food grade product line by Water Services Ltd

Common scales found in boilers.

The most common scales consist of calcium and magnesium salts and iron oxide.

a. Calcium carbonate. Calcium carbonate is caused by the breakdown of calcium bicarbonate by heat. It is found in untreated boilers, or improperly treated boilers.

b. Calcium sulfate. Calcium sulfate (gypsum) is found in boilers using high hardness, low alkalinity water, without proper treatment.

c. Calcium phosphate. Calcium phosphate is formed when the calcium in the feedwater reacts with the phosphate used for treatment. With proper treatment, this will form a sludge which can be removed in the blowdown. However, calcium phosphate can deposit as scale if the pH of the boiler water is below 11.0 and a sludge condition is not used.

d. Magnesium phosphate. Magnesium phosphate is caused by the reaction of magnesium salts in the feedwater with the phosphate used in the treatment. It will form only if the hydroxide content and the silica content of the boiler water are low.

e. Magnesium silicate. Magnesium silicate is formed from the magnesium and silica in the feedwater if the pH is above 11.0 and the silica level is more than half the phosphate level. Normally, it forms as a sludge that can be removed in the blowdown, but it may deposit on tubes as scale if a sludge conditioner is not present.

f. Iron oxide. Iron oxide is formed from the dissolved iron in the feedwater and hydroxide in the boiler water. Usually, the dissolved iron comes from the condensate return. This also should occur as a sludge which can be removed in the blowdown, but iron oxide can deposit as a scale on the boiler tubes if a sludge conditioner is not present.