Amanti del rigassificatore: studiatevi come funziona una bomba a metano

[Alessandro Marescotti <a.marescotti at peacelink.it>]

Fuel/Air Explosive (FAE)

U.S. Fuel-Air
Explosives 
BLU-73 FAE I
BLU-95 500-lb (FAE-II) 
BLU-96 2,000-lb (FAE-II) 
CBU-55 FAE I
CBU-72 FAE I

Fuel-Air Explosives [FAE] disperse an aerosol cloud of fuel which is
ignited by an embedded detonator to produce an explosion. The rapidly
expanding wave front due to overpressure flattens all objects within
close proximity of the epicenter of the aerosol fuel cloud, and produces
debilitating damage well beyond the flattened area. The main destructive
force of FAE is high overpressure, useful against soft targets such as
minefields, armored vehicles, aircraft parked in the open, and bunkers.

Fuel/air explosive represent the military application of the vapor cloud
explosions and dust explosions accidents that have long bedeviled a
variety of industries. 

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]:

D = C x (nE)1/3

where D is the distance in meters to a 1 psi overpressure; C is a
constant for damages associated with 1 psi overpressures or 0.15, n is a
yield factor of the vapor cloud explosion derived from the mechanical
yield of the combustion and is assumed to be 10 percent (or 0.1) and E is
the energy content of the explosive part of the cloud in Joules. E can be
calculated from the mass of substance in kilograms times the heat of
combustion (hc) in Joules per kilogram as follows:

E = mass x hc

Combining these two equations gives:

D = 0.15 x (0.1 x mass x hc)1/3

Vapor cloud explosion modeling historically has been subject to large
uncertainties resulting from inadequate understanding of deflagrative
effects. According to current single-degree of freedom models, blast
damage/injury can be represented by Pressure-Impulse (P-I) diagrams,
which include the effects of overpressure, dynamic pressure, impulse, and
pulse duration. The peak overpressure and duration are used to calculate
the impulse from shock waves. Even some advanced explosion models ignore
the effects of blast wave reflection off structures, which can produce
misleading results over- or under-estimating the vulnerability of a
structure. Sophisticated software used to produce three-dimensional
models of the effects of vapor cloud explosions allows the evaluation of
damage experienced by each structure within a facility as a result of a
primary explosion and any accompanying secondary explosions produced by
vapor clouds.

Fonte: 
http://www.fas.org/man/dod-101/sys/dumb/fae.htm