Seeking advice on the required location of a "free" vent opening on an AST used to store diesel fuel on a mine site. Tank is currently fitted with a free (atmospheric) vent installed onto a riser pipe. It's a self-bunded (double wall) horizontal cylindrical steel tank, with a max. capacity of approx. 600G. Installed outside, and not adjacent to a building. Cannot seem to find a requirement within NFPA 30 for the extended vent riser for such a tank and application? In this instance, does the discharge of diesel vapour need to occur at a certain distance from the ground?