Decision on the types of approach to take
When firefighters arrive at the scene of the fire, they have to make a crucial decision – whether to take an offensive or defensive firefighting approach. This decision determines how they will carry out their operations from then on.
Offensive approach
Defensive approach
What is it
Involving an aggressive attack to the fire source by entering the building and saving trapped victims/mitigating fire damage to property
An attempt to contain the fire from outside, usually involving sacrificing the building and its property/people trapped in the building
When is it employed
When there is a low chance of fire hazards occuring
When it is likely that fire hazards will occur
It is a crucial decision as making the wrong choice can mean subjecting themselves to life threatening fire hazards such as flashover and backdraft.
A flashover is the rapid development of fire of an area from floor to ceiling caused by heating by radiation. This causes an increase in the rate of combustion which results in flammable gases in the air entering their upper flammability range. (As more combustion products in the air can act as fuel)flashover will occur when heat radiated from the contents of the room raises the temperature of the contents, to their ignition temperature. Thus, all the contents ignite together instantaneously.
In a flashover, firefighters will be exposed to temperatures of more than 500°C. Exposure to temperatures of only 150°C will cause extreme pain and damage to all unprotected skin. The thermal barriers of bunker gear can protect firefighters for 15 seconds if subjected to flashover conditions hence firefighters are only able to escape if they are at the entrance of the room and it is unlikely they will survive flashovers.
Backdraft is a smoke explosion that can occur when extra air is introduced into a smoldering fire and heated gases enter their flammable range and ignite with explosive force. It occurs when fire has consumed the objects of the room and is now “burning itself out” by using all the oxygen present in the room. The room’s objects are at their ignition temperature and will explode in flames when oxygen enters, and the hot gases containing unburnt products can act as fuel and burn with an explosive force that can destroy the buildings and severely wound firefighters.
Firefighters must be able to make this decision quickly as time is a key factor in the success of their mission. Within 20 minutes of the fire, the building may already start to collapse (see Fig. 1) and the debris from the collapse can injure firefighters and victims, or obstruct the firefighters’ route.
Fire also uses up oxygen as it burns, hence within 2-10 minutes, depending on the level of oxygen in the room, trapped victims may already pass out or die from smoke inhalation.
As such, if firefighters take too long to make this decision, they may no longer have the option to take an offensive approach or it may already be futile to do so by then.
How Firefighters make the decision
Firefighters take the offensive approach to save lives and property. However, if fire hazards are present, the choice of offensive approach is ruled out and they will take the defensive approach. Hence, firefighters must identify fire hazards
Firefighters will choose to take the offensive approach in order to save lives and property. However, this decision is ultimately subjected to whether fire hazards are present. Regardless of whether there are people trapped inside the building or whether there is property to be saved, firefighters will take the defensive approach if they identify signs of fire hazards. Hence, firefighters must be able to quickly and accurately identify indicators of hazards to enable them to make the right decision of which approach to take.
Indicators firefighters consider
To identify the presence of these hazards, firefighters currently rely on a technique of reading the smoke emitted from the fire. They observe several key characteristics of smoke – color, flow, density and volume. However, these characteristics can be influenced by external factors, creating a situation whereby hazards may be present even though there are no obvious indicators from the smoke. This may result in firefighters making the wrong decision of taking the offensive approach and putting their lives in danger due to their inability to accurately identify the presence of these hazards.
Color
Color is a critical factor in determining the future of a fire. Smoke is fuel made up of particulates, aerosols and gases, which give it its color. Hence it can act as an indicator of what hazard may occur.
Black smoke is an indicator of flashover. (see fig. 1) Flashover occurs when the temperature of the room is at its peak as all materials in the room are ready to be ignited. Fire of greater thermal intensity would change greater amount of fuel into soot and discharged as black smoke.
Yellow brown smoke is an indicator of backdraft. (see fig. 1)the darker coloured smoke is due to the presence of large amounts of particles floating in the air of the room due to incomplete combustion. When there is incomplete combustion, it shows that the room is oxygen deficient as soot particles are not oxidised, a sign of backdraft. Due to incomplete combustion, there is less elemental carbon in the air, allowing sulfur to be seen which gives the smoke its characteristic yellow color. Unconverted carbon particles from incomplete combustion can also glow yellow.
Fig 1a. Black smoke indicating flashover conditions
Fig 1b. Brown smoke indicating backdraft conditions
However, as smoke flows, carbon content from the smoke will settle along surfaces and objects, this causes the smoke to be sieved which also lightens the smoke color. This creates uncertainty of whether the white smoke witnessed is an indication of early-stage heating which makes it safe to enter the building or if it is actually dangerous late-stage heating smoke that has flowed through some distance. (see fig 2) Firefighters may thus find themselves making the wrong decision of taking the offensive approach and exposing themselves to flashover and backdraft conditions if they rely on color of smoke as an indicator.
