An apparatus for extending the operational flow rate range of a turbine is described herein. Two or more removable sleeves may be used to change the cross-sectional area of a turbine. Each removable sleeve may define or eliminate the stator gap between a stator blade tip and an inner wall of the removable sleeve and a rotor gap between a rotor blade tip and an inner wall of the removable sleeve. A movable sleeve may be disposed in the turbine and may move between a first position and a second position in response to changes in the pressure differential across the turbine. The movable sleeve may define or eliminate a stator gap between a stator blade tip and the inner conical surface of the sleeve or a hub of the turbine and a rotor gap between a rotor blade tip and the inner conical surface of the sleeve.