UFC 3-240-13FN
25 May 2005
At what COC can the system operate scale free without water treatment?
(Assume that the estimated pH in the blowdown water is 8.5.)
b) Based on calcium carbonate:
110000
≡
≡ 4.3
EQUATION
(33)
COC
60 x 100
c) Based on calcium sulfate:
1250000
≡
≡ 14.4
EQUATION
(34)
COC
100 x 60
d) Based on silica:
≡
150 14 = 10.7
COC
EQUATION
(35)
The COC determined for calcium carbonate is lowest at 4.3, and this controls
the system operation. If the system is operated without water treatment, scaling should
not occur if the system is operated at less than 4.3 COC. Use of scale control treatment
will allow the number of allowable COC (for calcium carbonate) to be increased; you
can then determine the COC and blowdown by using a calculated scaling index.
4-3.4
Calcium Carbonate Scaling Indices. The scale found most commonly in
cooling tower water systems is calcium carbonate, present in the form of calcite
increase in temperature, is a complex function of temperature, TDS, calcium hardness,
total alkalinity, and pH. To predict if scale would form in the hotter sections of a cooling
water system, researchers have developed several scaling indices. Paragraphs 4-3.4.1
and 4-3.4.2 describe the predictive indexes that are used most commonly for cooling
water.
4-3.4.1
Langelier and Ryznar Indices. W.F. Langelier derived a method to
calculate the calcium carbonate scale-forming and scale-dissolving tendencies of
drinking water. The method is based on determining the saturation pH (pHs) at which
calcium carbonate scale will start to precipitate out of solution. If the measured pH
(pHactual) of the water is greater than its pHs, thus a positive value, the water has a
scale-forming tendency. If the measured pH (pHactual) of the water is less than its pHs,
thus a negative value, the water will have a scale-dissolving tendency. The pHactual
minus pHs is known as the Langelier Index or Langelier Saturation Index (LSI) (LSI =
pHactual pHs). This index was originally designed to predict calcium carbonate scale in
potable water. There are serious deficiencies in the accuracy of this index;
consequently, it has lost its practical application for cooling water systems.
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