Applications of Water Mist Systems: History
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Water mist systems have been proven to perform in many applications, such as enclosed compartments; open air; and large, well-ventilated spaces.

  • water mist
  • fire behaviour
  • fire suppression
  • fire protection

1. Machinery Spaces

In recent years, water mist has emerged as an alternative suppression medium over CO2 and halocarbon gaseous agents in machinery spaces for its ability to remove heat from hot surfaces without environmental impact. Machinery spaces such as large engine rooms on a ship or within a manufacturing process facility create flammable liquid hazards in the form of liquid fuel and lubricating and hydraulic oils. A ruptured hydraulic line can create fuel spray fires or liquid pool fires if the hot engine components ignite. There are many different test protocols established by IMO, Underwriters Laboratories (UL), and Factory Mutual (FM) related to machinery spaces for various fire scenarios that may occur in machinery spaces. These test fire scenarios are for various fire types, fire sizes, compartment obstructions, fuel types, ignition sources, compartment sizes, and ventilation conditions. The recent studies completed by Jeong et al. [1] and Choi et al. [2] applied water mist to engine diesel generators, shipboard engine rooms, and machinery spaces following IMO test protocols and found that water mist can effectively extinguish various fires, including spray fires, pool fires, and cascade fires that may occur in machinery spaces. A larger fire within the machinery space relative to the compartment size is easier to extinguish due to the larger amount of steam generated to displace the oxygen. According to these studies, using water mist to protect machinery spaces from fire is a preferable option. The quantity of water mist test protocols that are written specifically for machine space protection supports this use. The industrial environment is constantly changing as a result of new technology, so it is crucial that fire protection techniques such as water mist adapt as well to account for these new risks.

2. Power Generation Turbines

Turbine enclosures are seen as another type of machinery space, except the potential fire sources differ. The fire source in a turbine enclosure is mainly limited to the turbine itself and the associated fuel line. Water mist suppression is well suited for protecting turbine enclosures for its ability to extinguish fire rapidly and efficiently while producing minimal thermal shock and corrosion damage, post-fire clean-up requirements, and negative environmental impacts. A requirement of the FM Global Test Protocol 5560 approval for turbine protection is that the cooling of the turbine casing will not result in damage to the turbine blades. To overcome this, manufacturers have incorporated the cycling discharge method, which sprays momentarily on–off pulses to reduce the rapid cooling of the casing and still extinguish the fire within the required time [3]. Liu et al. [4] and Liu and Kim [5][6] established that cycling mist discharges into an enclosure resulted in a faster extinguishment and used less total water when compared with continuous applications. The increase in the compartment temperature during the off cycle of the water mist allowed for the evaporation of suspended droplets and the growing fire to further consume oxygen in the compartment. Once the water mist on cycle was reintroduced, it produced further cooling and turbulent mixing. The cycle is repeated until extinguishment is successfully achieved. This requirement makes turbines one of the more difficult fire hazards to protect effectively. Cycling discharge methods for turbines should be tested for other applications and fire risks to determine if the benefits can be applied to other fire risks. Because power generation is an expensive process with numerous potential safety hazards, fire protection methods are critical to turbine operators and insurers. Water mist has proven to be an effective method of fire suppression, but it will require ongoing research and development as new methods of power generation and storage are established. Water mist must be suitable for electrical equipment in order to be used in the power generation industry.

3. Electrical Equipment

Halon and CO2 gas agents are known methods for protecting electrical equipment from fires. There have been concerns regarding the use of water mist suppression as a replacement for halon and CO2 because of the electrical conductivity of the water causing damage or creating a risk of shock to personnel. Through various studies, water mist has since been proven to be a feasible protection method for electrical equipment in computer rooms, datacentres, and electrical switch gear cabinets [7][8][9]. Fires within these environments are typically slow-growing, where burning plastic-coated cables and circuit boards spread corrosive smoke. These studies showed that water mist could effectively extinguish fires without introducing any major electrical leakage across circuits or creating any critical damage to the equipment. The operation of the equipment remained unaffected during and after the studies. Water also proved to be superior to gas suppression for its ability to cool hot cable fires and avoid the reignition of plastics [10]. These findings show that water mist can be used safely in various applications, including control rooms for vital machinery or data storage facilities. Water mist suppression can do this as an environmentally safer alternative to previously used gases, which contribute to ozone depletion. Many industrial facilities place a high priority on the protection of this equipment to ensure fire safety in their normal operations. The ability to control fire with minimal water damage to the protected area also makes water mist an attractive alternative to fire sprinklers.

