Oxygen scavengers for Boiler Water

Oxygen scavengers are best fed to the storage section of the deaerator or feedwater tank. In most low-pressure industrial boiler systems, if oxygen remains in the feedwater and enters the boiler, it is too late to remove all of the oxygen and protect the boiler and condensate from oxygen corrosion. A residual of sulfite or other.

Oxygen scavenger in the boiler cannot chemically reduce all of the oxygen before it escapes with the steam or reacts with the metal surfaces.

The oxygen scavenger should be pumped either continuously or paced with makeup into the deaerator so that there is a positive residual at all times in the deaerator. Consideration should be given to see where the makeup and condensate enter the deaerator. Often times the high pressure condensate enters directly into the storage section. If the condensate contains oxygen, intermittently high dissolved oxygen levels in the feedwater may occur. Also sudden load changes can allow temporary high dissolved oxygen levels. Even with sulfite feed to the deaerator, but especially if sulfite is fed to the boiler, the boiler sulfite residual may not show a big change as oxygen is released into the steam or corrodes the boiler before it is reduced chemically. Dissolved oxygen testing in the feedwater would be the best tool to insure consistently low dissolved oxygen levels and adequate chemical feed and control.

How condensate is returned, the level controls on the deaerator, and the proportion of makeup to condensate can affect deaerator performance and therefore results. If there is a relatively small deaerator volume and the amount of condensate return varies throughout the day or night, load and performance can change. Dissolved oxygen content and feedwater oxygen scavenger concentration should be checked under the changing conditions.

These situations could explain why one system sees corrosion and another does not, even though it appears to be the same treatment and control.

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