Fires and Explosions. Definitions Flammability Flash Points Flammability limits Mixtures Temperature Dependence Pressure Dependence Minimum

Fires and Explosions

Fires and ExplosionsFires and ExplosionsDefinitionsFlammability

Flash PointsFlammability limitsMixturesTemperature DependencePressure Dependence

Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources

IntroductionIntroduction

We have been talking about source models for the release of materials and about dispersion models if the material is a toxicant.

Another concern is a release of flammable materials where we need to worry about fires and explosions.

Fire TriangleFire Triangle

Most are familiar with the Fire Triangle.

In order for a fire to start or be sustained you need to have a Fuel, an oxidizer and an ignition source.

If one of the three components is eliminated, then there will not be a fire (or explosion)

FuelFuel

Fuel must be present in certain concentrations.Typical cases where fuel occur are if there is a leak,

during filling operations, transfer operations, or excessive dusts.

Although we often cannot always eliminate these sources we can help by having good ventilation to keep vapors from building up.

Often we locate things out-doors, use grating on floors so vapors don’t build up.

OxidizersOxidizers

Oxygen is the most common oxidizer, especially that found in ambient air.

For oxygen, we often use “inerting” with nitrogen, helium blankets over flammable materials to reduce O2 content below that where you can have combustion.

Ignition SourcesIgnition Sources

Heat is a common ignition source.“Ignition sources are free!!!”Although we can eliminate ignition sources, it is

almost inevitable that an ignition source will be available if there is a large release of flammable material that cannot be diluted quickly.

Fire TetrahedronFire TetrahedronThe fire tetrahedron or fire

pyramid adds a fourth component—chemical chain reaction—as a necessity in the prevention and control of fires.

The free radicals formed during combustion are important intermediates in the initiation and propagation of the combustion reaction. Fire suppression materials scavenge these free radicals

DefinitionsDefinitions

Combustion – a chemical reaction in which a substance combines with an oxidizer and releases energy.

Explosion – rapid expansion of gases resulting in a rapid moving pressure or shock wave.

Mechanical Explosion – due to failure of vessel with high pressure non reactive gas.

ExplosionsExplosions

Detonation – explosion (chemical reaction) with shock wave greater than speed of sound

Deflagration – explosion (chemical reaction) with shock wave less than speed of sound

BLEVE – Boiling Liquid Expanding Vapor Explosion – when liquid is at a temperature above its atmospheric boiling point. Vessel ruptures – flammable liquid flashes and results in a fire/explosion


Confined explosion – an explosion occurring within a vessel or a building. Usually results in injury to the building inhabitants and extensive damage.

Unconfined explosion – an explosion occurring in the open. Usually results from spill of a flammable gas spill. These explosions are rarer than confined since dilution occurs.


Dust Explosions - This explosion results from the rapid combustion of fine solid particles. Many solid materials become very flammable when reduced to a fine powder.


Flash PointFlammability limitsMixturesTemperature DependencePressure Dependence


FlammabilityFlammabilityFlash Point (FP) – a property of material used to

determine the fire and explosive hazard. The lowest temperature of a liquid at which it gives off enough vapor to form an ignitable mixture with air.

Needs to be determined experimentally.Different methods to determine, open cup and closed

cup. Open cup is usually a few degrees higher.

National Fire Protection Association

Flammability classification

National Fire Protection Association

Flammability classificationFlammable IA – Flash point < 73°F, boiling point < 100 °FFlammable IB – Flash point < 73°F, boiling point > 100 °FFlammable IC – 73°F < Flash point < 100 °FCombustible II – 100 °F < Flash point < 140 °FCombustible IIIA – 140 °F < Flash point < 200 °FCombustible IIIB – Flash point > 200 °F

Mixture Flash PointsMixture Flash Points

Flash Points of mixtures can be estimated only IF one of the components is flammable. If more than one is flammable then need to determine experimentally.

For mixtures:Determine the temperature at which the vapor pressure of

the flammable in the liquid is equal to the pure component vapor pressure at its flash point.

Mixture Flash PointsMixture Flash PointsExample

Methanol FP=54°F, Vapor Pressure @ 54°F is 62 mmHgDetermine the flash point of a solution that is 75wt% MeOH in water.Solution:Since only one component is flammable, can estimate mixture FP:

Mixture Flash Point Example Continued


Raoult's Law

6298.4

0.63Now need the temperature that corresponds

to this . Use Antoine's equation (Append II)

ln

in Kelvin, in mmHg

sat

sat

sat

sat

sat

P xP

P mmHgP mmHg

x

P

BP A

C T

T P



Rearrange

- lnFrom Appendix II

A is 18.5875

B is 3626.55

C is -34.29

3626.5534.29 293.36

18.5875 ln 98.4

20.21 68.4

sat

BT C

A P

T K

T C F

Flammability LimitsFlammability Limits

There is usually a range of compositions of a flammable vapor and air where combustion occurs.

