What is a Water Mist?
Water Mist is a fire protection system that uses a combination of fine and coarse droplets allow the Water Mist to control, suppress or extinguish fires by cooling both the flame and surrounding gases by evaporation.
How does Water Mist work?
A Water Mist system uses Water Mist to control, suppress, or extinguish fires by cooling the flames and surrounding gasses through evaporation. Displacing oxygen by evaporation. Diminishing radiant heat by the small water droplets themselves.
What is the Water Mist System design standard?
The NFPA 750 “Standard on Water Mist Fire Protection Systems.”
NFPA 750 defines Water Mist as:
“A water spray for which the Dv0.99, for the flow-weighted cumulative volumetric distribution of water droplets, is less than 1000 microns (.0393 inches) at the minimum design operating pressure of the Water Mist nozzle.” Also,” A distribution system connected to a water supply or water and atomizing media supplies that are equipped with one or more nozzles capable of delivering Water Mist intended to control, suppress, or extinguish fires, and that has been demonstrated to meet the performance requirements of its listing and this standard.”
What are the types of Applications defined by NFPA 750?
NFPA 750 System Applications are classified in one of the three categories:
What are the main effects of Water Mist fire protection?
Water Mist provides a high degree of cooling from water being vaporized, and the water vapor produced hereof helps to weaken or even smothering the fire. In general, fire suppression or extinguishment by Water Mist application involves at least the following mechanisms:
Why is the water droplet size in a Water Mist System important?
Smaller droplet sizes result in more efficient use of the water. Water Mist works by flashing the droplets to steam, resulting in a reduction of the Oxygen content of the remaining air and cooling of the fire by causing loss of energy (steam production) and continued application of the Water Mist droplets. Smaller droplets have more surface area, which increases the heat transfer rate from the fire, the water flashes to steam rapidly. In addition to this heat absorption and resultant cooling, the steam displaces oxygen at the fire, thereby accomplishing extinguishment.
What is CO2 activation?
A Water Mist nozzle discharge may be activated by the CO2 detector response. When the CO2 detection reaches a predetermined level, it will alarm or/and trigger the nozzle; the nozzle begins to discharge water. CO2 detection can also be used to activate section valves in zone protection systems.
What can be done to avoid the clogging of nozzles?
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.
What are normal maintenance requirements for Water Mist systems?
This depends very much on the systems and applications. 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.
Can Water Mist harm people?
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 resulted in low oxygen concentration may make the fire environment untenable.
What is the difference between system operating pressure and system working pressure?
System operating pressure is the designated nozzle discharge pressure, while system working pressure is the maximum pressure designed for a system.
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 to protect machinery and combustion turbine enclosures.
What is the lifetime of the system?
Many years if the system is in good condition and that annual maintenance is done.
What are Water Mist’s main applications?