PURPOSE: To aim at economic operation free from overheating by setting up overheat preventing escape piping provided with a built-in solenoid valve as well as a flow meter in a pump discharge line, while providing the pump with a rotational speed signal generator, through which each signal of flow and rotational speed is inputted into an arithmetic unit and controlling the valve so as to release a minimum flow of liquid discharge which satisfies a fixed formula via the said escape piping.

CONSTITUTION: A rotation signal Ni from the rotational speed signal generator installed in the pump and a flow signal Qi from the flow meter installed in the discharge line are inputted to an arithmetic unit and when these relationships between them are found to be Ni≥αQi (α: constant), a solenoid valve is fully opened, then the valve is kept up in a full-open state till the condition of Ni≤ βQi (β: constant) is satified, and when it is satisfied, the solenoid valve is, in turn, fully closed. On the other hand, when the case is of Ni<αQi, the valve is left without being fully opened; solenoid valve operation is achieved after some time waiting for finding of the condition of Ni<βQi.