The condensate is heated to saturated conditions in the deaerator, usually by the steam extracted from the steam turbine. The extraction steam is mixed in the deaerator by a system of spray nozzles and cascading trays between which the steam percolates. Any dissolved gases in the condensate are released in this process and removed from the deaerator by venting to the atmosphere or the main condenser. Directly below the deaerator is the feedwater storage tank, in which a large quantity of feedwater is stored at near saturation conditions. This feedwater can be supplied to steam generators to maintain the required water inventory during a transient in the turbine trip event. The deaerator and storage tank are usually located at a high elevation in the turbine hall to ensure an adequate net positive suction head (NPSH) to the feedwater pumps at the inlet. NPSH is used to measure how close a fluid is to saturated conditions. Lowering the pressure at the suction side can induce cavitation. This arrangement minimizes the risk of cavitation in the pump.
In general, a deaerator is a device used to remove oxygen and other dissolved gases from the feedwater to steam generators. The deaerator is part of the feedwater heating system. It is usually situated between the last low-pressure heater and feedwater booster pumps. In particular, dissolved oxygen in the steam generator can cause serious corrosion damage by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Furthermore, dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion.