review of explosion and fire hazard of liquefied petroleum gas

Fire Safety Journal, 2 ( 1 9 7 9 / 8 0 ) 223 - 236 223 © Elsevier Sequoia S.A., Lausanne - - P r in ted in the N e the r l ands

Review of Explosion and Fire Hazard of Liquefied Petroleum Gas

D. J. R A S B A S H

Department of Fire Safety Engineering, School of Engineering, The King's BuiMings, University of Edinburgh, Edinburgh EH9 3JL (Gt. Britain)

(Received S e p t e m b e r 13, 1 9 7 9 )

S U M M A R Y

The properties of LPG that contribute to its fire and explosion hazards are outlined. The hazards include those associated with small vapour leaks from appliances and gas cylinders as well as major hazards associated with loss of containment and rapid vaporisation of tonnage quantities of the liquefied gas. Estimates are given of the magnitude of effects that might follow the latter type of release. In the UK the record of LPG is poor with regard to general fire occurrence compared with that of natural gas or town gas. This performance will need to be improved if the quant i ty of LPG used is to be increased. These improve- ments might be achieved by developing its use as a piped fuel and in motor vehicles, but both these would require careful hazard analysis. Although the record for major hazard with LPG in the UK is good, quantitative studies indicate tha t handling and transportation of tonnage quantities by road and sea are not safe enough. Methods are suggested for reducing the risk.

I N T R O D U C T I O N

Liquefied petroleum gas is a fuel resource which is going to become increasingly available in the next decade. It is important, before major expansion in the use and handling of this fuel comes about, that a critical assess- ment is made of its fire and explosion risk. This paper was prepared by the author at the request of the Essex Branch of the Environ- mental Health Officers Association in the UK, and reviews these risks with particular refer- ence to the UK. Large quantities of LPG are handled at present at Canvey Island which is situated in Essex, and there are proposals for

handling even more there in future. For this reason, particular at tent ion has been paid to the features and probability of major disasters that might follow loss of containment of the fuel.

Liquefied petroleum gas contains two main constituents, butane and propane. Commer- cial butane and propane boil, respectively, at about --2 and --45 °C. For the most part, the material is transported and stored at room temperature in the form of pressurised liquids, the pressure at the highest expected ambient temperature of 60 °C being about 7 bar for butane and 22 bar for propane. However, quantities in excess of one thousand tons tend to be stored as a refrigerated liquid at, or near, the boiling point. Ships' cargoes may be either pressurised liquids or refrigerated liquids. In addition, in parts of LPG installations the liquid is vaporised and is transported as a gas either by itself or, occasionally, as a mixture with some air.

G E N E R A L F I R E AND E X P L O S I O N P R O P E R T I E S

OF LPG

For the most part, the fire and explosion properties of LPG do not become manifest until the liquid is vaporised in the presence of air. However, the possibility of pressure bursts in the pressurised vessel is also a hazard, particularly in the presence of heating or fire, and this will, of course, produce physical explosive effects. In a manner similar to other flammable gases or vapours, the gas will not propagate flame outside certain limits of concentration, which for propane is about 2.2 - 10 per cent. and for butane 1.8 - 9 per cent. The maximum explosive pressures occur when the concentration is about twice the lower limit value. Con- centrations of flammable vapour and air with-

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in these limits may be ignited by a small source of ignition such as a small flame or a spark with an energy as low as 0.3 mJ. Outside these limits of concentration, not even a large flame would be able to produce flame propagation substantially beyond the driving influence of the flame. At any interface with air on one side and a flammable vapour above the upper flammable limit on the other, including particularly the neat vapour, there will be a zone within the limits which can be ignited and produce a continuing fire until the vapour has burned away. Refrigerated, liquefied petroleum gas in a vessel may be ignited in the same way as petrol and will burn in a similar manner, in that the flames above the liquid surface will supply the latent heat of vaporisation of the vapour being fed into the flame.

Any spillage of liquid from a pressurised container will result in almost instant total dispersion and evaporation. In general, a small spillage of refrigerated, liquefied gas will also vaporise quickly. If large quantities of refrigerated LPG are spilled, then the surrounding area will be rapidly cooled and vaporisation rates will be greatly reduced. However, if spillage takes place in, or on, an unlimited area of deep water, the vaporisation rate will be much higher and will tend to continue un- abated. I am not aware of any measurement of this rate; it is probably of the same order for liquid propane as it is for LNG, which is about 0.18 kg/m 2 s.

If a pocket of flammable vapour-air mixture builds up, particularly in an enclosed space, then the propagation of flame through the mixture can give rise to pressure effects. For a mixture that is completely enclosed and is approximately cubic in shape, the maximum pressure that may be developed is of the order of 7 bar gauge. A flammable vapour-air mixture established in a long duct may give rise to a detonat ion which might give pressure effects locally well in excess of 7 bar. Buildings may be severely damaged at internal pressures in excess of about 0.1 bar, and it should be remembered that for a complete enclosure the pressure tha t builds up is approximately pro- portional to the size of the pocket occupied by the flammable gas-air mixture in it. Thus, for a volume of, say, 10 m × 10 m × 4 m (i.e., 400 cubic metres) a pocket of flammable vapour-air mixture only one-seventieth the total volume, i.e., a volume of 6 cubic metres,

can, if ignited, give rise to major damaging effects. To fill such a volume with the stoichiometric mixture would require about 500 g of either propane or butane vapour. Thus a small leak can go quite far in producing damaging effects. In practice, windows and doors that may be present in most spaces could provide some relief. These would burst open, limiting the damage. The violence of the explosion would depend on the configuration of the items in the space through which flame is propagating, since the flame is accelerated by turbulence.

