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[Bandit127]

[edit]Skip to - Why do I need to know - if you are in a hurry[/edit]

The boring bit:
Boiling
Liquid
Evaporating (or Expanding)
Vapour
Explosion

The exciting bit:
http://uk.youtube.com/watch?v=GWjxrAhpBQk
http://www.metacafe.com/watch/166764/big_explosion/


What do I need to know?
The boilng point of a liquid depends on it's pressure. In a vacuum, water boils at 0 deg C. On top of Everest it boils at about 70 deg. In your engine (which is 5-7 psi above atmosphere) water boils at about 110 deg. (A 50:50 mix about 116 deg).

LPG (for example) is stored below it's boiling point by elevating the pressure it is under. Which is mostly fine. But when a tank is in a fire, it heats to over the boiling point for its pressure. So it boils. More pressure builds up. The pressure relief valve lets go and a jet of high pressure gas is released, which is then ignited. This is a sure sign of an impending explosion. You can see this in both videos and hear it scream in real life.
Not enough gas is released and critcally, the pressure continues to build.

At some point, something big lets go. Maybe a valve blows off, or the tank ruptures. Pressure drops instantly. Again crucially, what was mostly liquid happily boiling away has a sudden drop in boiling point - and the whole lot instantly turns to vapour and this creates huge pressure spike. KABOOM!

You can see better in the second video, but the explosion is caused by pressure. The vapour escapes and only then does it catch fire.

Why do I need to know?
1. Water does the same thing. If your engine is blowing steam the coolant left in there has a boiling point of about 116 deg. Do not reduce the boiling point of it by removing the cap...! You will get the lot in the face.
2. If you ever see (and hear) a liquid pressure vessel violently venting, it's time to leave. Quite quickly. Even if it is not flammable, it will still go KABOOM!