The goal of decreasing the combustion process to zero is accomplished via cooling as well as minimizing the material transformation and activation energy required for combustion. In other words, the objective of fire-extinguishing agents is to reduce the rate of combustion to zero.
In order to extinguish a fire, one of the fire triangle element must be removed. The primary extinguishing effect of carbon dioxide (CO2) is the dilution of O2 in air, which reduces the oxygen level below the minimum value required for combustion.
The shipping industry currently favors CO2 as a fire-extinguishing agent due to its great reliability and assured effectiveness, few residues, and low cost. Among all of these advantages, the rapidity with which CO2 can reach life-threatening concentrations is the most significant drawback.
On high pressure systems CO2 is stored in a liquid state in steel pressure cylinders and the pressure is around 57 bar at 20 deg. C. The CO2 reaches the nozzles in a mixed phase expanding thereafter to the ambient pressure. On low pressure systems the CO2 is stored at a pressure of 18 to 22 bar and to be able to store it in liquid from, the physical properties require a storage temperature of approx. -22 deg. C and therefore is equipped with an independent refrigeration system of which function is connected to the CO2 tank pressure control.
Depending on the application, CO2 concentrations between 35 and 50 percent by volume are typically used to create an oxygen deprivation and extinguish a fire, but this degree of oxygen depletion can potentially result in asphyxiation. Therefore, fixed systems are designed with safeguards that prevent the automated release of CO2 when the protected area is inhabited. CO2 is typically not thought to have a strong intrinsic toxicity, nor is it considered to form combustion products in a fire setting.
When in touch with the skin, CO2 may produce low-temperature burns and the damaged region should then be carefully irrigated with clean water and dressed by a competent professional.
The vessel must carry a minimum amount of CO2 sufficient to extinguish a fire in the vessel’s largest space (which is usually engine room space).
The CO2 cylinders must be stored in areas where the ambient temperature does not exceed 46 degrees Celsius, and they must not be exposed to direct sunlight, therefore the cylinders are located outside vessel accommodation on main deck or close to steering gear room on large vessels and the cylinders are equipped with pressure-relieving safety systems to alleviate excessive pressure induced by high temperatures.
In case of bulk CO2, the CO2 storage tank is chilled by refrigeration equipment that maintains an internal working pressure of 2.0MPa. The refrigeration system consists of two separate units, each with adequate cooling capacity to maintain the tank’s temperature. When activated by the tank pressure controls, each unit’s pressure switch starts and stops the refrigeration compressor. The refrigerant in the refrigeration system is Freon 134a. Each refrigeration unit is connected to its own evaporator, which is integrated into the top of the CO2 tank. To regulate operation, the compressors are equipped with intake and discharge pressure switches. On the refrigeration units, condensers for chilling with fresh water or fresh air are installed.
Normally the areas protected by the CO2 fire-extinguishing system are:
- engine room including engine control room
- vessel cargo holds
By means of cylinder manifolds and distribution pipes, these cylinders are linked to discharge nozzles within the protected spaces.
The distribution pipes in the hold spaces double as transmission pipes for the smoke detection system; operation of three-way valves is necessary to switch from the normal smoke detection mode to the CO2 release mode. The three-way valves for smoke detectors are installed in the fire station and explanation for this system can be found in here. Separate release systems safeguard the engine room and engine control room.
As specified above the amount of CO2 required to be carried on board the vessel is based upon the volume of the largest protected compartment and usually the engine room is the largest compartment to be protected and this normally requires that all bottles to be released in case of a fire. All other protected spaces require fewer cylinders than the engine room and the holds require different numbers of cylinders, also depending upon whether they are full or empty.
The chart with the release instructions is posted inside fire station or CO2 room close to the releasing cabinet.
Cylinders are connected in groups or blocks which are numbered and are released in these blocks by means of a separate hand levers which are located at the CO2 release cabinet in the fire station or CO2 room.