A major difference between petroleum gas and natural gas or town gas is that it is heavier than air. This increases the risk in a number of ways. Thus, there tends to be more ignition sources in the lower parts of a room than in the upper parts. Also LPG appliances tend to be in the lower parts of working or living areas, and leakage falling from the appliance itself does not have much opportuni ty to become diluted to safe concentrations below the lower limit. On the other hand, leakage rising from natural gas appliances might entrain air for a substantial distance prior to reaching a ceiling. There also tend to be more pockets, recesses, and inaccessible spaces near the floor or the ground where flammable vapours might accumulate. Finally, when ignition does take place there are more obstacles in the path of the flame near the ground than under a ceiling, causing the flame to become accelerated and the explosion more violent.

MAJOR FIRE AND EXPLOSION HAZARDS ASSO-

CIATED WITH LPG

The above gives a summary of the normal fire and explosion hazards associated with LPG. However, experiences over recent years have indicated that certain major and even disastrous hazards may be associated with this material due to the ability of large quantities of the liquid gas to flash rapidly into the vapour form. There are two major types of hazard according to whether ignition takes place soon after spillage or there is a substantial time gap. If the vapours are ignited soon after the spillage has taken place, a fireball is produced which consumes the vapours very quickly; far more quickly than if they were burning as a liquid over the initial surface.

Fig. 1. LPG burning in 35 m m vessel.

Fig. 2. Vapours f rom 4 x 10 - 5 m 3 LPG burning af ter release under water .

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This is illustrated in Figs. 1 and 2 which show (a) 4 × 10 -5 m a of liquid propane gas burning from a cylinder 35 mm diameter, and (b) the flame that is formed if vapours from a spillage of the same amount of liquid propane under water, are ignited. In the former case the fire takes three minutes to burn out: in the latter case the fuel is consumed in two seconds. However, it can be even more dangerous if the flammable gases evolved from the flashing of spilled LPG are not ignited. They may then be carried by either their own flow or by outside wind to an area where they can produce much damage when ignited.

Table 1 gives some information on these hazards. There is a summary of the maximum size of fireball that may be obtained from the rapid spillage and vaporisation of different quantities of LPG. Also given is the distance at which a cellulosic material may become ignited spontaneously or by pilot flame when under the influence of heat radiated by the fireball, and also the distance at which a per- son with parts of the body exposed to the radiation of the fireball might feel unbearable pain.

The fireball radius and durat ion were based on information with small quantities of fuel by Hasegawa and Sato [ 1], although the NFPA film on BLEVES appeared to indicate a dura- t ion of about 8 s for some 50 tonne of LPG, which is in line with the predictions. The radii for various effects of the fireball were based on the assumption that 20 per cent. of the total energy in the fire is radiated during the durat ion t ime of the fireball. Informat ion in Hasegawa and Sato's paper implies a peak radiation of about 22 per cent. of this energy ou tpu t and a mean radiation of about 15 - 20 per cent. However, experiments with radiant flames usually indicate a much higher mean radiant output . Thus Markstein [2] obtained a mean radiant ou tpu t of 24.4 per cent. for a range of turbulent diffusion flames. Relevant threshold values are summarised in Table 2. Criteria for pilot and spontaneous ignition were obtained from Lawson and Simms [ 3] and apply to fibreboard, whereas that for unbearable pain was obtained from Simms and Hinkley [4] . All the criteria are t ime depen- dent and the t ime taken was the calculated durat ion of the fireball.

The calculated radii of effects in Table 1 are perhaps larger than may have been actually

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Threshold criteria used to calculate fireball effects

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Fuel in Durat ion Radiant Pilot Spontaneous fireball t ime (s) ou tput , F* ignit ion ignit ion (tons) (W) threshold threshold

(W/m 2 ) (W/m 2 )

Unbearable pain threshold (W/m 2 )

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200 9.76 18.4 2.63 4.18 0.60 2 000 14.8 121 2.22 3.68 0.50

20 000 22.5 796 1.88 3.34 0.36

*Radius of ef fec t is given by r = 0.28 ~ where T is the relevant threshold value.

experienced in practice. There are a number of reasons for this. Thus, people exposed will tend to shelter bare skin and not experience pain at the distances indicated. Moreover, the ignition of materials inside domestic premises will usually be at distances about 0.6 times smaller than those shown in Table 1 since windows will absorb two-thirds of the radiation. Also, spontaneous ignition may be a more appropriate criterion for fire than pilot ignition in this situation. In addition, normal vegetation would contain too much moisture to be ignited at the distances shown and thick, iso- lated material, even if ignited, will tend to self-extinguish once the fireball has burned out. Humidity in the atmosphere and rain would reduce considerably the extent of fire spread. However, on a clear, dry day or night, it would be expected that dry vegetation and thin, cellulosic materials would be subject to pilot ignition up to the distance stated, the ignition sources for the latter being fires started by spontaneous ignition nearer to the fireball.

Table 1 also gives estimates of the maximum distance that flammable vapours may be carried and still be dangerous at points down- stream of spillages, and the maximum area of a flashing fire that might occur when such spillage vapours are ignited when carried down- wind. The distances were based on information provided by Kaiser [5] for travel of vapour from dumped quantities of LNG. Areas were based on information on plume widths also from experiments with LNG [6, 7].

