FIRES AND

EXPLOSION

OBJECTIVES:

TO DEFINE AND DISCUSS THE FIRE (FIRE TRIANGLE)

TO DISTINGUISH BETWEEN FIRES AND

EXPLOSION AS WELL AS THE FLAMMABILITY

CHARACTERISTICS OF LIQUIDS AND VAPORS

AND THE EXPLOSIONS CHARACTERISTICS

ENGINEERS MUST BE FAMILIAR WITH:

The fire and explosion properties of MATERIALS The nature of the fire and explosion PROCESS PROCEDURES TO REDUCE fire and explosion hazards

THREE MOST COMMON CHEMICAL PLANT ACCIDENTS:

FIRESEXPLOSION

TOXIC RELEASES

THE 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

The essential elements of combustion are:

Figure 6-1 The fire triangle.

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

Confined explosion: An explosion occuring within a vessel or a building. These are most common and usually result in injury 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 expending-vapor explosion (BLEVE): A BLEVE occurs if a vessel that contains a liquid at a temperature above its atmospheric pressure boiling point ruptures. 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. If the tank ruptures, the hot liquid volatilizes explosively.

FLAMMABILITY CHARACTERISTICS OF LIQUIDS AND VAPORS (PROVIDED IN APPENDIX B)

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

FLAMMIBILITY LIMIT DEPENDENCE ON TEMPERATURE

T 25c

0.75LFL LFL T 25

H

net heat of combustion (kcal/mole)

cH

cH

(Available for vapors)

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

P is the pressure (Mpa absolute) UFLP is the upper flammable limit (volume % of fuel plus air at 1 atm)

Example 6.3

If the UFL for a substance is 11% by volume at 0.0MPa gauge, What is the UFL

at 6.2MPa gauge?

ESTIMATING FLAMMABILITY LIMITS

stLFL 0.55C

stUFL 3.50C

Eq. 6.7

Eq. 6.8

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

Substituting z and applying Equations 6.7 and 6.8 yields;

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

Limiting O2 Conc. (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 , P ) 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 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 exceeds 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 temperature

Ti is the initial absolute temperature

Pf is the final absolute pressure

Pi is the initial absolute pressure

p vC /C

1 /

ff i

i

PT T

P

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 by a variety of mechanism~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

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 conduction and molecular diffusion.

These processes are relatively slow, causing the reaction front to propagate at speed less than sonic velocity