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.
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