US3846178A

US3846178A - System for absorption of explosive energy by pressure mitigation

Info

Publication number: US3846178A
Application number: US00257694A
Authority: US
Inventors: J Evjen; J Herrin
Original assignee: General Electric Co
Current assignee: Gates Energy Products Inc
Priority date: 1972-05-30
Filing date: 1972-05-30
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05

Abstract

1. A system for absorbing excessive pressures resulting from explosive forces generated by exploding material comprising. (a) a container; (b) a housing disposed in said container and occupying a first volume; (c) a source of explosive material disposed in said housing; (d) a second unfilled volume within said container and exclusive of said first volume; (e) a compressible closed-cell material occupying a third volume within said container and exclusive of said first and second volumes, said third volume being interfaced with said second volume; (f) venting means in said housing for permitting transmission of said explosive material from first volume to said second volume.

Description

Nov. 5, 1974 J. M. EVJEN ETAL SYSTEM FOR ABSORPTION OF EXPLOSIVE ENERGY BY PRESSURE MITIGATION 2 Sheets-Sheet 1 Filed May 50, L972 Nov. 5, 1974 J. M. EVJEN ETAL SYSTEM FOR ABSORPTION OF EXPLOSIVE ENERGY BY PRESSURE MITIGATION 2 Sheets-Sheet :3.

Filed May 50, 1972 United States Patent 3,846,178 SYSTEM FOR ABSORPTION OF EXPLOSIVE ENERGY BY PRESSURE MITIGATION John M. Evjen, Gainesville, and Jerry Herrin, Alachua, Fla., assignors to General Electric Company Filed May 30, 1972, Ser. No. 257,694 Int. Cl. H01m 1/04 US. Cl. 136-166 Claims ABSTRACT OF THE DISCLOSURE Excessive pressures resulting from energy generated by exploding material such as gases in a container such as, for example, a battery case, are absorbed by compressible materials such as low density closed cell materials. Preferably, the volume of free space within the container is kept as small as possible with respect to the volume of the compressible material space both to reduce the volume in which the explosive materials may accumulate and to provide a large ratio of expansion to provide maximum pressure reduction.

BACKGROUND OF THE INVENTION This invention relates to a system for absorbing explosive energies. In one aspect, the invention relates to a system for absorbing energies generated by exploding gases in a closed container.

Explosive substances such as gases, liquids, or solids may sometimes be generated and thereafter accumulate in closed devices such as a battery containing a plura'ity of cells. The substances may be gases, or liquid or solid explosive substances. These substances may be generated by chemical reaction of other materials within the enclosure. Explosion of such material causes a sudden rise in pressure in the free space within the container which, if not in some manner contained or absorbed, will in turn burst containers not normally designed to withstand such abnormal pressure. This is true not only of sealed devices but vented containers as well where, for example, a pressure relief vent is intended to prevent gradual pressure buildup. The problem can also be encountered in a system containing purging means to prevent accumulation of explosive gases-even at atmospheric pressureswhere the purging mechanism fails to purge the container for one reason or another. While containers can be made heavier in anticipation of such pressures, this can add considerable weight to the device which, in the case of an aircraft battery, is highly undesirable.

In one particular example a sealed battery case containing a number of cells may accumulate a mixture of hydrogen and oxygen gases given off by the cells during the course of charging or discharging. While pressure relief valves may be employed in the casing to vent accumulated gases which build up pressure, the gases, even at ambient pressure, may still represent an explosive mixture which is capable, upon ignition, of generating pressures many times the original ambient pressure.

Several solutions have been proposed including the construction of a container having suflicient strength to withstand such pressures. However, such a container would add to both the cost and weight of the device. It has also been suggested to employ breakaway sections in the container in the manner of safety pressure relief type vents. However, small vent-like ports which would merely vent the gas may not reduce the pressure rapidly enough to prevent explosion of the entire container while larger breakaway portions may still result in spewing out the contents of the device. It has also been proposed to increase the flexibility of the container walls so that they may expand, thus increasing the container volume to contain the gases resulting from the explosion at more moderate pressures.

However, it would be desirable to contain any explosion with little or no change in the design of the container. It is therefore an object of the invention to provide a system of absorbing abnormal pressures generated within a sealed device by the explosive energy generated by ignition of explosive substances therein. It is another object of the invention to provide means for absorbing abnormal pressures within a device without substantially increasing the weight of the device. It is yet another object of the invention to provide means for absorbing abnormal pressures generated by explosions within a device without substantially increasing the volume of the device. These and other objects of the invention will be apparent from the drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cutaway isometric view of a battery case containing the system of the invention.

FIG. 2 is a top sectional view of the case of FIG. 1.

