You have to have the ...

You have to have the effective size (d10 screen size) and the uniformity coefficient (d60/d10) to calculate the flow rate required for fluidization. You also need to know how much freeboard you have above the top of your filter media. Assuming you want to expand the bed 30% (the generally accepted percent expansion that reflects adequate fluidization) then you need to know you have enough freeboard in the tank to allow for this expansion percentage without discharging filter media out the backwash outlet line.