EXPLOSION RESISTANT TANK FOR LIQUID FUEL

TECHNICAL FIELD

This invention relates to pressurized tanks for con¬ taining liquid vaporizable fuel, and particularly to such tanks that are resistant to explosion.

BACKGROUND ART

It is well known that liquid fuel for engines, heating, or cooking is readily flammable and thus poten¬ tially an explosion source. Tanks of gasoline frequently explode when a fire causes the gasoline tank to get hot, raising the pressure inside the tank until it ruptures, and the gasoline spews out to cause an explosion. In U.S. 4,149,549 there is disclosed a means for suppressing the above type of explosion by filling the interior of the tank with a mass of layers of expanded metal where adjacent layers are arranged so that the direction of inclination of the expanded metal is different in adjacent layers. While this patented invention has provided a certain amount of protection against explosions of gasoline tanks it has not provided the ultimate protection and it has been totally unsuccessful with tanks of propane or similar hydrocarbon that is normally a gas but is stored under sufficient pres¬ sure to liquefy the hydrocarbon.

It is an object of this invention to provide an im¬ proved explosion resistant metal tank for liquid fuel. It is another object of this invention to provide an explosion resistant metal tank for liquid propane. It is still an¬ other object of this invention to provide an improved metal tank -.structure. Other objects will become apparent from the more detailed description which follows.

DISCLOSITRE OF THE INVENTION

This invention relates to an explosion resistant metal tank filled with liquid fuel in which the tank en¬ closes a mass of expanded metalc The improvement provided by this invention comprises a metal tank containing a nor¬ mally vaporous hydrocarbon under sufficient pressure to maintain the hydrocarbon in the liquid phase _r and a mass of expanded metal sheets packed together to form a structure of substantial thickness adjacent to and in heat conductive contact with substantially all of the internal surface of the tank; the mass having sufficiently high heat conductivity to transfer heat from the tank to the cool liquid fuel fill¬ ing the interstices of the mass of expanded metal so as to minimize the tendency for the tank to rupture due to the high temperature of the tank.

Another embodiment, of the invention relates to a process for making the above-described metal tank by pre¬ paring two integral, cup-shaped metal pressure heads having a cylindrical portion joined at one end to a closed substan¬ tially hemispherical head portion and terminating at the other end in a circumferential edge portion, attaching a manually operable valve to one pressure head with a conduit extending from the valve inwardly of the tank along the in¬ side surface of the one head and its adjoining cylindrical portion parallel to the longitudinal axis of the tank and beyond the circumferential edge portion and terminating with an open end at a point on the hemispherical surface of the other pressure head adjacent to its juncture with its re¬ spective cylindrical portion; preparing the mass of expanded metal sheeting to fit snugly against the inside contours of the two heads when joined at their respective circumferential edge portions so that the mass of expanded metal is in heat conductive contact with substantially all of the inside sur¬ faces of the two pressure heads; assembling the two pressure heads with the mass of expanded metal enclosed within both of the heads, welding the two pressure heads along the

cirσumferential edge portions to produce a pressure tight container, and stress-relieving the welded joint without melting any substantial portion of the mass of expanded metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed to be characteristic of this invention are set forth with particularity in the ap¬ pended claims. The invention itself, however, both as to its organization and method of operation, together with fur¬ ther objects and advantages thereof, may best be understood by reference to the following description taken in connec¬ tion with the accompanying drawings in which:

FIG. 1 is an elevational view, partially in cross- section, of the preferred embodiment of the tank of this invention.

FIG. 2 is a top plan view of the tank of FIG. 1. FIG. 3 is an elevational view of the upper half section of the tank of FIG. 1.

FIG. 4 is an elevational view of the lower half section of the tank of FIG. 1.

FIG. 5 is an enlarged view of a piece of expanded metal.

FIG. 6 is a perspective view of a tightly rolled coil of expanded metal for use in this invention with the container of FIG. 1.

FIG. 7 is an elevational view of a second embodiment of a tank for use in this invention.

FIG. 8 is an elevational view of a third embodiment of a tank for use in this invention.

FIG. 9 is a top plan view of the tank of FIG. 8. FIG. 10 is an elevational view of the cap on the inlet to the pressure relief valve.

FIG. 11 is a top plan view of the cap of FIG. 10.

