Explosion risk within the built environment or process facilities comes in various forms, but one of the least discussed, and consequently poses an unknown risk, is the possibility of an Aerosol Explosion, even below the liquid fuel’s flashpoint.

As per Gant, in a report for HSE, the aerosol is defined as “colloidal dispersion of solid or liquid particles mixed in gas”. One important aspect of aerosols is that we can observe and see them in our everyday lives, such as spray deodorant, air freshener, furniture polish, etc. Due to the fact that aerosol is basically a mixture of liquid and gas, liquid hydrocarbon fuel is able to mix with air to form hydrocarbon aerosol, and these hydrocarbon aerosols pose not only fire risk but also explosion risk, subject to dispersion conditions and containment of the system i.e. Lower Explosive Limit / Lower Flammability Limit.

There are several ways aerosol can be formed from liquid fuel below its flashpoints. As suggested by Gant and HSE, which includes: high-pressure release, condensation, air stripping, and agitation/splashing,

Due to the various ways aerosol can be formed, it is clear to see why aerosol explosions pose a threat, especially in chemical and process industries. In 2009, Santon conducted an incident survey related to the ignition of mist/aerosol formed from flammable liquids at temperatures below their flashpoints, and conclude that the frequency of incidents is far greater than previously thought.

One possible explosion scenario for Aerosol Explosion is a pinhole rupture of a pressurized, hydrocarbon fuel line, which produces a leak and hydrocarbon aerosol release. The size of the orifice or rupture dictates the dispersion of liquid fuel, and whether or not Aerosol is produced. Dispersion is also an important factor as it dictates the particle size which affects the overall Minimum Ignition Energy. Therefore, a pinhole rupture of a pressurized, diesel fuel line could produce Diesel Fuel Aerosol which poses an explosion hazard – even below the liquid fuel’s flashpoint.

This is the main topic of my Master’s Degree Dissertation during my studies at the University of Leeds – Fire & Explosion Engineering, and the limitations of experiment results and computer-based simulation of Aerosol Explosion drove me into pursuing a new methodology of testing, using Hartmann Dust Explosion Apparatus – known widely for dust explosion experiment only.

As it turns out, Hartmann Dust Explosion Apparatus could be utilized as a basis of analysis for the Aerosol Explosion phenomenon, albeit with some modification for the liquid fuel dispersion method, and limitations in terms of repeatability. The experiment was successful in terms of results, with Diesel Aerosol Explosion seen below its flashpoint. The lean limit can be determined at 0.15 ∅ for the assumed burned diesel, with good repeatability especially at stoichiometric, and at upper flammability range, the mixture kept exploding, suggesting that there is no rich limit although further research is needed.

Other possible hydrocarbons aerosol production, as per Gant and HSE, also needs to be considered, specifically for chemical / industrial facilities, process engineering, and its environmental and surrounding factor. Condensation needs to be taken into account, specifically for facilities with temperature differences, or situated in climate within the condensation limit of the fuel. Air stripping and agitation need to be taken into consideration for areas where spillage or liquid pool occurs, and areas where pressurized air is utilized. In all scenarios, it is also important to always check on the congestion level or containment of liquid fuel – related to its Minimum Concentration and Lower Flammability / Explosive Limit. Also worth noting that prevention is the best solution, and keeping the availability of fuel and ignition sources is the basic first step.

To read more about my research, which focused mostly on experimental methodology using Hartmann Apparatus, you can search for the paper titled: Mist Explosion using the Hartmann Dust Explosion Equipment, published in XII ISHPMIE. 12th International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions – XII ISHPMIE, 12-17 Aug 2018, Kansas City, MO, USA. ISHPMIE.

 Writer: Fahri Ali Imran - CEO Ignis Fire & Risk