Part III:  Fuel Gas and Dust Explosions

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Gas/Vapor Explosions (Diffuse Fuels)

Dusts (Diffuse Fuels

A. Combustion Explosion

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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.

The Company officer is more likely to be required to respond to a fuel gas explosion, than any other type of explosion.

B. Characterization of Explosion Damage
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

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a. Characteristics
Small pulverized, splintered debris

Long Missile Distances

High Rate of Pressure Rise

Negative Pressure Phase is powerful

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b. Fuel - Air Relationship
Optimum Mixture

Near or just above stoichiometric

Most efficient burning

Little following fire

2. Low Order Damage

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a. Characteristics
Large debris pieces

Short Missile Distances

Slower Rate of Pressure Rise

Pushing or Heaving

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b. Fuel - Air Relationship

Near LEL or UEL

Inefficient burning

Near LEL, little following fire

Near UEL, greater potential for following fire

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C. Vapor Density of Fuel
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.

Location of damage is not indicative of the vapor density, a common misconception, damage is

More a function of wall strength or,

Height of explosive range

2. Lighter-than-air gases
Collect in upper areas

Pocketing at ceilings

Migrates through openings

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3. Heavier-than-air gases

Collect in lower areas

Burns at high levels when ignited

Low pocketing is unusual

D. Ignition Source
1. Gas/Vapor Explosions
a. Minimum Ignition Energy
Most easily ignitable fuels

Ignition Temperatures 700 - 1100 F

Ignition Energies 0.20 - 0.25 millijoules

b. Location of the Ignition Source
May not be the same location as the fuel source

Will have an effect on the strength of the reaction

E. Underground Migration
Fuel Gases
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.
a. 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.

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g. May pass through concrete walls
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h. 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.

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F. Multiple ("Cascade") Explosions
As a result of migration and pocketing, multiple or cascade explosions often occur. Generally the subsequent explosions are called secondary 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

G. Dust Explosions

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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.

1. Finely divided solids as fuels

a. Suspended or

b. Layered

2. Wide variety of materials
Combustible and Non-Combustible

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 products

3. Particle size
a. Rates of combustion related to surface area

b. Violence of explosion is inversely proportional to particle size

c. 840 microns (0.033") diameter or less for explosion hazard

4. Concentration
a. Profound effect upon ignitability and violence of blast pressure wave

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 concentrations

5. Turbulence
a. Turbulence increases rate of combustion

b. Shape and size of vessel effects turbulence (i.e. pouring grain from a great height into an empty bin)

6. Moisture
a. In dust itself, affects minimum ignition energy

b. In surrounding air, has little effect

7. Minimum Ignition Energy
a. Ignition temperatures generally 572-1112 F

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 millijoules

8. Progression of Dust Explosions
a. Usually occur in series

b. Initial explosions usually less violent than subsequent

c. Subsequent explosions are fueled by additional dust put into suspension

H. Backdraft (Smoke or Heat) Explosions

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 combustion

a. Smoke

b. Airborne Particles

c. Carbon Monoxide

d. Flammable gases (products of combustion)

3. Introduction of oxygen creates rapid combustion

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