NO790299L - PRESSURE CONTAINER FOR GASES. - Google Patents

Description

"Pressure container for gases"

The present invention relates to containers which are primarily intended for liquid petroleum gases, such as butane, propane, ethylene oxide, hydrazine, vinyl acetate and many others. The invention is particularly directed to a container construction which will either completely prevent explosion or will provide a considerable delay before such a container explodes, when it is delayed

for intense heat, for example during a fire.

In recent years it has been found that it will be possible to protect containers containing flammable gas/air or steam/air mixtures against explosive combustion by filling the interior of the container with an expanded metal aluminum foil. Such containers, such as vehicle gasoline tanks, usually contain enough oxygen to sustain combustion and allow the flame front to reach detonation velocities, and the expanded metal foil will prevent explosive ignition of gas or vapor mixtures by conducting the heat away from the point of combustion so quickly that the flame front cannot spread.

Containers with liquefied petroleum gases (LPG) always have superatmospheric pressure to keep the gas in the liquid phase as a liquid, in which case it is not possible for air to penetrate and form an explosive mixture. However, there have been a number of serious accidents when LPG containers, such as train tank wagons, have been exposed to fire, and the resulting explosions have had frightening dimensions, for example by producing "fireballs" with a diameter of 60 meters or more. Such explosions are known as explosions from expanding steam in boiling liquids (boiling liquid expanding vapor explosions, BLEVE) and investigations of such explosions have shown that they are the result of a special course of events. First, an accident occurs, for example when an LPG container train derails. A fire then occurs when the leaked LPG or the contents of one or another carriage in the train ignites. This will in turn cause heating from the outside of one or more of the LPG containers. When the temperature rises, the liquid phase will boil and the pressure in the container will rise until a normally occurring safety valve will open. The gas flow from the safety valve will then be ignited. The ignited gas flow from a leak or from a safety valve on one container will also affect another container. As the heating of the pressurized space above the liquid surface in the container continues, the container wall (usually made of steel) will be heated to a temperature where the tensile strength is reduced to the extent that the wall cracks.

When such a crack occurs, the container pressure will be reduced rapidly and the liquid will evaporate very quickly and expand to a

large cloud of gas which is ignited by the fire and causes a very destructive "fireball". At the same time, the burning gas that is sprayed out from the ruptured container will very often be driven more than a hundred meters away. If for some reason the safety valve should not open or, due to the container being overturned, is below the liquid level, the rupture may occur rather as a simple pressure break, but the same destructive result is obtained. This also applies to containers that do not have a safety valve.

A container, especially a pressure container for liquefied petroleum gas, according to the invention has at least partially a filling of expanded metal and an external swellable coating.

Small containers, such as gas cylinders for portable cooking appliances, can be completely filled with expanded metal foil, if it is easier to fit it in this way. Larger containers can be partially filled and still provide the same effective protection against BLEVE. This partial filling will be cheaper, will occupy less volume and will weigh less.

Depending on the thermal conductivity of the expanded metal used inside the container, it may simply be possible to line the inside of the container with the expanded metal, while the central part of the container is free of filler material. Thereby, the volume occupied by the expanded metal will be reduced, and if necessary it may then be possible to use a thicker material.

The intumescent coating that is used especially for mobile and portable containers should be highly shock-resistant and wear-resistant.

Intumescent paints and varnishes are known and are used especially in buildings to delay the combustion of wooden materials and to delay the overheating of steel structures. The swellable coatings will foam up and char so that a heat-insulating barrier is formed. The intumescent coating system may comprise a single layer or may be applied in several layers, including primer layers and cover layers depending on what is required for the protection of the particular container etc.

Swellable coatings typically comprise carbon compounds, a foaming agent, a catalyst and a resinous binder. The carbon compound is a chemical compound which, by reaction with the catalyst, forms a large volume of charred mass. This swellable carbon provides the non-flammable barrier so characteristic of swellable coatings. A typical carbon compound is di/tri-pentaerythritol. The blowing agent or "blowing agent" does not release flammable gases upon decomposition which occurs at a certain temperature. Suitable foaming agents are chlorinated paraffin and crystalline melamine. The catalyst is a material containing a high percentage of phosphorus which is decomposed by the application of heat into phosphoric acid below the decomposition temperature of the carbon compound. Commonly used materials are ammonium polyphosphate and melamine phosphate. Suitable binders are chlorinated rubber and vinyl toluene/acrylate.

The sequence of events in the swelling involves a number of steps which may occur simultaneously: cleavage of the catalyst, reaction between the resulting acid and the carbon compound to produce the charred mass, softening of the binder to form a skin which prevents the release of non-flammable gases from the blowing agent escapes, foaming the carbon material to provide a honeycomb-like blanket resulting in very effective insulation.

