CO₂ Total Flooding Systems

CO₂ total flooding systems are based on creating an extinguishing concentration of CO₂ within an enclosed space containing the combustible materials. The quantity of CO₂ is determined by applying an appropriate flooding factor to the volume being protected.

The efficiency of a total flooding system depends upon maintaining the concentration for as long as possible, so before total flooding can be considered as a method of extinguishing, the protected space must be reasonably well enclosed. It is always advisable for an integrity test to be conducted to verify the rate of leakage.

A fixed supply of CO₂ is permanently connected to fixed piping and discharge nozzles are arranged to discharge CO₂ into the protected space.

Examples of Hazards

Rooms, vaults, enclosed machines, ovens, dust collectors, floor and ceiling voids and fume extraction ducts.

Type of Fires

Fires that can be extinguished by total flooding methods are:

(a)    Surface fires that can be extinguished quickly, such as those involving flammable liquids and vapours.

(b)    Deep seated fires that require cooling time in order to be extinguished, e.g. fires involving bulk paper and other solids.

Where CO₂ is NOT Effective

(a)    Materials that contain their own oxygen supply and liberate oxygen when burning, e.g. cellulose nitrate.

(b)    Reactive metals e.g. sodium, potassium, magnesium, titanium, zirconium, uranium and plutonium.

(c)     Metal hydrides.

While CO₂ may not extinguish these fires, it will not react dangerously or increase the burning rate. CO₂ will protect adjacent combustibles and will also extinguish fires of other materials in which the reactive metals are often stored.

Example:

·         Sodium stored or used under Kerosene.

·         Cellulose nitrate in a solvent.

·         Magnesium chips covered with heavy oil.

Volume of CO2 required (for surface fires only)

Volume of Space (in m3)		Volume Factor kg CO2/m3)	Calculated Minimum kg
>4 >14 >45 >126 >1400	<4 <14 <45 <126 <1400	1.15 1.07 1.01 0.90 0.80 0.74	4.5 16.0 45.0 110.0 1100.0

Example:

Room: 6 m x 9 m x 3m = 162 m³

162 m3 x 0.80 kg/m³ = 129.6 kg

Uncloseable Openings

Openings shall be arranged to close automatically before or simultaneously with the start of the CO₂ discharge. This can be done by self-closing door devices, fire curtains or steel shutters.

If it is not possible to seal the opening it is permissible for small openings to remain open provided they do not exceed the limits shown below, and are compensated by the addition of extra carbon dioxide.

Limits of Uncloseable Openings

The maximum area permitted is the smaller result of the following calculations:

(a)    An area in square metres, which is numerically equivalent to 10% of the volume in cubic metres.

(b)    10% of the total area of all sides, top and bottom in square metres.

When uncloseable openings exceed this limitation, the system should be designed by a local application method.

Compensation

Additional gas at the rate of 5 kg/m2 of opening.

Where necessary this quantity should be multiplied by the appropriate Material Conversion Factor (MCF).

The additional quantity should be discharged through the regular pipework system and the flow rate increased accordingly so that the additional quantity is discharged within the time.

Material Conversion Factor

For materials requiring a design concentration over 34%, the basic quantity of carbon dioxide calculated, i.e. the result of using Table 1, plus the addition for losses through limited openings, shall be increased by multiplying this quantity by the appropriate conversion factor.

The most hazardous material in the enclosure must be selected no matter what the quantity of that material.

Example:

Room: 6 m x 9 m x 3 m high = 162 m³

162m3 x 0.80kg/m³ = 129.6kg

Uncloseable opening = 1.0 m2 = 5.0 kg

Basic quantity = 134.6 kg

If room contains butadiene as the most hazardous material: MCF = 1.3

134.6 kg x 1.3 = 175kg

Temperature Correction

Additional quantities of CO₂ are needed to compensate for the effects of abnormal temperature. Hazards which operate at temperatures above 100°C may be more likely to re-ignite so it is necessary to hold the extinguishing concentration for a longer period to assist cooling.

Add 2% carbon dioxide for each 5°C above 100°C.

Example:

Oven: 3 m x 1.5 m x 1.8 m = 8.1 m3

If the normal working temperature is 204°C:

204-100 = 104/5 = 20.8

20.8 x 2% = 41.6%

8.1m3 x 1.07 kg/m3 = 8.66 (basic quantity) x 1.416 (temp correction)

= 12.26 kg

CO₂ has a lower expansion ratio at lower temperatures so it will be denser and leakage would be greater than normal.

Where the normal temperature of the enclosure is below -20°C, add 2% of CO₂ for each 1°C below -20°C.

Example:

Refrigerated space:

3 m x 6 m x 3 m = 54 m3 with a normal operating temperature of -23°C.

23°C - 20°C = 3°C x 2% = 6%

54 m3 x 0.90 kg/m3 = 48.6 kg (basic quantity) x 1.06 (temp correction)

= 51 .5 kg

If an addition has been made to the basic CO2 quantity to compensate for openings or application of an MCF, the total quantity should be used in place of the basic quantity in the above examples.

Forced Ventilation

When forced air ventilation systems are used, they shall, if possible, be shutdown before, or simultaneously, with the start of the CO₂ discharge. If this cannot be done, additional CO₂ must be applied.

If there is a short run-down time but the quantity of air removed is significant, additional CO₂ must be applied. The additional CO₂ must be discharged within the time specified.

For calculation purposes the volume of air removed in one minute will be replaced with CO₂ at the design concentration being used.

Example:

Assume the room has 30 m³ of air removed by the ventilation system in one minute.

30 m³ x 0.80 kg/m³ = 24 kg x 1.3 (MCF) = 31.2 kg + 175.0 kg (original) = 206.2 kg

Services such as heating, fuel supplies, paint spraying, conveyors etc. must also be shutdown before or simultaneously, with the CO₂ discharge.

Interconnected Volumes

In two or more interconnected volumes where free flow of CO2 can occur, the CO2 quantity shall be the sum of the quantities calculated for each volume, using its respective volume factor. If one volume requires greater than normal concentration, the higher concentration shall be used for all interconnected volumes.

Venting for Surface Fire Systems

Leakage around doors and windows often provides sufficient pressure relief without special arrangements being required. It is possible to calculate the area of free venting needed for very tight enclosures but it is recommended you provide the customer with the formula and CO2 flow rate so that his architect can take the responsibility.

where:

X is the free venting area (in mm²).

Q is the calculated carbon dioxide flow rate (in kg/min).

P is the permissible strength (internal pressure) of enclosure (in bar).