Compressed air foam

Compressed air foam is a process for the production of low expansion foam. The compressed air foam is generated by the fire fighting centrifugal pump. The English name for these systems is: Compressed Air Foam System (CAFS). Compressed air foam can be produced with different expansion coefficients (VZ). Dispersions with an expansion ratio (expansion ratio) of 4 to 11 are referred to as wet compressed air foam, and an expansion ratio of more than 11 is referred to as dry compressed air foam.

The common abbreviation CAFS (Compressed Air Foam System) describes the system that generates the compressed air foam (compressed air foam system, abbreviation: DLS ).

In contrast to conventional foam, DLS is not first foamed in the jet pipe , but rather when the water is mixed with the foaming agent with the aid of compressed air . Because of the expansion of the compressed foam on the jet pipe and the fact that no energy is used for air admixture, this foam can be thrown much further.

Compressed air foam was originally developed in Germany in the 1930s . In Europe, CAFS was first introduced and tested in practice in 1997 by the Ingolstadt professional fire department in a pilot project. Due to the positive results, Bavaria decided to be the first federal state to promote the new extinguishing agent .

function

Scheme of a DLS system

A pressure proportioning system conveys the foam concentrate from a storage tank into the pressure line of the centrifugal fire pump. The compressed air generated by the compressor is then fed to the water / foam mixture. While the foam concentrate mixes with the water without any problems, the supply of compressed air does not lead to foaming without additional effort. A dispersion with a largely homogeneous foam bubble structure is only created by generating sufficient turbulence.

nomenclature

Compressed air foam systems are described in DIN EN 16327. The standard differentiates between four system sizes (abbreviated designation): DLS 200, DLS 400, DLS 800 and DLS 1600. A compressed air foam system DLS 800/2400 describes a system with an extinguishing agent flow rate of 800 liters / minute and an air flow rate of 2400 liters / minute produces a dispersion (low foam). Systems according to this standard must meet several safety requirements: The switching operations must be reduced to a minimum. If the pressure admixture fails, the compressed air supply is blocked. If the admixture of pressure and the air supply fail, the centrifugal fire pump still represents the fallback level. Switching the system on and off must not lead to any pressure pulses. The system pressure is limited to 10 bar with compressed air foam operation.

Extinguishing effect

The addition of surfactants reduces the surface tension to such an extent that water no longer rolls off, but rather wets the surface of the material to be burned and can penetrate into narrow gaps with cooling. The resulting increased contact area between the fire and extinguishing water also improves the heat transfer. With air, water-surfactant mixtures can be foamed into a more or less homogeneous water / air dispersion depending on the manufacturing process, which improves the extinguishing success in several ways:

• The foam adheres to sloping surfaces and thus increases the contact time between the material being burned and the extinguishing agent,
• the better wetting increases the contact area and increases the heat transfer between the material being burned and the extinguishing agent
• the surface / volume ratio, which is higher than that of water, also improves the heat transfer between the material being burned and the extinguishing agent.
• In addition, the foam is able to separate the burning surface from the oxygen in the air (= main extinguishing effect in class B fires), to hold back the rising, flammable gases and to cool them by separating its water content.

Compressed air foam only enables foam to be used in internal attack. For jet pipe foamed foam, clean air is required at the location of the foam pipe, which is not available there in the internal attack.

Instructions for use

Fire fighting vehicles should be equipped with a DLS 800 to fight fires in buildings.

Fire fighting with wet DLS

Protect a surface with dry DLS

Fires must be fought with wet compressed air foam, as more extinguishing agent can be evaporated.

The dry compressed air foam is suitable for post-extinguishing work and for preventive covering of exposed surfaces with foam.

In order to be able to produce compressed air foam, minimum cross-sections must be observed, especially with the jet pipes. The setting range for hollow jet pipes should not be less than 300 l / min and for round jet pipes a minimum cross-section of 19 mm is required. These minimum cross-sections are necessary in order to be able to convey the compressed air in addition to the extinguishing agent flow. Only wet compressed air foam can be produced with hollow jet pipes. The expansion ratio here is usually 5. If compressed air foam is used for an internal attack, the available extinguishing agent flow rate should not fall below 200 l / min for tactical reasons.

The primary goal is to turn off the heat source. This can only be done by extinguishing the fire directly. In addition, make a circular movement into the fire area for several seconds. If necessary, fan out the full jet. The nozzle has to be closed again and again to check the success of the extinguishing.

When fighting a fire, make sure that the compressed air foam is applied to as large an area as possible. The mobility of the nozzle operator and his hit rate determine the extinguishing success significantly.

