Chapter 6 Fires and Explosions
Transcript of Chapter 6 Fires and Explosions
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Chapter 6
Fires and Explosions
Ref: Chapter 6, D. A. Crowl, J. F. Louvar, Chemical Process
Safety: Fundamental with Applications, 3rd edition,
Prentice-Hall, 2011.
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Introduction Chemicals present a substantial hazard in the form of fires and explosions.
Ex: 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 plantenvironments are even greater.
Organic solvents are the most common source of fires and explosions in the
chemical industry. Chemical and hydrocarbon plant losses resulting from
fires and explosions are substantial
To prevent accidents resulting from 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. 2
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Definitions
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.
Ignition: Ignition of a flammable mixture may be caused by a
flammable mixture coming in contact with a source of ignition
with sufficient energy or the gas reaching a temperature high
enough to cause the gas to autoignite.
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Definitions
Autoignition temperature (AIT): A fixed temperatureabove which adequate energy is available in theenvironment to provide an ignition source.
Flash point (FP): The flash point of a liquid is the lowesttemperature at which it gives off enough vapor to forman 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 increasingpressure.
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Definitions
Fire point: The fire point is the lowest temperature at which a
vapor above a liquid will continue to burn once ignited; the
fire point temperature is higher than the flash point.
Flammability limits: Vapor-air mixtures will ignite and burn
only 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 for combustion.
The mixture is also not combustible when the composition is
too 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. 5
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Definitions
Explosion: An explosion is a rapid expansion of gasesresulting in a rapidly moving pressure or shock wave.
The expansion can be mechanical (by means of a suddenrupture of a pressurized vessel), or it can be the result ofa rapid chemical reaction. Explosion damage is caused bythe pressure or shock wave.
Mechanical explosion: An explosion resulting from thesudden failure of a vessel containing high-pressurenonreactive gas.
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Definitions
Deflagration : An explosion in which the reaction front moves at a speed lessthan the speed of sound in the unreacted medium.
Detonation: An explosion in which the reaction front moves at a speedgreater 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 buildinginhabitants and extensive damage.
Unconfined explosion: Unconfined explosions occur in the open. This type of
explosion is usually the result of a flammable gas spill.
Dust explosion: This explosion results from the rapid combustion of fine solidparticles. Many solid materials (including common metals such as iron andaluminum) become flammable when reduced to a fine powder.
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Boiling-liquid expanding-vapor
explosion (BLEVE):A BLEVE occurs if a vessel that contains a liquid at a temperature aboveits atmospheric pressure boiling point ruptures.
The subsequent BLEVE is the explosive vaporization of a large fractionof the vessel contents; possibly followed by combustion or explosion
of the vaporized cloud if it is combustible.
This type of explosion occurs when an external fire heats the contentsof a tank of volatile material.
As the tank contents heat, the vapor pressure of the liquid within thetank increases and the tank's structural integrity is reduced because ofthe heating. If the tank ruptures, the hot liquid volatilizes explosively.
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Shock wave: An abrupt pressure wave moving through a gas.
A shock wave in open air is followed by a strong wind; the
combined shock wave and wind is called a blast wave.
The pressure increase in the shock wave is so rapid that the
process is mostly adiabatic.
Overpressure: The pressure on an object as a result of an
impacting shock wave.
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The Fire Triangle
The essential elements for combustion are fuel, an oxidizer, and
an ignition source.
Fire, or burning, is the rapid exothermic oxidation of an ignitedfuel.
Two common examples of the three components of the fire
triangle are wood, air, and a match; and gasoline, air, and aspark.
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When fuel, oxidizer, and an ignition source are present at the
necessary levels, burning will occur.
This means 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.
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Common Fuels, Oxidizers and Ignition
sourcesProcess Industry
Fuels
Liquids: gasoline, acetone, ether, pentane
Solids: plastics, wood dust, fibers, metal particles
Gases: acetylene, propane, carbon monoxide, hydrogen
Oxidizers
Gases: oxygen, fluorine, chlorine
Liquids: hydrogen peroxide, nitric acid, perchloric acid
Solids: metal peroxides, ammonium nitrite
Ignition sources
Sparks, flames, static electricity, heat
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Distinction between Fires and
Explosions
The major distinction between fires and explosions is the rate
of energy release.
Fires release energy slowly, whereas explosions release
energy rapidly, typically on the order of microseconds.
Fires can also result from explosions, and explosions can
result from fires.
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Le Chatelier equation for Prediction of
LFL and UFL of mixtures
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Where LFLi 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.
where UFL, is the upper flammable limit for component i (in volume %) of
component i in fuel and air.
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Limiting Oxygen Concentration and lnerting
The LFL is based on fuel in air.
However, oxygen is the key ingredient and there is a minimumoxygen concentration required to propagate a flame.
This is an especially useful result, because explosions and firescan be prevented by reducing the oxygen concentrationregardless of the concentration of the fuel.
This concept is the basis for a common procedure calledinerting.
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LOC or MOC or MSOC
Below the limiting oxygen concentration (LOC) the reaction
cannot generate enough energy to heat the entire mixture of
gases (including the inert gases) to the extent required for the
self-propagation of the flame.
The LOC has also been called the minimum oxygen
concentration (MOC), the maximum safe oxygen
concentration (MSOC),
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Flammability Diagram A general way to represent the flammability of a gas or vapor is by the
triangle diagram .
Concentrations of fuel, oxygen, and inert material (in volume or mole %)
are plotted on the three axes.
Each apex of the triangle represents either 100% fuel, oxygen, or nitrogen.
The tick marks on the scales show the direction in which the scale moves
across the figure.
Thus point A represents a mixture composed of 60% methane, 20%oxygen, and 20% nitrogen. The zone enclosed by the dashed line
represents all mixtures that are flammable. Because point A lies outside
the flammable zone, a mixture of this composition is not flammable.
Clearly, any gas mixture containing oxygen below the LOC is not
flammable. 18
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Flammability diagram for methane at 1 atm and 25 oC
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Minimum ignition energy (MIE)
The minimum ignition energy (MIE) is the minimum energy
input required to initiate combustion.
All flammable materials (including dusts) have MIEs.
The MIE depends on the specific chemical or mixture, the
concentration, pressure, and temperature.
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Characteristics of MIE
1. the MIE decreases with an increase in
pressure,
2. the MIE of dusts is, in general, at energy
levels somewhat higher than combustible
gases,
3. an increase in the nitrogen concentration
increases the MIE.
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Autoignition Temperature
The autoignition temperature (AIT) of a vapor, sometimes
called the spontaneous ignition temperature (SIT), is the
temperature at which the vapor ignites spontaneously from
the energy of the environment.
The autoignition temperature is a function of the
concentration of vapor, volume of vapor, pressure of the
system, presence of catalytic material, and flow conditions.
It is essential to experimentally determine AITs at conditions
as close as possible to process conditions.
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Auto-oxidation
Auto-oxidation 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 problem.
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 auto-oxidation, referred
to as spontaneous combustion. Ex: spontaneous combustion
include oils on a rag in a warm storage area23
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Detonation and Deflagration
The damage effects from an explosion depend highly on
whether the explosion results from a detonation or a
deflagration.
The difference depends on whether the reaction front
propagates above or below the speed of sound in the
unreacted gases.
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Detonation
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Deflagration
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Exercise
1. What are the LFL and UFL of a gas mixture
composed of 0.8% hexane, 2.0% methane,
and 0.5% ethylene by volume?
2. Estimate the LOC for butane (C4HI0).
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