6.0 Fires and Explosions
Chemicals present a very substantial hazard due to their potential to generate fires and
explosions. The combustion of one gallon of toluene can destroy an ordinary
chemistry laboratory in minutes; persons present may be killed. The potential
consequences of fires and explosions in pilot plants and process plant environments
are even greater.
The three most common chemical plant accidents are fires, explosions and toxic
releases, in that order. Organic solvents are the most common source of fires and
explosions in the chemical industry.
Yearly losses due to fires and explosions are substantial. Property losses for
explosions in the United States are estimated at over 200million. Additional losses
due to business interruptions are estimated to exceed 250million annually. To prevent
accidents due to fires and explosions, engineers must be familiar with:
• the fire and explosion properties of materials,
• the nature of the fire and explosion process, and
• procedures to reduce fire and explosion hazards.
6-1 THE FIRE TRIANGLE
The essential elements for combustion are fuel, oxidizer, and an ignition source.
These elements are illustrated by the fire triangle shown in Figure 6-1.
Fire, or burning, is the rapid, exothermic oxidation of an ignited fuel. The fuel can
be in solid, liquid or vapor form, but vapor and liquid fuels are generally easier to
ignite. The combustion always occurs in the vapor phase; liquids are volatized and
solids are decomposed into vapor prior to combustion.
When fuel, oxidizer, and an ignition source are present at the necessary levels,
burning will occur. The fire triangle tells us a fire will not occur if (1) fuel is not
present or is not present in sufficient quantities, (2) an oxidizer is not present or is not
present in sufficient quantities, and (3) the ignition source is not energetic enough to
initiate the fire.
Two common examples of the three components of the fire triangle are wood, air,
and a match; or gasoline, air, and a spark. However, other, less obvious combinations
of chemicals can lead to fires and explosions. Various fuels, oxidizers and ignition
sources common in the chemical industry are:
FUELS
Liquids
Solids
Gases
Gasoline, acetone, ether, pentane.
Plastics, wood dust, fibers, metal particles.
Acetylene, propane, carbon monoxide, hydrogen.
OXIDIZERS
Gases
Liquids
Solids
Oxygen, fluorine, chlorine.
Hydrogen peroxide, nitric acid, perchloric acid.
Metal peroxides, ammonium nitrate.
IGNITION SOURCES
Sparks, flames, static electricity, heat
6-2 DISTINCTION BETWEEN FIRES AND EXPLOSIONS
The major distinction between fires and explosions is the rate of energy release.
Fires release energy slowly, while explosions release energy very rapidly, typically on
the order of microseconds. Fires can also result from explosions and explosions can
result from fires.
6-3 DISTINCTION BETWEEN FIRES AND EXPLOSIONS
Some common definitions related to fires and explosions are given.
Combustion or Fire: Combustion or fire is a chemical reaction in which a substance
combines with an oxidant and releases energy. Part of the energy released is used to
sustain the reaction.
Autoignition temperature (AfT): A fixed temperature above which a flammable
mixture is capable of extracting enough energy from the environment to self-ignite.
Flash Point (FP): The flash point of a liquid is the lowest temperature at which it
gives off enough vapor to form an ignitable mixture with air. At the flash point, the
vapor will burn, but only briefly; inadequate vapor is produced to maintain
combustion. The flash point generally increases with increasing pressure.
There are several different experimental methods used to determine flash points.
Each method produces a somewhat different value. The two most commonly used
methods are open cup and closed cup, depending on the physical configuration of the
experimental equipment. The open cup flash point is a few degrees higher then the
closed cup.
Flammability Limits (LFL and UFL): Vapor-air mixtures will only ignite and burn
over a well-specified range of compositions. The mixture will not burn when the
composition is lower than the lower flammable limit (LFL); the mixture is too lean
rich; that is, when it is above the upper flammable limit (UFL). A mixture is
flammable only when the composition is between the LFL and the UFL. Commonly
used units are volume percent fuel (percent of fuel plus air).
