NFPA Standard : NFPA 12 for CO2
2-6.1 General.
The venting of flammable vapors and pressure buildup from the discharge of quantities of carbon dioxide into closed spaces shall be considered. Venting of flammable vapors is covered in 2-2.1.4. The pressure venting consideration involves such variables as enclosure strength and injection rate.
2-6.2 Pressure Relief Venting.
Porosity and leakages such as at doors, windows, and dampers, though not readily apparent or easily calculated, have been found to provide sufficient relief for the normal carbon dioxide flooding systems without need for additional venting. Record storage rooms, refrigerated spaces, and ductwork have also been found to need no additional venting when tested under their average system conditions.
2-6.2.1
For very tight enclosures, the area necessary for free venting shall be calculated form the following formula. Assuming the expansion of carbon dioxide to be 9 ft.3/lb (0.56 m.3/kg) will give satisfactory results.
X = Q
1.3
Where X = Free venting area in in.2
Q = Calculated carbon dioxide flow rate in lbs./min.
P = Allowable strength of enclosure in lbs/ft2
For SI Units
XM = 23.9 QM/
XM = Free venting area mm2
QM = Calculated carbon dioxide flow rate in kg/min.
PM = Allowable strength of enclosure bars, gauge.
2-6.2.2
In many instances, particularly when hazardous materials are involved, relief openings are already provided for explosion venting. These and other available openings often provide adequate venting.
