J Fail. Anal. and Preven. (2010) 10:322–333 DOI 10.1007/s11668-010-9360-9

TECHNICAL ARTICLE—PEER-REVIEWED

Risks of Fire and Explosion Associated With the Increasing Use of Liquefied Petroleum Gas S. M. Tauseef • Tasneem Abbasi • S. A. Abbasi

Submitted: 17 March 2010 / in revised form: 5 May 2010 / Published online: 9 June 2010  ASM International 2010

Abstract Liquefied petroleum gas (LPG) has been in use as household fuel all over the world for several decades. Until the late 1980s, its use in the developing world was largely confined to the economically well-off strata of the society but it has since spread over a much larger catchment. The increasing use of LPG has enhanced and generalized the risk of a ‘‘boiling liquid expanding vapor explosion’’ (BLEVE). This is evidenced from the reports which appear now and then of a LPG cylinder having exploded in a household, some workshop, or on a bus or a train. In fact some very major tragedies have been triggered by such explosions which also set off fires and cause secondary accidents. This paper describes what BLEVEs are and how can they be controlled. The paper focuses on hazards of BLEVE in large installations which deal with LPG, and other pressure-liquefied gases and discusses the nature, mechanism, and means of control of BLEVEs. Keywords Pressure-liquefied gas  BLEVE  Explosion  Superheat  Control

What Is BLEVE? The boiling liquid expanding vapor explosion (BLEVE) is among the most fearsome of accidents that can occur wherever a pressure-liquefied gas (PLG) exists. If a container with a PLG suffers structural failure—be it due to creep, fatigue, or fire-induced or any other form of

S. M. Tauseef  T. Abbasi  S. A. Abbasi (&) Center for Pollution Control and Environmental Engineering, Pondicherry University, Pondicherry 605 014, India e-mail: [email protected]

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failure—it may lead to a sudden depressurization of the container. As a result, the PLG will suddenly be transformed into a fluid which is ‘‘superheated’’ with respect to the precipitously lowered pressure. Depending on the nature of the chemical, quantity of superheated liquid present, and the mechanism of the container failure, such a situation can lead to instantaneous and violent vaporization of the contents, causing a ‘‘boiling liquid expanding vapor explosion’’—a BLEVE.

Illustrative Case Studies Some of the biggest process industry disasters have involved BLEVEs. Indeed the second largest process industry disaster in terms of fatalities—next only to the Bhopal gas tragedy of 1984—involved a succession of BLEVEs. The resulting explosions and fires destroyed an entire refinery at San Jaun Ixhaautepec, Maxico city, killing over 600 persons. We present below illustrative examples of accidents in which BLEVEs, accompanied by fires, were the cause of major catastrophes. Illustrative case studies are also presented where BLEVEs occurred in small household-scale LPG cylinders. These BLEVEs invariably lead to fires which caused as much, and often greater, damage than the explosion itself. Feyzin, France 4, January 1996: The episode at Feyzin is a tragic example of the tendency of BLEVEs to create ‘‘domino effect,’’ i.e., causing secondary accidents which may be as destructive as the initiating accident. A leak in a propane storage sphere occurred on 4 January 1996 at Feyzin, France. The continuing propane leak soon formed a visible cloud of vapor, 1 m deep. It spread for 150 m and was ignited 25 min after the leak started. Ignition was caused by an automobile that had stopped on a

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nearby road. The fire flashed back to the sphere and began heating it even as fire fighters struggled to douse the fire. Ninety minutes after the fire started, the sphere went through a BLEVE. Ten out of 12 firemen within 50 m of the sphere were killed. Men 140 m away were badly burned by a wave of propane which came over the compound wall. Altogether 15–18 men were killed and about 80 injured. Flying debris broke the legs of an adjacent sphere which fell over. Its relief valve discharged liquid which added to the fire, and 45 min later it initiated another BLEVE which led to a sequence of BLEVEs. Altogether five spheres and two other pressure vessels burst and three were damaged. The fire also spread to gasoline and fuel oil tanks. Mexico City 19, November 1984: The Mexico city disaster is another example of the tendency of a BLEVE to cause other BLEVEs and secondary fires. When a vessel undergoes a BLEVE, the fragments of the shattered vessel are propelled outward at great velocities in all directions. When the rocketing fragments hit other vessels, it results in secondary accidents. The fragments are also implicated in enhancing the toll of accidents in terms of death, injuries, and property damage. The disaster at the PEMEX LPG terminal in San Juan Ixhuatepec, Mexico City, was aggravated by these characteristics. The terminal was a large installation which received supplies from three gas refineries every day. On the morning of 19 November 1984, when the vessels at the PEMEX terminal were being filled with LPG arriving in a pipeline from a refinery 400 km away, a drop in the pipeline pressure was noticed by the control-room and a pumping station. It occurred because an 8 in. pipe connecting one of the spheres to a series of cylinders had ruptured. However, the operators did not think of this possibility and the release of the LPG from the leaking pipeline continued for 5–10 min. The escaping gas formed a 2-m high cloud which then drifted toward a flare tower, caught fire, and precipitated the first BLEVE. The explosion hurled vessel fragments wrapped in burning LPG in all directions. Some of the projectiles hit other vessels, damaging them, or caused local fires which engulfed other vessels. This led to the failure of one vessel after another and many exploding vessels caused nearby vessels to fail. Four LPG spheres, each containing 1500 m3 of LPG, and several other smaller cylinders holding between 45 and 270 m3 of the liquid suffered BLEVEs. Each BLEVE generated a fireball; such fireballs raged through the streets of Ixhuatepec for about 90 min. A block of perhaps 200 houses built mostly of wood, cardboard, and metal sheets was demolished by these fireballs. Masses of fragments of tanks and pipes, some of them weighing 40 ton, were blown into air and landed as far as 1200 m away. The PEMEX terminal was devastated.

