FIRST: RIP and Prayers to the truck driver and three (3) citizens who lost their lives in this event. We wish a full and speedy recovery to the 17 that we seriously injured, five (5) of whom have life-threatening injuries.
This week, we saw a catastrophic failure of an LPG tanker truck in Chile (see video below). This video, ever how unfortunate the event was, is a TEACHABLE moment for those who work with or around LPG or respond to LPG events.
News Video: https://www.youtube.com/watch?v=34VG-8H2uoc
NOTE: this was NOT a BLEVE; this was just a good old fashion catastrophic failure of a pressure vessel containing a Liquefied gas under pressure that was also a Flammable Gas.
When we discuss Vapor Pressure, Boiling Point, and Expansion Ratio… sit up and pay attention!
Here are three (3) critical data points regarding LPG.
LPG boils at around -40°F
The VP of LPG is 7,200 mm Hg (at 70°F); compare that to water’s VP at 70°F of around 22 mm Hg
The expansion ratio of LPG is 270:1, meaning 1 container of liquid will make 270 containers of the same size in gas
Let’s see what this looks like once its container is breached
LPG is a Liquefied Gas under Pressure and is shipped in a liquid state under-pressure. It is the PRESSURE of the gas in the headspace of the container that keeps it in a liquid state.
Once the container is breached or catastrophically fails and atmospheric conditions are present at the liquid surface, the LPG begins to boil immediately, just like that pot of water on our stove does when it reaches 212°F. The LPG will do it when it’s warmer than -40F outside. And as we can see, it does it “very efficiently”.
This was NOT a BLEVE as some on-line have claimed. This was a catastrophic failure of a transportation container (as can be seen in the video), followed by ignition of the gas cloud.
As we watch the gas rapidly expand, some may wonder why it does explode right away. This can be explained by the LEL/UEL of LPG, which is 1.8% to 10.0%. In that dense cloud, the concentration of the LPG gas is WAY OVER its Upper Explosive Limit (UEL) of 10%. As the gas continues to expand, it becomes diluted by atmospheric air, and once it’s diluted to a concentration LESS THAN 10% but more than 1.8%, AND it finds an ignition source, we have an explosion. And as we can see in the video, there are plenty of ignition sources available. (See discussion of MIE below)
Another physical characteristic we can see in the video is the Vapor Density of LPG. Notice how the gas immediately expands upward to a point, then quickly settles back to the ground. On the left side of the video, we can see it traveling down hill quite rapidly. Understand that more is happening than simple Vapor Density is heavier than air. COLD gas settles, and this LPG will be very cold immediately after the event (from the liquid phase to the gas phase). So both the gas’s temperature and its weight cause this pronounced behavior.
The Minimum Ignition Energy (MIE) of LPG is LESS THAN 0.5 mJ. To put this amount of energy into perspective, we can look at what it feels like to get shocked by touching something on a cold/dry winter day. Humans can feel a static shock at around 15 mJ. Some may feel a shock as low as 10 mJ; at 15 mJ, we usually react by pulling our hand away. So that static shock has at least 20 times the energy needed to ignite that large gas cloud! We just need the cloud to be within its FLAMMABLE range, as discussed above.