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