FIG. 3 is a rear sectional view of the case of FIG. 1.

FIG. 4 is a partially exploded and cutaway isometric view of the battery of FIG. 1.

DESCRIPTION OF THE INVENTION Referring now to the figures, the system of the invention is illustrated as applied to a battery comprising nickelcadmium cells used, for example, in an aircraft. A battery is generally indicated at 2 comprising an outer casing 4 having a cover 6 thereon. Battery 2 contains a plurality of cells 10 therein which are series-connected together to provide a preselected voltage at terminals 22 on the exterior of casing 4 of battery 2. Each of the cells is provided with a vent 12 thereon which permits gases generated within the cell to vent into the casing at a given pressure. The casing may be provided with conventional venting means a and 80b for mitigation of pressure buildup due to these gases.

While the cells occupy most of the volume of the casing, a small volume 40 on top of the cells is open to permit access to each of the cells and the cell terminals for interconnection of the cells together. This space, together With other spaces between and around the cells, etc., and hereinafter called the free volume, while small with respect to the entire volume of the battery case, is sufficiently large to allow hydrogen and oxygen gases to accumulate therein. These gases are liberated by the cells on overcharge in the proper proportion to provide an explosive mixture which can, upon ignition, raise the internal pressure to greater than p.s.i.a pressure much higher than casing 4 is normally designed to withstand. It should be noted here that the ignition and explosion of accumulated gases within a device is a very abnormal 0ccurrence and does not occur with suflicient regularity as to warrant the added expense and weight of increasing the thickness of all battery casings as this would add undesirable weight to the aircraft, thus decreasing the payload.

It should be noted here that the system of the invention does not necessarily prevent such an explosion or prevent damage to the contents of the container. However, the system of the invention will prevent damage to external surroundings in the unlikely event of such an explosion by absorbing the generated energy within the container.

In accordance with the invention, however, a portion of the battery case is filled with a compressible cellular material 50 which is preferably a closed cell foam material. As best seen in FIG. 4 this material, in a preferred embodiment, comprises a series of

foam blocks

52, 54, and 56 which have been cut and shaped to fit respectively into the volume between the two rows of cells; the space in front of the short row of cells under the leads to the external terminals 22; and in the cover, i.e., over the cells.

In accordance with the invention, a substantial portion of the free volume within the battery not taken up by the cells, preferably as much as possible, is occupied by the compressible cellular material to provide a minimum volume of open space available for accumulation of gases. The use of the term substantial portion is intended to define at least by volume of the free volume and preferably over 50%. Upon explosion of the gases, the gassuch as air-within the closed cells of the cornpressible cellular material is compressed by the pressure of the expansion of the ignited gases until a pressure equilibrium is achieved. When the volume of space within the battery casing occupied by the compressible cellular material is large with respect to the volume of free space in which the gases accumulate, the effect of Compression of the cellular material results in a large reduction in the pressure which would otherwise be developed by the explosion since the gases generated by the explosion expand into a volume (due to compression of the cellular material) which is much larger than that of the initial volume. Thus, for example, the pressure developed Within casing 4 using the system of the invention could be reduced from 90-150 p.s.i. to about 50 p.s.i. if the volume of compressible material was twice the volume of free space in casing 4 (not considering any expansion of the container walls and assuming compression of the cellular material With negligible inertia and elastic effects).

While the foregoing refers specifically to a battery in which oxygen and hydrogen gases are present, the invention can be applied to other devices in which other explosive substances can form, accumulate, or be stored. Depending upon the types of gases or other materials which may be generated or reacted together to form ignitable substances and the free volume to which gases resulting from such an ignition will be exposed (before compression of the cellular material), the volume of compressible cellular material may be increased or decreased. That is, if the particular explosive substances known to exist within a given device are capable, Without expansion, of increasing the pressure, upon ignition, to, for example, three times that of ambient pressure and the container is only capable of withstanding pressures of, for example, twice ambient pressure, the volume of compressible cellular material-before compressionneed only be approximately one-half the volume of the unoccupied free space within the container or /3 of the total, otherwise unused, volume. After ignition, the pressure generated will be effectively lowered by /3 due to compression of the compressible material-because the volume will have been increased by 50%. On the other hand, if the pressures capable of being generated are much larger, a larger ratio of volume of compressible material to volume of free space will be necessary.

The compressible cellular material to be used within the sealed container comprises closed cell, low density material such as foamed polymeric materials such as, for example, foam polyethylene, polyurethane, or other easily compressible material. In a preferred embodiment, the closed cellular material may be made from a polymer which also contains fire retardant additives and which has been expanded by the use of gases which are themselves not easily ignitable. The foams may be made, for example, using carbon dioxide liberating blowing agents well known to those skilled in the art which result in a foam structure containing carbon dioxide gases which are not as combustible as would be foams using other blowing agents.

