Fire & Explosion Hazards

By

Khial Zada Khattak

Contents

• Fire– What is fire– Fire Triangle – Factors effecting fire – Flash point & ignition temperature– Types of Fire – Fire prevention– Choice & use of fire-extinguisher– Fire prevention checklist

Contents (contd.)

• Explosion– What is explosion

– Types of explosion

– Explosive limit

– Dust explosions

– Back Firing

– Prevention of explosions

– Case Histories

Fire

• A combustible chemical reaction between oxygen and any other substance accompanied by the evolution of heat, light and flame is called fire.

– The substance which takes part in combustion reaction is termed as fuel.

– The temperature at which the combustion reaction takes place is called ignition temperature.

Fire Triangle

Fuel

• Fuels are classified as:– Solid fuels.

• Such as coal, wood, paper, plastic etc.

– Liquid fuels.• Such as Petrol, kerosene oil,

lube oils etc.– Gaseous fuels.

• Such as Natural Gas, Hydrogen, Acetylene etc.

Oxygen

• Oxygen

– For most of the fuels the required oxygen to

proceed for combustion is present in air

(21%).

Heat

• It is not necessary that fire is started by an open flame. There are others heat sources which can ignite the fuel such as:– Friction

– Sparks

– Grinding

– Pneumatic drilling

– Hot surface

– Static electricity

Factors Effecting Fire

• There are four main factors which effect the combustion process.

– Particle size of fuel.

– Oxygen concentration

– Ignition temperature

– Flash point (for flammable liquids)

Factors Effecting Fire (contd.)

• Particle Size of Fuel– If a fuel (solid or liquid) is finely divided into

tiny particles it achieves ignition temperature more quickly.

– This is become of increased contact surface area of the fuel e.g;

– Saw dust and wood shaving catches fire very quickly as compared to a big piece of wood.

– It concludes that, as we go on decreasing the particle size of fuel the, flammability goes on increasing.

Factors Effecting Fire (contd.)

• Oxygen Concentration

– Oxygen increases combustibility. As the quantity or concentration of oxygen increases the combustibility also increases.

– A steal wire can not be ignited in air but it burns readily in pure oxygen.

– Cotton cloths and overalls instantly burn with a welding spark at 26% Oxygen level which at 21% Oxygen (air) does not.

Factors Effecting Fire (contd.)

• Ignition Temperature

– The ignition temperature is lowest temperature at which a substance can be ignited in presence of Oxygen (Air). e.g.

• Wood 300 C

• Paper 200 C

• Match Head 80 C

– Higher is the ignition temperature lower is the risk of fire and vice versa.

Factors Effecting Fire (contd.)

• Flash Point

– Flash point is the lowest temperature of a flammable liquid at which sufficient vapours are released by the liquid to form a flammable Vapour-Air mixture, which can be ignited under given conditions.

– Lower is the flash point of a flammable liquid higher will be the risk of fire.

Factors Effecting Fire (contd.)

• Flash Point

• If flash point of a flammable liquid is less than ambient temperature, it is highly flammable & Hazardous.

• Avoid using it. Find out a substitute with higher flash point.

Factors Effecting Fire (contd.)

• Relative vapour density

– Relative vapour density is another important property of a flammable gas or vapour that influences the risk of fire. It is density of gas or vapour relative to air (Air is taken as 1).

– Lighter gases diffuse quickly and travel upwards.

– Heavier gases travel along ground and hence distant ignition is possible.

Relative Vapour DensitySubstance Substance Relative Vapour Density Relative Vapour Density

Methane Methane 0.550.55 Methanol Methanol 1.111.11 Ethanol Ethanol 1.601.60 Petrol Petrol 4.004.00 Ether Ether 2.002.00 Oxygen Oxygen 1.11.1 Hydrogen Hydrogen 0.10.1 Carbon dioxideCarbon dioxide 1.51.5 Carbon MonoxideCarbon Monoxide 0.960.96 Ammonia Ammonia 0.600.60 AcetyleneAcetylene 0.980.98

Types of Fire• Type A Fire

Fire on solid fuels e.g. wood, paper, plastic etc.

• Type B Fire

Fire of flammable liquids & Gases e.g. Petrol, Lube oils, Methane, Hydrogen.

• Type C Fire

Fire on energized electrical circuits, Fuse boxes, switches etc.

Fire Prevention

• Three pre requisites of fire triangle should not Combine.

Fire Prevention Remove fuel if Heat is present.Remove fuel if Heat is present.

Fire Prevention Remove Heat if Fuel is present .Remove Heat if Fuel is present .

Fire Prevention Remove Oxygen in presence of Fuel and Remove Oxygen in presence of Fuel and

HeatHeat

Choice and use of Fire extinguishers

Types of Fire Types of Fire Extinguishers Extinguishers

Types of Fire Types of Fire

AA BB CC

Dry Powder Dry Powder Yes Yes YesYes Yes Yes

COCO22 NoNo Yes Yes YesYes

BCF Halon 1211/Halotron-1BCF Halon 1211/Halotron-1 YesYes Yes Yes YesYes

Chemical FoamChemical Foam Yes Yes YesYes No No

Fire Extinction

• Remove any pre requisite of fire triangle.

Fire Extinction

• Remove Fuel.

Fire Extinction

• Remove Heat.

Fire Extinction

• Remove Oxygen (Air).

Fire Prevention Checklist

• Electrical Equipment– No make shift wiring– Extension cords serviceable – Motors and tools free of dirt and grease – Lights clear of combustible materials – Safest cleaning solvents used – Fuse and control boxes clean and closed – Circuits properly fused – Equipment approved for use in hazardous

areas (if required) – Ground connection clean and tight

Fire Prevention Checklist (contd.)