Lower explosive limit (LEL) and upper explosive limit (UEL) are used
interchangeably with LFL and UFL.
Explosion: An explosion is a rapid expansion of gases resulting in a rapidly moving
pressure or shock wave. The expansion can be mechanical (via the sudden rupture of a
pressurized vessel) or it can be the result of a rapid chemical reaction. Explosion
damage is caused by the pressure or shock wave.
Deflagration: An explosion with a resulting shock wave moving at a speed less then'
the speed of sound in the unreacted medium.
Detonation: An explosion with a resulting shock wave moving at a speed greater than
the speed of sound in the unreacted medium.
Confined explosion: An explosion occurring within a vessel or a building. These are
most common and usually result in injury to the building inhabitants and extensive
damage.
Unconfined explosion: Unconfined explosions occur in the open. This type of
explosion is usually the result of a flammable gas spill. The gas is dispersed and
mixed with air until it comes in contact with an ignition source.
Boiling liquid expanding vapor explosion (BLEVE): A BLEVE occurs if a vessel
ruptures which contains a liquid at a temperature above its atmospheric-pressure
boiling point. This type of explosion occurs when an external fire heats the contents of
a tank of volatile material. As the tank contents heat, the vapor pressure of the liquid
within the tank increases and the tank's structural integrity is reduced due to the
heating. If the tank ruptures the hot liquid volatilizes explosively.
Dust explosion: This explosion results from the rapid combustion of fine solid
particles. Many solid materials (including common metals such as iron and aluminum)
become very flammable when reduced to a fine powder.
6-4 FLAMMABILITY CHARACTERISTICS OF LIQUIDS AND VAPORS
Flammability characteristics of some important organic chemicals (liquids and gases)
are shown in Table 6.1.
Liquids
The flash point (FP) is one of the major physical properties used to determine the fire
and explosion hazards of liquids. Flash points for pure components are easily
determined experimentally. Table 6-1 lists flash points for a number of substances.
Flash points can be estimated for multicomponent mixtures if only one component is
flammable and if the flash point of the flammable is known. In this case the flash
point temperature is estimated by determining the temperature at which the vapor
pressure of the flammable in the mixture is equal to the pure component vapor
pressure at its flash point. Experimentally determined flash points are recommended
for multicomponent mixtures with more than one flammable component.
Example 6-1
Methanol has a flash point of 11℃ and its vapor pressure at this temperature is 53 mm
Hg. What is the flash point of a solution containing 75% methanol and 25% water by
weight?
Solution
The mole fractions of each component are needed to apply Raoult's Law.
Assuming a basis of 100 pounds of solution
Raoult's law is used to compute the vapor pressure (psat) of pure methanol, based on the
partial pressure required to flash.
p = xpsat
psat= p / x = 53/0.63 = 84.1 mm Hg
Using a graph of the vapor pressure versus temperature, shown on Figure 6-2, the flash
point of the solution is 17°C
Vapors
Flammable limits for vapors are determined experimentally. Vapor-air mixtures of
known concentration are added to a closed vessel and then ignited. The maximum
explosion pressure is measured.
This test is repeated with different concentrations to establish the range of
flammability for the specific gas. Figure 6-3 shows the results of one such
experimental run; this particular substance has an LFL of 2.2 per cent and a UFL of
7.8 per cent.
Vapor Mixtures
Frequently LFLs and UFLs for mixtures are needed. These mixture limits are
computed using the Le Chatelier equation'
Where
LFL; is the lower flammable limit for component i in volume % of component
i in fuel and air,
yi is the mole fraction of component i on a combustible basis, and
n is the number of combustible species.
Similarly,
Where
UFL; is the upper flammable limit for component i in volume % of component i in
fuel and air.
The Le Chatelier's equation is an empirically derived equation which is not
universally applicable. The limitations are covered in the literature.
Example 6-2
What is the LFL and UFL of a gas mixture composed of 0.8% hexane, 2.0% methane, and
0.5% ethylene by volume?
Solution
The mole fractions on a fuel only basis are calculated below. The LFL and
UFL data are obtained from Table 6-1.
Equation 6-1 is used to determine the LFL of the mixture.
Equation 6-2 is used to determine the UFL of the mixtures,
Since the above mixture contains 3.3% total combustibles, it is flammable.
6-5 MINIMUM OXYGEN CONCENTRATION (MOC) AND INERTING
The LFL is based on fuel in air. However, oxygen is the key ingredient and there is a
minimum oxygen concentration required to propagate a flame. This is an especially
useful result, because explosions and fires are preventable by reducing the oxygen
concentration regardless of the concentration of the fuel. This concept is the basis for
a common procedure called inerting
Below the MOC, the reaction cannot generate enough energy to heat the entire
mixture of gases (including the inerts) to the extent required for the self propagation
of the flame.
The MOC has units of per cent oxygen in air plus fuel. If experimental data are
not available, the MOC is estimated using the stoichiometry of the combustion
reaction and the LFL. This procedure works for many hydrocarbons.
Example 6-3
Estimate the MOC for butane (C4HIO).
Solution
The stoichiometry for this reaction is
C4H10 + 6.5O2  4CO2 + 5H2O
The LFL for butane (from Table 6-1) is 1.6% by volume. From the stoichiometry,
By substitution
The combustion of butane is preventable by adding nitrogen, carbon dioxide or even water
vapor until the oxygen concentration is below 10.4 %. The addition of water, however, is not
recommended because any condition which condenses water would move the oxygen
concentration back into the flammable region.
6-6 IGNITION ENERGY
The minimum ignition energy (MIE) is the minimum energy input required to initiate
combustion. All flammables (including dusts) have minimum ignition energies.
The MIE depends on the specific chemical or mixture, the concentration, pressure,
and temperature. A few MIEs are given in Table 6-2.
Experimental data indicates that
• The MIE decreases with an increase in pressure,
• The MIE of dusts are, in general, at energy levels comparable to combustible
gases, and
• An increase in the nitrogen concentration increases the MIEs.
Many hydrocarbons have MIEs of about 0.25 mJ. This is low when compared to
sources of ignition. For example, a static discharge of 22 mJ is initiated by walking
across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic
discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of
flammables and can provide an ignition source, contributing to plant explosions
6-7 AUTOOXIDATION
Autooxidation is the process of slow oxidation with accompanying evolution of heat,
sometimes leading to autoignition if the energy is not removed from the system.
Liquids with relatively low volatility are particularly susceptible to this oroblem.
Liquids with high volatility are less susceptible to autoignition because they self cool
as a result of evaporation.
Many fires are initiated as a result of autooxidation, referred to as spontaneous
combustion. Some examples of autooxidation with a potential for spontaneous
combustion include:
• Oils on a rag in a warm storage area,
• Insulation on a steam pipe saturated with certain polymers, and
• Filter aid saturated with certain polymers. Cases have been recorded where ten
year old filter aid residues were ignited when the land-filled material was
bulldozed, allowing autooxidation and eventual autoignition.
These examples illustrate why special precautions must be taken to prevent fires
due to autooxidation and autoignition.
6-8 ADIABATIC COMPRESSION
An additional means of ignition is adiabatic compression. For example, gasoline and
air in an automobile cylinder will ignite if the vapors are compressed to an adiabatic
temperature which exceeds the autoignition temperature. This is the cause of
preignition knock in engines which are running too hot and too lean. It is also the
reason why some over-heated engines continue to run after the ignition is turned off.
Several large accidents were caused by flammable vapors being sucked into the
intake of air compressors; subsequent compression resulted in autoignition. A
compressor is particularly susceptible to autoignition if it has a fouled after-cooler.
Safeguards must be included in the process design to prevent undesirable fires due to
adiabatic compression.
6-9 IGNITION SOURCES
As illustrated by the fire triangle, fires and explosions are preventable by eliminating
ignition sources. Various ignition sources were tabulated for over 25,000 fires by the
Factory Mutual Engineering Corporation and are summarized in Table 6-3. The
sources of ignition are numerous; consequently it is impossible to identify and
eliminate them all. The main reason for rendering a flammable liquid inert, for
example, is to prevent a fire or explosion by ignition from an unidentified source.
Although all sources of ignition are not likely to be identified, engineers must still
continue to identify and eliminate them.
Some very special situations might occur in a process facility where it is
impossible to avoid flammable mixtures. In these cases a very thorough safety
analysis is required to eliminate all possible ignition sources in each of the units
where flammable gases are present.
6-10 SPRAYS AND MISTS
Static electricity is generated when mists of sprays pass through orifices. A charge
may accumulate and discharge in a spark. If flammable vapors are present, a fire or
explosion will occur.
Mists and sprays also affect flammability limits. For suspensions with drop
diameters less than 0.01 mm, 'the lower flammability limit is virtually the same as the
substance in vapor form. This is true even at low temperatures where the liquid is
nonvolatile and no vapor is present. Mists of this type are formed by condensation.
6-11 EXPLOSIONS
Explosion behavior depends on a large number of parameters. A summary of the more
important parameters is shown in Table 6-4.
Detonation and Deflagration
Explosions are either detonations or deflagrations; the difference depends on the
speed of the shock wave emanating from the explosion.
Suppose a combustible mixture is placed within a long pipe. A small spark, flame,
or other ignition source initiates the reaction at one end of the pipe. After ignition, a
flame or reaction front moves down the pipe.
In front of the flame front is a pressure or shock wave. If the pressure wave moves
faster than the speed of sound in the unreacted medium the explosion is a detonation;
if it moves at a speed less than the speed of sound it is a deflagration.
Confined Explosions
A confined explosion occurs in a confined space, such as, a' vessel or a building. The
two most common confined explosion scenarios involve explosive vapors and
explosive dusts. Empirical studies have shown that the nature of the explosion is a
function of several experimentally determined characterisitics. These characteristics
are dependent on the explosive material used and include flammability or explosive
limits, the rate of pressure rise after the flammable mixture is ignited, and the
maximum pressure after ignition.
Explosion characteristics. The explosion characteristics determined using the vapor
and dust explosion apparatus are used in the following way.
1. The limits of flammability or explosivity are used to determine the safe
concentrations for operation or the quantity of inert required to control the
concentration within safe' regions.
2. The maximum rate of pressure rise is indicative of the robustness of an
explosion. Thus, the explosive behavior of different materials can be
compared on a relative basis. It is also used to design a vent for relieving a
vessel during an explosion before the pressure ruptures the vessel, or to
establish the time interval for adding an explosion suppressant (water, carbon
dioxide, or Halon) to stop the combustion process.
A plot of the log of the maximum pressure slope versus the log of the vessel
volume frequently produces a straight line of slope -1/3
This relationship is called the "Cubic Law."
(dp / dt )max V 1/ 3  constant  K g
(dp / dt )max V 1/ 3  K St
(6-3)
(6-4)
where Kg and Kst are called the deflagration indices for gas and dust respectively. As
the robustness of an explosion increases, the deflagration indices Kg and Kst increase.
The cubic law states that the pressure front takes longer to propagate through a larger
vessel. A few values for Kg and Kst are given in Tables 6-5 and 6-6. Dusts are further
classified into four classes, depending on the value of the deflagration index. These St
classes are shown in Table 6-6.
Dust explosions demonstrate --unique behavior. These explosions occur if finely
divided particles of solid material are dispersed in air and ignited. The dust particles
can be either an unwanted by-product or the product itself.
