Gas Discharge from a Tank
 

I'm trying to model a pressurized tank discharging it's pressure into a room at atmospheric pressure in 3D. I know I need to use a density based model since this is compressible but am having trouble with the geometry. I know I need to put an enclosure around the pressure vessel that will be considered the room but for the vessel is it supposed to be a 3D solid or just a shell? Also, should I use patching to define the different pressures and how would I define the boundaries? All advice is appreciated!

Also, I know that the time it takes for the tank to discharge is essentially proportional to the ratio of the volume of the tank to the area of the opening. Does this mean that I could make this 2D by making the ratio of the tank area to opening length the same ratio?