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:

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

mix ni

i 1 i

1LFL

yLFL

mix n

i

i 1 i

1UFL

y

UFL

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?

T 25c

0.75UFL UFL T 25

H

T 25c

0.75LFL LFL T 25

H

Flammibility limit dependence on temperature

net heat of combustion (kcal/mole)

cH

cH

Flammibility limit dependence on pressure Pressure has little effect on LFL except at very

low pressure (<50 mm Hg absolute)

PUFL UFL 20.6 log P 1

Estimating flammability limits

stLFL 0.55C

stUFL 3.50C

Cst = stoichiometric concentration Stoichiometric concentration for most organic

compounds is determined using the general combustion reaction

m x y 2 2 2

xC H O zO mCO H O

2

x yz m

4 2

0.55 100LFL

4.76m 1.19x 2.38y 1

3.50 100UFL

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:

Tf is the final absolute temperatureTi is the initial absolute temperaturePf is the final absolute pressurePi is the initial absolute pressure

p vC /C

1 /

ff i

i

PT T

P

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 JF? The AIT of hexane is 487 JC (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