Explosions involving dusts are most common in the flour milling, grain storage,
and coal mining industries. Accidents involving dust explosions can be quite
substantial; a series of grain silo explosions in Westwego near New Orleans in 1977
killed thirty-five people.
An initial dust explosion can cause secondary explosions. The primary explosion
sends a shock wave through the plant, stirring up additional dust which may result in a
secondary explosion. In this fashion the explosion "leapfrogs" its way through a plant.
Many times the secondary explosions are more damaging than the primary.
To be explosive, a dust mixture must have the following characteristics.
• The particles must be below a certain minimum size:
• The particle loading must be between certain limits.
• The dust loading must be reasonably uniform.
For most dusts, the lower explosion limit is between 20 and 60 gm/m3 and the upper
explosion limit between 2 and 6 kg/m3.
Vapor Cloud Explosions (VCE)
The most dangerous and destructive explosions in the chemical process industries are
vapor cloud explosions (VCE). These explosions occur by a sequence of steps:
1. Sudden release of a large quantity of flammable vapor. Typically this occurs
when a vessel, containing a superheated and pressurized liquid, ruptures.
2. Dispersion of the vapor throughout the plant site while mixing with air.
3. Ignition of the resulting vapor cloud.
The accident at Flixborough, England is a classic example of a vapor cloud
explosion. 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 twenty-nine vapor cloud explosions over the period 1974 through
1986 shows property losses for each event of between $5,000,000 to $100,000,000
and 140 fatalities (an average of almost 13 per year).
Some of the parameters that affect VCE behavior· are
Quantity of material released,
Fraction of material vaporized,
Probability of ignition of the cloud,
Distance travelled by the cloud prior to ignition,
Time delay before ignition of cloud,
Probability of explosion rather than fire,
Existence of a threshold quantity of material,
Efficiency of explosion, and
Location of ignition source with respect to release.
Methods which are used for preventing VCEs include keeping low inventories of
volatile, flammable materials; using process conditions which minimize flashing if a
vessel or pipeline is ruptured; using analyzers to detect leaks at very low
concentrations; and installing automated block valves to shut systems down while the
spill is in the incipient stage of development.
Boiling Liquid Expanding Vapor Explosions (BLEVE)·
A boiling liquid expanding vapor explosion (BLEVE, pronounced ble'.-vee) is a
special type of accident that can release large quantities of materials. If the materials
are flammable, a VCE might result; if toxic, a large area might be subjected to toxic
materials. For either situation, the energy released by the BLEVE process itself can
result in considerable damage.
A BLEVE occurs when a tank containing a liquid held above its atmospheric
pressure boiling point ruptures; resulting in the explosive vaporization of a large
fraction of the tank contents.
BLEVEs are caused by the sudden failure of the container due to any cause.
The most common type of BLEVE is caused by fire. The steps are as follows.
1. A fire develops adjacent to a tank containing a liquid.
2. The fire heats the walls of the tank.
3.
4.
5.
The tank walls below liquid level are cooled by the liquid, increasing the liquid
temperature and the pressure in the tank.
If the flames reach the tank walls or roof where there is only vapor and no liquid
to remove the heat, the tank metal temperature rises until it loses it structural
strength.
The tank ruptures, explosively vaporizing its contents.
If the liquid is flammable and a fire is the cause of the BLEVE, it may ignite as
the tank ruptures. Often, the boiling and burning liquid behaves as a rocket fuel,
propelling vessel parts for great distances. If the BLEVE is not caused by a fire, a
vapor cloud might form, resulting in a VCE. The vapors might also be hazardous to
personnel via skin burns or toxic effects.
When a BLEVE occurs in a vessel, only a fraction of the liquid vaporizes; the
amount depends on the physical and thermodynamic conditions of the vessel contents.
The fraction vaporized is estimated using Raoult’s Equation and the Antoine
Equation.