The feedwater is usually stored, pre-heated and conditioned in a feedwater tank and forwarded into the boiler by a boiler feedwater pump.
The feedwater has to be specially conditioned to avoid problems in the boiler and downstream components:
Corrosion - Corrosive components, especially O2 and CO2 have to be removed, usually by use of a deaerator. Remnants can be removed chemically, by use of oxygen scavenger. Furthermore feedwater has to be alkalized to a pH of 9 or higher, to reduce oxidation and to support the forming of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkalic agents into the feedwater, like sodium hydroxide (caustic soda) or volatile ammonia.
Deposits / Sediments / Fouling - Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile salts and minerals that would remain in soluted form when the feedwater is evaporated have to be removed, because these would be concentrated in the liquid phase and require excessive "blow-down" (draining) to avoid that the liquid eventually becomes saturated and solid crystals fall out. Even worse are minerals that form limescale. Therefore, the make-up water added to replace any losses of feedwater has to be demineralized/deionized water.