Posted By Jane Blunt It's pretty straightforward physics. The air exerts a force of 14 pounds for every square inch. If the tank has liquid or vapour inside exerting a force back in return there is no problem. If you manage to drain out the contents without letting air into the vessel to replace it, the vessel is trying to withstand the full force of the atmosphere all by itself, and unless it was designed to be a vacuum vessel it will collapse. We used to do an experiment on this when I was at school. A metal can (about the size of an old petrol can) was taken and some water was boiled inside it. When it had boiled for a minute or two, the heat source was removed, then immediately the can top was screwed back on. We then watched as the steam inside condensed back into water and the can collapsed with lots of creaking and groaning. Jane