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The accident was responsible for 650 deaths and over 6400 injuries. Damages due to the explosion and the resulting fire were estimated at approximately $31 million. BLEVEs Occurring During the Transportation of LPG Hundreds of accidents that have occurred all over the world during transpiration of LPG in recent years and some illustrative examples are presented below. At least 22 people were killed while 10 others were wounded when a truck carrying an LPG tank overturned, in Tigbao, Philippines on 2 February 2007. The truck’s brakes failed before the accident. The break failure caused the driver to lose control and hit the side of the road. The impact caused the truck to turn turtle and damaged the tank, causing a BLEVE. The explosion impacted a bus beside the truck, and the fatalities were mostly passengers of the bus. A light truck was carrying LPG cylinders on Rock Road, Australia, on 12 August, 2007 when one of the cylinders ignited and went through a BLEVE. Subsequently the other cylinders also experienced BLEVEs, destroying the truck and causing damage to surrounding houses. At Jaipur, India, on 10 November 2008 an LPG tanker suffered a BLEVE during the process of unloading, killing 3 persons. Apparently, the driver of the LPG trailer tanker attempted to drive off in the tanker without evacuating and without disconnecting the hose from the fill pipe of the underground LPG tank. As the tanker moved the control valve provided to liquid line was sheared and the whole assembly of the valve and the adaptor came out of LPG tanker creating a small hole in the bottom of the tanker. LPG gushed out of the hole and caught fire which engulfed the tanker. The tanker soon suffered a massive BLEVE with parts of the tanker being propelled to a distance of 150 m. An LPG tanker was on its way to Jammu from Ghaziabad, India, on April 12, 2009 when a car hit it from the rear leading to a gas leak from one of its valves. The driver of the tanker drove the truck for 2–3 km before her abandoned it on the road, raising an alarm. After 10 min, the tanker suffered a massive BLEVE. The blast was so powerful that the pieces of the tanker were hurled to a distance of 300–400 m. There was a railway track parallel to the accident spot and the flames also reached a railway guard who was asleep, causing him serious burn injuries. Nevertheless, a much greater tragedy was averted, as there were two more LPG tankers and a diesel truck parked near the accident and these trucks were removed just before the blast. Four people were killed and 44 were injured after two LPG tank trailers exploded in a train collision on August 11, 2009 near Brownsville, TX, USA.

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Two people were killed as a 16-ton LPG gas tanker suffered a BLEVE at Pune, India, on 26 August 2009. The explosion tore the tanker into pieces and caused a huge fire which gutted five vehicles including one container, two trucks and a car. Two persons were lethally burned on the spot. The explosion was so big that its bang was heard at a distance of 5 km. Locals residing nearby fled into the fields as the smoke enveloped the entire surrounding area. An LPG tanker was on transit when the driver reportedly lost control over the vehicle near a village at around 10.30 pm on 23 October, 2009. The speeding vehicle hit the highway railing, overturned and rolled over. A BLEVE followed, killing three villagers who were relaxing on the road side after dinner. The driver of the vehicle and his help were also burned to death.

Incidents of BLEVEs in Small LPG Cylinders One of the worst disasters perpetrated by a portable household-scale LPG cylinder which underwent a BLEVE occurred in a passenger train going from Cairo to Luxor in Egypt on February 20, 2002. It occurred in the fifth carriage of the 11-carriage passenger train and the resulting fire spread as the train ran. Seven of the carriages were burnt almost to cinders. According to the official figure given at the time, 383 people died. However, considering that seven carriages were burned to the ground and each carriage was packed with at least double the maximum carrying capacity of 150, this figure is probably an underestimate. The Cairo incident was a particularly macabre one, but BLEVEs involving small LPG cylinders is a fairly common phenomenon as may be seen from illustrative examples taken from the events that have occurred during 2008–2009. A man was killed and three others seriously injured when a cooking gas cylinder exploded in a house in Balasore, India, on 15 February, 2008. Four people were killed when an LPG cylinder being used for cooking went through a BLEVE near Patna, India, on 10 March, 2008. On the same day at Kolkata, India, the explosion of an LPG cylinder started a fire which engulfed nearby dwellings. One after another LPG cylinders exploded as the fire kept spreading before it was brought under control. Approximately 50 cylinders were thought to be involved in the accident. A leaking LPG cylinder exploded on 20 April, 2008 at Bangalore, India, killing a four-year-old boy, injuring 11, and damaging houses in the neighborhood. An LPG cylinder exploded in Dimapur, India, on 23 April, 2008 causing a devastating fire which led to the explosion of several more LPG cylinders. As many as 300

