Fire fighting
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Fire fighting is the knowledge and techniques required to extinguish a fire.
Historically, fire scientists created a graphical representation detailing the three elements fire needs to start (fire triangle). However, in recent years, one more point has been added to the triangle, creating the fire tetrahedron. This fourth point represents a chemical chain reaction needed to sustain fire. The three elements needed for the initial start of combustion are:
- combustible matter (fuel): paper, wood, gasoline, oil, gas, etc.;
- a combustive agent, i.e. an oxidant, usually the oxygen of the air;
- activation energy: heat, spark (electricity), etc.
(See also Combustion). To extinguish a fire, it is necessary to remove at least the fuel or the combustive agent (once it is started, the fire produces its own energy). Once the fire is suppressed, it is necessary to cool down the surrounding objects so the fire does not start again.
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Risks of a fire
The first risk that comes to the mind is the heat. Even if a person is not "in" the flames, he/she can be burnt by the infrared radiations, the contact with a hot object, by the hot gases (heated air, but particularly water vapour produced by the spraying), and by the smoke (hot particles), which are indeed the most dangerous thing. The firefighters are equipped with personal protective equipment (PPE) that includes fire-resistant clothing and helmets that slow down the diffusion of the heat towards the skin.
The primary risk to people in a fire is inhalation (breathing). A vast majority of the victims in fires die from smoke inhalation, not from burns. These risks include :
- the fire uses oxygen of the air, so there is no more oxygen to breathe;
- the fire can produce poisonous gases;
- the fire produces smoke (fine particles) that can burn inside the lungs.
For example, the plastics inside a car can generate 200 000 m3 of smoke at a rate of 20 to 30 cubic meter per second. For this reason, firefighters carry breathing apparatus (SCBA).
The heat can make pressurised gas cylinders and tanks explode, as well as some chemical products such as ammonium nitrate fertilizers. These explosions can bring physical traumas when someone falls down, by shrapnel, or by the over-pressure (blast).
There are two additional risks inside a building:
- the vision can be obscured by the smoke: you can not see where you go and can thus fall down or get lost;
- the building can collapse.
Additionally, it is important to note that firefighters are victims of road accidents when they drive to the fire station when they are called, especially at night (combination of stress, tireness, neglected road safety rules).
Means to extinguish a fire
Suppressing the fuel and the energy
The first method is to remove the fuel, e.g. cut off the gas, moving the combustible objects away.
When the activation energy is still present, it is also useful to switch it off; this will not stop a fire, but will help controlling a starting fire and will prevent a new fire to occur.
The first action is thus to "cut off the energies": gas and power supply, switch off the working machines (motors). It is also important to switch off the mechanical ventilation and air conditioning that can dangerously change the behaviour of the fire.
Reconnaissance and reading the fire
The first step of the operations is a reconnaissance to search for the origin of the fire (which is not obvious for an inside fire, especially when there is no witness), and spot the specific risks and the possible casualties. An fire occurring outside may not require reconnaissance; on the opposite, a fire in a cellar or an underground car park may require a long reconnaissance to spot the sat of the fire with only a few centimeters of visibility.
The "reading" of the fire is the analysis by the firefighters of the forewarnings of a thermal accident (flashover, backdraft, smoke explosion), which is performed during the reconnaissance and the fire suppression maneuvers. The main signs are:
- the research of the hot zones with the hand (protected by a glove!), especially touching a door before opening it;
- the presence of soot on the windows, which usually means the combustion is incomplete and thus there is a "lack" in combustive agent (air);
- the smoke goes in and out from the door frame, as if the fire breathes, which usually means a "lack" in combustive agent (air);
- spraying water on the ceiling with a short pulse of a diffused spray (e.g. cone with an opening angle of 60°) to test the heat of the smoke;
- when the temperature is moderated, the water falls down in drops with a sound of rain;
- when the temperature is high, it vaporises with a hiss.
Use of water
The main way to extinguish a fire is the water. The water has two roles:
- in contact with the fire, it vapourises, and this vapour pushes the air away (the volume of water vapour is 1 700 times bigger than liquid water); the fire has no combustive agent anymore;
- the vapourisation of water absorbs the heat; it cools the smoke, air, walls, objects, etc. and prevents an extension of the fire.
The extinction is thus a combination of "asphyxia" and cooling. The flame itself is suppressed by asphyxia, but the cooling is the most important element to master a fire in a closed volume.
Open air fire
Outside, the seat of the fire is sprayed with a straight spray: the cooling effect follows directly the "asphyxia" by vapor, it thus reduces the amount of water required. A straight spray is used so the water arrives massively to the seat is is not vapourised before. The spray may also have a mechanical effect: it can disperse the combustible product and thus prevent the fire to start again.
The fire is always fed with air, but the risk is limited for the people, except for wildfires or bushfires where they can be surrounded by the flames. But there might be a big risk of expansion.
It is a surface (object) that is sprayed; for this reason, the strategy is sometimes called two-dimensional attack or 2D attack.
It might be necessary to protect specific devices (house, gas tank) against infrared radiation, and thus to use a diffused spray between the fire and the object.
Even if the atmosphere is always renewed around the firefighters, there is still a risk of breathing poisonous gases or smoke; the breathing apparatus are thus often required.
Closed volume fire
Until the 1970s, the fires were attacked while they declined, so the same strategy as the open air fires was effective. In the modern cities, the fires are now attacked in their development phase:
- the firefighters arrive sooner;
- the thermal insulation of the houses confine the heat;
- the modern materials, especially the polymers, produce a lot more heat than the traditional materials (wood, plaster, stone, bricks, etc.).
Additionally, in these conditions, there is a greater risk of backdraft and of flashover.
The direct spray of the fire seat with a straight spray can have dramatic consequences: the water pushes air in front of it, so the fire is over fed before the water reaches it. This activation of the fire, and the mixing of the gases produced by the water flow, can created a flashover.
The most important is not the flames, but the control of the fire, i.e. the cooling of the smokes that can spread and start distant fires, and that endanger the life of the firefighters and casualties. The volume must be cooled before the seat is treated. This strategy is thus called three-dimensional attack, or 3D attack.
The first who proposed the use of a diffused spray was the chief Lloyd Layman of Parkersburg W V Fire Department, at the Fire Department Instructor's Conference (FDIC) in 1950 (Memphis, Tennessee).
The ceiling is first sprayed with short pulses of a diffused spray:
- it cools the smoke, thus the smoke is less likely to start a fire when it moves away;
- the pressure of the gas drops when it cools (law of the ideal gases), thus it also reduces the mobility of the smoke and avoids a "backfire" of water vapour;
- it creates an inert "water vapour sky", and thus prevent the roll-over (rolls of flames on the ceiling created by the burning of the hot gases).
It is necessary to make only short pulses, otherwise the spraying modifies the equilibrium (gas stratification) and mixes all the gases: the hot gases (initially at the ceiling) move all around in the room and the temperature rises at the ground, which is dangerous for the firefighters. An alternative is to cool all the atmosphere by spraying the whole atmosphere, e.g. drawing letters in the air ("pencilling").
The modern methods for an urban fire dictate the use of a massive initial water flow, e.g. 500 L/min for each fire hose. The aim is to absorb the maximum of heat at the beginning to stop the expansion of the sinister, and to produce a contraction of the smoke. When the flow is too small, the cooling is not sufficient, and the vapour that is produced can burn the firefighters (the drop of pressure is to small and the vapor is pushed back). Although it may seem paradoxical, the use of a strong flow with an efficient fire hose and an efficient strategy (diffused sprayed, small droplets) requires a smaller amount of water: once the temperature is lowered, only a limited amount of water is necessary to suppress the fire seat with a straight spray. For a living room of 50 m² (60 square yards), the required amount of water is estimated to 60 L (15 gallons).
French fire-fighters used an alternative method in the 1970s: they sprayed water on the hot walls to create a water vapour atmosphere and asphyxiate the fire. This method is no longer used because it happened to be dangerous: the pressure created pushed the hot gases and vapour towards the firefighters, causing severe burns, and pushed the hot gases in other rooms where it could start a new fire.
Asphyxiating a fire
In some cases, the use of water is harmful and thus contra-indicated:
- some chemical products react with water and produce poisonous gases, or even burn in contact with water (e.g. sodium);
- some products float on water, e.g. hydrocarbon (gasoline, oil, alcohol, etc.); a burning layer can then spread and extent;
- in case of a pressurised gas tank, it is necessary to avoid heat shocks that may damage the tank: the resulting decompression may give a BLEVE.
It is then necessary to asphyxiate the fire. This can be done with two means:
- some chemical products react with the fuel and stop the combustion;
- a layer of water-based foam is projected on the product by the fire hose, to separate the fuel from the air.
Ventilation or isolation of the fire
One of the main risk of a fire is the smoke: it carries heat and poisonous gases, and obscurs the vision. In the case of a fire in a closed location (building), two opposite strategies may be used: the isolation of the fire, or the positive pressure ventilation.
The isolation, or anti-ventilation, consists in closing all the openings to prevent the air from coming in and the smoke from going out. As the smoke is confined, this makes the rescue operations easier, and prevents the extention of the fire. But this also confines the heat and the gases produced by pyrolysis, giving a risk of backdraft if ever some air gets in, e.g. when opening a door to spray the fire.
The positive pressure ventilation (PPV) consist in using a fan to create an excessive pressure in a part of the building; this pressure will push the smoke and the heat away, and thus secure the rescue and fire fighting operations. It is necessary to have an exit for the smoke, to know very well the building to predict where the smoke will go, and to wedge the doors so they will stay opened and will not slam. The main risk of this method is to activate the fire, or even to create a flashover, e.g. if the smoke and the heat accumulate in a dead end.
Individual action
A starting fire is easy to extinguish: a thimbleful of water can extinguish a match, a bucket of water can extinguish a fire created by a match after one minute; but after a few minutes, tons of water are required. It is thus important to know how to fight a starting fire, but also to know that once it is started, the most effective action is to warn people to evacuate the building (if necessary) and call for help; any other action would be dangerous and harmful as it would delay the evacuation and the arrival of the firefighters.
- Fire of a pan or deep fryer:
- protect your hands with a wet cloth (e.g. a dish towel or a floorcloth);
- cut of the gas or the power supply;
- cover the pan or the fryer with a lid;
- place the cloth around the lid to make it damp.
- On other type of fires: use a fire extinguisher;
- otherwise try to suffocate the fire with a blanket or soil, sand, or spray water (except on a fat liquid).
- When the cloth of a person are on fire, the personn will usually panic and run; the wind created by the movement will activate the fire. It is necessary to tell the person to "stop, drop and roll" on the ground (or to force him/her to do so), and to roll him/her in a cloth when available. The use of a fire extinguisher is forbidden because the chemical agent will harm the person.
When the fire can not be fought, it is necessary
- in case of a building, to warn the occupants with calm to avoid panic (e.g. use the fire alarm), and to evacuate the building; in case of a vehicle, help the people to get out; this may necessitate an emergency movement;
- call for help (e.g. dial 9-1-1 in North America, 1-1-2 in the European Community);
- when it is not possible to get out (e.g.smoke fills the corridor): to get in a room and close the door, put cloth (possibly wet) under the door, spray the door with water when possible, and warn the firefighters by waving sign at the window; kneel or lay down to get fresh air.
During the evacuation, it is important:
- not to use an elevator;
- not to go in the smoke: it is easy to get lost, and smoke causes major damages (especially burns inside the lungs and asphyxia);
- always go towards the exit.
Appendix : Calculation of the amount of water required to suppress a fire in a closed volume
In the case of a closed volume, it is easy to compute the amount of water. Indeed, when the volume is tigh, the air can not come in; and the air is necessary for the combustion, the oxygen O2 (pure air contains 21% of O2). Whatever the amount of fuel available (wood, paper, cloth%u2026), the combustion will stop when the air becomes "thin", i.e. when there is less than 15% of oxygen.
This gives:
- the amount of water required to make the atmosphere inert, i.e. to prevent the pyrolysis gases to burn; this is the "volume computation";
- theamount of water required to cool the somke, the atmosphere; this is the "thermal computation".
These computations are only valid when considering a diffused spray. The diffused spray is not possible in the case of high ceiling: the spray is short and does not reach the upper layers of atmosphere. For this reason, the computations are not valid for big volumes such as barns or warehouses: a warehouse of 1,000 m² (1,200 square yards) and 10 m high (33 ft) represents 10,000 m3. Additionally, a warehouse ora barn can barely be considered as a closed volume, a great amount of water vapour can go away and will not replace the atmosphere of the building.
Volume computation
The fire needs air; if the water vapour pushes all the air away, the fuel can not burn anylonger. But the replacement of all the air by water vapour is harmfull for the firefighters and the possible casualties: the water vapour carries much more heat than the air at the same temperature (one can be burnt by water vapour at 100°C (212°F) above a boiling saucepan, whereas it is possible to put the arm in a oven at 270°C (518°F) without damage as long as one does not touch the walls of the oven). This amount of water is thus a maximum amount and should never be reached.
The minimal amount of water that could be used is the amount required to « diluate » the air so there is less than 15% of oxygen: below this concentration, the fire can not burn. This minimal amount will be called « optimal amount ».
The amount that is really used should be between these two values. Any additional water would just run on the floor and cause a water damage in the inferior levels, but would not contribute to the fire suppression.
Let us call:
- Vr the volume of the room,
- Vv the volume of vapour required,
- Vw the volume of liquid water to create the Vv volume of vapour,
then for an air at 500°C (773K, 932°F, best case concerning the volume, probable case at the beginning of the operation), we have
- <math>V_v = 3571 \cdot V_w<math> 1
and for a temperature of 100°C (373K, 212°F, worst cas concerning the volume, probable case when the fire is suppressed and the temperature is lowered):
- <math>V_v = 1723 \cdot V_w<math> 2
For the maximum volume, we have:
- <math>V_v = V_r<math>
considering a temperature of 100°C. To compute the optimal volume (dilution of oxygen from 21 to 15%), we have
- <math>V_v = 0.286 \cdot V_r <math> 3
for a temperature of 500°C. The table below show some results, for rooms with a height of 2,70 m (8 ft 10 in).
| Amount of water required to suppress the fire volume computation | |||
| Area of the room | Volume ofthe room Vr | Amount of liquid water Vw | |
|---|---|---|---|
| maximum | optimal | ||
| 25 m² (30 sq.yd) | 67,5 m3 | 39 L (9.4 gal) | 5.4 L (1.3 gal) |
| 50 m² (60 sq.yd) | 135 m3 | 78 L (19 gal) | 11 L (2.7 gal) |
| 70 m² (84 sq.yd) | 189 m3 | 110 L (26 gal) | 15 L (3.6 gal) |
Note that the formulas give the results in cubic meters; they are then translated in liters.
Of course, a room is never really closed, gases can go in (fresh air) and out (hot gases and water vapour) so the computations can not be exact.
- Notes
- Note 1: indeed, the mass of one mole of water weights 18 g, a liter (0.001 m3) represents one kilogram i.e. 55,56 moles, and at 500°C (773K), 55.56 moles of an ideal gas at the atmospheric pressur represents a volume of 3.571 m3
- Note 2: same as above with a temperature of 100°C (373K), one liter of liquid water produces 1.723 m3 of vapour
- Note 3: we considere that only Vr-Vv of the original room atmosphere remains (Vv have been replaced by water vapour). This atmosphere contains less than 21% of oxygen (some was used by the fire), so the remaining amount of oxygen represents less than 0,21·(Vr-Vv). The concentration of oxygen is thus less than 0,21·(Vr-Vv)/Vr, and we want this fraction to be 0.15 (15%)
Thermal computation
In the case of a fire in a closed volume, the first concern is to lower the temperature. In the worst case, we can considere that it is necessary to absorb all the heat produced by the fire (in fact, only a part of this heat must be absorbed to allow the extinction, not the whole). The heat is transferred to the smoke, walls, ceiling, floor, and a part of it go away woth the smoke by the outlets or through the wall when the insulation is weak. The most important is to absorb the heat of the smoke inside the room, and to lower the temperature but certainly not to set it back to 20°C (68°F). The computation made with this hypothesis is thus the calculation of a maximum, the amount that is really required is smaller.
If the room is totally tigh, the fire will stop spontaneoulsly when the concentration in oxygen is less than de 15%. The volume of oxygen used for this is 0,06·Vl 4.
A cubic meter of oxygen combined with a fuel typically produces 4,800 kCal, i.e. 20 MJ 5. The rise of temperature from 20 to 100°C (68 to 212°F) and the vapourisation of one liter water absorbs 539,000 kCal i.e. 2,260 MJ.
The volume of water Vw' that is required to absorb the heat is thus:
- <math>V_e' = 0,00053 \cdot V_l<math> 6
| Amount of water required to suppress the fire thermal computation | ||
| Area of the room | Volume of the room Vl | Amount of liquid water Vw' |
|---|---|---|
| 25 m² (30 sq.yd) | 67,5 m3 | 36 L (8.6 gal) |
| 50 m² (60 sq.yd) | 135 m3 | 72 L (17 gal) |
| 70 m² (84 sq.yd) | 189 m3 | 100 L (24 gal) |
Note that the formula give the result in cubic meter; it is then translated in liter for the table.
- Notes
- Note 4: the concentration of oxygen dropped from 21% to 15%, the volume of oxygen involved represents 21-15 = 6% of the volume of the room
- Note 5: for example, the combustion of 1 m3 of methane requires 2 m3 of pure O2 and generates 35.6 MJ ; 1 m3 of O2 thus contributes to the creation of 17.8 MJ (4,250 kCal);
- Note 6: Vw'·2260 = 0.06·Vr·20 in megajoules, thus Vw' = 5.31·10-4·Vr ;
Vw'·539000 = 0.06·Vr·4800 in kilocalories, thus Vw' = 5.34·10-4·Vr ;
the difference of 0,6% between the values is due to the approximations, and is negligible
Conclusion
Let us compare the calculated values:
| Amount of water required to suppress the fire comparison of computations | ||||
| Area of the room | Height of the room | Amount of water | ||
|---|---|---|---|---|
| Volume computation | Thermal computation | |||
| Maximum | Optimal | |||
| 25 m² (30 sq.yd) | 2,7 m (8 ft 10 in) | 39 L (9.4 gal) | 5.4 L (1.3 gal) | 36 L (8.6 gal) |
| 50 m² (60 sq.yd) | 2,7 m (8 ft 10 in) | 78 L (19 gal) | 11 L (2.7 gal) | 72 L (17 gal) |
| 70 m² (84 sq.yd) | 2,7 m (8 ft 10 in) | 110 L (26 gal) | 15 L (3.6 gal) | 100 L (24 gal) |
We can see that both computations give close values. This means that the amount of water required to cool the smoke is sufficient to make the atmosphere inert, and thus to suppress the fire.
See also
External links
- Firetactics.com (http://www.firetactics.com/)
- Pictures of the Fortworth FD (http://www.fortworthfire.com/) by Glen E. Ellman
- Fire.gov (http://www.fire.gov/): Building and Fire Research Laboratory (NIST)
- Firefighting flow rates (http://www.fire-flows.com/)de:Brandbekämpfung fr:lutte contre l'incendie