GB2029215A

GB2029215A - Prevention of explosions

Info

Publication number: GB2029215A
Application number: GB7835021A
Authority: GB
Original assignee: FIELD HAZAARD CO Ltd
Current assignee: FIELD HAZAARD CO Ltd
Priority date: 1978-08-30
Filing date: 1978-08-30
Publication date: 1980-03-19
Also published as: GB2029215B

Abstract

A method is described of preventing or reducing the likelihood or force of an explosion in an unconfined or partially unconfined cloud containing a mixture of air and a flammable on vapour, gas, dust, mist, fog or spray, which comprises releasing into the cloud a non-flammable non-toxic gas having a density lower than that of the cloud, and/or releasing into the surrounding atmosphere a non-flammable non-toxic gas having a density higher than air. The method is particularly applicable in the protection of chemical plant in which there is a risk of accidental release of flammable material, e.g. hydrocarbon vapour. Apparatus for performing the method is also described, comprising for example means such as discharge towers (1) spaced over the area at risk for release of the modifying gases from reservoirs (3 and 4). Gas detectors are provided to detect the formations of the cloud. <IMAGE>

Description

SPECIFICATION Prevention of explosions This invention concerns a method of preventing or reducing the likelihood or force of explosions in unconfined or partially unconfined clouds of flammable gas, vapour or other flammable material.

Unconfined vapour cloud explosions are phenomena which are causing increasing concern in the chemical industry. They are likely to occur when flammable chemicals are released into the atmosphere following plant failure or malfunction, and it has become clear in recent years that explosions of this kind can be extremely powerful and are capable of causing considerable damage. In most instances where these explosions have occurred, the gas or vapour concerned has been a hydrocarbon, usually a petroleum fraction or cyclohexane, but other gases and vapours have also been known to explode and flammable dusts, mists, fogs and sprays can behave in a similar manner.

The risk of such explosions occurring in any given plant is small, but it has nevertheless become desirable to seek to develop systems for preventing or reducing the risk of them happening or their force when they do. The underlying causes of these explosions have however been hitherto little understood and few if any practical systems have been proposed.

Studies which we have carried out indicate that it is the rapid flame acceleration effects which can take place in mixtures of flammable chemicals and air which are responsible for the high pressures produced in unconfined vapour cloud explosions. Our studies also indicate that it is flammable mixtures in which the velocity of sound is lower than in the surrounding atmosphere which will after ignition allow the creation of the differential pressure forces which cause rapid flame acceleration.

Our method of preventing or reducing the risk of these explosions occurring in clouds in which the velocity of sound is lower than in the surrounding atmosphere thus involves modifying the relative velocity of sound within and outside the cloud to minimise or reverse the flame acceleration which could otherwise occur. Thus in our method, modifications are made to the cloud and/orthe surrounding atmosphere such that difference in the velocity of sound within the cloud and that in the surrounding atmosphere is reduced or eliminated, or such that the velocity of sound within the cloud is made higher than that in the surrounding atmosphere.

Our studies also indicate that in circumstances where the velocity of sound in the cloud is initially higher than in the surrounding atmosphere, a further increase in the velocity of sound within the cloud relative to the surroundings can also be beneficial.

Thus in general our method involves increasing the velocity of sound within the cloud and/or decreasing the velocity of sound in the surrounding atmosphere, such that the velocity within the cloud is increased relative to the surroundings. In all cir cumstances, the greater is the increase in the veloc ity of sound the better is the effect, and thus our method is preferably operated such as to increase the relative sonic velocity as quickly and to as great an extent as is practically possible.

The velocity of sound in a gas varies inversely with the square root of the density of the gas, and in our method this relationship is used to effect the required modification of the cloud or surrounding atmosphere. A low density gas can be released into the cloud to decrease its average density to increase the velocity of sound within the cloud, and a high density gas can be released into the surround atmosphere to increase the average density of the surroundings to decrease the velocity of sound in the surroundings.

The invention thus provides a method of preventing or reducing the likelihood or force of an explosion in an unconfined or partially unconfined cloud containing a mixture of air and a flammable vapour, gas, dust, mist, fog or spray, which comprises releasing into the cloud a non-flammable non-toxic gas having a density lower than that of the cloud, and/or releasing into the surrounding atmosphere a nonflammable non-toxic gas having a density higher than air.