It was first thought that applying an intumescent coating to a container that already contained an expanded metal grid would reduce the time to an explosion during a fire because the insulation would prevent heat from being conducted out of a colder part of the container wall.

However, it has been found that the heat is transferred into LPG, where the energy is consumed to transform liquid into gas, and a limitation of the amount of heat that passes through the container wall is therefore of the greatest importance.

As the insulating capacity of the coating before foaming is small during use, where heat penetrates through the container wall to evaporate the liquid, the coating will not prevent normal operation when the container is used.

Containers to be protected by the proposed method can of course vary in size from buried storage tanks to gas containers for camping cooking appliances and aerosol containers. Freon, which has usually been used as propellant gas in aero-solar containers of all kinds, has been criticized for various reasons in many countries, and it is therefore proposed to replace Freon with, for example, butane, but the explosion risk for such containers naturally becomes a serious problem , which is not present with Freon as Freon is not flammable.

The need for large storage facilities for LPG has resulted in many more potential dangers where aerosol filling facilities are located.

It is believed that in the conditions involved in preventing BLEVE in containers according to the present invention during a fire, there is first a delay in the rate at which heat is supplied to the container's exterior caused by the swellable coating, and then there is a rapid conduction of heat away from the container wall by means of the expansion metal into the liquid, which absorbs a lot of heat energy during evaporation.

When the container also has a safety valve and when this opens, some liquid gas will evaporate and thereby use a large amount of heat energy which is conducted through the wall of the container, through the expanded metal grid and into the liquid. due to the exterior of the container being insulated by the foamed, swellable coating, the temperature of the container wall will be lowered significantly from its previous temperature. Thereby, it is prevented that the walls of the container get such a high temperature that there is any significant weakening, and the internal pressure in the container is therefore not great enough for the wall to burst. The cooling effect will therefore increase the time between opening and closing the safety valve, thereby reducing the amount of gas that is released.

With non-stable gases such as ethylene oxide, which can be explosive as a 100% gas without the supply of oxygen and which can self-ignite at relatively low temperatures, perhaps as a result of fission, in addition the expanding metal may prevent the liquefied gas at some point reaches a self-igniting temperature. Ethylene oxide in particular is extremely dangerous because as soon as it has reached a temperature of 560°C, an explosive decomposition occurs which continues even if the source causing the temperature is removed. The fission can therefore continue without this being obvious, until self-ignition occurs. It should be pointed out that the expanded metal, due to its inherent ability to spread heat quickly, will be able to prevent the ignition temperature from being reached, so that the danger is greatly reduced.

A pack of aluminum stretch metal grids with a weight of 25 grams per liter will have up to 500 times the thermal conductivity of petroleum gas and will therefore transfer heat from the container wall 500 times faster than if only the gas were present. due to the gas being in contact with the container wall at all points thereof

surface, except where the tensile metal is in contact with the wall, is assumed

however, the majority of the heat is conducted from the wall in

in the gas and then immediately to the expanded metal grid. Tests have shown that the expanded metal grid can be in contact with the wall of the container at relatively few points. This is of great importance for transportable containers, where there can be a displacement of the expansion metal away from the wall in the container due to, for example, vibration, and if the expansion metal was the primary heat conductor from the wall, this could be a serious problem.

Various tests have been carried out to find out how much protection containers according to the invention provide and fig. 1 shows a test device used during these tests.

Tests were carried out with standard dome-shaped non-valve-only gas containers made of steel for camping use, containing 200 grams of liquid butane. The test device is simple in itself and comprises a protective steel plate A, through which a gas burner B protrudes. A holder C is also attached to the plate A

for gas containers D. The flame E from the gas burner B is arranged so that it can be directed towards the top and side of the container at a point above the liquid level.

A first test was carried out with a gas container which is usually supplied without any protection. After 31 seconds, the curved bottom F in the container was pushed out, and the container burst after 2 minutes and 26 seconds. The container did not burst due to weakening of the container wall, but simply due to very high pressure developed by the expanding gas.

Another container was completely filled with expanded metal mesh of an aluminum alloy. The grid, which was 0.04 mm thick,

and this container was tested. The time before the bottom was pressed out was 54 seconds, while a pressure break occurred after 4 minutes and 18 seconds.

Two additional tests were performed. First it was tried

a container with the same filling of expanded aluminum, but with a 15 mm thick insulating coating (the coating used is manufactured by Sigma Coatings B.V, Holland and is known under the name "Firescreen"). The bottom of the container was not pushed out until after 23 minutes and 51 seconds had elapsed and the container did not burst.

The fourth container was essentially the same as the third, but had an additional swellable cover layer (Firescreen Finish") with a thickness of 700 microns. Expression of the bottom occurred after 33 minutes and this time too no breakage occurred.

