For the 20 the wing tanks have sequence valves. Basically blow off valves. The ctr pumps run for the start until 2 mins or slats extend as per the fcom. The higher pressure from the ctr pumps wins the pressure fight and you get ctr fuel. The flscu will off the pumps as described and will restart them at slat retraction until one of the ctr low level sensors is dry then I think it’s 2 or 5 mins for scavenge then they shut off. If my memory serves me well. Hope that helps some.