Understanding Water Balance: Langelier Saturation Index (LSI) & Ryznar Stability Index (RSI)
The water in your cooling system is not just a passive liquid. It has a ‘personality’ that can be destructive or form scale. LSI and RSI are two vital diagnostic tools that engineers use to ‘read’ the personality of the water and prevent problems before they occur.
What is the Langelier Saturation Index (LSI)? Scale & Corrosion Potential Indicator
Brief Definition:
LSI is a calculation that compares the actual pH of the water with its saturation pH (the pH at which the water is exactly saturated with Calcium Carbonate - CaCO₃). This index answers the fundamental question: ‘Is this water likely to deposit scale or dissolve metals (corrosive)?’
Function & Why It Matters:
Predicting Water Tendencies: LSI helps determine whether the water in your system is scale-forming or scale-dissolving (meaning corrosive).
Basis for Decision Making: LSI calculation results serve as a guide for adjusting water chemical treatment, such as raising or lowering pH, controlling hardness, or adjusting the dosage of corrosion and scale inhibitors.
How it Works & How to Read LSI Values:
LSI calculations are based on several water parameters: pH, Temperature, Total Alkalinity, Calcium Hardness, and TDS.
LSI Formula: LSI = pH (actual) - pHs (saturation pH)
Here is the key to interpreting the calculation results:
LSI > 0 (Positive): Water is SUPER-SATURATED with CaCO₃.
Meaning: Water is Scale-Forming. It tends to deposit Calcium Carbonate (CaCO₃) scale on pipes, heat exchangers, and fill packing. This inhibits heat transfer and clogs flow.
Action: Add scale inhibitors or adjust operating conditions (such as lowering the pH).
LSI = 0 (Zero): Water is PERFECTLY SATURATED.
Meaning: Water is in equilibrium. It does not tend to form significant new scale and is not too aggressive to cause corrosion. This is the ideal condition that is often sought.
LSI < 0 (Negative): UNDER-SATURATED water.
Meaning: The water is corrosive. Due to a lack of CaCO₃, the water will ‘search’ for a source of calcium carbonate by dissolving the protective layer on the surface of metals (such as iron pipes), causing corrosion.
Action: Add corrosion inhibitors or adjust operating conditions (such as increasing the pH).
What is the Ryznar Stability Index (RSI)? A More Empirical Prediction of Water Behaviour
Brief Description:
RSI was developed as an improvement and alternative to LSI. This index provides a more empirical (observation-based) prediction of water behaviour, especially in conditions where LSI provides ambiguous results.
Function & Why It Matters:
Strengthens LSI Predictions: RSI is used in conjunction with LSI to obtain a more accurate picture of water stability.
More Specific in Certain Ranges: RSI is known to be more reliable in predicting the potential for corrosion in water with a negative LSI.
How It Works & How to Read RSI Values:
RSI uses the same basic parameters as LSI.
RSI Formula: RSI = 2(pHs) - pH (actual)
Here is the key to interpreting RSI calculation results:
RSI < 5.0: Very high potential for SCALING.
RSI 5.0 - 6.0: Potential for MILD SCALING.
RSI 6.0 - 7.0: Water is in a STABLE condition. The water is neither too scale-forming nor too corrosive. This is the target zone for many systems.
RSI 7.0 - 7.5: Water is Slightly Corrosive.
RSI 7.5 - 9.0: Water is Corrosive.
RSI > 9.0: Potential for SEVERE CORROSION.
LSI vs RSI: Which Should Be Used?
The two are not competitors, but rather complementary partners.
Use LSI to answer theoretical questions: ‘Is this water saturated with CaCO₃?’
Use RSI to get a picture of the practical behaviour of water based on field experience.
Conclusion for Your Operations:
A healthy cooling tower system requires regular monitoring of both indices. Aim for conditions where LSI approaches zero (balanced) and RSI is in the range of 6.0 - 7.0. Achieving this balance indicates that your system is operating efficiently, with minimised scaling and corrosion risks, ultimately saving on energy costs, repairs, and equipment replacement.