As stated before, if any of the 3 elements viz fuel, oxygen, heat is removed, the fire will die out. If the chain reaction is broken, resulting reduction in vapours and heat production will rapidly extinguish the fire.
Removal of Fuel Starvation:
a. Remove combustible material away from seat of fire. e.g. drain fuel burning fuel tank or discharge cargo from a ship on fire.
b. Cut off liquid or gaseous fuel supply to seat of fire by shutting off proper valves.
Removal of Oxygen or Smothering:
Any fire will be extinguished if oxygen level in the air surrounding the seat of fire drops below 16% by volume.
Carbon dioxide or foam will smother the fire and starve the fire off oxygen.
Exceptions of this phenomenon are oxidising substances that release oxygen when heated or when they come in control with water. These substances include hypo chlorites, chlorates, nitrates, chromates, oxides and peroxides. Thus burning oxidisers cannot be extinguished by smothering them. Instead copious amount of water will have to be used for this purpose.
Removal of Heat or Cooling:
Fresh as well as seawater is an excellent heat absorber, if applied directly to the seat of fire, quickest heat reduction can be achieved.
Breaking the Chain Reaction or Inhibition:
Fire will be extinguished rapidly once the chain reaction sequence is broken. Dry chemicals are commonly used to attack chain reaction and inhibit combustion. These chemicals rapidly and directly attack the molecular structure of compounds formed during the chain reaction sequence. Break down of these compounds adversely affect the flame producing compatibility of the fire.
Point to note is these chemicals do not cool a smoldering fire or a liquid container heated above the liquid’s ignition temperature. So a cooling medium like water must be subsequently used to cool the smoldering embers or the sides of the container.
Fire Extinguishing Agents
8.1 Water
Water acts as a cooling agent, absorbs heat and cools burning material more effectively than any other fire-extinguishing agent.
Water when used to douse a fire will keep absorbing heat till its temperature rises to 100o c. at this temperature, water will absorb more heat to convert itself into steam thus moving a large chunk of absorbed heat away from burning material.
Steam being almost 1700 times more voluminous than water from which it generates, acts as smothering agent, smothers the burning material and starves it off oxygen.
Seawater is just as effective for Fire Fighting as Fresh Water
Onboard a ship, water is moved to a seat of fire in 2 ways.
a. Using ship’s firemain, fire hydrants, fire hoses and nozzles controlled by ship’s crew.
b. Using fixed automatic sprinkler system.
Water is easily available in large quantities.
Disadvantages of water medium:
a. It is a conductor of electricity.
b. Can damage cargo or machinery.
c. If use in large quantities, may cause stability problems for vessel.
d. Cannot be used on cargoes that flammable gases in contact with water.
8.2 Foam
Foam is the most suitable for extinguishing fires involving flammable liquids.
Foam forms a blanket of small bubbles on the surface of burning liquid preventing fuel vaporising and restricting oxygen supply. Water portion in the foam also cools down the surrounding structure below the ignition temperature of the flammable liquid and hence it is important that the foam blanket is maintained even after the flames have been extinguished.
Ideally, consistency of foam solution should be such that it should be able to flow freely to cover burning liquid rapidly, maintain a vapour tight blanket and must retain water to provide a long lasting seal.
The foam solution should be light enough to float on the material on fire and heavy enough to resist winds.
Chemical Foam
Chemical foam is the product of a chemical reaction when an alkali solution (like sodium bicarbonate, Na2Co3) is mixed with an acid solution (aluminium sulphate, Al2So4). Carbon dioxide gas is also a by product of this reaction but has no fire extinguishing value. Only purpose it serves is to inflate foam bubbles.
For each unit volume of water, 7 to 16 volume units of foam is produced.
This type of foam routinely used in portable fire extinguishers.
Mechanical Foam
A foam consternate is mixed with water to form foam solution, this mixed turbulently with air will form mechanical air foam. As this term suggests, bubbles are filled with air, degree of mixing decides the foam quality and design of equipment will control of the foam quality produced.
Mechanical foam is produced from proteins, synthetic detergents and surfactants.
8.2.1 Types of Foam
1. Protein Foam
Derived from natural protein solids. Produce dense viscous foam of high stability, high heat resistance and good resistance to burn back. It is non toxic, biodegradable after dilution and cheapest in cost. Either 3% or 6% concentrates are available for shipboard use.
