25 May 2005
All three methods are authorized for use on military installations and can be
used in combination with one another.
Calcium Carbonate Scale Control Using Solubilizing Chemicals.
Acids and phosphonates are chemicals that keep scale from forming. The use of acid in
cooling towers may not be appropriate for use at military installations due to the
associated risk of corrosion.
Acids. The acid most commonly used is sulfuric acid used as a diluted
solution (e.g., 40% sulfuric acid in water). The use of acids requires adequate pH
Phosphonates. The phosphonates used most frequently for calcium
carbonate scale control in recirculating cooling water systems are AMP (amino-tri
[methylene] phosphonic acid); HEDP (1-hydroxyethylidene 1,1-diphosphonic acid); and
PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid).
The chemical reaction of all phosphonates is similar; however, their stability
varies greatly. The presence of chlorine or other oxidants in treated cooling water favors
the use of PBTC, which is very resistant to decomposition, followed by HEDP, and
finally AMP. An active dosage of 3 to 5 ppm of either AMP or HEDP, or 1.5 to 2.5 ppm
PBTC, will increase the solubility of calcium carbonate by a factor of 3 or more relative
to using no chemical treatment. Rather than operating at a PSI of 6.0 (stable water, no
scale) in an untreated system, the cooling tower water can be used at a PSI of 4.0
without the occurrence of scale (see paragraph 4-3.4.2 and Appendix B); however, in
the absence of calcium scaling conditions, phosphonates can increase the corrosion of
both mild steel and copper.
Calcium Carbonate Scale Control Using Solubilizing Polymers. Many
different polymers are used in water treatment. For the most part, they have multi-
faceted performance capability; they can inhibit various types of scale formation as well
as disperse SS. Often water treatment products will include more than one type of
polymer in the product formulation. For control of calcium carbonate, homopolymers
such as polyacrylate, polymethacrylate, and polymaleate are used to keep calcium
carbonate in solution. Dosages of 3 to 5 ppm of active polymer in the cooling tower
water can control calcium carbonate scale formation to a PSI value as low as 4.5.
Calcium Carbonate Scale Control Using Sludge-Forming Polymers.
Certain homopolymers and copolymers act as crystal modifiers by distorting calcium
carbonate crystals such that they do not attach themselves to heat exchange surfaces,
but instead the crystals become SS that can be removed through filtration or blowdown.
Usually dosages of 1 to 3 ppm of active polymer in the cooling tower water will control
calcium carbonate scale. Due to formation of sludge, rather than the stabilization of
carbonate in solution, the PSI is not meaningful under these conditions.
Calcium Phosphate Scale Control Using Solubilizing Inhibitors. Often
calcium phosphate scale is formed in cooling water systems treated with a phosphate-