CO2 system of ship( Complete explanation & diagram)

                

                 

  

 CO2 Flooding System:

• CO2 flooding system or is one of the common fixed fire fighting system installed on most of the ships. It releases carbon dioxide (CO2) in bulk quantity to a protected space (such as engine room, cargo hold, purifier room, pump room, etc.) under fire. 

• Smothering action of CO2 extinguishes the fire thereby preventing spreading of fire to other parts of the ship.

 It consist of several CO2 cylinders located in a separate room, called CO2 room. These bottles contain CO2 in liquid state. When the fire in a protected space goes out of control or in a situation when the fire is not able to extinguish by local fire fighting media, CO2 flooding system comes into picture. 

• CO2 from the bottles directed via common manifold, main valve and distribution pipe lines to nozzles through which it is released to the protected space for fire extinction. Due to safety reasons, the CO2 flooding system is manually released from a release cabinet located outside the protected space.

                  Why CO2 is used?

Carbon dioxide is the fire extinguising medium used in CO2 flooding system. It has a high rate of expansion which allows it to work fast. When applied to a fire, CO2 provides a heavy blanket of gas that reduces the oxygen level to a point where combustion cannot occur. Since carbon dioxide is a gas, there is no clean up associated with a system discharge. CO2, a compound of carbon and oxygen, is a colorless gas which cause coughing to occur when inhaled. At higher concentrations it is acutely toxic. Low and high pressure CO2 is used for the fire protection of machinery spaces, pump rooms, cargo holds, paint stores and galley exhaust ventilation ducts on board ships.

Advantages of CO2 as fire fighting medium:

• Density is 1.5 times higher than air. So CO2 settles down and displaces air.
• It can be easily liquefied and bottled.
• 20% – 30% CO2 concentration extinguishes fire by smothering
• Non-corrosive
• Non- conductor of electricity
• No residues left after application
• No deterioration with age.
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Disadvantages of CO2 as fire fighting medium:

• CO2 is highly asphyxiating. 9% concentration causes unconsciousness within minutes
• Very little cooling effect. So there is danger of re-ignition.
• When discharged, solid CO2 particles present and generate sufficient static electricity to produce spark.
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Important Note:

[Main CO2 bottles contain carbon dioxide in liquid state with a pressure of 56 bar at 20 degree Celsius. Pressure of CO2 at 25 and 30 degree Celsius are 64 bar and 71 bar respectively. So it is important to keep the temperature of CO2 bottles low for limiting the pressure inside bottles.]

Explanation :

•CO2 from the main bottle is released by a ‘CO2 release cabinet’.  As soon as the release cabinet door is opened, a micro switch is activated.

• The micro switch will ensure the activation of CO2 warning alarms and ventilation shut off.

 • CO2 release cabinet or release box consist of two pilot CO2 cylinders or bottles containing CO2 gas inside. 

• The pressure of CO2 inside these pilot bottles is same as that of main CO2 bottles. Only quantity of gas is different. 

• For releasing CO2 to the protected space, one of the pilot bottle valve is opened. Now CO2 reaches two valves marked 1 and 2. First valve 1 is to be opened. Then CO2 passes through a non return valve and opens pneumatically operated master valve. 

• Now master valve is opened. Next, open valve 2 in the release cabinet, which supply CO2 to main CO2 bottle head assembly through a non return valve and ‘time delay unit’. Function of the time delay unit is explained below. 

• Head assembly consist of a pressure operated cylinder valve. This pressure operated cylinder valve prevents CO2 from bottle coming to the common manifold. 

• As CO2 reaches the head assembly from pilot bottle, pneumatic actuator in the head moves and opens the pressure operated cylinder valve. 

• Then CO2 from main bottle escapes to the common manifold via non return valve as shown. Each of the main CO2 cylinder have a head assembly and non return valve. 

• The pilot CO2 line is connected to all these heads. All the CO2 from the main bottles now release to common manifold. Since the master valve is already open before, CO2 from manifold is released to the protected space through distribution pipe lines and nozzles.

• There are two CO2 release cabinets. One is local release cabinet and is located in the CO2 room itself. One more remote release cabinet is used, which is located in a remote place such as fire control station. This facilitates operation of CO2 flooding system from a remote place other than CO2 room. Both release cabinets are connected in parallel and non return valves fitted in the lines prevent back flow of CO2. Also two pilot CO2 bottles are placed in the release cabinet. Only one is sufficient for the operation of both master valve and CO2 bottle head assembly.

Time Delay Unit :

CO2 from the pilot bottles in release cabinet reaches main CO2 bottle head assembly through a time delay unit. Function of this time delay unit is to delay the supply of CO2 to the head assembly for 60 – 90 seconds. In other words, when valve 2 in the release cabinet is opened, CO2 reaches only after 60 to 90 seconds to the head assembly. Reasons for providing such a s delay in CO2 flooding system are given below.

