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Design of fire fighting systems with carbon dioxide Calculation of necessary quantitiesNFPA 12 is perhaps the most widely accepted standard f or the design, installation, operation and maintenanceof f ire f ighting systems using carbon dioxide as the extinguishing medium. This standard deals with two (2)types of systems:

- high pressure systems: In these systems, carbon dioxide is stored in pressure containers (cylinders) atambient temperatures. High pressure systems are mostly used nowadays.

- low pressure systems: In these systems, carbon dioxide is stored in pressure containers at a controlled lowtemperature of 0 degF (or -18 degC). Low pressure systems are used in special applications, especially whenwe want to maximize the density of f ire f ighting medium per storage space, like f or example f or f ire- f ightingpurposes of gas turbines enclosures.

Due to its toxicity, carbon dioxide is not to be used in normally occupied spaces like of f ices, libraries, computerrooms etc. However, it is widely used f or f ight f ighting purposes in unoccupied and/or remote switchgearrooms, battery rooms, data rooms, cable tunnels.

Basic things to consider during design of a carbon dioxide f ire f ighting system

Af ter we determine which areas will be f ire protected with carbon dioxide, we must estimate the most probabletype of f ire that will develop upon f ire init iation. Based on NFPA 12, two (2) major types of f ire exist:

- surf ace f ires

- deep-seated f ires

Surf ace f ires are relatively easy to extinguish. They are mostly f ires which are caused by f lammable solids, gasor liquids.

On the other hand, deep-seated f ire are more dif f icult rivals. Deep-seated f ires are mostly smoldering f ires,like f or example a cable f ire. Bigger quantit ies of carbon dioxide are necessary f or their extinguishmentcompared to surf ace f ires, since exposed material will also have to be cooled to a temperature that will notallow its re- ignit ion.

Once we have decided the type of f ire, we can proceed to calculate the necessary quantit ies of carbon dioxide.

For this purpose, we must calculate the net volume of the protected space. This calculation does not normallytake into consideration f alse ceilings and/or f alse f loors.

Carbon dioxide requirements for surface f ires

Once the net volume is known, we proceed to determine the design concentration of carbon dioxide that isrequired f or the type of f lammable material involved. In no case shall a concentration less than 34% be used.Design concentrations are typically calculated by adding a saf ety f actor of 20% to the minimum concentrationf actors shown at Table 5.3.2.2 of NFPA 12, i.e. design concentration = 1,2 * minimum concentration

For a design concentration of 34%, NFPA 12 stipulated f looding f actors will have to be applied as a minimum:

Picture 1 Carbon dioxide volume f actors f or surf ace f ires

We take note that the smaller a space, the bigger the necessary quantity of carbon dioxide. For materialsrequiring a design concentration bigger than 34%, the quantit ies calculated until now will have to be multipliedwith the volume f actor given in Figure 5.3.4 of NFPA 12

Minimum calculated quantit ies will have to be increased in order to take into consideration any of the f ollowingreasons:

- openings that cannot be closed

- ventilation systems that cannot be shut down during carbon dioxide discharge

- a small percentage of carbon dioxide is vaporised during discharge without contributing to the putting out ofthe f ire

Although no specif ic rules exist, it is usual that minimum calculated quantit ies are multiplied by a f actor of 1.1 inorder to take into consideration all these parameters.

Carbon dioxide requirements for deep-seated f ires

Here, the calculation is more straight- f orward. Knowing the protected space net volume, we use the volumef actors of Table 5.4.2.1 of NFPA 12

Tab le 5.4.2.1 o f NFPA 12 (2005 e d itio n)

Picture 2 Design carbon dioxide concentration f or deep-seated f ires

Additional saf ety f actors, similar to surf ace f ires are also used here in order to take into considerationuncloseable openings, ventilation systems that cannot be shut down etc.

Selection of number of cylinders

Individual cylinders shall be used having a nominal weight capacity of 5, 10, 15, 20, 25, 35, 50, 75, 100, or 120 lb(2.3, 4.5, 6.8, 9.1, 11.4, 15.9, 22.7, 34.1, 45.4, or 54.4 kg respectively).

Depending on the calculated quantit ies of carbon dioxide that is necessary f or each space, we proceed toorder the f inal amount of carbon dioxide cylinders f or all protected spaces, taking into consideration thef ollowing:

- For redundancy reasons, overall selected amount of carbon dioxide cylinders is divided in two (2) banks ofcylinders: the main bank of cylinders and the reserve or auxiliary bank of cylinders.

- Calculation of the overall amount of cylinders is not done by adding the number of necessary cylinders perspace, since it is very unlikely that a f ire develops simultaneously in all spaces. If f or example, space A needsseven (7) cylinders, space B needs ten (10) cylinders and space C needs f if teen (15) cylinders, then we shallorder thirty (30) cylinders, 15 f or each bank so as to cover the worst case scenario: f ire outbreak at space C.

Design of fire fighting systems with carbon dioxide Calculation of necessary quantities