Car alarms of doom

You know that moment, immediately after an earthquake or explosion, when all the car alarms go off? Hold that thought…

In the UK in 2005, about three hundred tonnes of petrol spilt from an overflowing tank at the Buncefield fuel storage site. Most of that formed a vapour cloud in the still air and a spark eventually caused the whole lot to explode, flattening nearby buildings and leading to three days of fires that burned fifty thousand tonnes of petrol.

Amazingly, no-body died or was seriously injured, mostly because it happened first thing on a Sunday morning. People who like to blow shit up Explosion mechanism experts are still studying this, as it was a strange explosion. It was a complicated mixture of deflagration and detonation. Deflagration means rapid burning, giving you a boom; detonation means a proper explosion, giving you a bang. In a deflagration, the flame front rushes through the vapour cloud, setting it on fire and giving you a lovely fireball climbing into the sky. In a detonation, shock waves expand supersonically, kicking the whole lot off pretty much in one go.

Normally, with a big cloud of gasoline vapour in open space, you get deflagration. You only get detonation if you have the cloud confined so that pressures can build up (some of you can think of an obvious example). Buncefield was a mix of the two. Some of the trees and hedges provided enough confinement to turn deflagrations into detonations. However, the (sadly paywalled) summary report on the disaster also contains this theory:

“[The first] deflagration led to a fireball that … created a pressure wave … that moved across the car parks. This activated the remote keyless entry anti-theft alarms in cars at the western edges. These caused ignition and vented deflagrations from the cars … This suggests that it would be prudent to test the propensity, or otherwise, for activated antitheft alarms to ignite a flammable mixture.”

So yeah, the cars were sitting in the vapour cloud, the initial blast from the first burning set off the car alarms, the alarms triggered the cars to go boom.

I don’t know about you, but I find that hilarious.