A disposable blood separation set and a centrifugal blood processing system comprising a blood processing chamber adapted to be mounted on a rotor of a centrifuge; a frustro-conical cell separation chamber in fluid communication with the processing chamber, the cell separation chamber having an inlet, an outlet and a gravity valve inside the cell separation chamber. The valve is responsive to the gravitational field created by the speed of the rotor. When the rotor spins at high speed, the gravity valve may open the outlet at a location proximal to an axis of rotation of the rotor. When the rotor spins at a lower speed, the gravity valve may open the outlet at a location distal from the axis.