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DETAILED DESCRIPTION AND BEST MODE OF THE INVENTION

The container of the preferred embodiment and best mode of this invention is best illustrated in FIGS. 1-4 of the attached drawings. Tank 10 is comprised of an upper half section 11 and a lower half section 12 joined around the midsection by a welded seam 13. Half section 11 in¬ cludes a cylindrical portion 14 and an upper hemispherical pressure head portion 15. Similarly, half section 12 in¬ cludes a cylindrical portion 16 and a lower hemispherical pressure head portion 17. It is to be understood that pressure head portions 15 and 17 may not be truly hemi¬ spherical, from a geometrical point of view, but for pur¬ poses of this invention the word "hemispherical" is to be understood to include any σonvexly curved section that might be a suitable shape for the head of an internally pressurized tank. Upper pressure head portion 15 is fitted with one or more valves and/or- fittings. ^' designed to ^" fill the ^" tank, dispense its contents., and indicate the internal conditions of pressure and/or temperature. Discharge valve 18 is a pressure reducing valve designed to receive liquid from inside tank 10 and allow it to expand and transform itself into a gas as it leaves valve 18. A manually oper¬ ated needle valve is one of the well known types employed for this purpose because it permits easy adjustment of the rate of discharge of the contents as a gas. Another type of valve is a pressure relief valve 19 which automatically opens to relieve any unduly high pressure inside tank 10, preferably from areas near the inside wall of container 10 as will be explained in more detail below. Head portion 15 preferably includes a liquid inlet valve 20 which functions as a check valve in that it opens to admit liquid flow under pressure to fill tank 10 but otherwise remains closed, meas¬ urement tube 39 is employed in filling tank 10 to determine when the tank is full, which is when 80% of the internal volume is filled with liquid. Tube 39 extends downward such

that the end of the tube will be at the liquid level for 80% full in the vertical position. Similarly, tube 39 is placed such that when tank 10 is filled in the horizontal position the tube will be at the liquid level when 80% full.

In order to make tank 10 capable of standing verti¬ cally with protection for the valves on pressure head 15, guard ring 21 is welded to the outside of hemispherical por¬ tion 15 and foot ring 22 is welded to hemispherical portion 17. These two ring portions 21 and 22 permit tank 10 to stand on end in either direction and to protect valves 18, 19, 20, and tube 39 from being damaged in use.

Liquid discharge line 23 is operatively connected to discharge valve 18 to lead liquid from the interior of tank 10 to valve 18 for use by the consumer. Line 23 extends along cylindrical portion 14 and cylindrical portion 16 and terminates as an open end 24 along hemispherical portion 17. The exact location of end 24 is such that it will notify the consumer when the tank is almost empty, i.e., when the re¬ maining liquid is about 10% or less of the volume when the tank is filled. When liquid fuel level reaches 25 when tank 10 is in a vertical position, there will be a noticeable sputtering of liquid entering end 24. Similarly, when tank 10 is in the horizontal position and the liquid level reaches 26 the sputtering noise will occur. Thus line 23 is positioned to lie along cylindrical portions 14 and 16 and generally be parallel to the vertical axis 27 of tank 10. In order to keep line 23 in a fixed position it is preferred to fasten it by spot welding or any other equivalent means at 28. This also provides some facility when assembling the various parts of tank 10 as it is manufactured for sale and use.

In FIGS. 3 & 4 there are shown half sections 11 and 12 before being assembled. Section 11 has an inwardly stepped portion 29 to provide a good attachment seat when sections.11 and 12 are placed together and welded shut.

Inside tank 10 is placed a mass of expanded metal sheets packed together to form a structure of substantial

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thickness; preferably the mass is a tightly rolled coil 30 of expanded metal. An alternate embodiment to coil 30 is a hollow structure of expanded metal layers tightly packed against each other, the total thickness of the packed layers being 1-4 inches. The layers may be wrapped around an in¬ ternal support or attached to the wall of tank 10 by any convenient means. The position of coil 30 in tank 10 is shown in FIG. 1 and its original structure is seen in FIG. 6. Coil 30 is rolled tightly without crushing the expanded structure of the sheet (as seen in FIG. 5) and is made with little or no central space 38 along its vertical axis. Coil 30 is originally made in the form of a cylindrical roll with flat ends (see FIG. 6) and is then cut to contour it to fit the inside shape of tank 10. Cutout portions for valves 18, 19, and 20, and for line 23 are removed from coil 30. The flat ends of coil 30 are shaped into a hemispherical contour so that maximum amount of contact is made between tank 10 and coil 30 as shown in FIG. 1. The contact should be posi¬ tive contact- so as to provide the maximum heat conduction between tank 10 and coil 30. Contoured coil 30 must be placed inside sections 11 and 12 before they are assembled for the welding of seam 13. Coil 30 is made of an excellent heat conductor. For this reason, as well as because of its availability and low cost, aluminum is preferred. Many other metals, such as stainless steel, titanium, magnesium, copper, silver, and various alloys are also excellent heat conductors and can be used here, but are not preferred be¬ cause of the higher cost involved. The thickness of metal in the coil must be sufficient for the coil to have some stiffness so as to retain its shape inside tank 10 touching a maximum amount of its inside surface. Aluminum should be about 0.003 inch thick for this purpose, while stainless steel may be as thin as 0.001 inch thick to accomplish the same purpose.

