Firefighting
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- Distinguish from a firefight, which means a battle with firearms.
Firefighting is the act of extinguishing destructive fires. A firefighter fights these fires to prevent destruction of life, property and the environment. Firefighting is a highly technical profession which requires years of training and education in order to become proficient.
[edit] Science of firefighting
Historically, physicists created a graphical representation detailing the three elements of fire (fire triangle). In recent years, one more point has been added, creating the fire tetrahedron. The four elements needed to sustain combustion are:
To extinguish a fire, it is necessary to remove one or more of the four components of combustion. Removing any of these will not allow combustion to continue. Firefighters work by
- Eliminating exposure of fuel that may be ignited by nearby flames or radiant heat
- Cooling the fuel below flashpoint (The temperature when a fuel is combustible)
- Containing and extinguishing the fire
- Removing debris and extinguishing all hidden fires to prevent rekindling
[edit] Firefighters' duties
Firefighters' goals are to save life, property and the environment. A fire can rapidly spread and endanger many lives; however, with modern firefighting techniques, catastrophe is usually, but not always, avoided. To prevent fires from starting a firefighter's duties include public education and conducting fire inspections. Because firefighters are often the first responders to people in critical conditions, firefighters provide basic life support as emergency medical technicians or advanced life support as licensed paramedics.
[edit] Hazards Caused By Fire
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;
- aspirating heated smoke that can burn the inside of the lungs.
As an example, plastics inside a car can generate 200,000 m3 of smoke at a rate of 20-30 m3/sec.[citation needed]. Firefighters carry self-contained breathing apparatus (SCBA) (an open-circuit positive pressure compressed air system) to prevent smoke inhalation.
Obvious risks stem from the effects of heat. Even without contact with the flames (conduction), there are a number of comparably serious risks: burns from radiated heat, contact with a hot object, hot gases (e.g., air), steam and hot and/or toxic smoke. Firefighters are equipped with personal protective equipment (PPE) that includes fire-resistant clothing (nomex or polybenzimidazole fiber (PBI)) and helmets that limit the transmission of heat towards the body.
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.
Heat causes human flesh to burn as fuel causing severe medical problems. Depending upon the heat of the fire, burns can occur in a fraction of a second. A first degree burn (on the skin surface) is extremely painful. A second degree burn is a burn into the skin, and can cause shock, infections, and dehydration and if left untreated often results in death. Second degree burns compromise nerve tissue and are not painful. Third degree burns leave muscles and internal organs exposed from completely destroyed skin. If the person survives the shock and exposure to germs, medical treatment is extremely difficult.
Additional risks of firefighting encompass the following:
- vision can be obscured by the smoke: a person inside the building may not be able to see, can fall, or become disoriented and lost; becoming trapped and killed by the smoke or fire.
- the building can collapse on its occupants.
[edit] 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 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 vaporizes with a hiss.
Ideally, part of reconnaissance is to consult an existing Preplan for the building. This would provide knowledge of existing structures, fire fighter hazards, and can include strategies and tactics.
[edit] 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 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.
[edit] 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); leaving the fire with not enough combustive agent to continue, and it dies out.
- the vaporization of water absorbs the heat; it cools the smoke, air, walls, objects in the room, etc., that could act as further fuel, and thus prevents one of the means that fires grow, which is by "jumping" to nearby heat/fuel sources to start new fires, which then combine.
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.
Water may be accessed by pressurized fire hydrant, pumped from water sources such as lakes or rivers, delivered by tanker truck, or dropped from aircraft tankers in fighting forest fires.
[edit] 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.
[edit] Closed volume fire
Until the 1970s, fires were usually attacked while they declined, so the same strategy that was used for open air fires was effective. In recent times, fires are now attacked in their development phase as:
- firefighters arrive sooner;
- thermal insulation of houses confines the heat;
- modern materials, especially the polymers, produce a lot more heat than traditional materials (wood, plaster, stone, bricks, etc.).
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 supplied 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 lives 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.
[edit] 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 fire retardant foam is projected on the product by the fire hose, to keep the oxygen in air separated from the fuel.
[edit] Tactical 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.
Paul Grimwood introduced the concept of tactical ventilation in the 1980s to encourage a more well thought out approach to this aspect of firefighting. Following work with Warrington Fire Research Consultants (FRDG 6/94) his terminology and concepts were adopted officially by the UK fire service and are now referred to throughout revised Home Office training manuals (1996-97).
Paul Grimwood's original definition of his 1991 unified strategy stated that ....
'tactical ventilation is either the venting, or containment (isolation) actions by on-scene firefighters, used to take control from the outset of a fire's burning regime, in an effort to gain tactical advantage during interior structural firefighting operations'.
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.
[edit] Categorizing fires
Fires are sometimes categorized as "one alarm", "two alarm", "three alarm" (or higher) fires. There is no standard definition for what this means quantifiably, though it always refers to the level response by the local authorities. In some cities, the numeric rating refers to the number of fire stations that have been summoned to the fire. In others, the number counts the number of "dispatches" for additional personnel and equipment.[1][2]
[edit] 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.
[edit] 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:
- Vr be the volume of the room,
- Vv be the volume of vapour required,
- Vw be 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 have
and for a temperature of 100 °C (373 K, 212 °F, worst case concerning the volume, probable case when the fire is suppressed and the temperature is lowered):
For the maximum volume, we have:
- Vv = Vr
considering a temperature of 100 °C. To compute the optimal volume (dilution of oxygen from 21 to 15%), we have
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) | 67.5 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.
[edit] Notes
- ^ 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.
- ^ same as above with a temperature of 100 °C (373 K), one liter of liquid water produces 1.723 m³ of vapour
- ^ 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, and we want this fraction to be 0.15 (15%).
[edit] See also
- Main list: List of basic fire fighting topics
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