FIRE FIGHTING :

HIGH PRESSURE WATER SPRAY SYSTEM :-

This can be a completely separate system or it can be interconnected with the sprinkler system that is available for fire extinguishing in accommodation spaces. The system incorporates a compressed air vessel, fresh water pump and salt water pump all connected to piping which is led to sections, each section having its own shut-off valve and sprayer heads, which unlike the sprinkler system have no quartzoid bulbs or valves but are open. The system is kept charged with full of fresh water under pressure from the compressed air in the air vessel. When a section valve is opened in the event of fire in that section, water will be discharged in that section through spray nozzles to give the correct droplet size for fires in flammable liquids. The spray nozzles are located so as to give adequate water spray distribution on tank top and all fire risk areas in machinery space. At least 5 ltr/m" per minute water spray should be provided. The pressure drop in the line operates a pressure switch

which automatically starts a salt water pump which will continue to deliver water to the sprayers till the section valve is closed. The pumps must have an independent drive or an electric motor with a supply via the emergency generator switchboard. The pumps, its air supply, power source and controls must be outside the protected space and must not be affected by a fire in that space.

Water-spray is a potentially good fire fighting medium because of the following advantages:-

(a)it produces a large quantity of steam which has smothering action.

(b)in producing the steam, it absorbs a large amount of latent heat and thus giving a
cooling effect.

(c)the water spray protects fire-fighting personnel in the compartment.

(d)water is readily available.

Corrosion of the system is minimized by keeping the system charged with fresh water. After operation, the pipework is drained of salt water and refilled with fresh water after washing through. The multi-spray, sprinkler or drencher system can be connected through a cross connection which are normally locked shut when not in use. The main difference between the machinery space water spray system and the sprinkler system is that whilst the sprinkler system is automatic in operation, the water spray system is required to be manually operated. The sprayer head is similar to the sprinkler head but has no glass bulb, different types of deflector base being used to vary the spray pattern as required.

Disadvantages -

(a)when used in cargo spaces, the water spray might damage the cargo which might otherwise be salvaged.

(b)water spray on electrical equipment and switchboards are also damaging.

(c)water spray on some chemicals may cause explosive mixture.

(d)larger droplets of water extract little or no heat and therefore largely ineffective.

(e)larger droplets of water when explodes due to heat may disperse oil and spread fire elsewhere.

MICROMIST WATER FOG SYSTEM OF FIRE EXTINGUISHER :-

Micromist concept is developed by Michael O'connell , the owner and Managing Director of Micromist Ltd. The technology uses hot water which when injected through the nozzles produces smaller finer water micro-droplets (<50j_i). The mist turns into steam and starving the fire of oxygen and large cooling effect minimizes the risk of re-ignition. Hot boiling water with vapour bubbles when released through orifice, explodes into finely dispersed water particles and form fine mist. Rapid vaporization and subsequent cooling ,reduces the temperature to near ambient and perform quenching, inerting and oxygen reduction. Smaller water particles vaporize in no time, expanding to 1760 times the original volume and very effective in absorbing heat and starving the fire of oxygen. In comparison to water spray where the droplet size is about lmm, in watermist system the droplet size of 50u,(50 micron) does the vaporisation and cooling 300 times faster.

AUTOMATIC WATER SPRINKLER SYSTEM :

The system incorporates fire detection, alarm, and fire-fighting capability by water sprinkler system for structural protection of accommodation spaces. The standard fire protection is based on zones separated by the proof bulkheads and having fire proof divisions within them. A network of sprinkler heads is arranged throughout the spaces to be protected. Each sprinkler head is normally kept closed by a quartzoid bulb which is almost filled with a liquid having a high expansion ratio. When the bulb is experiencing heat due to fire, the liquid expands and shatters the bulb. Water maintained under pressure by compressed air, is then released from .the sprinkler head or heads in the form of heavy spray. Each head adequately covers a deck area of 16m² and sprinkler heads are arranged so that every part of each space are covered under water spray. The system has pressure tank which is part filled with fresh water and pressurized to 8 bar by compressed air. When the pressure drops below 5.5 bar, a salt water pumps cuts in automatically and maintains water pressure. The pressure at the highest sprinkler head should not be less than 4.8 bar. Sprinkler heads are grouped in sections with not more than 150 heads per section and each section has an alarm system. When the quartzoid bulb is manufactured, a small gas space is left inside the bulb so that if the bulb is subjected to heat, the liquid expands and the gas space diminishes. This will generate pressure inside the bulb and the bulb will shatter once a predetermined temperature and hence pressure is reached. Generally the operating temperature range permitted for these bulbs is 68°C to 93°C but the upper limit of temperature can be increased this would depend upon the position where the sprinkler head or heads are to be sited. Quartzoid bulbs are manufactured in different colours, the colour indicates the temperature rating for the bulb.

