, so that a propane emulsion will impinge against the annular bafile 45 and be directed thereby towards the upper opening 1" of the inner wall 1. While the largest drops of liquid fall back through this aperture 1" into the inner space of wall 1, the lighter droplets are entrained by the gas towards the separator means 13, 14; the operation of these separator means (which is already known and therefore needs not be described in detail) is such that the fine liquid droplets carried along by the vaporized gas are returned through the drain orifice of funnel 13 to the inside of wall 1, the practically dry gas accumulating at the top of the outer wall 5, above the annular trough 14; when the pressure, for instance in the example being described, attains a value approximately 140 p.s.i., the safety valve 10 opens and the dry gas is vented to and expands in the atmosphere through said valve and the open-air burner systern (not shown). This procedure continues as long as heat is transferred from the outside through the outer wall 5, provided that the temperature of the liquefied gas contained in the tank remains below 122 F. (50 C.) corresponding in this example to the vapour tension for which the safety valves 10 are gaged. This process corresponds for example to the case in which the tank was subjected to intense sun radiation capable to heating its content to a temperature of 85 F. to 122 F. (30 to 50 C.). However, if the temperature of the content of the tank according to this specific form of embodiment attained a value equal to or in excess of 122 F. (50 C.) which is likely to 'be attained only in case of a fire breaking out in close vicinity of the tank, the internal pressure will `reach 230 p.s.i. and the two safety valves 10 of the second -group will open in turn, and owing to their'crosssectional passage areas considerably greater than that of valve 10', they will permit the escape of the much greater mass of gas resulting from the considerably more intense evaporation caused in the annular chamber 7 by the heat radiation emitted from the seat of the fire. However, if the transfer of heat from the outside through the external wall 5 decreased or were even stopped completely, for example in case of decrease of the sun radiation or if the re were extinguished, the internal pressure will decrease again, thus causing all the valves previously open to close, inasmuch as, during the above-described procedure, only dry gas has passed through these valves, due to the provision of separator means 13 and 14, and accessorily of baflie 4S, so that these valves have been safely protected against flooding and possible damages, and also against insuficient closing for instance as a consequence of an intense bombardment thereof by a liquid. Thus, any loss of gas ceases immediately as the safety valves are reclosed and therefore ready to operate again in case of a subsequent increment in the external temperature.

Like the one of the tank illustrated in FIGURE 1, the external wall 5 of the tank illustrated in FIGURE 4 can be covered externally with an asbestos layer applied and secure for examples by spraying. In FIGURE 4, this asbestos layer 6 is provided only on the outer surface ot' the upper portion of the external wall 5 which is normally exposed to the more intense heating due to the fact that it is not cooled lby the evaporation of liquefied gas, notably in case the open-air burner system through which the gas escaping from the

safety valves

10 and 10 were relatively short in proportion to the diameter of the spherical tank.

The cooling of the legs 2 of the tank illustrated in FIG- URE 4 is also contemplated by circulating liquefied gas therein, this liquefied gas being taken from the annular chamber 7 and directed through the double tubular path formed in each leg, as illustrated in section in the lower left-hand portion of FIGURE 4.

The relatively thin internal wall 1 may consist of sheet metal, for example .08 thick. Of course, the shape, thickness, dimensions and relative disposal of the two

walls

1 and 5, as well as the number, arrangement and dimensions of the various safety valves 10 and 10', of the separator means 13 and 14, and also of baffle member 45 are susceptible of practical embodiments differing more or less from that illustrated in FIGURE 4.

The essential component elements of the spherical tank illustrated diagrammatically in FIGURE 5 are akin to those of the spherical tank shown in FIGURE 4; therefore, the homologue elements of these two structures are designated by the same reference numerals in these two figures, in order `to facilitate their identification. However, in the modified form of embodiment illustrated in FIG- URE 5 the lower aperture 1 of the thin internal wall 1 has not a neck-like lower extension opening into a vat forming the extension of the external Wall 5 below the tank (elements 9 and 35 of FIGURE 4). On the other hand, the form of embodiment illustrated in FIGURE 5 comprises in addition a series of preferably squareor rectangular-sectioned ducts or pipes 46, 46', as shown in the fragmentary section of FIGURE 7. These ducts extend in vertical meridian planes of the concentric spherical walls land 5 so as to engage the inner surface 0f the external wall 5 to which they are secured preferably by welding, as shown in FIGURE 7. These

ducts

46 and 46 have preferably a constant cross-sectional passage area throughout their length, as contrasted with the horiz-ontal sections of the annular chamber 7 formed by the concentric

spherical walls

1 and 5 which decrease towards the equatorial plane of these walls, as already explained hereinabove in connection with the form of embodiment illustrated in FIGURE 5; the internal wall 1 of relatively thin metal sheets consists preferably of separate segments (la, 1b, etc. in FIGURE 7) having their vertical edges secured to saidducts 46 and 46 at a suitable distance from the external wall in order to obtain the desired width of the annular chamber 7, whereby each compartment of this chamber 7 which is formed between any pair of adjacent ducts 46, 46' constitutes a fiuid-tight enclosure; thus, for instance, the edges 1a, '1b etc. of the internal wall 1 are bolted or screwed to lateral angle members rigid in turn with the radial faces of the square-sectioned ducts 46, 46', as shown diagrammatically at 46a and 4611 in FIGURE 7. The lower ends 47, 47' of the various, substantially

vertical ducts

46, 46 open near the bottom aperture 1' of the internal spherical wall 1, and their upper ends 48, 48 open each into a vent or exhaust member 49, 49' communicating `with the atmosphere (FIGURES 5 and 6). In the form of embodiment illustrated in FIGURES 5 t-o 7, this exhaust or vent member consists essentially of a pipe section 49 mounted in a uid-tight manner through t'ne external wall 5 and normally closed by a disk S0 of suitable fusible material; a side neck 51 connects this exhaust or vent member with the surrounding atmosphere either directly or through the medium of the open-air burner system 12 (not shown in FIGURE 5), to which the safety valves and 10' are also connected as already explained in detail hereinabove. On the other hand, the wall of each internal duct 46, 46' has secured therein, preferably by welding, near its upper end 48, a non-return ball-valve 52 normally connecting the upper end 48 of the relevant duct 46, 46' with the inner space of the tank, that is, the interior of its inner wall 1.

If a fire breaks out in close vicinity of the tank illustrated in FIGURE 5, and assuming that this tank is filled for example up to the topmost level indicated by the dash line 16, the protection of this tank against any risk of explosion is ensured firstly exactly under the same conditions as already explained in detail hereinabove. As the liquefied gas has the same level in the internal space 1, in the annular chamber 7 and in the aforesaid ducts 46, 46' having their lower ends 47, 47 constantly open, the same pressure prevails in the topmost portion of the tank and also in the upper portions of said ducts 46, 46', so that all the ball valves such as 52 are open as shown in FIGURE 6. As the liquefied gas circulating upwards through the annular chamber 7 is partially evaporated and subsequently separated and exhausted to the atmosphere in its vaporized state through the safety valves 10, 10', thus reducing the level of liquefied gas in the tank, the active circulation in said annular chamber 7 decreases gradually; if the liquefied gas in the tank further decreases down to a mean level 16' shown in thick line in FIGURE 5, the active circulation in annular chamber 7 remains sufficient to cause a mixture of liquid and moist vapour to continue to emerge from the upper end to the annular chamber 7; it is only if the fire continues, together with the procedure set forth hereinabove, and as the liquid level in the tank drops below the mark shown by the dash line 16" in FIGURE 5, which corresponds for example to a residual level of liquefied gas of about 40" in the spherical tank having a diameter of about 40 feet, and if the

safety valves

10 and 10 open for example under a rated internal pressure of 230 p.s.i. and recloseunder a rated internal pressure of 115 p.s.i., that vapour beginning to be overheated escapes through the upper end of annular chamber 7, this vapour being thereafter unable to absorb the heat continuously transferred through the external wall S from the seat of the fire. Of course, the degree of vapour overheat and its inability to absorb the heat iiux from the exterior increase at a given moment from the lower portions to the upper portions of the annular chamber 7, and at a given point of this annular chamber, as the level of residual liquefied gas decreases in the tank. Under these conditions, the material (as a rule steel) constituting the outer wall 5 will nearly inevitably attain near the top of annular chamber 7 local temperatures so high that it will not be strong enough to withstand the rising internal pressure. However, the risks of explosion in connection with a possibly breaking of the external wall 5, at least in the upper portions thereof, are eliminated by the present invention because the temperature increment in the portions of the external wall 5 which are adjacent to each exhaust member 49, 49' causes the melting of the fusible disk 50 closing the upper end of each substantially vertical duct 46, 46 the vacuum or suction resulting therefrom in the upper end 48 of each duct 46, 46' will immediately reclose the corresponding ball valve 52; the overpressure prevailing in the tank will then force out the residual mass of liquefied gas still contained in the bottom of this tank, through the lower ends 47, 47' of the substantially vertical ducts 46, 46' in which this still liquefied gas rises so as to be subsequently exhausted either directly through the side pipe 51 of exhaust members 49 or through the medium of the open-air burner system 12 (not shown), to the atmosphere where this still liquefied gas is burned off; in case this residual mass of still liquefied gas were burned in the atmosphere, no additional risk would result for the tank structure, provided that this combustion takes place at the end of an open-air burner system located at a relatively great distance from the upper portion of the tank in proportion to the tank diameter. Under these conditions the tank contains only gas of which the pressure constantly decreases slowly due to the provision of the exhaust -members 49 left open; any `breaking of the external wall S of the tank is thus safely prevented.

