Guidelines for design of fixed water mist firefighting systems – Part 2

Authors:
Gniewosz Siemiatkowski, VID FIREKILL
Diana Georgieva, TechInfo

The topic of securing underground garages has become increasingly relevant in recent years. Several major fires in this type of facility, as well as the introduction of electric cars, which create an entirely new category of fire hazards, are factors that draw attention to the subject. The article reviews the latest research on the topic of underground garage safety and discusses the design principles of water mist systems based on the draft standard prEN14972-5 and the VdS 3883-4 protocol, using as an example the FIREKILL™ low pressure water mist system.

In the media, it is easy to find CCTV footage showing the extremely rapid development of fire in electric vehicles – from scooters [1] to cars [2] and buses [3]. Therefore, it is no wonder that researchers from all over the world are trying to find answers to the hot questions about how dangerous fires in electric cars are and how to deal with them. The nature of a battery fire is that after crossing the limit of thermal reaction, energy is transferred from one cell to another, resulting in a fire that no longer needs an external source of oxygen to continue growing. This fact strongly influences the decisions of fire departments and designers of fire extinguishing and ventilation systems.

Wojciech Węgrzyński’s Science Fire Show podcast [4] (https://www.firescienceshow.com) covers electric car battery fires in 2 episodes. The first of them is a conversation with an expert from the RISE (Research Institutes of Sweden). According to them, the fires shown on YouTube represent cases of dynamic fire development, which they have rarely encountered during their research. It often takes 10-20 minutes for a battery fire to develop, and when the battery is heated from the outside, the battery overheat time is about 15 minutes. The test results show that the maximum and peak fire powers of an electric car are not higher than the values of cars with an internal combustion engine. In addition, higher fire dynamics is achieved in internal combustion vehicles with plastic fuel tanks when the ignition source is located in this part of the vehicle. A rapid rupture of the plastic fuel tank would cause a fire from the spilled fuel over a large area, thus spreading the fire to neighboring cars.

Water mist systems have been used with great success to fight liquid fuel fires. They also absorb large amounts of heat energy, making them a suitable technology for fire control in internal combustion or electric vehicles.

However, there are some significant differences between an electric car fire and a traditional car fire. Before the battery cell ignites, dangerous fumes are released into the atmosphere. A major component of the smoke is hydrogen fluoride (HF), which is highly flammable and toxic to the environment and dangerous to humans. Therefore, a burning battery is much safer than one that emits flammable and toxic gases. Therefore, automatic fire extinguishing systems should not be used at all costs to extinguish a fire. Their task is to control and limit the fire. Extinguishing the fire remains in the hands of professional firefighters equipped with protective clothing and oxygen apparatus. Other components of combustion smoke to be aware of are metal shavings and electrolyte droplets suspended in the smoke and air. And in this case, the water mist is a good solution, as it helps bind these air particles into the mist droplets and settle them on the floor. The dust-trapping properties of water mist have long been used in the mining industry and cargo ports.

The next part of the podcast [5] discusses the results of the analyzes made at the Institute for Construction Research. A series of CFD (Computational Fluid Dynamics) simulations shows that the height of the garage and the use of fire protection in it are more important for fighting the fire and providing evacuation conditions than the type of vehicles parked. It turns out that garages with a ceiling height of less than 2.7 m have no chance to meet the evacuation conditions. It is therefore good practice to design car parks with ceiling heights between 2.7m and 3m. In the case of electric cars, the best technique to control the spread of fire to other cars is fire barriers between parking spaces or the use of a fixed water extinguishing system such as a low pressure water mist system.

In 2021-2022, a study of electric cars was conducted in Denmark, the aim of which was to determine the associated fire risks and the best techniques for extinguishing these fires.

