GB2292997A - Improvements in and relating to explosion suppression - Google Patents

Abstract

The invention relates to a method of limiting the environmental disturbance of an explosion by generating an airborne liquid dispersion (23) which at least partly surrounds a body (22) of explosive material and detonating the explosive material (22) into the dispersion (23). A waterfall or liquid-filled mortors can be used to create the dispersion. The invention can be used in the disposal of munitions. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO EXPLOSION SUPPRESSION This invention relates to a method and apparatus for limiting the environmental disturbance of an explosion by generating an airborne liquid dispersion in the vicinity of the explosion.

In many countries military explosives in long term storage are no longer needed. It is expensive to guard them and to move them to the most remote disposal sites.

Controlled burning may cause more pollution and can lead to inadvertent high-order explosions. Intentional explosion under controlled conditions is the best option. However the quantities of explosives to be disposed of are enormous and the civilian irritation threshold for a long series of repeated explosions at random times is very low. Accordingly one aim of this invention is to obtain maximum reduction of long-range noise and ground-shock during controlled disposal with minimum cost.

My International patent application PCT/GB94/02079 and GB Patent Application 9505825.1 describe methods of generating an airborne liquid dispersion by the disruption of liquidfilled bags of thin-walled plastics material but this invention is concerned with a method of limiting the environmental disturbance of an explosion by generating an airborne liquid dispersion which at least partly surrounds a body of explosive material and detonating the explosive material into the dispersion.

In one embodiment the liquid is contained in at least one bag of thin-walled plastics material placed adjacent to the site where the body of explosive material is located and said at least one bag is disintegrated to form the airborne liquid dispersion at an appropriate brief interval before said explosive material is detonated.

Suitably, clusters of liquid-filled bags of plastics material, some of which contain gas bubbles, are used to generate the airborne liquid dispersion. The clusters of bags can be assembled into an array of building elements, some of which building elements include two or more bags in a common casing.

Alternatively, the airborne liquid dispersion can be the result of a waterfall such as one created by pumping water up to an elevated discharge location over a period which is long compared to the time taken to discharge the pumped water from the discharge location.

As a further alternative, the airborne liquid dispersion can be created by projecting liquid towards the body of explosive material from at least one discharge tube (eg a tube which ejects liquid therefrom explosively).

If desired, the liquid is water containing an additive to enhance its environmental disturbance - limiting properties.

To assist in the removal of chemical or radioactive products from the blast cloud created on detonation of the body of explosive material it is possible to include useful agents in the liquid forming the dispersion. Amongst such agents can be mentioned neutralising agents to counteract poisonous chemicals, sterilising agents to counter biological materials and capture media for limiting the spread of radioactive materials. The agents can, for example, be preadded to empty bags (so that they become active ingredients in the bags consequent to filling of the same with water), be introduced into the water used to fill the bags or be provided in rupturable canisters inserted into bags prior to, during, or after filling with liquid or can be inserted between bags after filling the latter.

It has been noted that on detonation of an explosive charge surmounted by an array of destructible thin-walled liquid-filled bags, a large volume of water becomes aerozolised (10keg of even cheap commercial explosive such as ANFO is capable of throwing more than five tonnes of water into the air - to a height of over 30 metres - in an aerozolised state) and thus any neutralising agent included in the bags gets widely dispersed throughout the products dispersed by the explosion.

In a chemical combating role suitable neutralising chemicals can be included (eg singly or as a "cocktail"). In a biological combating role - an enzyme or biocide (eg chlorine powder) can be used and in a radiation combating role a glue such as PVA could be included as a capture medium to trap radioactive particles from the air and deposit them in a limited region around the site of the explosion.

A preferred material for the bag walls is polythene with a thickness of between 100 and 500 microns. Material with a yield stress of 17 MPa has proved to be very suitable. 250 microns thickness material has also proved to be very suitable. A layflat dimension of between 250 and 1000 mm is convenient and 750mm gives a good sized tubular bag for many application having a flattened cylindrical shape of a nominal diameter of just under 500mm. This can accommodate a head of water of some 1.8 metres and accordingly can safely be used with water heads of 1000mm.

