I´m not sure if I understand you right... a float valve in the string prevents a kick from entering the drill pipe at the bottom end. For circulating the kick out through the annulus mud is pumped down through the string at low flow rate, here the float valve is open.
If you have a float in the string but need to fill the drill pipe (top fill) how do you do it and at what rate (GPM) do you fill the pipe?
I´m not sure if I understand you right... a float valve in the string prevents a kick from entering the drill pipe at the bottom end. For circulating the kick out through the annulus mud is pumped down through the string at low flow rate, here the float valve is open.