Hi,The setting of the pressure vacuum valve is a bit more complicated than that. Although the physical properties play a role, and the higher API indicates a higher evaporative loss during out breathing, the following shall also be undertaken when sizing the PVV (also refer to API 2000):
normal inbreathing: pumping out of tank as well as thermal inbreathing (to account for condensation of fluid)normal outbreathing: pumping into tank as well as thermal outbreathing (to account for evaporation of fluid)emergency venting: for example when the tank is on fire, an overpressure scenario arisesThe above will indicate the capacity of the PVV(s). For the pressure and vacuum settings:Pressure: the failure pressure of the tank shall be used as a guide to set the pressure, refer API 650 annex FVacuum: reference can be made to API 650 to determine any vacuum pressure limitationsTo clearly answer your question, however, I would use the higher API as this is the more volatile liquid and will result in more evaporative losses. Hope this helps.