PURPOSE: To reduce the variation of a discharging pressure by a method wherein pressure detectors for a high pressure side and a low pressure side, and a means for controlling the driving of a pump, which is effecting a waiting motion for a period of time longer than a time necessary for stopping, starting or changing the speed of the pump, are provided in the system.
CONSTITUTION: When a perssure condition is decided that it is lower than an objective water feeding pressure based on the detecting signals of the low pressure side pressure detector PSL and the high pressure side pressure detector PSH, the starting operations for the stopping pumps P, P1, P2 or the speed increasing operations of the pumps P, P1, P2 are effected. On the other hand, when the pressure condition is decided that it is higher than the objective water feeding pressure, the speed decreasing operations or the stopping operations of the operating pumps P, P1, P2 are effected while the waiting motions for a period of time longer than a time necessary for starting, stopping or changing the speeds of the pumps P, P1, P2 are effected before or after the decision of the pressure condition.
MITA YASUHIRO
TAGAWA YUKIO
