1. Basics of a Fire
- Fire is a phenomenon with which everyone is familiar.
- We use it daily to heat our homes and cook our meals. When harnessed, the power and energy from fire serves us well; however, when it is uncontrolled, a fire can quickly consume and destroy whatever lies in its path.
- While we are all familiar with fire, few of us are aware of its nature and complex processes.
v The Fire Triangle
There are three (3) components required for combustion to occur:
• Fuel – to vaporize and burn
• Oxygen – to combine with fuel vapor
• Heat – to raise the temperature of the fuel vapor to its ignition temperature
There are two important factors to remember in preventing and extinguishing a fire:
i) If any of the three components are missing, then a fire cannot start.
ii) If any of the three components are removed, then the fire will go out.
It is important to have a clear understanding of these three components and their inter-reactions in a fire.
2. Classification of fires
Fires are classified according to the type of fuel and if live electrical equipment is present. The classification of a fire is important, as it influences the selection and use of the correct extinguisher required to extinguish a fire. The six classes of fire are:
- Class A - Ordinary Combustibles (such as paper, wood, plastics, etc.)
- Class B - Flammable and combustible liquids
- Class C - Flammable gases
- Class D - Combustible metals
- Class E - Electrically energised equipment
- Class F - Cooking oils and fats
These classes of fire are also illustrated by a pictogram to assist people quickly identify the type of fire.
<Shipboard Firefighting>
3. Methods and Procedures
- Pre‐action preparations and training is of essence to combat fires successfully.
- Preparations also cover structural protection, fixed fire suppressing systems, equipment control, awareness and readiness.
- On live incidents, standard procedures for firefighting tactics onboard conventional vessel include;
- four main actions:
(1) Early Detection ‐ Alarm,
(2) First Attack,
(3) Containment, Control,
(4) BA‐Attack ‐ Safe Re‐entry Procedure.
- Primarily, early detection is of essence to extinguish the fire in its growth stage, before the fire has fully developed.
- Secondly, immediately after detection and alarm, the first attack is made by personnel detecting the fire.
- By using fire extinguishers or other means to suffocate the fire and/or removing the fuel, crew and the ship might avoid a larger incident.
- Third step, if the initial procedures fail, is to contain the fire in the fire compartment.
- Sealing off the area to prevent the fire to spread, removing fuel, and to minimize oxygen supply, is made to buy time for the fourth step to muster.
- To contain the fire, automatic, semi‐automatic or manual fixed installed fire suppression systems, if present and deemed proper action, should be engaged If the fixed installed fire suppression systems fail, boundary cooling of the ship structure is of essence. Since conventional ships normally is constructed with mild steel, a highly heat conductive construction material, the heat from the original fire is likely to travel through the construction and ignite other compartments. Boundary cooling requires vast amounts of water applied to the decks and bulkheads fire compartment rounding the initial fire compartment. Depending on the size of the initial, a sufficient number of personnel are required to operate the nozzles applying water for boundary cooling.
- Forth step is the re‐entry procedure, BA‐attack on the fire compartment
- This cannot be done in a safe way until the fire has been suppressed or reached its decay stage. The latter adds time to the total lapsed time to get in control of the fire. During this time, boundary cooling must be applied continuously
4. Fixed Gas Fire-extinguishing Systems
(1) Principles of Fixed Gas Fire-extinguishing Systems
Fixed gas fire-extinguishing systems typically suppress fires by reducing the available oxygen in the atmosphere to a point where combustion can no longer take place or by interrupting the chemical reaction necessary for the progression of the fire.
Advantages of fixed gas systems over water-based systems are that:
• Damage to sensitive equipment can be avoided, especially in the case of electronic equipment.
• Clean up time and equipment down time is substantially reduced.
Disadvantages are that:
• Some gaseous agents are hazardous to personnel.
• Cooling effect of gas systems is significantly less than water-based systems.
• Unlike the unlimited supply of water for fire-fighting systems, the quantity of gas available is limited to that carried in the cylinders protecting the space.
Due to the above disadvantages, it is essential that fixed gas fire-fighting systems be deployed as quickly as possible to minimize heat buildup. Also, care should be taken to avoid the possibility of a fire being restarted due to dissipation of the fire-extinguishing gas and the introduction of fresh air from protected compartments being prematurely opened after a fire.
In new installations, the most common fixed gas extinguishing systems encountered are either high/low pressure CO2 systems or those utilizing Halon “alternatives”.
(2) General Principles of Fixed Water Fire-extinguishing Systems
Water is an ideal extinguishing medium for many shipboard applications. It is readily available, has great heat absorbing capabilities and can be used on a variety of fires. There are several mechanisms involved in the extinguishment of a fire with water.
