An improved pump for efficient operation at low flow rates to avoid undue power consumption and heating of the fluid being pumped. There is provided a centrifugal pump including a housing with an inlet. A radial or mixed flow discharge impeller is rotatably mounted within the housing and an outlet volute extends about the impeller. First and second axially spaced diffusion passages establish fluid communication between the impeller and the volute and a valve is provided for closing one of the passages when low flow rates are demanded of the pump to minimize recirculation, leakage and churning losses consuming the power and leading to heating of the fluid.
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