Fire Fighting Media

Introduction

          Firefighting media/agents refers to the substances used to extinguish fires.

1. Water.
2. Foam.
3. CO2.
4. Halon.
5. DCP.
6. Combustible Metal Extinguishing Agent.

1. Water: Water is being used extensively in fighting fires in one way or the other.

          Merits

1. It acts as a coolant which keep cool and maintain the temperature during
   firefighting.
2. It is cheap and abundantly available.
3. It is a good solvent. It dissolves and washes many products of combustion such as ash so that the seat of the fire can be reached.
4. Its viscosity change with temperature is small. It can be easily pumped and conducted in hoses and pipes with ease. 
5. It has high latent heat. The latent heat of vaporization of water is 540 calories per gram. Hence, it can absorb tremendous amount of heat when converting from boiling liquid to steam.
6. Its expansion with heat is high. It expands 1600 times (ratio 1:1600) at 1000 C (steam). Thus, it is a very good smothering agent.
7. It has high Surface Tension. The high surface tension (72 dynes per cm) enables it to issue in a consolidated stream from the so called ‘fog’ nozzle or spraying devices.
8. Its density is reasonably high. The stream from a nozzle serves penetration purposes.
9. It has high molecular stability. It does not break down to any appreciable amount in temperatures approaching 16500⁰C which are lighter than the ordinary flame temperature.

          Demerits

1. Freezing: It freezes at sub-zero temperatures. Thus, cannot be pumped and used in such situations.
2. High Surface Tension: It restricts water from penetrating and wetting various tightly packed combustibles where this action is vitally needed for deep seated fire extinction.
3. Low Viscosity: It runs off quickly from non-horizontal surface leaving a very thin coating of heat removing liquid.
4. High Friction: It gives high friction loss in its transportation through pipes and hoses. Thus, the pressure drops while passing through pipes.
5. High Density: It readily sinks into most liquid fuels without providing the expected cooling.
6. Good Conductor of Electricity: It cannot be used for live electrical fires.
7. Metal Reactive: It reacts with several metals such as Magnesium, Sodium and Aluminium with the production of Hydrogen and evolution of heat. Hence it cannot be used on metal fires.

2. Foam:    Foam is mass of gas-filled bubbles formed by various methods from aqueous solution of specially formulated foaming agents. Being lighter it floats on all flammable or combustible liquids. It excludes air at the surface of the burning liquid and provides little cooling.

It also gives a layer as it has high viscosity. Foam has water and surfactant which forms foam post combination. Water as cooling and surfactant as blanketing effect as it has high viscosity (water – 97% and surfactant – 3%). Foam is of two types:

a)    Chemical Foam: Chemical foam is produced by mixing of Sodium Bi-Carbonate and Aluminium Sulphate with stabilizer has already been discontinued from firefighting foams and declared an obsolete technology. Chemical foam is the combination of chemicals (not using now).

                6NaHCO₃   +   Al₂(SO₄)₃         →           3Na₂SO₄ + Al(OH)₃ + 6CO₂

Sodium bi        Aluminium                          Sodium       Aluminium     Gas

Carbonate        Sulphate                            
Sulphate        Hydroxide

(Baking Soda)

    b)        Mechanical Foam: These are created by turbulence or by mechanical force (by Ventury effect). Types of mechanical foams are:

i) Protein type – A foam concentrate that has a “hydrolysed protein” base. These concentrates produce dense, viscous foams in high stability, high heat resistance but foam breaks down quickly into burning liquid. It also clogs in pumps, valves, etc. in atmospheric air.

ii) Fluoro protein – It is also called Film Forming Fluoro Protein Foam (FFFP). Fluorine is high firefighting agent. A foam concentrate that has protein base and one or more fluorinated surfactant. This is improvised form of Protein foam by its partial fluorination. Its surface tension is reduced for quicker spread. Its surface viscosity is increased for stable bubble.

iii) AFFF: Formed by blending of Fluoro Surfactant into synthetic detergents, this foam gives precipitation of a very thin film. These agents are capable of forming water solution film on the surface of the flammable liquids, so they are known as Aqueous Film Forming Foam.

