After the removal of calcium and magnesium hardness, iron, copper, colloidal silica,and other contaminants, the water is further treated for making it boiler feed water.The first step involves removal of corrosive gases. Dissolved oxygen and carbon dioxide are the targets. The corrosion caused by these gases is frequently the lesser problem; deposition of metal oxides in the boiler is a bigger problem.
Products of corrosion will become concentrated within the boiler, especially in areas where the heat transfer is high. Metal oxides will cause resistance to heat transfer in the most vulnerable part of the system. This can lead to local overheating and failure of the components. The deposits will also become heavier over time, causing pipe restriction and reduced circulation. The easiest way to deal with the problem is to avoid it, get rid of the oxygen and the carbon dioxide in the feed water.
The removal of oxygen, carbon dioxide, and other gases from feed water can be accomplished in more than one way. The feed water can be heated to reduce the solubility of the gases. This will also increase the efficiency of the boiler.Small amounts of oxygen in feed water can cause localized pitting that can cause boiler failure even when only a small amount of corrosion took place. Boilers areconstructed from carbon steel, and water is the heat transfer medium. The potential for corrosion is, therefore, high. Iron in feed water is normally in the form of an oxide. The two types of oxides are red iron oxide (Fe 2 O 3 ) and black magnetic oxide(Fe 3 O 4 ). Red oxide is formed in oxidizing conditions, while black oxide is formed under reducing conditions. Red iron oxide is converted to an insoluble hydroxide as soon as it gets into the high temperature, high alkalinity area of the boiler.
Black iron oxide is a normal product of corrosion. In new boilers, this reaction will occur and a film of iron oxide will cover the surface of the boiler. This film of magnetite will inhibit further reaction and will protect the boiler plate. The layer will be about 0.025 mm in thickness when further oxidation is inhibited. Obviously,oxidation will still take place, and a boiler will show about 1 mm of corrosion per year. This is one of the reasons why we use boiler plate that is much thicker than what is warranted when new. Water is frequently deaerated in a system where a fine spray of water comes in contact with steam. The steam will heat the droplets and sufficiently to remove the dissolved gases. The gas is then vented, and the deaerated water is ready for use.
Products of corrosion will become concentrated within the boiler, especially in areas where the heat transfer is high. Metal oxides will cause resistance to heat transfer in the most vulnerable part of the system. This can lead to local overheating and failure of the components. The deposits will also become heavier over time, causing pipe restriction and reduced circulation. The easiest way to deal with the problem is to avoid it, get rid of the oxygen and the carbon dioxide in the feed water.
The removal of oxygen, carbon dioxide, and other gases from feed water can be accomplished in more than one way. The feed water can be heated to reduce the solubility of the gases. This will also increase the efficiency of the boiler.Small amounts of oxygen in feed water can cause localized pitting that can cause boiler failure even when only a small amount of corrosion took place. Boilers areconstructed from carbon steel, and water is the heat transfer medium. The potential for corrosion is, therefore, high. Iron in feed water is normally in the form of an oxide. The two types of oxides are red iron oxide (Fe 2 O 3 ) and black magnetic oxide(Fe 3 O 4 ). Red oxide is formed in oxidizing conditions, while black oxide is formed under reducing conditions. Red iron oxide is converted to an insoluble hydroxide as soon as it gets into the high temperature, high alkalinity area of the boiler.
Black iron oxide is a normal product of corrosion. In new boilers, this reaction will occur and a film of iron oxide will cover the surface of the boiler. This film of magnetite will inhibit further reaction and will protect the boiler plate. The layer will be about 0.025 mm in thickness when further oxidation is inhibited. Obviously,oxidation will still take place, and a boiler will show about 1 mm of corrosion per year. This is one of the reasons why we use boiler plate that is much thicker than what is warranted when new. Water is frequently deaerated in a system where a fine spray of water comes in contact with steam. The steam will heat the droplets and sufficiently to remove the dissolved gases. The gas is then vented, and the deaerated water is ready for use.
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