The form of embodiment illustrated in FIGURE 5 lends itself to many modifications and variations of which a great number `will readily occur to those skilled in the art; more particularly, the members 49 fory venting the gaseous products to the atmosphere, which are connected to the corresponding upper ends of the lvarious vertical ducts 46, 46'- can be constructed with many modifications and differ considerably from the structure illustrated in detail in FIGURE 6; thus, one-way valves, non-return valves, relief valves, etc. of very different types, and responsive to a properly adjusted thermometric pickup, and so disposed as to be sensitive to the temperature of the external wall 5, notably in the portions most exposed to a thermal overload, that is, in the vicinity of the upper end of the annular chamber 7, may be used. The valve 52 is also adapted to be constructed in many different manners. The arrangement of the aforesaid vertical ducts 46, 46' is also optional; thus, more particularly, it is not absolutely necessary that these ducts be regularly spaced along the circumference of the annular chamber 7; their number and also their shape and crosssectional dimensions are also optional.

The alternate form of embodiment of the spherical tank according to this invention which is illustrated in FIGURE 8 of the attached drawings departs from the structure shown in FIGURE 4 only by the following points (the same reference numerals designating in both figures the homologue component elements): a cylindrical vat 9, closed by a possibly detachable bottom 35 coated with an asbestos layer 6', is connected directly with the bottom of the outer wall 5, has an external extension beneath the spherical tank proper, supported above the ground by legs 2, of the length relatively considerable in proportion to the diameter of said spherical tank, this length being for example eight feet in the case of a tank about 40 feet in diameter, the diameter of this cylindrical extension being of 20" to 28"; similarly the neck 61 whereby the thin inner wall 1 opens at its lower end into said vat 9 has an extension within the vat of nearly the same vertical length so as to form a chamber in the form of a cylindrical ring 7 of which the upper end communicates directly with the lower end of the annular chamber 7.

FIGURE 8 shows clearly that in case of prolonged fire externally of the tank there eventually remains in the double-walled cylindrical element 9, 61 a reduced mass of still liquefied gas, for example 35 cubic feet, whereas the spherical tank proper has been drained nearly completely of the liquefied gas previously contained therein, due to the gradual vaporization of this liquefied gas and to its discharge into the surrounding atmosphere through the safety valves 10" and possibly the open-air burner system 12 (not shown). On the other hand, the small residual mass of still liquefied gas then contained in the lower double-walled cylindrical vat 9, 61 will eventually be vaporized completely; this vaporization taking place rvery rapidly if the fire is continued; therefore, any risk of violent explosion is safely avoided in case of a possible perforation of the upper portion of the external wall 5 of the tank.

In the form of embodiment illustrated in FIGURE 8, means notably in the form of radial pipes 62, are also provided for diverting towards the inner ducts formed in the legs 2 of the tank structure one fraction of the liquefied gas contained in the lower portion of the double-wall cylindrical element 9, 61, in order to cool these legs 2 in case of fire, due to a liquefied-gas circulation subsequently returned to the annular chamber 7 at the upper ends of these legs. However, this last-described pipe arrangement is optional; it is nevertheless advantageous in that it will further accelerate the complete vaporization of the relatively small residual mass of liquefied gas remaining in the cylindrical double-wall element 9, 61, for, as illustrated by the arrows in FIGURE 8, it promotes the circulation in the lower portion of the annular chamber 7.

In all the above-described forms of embodiment the following improvements may further be provided:

Venting members connected for example to the openair burner system are mounted through the external wall, preferably in the upper portion thereof, the opening of these venting members, for example normally closed by fusible disks, being controlled by the temperature attained by the external wall in case of fire.

With this arrangement it is also possible to reduce considerably the pressure prevailing in the tank, notably when the latter has been drained completely, in case of fire, of its liquefied-gas content, by the means described hereinabove.

In this case and preferably a gaged valve is mounted in series with each `venting member leading to the atmosphere, this Agaged valve being adapted to open at an internal pressure lower than the pressure necessary for reclosing the aforesaid safety valves; this specific arrangement is of course also applicable to the exhaust members 49 of FIGURES 5 and 6. These two last-named arrangements are such that they permit of maintaining a residual pressure, in the tank from which all liquefied gas has been drained out, which is only slightly in excess of the atmospheric value, and therefore of rendering practically harmless a possible perforation of its external wall weakened by the heating in case the fire continued after the tank has -been drained out completely.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, -as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What I claim is:

1. A tank for a liquefied gas, comprising an inner wall and a closed, outer wall, supported in a close relationship to each other with a narrow, substantially annular space extending there-between substantially from the lower ends of said walls to the upper ends of the same, the liquefied gas being contained at least inside of said inner Wall, means to connect to each other the respective lower ends of said annular space and of the inside of said inner wall at least when the temperature outside of the tank increases dangerously, whereby the liquefied gas rising upwardly through said annular space is partly vaporized by absorbing the heat transmitted from the outside of the tank, and, near to the upper end of said annular space, means comprising bafiies to separate the vaporized gas from the still liquefied gas, means to exhaust the vaporized and separated gas in the atmosphere, collector means to collect the separated, still liquefied gas and tube means to return the collected, still liquefied gas down to the lower end of said annular space.

2. A tank according to claim 1, in which said gas exhausting means is connected to an open-air burner system.

3. A tank according to claim 1, comprising further supporting legs, inside of said legs passages for the liquefied gas being cir-culated therethrough, and means to connect said inner passages of the legs with said annular space.

4. A tank according to claim 1, in which a layer of asbestos is provided on the outer face of at least the upper portion of said outer wall.

5. A tank according to claim 4, in which a thin shell of a polished metal is provided on the asbestos layer.

6. A tank for a liquefied gas, comprising a closed inner wall adapted to withstand a predetermined, inner overpressure, and containing the liquefied gas, a substantially thinner, closed, outer wall supported around said inner wall in a close relationship thereto with a narrow, substantially annular space extending between said inner and outer walls substantially from the lower ends of said walls to the upper ends of the same, said annular space being normally closed and containing an inert gas, means to connect to each other the respective lower ends of said annular space yand of the inside of said inner wall only when the temperature outside of the tank increases dangerously, whereby the liquefied gas rising upwardly through said annular space is at least partly vaporized by absorbing the heat transmitted from the outside of the tank, and, near to the upper end of' said annular space, means to separate the vaporized gas from the still liquefied gas, and to freely exhaust in the atmosphere first the inert gas, then the vaporized and separated gas, and duct means for returning the still liquefied gas from the upper end of said annular space down to the lower end of the same.

7. A tank according to claim `6, in which said return duct extends vertically through the inside of said inner wall.

8. A tank :according to claim 6, in which said means to connect to each other the respective lower ends of said annular space and of the inside of said inner Wall consists of at least one valve adapted to said inner wall, and of at least one pickup device for controlling the opening of said valve.

9. A tank according to claim 8, in which the pickup devices are sensitive to the temperature outside of the tank.

10. A tank according to claim 8, in which the pickup devices are sensitive to the pressure inside of the inner wall of the tank.

11. A tank according to -claim 10, in which said connecting means consists of at least one safety valve with a breaking disk.

12. A tank according to claim 6, in which said means to connect to each other the respective lower ends of said annular space and of the inside of said inner wall comprises at least one duct having a section extending outside of said outer wall, at least one valve inserted in said outer duct section, and at least one pickup device sensitive to the temperature outside of the tank and incorporated to said valve.

13. A tank according to claim 12, in which the valve is normally closed by a fusible element.

14. A tank according to claim 6 in which a dome is mounted externally on the top portion of said outer wall,

with a free communication 4between the lower end of the inside of said dome and the upper end of said annular space, said gas from liquid separating means and means to guide the separated liquid into the return duct are disposed inside of the lower part of said dome, and said gas exhausting means is arranged at the upper end f said dome.

15. A tank according to claim 6, in which said gas exhausting means is normally closed by a pressuregaged member.

16. A tank for a liquefied gas, comprising a closed, outer wall adapted to withstand a predetermined, inner overpressure, a substantially thinner, inner wall with large openings at its upper and lower ends, said inner wall being supported inside of and in a close relationship to said outer wall with a narrow, substantially :annular space extending between said inner and outer Walls substantially from the lower ends of said walls to the upper ends of the same, the respective upper and lower ends of said annular space and of the inside of said inner Wall being permanently connected to each other through said large openings in said inner wall, whereby the liquefied gas normally contained in said annular space is at least partly vaporized by absorbing the heat transmitted from the outside of the tank and lthereby -rises in said annular space up to its upper end, and, near to said upper end of the annular space, means to separate the vaporized gas from the still liquefied gas and to return said still liquefied gas into the inside of said inner wall through its upper opening, and at least one safety valve gaged to open into the atmosphere at a first, predetermined, inner pressure and to re-close at a second predetermined, inner pressure, lower than said first,- predetermined, inner pressure.

