By Elke Pantazis. High-rack warehouses for flammable chemicals conceal numerous risks. Escaping liquids can sometimes self-ignite under certain conditions due to their chemical composition. Due to the rapid and fierce development of heat in the high-rack storage, the flames can spread in no time at all to other storage containers or the supporting structure. If the fire suppression system is triggered too late or if the extinguishing technology is inefficient, the situation can quickly become dangerous for people, the building and the environment.
Conventional extinguishing solutions
Conventional extinguishing technology relies on sprinkler or high expansion foam systems. Individual sprinkler heads in the sprinkler system can be relied on to trigger when a predefined temperature is reached. However, these are reactive systems where each sprinkler only triggers when the fire has already spread to that section. In addition, many flammable liquids can only be brought under control by mixing them with foaming agents.
In contrast, high expansion foam generators flood the entire extinguishing area which is confined by walls, removing the oxygen that the fire requires for it to burn. However, this can pose a risk of suffocation for people present in the danger area. Furthermore, the foam released from the extinguishing system can hinder the fire brigade in extinguishing the fire. Removing the foam and cleaning the affected hall also requires a relatively large amount of effort.
In the search for an alternative extinguishing method, Rosenbauer carried out a large-scale fire test with compressed-air foam extinguishing system at the request of a German manufacturer of thermosetting plastics.
What is compressed-air foam?
Compressed-air foam is a mixture of water and foaming agent that is held in a mixing chamber protected from environmental influences. Pressurised air is then injected into the chamber. This creates a homogeneous and stable foam in the pipes even before it reaches the nozzles so that the quality cannot be influenced by e.g. smoke gases.
The foam contains small air bubbles which allow it to stick very well to smooth surfaces – even vertically – and form a stable layer that lasts for a longer period of time (than standard low-expansion foam). This layer of foam sustainably cools the burning material and isolates it from oxygen to prevent any further reaction. Due to these particularly good extinguishing properties, the NFPA 11 (National Fire Protection Association) allows lower application rates than for standard low-expansion foam.
The test setup comprised a section of a warehouse (see Figure 2) with three rows of shelves, with a length of around 12m and a height of 7m. The third row of shelves stood at a distance of around 2m from the shelves shown in Figure 2. It was only partially filled with containers to see whether the flames could also pass over an aisle (see Figure 4). Plastic containers (Intermediate Bulk Containers (IBC)) with a volume of 1m3 were stored in the bottom two levels of the shelves, while the top two levels of the shelves held metal containers each with a capacity of 200l. Concrete tiles were laid around the base of the shelves to collect the escaping liquid. The fire was ignited using n-heptane that was placed in a bath in the collection troughs underneath the shelves. A fluorine-free foaming agent was selected for the compressed-air foam extinguishing system that was mixed into the water flow at a constant rate of 3%. Compressed air was added to the water-foam mixture in the CAFS (Compressed Air Foam System) unit with the help of a compressor.
The resulting compressed-air foam was then transported via pipes to the nozzles that were positioned above each of the four levels of the shelves.The aim of the test was to extinguish a fire, which was caused by a leak in one of the containers and self-ignited, with the help of a compressed-air foam extinguishing system. The success criteria were defined by experienced fire research experts from the RISE institute (Sweden) in cooperation with experts who were responsible for approving the project on behalf of the customer. The extinguishing system was triggered in the various tests between 40 and 60 seconds after the fire had ignited. On the one hand, the different times were selected to simulate the delay until detection by the detection system, while on the other hand, it enabled the limits of the system to be tested by investigating whether the compressed-air foam extinguishing system could also be used to successfully fight more intense fires.
In the first test scenario, a flammable liquid (styrene) escaped from a container in the lowest level of the shelves and hit the burning heptane in the bath. The flames in the three tests reached a maximum height of around 8 m above the ground, which was 2 m above the steel containers stored on the highest level of the shelves. Temperatures of up to 650°C and a heat release rate of almost 4,000kW were measured. The flames were brought under control and reduced to a height of 1.5m above the ground at the latest 60 seconds after the CAFS extinguishing system was triggered.
In the second scenario, the container with the escaping liquid was placed on the fourth level of the shelves.
Due to the higher position of the container (compared to scenario 1), the fire grew significantly quicker: The volume flow was more fragmented so that a larger surface area of the fuel was exposed to the flames. In turn, this resulted in faster evaporation and the development of flammable gases – and thus good conditions for more rapid ignition. A heat release rate of almost 7,000kW and temperatures of more than 600°C were reached.
Amongst other things, this was due to the fact that in this scenario the fire was able to burn along the flow of liquid escaping from the container and up the entire height of the shelves: The flames in this test reached a height of 9m. Scenario 2 was carried out twice. In the second test, the extinguishing system was triggered 20 seconds earlier because this corresponds better to the speed at which a detection system would trigger. As a result, it was possible to bring the fire under control more than one and a half minutes (90 seconds) earlier.
The two scenarios and the total of five completed tests demonstrated that a fire caused by a leak in a container can develop very quickly and cause a large amount of heat to be released. The greater the height from which the burning liquid escapes, the more intensive the fire becomes. The most important points for the rapid suppression of the fire are:
- Early detection of the fire
- Quick activation of the extinguishing system
- An extinguishing system whose design enables compressed-air foam to be distributed over the storage containers within 30 to 45 seconds
The collection troughs under the shelves should be split into sections no larger than 2 to 3m2. This reduces the surface area of the escaping and burning liquid and thus also the intensity of the fire. The burning liquid in the collection trough is the main source of fire that is constantly fed due to the leak. Therefore, it is essential that this fire is extinguished as quickly as possible. This can be achieved by positioning extinguishing nozzles underneath the lowest level of the shelves.
In both scenarios, compressed-air foam was demonstrated to be a very good extinguishing agent for high-bay warehouses storing flammable liquids. The requirements of the expert assessors were also met by the extinguishing system. Overal less water is required than for sprinkler systems due to its high extinguishing power. Alongside the higher efficiency of this extinguishing agent, another positive aspect is that the size of the extinguishing sectors – similar to a deluge extinguishing system – can be adapted depending on the protective goals, so that a potential fire can also be fought over a large area.
The advantages are clear when compared to high expansion foam systems:
- The entire hall does not need to be flooded
- The system is safer for people that are located in the hall
- Less effort is required to dispose of the extinguishing agent: it is applied locally in limited quantities and in the best case scenario is collected in the troughs under the shelves
Elke Pantazis, Msc.: Product Manager for Stationary Extinguishing Systemsat Rosenbauer Brandschutz GmbH
The article was published in FeuerTrutz International, issue 1.2019 (January 2019).
More information about eMagazine FeuerTrutz International