Extinguishing foam

from Wikipedia, the free encyclopedia
Fighting a car fire with heavy foam from a hollow jet pipe with an attachment nozzle
Foam release from the roof monitor of a tank fire engine
Use of extinguishing foam
Use of extinguishing foam

Extinguishing foam is a special foam that consists largely of filler gas (usually air ) as well as water and a foam concentrate. Due to its composition, it is also called air foam. Extinguishing foam is - usually by the fire brigade as - extinguishing agent for fighting of fires of fire classifications used A (solids) or B (liquids or materials which become liquid).

Historical

Foam compound trolley with compressed air bottle and comet tube in the Munich Fire Brigade Museum

In 1877, the Englishman John Henry Johnson developed a foam production process based on a chemical reaction. For this he received the British patent No. 560 on a fire extinguisher. Saponin was added to an aqueous solution . In this solution, aluminum sulfate was allowed to act on sodium hydrogen carbonate . The carbon dioxide produced during the reaction caused the solution to foam. Foam production using this process was laborious and only very limited amounts of extinguishing agent could be produced. Nevertheless, the process was further developed until the 1930s.

The air foam process, which has been used almost unchanged to this day, goes back to the Berlin patent attorney Clemens Wagner. In 1923 he invented the air foam jet pipe , which was then known as the comet pipe . He implemented his idea that normal air could be used to generate foam instead of nitrogen gas such as carbon dioxide. In 1932 Wagner had his invention patented.

In 1928, Dr. Wilhelm Friedrich in Berlin with fire extinguishing foam. He developed a foam concentrate that made it possible to produce foam by spraying the foam concentrate-water mixture through a jet pipe.

Extinguishing effect (s)

Extinguishing foam has different extinguishing effects depending on the type (see next chapter). For each type of foam, a distinction is made between the main and secondary extinguishing effects.

  • The separation effect is the phenomenon that a closed foam cover separates the fire from the surrounding atmosphere and thus removes the oxygen necessary for combustion.
  • foam is also destroyed during the use of foam. This creates tiny water droplets that absorb heat and evaporate. This cooling effect removes energy (heat) from the fire.
  • Applied as a foam carpet to flammable liquids, it prevents the liquid from evaporating. The covering effect prevents the formation of explosive vapor / air or gas / air mixtures.
  • Light foam in particular (see below) is used to flood rooms. With this displacement effect , both flammable gases and oxygen can be pushed out of rooms.
  • as a gas / water mixture, foam conducts heat only to a very limited extent. This is used in the insulation effect , in which the foam prevents the fire from spreading through thermal radiation .

Terminology

Expansion rate

The foaming number (VZ) is the ratio (the quotient) between the volume of the finished foam and the volume of the original water-foam compound mixture. The foaming rate depends on the foam nozzle used and cannot be changed.

Proportioning rate

The proportioning rate (ZR) indicates how large the proportion of foam compound is in the water-foam compound mixture. With a 3% admixture, 100 liters of water-foam concentrate make 3 liters of foam concentrate . The proportioning rate is set on the proportioning mixer , usually values ​​from 0.5% to 6% are possible. Modern foam concentrates (concentrates) sometimes require lower concentrations. These start at 0.1–0.3% for wetting agents and at 0.3–0.5% for foaming. Corresponding attachments to make commercial mixers even for these low proportioning rates are available in stores. The proportioning rate to be set is specified by the foam concentrate manufacturer. In general, however, the higher the proportioning rate, the more stable the foam becomes.

Destruction rate

The destruction rate indicates how high the proportion of the extinguishing foam is that is destroyed directly when it hits the fire. In addition to the heat of the fire, this can also be caused by chemical reactions with the fuel. When calculating the required amount of foam, a destruction rate of 50% is generally assumed, but in individual cases (such as with very hot mineral oil fires) this can be up to 70%.

Water half-life

The water half-life (WHZ) indicates the time in which half of the liquid originally contained in the foam (foam concentrate and water) escaped. The leakage of the liquid leads to an increasing drying of the foam with a decreasing extinguishing effect.

Types of foam

Tube for producing medium foam

In the fire brigade , the extinguishing foams are primarily divided into three categories according to their expansion rate: heavy foam, medium foam and light foam.

A water-foam compound mixture can also be used unfoamed to achieve better wetting in solid fires. The foam concentrate is dosed lower than when used as foam, which is called mains water.

Foam type Expansion rate Main extinguishing effect Secondary extinguishing effect
Heavy foam 4 to 20 cooling down Suffocation
Middle foam 21 to 200 Suffocation cooling down
Lightweight foam 201 to 1,000 Suffocation

Heavy foam

Heavy foam is a relatively wet foam with which you can achieve good throwing distances. It is used for fires involving solids or liquids ( fire classes A and B). The most important effects that contribute to fire fighting are the cooling effect and the separating effect. In addition, heavy foam can "stick" to vertical surfaces to a certain extent (depending on the foam compound, foaming and, last but not least, the structure of the surface).

