Why it’s so hard to outrun a WHOOSH
Propane
A food truck explosion critically injuring 2 people was recently caught on video in Philadelphia. Preliminary reports point to the catastrophic failure of a small propane tank. The video demonstrates the accidental energy release of one of the most common, misunderstood hazardous material on the planet. Let’s take a closer look.
Gas grills have on average been responsible for 7,200 fires per year causing upwards of $27 million in damages. Its relevance in homeland security is propane is an inexpensive, relatively stable hydrocarbon energy source capable of doing a substantial amount of damage that isn’t going away. You can purchase cylinders of varying sizes with no ID or background check. Hell, you can even have it delivered to home or picked it up at 7-Eleven. On the micro scale, propane is on the light end of the hydrocarbon spectrum with just 3 carbon and 8 hydrogen atoms. It is a gas above -44 degrees F, meaning it is already in the right state of matter to burn.
In order to get enough propane into a container that would last several uses and make it worth your while to buy, the product is pressurized to the point that the gas becomes a liquid. The small propane tank generating the blast, most likely a 20 lb or 30 lb cylinder, are designed to hold 4.7 and 7 gallons respectively of liquefied propane gas (LP). The expansion ratio of LP 1:270, meaning one drop of liquid equals 270 times that volume in air. To put that into perspective, if we took the propane container from underneath a gas outdoor drill and filled it with gas only, we could fill 270 of those 20lb cylinders. In other words, there’s a lot of boom in one of those thingies.
Domestic production of propane is booming. Many suspect the recent fires and explosion of rail cars is due to an abnormal amount of light, natural gas liquids (propane included) being left in the bulk sweet crude. While this practice should be stopped, it is in the producer’s best interest not to. This “fluffing” technically should change the transportation requirements of the product from a flammable liquid, a fuel which needs preheating to produce enough vapors to support combustion, to a flammable gas, a product ready to burn. But since it hasn’t, it aides the producers in boosting the volume of product, filling up more rail cars while filling up the pockets of the producers.
What we presumably see the video is the unintentional release of the gas for a time period before the introduction of an ignition source. The heavier than air gas, after leaking undetected from the cylinder, settles and accumulates in a range from 0-100% of the surrounding atmosphere. The gas will establish a concentration in air of between 2 and 9%, the lower and upper flammable limit for the gas, and when an ignition source is present, Yahtzee!
So what we have here is a cheap, readily available, and heavier than air flammable gas capable of fast deflagration. It is pressurized already and stored in a steel tank capable of holding a lot of pressure until catastrophic failure occurs. In other words, a pretty sizeable boom. Not the kind that is going to take a building down, but if your intent is otherwise……
