Theory on Fire Fighting

Combustion -Principles

Combustion is a chemical reaction or series of reactions , in which heat and light are produced . Combustion is a rapid rate reaction in which light is emitted as heat.

When combustion takes place it is the vapour given off by a substance that burns , and not the substance itself . Most normal substances can be made to give off vapour that can be ignited even steel if heated sufficiently ) and the temperature at which this vapour is
emitted , in sufficient quantity to support combustion , is known as the FLASH POINT of that substance.

Materials with low Flash Points give off sufficient flammable vapour at ambient temperatures , but others with high Flash Points require heating before they will produce sufficient vapour to support combustion .

Fuel may exist in solid , liquid or vapour-form . The burning of most materials produces a flame , this occurs when gases or vapours given off by a liquid or solid material are ignited . The majority of materials which surround us will , or can be made to , burn if the conditions are right . Fuel in one form or other may be considered to be ever present.

Once this vapour is ignited , the heat from the flame will increase the temperature of the substance causing more and more vapour to be formed to feed the fire .

FLASH POINT AND IGNITION TEMPERATURES

Flash Point is the lowest temperature at which sufficient vapour is being given off for there to be a flash if an ignition source is introduced .

Ignition Temperature , sometimes known as Fire Point , is the lowest temperature at which the introduction of an ignition source would result in a flash followed by afire .

Self Ignition Temperature , sometimes referred to as Spontaneous Ignition Temperature , is the temperature at which a substance will ignite without the introduction of an external ignition source .

The following are some examples of Flash Points and Self Ignition Temperatures
Fuel    Flash Point    Self Ignition Temp
Methane    - 175 C    536 C
Gasoline    - 40" C    468 C
Diesel Oil    71 C    338 C
Fuel Oil    82 C    255 - 410 C
Lubricating Oil    148 C    260 C
Cooking Oil    250  C    370 C
Wood    232  C

The following temperatures have been included for purposes of comparison
    Match Flame    900 c
    Cigarette    300 C
    Open Light Bulb    120 C

The vapour cannot be ignited unless its proportion with oxygen ( air lies between two concentrations , known as the Lower Flammable or Explosive ( L.F.L. or L.E.L. ) and the Upper Flammable or Explosive Limit ( U.F.L. or U.E.L. ) . Concentrations below the L.F.L. will also be too lean to be ignited , while concentrations above the U.F.L. will be too rich . Over-rich concentrations may be readily brought into the Flammable Range by dilution with air.

Combustion cannot take place without the presence of oxygen , in a quantity not far less than the amount of oxygen available in the atmosphere .

There is 21 % oxygen in the atmosphere . When the oxygen in the atmosphere around a fire has been reduced to about 16 % , a fire will " glow " using up less oxygen , and will " reflash " as soon as an access door is opened . A fire will die when the oxygen content or the surrounding air has been reduced to 12 % .

The Fire Triangle

For a fire to ignite the following ingredients are essential

Oxygen
Heat
Fuel
Removal of one or more of the sides of the triangle will result in the extinguishing of the fire . This can be accomplished in the following ways:

1    REMOVAL OF HEAT

Reduce the temperature of the burning substance to below its Flash Point by cooling.

2.    REMOVAL OF OXYGEN

Reduce the oxygen content of the atmosphere in contact to below 12 % by smothering.

3.    REMOVAL OF FUEL

Usually this means preventing further supplies of fuel from reaching the fire.

For a fire to continue to burn there is a fourth essential ingredient , a chemical process called PYROLOSIS Removal of this ingredient , and consequent estinguishment of the fire is accomplished by chemical dry powder or vapurising liquids such as Halon , BCF, etc.

 
 Classification of Fires

Fires may be grouped into five categories as follows

Class    Material                              Extinguishing Method

A    Carbonaceous materials,wood,    Cool below Flash Point
    cloth , paper etc.

B    Flammable liquids and solvents ,    Cut off oxygen or , in case of high
    oils , paints , fats etc.    Flash Point ( above 43' C ) oils cool
        with water spray or fog .

c    Liquefied gases    Shut off supply of gas then extinguish
        with chemical powder.

D    Metals    Cool by drenching with water. This is
        not a common type of fire .

