UFC 3-240-13FN
25 May 2005
where
C
= COC, no units
MSu = sulfate (as SO4) in makeup, ppm
4-3.2.5
Silica Scale. Silica scale can occur when the concentration of silica
exceeds its maximum solubility limit in water. A safe, very conservative value to assume
for the solubility limit is 150 ppm (as SiO2); thus, the maximum COC can be calculated
with this formula. However, silica solubility depends on pH and temperature and is in the
range of approximately 150 to 180 ppm (as SiO2) at the temperature range encountered
in most cooling towers (26 C to 54 C [80 F to 130 F]). As the pH increases in the
cooling tower water, silica becomes more soluble; thus, if cooling tower water pH is 9.0,
approximately 250 ppm silica (as SiO2) is the maximum. Using 150 ppm as the upper
limit, the allowable COC is represented by this equation:
150
≡
EQUATION
(32)
C
M
si
where
C
= COC, no units
150 = assumed maximum solubility of silica, ppm
M Si = silica (as SiO2) in the makeup, ppm
4-3.3
Determining COC to Control Operations. In cooling water systems, the
lowest calculated COC allowable, as determined by the relationships for these salts, is
the controlling factor for operations. This is because as the system operates, the
material that has the lowest calculated COC will be the first to come out of solution
(precipitate) and the most likely to form a scale deposit in the system. To prevent these
materials from forming a deposit on cooling water equipment, you must keep the COC
in the system at a level that is lower than the lowest COC calculated for calcium
carbonate, calcium phosphate, calcium sulfate, and silica. Using appropriate water
treatment chemicals will allow higher COC, depending on which chemical is used.
EXAMPLE 4-3:
a) A cooling tower makeup has the following composition:
Calcium hardness
100
ppm as CaCO3
Total (M) alkalinity
60
ppm as CaCO3
Phosphate
3
ppm as PO4
Sulfate
60
ppm as SO4
Silica
14
ppm as SiO2
pH
7.2
109
Previous Page
