Abstract: In a steam generator or boiler of the type having a pressure vessel having a zone in which heated water and steam can be separated, an outlet for the flow of pressurized steam and an outlet for the flow of liquid, a riser section in which fluid passes for heating therein and flow into the vessel zone, and a downcomer to receive the recirculated liquid from the vessel zone and feedwater for flow to the inlet of the riser section, the system includes feedwater control apparatus for sensing the mass flow of liquid in the downcomer, determining the liquid mass in the vessel zone, downcomer and riser section and controlling the feedwater rate in relation to such mass and the respective power conditions of the system, thereby providing better stability in the system operation. Trips and other problems caused by shrink and swell are thereby avoided and other benefits achieved.