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(B.L.E.V.E) Boiling Liquid Expansion Vapour Explosion & UVCE Explosion.

👉B.L.E.V.E. (Boiling Liquid Expanding Vapour Explosion). 
A BLEVE (Boiling Liquid Expanding Vapour Explosion) is experienced when 
containers of liquefied gases, are exposed to fires. It results from the failure of a vessel containing a liquid or a liquefied gas at a temperature significantly above its boiling point at normal atmospheric pressure. In case of container failure, a large fraction of 
the superheated liquid flashes to vapour, resulting in the sudden generation of a large amount of flammable gas.


Considering the large amount of liquefied gases which are in use both in the industry and society, the risk of BLEVEs is high, as is also evident from large number of instances of BLEVEs, both within and outside industrial premises. Amongst the most 
dangerous of such explosions are the rupture of liquefied gas containers such as large Horton spheres, bullets (both fixed and mobile), smaller cylinders, etc. If such a container is engulfed in fire, its metal is heated and loses mechanical strength. If the flames are in contact with the shell of the container, above the liquid level the vessel can rupture violently. Below the liquid level, the liquid conducts the heat away from the shell, thus helping to maintain its integrity. However, above the liquid level, the 
vapour is unable to conduct away heat from the shell, causing its temperature to rise. 
The critical temperature for most steels is around 4500C, and once this temperature is exceeded, the shell starts to lose its strength, and begins to swell outward. The metal thins at this area, and the bulging will accelerate till rupture occurs.This failure can occur in as little a time as 10 to 30 minutes of direct flame exposure, depending on the thickness of the metal, and the intensity of heating.
The rupture causes the release of superheated liquid into the open atmosphere, which immediately flashes to vapour. This sudden conversion of liquid to vapour also generates its own pressure disturbance in the atmosphere. However, if the material is material is flammable as is the case with most LPGs, another dangerous situation 
called a fireball is most likely to arise.

BLEVE Examples:


A chlorine bomb is a small-scale explosive device using the pressure of chemically evolving chlorine gas to produce an explosion. It is made with an airtight container part-filled with rubbing alcohol or similar solvent. When a chlorine tablet is added, it produces an expansive pressure increase, shattering the container.
A dry ice bomb is a simple bomb-like device. While the simplicity and ease of 
construction, high bursting pressure, and sound make this dry ice activity appealing for recreational purposes, however it can be unpredictable and dangerous.


👉UVCE (Unconfined Vapour Cloud Explosion):

• An overpressure caused when a gas cloud detonates or deflagrates in open air rather than simply burns.
• Cloud will spread from too rich, through flammable range to too lean.
• Edges start to burn through deflagration (steady state combustion).
• Cloud will disperse through natural convection.
• Flame velocity will increase with containment and turbulence.
• If velocity is high enough cloud will detonate.
• If cloud is small enough with little confinement it cannot explode.
• Increasing unsaturation will increase chance of explosion (flame speeds 
higher).
• Effect of explosion readily modelled by analogy with TNT.
• Factors favouring high over pressures.

Confinement:

•Prevents combustion products escaping, giving higher local pressures even with deflagration.
• Creates turbulence, a precursor for detonation.
• Terrain can cause confinement.
• Onsite leaks have a much higher potential for UVCE than offset leaks.
• Cloud composition.
• Highly unsaturated molecules are bad.
• High flammable range. 
• Low ignition energy. 
• High flame speeds. 
• Most UVCE C2 - C6 light gases disperse readily, heavy materials do not form 
vapour clouds easily.

 Weather. 

• Stable atmospheres lead to large clouds.
• Low wind speed encourages large clouds.
• Vapour Cloud Size impacts on
• probability of finding ignition sources
• likelihood of generating any overpressure. 
• magnitude of overpressure. 

 Source

• flashing liquids seem to give high overpressure
• vapour systems need very large failures to cause UVCE. 
• slow leaks give time for cloud to disperse naturally without finding an ignition source. 
• high pressure gives premixing required for large combustion. 
• Equipment failures where leak is not vertically upwards increases likelihood of large cloud.

Impact of Vapour Cloud

• World of explosives is dominated by TNT impact which is understood Vapour 
clouds, by analysis of incidents, seem to respond like TNT if we can determine 
the equivalent TNT.
• 1 pound of TNT has a LHV (Lower Heating Value) of 1890 BTU/lb
1 pound of hydrocarbon has a LHV of about 19000 BTU/lb
• A vapour cloud with a 10% efficiency will respond like a similar weight of 
TNT
• Guggan analysis of vapour clouds plotted efficiency against cloud size and 
several other theoretical factors and reached no effective conclusions. 
Efficiencies were between 0.1% and 50%
• Traditional EMRE view was 3% for offsite leak and 10% for onsite leak 
Pressure is function of distance from the blast and (blast size). 

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