The Danish Institute of Fire and Security (DBI) has implemented a project entitled “Electric vehicle fires at sea – new technologies and methods for suppression, containment and extinguishing for battery car fires on board ships (ELBAS)”. The research focused on the dangers associated with electric cars on ferries, where a possible fire could have serious consequences for people’s lives due to the inability to quickly evacuate. Project assumptions and preliminary results were published in the July 2022 DBI newsletter [6]. After preliminary studies, the DBI concluded that the risk of fire in an electric or hybrid car is lower than in an internal combustion car. If it does happen, the consequences can be more serious, as a lithium battery fire is difficult to put out and can reignite long after it has been extinguished. Re-ignition, similar to a burning lithium battery in the initial phase, releases large amounts of toxic and corrosive gas – hydrogen fluoride, which makes this area dangerous for people, and fire extinguishing is difficult.

DBI tested different extinguishing methods, ranging from stationary water mist extinguishers, fire blankets, side nozzles placed between cars (photo 3), spraying water under the car – to traditional water extinguishing with fire hoses. Low pressure water mist spraying has proven very effective in controlling and limiting the fire spread. Combined with traditional firefighting by professional firefighters, this system will be recommended by the DBI for decked ferries carrying cars.

Another very important finding of the DBI is the relationship between overheating of consecutive neighbouring cells in the battery and the model of the electric car. The newer the test cars, the slower the thermal response of the sequential cells. In other words, the battery of the new Tesla, for example, is less likely to catch fire than the old Nissan Leaf. This gives a very good perspective that today’s problems with electric cars may be just a short episode in the development of electromobility.

Bearing in mind the above full-scale tests and analyses carried out by independent organizations, it can be safely assumed that for the designer of a fixed water mist fire extinguishing system, it does not matter whether there will be electric cars in the garage or not, and whether there will be special places for charging or the cars are parked anywhere. This will matter to the fire services, whose task is to extinguish the vehicle. Of course, this depends on the guidelines of the authorities, legal regulations and administrative decisions in the specific cases.

The EN 14972 series of standards will soon include another test protocol dedicated to the design of underground garages. This is prEN14972-5 [7], which was developed on the basis of VdS 3883-4 [8] without significant changes. For now, due to the lack of official publication of the European protocol, the focus should be on the VdS requirements. However, the design of the installations can be based on the BDS EN 14972-1 standard [9] thanks to the mechanism described in Annex A to this standard, discussed in the previous article [10].

Protocol VdS 3883-4 [8] contains performance criteria defined for the latter system, suggesting that the water mist system manufacturer will first carry out extinguishing tests using sprinklers in their standard configuration, and then carry out tests on water mist nozzles . The water mist result in each of the three tests must be better than that obtained with the most favourable sprinkler configuration. The fire must not spread to neighbouring cars and their window panes must remain intact. The series of tests is described in Table 1, and for a system that we want to validate on, a total of 6 full-scale tests should be conducted. The above assumptions mean that a system approved in accordance with these criteria is more suitable for use in underground garages than ordinary sprinklers.

The tolerances of the protocol VdS 3883-4 [8] require reference to the guidelines in the guide VdS CEA 4001 [11] for the remaining design parameters not established by testing. The future standard prBDS EN14972-5 [7] will instead refer to BDS EN 12845 [12]. The missing element that should be included in the DIOM (Design, Installation, Operation and Maintenance) of the water mist system is the run time and the design work area (or the number of simultaneously operating nozzles). Since these values are not specified in the protocol, the normative values should be used. Therefore, for the underground garages with OH2 hazard, we accept the parameters from Table 2.

Therefore, if we refer to a typical sprinkler system, we will find many similarities and common points. The design and calculation process for both installations will be the same as for low pressure water mist systems. Figure 4 shows part of a sprinkler system project in the underground parking of the Vector office building in Warsaw.

For the purposes of this study, only the most unfavourable action zone determined by the designer is analysed. Due to the lack of heating in the garage, the system was designed as a dry installation with standing K80 sprinklers. To make a simple comparison between the two systems, the low pressure water mist nozzles of the OH-UPR type are plotted on the same projection (Figure 5). The shape of the design area and the diameters of the pipelines are adapted to the field of action of the nozzles and flows. Table 3 summarizes the main parameters of the two systems.

