'''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.
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 many other valuable services to the community they serve, such as:* Emergency medical services, as emergency medical technicians or as licensed paramedics, staffing ambulances.
In addition, firefighters also service in specialized fields, such as:
As an example, plastics inside a car can generate 200,000 m3 of smoke at a rate of 20-30 m3/sec.. Firefighters carry self-contained breathing apparatus (SCBA) (an open-circuit positive pressure compressed air system) to prevent smoke inhalation. These are not oxygen tanks. They carry compressed air.
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. Third degree burns compromise nerve tissue and are not painful, however the edges of a third degree burn are usually second and first degree burns which cause severe pain. 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:
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:
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.
There are four elements needed to start and sustain a fire and/or flame. These elements are classified in the “Fire Tetrahedron”. These four elements of the “Fire Tetrahedron” are:
The reducing agent, or fuel, is the substance or material that is being oxidized or burned in the combustion process. The most common fuels contain carbon along with combinations of hydrogen and oxygen. Heat is the energy component of the fire tetrahedron. When heat comes into contact with a fuel, it provides the energy necessary for ignition, causes the continuous production and ignition of fuel vapors or gases so that the combustion reaction can continue, and causes the vaporization of solid and liquid fuels. The self-sustained chemical chain reaction is a complex reaction that requires a fuel, an oxidizer, and heat energy to come together in a very specific way. A chain reaction is a series of reactions that occur in sequence with the results of each individual reaction being added to the rest. This happens in the science of fire, but is self-sustaining in that it continues without interruption. An oxidizing agent is a material or substance that when the proper conditions exist will release gases, including oxygen. This is crucial to the sustainment of a flame or fire.
A fire can be extinguished or put out by taking away any of the four components of the “Fire Tetrahedron”. One method to extinguish a fire is to use water. The first way that water extinguishes a fire is by cooling, which heat from the fire. This is possible through water’s ability to absorb massive amounts of heat by converting to water vapor. Without the heat the fuel no longer has the conditions required to produce oxygen to sustain the fire. The second way water extinguishes a fire is by smothering the fire. When water is heated to its boiling point it converts to water vapor. When this conversion takes place it dilutes the oxygen in the air, thus removing one of the elements that the fire requires to burn. This can be done with water by adding foam.
Another way to extinguish a fire is fuel removal. This can be accomplished by stopping the flow of liquid or gaseous fuel or by removing solid fuel in the path of a fire. Another way to accomplish this is to allow the fire to burn until all the fuel is consumed, at which point the fire will self extinguish.
Another way to extinguish a fire is by chemical flame inhibition. This can be accomplished through dry chemical and halogenated agents. These agents interrupt the combustion reaction and stop flaming. This method is effective on gas and liquid fuels because they must flame to burn.
Often, the main way to extinguish a fire is to spray with water. The water has two roles:
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.
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.
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:
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 endangers 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:
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 fire, 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.
In some cases, the use of water is undesirable:
It is then necessary to asphyxiate the fire. This can be done in different ways:
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 ....
Ventilation affects life safety, fire extinguishment, and property conservation. First, it pulls fire away from trapped occupants when properly used. It may also "limit fire spread by channeling fire toward nearby openings and allows fire fighters to safely attack the fire" as well as limit smoke, heat, and water damage.
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.
Hydraulic ventilation is the process of directing a stream from the inside of a structure out the window using a fog pattern. This effectively will pull smoke out of room. Smoke ejectors may also be used for this purpose.
Fires are sometimes categorised as "one alarm","all hands", "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.
Alarms are generally used to define the tiers of the response by what resources are used.
Structure fire response draws the following equipment: (Urban fire)
3 Engine/Pumper Companies
1 Truck/ladder/aerial Company
This is referred to as an Initial Alarm or Box Alarm.
Working fire request (For the same incident)
Other Specialized Rescue Units
Chief Officers/Fireground Commanders (If not on original dispatch)
Note: This is the balance of a 1 alarm Fire.
Second and subsequent alarms:
2 Engine Companies
1 Truck Company
The reason behind the "Alarm" is so the Incident Commander doesn't have to request each apparatus with the dispatcher. He can say "Give me a second alarm here" Instead of saying "Give me a Truck company, and 2 Engine companies" Along with requesting where they come from.
Keep in mind that categorization of fires varies between each fire department. A single alarm for one department may be a second alarm for another. Response always depends on the size of the fire and the department.
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:
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.
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.
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:
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
|Area of the room||Volume of the room Vr||Amount of liquid water Vw|
|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.