Mist Explosions using the Hartmann Dust Explosion Equipment

Explosion hazards in the USA and EU include gases, vapors, mists and dusts. There is guidance on the measurement of lean flammability limit and reactivity for gases, vapors and dusts but not for mists explosions. This work explores the use of a modified Hartmann explosion tube for investigating the lean flammability limit and reactivity of mist explosions. Flammable liquids below their flash point can explode if the liquid is atomized and pump diesel fuel was investigated in this work, as the flash point is above 60oC. Each liquid has a critical drop size necessary for it to explode and this is related to the liquid volatility and viscosity. It is about 70µm for kerosene and 10µm for tetralin. For a liquid mist to behave close to a gas in an explosion the drop size has to be small enough to be heated and vaporized in the time it takes the droplet to travel through a laminar flame preheat zone, which is about 1mm at stoichiometric conditions. The use of high pressure air injection in dust explosion test equipment was investigated using the ISO 1m3 dust explosion vessel, but this could not be made to work, nor could the standard Hartman equipment. However, a simply modification to the Hartman equipment enabled repeatable mist explosions to be generated for diesel. The air deflector was removed from the Hartmann equipment and the liquid injected down the air delivery tube. The reactivity of diesel mist was determined from the rate of pressure rise prior to the explosion tube vent bursting and from the measured initial flame speed. The lean flammability limit of diesel mists was determined as 0.15∅ and no rich limit was found, with tests up to an Ø of 5. It was concluded that the Hartmann equipment provides a good method for the characterization of mist explosions for diesel like aerosols