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Fires and ExplosionsFires 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
ExplosionsExplosions
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.
ExplosionsExplosions
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.
Fires and ExplosionsFires and ExplosionsDefinitionsFlammability
Flash PointFlammability limitsMixturesTemperature DependencePressure Dependence
Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources
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
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
Mixture Flash Point Example Continued
Mixture Flash Point Example Continued
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
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
Mixture Flammability Limits
Mixture Flammability Limits
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
Fires and ExplosionsFires and ExplosionsDefinitionsFlammability
Flash PointsFlammability limitsMixturesTemperature DependencePressure Dependence
Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources
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
Fires and ExplosionsFires and ExplosionsDefinitionsFlammability
Flash PointsFlammability limitsMixturesTemperature DependencePressure Dependence
Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources
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
Fires and ExplosionsFires and ExplosionsDefinitionsFlammability
Flash PointsFlammability limitsMixturesTemperature DependencePressure Dependence
Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources
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
Fires and ExplosionsFires and ExplosionsDefinitionsFlammability
Flash PointsFlammability limitsMixturesTemperature DependencePressure Dependence
Minimum Oxygen ConcentrationMinimum Ignition EnergyAdiabatic CompressionIgnition Sources
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