The Wily Coyote Lecture
Fires and Explosions
CME 470
4/13/2015
Eric Grulke. fires & explosions.
CME 470
1
Caution!
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Fire vs. Explosion
• Fires: release energy slowly
• Explosions: rapid release of energy
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Fire or Explosion?
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Fire Triangle
Fuels
•Solids: plastic, wood dust fiber, metal particles
•Liquids: gasoline, acetone, ether, pentane
•Vapors: acetylene, propane, CO, H2
Oxidizers
•Solids: metal peroxides,
ammonium nitrate
Ignition sources
•Sparks
•Liquids: H2O2, nitric acid,
perchloric acid
•Flames
•Vapors: O2, F2, Cl2
•heat
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•Static electricity
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The Fire Triangle
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Definitions
• Combustion: chemical reaction in which
fuel combines with oxidant and releases
energy
• Ignition: start of the burning process
• Autoignition temperature: T such that
mixture can self-ignite
• Flash Point: lowest temperature at which
the liquid will volatilize enough vapor to
form an ignitable mixture
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Definitions
• Fire point: lowest T at which vapor above
a liquid will burn
• Flammability limits: burning occurs
between LFL and UFL (LEL and UEL)
• Explosion: rapid expansion of gases with
fast pressure or shock wave
• Mechanical Explosion: explosion due to
vessel failure, high pressure non-reactive
gas
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Definitions
• Deflagration: explosion with shock wave
moving at a speed lower than speed of
sound
• Detonation: explosion with shock wave
moving faster than speed of sound
• Confined explosion: explosion inside
vessel or building
• Unconfined explosion: flammable gas spill
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Definitions
• Boiling liquid expanding vapor explosion
(BLEVE): vessel containing liquid at T>Tb ;
explosive vaporization of vessel contents
• Dust explosion: rapid combustion of fine
particles
• Shock wave: pressure wave moving
through a gas
• Overpressure: P as f(shock wave)
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Flammability Characteristics of
Liquids and Gases
• Liquids – use flash point temperature to
characterize the fire and explosion
hazards
• FPT – determined in open-cup apparatus;
open flame over liquid which is heated;
closed-cup apparatus gives lower T
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Flammability Characteristics of
Liquids and Gases
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Flammability Characteristics of
Liquids and Gases
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Flash Point
• FP’s are tabulated
• Multicomponent mixtures:
one component is
flammability and its
characteristics are known
• Estimate is based on the
partial pressure of the
flammable component
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TFP
c
 c
b     exp  
Tb 
Tb 


a
2



1  exp  c  
 T 

 b 

K. Satyanarayana, P. G. Rao,
Improved equation to estimate flash
points of organic compounds, J.
Hazardous Materials, 32, 81-85
(1992). Coefficients tabulated for
chemical groups.
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Example 6-1. Flash point
of MeOH solution
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Saturation Vapor Pressure
for Methanol
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Concentration of Flammable Gas
(vol%)
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Vapor Flammability
• LFLs and UFLs can
be computed for
mixtures using an
equation by Le
Chatelier
LFLmix 
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1
n
yi

i 1 LFLi
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LFL = f(T, P)


T  25
LFLT  LFL25  1  0.75

H c 

UFLP  UFL1  20.6  (log P  1)
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Estimating LFL, UFL
LFL  0.55 Cst
UFL  3.50  Cst
Cst is the volume % fuel in fuel plus air
x
Cm H x O y  z  O2  m  CO2   H 2O
2
x y
z  m 
4 2
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Minimum Oxygen Concentration
• LFL is based on fuel in air
• A minimum oxygen level is needed to
propagate a flame
• Below the MOC, the flame cannot
generate enough energy to heat the
mixture for self-propagation
• MOC is estimated using the stoichoimetry
of the combustion and the LFL
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Minimum Ignition Energies
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Ignition Sources of Major Fires
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Reaction and Pressure Fronts
Propagating Through a Pipe
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Test Apparatus for Acquiring Vapor
Explosion Data
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Typical Pressure Versus Time Data
Obtained from Explosivity Apparatus
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Pressure Rate and Maximum Explosion Pressure
as a Function of Vapor Concentration
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Typical Explosion Data
Exhibiting the Cubic Law
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Average Kg Values for
Selected Gases
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Average KSt Values for
Selected Dusts
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Effect of Initial Pressure on Maximum
Explosion Pressure and Rate
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Explosion Data for Propane Showing Peaks
Indicative of the Onset of Detonation
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Damage Produced by Overpressure
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Correlation Between Overpressure and
Scaled Distance, English Engineering Units
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Correlation Between Overpressure and
Scaled Distance, SI Units
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Maximum Horizontal Range of
Blast Fragments
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Images from Wikipedia.org
CASE STUDY: TNT
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2,4,6-trinitrotoluene
CAS Reg # 118-96-7
Formula: C7H5N3O6
Fw = 227.13 kg/kmol
Names: TNT, Trotyl, Triton, …
Density: 1654 kg/m3
Melting point: 80.35 C; boiling point: 295 C (decomposition)
Solubility: 0.13g/L in water; soluble in ether, acetone, benzene,
pyridine
EU classification: explosive (E), toxic (T), environmental hazard (N)
NFPA 704
TRINITROTOLUENE
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background
• Common explosive with convenient handling
properties
• C6H2(NO2)3CH3
• Standard measure of explosive strength
• Synthesis:
multi-step process. Nitration of toluene (nitric +
sulfuric acid) to MNT/separation/nitration to DNT then nitration to
TNT in anhydrous mixtures of nitric acid + oleum. NOX in feed nitric
acid must be controlled to prevent oxidation of methyl group.
• Stabilization: aqeous sodium sulfite to remove less stable
isomers and other byproducts. Rinse water is a significant pollutant.
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applications
• Common explosive for military and
industrial applications
• Low sensitivity to shock & friction; ignition
temperature is well above the melting
point
• Does not sorb water, relatively stable.
• Block sizes: 0.25, 0.5 and 1 kg.
• Synergistic blends with other exposives
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Explosive characteristics
•Explosives decompose to elements, stable molecules (mostly) without the
aid of external oxidizing agents.
•Exothermic, high activation energy
•Carbon is a product, leading to sooty appearance of explosions
•Ignition with a high velociy initiator or by concussion
•Reference point – Figure of Insensitivity
•The Figure of Insensitiveness is determined from impact testing, typically using a drop-weight tower. In this test,
a small sample of the explosive is placed on a small steel anvil which is slotted into a recess in the base of the
drop tower. A cylindrical, 1 kilogram steel weight (mounted inside a tube to accurately guide its descent to the
impact point in the centre of the anvil) is then dropped onto the test specimen from a measured height. The
specimen is monitored both during and after this process to determine whether initiation occurs. This test is
repeated many times, varying the drop height according to a prescribed method. Various heights are used,
starting with a small distance (e.g. 10 cm) and then progressively increasing it to as high as 3 metres. The series
of drop heights and whether initiation occurred are analysed statistically to determine the drop height which has a
50% likelihood of initiating the explosives. The intention of these tests is to develop safety policies/rules which will
govern the design, manufacturing, handling and storage of the explosive and any munitions containing it.
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Energy content
• 4.6 megajoules/kg (energy density)
– Nuclear weapons are measured in megatons
of TNT
– Gunpowder: 3 MJ/kg
– Dynamite: 7.5 MJ/kg
– Gasoline: 47.2 MJ/kg (gas+O2=10.4 MJ/kg)
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500 ton TNT
explosion,
1965,
wikipedia.org
Note white blast
wave at water
surface and
condensate
cloud caused by
shock wave.
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