VAPOUR EXPLOSIONS

When the liquid is disturbed by, for example, the transmission of building vibration, or by accidentally knocking the liquid container, the evaporation rate rises rapidly to a high value, and then drops back again just as quickly. The surface evaporation impedance mechanism has broken down quite suddenly and then rapidly repairs itself (see Fig. 4.9). 

This evaporation spike can also be reproduced repeatedly by tapping the Uquid container at fairly lengthy intervals. Frequently, the flow meter goes off-scale during the spike, but rapidly came back on scale towards the previous reading. Rocking the container so as to cause the liquid to slosh, will also create a boil-off spike. 

These boil-off events are called vapour explosions and demonstrate how sensitive the evaporation impedance of the surface is to disturbance. 

We have found that a demonstration dewar, with a narrow vapour exit into the atmosphere and containing some 5 L of LIN, exhibits puffs of vapour, equivalent to mini-vapour explosions, aU the time. We have frequently demonstrated this phenomenon in lectures, and it works most obligingly to confirm the irregular surface evaporation of cryogenic liquids. 

It is possible for agitation of the surface sub-layer to be maintained continuously over a longer period of time, several seconds, or minutes, or hours in length, so as to prevent the self-repairing mechanism re-establishing the equilibrium sub-layer structure. 

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BLEVE, BOILING LIQUID EXPANDING VAPOUR EXPLOSION Instantaneous release and ignition of flammable vapour upon rupture of a vessel eontaining flammable liquid above its atmospherie boiling point. 

Explosion a confined vapour cloud explosion (CVCE) can result from ignition of vapour within a building or equipment a boiling liquid expanding vapour explosion (BLEVE) can result when unvented containers of flammable chemicals burst with explosive violence as a result of the build-up of internal pressure unconfmed vapour cloud explosion (UVCE) can result from ignition of a very large vapour or gas/air cloud. 

Boiling liquid expanding vapour explosion follows failure of a pressurized eontainer of flaimnable liquid, e.g. LPG, or a sealed vessel eontaining volatile flammable liquids, under fire eonditions. Ignition results in a fireball and missiles. 

This type of explosion results from the release of a considerable quantity of flammable gas, or vapour, into the atmosphere, and its subsequent ignition. Such an explosion can cause extensive damage, such as occurred at Flixborough, HMSO (1975). Unconfined vapour explosions are discussed by Munday (1976) and Gugan (1979). 

Boiling liquid expanding vapour explosions (BLEVE)... 

Boiling liquid expanding vapour explosions occur when there is a sudden release of vapour, containing liquid droplets, due to the failure of a storage vessel exposed to fire. A serious incident involving the failure of a LPG (Liquified Petroleum Gas) storage sphere occurred at Feyzin, France, in 1966, when the tank was heated by an external fire fuelled by a leak from the tank see Lees (1996) and Marshall (1987). 

Note, the design and location of control rooms, particularly as regards protection against an unconfined vapour explosion, is covered in a publication of the Chemical Industries Association, CIA (1979a). 

Munday, G. (1976) Chem. Engr. London No. 308 (April) 278. Unconfined vapour explosions. 

If the mixture (or a dust cloud) is confined, even if only by surface irregularities or local partial obstructions, significant pressure effects can occur. Fuel-air mixtures near to stoicheiometric composition and closely confined will develop pressures of several bar within milliseconds, and material damage will be severe. Unconfined vapour explosions of large dimensions may involve higher flame velocities and significant pressure effects, as shown in the Flixborough disaster. 

The lime-alcohol residue from preparation of anhydrous alcohol ignited on discharge from the still and caused a vapour explosion. The finely divided and reactive lime may have heated on exposure to atmospheric moisture and caused ignition. See Water, below... 

The vapour explosion which occurs when liquid copper is dumped into water has... 

An extremely reactive liquid, solid or vapour with a dangerously high thermal capacity in both liquid and vapour state. When heated, the commonest cause of mid-19th century industrial explosions it also bursts containers on cooling to low ambient temperatures. Still a frequent cause of vapour explosions today. 

A Boiling Liquid Expanding Vapour Explosion, or BLEVE, is an industrial event related to the laboratory bump occasioned when the inadequately mixed bottom of a vessel of liquid becomes superheated, then explosively boils. In the industrial version, rupture of a pressurised container is usually involved. Although strictly speaking a non-reactive physical hazard, chemical fires and explosions, with fatalities, often follow. Means of estimating risk and prevention, with a fist of incidents are given[l], A more ferocious version, the Boiling Liquid Compressed Bubble... 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

See MOLTEN METAL EXPLOSIONS, SMELT, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

Use of carbon tetrachloride to extinguish a wax fire caused an explosion. This was attributed to a violent reaction between unsaturated wax components and carbon tetrachloride initiated by radicals from decomposing peroxides [1], Perhaps a more likely explanation could be that contact of the cold low-boiling liquid with the hot molten wax led to the physical effect of a vapour explosion, rather than the chemical explosion postulated [2],... 

Briscoe, F. (1978). LNG Water Vapour Explosions—Estimates of Yields and Pressures, Sess. 2A. LNG/LPG Gas Conf., 6th, 1978, Monte Carlo, Monaco. 

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