2. Fluoroproteins Foam
Contains fluorinated surface active agents and ideal for subsurface injection for tank fire fighting. Non toxic and biodegradable but more expensive than a protein foam. Used for fire fighting in oil refineries and short storage tanks.
3. Alcohol Resistant Foam
In case of fire, water soluble organic flammable liquids such as alcohols, enamel and lacquer thinners, Ketones, Ethers, Acrylonitrile, acetates, amines and Aldehydes will break down ordinary protein foam if used.
Alcohol resistant foam concentrates are proprietary compositions of several types containing protein, flour protein or AFFF base and are blended with an insoluble soap to permit its use on water soluble organic flammable liquids.
This type of foam concentrate is used on a chemical tanker and its application rates are higher than a protein foam.
4. Aqueous Film Forming Foam (A F F F)
Surfactants include detergents, wetting and liquid soaps and are widely used to produce AFFF or aqueous film forming foam. This product was initially developed by us naval research laboratory.
AFFF controls the vaporisation of flammable liquids by means of a water film that foam as the foam applied. It also cools and blankets the material on fire like any other foam. This double action gives a highly efficient quick acting foam cover for combustible liquid spills.
One end of surfactant molecule is polar and water soluble whereas other end is non polar and oil soluble. When a surfactant is mixed with water before application, polar end will dissolve in water but the non-polar end would remain intact. Once sprayed on a flammable liquid, this non-polar end dissolves in the liquid fuel and polar end drags water along with it.
As water is always heavier than fuel, this polar end should sink in the flammable liquid as per law of gravity. But the surface tension holding non-polar end being greater than the force of gravity on polar end, an extremely thin water film (to the tune of 003 cm) floats on top of flammable liquid.
Remainder free water in the solution would sink below the fuel surface to the bottom of the container. This thin water film keeps flammable vapours beneath its surface and prevents them to reach flames, ceasing flame production.
On ships, AFFF is used normally in Portable Fire Extinguisher.
8.2.2 Foam: Advantages and Disadvantages
Advantages:
a. Effective smothering agent provides cooling as secondary effect.
b. Can be used on class A and class B fires.
c. Prevents flammable vapours from rising.
d. When applied on the upstream side of oil spill, will effectively blanket the spill, does not break down readily, stays put and absorbs heat which may cause resignation.
e. Uses water economically, does not tax ship’s pumps, do not need much storage space.
Disadvantages:
a. Effective only in horizontal plane. In case fire is burning higher up in a protected space, foam will not be effective.
b. Foam compounds are expensive and deteriorate over a time period.
c. Do not offer any protection to personnel trapped in fire. If trapped in foam blanket, personnel find difficult to escape.
d. Foam systems are difficult to test.
8.3 TYPES OF FOAM CONCENTRATES
Low Expansion Foam:
Have an expansion ratio upto 50:1
Typical expansion ratio for low expansion foams used onboard ships is between 5:1 and 15:1
This type of foam is widely used onboard oil and chemical tankers. Foam concentrate is mixed with seawater and supplied to cargo block with fixed foam monitors located on main deck as well as with potable foam applicators.
Medium Expansion Foam:
Have an expansion ratio upto 500:1
Typical expansion ratio for low expansion foams used onboard tankers operating in special trades ranges between 75:1 and 150:1
Foam concentrate is mixed with seawater and delivered to cargo block with seawater and delivered to cargo block with monitors that have enlarge outlets with nets allowing more expansion.
High Expansion Foam:
Have an expansion ratio upto 15000:1
Typical expansion ratio onboard use 600:1
High expansion foam is designed for fires in confined spaces like engine rooms. Heavier than air but lighter than water or oil, it flows down openings and fills the compartment on fire, replacing air from corners and crevices. Effective on both class A and class B type fires. Produced by foam generators with air fans, nets and ducts.
8.4 CARBON DIOXIDE
Carbon dioxide is a non-flammable. Colourless, slightly acidic gas about 1.5 times heavier than air and can penetrate and spread to within the fire area effectively to smother the fire.
Being non-combustion, carbon dioxide does not chemically react with most substances.
Carbon dioxide can be compressed easily to liquefy it. Its critical temperature is 31o c, which means above this temperature, it will only remain as gas regardless of pressure.It is a non-conductor of electricity and does not deteriorate in quality with age.
To be effective, quantity of carbon dioxide required amounts to 30% volume of the space to be protected.
Can be effective on fires involving flammables oils and greases, electrical & electronic equipment paints and thinners.
It can be used for total flooding of machinery space, pump room, paint locker and bow thrusters space.
Disadvantage:
Has a limited cooling effect.
Although CO2 is non poisonous for humans, it is suffocating in the concentrations required for extinguishing fires. A person exposed to CO2 concentration of more than 9% by volume in air will suffer dizziness and will become unconscious. Unless removed to fresh air quickly, a person could be
Cylinders are heavy and maintenance difficult. Once used, CO2 cylinders will have to be recharged ashore prior another use and this facility may not be available or expansive if available at many ports.
8.5 DRY CHEMICAL IN POWDER FORM
May be installed in a fixed system or in portable and semi-portable fire extinguishers.
Dry chemical agents extinguish fires to the greatest extent by breaking the combustion chain.
They offer extremely little cooling, smothering and shielding of radiant heat.
Sodium Bicarbonate:
Very economical dry chemical, which is extensively used for fire fighting in galley, range, hood and duct fires as it is particularly effective on animal fats and vegetable oils.
If used on fires involving inflammable liquids, there is a possibility of fire flash back.
Potassium Bicarbonate:
Most effective on liquid fuel fires in driving flames back and eliminating flash back. More expensive than sodium bicarbonate.
Potassium Carbonate:
Fire extinguishing properties are as good as that of potassium bicarbonate. It however causes corrosion after it has extinguished fire.
Urea Potassium Bicarbonate:
Very effective fire fighter but not used widely due to high costs.
Monoammonium Phosphate (A B C Multipurpose)
Effective on class A, class B and class C fires.
Once in contact with the fire, the phosphate changes into met phosphoric acid, a glassy fusible material and covers solid surfaces with a fire retardant coating.
Disadvantage:
1. Forms an opaque cloud on release affecting visibility.
2. May cause breathing difficulty for fire fighters.
3. Not effective on materials which release oxygen if heated as well as combustible metals likes sodium, magnesium, potassium etc.
4. May deposit an insulated coating on electronic or telephonic equipment on fire.
5. If moisture is present, dry chemical may corrode or stain surface on which it settles.
6. May not be able to extinguish a deep-seated class a type fire.
8.6 DRY POWDERS
Dry powders were developed to control fires involving combustion metals like sodium, potassium, magnesium etc.
They compose of mainly graphite, which cools the fire and creates very heavy smoke, which will smother the fire.
Applied to seat of fire by shovels or scoops.
8.7 SAND
Can be used for fighting small fires and to be applied by shovels or scoops. Will another the fire once applied.
8.8 HALOGENATED HYDROCARBONS
Halogenated extinguishing agents consist of carbon and one or more of halogen elements like fluorine, chlorine, bromine and iodine.
IMO prohibits new fixed fire extinguishing installations, which use halogenated hydrocarbons.
Also as of now many countries have prohibited non-maritime use of any of the halogenated hydrocarbons.
Amongst halogenated hydrocarbons, Halon 1211 and Halon 1301 are most used.
Halon 1310 is an odourless and colourless gas as well as a nonconductor of electrically. It is stored and shipped as a liquid under pressure. Once released onto a seat of fire, it is propelled by its storage pressure and acts as a chain inhibitor.
Halon 1211 is colourless but has a faint sweet smell. It is non-conductor of electricity. It is stored and shipped as a liquid, pressurized by nitrogen gas. Once released upon a seat of fire, it will act as a chain inhibitor.
Advantages:
1. Effective on class A, class B and class C fires.
2. Non-corrosive and non-abrasive.
3. Leaves no residue to clean up. Thus can be used on fires in electrical and electronic equipment.
Disadvantages:
1. Difficult to obtain and difficult to dispose of.
2. Relatively toxic.
3. Cannot be used on combustible metals and hydrides.
8.9 FIXED FIRE EXTINGUISHING SYSTEMS: REQUIREMENTS