First, there is the cooling of the flame temperature when water passes through the combustion zone and absorbs heat through evaporation. Cooling of the flame temperature results in a reduction in the amount of radiant heat released by the fire, and therefore, a reduction in the amount of heat radiated back to the fuel surface.
Secondly, there is the cooling effect of the fuel surface by the direct impingement of water droplets on the surface. With a reduction of the radiant heat received at the fuel surface and the additional cooling of the fuel surface by direct contact with the water droplets, there is a reduction in the amount of combustible gases released. With sufficient cooling of the flame temperature and/or the fuel, the rate of vaporization of combustible vapors will be reduced to a point which combustion will no longer be self-supporting. Water has the important additional effect of when it evaporates it turns into steam.
The steam, which is in the immediate vicinity of the chemical reaction, displaces the air that supplies oxygen for the combustion process and results in a smothering of the fire.
Fixed water extinguishing systems are normally considered to include water spray, water sprinkler and water mist systems. These systems utilize fixed piping systems with distributed arrays of nozzles located in the overhead, which are supplied from dedicated pump(s).
However, the particular fire hazards and safety concerns vary depending on the particular type of space being protected. For example, in a machinery space, one would anticipate Class “B” combustibles to be involved, while in an accommodation space, one would anticipate the involvement of Class “A” combustibles.
Even the degree of anticipated supervision has a role. There are many locations in the accommodation spaces and service spaces that are not continuously supervised (cabins, storage closets, etc.) and a small initial fire could easily go unnoticed by shipboard personnel.
There are also certain differences in the extinguishing mechanisms at work for a water mist system as compared to those involved in a water spray or water sprinkler system. Accordingly, the system designs, as well as the requirements, vary depending upon the space to be protected and the type of system to be installed. The following provides a brief discussion regarding the individual types of systems.
5. Water Spray System
Water spray systems are manually operated, open “deluge” type systems and are typically used to protect open ro-ro spaces, “Special Category” spaces and cargo pump rooms. Water spray systems are also permitted for use in protecting machinery spaces, but such arrangements are seldom proposed due to the hazards involved and the availability of suitable alternatives. Like other types of fixed water extinguishing systems, the water spray system consists of pump(s), a fixed piping system and a distributed array of nozzles. However, the one distinctive characteristic of a water spray system is that it utilizes “open” type nozzles. When this type of system is activated, water will be discharged simultaneously through all of the branch nozzles, which can result in very high water demand rates. Also, these systems are normally required to be manually actuated, and therefore, personnel must initiate the discharge of the system.
6. Water Sprinkler Systems
Water sprinkler systems also utilize a distribution system consisting of fixed supply piping and overhead nozzles. However, unlike water spray systems, water sprinkler systems are designed for automatic activation. Since these systems are automatically actuated, the distribution system must be pressurized at all times. To accommodate the automatic release, the sprinkler nozzles used are of the “closed” type and fitted with individual heat sensitive links or bulbs that allow the nozzle to open when the temperature of the air in the vicinity of a particular nozzle exceeds a certain “activation” temperature. Since each individual nozzle has its own activation mechanism, only those nozzles in the immediate vicinity of the fire will see temperatures sufficient for activation and will open. Normally, the nozzles located directly above or next to the fire source, as well as the nozzles around the outside perimeter of the fire base, will be opened. Those nozzles located directly over or near the fire source serve to control the fire by wetting the flames and fuel source, while the nozzles around the perimeter of the fire serve to pre-cool any surrounding combustible materials. These systems are normally used to protect accommodation spaces and service spaces.
7. Foam Fire-extinguishing Systems
Foam
Foam is produced by the combination of three materials:
• Water
• Air
• Foam making agent
General Principals of Foam Extinguishing Systems
Firefighting foam is used to form a blanket on the surface of flaming liquids. The blanket prevents flammable vapors from leaving the surface and prevents oxygen from reaching the fuel. A fire cannot exist when the fuel and oxygen are separated, and therefore, a properly placed foam blanket will smother the fire. In addition, the water in the foam also has a cooling effect, which gives foam the ability to cool surrounding structure to help prevent flash back.
The ideal foam should flow freely enough to cover a surface rapidly, yet have adequate cohesive properties to stick together sufficiently to establish and maintain a vapor tight blanket. In addition, the solution must retain enough water to provide a long-lasting seal. Rapid loss of water would cause the foam to dry out and break down (wither) from the high temperatures associated with fire. The foam should also be light enough to float on flammable liquids, yet heavy enough to resist winds.
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