Fire fighting

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Image:Porte-en-feu-p1010040.jpg Fire fighting is the act of carrying out procedures to extinguish an unwanted fire

In all but the most trivial cases, knowledge and expertise are necessary for successful and safe fire-fighting.

Historically, fire scientists created a graphical representation detailing the three elements fire needs to start (fire triangle). In recent years, one more point has been added, creating the fire tetrahedron.

The three elements needed for the initial start of combustion (a form of oxidation; see the article on combustion) are:

To extinguish a fire, it is necessary to remove either the fuel or the combustive agent (once started, the fire requires no further activation). Once the fire has been suppressed, it is necessary to reduce the temperature of the surrounding objects so the fire does not start again.

Contents

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Risks of a fire

The obvious risk is heat. Even without contact with the flames, there are a number of comparably serious risks: burns from infrared radiation (radiated heat, like a domestic grill), contact with a hot object, hot gases (e.g., air), steam produced by spraying, and hot or toxic smoke. Firefighters are normally 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 smoke inhalation (breathing in smoke); most of those killed in fires die from this, not from burns. The risks of smoke include :

  • suffocation due to the fire consuming or displacing all the oxygen from the air;
  • poisonous gases produced by the fire;
  • smoke (fine particles) that can burn inside the lungs.

As an example, plastics inside a car can generate 200,000 m3 of smoke at a rate of 20 to 30 m3/sec.Template:Citeneeded. For this reason, firefighters carry breathing apparatus (SCBA).

The heat can make pressurised gas cylinders and tanks explode, producing what is called a BLEVE (Boiling Liquid Expanding Vapor Explosion). Some chemical products such as ammonium nitrate fertilizers can also explode. Explosions can cause physical trauma or potentially serious blast or shrapnel injuries.

There are two additional risks inside a building:

  • vision can be obscured by the smoke: you can not see, can fall, or become disoriented and lost;
  • the building can collapse.
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Means to extinguish a fire

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Suppressing the fuel and the energy

The first method is to remove fuel for the fire by, for example, cutting off the domestic gas supply and moving combustible objects from the path of the fire. When the activation energy is still present, it is also useful to switch it off; this will not stop a fire, but will help in controlling a starting fire and will prevent a new fire from occurring.

The first action is thus to "cut off the energies", such as domestic gas and electricity, and switch off working machines (motors). It is also important to turn off ventilation and air conditioning, as they supply oxygen which supports combustion and can dangerously change the behaviour of the fire.

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Reconnaissance and reading the fire

The first step of the operations is a reconnaissance to search for the origin of the fire (which may not not be obvious for an indoor fire, especially when there are no witnesses), and spot the specific risks and the possible casualties. Any fire occurring outside may not require reconnaissance; on the other hand, a fire in a cellar or an underground car park with only a few centimeters of visibility may require a long reconnaissance to spot the seat of the fire.

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:

  • hot zones, which can be detected with a gloved hand, especially by touching a door before opening it;
  • the presence of soot on the windows, which usually means that combustion is incomplete and thus there is a lack of air
  • smoke goes in and out from the door frame, as if the fire breathes, which usually means a lack of air to support combustion;
  • 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 moderate, the water falls down in drops with a sound of rain;
    • when the temperature is high, it vaporises with a hiss.
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Use of water

Often, the main way to extinguish a fire is to spray with water. The water has two roles:

  • in contact with the fire, it vaporizes, and this vapour displaces the oxygen (the volume of water vapour is 1,700 times greater than liquid water); the fire has no combustive agent anymore;
  • the vaporization 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 area.

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Open air fire

For fires in the open, the seat of the fire is sprayed with a straight spray: the cooling effect immediately follows the "asphyxia" by vapor, and reduces the amount of water required. A straight spray is used so the water arrives massively to the seat without being vaporized before. A strong spray may also have a mechanical effect: it can disperse the combustible product and thus prevent the fire from starting again.

The fire is always fed with air, but the risk to people is limited as they can move away, except in the case of wildfires or bushfires where they can be surrounded by the flames. But there might be a big risk of expansion.

Spray is aimed at a surface, or object: for this reason, the strategy is sometimes called two-dimensional attack or 2D attack.

It might be necessary to protect specific items (house, gas tank) against infrared radiation, and thus to use a diffused spray between the fire and the object.

Breathing apparatus is often required as there is still the risk of breathing in smoke or poisonous gases.

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Closed volume fire

Until the 1970s, fires were usually attacked while they declined, so the same strategy as for open air fires was effective. In recent times, fires are now attacked in their development phase as:

Additionally, in these conditions, there is a greater risk of backdraft and of flashover.

Spraying of the seat of the fire directly can have unfortunate and dramatic consequences: the water pushes air in front of it, so the fire is suppled with extra oxygen before the water reaches it. This activation of the fire, and the mixing of the gases produced by the water flow, can create a flashover.

The most important issue is not the flames, but control of the fire, i.e. the cooling of the smoke that can spread and start distant fires, and that endanger the life of people, including firefighters. The volume must be cooled before the seat is treated. This strategy originally of Swedish (Mats Rosander & Krister Giselsson) origin, was further adapted by London Fire Officer Paul Grimwood following a decade of operational use in London's busy west-end district between 1984-94 (www.firetactics.com) and termed three-dimensional attack, or 3D attack.

Use of a diffused spray was first proposed by Chief Lloyd Layman of Parkersburg, West Virginia Fire Department, at the Fire Department Instructor's Conference (FDIC) in 1950 held in Memphis, Tennessee, U.S.A.

Using Grimwood's modified '3D attack strategy' 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 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" which prevents roll-over (rolls of flames on the ceiling created by the burning of hot gases).

Only short pulses of water must be sprayed, otherwise the spraying modifies the equilibrium, and the gases mix instead of remaining stratified: the hot gases (initially at the ceiling) move around the room and the temperature rises at the ground, which is dangerous for firefighters. An alternative is to cool all the atmosphere by spraying the whole atmosphere as if 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 as much heat as possible at the beginning to stop the expansion of the sinister, and to reduce the smoke. When the flow is too small, the cooling is not sufficient, and the steam that is produced can burn firefighters (the drop of pressure is too 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 as 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 was risky: the pressure created pushed the hot gases and vapour towards the firefighters, causing severe burns, and pushed the hot gases into other rooms where they could start a new fire.

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Asphyxiating a fire

In some cases, the use of water is undesirable:

  • 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 extend;
  • in case of a pressurised gas tank, it is necessary to avoid heat shocks that may damage the tank: the resulting decompression may produce a BLEVE.

It is then necessary to asphyxiate the fire. This can be done in two ways:

  • 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 keep the oxygen in air separated from the fuel.
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Ventilation or isolation of the fire

One of the main risks of a fire is the smoke: it carries heat and poisonous gases, and obscures vision. In the case of a fire in a closed location (building), two different strategies may be used: isolation of the fire, or positive pressure ventilation.

Isolation, or anti-ventilation, consists of closing all the openings to prevent the air from coming in and the smoke from going out. As the smoke is confined, this makes rescue operations easier, and prevents the extension 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.

Positive pressure ventilation (PPV) consists of using a fan to create excess 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 the building very well to predict where the smoke will go, and to ensure that the doors remain open by wedging or propping them. The main risk of this method is that it may activate the fire, or even create a flashover, e.g. if the smoke and the heat accumulate in a dead end.

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Before the fire brigade arrives

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 has 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.

In case of a fire starting on your house:

  • Fire of a pan or deep fryer:
    1. cut off the gas or electricity;
    2. cover the pan or the fryer with a lid;
    3. place a cloth, preferably wet, over the lid to suffocate the fire.
  • In other type of fires: use a fire extinguisher;
  • otherwise try to suffocate the fire with a blanket or soil or sand, or spray water (but not on oil or liquid fat fires).
  • When a person's clothes are on fire, the person 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. You should not use a fire extinguisher because the chemical agent may harm them.
  • NEVER try to put a wet cloth on any part of your body to protect it from fire. The water from the cloth will heat up and can create steam and scald the skin where the cloth is placed.

When the fire cannot be fought, it is necessary

  • in case of a building, to calmly warn the occupants 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 and UK, or 9-9-9 in the United Kingdom or 0-0-0 in Australia);
  • when it is not possible to get out (e.g., if the corridor is full of smoke): get in a room and close the door, seal the crack under the door with clothes (wet, if possible), spray the door with water if possible, and warn firefighters or onlookers by waving at the window; kneel or lie down to get fresh air, as the hot smoke will rise.

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 damage (especially burns inside the lungs and asphyxia);
  • always go towards the exit.
  • not to leave the scene completely, so that firefighters and others know who is safe, and who is possibly trapped.

As soon as possible it is vital to find out with certainty who was in the building, and who has been evacuated, so that lifesavers know what to do. Evacuated people should stay together to facilitate this.

An exception to avoidance of the elevator may be necessary in the most extreme situations. In the case where you have an out-of-control and spreading fire in a lower floor, all staircases are blocked, and the possibility of rescue from outside is remote, the elevator or elevator shaft may be the only possible escape route. It is extremely, very, totally, dangerous and risky, but it may be the only chance. Conceivably this could have saved some lives in the fires in the World Trade Center on 11 September 2001.

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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 needed. The oxygen (O2) in air (21%) is necessary for combustion. Whatever the amount of fuel available (wood, paper, cloth), combustion will stop when the air becomes "thin", i.e. when it contains less than 15% oxygen. If additional air cannot enter, we can calculate:

  • The amount of water required to make the atmosphere inert, i.e. to prevent the pyrolysis gases to burn; this is the "volume computation";
  • The amount of water required to cool the smoke, the atmosphere; this is the "thermal computation".

These computations are only valid when considering a diffused spray which penetrates the entire volume; this is not possible in the case of a high ceiling: the spray is short and does not reach the upper layers of air. Consequently the computations are not valid for large 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. In practice, such large volumes are unlikely to be airtight anyway.

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Volume computation

Fire needs air; if water vapour pushes all the air away, the fuel can no longer burn. But the replacement of all the air by water vapour is harmful for firefighters and other people still in the building: the water vapour can carry much more heat than 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 an arm in an oven—without touching the metal!—at 270 °C (520 °F) without damage). This amount of water is thus an upper limit which should not actually be reached.

The optimal, and minimum, amount of water to use is the amount required to dilute the air to 15% oxygen: below this concentration, the fire cannot burn.

The amount used should be between the optimal value and the upper limit. Any additional water would just run on the floor and cause water damage without contributing to 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 (773 K, 932 °F, best case concerning the volume, probable case at the beginning of the operation), we haveTemplate:Fn

<math>V_v = 3571 \cdot V_w</math>

and for a temperature of 100 °C (373 K, 212 °F, worst cas concerning the volume, probable case when the fire is suppressed and the temperature is lowered):Template:Fn

<math>V_v = 1723 \cdot V_w</math>

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 haveTemplate:Fn

<math>V_v = 0.286 \cdot V_r </math>

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 of the room Vr Amount of liquid water Vw
maximum optimal
25 m² (30 yd²) 675 m³ 39 L (9.4 gal) 5.4 L (1.3 gal)
50 m² (60 yd²) 135 m³ 78 L (19 gal) 11 L (2.7 gal)
70 m² (84 yd²) 189 m³ 110 L (26 gal) 15 L (3.6 gal)

Note that the formulas give the results in cubic meters; which are multiplied by 1,000 to convert to 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 will not be exact.

Notes
Template:Fnb indeed, the mass of one mole of water is 18 g, a liter (0.001 m³) represents one kilogram i.e. 55.6 moles, and at 500 °C (773 K), 55.6 moles of an ideal gas at atmospheric pressure represents a volume of 3.57 m³.
Template:Fnb same as above with a temperature of 100 °C (373 K), one liter of liquid water produces 1.723 m³ of vapour
Template:Fnb we consider that only Vr - Vv of the original room atmosphere remains (Vv has 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; we need it to be 0.15 (15%)
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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 consider that it is necessary to absorb all the heat produced by the fire (in practice, only a part of the heat must be absorbed to extinguish the fire). The heat is transferred to the smoke, walls, ceiling, floor; part of it is carried away with the smoke by ventilation, or through poorly insulated walls. The most critical point is to absorb the heat of the smoke inside the room, and to lower the temperature, although not down to the normal ambient temperature of 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 airtight, the fire will stop spontaneously when the concentration of oxygen drops below 15%. The volume of oxygen used for this is 0.06·Vl.Template:Fn

A cubic meter of oxygen combined with a fuel typically produces 4,800 kcal, i.e. 20 MJ.Template:Fn The rise in temperature from 20 to 100 °C (68 to 212 °F) and the vaporization of one liter of water absorbs 539,000 kcal (2,260 MJ).

The volume of water Vw' that is required to absorb the heat is thus:Template:Fn

<math>V_e' = 0.00053 \cdot V_l</math>
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 yd²) 67.5 m3 36 L (8.6 gal)
50 m² (60 yd²) 135 m3 72 L (17 gal)
70 m² (84 yd²) 189 m3 100 L (24 gal)

Note that the formula gives the result in cubic meters; it is multiplied by 1,000 to convert to liters.

Notes
Template:Fnb the concentration of oxygen dropped from 21% to 15%, the volume of oxygen involved represents 21-15 = 6% of the volume of the room
Template:Fnb 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);
Template:Fnb 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
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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 yd²) 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 yd²) 2.7 m (8 ft 10 in) 78 L (19 gal) 11 L (2.7 gal) 72 L (17 gal)
70 m² (84 yd²) 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 closely similar 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.

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See also

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External links

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