Fuel tank Inerting- Vent System
In a vented fuel tank the vent has to be open at all times.
1. To discharge hydrocarbon laden ullage gas when the inerting system i operating. A cubic feet of ullage mixture has to vent out for each cubic feet of NEA supplied to the tank.
2. To discharge ullage moxture during climb to depressurize the tank so the tank remains at the same pressure as outside ambient
3. To repressurizethe tank during descent when the outside pressure increases.
I can not think any instant or reason when the vent will not be allowed to do its intended job; vent ullage mixture out or to allow outside air in.
I think the vent modification is to minimize flow through the tanks from one wing tip vent to the other.
Hope the above helps.