No oxygen is entering the boiler with the feed water when hydrazine is present in the water.
 

The objective is to maintain a hydrazine residual between 0.05-0.2 ppm depending on operating pressure and boiler designe. Actual consumption is determined under operating conditions.  A normal dosage is approximately 1 ltr. per day, depending of system layout.

Hydrazine is a convenient reductant because the by-products are typically nitrogen gas and water. Thus, it is used as an ANTIOXIDANT, an oxygen scavenger, and a corrosion inhibitor in water boilers and heating systems. 

Hydrazine is mainly used as a foaming agent in preparing polymer foams. 

No oxygen is entering the boiler with the feed water when hydrazine is present in the water.