FIRE: STAGES OF DEVELOPMENT

Shreya Sahajpal

What is Fire?

A chemical reaction initiated by presence of heat energy

in which a substance combines with oxygen in the air and

emits energy in the form of heat, light and sound.

Fire is a fast oxidation process.

Fire triangle:

1. fuel (something that will burn)

2. heat (enough to make the fuel burn)

3. oxygen

Fire Triangle / Tetrahedron

Fire triangle Fire tetrahedron

• Chain reaction is the feedback of heat to fuel to produce gaseous fuel used in flame.• Chain reaction provides the heat necessary to maintain the fire.

Ignition

Ignition occurs when sufficient heat is produced to cause combustion.

Chemical Reactions Electric Sparks Lightning Flame Heat of compression: If a flammable mixture is compressed

rapidly, it will be ignited when the heat generated by the compression is sufficient to raise the temperature of the vapour to its ignition point.

Hot Surfaces: Welding slag, engine exhaust, fired vessel stacks

Flash Point

The lowest temperature at which the vapor pressure of the liquid is just sufficient to produce a flammable mixture at the lower limit of flammability.

Stages of Fire

1. Incipient2. Growth3. Fully Developed (Flame)4. Decay

Ignition

Fuel, oxygen and heat join together in a sustained chemical reaction. At this stage, a fire extinguisher can control the fire.

During this initial phase of fire development, radiant heat warms adjacent fuel and initiates the process of pyrolysis.

A plume of hot gases and flame rises from the fire and mixes with the cooler air around.

Growth

With the initial flame as a heat source, additional fuel ignites.

Convection and radiation ignite more surfaces. The size of the fire increases and the plume rises. In a closed room, hot gases collecting at the ceiling

transfer heat, allowing all fuels in the room to come closer to their ignition temperature at the same time.

Fully Developed / Flame Stage

A region of first reaction which covers the period from initial occurrence of flame to a fully developed fire.

Energy release is at its greatesta) Flame Radiation: Radiant energy emitted by the flame

is transmitted hemi-spherically to distant locations independently of the convection movement.

b) Convective Heating: It becomes important only in the later phases of Fire development where large quantities of thermal energy have been released. Appreciable temperature rise at the ceiling level is produced.

Decay (Burnout)

The fire consumes available fuel, temperatures decrease, fire gets less intense

Rollover

Rollover occurs when ignited fire gases, or incompletely burned fuels, rise to the ceiling, and spread out horizontally. Then smoke appears to suddenly start burning. If nothing is done to ventilate the room or cool the air, this condition leads to flashover.

Flashover

Flashover is the sudden, simultaneous ignition of

everything in a room. Hot gases rise to the ceiling and spread out across to the

walls.

Heat radiates downward and intensifies until all combustible

items reach their ignition temperatures and burst into flames.

Temperatures soar to as much as 1,000 degrees Fahrenheit in

a few seconds.

Backdraft

Backdraft is an explosion that occurs when oxygen is introduced

into a room full of hot gases. A fire burning in a confined area consumes all the oxygen.

Visible flames disappear. Solid fuels smoulder, and hot flammable

gases accumulate and fill the room.

The temperature increases, the gases expand, and pressure builds,

pulsing against doors and windows.

If an opening is made to admit oxygen, the hot vaporized fuel bursts

into flames, and the pressurized gases explode through the opening,

resulting in a rolling fireball.

How fire spreads

Conduction: The passage of heat energy through or within a material because of

direct contact, such as a burning waste heating a nearby object, which ignites and

heats the objects hanging/placed behind, until they too burst into flames.

Convection: The flow of fluid or gas from hot areas to cooler areas. The heated air is

less dense, and rises, while cooler air descends. A large fire in an open area produces

plume or column of hot gas and smoke high into the air. But inside a room, those

rising gases encounter the ceiling. They travel horizontally along the ceiling forming

a thick layer of heated air, which then moves downward.

Radiation: Heat travelling via electromagnetic waves, without objects or gases

carrying it along. Radiated heat goes out in all directions, unnoticed until it strikes an

object. Burning buildings can radiate heat to surrounding structures, sometimes

even passing through glass windows and igniting objects inside.

Fire classifications

Class A: Ordinary combustible materials, such as wood, cloth, paper, rubber and many

plastics. They burn with an ember and leave an ash. Extinguish by cooling the fuel to a

temperature that is below the ignition temp. Water and other extinguishing agents are

effective.

Class B: Flammable liquids (burn at room temperature) and combustible liquids

(require heat to ignite). Petroleum greases, tars, oils, oil-based paints, solvents,

lacquers, alcohols, and flammable gases. High fire hazard; water may not extinguish.

Extinguish by creating a barrier between the fuel and the oxygen, such as layer of

foam.

Class C: Fuels that would be A or B except that they involve energized electrical

equipment. Special techniques and agents required to extinguish, most commonly

carbon dioxide or dry chemical agents. Use of water is very dangerous because water

conducts electricity.

Class D: Combustible metals, such as magnesium, titanium,

zirconium, sodium, lithium and potassium. Most cars contain

numerous such metals. Because of extremely high flame

temperatures, water can break down into hydrogen and oxygen,

enhancing burning or exploding. Extinguish with special

powders based on sodium chloride or other salts; also clean dry

sand.

Class K: Fires in cooking appliances that involve combustible

cooking media (vegetable or animal oils and fats).

Flame Detector

Flame detectors can provide a high-speed response in the detection of fires. Flame detector installations should consider the likely source of flame, detector cone of vision, and physical obstructions

Fire Extinguisher Types

Dry Chemical Extinguishers: Dry chemicals extinguish by interrupting the chemical reaction of the fire.

1. Stored Pressure. a) Disposable Shell. The agent and pressurizing gas are in a

sealed cylinder which is threaded into a valve and nozzle assembly. After use, the spent cylinder is discarded and a new one attached to the valve nozzle assembly. Disposable shell extinguishers usually have a capacity of 5 lb (2.27 kg) or less.

b) Rechargeable Shell. The propellant gas (usually Nitrogen) and the agent are stored in the extinguisher shell.

2. Cartridge Operated: The agent is stored at atmospheric pressure in a chamber with a large fill opening. An activating assembly consisting of a cylinder of propellant gas (C02 or Nitrogen) with a valve and gas tube assembly is connected to the chamber. Activating assemblies provide a means of releasing the propellant gas into the dry chemical chamber. The dry chemical agent is fluidized and flows from the tank to a hose and nozzle assembly.

Gaseous Extinguishers

1. Compressed Gas Units. C02 extinguishers are intended

for use on Class B-C fires. The extinguishers consist of a

pressure cylinder, a siphon tube and valve for releasing

the agent.

2. Liquefied Gas Units. Liquefied gas extinguishers have

features and characteristics similar to C02 extinguishers.

Water-base Extinguishers.

Water spray systems extinguish fires by rapid cooling effect, combined with

localized displacement of oxygen at the flame source as the water is flashed

into steam.

Two common types of extinguishers that use water. They are stored

pressure type and the pump type.

The stored pressure type is preferred over the pump type because of its ease

of operation. These extinguishers are suitable only for Class A fires, but may

be more effective than other Class A extinguishers on deep seated fires.

Foam

Foam is suitable for use on class A and class B fires, but is specifically designed for class B fires. Foam involves several of the extinguishing methods as it is mostly water, so it offers cooling capability, to float on the surface of a flammable liquid, forming barrier between the fuel surface and the air, so it excludes oxygen, this barrier acts to remove the fuel from the fire situation.