The method described by Tom is a fairly well controlled experiment,if the sealed vessel is dry. The temperature inside the vessel is limited by the boiling point of water. So as long as there is water in the pot, the temperature will not exceed 100C. Based on this, the max pressure (differential) that can be reached inside the can is about 3.5 psi above room pressure. (But if there's any liquid inside the can, it would get a lot higher.)
The problem is the rupture strength of the can in question. 3.5 psi is actually a pretty large internal pressure differential for a thin sheet metal can with a rectangular geometry. To put this in perspective, if a can wall is something like 9" x 7", the force exerted on it would be over 200 lbs, equally distributed. Add in the age, rust and the dent-induced stresses on the metal and you have a good chance for catastrophic failure if you seal the can tightly.
Edited by jecos (Thu May 18 2017 10:57 AM)