Langelier Saturation Index

Langelier Saturation Index.

The Langelier Saturation Index (LSI) of water is useful for determining corrosive or fouling trends in water for domestic and industrial use. It is a tool to calculate the indices needed to balance the water and its saturation levels, the objective is to reduce its tendency to be corrosive or fouling.

To calculate this index, it is necessary to have the following information:

  • pH of the water.
  • Temperature (T): is measured at the time of sampling, in order to obtain the real value of T or, if necessary, the operating temperature.
  • Calcium hardness (DCa): Determine the calcium hardness in the water sample.
  • Total alkalinity (FA): determine the total alkalinity (FA) of the water.
  • Conductivity as SDT.

The interpreted values of the ISL are generally (1):

ISL > 0 The analyzed water is expected to have fouling tendencies, this means that it is oversaturated and tends to precipitate CaCO3
ISL = 0 The analyzed water is considered saturated with CaCO3; no precipitation or dissolution of CaCO3 is expected, the water is balanced.
ISL < 0 The water is expected to have corrosive tendencies, this is a dissolution of the CaCO3 solid in the analyzed water.

The ideal range for the ISL is -0.3 to +0.3.

The negative values of the ISL (i.e., -0.6).

These are considered corrosive and measures should be taken to adjust the pH, total alkalinity or calcium hardness to avoid the effects of corrosion. In this case, calcite or corosex equipment may be necessary.

The positive values of the ISL (i.e. +0.6).

They indicate a tendency to scale and the pH, total alkalinity or calcium hardness should be adjusted in order to avoid scale formation in installed pipes or equipment.

How is the Langelier index calculated?

The following formula is used to calculate the Langelier Saturation Index:

  • 12.1* is a constant used in the calculation of the ISL.
  • The ISL can also be calculated with a difference between pH and pHs.

Derived from the pH equation, pHs is the point where water is saturated with calcium carbonate. The pHs is calculated as follows (2):

Where:

  • It should be remembered that Ca+2 and alkalinity values are reported in mg / l, and temperature (T) in °C.

 

By obtaining the pHsvalue, the ISL can be calculated as:

The results give the same indications as mentioned above:

pH – pHs > 0 The analyzed water is expected to have fouling tendencies, this means that it is oversaturated and tends to precipitate CaCO3
pH = pHs The analyzed water is considered saturated with CaCO3; no precipitation or dissolution of CaCO3 is expected, the water is balanced
pH – pHs < 0 Water is expected to have corrosive tendencies, this is a dissolution of the CaCO3 solid in the analyzed water

How to correct the water?

In waters with a corrosive tendency it is necessary to add an alkali. Corrosive tendency can be reduced by placing a Corosex or Calcite filter, and with these equipment’s we increase the pH of the water. In incrusting waters, it is necessary to place a softener. 

We can help you establish the appropriate equipment for the type of water you need to condition. In some cases, an acid can be dosed by means of dosing pumps to balance the pH, as long as the process and handling is safe.

Below is a support calculator for determining the ISL:

Langelier Index Calculator

pH *

Conductivity in TDS

*
[Ca2+] *
[HCO3-] *
Water temperature *

Langelier Saturation Index Results
pHs
LSI
Indication based on Langelier (1936)
Indication based on Langelier improved by Carrier (1965)

If you need more information, write to us.

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