Rating                                 Colour

                                          68°C                                                      Red

80°C                                     Yellow

93°C                                     Green

Once the bulb is shattered, the valve assembly falls permitting water to be discharged from the head, which strike the deflector plate and sprays over a considerable area. When a head comes into operation, the non-return alarm valve

for the section opens and water flows to the sprinkler head. This non-return valve also uncovers the small bore alarm pipe lead and water passes through this small bore alarm pipe to a rubber diaphragm. The water pressure acts upon the diaphragm and operates a switch which causes a break in the electrical circuit. Alarms both visible and audible fitted in engine room, bridge and crew space are then automatically operated.

Maintenance and testing :-

(a)Greasing the various valves and checking their freedom of movement, logging the pressure gauge reading before and after each alarm valve, thus checking the tightness of the non-return valve and checking the alarm system.

(b)Stop valves, A and B, are locked open and it either of these valves are inadvertently shut, a switch will be operated that brings alarm into operation.

(c)The alarm system can be tested by opening drain valve C, which reduces the system pressure , the pressure switch operated and gives alarm.

(d)The pressure tank level is checked and recharged, if necessary, with fresh water and air.

(e) The  centrifugal salt  water pump  (electrically operated having connection from emergency switchboard) should also be tested by closing the isolating v/v and draining the pressure switch circuit, when the pump should start automatical!}'. Delivery pressure should be logged.

(f)      In the event of a fire, when the normal situation is recovered the section and system are drained and flushed out, then recharged with fresh water and air.

(g) The hose connection line which is provided so that water can be supplied to the system from shore when the vessel is in dry dock is also to be checked for operation.

ANALYSIS OF FIRE EXTINGUISHING MEDIUMS :-

WATER: It is available in plenty, non-toxic, safe to use on most fires, can be easily directed over considerable distance. It has high latent heat, 2256.7 kJ/kg at atmospheric pressure hence it has very large cooling effect. It expands by 1700 times while converting from liquid to steam by absorbing heat and creates a smothering atmosphere.

When used on oil fires, all the oil surface is covered by water spray and surrounding hot metal should be cooled to prevent re-ignition. If water droplets enter the hot oil, they will be converted to steam- this rapid conversion from water to steam leads to spluttering and possible spread of the fire. Water droplets should be fine enough(mist or fog), so that they can convert to steam by taking heat from burning vapour and cool them in turn.

STEAM: Has a very limited cooling effect. Its higher temperature makes the control of smoldering fires somewhat protracted. It is expensive to produce and large quantities are required. Steam should not be used in conjunction with CO2 since it will reduce the effect of good fire fighting medium like CO2, Steam smothering is also not a recommended means.

FOAM: It is used principally for extinguishing oil fires and may be generated chemically or mechanically. Chemical generation of foam is accomplished by mixing together a solution of sodium bicarbonate and a solution of aluminium sulphate in the presence of a stabilizer (e.g.soap or liquorice). The result of the chemical reaction which takes place between these two solutions, is a mass of tough skinned bubbles containing carbon dioxide. Increase in volume is one of solution to eight of foam. Mechanical generation of foam is done by either mixing, using a suitable agitator, a dry powdered protein compound of hoof, horn and hydrolised blood with water and air or mixing a synthetic detergent concentrate with water and air.

CO2 Plant Maintenance :-

For machinery spaces containing diesel propelling machinery, or auxiliary machinery whose total power is 746 KW or more, a fixed fire fighting installation has to be provided. One such system is CO₂ total flooding system. The quantity of gas carried

a) must be sufficient to give a free volume equal to 40% of the gross volume of the largest cargo space or machinery compartment, whichever is greater, except where the horizontal casing area is less than 40% of the general area of the space or

b)    must give a free volume equal to 35% of the entire space, whichever is greater.

The free air volume of air receivers may have to be taken into consideration. Since rapid injection of CO2 is necessary, 85% of the gas must be released within two minutes, of actuating the system release. The volume of free CO2 is to be calculated at 0.56m7kg.

Maintenance and Testing

(a)To ensure that all moving parts are kept clean, free and well lubricated. Wires must be checked for tightness, toggles and pulleys must be greased.

(b)With the use of compressed air the CO₂  distribution pipes should be blown through periodically.

(c)The CO₂ are to be weighed periodically. Normally each bottle is charged with 45kgs of CO₂ weighing by spring balance (after disconnecting the toggle switch connecting pin with  the wire) or by means of ultrasonic or radioactive isotope unit detector are done. Recharging is necessary if there is a 10% weight loss.

