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FAQ |
1. What are the main effects of water mist fire protection?Water Mist provides a high degree of cooling from water being evaporated, and the steam hereof provides an inactive gas which helps smothering the fire. In general, fire suppression or extinguishment by water mist application involves at least the following mechanisms: 1) reduction of heat feedback to the fuel surface by cooling the fire gases, 2) reduction of oxygen concentration adjacent to the fuel surface by air displacement, air dilution and oxygen depletion, 3) attenuation of radiant heat flux to the fuel surface by water mist and water vapor, 4) dilution of fuel vapor by fire products, vaporized water and air entrainment, and 5) flame stretching and blow-off by gas current induced by water mist sprays. 2. Where are the differences between water spray systems, sprinklers and water mist systems?Water spray and sprinkler systems fight fires and protect the surroundings primarily by wetting the surfaces of the fuel and the surrounding structures to cool the structures and the fuel surfaces, and thereby suppressing or controlling the fires. Therefore water spray and sprinkler systems have an increasing efficiency with higher water densities and larger water drops. Water mist systems fight fires and protect the surroundings primarily by evaporation of water in and around the fire plume. The evaporation of water takes large quantities of energy which cools the flames and reduces the net heat output of the fires, and the steam created helps smother the fires. Therefore, for fires in open space, water mist systems perform best when delivering water droplets in sizes which are large enough to penetrate into the fire plume, and small enough to evaporate inside the fire plume. For fires in enclosures in particular, a higher temperature in the enclosure increases the fire protection effectiveness of a total flooding water mist system by enhancing mist vaporization. As a result, the effectiveness will eventually level off, and will not increase indefinitely as the total water mist discharge rate is increased. The effectiveness is also very dependent on the size of the fire relative to the size of the enclosure. Not only because larger the fires generate more heat and correspondingly higher temperatures, but also because larger fires reduce the oxygen level inside the enclosure faster. The design engineers therefore should have a high understanding in the required characteristics for a water mist system to be effective, as well as the fire hazards to be protected. 3. What are the advantages / disadvantages of water mist compared to gas systems?The advantages of water mist compared to gas systems are: 1) Water mist is non-toxic; 2) Water mist provides cooling; 3) Pump-equipped water mist systems are able to provide continued fire fighting, and multiple system activations; 4) Water mist systems are less sensitive to ventilation and openings. As a result, water mist systems are more suitable for installation in open rooms than gas systems and could be configured in zones like sprinkler/spray systems; 5) Water mist systems may in many cases be activated before gas systems normally are activated causing less damages due to fire. The disadvantages of water mist compared to gas systems are: 1) Water mist systems are most efficient when the fire is directly exposed to water mist sprays; 2) Water mist systems have larger and more complex piping configurations; 3) Water mist systems may cause water damage; and 4) Since heat is required to vaporize water mist, small fires in enclosures may not be extinguished; 5) Water mist systems may take longer time to extinguish the fire. 4. Can water mist systems wash out smoke?Experiences from fire tests conducted in tunnels and corridors, and from smoke scrubbing installations have shown that water mist, at the right conditions, are able to scrub soot particles from smoke. The efficiency of this performance depends very much on flux density and droplet size of the water mist system and on the time the smoke particles stay within the water mist atmosphere. Therefore, the smoke-scrubbing capability cannot be generalized and has to be evaluated case-by case. It is also important to realize that water mist is not able to scrub out non-water soluble gases like carbon monoxide (CO) and carbon dioxide(CO2). Some toxic gases, however, e.g. hydrogen chloride (HCL) are water soluble and would be scrubbed out to a certain degree. 5. If a system uses water mist and gas at the same time, should the system be installed according to gas guidelines or water mist guidelines?Water mist systems should always be installed in accordance with the guidelines with which the systems have been tested. The question is rather "should water mist systems which use water mist and gas at the same time" be tested in accordance with the gas test standards or in accordance with the water mist standards, or in accordance with both?" To that question, systems being marketed as water mist systems should be tested to the standards applicable for water mist systems, systems marketed as gas systems should be tested to standards applicable for gas systems, and systems being marketed as hybrid systems should be tested to some appropriate standards for the intended applications. 6. In case that the water mist system uses gas (e.g. Nitrogen) as a drive medium, how has the oxygen content in the protected room to be taken into account in regard of life safety?For such systems it must be the responsibility of the manufacturers, based on fire testing or assessment using a validated calculation methodology, to assure that the fire environment which their systems will create is safe for people to be in the environment indefinitely or for a period of time. 7. Often steel cylinders are used to store water. Are there any special requirements in regard of an inner lining of such cylinders?First of all the cylinders should in this respect be in accordance with the requirements for hand extinguishers and cylinders for gas extinguishing systems. Secondly the life time of the cylinders should be proven to be significantly longer than the date of expire which should be clearly marked on the cylinder, and the customers should clearly be made aware of through instructions and marking etc. that the cylinders should be changed with new ones before the expire date. Also, the people responsible for the servicing of systems should be made aware of this aspect. 8. What is the difference between high / and low pressure water mist systems?High pressure and low pressure systems are tested and approved to the same standards, and systems being approved in accordance with the test standards do also in general have comparable fire fighting capability within the type of occupancies, which the approval standards cover. The difference between high / and low pressure water mist systems is primarily to be found in the "nozzle" designs where different means of atomising water require different water pressures. The different designs of "water mist nozzles" and the required water pressures set different requirements to water/gas supply, pump, control, and piping for low pressure and high pressure systems. Comparisons of typical parameters between high and low pressure systems are given in the table below. The actual system parameters depend on system brands and designs.
9. What can be done, to avoid clogging of nozzles?Will water mist nozzles still be working after many years of exposure to sometimes harsh environments? Water mist nozzles and other system components are made of corrosive-resistant materials to prevent clogging due to corrosion. In addition, different corrosion-resistant tests are required for nozzle approvals. To assure product reliability, the manufacturing is also undergone periodical audit by approval/listing agencies. Filters are typically installed at nozzles, in the riser pipes and water intakes to prevent particulates from clogging the nozzles. Finally, clean water is required for water mist systems. 10. Where can water mist systems be tested?Water mist systems usually undergo two kinds of approval tests. Fire fighting performance tests and component reliability tests. Both kinds of tests are conducted by accredited laboratories such as FM Approvals (FM), Underwriters Laboratories (UL), SP Technical Research Institute of Sweden, Sintef, VTT, Danish Fire Laboratories (DFL), Danish Fire Institute (DBI) - just to mention a few. Laboratories accredited only for component tests are: FM Approvals, Underwriters Laboratories, VDS, DNV and others. 11. Are there any consultants /engineering offices being familiar with water mist systems?Since water mist systems provided by different manufacturers tend to differ significantly, the manufacturers will be the best sources for consultation and advice. The consulting firms that are IWMA members can be reached through the IWMA office. 12. Can carbon steel, zinc plated or galvanized pipe instead of stainless steel and other approved corrosion-resistant materials for water mist applications be used for water mist systems?The general answer is "NO". However, for water mist systems using nozzles with relatively large orifices comparable to those of spray nozzles, electro galvanized steel piping may in some cases be accepted by some authorities having jurisdiction, provided that piping is approved for its use and that regular inside inspections are undertaken. 13. What are normal maintenance requirements for water mist systems?This depends very much on the systems. The manufacturers, installers and authorities having the jurisdictions should clearly state the maintenance requirements. The maintenance requirements should under no conditions be less than those of sprinkler and gas systems. 14. What kind of redundancies should be foreseen for water mist systems compared to sprinklers / gas systems?See answer to no 13 15. In case of pump systems, how many of such systems shall be used as a maximum?Is it differently to sprinkler and water spray systems acceptable to use systems with, say, 10 relatively small pumps despite the relatively large number of moving parts etc? Redundancy is the key parameter. A system pump unit should be designed such that the total required flow rate is maintained despite the loss of a single pump. Equally, should the system pump unit provide its desired capacity if a single driver, e.g. electrical motor malfunctions. For the case where a very high degree of reliability is asked for, redundant sets of complete pump units may be used. 16. Can water mist harm humans?Can the very small droplets reach into the lungs? People should in general leave rooms when fire occurs. Although water mist itself is non-toxic, the toxic fire gases and sometimes the resulted low oxygen concentrations make the fire environment un-tenable. 17. Are there universal layout parameters available for water mist systems?As opposed to sprinkler or spray systems, water mist systems tend to be very different from brands to brands. Even for the same brand, system configurations could be different for different applications. 18. In case that the water mist system has been designed by one manufacturer, can the same design be applied to another make?No. 19. Is it possible to orientate where water mist is released?When water mist is released, it mixes with smoke. Both smoke and mist attenuate light. Therefore, it may be difficult for people to orientate in fire rooms when a water mist system is activated. 20. Does water mist work with foam additives, and what are the benefits?Most water mist systems will work with foam additives. AFFF additive does not provide noticeable effect on fuel spray fires. However, AFFF additive does in most cases improve the performance for surface fires. AFFF tends to reduce the evaporation of water droplets. Therefore, AFFF additive may decrease fire protection performance for enclosure fires because shielded fires are mainly extinguished by water vaporization. On the other hand, foam may flow in to shielded areas and provide suppression of surface fires. AFFF should therefore only be applied on the recommendations of the manufacturer of the water mist system. 21. Can high pressure water mist be a danger to people?Since the momentum of water mist dissipates quickly after it is discharged, the disperse water mist sprays in general are harmless a few decimeters away from the nozzle. 22. Can water mist fight fires in areas with draft or ventilation?Yes, depending on the nozzle layout. Water mist systems have been successfully tested in tunnel fires and outdoor fires with significant ventilation and draft conditions. However water mist systems should only be installed in locations and conditions for which they are designed and tested. 23. What is the difference between 'system operating pressure' and 'system working pressure'?System operating pressure is the designated design pressure when water mist is being discharged, while system working pressure is the maximum pressure that would occur in the system. 24. What is the maximum installation height?The maximum nozzle installation height for a system should be determined by fire testing. To date, the maximum nozzle height tested is about 10 meters. |