E    Electrical    Remove oxygen or extinguish with
        chemical agent.


CLASS " A " FIRES

Those fires involving Solid material - Wood , Paper, Cloth etc.

Under ambient conditions Class A material does not give off vapour that can be ignited but the Flash Point a Class A material is relatively low . It is only necessary to heat wood and paper a few hundred degrees to raise it to its Flash Point when combustion can take place , provided an ignition source is available . Hence the easy lighting of a match . The temperature of a match head after striking is about 1100' C , the flash point of the wood matchstick about 240 C .

Fortunately solid material with a relatively low flash point such as wood , paper cloth etc. is also fairly easily cooled to below its flash point when it will stop giving off flammable vapour and combustion will cease.

Therefore the best method of extinguishing a fire involving Class A materials is with water applied in the form of a spray.

 
As it is the latent heat of vaporisation of water which produces the cooling effect that extinguishes fire , its application in the form of a fine mist or spray will serve to maximize this effect.

In addition , the wide angle provided by the spray will protect the fire fighter from the heat of the fire and allow safe and comfortable close quarters contact .

A continuous spray applied by a jet / spray nozzle will provide complete protection from heat and flames in excess of a temperature of 2,000' C a few inches on the other side of the spray .

Water in the form of a jet is only required to combat a fire that cannot be reached by a spray or where the force of a jet is needed to break up material in a deep seated fire such as may be found in a hold fire .

CLASS " B " FIRES

Those fires involving liquids - oils, chemicals and paint

Oils have a wide range of flash points . As far as ships are concerned they may vary from - 45 C ( petroleum products ) to 65.5 C ( heavy fuel oil ) .

Low flash point oils cannot be extinguished by the application of water spray as the cooling factor is insufficient . In fact , the use of water spray on burning fats or other liquids with high boiling points is positively dangerous as water droplets falling into the liquid are instantly converted to steam and produce a violent eruption .

Fires involving relatively high flash point oils , such as fuel oil ( 65' C ) and diesel oil ( 43 C ) can be extinguished with water fog or spray by sweeping rapidly over the burning surface and driving the flames back.  There is , however , always the possibility of re-ignition until the extinction of the flames is complete . This technique also requires the surface of the burning oil to be free of obstructions and easily accessible.

For this reason the most effective method of dealing with oil fires is by cutting off the oxygen supply by covering the surface with foam . This method has the additional advantage in that foam can readily be induced to flow over burning oil surfaces that may not be accessible to water spray . Foam must not , however , be used on fat fires as it contains sufficient water to produce the reaction described above - dry powder, C02 , or Halon are suitable for this type of fire .

When foam is used it should be directed against a convenient vertical surface or projection and allowed to slide gently onto and flow over the burning surface .

Immediate reduction of the flame area is not apparent when combatting an oil fire by foam , which is frequently the case when applying water spray to a Class B fire . Perseverence is therefore necessary , together with an essential uninterrupted supply of foam compound .

It has been reported that over 50 % of engine room fires were abandoned too early where
perseverance would undoubtedly have succeeded.

Class B fires may be satisfactorily extingiushed by inhibiting combustion with Dry Chemical . The main disadvantage of this method is that Dry Chemical has no cooling effect and fires may reflash after they appear to have been satisfactorily extinguished . This method is most effective as a first line of attack but must be followed up by foam or water spray.

CLASS " C " FIRES

Liquefied gases .

Fires involving liquefied gases are not likely to be encountered in the Fleet . This type of fire has to be tackled by first isolating the source of gas and then extinguishing with chemical dry powder . Extensive boundary cooling is necessary and the fire area should if possible be contained by a certain of water spray .

Certain chemicals are included in this class of fire and many of these evolve toxic products of combustion . Breathing apparatus and the correct type of protective clothing is particularly important when dealing with this type of fire .

CLASS " D " FIRES

Steel and other metals will burn if subjected to greatly elevated temperatures . They are relatively uncommon but have been experienced in boilers and funnel uptakes .

Due to the very high temperatures involved there is a real risk that steam or water spray will break down to form hydrogen and produce an explosion . The only way to tackle fires of this nature is by drenching them with massive quantities of water using jets or flooding techniques .

