A B747-400 was MEL'ed and required to carry center tank fuel as ballast. In flight the ballast fuel was pumped out by the scavenge pump requiring a diversion to address the weight and balance problem.

Narrative

Flight had numerous write-ups. As a result; the operations placard required that the forward cargo compartment remain empty causing our flight segment to carry an extra 15;000 LBS of payload fuel in the center tank; note 'not to be burned' due to center of gravity limitations. The Dispatcher's comments referenced this in the flight plan. The Captain correctly set up the fuel panel for tank to engine as per the Flight Manual. He left the center tank pumps off due to payload fuel. Shortly after flight was enroute to ZZZ1; the Captain and I noticed that the center tank scavenge pump started transferring the payload fuel from the center tank to the left wing tank. This normally occurs inflight to scavenge the remaining fuel in the center tank when the main center fuel pumps are turned off. We contacted the Dispatcher and Maintenance to develop a solution or an alternate plan. Additionally; we directed Dispatch to contact the Fleet Manager and Load Planning to get us data of the weight and balance and center of gravity concerns. First; with Maintenance; we attempted to shut off the scavenge pump via the covered scavenge pump switch located on the overhead maintenance panel. The cap was closed in its proper position. Note the default position (cap closed) is the off position. No CBs are located on the flight deck for this pump. Maintenance provided us with the information that the pump should only run for 2.0 hours and only transfer 1;700 LBS per hour for a total to 3;400 LBS. In the meantime; the Captain and I discussed plans for a possible diversion back to ZZZ2 or into ZZZ3. We were now about 1.5 hours into the flight and transferred about 3;000 LBS of payload fuel. The Dispatcher and Load Planning were unable to come up with any data at all! The Flight Manager was contacted and briefed on the situation and provided no solution. So; with no additional information on payload fuel; the Captain and I decided to divert to ZZZ3 which was our best option. We made an uneventful landing in ZZZ3. The center tank payload fuel was about 11;500 LBS. The flight was planned to continue from ZZZ3 to ZZZ1. After landing in ZZZ3; the Dispatcher and Load Planning came up with data; which stated that the payload fuel had a range between 20;000-10;000 LBS. We loaded the center tank to 20;000 LBS of fuel for payload and possible scavenge from the center tank. We completed our checklists and the fuel panel was set up for tank to engine and the center tank pumps were left off due to the payload fuel; we were now continuing to ZZZ1. After we departed ZZZ3; we noticed that the center tank was again scavenging fuel with 20;000 LBS in the tank. The Captain decided to cycle the center tank fuel pumps. After cycling the center tank pumps; the scavenge pump went off the remainder of the flight. We made an uneventful landing at ZZZ1 -- mission complete. This brings up a question on fuel system logic or some other kind of anomaly. There are no procedures in the Flight Manual on carrying payload fuel. More importantly; why was it so difficult for Load Planning to give use data on the weight and balance?