It will be appreciated that the velocity of sound in a gas depends on other factors in addition to its density. There are for example instances in which the velocity of sound in one gas is higher than in another even though its density is lower. The requirements for the gas r leased into the cloud to be less dense than the cloud and for the gas released into the surrounding atmosphere to be more dense than air are however appropriate for practical purposes, although the precise criterian is that the velocity of sound within the modifying gas must be higher than that in the cloud or lowerthan that in the atmosphere, as appropriate.

Our method is principally applicable to flammable clouds in which the velocity of sound is lower than in the surrounding atmosphere. The mixtures forming these clouds are usually, but not always, denser than air. The method is in particular applicable to clouds of hydrocarbon vapour or gas which are denser than air, e.g. petroleum fractions and other hydrocarbons produced in the chemical industry, such as cyclohexane. As indicated above, however, our method may also be applied to clouds in which the velocity of sound is initially higher than in air, for example to clouds of hydrogen, deuterium, deuterium hydride, methane, borane, ammonia, acetylene, ethylene and carbon monoxide. It will be appreciated that with some of these gases, the velocity of sound in them is not always higher than in air but depends on the temperature.A more detailed discussion of the flammable materials to which the method may be applied is given below.

In general terms, the density of the cloud and/or surrounding atmosphere should preferably be modified to reduce the density of the cloud relative to the atmosphere (i.e. to increase the relative sonic velocity) to as great a degree as practical. Where the cloud density is initially greater than that of air, the density of the cloud and/or the surrounding atmosphere may for example be modified such that the average density within the cloud becomes approximately the same as or less than that of the surrounding atmosphere. The average density may for example be modified such that the density within the cloud is from 70 to 90% or as little as half that of the sur- rounding atmosphere.

The desired change in density is preferably effected by releasing into the cloud a gas having a lower density than the cloud, this being easierto effect in practice than increasing the average density of the surrounding atmosphere. In another preferred method, a combination of the two techniques may be used, i.e. releasing both a low density gas into the cloud and a high density gas into the surroundings.

The gases which may be used in our method should of course be non-toxic at the concentration used. As a general guide, the gases should not be capable of killing humans at concentrations in excess of 10% in air in less than one hour, otherwise than by suffocation. Examples of low density gases which may be used to reduce the average density of the cloud are helium, neon, water vapour or steam and nitrogen. Helium is preferably used in view of its low density. Examples of high density gases which may be used to increase the density of the atmosphere surrounding the cloud are non-flammable halogenated hydrocarbons (e.g. gaseous fluorocarbons and chlorofluorocarbons such as CCl2F2, CHCIF2 and CCIF2CCIF2), carbon dioxide, sulphur hexafluoride, argon, kyrypton, and xenon. In each case, mixtures of gases can be used.

Flammable clouds are likely to be ignited and explode very quickly aftertheirformation and for our method to be successful it is therefore necessary for the cloud to be detected and the density changes effected extremely rapidly, e.g. within 30 seconds.

This means in particular that the non-flammable gases used must be released generally at a high mass velocity in the shortest possible time to ensure that they are dispersed effectively into the cloud or surrounding atmosphere.

The actual quantity of gas required to effect the desired density changes will of course depend on the particular circumstances in question. The amount will for example depend on the size of the flammable cloud and its temperature, and the density or molecular weight of the flammable mater ial; it will also depend on the ambient temperature and the temperature and density or molecular weight of the non-flammable gas. The minimum amount likely to be required in any given risk area can however be calculated for any given combina tion of flammable material and non-flammable gas, on the basis of the probable volume of flammable material which might be released and the likely temperatures.The calculation can either be simply based on modifying the average density of the cloud relative to the surroundings or can be made on the basis of modifying the velocity of sound in the cloud relative to the surroundings, using the well known relationship between the velocity of sound in a gas and its density and pressure or temperature. An explosion prevention system can then be established in the risk area on the basis of the release of this minimum amount of non-flammable gas.