It should be emphasised that the figures given are based on very meagre experimental work and an incomplete understanding of the processes involved. However, the Table does

give an indication of the dimension of disaster that might result from a spillage of even mo- derate quantities of these liquid gases. Thus a 20-ton spillage from a road tanker could produce a flash fire over 5 hectares (about 10 acres) with transient flashes up to about 0.9 km, and a 2 000 ton spillage and vaporisation might produce a fire of 2 square kilometres with transient flames reaching a distance down- wind of 8 kilometres from the point of spillage. These areas would contain a substantial fraction above the stoichiometric or even the upper limit concentration and could burn for tens of seconds in a given locality.

In order to produce the effects outlined in Table 1, not only does the spillage of the liquid have to take place but also it must be vaporised or dispersed in a relatively short time. An estimate of this time for the condition of no rapid ignition is given in the Table and is based on the time when a lower travel distance would be expected from the mean rate of spillage than from the dumped spillage; it will be seen that it varies from 1.5 min for 2 tons to 13 min for 2 000 tons. To produce the maximum diameter fireball under conditions of rapid ignition, the time of release and dispersion need to be less than about twice the time of burning of the fireball. The latter is shown as varying from 4 to 22.5 s over the range of spillage sizes in Table 1.

The question which arises is, how possible is it for spillage with these characteristics to occur? Experience has shown that for LPG kept under pressure in a storage vessel it is possible for the vessel to become ruptured and to disgorge all its contents in a very short time. Moreover, the bulk of its contents when disgorged will almost immediately be either

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flashed into vapour in the atmosphere or dispersed as a fine mist which is just as dangerous. The most common way for this to occur is as a result of a fire, possibly due to a leak, which surrounds the storage vessel and heats it, thus weakening the steel. Occasion- ally, however, pressure bursts have occurred, either because of local damage to the storage vessel or expansion of the liquid following over-filling.

The mechanisms of spillage with pressure vessels are also likely with semi-refrigerated liquids, but are much less likely with fully refrigerated liquids. The latter are stored in insulated vessels which are much weaker than pressure vessels, in fact not very dissimilar to ordinary tanks for large scale storage of flammable liquid. It is rare, but certainly conceiv- able, that a simple metallurgical failure condition might occur in a tank, causing it to jettison its contents. It is possible also under certain process or accidental conditions for substantial pressures to build up within a refrigerated tank. The main process condition is known as "roll over" and is due to the varia- tion in product present in a tank. Thus a heavier, less volatile product may be allowed to flow above a lighter, more volatile product. A sinking of the upper layer into the lower one can cause a rapid evolution of vapour from the lower layers which might overcome the relief and burst the vessel. An accidental condition which might give rise to a similar sudden discharge of the bulk of the contents of a tank, and which is liable to occur with ships' cargoes, would follow the puncture of a tank below the waterline, either through stranding, collision or some other mechanism. It is then possible for water to enter the cargo space. Water being warmer than the cargo, particularly if it is liquid propane, can cause a rapid vaporisation which could split the tank. As far as the author is aware, this has never happened in practice, nor has any research work been carried out to define the conditions under which it might occur, but the possibility cannot be dismissed. Refrigerated gas tankers might also experience roll-over and metallurgical failure.

If a spillage occurs of a fully refrigerated liquid it still needs to be vaporised with sufficient rapidity. This is less likely on land for the reasons given above. On the sea there is a continuing supply of heat from the water to

the spreading vaporising fluid. There is also a tendency for the cooling of the water below the spreading vaporising liquid to cause the water to sink and be replaced by warmer water from underneath. Thus, the possibility of a catastrophe of dimensions that are indicated in Table 1 following an accident involving one or possibly two liquid flammable gas tanks in a ship, is a very real one. However, it is more difficult to conceive of the production of a fireball of the indicated size. A possible mech- an.ism might occur if the tank concerned were already surrounded by a substantial fire, which would act as a dispersive and vaporising agent to the contents if these were released with sufficient rapidity by disruption of the tank. A well known analogous situation is the "boil- over" which may be caused when the hot zone formed in a tank of burning heavy fuel oil reaches a layer of water that may be present at the floor of the tank.

For a 20 000 ton gas tanker the involve- ment of all the tanks in one fireball situation can probably be ruled out. Even the extended spill hazard is very unlikely since there will need to be a mechanism present which gives rise to the disgorgement of the contents within a number of different tanks in the limited time available. With LNG a mechanism has been suggested that local leakages and vaporisation of LNG within the cargo space, even of LNG spilled at a terminal, might stress the metal of the hull by cooling, which could then fail to support all of the other tanks [ 8]. l am not aware of any calculation which shows this can happen in the necessary time to produce effects as in Table 1. This mechanism however would not apply to LPG since the hull should be able to resist the low temperature for a sufficient time. On the other hand, a single land- based refrigerated tank could, if it collapsed, give rise to the disaster condition approaching those outlined in Table 1 if there were no available mechanism of channelling or other- wise controlling the released spillage.