1. As soon as CO2 release cabinet opens, alarm is sounded in the protected space. A time delay of 60 to 90 seconds give sufficient time for any personnel in the protected space to escape, even after the operation of both valves in the release cabinet.
2. If CO2 is released from main bottles to the common manifold before opening master valve, there may be difficulty in opening master valve because of the bulk CO2 pressure acting on it. Because of the time delay, positive opening of the master valve is ensured before releasing CO2.
3. Regulation per International Code for Fire Safety System (FSS Code) 2.1.3.2 says, “The pre-discharge alarm can be automatically activated (e. g. by opening the release cabinet door). The alarm shall operate for the length of time needed to evacuate the space, but in no case less than 20 seconds before the medium is released.”

The time delay can be achieved in different ways. Out of these, two types of time delay unit is commonly used in CO2 flooding system on board ships. One is electrical type, which has an AC 220 V power supply, a pressure switch, timer and a solenoid valve.

 

CO2 Main Bottle Head Assembly :

• Head assembly mainly consist of a cylinder valve, which is operated by a pneumatic actuator and linkages as shown in the figure below. CO2 from the release cabinet reaches the pneumatic actuator through time delay unit and actuator operates the cylinder valve to release CO2 from main bottles to the common manifold.

• The pressure operated cylinder valve shown in the figure above can also be manually operated by use of a lever inserted into the top of the valve

Bursting Disc in CO2 Flooding System :

• The pressure of CO2 inside the bottle depends on the temperature of the surroundings. As temperature increases, pressure also increases. Therefore to safeguard against abnormally high pressures inside the bottle, a bursting disc is provided on the head assembly.

• When the CO2 pressure inside the bottle increases to about 200 bar, bursting disc burst and releases CO2 to the CO2 room. Pressure inside the bottle build up to such a high value means there is a fire inside CO2 room. Here, CO2 released from the bottle itself extinguishes fire inside the room.

Pressure Switch, Pressure Gauge in Manifold

Pressure switch is fitted in the common manifold to identify any leakage of CO2 gas from bottles to the manifold. The pressure switch activates warning alarm for the leakage. Similarly pressure switch will activate alarms while actual operation of CO2 flooding system. A pressure gauge is also fitted on the manifold to read any gas pressure inside. A valve for pressure testing of the system is also connected to the pressure gauge connection.

                    Relief Valve 

There is a relief valve fitted in the manifold of CO2 flooding system. Function of the relief valve is to release the CO2 pressure in the manifold to the atmosphere outside CO2 room. Such a situation arises when CO2 get released from the bottle and master valve is still closed. Set pressure of relief valve is about 180 bar. 

Dip Tube in CO2 Extinguisher 

Whether it is portable CO2 fire extinguisher or cylinder in CO2 flooding system, a siphon tube or dip tube is necessary inside bottle.

The function of dip tube is to deliver liquid carbon dioxide outside the bottle. If dip tube in not provided, CO2 evaporates from the surface while discharge taking latent heat of vaporization and remaining liquid CO2 will freeze inside and hence failure of CO2 discharge. Therefore it is necessary to operate CO2 fire extinguishers in up right position.

What are the requirements of CO2 Room

In CO2 flooding system, carbon dioxide bottles are placed in a separate room called CO2 room. The requirements for location, accessibility, use and ventilation of CO2 storage spaces as per IMO are:

• Spaces for storage of cylinders or tanks for extinguishing gas should not be used for other purposes.
• These spaces should not be located in front of the forward collision bulkhead.
• Access to these spaces should be possible from the open deck.
• Spaces situated below the deck should be directly accessible by a stairway or ladder from the open deck.
• The space should be located no more than one deck below the open deck.
• Spaces where entrance from the open deck is not provided or which are located below deck are to be fitted with mechanical ventilation.
• The exhaust duct (suction) should be lead to the bottom of the space.
• Such spaces should be ventilated with at least 6 air changes per hour.

Requirements of CO2 Flooding System

1. Discharge requirement is, at least 50% of CO2 discharge to be carried out in 1 minute and at least 85% discharge in 2 minutes.
2. Capacity of CO2 in the system to be, 1.) 30% of the gross volume of the largest protected cargo space, 2.) 40% of the gross volume of machinery space excluding engine casings, 2.) 35% of the gross volume of machinery space including engine casings for vessels GT < 20000. Total amount of CO2 cylinders depends on the highest gross volume out of above 1,2,3 in a particular ship.
3. Safety procedures must be there against unauthorized use of the system.
4. Machinery space to be fitted with audio-visual alarm and ventilation blower trip.
5. Alarm must trigger well before operation of CO2 flooding system.
6. Permanent piping arrangements should be made.
7. Manifold, distribution piping to be pressure tested. See Pressure Testing of CO2 Flooding System below.
8. Diameter of associated pipe lines in the system should not be less than 20 mm.
9. Copper and flexible pipes are allowed between CO2 cylinder and common manifold.
10. Distribution pipes to cargo spaces should not pass through engine room.
11. All stop valves to be checked every month to ensure their working and position.
12. The CO2 flooding system installation to be checked monthly for any leakages.
13. All control valves to be tested annually.
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