Ryznar

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. 

Also several other indices, esp. Ryznar Index are now used. 

Note LSI and all derivations (such as Ryznar, Puckorius) are simply based on total analytical values rather than free ion species (those ions remaining after ion association pairs are deducted from the total analytical value (TAV). 

Ryznar, John W. 1954. Role of chemical additives in the use of lower cost diesel fuel oils. Proceedings from presentation at the Southeastern Railway Diesel Club, August. 

LANGELIER AND RYZNAR EQUATIONS SATURATION AND STABILITY INDEX... 

The Ryznar equation was developed to provide a closer correlation between the calculated prediction and the quantitative results obtained in the field. 

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. 

The most widely known of these alternative indices is probably the Ryznar Stability Index (Ryznar Index, Stability Index) (SI), which is a direct derivation of LSI and is based on observed field data. It is generally considered more useful than the LSI in water systems flowing at or above 2 ft/sec (0.6 m/sec), such as occurs in most cooling systems. 

All the indices developed since the introduction of the Langelier and Ryznar indices have serious flaws and, while remaining useful, are of limited practical applicability. 

Nevertheless, and despite some marketing rhetoric and the limitations of simple indices, these computer programs should not be regarded as replacements for Langelier and Ryznar in the field, but rather as additional tools for those occasions when more detailed and investigative work is required or where a more rounded view is sought, such as in a formal review between the cooling system water treater and its owner/operator. 

However, despite all the problems identified with the earlier simple indices and the question of whether the entire premise for pHs calculations and subsequent scale and corrosion risk prediction is valid, or even particularly relevant to modern day cooling systems, is to miss a vital point. The fact is that Langelier and Ryznar have been with us for more than 50 years and, despite their obvious limitations, there are no other easy to use or easy to understand replacements currently available ... 

Are the calculations for Langelier Saturation Index (LSI) or Ryznar Stability Index (SI) regularly undertaken and interpreted ... 

The corrosivity of hard waters is defined by the stability index or Ryznar index (RI) defined as ... 

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 RSI = 2pH - pH Total alkalinity, mg/L as CaCOj RSI < 6.5 = Water is supersaturated tends to... 

Kemmer [1988] states that the Langelier index is most useful in predicting likely events for low flow situations, e.g. in storage tanks, and the empirical Ryznar index is applicable only to flowing systems, where conditions at the wall are quite different from the conditions in the quiescent bulk. 

These indices have been used with success for many years. Recently Miiller-Steinhagen and Branch [1988] have shown that the Ryznar Index may indicate scaling for conditions where the water is not saturated with CaCO Such observations underline the difficulties that may attend the use of these indices. 

It is of interest to gauge performance of additives against the Ryznar Index for the water. Fig. 14.7 shows the effect of the threshold additive acetodiphosphonic acid and the crystal modifier polymaleic acid at different Ryznar indices. As would be expected from the definition of the Ryznar Index, the scaling is increased as the index is reduced when there is no additive present. In the presence of the threshold agent the rate of scaling is also reduced but below a Ryznar Index of about 4.5, the threshold effect is exceeded and the scaling rate rapidly rises. 

FIGURE 14.7. Scaling rate as a function of Ryznar Index in the presence of a threshold and crystal modifying agents... 

Atkinson, Gerard and Harris [1979] have provided a useful chart that sets out the opportunities for scale (and corrosion) control based on the Ryznar Index. Fig. 14.8 shows how the various additives may be used to advantage depending on the scaling/corrosion potential. 

FIGURE 14.8. Additive selection based on Ryznar Index... 

Ryznar, J.W., 1944, A new index for determining the amount of calcium carbonate formed by water. J. Am. Water Works Assn. 36, 472 - 486. 

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