Finally, there are two further factors which might make the situation worse than is appar- ent in Table 1. First, under some conditions, explosion effects over a large area have been experienced following a spillage greater than, say, some tens of tons of liquefied flammable gas in the open, even if the ignition source is a small one. This phenomenon is, perhaps, most likely when ignition is delayed, but cannot be

229

ruled out for immediate ignition with release from pressurised vessels [9] . We still know very little about this phenomenon but I be- lieve that it is more likely where pockets of flammable vapour-air mixture may be allowed to accumulate in enclosed spaces and where obstacles, plant or buildings, are present within the flammable vapour cloud which can increase the rate of flame spread by turbulence and possibly local detonation. There is much dis- cussion and some work proceeding on this topic but as far as I am aware, it has not been possible to reproduce yet the development of substantial pressures in the open under experimental conditions. If such pressure effects were, for example, manifested over a 1- 2 km 2 area over which a flashover has taken place following a 2 000 ton spillage, then certainly windows within the flash fire area and substantially beyond it would break, and much damage would be caused to buildings within the area. This would allow the flames to ignite easily combustible material within the buildings: thus, one might expect in a built up area items such as curtains and foam furniture and bedding would be universally ignited in most rooms. The breakage of the windows beyond the initial burning area would assist in developing fire in this area by radiation. For this reason, the transport of a large volume of flammable vapour over a built-up area is the greatest of the hazards of LPG in that it is liable to cause the largest number of deaths and most damage if it does occur. One cannot with confidence recommend to people that they close all doors and windows and stay in- doors, as one can with toxic or even radio- active release hazards, since damage caused by a blast accompanying the flash fire can rup- ture the houses and allow them to become ignited in many places. It has been loosely stated that there are many ignition sources in a built up area and that an incoming cloud of flammable gas will be ignited before the flammable vapours can encroach extensively into the area. It is difficult to accept this for a night time situation in purely residential areas. However, as will be indicated later, the controlled ignition of such clouds before they reach such areas could be a major defence against the hazard.

The second possible effect is the development of a fire storm as was caused by the bombing raids in World War II. In this type

of fire the flames coalesce to form a large column of flame above the area of the fire. Gale force winds are established towards the fire and the upward buoyant column of flame may throw burning material to distances at points beyond the fire area. In general, to obtain a fire storm it is necessary to have a burning area of about 1 km 2 and for buildings to be high and distances between buildings comparatively small, i.e., a fairly dense built up area [10]. On the other hand, the furnishing at present in most dwellings, if ignited, can burn very rapidly, and it may be possible that under these conditions even a mainly residential area of typical two storey high dwellings could allow a fire storm to be established.

L I K E L I H O O D OF F I R E AND E X P L O S I O N

H A Z A R D

The fact that one can postulate catastrophic happenings as indicated above does not mean that storage and transport in the quantities indicated should be eliminated. Thus, a possible catastrophic fire hazard exists for almost every shopping complex in the country. There are hazards of this kind also for hospitals, the- atres, cinemas, etc. The question that arises is, is it really necessary to expose people to possible major hazards and, if this cannot be avoided without great difficulty, can the hazard be sufficiently reduced so that it becomes at least endurable ? It must be remembered here that a major factor in the capacity to endure a hazard is the extent to which those at risk receive benefit from the hazardous activity. Thus, in the above quoted examples of catastrophic hazard, those at risk are those using the premises, not those outside.

Few would deny that the communi ty as a whole obtains great benefit from the availability of LPG fuels; the use of them in the liquefied form is very convenient, particularly in small quantities in places which piped fuels cannot reach. Moreover, in these days of energy scarcity, one must be mindful of the value of these liquefied gases as a substantial contr ibutor to our energy resources. This is particularly relevant in the U.K. since North Sea resources have an unusually high con- tent of these volatile gases. Indeed, for this reason, there is a good case for the con-

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TABLE 3

Fire stat ist ics - - LPG, natural gas (NG) and t o w n gas (TG)

(Taken f rom UK Annual Fire Stat ist ics for the year conce rned publ ished by Building Research Es tab l i shment 1970 - 3 and Home Office 1974 - 7).

Number o f fires, material first ignited:

In buildings LPG TG + NG

Not in buildings LPG TG + NG

Number o f fires, source o f ignition:

In buildings LPG TG + NG

Not in buildings LPG TG + NG

1977 1976 1974 1973 1972 1971 1970

1 113 894 912 844 967 541 632 1 302 1 357 1 608 1 644 1 846 1 691 1 613

n.a. $ n.a. p.a. 624 502 503 427 n.a. n.a. n.a. 346* 560* 622* 558*

1 710 1 631 1 573 1 406 1 270 1 088 1 080 7 588 7 577 8 776 8 978 8 333 7 333 7 107

n.a. n.a. n.a. 910 746 650 675 n.a. n.a. n.a. 26 14 19 23

Number o f fires, both source o f ignition and material first ignited:

In buildings LPG 570 559 567 418 TG + NG 769 848 964 914

Not in buildings LPG n.a. n.a. n.a. 282 TG + NG n.a. n.a. n.a. 0

424 288 287 817 859 760

207 217 178 3** 2** 3**

Fire rate in buildings (material first ignited)~109 therms

t N G and TG used in UK (109 Th) 14.549 13.969 12.634 10.700 9.757 7.440 5.750

Number o f fires per 109 Th for NG and TG 89.5 97.1 127 153 180 225 280

t L P G u~ed in UK (109 Th) (o ther than suppl ied to gas works) 0.612 0.605 0.631 0.627 0.516 0.356 0.268

Number o f fires per 109 Th for LPG 1 818 1 477 1 445 1 346 1 874 1 519 2 358

*TG only - - nearly all in gas works. **TG only.

t T a k e n f rom Annual Abst rac ts o f Stat ist ics 1974 and 1979 publ i shed by HMSO. $ no t available.

sumption of liquid flammable gas to increase by a factor of five or more over the next decade or so. For this reason also, it is very appropriate that one should take a very rigorous look at the hazard presented by LPG at the present t ime and particularly to look for areas where safety might be improved.

