Flame extinguishing in the cup-burner by inert gases

National and international standards on gaseous fire extinguishing systems specify the minimum design concentrations of gaseous agents for flammable liquid hazards based on extinguishing concentrations determined by the cup-burner method. Extinguishing concentration values, XG, reported in standards documents do not correlate as well as expected with heat capacity, the underlying figure of merit for inert gas agents, a result that is likely due to inconsistent application of the test method. In this paper an explicit relation is developed for inert gas extinguishing concentration in terms of heat capacity and fuel properties. Equilibrium stoichiometric combustion at the fuel-air interface is assumed. A unified set of extinguishing concentration data for six inert gas agents is used to validate the model. For n-heptane flames the characteristic extinction limit temperature, TEx, was found to be essentially the same for each inert gas agent, though TEx may depend on details of the test apparatus and procedure, variations of which may account for the observed inconsistencies in data submitted by different laboratories. The measured XG for CO2 was ∼5% less than expected based on heat capacity effects alone, a result qualitatively consistent with elevated heat loss due to radiation by CO2. The results of this work are expected to be useful in eventually harmonizing data used fire protection standards.