25 May 2005
based corrosion inhibitor program or when phosphate is present in the makeup water
(i.e., potable or recycled water). Calcium phosphate is much less soluble in water than
is calcium carbonate. If the calcium hardness is 500 ppm and the pH is above 7.0,
without any polymer treatment calcium phosphate scale will likely form, even at the low
level of 10 ppm phosphate (as PO4) in the cooling water (see paragraph 4-3.3).
Calcium phosphate solubility can be increased by a factor of a little less than 3 by the
addition of 4-ppm phosphonate (HEDP/PBTC) or by the use of 6 to 8 ppm of a
copolymer or terpolymer specific for calcium phosphate inhibition.
Calcium Sulfate Scale Control Using Solubilizing Polymers. Calcium
sulfate formation can result from high concentrations of calcium ions and sulfate ions in
the recirculating water; however, calcium sulfate is the most soluble of the scale-forming
calcium salts found in cooling tower waters having pH levels of greater than 8.0. This
means that calcium sulfate scale will not form unless some calcium ions (hardness)
remain in solution after the calcium reacts with all the carbonate and phosphate in the
water. Calcium sulfate scale may occur when the recirculating water contains calcium
hardness in the range of 500 to 700 ppm as CaCO3 and sulfate in the range of 500 to
700 ppm SO4. (See the predictive index in paragraph 4-3.4) The addition of 3 to 5 ppm
of a copolymer of acrylate and acrylamide will allow calcium sulfate to remain in solution
at a level almost 3 times the level allowed when using no treatment. Calcium sulfate
scale rarely forms at pH levels above 8.0 in the cooling water.
Magnesium Silicate Scale Control. Formation of magnesium silicate is
possible in cooling systems, but only under certain rare conditions. Magnesium ions
(hardness) first react with hydroxyl ions (OH-) to form magnesium hydroxide, which then
reacts with (absorbs) dissolved or colloidal silica. A deposit analysis often reports this
material as magnesium silicate. Since magnesium hydroxide solubility decreases at pH
levels above 9.0, this scale will usually occur only at a pH level above 9.0 and when the
magnesium hardness concentration is greater than 100 ppm.
Silica Scale Control. Silica solubility is dependent upon temperature and
pH. At pH levels greater than 8.5, silica remains soluble (no scale) at a concentration of
250 ppm as SiO2. At pH levels of 7.5 or below, maximum silica solubility is 150 ppm as
SiO2. At maximum silica levels, silica will first deposit on the cooling tower slats rather
than in the heat exchanger because silica is more soluble in hot water than in cold
water. The slats will become coated with a white, sometimes sparkling, deposit. If this
occurs, blowdown should be increased to decrease COC by at least 1 unit. This
procedure should stop additional scale formation. If the concentration of silica in the
makeup water is above 30 ppm, it will usually be the parameter that controls the
adjustment of cooling water system COC. If the silica concentration is high, external
treatment can reduce the level of silica in the makeup water (see Chapter 2). The
introduction of water treatment chemicals based on new polymer technology may allow
the solubility of silica to be increased above the old recognized limit of 150 ppm.