Too little fuel (lean mixture) not enough fuel to burn.

Too much fuel (rich mixture) not enough oxygen to burn

Flammability LimitsFlammability Limits

Table 6-1 gives upper flammability limits and lower flammability limits for several common substances.

Experimentally determined.LFL can be estimated from Flash Point:.

vapor pressure at flash point

760 mmHg

Determine vapor pressure using Antoine Equation

LFL

Mixture Flammability Limits


If you have a mixture of flammable components you can calculate Lower Flammability Limit of the mixture LFLmix using Le Chatelier’s relationship:

1

1

is flammability limit for component

is mole fraction of on combustible basis

is the number of combustible species

mix ni

i i

i

i

LFLy

LFL

LFL i

y i

n



You can also calculate an Upper Flammability Limit of the mixture UFLmix using Le Chatelier’s relationship:

1

1mix n

i

i i

UFLy

UFL

Flammability Limits – Temperature effect

Flammability Limits – Temperature effect

Table 6-1 gives flammability limits for 25°C and atmospheric pressure. If you are at a different temperature you can modify flammability limits

25

25

1 0.75( 25) /

1 0.75( 25) /

is heat of combustion for component

T is in C

T c

T c

c

LFL LFL T H

UFL UFL T H

H

Flammability Limits – Pressure effects

Flammability Limits – Pressure effects

LFL is not affected by pressure UFL does depend on the pressure

ProcedureCorrect for TemperatureCorrect for PressureCalculate for mixture

1020.6(log 1)

is in MPa absolutePUFL UFL P

P




Minimum Oxygen Concentration (MOC)

Minimum Oxygen Concentration (MOC)

LFL is based on “air” but actually it is O2 that is important. Often in industry they “inert” to dilute the O2 concentration.

Below the MOC the reaction cannot generate enough energy to heat the entire mixture to the extent required for self propagation.

MOCMOC

2

2

2 2 2

2

Moles Fuel Moles O

Moles Fuel & Moles Air Moles Fuel

Moles O

Moles Fuel

Need to balance stoichiometry

2

4 2Moles O

Moles Fuel

m x y

MOC

MOC LFL

xC H O zO mCO H O

x yz m

z




Minimum Ignition Energy (MIE)

Minimum Ignition Energy (MIE)

Minimum energy input needed to initiate combustionMost hydrocarbons have low MIE~0.25 mJWhereas the “spark” from walking across the room is

22mJ (almost 100X too much)Again, we always assume that an ignition source will

existTable 6-2 gives MIEs for some substances




Adiabatic CompressionAdiabatic Compression

When gases are compressed they heat up and can ignite (this is how a diesel engine works, also the cause of “knocking” in gasoline engines)

The adiabatic temperature rise is:1

and absolute

ff i

i

PT T

P

T P




Ignition SourcesIgnition Sources

Ignition sources are free!!!Table 6-3 gives the

results of a study by Factory Mutual Engineering Corporation who studied over 25,000 industrial fires to determine the source of ignition.

In Class ProblemIn Class Problem

What is the UFL of a gas mixture composed of 1% methane, 2% ethane and 3% propane by volume at 50°C and 2 atmospheres:

Data:Component MW Heat of Combustion

(kcal/mol)Methane 16.04 212.79Ethane 30.07 372.81Propane 44.09 526.74

SolutionSolution

Procedure:Correct for temperatureCorrect for pressure (only for UFL)Find for mixture.

SolutionSolution

Correction for Temperature : UFL from Table 6-1

25

50

50

50

Eq. 6-4 1 0.75( 25) /

Methane 15 1 0.75(25) / 212.79 16.32

Ethane 12.5 1 0.75(25) / 372.81 13.13

Propane 9.5 1 0.75(25) / 526.74 9.84

T cUFL UFL T H

UFL

UFL

UFL

Solution cont.Solution cont.Correction for Pressure (UFL only)

10

2 1 10

2 1

Propane

Eq. 6-5 20.6(log 1)

1012 0.202

1000

20.6(log (0.202 ) 1)

6.290

22.61

19.40

16.13

P

atm atm

atm atm

Methane

Ethane

UFL UFL P

kPa MPaP atm MPa

atm kPa

UFL UFL MPa

UFL UFL

UFL

UFL

UFL

Solution cont.Solution cont.Mixture calculation

Equation 6-2 for mixtures

Mixture Vol% Mol frac Comb

Methane 1 0.1667

Ethane 2 0.3333

Propane 3 0.5000

Combustibles 6

1

1mix n

i

i i

UFLy

UFL

Solution ContinuedSolution Continued

Since total combustibles in air 1+2+3=6 < 18 then the system is in the combustible range (below UFL)

118.0 %

0.1667 0.3333 0.522.61 19.40 16.13

MixtureUFL vol

Documents

Fires and Explosions. Definitions Flammability Flash Points Flammability limits Mixtures Temperature Dependence Pressure Dependence Minimum