UFC 3-240-13FN
25 May 2005
level that is equal to the lowest COC allowable for whichever salt has the lowest
solubility. The salt of concern is often calcium carbonate or calcium phosphate, but it
may be silica. The operating COC can be increased substantially with use of the cooling
water treatment chemicals described in this paragraph.
4-3.2.2
Calcium Carbonate Scale. Calcium carbonate scale results from the
breakdown of calcium bicarbonate, a naturally occurring salt. The degree of scaling
depends primarily on the calcium levels, bicarbonate alkalinity levels, and water
temperature in the cooling water system. The most accurate prediction of scale can be
developed using the Practical (Puckorius) Scaling Index (PSI) (see paragraph 4-3.4.2).
A rough prediction of calcium carbonate scale potential can be developed using this
formula:
110000
≡
C
EQUATION
(29)
TA x M ca
where
C
= COC
TA = total (M) alkalinity (as CaCO3) in makeup, ppm
M Ca = calcium hardness (as CaCO3) in makeup, ppm
4-3.2.3
Calcium Phosphate Scale. Calcium phosphate scale results when
calcium hardness reacts with phosphate. This will occur when more than 10 ppm of
orthophosphate are present in the circulating water and when the calcium hardness is
sufficiently high. The following formula can provide a very rough prediction of the
potential for calcium phosphate scale:
(105) x (9.8 - B pH )
≡
C
EQUATION
(30)
M ca
where
C
= COC, no units
BpH = measured pH in blowdown, pH units
MCa = calcium hardness (as CaCO3) in makeup water, ppm
4-3.2.4
Calcium Sulfate Scale. Calcium sulfate scale results when the calcium
hardness reacts with the sulfate. The potential for calcium sulfate scale can be
predicted using this formula:
1250000
≡
C
EQUATION
(31)
M ca x M
su
108
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