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Fire Safety Management > Principles of Fire Science > Flashcards

Principles of Fire Science Flashcards

1
Q

Fire

A

process involving rapid oxidative, exothermic or heat releasing reactions in which part of released energy sustains the process.

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2
Q

what does fire tetrahedron show?

A

four components required for the existence of a fire.

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3
Q

Fire Tetrahedron

A
  • Fuel: necessary to provide a source of material for exothermic reaction.
  • Oxidant: usually oxygen in air, must be present.
  • Source of heat: must also be present for a fire to be initiated and sustained.
  • Relative concentrations of fuel, vapours & oxygen entering flame must be appropriate to initiate / sustain the complex chain reactions that characterise flame chemistry.
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4
Q

source of heat can be in the form of…

A

a spark, a flame or perhaps just a heated environment.

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5
Q

Ways to stop a fire

A
  • Starvation
  • Smothering
  • Cooling
  • Inhibition
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6
Q

Starvation

A
  • Removal of Fuel
  • Flaming will clearly stop if fuel vapours are eliminated
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7
Q

Example of Starvation

A

shutting off the gas supply system in event of an unwanted fire involving gas stove.

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8
Q

Smothering

A
  • Removal of Oxygen
  • Fire can also be suppressed by preventing oxygen from reaching a flame
  • in practice, this is the most difficult to achieve
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9
Q

Example of smothering

A

covering it with a blanket

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10
Q

Cooling

A
  • Removal of Heat
  • Heat is routinely removed from a fire by the application of water streams by fire-fighters or automatic sprinkler systems.
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11
Q

Inhibition

A
  • Interference by Removal of Free Radicals
  • Chain reactions within the flame can be inhibited by the suitable application of special chemicals such as clean agents (Halon, now banned) and to a certain extent, dry powders.
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12
Q

Fire as a chemical reaction

A
  • Fire can be represented by a word equation for a hydrocarbon
  • As O2 is restricted, combustion is generally incomplete.
  • Results in formation of CO, soot and other chemical species.
  • Visible smoke and appearance of a flame is due to presence of soot due to incomplete combustion.
  • Well-ventilated flaming combustion does not generate much CO to be a threat to occupants.
  • Hence, the less well-ventilated a fire is, the larger the yield of CO and soot.
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13
Q

Forms of Fuels

A
  • Fuel for fires can be in the form of a vapour, liquid or solid.
  • However, flaming combustion has to be in the gaseous phase.
  • Hence, liquids and solids must first be vaporised to generate vapours for the flame.
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14
Q

How a Liquid Fuel burns

A
  • Flammable liquid fuel itself does not burn.
  • At pool surface, liquid molecules have enough energy to vaporise & these liquid vapours will mix with oxygen from the air.
  • If vapour mixture is within flammability limits & ignited by a pilot heat source, burning will begin.
  • Reach flashpoint temp, flames will flash across the surface of liquid, but flames will not be sustained.
  • Flash point is defined as the minimum temperature at which liquid forms a vapour above its surface in sufficient concentration that it can be ignited.
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15
Q

Fire Point of a Liquid

A
  • For flaming to continue, temperature of mixture has to be raised to fire point.
  • Fire point is at higher temperature than flash point at which burning will be sustained once vapours have been ignited.
  • Reason is cuz when fire point is reached, energy which is generated in reaction in gaseous phase is transferred back to liquid surface, thereby vaporising more potentially combustible material & burning continues.
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16
Q

How is flammability of liquids classified?

A

its flashpoint

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17
Q

what is the flashpoint?

A

lowest temperature at which an air and vapour mixture will combust at its surface.

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18
Q

Classification of Flammability of
Liquids: Flashpoint

A
  • Flash point is directly related to liquid’s ability to generate vapour, AKA its volatility.
  • Vapour generation is the primary factor in determining fire hazard.
  • Hence expression ‘low flash point - high hazard’ applies & flashpoint temp. is taken as measure for classifying flammability & volatility of liquid fuels to ensure their storage & safe handling.
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19
Q

What happens when a liquid is heated up to the auto-ignition temperature?

A

liquid will burn without any external heat source.

