1. IN COMBINATION WITH A HYDRAULIC LOAD, A MAIN PUMPING AND METERING UNIT INCLUDING A POSITIVE DISPLACEMENT PUMP AND DRIVE MEANS THEREFOR, WHICH UNIT IS ALONE ONLY CAPABLE OF PUMPING AT REQUIRED FLOW RATES UP TO A PREDETERMINED HIGH LOAD PRESSURE, AN AUXILIARY PUMP OF THE CENTRIFUGAL TYPE WHICH IS CAPABLE OF PUMPING AT SAID REQUIRED FLOW RATES AGAINST LOAD PRESSURES IN EXCESS OF SAID HIGH LOAD PRESSURE, MEANS DEFINING TWO FLOW PATHS BETWEEN THE MAIN PUMPING AND METERING UNIT AND THE LOAD, AT LEAST ONE OF WHICH PASSES THROUGH THE CENTRIFUGAL PUMP, AND VALVE MEANS OPERATIVE TO ROUTE THE MAIN UNIT FLOW THROUGH THE OTHER OF THE FLOW PATHS SO LONG AS THE PRESSURE APPLIED BY THE UNIT REMAINS BELOW SAID HIGH LOAD PRESSURE AND TO DIVERT FLOW THROUGH SAID ONE FLOW PATH WHEN THE PRESSURE EXCEEDS SAID HIGH LOAD PRESSURE.