Water mist is defined by droplet size and the smaller the droplets the larger the overall surface. The small droplets are created by the pressure and nozzles that are designed especially for this purpose. After a system has been activated, the released droplets evaporate and turn into steam. This steam or mist interacts with the fire. Due to the enormous increase in the volume of the water droplets (as they evaporate) the oxygen is displaced at the source of the fire and the flames are suppressed and / or extinguished.
Water Mist provides a high degree of cooling from water being vaporized, and the water vapor produced hereof helps weakening or even 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 and enhancing the attenuation of radiant heat flux to the fuel surface,
2) reduction of oxygen concentration adjacent to the fuel surface by air displacement, air dilution and oxygen depletion,
3) dilution of fuel vapor by fire products, vaporized water and enhanced air entrainment, and
4) aerodynamics effects such as flame stretching and blow-off by gas current induced by water mist sprays.
Besides the high cooling effect, water mist technology fights fires by reducing the oxygen concentration in the vicinity of the fire source. Many small droplets form a very large total water reaction surface compared to an equal volume of large droplets. The large surface area enables a high evaporation rate that extracts large portion of energy from the fire, but also increases the volume of water by 1600 times during transition from water to steam. The steam barrier hinders and blocks the access of air and oxygen supply to the fire, which causes the oxygen concentration to drop in the direct vicinity of the fire source. This effect is called inerting and takes place only directly at the fire source. The oxygen content in the surrounding of the fire generally does not decrease. Therefore, the effect is called local inerting effect. The significance of the oxygen displacement varies depending on the fire type and size of fire. Larger fires naturally produce more vapour than smaller fires. Smaller water mist droplets convert faster into steam, thus generate a higher local inertig effect.
High-pressure and low-pressure systems are tested and approved to the same standards, and systems approved in accordance with the same test standards do in general have comparable firefighting effectiveness for the occupancies covered by the approval standards. The difference between high- and low-pressure water mist systems is primarily their system operating pressure. Conventionally, high-pressure systems operate at 34.5 bar or greater, low-pressure systems operate at 12.1 bar or less, and intermediate-pressure systems operate at pressures greater than 12.1 bar but less than 34.5 bar. In general, for systems employing single-fluid nozzles, high-pressure systems produce smaller water droplets than low-pressure systems for the same design water mist flux. However, small water droplets produced by single-fluid high-pressure systems can also be produced with low-pressure systems employing dual-fluid nozzles. The different designs of water mist nozzles and the required water pressures set different requirements to water/gas supply, pump, control, and piping for different systems.
Water mist is defined in NFPA750 3.3.24 as a water spray for which the droplet diameter (Dv0.99), for the flow-weighted cumulative volumetric distribution of water droplets is less than 1000 μm within the nozzle operating pressure range. Droplet size will be different between manufactures and dependent on systems pressure and application.
Research indicates that fine (i.e., smaller than 400 microns) droplets are essential for extinguishment of Class B fires, although larger drop sizes are effective for Class A combustibles, which benefit from extinguishment by fuel wetting.
Drop size distribution alone does not determine the ability of a spray to extinguish a given fire. It is essential that any water mist system has been tested and validated in accordance with the correct fire test protocols for the given application. [Source: NFPA 750 A.3.3.24]
System operating pressure is the designated nozzle discharge pressure, while system working pressure is the maximum pressure that a system is designed for.
Open nozzles containing open orifices and are used in deluge water mist systems. The pipework between the water source or the section valve up to the nozzles is dry in stand-by condition. Deluge systems always need an external activation means to operate. When activated, the pipework is filled with water which finally discharges through all nozzles in the respective section. Deluge systems are designed to operate all nozzles in a pre-defined area.
In contrast closed nozzles, also called automatic nozzles, are closed by a closing mechanism (e.g. piston) in the nozzle in stand-by condition. These nozzles are equipped with a thermo-sensitive device designed to react at a predetermined temperature by automatically releasing the water mist independently of other nozzles. The pipework between the water source or the section valve up to the nozzles is pressurised either by water or gas in stand-by condition. System activation is triggered by rupture thermo-sensitive device by the heat of a fire.
