Boiling liquid expanding vapor explosion (BLEVE)

This entry was compiled, edited and written by: Cutler Cleveland

 MWHA boiling liquid expanding vapor explosion (BLEVE). Credit: MWH

BLEVE, pronounced "blevvy", is an acronym for "boiling liquid expanding vapor explosion". This is a type of explosion that can occur when a vessel containing a pressurized liquid is ruptured. Such explosions can be extremely hazardous.A BLEVE can occur in a vessel that stores a substance that is usually a gas at atmospheric pressure but is a liquid when pressurized (for example, liquefied petroleum gas (LPG)). The substance is stored partly in liquid form, with a gaseous vapor above the liquid filling the remainder of the container.

The steps involved in a typical BLEVE are:1

1. 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 heat radiation emanating from a nearby fire – the pressure 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 vapour to the atmosphere. The liquid is effective in cooling that part of the vessel wall which is in contact with it, but the vapour 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 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 – or other forms of mechanical failure such as gland/seal loss, sample line breakage, fatigue, or corrosion.

2. 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 not, vert, similar650 °C and its pressures reaches not, vert, similar15 atm. The vessel may also rupture due to mechanical failure as stated in the preceding paragraph.

3. 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. But 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 vapour explosion’ (BLEVE). This would occur within 1 ms of depressurization, causing a massive release of liquid–vapour 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.

4. 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 vapour, generate a powerful overpressure blast wave. The magnitude of the blast wave is much higher than the one caused by a vapour cloud explosion occurring in an identical quantity of material. The vessel is shattered and its pieces are propelled outwards. 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’ and 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.

5. 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 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 (the mixture being outside the flammability limits).

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 towards an approximately spherical shape that rises, leaving a wake of variable diameter. Such fireballs can be very large, causing a very strong thermal radiation.

1 This section is drawn from Abbasi and Abbasi (2006).

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