Oxygen is the main cause of corrosion in hot well tanks, feed lines, feed pumps and boilers. If carbon dioxide is also present then the pH will be low, the water will tend to be acidic, and the rate of corrosion will be increased. Typically the corrosion is of the pitting type where, although the metal loss may not be great, deep penetration and perforation can occur in a short period. Elimination of the dissolved oxygen may be achieved by chemical or physical methods, but more usually by a combination of both.
The essential requirements to reduce corrosion are to maintain the feed water at a pH of not less than 8.5 to 9, the lowest level at which carbon dioxide is absent, and to remove all traces of oxygen. The return of condensate from the plant will have a significant impact on boiler feed water treatment - condensate is hot and already chemically treated, consequently as more condensate is returned, less feed water treatment is required.
Water exposed to air can become saturated with oxygen, and the concentration will vary with temperature: the higher the temperature, the lower the oxygen content.
The first step in feed water treatment is to heat the water to drive off the oxygen. Typically a boiler feed tank should be operated at 85°C to 90°C. This leaves oxygen content of around 2 mg / litre (ppm). Operation at higher temperatures than this at atmospheric pressure can be difficult due to the close proximity of saturation temperature and the probability of cavitations in the feed pump, unless the feed tank is installed at a very high level above the boiler feed pump.
The addition of an oxygen scavenging chemical (Sodium Sulphite, Hydrazine or Tannin) will remove the remaining oxygen and prevent corrosion.