(d)The safely valve fitted in the main common manifold must be checked for proper operation. CO₂ bottle pressure is normally about 52 bar but this varies with temperature. Bottles should not be stored where the temperature is likely to exceed 55°C.  The (seal/bursting disc) is designed to rupture spontaneously at  177 bar produced by a temperature of about 63°C. In the event of fire in the CO₂ room space, gas is released by the relief arrangement on to the manifold and into the CO₂ room space thus extinguishing fire. The master valve prevents CO₂ release into engine room or any other space.

(e)CO₂ cabinet door opening alarm should be checked for functioning.

(f) Accidental discharge of any CO₂ bottle or leakage of CO₂ occurs from any bottle either in the starting section or in the main battery, a pressure switch in the line sounds alarm, and then vent to atmosphere valve can be opened. -

(g) The CO₂ nozzles fitted at the end of CO₂ discharge pipes normally close to the bilges must be kept clear for efficient discharge. The CO₂ cylinders are fitted with internal pipes which permit the CO₂ to pass through the distribution pipe work to the nozzles in the liquid state and it only evaporates on discharge from nozzles. The internal pipe therefore prevents evaporation of the liquid taking place on operation of the system, as the resultant drop in pressure and temperature would cause the vapour to freeze and deposit as snow in the valve and pipework, distribution pipes should not be less than 20mm bore.

Testing :-

CO₂ bottle are solid drawn steel or manganese steel hydraulically tested to 228 bar CO₂ bottles and associated pressure components are to be tested to codes of practice having regard to their locations and the maximum ambient temp, expected in service.

FIRE PUMPS AND FIRE MAIN SYSTEM :

Capacity of fire pumps -

The total capacity of the main fire pumps is to be not less than that required by the following formula;

Q = [0.15L(B+D) + 2.25]²

Where Q = total capacity in m³/hour.

B = greatest moulded breadth of ship, in mtr.

D = moulded depth to bulkhead deck in mtr.

L = Rule length of ship.

Any pump designated as a fire pump other than any emergency fire pump is to have a capacity not less than 80% of the total required capacity divided by the minimum number of required fire pumps and each such pump is in any event to be capable of delivering at least one jet of water. These fire pumps are to be capable of supplying the fire main system under the required conditions.

FIRE PUMPS :-

(a) In cargo ships of 150 tons gross and upwards, not less than two power pumps are to be provided, one of which is to be an independent pump.

(b) Sanitary, ballast, bilge or general service pumps may be accepted as fire pumps, provided that they are not normally used for pumping oil and that, if they are subjected to occasional duty for the transfer or pumping of fuel oil, suitable changeover arrangements are fitted.

(c) In cargo ships and fishing vessels, if a fire in a category 'A' machinery space could put the fire pumps out of action, there is to be an alternative means consisting of a fixed independently driven emergency fire pump which is to be capable of supplying at least one satisfactory jet of water.

(d) Relief valves are to be provided in conjunction with all fire pumps if the pumps are capable of developing a pressure exceeding the design pressure of the water service pipes, hydrants and hoses.

(e) Where centrifugal pumps are provided, a non-return valve is to be provided in the pipe connecting the pump to the fire main.

Emergency fire Pump :—

(a)The emergency fire pump, its source of power and its sea connection are to located in accessible positions outside the category 'A' machinery space.

(b)The sea valve is to be capable of being operated from a position near the pump.

(c)The room where the emergency fire pump prime mover is located is to be illuminated from the emergency source of power supply and to be well ventilated.

(d)If the emergency fire pump is required to supply water for a fixed fire-extinguisting system in the space where the main fire pumps are situated, it is to be capable of simaultaneously supplying water to this system and the fire main at the required rates.

FIRE MAIN:-

The diameter of the fire main is to be based on the required capacity of the fire pumps and the diameter of the water service pipes are to be sufficient to ensure an adequate supply of water for the operation of at least one fire hose.

PRESSURE IN THE FIRE MAIN :-

(a)The pressure to be maintained at any hydrant is to be sufficient to produce a jet throw at any nozzle of about 12 m.

(b)A power operated emergency fire pump is to maintain a pressure at any hydrant sufficient to produce a jet throw at any nozzle of about 12 m.

NUMBER & POSITION OF HYDRANTS :- The number and positions of hydrants are to be such that at least

one jet of water may reach any part normally accessible to the crew while the cargo ship or fishing vessel is being navigated and any part of any cargo space when empty. Furthermore such hydrants are to be positioned near the accesses to the protected spaces. At least one hydrant is to be provided in each machinery space.

FIRE HOSES :-

(a) Fire hoses are to be approved non-perishable material. The hoses are to be sufficient in length to project a jet of water to any of the spaces in which they may be required to be used. Their length in general is not to exceed 18 m. Each hose is to be provided with a nozzle and necessary couplings.

(b)The number of fire hoses to be provided, each complete with couplings and nozzles, is to be one for each 15 m length of Cargo ship or fishing vessel but in no case is there to be less than three. These numbers do not include any hoses required in any engine room.