Table 3 gives information on the occurrence of fire in the UK in which LPG was a contri- bu tory factor. For comparison, information is also given on fires in which ei ther natural gas or town gas was a con t r ibu tory factor. It con-

tains numbers of fires in which the fuel appliance was the ignition source and also numbers in which the fuel itself was the material first ignited. In addition, numbers are also given where both the fuel appliance was the ignition source and the fuel itself was the first ignited. In themselves, these figures are not very meaningful. They need to be compared in each case with the amount of fuel used in the UK. This information may be obtained from the annual abstract of statistics [ 11] and is also given in Table 3. It may be shown

that, in all categories, for all the years, liquefied petroleum gas produced many more fires per million therms of fuel used than did natural gas or town gas. Perhaps the most relevant comparison is for fires in which the fuel gas was the material first ignited. Table 3 shows that throughout the whole period, LPG was about ten times more hazardous than the other gases. Indeed, the figures in the Table are for fires in buildings only; taking account of outdoor fires would increase the LPG hazard by some 50 per cent. Another disturbing feature is that there is little evidence that this fire rate is decreasing as it undoubtedly is for natural gas. There are, of course, some good reasons for this, particularly, increasing amounts of natural gas, which has become much more available, are being used for central heating which is a comparatively safe way of using fuel. Moreover, all natural gas is piped, whereas the bulk of liquid fuel gases used are conveyed in stored form to the appliances. Also, as indicated above, liquefied petroleum gases suffer from the disadvantage of being heavier than air. The figures nevertheless point to the need for a substantial improvement in fire safety of liquefied fuel gases if their use is going to increase.

Turning to the major hazards, for tunately our experience in this country of such disasters with LPG as is indicated is virtually non-exis- tent although there have been a significant number of near misses. The sort of thing which could happen is exemplified by the Flix- borough disaster, but this, of course, involved cycl0hexane , not liquefied petroleum gas. A major feature of this type of hazard is that, unlike the individual LPG appliances relevant to most of the statistics in Table 2 and, indeed, also unlike the catastrophic fire hazard associated with buildings, those exposed to the hazard may be obtaining only marginal, or even sometimes negative, benefit from the activity. This applies particularly where large quantities of flammable liquid gas are transported at terminal areas; a risk can be stated to be almost focussed at those people living near these places.

In order to make design and planning deci- sions, we need quantitative information on how safe we are from catastrophies of this kind. Techniques are now becoming available to allow this to be estimated through proce- dures known as hazard analysis and fault tree

231

analysis. In the latter procedure, the projected catastrophe is the final incident in a chain of unlikely events which lead first to the spillage then the evaporation, with then ignition and either flaming or explosion, or both, taking place in an area where great harm can be done. Under the Health and Safety at Work Etc. Act, it is being proposed that organisations which store quantities of liquefied flammable gases in excess of 300 tonnes shall carry out hazard surveys and, in certain instances where com- munities might be at risk, to follow this up with a more detailed study which might include fault tree analysis [12]. A recent example of a detailed survey has been carried out by the Safety and Reliability Directorate on the Canvey Island situation [5]. In my department at the University of Edinburgh, for over the last four years we have carried out fault tree analyses for three special situations; two of these are listed and summarised in Table 4. The third is an a t tempt to obtain fault trees for a major spillage in road and rail transport, which is still in progress, this latter s tudy suggests that under present conditions a major disaster will occur about once in 10 - 20 years.

UNACCEPTABLE AND ACCEPTABLE HAZARDS

Having obtained estimates of the above, the question arises, under what conditions is the hazard unacceptable? There is much contro- versy as to how one should approach this problem at the present time. One approach is that if the analysis shows a hazard which is not greater than that of a similar kind which has been endured in the past, then this is evidence that the hazard is not unendurable. However, one must be very careful in postu- lating the factors which will allow us to say whether the known, endured type of incident is, indeed, one of a similar kind. The characteristics of a major incident involving fire or explosion with LPG are that it will kill by fire in a short time many of the people exposed to the risk; but, in some cases, those facing the risk welcome the activity, in other cases they are neutral, and in others antipathetic. Data which would assist in developing quantitative criteria may be obtained by studying our present risk of exposure to catastrophic fire and explosion hazards. The evidence from

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TABLE 4

Summaries of fault tree analyses involving major LPG risks

Type of risk Projected disaster Estimated probability of disaster

Horton sphere 600 ton capacity. Protected by water spray system and an available fire fighting crew. Situated 150 m from main chemical plant [20].

Liquefied gas marine terminal handling pressurised LPG [21].

(1) Catastrophic fireball at sphere. (2) Open Ilammable cloud explosion on plant following drifting of cloud of unignited vapours

Major fire or explosion in the terminal area involving either catastrophic fire ball or large open flammable cloud explosion or fire

(1) 1.7 × 10 8/annum

(2) 2.2 X 10 6/annum

2.3 x 10-3/annum

such a study that I have carried out indicates that, at present, a communi ty of some 10 000 people, for example, might expect a fire disaster that would kill more than one hundred people, caused by a risk to whose benefit they are neutral, about once in 10 million years. A difficulty with probabilities as low as this is that it is below the range of probabilities that may be confidently predicted by fault trees at the present time. However, it suggests that a criterion for acceptability of the order of once in 10 000 years, implied by the Canvey Island report [15], is too high.

