Water mist demystified
Published: 01 September, 2006
Comments Andy Shiner: “While water mist systems have been used for some time in industrial and certain types of commercial premises - and are also increasingly being used in maritime applications, the significant contribution they can make to fire safety is still not fully appreciated.”
Comments Andy Shiner: “While water mist systems have been used for some time in industrial and certain types of commercial premises - and are also increasingly being used in maritime applications, the significant contribution they can make to fire safety is still not fully appreciated.”
High-pressure water mist suppression systems sound simple - and indeed they are. The new Tyco HPWM system, for example, depends upon water under a pressure of 100bar. When discharged it becomes a fine atomised mist. This fine mist evaporates very quickly and is converted into steam that smothers the fire and prevents further oxygen from reaching it.
At the same time, the evaporation of the water creates a significant cooling effect. So the fire is extinguished by oxygen depletion and cooling. This combines the fire suppression characteristics of both conventional water-based suppression systems - deluge or sprinkler systems - and gaseous fire suppression systems.
Comments Shiner: “Obviously, water mist is extremely popular from an environmental standpoint, as it contains nothing but pure, potable water and so is completely safe for people and harmless to the environment. Also, when compared with conventional water sprinkler systems, the HPWM system uses only 10% of water for a given area. This has the benefit of minimising the potential for water damage on discharge or in the unlikely event of an unwanted system release.
“Smaller pipework dimensions result in a considerable reduction in weight, and enable the system to be installed in confined spaces,” he reports.
The shocking effects of water
Water-based systems in general have not been overly popular in offices, archives and museums for wholly understandable reasons. With the unprecedented growth of electronic IT and communications technology in the past few decades, businesses have become entirely dependent upon it for their very survival. And, as every schoolchild is taught: water and electricity do not mix. Water also has the potential to irreparably damage documents and destroy age-old manuscripts and historic artefacts and treasures.
Shiner explains: “These concerns are very real indeed when considering conventional sprinkler systems and water deluge systems due to the massive volumes of water they use to attack a fire. In reality, the water can do as much collateral damage as the fire itself. However, the high-pressure water mist option overcomes these challenges.”
The 90% reduction in the volume of water when compared with a typical sprinkler system means that the risk of water damage is considerably reduced, he reports. Additionally, the fine mist traps embers and smoke particles, washing them out of the contaminated air, so minimising damage to equipment, the interior of the building and its contents.
“The HPWM system offers many benefits - installations demand much less space, either very little or no water is required from the public water supply, and the solution severely limits the damage caused by heat. Systems can be installed with dry piping that ensures that there is neither pressure nor water in the protected area, so avoiding the risk of water damage in the event of leaking pipes.”
Industry applications
The HPWM system is also extremely useful for protecting industrial areas that traditionally have been protected by carbon-dioxide (CO2) suppression system installations which require the immediate area to be sealed off.
Paint spray booths and paint drying ovens are typical examples of locations which need an extinguishing system that can safely suppress flammable liquid fires.
“Water deluge systems have been used, but have proven to be less than ideal due to the volume of water that is discharged and the inevitable damage that is done to electronic equipment and the surrounding areas,” admits Shiner. “There are also question marks about deluge systems’ suitability to extinguish certain types of hazardous liquid fires.
“For these applications, the HPWM system uses traditional nozzles to cover the hazardous area itself, while a row of micro-nozzles mounted on a pipe is used to create a curtain of water mist in front of any large openings to the booth. This curtain helps to minimise the amount of oxygen that would otherwise be sucked into the fire; it also reduces the amount of toxic smoke and gases that would escape into the surrounding production environment.”
Marine applications
“Particularly suitable for protecting accommodation and service areas on both cargo and passenger-carrying ships, the Tyco HPWM system is approved to the International Maritime Organisation [IMO] Resolution A 800, and by all of the major international classification societies,” reports Shiner.
“Effectively, the high-pressure water mist system creates a high pressure only during the extinguishing process. This high pressure ensures that areas around any hot surfaces are cooled but - as the water mist evaporates before it reaches the hot metal - there is no risk of shock cooling and so changes in the metal structure are avoided,” he says.
The water mist nozzles in accommodation areas are usually the closed type that incorporate a glass bulb that fractures at a predetermined temperature, releasing the high-pressure water mist, says Andy Shiner. However, a more sophisticated system is available that allows remote temperature reading and remote activation of the nozzles. This enables the officer in charge to instigate proactive firefighting measures by creating a water mist barrier around the affected area.
One major advantage of high-pressure water mist suppression for the total flooding protection of ships’ engine rooms is that the relatively small quantity of water needed to extinguish a fire is harmless to equipment.
Andy Shiner feels this is key to the system’s useability: “Tyco HPWM can be used to replace Halon systems and is a much safer first-attack option to supplement the traditional carbon-dioxide suppression systems that are unsuitable for applications where people are likely to be exposed to the gas, as CO2 represents a serious toxic hazard and risk to life.
“The system is approved by all of the major international classification societies to IMO 668 [Alternative Arrangements for Halon Firefighting Extinguishing Systems in Machinery Spaces and Pump Rooms] for engine rooms up to 3,000 cubic metres.” Shiner concludes.
More information? E-mail: ashiner@tycoint.com
