DESCRIPTION AND APPLICATION

The carbon dioxide gas fire suppression systems belong to the Clean Agent category because carbon dioxide is a gas stored in cylinders in a liquid state and is delivered through special nozzles producing its vaporisation.

Traditionally these systems are treated differently from other systems probably due to the physical and chemical behaviour of the CO2 changing directly into gaseous and solid state.

As a complete oxidized substance, carbon dioxide does not burn and by reducing the oxygen level in the atmosphere tends to suffocate fire. Furthermore it is electrically non conductible and does not leave slag, so it is a good fire suppressor for electrical and electronic equipments. When delivered on a burning surface, it also exercises a cooling effect combined with the fire suppression effect. Its use is not recommended on materials sensitive to sudden temperature variations. Since it is volatile, if used outdoor, it is effective for small fires only.

The prohibition of Halons has strongly favoured the use of carbon dioxide systems.

The equipment is made of storage (cylinders) of pressurised C02 connected through valves to a pipe network reaching final destination (collector or distribution pumps). At the end of the pipes, nozzles provide a blanket of gas.
Inside the pipes there are different conditions of temperature and pressure so both vapour and liquid are present, thus making difficult the calculation of loss heads and the planning of the system.


The cylinders are only partially loaded with liquid in order to allow the expansion due to variation of temperature. For security reasons before the head valve a rupture disc is fitted that will burst in case of an occurring overpressure.

The colder is C02 in the cylinders the longer it takes to evaporate, so according to standard NFPA 12 the temperatures of assembled rack cylinders must stay between 0 and 50 °C so as to keep them at the normal pressure of 55 -60 bar.

Carbon dioxide suppression systems exist both in the high pressure version (with the extinguishing agent stored in cylinders at a pressure of up to 70 bar) and in the version low pressure (with the extinguisher stored in special refrigerated tanks at 18 bar that are supposed to feed the plants).

The low pressure plants are used for big systems with refrigerated tanks kept at the temperature of - 18 °C corresponding to a pressure of 20 bars.

The applications of these systems are manifold, the more so for machines using inflammable products such as rotary presses.

The main limitation is due to dangers provoked by the use of C02 at the necessary concentration which causes stifling problems to people for insufficient oxygen content.
This fact has practically banned its use in our Country in all places normally occupied by people.

The carbon dioxide systems can be of two kinds:

Total saturation;
Local application;
Mixed type.

Total expansion carbon dioxide systems

They are made of the following parts:

Cylinder rack assembly, with possibly one spare, filled with C02 with a flood pipe and selected valves, with a
remote control and a mechanical local control for quick release;
Collector channelling the carbon dioxide in the distribution network;
Pressure trip installed on the collector indicating to the control station the release of the cylinders;
Distribution network pipes;
Total flood nozzles placed in the areas to be protected (under the ceiling of electric cabins, in underground
passages of cables, in rooms with electric cabins etc);
A control station from which the release signal is launched;
Acoustic and optical signs fitted inside and outside the premises to be protected;
Rack assembly with springs and electric relay to indicate empty cylinders.

They are normally operated by a detector fire system sending a signal to the control station from which the release of the cylinders is operated. When the extinguisher is sent into rooms where people can be present, the opening command has a time delay to allow evacuation.

The control station together with the opening command also sends a signal to stop all ventilation systems and to shut all the openings so as to keep the concentration level in the room where the extinguisher is operating. In fact for these systems based on total saturation the sealing of premises is fundamental. The openings located in a high position do not endanger the saturation because are useful for the exit of air.

The arrival of cold C02 first creates a they depression but subsequently the mixture air-carbon dioxide absorbs heat from the walls and a significant increase of volume is obtained in a sealed condition of the rooms.

Carbon dioxide suppression systems in local applications

They are used where fire hazards are limited to small equipments placed in larger rooms and are of two types:

Systems made of a rack assembly of cylinders fitted with a flood pipe and rapid opening valves, of a
conveyance collector and of fixed output pipes ending with low speed nozzles;
Systems supplying a hose reel fitted at the end with an interception valve and ending with low speed nozzles
cone shaped as those of fire extinguishers.