Valves can be matched to normal vapour recovery system and their use compensates for the lack during loading phase (depot) or unloading phase (service stations) of the relative recovery plant.
For normal art. 54.33.65 without air is closed. But only for special requests, art. 18.104.22.168 is normal open.
Valves pneumatically open releasing gas into the atmosphere during loading while allow air to enter in compartments avoiding possible implosion of tank due to the vacuum during unloading.