It can protect the tank when involving in an emergency. Under
normal practice, vent is closed when loading and offloading and
temperature variation, it will cause the pressure variation inside
the tank, such as air pressure and negative vacuum pressure, the
vent of manhole cover will be closed automatically in order to keep
the tank under a normal pressure. When involving in emergency, such
as a car accident, the emergency exhaust valve will open the inside
tank, meets the setting, in order to prevent explosion. Setting
pressure of vent