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houses were razed to the ground and two children were killed. One woman and two children were killed and 10 other people were injured when a cooking gas cylinder exploded in a house in southwest Delhi, on March 2, 2008. At least three people were killed and 20 injured when a cooking gas cylinder exploded in Malegaon, India, on 29 September, 2008. The cylinder went off in a crowded market and the panic after the blast led to a stampede in which 60 people, including three policemen were injured. An LPG tank which underwent a BLEVE in a residential apartment in Cebu City, Philippines, on 5 November, 2008. The explosion blew the kitchen’s roof some 10 m away. The apartment’s second floor also caved in. Three other apartment units were damaged and four persons were hurt. A 150-kg LPG cylinder at Raymond Terrace, Australia, was engulfed in an accidental bin fire on 3 January, 2009. Soon the engulfed cylinder went through a BLEVE; the resulting explosion sent the fragments of the shattered cylinder smashing through trees and onto windows and walls, damaging them. The main body of the cylinder itself was propelled 80 m away. Two women, trapped in a station wagon which met with a traffic accident near Dalby, Australia, died when an LPG tank in the back of the car suffered a BLEVE. The accident which took place on 19 March 2009, had initially led to a fire. The fire then engulfed the LPG cylinder causing it to undergo BLEVE even as rescuers tried frantically to get the trapped women out. A father and son were injured in Melbourne, Australia on 27 April 2009 when the LPG tank on the vehicle they were repairing met with a BLEVE. A 6-year-old child died while six others were injured when an LPG cylinder suffered a BLEVE in a shop in Kolkata, India, on 2 February, 2009. At least 13 persons, including two children, were wounded when an LPG cylinder exploded at a roadside canteen in Malabon city, Philippines, on 22 June, 2009. Several patrons of the restaurant were also hurt. The blast triggered a fire that destroyed the ceiling of the canteen. Two people were killed in New Delhi when the LPG cylinder in their home underwent a BLEVE on 8 July, 2009. Four people suffered burn injuries in an LPG cylinder BLEVE at Mumbai on August 25, 2009. The blast also damaged two houses nearby. In Delhi on October 25, 2009, five people were injured in an LPG cylinder BLEVE. A fire broke out following the blast. On November 17, 2009, more than three dozen pilgrims and priests were injured when an LPG cylinder blew up in a temple near Agra.

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A gas contract worker was killed and 20 people were injured, 6 critically, in an LPG BLEVE on 14 December, 2009 at a shopping complex in Malacca, Malaysia. Two motor engineers were left with facial and upper body burns when a BLEVE occurred while they were working on the installation of the LPG tank of a car in a garage in Jagodina, Serbia, on 16 December, 2009.

Definition of BLEVE The Centre for Chemical Process Safety [1] has defined BLEVE as ‘‘a sudden release of a large mass of pressurized superheated liquid to the atmosphere.’’ The sudden release can occur due to containment failure caused by fire engulfment, a missile hit, corrosion, manufacturing defects, internal overheating, etc. According to Birk and Cunningham [2] ‘‘a BLEVE is the explosive release of expanding vapor and boiling liquid when a container holding a PLG gas fails catastrophically.’’ They have further defined ‘‘catastrophic failure’’ as the sudden opening of a tank to release its contents nearly instantaneously. The sudden release from confinement of a hitherto pressurized and liquefied vapor causes instantaneous and explosive boiling–vaporization, leading to a series of cataclysmic events. A BLEVE may give rise to the following [1, 3–5]: •

• • •

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formalin and phenol, and had concluded that the container had suffered a ‘‘boiling liquid expanding vapor explosion’’ [7]. Later, Walls [8, 9] defined BLEVE as ‘‘a failure of a major container into two or more pieces, occurring at a moment in time when the contained liquid is at a temperature well above its boiling point at normal atmospheric pressure.’’ Reid [10] had defined BLEVE as ‘‘the sudden loss of containment of a liquid that is at a superheated temperature.’’ Even though this definition served as a reference point for BLEVE through several years, the Centre for Chemical Process Safety [6] has revised Wall’s definition with the modification that ‘‘failure of a major container into two or more pieces’’ has been replaced by ‘‘failure of a vessel’’ [6]. Venart et al. [11] coined the term boiling liquid collapsed bubble explosion (BLCBE) to describe one of the variants of BLEVE.

Splashing of some of the liquid to form short-lived pools; the pools would be on fire if the liquid is flammable. Blast wave. Flying fragments (missiles). Fire or toxic gas release. If the pressure-liquefied vapor is flammable, as is often the case, the BLEVE leads to a fireball. If the material undergoing a BLEVE is toxic, as in the case of ammonia or chlorine, the adverse impacts include toxic gas dispersion.

