Page 256 - Introduction to Marine Engineering

232 Fire fighting and safety

Fire fighting at sea may be considered in three distinct stages,
detection—locating the fire; alarm—informing the rest of the ship; and
control—bringing to bear the means of extinguishing the fire.

Detection
The use of fire detectors is increasing, particularly with the tendency to
reduced manning and unmanned machinery spaces. A fire, if detected
quickly, can be fought and brought under control with a minimum of
damage. The main function of a fire detector is therefore to detect a fire
as quickly as possible; it must also be reliable and require a minimum of
attention. An important requirement is that it is not set off by any of the
normal occurrences in the protected space, that is it must be
appropriately sensitive to its surroundings. Three phenomena associ-
ated with fire are used to provide alarms: these are smoke, flames and heat,
The smoke detector makes use of two ionisation chambers, one open
to the atmosphere and one closed (Figure 13.1). The fine particles or
aerosols given off by a fire alter the resistance in the open ionisation
chamber, resulting in the operation of a cold cathode gas-filled valve.
The alarm sounds on the operation of the valve to give warning of a fire.
Smoke detectors are used in machinery spaces, accommodation areas
and cargo holds.
Flames, as opposed to smoke, are often the main result of gas and
liquid fires and flame detectors are used to protect against such hazards.
Flames give off ultra-violet and infra-red radiation and detectors are
available to respond to either. An infra-red flame detector is shown in
Figure 13.2. Flame detectors are used near to fuel handling equipment
in the machinery spaces and also at boiler fronts.

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Figure 13.1 Smoke detector