17. A tank according to claim 16, in which the gas from liquid separating means consists of a funnel disposed above the upper opening in said inner wall and of an annular trough disposed just yabove the upper rim of said funnel.

18. A tank according to claim 16, comprising further an annular bafiie member disposed just above the rim of the upper opening in said inner wall to direct thereinto the -gas and liquid mixture rising up to the upper end of said annular space.

19. A tank according to claim 16, comprising a plurality of safety valves, gaged to open into the atmosphere at predetermined, stepped, inner pressures, and having also likewise stepped, cross-sectional passages.

20, A tank according to claim 16, comprising further a set of substantially vertical ducts, disposed on the inner face of said outer wall and extending substantially from the lower end of said annular space up to the upper portion of said annular space, the lower end of each said duct being permanently open, exhaust members t0 the atmosphere, into which the upper ends of said ducts respectively open, [and at least one pickup device, sensitive to the temperature of the upper portion of said outer wall, for controlling the opening of said exhaust members at a dangerous, predetermined value of said temperature.

21. A tank according to claim 20, in which said exhaust members to the )atmosphere are mounted through said outer wall, and said temperature pickup devices are fusible elements, normally closing said exhaust members.

22. A tank according to claim 20, in which the upper end of each said duct is further provided with a valve opening into the inside of said inner wall.

23. A tank according to claim 20, in which said substantially vertical ducts have rectangular cross-sections, and said inner wall consists of separate sheet panels having their vertical edges tightly secured to the side walls of said ducts.

24. A tank according to claim 16, which further comprises supporting legs, and in which said inner and outer walls are provided, beneath and outside of the tank, with vertical, coaxial extensions having much smaller horizontal cross-sections than said inner and outer walls, the lower end of said outer wall extension being closed, whereas the lower end of said inner wall extension opens freely inside of said outer wall extension.

25. A tank for a liquefied gas, comprising a closed outer wall adapted to withstand a predetermined, inner overpressure, and having a downward closed bottom extension of much smaller horizontal cross-section, a substantially thinner, inner wall having a large opening at its upper end and a downward open bottom extension of still smaller horizontal cross-section, said inner and outer walls being supported one inside of the other and in a close relationship to one another, with their respective bottom extensions being coaxial, and 1a narrow, substantially annular space extending therebetween substantially from the upper, large opening in said inner wall to the open, lower end of its bottom extension, whereby the liquefied gas normally contained in said annular space is partly vaporized by absorbing the heat transmitted from the ambient atmosphere and thereby rises in said annular space, near to the upper end of said annular space, means to separate the vaporized Igas from the still liquefied gas, to collect the separated, still liquefied gas and to return the same into the inside of said inner wall through its upper large opening, above said separating means at least one safety valve gaged to open into the atmosphere at a first, predetermined, inner pressure and to re-close at a second, predetermined, inner pressure, lower than said first, predetermined, inner pressure, piping means inserted between the closed, lower end of said outer wall bottom extension and regions of said annular space at a predetermined level substantially above said wall bottom extensions, said piping means being arranged outside of the bottom extension of said outer wall, to be in thermal exchange with the ambient atmosphere, whereby the liquefied gas, when contained in Said annular space only at a level substantially under said predetermined level, is also partly vaporized in said piping means by absorbing the heat transmitted from the iambient atmosphere, and thereby rises in said piping means, the still liquefied gas being partly separated from the vaporized gas at said predetermined level, and being returned downward through said annular space into said outer wall bottom extension, whereas the vaporized gas and the still liquefied, not separated gas rise again in said annular space up to said separating means.

References Cited UNITED STATES PATENTS 1,544,854 7/ 1925 Mueller et al. 62-50 1,979,221 10/1934 Dana 62-50 2,190,366 2/1940 Mead 62-50 2,211,005 8/1940 Dick 62-50 2,242,108 5/ 1941 Bullowa et al. 6250 2,293,263 8/1942 Kornemann et al. 62-50 2,687,618 8/ 1954 Bergstrom 62-50 2,986,891 6/ 1961 McMahon 62-45 3,087,311 4/1963 Rousseau 62-52 X LLOYD L. KING, Primary Examiner.

U.S. Cl. X.R. 62-50