Does there exist a vapor pressure limit at which a floating roof is required for a supported cone roof tank in the states? Do diesel fuel storage tanks typically require some measure of vapor reduction (floating roof, vapor recovery, etc...)?
Between 3.5 and 15 kPa (1.5 to 11.1 psia) TVP you need a floating roof (there are exceptions for small tanks, etc). See 40 CFR 60 Subpart Kb and 40 CFR 63 Subpart WW. Some EPA regions are more restrictive. Floating roofs are also used to keep the product clean and minimize odors.