FIRE FIGHTING MEDIAS

1) WATER:

Advantages:

· good solvant

· viscosity change with temperature is small so can be easily pumped.

· high latent heat of vaporizing (540 Cal/gm) so it absorbs large amount of heat.

· expansion with heat is too high: 1lit water = 1600 lit steam which provide good smothering effect.

· high surface tension: 72 dyne/ cm. so it provides solid jet for spray.

· high density provide penetration in fire seat.

· high molecular stability: at high temp water converted in steam rather than  H+ and OH-. AT 16500 C.

· cheap and easy available.

Disadvantages:

· freeze at 0 C so can’t be pumped.

· high surface tension: can’t go to tight packed combustible material.

· low viscosity: can’t be use in non-horizontal surface.

· high friction: there is high friction losses in transporting water from pipes so the pressure at delivery will always less then at supplied from source

· high density: can’t be use in liquid fire because most of combustible liquid are lighter then water.

· good electric conductor: can’t be use in electric fire.

· metal reactive: can’t be use in metal fire .(if u spray water on sodium there will be big blast due to reaction).

2) FOAM:   

I) CHEMICAL FOAM

II) MECHANICAL FOAM

                        a)  protein foam

                        b)  floro protein foam

                        c)  AFFF- aquas film forming foam.

                        d)   high expansion foam

                        e)  alcohol resistance foam

I) CHEMICAL FOAM:

sodium bicarbonate and aluminum sulphate which is stored in  extinguisher reacts and gives foam and CO2

                        6NaHCO3 + AL2 (SO4)3   ----------> 3Na2 SO4 + 2 AL(OH)3 + 6 CO2

      II) MECHANICAL FOAM:

                      A) PROTEIN FOAM: also known as hydrolyzed protein utilized aqueous liquid                                                      concentrates proportioned with water for generation.

- which produce dense, viscous foam of high stability, high heat resistance and better resistance to reignition.

- but it breakdown quickly if these are ploged in to the burning liquid

- it need protection against atmospheric air with which it forms scales that would clog pipes, pumps, valve or orifice

- compatibility with other chemical is low.

B) FLUORO PROTEIN FOAM:

- is improvised form of protein foam by its partial fluorine nation.

- its surface tension is reduced for quicker spread.

- its surface viscosity is increased for stable bubble formation.

C) AFFF (AQUAS FILM FORMING FOAM):

- formed by blending flauro surfactance in to synthetic detergents.

- this foam gives precipitation of very thin film.

- because these agents are capable of forming water solution film on the surface of the flammable liquids, these are known as aqueous film forming foam.

- has fast spreading and leveling characteristic.

- fil, thickness = 0.0127 mm.

- they are used in 3-6 % proportion with water.

D) HIGH EXPANSION FOAM:

- -use to control class A and class B fire.

- this made up with synthetic hydrocarbon surfactance of a type will foam copiously with a small input of turbulent action

- usually used in 2 % proportioned in water.

E) ALCOHOL TYPE FOAM: (FFFP)

- it is based on aqueous hydrolyzed protein film forming surfactant and alcohol resistance

- it is opaque dark brown liquid, environmentally acceptable and biodegradable.

- it is multipurpose film forming fluoro protein foam (FFFP).

- high effective for alcohols and acetones etc.

3) CARBON DIOXIDE (CO2) :

- it is an inert gas.

- does not react with fuels.

Characteristics:

It can be liquefied at low temperature and pressor so can easily filled in CO2extinguishers.

                     - very high expansion ratio of 1 : 450.

                     - produce a cooling effect.

                     - immediately toxic beyond 5% concentration in air.

                     - 1.5 time heavier then air which makes smoothering effect.

                     - bad conductor of electricity.

4) HELOGENATED HYDROCARBONS:

- like carbon tetra chloride, methyl bromide, methyl iodide and chlorobromo methane.

- more effective then co2

- but highly toxic and BANNED BY LEGISLATION.

- innovative research led to a compound that retained the bromine content but had hydrogen atoms in the methyl portion replaced with fluorine, the result was effective with improved thermal stability.

             the chemical , bromo trifuro methane, CF3Br because of its low toxicity and its fire extinguishing efficiency, was officially recognized by NFPA(national fire protection association).For a fixed volume only about one-third as much weight is needed compared to C02. Since its liquid density is about twice that of C02 (at 700 F), only one cylinder is needed to do the job of six heavier cylinders of C02.

            Halon 1301 is a clean agent. It resists decomposition until about 8000 F. It inter-

rupts the chain reaction of fire.                                

            Similar gas bromo-chloro-di-fluoro methane (Halon 1211) is also as effective as Halon 1301 but is somewhat more toxic. It has, though, economic advantage.

Both Halon 1301 and Halon 1211 are considered most suitable for firefighting.

5) DRY CHEMICAL POWDER:

                   Dry Chemical Powder is important because of its being three-dimensional medium s compared to two-dimensional medium of Foam. At the same time, it does not provide any sealing to the surface of fuel, preventing evaluation of flammable vapor. Hence it leaves chances of re-ignition, in case the fuel is not sufficiently cooled to below its ignition temperature.

IMPORTANT DRY POWDERS IN USE ARE:

(a) SODIUM BICARBONATE:

                                         Sodium bicarbonate together with flow promoting additives it gives rapid fire extinguishment of B & C class of fire.

(b) POTASSIUM BICARBONATE:

                                         It is twice as powerful as Sodium bicarbonate but is more expensive.

        (c)   POTASSIUM SULPHATE:

                                        It is as good as Sodium bicarbonate, but it is compatible with foam

(d)   POTASSIUM CHLORIDE:

  It is more powerful than Potassium, bicarbonate but is corrosive

(e)   MONNEX BC POWDER:     

           It is an inclusion complex with urea bound up with potassium bicarbonate. It can be used very effectively in portable extinguishers. Small quantity of powder can extinguish, sometimes, a big fire depending upon the operational skill of the operator.

(f)  GENERAL PURPOSE ABC POWDER:

              All powders when used on A class fires, smoldering of the fuel will continue even after the powder cloud is settled. ABC Powders overcome this draw back by forming a fire proof coating on the burning fuel.

Such powders are based on mono-ammonium phosphate. It is useful for class A, B and C fires.

6) COMBUSTIBLE METAL EXTINGUISHING AGENTS:

                The flame zone in the combustion of metals is a high temperature and highly reactive area. The following extinguishing agents are used for effective contro1 of metal or 'D' class fires.

 (a) G-l POWDER:

                                  It is a mixture of graphite and foundry coke, sized to facilitate easy handling with an organic phosphate. This is carefully applied to the metal fires with a shovel.

(b)       MEL - L - X POWDER:

                                                   It is composed of Sodium Chloride, Tri calcium phosphate, Zinc Stearate and powdered Thermo Plastic material.

                                                  It is suitable for all types of metal fires.

(c)       INERT GAS:

                                                  Only the complete inert gases such as.Argon, Helium, Neon etc. may be used for blanketing purposes.

(d)    TEC POWDER:

                                                 Ternary Eutectic Chloride containing Sodium Chloride! Potassium Chloride and Barium Chloride is very effective on all metal fires. When applied it melts and forms a crust over the metal exclude air.