CO2 flooding system or central bank CO2 system 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 is an effective fire suppression agent applicable to a wide range of fire hazards. 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.
There are several advantages for the CO2 as a fire fighting medium. They are:
- 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- conductor of electricity
- No residues left after application
- No deterioration with age
Similarly there are some disadvantages also, such as:
- 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.
Explanation of CO2 Flooding System
As mentioned earlier CO2 flooding system floods the protected space under fire with carbon dioxide, which displaces air, thereby removing one leg of fire triangle for the extinction of fire. CO2 flooding system consist of main CO2 bottles, common manifold, master valve or distribution valve and distribution pipe lines with nozzles as shown in the figure below. Click on the diagram to enlarge.
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. CO2 from the main bottle is released by a ‘CO2 release cabinet’ as shown in the figure. 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.
As you can see in the diagram, 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.
CO2 flooding system explained here protects only a single space. There are systems which can protect multiple spaces such as engine room and pump room together. In that case separate release cabinets will be there for engine room and pump room. Main CO2 bottles are shared for engine room and pump room according to the volume of the spaces.
Time Delay Unit in CO2 Flooding System
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.
- 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.
- 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.
- Regulation per International Code for Fire Safety System (FSS Code) 126.96.36.199 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 as shown in the figure below.
Power source is always available for the unit. Solenoid valve is normally in closed position. When CO2 from pilot cylinder (CO2 release cabinet) reaches the time delay unit, the pressure acts on the pressure switch. This closes the pressure switch. But the timer allows power supply to the solenoid only after 60-90 seconds, the timing can be adjusted as required. As soon as the specified time is reached, solenoid becomes energize and the solenoid valve will open. This allow passage of CO2 to the pressure operated cylinder valve of main bottle. Normally a by pass valve is also fitted across this time delay unit which can be used in case the unit goes defective.
Another type of time delay unit uses an empty bottle fitted just before the head assembly. This empty bottle fills first before supplying pressurised CO2 to the head assembly. This is more reliable since there is no electrical components involved. Figure below 000000shows two time delay units, one for engine room and the other for purifier room.
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.
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 as shown in the figure above. 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. For better understanding have a look at the pressure- temperature relationship at different filling densities from the graph below.
Function of 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 as shown in CO2 line diagram.
Why Relief Valve is Required?
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. Again, set pressure varies with the system design and regulations. A relief valve is required for two purposes:
- If the pressure of fluid in a pipe line increases beyond the designed working pressure, there must be a pressure relief mechanism for bringing down the pressure to safe guard the system. In the case of CO2 flooding system, CO2 will accumulate in the manifold when it is released from the bottle and master valve is closed. Pressure of CO2 is 55 bar at 20 degree Celsius. As CO2 takes temperature from surroundings, its pressure also increases to dangerous levels. Since these pipes are pressure tested to 190 bar only, a relief valve is necessary in the manifold.
- Any pressure accumulate in the manifold may release other CO2 bottles which are intact, if the non return valve between bottle and manifold is damaged.
What is the Purpose of 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
- Discharge requirement is, at least 50% of CO2 discharge to be carried out in 1 minute and at least 85% discharge in 2 minutes.
- 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.
- Safety procedures must be there against unauthorized use of the system.
- Machinery space to be fitted with audio-visual alarm and ventilation blower trip.
- Alarm must trigger well before operation of CO2 flooding system.
- Permanent piping arrangements should be made.
- Manifold, distribution piping to be pressure tested. See Pressure Testing of CO2 Flooding System below.
- Diameter of associated pipe lines in the system should not be less than 20 mm.
- Copper and flexible pipes are allowed between CO2 cylinder and common manifold.
- Distribution pipes to cargo spaces should not pass through engine room.
- All stop valves to be checked every month to ensure their working and position.
- The CO2 flooding system installation to be checked monthly for any leakages.
- All control valves to be tested annually.
