Sequence valves are pressure relief valves attached to the secondary outlets on the wing tank pumps only. These valves prevent the pump outlet pressure rising above 25 psi. This pressure is lower than center tank pump output pressure and therefore permits center tank delivery to predominate.
The main engine fuel pump is powered by the accessory gearbox driven by the engine, and this is what delivers fuel to the engine even if the tank pumps have failed.
Last edited by tubby linton; 19th Apr 2017 at 20:45.