Page 357 - Corrosion Engineering Principles and Practice

326 C h a p t e r 8 C o r r o s i o n b y W a t e r 327

The study revealed that calcium carbonate (calcite) scale formed
most readily on heat-transfer surfaces in systems operating in a calcite
saturation level range of 20 to 150 mg/L, the typical range for
chemically treated cooling water. At much higher saturation levels, in
excess of 1000, calcite precipitated in the bulk water. Because of the
overwhelming high surface area of the precipitating crystals relative
to the metal surface in the system, continuing precipitation would
lead to growth on crystals in the bulk water rather than on heat-
transfer surfaces. The presence of ozone in cooling systems did not
appear to influence calcite precipitation and/or scale formation.

8.8.3 Optimizing Calcium Phosphate Scale Inhibitor
Dosage in a High-TDS Cooling System
A major manufacturer of polymers for calcium phosphate scale
control in cooling systems has developed laboratory data on the
minimum effective scale inhibitor (copolymer) dosage required to
prevent calcium phosphate deposition over a broad range of calcium
and phosphate concentrations, and a range of pH and temperatures.
The data were developed using static tests, but have been observed to
correlate well with the dosage requirements for the copolymer in
operating cooling systems. The data were developed using test waters
with relatively low levels of dissolved solids. Recommendations from
the data were typically made as a function of calcium concentration,
phosphate concentration, and pH. This database was used to project
the treatment requirements for a utility cooling system that used
geothermal brine for makeup water. An extremely high dosage (30 to
35 mg/L) was recommended based upon the laboratory data [22].
It was believed that much lower dosages would be required in
the actual cooling system because of the reduced availability of
calcium anticipated in the high-TDS recirculating water. As a result, it
was believed that a model based upon dosage as a function of the
ion-association model saturation level for tricalcium phosphate
would be more appropriate, and accurate, than a simple lookup table
of dosage versus pH and analytical values for calcium and phosphate.
Tricalcium phosphate saturation levels were calculated for each of
the laboratory data points. Regression analysis was used to develop a
model for dosage as a function of saturation level and temperature.
The model was used to predict the minimum effective dosage for
the system with the makeup and recirculating water chemistry found
in Table 8.21. A dosage in the range of 10 to 11 mg/L was predicted,
rather than the 30 mg/L derived from the lookup tables. A dosage
minimization study was conducted to determine the minimum
effective dosage. The system was initially treated with the copolymer
at a dosage of 30 mg/L in the recirculating water. The dosage was
decreased until deposition was observed. Failure was noted when
the recirculating water concentration dropped below 10 mg/L,
validating the ion–association-based dosage model.