Scaling indices Ryznar stability index

Calcium—In general, calcium (as CaCOs) below 800 ppm should not result in calcium sulfate scale. In arid climates, however, the critical level may be much lower. For calcium carbonate scaling tendencies, calculate the Langelier Saturation Index or the Ryznar Stability Index. 

When the number of concentrations of the circulating water is in the order of 3-7, some of the salts dissolved can exceed their solubility limits and precipitate, causing scale formation in pipes and coolers. The purpose of the treatment of the cooling water is to avoid scale formation. This is achieved by the injection of sulfuric acid to convert Ca and Mg carbonates (carbonate hardness) into more soluble sulfates. The amount of acid used must be limited to maintain some residual alkalinity in the system. If the system pH is reduced to far below 7.0, it would result in an accelerated corrosion within the system. As stated earlier, scale formation and/or corrosion tendency is defined by the Saturation Index (Langelier Index) and Stability Index (Ryznar equation). 

Over the years, the use and application of several different saturation indices that aid the prediction of scaling and corrosion potential in different waters have evolved to become an integral part of the business of treating and managing cooling water systems. The two most popular are the Lange-lier Saturation Index (LSI) and the Ryznar Stability Index (SI), which were both introduced more than 50 years ago. 

I I Water treatment engineers use the Langelier saturation index (LI) and Ryznar stability index (Rl) [7,2] to evaluate the scaling and corroding tendencies of water. Here is a graphical method for determining these pa-... 

RYZNAR STABILITY INDEX - An index based on calcium carbonate pH of saturation vs. actual pH to determine scaling or corrosion tendencies of a water (R.I. = 2 pHs). 

Water with a Ryznar stability index less than 6.0 is definitely scaleforming indices of 6.0-7.0 are regarded as good for domestic waters possibly at values above 7.0, and certainly with indices above 7.5, the waters do not form a protective scale. One of the reasons for the discrepancies noted in the performance of waters with identical saturation indices lies in the buffer capacity of a water. 

The Ryznar stability index (RSI) uses a correlation established between an empirical database of scale thickness observed in municipal water systems and associated water chemistry data. As with the LSI, the RSI has its basis in the concept of saturation level. The RSI takes the following form ... 

John Ryznar of NALCO developed a useful modification called the Stability Index, based on empirical data from field locations. The Saturation pH is determined exactly the same way, but the Index is obtained differently. The Stability Index is calculated from (2 pH - pH). It always will be a positive whole number. stability Index numbers around 7 show the water is quite stable, neither corrosive nor scaling. As the Stability Index drops below 6, water becomes progressively more scaling. On the other hand, as the Stability Index rises above 7, water becomes progressively more corrosive. A cooling water with a Stability Index of 10 might produce severe to intolerable corrosion conditions. 

Equilibrium curves can be determined and, on the basis of a given water composition, used to establish whether a water is scaling (not corrosive) or non-scaling (corrosive). Many different methods have been proposed for prediction of the formation of CaCOa. Commonly used equations or indices are the Langelier Saturation Index, LSI (Langelier, 1936) and the Stability Index, RSI, a modification of the LSI proposed by Ryznar (1944). In the German literature the LSI is known simply as the Saturation Index, Is (Morbe et al., 1987). These indices describe the algebraic difference between the actual pH of the water and the calculated pH at which it would be saturated with calcium carbonate ... 

The tendency of water to form scale or cause corrosion is measured by either the Langelier Scaling Index (LSI), which is also called the Saturation Index, or the Ryznar Stability Index (RSI), which is also called the Stability Index (Table 4.2). 

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