Calculation of CO2 System

GENERAL
Fire-extinguishing medium
Where the quantity of the fire-extinguishing medium is required to protect more than one space, the quantity of medium available need not be more than the largest quantity required for any one space so protected.
The volume of starting air receivers, converted to free air volume, shall be added to the gross volume of the machinery space when calculating the necessary quantity of the fire-extinguishing medium.
Alternatively, a discharge pipe from the safety valves may be fitted and led directly to the open air.
Means shall be provided fore the crew to safely check the quantity of the fire-extinguishing medium in the containers.
Containers for the storage of fire-extinguishing medium and associated pressure components shall be designed to pressure codes of practice to the satisfaction of the Administration having regard to their locations and maximum ambient temperatures expected in service.

Installation requirements
The piping for the distribution of fire-extinguishing medium shall be arranged and discharge nozzles so positioned that a uniform distribution of the medium is obtained.
Except as otherwise permitted by the Administration, pressure containers required for the storage of fire extinguishing medium, other than steam, shall be located outside the protected spaces.

Spare parts for the system shall be stored on board and be to the satisfaction of the Administration.

System control requirements
The necessary pipes for conveying fire-extinguishing medium into the protected spaces shall be provided with control valves so marked as to indicate clearly the spaces to which the pipes are led.

Suitable provision shall be made to prevent inadvertent release of the medium into the space. Where a cargo space fitted with a gas fire-extinguishing system is used as a passenger space, the gas connection shall be blanked during such use. The pipes may pass through accommodations providing that they are of substantial thickness and that their tightness is verified with a pressure test, after their installation, at a pressure head not less than 5N/mm². In addition, pipes passing through accommodation areas shall be joined only by welding and shall not be fitted with drains or other openings within such spaces.
The pipes shall not pass through refrigerated spaces.

Means shall be provided for automatically giving audible warning of the release of fire-extinguishing medium into any ro-ro spaces and other spaces in which personnel normally work or to which they have access. The pre-discharge alarm shall be automatically activated (e.g., by opening of the release cabinet door). The alarm shall operate for the length of time needed to evacuate the space, but in no case less than 20s before the medium is released.

Conventional cargo spaces and small spaces (such as compressor rooms, paint lockers, etc.) with only a local release need not be provided with such an alarm.
The means of control of any fixed gas fire-extinguishing system shall be readily accessible, simple to
operate and shall be grouped together in as few locations as possible at positions note likely to be cut off by a fire in a protected space. At each location there shall be clear instructions relating to the operation of the system having regard to the safety of personnel.

Automatic release of fire-extinguishing medium shall not be permitted, except as permitted by the
Administration.

Carbon Dioxide as a Fire Extinguishing Agent
The advantages of carbon dioxide gas for fire extinguishing purposes have been long known. As early as 1914, the Bell Telephone Company of Pennsylvania installed a number of seven pound capacity portable CO2 extinguishers for use on electrical wiring and equipment. By the 1920's, automatic systems utilizing carbon dioxide were available. In 1928, work on the NFPA Standard for carbon dioxide extinguishing systems was begun.

Carbon Dioxide and the Fire Triangle
The mechanisms by which carbon dioxide extinguishes fire are rather well known. If we go back to the familiar fire triangle, we realize that an interaction between fuel, oxygen and heat is necessary to
produce a fire condition. When these three elements are present in a proper relationship, fire will result.
Carbon dioxide extinguishes fire by physically attacking all three points of the fire triangle. The primary attack is on the oxygen content of the atmosphere. The introduction of CO2 into the fire zone displaces sufficient oxygen in the atmosphere to extinguish the open burning. At the same time, the extinguishing process is aided by a reduction in the concentration of gasified fuel in the fire area. And finally, CO2 does provide some cooling in the fire zone to complete the extinguishing process.

With a surface-type fire, that is, a fire which has not heated the fuel to its auto-ignition temperature much beyond the very surface of that fuel, extinguishment is rapid. Such surface fires are usually the case when liquid fuels are involved. Unfortunately, there is no guarantee that all hazards will produce surface fires.
In fact, a great many hazards are more likely to produce fires which will penetrate for some depth into the fuel. Such fires are commonly referred to as deep-seated. When dealing with a so-called deepseated potential, it is necessary not only to remove the oxygen and decrease the gaseous phase of the fuel in the area, but it is equally important to permit the heat which is built up in the fuel itself to dissipate.
If the heat is not dissipated and the inert atmosphere is removed, the fire may very easily reflash. For
such hazards, it is necessary to reduce the concentration of oxygen and gaseous fuel to a point where
not only is the open flaming stopped, but also any smouldering is eliminated. To accomplish this, the
concentration of agent must be held for a sufficiently long time to permit adequate dissipation of built-up heat. The NFPA Standard 12 on carbon dioxide systems has long been a leader in prescribing thorough and conservative fire protection. The standard requires a mandatory 20-minute holding time, or soaking time, for any potentially deep-seated fire hazard. What this means is that the inerting concentration of carbon dioxide shall be maintained in a deep-seated hazard for a minimum of 20 minutes in order to permit cooling and complete extinguishment.

Application Methods of Carbon Dioxide in Fire Extinguishment
Over the years, two methods of applying carbon dioxide have been developed as follows :
- Total flooding application
- Local application.
As we are dealing with total flooding CO2 fire extinguishing system here, only the description on CO2 fire extinguishing system will be covered here.
The total flooding technique consists of filling an enclosure with carbon dioxide vapour to a prescribed concentration. This technique is applicable both for surface-type fires and potential deep-seated fires.

CO2 Concentration For Total Flooding System As Required by SOLAS
Cargo Spaces
For cargo spaces the quantity of carbon dioxide available shall, unless otherwise provided, be sufficient to give a minimum volume of free gas equal to 30% of the gross volume of the largest cargo space to be protected in the ship.

Machinery Spaces
For machinery spaces the quantity of carbon dioxide carried shall be sufficient to give a minimum volume of free gas equal to the larger of the following volumes, either:

1. 40% of the gross volume of the largest machinery space so protected , the volume to exclude
that part of the casing above the level at which the horizontal area of the casing is 40% or less of the
horizontal area of the space concerned taken midway between the tank top and the lowest part of the
casing; or

2. 35%of the gross volume of the largest machinery space protected, including the casing.

The percentages specified in paragraph 1. and 2. above may be reduced to 35% and 30%, respectively, for cargo ships of less than 2,000 gross tonnage where two or more machinery spaces, which are not entirely separate, are considered as forming one space.

For the purpose of this paragraph the volume of free carbon dioxide shall be calculated at 0.56m³/kg.

For machinery spaces the fixed piping system shall be such that 85% of the gas can be discharged into the space within 2 min.

 Diagramatic of CO2 system

CO2 Fire Extinguishing System for Marine and Offshore Applications
Fire extinguishing systems for marine and offshore applications can be divided into 2 main types :
- High pressure fire extinguishing system
- Low pressure fire extinguishing system

High Pressure CO2 Fire Extinguishing System
The term high pressure system is used because the CO2 gas is compressed and stored inside high
pressure cylinders of either 67.5L or 80L capacity seamless steel high pressure cylinders. These
cylinders are then connected by manifolds and led via distribution piping to the spaces which are
protected by the system.

Low Pressure CO2 Fire Extinguishing System
The term low pressure system is used because the CO2 gas is stored inside refrigerated low pressure
tanks where the pressure is the CO2 liquid is maintained at low pressure. The low pressure CO2 is led via outlet piping to the spaces which are protected by the system.

Both high pressure and low pressure systems had been used for offshore and marine applications, but because low pressure system can only be economical, as compared to high pressure system, when the space to be protected requires more than 500 high pressure cylinders, and due to high running and
maintenance costs for low pressure systems, high pressure systems are mostly used for fire extinguishing systems

High Pressure CO2 System – High Pressure CO2 cylinders
Design and equipment
1. In respect of their material, manufacture, type and testing, CO2 cylinders must comply with
the following requirements:
Gas cylinders are bottles with a capacity of not more than 150 l with an outside diameter of
<420 mm and a length of <2000 mm which are charged with gases in special filling stations
and are thereafter brought on board ship where the pressurized gases are used.

