One Mechanic insists that you have to pull CB's for the scavange pumps in order to keep ballast fuel in CTR tank, but the other nite, we loaded 8tons in CTR tank, inserted 8tons in FMC "Ballast Fuel". CTR pumps were OFF all the way, since flight was only 5 hours. When we landed we still had 8tons in CTR tank. No scavange, No CB's pulled.
I believe that if the reserve transfer valves open during the flight, this is also a trigger for the
electric scavenge pump to operate for 2 hours (or until the scavenge pump runs dry for 5 minutes). In this case, you would probably have to pull a CB. Mind you, my manuals say that the scavenge pump max output is 771Kg/hour, so it would take a while to shift 8 tonnes
Do you ever use ballast fuel AND have fuel in the reserve tanks?
Originally, the
hydromechanical scavenge pumps relied on fuel levels in the inboard mains
and the CWT (and fuel pressure in the fuel feed manifold). There are float valves stopping these "jet" scavenge pumps from operating until the conditions are right. However, the float valves in the CWT may be wired open (modification), leaving only the wing tank float valves active. These wing float valves operate at 27,270 Kg (inboard main level). Jet pumps scavenge at a rate of 545Kg/hr. I don't know how you would stop the wing-only float valve scavenge system from operating.
There are a lot of airline/aircraft specific details here, so I would advise caution.
Rgds
NSEU