An improved method to determine the minimum extinguishing concentrations of ultrafine dry powder agents: Taking NaHCO3 and KHCO3 as examples

Abstract

Ultrafine dry powder agents (UDPAs) are the most potential Halon substitutes due to the high extinguishing effectiveness and excellent environmental friendliness. Nevertheless, the method for obtaining minimum extinguishing concentration (MEC) is flawed and inaccurate. An improved method is proposed by developing the special laser measurement system without powder contamination, the concentration calibration test bench and the cup burner with uniform aerosol generating. The results show that the optimum calibration position is 0.35–0.65 m from the inlet in the y-axis coordinate. More accurate mathematical relationships between the transmittance of the laser measurement system and the NaHCO3 and KHCO3 powders are established, with the errors less than 12%. Upon reaching MECs in the modified cup burner, the flames pulsate, then fade and eventually die. Based on the phenomenon of the sudden change in the transmittance of the laser measurement system at the moment of flame extinguishment, i.e. the “step shape” curve, a more accurate critical criterion point for the MEC is proposed. The measured MECs for methane flames by NaHCO3 (32.2 g/m3) are approximately 4 times that of KHCO3 (8.2 g/m3), consistent with previous studies. The obtained MECs of NaHCO3 and KHCO3 for kerosene flame are verified by 1 m3 total flooding fire extinguishing experiments. The improved method may guide the design of industry fire protection systems based on UDPAs to achieve process safety.