The invention also provides apparatus or a system for putting our method into effect which comprises means for detecting the formation of the cloud and means for releasing the gas having a density lower than the cloud into the cloud and/or means of releasing the gas having a density higher than air into the surrounding atmosphere.

It is to be understood that at the point of release the modifying gases may be in either gaseous or liquid form.

The cloud detection means may be any suitable type of fast-acting flammable gas detector, for example one which is capable of detecting a cloud within 10 seconds of its formation. The detectors should be able to distinguish between the flammable material and the modifying gases, and detect the flammable material in the presence of either or both. Suitable detectors are manufactured by J. and S. Sieger (Poole) and Detection Instruments Ltd (Wokingham).

Preferably, a plurality of both detectors and gas release means are arranged on a three dimensional grid basis over the area at risk. They may for example be evenly spaced apart so as to protect sepa rarely unit areas (e.g. areas 30m x 30m, to a height of 30m) throughout the total area at risk. With this kind of system, the cloud detectors can be arranged to operate such that the size and position of the cloud is automatically estimated and the appropriate number of release means actuated automatically at the correct position, either within the cloud or in the air surrounding the cloud as the case may be. Manually activated systems can be used if desired, but computer-controlled automatic systems are preferred.

The gas release preferably takes the form of vertical discharge towers comprising pipes for the gas and orifices or nozzles through which the gas is discharged. Each tower can either have a single orifice or a plurality of them, arranged if desired at different elevations and pointed in different directions. They may for example be round or otherwise shaped and placed so as to discharge the gas horizontally orvertically or at any other angle.

As with gas detectors, the discharge towers are preferably arranged on a grid basis, e.g. by locating 30m towers at each corner of 30m squares. The gas detectors can conveniently be located on or adjacent the discharge towers.

As an alternative, the discharge towers may be sloping ratherthan vertical, and in generally they may be arranged either uniformly or randomly. They can for example be arranged linearly, radially or circularly, but as just indicated vertical towers on a square grid basis are preferred. In another alternative, the gases can be distributed and discharged from an elevated grid of pipes, without the use of discharge towers.

The modifying gases may supplied from reservoirs of the gas to the orifices in the discharge towers by a system of pipes of circular or other cross-section. The pipe system may in general be elevated, at ground level or subterranean. The elements of the pipework directly or indirectly serving the discharge orifices may be vertical or sloping. The pipe for the low and high density gases may be wholly independent or use, in part, the same pipework, and they may be concentric.

The gas reservoirs store the low and high density gases either as gases or liquids under pressure. They may be pressure vessels and spherical or cylindrical in shape and be located centrally or peripherally (or intermediately) over the area covered by the system.

They may be single units or in multiple (even up to one reservoir per orifice) and they may be located above or below ground level. The gases, particularly the dense gases, may be held in liquid form in their reservoirs and the discharge may be under vapour pressure alone or augmented by permanent gas pressurisation to maintain the maximum flow rate and inhibit cooling. Alternatively, the gas may be held under pressure in the gaseous state in the reservoir.

The gases may be released from the reservoirs by rupturing, e.g. explosively, a sealing or bursting disc on the outlet from the reservoirs A single gas reservoir for each gas can in relatively small systems be used to supply gas to the whole system, but it will usually be preferable to have a series of isolated reservoirs each serving a single orifice or just a small number of orifices. For example, a pair of reservoirs, one for each gas, can be located beneath each discharge tower. The reservoir for the denser gas is preferably situated directly below the tower in view of the high pressures occuring on release of the gas. This system obviates the need for large peripheral reservoirs, and a complete pipe grid, and the capacity of the individual reservoirs is of course only a small proportion of the total capacity.

The system may also be provided with wind vanes and anemometers to estimate the likely speed and direction of movement of the cloud and to identify the particular discharge towers to be actuated. Valves can also be provided to terminate the gas discharge when the cloud has moved away, orto change from lowto high density discharge (or vice versa) as the cloud moves across the grid.

The operation of the system is best made fully automatic; For economy, as well as effectiveness of operation, the pattern of operation of discharge towers should be correlated not only with detector response but also with wind speed and direction.

Such a system is best left to a computer to control.