METHODS OF IMPROVING FIRE SAFETY WITH

LPG

General hazards The poor record of LPG with regard to fire

occurrence compared with piped gas prompts one to look for possible ways of improving its safety. Unfortunately, it is not obvious to find these. The high fire rate would appear to be inherent in the places and manner where LPG is used, and arise because of the sheer convenience of using bottled gas. Good Codes of Practice, such as the Home Office Codes [ 13, 14], have been available since 1971, and LPGITA have provided much guidance on this matter [15] . Recently Kemp and Drysdale have produced a design guide for architects [ 16]. The effect of these might well begin to

show in future years. The situation should also improve when regulations under the Health and Safety at Work, Etc. Act are produced with accompanying Codes of Practice for different industrial situations. A possible general improvement that might be effected is to recommend that flammable gas detectors be employed in the vicinity of LPG appliances. The provision of a mass market for such detectors could well result in a cheap, reliable instrument in a way similar to that in which the mass market for smoke detection has developed in the United States. They could actually be fit ted on appliances and set to give a warning or shut off a supply or even open extra ventilation if a high concentration of flammable gas is detected.

A major step forward, particularly if the use of LPG is to be increased, is to move the use into the direction where there would be less handling of cylinders, particularly towards piped gas. Another possibility is to use LPG for road vehicles. Here, the hazard would need to be comparable with that of petrol if such a use was not to give rise to special anxiety on fire safety. There are reasons for believing that in some respects the fire hazard would be greater than petrol, e.g., the influence of a fire near the tank as a result of a leak. Even though the LPG tank may be vented, this is unlikely to prevent a substantial fire inside a vehicle when the vent operated. Also, overfilling is more likely to lead to accidents. However,

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the tank would be more robust than a petrol tank and less likely to be damaged in a collision. A detailed hazard survey would be valuable before this use were to become extensive in the UK, perhaps with some full scale tests, as well, on the effect of fire. LPG is being used widely for cars in Europe, particularly in Hol- land where 450 000 tons per annum are being used for road transport. An enquiry to the Dutch Research Institute for Road Vehicles has indicated that experience in Holland has not given reason for special concern.

Major hazards What can we do about the major hazards

associated with storage a~d transport of LPG in quantities of a few tons upwards? The relatively clean record in the UK so far should not blind us to the risk that exists, not only to the population at large but also, and perhaps particularly, to emergency services called to deal with an incident. As far as road tankers are concerned, the recent regulations requir- ing them to carry a warning panel which in- cludes instructions on emergency action [ 17], goes some way to reducing the hazard to which they are exposed. The code 2WE for butane and propane indicates that there is danger of a violent explosion, they should wear full protective clothing and use water fog as a controlling medium; they should also consider evacuation of the public. However, it contains no indication of how far away danger would be manifest if there is a "violent explosion".

Thus, American experience of incidents involving a road or rail tanker and accompanied by fire, is that there may be only about 10 minutes available before a BLEVE takes place. The latter is an acronym for "boiling liquid expanding vapour explosion" but really cor- responds to the fireball situation outlined in Table 1. The information in the Table suggests that if firemen are called to a fire incident on such a tanker, they would be unwise to approach within 300 metres, especially as the so called full protective clothing is not a protection against radiant heat. They almost certainly would approach in order to set up some cooling onto the tank, particularly if many people were at risk, but in doing so they would themselves be taking a risk. Even if there is no leak following an incident in which an LPG tanker is involved in violent collision, there may be sufficient damage, possibly unseen,

to the tanker which could result in its strength being reduced below that of the pressure release, and its sudden bursting. Thus, there is evidence that the unexpected BLEVE at Waverley, Tennessee, last year was due to the scoring of a rail tanker which had been de- railed. It is my opinion that a major step towards reducing the possibility of incidents of this kind would be to transport LPG, and, indeed, other dangerous materials like it, in the refrigerated state in insulated containers. The safety would be improved by reducing the temperature to well below the normal boiling point. This practice should eliminate BLEVES and pressure bursts of the above kind. Also, in spite of the mistake of overfilling which appears to have been made in the recent Spanish disaster, it almost certainly would not have occurred if the liquid had been transported in an insulated container. Such a change would bring about some changes in the fuel as used in practice and would cause extra expense. However, a fundamental reas- sessment of the way LPG is handled is t imely if there is likely to be a sharp increase in its use. For example, if LPG is to be used as an automotive fuel, it could be transported in the refrigerated state, stored at the retailing garage in this state, and warmed sufficiently on it being delivered to the fuel tanks of vehicles, if this is indeed necessary. Another in- teresting possibility is to fill the tank with a metal mesh. This has been used in the past to suppress vapour-air explosions in a tank, a risk which is mostly irrelevant for LPG. How- ever, the presence of the metal mesh may reduce the rate of ejection of the fluid following a burst, but this needs to be investigated.

With regard to very large tonnage stored in refrigerated tanks on land, here, at least, a step towards safety has been taken by reducing the liquid temperature to its boiling point. One still has the possibility of failure of the tank but the state of the liquid allows a very important safety step to be taken, i.e., that of redundant secondary containment. This means putting a second container round the first, which will hold all the liquid if the first container fails. The structure of the redundant containment should be such as not to fail by mechanisms which cause failure of the first (mainly rollover and metallurgical failure). An outer shell of concrete is therefore appropriate for the task.

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GAS TANKERS ON SHIPS

This still leaves the thorny question of liquefied gas tanks on ships. In addit ion to the mechanisms of failure indicated above, these have certain more likely failure mechanisms. These ships move about in inshore waters where they may be subject to collision and stranding, and they are of ten in direct contac t with a terminal where there is a significant probabil i ty there could be a leak or overfilling, or some o ther maloperat ion that might cause spillage. There may also be ignition sources on or near ships. In addition, the management of ships tends to be heterogeneous and not under the same degree of discipline and control as land based management. Finally, there exists, in close proximity , much water with which these liquefied gases are thermally in- compatible and which could enhance a spillage hazard by causing rapid vaporisation.

A simple solution would be to keep these tankers away from highly populated areas. Table 1 suggests that a distance of about 6 km or 4 miles would be reasonable. Although this is within the maximum distance o f spread to the LFL, it is unlikely that a large area that would maintain a flash fire would build up outside this distance for a leakage up to 1000 tons . What is perhaps more impor tant is that this distance will allow a flexibility o f emergency operat ion if there is a dangerous incident which results in a tank being punctured, particularly under water. Thus, if the vapours of such a leak are not threatening people, the safest way of dealing with them is to allow them to disperse on their own. Owing to the possibility of a sudden intensification of the incident, resulting from the possible entry of water into a tank, I would recommend immediate evacuation of those on board and exposed to the vapours, and would advise that emergency services should not approach un- less they are protec ted against being surrounded by fire and exposed to explosion forces. However, if the vapour leak becomes so large as to develop into a threat to people on shore, particularly in a built-up area, then I think it would be necessary to ignite the leak. This will result in a flash back to the ship and a fierce fire there. The evacuation of per- sonnel is a necessary precaut ion to allow this opt ion to be exercised. However, the dimensions of the fire near the ship, although great,

would be much less than the maximum flash area indicated in Table 1. Thus, for 2 000 tons of LPG leaking in 13 minutes, the fire size would be about 130 m diameter and 360 m high; for 20 000 tons escaping in 26 minutes, 300 m in diameter and 600 m high. Even the occurrence of a fireball during such a fire as a result of a 2 000 ton tank suddenly disrupting is unlikely to endanger people at a distance of 6 km. However, the liability of such a tank to disrupt at any t ime if water can gain access to the tank will compromise the ability of emergency services to call for an evacuation to a safe distance in an emergency situation for distances less than about 4 km.

As things are at the moment , these tankers move well within this 6 km distance. Can the risk in this situation be reduced to endurable levels ? Management at terminals and tankers could be tightened, e.g., only one management could be responsible for both. Also, tankers could be used which are designed so as not to puncture or spill contents into the sea, under any conditions of stranding, collision or process maloperation. Judging from the Canvey Island repor t and our own hazard analysis for an LPG terminal given in Table 4, there appears to be a long haul before this approach becomes convincingly viable, bearing in mind the criterion for acceptabili ty implied above rather than the one adopted in the Canvey Island report . It should be noted here that the 1975 International (IMCO) Standard [18] allows both butane and propane tanks to be situated only 0.76 m from the hull, and actually visualises the puncturing of tanks and their becoming filled with water as a possible condi- t ion for the design of the stability of the ship.

Contributions to safety might be made by chilling a liquefied gas cargo to a tempera ture well below its boiling point and surrounding the tanker at the terminal by a boom which will reduce the extent of spread of spilled liquefied gas. If there is a substantial spillage this would cause ice to form under the spillage and considerably reduce the rate of vaporisation which is the dangerous factor in this situation. Thus, Reid and Smith report [19] a rate of vaporisation for LPG on a limited area of ice about one-third that on a limited area of water, and, in both cases, the rate drops off inversely as the square roo t of the time. Moreover, if there is a puncture in a tank which results in water invasion, the lower

235

temperature of the LPG would encourage the rapid formation of ice without vapour formation soon after the water ingress, and therefore considerably reduce the possibility of explosive formation of flammable vapour in the tank. However, much research is required to establish opt imum conditions for this reduction. Even without extra chilling, a boom set up some 20 metres away from the ship's side, attached to each end of the ship and strengthened and perhaps compartmented with supporting spokes to the ship's side, would provide valuable independent secondary containment for many spillage situations. The vapour would not be held back but its rate of production would be reduced and would probably become controllable by emergency measures, e.g., the automatic action of medium or high expansion foam generators. Indeed, it would be desirable for such ships to carry such a boom while moving near populated areas. Although this may result in struc- tural and navigational difficulties it could be made to act as a first line of defence against a lateral collision. The boom may be kept on deck for passage through the high seas and lowered for inshore waters. An inner hull, as is common for LNG ships, would also be beneficial.

Beyond this, if there is any residual, unacceptable risk to exposed residential areas, there is little one can recommend other than to give the houses the necessary degree of fire and explosion protection and to compensate injured parties for exposure to excessive risk and anxiety due to risk. It must be remembered that anxiety is caused not only by possible accidents affecting the injured parties directly, but by catastrophic accidents that may happen anywhere in the world -- particularly if it cannot be clearly demonstrated that conditions are significantly different. The type of protection that would be relevant would be:

(a) A warning system which would give warning of imminent danger due to a major spillage having occurred, or some other reason.

(b) Emergency procedure including notice of what people should do in the event of a warning. This would include instructions on how to seek refuge or to evacuate.

(c) Provision of a controlled flammable gas detector-ignit ion source system to ignite flammable gases if they approach the shore.

(d) Provision of fire retardant shutters or curtains for windows of premises under threat of fire.

(e) Provision of t reatment for splintering of all glass under threat of explosion pressures and not protected under (d).

(f) Provision of sprinkler installations for all premises under threat of fire.

(g) Provision of safe refuges against explosion or fire effects.

CONCLUSIONS

(1) Liquefied fuel gas is a valuable fuel resource which is likely to increase greatly in availability in future years. However, special efforts are needed to make its use sufficiently safe with regard to fire and explosion. (2) The fire and explosion hazard of LPG in its everyday use is intrinsically more difficult to counter than that of town gas or natural gas. The reasons are:

(a) that LPG vapours are heavier than air; (b) the gas is provided as an article of com-

merce rather than in a piped system; (c) because of its special convenience it is

used in situations where it is not easily subject to control. (3) Available statistics on fire indicate that LPG is more than ten times more likely than natural gas to cause fires and explosions per unit energy use of fuel. (4) Future major expansion of LPG should take place in the direction of providing piped fuel or into situations which do not require exchange and trading of gas cylinders, A detailed hazard survey is justified prior to any extension in its use. More extensive regulations and codes are also required. (5) LPG as used at present has attached to it a very serious propensity for major fire and explosion hazard. This is due to the ability of the liquefied gas to vaporise very rapidly when containment has been lost and thus produce massive vapour clouds in a short time. This applies particularly to the pressurised liquid at ambient temperatures for which secondary containment is generally impracticable, but also, under some conditions, to the refrigerated liquid at ambient pressure for which secondary corrtainment is generally practicable. (6) The hazards result from the possibility of a large fireball which would occur if ignition followed soon after release or fire or explo-

236

sion, in an open flammable cloud, if ignition was delayed. Perhaps the greatest hazard to people would occur if a flammable cloud drifted over a populated area and became ignited. (7) In the UK there is no experience of a major disaster of the type outlined in (6). However, hazard surveys available indicate that the likelihood of such a disaster is quite credible. (8) Reduction of major hazards in road and rail transport could fol low if LPG were transported in the refrigerated state -- particularly if the temperature was well below the boiling point at ambient temperature. There would, however, be economic disadvantages of doing this. (9) Both pressurised and refrigerated tankers (ships) should, if possible, be kept well away from populated areas, both to prevent a disaster and to give flexibility in an emergency situation. If, in the national interest, it is not possible to do this in certain instances, then it is necessary to reduce the risk o f a major disaster due to fire or explosion to a level below that which appears to be the case at the present time. This can be done by:

(a) Improving management and design of tankers and terminals so that spillages will occ, ur at a far lower expected frequency.

(b) Devising methods of reducing the rate of vapour formation from the spillage, particularly by containment.

(c) Providing special fire and explosion protection to those exposed to the risk. There may also be a case fgr providing com- pensation to some of those exposed to the risk.

REFERENCES

1 K. Hasegawa and K. Sato, Experimental investigation of the unconfined vapour-cloud explosions of hydrocarbons, Tech. Mere. Fire Res. Inst. No 12, Tokyo, 1978.

2 G. H. Markstein, Scaling of radiative characteristics of turbulent diffusion flames, FMRC No 22361-4, 19 76, Factory Mutual Research.

3 D. I. Lawson and D. L. Simms, The ignition of wood by radiation, Br. J. Appl. Phys., Vol. 3, September ( 1952 ).

4 D. L. Simms and P. L. Hinkley, Protective clothing against flames and heat, Fire Res. Spec. Rep. No 3, 1960.

5 Canvey -- An Investigation of Potential Hazards from Operations in the Canvey Island/Thurrock Area, Her Majesty's Stationery Office, London, 1978.

6 A. Kneebone and L. R. Prew, Shipboard jettison tests of LNG onto the sea, 4th Int. Conf. on Liquefied Natural Gas, Algiers, 1974.

7 W. G. May, W. McQueen and R. H. Whipp, Dis- persion of LNG spills, Hydrocarbon Process., May (1973) 105.

8 J. A. Fay and J. J. MacKenzie, Cold cargo, Envi- ronment, 14 (9) (1972) 21.

9 J. I. Cox, Canvey -- comment on the report, Chem. Eng. (London), October, 1978, p. 747.

10 R. Baldwin and M. A. North, The firestorm -- its size and importance, Fire Res. Note No 645, Fire Research Station, Borehamwood, U.K.

11 Annual Abstract o f Statistics, Central Statistical Office, 1974/79, Her Majesty's Stationery Office, London.

12 Hazardous Installations (Notification and Survey) Regulations 1978, Health and Safety Commission Consultative Document.

13 Code o f Practice for the Storage o f LPG at Fixed Installations, Her Majesty's Stationery Office, London, 1971.

14 Safe Use and Storage of LPG in Residential Pre- mises, Fire Prevention Guide No 4, Her Majesty's Stationery Office, London.

15 Codes o f Practice (various). Publications on be- half of Liquefied Petroleum Gas Industry Tech- nical Association (UK). Printed by and obtainable from Wm Culross & Son Ltd., Coupar Angus, Perthshire, U.K.

16 N. Kemp and D. D. Drysdale, Designing for flammable atmospheres in buildings, Architects' J., (March 7) (1979) 499; (March 14) {1979) 557; (March 28) (1979) 657.

17 A Guide to Tanker Marking Regulations, Health and Safety Executive, Her Majesty's Stationery Office, 1979.

18 Code for the Construction and Equipment o f Ships Carrying Liquefied Gases in Bulk, Inter- Government Maritime Consultative Organization, London, 1976.

19 R. C. Reid and K. A. Smith, Behavior of LPG on water, Hydrocarbon Proc., April, 1978.

20 D. D. Drysdale and G. J. David, Int. Symp. on Fire Safety Evaluation in Industry, Stockholm, 1979, pp. 1 - 25, to be published in Fire Safety Journal.

21 R. J. Beckett, Fault tree analysis for the opera- tion of a liquefied gas marine terminal, MSc Dis- sertation, Univ. Edinburgh, 1978, paper in prepa- ration.

review of explosion and fire hazard of liquefied petroleum gas

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