To make matters even more complicated, the color of the smoke can also be affected by the type of fuel as this would result in different particles in the air, giving the smoke a different colored appearance.almost all solid materials give off white smoke when heated as that is moisture. This white smoke will mix with black smokes which causes the black smoke to appear grey or white. As material decomposes and dries, so will the colour of smoke produced. Natural materials will produce white or brown smoke while plastics produce grey smoke when combusted. Hence, smoke containing high particulate content may not necessarily appear black. And even if firefighters know what fuel is being burnt and are aware that they need to take in this factor when considering the color of the smoke, it will still be difficult for them to do so as the definite colors only apply for single-fuel or single-commodity fires.in common residential and commercial fires, the smoke seen leaving a building is usually a mixture of different coloured smoke and is rarely from a singel fuel source.
Hence smoke color can vary enormously and is not a reliable indicator.
Fig 2a. White smoke from a building that is experiencing flashover conditions, absence of black smoke
Fig 2b. White/Black smoke from a building experiencing backdraft conditions, absence of yellow/brown smoke
2. Flow of smoke
The flow of smoke can either be described as turbulent or laminar. Turbulent smoke is caused by serious heat which results in the fast molecular spread of the gases in the smoke. The molecules of the smoke are bouncing around, colliding and expanding in a violent fashion. This spread is due to radiant heat feedback from the room when no more heat can be absorbed by the room. If the room is still absorbing heat, the heat of the smoke is then absorbed, generating a more stable and calm flow referred to as “laminar” smoke flow. Hence, an indicator of flashover would be turbulent flow of smoke as flashover occurs when the contents of the room can no longer absorb anymore radiant heat. (See fig. 4)
Fig 4b. Laminar smoke from non flashover
Fig 4a. Turbulent smoke from flashover
Looking at the circled regions of smoke in fig 4a and 4b. Fig 4a smoke is much more textured and appears cloudier than fig 4b smoke. Fig 4b smoke flow is more linear than fig 4a. Fig 4a smoke is turbulent and fig 4b smoke is laminar.
However, once again, this may not necessarily be the best indicator of potential hazards as it can be influenced by outside factors such as the weather. Cold weather causes the smoke to cool faster, stalling its flow and making it appear less turbulent. Humid weather can increase air resistance to the smoke, resulting in a slower and calmer flow as well. Hence, these indicators of flashover may no longer be present and firefighters may mistakenly choose to take the offensive approach when turbulent smoke flow is not observed even though the hazard is still present. (see fig. 5)
Fig 5. Flashover conditions but no turbulent smoke present due to cold weather in Canada
In identifying backdraft, one of the characteristics of the smoke flow observed is the suction back of puffs of smoke back into the enclosed room where they are emitted from. This is due to the fire trying to suck in oxygen from outside the room in order to sustain itself.
Figure 6
However, air pressure around the building can be influenced by wind conditions which creates regions where the pressure is either above or below the the pressure in the undisturbed air stream. Cold weather could also create an environment of lower pressure outside the building. In regions of low pressure outside the building, there may not be the characteristic sign of smoke being drawn back in even though a backdraft is occuring. In certain building structures where openings are very tightly sealed, such as in factories and warehouses, it may also be difficult for smoke to be drawn back into the building. Hence, firefighters may be unaware of the hazard occurring and end up entering the building under such dangerous conditions.
3. Buoyancy
Buoyant smoke would ascend swiftly and not accumulate at the floor. (Fig. 7) How buoyant and fast this smoke rises mainly depends on the temperature. This is because smoke temperature increase would expand and decrease in density, thus rise above the denser gases in the atmosphere. Now in the initial stage of fire growth smoke is most likely to be less buoyant because it is less heated. However as the fire grows steadily to be a flashover, heat released increases causing the temperature of the smoke increases. Hence one flashover indicator is buoyant smoke.
Fig 7 Buoyant smoke due to flashover
Buoyancy of smoke can depend on the weather conditions as cold weather can cause the smoke to cool and hence become denser and less buoyant. Smoke that has travelled a long way from the original fire source, could also have cooled significantly and affecting its buoyancy. This can once again be misleading to firefighters if they rely on the buoyancy of the smoke to identify hazards.
Fig 8. Flashover smoke that is not as buoyant
Case Study : Forecast of Hurricane course and magnitude
Forecast of Hurricane course and magnitude
Point of Comparison
Predicting presence of flashover and backdraft
Forecasting the future track and intensity of hurricanes to determine if there is a need for evacuation
Prediction of a hazard to make a crucial decision
Predicting presence of flashover and backdraft in order to determine if an offensive or defensive approach should be taken
Observation of cloud pattern around the eye of the hurricane from satellite images can be used to predict future direction and intensity.
Outer cloud bands of cumulonimbus clouds indicate the future direction, and the cloud pattern surrounding the cyclone eye indicates the future intensity of the cyclone.
This method of intensity analysis is based on the degree of spiraling in the cloud bands. The more spiral the cloud pattern is the more intense is the cyclone. Changes in the track of cyclones correlate with the angular rotation of the overall features.
Use of qualitative analysis in the prediction of an event
Observation of physical attributes of smoke such as its color, flow and buoyancy can be used to predict whether fire hazard will occur
Black, buoyant smoke with turbulent flow is a sign that flashover is going to occur
Brown, buoyant smoke that is being sucked back into the burning building is a sign that backdraft is going to occur
The inexactness in the eye position and cloud pattern due to dense, less regularly arranged clouds obfuscating surface attributes in early stages of hurricane development.
Satellite images are sensitive to cloud droplets and ice particles, not just precipitation which would affect observation of cloud patterns from these images.
Qualitative analysis results in uncertainty as the characteristics observed can be influenced by other factors
Color of smoke can be influenced by type of fuel rather than just presence of hazards.
Flow and buoyancy can be influenced by weather conditions.
Cloud top of the hurricane prevents scientists from making accurate observations to distinguish the shape of rainbands and the eye wall in the hurricane. If details of the inner core and vertical structure of tropical cyclones cannot be accurately depicted by these images, scientists will be unable to determine the position of the low-level center of the hurricane and monitor its structural changes in order for predictions to be made.
If the path and intensity is predicted inaccurately, evacuation orders may not be given even though the hurricane warrants an evacuation.
Uncertainty prevents right decision from being made, resulting in negative outcomes.
Ambiguous characteristics due to external factors influencing smoke patterns prevents firefighters from accurately classifying the type of smoke and predicting whether the hazard is imminent.
They may deduce that no hazards are imminent and enter the building even though dangerous conditions may be present
What can we learn from the solution:
Nature of the solution
In the context of predicting Hurricanes
How we can adapt it to predict fire hazards
Currently, qualitative assessment of physical appearance allows for risk assessment of hazard
Looking at satellite images allows for observation of cloud pattern to predict path and intensity of hurricane
Color of smoke indicates how much complete/incomplete combustion products are in the smoke and hence the likelihood of fire hazards occuring
Factors that can be measured quantitatively, which influence the physical indicators
Wind velocity, pressure, temperature, and moisture influence the observed cloud pattern of hurricanes
Amount of carbon and sulfur products in smoke influences the color of the smoke
Allows for the establishment of a relationship between the quantitative measurements and the hazard
Aircraft reconnaissance missions conducted to collect these quantitative data which can be put into computer models to generate more accurate cloud patterns and predict future behaviour of the hurricane
This microphysical information about the clouds in the hurricane allows cloud water content to be distinguished from ice and liquid particles, hence the issue of observed cloud pattern being influenced by these external factors is resolved, allowing for more accurate predictions through quantitative collection of data.
Using a multi-particle-meter to quantitatively determine levels of carbon and sulfur in the smoke
This allows firefighters to know how much combustion/incomplete combustion products are present in the smoke even if the color is contaminated by type of fuels
In predicting hurricanes, quantitative data of a number of factors are collected as they are all interrelated in determining the overall cloud pattern. However, in the case of factors that influence the physical characteristics of smoke, traits such as color, flow and buoyancy are all independent factors that are a result of increased rate of combustion/incomplete combustion. Increased rate of complete/incomplete combustion leads to more particulate matter in the air, influencing the color of the smoke and higher temperatures/lack of oxygen which affect the flow of the smoke and its buoyancy. Hence as long as any one of these physical attributes can be defined quantitatively, the rate of combustion/incomplete combustion can be deduced, allowing for the prediction of flashover/backdraft. Thus there is only the need to measure any one of these factors.
To quantify the buoyancy and degree of turbulence in the flow of smoke, the temperature of smoke would thus have to be measured as it is what influences these physical characteristics that are observed. However, this method would still face the same issue of the smoke’s temperature being affected by external factors such as weather. Thus, temperature of smoke recorded on the outside might not accurately reflect the fire conditions on the inside of the building. Hence it is not a viable method. The same goes for measuring pressure differences inside and outside the building to quantitatively determine if air is being sucked in during a backdraft. External weather conditions would also influence the pressure measured, hence even if it can be quantified, it would not aid in determining whether a backdraft is about to occur.
Instead, a method of quantifying the true color of the smoke by measuring the amount of complete/incomplete combustion products in it would be a better option. Flashover and backdraft occurs when the rate of combustion increases and the critical mass fraction of fuel in the smoke reaches a certain level causing it to enter the flammable range. This allows for all contents in the room to simultaneously ignite as in the case of a flashover, or for an explosion to occur when incomplete combustion products are ignited under conditions of insufficient oxygen during a backdraft. However, the critical mass fraction varies under different conditions, depending on the type of fuel being burnt and whether some of the combustion products are being deposited on the surfaces of objects in the building. Generally, however, flashover and backdraft occurs when the percentage mass of combustion products from complete and incomplete combustion respectively are between 15%-18%. Above 18%, it is more or less certain that flashover/backdraft will occur. However, there is still some uncertainty in these values which is undesirable. Thus, it is proposed that firefighters should look at the rate of change of these combustion products rather than just their percentage composition in smoke as an increase in the rate of complete/incomplete combustion leading up to flashover/backdraft is directly related to an increase in the rate of production of complete/incomplete combustion products. Thus, looking at the rate of increase rather than just the critical mass fraction alone would provide more accurate deductions since rate is independent of these external factors.
A device can be used to measure levels of combustion products in the smoke and firefighters can calibrate the device by using it during their flashover/backdraft training practices where they can determine what is absolute increase in the rate of change of combustion products that indicates a flashover/backdraft event is about to occur. This rate of increase can then be used as a reference in actual situations, providing an even more accurate indicator than just looking at the composition of smoke alone.
As seen in the diagram above, the exponential increase in rate of production of combustion products indicates a flashover happening. During combustion, heat from the fire converts fuel into elemental carbon. Hence, an increased rate of combustion can be identified by an exponential increase in the amount of elemental carbon found in smoke. Identifying an increase in the rate of combustion would then allow for flashover to be predicted in advance.
As seen in the diagram above, rate of combustion is stalled when there is insufficient oxygen. This means that more incomplete combustion is taking place, resulting in an increase in the rate of production of incomplete combustion products. Products of incomplete combustion include carbon monoxide and elemental carbon but more notably, elemental sulfur. Sulfur compounds are present in a variety of common materials found in buildings. They can be found in cement, rubber and plastics, all of which make up a large percentage of the materials found in a building. When these materials are burnt, the sulfur compounds would be oxidized to sulfur dioxide. However, under conditions with insufficient oxygen, sulfur will not be able to be converted to this form. Hence, large amounts of elemental sulfur would be present in the smoke and measuring the rate of increase of sulfur composition would allow for backdraft to be predicted.
Sulfur content in cement
Sulfur content in rubber
Sulfur content in fossil fuels which are used to make plastics
GI:
Multi-particle meter
As such, a device that is able to measure the levels of carbon and sulfur in the smoke can be employed to chart out the rate of increase in combustion, giving firefighters a clear indicator when backdraft/flashover is going to occur.
How the device works:
A light source emits light of two specific wavelength values which correspond to maximum absorption wavelength values of elemental carbon (880 nm), elemental sulfur (256 nm).
Electromagnetic waves of the specific wavelengths are transmitted from the emitter to the receiver. As the electromagnetic waves pass through the smoke that flows through, the light of specific wavelengths will be absorbed by the respective carbon and sulfur particles found in the smoke, and the receiver detects and measures the loss in electromagnetic waves due to the absorption by the particles. The decrease in light transmittance indicates an increase in concentration of carbon/sulfur particles in the smoke and a graph showing the concentration of particles measured will be generated in a data logger.
An exponential increase in concentration of carbon or sulfur particles indicates that a flashover or backdraft may be imminent, enabling firefighters to predict such hazards and make the right decision in choosing to take an offensive or defensive approach.
When firefighters arrive to the scene, they can first observe the physical characteristics of the smoke. If there is no ambiguity in determining that these characteristics indicate the presence of flashover/backdraft, firefighters can instantly make the decision to take the defensive approach. However, if the smoke is not clearly black/brown and there is the absence of the other indicators based on observing the flow and buoyancy, firefighters can use this device to get rid of the dangerous uncertainty as it would provide a definitive answer as to whether a fire hazard is imminent.
This is a handheld device for firefighters to directly measure the composition of smoke from outside the building. If the smoke rises and is out of reach, the device can be attached to an extendable pole. For fires on levels above the ground floor where the smoke would be much higher up, the device can also be attached to drones which can fly up and collect the smoke samples. Wireless transmission from the device would then allow for a graph to be generated using a data logger and firefighters will be able to identify based on the rate of increase in concentration of combustion products whether flashover/backdraft is about to occur.
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