Technical Details

Introduction

In recent years there have been radical changes in the methods and materials used for fire detection, prevention and extinguishing as well as in the development of efficient equipment and more convenient operation.

At the same time, public awareness of the damage of fires and the need for early detection and prevention of fires has greatly increased. Additionally, awareness about the damage to the environment caused by the use of traditional extinguishing materials has also been brought to the fore. As a result, there is a growing tendency to move towards using firefighting materials that are both environmentally and ozone-layer friendly.

The most important factor in the prevention of fires is public awareness of the risks of fire occurring and adherence to prevention mechanisms. Protection from fire can be identified in two stages: the first is to prevent the formation of fire. The second – extinguishing the fire while ensuring minimum damage from the fire and firefighting operations. Both of these areas are covered comprehensively and in great details in professional literature.

In addition, there is a set of rules, regulations and guidelines that define the minimum requirements to ensure the protection against fire and its efficient extinguishing. Primary attention must be given to the prevention of fire. Fire, no matter how small and even if extinguished quickly, cause some damage. One of the first to prevent the formation of a fire in residential and industrial structures alike, is “good housekeeping”. This definition includes, first and foremost, tidy environment, removal of accumulated waste, proper maintenance of electrical and mechanical equipment including periodic inspection, removal of any combustible matter in and around the vicinity of flammable electrical installations or facilities etc.

In other words – public awareness of fire risks and how they develop. This goal can be achieved by proper guidance and training of both permanent and temporary staff, whether in a residential environment, office, store or in a factory, in addition to having a dedicated person or team responsible for enforcing the rules regarding fire prevention in the facility. If despite all the preventative measures a fire does occur, it is important to detect it and put it out as quickly as possible. As already explained in the discussion on the nature of fire, it spreads at a rapid rate so the sooner it is dealt with, the easier and more efficient it will be and the lesser the damage caused.

Most fires are not caused directly by people. They are usually the result of neglect or failure of equipment, or leaving a potential fire hazard unattended. Therefore, one of the most important factors that make it easier to control the fire, is its early detection. For that purpose, many sophisticated detection methods were developed. They usually detect either flames, smoke, heat or other by-products of combustion. They can also activate automatic extinguishing devices, both when people are present, but especially at times or places where people are not present, or those who are present, are not able to effectively deal with the situation.

Portable equipment, suitable for the type and content of the site, and used by trained personnel, is the most effective means of extinguishing the fire in its early stages. However, in every case of a fire, it is mandatory to call upon the services of professional firefighters, with appropriate equipment and professional know-how, who are able to control fires at more advanced stages. In many cases, catastrophic outcomes developed as a result of delaying calls to professional firefighters. Therefore, with any fire situation, local extinguishing efforts and calls to professional fire fighting services must be performed in parallel.

Combustion process

Fire as we know it, is a visible expression of the combustion process, which is a chemical process, which consists of a chain of heat producing reactions which accelerate themselves and occur in solid fuel or fumes. The result from these reactions is usually but not necessarily, the process of oxidation of molecules of gas by atmospheric oxygen. So that the gas molecule react chemically with the molecule of oxygen, it should be given energy, which will result in a collision between the molecules and chemical change, accompanied by energy (heat) again.

The minimum energy required to create such a reaction is called the “threshold energy” and it is higher in most cases than the average energy that molecules of material at ambient temperature. The resulting temperature in fuel – when given the threshold energy – is called “ignition temperature”. The heat generated following the ignition, provides additional energy to the gas molecules, increases the number of molecules with energy threshold and increases the combustion process. With further increase of temperature, energy is released, allowing the continuation of the combustion process until running out of one of the components of the reaction (oxygen or fuel) or until the energy produced would transfer to an external source or until an external intervention in the combustion process stops the chain reaction.

Threshold level of energy depends on the initial heat source, the physical and chemical nature of the fuel and the composition and pressure of the atmosphere in which the ignition is created. Regardless of the initial physical state of the fuel (solid, liquid or gas), ignition only occurs when the fuel is in a state of gas or vapor. In the case of combustion of solid or liquid, heat should turn them into gas first. When the ambient temperature rises, the fuel-oxygen mixture heats up and the additional threshold energy required for ignition, decreases. Combustion can start without an external ignition factor. Temperature at which the fuel-oxygen mixture may ignite itself is called “self-ignition temperature”.

Summary – combustion

1. Oxydising agent, combustible materials and ignition heat are necessary for combustion.

2. Combustible material must heat up to ignite, either via external source or by self heating.

3. Continued combustion depends on the sufficient production of heat that

4. Combustion continues – a. Until fuel runs out, or b. Until the concentration of oxygen in the immediate environment falls below what is required or c. To the amount of oxidation is absorbed by external factors or prevented from reaching the fuel, or d. External intervention with the combustion process breaks the chain reaction. Required for combustion: fuel, oxidation and heat, and ensuring the continuity of the process.

Interrupting any of these factors lead to cessation of combustion and extinguished fire.

Extinguishing Process

Combustion conditions point to the possible ways of extinguishing the fire. Interruption of one of the basic factors reduce the intensity of combustion, leading to it being extinguished. Compared with the four basic conditions of fire which are:

  • Gas
  • Heat
  • Oxygen
  • Uninterrupted combustion process

we can line up the four fundamental methods of extinguishing:

  • Disconnect the fuel
  • reducing heat (cooling)
  • disconnecting or reducing oxygen
  • inhibition of the combustion process

Combustion and extinguishing are very complex processes, hence there is not one extinguishing substance operating in any of these modes. In order to make the best use of firefighting materials, it is necessary to thoroughly understand all the extinguishing methods and recognize the method by which each of these materials work.

Combination of extinguishing materials

Extinguishing operations open up many possibilities for the integration of materials. For example, in order to extinguish a fire in a plane or a car – which contain large amounts of fuel – it is essential to have a powerful and quick extinguishing method which allows removal of passengers and prevents possible explosion of the fuel tanks. At the same time, it is necessary to prevent recurring flare. In this example, a combination of powder or halon – which have an almost instantaneous extinguishing effect – is used, together with foam which cools down the fire and isolates it from the environment thus securing the area against flare back. Ordinary extinguishing powders actually destroy the foam, so usually in such cases, special powders are used that allow integration with the foam without breaking it. In cases where ordinary powder was first used, the amount of foam required for efficient covering of the affected area will be roughly double the amount required in cases where a combination powder is used.

Extinguishing materials – powders

Powder extinguisher is a quick and highly effective means against emanating from flammable liquids, gases, fuel, as well as electrical fires. Several types of powder are also suitable for extinguishing fires emanating from flammable solids and even light metals. Powdering the flames is a much faster means than any other extinguishing materials, however it must encompass the entire volume of the flame at once. Its extinguishing ability is high and it does not develop bad substances that may be harmful to humans.

Unlike water and foam, it does not conduct electricity, or causes damage to equipment and materials. The main disadvantages of powder are its inability to prevent repeat flare as well as generating a lot of dirt, requiring cleanup and restoration. Many types of powders have been developed and they occupy a respectable place among the many extinguishing materials and substances on the market due to their efficiency, speed and safety. In many places, Powder extinguishers replace water and foam extinguishers. The development of multi-purpose powders – capable of extinguishing a variety of solid oriented fires (e.g. wood, cloth, rubber) allows the utilisation of multi-purpose fire extinguishers to counter the risks . In places where the fire risks are of a combined nature, one multi-purpose fire extinguisher can replace several single-purpose ones.

The primary extinguishing feature of the powder is delaying the chain reaction nature of the combustion process. Tiny grains of powder forming a very large contact area with gas burning, and mass dispersion allows almost immediate capture of entities operating in the combustion process. Interference caused by the act of powdering leads to the fire’s being immediately extinguished. Extinguishing operation has secondary effects, such as the release of carbon dioxide gas from the air that causes an isolation effect, physical disruption to the flow of fuel vapors by the jet pressure of the powder and a cooling effect. These effects are attributed only a secondary weighting in the extinguishing process.

Characteristics of powder extinguishers

Various powder properties determine its nature and suitability for the purpose. The main features of the powder and their importance to its proper operation are: – flows The importance of this feature is ensuring effective dispersion of the powder, covering the entire volume of the burning area; A continuous flow of material from the equipment and the prevention of the formation of lumps inside the fire extinguishing tanks, even after shaking or prolonged storage.

Flow smoothness is achieved by the addition of smoothing substances and by finely  grinding the material itself. Physical behavior of the powder should be similar to that of a liquid. – Moisture resistance Absorption of moisture from the air may stimulate the formation of clumps in the powder and lead to a reduction in its capabilities. To prevent this, the powder is mixed with moisture rejecting substances as well as with paraffin compound. Hydropovisation creates a shell like layer on the powder particles, which can absorb certain amount of moisture without damaging the content of the particle itself. Despite this, exposure to moisture or humidity can ruin the powder by creating lumps that prevents it from flowing smoothly.

– Particle uniformity – The efficiency of the powder depends on the size of the contact area between the burning substance and how effectively it can be sprayed inside the volume of the flame. Therefore its particles should be very small, yet heavy enough to penetrate the flame space and not be pushed away by hot air or wind. Optimal size of the particles of the powders is about 45 μ (microns) and the effective range is within 10-90 μ.

Multi-purpose powder – suitable for fighting fire from solids – contains a controlled number of larger and heavier particles, designed to penetrate through the turbulence of the burning flame. – Besides these features, associated with the extinguishing capacity, powder extinguishers should also be safe in use, and not cause collateral damage to the environment in which it is activated. To ensure this is required, the powder does not contain abrasive materials that would not toxic or corrosive and will be dielectric – not conduct electricity

– that would not endanger the employee and not short-circuit electrical equipment. Important feature of powder extinguishers is weight, which is set in three different conditions:
– Weight volumes after loose pouring.
– Weight Capacities after prolonged shaking.
– Specific gravity, which is the net weight of the material (without the air space between the grains), and can be determined only in a laboratory.

The first two values ​​give an idea of the extent of the amount of powder the extinguisher container can hold and the degree of tightening and mixing needed in order for smooth operation of the extinguisher. Specific gravity allows determining the size of the extinguisher and its operational characteristics for the given powder.

Extinguishing action of the powder

Most types of powder extinguishers contain 90% – 95% finely ground sodium bi-carbonate (“baking soda), plus various materials that ensure its special characteristics. The fire extinguishing ability of sodium bi-carbonate has been known for a very long time. Over time it became clear many compounds containing elements from the alkali metals (lithium, sodium, potassium), or from halogens (fluorine, bromine, chlorine) show good fire extinguishing abilities. Both groups of these elements belong to one column values ​​in cyclical basics. The higher its position in the column and the heavier its atomic weight – the higher the element’s level of activity and its extinguishing effectiveness. In the metals group, the extinguishing ability goes from lithium (lowest), to sodium to potassium (highest).

similarly, in the halogens group, the effectiveness of bromine and iodine exceeds that of fluorine and chlorine. The groups which are active in the combustion process are ions of hydrogen (+ h) and hydroxyl (-ho). Their energy can be absorbed by active groups of the extinguishing compounds. The heavier and larger the material’s atoms are, it attacks more easily the active ions to create neutral, inactive compounds. For example, a group of ammonia can absorb an active hydrogen ion:

+4 h n + h + 3nh

and later active hydroxyl ion

o2h + 3nh-ho + + nh

In this process, a group of ammonia 3nh is released again, which can continue to adsorb active ions in substances with high extinguishing capability e.g. bi-carbonate, carbonate, oxalate and more. Many of these materials do not show high efficacy as fire extinguishing substances. Others show great value for this purpose. For example, potassium bi-carbonate is twice as effective as a fire extinguishing agent as sodium bi-carbonate. This is explained by its higher place in the alkali metals column.

Types of powder

Most, if not all powder manufacturers give their powders special features by mixing them with various additives which usually constitute trade secrets. In general though, there are several main types of powders: Standard powder (B), suitable against fires in Class B and C as well as surface fires in Class A. This is a standard powder for extinguishing liquid based fires. It contains 95% Sodium bi-carbonate plus a smoothing agent which allows it to flow easily. By finely grinding it and with the addition of magnesium, the powder is given a moisture rejection ability. It is very durable and its extinguishing ability can be maintained for quite a long time. It is highly effective in all types of liquid based fires, electricity based fires and pressured gas based fires. It can also extinguish soild based fires, as long as the fire did not penetrate the inner layers of the material that’s burning. However this powder is ineffective in deep fires (as in solids) and in cases of burning light metals.

Fortified powder is suitable for the same types of fire as the standard powder, but is based on potassium carbonate. It is similar to the standard powder, but its extinguishing capacity is higher in liquid based fires. This powder can also prevent recurring fire. It is suitable for use in high risk places where it is important to ensure maximum protection. It can be used in medium sized fire extinguishers even by untrained personnel. Multi-purpose powder – (for fire types A, B, C and in some cases, type D) is based on ammonium – phosphate, a material similar in composition to water which suppresses the flames. It operates like a normal powder in liquid based fires. Its special ability is in fighting fires emanating from  solids such as wood and some types of light metals. This powder allows extinguishing fires in vehicles and in places where there is a mixture of flammable liquids and solids.

Multi-purpose powders extinguishes liquids in a similar manner to other powders, but in a completely different manner when solids are involved. Here, powder particles come into contact with the burning material, dissolve and form fire resistant compounds; They coat the burning material and prevent oxygen from penetrating and reaching the fuel vapors. The coating maintains its suppleness when its hot, but when it cools down it cracks and peels. The main effect of this powder is the isolation of the burning solid material. Cooling and inhibition factors in such cases are minimal, and if the shell surrounding the hot metallic body, breaks, fire may flare up again by itself. Hence, after extinguishing the fire, it is vital to ensure the area is allowed to self-cool without physical intervention, because the coating formed on the extinguished surface prevents a re-flare of the material. After using this type of extinguishing powder, even substances which easily ignite such as cotton or paper cannot be ignited. The additives mixed with the basic material of the multi-purpose powder include smoothing and moisture resistant substances, as well as resins that facilitate adherence of the powder to the burning material, allowing coverage from the bottom up.

Some powder particles, with increased size and weight, help its penetration into the burning material through hot air currents and the creation of an inner lining. This powder is greatly affected by moisture and needs to be well protected by being kept in suitable cases and in well sealed fire extinguishers. Combined powder (also known as “Powder Q”), is appropriate for fire types B & C as well as combined extinguishing with foam. It is similar in its composition to sodium bi-carbonate based powders, but instead of magnesium – used to improve flow properties and rejection of moisture – it contains aluminum or silicon compounds, which do not reduce the surface tension of water. The main problem in using foam and powder together is that normal powder contains materials which reduce the surface tension of the water, causing rapid disintegration of the foam bubbles.

By changing the composition of the additives, it can be incorporated into the foam without reducing its stability. There is a whole series of powder based extinguishers featuring substances based on chloride potassium, sodium bi-carbonate with the addition of urea and more. Each with its special features, such as increased extinguishing ability, Integration with foam and extinguishing capability of solids. There are powders, intended for specific fire risks which require specific features. Among these are powders based on graphite designed for light metals. It can not be sprayed via a fire extinguisher and must be thrown directly on the fire with a spoon or a shovel. Chloride-based powder extinguisher designed to put out molten metals are also suitable for extinguishing light metals. You can use it with special fire extinguishers that have a device allowing soft dispersion.

Powder extinguishers

Ammonium phosphate based powders is produced in three levels of concentration: 40%, 75%, 90%, mixed with ammonium sulfate as supplementary material. Extinguishing efficiency figure refers to the concentration of 94%. Relative efficiency is the ratio of the surface area of the fuel and the weight of the powder extinguishing this area, while the standard – sodium bi-carbonate based – powder is used as a reference value of 1.0.

Powder Use – volumes and flow rates

In order to stop the combustion by capturing all the active groups at once, one needs you have to insert a large amount of powder particles into the flame and disperse them uniformly across the the  entire volume of the flame. This requires powerful output capacity. To achieve efficient extinguishing action, the entire flame volume needs to be affected because the groups which did not bind during the extinguishing process, can remain active and reignite the hot gases. The speed of the extinguishing process and the distribution of the powder must be greater than the rate of the re-ignition of the fuel vapors, which continue to rise from the area of the evaporating fire.

The flow of powder from the extinguishing device should fit the size of the area burning. The amount of powder emitted per second, which can be used to extinguish the fire using the minimum amount of powder, is considered as optimal flow, as long as the dispersion of the powder is uniform over the entire burning area. Extinguisher sizes and flow rates are listed in the chapter dealing with fire extinguishers. The powder flows out of the extinguishing device as a concentrated jet stream. Having traveled a given distance, it is dispersed in such a way that in the entire volume of the powder ‘cloud’, the particle concentration is uniform. If the distance from the flame is too short, the powder ‘cloud’ is not sufficiently developed and does not allow effective extinguishing. Conversely, if the distance between the extinguisher and the fire is too far, the powder ‘cloud’ becomes too thin, and the concentration too low to ensure an effective extinguishing action.

The effective range of fire extinguishing powder depends on the structure and the ejection mechanism and the output pressure. Fire extinguishers in the range of 6-12 kg in weight, should have an effective extinguishing range – i.e. the distance between the extinguisher and the fire – of 2-3 meters. Efficient distribution of the powder is achieved by rapid swinging movements of the extinguishing device. Proper flow and distribution of the powder ensure maximum extinguishing efficiency. Ejection speed and the need to complete a full extinguishing of the fire in a short time, require a degree of expertise and confidence in the operation of fire extinguishers. While the extinguishing capacity of this material is very high, it is best to keep a reserve of powder, and use fire extinguishers which are of larger capacity than that which is seemingly appropriate. Uneven dispersion of the powder, ejection range which is too big or small and low jet pressure, require increased amounts of powder and decrease the efficiency of firefighting. Slow powder flow does not guarantee complete extinguishing and allows the fire to flare up again. Example, when the fire requires 24 kg powder, using two fire extinguishers of 12 kg each – one after the other – is not effective enough because the powder flow is lower than required. However, operating two fire extinguishers in parallel at once – by trained firefighters – the fire can be effectively overcome.

Extinguishing powder rules/guidelines

In order to get the maximum benefit from powder based fire extinguishers, these rules must be adhered to:

– Always work in the wind’s direction. – To extinguishing burning liquid fuel, spread the powder from a distance 2-3 meters in rapid swinging movement from side to side. have the powder jet as parallel as possible to the fuel surface. “Sweep” the flame above the fuel. When the fire is out, stop the powder jet immediately, observe any re-flares and put these out immediately. Avoid aiming the powder jet from above in order to prevent splashing the burning fuel outside the fire zone. Adhere to a fixed powder spraying distance from the fire to allow the powder cloud to develop properly.

– To extinguish carbon-containing solids (wood, cloth, paper), use a multi-purpose powder. Direct the jet of powder into burning areas and cover them well with powder. After the flames have been put out, cover any hot surface with powder. Notice any re-flare and put it out immediately. Remember that multi-purpose powder creates a hard impenetrable layer over the hot surface and isolates it from contact with air so make ​​sure to completely cover the burning object with powder.

– To extinguishing superficial, fiber based fires, use Type I or II powder. Spray the powder first as a soft cloud from a distance of about 3 meters and about 1-1.5 m above the fire. Then continue with short bursts into the core of the fire, by pressing lightly and repeatedly on the extinguisher’s lever. Get the well into the burning material. By spraying the powder in front of the front of the fire, its spread can be avoided.

– Initial extinguishing of bundles of cotton & fabrics based fire is achieved by using multi-function powder (A) on the bundles, then complete the operation by flooding it with water.

– To extinguish straw based fires, use multi-function powder (A). Cover the burning area well with the powder. Take advantage of the wind blowing off the back for good insertion of the powder into the core of the fire.

– To extinguishing a burning car tire, spray the tire with multi-function powder (A) in frequent short bursts, extinguishing the flames. Remove the tire from the vehicle as soon as practical and soak it in water.

– To extinguish a pressurised gas jet, spray the powder in the same direction as the gas flow. Stop the flow of gas by detecting and closing the main valve. Under no circumstances direct the flow of powder against that of the gas stream. It is both ineffective and extremely dangerous to the fireman. In cases where there is no way to stop the flow of gas, it is best to limit firefighting operations to locating the source of the fire and preventing it from spreading. Unburnt gas may generate toxic or explosive fumes in the environment, therefore it is better to let it burn.

– To extinguish dust fires, spray powder in a soft jet and cover the burning area until it is completely extinguished. Avoid spraying a concentrated jet of powder so as not to spread the fire. Use a fire extinguisher with a special adapter which allows a softer type of flow.

– To extinguishing light metals based fires, follow the rules regarding a dust fire, but use only the appropriate powder for light metals. To extinguish metal objects which are on on fire (aircrafts, missiles), attempt to cover the entire burning area with multi-purpose Powder to create an  insulating layer, impermeable to air.

Metals based fires

Metals can catch fire and burn when they are processed in a special way e.g. steel wool, copper shavings, tin, zinc, chromium or lead. Metals associated with mining and nuclear industry are a special risk group: hafnium, thorium, plutonium, zirconium and uranium. These metals flare up easily, even without an external ignition source and upon contact with water or powder extinguishers, they respond strongly and increase their burn. They can burn even at seemingly inert atmosphere e.g. at 2co or nitrogen.

Metal based fire creates higher temperatures than those of flammable liquids and other solids. As a result, it can cause combustion of various other materials in the immediate vicinity or if it comes into contact with water, the breakdown of the water, creating a mixture of free oxygen and hydrogen (blaring mix) and explosion. Hence, metal based fires must not use water jet or mist form of both.

Extinguishing metal fires of any kind, whether with chunks, chips or sawdust, requires knowledge and expertise. Special extinguishing materials need to be used because conventional ones such as water, gases, volatile liquids and powders are ineffective and may be even dangerous for this type of fires.

The most preferred material in such scenario is sifted graphite powder, plus organic compounds of phosphorus. The large surface area of the small graphite particles effectively absorbs the heat of combustion, while phosphorus compounds create a gas that fills the cavities of the burning material and reduces the amount of oxygen in them. The graphite powder is spread manually, using shovels or mechanical equipment. Being a very fine powder and having the tendency to develop lumps, prevents its use in fire extinguishers.

Additional material is metal extinguishing powder based on Nitrite chloride (common salt) with special additives, which can be dispersed on the burning metal with fire extinguishers. The powder forms a layer of continuous membrane on the surface that isolates the combustion from air. The powder extinguisher should be equipped with a special fitting allowing soft dispersion on the fire.

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