I was reading Matthew Reilly’s Ice Station recently – a great book with lots of action – but one thing really had me gritting my teeth. At one point in the story the tanks containing the station’s refrigerant (to keep the walls frozen) are ruptured. In the book, this flammable gas hung around, making local fireballs every time someone had to fire a gun (and incinerating them as well). It was a convenient plot device so that the gunplay was replaced with hand-to-hand combat and scenes with little hand-held crossbows, but not realistic in my book – hence this post.
There were a couple of things wrong here. In Reilly’s scenario, the shooters who triggered the explosive gases were at the lower levels of the station. So what is a released refrigerant gas doing lurking down there? I have not looked up the specs of the gas he mentions, but refrigerants are notoriously volatile for a start – the whole reason they are used in refrigerant cycles in the first place. The released refrigerant would have immediately gone UP!
The second thing is – if the muzzle flare was capable of igniting the gas, it would have triggered a continuous explosion. The flame would have raced through the explosive mixture in all directions, creating a pressure wave from the rapidly expanding combustion gases. Basically at the first ignition, there would have been an explosion at the TOP of the facility that would have probably blown the roof off. Then the flammable gas would have been combusted – i.e. gone.
That’s if the gas actually ignited in the first place.
All flammable gases have an explosive range in air. Too little of the gas and it will not ignite – too MUCH of the gas and it will not ignite either (generally the lowest ignition energy is required in the middle of this range). These are called the Lower Explosive Limit and Upper Explosive Limit respectively. These are usually expressed as a percentage concentration of the gas in air. For example – methane (the major constituent of natural gas) – has a LEL of ~5% and a UEL of ~16%. A fairly narrow range. I also grit my teeth in all those Hollywood films where someone sets the gas on the stove and flicks a match behind them. If they do this immediately the methane concentration is unlikely to be 5%. If they wait too long no amount of flame will create an explosion at all. The one scenario I will credit is where an open flame is left burning with the gas filling the room gradually. In this case it will explode when it reaches around 5%.
Now consider the explosive range of a gas like Hydrogen – notorious for being dangerous to handle. This has an explosive range from 4 to 75% in air. And people want to have Hydrogen bowsers at the fuel station! Hydrogen also has a very high flame speed – which means a more intense pressure wave.
Acetylene (used in welding) is even more fun – with an explosive range of 2.5 to 82% in air and an autoignition temperature of only 305C (Hydrogen and Methane need temperatures approaching 600C).
Powders can also be explosive.
When dealing with powders, it’s the Minimum Ignition Energy that is critical. Generally if a powder (or vapour) has a MIE lower than 30 mJ (milli-Joules), then a person needs to be earthed to an industrial safety system to deal with it.
If you thought paracetamol could cure headache – it can cause them too. Here are some examples of the MIE of powders:
- Paracetamol <10 mJ (this is produced as powder than pressed into tablets)
- Magnesium 20 mJ
- Sugar 30 mJ
- Wheat Flour 50 mJ
- Zinc 200 mJ
- PVC 1500 mJ
Suger dust, magnesium and paracetamol powders can all be very explosive. Of course the MIE of powders also depends on the particle size.