Guidelines for design of fixed water mist firefighting systems – Part 3

Authors:
Gniewosz Siemiatkowski, VID FIREKILL
Diana Georgieva, TechInfo

The design of water mist systems should not be limited to applications where fixed water systems are required by law. It is worth noting the possibilities that water mist technology provides for the protection of life and property. The recently published standard BDS EN 14972-3:2022 [1] is a water mist test protocol for hotel and office buildings, which is also applicable to such important public buildings as hospitals and homes for the elderly. The article examines the impact of water mist systems on the evacuation of people with special needs and presents examples of realized projects of low pressure water mist systems in two hospitals in Europe, showing the VID FIREKILL system as an example.

The evacuation of people from the premises of public buildings during a fire is a huge effort for the responsible officials in the respective building, but also a multidisciplinary challenge for experts, architects and designers of ventilation and fire protection systems. In Bulgaria, fire protection systems are usually not associated with the protection of human life, but rather with the protection of property. In contrast, in the United States, Western Europe, and Scandinavia, the importance of fixed water-based firefighting systems in the evacuation strategy of people, especially people with limited mobility, has long been recognized. For example, the National Sprinkler Association (NFSA) in the US promotes the use of sprinklers in care homes, hospitals and residences with the motto: „Fire sprinklers buy time. Time buys life“.

It is difficult to disagree with such a statement, considering that the sprinkler (classical or water mist) is designed to be activated at an early stage of fire development and to control and extinguish the fire. That is why its main function allows to “buy time”, i.e. to stop the fire and limit the formation of deadly smoke. The time frame for evacuation from an unprotected building is approximately three minutes from ignition to inflame. In many cases, safe evacuation without a fire protection system is simply impossible. In this case, the water mist has another important advantage – cooling the environment. The ability to absorb a large amount of heat energy means that the fire is separated by a curtain of water mist that protects people from the high temperatures.

The smaller droplets in the water mist jet evaporate faster than with classic sprinklers and thus the water resource is used more efficiently. Less water means not only smaller, hard-to-place fire tanks, but also less water loss when the system is activated.

Sometimes the fire extinguishing system can be activated intentionally or accidentally when the sprinkler nozzle/ampoule fails. To illustrate the difference in water discharge in such a situation, the flow from the sprinkler and water mist nozzle should be accounted for by the available pressure. A nozzle will start the main pumps, but the flow rate will be much less than for the design area, so losses will be almost negligible. Sprinkler pumps can operate at the beginning of their characteristic, delivering a higher pressure than assumed for the calculation of the worst zone. For the purposes of this article, the actual design conditions for the National Pulmonology Center in Budapest, Hungary are analysed.

The design was carried out with low pressure water mist nozzles type OH-VSO and OH-PX2 of the VID FIREKILL system. The pumping plant is sized for 1100 l/min at 14 bar pressure with a speed control system to maintain constant pressure (Figure 2). Alternatively, sprinklers of the K80 type were considered, where the water demand for the OH3 adverse zone is 2100 l/min. It is assumed that the pressure in the pump is 8 bar (Figure 1). Design details will be discussed later in the article.

The water flow calculation requires the coefficient K (k-factor) and the operating pressure of the nozzle used. The equation used is the following:

q = K √ p

where: q is the flow rate (l/min), p is the working pressure (bar), K is the k-factor coefficient (metric).

For sprinkler K80
q = 80 √ 8= 226 l/min

For OH-VSO nozzle with K16.7
q = 16.7 √ 14= 62.5 l/min

This example shows that random sprinkler actuation produces almost four times the flow rate of a low-pressure water mist nozzle actuation. Therefore, this is one of the factors that an investor should consider when choosing a fire extinguishing system.

Another important factor in protecting the health of people evacuated from a burning building is dealing with smoke and hazardous gases, which are far more likely causes of death in a fire than the fire itself. We can imagine that in the case of a hospital or nursing home where people have limited mobility, the time required for the smoke to cover more victims is shorter than for example in a shopping mall. Fixed water extinguishing systems reduce smoke generation by controlling and containing the fire itself. Water mist has an additional mechanism that was already mentioned in the previous article [2]. The better atomized water spray absorbs the dust and partly the gases contained in the smoke, becoming a kind of filter.
In 2022, the Polish foundation cfbt.pl organizes fire workshops aimed training and supporting the development of fire rescue in Poland. Part of the training program was demonstrations of burning furniture foam simultaneously in two identical test rooms (Picture 3).

A single FIREKILL OH series water mist nozzle is installed in the right test chamber in the ceiling, located at a height of 2.4 m. Water is supplied with a DN25 fire hose from a fire pump located in a nearby vehicle. The nozzle pressure is approximately 6 bar. There is no protection in the left test chamber. The purpose of the test is to show the difference in fire development between the two scenarios. Given that most hospitals and nursing homes do not have any active automatic fire protection and usually use foam mattresses, this material was used for the demonstration. Although the operating conditions of the fire extinguishing installation are unfavourable and incompatible with real conditions, due to air penetration through one of the walls of the modelled room and the entry of large amounts of oxygen fuelling the fire, the results of the experiment clearly show how the water mist installation “buys time” to evacuate people.