More expensive bag materials are not ruled out and sheeting having a layer of nylon bonded to a layer of polyurethane can be used, particularly where a cluster of bags may be deployed on a more long term basis such as in the field storage of ammunition or when deployed in a battlefield around sensitive installations, fuel tanks, aircraft or vehicles.

The lengths of the bags can vary considerably but in a typical case will be at least one and probably at least three times the nominal diameter of the bag.

Closing the open end of a length of layflat plastic tubing after filling with water can be effected in a variety of ways. Small diameter bags (say of 100mm filled diameter) can be tied off with two thumb knots or twisted round to create a closed end which is then secured closed using a length of string, a twisted wire a proprietary ratchet tie (eg a polyurethane cable tie) or a clip (eg a plastics clip of the kind mass-produced for closing freezer bags).

It is also possible to partially weld closed the open end of a layflat tube, thus facilitating the filling and making unwanted spillage of water less likely. Valves made from further lengths of plastics sheeting can be used and one form of bag incorporating such a valve is described in my aforementioned PCT application.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a charge of explosive material surrounded by an array of tubular water-filled bags, Figure 2 shows some examples of pluri-tube building elements for making an array as shown in Figure 1, Figure 3 shows the use of explosions to suppress later explosions in a first method according to the invention; Figure 4 shows the use of a waterfall to suppress explosions in a second method according to the invention, and Figure 5 shows the use of discharge tubes to eject an airborne liquid dispersion according to a third method according to the invention.

Figure 1 shows an explosive charge 10 (e.g. a bomb, shell or boxes of munitions) surrounded by an array of water-filled tubes 11 made from plastics tubing. Each bag 11 contains around 250 kg of water so that a total of some 7250kg of water is shown in the illustrated embodiment. Between a few hundred kilograms and a few thousand kilograms of water would typically be massed around a charge containing some ten kilograms of high explosive but these figures are examples only and are provided to give an indication of the scale on which water is used in the method of the invention rather than guidelines for specific use. However contrary to what the prior art suggests, I find that large volumes of water are desirable, it being preferable to use an excess of water than to use less than is required to properly quench the explosive blast.

An important practical matter is how to efficiently handle water-filled bags of this size when they are made from thin-walled, low cost plastics tubing. Tubular bags which contain more than a few tens of kgs of water are very difficult to move in a filled condition and they are very hard to drag across a level surface. Bags which are several times their diameter in length roll down the most imperceptible incline. Only fairly short bags filled to taughtness with well-formed square "pillow-case" ends provide roll stability.

Long single bags not fully filled can roll in several directions at once. For this reason the bags shown in Figure 1 are first located in the desired position and then are filled in situ. Figure 2 shows one useful way of arranging the bags so that the array of Figure 1 can easily be secured.

By locating two or more tubes in a common casing 12 to form building elements 13 the problem of tubes rolling out of place can be effectively prevented. Once in a common casing 12, rolling requires relative motion between adjacent tubes 1 which is prevented by friction between bag materials along the lines of contact. If required these frictional forces can be enhanced by double-sided adhesive tape located between bags 11 and/or between the casing 12 and the bags 11.

Figure 2 shows a dual pack 13a, two forms of triple pack 13b,l3c and two forms of quad-pack 13d and 13e. The triple pack 13c requires careful filling if it is not to convert into the configuration of pack 13b but once the outer casing 12 is taut the equilateral triad 13c is quite stable even on sloping ground and can resist three men jumping on it. Triads 13c are a good way of building height in the cluster of bags which is needed to cover bigger explosive charges.

The tensions in the outer casing 12 will share the load and so reduce those of the inner bags 11. This does not help material near the "hemispherical" ends of the inner tubes but this region experiences stresses which are half those of the hoop stress in the cylindrical region. If the circumferential length of the casing 12 is correct, the safe head of water can be nearly doubled and nearly constant stress in all directions of the material can be achieved. In addition to the triad, flat groups of two or more tubes can interlock with one another and so also be used to build walls as shown in Figure 1.

