6/9/2015Eric Grulke. fires & explosions. CME 470 1 The Wily Coyote Lecture Fires and Explosions CME 470

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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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

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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.

Documents

6/9/2015Eric Grulke. fires & explosions. CME 470 1 The Wily Coyote Lecture Fires and Explosions CME 470