Pool fire

Ignition of pool fire in a test of a water mist system according to IMO 1165. Kristian Hox, SINTEF NBL

Water mist for large machinery spaces

Published:  07 October, 2013

Chief Scientist at SINTEF to start research work for an optional extinguishing system to CO2.

Water mist systems are extensively used as fire protection for machinery spaces onboard passenger ships, and have almost taken over as fire extinguishing systems following the standarisation of such systems by the International Maritime Organisation (IMO).

Water mist systems are documented through full-scale fire testing of manufacturers’ solutions. In practice it has been shown that spaces with a ceiling height above approximately 10 m need more water mist nozzles in order for low-seated fires to be extinguished.

The IMO method describing the test procedures requires that all nozzles are mounted at ceiling level, but provides no opportunity to extrapolate results from fire tests to spaces with higher ceiling heights.

As conventional merchant vessels have large machinery spaces with ceiling heights above 10 m, it is still common to install CO2 systems in these ships - given there are no alternative extinguishing systems approved for this application area.

Several manufacturers of water mist systems have documented that it is possible to extinguish fires in machinery spaces with heights up to 10 m and with a volume exceeding 3 000 m3. A research project initiated by IWMA (International Water Mist Association) resulted in a change of IMO's regulations, which has meant that test results are now valid also for spaces up to twice the tested volume. However, increasing the ceiling height is not allowed.

If it could be shown through testing that water mist systems had sufficient ability to fight fires in larger machinery spaces, such systems would challenge and replace CO2 systems.

In practice this would mean that fire extinguishing could start immediately after a fire has been detected. Today the start of the extinguishing process can be delayed by 10-15 minutes because CO2 represents a toxic hazard to persons in the machinery space. The CO2 cannot be released before people are evacuated from the engine room and all air supply is stopped and all openings are closed. Every minute with a fire in the machinery space increases the damage to equipment and cables; it has been indicated that the damage increases by 12,000 Euros per minute.

Water mist systems can be released immediately after fire detection, and will quickly cool down the machinery space. Most fires would be extinguished after 10-15 minutes. In addition, water mist systems have become popular amongst engineers and crew because - aside from posing no hazards to people - they are easy to test regularly. Water mist systems on passenger ships are often tested every week, and are simultaneously used for cleaning the machinery space. The costs associated with refilling the water reservoir are negligible in comparison with the replacement of CO2 bottles following the release of a CO2 system.

SINTEF NBL is now looking to start a project that could open up the use of water mist systems in large machinery spaces, for which purpose we will use our large test hall. The test hall is over 20 m high and has a total volume of approximately 14 000 m3.

For more information contact me on ragnar.wighus@sintef.no

This article was first published in Brandposten, the magazine of SP Fire Technology, Sweden.

  • Operation Florian

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