The engines have mechanical low and high pressure fuel pumps, and they can be gravity fed (with some altitude restrictions) from the wing tanks if the (electric) tank pumps are not working.
Some fuel - about 2 tonnes - from the centre tank is unuseable without the electric pumps.
.
Last edited by Uplinker; 1st May 2016 at 15:56.
Reason: Removed the bit about the valves that I am not sure about.