www.saVRee.com Deaerators Explained| pg. 2
A typical deaerator will remove almost all dissolved oxygen and CO2, with the remainder
being removed by oxygen scavengers (sodium sulphite, hydrazine etc.) and CO2
scavengers (neutralising amines, bicarbonate etc.).
Most deaerators are designed to reduce dissolved oxygen levels to 0.05 cc/l (7 ppb), with
oxygen scavengers removing the remainder.
Why remove oxygen and carbon dioxide?
Corrosion of boiler components exposed to water will occur if dissolved oxygen is
present, or, the water pH is low.
Boilers and their ancillary systems are mostly constructed of carbon steel. As steel is iron
based and oxygen reacts with iron to form red iron oxide (rust), the potential for
corrosion is high. For this reason, it is imperative that the boiler feedwater dissolved
oxygen content is as low as possible.
The amount of dissolved carbon dioxide in water dictates how acidic the water is. The
greater the dissolved CO2 in the water, the lower the water’s pH i.e. the more acidic the
water is. Low pH values will cause corrosion of boiler parts and consequently must be
avoided. A typical boiler will operate with a pH value of between 8 to 11 (approx.), but
this depends heavily upon the boiler system.
The corrosion rate is not only dependent upon the dissolved oxygen and dissolved
carbon dioxide levels, it is also dependent upon temperature. High temperatures cause
high corrosion rates, even with low amounts of dissolved gases. For this reason, low
temperature steam systems can tolerate much higher levels of dissolved oxygen and
carbon dioxide than high temperature systems.
What is corrosion?
Corrosion can be classed as general, localised, or stress.
General corrosion occurs within a single system component, or throughout the entire
system, and is usually considered light corrosion. A thin red oxide layer covering the water
side heat transfer surfaces of a boiler is an example of general corrosion. General
corrosion is often red (iron oxide) or black (magnetite oxide) in colour. If waterside
metal surfaces are red, the metal is corroding, and corrective action must be taken. Black
surfaces are desired as magnetite oxide deters further corrosion.
Localised corrosion relates to corrosion within a speci�c area; this type of corrosion is
usually moderate to extensive. Oxygen pitting (small holes in a metal surface caused by