Boksburg Tanker Explosion: The Physics Behind it

LPG (liquid petroleum gas) has a boiling point of -44°C at atmospheric pressure at sea level. That means at room temperature (20°C) it occurs in gaseous form.

However, remember that the boiling point is dependent on pressure. The higher the ambient (surrounding) pressure, the higher the boiling point.

Thus to keep LPG in liquid form it has to be highly pressurized.

At typical room temperature (20°C) LPG needs 836 kilopascals to turn it into liquid form. That is 8 times the atmospheric pressure of 101kPa. Given that Boksburg is 1700m above sea level the actual atmospheric pressure would be less than 101kPa.

In order to keep the LPG liquified, a very strong container is needed to withstand this high pressure.

One may ask why liquify the LPG? To make it easier to transport. Liquid occupies far much less volume than gas. Your 9 kg cooking gas would need a container the size of a pickup truck if it were not liquified.

When a high-pressure vessel containing a liquid is suddenly compromised, the liquid will instantly turn into vapour leading to a BLEVE (boiling liquid expanding vapour explosion). The BLEVE is not exothermic (generating heat) but it is forceful and will spread the vapour over a wide area. If the vapour is flammable (such as petroleum gas) it will then mix with air (which contains oxygen) and ignite causing another exothermic explosion within milli-seconds of the BLEVE.

Thus a typical tanker explosion is a 2 stage explosion. First, there is a BLEVE which envelopes a wide area with flammable vapour. If there is a flame in the area that ignites the vapour then there is an exothermic explosion that burns everything that is now enveloped in petroleum vapour.

The likely sequence of events in Boksburg. When the tanker scrapped its top against the bridge, the compromise did not cause an instant complete loss of pressure. There was a continuous escape of gas which caught fire and burnt continuously. It is likely the continuous flame heated a portion of the tanker. Heat weakens metal. At some point about an hour after the initial accident, the casing of the tanker got weak enough to rupture and cause a BLEVE.

Within milliseconds people standing near the tanker would have been enveloped in highly flammable vapour. Because there was a flame already present within milliseconds that vapour ignited burning people. Victims of the secondary explosion are likely to have been burnt all over the body. Victims of a purely exothermic explosion are usually burnt only on the side facing the explosion with the worst injury being caused by the blast wave, not by the heat (broken bodies rather than burnt bodies).

BLEVEs are not uncommon in the LPG industry. This video (https://www.youtube.com/watch?v=NuPVEsQaGB0) documents some of the worst BLEVEs. Most of these BLEVEs caused few fatalities. In one involving a railway tanker train, no one died even though a rail tanker was tossed several hundred meters by the explosion.

The reason for the very high number of casualties in Boksburg was the lack of management of the scene. The only person who understood the risk and tried to keep people as far away as possible from the scene was the driver.

State authorities did not appreciate the risk. First, they send out the wrong type of responders, firemen with normal fire tenders. Second, they did nothing to manage the vicinity. An evacuation order should have been issued for several hundred metres around the scene.

BLEVE-initiated explosions can toss very large projectiles for several hundred metres. In this case, it looks like the tanker trailer was not tossed several metres because of the weight of the bridge above it. If that bridge and railway tracks had not been there that tanker could have landed on someone's house.

The media and public sentiment seem to be focusing on the person who made the initial mistake, the driver. Yet the biggest culprits here are the authorities. They clearly lacked the knowledge to appreciate the risk and expertise to manage the scene. Forces should have been sent to evacuate people and keep them several hundred metres away from the scene. Fire tenders capable of throwing fire suppressants several hundred metres should have been sent, not ones that need a man to take a hose close to the fire.

Naledi Pandor is right that it does not help that the politics of Zimbabwe are vindictive, rancorous and full of "if I can't have it, you can't have it too" attitude.

But I do not agree those politics are the reason for the tanking economy.

The reason for the tanking of the economy is corruption, cronyism, overregulation and extortionist mentality by government.

The government has been using regulation to close down economic space and create monopolies controlled by the cronies of politicians. For example, private transport operators have been forced to close or subcontract to ZUPCO (Zimbabwe United Passenger Company) are government owned entity so corrupt that it has only survived through a dripline of handouts, and now the forced redirection of private transport investment for its benefit.

Even the cornerstone of Zimbabwe's household economy, small to medium scale rural agriculture has not been spared. Farmers are forced to deliver produce to government owned entities such as GMB (Grain Marketing Board) who pay in fits and starts or not at all. Yet such entities "loan" the produce delivered to the companies of politicians and their cronies who then engineer ways to get their "loans" and "accounts" written off.

An example of the "writting off" of debt is when foreign currency debt owed to private players such as Miekles Hotel was "taken over" by government which then did not bother to pay despite Miekles hotel winning a court case.

The closing of economic space for small businesses means that the owners and their employees often go to look for ecomic space in neighbouring countries particularly South Africa.

For example before I came to South Africa I was running a transport company in Zimbabwe. I parked and sold the trucks after I was approached by RBZ officials demanding that I reptriate ALL foreign earnings that I had invoiced for. This is despite that transporter brokers deducted supplies such as diesel, tyres and driver allowances before paying the invoice. For example if a broker provided material worth R7000 and I invoices R10000 for the trip that meant that I would only get paid R3000 by the broker. But that government of Zimbabwe wanted all R10000 "repatriated" to Zimbabwe at the penalty of prison. I chose to leave the business than regularly haggle with and bribe RBZ officials.

In the same yard I parked my trucks in Zimbabwe there were people who started off selling diesel stolen by truck drivers around Harare. Soon very high ranking officials (an army colonel and senior intelligence official) started supplying diesel from government "allocations" to those young men. Truckloads of diesel taken straight from NOCZIM (National Oil Company of Zimbabwe) were offloaded into drums and sold on the black market at a time the country was facing severe fuel shortage. Needless to say in the space of a year the sigubhu pushers had more than 10 trucks.

As long as government services are provided at extortionist daylight robbery proportions economic space will remain closed for small and medium enterprises and small business owners and their potential employees will continue flocking to neighbouring countries particularly South Africa, for the economic space they are denied at home.

South Africa needs to tell Munangagwa to stop the corruption and cronyism. Sanctions are an irritation but not an economy killer. Chamisa is largely irrelevant.

If South Africa wants to stop the tide of economy refugees from Zimbabwe, the former needs to be very direct and frank that corruption and overregulation need to end. Particularly the forced surrender of foreign earning to government is a nuclear bomb to the economy. Stop that and Zimbabwe's economy will start to turn around.