ship board fire_detection

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Certificate of Competency - 5 G e n e r a l E n g i n e e r i n g K n o w l e d g e

© 2008 Singapore Maritime Academy | D33| Version 01| October 2008

Fire Detectors

A fire detector detects a fire and gives warning to the occupants. Warnings are both visual and audible. It should be heard throughout the ship with light indicators on the bridge and engine room.

Early discovery of fire is very important as the fire can go out of control very quickly. Containing and extinguishing the fire in the early stages is critical as well as easier.

Automatic Fire Detection SystemThe fire detection system consist of the following

Normal and emergency power supplies1.

Fire detection control unit2.

Fire detectors3.

Alarms4.

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Normal and Emergency power supplies.During normal operation, power to run the fire detection system comes from the common generators switchboard supply.

In an emergency, when the normal generator supply is interrupted, the emergency power supply will come from storage batteries or independently driven emergency generator.

Fire Detection Control Unit It is a drip-proof enclosed panel containing the following

Signal indicating fire alarm1.

Fault alarm2.

Power failure alarm3.

The Fire Detection Control Unit is equipped with audio and visual signals. It contains:

Fire alarms �

Fault alarms �

Power failure alarms �

When there is a fire break-out, the fire sensors detect the fire and activate the fire alarm. The fire control panel indicates the exact zone on fire. Once the fire alarm is activated, it gives off audio and visual signals. The audio signals come from an electric bell, which is audible in all places inside the ship. It stops ringing, when you activate the alarm bell cut-off switch. However, this will not shut off the alarm lights in the control unit, until the reset button is activated. The reset button should not be activated, until the fire is located and extinguished.

Some ships have the fire alarm signals to activate closure of doors, ventilation fans, pumps and dampers and also automatic extinguishing systems like automatic sprinkler system.In an automatic sprinkler system, when the fire detectors are activated, water immediately rains down on the area, extinguishing the fire. The fire detectors are quartzoid bulb type and are often installed in passenger vessels.

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Types of Fire Detectors

There are 3 main types of automatic fire detectors namely,

Heat detectors �

Smoke detectors and �

Flame detectors �

They are installed in ship’s various locations appropriately to ensure maximum coverage and quickest response to a fire.

Heat detectorsWhen there is heat generated from the scene of the fire, heat detectors are activated. Their principle of operations could be based on

Bimetallic strip �

Fusible links or �

Quartzoid bulb �

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Quartzoid bulb type heat detectorIn the quartzoid bulb type heat detector, the quartzoid contains a liquid with high expansion coefficient. At a specific temperature, the bulb breaks under the pressure of the expanding liquid, causing water to flow through. Detectors based on quartzoid bulb cannot be tested, as they are destroyed by the tests themselves.

Heat Detector - Quartzoid bulb type

Threeded for connection to pressuerised pipe

Valve

Quartzoid bulb

Fusible link detectorA fusible link detector holds a spring device in an open position. When the device reaches its melting temperature, it drops out and the spring releases. As a result, circuit is completed and alarm signals are sent. The link must be replaced to restore the alarm. Detectors based on fusible links cannot be tested, as they are destroyed by the tests themselves.

Heat detectors are normally fitted on ceilings. The disadvantage of a heat detector is that it is unable to sense the fire until the temperature of the room is sufficient to activate the fire detector - by which time the fire would have gone out of control. This is known as the ‘thermal lag’.

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Connecting terminal

Plastic base moulding

Insulating bushPlug assembly

Fusible alloyContral conductorFinned caseInsulating pip

Fusible alloy contacting central conductor

Fusible Link Detector

Smoke DetectorsThey work on the principle that smoke particles will obscure a light source falling on a photo electric cell (receiver). This affects the current generated and this is sensed by the fire detecting system and an alarm is activated.

Smoke detectors are designed to detect smoke particles. It is most useful for Class A fires where a lot of smoke is generated. It is also suitable for smoldering fires.

The disadvantage of these detectors is that it cannot distinguish between smoke, dust particles, oily vapours or even steam. This can result in frequent false alarms.

Flame DetectorsFlame detectors detect the infra-red waves and the flickering motion of a flame generating from fire. These are very sensitive fire detectors and they send alarm signals immediately on fire detection.

The disadvantage of flame detector is that it cannot detect a fire that is behind an obstacle, for example, behind a generator or a switchboard. It is also very susceptible to false alarms from welding works, flickering sunlight reflections, flickering lights, etc.

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Engine RoomEngine rooms normally have flame and smoke detectors.

Engine room fires are of the Class B type - oil, gas and paint fires.

Fires due to oil dripping on hot surfaces, fuel vapour accumulations in purifier rooms are instantaneous and are identified immediately by flame detectors.

Intense smoke generates from oily burning rags, overheated electrical machinery and hence smoke detectors are suitable.

Accommodation SpacesSmoke detectors are fitted in these locations.

These areas generally have Class A materials, which emit a lot of smoke, when on fire.

Bridge and other control roomsSmoke detectors are priority, but flame detectors are suitable as additional detecting devices.

These areas generally have the risk of Class A and Class C fires.

Cargo SpacesSmoke detectors are generally installed in cargo spaces.

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