Langelier Saturation Index

Langelier Saturation Index

What is Langelier Saturation Index?

Langelier Saturation Index(LSI) was developed by Scientist Langelier in 1936.

The LSI is used to determine the need for calcium carbonate precipitation scale control in water sources containing a TDS concentration of less than 10,000 mg/L

LSI is most widely used tool to predict tendency of cooling water. In order to calculate LSI five values namely TDS(Total Dissolved Solid), Temperature, Calcium Hardness, M-Alkalinity & pH are  very important.

Based on these five values you can calculate saturation pH called pHs & then subtract it from actual pH to get LSI.

Actually, LSI predicts tendency of CaCO3(Calcium Carbonate), whether it will precipitate it or dissolve it.

Basically,This information is important at the time of designing a cooling water chemical treatment program, especially in water that have a tendency to form calcium carbonate scale.

LSI Prediction
If LSI is Negative; No potential to scale,Water will dissolve CaCO3
If LSI is Positive; Scale will form & CaCO3 precipitation occurs
If LSI is ZERO; Water is considered as stable water

Langelier Saturation Index Calculator:

LSI Potential For CaCO3 Deposition
3.0 Extremely Severe
2.0 Very Severe
1.0 Severe
0.5 Moderate
0 Stable Water
-0.5 Non-scaling,Slight tendency to dissolve CaCO3
-1.0 Non-scaling,Moderate tendency to dissolve CaCO3
-2.0 Non-scaling,Strong tendency to dissolve CaCO3
-3.0 Non-scaling,Very strong tendency to dissolve CaCO3

LSI Calculation Formula:

LSI Formula

A =( LOG10 * (TDS)-1) / 10

B = (-13.12 * LOG10(temp +273)) + 34.55

C = LOG(CaH) – 0.4

D = LOG10(M-Alk)

pHs = (9.3 + A + B) – (C + D)

LSI = pHa – pHs

Where;

TDS = Total Dissolved Solids(ppm)

temp = Temperature(dEG C)

CaH = Calcium Hardness(ppm as CaCO3)

M-Alk = M-Alkalinity(ppm as CaCO3)

pHs = Saturation pH

pHa = Actual pH

LSI Calculation Example:

pH 8.0
Total Dissolved Solids 500 mg/L
Temperature 30 dEG C
Calcium Hardness 250 ppm as CaCO3
M-Alkalinity 100 ppm as CaCO3

Lets Calculate..

Put cooling water analysis values in above LSI Formula. You get below values for constants A,B,C & D as per below table:

A 0.17
B 2.02
C 2.0
D 2.0
pHs 7.49

Finally, LSI = pHa – PHs

 LSI = 8.0 – 7.49 = 0.51 

The tendency of this water is slightly scale forming.

 

 

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