In operation, the requirementfortriggering may be the near simultaneous (within say 10 seconds) and continuous registration of levels of flammable gas in excess of the appropriate lower flammability limit by two or more nearby detectors or by one detector and a manual operation. Triggering causes the control system to discharge low density gas into the cloud at the nearest tower or towers and high density gas into the atmosphere at the next-but-one group upwind (electrically registering wind anemometers and vanes are necessary for this function). Subsequently, as detectors further downwind respond-assuming none upwind do-further towers set within the developing flammable cloud discharge their low density gas and those towers closely surrounding the area defined by the detectors and the wind as flammable release high density gas.This sequence continues until the cloud moves beyond the boundary of the system or general fire breaks out.

The explosion prevention system can also be designed to act additionally for the dispersion of conventional fire extinguishing chemicals. Use of the system in this way has the advantage of reducing the overall capital cost. The system can be used to protect chemical plant both on land and off-shore.

The system of the invention will be described in further detail with reference to the accompanying drawings in which: Figure lisa perspective view of a series of discharge towers arranged on a grid basis; Figure 2 shows a pair of gas reservoirs located beneath a discharge tower; Figure 3 shows a single discharge tower in elevation; Figure 4 shows cross-sections of the tower of Figure 4 along A-A and B-B; Figure 5 is a diagram showing the shape of the plumes of discharged gas in horizontal section; and Figure 6 is a diagram showing the interrelationship of the main elements of the system.

Figure 1 shows a system comprising 35 discharge towers (1), each 30m high, arranged on a grid basis at the corners of 30m squares. The area protected by the system thus measures 120 x 180m. Beneath each tower is an underground cell (2) containing a reservoir (3) for the low density modifying gas and a reservoir (4) for the high density modifying gas. The reservoirs are connected to the tower by a pipe system (5).

Figure 2 shows in greater detail an underground cell (2) beneath one of the discharge towers (1). A reservoir (4) for the high density modifying gas (e.g.

SF6) lies directly below the tower (1) which is formed of concentric pipes (10) and (11). The outer concentric pipe (10) leads directly from the reservoir (4) to allow discharge of the high density gas and the reservoir is sealed off from the pipe buy a bursting disc (12) which can be explosively ruptured to release the gas. The second reservoir (3) is for the low density modifying gas (e.g. He) and is connected to the discharge tower (1) by a bifurcated pipe (13), the two limbs of which pass through the outer concentric pipe (10) to the lower end (14) of the inner concentric pipe (11). The pipe (13) is provided with a bursting disc (15) which can be explosively ruptured to release the gas.

As shown in Figure 3 each discharge tower (1) is essentially formed by the free-standing concentric pipes (10) and (11) which extend vertically for 30 m from the underground cell (2). The outer pipe (10) is provided at intervals with flammable gas detectors (20) directed into the area protected by the tower. A series of sets of discharge nozzles for the low and high density gases is provided at intervals along the length of the tower. The first (21) of these sets of nozzles is located 2.5 m above ground level and the other five sets (22) are at 5 m intervals. As shown in the cross-sections of Figure 4, each set of nozzles (21) and (22) has four nozzles (23) for discharging gas from the inner concentric pipe (11) and another four nozzles (24) for discharging gas from the outer concentric pipe (10).Figure 4 also shows that the nozzles of each level are offset by 45" are compared to those of the adjacent level or levels, and the noz zles on adjacent towers are also arranged such that nozzles on the same level are offset by 45" from their neighbours. This arrangement ensures that the gases are discharged evenly over the area protected by the grid of towers.

In general, the size and number of the nozzles should be such as to allow discharge of the gases from the reservoirs within about 30 seconds.

The nozzles should befitted to allow for the contraction which takes place when the gases are discharged.

Figure 5 shows diagrammatically the shape of the plumes of gas discharged from two adjacent levels of nozzles (22) into a 30 m square area protected by four discharge towers (31,32, 33 and 34). The heavy lines show the plumes from one level and the lighter lines those from the adjacent level. Nozzles at the same level on adjacent towers are offset by 45" so that they do not discharge directly towards each other, as shown by the pairs of plumes (35) and (36) from towers (31) and (34). The nozzles of the adjacent level are similarly offset in relation to each other and in addition are offset by 45" in relation to the first level. Thus the plume (35) from the first level of tower (31) is offset by 45" from the plumes (36) and (37) discharged from the adjacent level of the same tower.

