Dissolved oxygen eats into iron and corrodes it leading to steam leaks. High feedwater temperature drives out dissolved oxygen, thus preventing leaks and saving fuel.
The importance of the boiler feedtank, where boiler feedwater and make-up water are stored and into which condensate is returned, is often underestimated. It is important that the water in the feedtank is kept at high temperature to minimize the content of dissolved oxygen and other gases.
Eliminating dissolved oxygen (D.O) is important because D.O in water is primarily responsible for corrosion of iron. This fact is duly recognized by plant personnel and oxygen scavenging chemicals are usually added to condition the boiler feedwater. As per BIS std 845, a D.O level of 2 ppm in boiler feedwater is acceptable.
As seen in the above graph, this level of 2ppm is achieved if the feedwater temperature is raised to 90°C. At this temperature D.O is driven out of the water, thus leading to savings in the quantity of oxygen scavenging chemicals required to be added. Reduced chemical addition translates to reduced scaling and boiler blowdown. Returning condensate and flash to feedwater tank raises the feedwater temperature in an energy efficient manner.