In practice it may be useful to have available, when creating an array of water filled bags around a device to be detonated, bags (known as "sausages") which are some 1.3 metres long and some 100mm in diameter. These can be used for filling gaps which might arise between building elements 13 or which might be needed to "wedge up" other bags or bagfilled elements. A partially filled "sausage" is also useful for filling gaps close to the charge, since it has been found to be desirable to close-couple the water volumes to the blast source and a partially-filled bag can be bent around munition to do just that.

To prevent excessive ground cratering, the munition can be placed on a non-destructible baseplate and the array of water-filled bags assembled over and around the baseplate.

Figures 1 and 2 show the use of some bags 14 which contain air bubbles. I have found the inclusion of pockets of air in the array of bags to be beneficial for blast/noise suppression and one way of including air within a bag 11 is to include a roll of "bubble-pack" 15 in the bag. Also, the use of building elements 13 by including bags in a containing casing 12 also creates air compartments 16 between the tubes 11.

Arrays of water filled bags can also be used in a protective role to create a target zone for Incoming ballistic devices (shells or mortors) to reduce the explosive impact such devices would otherwise produce. Filled arrays can thus be used to protect strategically important areas likely to be targets for terrorist attach or sensitive areas or valuable equipment in a battlefield situation.

The behaviour of explosives and the transmission of shock waves through air and through water have been the subject of intensive study for many years and the results are now well known. Except in the region very close to the explosive charge, the velocity of propagation of a shock wave depends on the square root of bulk modulus over density. For water this is about 1500 metres per second. For a gas the bulk modulus is the product of pressure and the specific heat ratio (1.4 for air). Both the density and the bulk modulus of a gas rise directly with pressure so this has no effect on the speed of sound. Temperature changes at constant pressure do change the density and so the speed of sound rises with the square root of absolute temperature. At 0 C the velocity in dry air is 331 metres/sec.At 30000C, about 11 times hotter on the absolute temperature scale, it would be 3.3 times faster i.e.

1100 metres per second. Higher speeds occur for the lighter gases like carbon monoxide and steam which are produced by explosions.

Things get more interesting if there are bubbles of air in water or drops of water in air. If these are small compared to the wavelengths of sound, the air bubbles gixe a great reduction of bulk modulus but not so much in the density.

Shock waves with the magnitude of explosions squash the bubbles to very small volumes but the water around them has to be given kinetic energy to move into the bubble space and then again when the bubbles bounce back. Furthermore squashing bubbles makes the air in them very hot so water can be evaporated. There is also the interesting result that the back of the shock wave, where compression has reduced the column of bubbles ought to be travelling faster than the front where the bubbles have not yet been compressed. This makes for very high pressure gradients which are associated with large internal losses. Hence theory suggests that useful energy absorbtion will occur by detonating explosive charges into mixed air/liquid enviroments such as an airborne liquid dispersion.

The detonation of one explosive charge buried below a set of water bags can be suppressed well. Suppression can be improved by the inclusion of air spaces at carefully chosen points in the pile of bags as discussed above and in my other patent applications previously referred to. But there is a way in which the interaction of hot gases and water droplets can be still further improved. The first problem for the simplest bag arrangement is that when gases are hottest and when the enhancements of heat-transfer coefficient by high pressure is highest, the surrounding water has not yet been disturbed and so the evaporating surface is still just an expanding shell with a rather low internal area. The second problem is that later in the process, the water droplets and the cooler gases are moving in more or less the same direction so that the relative velocity is lower than it might be.

For the disposal of many thousands of items of military stores it is possible to achieve much greater reductions by using small suppressed explosions to project large quantities of dispersed water droplets in a cloud 23 towards larger explosions. The smallest items, perhaps propellants and uncased explosives 20, could be placed at the perimeter of a large circle and encased in packed water bags 21 with carefully chosen air inclusion. The bags 21 could be arranged in a wider stack on the outside as shown in Figure 3. This would bias the water cloud 23 to move towards the centre where there could be bigger charges 22 such as bombs and larger mines. These would be closely packed, with multiple detonation routes which would be delayed by the fall time of the main body of water.The bigger charges would thus explode into a deluge of fast, incoming water drops so that the highest pressures and temperatures would occur with maximum counter-flow velocity and large surface area.

