(134c) Investigation of the Lower Explosion Limit of Hybrid Mixtures

Hybrid mixture explosions are the type of explosion that involves at least two combustible materials of different state of aggregation for example, mixtures of combustible dust with a flammable gas, vapor or spray. They are usually encountered in industrial processes that handle dust and flammable gases, sprays or vapors such as paint factories (pigments and solvents), mining (dusts and gas), grain elevators (small grains and fermentation gases) or pharmaceutical industries (dust and solvent vapor). It has long been known that the explosion sensitivity and severity of hybrid mixtures differ from that of the single components.

The present paper reports on the experimental and theoretical investigations of the lower explosion limits of two and three phase mixtures such as gas-dust, spray-gas, dust-spray and dust-gas-spray. The materials used were corn starch, lycopodium, brown coal as combustible dust, methane and hydrogen as combustible gases and acetone, ethanol and isopropanol as sprays.

The experiments were performed in the standardized 20 Liter spherical explosion chamber where modifications were done to allow the input of solvent and gas. The test protocol was in accordance with the European standard, EN 14034 using electrical spark as an ignition source. The experimental results demonstrate significant enhancements in the explosion likelihood brought about by gas or spray admixture with dust and vice versa. They also confirm that a hybrid mixture explosion is possible even when both dust and spray or gas concentrations are respectively lower than their minimum explosion concentration (MEC) and lower explosible limit (LEL). For example, starch with MEC of 250 g/m³ decrease to 20 g/m³, 30 g/m³ and 125 g/m³ when small amount of isopropanol spray, methane gas and hydrogen gas respectively, were added even though these concentrations of gases and solvents were all below the LEL of the individual substances.

Comparisons have been made between the lower explosible limit of the experimental data and classical models such as those developed by Bartknecht, Le Chatelier, Jiang et al and a newly developed model by Addai. With the exception of the Le Chatelier model, all the other model was in agreement with the experimental result from a safety point of view. This research provides safety practitioners with a practicable means to characterize material hazards without the need to perform a large number of tests.