As fully explained in U.S. Patent 4,149,649 coil 30 may be made such that adjacent layers of expanded metal sheets in coil 30 have opposite directions of inclination

produced by the process of making the expanded metal. This presumably provides a lower bulk density for coil 30 that would be the case if the adjacent layers have the same di¬ rection of inclination. Insofar as this present invention is concerned coil 30 may have either orientation and the results of explosion resistance are substantially the same. In other words, it makes no difference whether coil 30 has adjacent layers of expanded metal with the same or opposite directions of inclination.

In order for a welded tank to perform well when in¬ ternally pressurized, all welded seams must be given a stress relieving treatment. Normally this is accomplished by heating the welded tank to an elevated temperature for a brief time and then cooling before being put into use. The elevated temperature needed for stress relief of a welded steel tank is above the softening point of aluminum (approx¬ imately 1200°F) and so such a treatment cannot be employed for the containers of this invention having a mass of ex¬ panded aluminum inside. If the expanded metal is stainless steel or other material higher melting than aluminum, the high temperature stress. ^" relief ^•' is: ^' permissible, and is pre¬ ferred. Similarly,, if the tank is made, of aluminum and. the expanded metal is also aluminum, the weld may be stress re¬ lieved by heat treatment without damaging the expanded metal structure. When the expanded metal is aluminum, and the tank is steel, the stress relief is best accomplished by other means, e.g., shot peening, which do not involve ele¬ vated temperatures that would soften aluminum.

When half sections 11 and 12 are welded with coil 30 inside, there is a localized softening or melting of coil 30 where it touches the inside of the welded seam and the adjacent portions of sections 11 and 12. This is not dam¬ aging to coil 30, however, and to the contrary, is believed to be advantageous. The interior surfaces of sections 11 and 12 where they meet around edge 29 (see FIG. 3) extends inwardly somewhat and pinches the midseσtion of coil 30. This causes a small portion of the inside walls of sections

11 and 12 adjacent to the joined edges ^' at the welded seam 13 to be out of physical contact with coil 30. The heat of welding softens a very narrow band around coil 30 suffi¬ ciently to permit coil 30 to fit itself more intimately to the inside wall contours and thus provide much better heat conduction contact.

In FIG. 7 there is shown a prior art tank 31 having a central cylindrical portion 32, an upper hemispherical pres¬ sure head portion 33 and a lower hemispherical pressure head portion 34. This container is formed by rolling a steel sheet into a cylindrical form and welding the joining edges of the sheet along seam 35. Upper pressure head portion 33 is welded to cylindrical portion 32 along seam 36. Lower pressure head portion 34 is welded to cylindrical portion 32 along seam 37. This prior art tank is made explosion resis¬ tant by being fitted with an internal mass or coil of ex¬ panded metal 30 which is in heat conducive contact with sub¬ stantially all of the inside surface of tank 31. The prior art tank, however, is not as strong as tank 10 of FIG. 1. Under the elevated temperature of a fire, both tanks 10 and 31 tend to bulge outward because of the increasing internal pressure. Any bulge in the tank wall may leave a pocket of vapor or liquid between the tank wall and coil 30, and such a pocket is a potential explosion site. Thus it is impor¬ tant to make tank 10 as resistant to bulging as possible. Tank 31 will bulge much more than container 10, because seam 13 of tank 10 provides an additional circumferential stiff¬ ening that is not found in tank 31.

Fuel tanks which are filled only with fuel and do not have an internal mass of expanded metal readily explode when exposed to fire. The fire heats up the tank and its fuel, which increases its vapor pressure until the tank ruptures. Vapor and liquid spew out and are ignited explosively blowing fuel in all directions and sucking the surrounding air into the explosion-produced vacuum to support the vio¬ lent combustion. Furthermore, there is no flame barrier to prevent the flame from spreading to the interior of the tank.

