40 Questions
What is the critical rate of application of a fire extinguishing agent dependent on?
The capacity to quench Free Radicals
Why are Dry Chemical Agents not suitable for use in telephone exchanges and computer equipment rooms?
Because they can render electrical equipment inoperative
What is the disadvantage of using Dry Chemical Agents in conjunction with water?
It reduces the effectiveness of the Dry Chemical Agent
What property of Dry Chemical Agents prevents them from producing a lasting inert atmosphere?
Their insulating properties
What is the result of using Dry Chemical Agents in the presence of re-ignition sources?
Temporary extinguishment of the fire
What is the benefit of using Halon extinguishing agents?
They create an inert atmosphere
What is unique about the flame-inhibiting property of Halon extinguishing agents?
It creates an inert atmosphere
Which type of Halon is more effective than others?
Bromine-based Halon
What is the fourth component of the Fire Tetrahedron?
Free Radicals
What is the combustion reaction of Octane with oxygen?
C8H16 + 12O2 → 8CO2 + 8H2O
According to the Collision Theory, how many molecules can collide simultaneously?
3-4 molecules
What is the reason for the reaction of Octane with oxygen not taking place in a single step?
Not more than 3-4 molecules can collide simultaneously
What is the characteristic of the products of the first stage of the reaction?
They have incomplete molecular structures
Why do the products of the first stage of the reaction react immediately in the next stage?
Their activation energy levels are low
What is the result of the reaction of Octane with oxygen in the first stage?
All of the above
How does the reaction of Octane with oxygen take place?
In several stages
What type of fires are Halons not effective against?
Class D fires
What is the primary mechanism by which Halons extinguish fires?
Choking off oxygen
What does the first digit in the Halon nomenclature represent?
Number of carbon atoms
What is the deadline for phase out of Halons in developed countries according to the Montreal Protocol?
01 Jan 1995
What is the purpose of creating a Halon bank in the Navy?
To procure and store Halon for use until 2010
What is the trend in toxicity and extinguishing properties of Halons from F to I?
Increase in toxicity and extinguishing properties
What type of atom is not numbered in the Halon nomenclature?
Hydrogen
What international agreement aims to phase out ozone-depleting substances, including Halons?
Montreal Protocol
What is a characteristic of electrically non-conducting fire extinguishing agents?
They do not leave a residue upon evaporation
What is the chemical name of the trade name PFC-410?
PerFluoroButane (C4F10)
What is the primary way HFC-227ea achieves effective fire extinguishment?
By physical cooling of the fire
Why is HFC-227ea safe to use on energized electrical equipment?
Because it is non-conductive
What is the trade name of the blend of HydroChloroFluoroCarbons?
NAF S-III
Why is R-595 Blend classified as a transitional substance?
Because it is a HydroChloroFluoroCarbon (HCFC)
What is the chemical name of the trade name FE-13?
TriFluoroMethane (CHF3)
How does the volume required for the stowage of HFC-227ea compare to that of Halon 1301?
It is approximately 1.6 times more than that of Halon 1301
Why are inert gases not suitable for rapidly escalating hydrocarbon fires or explosion inertion in occupied areas of a ship?
Due to their long discharge time and large agent volume
What is the droplet size of water mist?
1000 microns
How does water mist combat fires?
By cooling, oxygen displacement, and radiant heat attenuation
What happens to the water mist when it absorbs heat from the fire?
It gets converted to steam expanding up to 1700 times by volume
What is an additional benefit of water mist systems?
They clean, absorb smoke and irritant gases, and particulate
Why are water mist fire protection systems complex?
Due to their high pressure requirements and nozzle design technology for providing fine atomization
Where do water mist fire protection systems find application?
In areas such as Turbine Hoods, Flammable Liquid Storage, Machinery Rooms, Emergency Generator Rooms, Switch Gear Rooms, and Engine Rooms
What is an advantage of water mist systems?
They are environmentally acceptable and do not contribute to ODP or Global Warming
Study Notes
Free Radicals
- Free Radicals are the fourth component of the Fire Tetrahedron
- They form in the combustion of fuels like Octane, which combines with oxygen to produce carbon dioxide and steam
- The reaction involves multiple stages, not a single step, due to the limited number of molecules that can collide simultaneously
Chain Reaction
- The combustion of Octane involves a chain reaction, with the initial reaction producing highly unstable products with incomplete molecular structures
- These products react immediately in the next stage, completing the reaction in stages
Extinguishing Agents
- The critical rate of application of an extinguishing agent depends on its capacity to quench Free Radicals
- Faster quenching requires less powder to extinguish the fire, and smaller particle size helps cover a larger area
Limitations of Dry Chemical Agents
- Dry Chemical Agents do not produce a lasting inert atmosphere above a flammable liquid's surface
- They should not be used in areas with relays and delicate electrical contacts, as they can render equipment inoperative
- They can be slightly corrosive and should be removed from undamaged surfaces after fire extinguishment
- The use of water and dry chemical agents should be avoided
Halons as an Extinguishing Agent
- Halogenated extinguishing agents are hydrocarbons with one or more hydrogen atoms replaced by atoms from the halogen series
- They are non-flammable, non-toxic, and leave no residue
- Halons create an inert, survivable atmosphere in the compartment, and are effective on Class B, C, and E fires, but less effective on Class A fires
- They are not to be used on Class D fires
Halon Nomenclature
- The first digit of the number represents the number of carbon atoms in the compound molecule
- The second digit represents the number of fluorine atoms, the third digit represents the number of chlorine atoms, and the fourth digit represents the number of bromine atoms
- The fifth digit represents the number of iodine atoms (if any), and hydrogen is not numbered
Halon Substitutes and Alternatives
- The Montreal Protocol has led to the phase out of ozone-depleting substances, including Halons
- The international community has formulated alternatives to Halons, including:
- FC-3-1-10 (PerFluoroButane, C4F10)
- HFC-227ea (HeptaFluoroPropane, CF3CHFCF3)
- R-595 Blend (a blend of HydroChloroFluoroCarbons)
- HFC-23 (TriFluoroMethane, CHF3)
- Water Mist (an environmentally acceptable solution)
Water Mist
- Water Mist is defined as water having a droplet size of 1000 microns
- It combats fires by cooling, oxygen displacement, and radiant heat attenuation
- Water Mist systems are effective, clean, and absorb smoke and irritant gases, making them suitable for various applications, including Turbine Hoods, Flammable Liquid Storage, and Machinery Rooms.
Understanding the role of free radicals and chain reactions in combustion processes, including the combustion of fuels like octane.
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