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CREW’S VIEWS Mist opportunity


Cars on a ro-ro ship with the Marioff Hi-fog Sprinkler system visible in the ceiling.


F


ires at sea can be catastrophic; there are no fire brigades to call and no extra man power to rely


on. Even if help was available, it is oſten hours away. One only has to glance at the archives of Lloyd’s casualty reports to find numerous examples of fires that have overwhelmed crews due to the inadequacy of the fire preventative measures onboard, resulting in huge casualties in monetary terms and even loss of lives. Water mist suppression systems


are an effective method of preventing such accidents. It uses a fine water mist combined with high pressure in the region of 80 – 120bar to extinguish fires without any danger to people or the environment. For the fire suppression systems to work correctly and effectively a clear distance around the head has to be maintained. If items are stored too close or too high there is not enough room for the fog/mist cloud to form, impairing the effectiveness of the system. When a sprinkler head is activated,


several small high pressure jets of water are produced by the specially designed spray heads which causes the water to enter the space as fine fog or mist. Te small water droplets rapidly evaporate, displacing the oxygen at the seat of the fire, which then quickly cools the surrounding air efficiently suppressing and controlling the fire before it can spread or reignite. The rival to the fog system is the traditional and much cheaper water


The Naval Architect July/August 2011


sprinkler. Each sprinkler head has an individual heat sensitive bulb filled with a fluid consisting of a non-toxic proprietary glycerine solution. When the sprinkler head is heated by a fire, the fusible element will break at the preset temperature, releasing the cap which seals the water orifice and water is immediately discharged out onto the fire. The rose plate at the bottom of the sprinkler ensures some of the water droplets hit the deckhead before falling like rain on to the surfaces around, ensuring a complete spray pattern is achieved. As the sprinkler heads operate individually, the number of sprinklers activated is limited to those near the fire, thereby maximising the available water pressure over the point of the fire’s origin. However, as the pressure required for the system is a low one, usually 8bar, a lot more water is needed compared with the mist system. The extraction of the heat too is at a far slower rate further reducing its effectiveness. One of


the favoured water mist


suppression systems in the cruise industry is that supplied by Marioff. Their Hi-fog sprinklers


are used


wherever automatic fire prevention activation is required. The sprinklers are compact, precision manufactured components made of chrome-plated brass and equipped with seven nozzles. Each sprinkler is protected by its own dedicated strainer to prevent clogging of the nozzles. The sprinkler bulb is


protected from outside damage by a metal cover with narrow openings which has a separate socket attached to the piping. As the heads are sensitive to contamination and dirt, care needs to be exercised when handling and cleaning them as both the bulb and spray head nozzles are fragile. When the temperature in a space reaches the response temperature of the heat sensitive sprinkler bulb causing the bulb to break, a valve spool is released. The water pressure and a spring push the valve spool down, causing the water to flow into the sprinkler nozzles. The fog created by the 1760 times expansion is propelled out at high speed, its strong cooling effect serves not only to fight the fire but also to protect people and property against the radiated heat. The high pressure water mist rapidly suppresses the fire while limiting the spread of smoke. This is achieved by the water droplets binding to the particles of smoke so that they fall to the floor, effectively halting the spread. Since the Hi-fog system also uses far less water than traditional sprinkler systems, up to 90% less, two more benefits are obtained. The damage from water and the subsequent clean up is less extensive. Also, the reduced volume means vessel stability is not an issue. With all the positive aspects of the


hi-fog system, the writing may well be on the wall for the traditional water sprinkler system. NA


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