Quote Originally Posted by JahJahwarrior View Post
Think of the laws of physics and how they affect guns. force=mass*acceleration. When a valve is shot out (if threads are stripped, as FW says, for example), an equal force acts on both the tank and the valve. But, the mass of the valves is much less than the mass of the tank, so the acceleration of the valve will be much greater.

You see this in guns as well: the exploding powder puts equal force on the bullet and the gun. However, the bullet goes much further than the gun, because it has much smaller mass, especially when you consider that the mass of the shooter might well be considered to be part of the mass of the gun, as it must also be moved with the gun, the two are like a connected system.
You are right about the mass difference between tank and valve, but you neglected the mass of the gas trying to make that tank into a "rocket". Someone with more time than I have can calculate just how much acceleration that might give a tank