The ‘‘Discovery’’ of BLEVE Boiler explosions have been common ever since boilers came to be used, especially during the era of the industrial revolution, and it is likely that a large fraction of boiler explosions were BLEVEs. BLEVEs must also have been occurring from time to time since mankind has learned how to liquefy and store gasses under pressure. But those explosions were not recognized as a phenomenon distinct from other types of explosions till 1957. In that year, the acronym BLEVE was coined by three Factory Mutual Research Corporation workers J.B. Smith, W.S. Marsh, and W.L. Walls [6]. They had analyzed the likely mode of failure of a vessel containing an overheated mixture of

Jim Smith (left) and Bill Walls who, along with Bill Marsh had introduced the term BLEVE and had given the initial explanation of its mechanism (Photo courtesy Ms Roberta, daughter of Mr Walls)

The Mechanism of BLEVE McDevitt et al. [12], Prugh [13, 14], Leslie and Birk [15], Lees [3], Birk and Cunningham [2, 16], Casal et al. [5] and Venart [17], among others, have given elaborate descriptions of the occurrences which lead to a BLEVE. Based on their observations, and of Reid [18], Shebeko et al. [19] and

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Mechanical damage and/or

Vessel containing pressure liquefied gas (PLG)

Flame engulfment/ heat radiation and/or Fatigue, corrosion, etc.

Vessels fails, causing sudden depressurization of the contained liquid, rendering it superheated. The liquid undergoes instantaneous nucleation followed by explosive vaporization

Blast wave with shattering of the vessel

Are projectiles formed?

Sudden and violent release of vessel contents

A

No

No further escalation of consequences of BLEVE

No

Is the substance toxic?

No

Yes Yes

Is the substance flammable?

Yes

No Toxic dispersion may result in human and other casualties Any other unit in the zone of impact?

Yes Possibility of domino effect (knock-on-accidents)

Have pool fires also formed? Fireball

Yes

Yes

Is the pool fire of considerable size?

No A

Fig. 1 Precipitation of BLEVE and its consequences

Birk et al. [20], the steps involved in a typical BLEVE (Fig. 1) can be identified in following sections. A Vessel Containing Pressurized Liquid Gas (PLG) Receives Heat Load or Fails Due to a Missile hit, Fatigue, or Corrosion If a vessel containing ‘‘pressure-liquefied gas (PLG),’’ in other words a liquid confined at a temperature above its atmospheric pressure boiling point, gets accidentally heated—say from the from a nearby fire—the pressure

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inside the vessel begins to rise. When this pressure reaches the set pressure of the pressure relief valve, the valve operates. The liquid level in the vessel falls as the valve releases the liquid vapor to the atmosphere. The liquid is effective in cooling that part of the vessel wall which is in contact with it, but the vapor is not. The proportion of the vessel wall which has the benefit of liquid cooling falls as the liquid vaporizes. After a time, the portion of the metal which is not cooled by liquid also becomes exposed to the heat load, weakens, and may then rupture. This can occur even though the pressure relief valve may be operating

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correctly. A vessel may also fail even in absence of fire engulfment if it is accidentally hit by missiles originating from another vessel exploding nearby—as it happened during the serial explosions in the LPG facility at Mexico City [4, 21, 22]—or other forms of mechanical failure such as gland/seal loss, sample line breakage, fatigue, or corrosion [23]. The Vessel Fails A pressure vessel is designed to withstand the relief valve set pressure, but only at the design temperature conditions. If the metal has its temperature raised due to heat load exerted by a nearby fire, it may lose strength sufficiently to rupture. For example, the steel normally used to build LPG vessels may fail when the vessel is heated to *650 C and its pressures reaches *15 atm. The vessel may also rupture due to mechanical failure as stated in the preceding paragraph. There Is Instantaneous Depressurization and Explosion When a vessel fails, there is instantaneous depressurization. The liquid inside the vessel, which hitherto was at a temperature corresponding to a high pressure, is suddenly at atmospheric pressure but at a temperature well above the liquid’s atmospheric pressure boiling point. In other words, the liquid is superheated. However, there is a limit—different for different liquids—up to with liquids can withstand superheating. If the temperature of the liquid in the suddenly depressurized vessel is above this ‘‘superheat limit temperature’’ (SLT), there will be instantaneous and homogeneous nucleation. It would cause a sudden and violent flashing of a large portion of the liquid, resulting in a ‘‘boiling liquid expanding vapor explosion’’ (BLEVE). This would occur within 1 ms of depressurization, causing a massive release of liquid–vapor mixture. If the liquid in a suddenly depressurized vessel is below its SLT, but is in a state of ‘‘significant superheat,’’ a BLEVE would still occur because factors such as depressurization waves agitating the liquid when the vessel first develops a crack, and presence of likely heterogeneous nucleation sites, would cause the explosive boiling-cum-vaporization which triggers BLEVE [23]. Prugh [13, 14] was among the firsts who stressed that a BLEVE can occur even for initial temperatures below the superheat limit, but stated that the higher trinitrotoluene (TNT) equivalent for BLEVE occurs near or above the SLT. Birk and Cunningham [2] state that BLEVEs have been observed with propane when the PLG was at ambient temperature (20 C), well below its atmospheric SLT of 53 C. However, like Prugh [13, 14], they, too, recognize that for violent, explosive, boiling to take place, there must be the potential for superheat in the liquid

