Understanding Fire and Explosion: Chemistry, Heat Transfer, and Safety

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welcome to this fire and exclusion module and in this particular module we are going to discuss about the fire and

exclusion what are the basic definition of fire and exclusion and what is the difference between the fire and

exclusion what kind of and conditions required to produce any kind of fire what how the

heat being generated in fire or explosion being transmitted to some other places that is modes of heat

transfer we will discuss about the fire triangle what are the necessary ingredients and what are their necessary

arms of this fire triangle we will discuss about the various kind of a flammability characteristics now we know

that fire is extremely dangerous not only for our domestic affairs but for the various industrial establishment now

sometimes this fire may lead to explosion because explosion is the rapid generation of shockwaves maybe because

of fire may or may not be because of some other reasons so in this particular module we will discuss that what are the

different conditions required for the fire how the fire may lead to the explosion what kind of explosion and are

dangerous for the industrial establishments because ultimately whenever there is a fire then definitely

there is a law loss of not only to the human being but also for the vegetation animal and above all the property and

sometimes explosion may lead to the hidden danger to the nearby buildings to the vegetation to the human being

etcetera now question arises what is fire now fire is very common we cannot imagine our sustainability without fire

we cannot cook food we cannot have several other aspects we cannot reduce is three etcetera so what is fire the

wrath this is the rapid chemical process that produces heat and usually light now fire

is a visible effect of the process of combustion now for combustion what kind of different conditions required we will

discuss in this particular module now it occurs between oxygen in the air and some sort of fuel this is known as fire

triangle now before we go into detail of this fire triangle let us have a thing that what is fuel now fuel is that what

is actually being burned because everything is not having the probability of fire generation ability now this fuel

is available in three physically stated States solid liquid and the gas was best example of solid is cool liquid your

gasoline diesel gas was natural gas etc so combustion usually occurs when fuel presents in gas was state now let us

have a discussion about the chemistry of combustion now usually oxygen and fuel they are

having the affinity in three aspects one is oxidation another one is a combustion now if

oxygen is not present then pyrolysis and usually whenever we discuss about fire then there are two three major issues

related to the sustainability of the fire so if there is a net release of heat then only the fire will sustain now

when there is a combustion then we can expect a variety of products for of combustion usually the combustion

produces smoke and other substances now this is smoke sometimes carry some dangerous or toxic materials so specific

products whatever product being generated during the combustion they are dependent on what kind of fuel being

used and what is the temperature and how much quantity of oxygen is available for that particular process

suppose there are unsaturated hydrocarbon then they may lead to the generation of co2 Cu etcetera and how

much fuel is being consumed during during the process of fire so these all the product of combustion they all

depend on these three criteria or these three three three systems sometimes you may experience that a few fires may

consume all available fuels now let us have a look of smoke because usually when there is a fire then definitely you

can expect a certain quantity of smoke being generated now usually the smoke U is airborne product of combustion

usually consists of ashes some gases some unburned hydrocarbons or some unburned fuel different aerosols and

usually these sometimes these ashes gases or aerosols they causes severe health hazards so

inhalation of smoke they may cause severe illness sometimes heaving it may lead to the dead and usually the

dangerous part is that the generation of this type of smoke cloud it may propagate to the to the nearby

population we are these ashes the people may inhale the ashes gases aero solar set at may create the health hazard now

the smoke contents they are usually particles solid metal consisting of some unburned partially or completely burnt

substances it can be very hot or toxic in nature sometimes certain vapors may be generated they are the small droplets

of liquid suspended in air oils from fuel or water from suppression efforts etcetera again this this type of effect

is extremely dangerous there there may be a generation of certain gases certain of gases and

usually these gases produced by fire they are toxic in nature see you co2 and unburned hydrocarbons in

sometimes if a polymer or a plastic products on catches fire then some sort of dioxins etcetera and these all gases

are extremely dangerous or extremely hazardous to the human being as well as they are the heat carriers so carbon

sorry common gases these usually includes the carbon monoxide carbon dioxide sometimes hydrogen cyanides

sometimes Foss genes so so there are so many accidents in past in the chemical industries they took place and they

generated these off or dangerous gases and they become the part and parcel of human being and causes a severe death

and a fatality to the human being so practically one must know that what kind of fuel what kind of source of fuel is

there and how the combustion took place so that the necessary action towards the safeguard of mankind or towards the

safeguard of environment can taking place now once we know that that the this chemistry of fire then question

arises if this is an exothermic reaction and if the net heat being liberated during the course of fire then how it