These nozzles are used in wet systems, pre-action systems or dry pipe systems and operate independently from an external activation means. These systems are designed for a maximum operational area which is defined by the fire hazard and the evacuation conditions.
Water mist offers many advantages in comparison to other firefighting technologies, but as any technology it also has limitations and application fields it should not be used for.
One of these limitation results from extreme ventilation conditions which can prevent the water mist reaching the source of the fire. A further limitation is unsuitable materials. Water mist should not be used for direct application on materials that strongly react with water, generating violent reactions or significant amounts of hazardous products. Such materials are the following (As defined in NFPA 750 18.104.22.168):
Water mist systems should also not be used for the direct application on liquefied gases at cryogenic temperatures, such as liquefied natural gas (As defined in NFPA 750 22.214.171.124).
Water mist system shall generally only be applied for applications where they have successfully demonstrated firefighting performance in full scale fire tests.
People should in general leave the fire room when a fire occurs. Although water mist itself is non-toxic, it may become unsafe if inhaled when harmful water-soluble fire products are dissolved in the water mist. Furthermore, fire gases and sometimes the resulted low oxygen concentration may make the fire environment untenable.
Since the momentum of water mist spray dissipates quickly after it is discharged, the disperse water mist sprays in general are harmless one meter away from the nozzle. For the safe distance pertaining to a specific nozzle discharge, consult with the system provider.
When water mist is released, it mixes with fire products. The mist, fire products and water vapor generated from mist vaporization reduce the visibility in the fire area. Furthermore, a person may become disoriented or incapacitated by toxic fire products or low oxygen concentration. Therefore, it is advisable to leave the fire area when a water mist system is activated.
Experiences from fire tests conducted in tunnels and corridors, and from smoke scrubbing installations have shown that water mist, at right conditions, are able to scrub soot particles from smoke. The efficiency of this performance depends very much on spray momentum, mist flux density, water mist droplet size and smoke particle property. 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 could be scrubbed out to a certain degree.
Water mist systems usually undergo two kinds of approval tests: Fire extinguishing performance tests and component reliability tests. Both kinds of tests are conducted by accredited laboratories such as FM Approvals (FM), Underwriters Laboratories (UL), RISE Fire Research AS, RISE Research Institutes of Sweden, (formerly SP Technical Research Institute of Sweden), VTT Technical Research Center of Finland (VTT), VdS Schadenverhütung GmbH (VDS), Danish Fire Laboratories (DFL) - just to mention a few.
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.
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 effectiveness with higher water densities and larger water drops if sufficient cooling is provided to prevent excessive number of water spray nozzles or sprinklers to activate. Water mist systems fight fires and protect the surroundings primarily by fast water vaporization in the fire environment. The water vaporization takes large quantities of energy which cools the flames, and reduces the heat transfer from the fire to the fuel surface, and the produced water vapor helps vitiating the air to suppress or extinguish the fire. Therefore, for fires in open space under situations where sufficient air vitiation cannot be achieved, water mist systems perform best when delivering sufficient spray momentum to penetrate the fire plume, and droplets large enough to deposit on the fuel surface. For fires in enclosures, a higher enclosure gas temperature increases the fire extinguishing effectiveness of a water mist system by enhancing mist vaporization. However, the effectiveness will eventually level off as the total water mist discharge rate is increased, due to the fact that the increase of water mist discharge rate reduces the enclosure gas temperature. The effectiveness is also very dependent on the fire heat release rate relative to the enclosure size. Not only because larger fires generate more heat and correspondingly higher enclosure gas temperatures, but also because larger fires deplete oxygen inside the enclosure faster. Still, to be effective, water mist sprays should be able to provide adequate mixing inside the enclosure. The design engineers therefore should have a high understanding of the requirements for a water mist system to be effective for the fire hazards to be protected.
Sprinkler heads are used in sprinkler systems and water mist nozzles are used in water mist systems. Both systems suppress or extinguish fires with water. Under certain situations additives are used with water to increase fire extinguishing effectiveness or to prevent freezing. Both systems can be configured in deluge or automatic water discharge operation, in local or area protection, and in wet-pipe, dry-pipe or pre-action systems. With the above similarities nonetheless, sprinkler and water mist systems do have the following differences:
1) Sprinklers typically have only single orifices. Water mist nozzles may have single or multiple orifices.