4. Transportation

Water mist is commonly used across many forms of transportation systems because it requires minimal water storage and is lightweight compared with other alternatives. Water mist fire suppression has been tested and approved for use on passenger ships, cargo ships, freight aircraft, military aircraft, road vehicles, and heavy machinery vehicles. In the 1980s, many lives were lost onboard during passenger ferry fires, leading to the International Maritime Organization (IMO) regulating sprinklers on all ships capable of carrying 35 or more passengers [3]. Due to the complexity of adding sprinklers to existing ships not designed for the weight and additional space required for the system, there was a large push in research and development to find a system equivalent to sprinklers that could be fitted. Testing showed that the water mist system was more effective at extinguishing shielded fire when compared with conventional sprinklers and provided equivalent or better protection for small cabins [11]. The system should be designed specifically for each fire compartment and risk. Most maritime authorities worldwide will accept a cabin water mist system that meets the IMO test protocol.
The cargo area of ships presents a large fire hazard due to the dense fuel loads within the compartment. Large-scale tests conducted by the Technical Research Institute of Sweden found that roll-on–roll-off (ro-ro) cargo decks on ships could be protected using a water mist system, which provided an increase in flow rate when compared with traditional water spray systems [12]. The system was effective for direct applications, though the system could not extinguish shielded fires when covered by shelving.
Water mist has also been tested for cargo fires on large aircraft. Full-scale testing was conducted by the Federal Aviation Administration [13][14] to determine if water mist is an alternative to halon on aircraft cargo fires. These fires are typically deep-seated, involving a wide variety of fuels. The tests showed that for water mist to be effective it required a high-pressure discharge where it could control a fire for up to 90 min. The performance objective for aircraft cargo fires is to control the fire for 180 min to allow for a safe landing before evacuation [10]. Water mist has not yet been proven to be an equivalent alternative to halon fire suppression on cargo or military aircraft, and, therefore, there will be an exception for the use of this ozone-depleting gas in the aviation industry until an effective substitute is found that meets the lightweight suppression requirements for aircraft safety.
The performance requirements for ground transportation are less onerous, and water mist has been used as a protection method for transport trucks, military combat vehicles, and heavy industrial machinery such as mining trucks and equipment [10]. Water mist is used to suppress diesel vehicle engine fires in a similar way as it is to suppress large industrial engine room fires. This research has significance because every type of transportation vehicle requires low-profile fire protection equipment. The equipment cannot significantly reduce the amount of space that can be used for cargo or passengers and must be installed within aircraft, sea vessels, and road vehicles without adding extra weight that reduces travel distances and increases fuel consumption. Comparing water mist with other fire suppression techniques, such as sprinklers, it can be said that water mist belongs to the category of low-profile equipment. The world is moving towards more freight due to import trade and online shopping, which requires support from the fire protection sector in the form of ongoing research and development. In big cities with a growing population and commuter problems, transportation safety is a crucial issue.
Burn-over protection for firefighting vehicles used externally for bushfire (wildland fire) purposes in Australia has recently incorporated cabin and vehicle spray systems for the protection of occupants and could have wider applications using mist systems (AFAC, 2021). The role of misting systems would need to be assessed in the context of wind conditions before using finer mists than those applied at present.

5. Road and Rail Tunnels

Water-based fire protection was originally discredited for use in tunnels until a series of serious highway and rail tunnel fires occurred between 1995 and 2005 [3]. This led to the development of water mist systems for road tunnels being one of the most active research areas. Fire tests carried out in Switzerland [15] involved burning passenger cars arranged to simulate an accident involving three (03) vehicles in a tunnel. The activation of the water mist system was able to control the burning of the three (03) vehicles and keep the fire from spreading to the adjacent vehicle until the fire consumed all the fuels available within the three (03) controlled cars. As fuel tanks were ruptured, the water mist was able to easily extinguish the burning liquid fuel and cool it as it spread across the roadways. Tunnel safety researchers claim that water mist also improves visibility within tunnels during fire scenarios and assists in safe evacuation [15][16]. The research is in furtherance of using water mist in road tunnels. As tunnel construction varies, it is crucial that it be used in each individual case. To prove its effectiveness in conjunction with other protection systems, such as extraction exhaust fans or fire detection techniques, research and testing need to be conducted. Current research on fire protection in tunnels is intense, and more studies on the use of water mist should be conducted.

6. Nuclear Power Generation

Fire protection systems for nuclear powerplants are designed with the concept of defence-in-depth. This involves fire prevention, rapid detection and suppression, and safe shutdown capability [12]. Ha et al. explained that water mist fire extinguishing systems are a very good candidate for protecting safety shutdown equipment in fire compartments because the application of water spray needs to be restricted [17]. Several recent studies were conducted to apply water mist systems to nuclear powerplants, typically in emergency diesel generator rooms or the electrical rooms of a reactor [18][19][20]. The findings were similar to existing research on water mist suppression in power generation and industrial occupancies. Nuclear power is often considered when discussing alternative energy generation methods. It is critical that fire suppression is performed safely if nuclear power is to be considered for use. Water mist fire protection should be investigated if it can improve nuclear powerplant safety. It may be useful for other aspects of nuclear power safety.