Figure 6 is a diagram showing the way in which the explosion prevention system is controlled and operated. The operation computer (40) receives information by electrical signals from the automatic gas detectors (20) on each tower (1), a wind vane (41) and an anemometer (42), and also from manual press button triggers (43). It is also capable of sending signals to the bursting discs (12) and (15) at the outlets of the reservoirs (3) and (4) to actuate their rupture to release the modifying gases. When an escape of flammable gas is detected, a signal is sent to the computer which then calculates the size of the cloud from these signals and also calculate its speed and direction of movement from the information received continuously received from the wind vane and anemometer.According to the size, position, speed and direction of movement of the cloud, the computer then signals instructions to explode the bursting discs on the reservoirs of the appropriate discharge towers.

A second computer (44) is also provided which continuously and automatically monitors all of the components in the system and the status of the stocks of modifying gases. It is also programmed to identify leaks, scrutinize the electrical systems and undertake periodical simulations of the operation of the system.

Our method is further illustrated by the following example of its application to a potential escape of hydrocarbon gas or vapour, using the system just described.

Theoretically, the average and approximate maximum densities of all rich hydrocarbon/air mixtures (those having the lowest velocities of sound but still capable of burning) are 300/o greater than air (atthe same temperatures and pressure). In our method, these mixtures should be made either less dense than the surrounding atmosphere, or around which the atmosphere should be increased in density, or both. Clearly the former may be satisfied with less very low molecular weight material than if the molecular weight of the diluent is not so low. The rule in this case is, if ML is the molecular weight of the light diluent, the volume of diluent required per unit volume of flammable cloud is: 0.3/ / (1-3.47 x 10-2.M,) (1) for achieving a nett density equal to air at the same temperature and pressure.Similarly, outside the cloud augmentation of the density of the surrounding atmosphere to that of the flammable cloud would require: 0.231/ (2.67 x 10-2.MH-1) (22) volumes of heavy diluent (molecular weight MH) per unit volume of the atmosphere. In this case, however, the atmosphere is virtually limitless and a limit must therefore be set.

Whilst it might be desirable to reverse the original density distribution, it is probably not essential. The volumes of light and heavy diluents required may be balanced in such a way that the 'internal' and 'exter nal' densities become equal. If X, and XH are the respective volumes of light and heavy diluents per unit volume of flammable cloud, or the atmosphere, then for this to be true: 1.3+3.47x10-2.X,.M, 1 1+3.47x10-2.XH.MH (3) 1 + XL 1 + XH must be satisfied.

These equations may be used as follows. The total weight of a potential hydrocarbon escape is assessed, for example, as 10 tonnes. This on admixture with air could produce as much as 300 tonnes of flammable mixture, or approximately 2.3 x 103 m3. )f helium (ML = 4) is used within the cloud and sulphur hexafluoride (MH = 146) outside it, the amount of helium required on the basis of equation (1) would be approximately 80,000 m3 or 14.3 tonne; the amount of sulphur hexafluoride, on the basis of equation (2) and assuming the volume of air so treated is equal to the cloud volume - about 18,300 m3 or 120 tonnes.The use of both simultaneously, equation (3), involves for example one of the following combinations: Helium (tonnes) SulphurHexafluoride 14.3 0 11.8 15 9.6 30 7.6 45 5.6 60 4.1 75 2.6 90 1.2 105 0 120 In the above the quantities of diluent or modifying agents per unit volume of flammable cloud have been based on rich limit densities (i.e. 4 x stoichiometric concentration) whereas the total volume of the flammable cloud has been assessed on lean limit concentrations (ie. 2 x stoichiometric concentration). This approach therefore probably overestimates the quantity of diluent beyond that which is strictly necessary.