can be transmitted to some other place in the japonic student one particular storage tank cache the fire caught fire

and then the heat was transmitted to some other tanks and through the auto ignition temperature all these tank the

temperature of those tank rose very rapidly the vapor vapors were formed and they took the temperature they acquired

a temperature of auto-ignition and then they catch the fire so he must know that what kind of different modes of heat

transfer be there so heat energy usually we know that this is a basic phenomena that heat energy always flows from

hotter to colder part so combustion gives off heat that can ignite other nearby fuel sometimes maybe because of

auto ignition and sometimes because of the conduction because of the convection or because of

radiative heat transfer so three methods of heat transfer use conduction convection and radiation now let us have

a brief look about these modes of heat transfer conduction now heat transfer from one molecule to another they are

the direct in contact and the conduct conductors usually transfer heat very well and you can experience that if you

have a metal plate and if you try to heat one one end of that particular metal plate and automatically it goes to

the other part of the metal plate now usually to overcome such type of problem in a positive manner we usually adopt

the concept of insulation and usually insulator do not transfer heat very well so in case if you wish to inhibit the

heat transfer so that it cannot go from one place to another then you must use the insulators the convective heat

transfer the convection movement of heat through a fluid medium such as air or liquid and usually they form the

convective currents you can see in this particular figure they usually form the convective current

and sometimes the heat may transfer from one place to another place through this convective layers now let us have an

example suppose these two are the tanks where say petrol or a gasoline is filled and by any means this particular tank

catches fire and because of the temperature rise because of the other availability of oxygen in the near in

the atmosphere the fire propagates and by this way a lot of heat being generated in due course of time to this

particular area the generation of heat may led to the formation of more and more vapors

that because oxygen is available in abundance because it is surrounded by air then the fire catches rapidly and if

it even if it is having the heat value is substantially high then the heat may transfer to this particular tank number

two this is tank number one this is tank number two now through this convective heat transfer the content of this this

particular tank is eaten up and it may it may produce enough vapor and if this particular tank is not equipped with

safety devices or sometimes it may get with the safety of all so so enough vapors may generated and there are two

possibility one possibility is that this the fire which is here it may come to this place and the the vapors being

generated may catch the fire another possibility is that the content of this particular tank may approach to the auto

ignition temperature so that it may ignite and you can imagine the scenario that both of the tanks may be in a

position of burning and the quantum of heat being generated in due course of time so you need to think about that

what should be the how we can eliminate the formation of convective convective heat transfer how we can card on it off

if once it is this particular tank catches fire and if it is not possible then how we can control the fire because

if there there is another tank then because the heat is propagated in such a way or heat is being liberated in such a

way that both the tank they they they catches they catch fire then the excess amount of heat may may may propagate the

fire to this tank number three so one must know that how to control this particular aspect sometimes when in our

domestic affair there is a fire and that is unwanted fire then sometimes they it may create like

suppose you are sleeping and you are having a room heater or somebody somebody ignited the the cigarette and

by any means the quilt or anything catches the fire and sometimes the convective convective layers may form

within this room and these and the reason of the formation of these convective layers is that hot gases they

are having the the density difference with cold gases so there may be a chance of formation of this type of convective

layers and these hot gases usually they travel horizontally so gas then banked down a wall or move outside and

sometimes horizontally or vertically etcetera and whatever toxic gas is being generated the somebody who is within

this this room the toxic gases those were generated in due course of time the the particular person may inhale this

thing and it may create a future problem or it may create the immediate problem radiation the radiation is again a very

good source of heat transfer and sometimes in past so many accidents attributed to the radiative heat

transfer so the transfer of heat in the form of an invisible wave is clubbed under the head of radiation so heat

radiated to nearby structure even it can ignite it radiated heat passing through a window can ignite an object so in

combination of radiative heat transfer and convective heat transfer sometimes major accident they took place

and passed now how we can prevent the accident there those who are attributed to fire so a prime of AC the engineers

must know or must be familiar with that the fire and explosion properties of the material it is quite obvious that if you

are handling the petrol and if you are going to a petrol pump or petrol station petrol

filling station then always you took a proper attention or you have to take the proper attention not to ignite any kind

of cigarette or sometimes there are so many advisories be written on those petrol pumps that not to do do and

don'ts etc because we all know that what are the flammable properties of petrol similarly if you are working in a

kitchen then you are familiar with with flammable properties of LPG so you cannot treat the petrol with simple

potable water or simple water so because we all know that it catches fire immediately and it produces off gases