In the case of a fuel, such as propane, which is kept liquid in a pressurized tank, the explosion capabilities are even worse than that of gasoline because of the high internal pressure normally in the tank. Such tanks are expected to contain 450-500 psi in normal usage and to have a burst strength of about 900-1000 psi. In the event of a fire the propane tank eventually blows the pressure relief valve emitting vapor to the surrounding fire, and normally the vapor cannot be released fast enough to keep the internal pressure from rising still higher. Eventually the tank will rupture and a violent explosion will occur. When the tank is filled with a mass of expanded metal it will hold approx¬ imately 98% of the volume of liquid propane held in a tank with no expanded metal structure. The expanded metal struc¬ ture provides good heat conduction between the top and the bottom of the tank, reducing the potential for a violent explosion. The top of the tank in a fire is hot because of the burning vapor escaping. The bottom of the tank is cooler because of the evaporation of the liquid propane to vapor. The heat conducted through the internal structure of expanded metal reduces the temperature of the walls of the tank which reduces the potential rupturing the walls and causing an explosion. Furthermore, when there is any opening in the tank walls, whether it be the pressure relief valve vent, a split in the wall, or the like, the outrushing liquid and vapor will cause the expanded metal to plug up that vent or split allowing some outward leakage of vapor and liquid, but the tiny openings in the expanded metal are an effective flame barrier. Even though vapor and liquid leak outwardly and burn, the potential for explosion is almost completely eliminated because of the flame barrier characteristic. Since air cannot flow into the tank, the fuel will not burn until it escapes from the tank. This is believed to be the explan¬ ation for the explosion resistance characteristics of this invention but it is not intended that this invention be lim¬ ited thereto.

The third embodiment of tank 10 is shown in FIGS. 8

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and 9 as a tank that is substantially-spherical. This em¬ bodiment comprises upper hemispherical pressure head 40 and lower hemispherical pressure head 41 joined by a circumfer¬ ential weld seam 42. Upper guard ring 43 and lower foot ring 44 permit the spherical container to stand on a flat surface. This container has a single multipurpose valve 45 which serves as a liquid inlet, gas outlet, pressure relief valve, and a measuring tube for filling. As in the case of the previously described embodiments, there is a liquid discharge line 23, an inlet line 46 to the pressure relief valve, and a cap on inlet line 46. A spherical tank is the strongest shape for an internally pressurized container and in use it is substantially free of bulges which are poten¬ tial explosion sites.

In FIGS. 10 and 11 there is illustrated cap 47 on inlet tube 46 leading to-pressure relief valve 19 on tank 10. The reason for having tube 46 is to have any gas flow through valve 19 to come from near the inside surface of tank 10. The flow of gas from the lower part of the tank to the upper pressure head of the tank provides some cooling effect which. is desirable in reducing the potential for ex¬ plosion. Upper pressure head will be hot from any outside flame burning fuel that is escaping from that head, while liquid and vapor in the lower part of the tank is much cooler. Furthermore, if there is any bulging in the walls of tank 10, the vapor from lower in the tank will flow up¬ wardly along the path of least resistance, which will be along the inside surface of the walls where coil 30 has been pulled away from the wall.

- Cap 47 has a plurality of inlet holes around its periphery so as to offer no substantial resistance to the flow of vapor therethrough. Because of the possibility of pieces of coil 30 plugging the entrance to tube 46, it has been found to be desirable to employ cap 47 which has many inlet holes 48. Any one of holes 48 may become plugged but because of the large number of those holes, it is not likely that the entire flow into tube 46 will be so plugged.

The fuel employed in the tank of this invention is any flammable liquid which exists in the gas phase at am¬ bient temperatures and pressures. The most commonly known of such a fuel is propane. Other fuels for which the tanks of this invention are useful include natural gas, methane, ethane, butane, acetylene, cutting gases of various compo¬ sitions, etc.

The preferred metal for tank 10 is steel because of its strength, availability, and cost. Aluminum is also a metal for tank 10 having the advantage of being light weight and of permitting stress relief of welds at temperatures that would not melt the internal mass of expanded metal. Many other metals are operable but are much too costly for everyday use, e.g., titanium and various alloys.

While the invention has been described with respect to specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What is claimed as new and what is desired to secure by Letters Patent of the United States is: