The explosion at GMB Lions Den Chinhoyi – An explanation of the likely cause

By Dr. Paul Stanford Kupakuwana| It was sad to learn about the GMB workers killed in Lions Den silo explosions. In the absence of the full knowledge on what exactly happened, I have decided to share information on why dust explosions occur (with the hope that it will help ease the investigation.)

Dust explosions happen when combustible dust with the correct particle size forms a cloud of concentration in air. This dust is known as ‘explosible.’ Combustible specks of dust include some commonly found materials, such as sugar, milk powder, coal, and grain. Dust explosions can occur in processes where there are specks of dust handled, such as grain silos, extraction systems, spray dryers and material transfer systems. The following conditions are needed for clouds of dust to become explosible:

• Particle size – combustible dust must have a fine enough particle size to sufficiently increase the surface area;
• Dispersal – the dust must be in a combustible mixture in the air;
• Explosive concentrations – unlike flammable vapours, specks of dust do not have a defined Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL), but as an indicative value, the LEL would be 25-50g per cubic metre of air, with UEL ten times the LEL. At these concentrations, the dust cloud would be substantial and apparent;
• Ignition energy – there is minimum ignition energy (MIE) required to ignite the dust. This data is usually available for each dust and is critical in the assessment of explosion risk;
• Heat and humidity – high temperatures can increase the risk of dust explosion as the hot surface can ignite the dust if they provide the MIE. High humidity (damping down), on the other hand, can reduce the likelihood of a dust explosion as dust clouds do not form as readily, and a greater MIE is needed to ignite the dust.

For a fire to start, the fire triangle is essential: heat, fuel and oxygen are needed. To create the conditions for a dust explosion, the ‘dust pentagon’ considers heat, fuel, oxygen, confinement of the dust (e.g. enclosing the dust within a building or silo) and dispersion of the dust (formation of the dust cloud).

As we can see, the conditions needed for dust explosions to occur are prescribed clearly. It is unusual for such conditions, e.g. explosive concentrations of dust, to be present in the general atmosphere as these would, at a fundamental level, need to be controlled for operator exposure and health issues. As a result, most dust explosions happen within processes or equipment where conditions for the explosion can be achieved more readily. These explosions are known as ‘primary explosions.’ The primary explosion can be quite minor, but the dust disturbed within the building or silo (e.g., on the floor, girders, lying on vessels, etc.) then forms an explosible mixture and is ignited, causing one or more secondary explosions that can be devastating.

Housekeeping and dust control are, therefore, essential in an area where process specks of dust are generated. It is essential that the conditions needed for an explosion are understood and the potential assessed so that controls can be implemented to prevent a dust explosion. This mitigation will occur through the development of a robust risk assessment process.

Dr. Paul Stanford Kupakuwana is a Chartered Professional Engineer in Chemical Engineering and a Process Safety Management specialist. He is Founder and CEO of Stanford Global Limited, a UK-based company which offers Process Safety Management Consultancy and Training. Paul can be reached through: info_at_stanford-global.com(spam protected)