BSI EN 14972 Fixed
firefighting systems - Watermist systems - Design and installation
The draft version of this CEN standard is currently under review. In the UK, the standard is set to become the major reference document for the design and installation of water mist systems.
EUROFEU Fixed Extinguishing Section,
GUIDANCE NOTES
FOR WATER MIST SYSTEMS
The purpose of these guidance notes is to assist all interested parties involved in the application, specification, design and installation of water mist systems. It is not intended to be a design code or approval guide but simply to provide fundamental information on the subject of water mist, which can be used to determine the best course of application for this technology.
NFPA 750 - Year 2000 Edition
NFPA 750 is the US Standard for the Installation of Water Mist Fire Protection Systems and, whilst it may not be entirely appropriate for systems in the UK, there are a number of useful considerations to be taken into account. Where applicable, NFPA 750 definitions have been used in our FAQ's.
Note: NFPA 750 is NOT a design standard !
Scotch Mist® systems comply with both standards where applicable.
A Scotch Mist® Deluge water mist system uses open nozzles attached to a piping system connected to a water supply that is controlled by means of a detection system installed in the same area as the mist nozzles. When the water delivery system is activated, water flows into the piping system and discharges through all nozzles attached to the system [see NFPA 750, Cl.3-4.1].
A Scotch Mist® Dry Pipe water mist system uses automatic nozzles attached to a piping system containing air, usually under pressure, that monitors whether any nozzles are opened by fire. When the water delivery system is activated by a drop in monitoring pressure, water flows into the piping systems and discharges through any opened nozzles [see NFPA 750, Cl. 3-4.3].
Due to the large quantity of water storage required by a deluge system, users often opt for a dry pipe system, which is acceptable because water mist systems are not effective against incipient fires. Adequate alarm notification features such as a Red Care link to the local brigade should be provided.
Scotch Mist® systems are designed with three basic objectives in mind;
For Fire Control using ;
- A reduction in the thermal exposure to the structure, where the primary objective is to maintain the structural integrity of the building (e.g., prevent flashover),
- A reduction in the threat to occupants, where the primary objective is to minimize the loss of life, and
- A reduction in a fire-related characteristic, such as heat release rate, fire growth rate or spread to adjacent objects [see NFPA 750, Cl.5-3.1.1];
For Fire Suppression, defined as the sharp reduction in the heat release rate of a fire and the prevention of its re-growth by a sufficient application of water mist [NFPA 750, Cl.5-3.1.2]; and
For Fire Extinguishment, defined as the complete suppression of a fire until there are no burning combustibles [NFPA 750, Cl.5-3.1.3].
If fire thermal loads are low, it is usual, as with a gaseous system, to opt for fire suppression and control for `the period' which is defined by the specifier.
Automatic supplies of water are adequate to serve the system for a duration designed to comply with the objectives of the system and can be provided sufficient for two complete discharges [see NFPA 750, Cl.7-3].
The time required to achieve suppression and fire control for any project will normally be defined by the specifying body or authority having jurisdiction, although a risk assessment will often be required.
Scotch Mist® cylinder systems are designed for short duration discharges and are ideally suited to local or small flooding applications where extinguishment can be guaranteed. Suppression systems require large scale water storage with pumps designed to cater for the greater flows involved and this type of system can handle long discharge durations.
Properly designed water mist systems can be effective on both liquid fuel (Class B) and solid fuel (Class A) fires. Research indicates that fine (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. For this reason, the definition of water mist in NFPA 750 includes sprays with Dv0.99 of up to 1000 microns.
The relationship between drop size distribution and extinguishing capability of a water mist is complex. In general, very fine particles enhance heat absorption and generation of water vapour. With liquid (Class B) fuels, too many "large" drops could agitate the surface of the fuel and increase burning intensity. On the other hand, larger drops can assist the spray to penetrate and wet charred, smouldering Class A fuels. Larger drops can also entrain finer drops in their wake and improve the transport of much smaller drop sizes into the seat of the fire, one of the attributes of the Scorch Mist® system.
Drop size distribution alone does not determine the ability of a spray to extinguish a given fire and Scotch Mist® is an advanced system whose nozzles produce a mist which benefits from a variety of droplet sizes.
Factors such as fuel properties, enclosure effects (which are a function of ventilation and heat confinement), spray flux density and spray velocity (momentum) are all involved in determining if a fire will be extinguished. The "momentum" of an element of spray is the product of its velocity and the mass of dispersed water droplets, i.e., the mass flow rate. It must be stressed that the term velocity implies direction as well as speed. It is the momentum of a mist in a particular direction, relative to the direction of flow of the hot fire gases, which enhances cooling and suppression effectiveness. Opposing directional flows bring about turbulent mixing, hence improved cooling. Therefore, all three variables - drop size distribution, flux density and velocity - are involved in determining the ability of a mist to extinguish a fire in a given scenario [NFPA 750, Cl. A-1-4.21], all of which are characterised by the Scotch Mist® system.