NOZZLES:-

(a)Standard nozzles are to be 12 mm, 16 mm or 19mm or as near thereto as possible, so as to make full use of the maximum discharge capacity of the fire pumps.

(b)For accommodation and service spaces, the nozzle size need not exceed 12 mm.

(c)The size of nozzles intended for use in conjunction with a manually operated emergency fire pump, need to be excess of 9.5 mm.

(d)For machinery spaces and exterior locations, the nozzle size is to be no less than 12 mm.

(e)All nozzles are to be of approved dual purpose type, i.e. spray/jet type, incorporating to a shut-off.

FIRE DETECTION TECHNOLOGIES

(1)     Short comings of ion-chamber detection:-

Ion-chamber detection depends on aerosol produced in combustible products reaching the detector, and therefore, detector location is very important. Fires which start some distance away from the detector, could take a number of minutes before smoke sets off the alarm, while slow smouldering fires could take well over thirty minutes before they are detected. So Ionisation chamber is only suitable where a large flame fire and enough of smoke evolves, may be in machinery space oil fire.

(2)  Short comings of optical detectors:-

Optical detectors suffer from high false alarm rates because of dust collection which scatter light and eventually leads to spurious alarms. However, with an aerodynamic design which encourages dust settlement on the less critical optical surfaces, the units are less susceptible to false alarms.

(3) Carbon monoxide Smoke Detector latest development :- When   a   slow   smouldering   fire  takes  place,   then  carbon

monoxide evolved, being a gas, is much more mobile than aerosols or particles and can move by diffusion, whereas smoke movement is largely constrained by connection currents created from the fire. So, in theory, carbon monoxide detector would have a faster detection rate where a strong smoke plume is not present. The risk areas better suited to carbon monoxide fire detection arrangement are:

(i) Rooms with partially compartmentalized ceilings, where the mobility of CO gas enables detection in situations where other fire products may not easily reach the detectors.

(ii) Areas with soft furnishing, where there is the likelihood of an undiscovered fire smouldering for a significant period before igniting. Carbon monoxide detectors would give several hours advanced warning.

(iii) Areas with steam problems or where CO2 smoke is used for special effects. As carbon monoxide detectors are not effected by aerosols they are resistant to spurious alarms in situations where these exists as part of the normal environment.

(4)     Very Early Smoke Detection Alarm System(VESDAS)

Unlike point detectors, the optical air sampling system does not rely on convection current to introduce smoke to the detector. Instead, air is drawn through a dust filter designed to remove particles more than 25u in diameter. The diameter is close to upper size limit for large smoke particles. On entering the chamber, the air sample is subjected to an intense flash of light (Xenon light), while an extremely sensitive photo-electric receiver detects lights scattered off the particles which are suspended in the air stream. The resulting signal is then processed to give an analogue signal of smoke intensity. VESDAS are using broad- spectrum light source* which are more sensitive in a much broader spectral band width and responds to smoke particles of all sizes. VESDAS   are  used  to   safeguard  high value  assets  such as informations stored in computer rooms, machinery control, navigation and safety systems.

 (5) Flame Detection :-

Ultra-violet technology based flame detectors have fallen into disrepute as they have a higher frequency of spurious alarms and if the lens is covered with a thin film of oil, the detector is rendered useless. Infra-red type systems are responsive to sunlight and other high intensity lights and blackbody radiation and not suitable on board. The most effective flame detector, is the single wave length type device consisting of a pyro-electric sensor with an interference filter to limit the Infra-red bandwidth.

Transmitting radiation in a narrow range of wavelengths centred on 43u, corresponds to an emission/absorption band of CO2 and therefore, the detector is sensitive to radiation produced by hot CO2 in flames. Yet solar radiation at 4.4u is absorbed by atmospheric CO2, so the detector is insensitive to sunlight. An additional advantage is that the output of the pyro-electric sensor is amplified and electrically filtered to pass a restricted range of 1 to 30 Hz -which is characteristic of the flicker from flames, thus providing discrimination against radiation arising from nearby hot bodies.

(6) Latest Single Channel flame detector :-

An optical filter is installed to enable a single electronic infra-red sensor to measure the radiated energy present in two separate wavebands placed on either side of the flame detection waveband, (3.8 and 4.8(a). The signal obtained from this channel is cross-corelated with the signal from the flame detection channel to provide an accurate prediction of non-flame energy. The use of such single channel optical processing technique, as opposed to the use of two separate electronic sensors, improves the overall reliability and eliminates the nuisance alarms from modulated black body sources (the energy given by a hot object). Although for most shipboard applications the carbon monoxide detector is perhaps the best solution, ion-chamber, optical or the new infra-red flame detection technologies are still best suited to machinery spaces.

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