Why is the degassing system in the water important?
Installing a deaerator in water is very important in treating deaerated water in order to meet industry standards for both oxygen content and allowable metal oxide levels in the feed water set.
Because the dissolved gases present in the water cause many corrosion problems. Dissolved oxygen in water causes surface pitting, carbon dioxide often causes corrosion in condensate systems. Especially here carbon dioxide gas combines with water to form weak carbonic acid, which will melt iron, when returning to the boiler this substance will precipitate, creating deposits in boilers and pipes. Water containing ammonium especially in the presence of oxygen will attack copper and copper alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and energy consumption.
Instructions for degassing boiler feed water:
Deaeration of boiler feed water can be done in the following ways:
1. Mechanical air removal:
Mechanical air removal is often used before adding oxygen bleach. This elimination is based on the laws of physics of Charles and Henry. This law indicates that oxygen and carbon dioxide can be removed by heating the boiler feed water thereby reducing the oxygen and carbon dioxide concentrations in the feed water.
This method is most effective at the boiling point of water at the pressure of the air removal device. Air removal equipment includes vacuum type and pressure type.
Vacuum deaerators operate at atmospheric pressure at about 820°C and can reduce the oxygen content of water to less than 0.02 mg/l. Need to use a vacuum pump or inflatable to maintain vacuum.
The pressurized air eliminator works by introducing steam into the feed water through a pressure control valve to maintain the desired operating pressure, so the temperature is at least 1050°C. . Steam raises water temperature releasing oxygen and carbon dioxide from the system. This device can reduce the oxygen content to 0.005mg/l.
Steam degassing is more commonly used in boiler feedwater degassing because:
Vapor is always available and free of oxygen and carbon dioxide
The steam heats the water and reduces the solubility of oxygen
Steam provides the necessary heating for the reaction to be complete
2. Chemical air removal:
Although mechanical de-aerators reduce oxygen to very low levels, even a very small amount of residual oxygen can have a corrosive effect on the system. Therefore, it is necessary to remove the remaining oxygen with an oxygen scavenger such as sodium sulfite or hydrazine. Sodium sulfite reacts with oxygen to form sodium sulfate which increases TDS in boiler water and improves the quality of treated water.
Hydrazine reacts with oxygen to form nitrogen and water. These are used with high pressure boilers when the solids in the boiler are low and do not increase the amount of TDS in the water.
Principles when degassing feed water:
– The solubility of any gas in a liquid is directly proportional to the partial pressure of the gas at the liquid’s surface.
– The solubility of a gas in a liquid decreases with increasing liquid temperature
– The removal efficiency is increased when the liquid and gas are well mixed
The solubility of a gas in a liquid is expressed as:
Ctotal = K x P.
Ctotal: Total concentration of the gas.
P: Partial pressure of the gas.
K: Correspondence constant.
It is thus possible to remove the gas from the water by reducing the partial pressure of the gas in the atmosphere in contact with the liquid. It can be done in 1 of 2 ways:
– Apply vacuum to the system.
– Introduce a new gas into the system.
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