Scaling indices Langelier saturation 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. 

Langelier Saturation Index—Ideally, maintain between -0.5 and +0.5 A negative LSI indicates corrosion tendencies. A positive LSI indicates CaCOs scaling tendencies. 

LSI (Langelier Saturation Index) an indication of the corrosive (negative) or scale-forming (positive) tendencies of the water. Hardness the total dissolved calcium and magnesium salts in water. Compounds of these two elements are responsible for most scale deposits. Units are mg/l as CaCOs. 

The Langelier Saturation Index (LSI) is a method for quantifying the scaling or corrosion tendency of water. It was originally applied to cooling water. The LSI is based on the pH and temperature of the water in question as well as the concentrations of TDS, calcium hardness, and alkalinity. 

Langelier Saturation Index (LSI) is used to determine the scaling potential of calcium carbonate. (Note that LSI is used up to about 4,000 ppm TDS higher concentrations rely on the Stiff-Davis Saturation Index.) The LSI is calculated using the following formulas... 

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-... 

LANGELIER SATURATION INDEX - An index (SI) based upon the pH of saturation of calcium carbonate used to determine the tendencies of a water supply toward corrosion or scaling. A positive index indicates scaling tendencies a negative one means corrosion tendencies. (Langlier Index = pH - pHs, where pH = actual pH of water and pHs = pH at which water having the same alkalinity and calcium content is just saturated with calcium carbonate. 

Langelier Saturation Index (LSI) It is a means of expressing the degree of saturation of water as related to calcium carbonate solubihty. The difference between the actual pH and the pH at which the water would be saturated with calcium carbonate, pHj. It quantifies CaCOs scaling potential. If LSI is positive. 

The scaling tendency of a given feed water is often evaluated using the Langelier Saturation Index (LSI) for brackish water and the Stiff and Davis Stability Index (S DSI) for seawater (LSI is acceptable for seawater). LSI and S DSI are defined below ... 

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... 

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 Langelier saturation index (LSI) is probably the most widely used indicator of a water scale potential. This index indicates the driving force for scale formation and growth in terms of pH as a master variable. In order to calculate the LSI, it is necessary to know the alkalinity (mg L as CaCOj or calcite), the calcium hardness (mg L Ca + as CaCOj), the total dissolved solids (mg L h TDS), the actual pH, and the temperature of the water (°C). If TDS is unknown, but conductivity is, one can estimate mg L" TDS using a conversion table (Table 8.15). LSI is defined as... 

Stiff-Davis index. The Stiff-Davis index attempts to overcome the shortcomings of the Langelier index with respect to waters with high total dissolved solids and the impact of common ion effects on the driving force for scale formation. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH at saturation (pHs) for a water is empirically modified in the Stiff-Davis index. The Stiff-Davis index will predict that a water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the Langelier saturation 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). 

The protective carbonate scale is not just a ftinction of the concentration of calcium and magnesium salts. It is also affected by the alkalinity of the water and concentrations of other salts. Saturation indexes have been developed for monitoring such concentrations. A popular saturation index is the Langelier index, which provides a simple method for determining the conditions and concentrations under which water will form this kind of protective film. Section 2.2 in Chap. 2, Environments, describes in detail the Langelier index and a few other indexes and methods to monitor scaling tendencies of waters. 

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). 

If the actual pH is below pH, the result is a negative index and CaCO will dissolve in the water. It is also generally assumed that this will indicate the water to be corrosive towards steel in the presence of oxygen. On the other hand if the Langelier index is positive (i.e. pH > pH,) and the water is saturated with CaCO scale formation is likely to occur. 

For calculating pHj in order to solve the equation, there are tables for values of pK - pK ) at various dissolved solids contents and various temperatures. The calcium content and alkalinity can be easily determined by the plant chemist. If the Langelier index (pH-pH,) is zero, the water is just saturated with CaCOj. If the index is positive, the water is supersaturated with and will tend to deposit CaC03 as well as be more or less scale forming and noncorrosive. If the index is negative, the water tends to be corrosive. 

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