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when it is suddenly exposed to a pressure below its saturation pressure as a result of the initial tank failure. Indeed the superheat aspect becomes implicit if the material under reference is a PLG—i.e., a substance which would have been in gaseous state at atmospheric pressure but is held as liquid in a pressurized container. Numerous industrial chemicals such as liquid petroleum gas, compressed natural gas, liquefied chlorine, etc., confirm to this definition. So does superheated water in a boiler. The Vessel Is Shattered The suddenly vaporizing liquid—with several hundred-fold to over a thousand-fold increase in its volume—plus the expansion of the already existing vapor, generate a powerful overpressure blast wave. The magnitude of the blast wave is much higher than the one caused by a vapor cloud explosion occurring in an identical quantity of material. The vessel is shattered and its pieces are propelled outward. Some of the liquid may be splashed and hit the ground nearby forming short-lived pools before vaporizing. These pools may be afire if the liquid happens to be flammable. The shattering of the vessel sends big and small fragments shooting at high velocities in all directions. The missiles can, and often do, damage other vessels storing liquefied gas under pressure, causing them to undergo BLEVE as well. This ‘‘domino effect’’ [24, 25] was witnessed at its most tragic worst at the Mexico City in 1984, causing the largest number of loss of lives ever occurred in an explosion-cum fire accident in a process industry. At times a large part of the vessel itself turns into a missile and is shot over long distances. For example, at port Newark, a portion of a sphere went flying over 800 m and demolished a petrol bunk on which it landed. There Is Fireball or Toxic Dispersion If the substance involved is not combustible or toxic, such as water in boilers—the pressure wave and the missiles are the only effects of the explosion. But if the substance is flammable, as is often the case, the mixture of liquid/gas released by the explosion catches fire, giving rise to a fireball. Past accidents analysis—such as the one reported by Prugh [13, 14] covering ‘‘notable BLEVE accidents (1926–1986)’’ and the compilation made by Lees [3]— reveal that over two-thirds of all BLEVEs involve flammable chemicals. With such chemicals, a BLEVE is almost always followed by a fireball, causing massive damage due to the intense thermal radiation that ensues. The shape, the size, and the heat load exerted by the fireball are a function of numerous factors. It may so happen that the whole mass of fuel can burn only at its periphery because there is no air inside the mass

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328 Fig. 2 Frequency of causative events (based on BLEVE incidents reported during 1995– 2010)

J Fail. Anal. and Preven. (2010) 10:322–333 Mechaniical failure 2% 2 Overhheating Vap our space 6% contaamination 2%

Runaaway reactionss 12%

Fire 36% %

Overffilling 20 0% Mechaanical damagee 22%

(the mixture being outside the flammability limits). Further, not all the fuel initially contained in the tank may be involved in the fire; some of the material might escape (from a crack or other opening in the vessel) before the explosion [5]. Some of the fuel may be entrained in the wake formed by the flying fragments. In the BLEVE disasters, such as the ones that occurred at Mexico City in 1984, and at Sydney in 1990—fragments of shattering vessels carried with them portions of the flammable liquid, causing fires all around and jeopardizing other vessels. As the fireball grows, the turbulence of the fire entrains air into the fireball. Simultaneously, the thermal radiation vaporizes the liquid droplets and heats the mixture. As a result of these processes, the whole mass turbulently increases in volume, evolving toward an approximately spherical shape that rises, leaving a wake of variable diameter. Such fireballs can be very large, causing a very strong thermal radiation [26, 27]. The size, the life, and the radiation intensity of a fireball may also depend on the temperature of the liquid lading [28], and whether the loss of confinement of the flammable material had occurred when the pressure inside was still rising [29]. The BLEVE fireballs are spheroidal when fully developed; on liftoff they acquire a mushroom-like shape. Fireballs resulting from two-step BLEVEs may be approximately ellipsoidal in shape [28]. Toxic dispersion: BLEVE accidents have occurred involving ammonia [5], chlorine [30], chlorobutadiene [31], and phosgene [30] wherein the explosion did not cause a fireball but was accompanied by dispersion of toxic material. Indeed of the one-third past BLEVE events not involving flammable liquids, the majority have been associated with toxic gases—chlorine (14%), ammonia (10%), and phosgene (2%) account for 76% of the BLEVEs involving nonflammables. With such chemicals, fatalities have been caused by the toxic clouds that accompanied the blast wave and the missiles when BLEVE occurred.

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Among major BLEVEs with toxic dispersion, the maximum cases have involved chlorine, accounting for 138 fatalities during 1926–1981 [13], followed by ammonia (49 fatalities in the corresponding period). According to Lees [3], it is possible, though not common, for a BLEVE event not caused by an engulfing fire to provide the source for a large vapor. BLEVEs without any fire or toxic dispersion have also occurred—involving carbon dioxide and water [4, 21, 22, 31]. The Relative frequency of events which have triggered BLEVEs in the past are shown in Fig. 2.

