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Transcript of PUIS-Fires and Explosions
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FIRES
EXPLOSIONS
AND
FUNDAMENTALS and DESIGNCONSIDERATIONS
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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
Oxidizers Liquids
Gases
Oxygen,
fluorine, chlorine hydrogen
peroxide, nitricacid, perchloricacid
Solids
Metal peroxides,ammoniumnitrate Ignition sources
Sparks, flames, static
electricity, heat
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Combustion or fire:
Combustion or fire is a chemical reaction in which a
substance combines with an oxidant and releases
energy. Part of the energy released is used to
sustain the reaction.
Ignition:
Ignition of a flammable mixture may be caused by a
flammable mixture coming in contact with a source
of ignition with sufficient energy or the gas reaching
a temperature high enough to cause the gas to
autoignite.
Definitions
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Autoignition temperature (AIT):
A fixed temperature above which adequate energy is
available in the environment to provide an ignition
source.
Flash point (FP):
The flash point of a liquid is the lowest temperature
at which it gives off enough vapor to form an
ignitable mixture with air. At the flash point the vapor
will burn but only briefly; inadequate vapor is
produced to maintain combustion. The flash pointgenerally increases with increasing pressure.
Definitions
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Firepoint:
The fire point is the lowest temperature at which a
vapor above a liquid will continue to burn once
ignited; the fire point temperature is higher than the
flash point.
Flammability limits:
Vapor-air mixtures will ignite and burn only over a
well-specified range of compositions. The mixture
will not burn when the composition is lower than the
lower flammable limit (LFL); the mixture is too leanfor combustion. The mixture is also not combustible
when the composition is too rich; that is, when it is
above the upper flammable limit (UFL).
Definitions
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A mixture is flammable only when the composition is
between the LFL and the UFL. Commonly used units
are volume percent fuel (percentage of fuel plus air).
Lower explosion limit (LEL) and upper explosion limit
(UEL) are used interchangeably with LFL and UFL.
Explosion:
An explosion is a rapid expansion of gases resulting
in a rapidly moving pressure or shock wave. The
expansion can be mechanical (by means of a
sudden rupture of a pressurized vessel), or it can bethe result of a rapid chemical reaction. Explosion
damage is caused by the pressure or shock wave.
Definitions
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Mechanical explosion:
An explosion resulting from the sudden failure of a
vessel containing high-pressure nonreactive gas.
Deflagration:
An explosion in which the reaction front moves at aspeed less than the speed of sound in the un-
reacted medium.
Detonation:
An explosion in which the reaction front moves at aspeed greater than the speed of sound in the
unreacted medium.
Definitions
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Confined explosion: An explosion occurring within a vessel or a building.
These are most common and usually result in injury
to the building inhabitants and extensive damage.
Unconfined explosion: Unconfined explosions occur in the open. This type
of explosion is usually the result of a flammable gas
spill.
Definitions
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Boiling-liquid expanding-vapor explosion (BLEVE):
BLEVE occurs if a vessel that contains a liquid at a
temperature above its atmospheric pressure boiling
point ruptures. The subsequent BLEVE is the
explosive vaporization of a large fraction of the
vessel contents; possibly followed by combustion orexplosion of the vaporized cloud if it is combustible.
Dust explosion:
This explosion results from the rapid combustion of
fine solid particles. Many solid materials (includingcommon metals such as iron and aluminum)
become flammable when reduced to a fine powder.
Definitions
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Shock wave:
An abrupt pressure wave moving through a gas. A
shock wave in open air is followed by a strong wind;
the combined shock wave and wind is called a blast
wave. The pressure increase in the shock wave is so
rapid that the process is mostly adiabatic.
Overpressure:
The pressure on an object as a result of an
impacting shock wave.
Definitions
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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 (NFPA)
Liquids with a flash point < 100F
Combustible Liquids (NFPA)
Liquids with a flash point 100F
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
AUTO
IGNITION
AIT
MISTFLAMMABLE REGION
TEMPERATURE
CONCENTRATIONOFFUEL
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 ofC7H16 at stoichiometric conditions
At 50% stoichiometric, C7H16 vol. % @ 0.9% Experimental is 1.1%
For 1 vol. %, vapor pressure is 1 kPatemperature = 23o F
Experimental flash point temperature = 25o F
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Flammability
Diagram1 Atmosphere
25C
FLAMMABLE
MIXTURES
HEL
LEL
LOC
Limiting O2
Concentration:
Vol. % O2 below
which combustioncant occur
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Flammability
Diagram1 Atmosphere
25C
FLAMMABLE
MIXTURES
HEL
LEL
LOC
Limiting O2
Concentration:
Vol. % O2 below
which combustioncant occur
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Flammable Limits Change
With:
Inerts
Temperature
Pressure
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Effect of Temperature on
Lower Flammability Limit (LFL)
L
E
L,
%
LELLower
Explosion
Limit
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Effect of Pressure of
Flammability
Initial Pressure, Atm.
NaturalGas,volum
e%
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 combustantType of compound
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Minimum Ignition Energy
Effects of Stoichiometry
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Autoignition Temperature
Temperature at which the vapor ignitesspontaneously from the energy of theenvironment
Function of:
Concentration of the vapor
Material in contact Size of the containment
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Flammability Relationships
AIT
MISTFLAMMABLE REGION
TEMPERATURE
CONCENTRATIONOF
FUEL
FLASH POINT
FLAMMABLE REGION
AUTO
IGNITION
AIT
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Material Variation Autoignition
Temperature
Pentane in air 1.50%
3.75%
7.65%
1018 F
936 F
889 F
Benzene Iron flask
Quartz flask
1252 F
1060 F
Carbon disulfide 200 ml flask1000 ml flask
10000 ml flask
248 F230 F
205 F
Autoignition Temperature
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Autoignition Temperature
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The process of slow oxidation with accompanyingevolution 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 arecompressed to an adiabatic temperaturethat 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 intocompressors; aluminum portable oxygensystem fires
Adiabatic Compression
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Ignition Sources of Major Fires
Source Percent of AccidentsElectrical 23Smoking 18
Friction 10
Overheated Materials 8Hot 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 ofdeflagration
Deflagration
Propagating reactions in which the energy transferfrom the reaction zone to the unreacted zone isaccomplished thru ordinary transport processessuch as heat and mass transfer.
Detonation / Explosion
Propagating reactions in which energy is transferredfrom the reaction zone to the unreacted zone on areactive shock wave. The velocity of the shockwave always exceeds sonic velocity in the reactant.
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Classification of Explosions
EXPLOSION = Rapid Equilibration of High PressureGas via Shock Wave
Physical Explosions Chemical Explosions
Propagating ReactionsUniform Reactions
ThermalExplosions
Deflagrations(Normal
Transport)
Detonations(Shock Wave)
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Potential Energy
PRESSURE, psig TNT EQUIV., lbs. per ft3
10
100
1000
10000
0.001
0.02
1.42
6.53
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 fixedvolume with pressure ratio of about 10.
CH4 + 2 O2 = CO2 + 2 H2O + 21000 BTU/lb
Initial Mols = 1 + 2/.21 = 10.52
Final Mols = 1 + 2 + 2(0.79/0.21) = 10.52
Initial Temp = 298oK
Final Temp = 2500oKPressure Ratio = 9.7
Initial 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
CharacteristicsDETONATION
VAPOR CLOUD DEFLAGRATION
TIME
OVERPRESSURE
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Two Special Cases
Vapor Cloud Explosion
Boiling Liquid /Expanding VaporExplosion
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V C EU
N
C
O
N
F
IN
E
D
A
P
O
R
L
O
U
D
X
P
L
O
S
IO
N
S
An overpressure caused when a gas clouddetonates or deflagrates in open air rather thansimply burns.
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What Happens to a Vapor Cloud?
Cloud will spread from too rich, through flammablerange to too lean.
Edges start to burn through deflagration (steady statecombustion).
Cloud will disperse through natural convection.
Flame velocity will increase with containment andturbulence.
If velocity is high enough cloud will detonate.
If cloud is small enough with little confinement it cannotexplode.
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What Favors Hi Overpressures?