Calculation of CO2 Required
- Quantity of free carbon dioxide calculation at 0.56 m3/ kg
- Mixing ratio of CO2 based on the volume of protected spaces to be as follows:
a) Gross volume of engine room space including casing = 35%
a) Gross volume of engine room space excluding casing = 40%
a) Gross volume of purifier room space = 40%
- Quantity of CO2 in kg = (GROSS VOLUME x MIXING RATIO) / 0.56
|NAME OF SPACE||GROSS MOLUME (M3)||QUANTITY OF CO2 (KG)||QUANTITY OF 45 KG CYLINDER|
|ENG ROOM SPACE INCL. CASING||7324||4577.5||102|
|2||ENG ROOM SPACE EXCL. CASING||6856||4897.14||
|3||PURIFIER ROOM SPACE||132||94.2||
Therefore required quantity of 45 kg CO2 cylinder for engine room = 109
Note: The air reservoir in engine room space is not included in the above calculation since the outlet of safety valve shall be led to atmosphere outside engine room.
Pressure Testing of CO2 Flooding System
Low Pressure Section
|Branch Pipe to Nozzles||Galvanized Seamless Manganese Steel||7 Bar Air Blowing Test|
Medium Pressure Section
|Master Valve to Main Pipe Branches||Galvanized Seamless Manganese Steel||
80 Bar Pressure Test
|High Pressure Section||CO2 Cylinder to Master Valve||Galvanized Seamless Manganese Steel||
190 Bar Pressure Test
|Pilot Control Line||Control Lines||Copper Tube||
58 Bar Pressure Test
Maintenance of CO2 Flooding System
Fixed carbon dioxide fire extinguishing systems should be kept in good working order and readily available for immediate use. Maintenance and inspections should be carried out in accordance with the ship’s maintenance plan having due regard to ensuring the reliability of the system. The on board maintenance plan should be included in the ship’s safety management system and should be based on the system manufacturer’s recommendations.
- Alarms, Machinery trips, Door alarm, Quick closing valves, Flaps, Skylights, Ventilation fan trips to be tested.
- General inspection of the installation, pipe lines to be checked.
- Verify CO2 contents by mechanical advantage lever, ultrasonic liquid level measurement or radioactive liquid level measurement. Equipment for measuring CO2 content must be available on board ships. A maximum reduction of 5% in CO2 content may be allowed provided total CO2 quantity on board is not less than the required amount.
- Blow through the system with 6-7 bar air with plastic air bags at the nozzle ends.
- The servo cylinders and any remote controlled stop valves to be tested by opening one pilot cylinder. The main CO2 bottles must be uncoupled for this purpose.
- Spring loaded relief valves to be checked and pressure tested at 180 bar.
- Checking of HP alarms which operate with gas pressure.
- Section of CO2 which can be shut off must be tested with air at 25 bar.
- CO2 bottles to be inspected internally. Siphon tube or dip tube to be checked. Cylinders to be pressure tested at 250 bar. If permanent change in volume is observed, those cylinders to be discarded. After 20 years of installation of CO2 flooding system, these tests to be carried out every 5 years thereafter.
15 yearly and 5 year thereafter
- Pressure test HP pipes, stop valves to engine room, pump room, purifier room,etc. at 190 bar (hydraulic), medium pressure pipes at 80 bar (hydraulic) and low pressure pipes at 7 bar air.
- After hydraulic pressure testing, lines to be blown through by dry air.
Service chart sample used by marine engineers for CO2 flooding system on ships is shown below. Click on the diagram to enlarge.
Procedure of Releasing CO2 to the Engine Room on Ships
- Muster and take head count.
- Master will take decision, depending upon the situation and as per the company policy.
- If total CO2 flooding of engine room is to be done, master will consult with chief engineer.
- Nearest coast guard to be informed as soon as possible.
- Ensure proper sealing of the engine room.
