I do not have much data on the B777 venting layout but it is only the centre tank that has the system fitted.
An extract from the AMM suggests that this tank is closed off to venting except during refueling for fuel expansion.
The cross vent check valve opens during overfuel conditions to relieve fuel tank pressure. The cross vent check valve is normally closed to prevent ambient air from going into the center tank.
The cross vent check valve opens during overfuel conditions to relieve fuel tank pressure. The cross vent check valve is normally closed to prevent ambient air from going into the center tank.
The Nitrogen generation is in fact an oxygen reduction system that increases the percentage of nitrogen in a given volume.
The Nitrogen Generation System (NGS) reduces the oxygen content of the air mixture in the center tank to a level which will not support combustion.
TW