A cargo tank must be certified as an ASME Code Section VIII pressure vessel with a design pressure between 100 and 500 psi, set no lower than the cargo's vapor pressure at 115°F. Which DOT cargo tank specification is this?

49 CFR 178.337-1 (MC 331 cargo tank design pressure requirement)

Propane and other liquefied compressed gases stay liquid only under real pressure, so the tank hauling them has to be engineered as a true pressure vessel rather than the thin-walled shell used for gasoline or chemicals. That difference in construction is exactly why MC 331 tanks look like the bullet-shaped, bolted-dome trailers seen hauling propane.