peteroznewman
Subscriber

No, don’t deactivate the hydrostatic pressure unless you want to see the stress in the tank when it has no liquid, but is pressurized. 

Just leave the hydrostatic pressure active in step 2 and also have a separate pressure load on all the tank wall faces, including the faces below the liquid free surface.

The air pressure is pushing on the free surface and adding a constant pressure offset to the liquid, which transfers it to the wall.