You say that the CTR tank water scavenge pumps are ALWAYS running.
But you also state that their "power supply" comes from the tank booster pumps outlets.
My question is then - how can the water scavenge pumps run if there is no fuel in the CTR tank?
I suppose that they will shut down automatically, when booster pumps stop producing pressure due lack of fuel (at approx. 900 kg) - you see what I mean?
Yes, took a peek in the AMM. It is not very clear but the Centre tank scavenge pump and the Centre tank Water scavenge pump are linked. It looks as though with the Centre Tank scavenge system running, liquid is sucked through the centre tank water scavenge lines as well. The centre tank scavenge jet pump motive power comes from the wing booster pumps.