Chapter 6_Fires and Explosions in chemical industries.ppt

Fires and Explosions

1. CHAPTER-6
Fires and explosions
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2. Organic solvents are the most common source of fires and explosions in
the chemical industry. The most common chemical plant accidents
are
 fires, (rapid exothermic oxidation of an ignited fuel)
explosions, and
 toxic releases
property losses in the United States estimated at almost
 $300 million dollars (1997).
Additional losses in life and
business interruptions
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6. 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.
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7. 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
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9. Distinction between Fires and Explosions
•Fires release energy slowly
•Explosions release energy rapidly
e.g. automobile tire
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11. DEFINITIONS
Combustion or fire: Combustion or fire is a chemical reaction in which a substance
combines with an oxidant and releases energy.
Ignition: 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.
Auto-ignition 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.
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.
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12. Flammability limits: Vapor-air mixtures will ignite and burn only over a well-specified
range of compositions.
Explosion: An explosion is a rapid expansion of gases resulting in a rapidly moving
pressure or shock wave.
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 a speed less than the
speed of sound in the unreacted medium.
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13. Detonation: An explosion in which the reaction front moves at a speed greater than the
speed of sound in the unreacted medium.
Confined explosion: An explosion occurring within a vessel or a building.
Unconfined explosion: Unconfined explosions occur in the open
Boiling-liquid expanding-vapor explosion (BLEVE): A BLEVE occurs if a vessel that
contains a liquid at a temperature above its atmospheric pressure boiling point
ruptures.
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14. Dust explosion: This explosion results from the rapid combustion of fine solid
particles.
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.
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17. Flammability Characteristics of Liquids and Vapors
Liquids: The flash point temperature is one of the major quantities used
to characterize the fire and explosion hazard of liquids.
Gases and Vapors: Vapor-air mixtures of known concentration are added
and then ignited. The maximum explosion pressure is measured.
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30. IGNITION ENERGY
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.
•the MIE decreases with an increase in pressure,
•the MIE of dusts is, in general, at energy levels somewhat higher than
combustible gases, and
• an increase in the nitrogen concentration increases the MIE.
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32. AUTOIGNITION
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
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33. 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.
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34. ADIABATIC COMPRESSION
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36. SPRAYS AND MISTS
Static electricity is generated when mists or sprays pass through orifices.
A charge may accumulate and discharge in a spark. If flammable
vapors are present, a fire or explosion will occur
For mechanically formed mists with drop diameters between 0.01 mm
and 0.2 mm the LFL decreases as the drop diameter increases.
When sprays have drop diameters between 0.6 mm and 1.5 mm, flame
propagation is impossible.
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40. CONFINED EXPLOSIONS
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(1)evacuating the vessel,
(2)adjusting the temperature,
(3)metering in the gases to obtain the proper
mixture,
(4)igniting the gas by a spark, and
(5)measuring the pressure as a function of
time.

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42. •used to determine the safe concentrations
for operation
•The maximum rate is also used to design a vent for relieving a vessel
during an explosion
Explosion suppressant: water, carbon dioxide, or Halon
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51. The constants KG and Kst are not physical properties of the material
because they are dependent on
(1)the composition of the mixture,
(2)the mixing within the vessel,
(3)the shape of the reaction vessel, and
(4)the energy of the ignition source.
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52. Explosions involving dusts are most common in the
•flour milling,
•grain storage, and
•Coal mining industries.
To be explosive, a dust mixture must have the following characteristics:
•the particles must be below a certain minimum size,
•the particle loading must be between certain limits,
•the dust loading must be reasonably uniform.
lower explosion limit is between 20 g/m
3
and 60 g/m
3
and the upper
explosion limit is between 2 kg/m
3
and 6 kg/m
3
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Blast Damage Resulting from Overpressure

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Figure 6-23

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61. ENERGY OF MECHANICAL EXPLOSIONS
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64. VCE
Parameters affecting VCE behavior
•Quantity of materials released
•Fraction of material vaporized
•Probability of ignition of the cloud
•Distance travelled by the cloud prior to ignition
•Time delay before ignition of cloud
•Probability of explosion rather than fire
•Existence of a threshold quantity of material
•Location of ignition source w. r. t release
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65. Qualitative study indicates
•Ignition increases as the VC size increases
•VC fires are more common than explosion
• Small explosion efficiency
•Turbulent mixing of air and vapor
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66. VCE prevention
•Stop material release
•Prevent ignition
•Keeping low inventories of volatile, flammable materials
•Minimize flashing
•Use analyzer to detect leak
•Installed automatic block to shut system
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67. BLEVE
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ENERGY OF CHEMICAL EXPLOSIONS
Follows two mechanisms:
1.Thermal heating of reaction products
2. Change in no. of moles by reactions
ii
ffi
f
Tn
TnP
P
2222283 18.8NO4H3CO18.8N5OHC 


2
1
ePdVW
-PdVdA
PdV-SdT- dA

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PROBLEMS

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