From the above data it appears that there is a significant reduction in the risk of breakage occurring in the container construction according to the invention.

Another series of tests was carried out with a flame burner which had a larger flame and produced more heat. The ambient atmosphere in which the second series of tests was conducted had a lower temperature of the order of 10 to 15°C. which causes a greater difference in the liquid/gas phase. An unprotected container split open in the side and produced a true BLEVE after 45 seconds, and a container filled only with stretch metal foil also ruptured and produced a true BLEVE after 50 seconds. In a third container, filled with stretch metal foil, a cover layer of "Firescreen" of 15 mm and a layer of "Firescreen Finish" of 700 micron, blue ste! the gas away at the bottom after 26 minutes and 45 seconds due to

high internal pressure. There was no splitting of the side of the container due to weakening of the container wall. The effect of the blow-off at the bottom is more similar to a pressure relief effect that occurs when a safety valve is provided and released.

However, it should be pointed out that there is a considerable delay before a pressure break occurs.

Examples of containers produced in accordance with the present invention will now be described with reference to the accompanying

the following drawings where:

Fig. 2A and B schematically show elevation and ground plan

of a portable LPG container of a type used for cooking appliances in caravans and the like. Fig. 3A schematically shows a section through a container used in train transport, Fig. 3B schematically shows a section through another type of container used in train transport and

fig. 3C shows a longitudinal section of the one in fig. 3B showed container.

The one in fig. The container shown in 2A and B is basically of a well-known type and comprises an elongated cylindrical wall 1 which has dome-shaped ends 2 and 3. On the lower end 3, an average cylindrical strut 4 with a lower flange 5 is attached so that the container could stand firmly on the ground. The upper end of the container has a gas outlet 6 with a valve unit 7 with one manual valve and one safety valve. These items on the container are standard. However, the container has an internal spiral-wound coil of aluminum alloy expanded metal foil which, although occupying only between 1 and 2 percent of the container's internal volume, extends throughout the container, so that it is able to conduct heat from the container wall into it liquefied gas, regardless of how the container is oriented. The expanded metal grid of aluminum alloy has a thickness of 0.04 mm and has a strand width of 1.78 mm. In the longitudinal direction of the lattice meshes, i.e. the length of the parallelogram-shaped openings that are provided in the lattice during the stretching process, the length of such an opening is 15 mm. The width of each opening is 8.4 mm in the extended state. When such a grid is wound into a coil it has a density of approximately 0.03 grams per cm.

On the container's outer surface, a swellable coating 9 is applied, which can be of the single-layer or multi-layer type, as used in the samples described above.

In fig. 3A is shown a section through a much larger container of the type usually fitted to a railway carriage. The container again has a substantially cylindrical wall 11 and dome-shaped spherical ends 12 and 13, as shown in fig. 3C. Unlike the one in fig. 2A and B shown portable container, the main axis of the cylinder of the railway car container according to the present example is mainly horizontal and includes a pressure relief valve (not shown). On the outside, the container has a single or multi-layer swellable coating 14 as in the preceding example, but in the interior there is only a partial filling with expanded metal grid 15 which is designed as large coils 16 which are supported separately in an annular row of radially extending expanded metal plates 17 and of two mainly semi-cylindrical expanded metal sheets 18.

Fig. 3B and C show an alternative example to that shown in fig. 3A. In this example, the expanded metal grid is arranged in a number of layers 1? and forms a ring shape against the wall of the container and is internally supported by a cylinder 20 of expanded metal with larger overall dimensions and which is braced internally by transverse rods 21.

It should be pointed out that the arrangement of the expanded metal inside the container can be varied from application to application and depending on the size of the container.

Claims (12)

1. Container, characterized in that it is at least partially filled with a stretch metal grid and has an outer swellable coating. 2. Container according to claim 1, characterized in that the container is a pressure container for liquid petroleum gas. 3. Container according to claim 1 or 2, characterized in that it is completely filled with an expanded metal grid. 4. Container according to claim 1 or 2, characterized in that the expanded metal grid filling is arranged as a lining in the container. 5. Container according to any one of the preceding claims, characterized in that the expanded metal grid is produced from an aluminum foil. 6.B container according to any of the preceding claims, characterized in that the container is a gas bottle. 7. Container according to any one of claims 1 to 5, characterized in that the container is a railcar or road transport tank. 8. Container according to any one of claims 1 to 5, characterized in that the container is a permanently mounted tank. 9. B container according to any one of claims 1 to 5, characterized in that the container is an aerosol can. 10.B eholder according to any of the preceding claims, characterized in that the swellable coating is applied in a single layer. 11. Container according to any one of claims 1 to 9, characterized in that the swellable coating is applied in several layers. 12.B eholder according to claim 1, essentially as described with reference to any of the examples and as shown in the drawings.