Figure 5: Example of water mist system in underground parking of an office building using VID FIREKILL nozzles. Source: own development based on materials of ATR ENGINEERING TOMASZ RECHNIO. The most unfavourable area is shown, dry system – 180 m2.

From the summary and drawings, it can be seen that there are several additional advantages to designing a water mist system for garages beyond operational and cooling efficiency. First of all, the need for water is reduced by at least two times, which means a smaller fire tank or no tank in the case of a reliable central water supply. At the same time, the saved space can be used for additional parking spaces, which are very valuable in an urban environment.

For smaller amounts of water, smaller pipelines are chosen, which are easier to distribute in low spaces, such as underground parking lots. It also makes it easier to avoid other ceiling installations that the pipes might interfere with. An example of piping routing and sizing is shown in Pictures 1 and 5.

No less important is the power supply. The starting current of the sprinkler pump will be five times higher than that of a two-pump set manufactured in accordance with BDS EN 14972-1. This is due to the fact that the pumps have a shorter power curve and the frequency converters are not prohibited or limited. An example of pumps with motor inverters are shown in Picture 5. These considerations are particularly important for existing buildings.

Another concept for using water mist in underground garages that are not covered by the obligation for fixed water extinguishing systems is to protect the electric car charging stations themselves. Then protection is not designed for the entire parking, but the requirements of BDS EN 12845 or BDS EN 14972-1 are partially followed. Using the example in Figure 7, it can be seen that the calculated actuation area covers the entire access and parking area of approximately 185 m2 with 13 nozzles. If the sprinkler directions are followed, 12 nozzles will be needed.

If the determination of the area of operation is approached according to the philosophy of FM Approvals, then as a margin of safety we will take the number of nozzles activated in the fire test +50%. This would mean 8 OH-UPR nozzles operating simultaneously as 5 nozzles were activated during the test in the worst case scenario. For the site under consideration, this would mean that we control the fire of the car parked in space number 5 and also protect the neighboring cars in spaces 3–4 and 6–8. Such a scenario seems reasonable, but in such a case the assessor and the designer must decide what actuation zone they can accept in the particular design, as this is an additional protection system that goes beyond the design regulations and standards. It is wise to use the knowledge gained from fire tests in the investment process.

Conclusion

Underground garages are critical infrastructure in urban spaces, whose importance continues to grow. Electric cars in parking lots are an important topic for the public, researchers, and the fire industry due to high-profile fire incidents. The growing number of studies of their characteristics and full-scale fire tests allow us to choose suitable passive and active protection systems. By designing a water mist system based on the BDS EN 14972-1 standard [9] and using the VdS test protocol [8], participants in the investment process can be sure that the chosen technology will meet their expectations and provide safe operation of the infrastructure.

See also:
Guidelines for design of fixed water mist firefighting systems – Part 1
Guidelines for design of fixed water mist firefighting systems – Part 3

References:
[1] https://www.youtube.com/watch?v=eTm5O044sv4
[2] https://www.youtube.com/watch?v=sAQlLu5ttOk
[3] https://www.youtube.com/watch?v=T71cVhxG_v4
[4] https://www.firescienceshow.com/005-battery-fires-with-roeland-bisschop/
[5] https://www.firescienceshow.com/006-evs-rapid-fires-and-car-parks/
[6] DBI report 2/2022 (brandogsikring.dk)
[7] prБДС EN 14972-5:2022 Fixed firefighting systems – Water mist systems – Part 5: Test protocol for car garages for automatic nozzle systems.
[8] VdS 3883-4 Fire test protocol for water mist systems, Part 4: Protection of car garages.
[9] BDS EN 14972-1:2021 Fixed firefighting systems. Water mist systems. Part 1: Design, installation, inspection and maintenance.
[10] https://fire-techinfo.com, 15 March 2023: Guidelines for design of fixed water mist firefighting systems – part 1. BDS EN 14972 – – Fixed firefighting systems. Water mist systems.
[11] VdS CEA 4001 Sprinkler Systems: Planning and Installation.
[12] БДС EN 12845:2015+A1:2020 Fixed firefighting systems. Automatic sprinkler systems – Design, installation and maintenance.

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