This air foam generated from AFFF solution’s possess low viscosity have fast spreading and levelling characteristics and act as surface barrier to exclude air and halt fuel vaporization just as other foams do.

iv) High Expansion foam (used in bubble toys): High Expansion Foam is an agent for control and extinction of A and B Class fires and is particularly suited as a flooding agent for use in confined spaces. These are made up with synthetic hydrocarbon surfactants of a type will foam copiously with a small input of turbulent action. 

v)  Alcohol type foam: It is based on aqueous hydrolysed protein film forming surfactant and alcohol resistant. It is opaque dark brown liquid, environmentally acceptable and biodegradable. It is a multipurpose Film Forming Fluoro Protein Foam (FFFP) highly effective against both hydrocarbon fuels and polar solvents such as alcohol, ethers and acetones, etc.

3. Carbon Dioxide (CO₂): CO₂ is a clean agent. Being an inert gas CO₂ does not react with fuels and is capable of dilution of Oxygen (air) in the atmosphere.

i) CO₂ is the only gas, which can be liquefied at ordinary temperatures, not under extreme high pressures and can be kept in small containers.

ii) CO₂ has very high expansion ratio of 1:450.

iii) It is 1.5 times heavier than air which makes it a good smothering agent.

iv) It is a bad conductor of electricity.

v) Doesn’t leave residue after use.

vi) It produces cooling effect.

vii) It is mildly toxic beyond 5% concentration in air.

4. Halon: It is also a clean agent. There are two types of Halons.

a. Halon 1301 – CF₃Br (Bromo Trifluoro Methane): Halon 1301 has low toxicity, and its firefighting efficiency is good. Hence it is officially recognized by NFPA (USA). For a fixed volume only about one-third as much weight is needed compared to CO₂. Since its liquid density is about twice that of CO₂, only one cylinder is needed to do the job of 6 CO₂ cylinders. It is a clean agent. It interrupts the chain reaction of fire.

b. Halon 1211 – CF₂ClBr (Bromo Chloro Difluoro Methane): It also effective as Halon 1301 but is somewhat more toxic. It has, though, economic advantage.

         Halon 1211                                             Halon 1301

i) Halon is equal to 6 CO₂ cylinders (ration – 1:6).

5. Dry Chemical Powder: DCP has three-dimensional medium as compared to foam which has two-dimensional medium. 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 DCP in use are:

a) Sodium Bicarbonate [NaHCO₃ (Baking Soda)]: It gives rapid fire extinguishment of B & C class of fire.

b) Potassium Bicarbonate [KHCO₃]: It is twice as powerful as Sodium Bicarbonate but is more expensive.

c) Potassium Chloride [KCl]: It is more powerful than Potassium Bicarbonate but is corrosive.

d) Monnex BC Powder [KHCO₃ + Urea]: It is an inclusion complex with urea bound up with Potassium Bicarbonate.

e) Mono Ammonia Phosphate (MAP) / General Purpose ABC Powder: It is very useful for A, B & C class of fires. All powders when used on ‘A’ class fires, smothering of the fuel will continue even after the powder cloud is settled. MAP Powder / ABC Powder overcome this draw back by forming a fireproof coating on the burning fuel.

DCP also has two types:    

i) Stored type:

· Gas is stored inside cylinder.

· To be send to the manufacturers to refill.

ii) Cartridge type:

· Gas is released by when cartridge puncture.

· Can refill manually.

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 control of metal i.e., ‘D’ class fires.

a) 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.

b) TEC Powder: Ternary Eutectic Chloride containing Sodium Chloride (NaCl), Potassium Chloride (KCl) and Barium Chloride (BaCl) is very effective on all metal fires. When applied it melts and forms a crust over the metal exclude air.

c) G1 Powder: It is a mixture of graphite and foundry coke, sized to facilitate easy handling with an organic phosphate.

d) Inert Gas: Only the complete inert gases such as Argon, Helium, Neon, etc. may be used for blanketing purpose.

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