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Water Quality and Blowdown
Maintaining Water Quality:
The steel used in MD towers has been galvanized with a heavy zinc coating
averaging 2.0 mils in thickness. The MD stainless steel option is even more
corrosion resistant than galvanized steel in certain environments. Other ma-
terials used (PVC fill, drift eliminators, and louvers, aluminum fans, etc.) are
selected to offer maximum service life in a “normal” cooling tower environment,
defined as follows:
Circulating water with a pH between 6.5 and 8; a chloride content (as NaCl)
below 500 mg/L; a sulfate content (SO
4
) below 250 mg/L; total alkalinity (as
CaCO
3
) below 500 mg/L; calcium hardness (as CaCO
3
) above 50 mg/L; a
maximum inlet water temperature not to exceed 125°F (51.7°C); no significant
contamination with unusual chemicals or foreign substances; and adequate
water treatment to minimize scaling.
• Startup Conditions: The water conditions during the initial tower operation
are crucial in preventing premature corrosion of galvanized steel (white
rust). For at least the first eight weeks of operation, pH should be controlled
between 6.5 and 8.0 with hardness and alkalinity levels between 100 and
300 mg/L (expressed as CaCO
3
).
• Chlorine (if used) shall be added intermittently, with a free residual not to
exceed 1 mg/L—maintained for short periods. Excessive chlorine levels may
deteriorate sealants and other materials of construction.
• An atmosphere surrounding the tower no worse than “moderate industrial”,
where rainfall and fog are no more than slightly acid, and they do not contain
significant chlorides or hydrogen sulfide (H
2
S).
• Many proprietary chemicals exist for control of scale, corrosion, and biologi-
cal growth and should be used prudently. Also, combinations of chemicals
may cause reactions which reduce treatment effectiveness, and certain
chemicals such as surfactants, biodispersants and antifoams may increase
drift rate.
Unless you purchased a stainless steel MD, the structure of your MD
tower consists primarily of galvanized steel, therefore your water
treatment program must be compatible with zinc. In working with
your water treatment supplier, it is important that you recognize
the potential effects on zinc of the specific treatment program you
choose.
Note
operation
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