Langelier Saturation Index (LSI) and its importance in water chemistry…

Langelier Saturation index(LSI) is an important term in water chemistry. LSI is a measure of a solution’s ability to dissolve or deposit calcium carbonate, is often used as an indicator of the corrosivity of water. It is used for finding ability of water for corrosion or scaling in heat exchanger where water is used as heat transfer media in either side of heat exchanger. The scaling in pipe or heat exchnager tubes can cause insulation for heat transfer or metal loss in case of corrosion tendency of water. LSI is an equilibrium model derived from the theoretical concepts of saturation and helps in indicating the degree of saturation of water with respect to calcium carbonate.

Langelier derived an equation for the pH at which water is saturated with calcium carbonate (pHs). This equation is based on the equilibrium expressions of calcium carbonate solubility and bicarbonate dissociation. To approximate actual conditions more closely, pHs calculations were modified to include the effects of temperatureand ionic strength

The Langelier Index isdefined as the difference between actual pH (measured) and calculated pHs. Themagnitude and sign of the LSI value show water’s tendency to form or dissolve scale,and thus to encourage corrosion. Although information obtained from the LI is not quantitative, it can be useful in estimating water treatment requirements for heat exchanges in process industry, low pressure boilers, cooling towers and its network, and water treatment plants, as well as a general indicator of the corrosivity of water.

The LSI is expressed as the difference between the actual system pH and the saturation pH:

LSI = pH (measured) − pHs

  • For LSI > 0, water is super saturated and tends to precipitate ascale layer of CaCO3.
  • For LSI = 0, water is saturated (in equilibrium) with CaCO3. A scale layer ofCaCO3 is neither precipitated nor dissolved.
  • For LSI < 0, water is under saturated and tends to dissolve solid CaCO3.

The LSIis most widely used indicator of cooling water scale potential, It is also used to decide qty of water treatment chemicals in the cooling tower or any water system involved in heat transfer. With LSI we can only predict about the nature of water as scaling or corrosion but quantification cannot be derived.

For calculating LSI, it is necessary to know the

  1. alkalinity (mg/l as CaCO3),
  2. calcium hardness (mg/lCa2+ as CaCO3),
  3. the total dissolved solids (mg/l TDS),
  4. the actual pH,
  5. the temperature of the water (oC).

LSI = pH – pHs

Where:

pH is the measured water pH

pHs is the pH at saturation in calcite or calcium carbonate and is defined as:

pHs = (9.3 + A + B) – (C + D)

Where:

  • A = (Log10 [TDS] – 1) / 10
    • B = -13.12 x Log10 (oC + 273) + 34.55
    • C = Log10 [Ca2+ as CaCO3] – 0.4
    • D = Log10 [alkalinity as CaCO3]
LSI Indication
LSI<0 Water is undersaturated with respect to calcium carbonate. Undersaturated water has a tendency to remove existing calcium carbonate protective coatings in pipelines and equipment.
LSI=0 Water is considered to be neutral. Neither scale-forming nor scale removing.
LSI>0 Water is supersaturated with respect to calcium carbonate (CaCO3) and scale forming may occur.
LSI (Carrier) Indication
-2.0<-0.5 Serious corrosion
-0.5<0 Slightly corrosion but non-scale forming
LSI = 0,0 Balanced but pitting corrosion possible
0.0<0.5 Sligthly scale forming and corrosive
0.5<2 Scale forming but non corrosive

Source: Lenntech

For Langelier Saturation Index (LSI) calculator click here,

For cooling tower approach and its importance click here

For cross and counter flow cooling tower, click here

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