There are two primary ways to feed water into boiler; off/on operation of pumps or modulating feedwater using continuous run pumps.

On/Off feedwater control utilizes a level control switch on the boiler to operate the pumps. When the water level drops typically a few inches in the boiler, the feed water pump is turned on and remains in operation until the water level is restored to the proper level. This method of control is less costly to install, service and maintain. The down side is the possibility of the boiler not being able to respond to rapid load swings without generating carryover. The carry over is created when 200-225 F water hits the 300+ F water in the boiler when the pump is replenishing water in the boiler.
< Modulating feed water control of a boiler uses an air or electrically operated valve to feed in the precise amount of feed water to maintain the level in the boiler. The modulating control valve is fed by a level control on the boiler which puts out either an air or electrical signal proportional to the water level in the boiler. Feed water pumps in this method run continuously with a pump bypass line to protect them against dead head conditions when the boiler requires no water. Although a bit more complicated and costly, modulating feed water control is a much better way to operate your boilers and eliminate carryover and wet steam problems to your plant.
In all cases, make sure you do not feed water into your boilers at a temperature less than 195-200 F. A boiler making steam at 100 psig will have water at a temperature of 338 F. If cooler water at less than 195 F enters the boiler, thermal shock will damage the boiler in the area where the cooler feedwater enters.
We can provide sales, service and installation of either method of feed water for your boilers