7. Explosion Hazard Mitigation

Hazardous explosion areas created by airborne gases can become extremely dangerous if ignited [21]. Water mist discharge into an unconfined environment with high momentum can entrain air into a gas cloud and dilute it below its lower flammable limit, reducing the potential for ignition and explosion. If ignited, during the explosive ignition of a gas cloud with a high flame speed and a shockwave, the spray/mist can reduce deflagration pressure and possibly extinguish the flame. The shockwave breaks up the droplets into a micro-mist that can quickly evaporate to absorb a significant fraction of the combustion energy released during the deflagration. To be effective, water mist must be produced at a smaller droplet size of around 1 µm, which is smaller than what is seen to be achievable for the existing water mist products for fire suppression that have been reviewed [22][23]. There was little research found on this topic. It appears to be an intriguing application that should be further developed.

8. Cooking Areas

Commercial and industrial cooking oil or fat fryers are difficult to protect from fire due to their high temperatures and ability to reignite easily [20]. In order to extinguish the fire, the entire surface of the oil must be suppressed simultaneously with the rapid cooling of the oil to prevent reignition [20]. Due to the chemical additives consistent in most fire suppression systems not being appropriate for the food production industry, there are limited options for protection. A common method is carbon dioxide (CO2), which can extinguish flames across the oil surface but is not able to cool the oil sufficiently and prevent reignition. Water mist has demonstrated good fire extinguishing and oil cooling capabilities for these fires [5][22][23][24]. Specifically, research by Liu et el. in 2006 [20] found that water mist effectively extinguished a large industrial oil fire and cooled the hot oil to prevent it from reigniting regardless of the oil depth. Water mist for oil fat fryers appears to have a significant advantage over alternative methods (such as foams). It is clearly a strong option because there are minimal methods for suppression that are acceptable for food processing. More food processing facilities are being built in metropolitan areas to allow for quick delivery to consumers, as the popularity of mass production of “ready-to-eat meals” has grown in recent years. Large fryer converters require fire protection in these facilities. Water mist should be researched further to see if it can be used on these conveyor-type fryers to protect the equipment and the people working around them. The primary motivation for fire protection is to protect people, so it is crucial to investigate the use of water mist in commercial structures as well.

9. Commercial Buildings

Water mist systems have been developed and used in commercial buildings, with FM Global and Underwriters Laboratories (UL) having test protocols for Light Hazard and Ordinary Hazard I and II, which are commonly protected by sprinkler systems. A water mist suppression system’s ability to suppress a fire and maintain minimal water damage to the protected area makes it an attractive option for protecting areas such as libraries, archives, heritage buildings, and museums [22][25][26][27]. Milke and Gerschefski [28] completed fire testing for library settings with shelving-type storage and book vaults for sensitive archives. A total compartment application was able to control the fire damage to the documents and extinguish shielded fires using 30% of the amount of water compared with a traditional sprinkler system. No damage occurred due to water, and only a minor amount of discolouration of documents was observed from smoke if stored closest to the shelf flue near the fire. It was noted that the water mist could not control the fire without discharging within 100 s of ignition. Sprinklers have been proven to be a reliable fire protection strategy in commercial structures. Before the industry switches from using standard sprinklers, the advantages of water mist systems over sprinklers would need to be further proved. Although more testing needs to be performed, water mist suppression has potential as an alternative.

10. Residential Buildings

Water mist can be suitable for residential occupancies in areas where water supplies are limited [29]. Fire testing on water mist in bedroom and kitchen fuel fires has found that water mist can control a fire with up to half the water required for a typical residential sprinkler [27][30][31]. The use of water mist as an alternative to sprinklers is limited by its reliability and cost [10]. Water mist systems, in combination with other life safety systems such as smoke detection, fire alarms, and passive systems, have been used in residential buildings [32]. These systems are used to mitigate the risk of life loss, protect areas where occupants are housed, increase tenable conditions where extended emergency exit travel distances are present, and protect high-value items or historic features of the building [31]. The use of these systems in residential buildings is limited by the information available for experts and approval authorities to apply and review. Aspects such as reliability, real fire historical data, and long-term maintenance requirements are largely unknown. There is a need for more research and development to enhance the understanding of these systems and how they can be used in all applications.
A further consideration is the protection of external building elements arising from bushfire events. For this application, consideration will need to be given to the “design bushfire” conditions, including ambient wind, when considering droplet sizes. The location and design of nozzles may need to be further researched in the use of misting systems for “real-world” bushfire protection purposes.

This entry is adapted from the peer-reviewed paper 10.3390/fire6020040

References

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