Lecture 6 Flashcards
The three most common chemical plant accidents are:
-fires
-explosions
-toxic releases.
___ are the most common source of fires and explosions in the chemical industry.
Organic solvents
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.
___ is a chemical reaction between ___, ____, and ____. it is a self-sustaining process of rapid oxidation or combustion of a fuel, which produces heat and light.
-Fire
- fuel, oxygen, and heat
The major distinction between fires and explosions is the rate of energy release.
-Fires release energy slowly
-Explosions release energy rapidly, typically on the order of microseconds.
___- can also result from ___, and ____ can results from ___.
-fires, explosions
- explosions, fires
a chemical reaction where a substance combines with an oxidant and releases energy. Part of the released energy is used to sustain the reaction.
Combustion or Fire
____ of a flammable mixture can be caused by:
Contact with a source of ignition with sufficient energy
The gas reaching a temperature high enough for autoignition
Ignition
The fixed temperature above which there’s enough energy in the environment to ignite a substance.
The temperature at which a product can spontaneously ignite.
Autoignition Temperature (AIT)
The lowest temperature at which a liquid releases enough vapor to form an ignitable mixture with air.
Flash Point (FP):
The lowest temperature at which vapor above a liquid will continue to burn after ignition.
Fire Point
The range of gas concentrations in air that can support an explosive process.
Bounded by measurable limits called:
Upper Explosive Limits (UEL) or Upper Flammable Limit (UFL)
Lower Explosive Limits (LEL) or Lower Flammable Limit (LFL)
Flammability Limits:
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 be the result of a rapid chemical reaction. Explosion damage is caused by the pressure or shock wave.
Explosion
An explosion resulting from the sudden failure of a vessel containing high-pressure nonreactive gas.
Mechanical explosion
An explosion in which the reaction front moves at a speed less than the speed of sound in the unreacted medium.
Deflagration
An explosion in which the reaction front moves at a speed greater than the speed of sound in the unreacted
medium.
Detonation
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.
Confined explosion:
Occurs in the open, often due to flammable gas spills.
Gas disperses and mixes with air until it ignites.
Less common than confined explosions due to wind dilution.
Destructive due to large quantities of gas and area involved.
Unconfined Explosion
Occurs when a vessel with a liquid above its atmospheric pressure boiling point ruptures.
Explosive vaporization of a large portion of the vessel contents.
May be followed by combustion or explosion of the vaporized cloud.
Caused by external fire heating the tank contents.
Boiling-Liquid Expanding-Vapor Explosion (BLEVE)
Results from the rapid combustion of fine solid particles.
Many solid materials become flammable when powdered.
Examples include common metals like iron and aluminum.
Dust Explosion
Results from the rapid combustion of fine solid particles.
Many solid materials become flammable when powdered.
Examples include common metals like iron and aluminum.
Dust Explosion
An abrupt pressure wave moving through a gas. In open air, it’s followed by a strong wind. The pressure increase is rapid and mostly adiabatic. It’s expected from highly explosive materials like TNT but can also occur from the sudden rupture of a pressure vessel.
Shock Wave
the combined shock wave and strong wind. A pressure wave propagating in air.
Blast Wave
The pressure on an object as a result of an impacting shock wave.
Overpressure
The minimum energy input required to initiate combustion. All flammable materials have MIEs, which depend on the specific chemical or mixture, concentration, pressure, and temperature.
Minimum Ignition Energy (MIE)
Explosions that occur due to the release of flammable vapor over a large volume. They are most commonly deflagrations.
Vapor Cloud Explosions (VCEs)
-is a simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT.
The approach is based on the assumption that
-an exploding fuel mass behaves like exploding TNT on an equivalent energy basis.
-A typical value for the energy of explosion of TNT is 1120 cal/g = 4686 kJ/kg = 2016 Btu/lb. The heat of combustion for the flammable gas can be used in place of the energy of explosion for the combustible gas.
TNT equivalency
The blast wave resulting from a chemical explosion is generated by the ____ of gases at the ____.
rapid expansion, explosion site.
Blast wave can be caused by two mechanisms:
-Thermal heating of the reaction products
-The change in the total number of moles by reaction.
The energy released during a reaction explosion is computed using ____
standard thermodynamics.
The released energy is equal to the work required to expand the gases.
This expansion work is a form of _____.
mechanical energy.
An explosion occurring in a confined vessel or structure can rupture the vessel or structure, resulting in the projection of debris over a wide area.
Missile Damage
_____ also create missiles by blast wave impact and subsequent translation of structures
Unconfined explosions
____, can cause appreciable injury to people and damage to structures and process equipment.
The debris, or missiles
people can be injured by explosions from ___ or ____effects
direct blast, indirect blast
____ are frequently a means by which an accident propagates throughout a plant facility.
Missiles
blast damage effects are estimated using ____
probit (probability unit) analysis
VCEs occur in a sequence of steps:
-Sudden release of a large quantity of flammable vapor (typically this occurs when a vessel,
containing a superheated and pressurized liquid, ruptures).
-Dispersion of the vapor throughout the plant site while mixing with air,
-Ignition of the resulting vapor cloud.
The most dangerous and destructive explosions in the chemical process industries are ____.
vapor cloud explosions (VCEs).
a type of accident that can release large quantities of materials. If these materials are flammable, a VCE (Vapor Cloud Explosion) can occur. If they are toxic, a large area can be subjected to toxic substances.
Boiling-Liquid Expanding-Vapor Explosion (BLEVE)
happens when a tank containing a liquid held above its atmospheric pressure boiling point ruptures. This causes a large portion of the tank’s contents to vaporize explosively.
BLEVE
BLEVES are caused by the sudden failure of a container, often due to fire. The steps leading to a fire-induced BLEVE are:
-fire starts near a tank containing a liquid.
-The fire heats the tank’s walls.
-The liquid inside the tank cools the walls below the liquid level, increasing the temperature and pressure inside the tank.
-If the fire reaches the top of the tank where there’s only vapor, the tank metal heats up until it loses its structural strength.
-The tank ruptures, causing the explosive vaporization of its contents.
if the liquid in the tank is flammable and the BLEVE is caused by a fire, the liquid may ignite as the tank ruptures. The boiling and burning liquid can act like rocket fuel, propelling vessel parts for significant distances.
Fire and Explosion
The vapors released during a BLEVE can be hazardous to personnel, causing skin burns or toxic effects.
Hazardous Vapors
To limit the potential damage from fires and explosions, a twofold strategy is employed:
-Prevent the initiation of the fire or explosion.
-Minimize the damage after a fire or explosion has occurred.
Several common methods are used to implement this twofold strategy:
-Inerting:
-Use of the flammability diagram:
-Controlling static electricity
-Ventilation:
-Explosion-proof equipment and instruments:
-Sprinkler systems:
___the process of adding an inert gas to a combustible mixture to reduce the concentration of oxygen below the limiting oxygen concentration (LOC). ____ gas is usually nitrogen or carbon dioxide, although steam is sometimes used.
Inerting, inert
For many gases the LOC is approximately ____, and for many dusts it is approximately ___.
10%, 8%
Inerting begins with an _____ of the vessel with inert gas to bring the oxygen concentration down to safe concentrations.
initial purge
An inerting system is required to maintain an_____in the vapor space above the liquid.
inert atmosphere
Inerting is the most effective and reliable method for preventing ____. It is always used when working with flammable liquids that are 5°C (or less) below the flash point (closed cup).
ignition
Three methods are used to achieve this objective: (preventing electrostatic)
-Prevent charges from accumulating to dangerous levels
-Prevent charges from accumulating to dangerous levels using low-energy discharges:
-Prevent the possibility of an ignition
When pumping fluids into a vessel through a pipe on top of the vessel, the separation process produces a streaming current (Is), which is the basis for charge buildup. It is possible to substantially reduce this electrostatic hazard by adding an enlarged section of pipe just before entering the tank. This hold provides time for charge reduction by relaxation.
Relaxation:
reduces the voltage of an entire system to ground level or zero voltage. This also eliminates the charge buildup between various parts of a system, eliminating the potential for static sparks.
Bonding & Grounding:
An extended line, sometimes called a dip leg or dip pipe, reduces the electrical charge that accumulates when liquids are allowed to free fall
Dip Pipes
is another method used to prevent fires and explosions
proper ventilation
purpose of ___ is to dilute the explosive vapors with air to prevent explosion and to confine the hazardous flammable mixtures
ventilation
Recommended due to high average wind velocities for safe dilution of chemical leaks.
Open-Air Plants
All electrical devices are potential ignition sources.
Special design features are needed to prevent ignition of flammable vapors and dusts.
The fire and explosion risk increases with the number and type of electrical devices in a process area.
Electrical Devices and Fire Hazards
2 Process Area Classifications
-XP (Explosion Proof)
Non-XP
Flammable materials (especially vapors) might be present at certain times.
XP (Explosion Proof)
Flammable materials are not present, even under abnormal conditions. Open flames, heated elements, and other ignition sources are allowed in non-XP areas.
Non-XP:
Effective for fire containment
Sprinkler Systems
System components of sprinkler
Array of sprinkler heads
Water supply
High-mounted heads (near ceilings)
2 Methods of Activating Sprinkler Heads:
-wet-pipe system
-deluge system
Individual head activation.
Fusible link melts, releasing plug.
Cannot be turned off without stopping main water supply.
Suitable for storage areas, laboratories, control rooms, and small pilot areas.
Wet-pipe System:
Activates entire sprinkler array.
Controlled by heat or smoke detectors.
May activate in unaffected areas.
Used for plant process areas and larger pilot plants.
Deluge System:
