Boksburg explosion – The science behind the blast

The massive Boksburg blast, which took the lives of 18 people and left dozens more badly burned and injured, was probably what is known as a BLEVE (boiling liquid expanding vapour explosion), one of the most devastating non-military explosions.

BLEVEs normally occur when a vessel containing pressurised fluid – like the Liquid Petroleum Gas (LPG) being carried by the truck which caught fire in Boksburg – ruptures after it reaches a temperature above the boiling point of its contents.

Why did it blow?

In the case of pressurised LPG, that boiling point is below zero degrees Celsius. When this happens, the liquid quickly becomes a gas.

The gas expands rapidly and sits close to the ground and can be further hemmed in by physical barriers, such as walls or, as in the case in Boksburg, by the structure of the bridge and the approaches to it.

The tanker truck carrying the LPG got stuck under the bridge because it was too tall.

The apparent metal-on-metal friction from the initial impact started the first fire.

The BLEVE would have ensued after the rupturing of the truck’s tanks and the explosion was of such force it destroyed the bridge, while the blast and flames killed and injured many of the people within the danger zone.

Also Read: WATCH: The aftermath of the Boksburg tanker explosion

Danger zone

Scientists have determined that if there is an explosion of a Liquid Petroleum Gas tank, those up to 1.3km away are considered to be at high risk from death or injury.

The National Wildfire Co-ordinating Group in the US directs fire fighters and first responders to move at least 750m away from the site, because projectiles or missiles from the explosion can travel that far.

“BLEVEs are a major hazard to emergency responders,” it adds.

The National Institute for Occupational Safety and Health in the US advises: “Fight fire from the maximum distance possible, or use unmanned hose holders or monitor nozzles.

“For massive fires, use unmanned hose holders or monitor nozzles; if this is impossible, leave the area and let the fire burn.”

The National Fire Chiefs Council in the UK says that in the case of major fires or gas leaks, “evacuation of the surrounding area should be considered a priority as debris may be projected over a kilometre from the fire site”.

Also Read: Boksburg explosion: Company launches investigation after its ‘experienced’ driver was in truck