Over time, new technologies have improved the arsenal of weapons that ship crews have in the battle against fire but fighting fire may well still involve manual means such as fire hoses and hand-held extinguishers, buckets and sand as well as advanced systems such as water mist, sprinklers and gas suppressants.
When it comes to fire extinguishers, the wrong choice can make matters worse. Part of a seafarer’s basic training will cover which type of extinguisher to use in different situations.
There are six different types of hand-held extinguishers with each type intended for dealing with one or more of the different types of fires and completely unsuited for others:
- Powder fire extinguishers are ideal for use in mixed risk environments. They are the only effective solution for fires involving flammable gases.
- Foam fire extinguishers are ideal for use on fire involving solid combustible materials and are highly effective on flammable liquid fires. The layer of foam applied by these extinguishers helps to prevent re-ignition after the fire has been extinguished.
- CO2 fire extinguishers are suitable for use on flammable liquid fires and are extremely effective at extinguishing fire involving electrical equipment.
- Water fire extinguishers are suitable for use in environments containing solid combustible materials such as wood, paper and textiles. They should not be used around electrical equipment (unless water extinguishers with additive are used).
- Wet chemical fire extinguishers are usually supplied with a special application lance. They are intended for tackling large burning oil fires and are ideally suited to the kitchen/galley environment.
- Water mist works on the basis of using microscopic particles of water to cool a fire, suffocate it and then cool the burning media to prevent re-ignition. Water mists extinguishers are ideal for covering areas where multiple fire risks can be found.
- Manual fire-fighting when not involving fire extinguishers will rely on pumps and hoses. A sufficient supply of water for fighting fires is not normally a problem for ships unless the pumps and hoses are damaged or inoperable or if the vessel is at a berth where it either lays aground or where the water depth is very shallow. In the latter cases it is always a wise precaution for the ship’s officers and crew to ensure that a shore hydrant is available nearby.
Unlike fighting fires ashore where the volume of water used is not an issue, at sea an excess of water is highly dangerous and cause the ship to capsize. Water can also react with some cargoes releasing hazardous gasses or even causing further fires and explosions. The latest requirement for container ships to carry a water lance capable of penetrating a container may well help extinguish some fires but some believe that it will be a matter of time before the use of such equipment will actually cause a fire to become much fiercer.
The FSS Code and SOLAS contain regulations that cover all aspects of the fire pumps, hydrants and hoses, including their capacity, placement and numbers. Exact details will be ship-specific and will also be dependent on ship type. The regulations also cover ventilation, dampers and fire doors.
A system will inevitably contain numerous valves to isolate parts of the system when maintenance or repair work is needed. The valves connecting the pumps to the sea chest should only be closed when work is being done so that at all other times there will be a ready supply of water to the system. Checking the status of valves is an essential part of regular inspections.
Piping is an often-neglected part of the fire system but its condition is as important as any other part since a damaged or leaking pipe can render the whole system useless. Pipes for the fire system are generally of steel construction and therefore subject to corrosion. This is especially true of pipes on deck exposed to the open air and corrosion on these pipes often goes undetected, especially if paint is concealing areas of wastage.
A suitable fire hose should be stored close to each hydrant together with appropriate connectors and nozzles. As with the pipes, hoses should be checked regularly for damage. All ships are required to carry an international fire hose connector so that in the event of failure of its own firefighting system, a ship in port can have its piping and hosing system connected to a shore water supply. The connector can also be used to connect the system to the pumps of another vessel when in port or at sea.
Sprinklers and water mist
Most modern ships are now equipped with a sprinkler or water mist/fog extinguishing system. In such systems, the sprinkler head is usually a combined detector unit. Sprinkler systems can also be activated manually if a fire is seen before the system activates. When heat or smoke activates the system, the head water is released to extinguish the fire. The types of systems are basically similar in that they use water released from overhead points when activated but the mist systems use less water and have other claimed advantages.
The water for the systems is supplied through the sea chest but there is also a tank of fresh water that is used in the first instance for priming the system so that the standing water in the pipes is not corrosive.
Sprinkler and water mist systems can be brought into action faster than gas systems since it is not necessary to close openings, shut down ventilation or evacuate the space before release. The time needed to extinguish fires with water mist can be longer than for gas, but water mist also cools the space and controls smoke in the process. An unlimited water supply is also usually available.
In a water mist system, the water is under pressure and released through a spray head. The small water droplets allow the mist to control, suppress or extinguish fires by cooling both flame and atmosphere and displacing oxygen by evaporation. The mist is more penetrative than water from sprinklers and also acts as a smoke suppressant thus preventing other heads from being activated by smoke and so reducing water demand.
From a safety point of view, the ship’s stability is far less likely to be compromised by the free surface effect of the amount of water used and, for those systems using fresh water, carrying less of it means more cargo capacity is available or less fuel is needed. Water mist has been shown to be highly effective at extinguishing fires in both demonstrations and actual operational circumstances.
Water mist systems come in both high-pressure and low-pressure variants. Over the years, the pressure needed to produce the fine droplets has reduced from around 100bar to levels around 7bar in some systems. There are, however, still plenty of manufacturers that continue to produce high-pressure systems.
Proponents of these argue that the higher pressure produces smaller droplets that aid in rapid extinguishing. The water droplets can expand to almost 2,000 times in size as they vaporise, depriving the fire of essential oxygen. The more droplets there are and the greater the area they occupy, the more effective will be the suppression.
Although considered an improvement over sprinkler systems, water mist installations are not without problems. After several vessels were detained in US ports as a result of inoperable systems, a number of flag states and P&I clubs considered it necessary to offer advice on maintenance and checking of systems. It appears that the majority of the detentions were due to systems being secured either by closed supply valves or by placing the system in a manual mode of operation.
When a system requiring an automatic operation capability is placed in manual mode, the sensors and alarms are not engaged and the system’s quick response capability is disabled. The chances of a fire spreading increase when the protected space is unmanned and the overall effectiveness of the water mist system could be reduced, particularly in terms of time needed to extinguish the fire.
In addition to water, there are other means that are used to fight fire on ships. Some of the most effective systems, such as those that use Halon, have been banned under the Montreal Convention because of their ozone-depleting effects. An alternative called Novec 1230 is now used instead of Halon.
Novec 1230 systems are individually designed and appropriate sized storage cylinders chosen according to the hydraulics and quantity of agent required. The components are designed and tested to operate in the temperature range 0°C to 50°C. The cylinders are generally stored outside the area being protected although under certain circumstances they can be kept in the same space. Novec 1230 systems are designed to hold both the Novec 1230 in the form of a liquid and nitrogen, which is used to super-pressurise the container to 24.8bar at 20°C.
When the system is activated the contents flow into the distribution pipework to the discharge nozzle(s) where it is dispersed as a vapour in less than 10 seconds. During the discharge the enclosure will be fogged which may reduce visibility. This normally clears rapidly and should not obstruct the ability of personnel to safely exit the protected area.
Under normal conditions, Novec 1230 is a colourless and low-odour fluid with a density around 11 times that of air. It decomposes at temperatures above 500°C and it is therefore important to avoid applications involving hazards where continuously hot surfaces are involved. Upon exposure to flames, Novec 1230 will decompose to form halogen acids.
Their presence will be readily detected by a sharp, pungent odour before maximum hazardous exposure levels are reached. Fire toxicity studies show that decomposition products from the fire itself, especially carbon monoxide, smoke, oxygen depletion and heat, may create a greater hazard.
The successful performance of a gaseous total flooding system is largely dependent on the integrity of the protected enclosure. It is essential that a room integrity test is performed on any protected enclosure to establish the total equivalent leakage area and enable a prediction to be made of the enclosure’s ability to retain Novec 1230. The required retention time will depend on the particulars of the hazard, but MSC/Circ.848 states that this should not be less than 15 minutes. Longer retention times may sometimes be necessary if enclosures contain hazards that may readily become deep seated.