To restrain the amount of eddy current of boiler feed water, which is generated by the sudden fluctuation of a flow rate, by a method wherein a feed water flow rate control valve is controlled in accordance with the number of operating sets of pump, in a feed water system provided with a plurality of sets of boiler feed water pumps and with the pumps controlled so that a differential pressure between the upstream side of the feed water flow rate control valve and the downstream side of the same becomes a predetermined value.

In a feed water device equipped with two sets of boiler feed water pumps when one set of pump is operated, a function operator 22 is connected by a switching device 21 while the lower value between the signals from a switching device 19 and the switching device 21 is selected by a low selector 20 to transmit the same to a feed water flow rate adjusting valve 14. In a case except the operating time of one set of the pump, a function operator 23 is selected to transmit the same to the low selector 20. In this case, the function operator 22 is provided with a limit curve so as not to exceed the limit flow rate with respect to the number of revolution of the pump. When one set of pump is tripped and a preliminary machine is started automatically, the feed water flow rate adjusting valve 24 is choked so that the flow rate is within the limit for one set of pump and when two sets of the pumps are operated, the valve 24 is controlled so that the flow rate becomes within a limit for two sets of the pumps.