Urea Storage Tank - Venting and Overflow

I'm installing some Urea storage tanks for a small power plant. The Urea manufacturer's users guide says to vent the tanks to the outside using pipe size twice the size of the fill piping. That seems big to me, but whatever, it's not that expensive. My real question is this:

I need to put in some kind of overflow on the tank. Normally I'd provide a tank nozzle 8" from the top of the tank and run piping from it to the floor of the contaiment area, but I'm concerned about vapors migrating down the vent pipe and accumulating in the containment. So far, available vapor info is fuzzy.

Anyone have any experience with this? Thanks for any feedback.