An even more rapid method of disposal uses the facility sketched in Figure 4. This requires a cliff or quarry 25 with a small pond 26 of water at the bottom. At the top of a cliff is a large tank 27 of water built over a short channel 28.

The floor of the tank has a trap door 29 which can be opened to release a deluge of water into the channel and then outwards over the end of the channel and down on top of a charge 30 suspended from two winches 31 a safe distance below.

The time of fall of the cloud of water drops can be accurately calculated and so the detonators can be triggered at the correct instant. The dispersed cloud of water drops provides ideal consitions for suppression. The water would run down into the pond 26 and could be pumped back to the tank 27 for the next shot. Chemical residues in the water could be neutralised. Soot and scrap steel could be collected.

If quarries or cliffs are not available the use of a "water mortar" shown in Figure 5 could be used. This would consist of a large steel tube 40 with a diameter of about 1 meter and a length of say 10 metres arranged as one edge of a tetrahedron as in Figure 5. At the lower end of the tube would be an upward pointing breech, fitted with a glow-plug, into which can be injected propellant fuel at a controlled rate. Propane and compressed-air laced with oxygen would be suitable.

The tube 40 would be partly filled with water and the propellant quantities and feed-rates chosen to project water and combustion products to a height of 30 metres or more. A slightly flared blunderbuss muzzle could be used to produce a spread discharge. The trajectory of the cloud of water drops 41 could be arranged to meet clouds from a number of other mortars aimed to converge with the charge 42 just at the moment of detonation. The firing signal could be inhibited if the time integral of the breech pressures were not all correct.

As an alternative to ejecting the water from the tube using an explosive charge, compressed gas (eg air) could be used, release of the gas at the correct moment being controlled by a conventionally obtainable dump valve. If the tubes used to project water at the explosive charge 42 are equidistant therefrom, simultaneous discharge from the tubes 40 will be acceptable (and probably desirable). If differing distances are involved, the timings of the separate tube "firings" need to be synchronised to ensure at least near simultaneous arrival of the clouds of water drops at the charge 42.

Claims (12)

Claims

1. A method of limiting the environmental disturbance of an explosion by generating an airborne liquid dispersion which at least partly surrounds a body of explosive material and detonating the explosive material into the dispersion.

2. A method according to claim 1, wherein the liquid is contained in at least one bag of thin-walled plastics material placed adjacent to the site where the body of explosive material is located and said at least one bag is disintegrated to form the airborne liquid dispersion at an appropriate brief interval before said explosive material is detonated.

3. A method according to claim 2, in which clusters of liquid-filled thin-walled bags of plastics material, some of which may contain gas bubbles, are used to generate the airborne liquid dispersion.

4. A method according to claim 2 or claim 3, in which there are a plurality of bags assembled into an array of building elements some of which building elements include two or more bags in a common casing.

5. A method according to claim 1, in which the airborne liquid dispersion is created by a waterfall.

6. A method according to claim 5, in which the waterfall is created by pumping water up to an elevated discharge location over a period long compared to the time taken to discharge the pumped water from the discharge location.

7. A method according to claim 1, in which the airborne liquid dispersion is created by projecting liquid towards the body of explosive material from at least one discharge tube.

8. A method according to claim 7, in which the or each discharge tube ejects liquid therefrom explosively.

9. A method of limiting the environmental disturbance of an explosion substantially as hereindescribed with reference to Figure 3,4 or 5 of the accompanying drawings.

10. A method as claimed in any preceding claim in which the liquid is water containing an additive to enhance its environmental disturbance-limiting properties.

11. A method as claimed in claim 10, in which the additives are at least one of a neutralising agent to counter a poisonous chemical, a sterilising agent to counter biological material, a capture medium for limiting the spread of radioactive material and a detergent to facilitate creation of the dispersion.

12. Apparatus for use in the method of any preceding claim which includes means to create an airborne liquid dispersion that can at least partly surround a body of explosive charge on its detonation.