Fire Detectors for unmanned machinery spaces of cargo ship

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Fire Detectors for unmanned machinery spaces of cargo ship

A variety of devices are available for detecting fire in unmanned machinery spaces but each has an ability to detect basically only one aspect. Thus, smoke detectors based on the ionization chamber are able to recognize combustion products but will not register radiation from a flame or heat.

A smoke detector based on the interruption of light reception by a photo-electric cell, will only identify the shadow effect of dark smoke when it passes through its chamber. It will not identify unseen combustion products, heat or flame. Heat sensors and rate of temperature rise sensors based on the differential expansion of bimetal strips will detect only heat. Flame detectors may not detect a flame which is hidden by equipment and are sometimes caused to operate by sources of radiation other than from a flame.

Each device has a limited ability to detect a feature associated with fire so that systems are likely to have more than one type of device. Watchkeepers can detect fire by seeing flames and smoke, smelling combustion products and feeling heat

lonization chamber combustion products detectors

Within machinery spaces, ionization chamber combustion products detectors (Figure 14.6) are the type most used. These devices monitor the electrical change which occurs when combustion particles reach the open chamber of the detector.

Figure : Ionization chamber combustion products

Air in the open and closed chambers conducts a small electric leakage level current from the low pressure d.c. supply, because it is ionized by alpha particles emitted from a small piece of enclosed radium. Any combustion products reaching the open chamber, reduce the conductivity. The increased resistance to current flow across the open chamber air gap, causes a rise in potential which triggers the electronic device and switches on a full current flow in the alarm circuit.

Operation of any or all of the devices will operate the one alarm. The indicator light shows which device has functioned so that the source of combustion products (sometimes not visible) can be located. The indicator light provided on the sensor body is also used for testing with gas from a test canister or cigarette smoke.

The appropriate number of detector heads are sited in strategic positions above equipment which is a fire risk and at deckhead level in the protected compartment.

Obscuration type smoke detectors

A simple photo-cell and light arrangement (Figure 14.7) can be used as a detector for visible smoke but this type of device would not detect invisible combustion products.

Figure : obscuration type smoke detector

Flame detectors

Photo conductors sensitive to infra-red light (Figure 14.8) or photo-emmissive cells (Figure 14.9) which are sensitive to ultra-violet light, can both be used for flame detection. Light may be directed through a filter, on to the detecting surface by a wide angle lens.

Figure : Infra red flame detector

Figure : Ultra violet flame detector

A rotating mirror can be incorporated to extend the area under surveillance. The radiation increases conductivity so that current flow increases and causes an alarm circuit to be triggered. To reduce the chance of false alarms from light sources other than flame, a capacitor and a coil are connected in series, such that only current fluctuations, with a frequency similar to that of flames, are passed.

Heat sensors

Bimetal strips made up from brass and INVAR will deflect when heated due to the high expansion coefficient of the brass and the low expansion coefficient of the INVAR, which is 36% nickel steel. Deflection increases with temperature and could in itself be used to complete an electrical circuit and operate an alarm at some specific figure. However, rate of rise can be detected using two bimetal strips (Figure 14.10) of different thickness. These are set up parallel to each other and arranged to deflect in the same direction.

Rapid temperature rise will cause the thinner element to deflect more quickly than the other, so causing contact to be made and the alarm circuit to be completed. A slow rise causes a similar deflection in both elements, so that a gap between the contacts persists, until a certain maximum temperature is reached when contact is made because of other differences in the make up of the strips.

Figure :Rate of rise heat detector (R. C. Dean)

Detection system

The control equipment associated with the installation may be accommodated in the wheel house or in a fire control centre and comprises a fire indicating cabinet to which the detector heads are connected, a power unit, to convert the incoming ship's supply to the voltage appropriate to the equipment and a standby battery unit. The cabinet will indicate in which space a fire has been detected and will also monitor the system and indicate whether a fault has developed. It will also instigate an audible alarm.

Summarized below general cargo ship fire protection equipments & guideline:

Fire main system & related mechanism

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CO2 fire extinguishing installation

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Fire fighting equipments

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Details of fire detectors

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Machinery space fire & use of Walter Kidde CO2 system

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Fire protection system for cargo holds

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Low pressure CO2 storage

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Halon system

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Multi-spray system for the machinery spaces

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Automatic sprinkler system

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Details various portable extinguishers & how to use ?

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