Middle foam

Due to the higher expansion rate, the middle foam is significantly lighter than heavy foam and can (if the foam cannot flow off) be piled up to a height of 5 meters. Its adhesion to surfaces is poor, which means that it can be used for flooding objects. In addition to the displacement effect, the separating and subordinate cooling effects are among the extinguishing effects of the central foam.

Lightweight foam

This type of foam cannot be thrown because it is so light that it is already blown away by the wind. The main place of use is therefore in closed rooms. In addition, in contrast to medium and heavy foam, a special generator is necessary for the production of light foam .

The main extinguishing effect is the displacement effect. In addition, the foam breaks down very quickly under the influence of heat into the finest water droplets and thus has a cooling effect.

Mains water

The net water is a specialty, because it consists of water and foam compound (used here as a wetting agent ), but is released in non- foamed form via multi-purpose or hollow jet pipes (is therefore not an extinguishing foam). The addition of foam compound reduces the surface tension of the water, which enables the mixture to penetrate better and deeper into (burning) objects - such as. B. paper or textile balls - to penetrate.

Foaming agent

The foam concentrates are mixed with the extinguishing water either in the vehicle or later by a mixer.

Various foaming agents are used by the fire service:

  • With multigrade foam concentrate (MBS) all types of foam (heavy, medium and light foam) can be produced.
  • Water-film-forming foam agents ( AFFF , also written "A3F", English for aquatious film forming foam ) also form a vapor-tight, aqueous liquid film between the foam and the burning liquid. However, some of them are prohibited due to their environmental hazards.
  • Class A foam foam concentrates (ClAFSM) are widespread in the USA, but are only emerging in Germany. ClAFSM were developed as a wetting agent and foam agent for fire class A, especially for vegetation fires. A great advantage is that, depending on the purpose, they only have to be added to the extinguishing water at 0.1% to 1.0%, so they are very economical to use.
  • In contrast, protein foaming agents have lost their importance . With the protein foaming agents made from animal raw materials, only heavy foam can be produced, which, however, has unsurpassed adhesion.
  • Fluoroprotein foams (FPS) and film-forming fluoroprotein foams (FFFP) are more likely to be used by plant fire departments.

The requirements for foam concentrate for heavy, medium and light foam for use on non-polar liquids and heavy foam for use on polar liquids are specified in DIN EN 1568.

Occasionally the term "foam concentrate" is also used. However, because concentrates must be diluted before use, this term is incorrect.

Foaming techniques

Foam foam

Any foam to which the air is only added in the foam jet lance (or high-expansion foam generator ) is referred to as "foam jet pipe" . So only a mixture of water and foam concentrate is conveyed in the hose.

Compressed air foam

With compressed air foam ("DLS" or "CAFS" for "Compressed Air Foam System"), the foam is already produced in the vehicle from water, foam compound and air and conveyed through the hoses in compressed form. The process itself dates back to the 1930s and was largely forgotten, but is currently experiencing a very controversial renaissance in Germany, returning from the USA.

The structure of compressed air foam is significantly more homogeneous than that of conventional air foam, it is similar to the consistency of shaving foam and adheres to vertical surfaces. This makes it possible not only to delete in the actual sense, but also, for example, to foam walls as a precaution to protect them. Depending on the setting of the system, the expansion rate of compressed air foam is between about 4 and 15 and thus in the range of heavy foam in the sense of air foam, but the extinguishing effect with a expansion rate of 15 is based on the separation rather than on the separation due to the different structure in contrast to air foam the cooling effect.

Process for foam production

The foam concentrate solution for the extinguishing foam can be produced using the common proportioners (DIN injector mixers) according to 3 different processes:

Straight ahead

Pump premixing process with a proportioner
Pump premixing process with two mixers and thus double the flow rate

In the classic process, the proportioner is located between the distributor and the jet pipe. The proportioner and foam nozzle must be directly coordinated with one another in terms of their flow rate. If necessary, several mixers can be used.

One problem with this method is that there are high pressure losses in the proportioner (according to DIN 14384, a maximum of 38% pressure loss is permitted) and that there may be a maximum pressure difference of 2 bar between the proportioner and the foam nozzle, as otherwise the proportioner will no longer work reliably. There are now also fire brigades that connect the proportioner directly to the pump outlet. This has the advantage that you don't have to carry the foam concentrate that far. You can also add foaming agent as a wetting agent.

Pump premix process

The pump premixing process works with two pumps connected in series, between which the proportioner is installed in the hose line. If necessary, several proportioners can also be used here. As the term "pump premix" suggests, the proportioner is installed upstream of the second pump. This allows high throw distances at the discharge fitting, because the pressure is only built up in the second pump with the water-foam compound mixture. This variant is ideal for bridging large distances. The total delivery rate of this structure, however, depends on the proportioner, through which only a certain amount of water (200 l / min, 400 l / min or 800 l / min) flows depending on the type (Z2, Z4 and Z8). This can be avoided by connecting several proportioners in parallel between the two pumps.

The main disadvantage is the contamination of the second pump. Another disadvantage is the decentralized mixing location.

Shunt process

Shunt proceedings

This method is a further development of the straight-ahead method, which works with only one pump and one proportioner. A proportioner is connected to an outlet on the pump, which delivers its mixture in a ring closure back to the pump via a claw . The mixture then reaches the jet pipe via the second outlet.

The swirling in the pump results in a very good mixing of extinguishing water and foam compound. However, the machine operator at the pump must ensure that the pressure in the two inflows to the pump is equal (as a guideline, the pump pressure should be around 4 bar above the hydrant pressure, which corresponds to the pressure loss in the proportioner). Otherwise, optimal mixing cannot be achieved and, in the worst case, no foam can be generated.

The effective admixture at the dispensing fitting can be calculated using the following formula:

Since the water cannot flow backwards due to pressure surges, a backflow preventer ( e.g. check valve ) must be installed between the hydrant and the pump to prevent a foam concentrate-water mixture from entering the drinking water network. Such pollution of the drinking water has already occurred several times when no backflow preventer was used.

Stationary foam extinguishing systems

Foam cup and foam manifold for the foam extinguishing system of a fixed roof tank
Skimmer cup and skimmer for fixed roof tanks

Stationary foam extinguishing systems are fire extinguishing systems that work according to the classic proportioning process, in which a proportioner is used after the pump. In Europe, EN 13565-2 regulates the planning, installation, commissioning and operation of heavy, medium or light foam extinguishing systems.

In the case of the stationary foam extinguishing systems, the foam can be released via special sprinkler nozzles, foam jet pipes, foam bends or high-expansion foam generators when it emerges from the pipe system. In the case of sprinkler systems with extinguishing nozzles, these are evenly distributed in the room to be protected. Foam extinguishing systems are preferably used where flammable liquids are to be expected, such as in tank farms or filling stations. In industrial plants or at airports, for example in hangars, so-called monitors are often used for selective extinguishing with foam. In these systems, the foam emerges from the monitor cannon under high pressure in order to achieve great throwing distances.

Foam extinguisher

View into the foam compound / water storage tank of a foam extinguisher with the propellant cartridge removed

Foam fire extinguishers partly work with separate foam and water supplies (charging extinguishers). After activating the extinguisher, CO 2 presses the foam compound from a gas cartridge into the container with water and then the water-foam compound mixture out of the fire extinguisher, where it is finally foamed. When replacing the extinguishing agent (usually every 5 to 8 years), usually only the cartridge has to be changed and not the entire extinguishing agent. This saves additional costs for fire extinguisher maintenance.

There are also foam extinguishers that already contain a prefabricated foam concentrate-water mixture (permanent pressure extinguisher). Here the extinguishing agent and the compressed gas are in a container that is constantly under pressure.

Foam extinguishers are manufactured for fire classes A (solids) and B (liquids) as well as special fat fire extinguishers for fire class F (fat fires).

Hazards and safety information

Hazards

Working with foam concentrate may only be done with eye protection. Foam concentrates are usually assigned to water hazard class 2, which is why a fire-fighting water retention system should be set up to prevent penetration into the sewer system or open waters. If this does not happen, a lack of oxygen ( hypoxia ) can cause fish to die.

In areas where foam is used, breathing protection that is independent of the ambient air may only be used, as otherwise breathing is no longer possible if you fall into the foam.

Foam must not be used in electrical systems as long as the system is not activated .

PFC in extinguishing foams

PFCs are per- and polyfluorinated alkyl compounds . Some individual compounds are suspected of being carcinogenic. Due to their properties, foam extinguishing agents containing PFOS were banned. Perfluorooctanesulfonic acid (PFOS) may not be placed on the market or used either as a substance or as a component of mixtures in a concentration of ≥ 10 mg / kg (0.001% by weight) (Regulation (EU) No. 757/2010).

In the past, the use of PFC-containing extinguishing foams was permitted and has led to some damage. Damage can occur at airports due to the foaming of runways or fire extinguishing exercises. At the airport Dusseldorf and Nuremberg renovations are already under way.

The Federal Court has ruled in 2018 for public liability in firefighting that (containing PFC) in unnecessary use of foam concentrates the municipality for environmental impact is liable.

From July 4, 2020, an EU-wide restriction applies to perfluorooctanoic acid (PFOA), its salts and substances that can be broken down into PFOA. The following limit values ​​must then be observed in new products: 25 ppb PFOA and salts, 1000 ppb for PFOA-related compounds (e.g. precursor compounds). Fire extinguishing foam mixtures that were on the market before July 4, 2020 may be used up.

literature

  • Roy Bergdoll, Sebastian Breitenbach: Die Roten Hefte, Issue 1 - Burning and Extinguishing . 18th edition. Kohlhammer, Stuttgart 2019, ISBN 978-3-17-026968-2 .

Web links

Commons : Erase Foam  - Collection of images, videos and audio files

Individual evidence

  1. ^ Roy Bergdoll, Sebastian Breitenbach: Die Roten Hefte, Heft 1 - Burning and Extinguishing . 18th edition. Kohlhammer, Stuttgart 2019, ISBN 978-3-17-026968-2 , pp. 84 .
  2. a b c extinguishing foam then and now. In: Feuerwehr-Magazin special issue 2006 - fire fighting with foam. P. 9.
  3. Wulf-Erich Damrau: Foam against fire. Dr. Sthamer, Hamburg 2006.
  4. Extinguishing agent foam. Training of volunteer fire brigades - training to become a squad leader, Neckar-Verlag 2004, p. 12.
  5. The 11 Biggest Misconceptions About Foam. Retrieved June 29, 2016 .
  6. ^ Institute of the Fire Brigade Saxony-Anhalt: Extinguishing flammable liquids with foams: Investigation of characteristic values . Heyrothsberge, undated, p. 7.
  7. a b c d The types of foam. In: Feuerwehr-Magazin special issue 2006 - fire fighting with foam. P. 16ff.
  8. Foam extinguishing agents through the ages. In: fire department magazine. 1/2011, p. 22ff.
  9. Which foam concentrates are there in: Feuerwehr-Magazin special issue 2006 - Fire fighting with foam. P. 26ff.
  10. Technical report compressed air foam ( Memento from June 7, 2012 in the Internet Archive ) (PDF; 1.6 MB), Association for the Promotion of German Fire Protection eV, 2010.
  11. a b Fire Brigade Service Regulations 1 Basic Activities - Extinguishing and Assistance (with editorial additions until 03/2007), Federal Office for Civil Protection and Disaster Relief , 2007.
  12. ^ Karl Ebert: Handbook of fire brigade fittings. Max Widenmann KG
  13. a b c proportioning technology. In: Feuerwehr-Magazin special issue 2006 - fire fighting with foam. updated edition. P. 58ff.
  14. Fire brigade magazine : Drinking water protection when using fire brigades: You have to pay attention to this! ( Memento of the original from October 18, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. by Michael Klöpper, accessed October 19, 2015. @1@ 2Template: Webachiv / IABot / www.feuerwehrmagazin.de
  15. DIN EN 13565-2: 2009-09 - standard. beuth.de, accessed on May 15, 2012 .
  16. Foam extinguishing systems , Minimax
  17. Hans-Joachim Gressmann: Defensive and technical fire protection: For architects, civil engineers and fire-fighting engineers . 2nd Edition. Expert-Verlag, Renningen 2007, ISBN 978-3-8169-2778-5 , p. 221-238 .
  18. a b Product data sheet foam extinguisher ( Memento from February 22, 2014 in the Internet Archive ), Total (fire protection)
  19. Difference between permanent pressure extinguisher and charging extinguisher. Retrieved March 11, 2017 .
  20. Recommendations. Gloria (fire protection) , accessed May 2, 2014 .
  21. Grease fire extinguishers. Gloria (fire protection) , accessed May 15, 2012 .
  22. Fish deaths after major fire: water samples evaluated. In: owl24.de. November 15, 2019, accessed November 18, 2019 .
  23. PFT-polluted areas on the airport premises are hydraulically shielded by three groundwater remediation systems. Düsseldorf Airport, July 6, 2016, accessed on November 24, 2019 .
  24. Wolfram Göll: Hazardous waste dump instead of north connection? Bayernkurier, June 22, 2016, accessed on November 24, 2019 .
  25. Federal Court of Justice: Communication from the press office No. 105/2018 Federal Court of Justice on official liability in the event of fire-fighting - judgment of June 14, 2018 - III ZR 54/17. June 14, 2018, accessed November 24, 2019 .
  26. PFC in fire extinguishing agents. In: www.umweltbundesamt.de. March 6, 2014, accessed November 24, 2019 .