and the fire may propagate and fire may destroy each and everything so so we we usually take some precautionary measures

so when in general perspective coming from the picture then ingenious must be familiar with the fire and explosion

properties of the material that what is the boiling point what is the flash point what is the flame point etcetera

the nature of fire and explosion process how it can catch as the fire what what kind of things may lead to fire then

procedure to reduce fire and explosion hazard ha if once it is ignited then how can reduce the fire sometimes you may

use the sprinkler system sometimes if you may you use to garden of air sometimes you may use to to cut the fuel

supply to the source etc so you must know and all three things which we have covered in this particular aspect they

are particularly the engineered things and how we can engineer all those things we will discuss in next couple of slides

now to be more precise and to be more in scientific matter the three essential elements are required for combustion or

you can say fire fuel oxygen heat or source of ignition so all three combined to form a fire triangle now if

any one of the arm is missing then there would be no fire and if all the arms of this particular triangle are combined

then there would be fire and suppose if you wish to extinguish the fire you need to eliminate any one of the arm so

usually whenever we remove oxygen or net release of heat or fuel supply then fire is distinguished so usually in in other

aspect this is the chemical chain reaction which keeps the fire burning now how we can assess that what are the

necessary conditions for the formation of fire triangle so a prime of AC first thing comes into our mind is the fuel

because usually when we used to go to the petrol pump or when you used to go to the kitchen our subconscious mind

says that there is a presence of LPG we don't bother about the source of ignition we don't bother about the

availability of oxygen at the source so fuel let us have a brief discussion about fuel fuel must be present in

certain concentration now remember there is a nice takeo metric demand of fuel let us have a basic equation of methane

burning this is the basic equation ch4 plus twice o2 co2 plus twice H 2 plus Delta H now for one mole of methane the

two moles of oxygen is required so if the few the oxygen is available suppose in a two mood and if there is a half

mole then this is takeo metric demand will not fulfill so fuel must be present in a certain concentration we will

discuss about the lower flammability limit upper flammability limit in due course

of time now typical cases we are fire occur or if there is a leak during the filling operation transfer operation or

excessive dust we will take up as a Jaipur accident as one of the case study and all aspects like filling operation

transfer operation they took place in the Jaipur accident often cannot always eliminate these sources sometimes it is

very impractical to eliminate all things like all arms of anyone off of fire triangle like if you have if you wish to

cook something in your kitchen how can you imagine the scenario that your fire triangle is not fulfilled you

you require the presence of fuel you require the presence of oxygen you require the presence of source of

ignition and above all you require that net chemical chain reaction be there so we cannot always eliminate these sources

we can help by having the good ventilation or we can have help of good housekeeping to keep the vapors from

building up now there are certain examples of liquid solid and a gas was fuels like gasoline

acetone ether pentane they are clubbed under the head of liquid solid plastic wood dust wood dust fiber metal

particles etcetera gas was acetylene propane carbon monoxide hydrogen etcetera so you can list it you can

enlist n number of a source of fuels so once you characterize the few then the oxidizers oxygen is the most common

oxidizer especially that found in ambient air so now why the oxygen or oxidizers are important the reason is

that suppose if we wish to extinguish the fire and sometimes it is not possible to Cardon of oxygen then we

should aleut the fuel concentration in such a way that the essential amount of oxygen

whatever required as per the requirement of stoichiometric nature then the concentration should be on the lower

side so for oxygen we often use in erting with the nitrogen or helium blankets over the flammability material

to reduce the oxygen content or to to Cardon it off from the fire so the oxygen content should be below the

required amount of a combustion go back to the previous example of ch4 plus twice o to co2 plus twice H 2o Delta H

now if for one mole of ch4 if the oxygen is suppose oxygen the availability of oxygen is say for one mole then

definitely there would be no fire because the mixture is too lean to combust so it is essential that the

oxygen must represent in sufficient quantity whether you are taking the fire in a positive manner all you are taking

the fire in a negative manner so a different type of oxidizers available as on date the liquid hydrogen peroxide

nitric acid perchloric acid there are certain solid oxidizes like metal peroxides ammonium nitrite etcetera they

are certain gas was obviously oxygen is most common source of oxidizer than fluorine chlorine etcetera then the last

thing in the fire triangle is this ignition source now usually heat is the common ignition

source and remember one thing that all the ignition sources are free even the static charge being developed or

accumulated on your here may be dangerous and if you are wearing some silk cloths etc sometimes you may

