Predicting Scale Formation by Calculation

The values obtained from these calculation procedures should be taken only as guidelines. They indicate the likelihood of scale formation. Many assumptions had to be made in developing the method of calculation, which may not apply to the specific water being evaluated. If scale formation is indicated by calculation, it serves as an alarm. If you are looking at a possible water source, you should avoid those that show scaling tendencies or make provision for treatment Similarly, you should avoid mixing waters where the blend exhibits scaling tendencies under system conditions. 

Some of the critical properties of water change very quickly after sampling. These properties should be determined in the field immediately after the sample is taken in order to determine an acceptable scaling tendency. Two properties that should be determined in this manner to have any value are pH and bicarbonate (HCOj ). The instant that the pressure is reduced, any dissolved acid gases (HjS and COj) will begin to escape from the water and the pH will begin to rise. The loss of dissolved CO2 will have a direct effect on the bicarbonate and carbonate concentrations. Generally only bicarbonate determination is needed carbonate concentration is small. 

Calcium carbonate precipitation is caused by a shift toward carbonates in the carbonate-bicarbonate-carbon-dioxide equilibrium. When the equilibrium shifts in the other direction, the precipitate goes back into solution. Since there is usually considerable delay between the establishment of an equilibrium and the precipitation or dissolution of calcium carbonate, unstable conditions exist in which water will precipitate or dissolve calcium carbonate on standing. 

Langelier developed an equation setting forth the conditions of the carbonate equilibrium. By the use of this equation, the pH of water at equilibrium can be calculated. If the pH is higher than the calculated pH, the water has a tendency to form scale if it is lower, the water has a tendency to be corrosive. Langeher s equation can be expressed in a single form as follows  

SI is the stabUity index. A positive index indicates scale formation. A negative index indicates corrosion. This equation was derived with many assumptions that do not apply in all cases. Because of this, the results should not be applied too adamantly. If SI is -1-0.5 or above, consider the system 

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