I need to determine the amount of time it takes for a pressurized tank to reach atmospheric pressure.  I understand this is a transient analysis, but I do not know how to simulate the pressure drop in the tank as the gases escape through an emergency release.  I cannot do a physical test, so I need to determine this by analysis.

I assume I need to do this by creating a fluid sub-domain in the tank, and a boundary condition at the emergency release.  Since this is venting to atmosphere, should I simply set the boundary condition to atmospheric pressure?  I am certain I am missing something.

Any help is appreciated.