Testing a water mist system.
Issue: 10/01

Water mist fire protection systems gained acceptance just a few years ago as a replacement in many applications for halon gas, which is now banned by most countries worldwide. The effective firefighting qualities of water mist have led to its use in a rapidly growing number of fire protection applications, including oil platforms, gas turbine enclosures, hotels, trains, ships and numerous manufacturing environments.

But, how does water mist differ from conventional sprinkler systems? And are there some applications where water mist systems are more appropriate for use than others? The following list is intended to help engineers determine the answers to these questions.

  • Replacement for halon and CO2 in many applications. The first major use of water mist protection systems was on ships as a replacement for CO2 and in response to regulatory requirements to provide sprinklers on passenger vessels. Water mist is environmentally benign and safe for occupied areas.

  • Safer than CO2. There have been numerous instances of injuries and even deaths as a result of accidental CO2 discharges during routine maintenance and other situations. Water mist is not only safe, but actually improves the livability in fire areas by cooling and scrubbing the air.

  • Cools the fire area dramatically. This allows firefighters to enter and extinguish fires. Cooling is accomplished due to the greater surface area presented by the quasi-gas created by water mist systems, and the blocking of radiant heat by the many microscopic droplets.

  • Uses less water than conventional sprinklers. This is a big benefit in many applications where run-off is a concern, such as "cutoff" rooms containing paint or flammable chemicals. Low water usage is critical in Europe where building owners must provide a collection area for all run-off.

  • Prevents reignition. Again, because of its cooling effect and room flooding ability, water mist systems are very good at preventing reignition, even of oil bath fires or other "pool" fires. In Factory Mutual Research tests, attempts to reignite fires with a torch were often impossible because the water mist put the torch out.

  • Works well against high heat release rate fires in enclosures when total "flooding" protection is used. For large fires in enclosures, the strong convection and high heat release rates are ideal for optimal water mist operation. In this application, the enclosure is flooded with water mist. The atomized droplets are drawn to the base of the fire and flash instantaneously to steam, expanding in volume by 1,700 times, thereby displacing oxygen required for combustion.

  • Works in partially ventilated areas. Unlike CO2 or halon, water mist will work in areas where a door or vent has been left open. Gas-based extinguishing systems typically require a sealed area so that a gas concentration can be achieved to put out the fire.

  • Has some smoke-scrubbing qualities. Some smoke and toxic gases are absorbed by the atomized spray and eventually settle with the water.

What's the Down Side?

Here's a list of some of the reasons that water mist may not be effective in all applications.

  • Fire testing and component reliability testing are required to verify that water mist protection systems are effective for specific hazards. There is currently no general design method recognized for water mist protection systems.

  • Doesn't do well on very small fires, such as trash can fires; however, it will contain small fires in an equilibrium state and cool the room enough to allow extinguishment by firefighters or facility personnel.

  • It's more expensive. While prices continue to fall with wider use, the fact remains that water mist systems tend to be somewhat more complicated and more expensive than conventional sprinkler systems.

  • It's new, and many authorities having jurisdiction (AHJs) are uncertain how to evaluate water mist systems. This situation is improving and the introduction of clear documentation, such as Factory Mutual Research's soon-to-be-released water mist approval standard, will help manufacturers and insureds gain acceptance of water mist systems.

  • Typically requires greater water pressure than conventional sprinklers. While some water mist systems have been developed to operate at relatively low water pressure, most water mist systems require compressed gas or high-pressure pumps to create the atomized spray necessary for proper operation.

Copyright 2000 Factory Mutual Insurance Company. Reprinted with permission from Approved Product News, Volume 16, Number 2, 2000.