A turbine in communication with a high pressure source and a low pressure source comprising: a housing having an inlet, a volute and an outlet, said inlet coupled to the volute through a primary fluid path and a secondary fluid path; and an impeller rotatably coupled to the housing; characterized by a hydraulically actuated valve assembly disposed within the secondary fluid path selectively communicating fluid from the inlet to the volute, wherein the valve assembly comprises a valve head positioned relative to a valve seat comprising a plurality of valve head positions between the first position and the second position; and a hydraulic actuator coupled to the high pressure source and the low pressure source, said hydraulic actuator coupled to the hydraulically actuated valve assembly moving the hydraulically actuated valve assembly from a first position communicating fluid from the inlet into the volute to a second position blocking flow from the inlet to the volute in response to selectively coupling the high pressure source and the low pressure source to the hydraulic actuator; wherein the valve assembly comprises a valve stem coupled to a piston within a piston housing, said piston and piston housing defining a control cavity within the piston housing; wherein said control cavity is fluidically coupled to a port through the piston housing; wherein the port is selectively coupled via a first valve to a high pressure source and via a second valve to a low pressure source to move the piston within the piston housing and to move the valve stem relative to the valve seat; and wherein the turbine further comprises a position sensor coupled to the piston generating a position signal and a controller coupled to the position sensor, said controller controlling the first valve and the second valve in response to the position signal. a housing having an inlet, a volute and an outlet, said inlet coupled to the volute through a primary fluid path and a secondary fluid path; and an impeller rotatably coupled to the housing; characterized by a hydraulically actuated valve assembly disposed within the secondary fluid path selectively communicating fluid from the inlet to the volute, wherein the valve assembly comprises a valve head positioned relative to a valve seat comprising a plurality of valve head positions between the first position and the second position; and a hydraulic actuator coupled to the high pressure source and the low pressure source, said hydraulic actuator coupled to the hydraulically actuated valve assembly moving the hydraulically actuated valve assembly from a first position communicating fluid from the inlet into the volute to a second position blocking flow from the inlet to the volute in response to selectively coupling the high pressure source and the low pressure source to the hydraulic actuator; wherein the valve assembly comprises a valve stem coupled to a piston within a piston housing, said piston and piston housing defining a control cavity within the piston housing; wherein said control cavity is fluidically coupled to a port through the piston housing; wherein the port is selectively coupled via a first valve to a high pressure source and via a second valve to a low pressure source to move the piston within the piston housing and to move the valve stem relative to the valve seat; and wherein the turbine further comprises a position sensor coupled to the piston generating a position signal and a controller coupled to the position sensor, said controller controlling the first valve and the second valve in response to the position signal.