SCALE CONTROL

As water heats and is converted into steam, contaminants brought into the system with makeup water are left behind in the boiler. The boiler acts like a distillation unit, taking pure water out as steam and leaving behind the minerals and other contaminants in the boiler. Scale forms as a result of the precipitation of normally soluble solids from solution because of heat that makes them become insoluble. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate.

OXYGEN PITTING CORROSION

Oxygen gets into the boiler system through a variety of sources, but primarily as a contaminant from the makeup water. Under the conditions of pressure and high temperature seen in boiler systems, very small amounts of oxygen getting into the boiler can cause corrosion. Oxygen pitting corrosion will cause isolated, deep craters in the metal surface. The deep pitting caused by oxygen corrosion can lead to sudden and early failure of boilers. Dissolved oxygen is removed either by mechanical deaeration, or by chemical oxygen scavenging.

Even with a deaearator working perfectly, oxygen scavenger is necessary to ensure that corrosion is minimized. Without a deaerator, generous amount of an oxygen scavenger may be necessary to assure complete oxygen removal. Sodium sulphite is by far the most common oxygen scavenger used. In a boiler system that operates without a deaerator, it is the oxygen scavenger that is of choice. In high-pressure systems, over 450 psig, alternative oxygen deaerator may be desirable.

STEAM AND CONDENSATE SYSTEM CORROSION

Corrosion in the steam and condensate system can be due to either oxygen attack or low pH corrosion. Low pH corrosion is due to the presence of carbon dioxide in the steam. This reacts with the Condensate to form carbonic acid, which attacks the piping.

The pH of untreated condensate can be as low as 3.5-4.0 which is extremely acidic. This acidic condensate attacks the metal in the piping, particularly along the bottom of horizontal runs, or in areas where the piping is thin, such as at threaded joints.

Corrosion is increased at weld locations, where the metal is heat stressed from the process of being welded.

Low pH corrosion is controlled by the use of neutralizing amines.

The neutralizing amine functions by reacting and neutralizing the acid in the condensate water. It elevates the pH above the corrosive range. Maintaining pH levels between 7.5 and 8.5 in the returned condensate controls neutralizing amines. If the pH is allowed to fall bellow these limits, corrosion is likely to occur. If condensate pH reaches 9.0 or above, there is a possibility of the formation of amine carbonates, which can cause temporary deposit problems.

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