The boiler Water system consists of ECONOMIZER AND DRUM & FURNACE WATER WALL.
1) ECONOMIZER
Feedwater is supplied from the feedwater system to the boiler and is preheated by the economizer, Feedwater quality shall be properly controlled by the feedwater chemical treatment system. The economizer is situated at downstream of the reheater located in back end of the boiler.
Since sufficient feedwater is not supplied into the economizer though the heat is absorbed from flue gas at the boiler start-up and low load operation, steaming may occur in the economizer. Therefore the economizer recirculation line is provided between the furnace downcomer and the economizer inlet feedwater piping. This line provides the recirculation flow between the drum, the downcomer and the economizer so as to prevent steaming in the economizer. Economizer recirculation valve is only opened at initial stage of the boiler start-up.
2) DRUM & FURNACE WATER WALL
Feedwater preheated in the economizer is supplied to the drum. At the drum, water flows downward through the downcomer by gravity, and then flows into the furnace water wall. Steam is generated at the furnace water wall and water/steam mixture flows upward to the drum because of its small gravity. In this way, natural circulation is formed at drum, downcomer and furnace water wall.
Drum water is treated by the phosphate injection system. And the drum has the continuous blowdown valve to discharge suspended solid from the drum water during normal operation. The drum also has cyclone separators and scrubber to separate the water from saturated steam. The startup blowoff valves are provided to the drum to handle the swelling phenomenon during the boiler start-up period.
“This description is FOR HITACHI Steam Generator “