Fires and Explosion Flashcards
Three most common chemical plant accidents
Fires, explosions and toxic releases
The most common source of fires and explosions in the chemical industry
Organic solvents
Engineers must be familiar with (3) to prevent accidents resulting from fires and explosions
- The fire and explosion properties of materials
- The nature of the fire and explosion process
- Procedures to reduce fire and explosion hazards
Elements of Fire
Oxygen
Heat
Fuel
Chemical Reaction between fuel, oxygen and heat
Fire
Self-sustaining process of rapid oxidation or combustion of fuel
Fire
Fire is the combustion of fuel, what is produced?
heat and light
Major distinction between explosion and fire
rate of energy release
Release of energy is slow
Fire
Release energy is rapid, in order of microseconds
Explosion
Fires can also result from explosion and explosion can result from fires (T/F)
T
Is a chemical reaction in which a substance combines with an oxidant and releases energy
Combustion or Fire
Part of the energy released in fire does what?
sustain the reaction
May be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy
Ignition
A gas reaching a temperature high enough to cause the gas to autoignite
Ignition
A fixed temperature above which adequate energy is available in the environment to provide an ignition source.
Autoignition Temperature (AIT)
The temperature at which a product can undergo spontaneous ignition
AIT
lowest Temperature of a liquid at which it gives off enough vapor to form an ignitable mixture with air
Flash Point
Lowest temperature at which vapor above a liquid will continue to burn once ignited
Firepoint
T/F: Flash point is higher than firepoint
F
The range of concentration of gases in air
Flammability Limits
Support the explosive process bounded by measurable limits called
UEL or UFL and LEL or LFL
Gasoline vapors has LEL of
1.4%
Gasoline vapors has UEL of
7.6%
A rapid expansion of gases resulting in a rapidly moving pressure or shock wave
Explosion
Expansion can be mechanical by means of?
Sudden rupture of pressurized vessel
Explosion resulting from sudden failure of a vessel containing high-pressure nonreactive gas
Mechanical
Explosion damage is caused by?
Pressure or shock wave
Pressure and shock wave can cause?
explosion damage
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
Confined explosion
THese are the most common and usually result in injury to the building inhabitants and extensive damage
Confined explosion
Occur in the open
Unconfined explosion
Result of a flammable gas spill
Unconfined explosion
The gas in this explosion is dispersed and mixed with air until it comes in contact with an ignition source
Unconfined explosion
Rarer to happen because the explosive material is frequently diluted below the LFL by wind dispersion
Unconfined explosion
Are destructive because large quantities of gas and large areas are frequently involved
Unconfined explosion
Occurs if the vessel that contains a liquid at a temperature above its atmospheric pressure boiling point ruptures
Boiling-Liquid Expanding-Vapor Explosion (BLEVE)
Subsequent BLEVE is what?
Explosive vaporization of a large fraction of the vessel contents
Possibly followed by combustion or
Explosion of the vaporized cloud (if it is combustible)
Occurs when an external fire heats the content within the tank of volatile material
BLEVE
During BLEVE, as the tank contents heat, the ____ of the liquid within the tank increases and the tank’s ______ is reduced because of the heating
vapor pressure, structural intergity
In BLEVE, when the tank ruptures, the hot liquid?
Volatilizes explosively
Results from rapid combustion of fine solid particles
Dust explosion
Many solid materials (common metals such as iron and aluminum) become flammable when reduced to fine powder (T/F)
T
Abrupt pressure wave moving though a gas
Shock wave
This is followed by a strong wind
Shock wave in open air
The pressure increase in the shock wave is ____ that the process is mostly _____
rapid, adiabatic
Is expected from highly explosive materials such as TNT but can also occur from sudden rupture of a pressure vessel
Shock wave
Combined shock wave and strong wind
Blast wave
Pressure wave propagating in air
Blast wave
Pressure on an object as a result of an impacting shock wave
Overpressure
Minimum energy input required ti initiate combustion
Minimum ignition energy
All flammable materials (including dust) have MIEs (T/F)
T
Where doe MIE depends (4)
Specific chemical or mixture
concentration
pressure
temperature
Explosions that occur because of the release of the flammable vapor over large volume
Vapor cloud explosion (VCE)
Are most commonly deflagrations
VCS
Simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT
TNT equivalency
Based on assumption that an exploding fuel mass behaves like exploding TNT on an equivalent energy basis
TNT equivalency
Typical values for energy of explosion of TNT (cal/g)
1120
Typical values for energy of explosion of TNT (kJ/kg)
4686
Typical values for energy of explosion of TNT (btu/lb)
2016
This can be used in place of the energy of explosion for the combustible gas
heat of combustion for the flammable gas
Results from a chemical explosion generated by the expansion of gases at the explosion site
Blast wave
Expansion can be caused by two mechanisms:
- thermal heating of the reaction products
- the change in the total number of moles by reaction
Energy released during a reaction explosion is computed using?
Standard thermodynamics
T/F: The released energy is equal to the work required to expand the gases
T
Expansion work is a form of mechanical energy (T/F)
T
An explosion occurring in a confined vessel or structure can rupture the vessel resulting in the projection of debris over a wide area.
Missile damage
Can cause appreciable injury to people and damage to structures and process equipment
Debris or Missiles
Also create missiles by blast wave impact and subsequent translation of structures
Unconfined ecplosions
A means by which an accident propagates throughout the plant
Missiles
When debris strikes storage tanks and other process equipment, what is the result?
Secondary fires or explosions
People can be injured by explosions from ?
Direct blast effects
Types of direct blast effects
overpressure nad thermal radiation
These are estimated using probit
Blast damage effects
Blast damage effects can be estimated using
probit / probability unit
The most dangerous and destructive explosions in the chemical process industries
Vapor Cloud explosions
Sequence of steps in the occurrence of VC Explosions
- Sudden release of a large quantity of flammable vapor (superheated and pressurized liquid)
- Dispersion of the vapor throughout the plant site while mixing with air
- Ignition of the resulting vapor cloud
Special type of accident that can release large quantities of materials
BLEVE
If the material is flammable, what might result?
VCE
Occurs when a tank containing a liquid held above its atmospheric pressure boiling point ruptures
BLEVE
The most common type of BLEVE is caused by ?
Fire
BLEVE occurrence steps
- Fire develops adjacent to a tank containing a liquid
- Fire heats the walls of the tank
- The tank walls below liquid level are cooled by the liquid, increasing the liquid temperature and pressure in the tank
- the flames reach the tank wall or roof where there is only a vapor and no liquid to remove the heat, the tank metal temperature rises until the tank loses its structural strength
- The tank ruptures, explosively vaporizing the contents
If the liquid is flammable and a fire is the cause of the BLEVE, the liquid may?
ignite as the tank ruptures
Often, the boiling liquid behaves as a ____ propelling vessel parts for great distances
rocket
If the BLEVE is not caused by fire, what might form?
Vapor cloud resulting in a VCE
The vapors also might be hazardous to personnel by means of ?
skin burns or toxic effects
When BLEVE occurs in a vessel, only a fraction of the _____ vaporizes
liquid
The amount of liquid that vaporizes when BLEVE occurs in a vessel depends on ?
physical and thermodynamic conditions of the vessel content
Twofold strategy used to limit the potential damage from fires and explosions
Prevent the initiation of the fire or explosion
Minimize the damage after a fire or explosion has occurred
Common method of F nad E prevention
Use of Flammability diagram
Sprinkler systems
Explosion-proof equipment and instruments
Ventilation
Controlling static electricity
Inerting
Process of adding an inert gas to a combustible mixture to reduce the concentration of oxygen below the limiting oxygen concentration (LOC)
Inerting
The inert gas is usually?
Nitrogen, CO2 or steam
For many gases the LOC is ?
10%
For many dusts the LOC is?
8%
Where does inerting begins?
- Initial purge of the vessel with inert gas to bring oxygen to safe concentrations
- Flammable material is charged
What is required to maintain an inert atmosphere in the vapor space above the liquid?
Inerting system
Charge buildup results
sparks
is an inevitable event if control methods are not appropriately used
ignition of flammable materials
Engineers recognize the problem and install special features to prevent (2)
- sparks by eliminating the buildup and accumulation of static charge
- inerting the surrounding
Most effective and reliable method for preventing ignition is?
Interting
This is always used when working with flammable liquids that are 5°C (or less) below the flash point (closed cup)
Inerting
Design objective for controlling static electricity is
- Prevent the buildup of charges on a product as well as the surrounding objects
Foe every charged object there exists its _____ counterpart
oppositely charged
Three methods for achieving the design objective:
- Reducing the rate of charge generation and increase rate of relaxation (liquids)
- Designing the system to include charge reduction by means of low-energy discharge (powders)
- Maintain oxidant levels below the combustible levels (inerting)
Maintain fuel levels below LFL or above UFL
Measures to mitigate the consequences of an explosion
deflagration venting
explosion suppression
Streaming current Is is produces during the _____ when pumping fluids into a vessel through a pipe on top
separation process
Basis of charge buildup
Streaming current
Adding an enlarged section of pipe just before entering the tank, providing time for charge reduction
Relaxation
Voltage difference between two conductive materials is reduced to zero during?
Bonding two conductive materials
If sets of the bonded materials have different voltages, it can be set to zero through
Grounding
Ground level or
Zero voltage
Eliminates the charge buildup between various parts of system, eliminating the potential for static sparks
Bonding and Grounding
An extended line that reduces the electrical charge that accumulates when liquid is allowed to free fall
Dip leg or Dip pipe
When using dip leg care is a must to avoid?
Siphoning back when the inlet is stopped
Common method used in dip piping is?
place a hole in the dip pipe near the top of the vessel
Another method for dip piping is?
Use angle iron instead of pipe and let the liquid flow down
These methods are also used in filling drums
Hole and angle iron
Another method to prevent fires and explosions to dilute the explosive vapors with air and confine the hazardous flammable mixtures
Proper Ventilation
Are recommended because the average wind velocity are high enough to safely dilute volatile chemical leaks that may exist within the plant
Open-Air Plants
If processes cannot be constructed outside, local and dilution ventilation are required
Plants inside the building
Most effective method for controlling flammable gas releases
Local ventilation
When the potential points of releases are numerous, what is used?
Dilution ventilation
T/F: All electrical devices are inherent ignition sources
T
T/F: The fire and explosion hazards is directly proportional to the number and type of electrically powered devices in the process area
T
Process areas are divided into two major types of environment:
XP or Explosion proof
Non-XP
Means flammable materials (vapors) might be present at certain times
XP
Means that flammable materials are not present, even abnormal conditions
Non-Xp
Designated open flames, heated elements and other sources of ignition may be present
Non-xp
Are effective way to contain fires
Sprinkler Systems
Consists of an array of sprinkler heads connected to water supply
Sprinkler systems
Are mounted in a high location usually near ceilings and disperse a fine spray of water over an area when activated
heads
2 Methods of Activating Sprinkler Systems
- Wet-pipe system
- Deluge System
Activates the head individually melting the fusible link holding a plug in the head assembly
Wet-pipe system
Once activated, the system cannot be turned off unless the main water supply is stopped
Wet-pipe system
Used for storage area, laboratories, control rooms and small pilot areas
Wet pipe systems
Activates the entire sprinkler array from a common control point
Deluge system
The control point is connected to an array of ___ and _____ detectors that start the sprinklers when an abnormal condition is detected
heat, smoke
Used for plant process areas and larger pilot plants
Deluge systems