The foamed structure may comprise small cell or large cell foams but in either event should comprise a low density foam structure; that is, the ratio of open space or gas-filled space to the volume of polymeric material should be high to provide a maximum volume of compressible substance, i.e., gaseous rather than solid. While the invention thus contemplates foamed plastics as the preferred compressible material, the cell size is not deemed to be important and may, indeed, comprise one or more sealed cells such as sealed bags of air or inert gas. The uase of the term foam structure is therefore intended herein to define a closed cell structure comprising at least one cell.

While the invention has been described with respect to utilization of such a system in a sealed aircraft battery containing cells which may generate hydrogen and oxygen, the use of such a system in other sealed containers is to be considered within the scope of the invention which is to be limited only by the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A system for absorbing excessive pressures resulting from explosive forces generated by exploding material comprising.

(a) a container;

(b) a housing disposed in said container and occupying a first volume;

(c) a source of explosive material disposed in said housing;

(d) a second unfilled volume within said container and exclusive of said first volume;

(e) a compressible closed-cell material occupying a third volume within said container and exclusive of said first and second volumes, said third volume being interfaced with said second volume;

(f) venting means in said housing for permitting trans mission of said explosive material from first volume to said second volume.

2. The system of claim 1 wherein said closed cell material comprises low density foamed polymeric substances.

3. The system of claim 1 wherein said second volume is less than the volume occupied by said compressible closed cell material.

4. An explosion-resistant rechargeable battery system comprising an enclosed casing having therein (a) a plurality of vented electrochemical cells capable of generating explosive gas mixtures of hydrogen and oxygen;

(b) a free volume within said casing not occupied by said cells; and

(c) pressure absorption means comprising a compressible, low density, closed cell foam ocupying a substantial portion of said free volume, said explosive gas mixture tending to collect in the unfilled volume of said free volume not containing said absorption means whereby upon an explosion of said explosive gas mixture, the pressure generated by said exploding gases in said unfilled volume will be redistributed into an increased volume due to the compression of said foam.

5. The battery system of claim 4 wherein said foam occupies at least 50% of said free volume.

6. The battery system of claim 4 wherein said foam comprises foamed polyethylene.

7. The battery system of claim 6 wherein said casing is further provided with (a) terminal means adjacent said cells;

(b) a removable cover;

(c) a first layer of said foamed polyethylene Within said casing adjacent said terminal means; and

(d) a second layer of said foamed polyethylene with said casing adjacent said removable cover.

8. The battery system of claim 4 wherein said casing is further provided with venting for mitigating pressure buildup.

9. An explosion-resistant rechargeable battery system comprising a rectangular parallelepiped casting of about 30-40 mils thickness having a removable cover thereon and having at least two parallel rows of rechargeable vented nickel-cadmium cells therein separated by a first layer of foamed polyethylene, terminal means on said casing adjacent one of said cell rows and a second layer of foamed polyethylene adjacent said terminal means, and a third layer of foamed polyethylene adjacent said removable cover, said vented cells capable of generating explosive mixtures of hydrogen and oxygen during overcharge, said three layers of foam occupying at least half of the free volume within said casing not occupied by said cells said cells generating explosive gas mixtures of hydrogen and oxygen during overcharge, said gas mixture tending to collect in the unfilled volume of said free volume not filled by said three layers of foam whereby upon explosion of said gases, the energy released thereby will be absorbed within said casing and thereby preventing rupture of said casing.

10. The system of claim 4 wherein said explosive gas mixture, upon ignition, is capable of generating a first pressure within a fixed volume equal to the unfilled volume, said casing has a structural strength capable of withstanding without rupturing up to a second predetermined internal pressure, said first pressure is greater than or equal to said second pressure, and the magnitude of said References Cited UNITED STATES PATENTS 3,597,279 8/1971 Branche 136173 2,812,378 11/1957 Barrett 136l66 3,314,823 4/1967 Balaguer 136173 OTHER REFERENCES Teach: Polystyrene, Reinhold Plastics Applications Series, New York, Nov. 15, 1960, pp. 128 and 134.

DONALD L. WALTON, Primary Examiner Us. 01. X.R. l36179 UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Patent No, 846, 178 Dated November 5, 1974 Inventor(s) John M. Evjen and Jerry Herrin It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, Column 4, Line 33, insert "said" between from and first Claim 9, Column 4, Line 73, "casting" should be casing Signed and sealed this 4th day of February 1975,

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents uscoMM-oc e037 e-pea FORM PO-1050 (10-69) w u.s. sovsnuusm PRINTING omc: lacs o-ase-su