Gas/Vapor Explosions (Diffuse Fuels)A. Combustion Explosion
Dusts (Diffuse Fuels
The most common of the chemical explosions are those that are a result of the ignition of a hydrocarbon fuel, industrial gases, smoke, and dusts. In combustion explosions elevated pressures are as a result of the ignition of the fuels in the presence of air which results in a propagating through the fuel air mixture.B. Characterization of Explosion Damage
The Company officer is more likely to be required to respond to a fuel gas explosion, than any other type of explosion.
Damage at the scene of a fuel gas explosion can often indicate the rate of reaction during the explosion. And, in the case of fuel gases indicate the relative relationship of the fuel to air relationship. The following indicators should assist the investigator in determining the rate of reaction and the fuel to air relationship. One modifier that should be considered when using this information is the effect of venting, especially if the building or vessel was designed with explosion vents as a "normal" component.
1. High Order Damage
C. Vapor Density of Fuela. CharacteristicsSmall pulverized, splintered debris
Long Missile Distances
High Rate of Pressure Rise
Negative Pressure Phase is powerfulb. Fuel - Air Relationship2. Low Order DamageOptimum Mixture
Near or just above stoichiometric
Most efficient burning
Little following fire
a. CharacteristicsLarge debris pieces
Short Missile Distances
Slower Rate of Pressure Rise
Pushing or Heaving
b. Fuel - Air RelationshipNear LEL or UEL
Near LEL, little following fire
Near UEL, greater potential for following fire
1. Relationship of vapor density to damage
A common misconception concerning fuel gas explosions is that lighter than air gases (VD = -1) will cause the vessel (often a structure) to fail near the ceiling and heavier than air gases and vapors (VD = +1) to fail closer to the floor. This concept is not true. Many of the older texts and reference materials support this concept. Consideration must be given to the location of the ignition source, flammable range, and the construction of the vessel instead of making generalizations.2. Lighter-than-air gases
Location of damage is not indicative of the vapor density, a common misconception, damage isMore a function of wall strength or,
Height of explosive range
D. Ignition SourceCollect in upper areas
Pocketing at ceilings
Migrates through openings
3. Heavier-than-air gasesCollect in lower areas
Burns at high levels when ignited
Low pocketing is unusual
1. Gas/Vapor ExplosionsE. Underground Migrationa. Minimum Ignition EnergyMost easily ignitable fuelsb. Location of the Ignition Source
Ignition Temperatures 700 - 1100 F
Ignition Energies 0.20 - 0.25 millijoulesMay not be the same location as the fuel source
Will have an effect on the strength of the reaction
The underground migration of fuels and the effect of that migration can be misunderstood. The following list provides some basic information concerning the underground migration of gases.F. Multiple ("Cascade") Explosionsa. Underground migration is common
b. Both lighter and heavier than air fuels
c. Follow exterior of underground pipes
d. May enter sewers, drain tiles, etc.
e. Odorant "Scrubbing"
f. May pass harmlessly into air unless obstructed by pavement, rain, snow, ice, etc.g. May pass through concrete wallsh. Migrations of hundreds of feet is not unknown in underground conduits. (Pipes)
i. Vegetation Survey, looking for dying or dead vegetation can be completed to determine the area where the leak is or where the gas is migrating underground.
As a result of migration and pocketing, multiple or cascade explosions often occur. Generally the subsequent explosions are called secondary explosions.G. Dust Explosions
1. Multiple pockets of gas
2. "Cascade" from room to room or floor to floor
3. Aeration of pockets over the UEL
4. Multiple explosions are very common
Finely divided solid materials (dusts and fines) when suspended in air and when provided a suitable ignition source and produce a violent reaction. Many associate this reaction with grains and similar products. However, a wide variety of products are susceptible to this reaction. There is a lower explosive limit associated with dusts. As a general "rule of thumb" the product is within the explosive range if the dust can be seen. Also, with dusts, secondary or multiple explosions are common, as the initial explosion puts additional fuel into the air.H. Backdraft (Smoke or Heat) Explosions
1. Finely divided solids as fuelsa. Suspended or2. Wide variety of materials
b. LayeredCombustible and Non-Combustible3. Particle size
a. Agricultural Products-Grain dust, Sawdust, etc.
b. Carbonaceous Materials-Coal, Charcoal
c. Chemicals-Aspirin, Ascorbic Acid, etc.
d. ) Dyes and Pigments-Paint, etc
e. Metals-Aluminum, Magnesium, etc.
f. Plastics and Resins-Synthetic Rubber, etc.
g. Explosives and Propellants, Gunpowder, explosive manufacturing by productsa. Rates of combustion related to surface area4. Concentration
b. Violence of explosion is inversely proportional to particle size
c. 840 microns (0.033") diameter or less for explosion hazarda. Profound effect upon ignitability and violence of blast pressure wave5. Turbulence
b. Minimum concentrations exist for specific materials
c. Minimum concentrations for most materials are from 0.015 to 2.0 oz./cu.ft.
d. Most common concentrations are less than 1.0 oz./cu.ft.
e. Generally no upper concentration limit
f. Reaction more controlled by surface area to air ration than by a maximum concentration
g. Rate of pressure rise and maximum pressures increase with concentration to stoichiometric and decrease in higher concentrationsa. Turbulence increases rate of combustion6. Moisture
b. Shape and size of vessel effects turbulence (i.e. pouring grain from a great height into an empty bin)a. In dust itself, affects minimum ignition energy7. Minimum Ignition Energy
b. In surrounding air, has little effecta. Ignition temperatures generally 572-1112 F8. Progression of Dust Explosions
b. Layered dusts have lower ignition temperatures than the same dusts in suspension
c. Minimum ignition energies are higher than for gases and vapors
d. 10-40 millijoulesa. Usually occur in series
b. Initial explosions usually less violent than subsequent
c. Subsequent explosions are fueled by additional dust put into suspension
Backdraft of smoke explosions can be a violent reaction. They are a result of a fire in post flashover conditions with limited oxygen supplies. Contained in the smoke are both combustion gases and carbon. Backdraft or smoke explosions often result in the introduction of a "fresh air" supply and the increase in the burning rate of the gases and solids in suspension and above their ignition temperature. This rapid increase in combustion rate causes an over-pressurization of the compartment.
1. Ventilation controlled fires
2. Fuels generated by incomplete combustiona. Smoke3. Introduction of oxygen creates rapid combustion
b. Airborne Particles
c. Carbon Monoxide
d. Flammable gases (products of combustion)
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