All approved low-expansion foam agents can be used to produce compressed air foam.

Due to a tragic accident, pressure hoses were examined with regard to their heat and pressure resistance. The main results are: When pumping water and compressed air foam, there are no differences up to 500 ° C. If the jet pipe is closed, the hose filled with DLS lasts for 30 minutes at an ambient temperature of 275 ° C. These extreme temperatures also endanger the emergency services. It should also be borne in mind that other interfering influences can limit or interrupt the supply of extinguishing agent. With regard to the quality of the pressure hoses, the technical recommendation of the Technical Committee of the German Fire Brigades (No. 2 of July 30, 2015) must be observed.

Sprinkler pipe for spaces that are difficult to access

The temperatures of a fire event in buildings can lead to a time-delayed renewed flare-up due to heat transfer despite apparent extinguishing success, especially in cavities. In order to prevent this, exploratory openings must be created and, if necessary, the dispersion must be applied through special pipes even in places that are difficult to access. Extremely high fire loads can also endanger the emergency services. Once the dispersion has evaporated on the surfaces, it flares up again. These problems apply regardless of the deletion method used. Only the time interval up to reignition is longer with compressed air foam.

The compressed air foam is not only suitable for combating flammable solid substances, but also for liquid fires. Please note, however, that the dispersion may only be applied indirectly to the burning liquid surface. This can be achieved, among other things, if exposed areas, such as the tank of a truck, are used as a reflective surface for the full jet. The dispersion simultaneously cools the heated surfaces and the extinguishing agent running off covers the burning pool. If the surfaces remain white, this signals to the nozzle operator that there is sufficient cooling effect. It is important that liquid fires are always fought with at least two pipes at the same time, so there is a risk of flashback.

Metal fires cannot be extinguished with compressed air foam, but the dispersion is also very helpful here to extinguish secondary fires sustainably and to protect adjacent objects from thermal radiation. The melt can then be covered with the dry compressed air foam. The development of steam is kept within limits and the heat radiation is significantly reduced. As with liquid fires, direct application into the melt must be avoided at all costs.

The use of compressed air foam makes work at the scene of the fire easier for the well-trained nozzle operator, but does not replace the usual and still necessary tactical and technical measures.

Taking into account the technical and tactical information of the compressed air foam working group of Unit 5 of the vfdb , a significantly higher extinguishing effect can be expected from the use of compressed air foam. At the same time, there is a safety gain due to a greater distance from the danger area, which is possible due to the greater range of the extinguishing agent jet. Even with extensive extinguishing work (roof truss fires, industrial fires) that were consistently carried out with the compressed air foam extinguishing process, no significant water damage occurred. This can definitely be seen as proof of the efficiency of the compressed air foam extinguishing process. The prerequisite is the exclusive use of compressed air foam for fire fighting.

Advantages and disadvantages of the procedure

The compressed air foam extinguishing technology is based on the advantages of pressure admixture. The foam generation takes place in the vehicle and not on the nozzle. This means that foam generation is independent of the harmful effects of fire smoke. The adhesion of the dispersion and the penetration of the liquid stored in the dispersion into the fire material lead to a very lasting extinguishing effect. The visible extinguishing or cooling effect leads to further savings due to the adhesive extinguishing agent (white surface). Water damage can almost be avoided with consistent application of this technology.

In addition to the extinguishing effect, the proportion of compressed air in the hoses leads to a reduction in weight of at least 25%, which users find particularly pleasant. With an extinguishing agent flow rate of 300 l / min, for example, burning objects can still be reached at a distance of 30 meters at a height of two to five meters. The 800 l / min are mainly required for an outside attack, for example in the event of a roof fire, from the rescue cage of the turntable ladder . With a lower flow rate, there would be the risk that too little extinguishing agent could reach the fire object.

The additional costs for the air compressor as well as the regulation and control technology are disadvantageous.

literature

• Ulrich Braun: Die Rote Hefte, Booklet 211 - compressed air foam . 1st edition. Kohlhammer, Stuttgart 2010, ISBN 978-3-17-019312-3 .
• Working group compressed air foam - work results - Unit 5 of the vfdb

Web links

• Evaluation of the DLS extinguishing process of the Research Center for Fire Protection Technology KIT on DLS (PDF file)
• Accident Commission "Tübingen" on www.atemschutzunfaelle.de: Report on the operation (Word file)

Individual evidence

1. ^ Association for the Promotion of German Fire Protection eV: Technical Report Compressed Air Foam (DLS). October 2010, accessed March 6, 2017 .