BLEVEs don’t Necessarily Lead to Fire Interestingly, even today, and despite the copious R&D that has gone into the understanding of the mechanism, impact, and management of BLEVEs, to a majority of the people including safety professionals and regulatory agencies, BLEVE is something restricted to pressureliquefied flammable gases such as LNG, LPG, and other petrochemicals which are used in pressure-liquefied form. For this reason, BLEVE is also generally perceived as something which is inevitably accompanied by a fireball. This notion is so prevalent that in most fora any mention of BLEVE occurring in a chlorine or ammonia storage vessel is received with surprise and skepticism. Some compendiums even classify BLEVE not under ‘explosions’ but under ‘fires’! But the fact that BLEVE can, and does, occur in any situation where a substance is existing as a liquid at a temperature at which it ought to have been a gas was recognized with great clarity by none other than the ‘‘discoverers’’ of BLEVE—Smith, Marsh, and Walls. As documented by Walls [8, 9], the hazard of BLEVE is present in many other situations including several much more common than the chemical reactor that was the subject at hand, any liquid that was at a temperature well

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above its normal boiling point (so-called superheated) at the moment of vessel failure, was susceptible to BLEVE. Liquefied gases were obvious candidates because, with the exception of certain low pressure cryogenic or low temperature applications, their temperatures were always above the normal boiling point at atmospheric pressure [9]. The trio went onto observe that any liquid could become superheated through fire exposure, provided the vessel design permitted a build-up in pressure. Cans and drums of flammable liquids were notable examples. Indeed as we have noted elsewhere, on the basis of available records of past BLEVEs, one-fifth of all BLEVEs have involved nonflammable substances! Most remarkably Smith, Marsh, and Walls associated the BLEVE hazard with a chemical which is the most common fire extinguisher and life-support substance known to mankind: water! According to them, water heaters and the steam side of boilers are prone to BLEVE [8, 9] After all, the April 24, 1957 explosion studied by the Factory Mutual Engineering Division team, which gave rise to the term BLEVE, was caused by a superheated aqueous solution: ‘‘essentially water at a temperature well above its 212 1F (100 1C) normal boiling point’’ [8, 9]. Very few, if any, boiler explosions have been recognized as BLEVE though it is obvious that quite a few of the boiler explosions, outside the ones which occur solely due to overheated steam and have no superheated water present at the time of explosion, or the ones due to the fire box, are essentially BLEVEs. Indeed if the boiler explosions which occur when holding superheated water are included in the BLEVE tally, it may well turn out that the substance most frequently involved in BLEVEs is none other than water!

BLEVE Prevention It is well-neigh impossible to say with certainty whether a jeopardized vessel will suffer a BLEVE or not. Likewise, it is not possible to forecast with any measure of confidence when a vessel will suffer a BLEVE after getting jeopardized. Some of the risk assessment indices developed in recent years are helpful in identifying process hazards and associated risks [32–35], but they have been found wanting in forecasting the ‘‘time-to-BLEVE’’ [36–38]. Nor can explosion suppressants or inertants work in case of BLEVE as they do for dust explosions [39]. These aspects, and the uncertainty associated with forecasting the size, range, direction, and momentum of missiles likely from a BLEVE, pose special challenges toward preventing a BLEVE or in containing the damage a BLEVE may cause [40, 41]. There have been several tragic incidents when fire fighters arrive to save a fire-engulfed vessel only to be

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killed by the expanding fireball or the rocketing fragments when a vessel suddenly bursts. The strategies required to minimize the occurrence and the adverse impact of BLEVE, have been reviewed by Prugh [13], Khan and Abbasi [4], and Casal et al. [5]. Pointers can also be drawn from the studies such as effect of pressure relief value (PRV) functioning, survivability of steel cylinders in comparison to aluminum cylinders and projectile range. The strategies can be broadly classified into three categories: (A)

(B) (C)

Reducing the probability of a vessel becoming jeopardized by impact with a fragment, being exposed to a fire, containing a pronounced structural weakness, experiencing a runaway reaction, or being involved in a transportation accident. Cushioning the impact of the above so that the perturbation does not escalate to a BLEVE. Minimizing the damage if a BLEVE does occur.

We summarize below the strategies possible under each of these categories. Preventing the Causes Which Can Make a Vessel Vulnerable to BLEVE Preventing Exposure to Fire Keeping the PLG-Containing Vessel a Safe Distance Away From Likely Source of Fire Fire engulfment being the most common of the causes due to which PLG vessels undergo BLEVE, it is imperative that a reasonably large distance should separate a PLG vessel from another vessel handling a flammable material or from other sources of fire. Of course this can at best reduce the probability of a PLG vessel being heated by the radiation load from another vessel which has caught fire. The PLG vessel may still be jeopardized by blast waves or projectile hits from another exploding vessel. Sloping of the Nearby Ground To prevent a pool fire occurring after an accidental spill from a PLG vessel, the ground radially away from a fixed installation should have a downward slope of not \1% so as to lead the spill away to a safe area. Water Barriers These may be installed close to the PLG containers. These consist of sprayer system which generates curtains of fine water mist. The barriers can capture flammable vapor if released from the PLG container and disperse it without getting ignited. Water mists can also dissolve some of the released material if it happens to be ammonia, chlorine, or some other water soluble substance, thereby reducing the toxic dispersion.