- Make sure emergency generator is running and on load. Power is required for running emergency fire pump for boundary cooling.
- All access doors, vent flaps, blower flaps, skylights, hatches, fire flaps, to be closed.
- Quick closing valves to be closed.
- Emergency stops for lube oil pumps, fuel oil pumps to be operated.
- All machinery in engine room to be stopped.
- Ensure all personnel vacated engine room.
- CO2 must be released by competent engineer.
- Break the glass and take out key for CO2 release cabinet from key box. Using local CO2 release cabinet is preferred.
- Open the release cabinet. Audio visual alarms will sound and ventilation blower trips will activate.
- In the CO2 release cabinet, open pilot cylinder valve first. Now open valve 1 for master valve first. Then open valve 2 for CO2 releasing mechanism. Co2 will be released after 60-90 seconds of time delay.
- Now the system pressure can be checked from the pressure gauge on the manifold.
- If CO2 is not released, then follow emergency releasing procedure. Open master valve manually and open each CO2 main bottle by manual actuation lever.
Important Notes on CO2 Flooding System
- CO2 flooding is the final course of action, used only when all else has failed. Proper sealing of the engine room is essential for effective extinction of fire. There were incidents like CO2 is released, effective smothering did not occur due to improper sealing of engine room and fire remained unextinguished.
- To obtain a total flooding of engine room, about 35% by volume of CO2 is required in 2 minutes. This will reduce the oxygen content of the air in engine room to less than 15% to extinguish the fire. At this CO2 concentration human life cannot be supported.
- Typically it takes about 15-20 seconds after release of CO2 before the concentration in engine room reaches a dangerous level.
Safety Precautions After CO2 Release
After CO2 flooding system is operated efficiently, engine room fire will extinguish by smothering action. But before engine room entry is made or space ventilated, it is recommended to obtain expert advice from shore. Nearest coast guard can be contacted for getting assistance for the same.
- The first question is, how to make sure that CO2 is actually released after the operation of CO2 flooding system. When Co2 is released there will be a loud noise of gas escaping to the protected space. CO2 bottles can be felt cold after releasing. Visual inspection of the operation of, pressure operated cylinder valves also can be carried out.
- CO2 has very little cooling effect. So there is a danger of re-ignition of fire when engine room is ventilated immediately. Keep the boundary cooling running to reduce the temperature of the engine room.
- Ventilation of engine room should not be started until it has been definitely established that the fire has been extinguished completely, which will take several hours.
- Engine room to be sufficiently ventilated before entering.
- Entry to be carried out by trained personnel wearing breathing apparatus.
- Even after fire has been extinguished completely, never bring bare flame such as candle light or lighted cigarette into the burnt room, other wise fire may break out again due to explosion of combustible gases, if any.
- In order to allow persons to get away quickly and safely in case of fire, entrances and exits shall be always kept clear.
- Back up team or support team to be ready in case any difficulties happens in side.
- An attendant should be instructed to remain at the entrance of the engine room.
- An agreed and tested system of communication to be established between attendant and team inside engine room.
- In case any emergency occur to the team inside engine room, the attendant is not supposed to enter inside before the help has arrived.
- In the event of ventilation system fails, the personnel in the space should leave immediately.
MEO Oral and Written Examination Questions and Answers on CO2 Total Flooding System on Ships
Why do you make CO room ventilation ? Where ?
- To remove the leakage CO2 from the CO2 room.
- CO2 gas is heavier than air and does not support to human life (Suffocate).
- CO2 room must be well ventilated before entry
- CO2 room is situated outside the machinery space usually at main deck and it must be a separate compartment.
A short note on CO2 room ventilation arrangement ?
- It is made by motor driven exhaust blower or fan, which suction is drown from CO2 room floor, because CO2 is heavier than air and leakage CO2 can accumulate on the CO2 room floor.
- Fan control switch is fitted outside the CO2 room, at the entrance.
