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Faculty of Chemical EngineeringUniversiti Teknologi MARA

Fires and Explosions: Part IIAcknowledgement to Dr Syed Shatir A. Syed-Hassan

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Explosion

• Explosion is a sudden and violent release of energy.• The violence of the explosions depends on the rate at

which energy is released.• The energy release must be sudden enough to cause a

local accumulation of energy at the site of location.• This energy is dissipated by a variety of mechanisms,

including formation of pressure wave, projectiles, thermal radiation, and acoustic energy.

• The damage from an explosion is caused by the dissipating energy.

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Explosion

• If the explosion occurs in a gas, the energy causes the gas to expand rapidly, initiating a pressure wave that moves rapidly outward from the blast source.

• The pressure wave contains energy, which results in damage to the surroundings.

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Explosion

• To understand explosion impacts, we must understand the dynamics of the pressure wave.

• A pressure wave propagating in air is called a blast wave because the pressure wave is followed by a strong wind.

• A shock wave or shock front results if the pressure front has an abrupt pressure change.

• The maximum pressure over ambient pressure is called the peak overpressure.

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Shockwave

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Shockwave

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Injury

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Injury

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Parameters Affecting the Behaviour of Explosions

• Ambient temperature• Ambient pressure• Composition of explosive material• Physical properties of explosive material• Nature of ignition source: type, energy and duration• Geometry of surroundings: confined and unconfined• Amount of combustible material• Turbulence of combustible material• Time before ignition• Rate at which combustible material is released

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Detonation and Deflagration• The explosions from combustion are of two kinds:

Detonation Deflagration

• A deflagration is a very fast moving and hot fire that moves as heated materials ignite cold ones.

• A detonation is an even faster-moving fire that can also create a shock wave

• Detonation: Reaction front propagates above the sonic velocity.

• Deflagration: Reaction propagates at a speed less than the sonic velocity.

• A detonation generates greater pressures and is more destructive than a deflagration.

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Detonation and Deflagration

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Detonation and Deflagration

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Detonation and Deflagration

DETONATION

VAPOR CLOUD DEFLAGRATION

TIME

OVE

RPRE

SSU

RE

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Blast Damage from Overpressure

• The explosions (either detonation or deflagration) results in a reaction front moving outward from the ignition source preceded by a shock wave or pressure front.

• After the combustible material is consumed, the reaction front terminates, but the pressure wave continues its outward movement.

• A blast wave is composed of the pressure wave and subsequent wind.

• It is the blast wave that causes most of the damage.

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Blast Damage from Overpressure

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Blast Damage from Overpressure

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Some damage approximations based on overpressure

psig kpa Damage0.150.40.725-7

1.032.764.813.834.5-48.2

Glass breakageLimited minor structural damageMinor damage to house structurePartial collapse of walls and roofs of housesNearly complete destruction of houses

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Estimation of overpressure

• Experiments with explosive have demonstrated that the overpressure can be estimated using an equivalent mass of TNT (mTNT) and the distance from the ground-zero point of the explosion (r).

• The empirically derived scaling law is:

3/1TNTm

rze …………………………………(1.1)

Where ze is the scaled distance (m kg-1/3)

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You can get scaled overpressure, (Ps) if you know ze

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Estimation of overpressure

a

os P

PP …………………………………(1.2)

Where reoverpressu scaled sP

reoverpressu on-side peak oP

pressure ambient aP

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You can also use this equation

222

2

35.11

32.01

048.01

5.411616

eee

e

a

o

zzz

z

P

P…(1.3)

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Example

One kg of TNT is exploded. Compute the overpressure at a distance of 30 meter from the explosion

3/1TNTm

rze

Question

Solution

1/3-

3/1kg m 30

kg 1.0

m 30ez

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You can get scaled overpressure, (Ps) if you know ze

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Example

From the figure, scaled overpressure is 0.055

kPa 3.101055.0 oP

3.101055.0 oP

psi 0.81 or kPa 6.5oP

This overpressure will cause minor damage to house structure (slide no. 15)

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Using Probit to estimate the impact of explosion

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Probit equation

Where:

VkkY ln21

Y = Probit Variablek1 and k2 = constantsV = Causitive Variable

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Example

A blast produces a peak overpressure of 47,000 N/m2.

a) What fraction of structures will be damaged by exposure to this overpressure?

b) What fraction of people exposed will die as a result of lung hemorrhage?

c) What fraction will have eardrums ruptured?d) What conclusions about the effects of this blast can

be drawn?

