Page 177 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.13

PREDICTING SCALE FORMATION
AND CORROSION TENDENCIES

GENERAL

A common and costly water-caused problem is the formation and deposit of mineral scale.
Although scale deposits may contain a complex mixture of mineral salts, the primary con-
stituent is calcium carbonate.
Most salts are more soluble in hot water than cold water. Calcium and magnesium salts,
on the other hand, dissolve more readily in cold water than hot. As a result, they tend to
deposit on surfaces when there is a rise in temperature. The following are the primary fac-
tors which affect this tendency:
1. Alkalinity
2. Hardness (calcium)
3. pH
4. Total amount of dissolved solids
5. Temperature

pH
The pH value reflects the ratio of bicarbonate to carbonate alkalinity. A higher pH value
indicates a greater carbonate content of the water, and the increased tendency of calcium
and magnesium carbonates to precipitate out of solution. Also, the higher the temperature,
the greater the tendency of dissolved solids to precipitate out of solution because of their
property of inverse solubility.

LANGELIER SATURATION INDEX (LSI)

In the 1930s, W. F. Langelier studied the primary factors that affected the tendency of water
to form deposits of mineral scale on heat transfer equipment. As a result of this work, the
Langelier index was created. The index is based on numerical values given to the factors
that affect deposits. This index is best known as the Langelier saturation index (LSI).
The LSI is actually a calcium carbonate saturation index. It is based on the assumption
that water with a scaling tendency will tend to deposit a corrosion-inhibiting film of cal-
cium carbonate and thus be less corrosive. A water with nonscaling tendency will tend to
dissolve protective films and thus be more corrosive. The interpretation of the LSI is based
on numerical values given in Table 4.4. The LSI is calculated as follows:
LSI = pH − pHs (4.2)

where LSI = Langelier saturation index
pH = value obtained from testing the water in question
pHs = value calculated for the pH of saturation for the calcium carbonate present in
the water in question

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