Briefly describe carbon dioxide flooding system on ships ?
- Steel cylinder, 67 liters capacity, each charged with 45 kg liquid CO2 under pressure of 55 bar.
- Provided with safety disc, bursts at a pressure of 175 to 195 bar.
- Quick release or total flooding cylinders are arranged that 85% of the capacity can be released within 2 minutes.
- Can be discharged individually.
- Manual release mechanism for individual release.
- Same manifold for different groups (engine room, pump room, etc.)
- The liquid when released produces about 450 times its original liquid volume in the gas form.
- One kg of liquid CO2 can produce 0.56 cubic meter gas.
- To be effective in smothering a fire the total carbon dioxide gas to be obtained at least 30% of the gross volume of the largest room of the ship and the total flooding system must release to obtain 40% of the gross volume of the machinery space and held until the fire is out.
- In the case of CO2 flooding system for cargo holds, the visual and audible smoke detecting cabinet (with smoke detectors) is located in the CO2 room and two exhaust fans are installed in the water tight galvanized steel box on top of it. The three way valves, under the smoke detecting cabinet, are normally closed to CO2 line and the blower sucks air from the holds through smoke detector will be closed and CO2 can be discharged to the required cargo hold.
What are the safety devices on CO2 flooding system ?
- Master valve with alarm switch.
- Relief valves at manifold.
- Stop valve and pull handle are in lock release cabinet and alarm switch.
- Safety bursting disc at each CO2 bottle.
- Leakage detecting pressure switch on manifold.
- Non return discharge valves after CO2 bottles.
What will you do for a cargo hold fire ?
- Remote detector fitted at the CO2 room can detect smoke on concerned cargo space.
- This operation must be done by master’s order.
- After ensuring no person left in cargo space, seal off the cargo space (closing of ventilation fan, fire damper, and hatch cover), operate 3 way valve for the concerned hold and manually open required CO2 bottles.
What are the requirements of CO2 bottles ?
- All bottles stamped at 52 bar pressure.
- Bursting disc fitted, operates at 177~ 193 bar at 63 °C
- Store in temperature less than 55 °C
- Recharge if 5 % loss.
- Clamped against movement and vibration(by wooden plank).
- Remote and manual operation possible.
- Hydraulically tested to 228 bar.
- Level tested (by radio active level indication).
- if > 10 years internal and external examination required.
What are the limitations on CO2 properties ?
- CO2 is heavier than air
- Less cooling effect
- Static electricity induced
- Heavily asphyxiating (suffocating)
- Ineffective if > 10 ~ 12 minutes
- “One off” no reserve
- Total evacuation required
- Possibility of thermal shock
- No protection of personal
- Expands 450 times its volume in liquid to produce gas.
What are general inspections carried out in CO2 room ?
- Check emergency light and all other lights.
- Check exhaust fan / ventilation.
- Check all bottles overall condition, clamps, valves etc.
- Check operating wire condition.
- Check CO2 alarms.
- CO2 room key should be in position.
- Check the operating instructions.
- Inspection to be recorded in log book and Saturday safety routine book.
What are survey items on CO2 flooding system on ships ?
- Check CO2 weight every 2 years
- Testing of cylinder at 228 bars
- Blow through the lines
- General inspection on Instructions, Key, Emergency lights, Ventilation, Alarms etc.
What are the CO2 room safety arrangement ?
Exhaust fan and suction duct
- Provided at the bottom of the CO2 room.
- Any accumulated CO2 from leakage at the bottom can be exhausted to atmosphere.
- Reason explained above “Why Relief Valve is Required ?” section.
- Fitted on connection pipe between each cylinder discharge valve and manifold, so that leakage of one cylinder cannot effect other cylinder.
- Each bottle has a combined bursting disc, which will rupture spontaneously at a pressure of 177 bar at 63°C
Pressure gauge and pressure alarm in the mainfold
- Reasons explained above.