Thus, the system may for example allow for the release of 15 tonnes of helium or 120 tonnes of sul- phur hexafluoride in 30 seconds or less. It will be appreciated that the volume requiring protection in this example may be much less than the volume which would include the whole plant. Therefore, what is discharged and where, will depend upon the information provided by the flammable gas detectors. In this case, and in most cases, the depth of the cloud priorto ignition would be less than 30 m. It is rational, but not essential, that this dimension should fix the grid spacing for the discharge system.

Precisely where the cloud will move, and what volume it will occupy, even if it occurs as forecast, will almost certainly not be known in advance. If it is not 30 m. deep it will occupy an area greater than 7,700 m2. which in turn will not be square and will therefore exceed, in at least one dimension, the three grid units (30 m. pitch) within which th 2.3 x 1 06my would otherwise just fit.

The approach set out above would therefore be necessary to allow for the cloud's behaviour.

Nevertheless, if the cloud for the moment is considered as a geometrical box within a regular grid, then up to 15 tonnes of helium would have to be discharged from all the discharge towers of the grid within and just outside the cloud (16 in all). Approximately one tonne of helium is therefore required per discharge tower throughout the plant, that is, at every discharge tower whether or not they are likely to function during any particular incident. Similarly, the same volume of air surrounding the 'cloud' would require to be protected by the discharge capacity of up to 15 tonnes of sulphur hexafluoride at every discharge tower. Again the same rationale applies, the provision of sulphur hexafluoride at all discharge points is necessary whatever is forecast for any particular incident.

In a grid, say 300 m. square, which might reasonably be considered for a medium size chemical plant, the necessary provision on the above basis would be about 120 tonnes of helium and over 6,000 of su Iphur hexafluoride. These are large quantities and are best kept in small reservoirs.

It will be appreciated that the suggested use of helium and sulphur hexafluoride is for illustrative purposes only. Also, the quantities are a factor of 8 x more than necessary to deal with a stoichiometric cloud. Some scope remains therefore for reducing the inventories if this is considered justifiable.

Naturally, for permanent gases the sizes of the reservoirs, and pipework and orifices, will depend upon the storage pressure adopted. For gases liquifiable by pressure alone the relevant component sizes are determined by the liquid gas density, although their standards of construction will depend upon the imposition of any permenentgas pressurisation over and above the appropriate saturation pressure.

In the system described above with reference to the drawings, the pipework and the nozzles are so sized and placed as to give total discharge in 30 seconds as uniformly as possible, and adequate coverage of the volume may be achieved by discharge through 50 mm. diameter nozzles. The inner concentric pipe (11) should be designed to contain 250 atmospheres and resist collapse under 40 atmospheres pressure, and should not be less than 500 mm in diameter.

The outer concentric pipe (10) should have a diameter of 700 mm. and its maximum pressure rating should be 40 atmospheres.

During the vapourizing liquid discharge severe transitory cooling of the system could arise which would require the use of steels or other materials with high ductility in the range 30"C to 40 C (or whatever range was appropriate to the modifying gases used) and a design which would minimise thermally induced stresses.

The reservoirs themselves, again on the basis of 1 tonne of helium and 15 tonnes of sulphur hexafluoride, would need to be of 27.5 m3. (at 210 atmospheres pressure) and 20.5 m3. (at 40 atmospheres pressure) capacities respectively. If each were cylindrical and 8 m. long then their respective diameters would be 2,100 m. and 1,850 m. The thickness of the shell of the larger and higher pressure reservoir on the basis of allowable stresses in carbon steel would be 240 mm.

Although our method has been described mainly in relation to the treatment of clouds of hydrocarbon vapour, it is applicable to clouds of flammable material generally. The following list gives the principle classes of flammable or combustible material to which the method may be applied.

1. Saturated or unsaturated, straight or branched acyclic hydrocarbons such as: paraffinic hydrocarbons, e.g. methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, etc. and their higher homologues and isomers; olefinic hydrocarbons having one or more double bonds, e.g. ethylene, propyiene, butylene (butane), pentene, propadiene, butadiene, pentadiene, etc. and their higher homologues and isomers; acetylenic hydrocarbons having one or more triple bonds e.g. acetylene, methyl acetylene (propyne), butyne, etc. and their higher homologues and isomers; and straight and branched chain hydrocarbons containing both double and triple bonds.