• Friction

– Machinery properly lubricated

– Machinery properly adjusted and / or aligned

• Special Fire Hazard Materials

– Storage of special flammables isolated.

Fire Prevention Checklist (contd.)

• Welding and Cutting

– Area surveyed for fire safety

– Combustibles removed or covered adequately.

– Work Permit issued.

– Hot pipes clear of combustible materials.

– Fire extinguishers placed near by.

– Ashes in metal containers.

Fire Prevention Checklist (contd.)

• Smoking and Matches

– “No smoking” and “ smoking” areas clearly marked

– Butt containers available and serviceable

– No discarded smoking materials in prohibited area

• Spontaneous Ignition

– Flammable waste materials in closed, metal containers & emptied frequently.

– Trash receptacles emptied daily

Fire Prevention Checklist (contd.)

• Static Electricity – Flammable liquid dispensing vessels be

grounded.– Moving machinery grounded.

• House Keeping– No accumulation of rubbish. – Passage ways clear of obstacles. – No leaks of fluids and floor free of spills. – Fire doors unblocked and operating freely.– Fire extinguishers functional & unblocked.

Explosions

A violent and rapid increase in

pressure in a confined space which

may occur as a result of a physical

change or a chemical reaction.

Types of Explosions

• Explosions are classified as:

– Physical Explosion

– Chemical Explosion

Physical Explosion

An explosion that occurs as a result of a physical change i.e. compression or heating is known as physical explosion

Bursting of pressure cooker or compressed gas cylinders are the examples.

Chemical Explosions

A chemical explosion is that which occurs as a result of a pressure increase caused by the energy released during a chemical reaction. When a piece of sodium metal is put in water, it reacts violently producing Sodium Hydroxide and Hydrogen. The temperature rises so high that the Hydrogen produced bursts into flame and explosion occurs.

Explosivity

The extent to which an explosion can occur by a

fuel-air mixture is explosivity of the fuel.

Not every fuel-air mixture are equally explosive.

Explosion can only occur if the concentration of a

flammable gas or vapour lies between upper and

lower explosive limits.

Lower Explosive Limit

It is the minimum percentage of a gas or vapour in air below which fuel is not enough to react explosively with the oxygen.

Lower explosive limit (LEL) of Methane is 5% by volume in air.

Lower is the value of LEL, Higher is the risk of explosion.

Upper Explosive Limit

It is the maximum percentage of gas or vapour in air that gives the explosive mixture. Above this concentration Oxygen is not enough to react explosively.

Upper explosive limit of Methane is 15% by volume in air. If Methane is present in higher concentration the explosion will not occur.

Explosive Range

The span between UEL & LEL is explosive

range.

Wider the explosive range of a fuel greater is

the risk of explosion. Methane has UEL 15%,

LEL 5% by vol. in air where as Acetylene has

UEL 81% & LEL 2%. So Acetylene having

quite bigger range is more explosive.

Dust Explosions

Finely divided organic materials such as

dust of sugar, flour, carbon, milk powder

may combine with oxygen violently, on

provision of ignition source resulting in

explosions.

Back Fire

Explosions may also occur as a result of

back fires. The person working on Boilers,

furnaces and closed vessels (containing

fuels) with vent stacks must take care of it.

To avoid back firing vent stacks should be

equipped with flame arresters.

Explosive Limits & ranges of some Fuels

FuelFuelLower Lower

Explosive Limit Explosive Limit in Vol. % LELin Vol. % LEL

Upper Upper Explosive Explosive

Limit in Vol. % Limit in Vol. % UELUEL

Explosive Explosive Range in Range in

Vol. %Vol. %

Carbon Carbon monoxidemonoxide 12.512.5 7474 61.561.5

MethaneMethane 55 1515 1010

HydrogenHydrogen 44 7474 7070AmmoniaAmmonia 1515 2929 1414AcetyleneAcetylene 22 8181 7979

Propane Propane 2.52.5 9.59.5 77

AcetoneAcetone 2.62.6 12.812.8 10.210.2

Table Shows Explosive Limits & ranges of some Fuels

Explosive Limits & ranges of some Fuels

FuelFuelLower Explosive Lower Explosive Limit in Vol. % Limit in Vol. %

LELLEL

Upper Upper Explosive Limit Explosive Limit in Vol. % UELin Vol. % UEL

Explosive Explosive Range in Range in

Vol. %Vol. %

Diethyl EtherDiethyl Ether 1.71.7 3636 34.234.2

PetrolPetrol 11 88 77

MethanolMethanol 66 3636 3030EthanolEthanol 33 1919 1616BenzeneBenzene 1.21.2 88 6.86.8

XyleneXylene 11 66 55

Carbon DisulfideCarbon Disulfide 11 6060 5959

Table Shows Explosive Limits & ranges of some Fuels

Prevention of Explosions

• When flammable gases or vapours are mixed with air, explosive mixtures are formed, which are sometime very easily ignited. Any electrical apparatus used on premises when such mixtures can be formed must be explosion proof.

• Flammable liquids must never be poured into the drains.

• Waste water that has come in contact with a flammable liquid may only be released into the drains through a separator which will remove any flammable residues.

Prevention of Explosions (contd.)

• Care must always be taken with apparently empty containers that have held a flammable liquid or gas. Such containers are not necessarily ‘EMPTY’ just because they look empty.

• All flammable liquids dispensing vessels should be grounded.

• All moving machinery should be grounded.• Premises should always be free from

unnecessary combustible materials.• All vessels, pumps motors and pipe lines should

be grounded.• Steam should be given in vent stacks during

rains.

THANK YOU &

Have a nice day