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20
Q

Auto-ignition temperature

A
  • Auto-ignition temp of fuel is the lowest temperature at which it will spontaneously ignite in normal atmosphere without an external source of ignition.
  • High enough for the liquid to supply the activation energy needed for combustion.
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21
Q

Petroleum products are divided into the following classes:

A
  • Class 0
  • Class I
  • Class II
  • Class III
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22
Q

Class 0

A

Liquefied Petroleum Gas

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23
Q

Class I Flash point

A

below 23°C

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24
Q

Class II

A

Flash point between 23°C and 60°C
(both inclusive);

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25
Q

Class III

A

Flash point above 60°C but not more
than 93°C

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26
Q

Who/How are Class 0, I & II petroleum are regulated

A

By SCDF

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27
Q

Licensable product of Class III patroleum

A

diesel

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28
Q

How a vapour fuel burns

A
  • Vapour fuel will ignite with heat source when have oxygen only when mixture is within UFL and LFL at a minimum temperature, the flash point.
  • Above the UFL the mixture is too rich in fuel to sustain combustion & below LFL too little fuel is present to maintain heat generation at level high enough to sustain the reaction.
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29
Q

LFL stand for

A

Lower Flammability Limit

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30
Q

UFL stand for

A

Higher Flammability Limit

31
Q

What does LFL mean?

A
  • the flame is too lean to burn
32
Q

LFL air & propane percentage

A

Air: 100% - 97.9%
Propane: 0% - 2.1%

33
Q

UFL air & propane percentage

A

Air: 0% - 90.5%
Propane: 9.5% - 100%

34
Q

How a vapor fuel burns

A
  • Vapour fuel will ignite with heat source in presence of oxygen only when mixture is within UFL and LFL at a minimum temperature, the flash point.
  • Above UFL: mixture is too rich in fuel to sustain combustion
  • Below LFL: too little fuel present to maintain heat generation at a level high enough to sustain the reaction.
35
Q

Common fire prevention measure for vapor burning

A
  • Provide adequate mechanical ventilation in areas where there is a leak of combustible vapors.
  • This ensures vapor concentration never reaches the LFL
  • Because mixture is diluted.
36
Q

What is piloted ignition

A

A mixture within its flammability limits can be ignited by a small ignition source.

37
Q

What happens to a vapor after (pilot) ignition?

A
  • Following ignition, flaming results in which chemical reactions & hence a flame propagates rapidly through mixture.
  • If there is no ignition source / pilot, a mixture within its flammability limits will ‘self-ignite’ if heated to its auto-ignition temperature. Again, flaming follows ignition.
38
Q

Auto-ignition

A
  • Lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark.
39
Q

How a Solid Fuel burns

A
  • It first requires solid to be converted to vapor at the solid surface before mixing with oxygen of air.
  • Combustible solids are generally polymeric, which are composed of macromolecules.
40
Q

What are Macromolecules ?

A
  • Too large to be vaporized directly,
  • So as heat is transferred to a solid surface from a flame
  • Vapors can only be generated if chemical bonds within macromolecules are broken & smaller molecular species are generated.
41
Q

What is Pyrolysis?

A
  • A process of thermal decomposition that requires much more energy than simple evaporation in liquids.
  • Hence, surface temperature of solid must usually be raised to a much higher level than a liquid before significant amounts of vapor are released & ignition is achieved.
  • Temperature at surface of burning solids is usually greater than 350°C.
42
Q

Burning of Wood

A
  • When wood is heated to temperatures around 100°C, water vapour is driven off.
  • At higher temperatures of 200°C to 250°C, wood will discolour as pyrolysis begins.
  • However, combustible vapors are not generated in sufficient quantity to cause wood to be ignited until wood reaches a temperature of 350°C to 390°C.
43
Q

Forming of Char

A
  • Layer of carbonaceous char is left in place due to flames
  • As wood continues to burn, pyrolysis zone recedes deeper into solid & char thickens.
  • Char layer insulates interior of wood & thereby slows down generation of vapors.
  • If not enough combustible vapor given off, flame will cease & allow oxygen to contact hot char & smouldering may begin.
44
Q

Burning of Plastics

A
45
Q

types of plastic

A

thermoplastics & thermosets.