A 1.8 x 0.3 m polycarbonate strip was attached to the upper front of both test rooms. This material melts at 230°C and the yield strength is 130°C. Picture 4 illustrates the temperature conditions prevailing in the unprotected room after less than 3 minutes. If we assume that the hot gases were able to escape quite freely from the room and that the only combustible materials were a piece of furniture foam and a 2 x 2 m lining on the back wall, achieving such a fire development in such a short time only confirms the previously stated thesis, that the evacuation of people with reduced mobility from a hospital without the protection of a fixed system would be impossible. At the same time, it should be noted that the polycarbonate in the protected room is not even deformed.

The previously mentioned properties of water mist to filter smoke are shown in Picture 5. Black smoke is coming out of the unprotected room, which is quickly blown away by the wind to the left. Black smoke is not observed in the space between the two test chambers. Only the water mist and water vapor come out of the protected room, as seen in the picture.

Similar conclusions were also drawn during full-scale fire tests carried out at the DFL accredited laboratory in Denmark on behalf of the fire protection engineering bureau RHT Sicherheitstechnik for compliance with the requirements of the Austrian Institute of Civil Engineering. The purpose of the tests was to prove the effectiveness of heat absorption and to prevent the vertical spread of the fire along the facade to the rooms located above the fire. The condition of the test was that the nozzle should be positioned so that it does not extinguish the fire, but only acts as a curtain. The power of the fire in the room was 15 MW, and the measurements lasted 30 minutes. The opening protected by the nozzle was 5 x 3 m in size and had no glass. Picture 6 shows a fire already burning and being maintained inside the room. The amount of smoke that escapes is negligible compared to free burning. Even more interesting are the results of the temperature measurements and the heat flux taken outside (Figure 3). The temperature in the room above the fire dropped to 117°C from the maximum 500°C observed in the free burning test. Heat flow is reduced by up to 87%. The flame does not go outside and there is no chance of the fire spreading to the facade. Water mist therefore proves its usefulness in reducing this type of risk as well as significantly improving evacuation conditions. Considering that hotels and hospitals are often high-rise buildings, the use of water mist nozzles to limit the fire spread along the facade is also justified, especially after the numerous building fires in the world related to combustible facades.

Therefore, water mist is sufficient to be accepted if it has the same result as sprinklers. This is a different criterion than the one set in the garage tests, where the water mist had to perform better than the sprinkler to be accepted. An additional condition is the number of nozzles activated during the test. No more than four nozzles may be activated per test and only one of the nozzles may be outside the nearest ring around the combustible material. This is an important design parameter as activation of additional nozzles may result in too large an actuation area under fire conditions and the pump may not be able to deliver the design flow and pressure parameters. This will cause the fire to spread beyond the water mist zone. The distance between the nozzles and the arrangement of the rings are shown in Figure 6.

1. Better cooling effect than classic sprinklers:

a. CFD analyzes were performed for the building and based on the positive results of the simulation of temperature conditions during the fire, it was possible to adjust the level of passive protection and the HVAC system. The savings justify the higher costs of the fire protection system.

b. A better cooling effect means easier evacuation and greater safety for patients and hospital staff.

2. Lower water losses in case of unjustified activation of the system.

3. Better water quality in the system thanks to the use of stainless steel pipes. Running the system will cause less damage and health risks than sprinkler pipes full of sludge and rust.

4. The possibility of using smaller tanks and pipes, which is especially important when renovating existing buildings, as is the case in the present example.

OH-VSO type nozzles tested according to EN 14972-3 protocol [1] were used for the corridor areas and recovery rooms. Assumes 6 nozzles operating simultaneously or as many as are required for an area of 72 m2 – whichever is greater. This resulted in a water requirement for each activated zone in the range of 280 – 320 l/min depending on the distance to the pumping station. The most unfavourable area turns out to be a large HVAC machine room located on the penultimate floor (Figure 8). For this area, nozzles tested for hazard category OH3 were used, which are not the subject of this article.

In another example from the Libourne Hospital Center in France, the water mist was used only in part of the renovated building for the needs of the underground garages. Demand for parking spaces has increased significantly, creating an additional fire hazard. Due to the lack of space for a large pumping station and water tank, the OH-UPR nozzles were used for the OH2 classification in accordance with the tests described in the previous article [2].

Conclusion

Hospitals and care homes require special attention to fire safety through the lens of people with reduced mobility and reduced resilience. Systems that are designed to fight fire can, more or less, “buy time” for their safe evacuation. Water mist fits perfectly into this scenario, leading to its growing popularity among designers and investors looking for efficient solutions for their sites. The standards BDS EN 14972-1 and BDS EN 14972-3 provide a solid basis for designing and building a safe fire extinguishing system.

See also:
Guidelines for design of fixed water mist firefighting systems – Part 1
Guidelines for design of fixed water mist firefighting systems – Part 2

References:
[1] BDS EN 14972-3:2022 – Fixed firefighting systems – Water mist systems – Part 3: Test protocol for office, school classrooms and hotel for automatic nozzle systems.
[2] Guidelines for design of fixed water mist firefighting systems – Part 2. Underground garages – design guidelines according to prБДС EN 14972-5: Test protocol for car garages for automatic nozzle systems.
[3] VID FIREKILL – DIOM CEN EN 14972 No. 210125-01-I OH-CEN DIOM version I.
[4] BDS EN 14972-1:2021 Fixed firefighting systems. Water mist systems. Part 1: Design, installation, inspection and maintenance.
[5] VdS 3883-1 Fire test protocol for water mist systems, Part 1: Protection of office spaces and accommodation areas.
[6] Guidelines for design of fixed water mist firefighting systems – Part 1. BDS EN 14972 – Fixed firefighting systems. Water mist systems.

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