PURPOSE: To maintain impellers fully opened by providing an emergency hydraulic circuit which includes an accumulator additionally to a hydraulic circuit for controlling the impellers, and by opening solenoid valves in response to a trouble signal from the impeller control circuit to actuate the emergency hydraulic circuit.

CONSTITUTION: If some trouble occurs in an impeller control circuit, an operation stop signal is issued and responding to the signal, solenoid valves 22 and 23 are opened and solenoid valves 26 and 27 are closed. Pressure oil in an accumulator 21 is then guided through the solenoid valve 22 and pilot check valve 24 into a hydraulic cylinder 3 so that a piston 14 and a lever 16 is forcibly pushed up under the influence of the pressure oil toward the impellers opening direction. A control rod 10 is similarly pushed up to maintin impellers 7 in the fully opened angle. Accordingly, when the control circuit contains a defective, it does not require instant correction and the pump is allowed to proceed its operation as a pump with fully opened fixed impellers.