25 May 2005
the boiler feedwater due to improper softener operation and when there is an
inadequate level of sludge or scale conditioner or dispersant.
Calcium Sulfate. Calcium sulfate (gypsum) is off-white or tan in color and
is formed in boilers that are using water of high hardness and low alkalinity without
proper treatment. Addition of a strong acid will dissolve the calcium sulfate scale with no
gas bubble formation or release of carbon dioxide. Calcium sulfate is much less
common than calcium carbonate, but it can form when there is calcium hardness in the
boiler feedwater due to improper softener operation and when there is an inadequate
level of sludge or scale conditioner or dispersant.
Calcium Phosphate. Calcium phosphate is formed when the dissolved
calcium in the feedwater reacts with phosphate treatment chemicals added to the boiler
water. With proper treatment controls, calcium phosphate forms a sludge that will be
removed in the blowdown. However, calcium phosphate can deposit as scale if the pH
of the boiler water is below 11.0 and if a sludge conditioner is not used. Addition of a
strong acid will dissolve this scale fairly easily with no gas bubble formation.
Magnesium Phosphate. Magnesium phosphate scale is an off-white
deposit formed by the reaction of magnesium salts from the feedwater with the
phosphate used in the boiler water treatment. It will form only if both the hydroxide
content and silica content of the boiler water are low. Addition of a strong acid will
dissolve this scale fairly readily with no gas bubble formation.
Magnesium Silicate. Magnesium silicate scale, an off-white deposit, is
formed from the magnesium and silica in the feedwater when the pH is above 11.0 and
the silica level is more than half that of the phosphate level in the boiler water. Normally,
it forms as a sludge that will be removed in the blowdown, but it may form scale
deposits on tubes if a sludge conditioner is not present. Most acids will not remove this
deposit. Caustic or special chemicals are needed to remove the magnesium sulfate
Iron Oxide and Iron Hydroxide. Iron oxide scales and iron hydroxide
scales are red/black deposits that are formed when the iron salts dissolved in the
feedwater react with hydroxide found in the boiler water. Usually, the dissolved iron is
introduced into the system from the condensate return due to corrosion. Iron oxide can
be deposited as a scale on the boiler tubes if the proper type of sludge conditioner is not
present. With proper water treatment, this deposit should form as sludge, rather than
scale, and can be removed by blowdown. The presence of iron oxide on the internal
boiler surfaces can be caused by oxygen corrosion of the boiler metal.
External Boiler Water Treatment. Specific technologies for external
treatment or pre-treatment are described in Chapter 2. The strategy for external
treatment is to remove unwanted impurities in the makeup water before they can enter
the boiler. Proper external treatment can eliminate, or at least minimize, scale- and