Abstract: 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.

Abstract: A rotating centrifugal impeller spins contaminated fluid at high speed around the outside of a stationary cylindrical particle barrier filter screen and by centrifugal action separates the solid contaminants for removal to a dirt accumulator. Simultaneously, the impeller pumps or propels the remaining clean fluid radially inward, causing it to turn abruptly from its tangential path and to flow through the screen openings to a central chamber from which the fluid is discharged. Even though some of the solid particles may be smaller than the screen openings, they cannot turn radially inward from their trajectories sharply enough to pass through the openings and instead will be contained in their tangential path until centrifuged outward. The barrier screen has a smooth external surface so that it may be washed clean by the high velocity tangential flow of the spinning fluid across the screen, thus preventing any particle build-up on the screen and clogging thereof during start-up and shut-down.