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Fire Detectors for unmanned machinery spaces

Fire protection on ships is provided by detection and fire-fighting equipment together with structural features which are intended to contain an outbreak of fire and the employment when required of non-combustible materials to prevent its spread.

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 associated with fire are used to provide alarms: these are smoke, flames and heat,

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.

Smoke detector

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.

The smoke detector makes use of two ionisation chambers, one open to the atmosphere and one closed (Figure ). 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.

Flame detector

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 . Flame detectors are used near to fuel handling equipment in the machinery spaces and also at boiler fronts.

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 are the type most used. These devices monitor the electrical change which occurs when combustion particles reach the open chamber of the detector. 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 can be used as a detector for visible smoke but this type of device would not detect invisible combustion products.

Flame detectors

Photo conductors sensitive to infra-red light or photo-emmissive cells 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.

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

Heat detectors can use any of a number of principles of operation, such as liquid expansion, low melting point material or bimetallic strips. The most usual detector nowadays operates on either a set temperature rise or a rate of temperature rise being exceeded. Thus an increase in temperature occurring quickly could set off the alarm before the set temperature was reached. The relative movement of two coiled bimetallic thermostats, one exposed and one shielded, acts as the detecting element (Figure ). Heat detectors are used in places such as the galley and laundry where other types of detector would give off false alarms.

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 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.

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.

Fire Alarm

Associated with fire detectors is the electric circuit to ring an alarm bell. This bell will usually sound in the machinery space, if the fire occurs there, and also on the bridge. Fires in other spaces will result in alarm bells sounding on the bridge. Any fire discovered in its early stages will require the finder to give the alarm and or make the decision to deal with it himself if he can. Giving the alarm can take many forms such as shouting 'Fire', banging on bulkheads or any action necessary to attract attention. It is necessary to give an alarm in order to concentrate resources and effort quickly onto the fire, even if the fire must be left to burn for a short time unchecked.

Types of portable fire extinguisher

There are four principal types of portable extinguisher usually found on board ship. These are the soda-acid, foam, dry powder and carbon dioxide extinguishers. Details as below :
  1. Soda acid portable fire extinguisher
  2. The container of this extinguisher holds a sodium bicarbonate solution. The screw-on cap contains a plunger mechanism covered by a safety guard.

  3. Foam type portable fire extinguisher
  4. The main container is filled with sodium bicarbonate solution and a long inner polythene container is filled with aluminium sulphate

  5. Dry powder fire extinguishers
  6. The outer container contains sodium bicarbonate powder. A capsule of carbon dioxide gas is located beneath a plunger mechanism in the central cap

  7. CO2 portable fire extinguisher
  8. A very strong container is used to store liquid carbon dioxide under pressure

Fixed fire extinguishing installations

A variety of different fixed fire fighting installations exist, some of which are specifically designed for certain types of ship. A selection of the more general installations will now be outlined.
  1. Fire main system for cargo ships

  2. An outbreak of fire requires a source of ignition, the presence of combustible material and ample oxygen. Of the three factors, oxygen is provided in large quantities in machinery spaces, accommodation, dry cargo holds and tanker pumprooms by ventilation fans. Air supply trunkings are not only a source for a supply of oxygen to feed the fire but also have potential for carrying smoke from one area to another....

  3. Automatic water spray & water mist system for machinery protected area

  4. The automatic spray or sprinker system provides a network of sprinkler heads throughout the protected spaces. This system may be used in accommodation areas, and in machinery spaces with certain variations in the equipment used and the method of operation. ....

  5. Automatic foam induction system for machinery space fire

  6. Foam spreading systems are designed to suit the particular ship's requirements with regard to quantity of foam, areas to be protected, etc. Mechanical foam is the usual substance used, being produced by mixing foam making liquid with large quantities of water. Violent agitation of the mixture in air creates air bubbles in the foam. ...

  7. CO2 fire extinguishing installations for machinery spaces

  8. Fire extinguishing installations employing CO 2 stored under pressure at ambient temperature are extensively used to protect ships' cargo compartments, boiler rooms and machinery spaces. When released the CO 2 is distributed throughout the compartment, so diminishing the relative oxygen content and rendering the atmosphere inert....

  9. Inert gas systems, inert gas generator

  10. Inert gases are those which do not support combustion and are largely nitrogen and carbon dioxide. Large quantities suitable for fire extinguishing can be obtained by burning fuel in carefully measured amounts or by cleaning the exhaust gases from a boiler. ....

  11. Fire fighting Halon system

  12. A Halon storage system would be very similar to one using carbon dioxide except that fewer cylinders would be required. The liquefied Halon is usually pressurised in the cylinders with nitrogen in order to increase the speed of discharge. ....

Other important fire & safety equipments

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