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Solution

VkkY ln21 Probit Equation

Structural damage:

oPY ln 92.28.23

Death from lung hemorrhage:

oPY ln 91.61.77

Eardrum ruptures:

oPY ln 93.16.15

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Solution

2o N/m 000,47P For

Structural damage:

61.7Y

Death from lung hemorrhage:76.2Y

Eardrum ruptures:163.5Y

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Percent Affected

From probit-percentage conversion tablePercent Affected (%)

Structural Damage 99.6

Death (Lung Hem) 0 (Y is negative)

Eardrum ruptures 56

The blast is not serious enough to expect fatalities, but serious enough to cause extensive damage to surrounding structures and to rupture eardrums of more than half of the people exposed.

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Vapour Cloud Explosions

• The most dangerous and destructive explosions in the process industries.

• The explosions occur in a sequence of steps: Sudden release of a large quantity of flammable vapour (e.g.

a vessel containing a superheated and pressurised liquid ruptures).

Dispersion of the vapour throughout the plant site while mixing with air.

Ignition of the resulting vapour cloud.

• Any process containing quantities of liquefied gases, volatile superheated liquid, or high-pressure gases is considered a good candidate for a VCE.

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Vapour Cloud Explosions

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Vapour Cloud Explosions

Example: Flixborough, England (1974)

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http://www.hse.gov.uk/comah/sragtech/caseflixboroug74.htm

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http://www.hse.gov.uk/comah/sragtech/caseflixboroug74.htm

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V C EUNCONFINED

APOR

LOUD

XPLOSIONS

• An overpressure happens when a gas cloud detonates or deflagrates in open air rather than simply burns.

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What Happens to a Vapor Cloud?

• Cloud will spread from too rich, through flammable range to too lean.

• Edges start to burn through deflagration (steady state combustion).

• Cloud will disperse through natural convection.

• Flame velocity will increase with containment and turbulence.

• If velocity is high enough cloud will detonate.

• If cloud is small enough with little confinement it cannot explode.

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What Favors Hi Overpressures?

• Confinement

• Prevents escape, increases turbulence

• Cloud composition

• Unsaturated molecules – ‘all ethylene clouds explode’; low ignition energies; high flame speeds

• Good weather

• Stable atmospheres, low wind speeds

• Large Vapor Clouds

• Higher probability of finding ignition source; more likely to generate overpressure

• Source

• Flashing liquids; high pressures; large, low or downward facing leaks

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Vapour Cloud Explosions• Some of parameters that affect VCE behaviour:

• The type and quantity of material released and vaporised• The time span from the onset of the leakage until the ignition• The configuration of the space where the leakage took place.• Probability of ignition of the cloud (position & number of ignition

sources)• Distance traveled by the cloud before ignition• Efficiency of explosion

• Studies have shown that:• Ignition probability increases as the size of vapour cloud increases• The explosion efficiency is usually small (approx 2%)• (i)Turbulent mixing of vapour and air and (ii) ignition of the cloud

at a point remote from the release increases the explosion’s impact.

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TNT Equivalency Method• According to this method, the power of the vapor

cloud explosion equates to an equivalent mass of TNT (tri-nitrotoluene) that would produce the same explosive power.

TNT

c

E

HMW

mm

1

TNT

Equivalent mass of TNT

Empirical explosion efficiency

Mass of hydrocarbon

Energy of explosion/ Heat of combustion

The energy of explosion of the TNT = 4686 kJ/kg

Molecular weight of hydrocarbon

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TNT Equivalency Method• The explosion efficiency is one of the major problems

in the equivalency method.• The method calculates the overpressure of an

explosion without taking into consideration the space configuration where the explosion takes place (degree of confinement/congestion) – an explosion in the middle of an area full of equipment, or in a closed space, will exhibit different power from an equivalent one in an open space.

• The advantage of this method is that it is easy to apply because the calculations are simple.

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TNT Equivalency Method

• Procedure to estimate the damage associated with an explosion using the TNT equivalency method: Determine the total quantity of flammable material involved

in the explosion. Estimate the explosion efficiency, and calculate the

equivalent mass of TNT . Use the scaling law to estimate the peak side-on

overpressure. Estimate the damage for common structures and process

equipment.

NOTE: The procedure can be applied in reverse to estimate the quantity of material involved based on damage estimates.

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Example 1

One thousand kilograms of methane escapes from a storage vessel, mixes with air, and explodes.

a) Determine the equivalent amount of TNTb) Determine the side-on peak overpressure at a

distance of 50 m from the blast, and estimate the possible impact to the structure.

Assume an explosion efficiency of 2%.

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Solution to Example 1

a)

TNT kg 214)3.802)(016.0/1(100002.0

TNT

TNTE

m

b) 1/33/1

m/kg 4.8TNT

e m

rz

From the figure, Ps = 0.25

So, Po= 25.3 kPa

This overpressure will demolish steel panel buildings or ruptures oil storage tanks

a

os P

PP

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TNO Multi-Energy Method

• TNO is the Netherlands Organization for Applied Scientific Research.