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Preventing Mechanical Damage Trucks and railroad cars carrying PLGs should be protected from accidental damage with double containers, equipped with insulation in the annular spaces. Collision or overturning during transportation damages the outer shall. This makes it essential to fabricate the outer container with a material which would provide protection for the inner tankage. Preventing Overfilling and Overpressure Rigid compliance with standards during the filling and weighing of the BLEVE-prone tanks alongside very careful installation and testing of relief devices have reduced the frequency of BLEVEs on account of overfilling. However, accidents continue to occur during pumping of PLGs as happened at Moombas, South Australia, on 16 June 2001, killing one person, injuring three, and damaging the infrastructure. The relief-devices are prone to plugging; this problem is circumvented by the installation of rupture disks in series as pluggage protection under relief valves. Rupture disks are also installed ‘‘in parallel’’ to relief valves as a last resort protection. Prevention of Runaway Reaction The accident which led to the coinage of the acronym BLEVE was a runaway reaction. However, BLEVEs due to runaway reactions are much less common than BLEVEs which occur when a PLG storage vessel suffers accidental damage. Instrumentation should be provided for continuous monitoring of temperature and pressure within all process equipment likely to contain self-reactive materials. Such equipment should have facilities for counteracting overpressure or overtemperature; for example, internal cooling coils or external jackets, remote-controlled venting valves, inhibitor–injection systems, and internal deluges, as well as high-temperature and/or high-pressure alarms for control-room and field personnel. Prevention of Vapor–Space Contamination with Reactive Material Vessels containing highly reactive gases such as hydrogen and chlorine in liquefied form should be safeguarded against contamination by other substances with which they can react. Inerting vapor spaces with nitrogen or other nonreactive gas and installing explosion–suppression systems may prevent vapor–space explosions thus reducing the risk of vessel damage and, consequently, a BLEVE [13].

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Prevention of Internal Weakening of Vessel Structure Due to Fatigue, Creep, Corrosion, Etc. Proper design and pre-use testing of containers can prevent distortion and possible rupture of containers. Periodic wallthickness measurements, internal inspection for corrosion, acoustic emission testing for the possible cracking of the container, etc., should be performed to ensure the fitness of the containers. Preventive maintenance should be done along with ‘‘predictive’’ maintenance [13]. General Protection from Fire As Well As Accidental Hits, by Container Burial Vessels containing PLGs can be protected from fire, or external hits, to a very great extent if they are partially or totally buried. However, such vessels are difficult to inspect and are particularly vulnerable to corrosion. Prevention of Excessive Superheat Which May Prevent Explosive Boiling Taking a cue from distillation systems and reactors in which nucleation devices such as sharp-edged ceramic material or an aluminum mesh is placed in the liquid being distilled to assist boiling and prevent superheating, similar devices have been explored for PLG containers. However, a well-tried and tested strategy along these lines is yet to evolve. Managing a Jeopardized Vessel to Prevent It From Undergoing BLEVE Thermal Insulation The PLG containers should be thermally insulated to the maximum extent possible as it would reduce the rate of heating of the vessel when it receives heat load and delay the pressure increase inside. If the container wall is protected with a steel jacket and a ceramic insulation of adequate thickness (13 mm or more), it provides substantial thermal protection. Even steel jackets with an air gap between the jacket walls can cut the wall heating rate to approximately half of the unprotected wall. However, such fire proofing cannot by itself prevent a BLEVE; it can at best delay the catastrophic event by 4–5 h giving time for the fire fighters to remove the heat load. In fixed installations, even the vessel support system should be insulated so that it does not cave in when subjected to heat. Likewise, the valves, pipes, and other safety elements used in the PLG vessel must have the ability to resist the action of fire and withstand the high temperatures that may be reached in a crisis situation. The thermal insulation system

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should be installed in such a way that it does not interfere with the periodic inspection of the tank surface and support systems. Fireproofing can be even more effective in delaying a BLEVE if the pressure relief valve (PRV) operates correctly. Directed Water Deluge To cut off the heat load once a PLG vessel gets engulfed in fire, it has to be subjected to what is called ‘‘directed water deluge.’’ Water must be applied as soon as possible, with a layer of adequate thickness which should totally cover the vessel wall, especially those areas directly covered with flame. A water flow rate of 10 m2 min1 is recommended, which should be upped to 15 m2 min1 in areas directly being licked by a flame. If the flame is highly turbulent, which can generate a heat flux of the order of 350 kW m2, flow rates even larger than 25 m2 min1 may be required. But if the PLG vessel is being impinged by jet fire the water deluge is less effective; it cannot be relied upon to maintain a water film over the whole tank surface. The dry patches, where the water film broke down got heated to about 350 C in 10 min during the course of full-scale tests. Rapid Depressurization Another step must be taken along with the start of ‘‘directed water deluge’’ to reduce the probability of a BLEVE—depressurization of the vessel with remote operated ‘‘fireproof’’ valves bypassing the installed PRV. Such devices should be able to reduce the vessel pressure to half of the design pressure within 15 min [5]. The released material should be eliminated in a safe manner, for example with a torch. The depressurization should not be too rapid either as it may lead to extremely low temperature and fragility in the steel. For a 54 m3 tank holding 23 ton of LPG which underwent a BLEVE at Alma-Ata, Kazakhastan, in 1989, Shebeko et al. [19] theorize that the tank would not have exploded if its safety value had a cross-sectional area not \77 cm2 and operating pressure not exceeding 1.6 MPa. Attempts to develop pressure relief devices specific to the substance stored as PLG have also been made, for example liquefied ammonia. The authors have modeled thermal response of a horizontal ammonia tank severely engulfed in fire.

