Centrifugal compressors can incorporate a side stream flow of intermediate pressure vapor between stages of that compressor. The side stream flow can be controlled by a side stream injection port controlled by a throttle ring disposed between stages of the compressor. The throttle ring can allow or obstruct flow through the side stream injection port. The throttle ring can extend and retract in a direction substantially perpendicular to the direction of flow from the first stage impeller to the second stage impeller. A method of operating a centrifugal compressor can include actuating a throttle ring by rotating a drive ring to adjust a flow of interstage fluid into the second stage impeller.