I'll just repeat most of what i said before !!! (Also add more !!!)
There is NO need for any fuel in the centre tank if the flight does not require it ! THe engine start uses same fuel as during any other phase of flight.
If the amount of fuel required is greater than the capacity of the wing tanks, THEN fuel is loaded in the centre tank.
If there is more than 453 Kgs of fuel in the centre tank, for stuctural reasons, the main (wing) tanks must be full before more fuel is loaded into the centre tank. The structural reason is that due to the position of the gear, when on the ground/during landing, the stresses upon the wing root are too great with all weight in centre (between gear) rather than in wings (outboard of gear, holding wings down, therefore relieving stress upon the wing root).
When all pumps (2 for each tank) are operating, the centre tank pumps (on the classic) operate at a higher pressure than the wing tank pumps. (on NG, the plumbing/valves create the higer pressure required) This encourages the engines to draw fuel from the centre tank. The fuel pumped by the wing tank pumps is simply returned to the main tanks. When the fuel in the centre tank is exhausted or the centre pumps are switched off, the system is plumbed to simply draw fuel from the wing tank pumps.
If the situation occurs where the pumps are no longer powered or the pilots switch off all fuel pumps then the engines have the capability of 'sucking' fuel from their respective wing tanks to maintain a constant fuel supply. If that were to happen, the fuel in the centre tank is left redundant and the structural implications can not be helped !
Now to consider the case when the wing tanks are full but the centre tank contains >453 kgs but <2300 kgs.
The fuel pumps/pressure 'sensors' are located towards the front of the centre tank. When there is less than 2300 kgs in the centre tank, at take-off and during the initial stages of the climb when there is a high body-angle, the pumps/sensors may become exposed.
This situation would cause a 'low pressure' caution on the flight deck. If this were to occur on take-off/early climb, the distraction may cause problems.
The reason for the large proportion of SOPs to ask for Centre pumps off with less than 2300 kgs in the centre tank is to avoid this undesired distraction - to the best of my knowledge, there is no other reason.
The centre tank pumps can still run without fuel in the centre tank without resulting in damage. Of course it is still advisable to turn off pumps with an empty tank to avoid wearing out pumps as they will be running without the colling/lubrication provided by the fuel !
The 737 also has a 'scavenge' feature. This means that when then centre pumps are switched off (regardless of contents of centre tank), the pressure created by the pumps in main tank 1 will draw out remaining fuel from the centre tank. On the classic, this process will run for 20 mins (although it can be restared and run for another 20 mins by recycling centre pump switches). On the NG, it will run for the remainder of the flight BUT will only begin once the quantity in main tank 1 drops below 50%.
Finally, unusable fuel quantites :
Classic Wing tanks (each) 272 kgs
Centre tank 419 kgs
NG Wing tanks (each) 152 kgs
Centre tank 398 kgs
If this doesn't help, read it again !!!
Hope this actually helps !!!!!!!!!!!
