FIRES AND EXPLOSION
LECTURE 10
Fire Triangle
Fuels:
Liquids:gasoline,acetone
Solids: plastic,wood dust,fibers
Gases: acetylene,propane,hydrogen
 Oxidizers:
Gases: oxygen,fluorine,chlorine
Liquids:hydrogen peroxide,nitric acid
Solids: metal peroxides,ammonium
nitrite

Ignition sources:
Sparks,flames,static electricity,heat

Difference between fires and
explosions




Rate of energy release
Fires release energy slowly, explosions release
energy rapidly
Fires can result from explosions, explosions can result
from fires
Analogy example: automobile tire. Compressed air
within tire contain energy. If energy is released
slowly through nozzle, tire is harmlessly deflated. But
if tire ruptures suddenly and all energy within the
compressed tire releases rapidly, the result is
dangerous explosion
Definition




Combustion or fire: chemical reaction which a substance
combines with an oxidant and releases energy
Ignition: ignition of a flammable mixture may be caused by
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
Autoignition temperature: a fixed temperature above which
adequate energy is available in the environment to provide an
ignition source
Flash point: flash point of a liquid is the lowest temperature at
which it gives off enough vapor to form an ignitable mixture
with air




Flammability limits: Vapor-air mixtures will ignite and burn only
over a well-specified range of compositions. Mixtures will not
burn when composition is lower than the lower flammable limit
(LFL). Mixture is also not combustible when it is above the
upper flammable limit (UFL). Mixture is flammable only when
the composition is between LFL and UFL
Explosion:rapid expansion of gases resulting in a rapidly moving
pressure or shock wave. The expansion can be mechanical or
can be the result of a rapid chemical reaction
Deflagration: an explosion in which the reaction front moves at
a speed less than the speed of sound in the unreacted medium
Detonation: an explosion in which the reaction front moves at a
speed greater than the speed of sound in the unreacted
medium
Flammability characteristics of
liquids and vapors
Liquids


flash point temperature is one of the major quantities used to
characterize the fire and explosion hazard of liquids
Flash points can be estimated for multicomponent mixtures if
only one component is flammable and if flash point of the
flammable component is known
Gases and vapors

Flammability limits for vapors are determined experimentally in
a specially designed closed vessel apparatus (see pg 255)
Vapor mixtures

Frequently LFL and UFL for mixtures are needed. These
mixture limits are computed using the equation:
LFLmix



1
 n
yi

i 1 LFLi
UFLmix
1
 n
yi

i 1 UFLi
LFLi is the lower flammable limit for component i
yi is the mole fraction of component I on combustible basis
N is the number of combustible species

Example 6.2
What are the LFL and UFL of a gas mixture
composed of 0.8% hexane, 2.0% methane, and
0.5% ethylene by volume?
Flammibility limit dependence
on temperature
0.75
LFLT  LFL 25 
 T  25
H c
H c
0.75
UFLT  UFL 25 
 T  25
H c
H c
net heat of combustion (kcal/mole)
Flammibility limit dependence
on pressure

Pressure has little effect on LFL except at very low
pressure (<50 mm Hg absolute)
UFLP  UFL  20.6  log P  1
Estimating flammability limits
LFL  0.55Cst
UFL  3.50Cst


Cst = stoichiometric concentration
Stoichiometric concentration for most organic compounds is
determined using the general combustion reaction
x
Cm H x O y  zO 2  mCO 2  H 2O
2
x y
z  m 
4 2
LFL 
UFL 
0.55 100 
4.76m  1.19x  2.38y  1
3.50 100 
4.76m  1.19x  2.38y  1

Example 6.4
Estimate the LFL and UFL for hexane, and compare
the calculated limits to the actual values determined
experimentally
Limiting oxygen concentration
and inerting




Explosions and fires can be prevented by reducing the oxygen
concentration regardless of the concentration of the fuel
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
LOC=MOC ~ minimum oxygen concentration
This concept is the basis for a common procedure called inerting
(Chapter 7)
Ignition energy








Minimum ignition energy (MIE) is the minimum energy required
to initiate combustion. All flammable materials (including dusts)
have MIEs.
MIE depends on specific chemical or mixture, concentration,
pressure and temperature
MIE decrease with increase of pressure
MIE of dusts is in general, at energy levels somewhat higher
than combustible gases
An increase in nitrogen concentration increases MIE
Many hydrocarbons have MIEs about 0.25mJ
Static discharge by walking across rug = 25mJ
Electrostatic discharges, as a result of fluid flow, also have
energy levels exceeding MIE of flammable materials and can
provide an ignition source  plant explosion
Autoignition



Autoignition temperature (AIT) of vapor=spontaneous ignition
temperature (SIT)
Temperature at which the vapor ignites spontaneously from the
energy of environment
Depends on concentration of vapor, volume of vapor, pressure
of system, presence of catalytic material and flow conditions
Auto-oxidation



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 susceptible to this
problem
Liquids with high volatility are less susceptible to autoignition
because they self-cool as a result of evaporation
Adiabatic compression



Gasoline and air in an automobile cylinder will ignite if the
vapors are compressed to an adiabatic temperature that
excceeds the autoignition temperature
It is the reason some overheated engines continue to run after
the ignition is turned off
Several large accidents have been caused by flammable vapors
being sucked into the intake of air compressor – subsequent
compression resulted in autoignition

The adiabatic temperature increase for an ideal gas is computed
from the thermodynamic adiabatic compression equation:
 Pf 
Tf  Ti  
 Pi 
 1 / 
Tf is the final absolute temperature
Ti is the initial absolute temperature
Pf is the final absolute pressure
Pi is the initial absolute pressure
  Cp / C v

Example 6.6
What is the final temperature after compressing air
over liquid hexane from 14.7 psia to 500 psia if the
initial temperature is 100 ̊F? The AIT of hexane is
487 ̊C (from Appendix B) and  for is 1.4
Explosions



Explosion results from rapid release of energy. Energy release
must be sudden enough to cause local accumulation of energy
at the site of explosion
Energy is dissipated~formation of pressure wave, projectiles,
thermal radiation, acoustic energy
Damage from explosion is caused by dissipating energy





If explosion occurs in gas, the energy causes the gas to expand
rapidly, forcing back the surrounding gas and initiating a
pressure wave that moves rapidly outward from the blast
source
Pressure wave contains energy~damage to surroundings
For chemical plants much of the damage from explosions is due
to pressure wave
A pressure wave propagating in air is called blast wave because
the pressure wave is followed by strong wind
Shock wave or shock front results if the pressure front has an
abrupt pressure change~highly explosive material-TNT
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
In some combustion reactions, the reaction front is propagated
by a strong pressure wave, which compresses the unreacted
mixture in front of the reaction front
This compression occurs rapidly, resulting in an abrupt pressure
change or shock in front of the reaction front - This is
classified as detonation, resulting in a reaction front and leading
shock wave that propagates into the unreacted mixture at or
above sonic velocity


For deflagration, the energy from the reaction is
transferred to the unreacted mixture by heat
conductiion and molecular diffusion.
These processes are relatively slow, causing the
reaction front to propagate at speed less than sonic
velocity