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which could be by several hours—such delay is a mixed blessing. If care is not taken to prevent emergency responders or bystanders from going within ‘‘striking distance’’ of the possible fireball, shock wave, or projectiles, the toll from a delayed BLEVE may even be higher than from the rapid one, as had happened at Feyzin, France [30, 31]. Damage from projectiles is of the greatest concern because their impact area is much larger then of the fireball or shock-wave. Cushioning the Missile Damage The first concern toward minimizing the damage caused by a BLEVE is to prevent the accident from triggering secondary and higher order accidents. Indeed the past accident analysis of BLEVEs tells us that very few BLEVEs occur as stand-alone accidents; in a large number of instances BLEVEs cause ‘‘domino effect,’’ triggering serial blasts [24, 25]. To prevent domino effect, other vessels, which may explode on being heated or mechanically damaged, should be kept as far away from PLG-containing vessels as possible. Barriers may also be placed around the vessels to cushion the impact of outgoing or incoming missiles. It is relatively easy to provide a barricade for vessels with energy contents in the range 103–105 J, but it becomes progressively more difficult as the energy content rises, and for energy contents capable of giving a shock wave of (50– 100)9106 J, putting a barricade would require sophisticated design and benefit–cost optimization. The preferred form of barricade is a closed cubicle. For protection against blast using the equivalent static pressure method, the High Pressure Safety Code (quoted in [3]) gives the relevant pressure as  0:72 E  106 P ¼ 7:6 ; P\70; V where E is the shock-wave energy (J), P is the equivalent static pressure (bar), and V is the volume of the enclosure (m3). This equation is applicable where the aspect ratio of the enclosure does not exceed two. Barricades can also take the form of thin-walled pressure vessels. Small enclosures can be made of angle iron and steel plate. Large barricades should be of reinforced concrete. Since the shock wave has positive and negative phases, reinforcement is required on both inner and outer faces. The barricade should have the provision to allow dispersion of small leaks by ventilation.

Minimizing the Damage if a BLEVE Does Occur

Fireball Suppression

If a vessel suffers a BLEVE within a few minutes after getting jeopardized, very little can be done to reduce the damage it would cause. But even if a BLEVE is delayed—

This is a possibility yet to be translated into practice, but its potential is obvious. If fire suppressants can be released in a way that they get mixed with the flashing material when a

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vessel suffers a BLEVE, the fireball formation can be tempered with and its intensity reduced significantly. Systems can also be put in place so that the fireball, at the moment of its formation, gets surrounded by a cloud of certain fire suppressant. Then, the suppressant would be sucked into the fireball by strong air entrainment. As a result, the flame may be completely suppressed, or at least the fireball size would be significantly reduced. The use of water mist as a fireball suppressant is an obvious possibility but the liquid droplets may completely evaporate before they are sucked into the fireball. This may reduce the suppressing effect. Aerosol fire extinguishing agents (AFEAs) may be a better substitute, because aerosol particles are not subjected to a phase change. They work by destroying the active centers which are necessary to sustain the flame. AFEAs can be generated by the combustion of solid propellants. How Close Emergency Responders Can Go to a Jeopardized Vessel It has been proposed that fire fighters should not go closer to a jeopardized vessel than four fireball radii (which can be estimated on-the-spot using the expression R = 3m0.33 where m is the lading mass in kg and R is the fireball radius in m), to a minimum of 90 m. If it is possible, the distance should be longer to reduce the hazard from rocketing fragments. Further, the emergency responders should be wearing protective clothing that can withstand radiation load of 21 kW m2 for the anticipated duration of a fireball (to be estimated as 0.15R, s). For large-scale tanks the ‘‘safe’’ distance may be too long to enable fire fighters from directing water onto fire-impinged tanks. For such large tank installations, water spray systems should already be in place and operating when responders arrive. However, delayed BLEVE remains a major risk to fire fighters dealing with uninsulated transport tanks and small stationary tanks; the responders are exposed to serious risk from fireball, blast, and projectile effects. Responder should also expect danger from potential secondary projectiles (such as attached pipes, nearby equipment, etc.) which can be sent large distances by the waves which accompany a BLEVE. Evacuation The public should be evacuated to a distance of at least 15 fireball radii, preferably 30 fireball radii away from jeopardized tanks. This distance should be increased downwind of a potential BLEVE. At this distance, there is little threat from the fireball thermal radiation or blast. As tank size increases above 5 m3, the 30R distance becomes more and more conservative and the 15R distance becomes more appropriate. If the PLG involved in a BLEVE happens

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to be toxic—such as chlorine, ammonia, methyl isocyanate, or phosgene—its initial dispersion would be influenced by the blast wave effects and would even carry it upwind to some distance before the usual meteorological factors and density effects become influential in the plume dispersion. Emergency preparedness for accidents involving such PLGs should factor in the blast-mediated dispersion. Acknowledgment S. M. Tauseef thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, for Senior Research Fellowship. Thanks are also due to Dr F. Tamanini of FM Global for putting us in touch with Mr Bill Walls, a pioneer in BLEVE studies.

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