3. Introduction
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.
4. Fire 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)
5. Fuel
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.
6. Oxidizers
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.
7. Ignition 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.
8. Fire Tetrahedron
The 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
9. Definitions
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.
10. Explosions
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
11. Explosions
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.
12. Explosions
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.
13. Fires and Explosions
Definitions
Flammability
Flash Point
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources
14. Flammability
Flash 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.
15. National Fire Protection Association
Flammability classification
Flammable IA – Flash point < 73°F, boiling point < 100 °F
Flammable IB – Flash point < 73°F, boiling point > 100 °F
Flammable IC – 73°F < Flash point < 100 °F
Combustible II – 100 °F < Flash point < 140 °F
Combustible IIIA – 140 °F < Flash point < 200 °F
Combustible IIIB – Flash point > 200 °F
16. Mixture 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.
17. Mixture Flash Points
Example
Methanol FP=54°F, Vapor Pressure @ 54°F is 62 mmHg
Determine the flash point of a solution that is 75wt% MeOH in water.
Solution:
Since only one component is flammable, can estimate mixture FP:
18. Mixture Flash Point Example Continued
Raoult's Law
62
98.4
0.63
Now 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 mmHg
P mmHg
x
P
B
P A
C T
T P
19. Mixture Flash Point Example Continued
Rearrange
-ln
From Appendix II
A is 18.5875
B is 3626.55
C is -34.29
3626.55
34.29 293.36
18.5875 ln 98.4
20.21 68.4
sat
B
T C
A P
T K
T C F
20. Flammability 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
21. Flammability 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
22. 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 n
i
i i
i
i
LFL
y
LFL
LFL i
y i
n
23. Mixture Flammability Limits
You can also calculate an Upper Flammability
Limit of the mixture UFLmix using Le Chatelier’s
relationship:
1
1
mix n
i
i i
UFL
y
UFL
24. 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
25. Flammability Limits – Pressure effects
LFL is not affected by pressure
UFL does depend on the pressure
Procedure
Correct for Temperature
Correct for Pressure
Calculate for mixture
10
20.6(log 1)
is in MPa absolute
P
UFL UFL P
P
27. 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.
28. MOC
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 2
Moles O
Moles Fuel
m x y
MOC
MOC LFL
x
C H O zO mCO H O
x y
z m
z
30. Minimum Ignition Energy (MIE)
Minimum energy input needed to initiate combustion
Most hydrocarbons have low MIE~0.25 mJ
Whereas the “spark” from walking across the room is
22mJ (almost 100X too much)
Again, we always assume that an ignition source will
exist
Table 6-2 gives MIEs for some substances
32. Adiabatic 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
f
f i
i
P
T T
P
T P
34. Ignition 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.
35. In 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 16.04 212.79
Ethane 30.07 372.81
Propane 44.09 526.74
39. 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
1
mix n
i
i i
UFL
y
UFL
40. Solution Continued
Since total combustibles in air 1+2+3=6 < 18 then
the system is in the combustible range (below
UFL)
1
18.0 %
0.1667 0.3333 0.5
22.61 19.40 16.13
Mixture
UFL vol
