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.
2
Q
what does fire tetrahedron show?
A
four components required for the existence of a fire.
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.
4
Q
source of heat can be in the form of…
A
a spark, a flame or perhaps just a heated environment.
5
Q
Ways to stop a fire
A
- Starvation
- Smothering
- Cooling
- Inhibition
6
Q
Starvation
A
- Removal of Fuel
- Flaming will clearly stop if fuel vapours are eliminated
7
Q
Example of Starvation
A
shutting off the gas supply system in event of an unwanted fire involving gas stove.
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
9
Q
Example of smothering
A
covering it with a blanket
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.
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.
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.
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.
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.
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.
16
Q
How is flammability of liquids classified?
A
its flashpoint
17
Q
what is the flashpoint?
A
lowest temperature at which an air and vapour mixture will combust at its surface.
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.
19
Q
What happens when a liquid is heated up to the auto-ignition temperature?
A
liquid will burn without any external heat source.
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.
21
Q
Petroleum products are divided into the following classes:
A
- Class 0
- Class I
- Class II
- Class III
22
Q
Class 0
A
Liquefied Petroleum Gas
23
Q
Class I Flash point
A
below 23°C
24
Q
Class II
A
Flash point between 23°C and 60°C
(both inclusive);
25
Q
Class III
A
Flash point above 60°C but not more
than 93°C
26
Q
Who/How are Class 0, I & II petroleum are regulated
A
By SCDF
27
Q
Licensable product of Class III patroleum
A
diesel
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.
29
Q
LFL stand for
A
Lower Flammability Limit
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.
