The NFPA 750 “Standard on Water Mist Fire Protection Systems” – 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.”
DV0.99 is the drop diameter such that 99% of the cumulative volume being discharged is as drops with a diameter less than this value. This is important because for a given quantity of water, the smaller the droplet, the larger the water surface area exposed for heat absorption becomes.
The water mist extinguishing mechanism is two fold.
- Removal of heat energy from the fire plume due to vaporization of mist into steam. The large surface area per unit volume inherent in smaller droplets results in a very efficient and rapid heat transfer process.
- Localized displacement of oxygen from the fire base due to the production of steam. When water changes into steam, it expands by a factor of 1650 times which results in oxygen displacement in the immediate fire area.
The SECURIPLEX Low Pressure (FIRE-SCOPE® 2000) and High Pressure (FIRE-SCOPE® 5000) produce a water mist with droplets in the Dv0.99 range of 200 microns.
Is a Water Mist System where the distribution piping is exposed to an operating pressure of 500 psig (34.5 bar), or greater.
Is a Water Mist System where the distribution piping is exposed to an operating pressure greater than 175 psig (12.1 bar) but less than 500psig (34.5 bar).
Is a Water Mist System where the distribution piping is exposed to an operating pressure than 175 psig (12.1 bar) or less.
A water mist system which utilizes a single piping system to supply each nozzle.
A water mist system in which water and atomizing media are separately supplied to and mixed at the water mist nozzle.
In general, high pressure water mist systems will produce a finer mist (with a lower Dv0.99) than either intermediate or low pressure systems. A notable exception to this is the SECURIPLEX FIRE-SCOPE® 2000, Low Pressure, Twin-Fluid, system.
- The SECURIPLEX (FIRE-SCOPE® 2000) is a Low Pressure System with minimum nozzle pressures of 70psig.
- The SECURIPLEX (FIRE-SCOPE® 2000) is a “Low Pressure” AND “Twin Fluid” Water Mist Fire Suppression System.
- The SECURIPLEX Semi-conductor (Wet Bench) Protection System is a Low Pressure System with minimum nozzle Pressures of 40 psig.
- The SECURIPLEX FIRE-SCOPE® 5000 is a High Pressure System with minimum nozzle pressures of 750 psig.
The “Twin-Fluid” system by using an atomizing media of compressed air (or nitrogen) provides mechanical mixing at the nozzle to produce the water mist. This allows for the production of very fine mist with a Dv0.99 value in the 200 micron range at low pressures. The high pressure systems utilize high pressure in combination with small discharge orifices to achieve the smaller droplet sizes.
High pressure piping or hydraulic tubing is not necessary.
Lower operating pressure allows the use of larger orifices in the nozzles reducing susceptibility to clogging and eliminating the need for special fine filtration of the system water supply.
Either compressed air or nitrogen can be used, both are approved.
NFPA 750 System Applications are classified in one of the three categories:
- Local Application Systems – designed and installed to provide complete distribution of mist around the hazard or object to be protected whether enclosed, unenclosed or open condition.
- Total Compartment Application Systems – designed and installed to provide complete protection of an enclosure or space (Total Flood).
- Zoned Application Systems – a subset of Total Flood above, design to protect a predetermined portion of the compartment, enclosure or space.
SECURIPLEX has FM Approval for Total Flood (Total Compartment Protection) for both the Low Pressure and High Pressure Systems. In addition, the Low Pressure System is FM Approved for Local Application Systems as Self Contained, Hybrid, and Plant Services configurations.
The Factory Mutual Approval Guide lists seven categories for the approval of Water Mist Systems. These seven categories currently are:
- Protection of Combustion Turbines, Machinery Spaces, Special Hazard Machinery Space and Flammable Liquid Processing Hazards in Enclosures with Volumes Not Exceeding 9175 ft3 (260 m3).
- Protection of Combustion Turbines, Machinery Spaces, Special Hazard Machinery Spaces and Flammable Liquid Processing Hazards in Enclosures with Volumes Exceeding 9175 ft3 (260 m3).
- Protection of Industrial Oil Cookers.
- Protection of Light Hazard Occupancies.
- Protection of Wet Benches and Other Processing Equipment.
- Protection of Computer Room Sub-floors.
- For Local Application Protection.
FIRE-SCOPE® 2000 (Low Pressure Dual Media System) has the following approvals:
- Protection of Combustion Turbines, Machinery Spaces, Special Hazard Machinery Spaces and Flammable Liquid Processing Hazards in Enclosures with Volumes not Exceeding 9175 ft3 (260 m3).
- Protection of Wet Benches and Other Processing Equipment.
- For Local Application Protection.
FIRE-SCOPE® 5000 (High Pressure, Single Fluid System) has the following approvals:
- Protection of Combustion Turbines, Machinery Spaces, Special Hazard Machinery Spaces and Flammable Liquid Processing Hazards in Enclosures with Volumes Exceeding 9175 ft3 (260 m3).
- Marine/VSCA
Smaller droplet sizes result in a more efficient use of the water. Water Mist works by flashing of the droplets to steam, resulting in 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.
In general, High Pressure Water Mist Systems discharge water in smaller droplets than do either Intermediate Pressure or Low Pressure Water Mist Systems. Because of the use of Dual Media, the Securiplex FIRE-SCOPE®2000 System produces a water mist in the 150 – 200 micron range which is in the same order of magnitude as most High Pressure Systems.
We have been asked to propose a Water Mist System for a room containing three (3) Emergency Diesel Generators. The room volume is 22,000 ft3, which exceeds the total flooding approvals. How would Securiplex propose to protect this hazard?
In order to choose the best System, several questions must be resolved:
- Ceiling height should be approximately 16 feet (5 meters) because water mist becomes less to non-effective at ceiling heights above 5 meters. Intermediate or level located nozzles may be required to retain effectiveness.
- Is FM Approval/Listing required for system? SECURIPLEX Systems are “scalable” and will provide effective temperature and fire control in Total Flood Applications and can be used in spaces greater than their Listings.
- Is there a sprinkler system installed in the Space? the Building? Some AHJ’s require that the required sprinkler system be retained in enclosures with alternate protection systems. Some AHJ’s will not consider Local Application Systems acceptable as replacement for a required sprinkler system.
- Is there a reliable water and/or air source available? Use of Facility Water and/or Air as a Hybrid or Plant Services System is a very cost effective way of providing a fire protection system to protect a hazard.
- Do Diesels run individually or all at the same time (or, is the hazard and individual Generator or all Generators)? A Total Flood System protects all simultaneously; a Local Application System can be designed to protect one or all (or any combination in between) units simultaneously.
We would propose a Self Contained, Local Application System protecting a single unit (possibly for 10 minutes flow to provide the option for protecting a second unit in the event of activation for the first) unless there is adequate water pressure and flow for a hybrid system (it is rare that adequate air flow and pressure are available!). We would suggest leaving the sprinkler heads in the space, but making them activate at much higher temperature or making this space a Pre-action.
We would propose a Self Contained Total Flood (low pressure) System, if there is no requirement for FM Approval for control and cooling only (4-5 hour rated fire wall enclosure) and the ceiling height was no more than 18 feet.