Fires and Explosions

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Fires and Explosions
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Fires and Explosions

• Definitions
• Flammability
• Flash Points
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Introduction

• We have been talking about source models for the
  release of materials and about dispersion models
  if the material is a toxicant.
• Another concern is a release of flammable
  materials where we need to worry about fires and
  explosions.

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Fire Triangle

• Most are familiar with the Fire Triangle.
• In order for a fire to start or be sustained you
  need to have a Fuel, an oxidizer and an ignition
  source.
• If one of the three components is eliminated,
  then there will not be a fire (or explosion)

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Fuel

• Fuel must be present in certain concentrations.
• Typical cases where fuel occur are if there is a
  leak, during filling operations, transfer
  operations, or excessive dusts.
• Although we often cannot always eliminate these
  sources we can help by having good ventilation to
  keep vapors from building up.
• Often we locate things out-doors, use grating on
  floors so vapors dont build up.

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Oxidizers

• Oxygen is the most common oxidizer, especially
  that found in ambient air.
• For oxygen, we often use inerting with
  nitrogen, helium blankets over flammable
  materials to reduce O2 content below that where
  you can have combustion.

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Ignition Sources

• Heat is a common ignition source.
• Ignition sources are free!!!
• Although we can eliminate ignition sources, it is
  almost inevitable that an ignition source will be
  available if there is a large release of
  flammable material that cannot be diluted quickly.

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Fire Tetrahedron

• The fire tetrahedron or fire pyramid adds a
  fourth componentchemical chain reactionas a
  necessity in the prevention and control of fires.
  
• The free radicals formed during combustion are
  important intermediates in the initiation and
  propagation of the combustion reaction. Fire
  suppression materials scavenge these free radicals

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Definitions

• Combustion a chemical reaction in which a
  substance combines with an oxidizer and releases
  energy.
• Explosion rapid expansion of gases resulting in
  a rapid moving pressure or shock wave.
• Mechanical Explosion due to failure of vessel
  with high pressure non reactive gas.

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Explosions

• Detonation explosion (chemical reaction) with
  shock wave greater than speed of sound
• Deflagration explosion (chemical reaction) with
  shock wave less than speed of sound
• BLEVE Boiling Liquid Expanding Vapor Explosion
  when liquid is at a temperature above its
  atmospheric boiling point. Vessel ruptures
  flammable liquid flashes and results in a
  fire/explosion

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Explosions

• Confined explosion an explosion occurring
  within a vessel or a building. Usually results
  in injury to the building inhabitants and
  extensive damage.
• Unconfined explosion an explosion occurring in
  the open. Usually results from spill of a
  flammable gas spill. These explosions are rarer
  than confined since dilution occurs.

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Explosions

• Dust Explosions - This explosion results from
  the rapid combustion of fine solid particles.
  Many solid materials become very flammable when
  reduced to a fine powder.

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Fires and Explosions

• Definitions
• Flammability
• Flash Point
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Flammability

• Flash Point (FP) a property of material used to
  determine the fire and explosive hazard. The
  lowest temperature of a liquid at which it gives
  off enough vapor to form an ignitable mixture
  with air.
• Needs to be determined experimentally.
• Different methods to determine, open cup and
  closed cup. Open cup is usually a few degrees
  higher.

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National Fire Protection AssociationFlammability
classification

• Flammable IA Flash point lt 73F, boiling point
  lt 100 F
• Flammable IB Flash point lt 73F, boiling point
  gt 100 F
• Flammable IC 73F lt Flash point lt 100 F
• Combustible II 100 F lt Flash point lt 140 F
• Combustible IIIA 140 F lt Flash point lt 200 F
• Combustible IIIB Flash point gt 200 F

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Mixture Flash Points

• Flash Points of mixtures can be estimated only IF
  one of the components is flammable. If more than
  one is flammable then need to determine
  experimentally.
• For mixtures
• Determine the temperature at which the vapor
  pressure of the flammable in the liquid is equal
  to the pure component vapor pressure at its flash
  point.

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Mixture Flash Points

• Example
• Methanol FP54F, Vapor Pressure _at_ 54F is 62
  mmHg
• Determine the flash point of a solution that is
  75wt MeOH in water.
• Solution
• Since only one component is flammable, can
  estimate mixture FP

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Mixture Flash Point Example Continued

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Mixture Flash Point Example Continued

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Flammability Limits

• There is usually a range of compositions of a
  flammable vapor and air where combustion occurs.
• Too little fuel (lean mixture) not enough fuel to
  burn.
• Too much fuel (rich mixture) not enough oxygen to
  burn

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Flammability Limits

• Table 6-1 gives upper flammability limits and
  lower flammability limits for several common
  substances.
• Experimentally determined.
• LFL can be estimated from Flash Point.

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Mixture Flammability Limits

• If you have a mixture of flammable components you
  can calculate Lower Flammability Limit of the
  mixture LFLmix using Le Chateliers relationship

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Mixture Flammability Limits

• You can also calculate an Upper Flammability
  Limit of the mixture UFLmix using Le Chateliers
  relationship

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Flammability Limits Temperature effect

• Table 6-1 gives flammability limits for 25C and
  atmospheric pressure. If you are at a different
  temperature you can modify flammability limits

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Flammability Limits Pressure effects

• LFL is not affected by pressure
• UFL does depend on the pressure
• Procedure
• Correct for Temperature
• Correct for Pressure
• Calculate for mixture

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Fires and Explosions

• Definitions
• Flammability
• Flash Points
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Minimum Oxygen Concentration (MOC)

• LFL is based on air but actually it is O2 that
  is important. Often in industry they inert to
  dilute the O2 concentration.
• Below the MOC the reaction cannot generate
  enough energy to heat the entire mixture to the
  extent required for self propagation.

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MOC

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Fires and Explosions

• Definitions
• Flammability
• Flash Points
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Minimum Ignition Energy (MIE)

• Minimum energy input needed to initiate
  combustion
• Most hydrocarbons have low MIE0.25 mJ
• Whereas the spark from walking across the room
  is 22mJ (almost 100X too much)
• Again, we always assume that an ignition source
  will exist
• Table 6-2 gives MIEs for some substances

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Fires and Explosions

• Definitions
• Flammability
• Flash Points
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Adiabatic Compression

• When gases are compressed they heat up and can
  ignite (this is how a diesel engine works, also
  the cause of knocking in gasoline engines)
• The adiabatic temperature rise is

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Fires and Explosions

• Definitions
• Flammability
• Flash Points
• Flammability limits
• Mixtures
• Temperature Dependence
• Pressure Dependence
• Minimum Oxygen Concentration
• Minimum Ignition Energy
• Adiabatic Compression
• Ignition Sources

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Ignition Sources

• Ignition sources are free!!!
• Table 6-3 gives the results of a study by Factory
  Mutual Engineering Corporation who studied over
  25,000 industrial fires to determine the source
  of ignition.

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In Class Problem

• What is the UFL of a gas mixture composed of 1
  methane, 2 ethane and 3 propane by volume at
  50C and 2 atmospheres
• Data
• Component MW Heat of Combustion
• (kcal/mol)
• Methane 16.04 212.79
• Ethane 30.07 372.81
• Propane 44.09 526.74

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Solution

• Procedure
• Correct for temperature
• Correct for pressure (only for UFL)
• Find for mixture.

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Solution

• Correction for Temperature UFL from Table 6-1

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Solution cont.

• Correction for Pressure (UFL only)

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Solution cont.

• Mixture calculation
• Equation 6-2 for mixtures

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Solution Continued

• Since total combustibles in air 1236 lt 18 then
  the system is in the combustible range (below UFL)