CLASS " E " FIRES

Fires involving electrical equipment

An electrical fire usually originates through a short circuit where the source of electrical energy is prevented from finding a natural earth . The build up of electrical energy will cause any combustible material that may come into contact with the electrical discharge to ignite .

The essential action in the event of an electrical fire is to cut off the source of electrical supply to the circuit affected .

This may not always be possible and even if the supply of electrical energy is removed the combustible material may still continue to burn .

To avoid the spread of fire it will frequently be necessary to combat an electrical fire before the supply of electrical energy has been removed , the fire fighting medium must therefore be a non-conductor of electricity .

C02 gas does not conduct electricity and is the most suitable medium for extinguishing electrical fires , operating on the principle of smothering the fire by displacing the oxygen .

Halon , BCF and Chemical Dry Powder are also effective when dealing with electrical fires . These media function on the principle of disrupting the chemistry of the fire although Halon and BCF also have a partial smothering effect


  Fire Precautions - Machinery Spaces


Due to the presence of oil and oil vapours in Machinery Spaces , a fire in the Engine Room , or other Machinery Space, if not due to oil in origin will very likely develop into an oil fire within a short time .

The most successful means of preventing a fire from developing in the machinery spaces may be listed as follows :

1 .     Safety awareness by the personnel working in these spaces

2.    Good housekeeping.

3.    Defining the high risk areas where additional precautions must be taken ,i.e. restricted smoking in certain areas ( Purifier Rooms ) etc.

4.    Frequent inspection of the Machinery Spaces for the sole purpose of removing fire hazards.

5.    Correct maintenance of machinery , mechanical and electrical , and fire fighting equipment.

6.    Keeping the bilges clean and free from oil and water.

7.    Keeping the spaces well ventilated and free from hydrocarbon gas at all times .


The Master must include the Machinery Spaces in inspections to ensure the correct maintenance of the safety and fire fighting equipment and must consult with the Chief Engineer Officer to ensure that all precautions listed above are being followed . It should be remembered that the Chief Officer is responsible for the correct maintenance of the fire fighting equipment in the machinery spaces . In the context of this regulation both the Transfer Pumproom forward and the Main Cargo Pumprooms should be treated as Machinery Spaces.

   Fire Precautions -Accommodation, Storerooms and Galley
An accommodation fire will involve Class A material ( fibrous material

The most likely causes of an accommodation fire may be listed as follows:
1 .    Careless smoking

2.    Incorrect use of electrical appliances

3.    Spontaneous combustin

The company's Regulations with regard to smoking must be made known to every member of the ship's company.

Every cabin should be provided with an ashtray of the cigarette self extinguishing type .
Any privately owned ashtray of the non self extinguishing type should be confiscated by the Master and returned to the owner when he leaves the vessel .

Smoking in bed is prohibited at all times regardless of whether or not the ship is gas - free .

Regular inspections should be made to ensure that electrical fittings are safe The removal of light glasses or the shading of electric light bulbs is dangerous and is forbidden .

The Deck Officer of the watch is to make rounds of the accommodation and outside the storerooms to check against the possibility of an undetected outbreak of fire after the
8 to 12 and 12 to 4 night watches . During these rounds the Officer is to inspect drying rooms to ensure that no clothing has been left over or in contact with heaters .

The Chief Cook is responsible for ensuring that the Galley stove and other electrical appliances are switched off before the Galley is vacated each night.  He will also ensure that the stoves , hot - plates and ventilation exhausts are kept free from accumulations of fat and grease at all times .

Electric lights and appliance must not be left switched on in unattended spaces.

 Use Of C02 in Enclosed Spaces


If a fire occurs in the Engine Room , Main Cargo Pumproom or some other space and the C02 system is used , the system should be operated from its remote position and after the necessary operating instructions complied with a check should be made in the C02 room to ensure that the necessary cylinders have in fact fired .

This can easily be ascertained by noticing whether the levers attached to the wire pulls have moved over to the fired position .

Once the system has been fully released and the fire extinguished no personnel are to enter the space without breathing apparatus until the main supply valve has been shut . The space must of course be well ventilated prior to entry in order to remove all traces Of C02 and ensure sufficient air has been introduced to make the space " safe.