Chapter 6 Fires and Explosions

Embed Size (px)

Citation preview

  • 7/24/2019 Chapter 6 Fires and Explosions

    1/29

    Chapter 6

    Fires and Explosions

    Ref: Chapter 6, D. A. Crowl, J. F. Louvar, Chemical Process

    Safety: Fundamental with Applications, 3rd edition,

    Prentice-Hall, 2011.

    1

  • 7/24/2019 Chapter 6 Fires and Explosions

    2/29

    Introduction Chemicals present a substantial hazard in the form of fires and explosions.

    Ex: combustion of one gallon of toluene can destroy an ordinary

    chemistry laboratory in minutes; persons present may be killed.

    The potential consequences of fires and explosions in pilot plants and plantenvironments are even greater.

    Organic solvents are the most common source of fires and explosions in the

    chemical industry. Chemical and hydrocarbon plant losses resulting from

    fires and explosions are substantial

    To prevent accidents resulting from fires and explosions, engineers must be

    familiar with the fire and explosion properties of materials, the nature of

    the fire and explosion process, and procedures to reduce fire and explosion

    hazards. 2

  • 7/24/2019 Chapter 6 Fires and Explosions

    3/29

    Definitions

    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.

    3

  • 7/24/2019 Chapter 6 Fires and Explosions

    4/29

    Definitions

    Autoignition temperature (AIT): A fixed temperatureabove which adequate energy is available in theenvironment to provide an ignition source.

    Flash point (FP): The flash point of a liquid is the lowesttemperature at which it gives off enough vapor to forman ignitable mixture with air.

    At the flash point the vapor will burn but only briefly;inadequate vapor is produced to maintain combustion.The flash point generally increases with increasingpressure.

    4

  • 7/24/2019 Chapter 6 Fires and Explosions

    5/29

    Definitions

    Fire point: 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 lean for combustion.

    The mixture is also not combustible when the composition is

    too rich; that is, when it is above the upper flammable limit

    (UFL). A mixture is flammable only when the composition is

    between the LFL and the UFL. 5

  • 7/24/2019 Chapter 6 Fires and Explosions

    6/29

    Definitions

    Explosion: An explosion is a rapid expansion of gasesresulting in a rapidly moving pressure or shock wave.

    The expansion can be mechanical (by means of a suddenrupture of a pressurized vessel), or it can be the result ofa rapid chemical reaction. Explosion damage is caused bythe pressure or shock wave.

    Mechanical explosion: An explosion resulting from thesudden failure of a vessel containing high-pressurenonreactive gas.

    6

  • 7/24/2019 Chapter 6 Fires and Explosions

    7/29

    Definitions

    Deflagration : An explosion in which the reaction front moves at a speed lessthan the speed of sound in the unreacted medium.

    Detonation: An explosion in which the reaction front moves at a speedgreater than the speed of sound in the unreacted medium.

    Confined explosion: An explosion occurring within a vessel or a building.These are most common and usually result in injury to the buildinginhabitants and extensive damage.

    Unconfined explosion: Unconfined explosions occur in the open. This type of

    explosion is usually the result of a flammable gas spill.

    Dust explosion: This explosion results from the rapid combustion of fine solidparticles. Many solid materials (including common metals such as iron andaluminum) become flammable when reduced to a fine powder.

    7

  • 7/24/2019 Chapter 6 Fires and Explosions

    8/29

    Boiling-liquid expanding-vapor

    explosion (BLEVE):A BLEVE occurs if a vessel that contains a liquid at a temperature aboveits atmospheric pressure boiling point ruptures.

    The subsequent BLEVE is the explosive vaporization of a large fractionof the vessel contents; possibly followed by combustion or explosion

    of the vaporized cloud if it is combustible.

    This type of explosion occurs when an external fire heats the contentsof a tank of volatile material.

    As the tank contents heat, the vapor pressure of the liquid within thetank increases and the tank's structural integrity is reduced because ofthe heating. If the tank ruptures, the hot liquid volatilizes explosively.

    8

  • 7/24/2019 Chapter 6 Fires and Explosions

    9/29

    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.

    9

  • 7/24/2019 Chapter 6 Fires and Explosions

    10/29

    The Fire Triangle

    The essential elements for combustion are fuel, an oxidizer, and

    an ignition source.

    Fire, or burning, is the rapid exothermic oxidation of an ignitedfuel.

    Two common examples of the three components of the fire

    triangle are wood, air, and a match; and gasoline, air, and aspark.

    10

  • 7/24/2019 Chapter 6 Fires and Explosions

    11/29

    11

  • 7/24/2019 Chapter 6 Fires and Explosions

    12/29

    When fuel, oxidizer, and an ignition source are present at the

    necessary levels, burning will occur.

    This means a fire will not occur if

    (1) fuel is not present or is not present in sufficient quantities

    (2) an oxidizer is not present or is not present in sufficient

    quantities, and

    (3) the ignition source is not energetic enough to initiate the fire.

    12

  • 7/24/2019 Chapter 6 Fires and Explosions

    13/29

    Common Fuels, Oxidizers and Ignition

    sourcesProcess Industry

    Fuels

    Liquids: gasoline, acetone, ether, pentane

    Solids: plastics, wood dust, fibers, metal particles

    Gases: acetylene, propane, carbon monoxide, hydrogen

    Oxidizers

    Gases: oxygen, fluorine, chlorine

    Liquids: hydrogen peroxide, nitric acid, perchloric acid

    Solids: metal peroxides, ammonium nitrite

    Ignition sources

    Sparks, flames, static electricity, heat

    13

  • 7/24/2019 Chapter 6 Fires and Explosions

    14/29

    Distinction between Fires and

    Explosions

    The major distinction between fires and explosions is the rate

    of energy release.

    Fires release energy slowly, whereas explosions release

    energy rapidly, typically on the order of microseconds.

    Fires can also result from explosions, and explosions can

    result from fires.

    14

  • 7/24/2019 Chapter 6 Fires and Explosions

    15/29

    Le Chatelier equation for Prediction of

    LFL and UFL of mixtures

    15

    Where LFLi is the lower flammable limit for component i (in volume %) of

    component i in fuel and air,

    yi is the mole fraction of component i on a combustible basis, and

    n is the number of combustible species.

    where UFL, is the upper flammable limit for component i (in volume %) of

    component i in fuel and air.

  • 7/24/2019 Chapter 6 Fires and Explosions

    16/29

    Limiting Oxygen Concentration and lnerting

    The LFL is based on fuel in air.

    However, oxygen is the key ingredient and there is a minimumoxygen concentration required to propagate a flame.

    This is an especially useful result, because explosions and firescan be prevented by reducing the oxygen concentrationregardless of the concentration of the fuel.

    This concept is the basis for a common procedure calledinerting.

    16

  • 7/24/2019 Chapter 6 Fires and Explosions

    17/29

    LOC or MOC or MSOC

    Below the limiting oxygen concentration (LOC) the reaction

    cannot generate enough energy to heat the entire mixture of

    gases (including the inert gases) to the extent required for the

    self-propagation of the flame.

    The LOC has also been called the minimum oxygen

    concentration (MOC), the maximum safe oxygen

    concentration (MSOC),

    17

  • 7/24/2019 Chapter 6 Fires and Explosions

    18/29

    Flammability Diagram A general way to represent the flammability of a gas or vapor is by the

    triangle diagram .

    Concentrations of fuel, oxygen, and inert material (in volume or mole %)

    are plotted on the three axes.

    Each apex of the triangle represents either 100% fuel, oxygen, or nitrogen.

    The tick marks on the scales show the direction in which the scale moves

    across the figure.

    Thus point A represents a mixture composed of 60% methane, 20%oxygen, and 20% nitrogen. The zone enclosed by the dashed line

    represents all mixtures that are flammable. Because point A lies outside

    the flammable zone, a mixture of this composition is not flammable.

    Clearly, any gas mixture containing oxygen below the LOC is not

    flammable. 18

  • 7/24/2019 Chapter 6 Fires and Explosions

    19/29

    Flammability diagram for methane at 1 atm and 25 oC

    19

  • 7/24/2019 Chapter 6 Fires and Explosions

    20/29

    Minimum ignition energy (MIE)

    The minimum ignition energy (MIE) is the minimum energy

    input required to initiate combustion.

    All flammable materials (including dusts) have MIEs.

    The MIE depends on the specific chemical or mixture, the

    concentration, pressure, and temperature.

    20

  • 7/24/2019 Chapter 6 Fires and Explosions

    21/29

    Characteristics of MIE

    1. the MIE decreases with an increase in

    pressure,

    2. the MIE of dusts is, in general, at energy

    levels somewhat higher than combustible

    gases,

    3. an increase in the nitrogen concentration

    increases the MIE.

    21

  • 7/24/2019 Chapter 6 Fires and Explosions

    22/29

    Autoignition Temperature

    The autoignition temperature (AIT) of a vapor, sometimes

    called the spontaneous ignition temperature (SIT), is the

    temperature at which the vapor ignites spontaneously from

    the energy of the environment.

    The autoignition temperature is a function of the

    concentration of vapor, volume of vapor, pressure of the

    system, presence of catalytic material, and flow conditions.

    It is essential to experimentally determine AITs at conditions

    as close as possible to process conditions.

    22

  • 7/24/2019 Chapter 6 Fires and Explosions

    23/29

    Auto-oxidation

    Auto-oxidation is 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.

    Many fires are initiated as a result of auto-oxidation, referred

    to as spontaneous combustion. Ex: spontaneous combustion

    include oils on a rag in a warm storage area23

  • 7/24/2019 Chapter 6 Fires and Explosions

    24/29

    Detonation and Deflagration

    The damage effects from an explosion depend highly on

    whether the explosion results from a detonation or a

    deflagration.

    The difference depends on whether the reaction front

    propagates above or below the speed of sound in the

    unreacted gases.

    24

  • 7/24/2019 Chapter 6 Fires and Explosions

    25/29

    Detonation

    25

  • 7/24/2019 Chapter 6 Fires and Explosions

    26/29

    Deflagration

    26

  • 7/24/2019 Chapter 6 Fires and Explosions

    27/29

  • 7/24/2019 Chapter 6 Fires and Explosions

    28/29

    Exercise

    1. What are the LFL and UFL of a gas mixture

    composed of 0.8% hexane, 2.0% methane,

    and 0.5% ethylene by volume?

    2. Estimate the LOC for butane (C4HI0).

    28

  • 7/24/2019 Chapter 6 Fires and Explosions

    29/29

    29