The object of the study is to know – reason of fire ( chemistry ) & to cut off the same to extinguish the fire. To need to know the fire it is required to have knowledge of chemistry of all fire & fire triangle.
The protection of fire is essential for people and property. Fire prevention can be increased by structural solution and by detection / supression systems. Solution must not, however, compromise safety aspects or be harmful to the environment.
Chemistry of fire
Combustion – The burning of a substance in air or oxygen is commonly known as combustion. But in wider sense any chemical reaction attended with evolution of heat and light is called Combustion.
Burning is a rapid form of oxidation.
Combustion is possible even in absence of oxygen. e.g., Powdered arsenic or antimony, if sprinkled over chlorine, burns spontaneously, when sparks are seen. Hence chlorine is the supporter of oxygen and the metal concern is combustible.
Slower form of combustion, such as rusting which is the oxidation of iron. But we do not think of these as fire.
Fire is an external sign of chemical action. Only gas can be ignited.
To start, the action it is necessary to apply a flame or spark having a certain minimum energy to a substance, which has been raised to a temperature sufficient to release flammable vapours to ignite.
Once started, heat energy released will radiate in all direction, heating the surface exposed to it. So the amount of gases liberated more and of the burning material increases and the fire spreads even more rapidly.
Fire triangle
The three basic requirements for a fire then are fuel, heat, air ( oxygen ), these can be represented by triangle with sides representing fuel, heat and oxygen.
Fire Tetrahedron
It is more appropriate to use a fire tetrahedron to illustrate the nature of fire.
Three sloping surfaces in the tetrahedron represents fuel, heat & oxygen and the base represents the chain reaction between the short lived chemical species following ignition.
The basic difference between the fire triangle and fire tetrahedron is this : The tetrahedron illustrates how flaming combustion is supported and sustained through the chain reaction. The chain reaction face keep other faces from falling apart.
As the temperature rises, the reaction rate increases resulting in a further rise in temperature. There is however an opposition influence due to the depletion of the fuel and its replacement by the combustion products.
In case of a flammable gas mixture, the speed of reaction is sufficiently high to result in explosion.
If the chain can be broken, the fire will be extinguished , i.e., the base of the tetrahedron (which may be considered as maintaining the sloping faces in place ) is destroyed.
The halone extinguishing agents and certain dry chemicals effectively break the chain reaction by attacking the structure of the active chemical species and preventing the reaction and killing the reaction in one hundredth of a second.
Heat
May be produced from
- Chemical energy – exothermic reaction, polymerization, de-composition, flame
- Mechanical energy – friction
- Electrical energy – sparks, heated conductors
Once formed may be transformed by
- Conduction – through solids e.g., steel bulk heads
- Convection – in circulation gases or liquids
- Radiation – directly through spaces.
Fuels
Fuels are classified as
- Solid
- Liquid
- Gaseous
- Metal
Solid – Example of wood fire :
Carbonaceous materials like wood, paper etc., consists mostly of cellulose ( C6H10O5).
Living plants create Carbohydrates ( glucose = C6H12O6 ) from water, CO2 and sun’s energy ( Photosynthesis ).
At 1710C wood begins to decompose
At 3020C this decomposition becomes violent
( C6H10O5) + 602 5H2O + 6CO2 + heat energy
Liquid – are mainly Hydrocarbons only
Grade A : F.P. below 23 0C , petrol, spirits Highly flammable
Grade B : F.P. below 23 0C – 66 0C, Kerosenes Highly flammable
Grade C : F.P. above 66 0C, cooking oil, lube oil, diesel oil safe for marine use
Volatile – Flash point below 600 C as determined by the closed cup method of testing.
Non-volatile – Flash point of 600 C or above as determined by the closed cup method of testing.
Gases – may be evolved from cargoes such as coal, oil, fermenting sugar etc.and also from LPG carrier. Depending on the vapour density, many are heavier than air and will collect in low spaces. Gas fires are explosive in nature. Different areas of a gas cloud may have different concentrations and be within the range of flammability in one place but not another.
Ignition Temperature
(a) Flash point
It is the lowest temperature at which the vapours will flash momentarily but not continue to burn, upon application of spark or flame.
Flash point is indicative of fire and explosion risk.
For example, flash point of the fuel oil carried shall not be less than 600 C.
(b) Flame point / Fire point
It is the lowest temperature at which the vapours will flash and continue to burn, upon application of spark or flame.
(c ) Spontaneous Combustion
Oxidation processes are exothermic, they give out heat and if this oxidation took place where the heat could not escape then the heat level may build up above ignition temperature and a fire is caused without spark or flame being necessary. This is called Spontaneous combustion .
The temperature at which the vapours will commence to burn without the application of spark or flame is called Spontaneous ignition temperature. This is some times referred to as Auto ignition temperature.
( The lowest temperature to which a solid, liquid or gas require to be raised to cause self-sustained combustion without initiation by a spark or flame is called spontaneous ignition temperature. This is sometimes referred to a auto ignition temperature. )
For example,
Pile of oily waste contains free carbon which will readily oxidize, but waste prevents the dissipation of the heat.
Action of bacteria on certain organic materials can cause a rise in temperature and can cause fire.
Rate of reaction and Calorific value
All combustion reactions are exothermic & heat releases heat according to their calorific value.
Other factors like rate of reaction must be considered in conjunction with the quantity of heat released in a chemical reaction.
For example, burning of magnesium produces less heat than the burning of carbon. But, magnesium has a rate of combustion than carbon so that heat released much more rapidly.
The nature of flame
Flame of a burning materials may be defined as a reaction having the ability to propagate ( or spread ) through an atmosphere with the emission of heat and light.
Propagation of flame front may be thought of transition region separating burnt from unburnt gases. Light is usually given out from this region. A fuel oxidant mixture which liberate enough energy on combustion to allow flame to spread through the ignited region of the mixture is called flammable. In a flammable medium a local ignition source produces a flame front, which passes into the next layer of gas and continue the cycle of operation like, runner in a relay race.
Combustion is a type of reaction known as chain reaction.
Explosion
There is no scientific definition of explosion. But in the wider sense, an explosion is an effect produced by sudden violent expansion of gases. This is a process of rapid physical and chemical transformation of a system into mechanical work, accompanied by a change of its potential energy may also be accompanied by shock waves and / or the disruption of enclosing materials or structure.
Oxidising Substances
An oxidising substances is a material that releases oxygen when it is heated ( or, when it comes in contact with water). For example, hypochlorites, chlorates, nitrates, chromates, oxides, etc. This burning oxiders cannot be extinguished by removing their oxygen. This can be extinguished by water.
Extinguishing
To extinguish a fire remove one side of the triangle
Cooling (water)
Starving(removal of combustible mat. )
Air
Smothering( inert gas, foam, restriction of air )
There are three basic ways of fighting a fire: –
- by removing a combusting material
- by preventing the supply of oxygen , and
- by using a cooling medium to reduce the temperature of material below ignition temperature.
Range of Flammability
A flammable gas or vapour will only burn in air if the composition lies between certain limits. There is a level of concentration of vapours below which it will be too lean to burn and above which too rich.
The lower limit of flammability is defined as the lowest concentration of fuel that will just support a self-propagating flame.
Similarly, higher limit of flammability is defined as the highest concentration of fuel that will just support a self-propagating flame