46
Q

physical different between thermoplastics and thermosets

A
  • thermoplastics can be remelted
  • thermoset plastics remain in a permanent solid state once hardened.
47
Q

Thermoplastics

A
  • Highly flammable.
  • Melt at temperature lower than ignition temperature.
  • Usually linear structures.
  • Since molecular structure is not cross linked, thermoplastic when heated will soften & start to melt and flow.
48
Q

Examples of Thermoplastics

A
  • Examples include polyethylene, polypropylene and polystyrene
49
Q

Secondary Hazards of Thermoplastics

A
  • Melt-flow & melt-drip when heated that results in fire spread,
  • Thus pose very serious secondary hazard in fires involving situations where polymeric materials are used in construction of doors, windows, ceilings, roofs & curtains.
50
Q

Thermoset Plastics

A
  • Have three-dimensional cross linked structures.
  • Do not melt but decompose to generate vapours & a carbonaceous char.
  • Susceptible to smouldering.
  • Do not melt but remain in a permanent solid state once hardened.
  • Char development on the solid surface may slow down heat transfer
    from the flame to the unburnt solid material & thereby reduce burning rates.
51
Q

Smouldering

A
  • The slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel.
52
Q

Common solid materials which can sustain a smouldering reaction

A

Fibre, wood, cotton and charcoal.

53
Q

Basic difference between smouldering
and flaming combustion

A

Smouldering occurs on the surface of the solid rather than in the gas phase.

54
Q

Other difference between smouldering
and flaming combustion

A
  • Smouldering is a surface phenomenon but can propagate to interior of a porous fuel if it is permeable to flow.
  • Temperature & heat released during smouldering are low compared to those in flaming combustion, that is about 600°C vs about 1,500°C.
55
Q

Characteristics of Smouldering

A
  • Propagates in a creeping fashion, around 0.1 mm/s, about ten times slower than flames spread over a solid.
  • Can be a significant fire hazard.
  • Emits toxic gases such as CO at a higher yield than flaming fires & leaves behind a significant amount of solid residue.
  • Emitted vapours are flammable and could later be ignited, triggering transition to flaming combustion.
56
Q

Definition of heat transfer in fire

A

The transport of heat energy from one point to another caused by a temperature difference between those points.

57
Q

three basic modes of heat transfer

A
  • Conduction
  • Convection
  • Radiation.
58
Q

Conduction

A
  • Takes place within solids when one side of an object is heated.
  • Energy is transferred from heated side of solid to unheated side at a rate dependent on diff in temperature, thickness, thermal conductivity (k) of material & area of solid.
59
Q

Example of Heat Conduction

A
  • Steel beam located in a room & passing through a wall to another room that is being impinged upon by a fire.
  • As the beam/truss heats, temperature increases along the steel beam to the cooler end.
  • The end opposite fire heats up, & if it reaches auto-ignition temperature of combustible materials close to it, a second fire will start.
60
Q

Convection

A

The transfer of heat energy by the movement of heated gases from the source of heat to a cooler part of the environment which could be a solid.

61
Q

Example of convection

A

Hot gases from a fire plume transferring heat to the ceiling or
walls of a room.

62
Q

Significance of Convection in Fires

A
  • Plays a significant role in fires because it is the major means for fire to spread by the heated gases and products of combustion spreading out into the upper portions of a building from room of fire origin.
  • Thus why soot & other products of combustion is observed to spread
    throughout a structure & even in rooms that are a significant distance away and have no direct flame impingement.
63
Q

Radiation

A
  • Transfer of heat energy from a hot surface or gas, the radiator, to a
    cooler material, the target, by electromagnetic waves without the need of an intervening medium.
  • Radiant energy can be transferred only by line of sight & will be reduced or blocked by intervening materials.
64
Q

Examples of Radiation

A
  • Heat from the sun being radiated to earth through vacuum.
  • Heat you can feel that is being radiated from a fire when standing in front of it.
65
Q

Four Stages of Fire Development in a Compartment

A
  • Ignition and Growth
  • Flashover
  • Fully Developed Fire
  • Decay
66
Q

Ignition and Growth (Ignition itself)