Confinement Prevents escape,
increases turbulence
Cloud composition
Unsaturated moleculesall ethylene cloudsexplode; low ignitionenergies; high flamespeeds
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
70
160290
470
670
940
1
2
510
15
20
3035
50
65
Peak
Overpressurepsi
Equivalent
Wind Velocitymph
Knock personnel down
Rupture eardrums
Damage lungs
Threshold fatalities
50% fatalities
99% fatalities
Effects
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Impact of VCEs on Facilities
0.5-to-1
1-to-2
2-to-3
3-to-4
57
7-8
Peak
Overpressurepsi
Glass windows break
Common siding types fail:
- corrugated asbestos shatters- corrugated steel panel joints fail
- wood siding blows in
Unreinforced concrete, cinder block walls fail
Self-framed steel panel buildings collapse
Oil storage tanks rupture
Utility poles snapLoaded rail cars overturn
Unreinforced brick walls fail
Typical Damage
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Vapor Clouds and TNT
World of explosives is dominated by TNT impactwhich is understood.
Vapor clouds, by analysis of incidents, seem torespond 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 respondlike 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 toomany unknowns for simple TNT model.
Key variables to overpressure effect are:
Quantity of combustant in explosion
Congestion/confinement for escape of combustionproducts
Number of serial explosions
Multi-energy model is consistent with modelsand pilot explosions.
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The result of a vessel failure in a fire andrelease of a pressurized liquid rapidly into
the fire A pressure wave, a fire ball, vessel
fragments and burning liquid droplets areusually the result
B L E VO
I
L
I
N
G
I
Q
U
I
D
X
P
A
N
D
I
NG
X
P
L
O
S
I
ON
S
EA
P
O
R
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BLEVE
FUEL
SOURCE
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Distance Comparison
1
2
510
20
50
100
200
500
1000
INVENTORY
(tons)
18
36
6090
130
200
280
400
600
820
BLEVE
120
150
200250
310
420
530
670
900
1150
UVCE
20
30
36
50
60
100
130
FIRE
Distance
in 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 Areasjkdj
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 Sparksjkfdkd fjkdjd
kdjfdkd
Lightning Handheld Electrical
Equipment
Typical Control Spacing
Procedures
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Inerting Vacuum Purging Most common procedure for inerting
reactors
Steps1. Draw a vacuum
2. Relieve the vacuum with an inert gas
3. Repeat Steps 1 and 2 until the desired oxidantlevel is reached
Oxidant Concentration after j cycles:
where PL is vacuum level
PH is inert pressure
jP
Poyj
yH
L )(
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Inerting Pressure Purging Most common procedure for inerting
reactors
Steps1. Add inert gas under pressure
2. Vent down to atmospheric pressure
3. Repeat Steps 1 and 2 until the desired oxidantlevel is reached
Oxidant Concentration after j cycles:
where nL is atmospheric moles
nH is pressure moles
jnn
oyjy
H
L )(
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Vacuum? Pressure? Which?
Pressure purging is faster becausepressure differentials are greater (+PP)
Vacuum purging uses less inert gas thanpressure purging (+VP)
Combining the two gains benefits of both
especially if the initial cycle is a vacuumcycle (+ 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.
25C
FLAMMABLE
MIXTURES
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 aflowing fluid or solid
The larger the pipe / the faster the flow, thelarger the current
Relaxation Time The time for a charge to dissipate by leakage
The lower the conductivity / the higher thedielectric constant, the longer the time
Controlling
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Controlling
Static 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
Controlling
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Controlling
Static ElectricityGROUNDING
BONDING
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Static Electricity Real Life
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Explosion Proof Equipment
All electrical devices are inherent ignitionsources
If flammable materials might be present attimes in an area, it is designated XP(Explosion Proof Required)
Explosion-proof housing (or intrinsically-safeequipment) is required
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Area Classification
NationalElectrical
Code (NEC)
defines area
classifications
as a function
of the nature
and degree of
process
hazardspresent
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 concentrationsnormally 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 (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 inmagnitude or even sometimesdesigned out, many of the hazards
that can lead to fires/explosions areunavoidable
Eliminating at least one side of theFire Triangle represents the bestchance for avoiding fires andexplosions
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END of PRESENTATION