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04/18/23 Eric Grulke. fires & explosions. CME 470
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The Wily Coyote LectureFires and Explosions
CME 470
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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 TriangleFuels
•Solids: plastic, wood dust fiber, metal particles
•Liquids: gasoline, acetone, ether, pentane
•Vapors: acetylene, propane, CO, H2
Oxidizers
•Solids: metal peroxides, ammonium nitrate
•Liquids: H2O2, nitric acid, perchloric acid
•Vapors: O2, F2, Cl2
Ignition sources
•Sparks
•Flames
•Static electricity
•heat
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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
2
2
exp1
exp
b
bbFP
Tc
Tc
Tc
b
aT
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
n
i i
imix
LFL
yLFL
1
1
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LFL = f(T, P)
c
T H
TLFLLFL
2575.0125
)1(log6.201 PUFLUFLP
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Estimating LFL, UFL
st
st
CUFL
CLFL
50.3
55.0
Cst is the volume % fuel in fuel plus air
24
2 222
yxmz
OHx
COmOzOHC yxm
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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
Case study: TNTImages from Wikipedia.org
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Trinitrotoluene
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
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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
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•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.
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.orgNote white blast wave at water surface and condensate cloud caused by
shock wave.
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