A
  • After ignition, fire grows & produces increasing amounts of energy, mostly
    due to flame spread.
  • Object burning in a room behaves as if it were burning in the open, which means that there is no restriction on the amount of oxygen, hence, a well-ventilated fire.
  • After a short period, however, confinement begins to influence fire development.
67
Q

Ignition and Growth (Formation of Fire Plume)

A
  • Smoke produced by burning object rises to form a hot gas layer below ceiling.
  • Cold gases surround hot gases in the flame and hotter, less dense mass will rise upward due to density difference or buoyancy. Buoyant flow including any flames is called a fire plume.
  • As hot gases rise, cold air will be entrained into the plume. This mixture of combustion products & air will impinge on the ceiling of the fire compartment and cause a layer of hot
    gases to be formed. This layer heats the ceiling and upper walls.
  • Thermal radiation from hot layer, ceiling & upper walls then heats all objects in lower part of room boosting both rate of burning of original object & rate of flame spread over its surface.
68
Q

Ignition and Growth (Formation of Hot Upper Layer)

A
  • Plume flow impinges on ceiling, gases spread across it as a circular jet.
  • Energy of jet will trigger smoke and heat detectors & sprinkler head on
    ceiling. Ceiling jet eventually reaches enclosure walls and is forced to move downward along wall.
  • However, as gases are still warmer than surrounding ambient air, flow will turn upward due to buoyancy.
  • Thus, layer of hot gases will be formed under the ceiling. Room is divided into two distinct layers: a hot upper layer consisting of combustion products and entrained air, and a cold lower layer consisting of air.
69
Q

Flashover

A
  • Hot upper layer will radiate heat on lower part of room, increasing rate of burning of original object including combustible materials on floor getting them ready for flashover.
  • Occurs when flames suddenly sweep across room, involving all combustibles in room.
  • The transition from the burning of one or two objects in room to full room involvement which occurs when temperature of the upper layer reaches 500 to 600°C. Radiation levels at floor level must reach roughly 20 kW/m2.
  • During flashover, it is no longer possible for occupants in the compartment to survive.
  • Hence it is important to tackle a fire before it reaches flashover.
70
Q

Fully developed fire

A
  • Always follows a flashover, where fire bums vigorously for some time until combustibles are mostly consumed.
  • This stage the fire is ventilation controlled, flames extend out through opening and all combustible materials in the enclosure is involved in fire.
  • Average gas temperature within a compartment during a fully developed fire ranges from 700°C to 1200°C.
  • Fully developed fire will burn as long as there is sufficient fuel & oxygen available for combustion.
  • Flaming eventually ceases, leaving a mass of glowing embers.
71
Q

Science of Ignition and Growth

A
  • Ignition produces heat releasing reaction & is characterised by an increase in temperature much higher than the surroundings.
  • Occurs mostly by application of a pilot heat source.
  • After ignition, fire is said to be fuel-controlled, since there is sufficient oxygen available for combustion & fire growth depends on fuel characteristics & geometry.
  • Growth stage can occur very rapidly especially with flaming combustion where heat from first burning material is able to ignite adjacent fuel packages & since there is sufficient oxygen
    available in the room.
72
Q

Science of Flashover

A
  • Transition from the growth stage to fully developed fire where there is total surface fire involvement of combustibles in the room.
  • Temperature in the room reaches 500°C to 600°C and radiation to the floor of the room is about 20 kW/m2.
  • Flames appear at room’s openings such as windows.
73
Q

Science of Fully Developed Fire

A
  • Energy released in the compartment is at its highest and is only limited by the availability of oxygen.
  • Ventilation-controlled phenomenon.
  • However, due to high temperature & pressure, the glass windows would have been broken to allow oxygen to enter compartment earlier.
  • Unburnt gases collect at ceiling level & as these gases leave windows they burn, causing flames to stick out through windows.
  • Average gas temperature in compartment is high, ranging from 700°C to 1200°C.
74
Q

Decay Stage

A
  • As fuel is consumed, energy release rate diminishes & thus average gas temperature in compartment declines.
  • Fire may go from ventilation- ontrolled to fuel-controlled during this period.

Fire Safety Management (7 decks)

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