Ok. NFPA 12 requires a inicial extinguishing concentration delivered in a quick way and then a 2nd application to prevent reignition that must be kept for some time. For example we bought a CO2 system for a electric generator (hydroelectric power plant) from a company and the design basis for this case was:
Initial extinguishing: The volume of the enclosure multiplied by a required extinguishing density (V=95m3, dens=1.3kg/m3 on 1 minute, mCO2=124kg). That lead to 3 bottles of 45kg of CO2 (135kg). Some people subtract the equipment included in the protected volume others don´t depending on the design concentration and other considerations or some add security factors.
There is a table in NFPA especial for electric rotating equipment that indicates the required kg of CO2 for the extended 20 min concentration. That lead to a quantity of 5 x 45kg botles.
So the system required 8 x 45kg botles, 3+5 that delivers CO2 with independent valve and pipe systems, one after the other.
Also, in our case we have two generator pits so the system was designed with a second 8 botles set as a back up battery.
This is just an example of the calculation included in the contract, but of course the correct aproach for each case must be designed by an expert due to the high risk of CO2 systems, type of fire/fuel expected, etc.
I know a person that have been involved in a CO2 accident and one guy almost die. This a serious matter.
One thing that I can critizice of NFPA 12 (almost for the 2003 NFC edition) is that the security signs and other security provisions have not been standarized jet as I would like.
I can read on the 2003 ed, that it is too open, the security provitions mentioned some are too tight and others too soft, at least I would like to see an appendix on NFPA 12 given more criteria on this for different aplications. If somebody knows more about the current edition, please tell.
