Cloud Explosions

The most dangerous and destructive explosions in the chemical process industries are vapor cloud explosions (VCEs). These explosions occur in a sequence of steps  

sudden release of a large quantity of flammable vapor (typically this occurs when a vessel, containing a superheated and pressurized liquid, ruptures), 

dispersion of the vapor throughout the plant site while mixing with air, 

The accident at Flixborough, England, is a classic example of a VCE. A sudden failure of a 20-inch cyclohexane line between reactors led to vaporization of an estimated 30 tons of cyclohexane. The vapor cloud dispersed throughout the plant site and was ignited by an unknown source 45 seconds after the release. The entire plant site was leveled and 28 people were killed. 

A summary of 29 VCEs24 over the period 1974-1986 shows property losses for each event of between 5,000,000 and 100,000,000 and 140 fatalities (an average of almost 13 per year). 

Detonations can occur in solids and liquids but are particularly frequent in petroleum facilities in mixtures of hydrocarbon vapors with air or oxygen. Detonations will develop more rapidly at initial pressures above ambient atmospheric pressure. If the initial pressure is high the detonation pressure will be more severe and destructive. 

Detonations produce much higher pressures than what be considered ordinary explosions. In most cases a process vessel or piping systems will be unable to contain detonation pressure. The only safe procedure is to avoid process system detonations is to preventing the formation of flammable vapor and air mixtures within vessels and piping systems. While the flame speed of explosions is at relatively slow speed, detonations travel at supersonic speeds and will be more destructive. 

In order for a vapor cloud explosion to occur in a hydrocarbon facility, four conditions have to be achieved  

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

Two types of open air explosions are possible, representing two different mechanisms for pressure buildup. 

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Vapor cloud explosions. Explosions which occur in the open air are vapor cloud explosions. A vapor cloud explosion is one of the most serious hazards in the process industries. Although a large toxic release may have a greater disaster potential, vapor cloud explosions tend to occur more frequently. Most vapor cloud explosions have been the result of leaks of flashing flammable liquids. 

Evaluating the Characteristics of Vapor Cloud Explosions, Elash Eires, and BLEVEs Technical Management of Chemical Process Safety (Corporate)... 

K. Gugan, Unconfined Vapor Cloud Explosions, Gulf Publishing, Houston, Tex., 1979. 

Dutch State Mines (Stamicarbon). Vapor-phase, catalytic hydrogenation of phenol to cyclohexanone over palladium on alumina, Hcensed by Stamicarbon, the engineering subsidiary of DSM, gives a 95% yield at high conversion plus an additional 3% by dehydrogenation of coproduct cyclohexanol over a copper catalyst. Cyclohexane oxidation, an alternative route to cyclohexanone, is used in the United States and in Asia by DSM. A cyclohexane vapor-cloud explosion occurred in 1975 at a co-owned DSM plant in Flixborough, UK (12) the plant was rebuilt but later closed. In addition to the conventional Raschig process for hydroxylamine, DSM has developed a hydroxylamine phosphate—oxime (HPO) process for cyclohexanone oxime no by-product ammonium sulfate is produced. Catalytic ammonia oxidation is followed by absorption of NO in a buffered aqueous phosphoric acid... 

Frank T. Bodurtha/ Sc D / E. I. du Pont de Nemours and Co., Inc., (retired) Consultant, Frank T. Bodui tha, Inc. (Gas Explosions Unconfined Vapor Cloud Explosions [UVCE.s] and Boiling Liquid Expanding Vapor Explosions [BLEVE.s])... 

Uucoufiued Vapor Cloud Explosions (UVCEs) and Boiling Liquid... 

Vapor cloud explosions can result if clouds of flammable vapor in air are formed. It is important to understand how hquids and gases flow through holes in equipment and how resulting vapor or gas clouds are dispersed in air. 

Storage Facilities The Fhxborough disaster (Lees, 1980) occurred on June I, 1974, and involved a large, unconfined vapor cloud explosion (or explosions—there may have been two) and Fire that killed 28 people and injured 36 at the plant and many more in the surrounding area. The entire chemical plant was demolished and 1821 houses and 167 shops were damaged. 

UNCONFINED VAPOR CLOUD EXPLOSIONS (UVCEs) AND BOILING LIQUID EXPANDING VAPOR EXPLOSIONS (BLEVEs)... 

Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

Investigate the potential for unconfmed vapor cloud explosions resulting from accidents at the flammable storage tank area. 

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. 

Unconfined Vapor Cloud Explosion (UCVE) Occurs when a sufficient amount of flammable material (gas or liquid having high vapor... 

In particular, great care must be take when evaluating tradeoffs for a containment building for a flammable and toxic material such as hydrogen cyanide. A leak or fire inside the building could cause a confined vapor cloud explosion which destroys the building. The total risk may actually increase. 

Vapor cloud explosions can cause damaging overpressures (CCPS, 1994b). 

The total cost of the 100 losses is 7.52 billion in 1997 dollars, The largest loss was 252,500,000, from a vapor cloud explosion at a gas processing plant at Cactus, Mexico. The average loss was 75,800,000. 

Vapor Cloud Explosions Explosions Fires Olher... 

On June 1, 1974, a vapor cloud explosion destroyed the 70,CKX) tons per year, Flixborough Nyprocyclohexane oxidation plant killing 28 people. Other plants on the site were seriously damaged or destroyed and the site was destroyed. 

Effects of vapor cloud explosions on people, houses, engineering structures, etc. 

Gas dispersion models provided the toxic effects of chemical releases, fire, or unconfined vapor cloud explosion. 

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