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Transcript of Class Lecture
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FIRESEXPLOSIONS
AND
FUNDAMENTALS and DESIGN CONSIDERATIONS
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The Fire Triangle Fuels:
– Liquids gasoline, acetone,
ether, pentane– Solids
plastics, wood dust, fibers, metal particles
– Gases acetylene, propane,
carbon monoxide, hydrogen
AIR
(OX
YG
EN
)FU
EL
Oxidizers– Liquids– Gases
Oxygen, fluorine, chlorine
hydrogen peroxide, nitric acid, perchloric acid
– Solids Metal peroxides,
ammonium nitrate Ignition sources
Sparks, flames, static electricity, heat
IGNITION SOURCE
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Flash Point– Lowest temperature at which a flammable
liquid gives off enough vapor to form an ignitable mixture with air
Flammable Liquids– Liquids with a flash point < 100°F
Combustible Liquids– Liquids with a flash point ³ 100°F
Liquid Fuels – Definitions
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Flammable / Explosive Limits– Range of composition of material in air
which will burn UFL – Upper Flammable Limit LFL – Lower Flammable Limit HEL – Higher Explosive Limit LEL – Lower Explosive Limit
Vapor Mixtures – Definitions
SAME
SAME
Measuring These Limits for Vapor-Air Mixtures– Known concentrations are placed in a closed
vessel apparatus and then ignition is attempted
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Flammability Relationships
UPPER LIMIT
LOWER LIMIT
VAP
OR
PR
ES
SU
RE
AUTOIGNITION
AIT
MISTFLAMMABLE REGION
TEMPERATURE
CO
NC
EN
TR
AT
ION
OF
FU
EL
FLASH POINT
FLAMMABLE REGION
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Flash Point From Vapor Pressure
Most materials start to burn at 50% stoichiometric For heptane:
– C7H16 + 11 O2 = 7 CO2 + 8 H2O
– Air = 11/ 0.21 = 52.38 moles air /mole of C7H16 at stoichiometric conditions
– At 50% stoichiometric, C7H16 vol. % @ 0.9%
– Experimental is 1.1%
– For 1 vol. %, vapor pressure is 1 kPa temperature = 23o F
– Experimental flash point temperature = 25o F
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Flammability Diagram
1 Atmosphere25°C
FLAMMABLEMIXTURES HEL
LEL
LOC
Limiting O2 Concentration:Vol. % O2 below which combustion can’t occur
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Flammability Diagram
1 Atmosphere25°C
FLAMMABLEMIXTURES
HEL
LEL
LOC
Limiting O2 Concentration:Vol. % O2 below which combustion can’t occur
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Flammable Limits Change With:
Inerts
Temperature
Pressure
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Effect of Temperature onLower Limits of Flammability
LEL,%
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Effect of Pressure of Flammability
Initial Pressure, Atm.
Nat
ura
l G
as,
volu
me%
Natural Gas In Air at 28oC
HEL
LEL
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Minimum Ignition Energy
Lowest amount of energy required for ignition– Major variable
– Dependent on: Temperature % of combustible in combustant Type of compound
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Minimum Ignition Energy
Effects of Stoichiometry
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Autoignition Temperature
Temperature at which the vapor ignites spontaneously from the energy of the environment
Function of:
– Concentration of the vapor
– Material in contact
– Size of the containment
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Flammability Relationships
UPPER LIMIT
LOWER LIMIT
VAP
OR
PR
ES
SU
RE
AIT
MISTFLAMMABLE REGION
TEMPERATURE
CO
NC
EN
TR
AT
ION
OF
FU
EL
FLASH POINT
FLAMMABLE REGION
AUTOIGNITION
AIT
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Material Variation AutoignitionTemperature
Pentane in air 1.50%3.75%7.65%
1018 °F936 °F889 °F
Benzene Iron flaskQuartz flask
1252 °F1060 °F
Carbon disulfide 200 ml flask1000 ml flask
10000 ml flask
248 °F230 °F205 °F
Autoignition Temperature
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Autoignition Temperature
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The process of slow oxidation with accompanying evolution of heat, sometimes leading to autoignition if the energy is not removed from the system
Liquids with relatively low volatility are particularly susceptible to this problem
Liquids with high volatility are less susceptible to autoignition because they self-cool as a result of evaporation
Known as spontaneous combustion when a fire results; e.g., oily rags in warm rooms; land fill fires
Auto-Oxidation
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Fuel and air will ignite if the vapors are compressed to an adiabatic temperature that exceeds the autoignition temperature
Adiabatic Compression Ignition (ACI)
Diesel engines operate on this principle; pre-ignition knocking in gasoline engines
E.g., flammable vapors sucked into compressors; aluminum portable oxygen system fires
Adiabatic Compression
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Ignition Sources of Major FiresSource Percent of AccidentsElectrical 23
Smoking 18
Friction 10
Overheated Materials 8
Hot Surfaces 7
Burner Flames 7
…
Cutting, Welding, Mech. Sparks 6
…
Static Sparks 1
All Other 20
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More Definitions Fire
– A slow form of deflagration
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.
