Saturday, 21 September 2013

Fire Science and Fire Extinguishers

Fire Since and Fire Extinguishers

What is Fire

Fire is a combustible chemical reaction between oxygen and any type of fuel resulting heat, smoke light called fire.

 

Fire Tetrahedron

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Fire is a rapid chemical reaction which starts rapidly in the presence of four basis element which are show in tetrahedron.
Oxygen
The oxidizer is a reactant in the chemical reaction of fire. In most cases, it is the ambient air, and in particular one of its components, oxygen (O2). In certain cases such as some explosives, the oxidizer and combustible are the same (e.g., nitroglycerin, an unstable molecule that has oxidizing parts in the same molecule as the oxidize able parts).
Fuel
Fuels are any materials that store potential energy in forms that can be practicably released and used as heat energy. The concept originally applied solely to those materials storing energy in the form of chemical energy that could be released through combustion but the concept has since been also applied to other sources of heat energy such as nuclear energy (via nuclear fission or nuclear fusion), as well as releases of chemical energy released through non-combustion oxidation.
Fuels are further classified in two three types depending on their physical condition or state.
  • Solid Fuel      Such as wood, coal, peat etc.
  • Liquid Fuel   Such as diesel, Gasoline, Kerosene, Coal tar, Ethanol etc.
  • Gaseous Fuel Hydrogen, Propane, Water gas, LPG, CNG, etc.
Heat
Heat is a form of energy required to start combustion reaction in various quantity for various type of fuel. Heat can be transferred by three modes.
  • Conduction
  • Convection
  • Radiation
Chemical Reaction
When fuel, oxygen and required amount of heat are met together resulting starts a chain reaction causing fire.

Portable Fire Extinguishers, Classifications and their Uses

Introduction to NFPA 

NFPA stands for National Fire Protection Association founded in 1896, NFPA grew out of that first meeting on sprinkler standards. The By laws of the Association that were first established in 1896 embody the spirit of the codes and standards development process.
“The purposes of the Association shall be to promote the science and improve the methods of fire protection and prevention, electrical safety and other related safety goals; to obtain and circulate information and promote education and research on these subjects; and to secure the cooperation of its members and the public in establishing proper safeguards against loss of life and property.”
The NFPA mission today is accomplished by advocating consensus codes and standards, research, training, and education for safety related issues. NFPA’s National Fire Codes are administered by more than 250 Technical Committees comprised of approximately 8,000 volunteers and are adopted and used throughout the world.
NFPA Fire Classification
Class A
Class A fires are ordinary materials like burning paper, lumber, cardboard, plastics etc.
Class B
Class B fires are involve flammable or combustible liquids such as gasoline, kerosene, and common organicsolvents used in the chemical industry.
Class C
Class C fires involve energized electrical equipment, such as appliances, switches, panel boxes, power tools,hot plates and stirrers. Water can be a dangerous extinguishing medium for class C fires because of the risk of electrical shock unless a specialized water mist extinguisher is used.
Class D
Class D fires involve combustible metals, such as magnesium, titanium, potassium and sodium as well aspyrophoric organometallic reagents such as alkyllithiums, Grignards and diethylzinc. These materials burn at high temperatures and will react violently with water, air, and/or other chemicals. Handle with care!!
Class K
Class K fires are kitchen fires. This class was added to the NFPA portable extinguishers Standard 10 in 1998. Kitchen extinguishers installed before June 30, 1998 are "grandfathered" into the standard.
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Basic Types of Fire Extinguishers

1.     Water extinguishers
Water fire extinguishers are suitable for class A (paper, wood etc.) fires, but not for class B, C and D fires such as burning liquids, electrical fires or reactive metal fires. In these cases, the flames will be spread or the hazard made greater! Water mist extinguishers are suitable for class A only.

2.     Dry chemical extinguishers (DCP)

DCP fire extinguishers are useful for either class A B C or class B C fires (check the label) and are your best all-around choice for common fire situations. They have an advantage over CO2 and "clean agent" extinguishers in that they leave a blanket of non-flammable material on the extinguished material which reduces the likelihood of reignition. They also make a terrible mess, but if the choice is a fire or a mess, take the mess Note that there are two kinds of dry chemical extinguishers:
  • Type BC fire extinguishers contain sodium or potassium bicarbonate.
  • Type ABC fire extinguishers contain ammonium phosphate.
Proper planning can avoid situations where you might have to make a choice between extinguisher types. Ensure that the extinguishers closest to your computers or aircraft are of an appropriate type (if local fire codes permit) and that workers in those areas are trained on when and how to use them. And remember, if your computer or airplane is fully engulfed in flames or a person is in danger, then possible added damage from an ABC extinguisher is moot.
3.      CO2 (carbon dioxide) extinguishers
Carbon dioxide fire extinguisher are for class B and C fires. They don't work very well on class A fires because the material usually reignites. CO2 extinguishers have an advantage over dry chemical in that they leave behind no harmful residue. That makes carbon dioxide a good choice for an electrical fire involving a computer or other delicate instrument. Note that CO2 is a bad choice for a flammable metal fires such as Grignard reagents, alkyllithiums and sodium metal because CO2 reacts with these materials. COextinguishers are not approved for class D fires! Carbon dioxide extinguishers do not have pressure gauges because carbon dioxide is a condensable gas. Thus, pressure does not tell you how much agent remains in the cylinder. Instead, the extinguisher should have a tare (empty) weight stamped on it. To determine the amount of carbon dioxide remaining in the extinguisher, subtract the tare weight from the current weight.
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4.      Metal/Sand Extinguishers
Metal and sand fire extinguisher are for flammable metals (class D fires) and work by simply smothering the fire. The most common extinguishing agent in this class is sodium chloride, but there are a variety of other options. You should have an approved class D unit if you are working with flammable metals.
I. Sodium chloride (NaCl) works well for metal fires involving magnesium, sodium (spills and in depth), potassium, sodium/potassium alloys, uranium and powdered aluminum. Heat from the fire causes the agent to cake and form a crust that excludes air and dissipates heat.
II. Powdered copper metal (Cu metal) is preferred for fires involving lithium and lithium alloys. Developed in conjunction with the U.S. Navy, it is the only known lithium firefighting agent which will cling to a vertical surface thus making it the preferred agent on three dimensional and flowing fires.
III. Graphite-based powders are also designed for use on lithium fires. This agent can also be effective on fires involving high-melting metals such as zirconium and titanium.
IV. Specially-designed sodium bicarbonate-based dry agents can suppress fires with most metal alkyls, pyrophoric liquids which ignite on contact with air, such as triethylaluminum, but do not rely on a standard BC extinguisher for this purpose.
V. Sodium carbonate-based dry powders can be used with most Class D fires involving sodium, potassium or sodium/potassium alloys. This agent is recommended where stress corrosion of stainless steel must be kept to an absolute minimum.
5.     Halotron-I extinguisher
Like carbon dioxide units, are "clean agents" that leave no residue after discharge. Halotron I is less damaging to the Earth's ozone layer than Halon 1211 (which was banned by international agreements starting in 1994). This "clean agent" discharges as a liquid, has high visibility during discharge, does not cause thermal or static shock, leaves no residue and is non-conducting. These properties make it ideal for computer rooms, clean rooms, telecommunications equipment, and electronics. These superior properties of Halotron I come at a higher cost relative to carbon dioxide.
6.     Hydrofluorocarbon (HCFC)
FE-36TM (Hydrofluorocarbon-236fa or HFC-236fa) is another "clean agent" replacement for Halon 1211. This DuPont-manufactured substance is available commercially in Clean guard® extinguishers. The FE-36 agent is less toxic than both Halon 1211 and Halotron I. In addition, FE-36 has zero ozone-depleting potential; FE-36 is not scheduled for phase-out whereas Halotron I production is slated to cease in 2015. A 100% non-magnetic Clean Guard model is now available (see the warning box below).

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