Where do the vapors go?

The vapor valve opening is the same size as the outlet plumbing so opening that line should prevent any vacuum. Vacuum is not good. Aluminum work hardens as it ages. I remember the owner of a small petroleum hauler being upset over a small mark in the side of the barrel after a guy got up against a phone pole on a tight place. He said the flexing at that spot from loading and unloading will eventually crack; any partial vacuum will accelerate that.

 

When I gravity drop into an underground tank my vapor return hose is 3 inches. My drop hose is 4.

 

Yes, for diesel.

 

The liquid fuel pushes the vapors out, the old vent lines were usually 2" and would whistle for a warning as the tank filled due to the pressure.

 

You don’t need as big a vapor hose, as the product hose. Liquids are 800+/- times more denser than air, thus the same volume of vapor will move easy thru a smaller hose.

 

The guidelines we use state that you use 1in diameter hose for vapor for each 2 inches diameter for liquid. So you can vent two 4” product hoses with one 3” vapor line.

 

And for jet fuel and AvGas. I have never used a vapor recovery hose to unload.