|U.S. Fuel-Air Explosives|
Accidental vapor cloud explosion hazards are of great concern to the refining and chemical processing industry, and a number of catastrophic explosion accidents have had significant consequences in terms of injury, property damage, business interruption, loss of goodwill, and environmental impact.
And every year, many serious explosions and fires occur in industrial plants as a result of dust. Many materials form dust clouds that can easily ignite and explode, injuring personnel and damaging plant. This is a well-known phenomenon in the coal mining, grain storage, and the woodworking and paper industries. Many miners have been killed and injured and massive production losses have resulted from coal dust explosions in underground coal mining operations. Of the 129 grain dust explosions that occurred nationwide between 1987 and 1997, about half involved corn. Eleven were caused by wheat dust and 10 by dust from soybeans. Billions of tiny, highly combustible particles of grain are generated by grain kernels rubbing together as they move along conveyer belts and shifted between bins. Inside the enclosed chambers, those particles rise in a cloud. When the dust gets in with the right mixture of oxygen and comes in contact with a spark or even an overheated bearing on a conveyer belt, it is extremely explosive.Almost all organic material in the form of a dust cloud will ignite at temperatures below 500 oC - approximately the same temperature as a newly extinguished match. Cotton, plastics and foodstuffs such as sugar, flour and cocoa can also, under the right conditions, act as explosives. In order for a dust explosion to take place, the dust particles must be of a certain size and the amount of finely granulated material per unit of volume must lie within certain critical values. There is generally a direct correlation between particle size and explosive hazard. The smaller the particle, the more reactive the dust. As the materials become smaller, they disperse and remain suspended more easily, increasing the potential for ignition and propagation of the reaction. Industrial explosion prevention measures include, where possible, providing nitrogen gas purging to ensure that the oxygen concentration is kept below that required for combustion. For vapor cloud explosion there is a minimum ratio of fuel vapor to air below which ignition will not occur. Alternately, there is also a maximum ratio of fuel vapor to air, at which ignition will not occur. These limits are termed the lower and upper explosive limits. For gasoline vapor, the explosive range is from 1.3 to 6.0% vapor to air, and for methane this range is 5 to 15%. Many parameters contribute to the potential damage from a vapor cloud explosion, including the mass and type of material released, the strength of ignition source, the nature of the release event (e.g., turbulent jet release), and turbulence induced in the cloud (e.g., from ambient obstructions). TNT generates well over 4,000 psi overpressure in close proximity to the source of the explosion, along with significant radiant heat effects from the explosion's fireball. Conventional high explosive munitions also produce fragments from the munition case, as well as fragments from material in the target area that is broken loose by the high blast overpressures.
There are dramatic differences between explosions involving vapor clouds and high explosives at close distances. For the same amount of energy, the high explosive blast overpressure is much higher and the blast impulse is much lower than that from a vapor cloud explosion. The shock wave from a TNT explosion is of relatively short duration, while the blast wave produced by an explosion of hydrocarbon material displays a relatively long duration. The duration of the positive phase of a shock wave is an important parameter in the response of structures to a blast.Although the detonation combustion mode produces the most severe damage, fast deflagrations of the cloud can result from flame acceleration under confined and congested conditions. Flame propagation speed has a significant influence on the blast parameters both inside and outside the source volume.
The blast effects from vapor cloud explosions are determined not only by the amount of fuel, but more importantly by the combustion mode of the cloud. Significant overpressures can be generated by both detonations and deflagrations. Most vapor cloud explosions are deflagrations, not detonations. Flame speed of a deflagration is subsonic, with flame speed increasing in restricted areas and decreasing in open areas. Significantly, a detonation is supersonic, and will proceed through almost all of the available flammable vapor at the detonation reaction rate. This creates far more severe peak over-pressures and much higher amounts of blast energy. The speed of the flame front movement is directly proportional to the amount of blast over-pressure. A wide spectrum of flame speeds may result from flame acceleration under various conditions. High flame front speeds and resulting high blast over pressures are seen in accidental vapor cloud explosions where there is a significant amount of confinement and congestion that limits flame front expansion and increases flame turbulence. These conditions are evidently more difficult to achieve in the unconfined environment in which military fuel-air explosives are intended to operate.Based on the known properties of flammable substances and explosives, it is possible to use conservative assumptions and calculate the maximum distance at which an overpressure or heat effect of concern can be detected. Distances for potential impacts could be derived using the following calculation method [described in Flammable Gases and Liquids and Their Hazards]: