Dissolved oxygen eats into iron and corrodes it. Thus dissolved oxygen should be removed from the feedwater tank else it can lead to corrosion of the tank and of the boiler tubes leading to a safety hazard.
The importance of the boiler feedtank, where boiler feedwater and make-up water are stored and into which condensate is returned, is often underestimated. Eliminating dissolved oxygen (D.O) is important because D.O in water is primarily responsible for corrosion of iron. High dissolved oxygen content leads to corrosion of the metal on the water side viz. the feedwater tank and the boiler drum and tubes. 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.
In addition to oxygen scavenging chemicals, high feedwater temperature also drives out dissolved oxygen. Thus it is important that the water in the feedtank is kept at high temperature to minimize the content of dissolved oxygen and other gases. This also has a simultaneous benefit on the boiler efficiency.
Leaks in the feedwater tank also result in hot feed water leaking into the open work environment. Additionally these leaks also reduce the quantity of water available for the boiler in case of emergency.
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.