The valve handles are positioned so that the control box door cannot be closed with a valve in the open position. It is also arranged that the control box door will operate two switches when it is in the open position, to initiate audible and visual alarms. Usually, operating lever No.1 for the engine room system will also initiate the sounding of the alarms; operating lever No.2 will shut down the engine room ventilation fans. There is no ventilation shut down signal for the cargo fans, therefore any fans, fan flaps or dampers must be stopped/closed separately on some systems. A pressure gauge is fitted to the control CO2 pipeline to indicate pilot CO2 pressure.
Control CO2 cylinders are used to activate the main CO2 cylinders’ discharge. Two control cylinders are located in the CO2 room and two are located in the fire station; one cylinder is a standby cylinder at each site. The system is operated by a supply of CO2 which is separate from the main fire extinguishing CO2. It is stored in two small control cylinders installed within the control boxes at the fire station and in the CO2 room. The control cylinders are connected to the hold release or engine room release control valves; from these valves control CO2 is directed to operate the main distribution valve for the engine room system or hold systems. The control CO2 also operates the release mechanism of the banks of CO2 cylinders for the required system.
A pressure operated distribution (POD) valve is installed into the CO2 discharge pipe work between the CO2 manifold and the discharge nozzles to direct the CO2 gas into the space where the fire is located.
Flooding the relevant room with extinguishing gas initially causes an increase in room pressure and, due to thermodynamic processes, a virtually homogeneous distribution of CO2 in the fire-affected room. The excess pressure causes smoke to escape from a room that is often not airtight.
If CO2 is released in any cargo hold compartment, the number of cylinders available to safeguard the engine room will decrease and the engine room cannot be protected. Therefore, they must be recharged or replaced immediately in order to restore the CO2 system’s fire fighting effectiveness. The cylinders must be recharged or replaced at the next port, or the ship must be diverted to the closest port having CO2 recharge/replacement capabilities. In the event of a fire in a hold and the release of CO2 into that hold, the hold must remain sealed until arrival in port, where the local fire department can provide aid.
Compressed air connections are installed on the main CO2 distribution lines to the engine room and cargo holds, allowing compressed air to be fed to the system in order to verify all outlet locations. The same pipe is utilized for the delivery of CO2 to the holds and the smoke detection system. This pipe is utilized for long distances and must be visually inspected at regular intervals of no more than one month to check for pipe failure. In the case of a pipe failure, CO2 will not be discharged into the hold, rendering fire suppression ineffective.
On above image the discharge of CO2 to the cargo holds is via a diverter three-way valve block to any cargo hold. Diverter valves are used where the CO2 system is combined with a cargo hold smoke sampling system and CO2 gas is required to be redirected down the smoke sampling lines to the cargo space affected. If a fire is detected in any hold, the smoke sampling system for that hold is shut down and the smoke sampling line becomes the distribution line for CO2.
When the control cabinet door for the cargo system is opened, either in the fire station or locally in the CO2 room, alarms are sounded in each hold. It will be necessary for the operator to stop the necessary ventilation fans and shut and dog the flaps.
As stated before the number of cylinders released into a hold depends upon whether the hold is
full or empty. The operator must activate the correct release levers for the hold in question and must check before release which levers are to be operated.
It is important to note that after releasing CO2 inside cargo holds and until arriving in port, keep all openings sealed and directional valves open. Do not open the hatches or other openings of the compartment until arrival in port and do not operate the smoke detection system for that hold.
Discharge of CO2 to the engine room and engine control room is direct from the cylinders to the distribution pipework and nozzles. The main distribution valve isolates the cylinders and control system from the engine room pipework unless a release is initiated. Indication that a cylinder has released its charge will be shown by snow and ice developing on the outlet line and the top of the cylinder.
It is important to note that after CO2 has been released to the engine room, you need to allow time for structural cooling before opening the engine room and at least 24 hours must be allowed before opening and ventilating the engine room. Also, do not enter a CO2 flooded space without using breathing apparatus due to possible asphyxiation. Moreover, when operating the CO2 locally from the CO2 room the operator must take precautions to prevent injury or danger. Breathing apparatus should be worn to safeguard against the effects of CO2 release into the CO2 room. Attention must also be paid to which valves are being opened to ensure that only bottles for the hold concerned are actually released.
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