PURPOSE: To aim at economic operation, by setting up overheat preventing escape piping provided with a built-in solenoid valve as well as a pressure signal generator in a pump discharge line, while providing the pump with a rotational speed signal generator so as to cause signals from both these generators to be inputted into an arithmetic unit, through which a minimum flow of liquid discharge corresponding to a rotational speed variation that satisfies a specific formula is released through the escape piping.

CONSTITUTION: A rotational speed signal Ni from the rotational speed signal generator installed to a pump and a pressure signal Hi from the pressure signal generator installed in the discharge line are inputted in to an arithmetic unit, and when these relationships between them are found to be Hi≥αNi2 (α:constant), a solenoid valve is fully opened, then the valve is kept up in a full-open state until the condition of Hi≤βNi2 (β: constant) is satisfied and when it is satisfied, the valve is, in turn, fully closed. On the other hand, when Hi<αNi2 is the case, the solenoid valve is kept up without being fully opened; solenoid valve operation is achieved after some time waiting for finding of the condition of Hi≤βNi2.