2) Sprinkler water discharge K-factors range from 43 to 475 lpm/bar1/2. The K-factors of water mist nozzles are much smaller, typically less than 20 lpm/bar1/2.
3) Sprinklers discharge only water or water with additives. In addition to water, water mist nozzles may also discharge additives, air and various gas agents.
4) Sprinkles typically operate in the range of 0.5 to 12 bar. On the other hand, single-fluid water mist nozzles operate in the range of 5 to 140 bar or higher, but dual-fluid water mist nozzles may operate at lower pressure levels.
5) For water mist systems, 99% of the discharged water droplets should not exceed 1000 µm in size. There is no such limitation for sprinkler systems.
6) Sprinkler systems suppress fires primarily by wetting the fuel surface. The key fire suppression mechanisms for water mist is fast water droplet vaporization, which leads to cooling, air displacement, air/fuel vapor dilution, and heat radiation attenuation, although wetting of the fuel surface could also be an important suppression mechanism for water mist.
7) For water supply, sprinkler systems typically use centrifugal pumps driven by diesel engines or electrical motors. For water mist systems, water supply is typically provided by positive displacement pumps matched with electrical motors or provided by the compression of high-pressure air or nitrogen.
Water mist sprinklers and water mist nozzles are frequently called interchangeably. But more specifically, water mist sprinklers are nozzles equipped with thermal elements for automatic operations of individual nozzles. On the other hand, water mist nozzles are meant open nozzles, not equipped with thermal elements.
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.
Adequate redundancy is the key. A system pump unit should be designed with a sufficient redundancy such that the total required flow rate is maintained with the expected reliability of the pump and driver.
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.
The advantages of water mist systems compared to gas systems are: 1) Water mist is non-toxic; 2) Water mist provides a much higher cooling capability; 3) Pump-equipped water mist systems are able to provide continuous firefighting and provide water supply to multiple system activations; and 4) Water mist fire protection is 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. The disadvantages compared to gas systems are: 1) Water mist systems are less efficient for shielded fires; 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; and 5) Water mist systems may take longer time to extinguish the fire.
There is no general requirement for a stand-by or redundant pump. A stand-by pump is used for critical or enhanced availability applications where a risk assessment shows the necessity. This special requirement is defined by the authority having jurisdiction.
For marine and offshore applications, the respective IMO standards require a redundant means of pumping.
For multiple pump units, typically for high-pressure water mist systems, a n+1 pump redundancy is the most common approach.
The manufacturers, installers and authorities having the jurisdictions should clearly state the redundancy requirements, if any, for the intended applications.
Yes, depending on the nozzle layout, spray characteristics and ventilation condition. Water mist systems have been successfully tested for tunnel fires and outdoor fires. However, water mist systems should only be installed in locations and conditions for which they are designed and tested.
The nozzle flow characteristics are given by the K-factor, which is a constant that indicates the dependence of the flow rate from the pressure at the nozzle. The nozzle manufacturer determines the K-factors for every nozzle type by modelling and confirming these values in experimental discharge tests. The flow rate is calculated by the following formula whereby Q is the flow rate in l/min and p is the pressure at the nozzle in bar. Q = K ∙ √p
A water mist nozzle discharge may be activated by the thermal response of its fusible bi-metal link, or glass bulb filled with a liquid or liquid mixture. When the thermal element reaches a predetermined temperature, it breaks, and the nozzle begins to discharge water. Thermal elements can also be used to activate section valves in zone protection systems.
Open systems: Systems employing open nozzles for water discharge. These systems are normally called deluge systems (see below for deluge systems), which may be of wet-pipe, dry-pipe or pre-action types.
Closed systems: Systems employing automatic nozzles equipped with thermal elements. These systems are normally called automatic systems, which may be of wet-pipe, dry-pipe or pre-action types.