Scotch Mist® systems can be designed to provide a variety of characteristics by adjusting the discharge water pressure and volume to suit differing risk heights and fire loads. For instance, decreasing system pressure increases droplet size and velocity or momentum. Increasing system pressure, on the other hand, decreases velocity and droplet size, leading eventually to an inerting mist.
Water mist systems have been proven to be effective in controlling, suppressing or extinguishing many types of fires [see NFPA 750, Cl.A-1-6]. Potential applications for Scotch Mist® include;
- Hazardous solids including fires involving plastic foam furnishings.
- Ordinary (Class A) combustible fires such as paper, wood and textiles.
- Electrical hazards such as transformers, switches, circuit breakers and rotating equipment.
- Electronic equipment including telecommunications equipment and cables.
Scotch Mist® is suitable for a variety of performance objectives, including fire control, suppression and extinguishment, temperature control and exposure protection.
Like other fine water spray systems, Scotch Mist® is unsuitable for direct application onto substances that react violently with water such as reactive metals. In addition, fine water sprays are not totally effective on fires with low thermal loads or on fires that are shielded from the direct effect of the spray. For these applications, Scotch Mist® will provide control or suppression, although extinguishment may be achieved at higher pressures.
Scotch Mist® has one very special and unique quality - the BLASTER (Boundary Layer and Surface Tension Energy Release) effect which causes the vaporising water from the nozzle to expand 1800 times!
At an expansion rate of only 1600, rarefaction of the air occurs leading to a reduced oxygen content around the fire and hence, the atmosphere is inerted. The Scotch Mist® system, therefore, inherently produces the rate of water expansion or vaporisation required for inerting without the need for direct contact with fire.
Current Droplet Measurement
Technology
Scotch Mist® has been tested using the method prescribed by NFPA 750, 2000 Edition and a report detailing the results is available for Download. There is no correlation between droplet size and pressure - it is the technology of the nozzle that determines the type and size of mist produced!
The history of changes to NFPA 750 between the 1996 and 2000 editions is described in an IWMA Newsletter by J. R. Mawhinney, Chairman of the NFPA 750 Water Mist Committee. In the Newsletter, a copy of which is available for Download, Mr. Mawhinney explains that;
An Appendix note in the 1996 edition [of NFPA 750] discussed the role of drop size distribution in fire extinguishment. That discussion included a proposed method of classifying water mist drop size distributions as Class I, II or III, as a means to simplify comparisons of the "coarser" or "finer" sprays produced by different nozzles. The discussion pointed out that a Class I spray was neither better nor worse than a Class II or III spray - any of the three classes could be the optimal choice for a particular application. Unfortunately, there was a tendency for some authorities having jurisdiction to interpret a superiority of Class I over Class III sprays, such that they specified that a Class I spray was required in all cases. For this reason the [NFPA 750 Technical] Committee decided to drop the described classification system from the appendix of the 2000 edition [of NFPA 750].
New technological means of establishing the characteristics of water mists are now described in NFPA 750 and in the same IWMA Newsletter, Mr. Mawhinney explains that;
In the 1996 edition [of NFPA 750], it was required that the drop size distribution be measured at several locations on a plane one metre below the nozzle. Given that drop size distribution measurements are statistical readings, not absolutely representative dimensions, this interpretation based on a single point reading was not realistic. The [NFPA 750 Technical] Committee had to work out a more statistically meaningful way of characterizing the drop sizes in a water mist.
For the 2000 edition, the Committee maintained the upper limit of Dv0.99 < 1000 microns, but they clarified that this value must be determined from a "flow-weighted" cumulative volumetric distribution measurement, not a single point measurement. The flow weighted value is a statistically corrected drop size distribution that blends the results of up to 24 measurements taken at specified locations distributed throughout the spray cone. The locations for taking the measurements and the method of doing flow-weighting are described at length in the appendix.
In short, the locations for measuring drop size are the centroids of regions of equal area in a plane through the spray cone taken one metre below the nozzle. The flux density at each of the regions of equal area is also measured. The flux densities are used to 'weight' the corresponding cumulative percent drop size distributions, so that areas of high flux density are more representative than areas of low flux density.
What emerges is a single cumulative percent volume distribution "S-curve" which is statistically representative of the overall drop size distribution at that distance from the nozzle. This methodology is similar to practices used in other industrial applications where drop size distribution is important to performance.
From an engineering sense, the rigorous method described for obtaining a weighted drop size distribution curve for water mist nozzles is much more "correct" than the 1996 approach. It requires time and careful work, however, and is therefore expensive.