Daejun Chang- Fire and Explosion: Basic Phenomena

Korea Advanced Institute ofScience and Technology

OSE551 Reliability and Risk Analysis for Offshore Plants

Daejun CHANG ([email protected])

Division of Ocean Systems Engineering

Fire and Explosion- Basic Phenomena

-1- Ocean Systems EngineeringProf. Daejun CHANG

Fire (Explosion) TriangleFire (Explosion) Triangle

Fuel

Ignition source

Air (oxygen)

Fire/Explosion

Sparks, flames, static electricity, heat


Fire (Explosion) TrianglesFire (Explosion) Triangles

Fuel

Ignition source

Air (oxygen)

Fire/Explosion

Fuel

Ignition source

Air (oxygen)

Fire or explosion Safe if one of the three is missing.


Properties of Methane (CH4)Properties of Methane (CH4)

Ø The MSDS for methane will give the information:

Item Unit ValueAppearance colorless odorless gasExplosion limits

LELUEL

% 5 – 155

15

Flash point ℃ -221Melting point ℃ -182Boiling point ℃ -164Auto-ignition temperature ℃ 537Density at 20℃ g/l 0.717 (Note 1.2 g/l for air)Water solubility at 20℃ slight (35 ml/l)


Liquid Fuel ClassificationLiquid Fuel Classification

Ø Flash Point§ The lowest temperature at which a flammable liquid gives off

enough vapor to form an ignitable mixture with air

Ø Fire Point§ The lowest temperature at which it will continue to burn

after ignition for at least 5 seconds.

Ø Flammable Liquids (NFPA)§ Liquids with a flash point < 100°F (38 °C)

Ø Combustible Liquids (NFPA)§ Liquids with a flash point ³ 100°F (38 °C)


Vapor Mixtures Vapor Mixtures –– DefinitionsDefinitions

Ø Flammable / Explosive Limits§ Range of composition of material in air which will burn

- UFL – Upper Flammable Limit- LFL – Lower Flammable Limit- UEL – Upper Explosive Limit = UFL- LEL – Lower Explosive Limit = LFL

Ø Measuring these limits for vapor-air mixtures– Known concentrations are placed in a closed vessel

apparatus and then ignition is attempted.


jet flowFlammableliquid

leak orifice

Flammable Vapor FractionFlammable Vapor Fraction

erature with tempincreasing :f(T)P

bar) /(1P/PP y

vapor

vaportotalvaporvapor

=

==

Ø Consider a liquid jet from a pressure vessel

§ The jet will be atomized into small mist particles.

§ Then, the fine particles will vaporized due to high surface area

§ The vapor fraction is proportional to the vapor pressure, which is a

function increasing w.r.t. temperature.


Flammability RelationshipsFlammability Relationships

Temperature

Con

c. o

f fla

mm

able

vap

orLiquid (Mist) Vapor


Flash Point From Vapor PressureFlash Point From Vapor Pressure

Ø Stoichiometric composition for perfect combustion with no reactants (including O2) left

Ø Most materials start to burn at 50% stoichiometric

Ø For heptane:§ C77H16 + 11 O2 = 7 CO2 + 8 H2O§ Air = 11/ 0.21 = 52.38 moles air /mole of C77H16 at stoichiometric

conditions§ At 50% stoichiometric, C7H16 vol. % @ 0.9%

(Experimental is 1.1%)§ 1 vol. % = vapor pressure 1 kPa = liquid temperature -5 oC

(Experimental flash point temperature = -3.9 oC)


Flammable Limits Change With:Flammable Limits Change With:

Ø Fire and explosion are a kind of chain reaction.§ A fuel molecule forms a radical.§ It reacts with oxygen to generate more radicals.§ In this way the reaction propagate.

Ø The chain reaction is affected by§ Temperature: thermal energy level§ Pressure: distance between molecules§ Inert gas: barrier to propagation or radical absorber


Effect of Temperature on Lower Limits of FlammabilityEffect of Temperature on Lower Limits of Flammability

LEL,%

Ø LEL decreases with temperature.è The gas becomes easier to burn

due to increased thermal energy level.


Effect of Pressure of FlammabilityEffect of Pressure of Flammability

Initial Pressure, Atm.

Nat

ural

Gas

, vol

ume%

Natural gas in air at 28oC

UEL

LEL

Ø LEL decreases with pressure.è The gas becomes easier to burn

due to decreased molecular distance.



Temperature

Flashingpoint

Con

c. o

f fla

mm

able

vap

or

Autoignitiontemperature (AIT)

Flammable region

Liquid (Mist) Vapor

Unflammable region

Unflammable region


Minimum Ignition EnergyMinimum Ignition Energy

Ø Lowest amount of energy required for ignition

§ Measured by the spark energy that causes ignition

§ Dependent on:- Temperature- % of combustible - Type of compound

§ Far smaller than usual energy sources like - sparks from rotating machines- sparks caused by dropped objects- Electrostatic sparks caused by operators


Minimum Ignition EnergyMinimum Ignition Energy

Effects of Stoichiometry


Autoignition TemperatureAutoignition Temperature

Ø Temperature at which the vapor ignites spontaneously from the energy of the environment

