The most frequently used system for fighting fire in cargo holds of a general cargo ship is the Co2 flooding system. The Co2 system consists of a fire detection system (smoke detectors) and an alarm system, along with Co2 cylinders. During an indication of fire in the cargo hold, the gang of co2 bottles are released depending upon the cargo permiability (how much space is empty over the cargo for co2).
The fire fighting system for cargo hold consists of a 20mm diameter sampling pipe in all cargo hold compartments of the ship. This is controlled from a cabinet placed on bridge or in ship’s control center. Air is continously drawn through these pipes to the cabinet with the help of suction fans which delivers air from the diverting valve into the wheel house.
When there is a fire in any of the cargo hold compartments, the smoke is sucked into the sampling pipes and is passed through diverting valves in to the wheel house, thus warning the bridge personnel about the fire. Simultaneously, the sample from the pipes is passed over a smoke detector which senses the smoke and activates the audio visual alarm indicating the outbreak of fire.
The advantage of audio-visual alarm is that even if the bridge is unattended for e.g in port, there still will be an alarm for the outbreak of fire.
In the cabinet, the sample is passed over small propellers made up of nylon, through a transparent tube of 13mm diameter, indicating the flow. If the propellers are not running it indicates that the pipe is choked and hence to be rectified as soon as possible.
When there is a fire, the sampling pipe turns dark which can be seen in the transparent pipe of the particular cargo hold. The same sampling pipe is connected to bank of co2 bottles with the help of changeover valve. The co2 is released by opening appropriate valve for the hold after confirmation and discussion with the captain of the vessel or ship.
Important point that is considered before releasing the co2 is that the amount of co2 to be release is calculated i.e free volume of affected cargo hold which is calculated as Total volume of hold – volume of cargo. 30 % of the volume calculated is to be flooded with Co2. For one bottle of co2 the volume is calculated as 45.2kg.