Monday, June 1, 2009

Carbon Dioxide (CO2) Extinguishing Systems

Carbon dioxide (C02) extinguishing systems have, for a long time, been approved for ship installation as well as for industrial occupancies ashore. Aboard ship, carbon dioxide has been approved for cargo and tank compartments, spaces containing internal combustion or gas-turbine main propulsion machinery and other spaces.

Properties of Carbon Dioxide

Carbon dioxide is normally a gas, but it may be liquefied or solidified under pressure. At -43°C (-110°F), carbon dioxide exists as a solid, called "dry ice." The critical temperature of carbon dioxide is 31°C (87.8°F). Above that temperature, it is always a gas, regardless of pressure. Carbon dioxide does not support combustion in ordinary materials. However, there are some exceptions, as when C02 reacts with magnesium and other metals. Carbon dioxide is about 1.5 times heavier than air. This adds to its suitability as an extinguishing agent, because CO2 tends to fall through air and blanket a fire. Its weight makes it less prone to dissipate quickly. In addition, carbon dioxide is not an electrical conductor; it is approved for extinguishing fires in energized electrical equipment.

Extinguishing Properties of Carbon Dioxide

Carbon dioxide extinguishes fire mainly by smothering. It dilutes the air surrounding the fire until the oxygen content is too low to support combustion. For this reason it is effective on class B fires, where the main consideration is to keep the flammable vapors separated from oxygen in the air. C02 has a very limited cooling effect. It can be used on class A fires in confined spaces, where the atmosphere may be diluted sufficiently to stop combustion. However, C02 extinguishment takes time. The concentration of carbon dioxide must be maintained until all the fire is out. Constraint and patience are needed.

Carbon dioxide is sometimes used to protect areas containing valuable articles. Unlike water and some other agents, carbon dioxide dissipates without leaving a residue. As mentioned above, it does not conduct electricity and can be used on live electrical equipment. However, fire parties must maintain a reasonable distance when using a portable C02 extinguisher or hoseline from a semiportable system on high voltage gear.

Uses of Carbon Dioxide

Carbon dioxide is used primarily for class Band C fires. It may also be used to knock down a class A fire. It is particularly effective on fires involving flammable oils and greases.

Electrical and electronic equipment, such as motors, generators and navigational devices.

Hazardous and semihazardous solid materials, such as some plastics, except those that contain their own oxygen (like nitrocellulose) machinery spaces, engine rooms and paint and tool lockers.

Cargo spaces where total flooding with carbon dioxide may be accomplished.

Galleys and other cooking areas.

Compartments containing high value cargo, such as works of art, delicate machinery and other material that would be ruined or damaged by water or water-based extinguishing agents.

Spaces where after-fire cleanup would be a problem.

Limitations on the Use of Carbon Dioxide

Effectiveness. C02 is not effective on substances that contain their own oxygen (oxidizing agents). It is not effective on combustible metals such as sodium, potassium, magnesium and zirconium. In fact, when C02 is used on burning magnesium, it reacts with the magnesium to form carbon, oxygen and magnesium oxide. The fire is intensified by the addition of oxygen and carbon, a fuel.

Outside Use

To be fully effective, the gas must be confined. For this reason, C02 is not as effective outside as it is in a confined space. This does not mean that it cannot be used outside. Portable CO2 extinguishers and hoselines have extinguished many fires in the open. An outside fire should be attacked from the windward side; the CO2 should be directed low with a sweeping motion for a spill fire, or down at the center of a confined fire. The effective range for a portable C02 fire extinguisher is about 1.5 m (5 ft).

Possibility of Reignition

Compared with water carbon dioxide has a very limited cooling capacity. It may not cool the fuel below its ignition temperature, and it is more likely than other extinguishing agents to allow reflash. (Its main extinguishing action, as noted above, is oxygen dilution.) When portable C02 extinguishers or hose lines from semiportable extinguishers are used, additional backup water hoselines should be brought to the scene. In case of live electrical equipment, an additional nonconducting agent must be brought to the scene.

When a space is flooded with CO2 the concentration must be kept up to a certain level After the initial application of a set number of C02 cylinders, additional cylinders must be discharged into the space periodically. These backup applications maintain the concentration of C02 for periods varying from hours to days. C02 works well in confined spaces, but it works slowly; patience is the watchword.

If a flooded space is opened before the fire is completely extinguished, air entering the space may cause reignition. Carbon dioxide cannot be purchased at sea. Reignition requires a second attack, at a time when less C02 is available.

Hazards. Although carbon dioxide is not poisonous to the human system, it is suffocating in the concentration necessary for extinguishment. A person exposed to this concentration would suffer dizziness and unconsciousness. Unless removed quickly to fresh air, the victim could die.

Carbon Dioxide Systems

Carbon dioxide extinguishing systems aboard vessels are usually not automatic. However automatic systems may be installed in certain ships and towing vessels with Coast Guard approvaL In the manual system, a fire detector senses fire and actuates an alarm. The engine room is alerted, and the bridge and CO2 room are notified as to the location of the fire (see Chapter 6). After it is verified that a fire actually exists, the amount of carbon dioxide required for the involved space is released from the C02 room.

Coast Guard regulations require that an evacuation alarm be sounded when CO2 is introduced into a space that is normally accessible to persons on board, other than paint and lamp lockers and similar small spaces. However on systems installed since July 1, 1957, an alarm is required only if delayed discharge is used. Delayed discharge is required where large amounts of C02 are released into large spaces. Delayed discharge may also be required for smaller spaces from which there are no horizontal escape routes.

The alarm sounds during a 20-second delay period prior to the discharge of carbon dioxide into the space. It uses no source of power other than the carbon dioxide itself. Every carbon dioxide alarm must be conspicuously identified with the warning "WHEN THE ALARM SOUNDS VACATE AT ONCE. CARBON DIOXIDE IS BEING RELEASED."

Portable and semiportable C02 extinguishers may be located in certain spaces. Small systems, consisting of one to four C02 cylinders, a hose and a nozzle, are often provided to protect against specific hazards. Those who work in the areas protected by these appliances should be familiar with their operation.