Ø Function of:§ Concentration of the vapor§ Material in contact§ Size of the containment



Temperature

Flashingpoint

Con

c. o

f fla

mm

able

vap

or

Autoignitiontemperature (AIT)

Autoignition regionFlammable region

Liquid (Mist) Vapor

Unflammable region

Unflammable region



Material Variation AutoignitionTemperature, oC

Pentane in air 1.50%3.75%7.65%

548502476

Benzene Iron flaskQuartz flask

678571

Carbon disulfide 200 ml flask1000 ml flask10000 ml flask

12011096




Adiabatic CompressionAdiabatic Compression

Ø Fuel and air will ignite if the vapors are compressed to an adiabatic temperature that exceeds the autoignition temperature

Ø Adiabatic Compression Ignition (ACI)

Ø Diesel engines operate on this principle; pre-ignition knocking in gasoline engines

Ø E.g., flammable vapors sucked into compressors; aluminum portable oxygen system fires


Ignition Sources of Major FiresIgnition Sources of Major Fires

Source Percent of AccidentsElectrical 23Smoking 18Friction 10Overheated Materials 8Hot Surfaces 7Burner Flames 7…Cutting, Welding, Mech. Sparks 6

…Static Sparks 1All Other 20


More DefinitionsMore Definitions

Ø Fire§ A slow form of deflagration

Ø Deflagration§ Propagating reactions in which the energy transfer from the

reaction zone to the unreacted zone is accomplished thru ordinary transport processes such as heat and mass transfer.

Ø Detonation / Explosion§ Propagating reactions in which energy is transferred from the

reaction zone to the unreacted zone on a reactive shock wave. The velocity of the shock wave always exceeds sonic velocity in the reactant.


DeflagrationDeflagration

Ø Combustion with flame speeds at non-turbulent velocities of 0.5 -1 m/sec.

Ø Pressures rise by heat balance in fixed volume with pressure ratio of about 10.

CH4 + 2 O2 = CO2 + 2 H2O + 21000 BTU/lbInitial Mols = 1 + 2/.21 = 10.52Final Mols = 1 + 2 + 2(0.79/0.21) = 10.52Initial Temp = 298oKFinal Temp = 2500oKPressure Ratio = 9.7Initial Pressure = 1 bar (abs)Final Pressure = 9.7 bar (abs)


DetonationDetonation

Ø Highly turbulent combustionØ Very high flame speedsØ Extremely high pressures >>10 bars


Pressure vs. Time CharacteristicsPressure vs. Time Characteristics

DETONATION

VAPOR CLOUD DEFLAGRATION

TIME

OV

ER

PRE

SSU

RE


Two Special CasesTwo Special Cases

Ø Vapor Cloud Explosion

Ø Boiling Liquid /Expanding Vapor Explosion


UVCEUVCE

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

Unconfined

Vapor

Cloud

Explosion


What Happens to a Vapor Cloud?What Happens to a Vapor Cloud?

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

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

Ø Cloud will disperse through natural convection.

Ø Flame velocity will increase with containment and turbulence.

Ø If velocity is high enough, the cloud will detonate.

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


What favors high overpressures?What favors high overpressures?

Ø Confinement

§ Prevents escape, increases turbulence

Ø Cloud composition

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

Ø Good weather

§ Stable atmospheres, low wind speeds

Ø Large Vapor Clouds

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

Ø Source

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


Impact of VCEs on PeopleImpact of VCEs on People

70160290

470670

940

12 5

10 1520 30 355065

PeakOverpressure

psi

EquivalentWind Velocity

mphKnock personnel down

Rupture eardrums

Damage lungs

Threshold fatalities50% fatalities99% fatalities

Effects


Impact of VCEs on FacilitiesImpact of VCEs on Facilities

0.5-to-11-to-2

2-to-33-to-4

57

7-8

PeakOverpressure

psi Glass windows breakCommon siding types fail:

- corrugated asbestos shatters- corrugated steel panel joints fail- wood siding blows in

Unreinforced concrete, cinder block walls failSelf-framed steel panel buildings collapseOil storage tanks ruptureUtility poles snapLoaded rail cars overturnUnreinforced brick walls fail

Typical Damage


Vapor Clouds and TNTVapor Clouds and TNT

Ø World of explosives is dominated by TNT impact which is understood.

Ø Vapor clouds, by analysis of incidents, seem to respond like TNT if we can determine the equivalent TNT.

Ø 1 pound of TNT has a LHV of 1890 BTU/lb.

Ø 1 pound of hydrocarbon has a LHV of about 19000 BTU/lb.

Ø A vapor cloud with a 10% efficiency will respond like a similar weight of TNT.

è It is a matter of reaction speed, not the energy density.


Ø The result of a vessel failure in a fire and release of a pressurized liquid rapidly into the fire

Ø A pressure wave, a fire ball, vessel fragments and burning liquid droplets are usually the result

BLEVEBLEVE

Boiling

Liquid

Expanding

Vapor

Explosion


BLEVEBLEVE


Liquid Fuel ClassificationLiquid Fuel Classification

FUELSOURCE

Check the Y-tube.

Documents

Daejun Chang- Fire and Explosion: Basic Phenomena