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.
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Classification of Explosions
EXPLOSION =Rapid Equilibration of High Pressure Gas via Shock Wave
Physical Explosions Chemical Explosions
Propagating ReactionsUniform Reactions
Thermal Explosions
Deflagrations(Normal
Transport)
Detonations(Shock Wave)
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Potential Energy
PRESSURE, psig TNT EQUIV., lbs. per ft3
10100
100010000
0.0010.021.426.53
TNT equivalent = 5 x 105 calories/lbm
Stored Volumes of Ideal Gas at 20° C
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Deflagration Combustion with flame speeds at non-
turbulent velocities of 0.5 - 1 m/sec. 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)
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Detonation
Highly turbulent combustion Very high flame speeds Extremely high pressures >>10 bars
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Pressure vs Time Characteristics
DETONATION
VAPOR CLOUD DEFLAGRATION
TIME
OV
ER
PR
ES
SU
RE
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CONSEQUENCES
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Bayway, NJH-Oil Incident 1970
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Two Special Cases
Vapor Cloud Explosion
Boiling Liquid /Expanding Vapor Explosion
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V C EUNCONFINED
APOR
LOUD
XPLOSIONS
An overpressure caused 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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Impact of VCEs on People
70160290
470670
940
12 5
10 1520 30 355065
PeakOverpressure
psi
EquivalentWind Velocity
mph
Knock personnel down
Rupture eardrums
Damage lungs
Threshold fatalities50% fatalities99% fatalities
Effects
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Impact 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
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Vapor Clouds and TNT
World of explosives is dominated by TNT impact which is understood.
Vapor clouds, by analysis of incidents, seem to respond like TNT if we can determine the equivalent TNT.
1 pound of TNT has a LHV of 1890 BTU/lb.
1 pound of hydrocarbon has a LHV of about 19000 BTU/lb.
A vapor cloud with a 10% efficiency will respond like a similar weight of TNT.
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Multi-Energy Models Experts plotted efficiency against vapor cloud
size and … reached no effective conclusions. Efficiencies were between 0.1% and 50%
Recent developments in science suggest too many unknowns for simple TNT model.
Key variables to overpressure effect are:– Quantity of combustant in explosion
– Congestion/confinement for escape of combustion products
– Number of serial explosions
Multi-energy model is consistent with models and pilot explosions.
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The result of a vessel failure in a fire and release of a pressurized liquid rapidly into the fire
A pressure wave, a fire ball, vessel fragments and burning liquid droplets are usually the result
B L E VOILING
IQUID
XPANDING
XPLOSIONS
EAPOR
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BLEVE
FUELSOURCE
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Distance Comparison
125
102050
100200500
1000
INVENTORY(tons)
18366090
130200280400600820
BLEVE
1201502002503104205306709001150
UVCE
2030365060
100130
FIRE
Distancein Meters
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DESIGN for PREVENTION
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Eliminate Ignition Sources
Typical Control– Spacing and Layout– Spacing and Layout– Work Procedures– Work Procedures– Sewer Design, Diking,
Weed Control, Housekeeping
– Procedures
Fire or Flames– Furnaces and Boilers– Flares– Welding– Sparks from Tools– Spread from Other Areas jkdj
dkdjfdk dkdfjdkkd jkfdkd fkd fjkd fjdkkf djkfdkf jkdkf dkf
– Matches and Lighters
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Eliminate Ignition Sources
Hot Surfaces– Hot Pipes and Equipment– Automotive Equipment
Typical Control– Area Classification– Grounding, Inerting,
Relaxation– Geometry, Snuffing– Procedures
Electrical– Sparks from Switches– Static Sparks jkfdkd fjkdjd
kdjfdkd– Lightning– Handheld Electrical
Equipment
Typical Control– Spacing– Procedures
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Inerting – Vacuum Purging Most common procedure for inerting
reactors Steps
1. Draw a vacuum2. Relieve the vacuum with an inert gas3. Repeat Steps 1 and 2 until the desired oxidant
level is reached Oxidant Concentration after j cycles:
where PL is vacuum level
PH is inert pressure
jPP
oyjyH
L )(
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Inerting – Pressure Purging Most common procedure for inerting
reactors Steps
1. Add inert gas under pressure2. Vent down to atmospheric pressure3. Repeat Steps 1 and 2 until the desired oxidant
level is reached Oxidant Concentration after j cycles:
where nL is atmospheric moles
nH is pressure moles
jnn
oyjy HL )(
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Vacuum? Pressure? Which? Pressure purging is faster because
pressure differentials are greater (+PP)
Vacuum purging uses less inert gas than pressure purging (+VP)
Combining the two gains benefits of both especially if the initial cycle is a vacuum cycle (+ VP&PP)
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Other Methods of Inerting
Sweep-Through Purging– ‘In one end, and out the other’– For equipment not rated for pressure, vacuum– Requires large quantities of inert gas
Siphon Purging– Fill vessel with a compatible liquid– Use Sweep-Through on small vapor space– Add inert purge gas as vessel is drained– Very efficient for large storage vessels
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1 Atmos.25°C
FLAMMABLEMIXTURES
Using the FlammabilityDiagram
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Static Electricity Sparks resulting from static charge buildup
(involving at least one poor conductor) and sudden discharge
Household Example: walking across a rug and grabbing a door knob
Industrial Example: Pumping nonconductive liquid through a pipe then subsequent grounding of the container
Dangerous energy near flammable vapors 0.1 mJ
Static buildup by walking across carpet 20 mJ
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Double-Layer Charging Streaming Current
– The flow of electricity produced by transferring electrons from one surface to another by a flowing fluid or solid
– The larger the pipe / the faster the flow, the larger the current
Relaxation Time– The time for a charge to dissipate by leakage– The lower the conductivity / the higher the
dielectric constant, the longer the time
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ControllingStatic Electricity Reduce rate of charge generation
– Reduce flow rates
Increase the rate of charge relaxation– Relaxation tanks after filters, enlarged section of
pipe before entering tanks
Use bonding and grounding to prevent discharge
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ControllingStatic Electricity
GROUNDING
BONDING
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Static Electricity – Real Life
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Explosion Proof Equipment
All electrical devices are inherent ignition sources
If flammable materials might be present at times in an area, it is designated XP (Explosion Proof Required)
Explosion-proof housing (or intrinsically-safe equipment) is required
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Area Classification National
Electrical Code (NEC) defines area classifications as a function of the nature and degree of process hazards present
Class I Flammable gases/vapors present
Class II Combustible dusts present
Class III Combustible dusts present but not likely in suspension
Group A Acetylene
Group B Hydrogen, ethylene
Group C CO, H2S
Group D Butane, ethane
Division 1 Flammable concentrations normally present
Division 2 Flammable materials are normally in closed systems
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VENTILATION
Open-Air Plants– Average wind velocities are often high enough to
safely dilute volatile chemical leaks
Plants Inside Buildings– Local ventilation
Purge boxes ‘Elephant trunks’
– Dilution ventilation ( ³ 1 ft3/min/ft2 of floor area) When many small points of possible leaks exist
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Summary
Though they can often be reduced in magnitude or even sometimes designed out, many of the hazards that can lead to fires/explosions are unavoidable
Eliminating at least one side of the Fire Triangle represents the best chance for avoiding fires and explosions
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END of PRESENTATION