Deluge systems: Systems employing open nozzles to discharge water through all nozzle simultaneously in the protected area. The water is held back from the piping by a deluge valve activated by a heat or flame detection system installed throughout the same protected area.
Pre-action systems: Wet-pipe or dry-pipe systems employing nozzles equipped with thermal elements. A system of fire detectors is installed throughout the protected area. When a detector is tripped, an automatic water control valve is opened to admit water into the piping system. Each nozzle discharges water when its thermal element is activated. The control valve opening may be configured in single- or double-interlocks and coupled with a manual override.
Total protection: Total protection is also called total flooding protection or total compartment protection. All the open nozzles in the protected enclosure discharge water simultaneously when a fire is detected.
Local protection: Fire protection is provided for specific areas in an occupancy which may or may not be fire protected.
Limited enclosure protection: This term arises from the misconception that water mist can only protect small rooms such as one- or two-person bedrooms or small offices, and that the rooms have to be completely sealed. As a matter of fact, fire performance test protocols for system certifications require that certified water mist systems, with the opening allowances set by the test protocols, have to suppress or extinguish the intended fire hazards in the intended maximum room size. So there is no such “limited enclosure protection” as long as certified systems are used within their proven fire protection capability.
Unlimited area protection: The protection area for a water mist system is not limited provided that with negligible oxygen concentration reduction, the system shows that it can suppress or extinguish an intended fire hazard within a specific area of nozzles discharging at a designated design water flux surrounding the fire origin.
First of all, the cylinders should in this respect be in accordance with the requirements for hand-held extinguishers and cylinders for gas extinguishing systems. Secondly, the lifetime of the cylinders should be proven to be significantly longer than the expiration date clearly marked on the cylinder, and the customers should clearly be made aware of through users’ manual instructions and marking etc. that the cylinders should be replaced with new ones before the expiration date. Also, the people responsible for servicing the systems should be made aware of this aspect.
No, unless the same water mist spray characteristics are employed, which is in practice extremely difficult without identical nozzle.
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 standards for the intended applications.
Most water mist systems will work with foam additives. AFFF additive does not provide noticeable effect on 3-D fuel spray fires. However, AFFF additive does in most cases improve the performance for 2-D surface fires. AFFF tends to reduce the vaporization of water droplets. Therefore, AFFF additive may decrease fire protection performance for shielded enclosure fires because shielded fires are mainly extinguished by water droplet vaporization to inert air. On the other hand, foam may flow to shielded areas and provide suppression of surface fires. AFFF should therefore only be applied on the recommendation of the manufacturer of the water mist system based on test results.
As opposed to sprinkler or spray systems, water mist systems for the same applications may be very different from brands to brands, in terms of system configuration, operating pressure, water mist flux and spray characteristics. Even for the same brand, system configurations could be different for different applications. As a result, before water mist spray characteristics are standardized for the same applications similarly to those of sprinkler and water spray systems, no common layout parameters are available for water mist systems.
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 for the protection of machinery and combustion turbine enclosures.
The room height below which a water mist system can provide the intended fire protection performance in the room based on the fire test results.
The maximum building height is sometimes used interchangeably with the maximum room height or maximum enclosure height. However, more specifically, the room or enclosure usually refers to an enclosed space in a building.
The enclosure size below which a water mist system can provide the intended fire protection performance in the enclosure based on the fire test results.
The letters DIOM stand for design, installation, operation and maintenance. Each water mist system is unique and all manufacturers provide DIOM manual specific to their systems.
This depends very much on the systems and applications. The manufacturers, installers together with the authorities having jurisdictions should clearly state the maintenance requirements. The maintenance requirements should under no conditions be less than those of sprinkler and gas systems. The reference, where the requirements are listed in detail, is the manufacturer’s DIOM (design, installation, operation and maintenance manual).
Water mist nozzles and other system components are made of corrosive-resistant materials to prevent clogging due to corrosion. In addition, different corrosion-resistant and clogging propensity tests are required for nozzle approvals. To assure product reliability, the manufacturing is subject to 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. If nozzles are installed in very harsh environments, e.g., paint booths, they typically have caps to protect the orifices. Finally, good water quality is required for water mist systems.