2. Cyclic hydrocarbons (which may be saturated or unsaturated and substituted, e.g. by alkyl groups) e.g. cyclopropane and cyclohexane; 3. Aromatic hydrocarbons (including both mononuclear, polynuclear and fused ring structures), e.g. benzene, toluene, xylene, styrene, diphenyl and naphthalene; 4. Organic oxides e.g. ethylene oxide and propylene oxide; 5. Alcohols e.g. methyl alcohol and ethyl alcohol; 6. Ethers e.g. diethyl ether and diphenyl ether; 7. Ketones, e.g. acetone and dimethyl ketone; 8. Aldehydes, e.g. acetaldehyde; 9. Organic acids, e.g. formic acid, acetic acid and maleic acid; 10. Esters, e.g. methyl formate and amyl acetate; 11. Peroxides, e.g. methyl ethyl ketone peroxides; 12.Substituted derivatives of all of the above classes in which the substituant is one or more of: (a) a fluorine, chlorine, bromine or iodine atom; (b) an amino or imino group; (c) a nitro, nitrate or nitroso group; (d) a nitrile group; (e) a cyanate or isocyanate group; (f) a sulphate, sulphite, sulphonyl or sulphinyl group; (g) a phosphorus-containing group, e.g. phosphino, phosphate, phosphite and phosphonium; 13. Substances generically classified as cellulosic, starches, sugars, fats, proteins, and waxes; 14. Inorganic combustibles and flammables including hydrogen, hydrogen sulphide, ammonia, phosphine, arsine, stibene, carbon disulphide, carbonyl sulphide, sulphur, carbon, phosphorus, metal dusts, hydrazine, the boranes, the silanes and the germanes; and 15. Any of the above in admixture with each other and/or inert diluents (such as nitrogen, helium, carbon dioxide, water) or with reactive materials (e.g. hydrogen, oxygen, ozone, fluorine, chlorine).

Claims

1. A method of preventing or reducing the likelihood orforce of an explosion in an unconfined or partially unconfined cloud containing a mixture of air and a flammable vapour, gas, dust, mist, fog or spray, which comprises releasing into the cloud a non-flammable non-toxic gas having a density lower than that of the cloud, and/or releasing into the Our rounding atmosphere a non-flammable non-toxic gas having a density higher than air.

2. A method as claimed in claim 1 wherein a relatively low density gas is released into the cloud and a relatively high density gas is released into the surrounding atmosphere.

3. A method as claimed in claim 1 or claim 2 wherein the cloud density is greater than that of air and the density of the cloud and/or the surrounding atmosphere is modified such that the average density within the cloud is approximately the same as or less than that of the surrounding atmosphere.

4. A method as claimed in any one of the preceding claims wherein the cloud is detected and the gas or gases released within 30 seconds of the formation of the cloud.

5. A method as claimed in any one of the preceding claims wherein the flammable material is a hydrocarbon.

6. A method as claimed in any one of the preceding claims wherein the relatively low density gas is helium.

7. A method as claimed in any one of the preceding claims wherein the relatively high density gas is a halogenated hydrocarbon, carbon dioxide, sulphur hexafluoride, argon, krypton or xenon.

8. Apparatus for performing a method as claimed in any one of the preceding claims, which comprises means for detecting the formation of the cloud and means for releasing the relatively low density gas into the cloud and/or means for releasing the relatively high density gas into the surrounding atmosphere.

9. Apparatus as claimed in claim 8 wherein a series of detectors and gas release means are arranged on a three dimensional grid basis over the area at risk.

10. Apparatus as claimed in claim 9 wherein the detectors are arranged to operate such that the size and position of the cloud is automatically estimated and the appropriate release means actuated automatically.

11. Apparatus as claimed in any one of claims 8 to 10 wherein the gas release means are vertical discharge towers connected by pipes to reservoirs for storing the non-flammable gases under pressure, the reservoirs having at their outlets rupturablediscs to allow release of the gas.

12. Apparatus as claimed in claim 11 wherein each discharge tower has beneath it a reservoir for the relatively low density gas and a reservoir for the relatively high density gas.

13. Apparatus as claimed in claim 8 substantially as described herein with reference to the accompanying drawings.