The minimum carbon dioxide design concentration for extinguishment of an ethylene fire is

NFPA 12 lists required design concentrations of carbon dioxide for total-flooding extinguishment of specific fuels. In Table 5.5.2 (2021 ed.) ethylene (a flammable gas) requires a minimum CO₂ concentration of 43 % by volume. Concentrations lower than this do not reliably reduce the oxygen content of the atmosphere to the level needed to stop ethylene flame propagation; higher values are allowed but are not the “minimum design” value the standard specifies.

A. 34 %—design value for many Class B liquids (e.g., heptane), but NFPA 12 shows it is inadequate for ethylene gas.

B. 40 %—still below the 43 % listed for ethylene; would not ensure extinguishment margin required by the standard.

D. 49 %—above the minimum; while effective, it is not the “minimum design concentration” asked for, so not the correct answer.

1. NFPA 12, Standard on Carbon Dioxide Extinguishing Systems, 2021 edition, Table 5.5.2 “Minimum Design Concentrations for Total-Flooding Systems,” entry for Ethylene – 43 % CO₂ (p. 27).

2. NFPA Fire Protection Handbook, 20th ed., Vol. I, Section 12, Table “Carbon Dioxide Extinguishing Concentrations,” Ethylene – 43 % (pp. 12-31 to 12-32).

3. G. Heskestad & D. Drysdale, Fire & Combustion (3rd ed., Wiley, 2019) citing NFPA 12 data for gaseous fuels, Section 15.4.3 (p. 487).