• This method suggests that damaging explosion can only occur when flame acceleration takes place within a plant structure – truly unconfined explosions are unlikely to occur.

• The basis for this model: the energy of explosion depends highly on the level of congestion and depends less on the fuel in the cloud.

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TNO Multi-Energy Method

• Main factors affecting vapour cloud explosions in chemical plant structures:• Turbulence effects causing accelerating flame speed as the

flame passes obstacles in a plant structure, eventually giving explosive overpressure effects.

• Highly congested plant structures give high overpressures• Flammable gases with high laminar burning velocity give high

overpressures• The degree to which leaks of flammable gases fill a structure

– larger the volumes of structure give higher overpressures• Longer flame path lengths through the structure give higher

overpressures.

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TNO Multi-Energy Method

• Procedures :• Determine the charge combustion energy, E, where E is

determined by multiplying the confined volume occupied by a vapour cloud with the heat of combustion of a stoichiometric hydrocarbon-air mixture (3.5 x 106 J/m3).

• Estimate the blast strength (between 1 – 10).• Determine the Sachs-scaled distance using the following

equation:

3/1/ aPE

rR

Ambient pressure (Pa)

Distance from the charge (m)

The charge combustion energy (J)

Sachs-scaled distance

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TNO Multi-Energy Method

• Procedures :• The Sachs-scaled blast side-on overpressure is read from the

blast chart.• The overpressure is given by:

aso PPP

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Example 2

Consider the explosion of a propane-air vapour cloud confined beneath a storage tank. The tank is supported 1 m off the ground by concrete piles. The concentration of vapour in the cloud is assumed to be at stoichiometric concentrations. A cloud of 2094 m3 confined below the tank, representing the volume underneath the tank. Determine the overpressure from this vapour cloud explosion at a distance of 100 m from the blast using the TNO multi-energy method. Assume the blast strength of 7 for this explosion.

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Solution to Example 2

J 10 329.7)J/m 10 5.3)(m 2094( 9363 E

From the blast chart,

4.2P 325,101/J 107.329

m 100

/ 3/1

a93/1

aPE

rR

13.0sP

psi 1.9 kPa 2.13kPa 3.10113.0 aso PPP

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BLEVE

• BLEVE, pronounced /ˈblɛvi/ ("blevvy"), is an acronym for “Boiling Liquid Expanding Vapour Explosion".

• It is the result of a liquid within a container reaching a temperature well above its boiling point at atmospheric temperature, causing the vessel to rupture into two or more pieces.

• A BLEVE can occur when fire impinges on the tank shell at a point or points above the liquid level of the contents of the tank.

• This impingement causes the metal to weaken and fail from the internal pressure.

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BLEVE

• BLEVEs can also be caused by an external fire near the storage vessel causing heating of the contents and pressure build-up.

• Such explosions can be extremely hazardous.• BLEVEs can result from mechanical damage to a tank,

as well. • This damage can be the result of a train derailment,

traffic accident, or other physical shock. • When a BLEVE occurs, debris may travel hundreds of

feet, with tremendous force, and the escaping fuel can ignite causing an expanding fireball.

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BLEVE

• If the vessel is ruptured — the vapour portion may rapidly leak, lowering the pressure inside the container and releasing a wave of overpressure from the point of rupture.

• This sudden drop in pressure inside the container causes violent boiling of the liquid, which rapidly liberates large amounts of vapour in the process.

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BLEVE

• The pressure of this vapour can be extremely high, causing a second, much more significant wave of overpressure (an explosion) which may completely destroy the storage vessel and project fragments over the surrounding area.

• If the substance involved is flammable, it is likely that the resulting cloud of the substance will ignite after the BLEVE has occurred, forming a fireball and possibly a fuel-air explosion, also termed a vapour cloud explosion (VCE).

• If the materials are toxic, a large area will be contaminated.

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BLEVE

• The most common type of BLEVE is caused by fire.• The steps are as follows:

• A fire develops adjacent to a tank containing a liquid.• The fire heats the walls of the tank.• The liquid-filled portion stays cool due to heat-sink effects,

but the steel around the vapor space rapidly heats up. • If the flames reach the tank walls or roof where there is only

vapour and no liquid to remove the heat, the tank metal temperature rises until the tank loses its structural strength.

• The tank ruptures, explosively vapourising its contents.

• Often, the boiling and burning of liquid behaves as a rocket fuel, propelling vessel parts for great distances.

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BLEVE

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Summary• Blast wave• Shock wave• Explosion injury• Explosion behavior parameters• Detonation and deflagration• Overpressure• Blast damage from overpressure

• Usage of probit to estimate impact of explosion• VCE

• TNT Equivalency Method• TNO Multi-Energy Method

• BLEVE