Ryznar index

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

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. 

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

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... 

When the Ryznar index (RI) is less than 6, scaling tendency increases, while its corrosion counterpart decreases. On the other hand, if RI is greater than 7.5, corrosion increases, and scaling will be very unlikely to occur. It is safe to work at RI values of between 6 and 7.5. 

The Ryznar index, which provides a more quantitative approach and also differentiates between waters with the same saturation index but with highly different properties, is given as follows ... 

As can be seen from the saturation index, it might be predicted that both waters would be equally scale-forming, yet the Ryznar index indicates otherwise. 

The solubility of elements in freshwater is limited and the solubility of calcium and magnesium carbonates are of particular importance in freshwaters. The solubility of carbonates is inversely proportional to the temperature of the water. In other words, as the water temperature increases, calcium and magnesium carbonates become less soluble. If the solubility decreases sufficiently, carbonates will precipitate and form a scale on the surfaces of the system. This scale can provide a protective barrier to prevent corrosion of the metallic elements in a system. Excessive scale deposits can interfere with water flow and heat transfer. The quality of the scale is dependent on the quantity of calcium that can precipitate as well as water flow and the chloride and sulfate content of the water. The tendency of water to precipitate a carbonate scale is estimated from corrosion indices such as the Langelier Saturation Index (LSI) and Caldwell-Lawrence calculations [6-8] which use calcium, alkalinity, total dissolved solids, temperature and pH properties of the water. Other indices, such as the Ryznar Index... 

Note on calcium carbonate equilibrium and the Ryznar index... 

The Ryznar index gives the scale-forming or corrosive tendency of one given type of water toward steel at a given temperature ... 

Far a closer definition of the problem, the Ryznar index is often calculated in three different places in the system ... 

Tlie Ryznar index gives a general tendency of course, but it does not show the influence of certain important parameters such as Cl and NOJ ions, NH3, miscellaneous pollution, etc. When the index is used, it should therefore be supplemented by information specific to the water, the system or its environment. 

The Ryznar index can haidly be used because it is based on the Langelier graph. The latter is valid only for salinity lower than 5 g l". The Stiff and Davis index must therefore be used. 

Ryznar stability index. The Ryznar stability index (RSI) attempts to correlate an empirical database of scale thickness observed in municipal water systems to the water chemistry. Like the LSI, the RSI has its basis in the concept of saturation level. Ryznar attempted to quantify the relationship between calcium carbonate saturation state and scale formation. The Ryznar index takes the form... 

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