observe that the static electricity being generated and sometimes you may also that there is a generation of spark

be dangerous the spar been produced by the shoe nails of your nails of your shoes again it's easily or it's the free

available source of ignition now one can eliminate ignition sources it is almost inevitable that the ignition source it

will be available if there is a large release of filament material that cannot be diluted quickly one more thing is

that whatever source of ignition you are using it must have a sufficient quantity of energy or Delta H within it so that

it can ignite that combustion mixture so once it is ignite then there is a formation of chemical chain reaction and

the net release based on its Delta H it is sufficient to sustain that particular file so the the various example of

ignition sources like a spark flame static electricity heat etc so you can modify the fire triangle with respect to

this chemical chain reaction because if you wish to sustain the fire then definitely you must have a chemical

change reaction within it now once we go into deeper in the fire and explosion we must know the various definition because

we will use different type of terminology in due course of time in all more deals of this fire in explosion so

we have divided the things into a couple of aspect so first we will discuss about the various definition the first

scientific definition of combustion is the chemical reaction in which a substance combined with an oxidant

and releases energy remember that particular release of energy is required for the sustainability of the fire

so if you you remove this particular energy being liberated during the course of fire you can extinguish the fire

explosion the rapid expansion of gases resulting in rapidly moving pressure or shockwaves the mechanical explosion this

is due to the sudden failure of vessel containing high-pressure non reactive gases there may be certain physical

explosion this results from the sudden failure of a vessel containing high pressure of non reactive gases then

detonation the explosion we will discuss this detonation and deflagration in the subsequent modules the detonation

explosion chemical reaction with a shockwave greater than speed of sound this is extremely dangerous because

sometimes the shockwave whatever being generated due to the explosion and may match with the natural frequency of any

object and a couple of times and that particular object may destroy or may collapse so this is the detonation is

extremely important then deflagration the explosion or the chemical reaction with the shockwave less than the speed

of sound then bleve the boiling liquid expanding vapor explosion when liquid is at a temperature above its atmospheric

boiling point sometimes vessel ruptures and flammable liquid flashes and results in fire or

explosion we will discuss because this levy in detail in the subsequent modules the confined explosions then explosion

occurring within a vessel or building usually results in injury to building inhabitants and extensively damaged the

unconfined explosion and explosion occurring in open usually result from a spill of a flammable gases spill these

explosions are rarer than confined since dilution occurs now you usually these unconfined explosions are extremely

dangerous because you don't have any control and confined explosions suppose you are sitting in a particular room and

sometimes was vessel rupture ruptures and the vapor cloud of the flammable vapor forms and it moves here and there

in that particular room then you are having the liberty or you are having some equipment or safety devices

through which you can control that particular vapor cloud which is not this facility is not available for the vapor

cloud explosion sorry unconfined explosion and in past of the flexible extrusion and and Jaipur accident they

took place because of this unconfined explosive just explosion now this explosion results from the rapid

combustion of fine solid particles and many solid material become very flammable when it reduces to a fine

powder another aspect is that since we we learn about the detonation and deflagration we must know that what is

shockwave because we utilize the term shockwave over there so shockwave and a blood pressure wave

moving through a gas and in open air a shockwave is followed by a strong wind the combination of a shockwave and winds

can result in a blast pressure wave so you can see there is a combined effect of all factors overpressure the pressure

of an explosion above atmospheric pressure and more specifically the pressure on an object resulting from the

shockwave we should know about the ignition usually a flammable material which may be ignited by the combination

of a fuel and oxidant in contact with an ignition source or if a flammable gas is sufficiently heated the gas can ignite

so must' where the the practical definition of ignition the minimum ignition energy the smallest energy

input needed to start the combustion so usually the typical minimum ignition energy of hydrocarbon is around 0.25

mega Joule now to place this in perspective the static charge from walking a Krauss

carpet is 22 milli Joule an automobile a spark plus plug is 25 lead you now you can imagine that this much

amount of minimum ignition energy is required for combustion of hydrocarbon and walking across a carpet generates

this much quantum of energy so how dangerous it is auto ignition temperature the temperature threshold

above which enough energy is available to act as an ignition source so in this to a particular chapter we

have discussed a particular module we have discussed about the various definition what is the chemistry of fire

what is the concept of a fire triangle because the knowledge of this fire triangle is extremely important to

extinguish the fire in the subsequent modules we will discuss about the flammability characteristics

what kind of definitions required what are the flammability limits how we can deal the mixtures etcetera thank you

very much [Music] [Applause]

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