These Rules are not valid for gas cylinders with
– a maximum allowable working pressure of maximum 0,5 bar, or
– a capacity <0,5 l.

These Rules are only valid in a limited range for gas cylinders with
– a maximum allowable working pressure of maximum 200 bar and
– a capacity > 0,5 l and < 4 l

2. CO2 cylinders may normally only be filled with liquid CO2 in a ratio of 2 kg CO2 to every 3
litres of cylinder capacity. Subject to the shipping route concerned, special consideration may be
given to a higher filling ratio (3 kg CO2 to every 4 litres capacity).

3. Cylinders intended for flooding boiler rooms, machinery spaces as well as cargo pump and
compressor rooms are to be equipped with quick-opening valves for group release enabling
these spaces to be flooded with 85 % of the required gas volume within two minutes. Cylinders
intended for the flooding of cargo spaces need only be fitted with individual release valves.
For cargo spaces for the carriage of motor vehicles with fuel in their tanks and for ro/ro spaces
CO2 cylinders with quick-opening valves suitable for group release are to be provided for
flooding of these spaces within 10 minutes with 2/3 of the prescribed quantity of CO2.

4. Cylinder valves are to be approved by a recognized institution and be fitted with an
overpressure relief device 19.

5. Siphons are to be securely connected to the cylinder valve.

Disposition
1. CO2 cylinders are to be stored in special spaces, securely anchored and connected to a
manifold.
Check valves are to be fitted between individual cylinders and the manifold. If hoses are used to
connect the cylinders to the manifold, they are to be type approved.
2. At least the cylinders intended for the quick flooding of boiler rooms and machinery spaces
are to be grouped together in one room.
3. The cylinders for CO2 fire extinguishing systems for scavenge trunks and for similar purposes
may be stored in the machinery space on condition that an evidence by calculation is provided
proving that the concentration of the free CO2 gas (in case of leakages at all cylinders provided)
relative to the net volume of the engine room does not exceed 4 %.

The diagram below shows the corresponding pressure at different temperatures, under
different Filling Ratios (which is the amount of CO2 filled (kg) divided by the capacity (dm2) of
the high pressure CO2 cylinders.

It is important to know the filling ratio as this will determine the maximum temperature that the
cylinders can be stored, because the high pressure cylinders stored inside a CO2 Room as
required by the Rules.

The CO2 Fire Extinguishing System is calculated in full compliance with latest SOLAS, flag Authority and Classification Society’s requirements.

Quantity of CO2 gas calculation:
- Quantity of free CO2 gas is calculated at filling density of 0.56 m3/kg
- Mixing ratio of CO2 gas based on the gross volume of different type of protected space:
1. Machinery space excluding casing* = 40%
2. Machinery space including casing* = 35%

Note:
Point 1, 2 provided that percentages may be reduced to 35% and 30% respectively for cargo
ships of less than 2,000 gross tonnage.

For engine room, select the largest quantity from 1 or 2.

Quantity of CO2 gas (kg) = Gross volume of space (m3) x Mixing ratio
                                                                  0.56 (m3/kg)

Note
* Casing is defined as that part of the engine room above the level at which the horizontal area is 40% or less then the horizontal of the engine room, taken midway between the tank top and the lower part of the casing.
** Cargo spaces including ro-ro deck should be calculated with mixing ratio of 45%.

Therefore required quantity of 45 kg CO2 cylinder for engine room = 8

 Tabel 1. Sample Calculation of CO2 Capacity in Engine Room

Remark:
1. The CO2 cylinders and equipments are to be located in dedicated CO2 rooms, constructed
according to class requirements.
2. The weight of each CO2 bottle above tabel are choosen 45kg.
3. The CO2 system is a central bank system and the number of CO2 cylinder is calculated based
on the largest protected area.


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