What is the Langelier Index? A Complete Guide to Predicting Water Sedimentation and Corrosion

Introduction:

In industries ranging from power plants and refineries to manufacturing plants and heating and cooling systems, water quality plays a vital role in the optimal performance of equipment. Water that appears clean may, over time, cause scale or corrosion of metals. These two phenomena not only reduce the life of equipment, but also significantly increase maintenance and repair costs.

But how do we know if our water has a tendency to scale or corrode? Is there a way to predict this behavior?

Yes! Using water stability indices such as the Langelier Index (LSI), the Reisnar Index, and the Pocurius Index, we can easily assess the condition of water.

In this article, we will introduce you to these indices in a simple and practical way, examine how to calculate and apply each one, and finally, provide solutions for smart water management in industrial systems.

Why is water's tendency to scale or corrode important?

Systems such as boilers, chillers, heat exchangers, cooling towers, and thermal desalination plants all use water circulation to transfer heat or produce steam. In this process, if the water contains dissolved minerals (such as calcium carbonate or magnesium sulfate), these materials may precipitate as solids as the temperature increases or the pH changes.

On the other hand, acidic or low-alkaline waters can dissolve metals and cause corrosion of pipes and equipment.

The result in both cases is the same:

• Reduced thermal efficiency
• Increased energy consumption
• Clogged pipes
• Costly breakdowns
• Production line downtime

Therefore, predicting the behavior of water before it enters the system is one of the most important preventive measures in industrial water management.

Deposits on thermal desalination pipes in industries

Langelier Index (LSI): The first step in assessing water stability

What is the Langelier Index?

The Langelier Saturation Index (LSI) is a numerical indicator that indicates the tendency of water under current conditions to dissolve or form calcium carbonate (CaCO₃) deposits.

This index is calculated by comparing the actual pH of the water with the saturation pH (pHs):

LSI = pH (real) - pHs

• If LSI = 0 :the water is in equilibrium and has no tendency to precipitate or corrode.
• If LSI > 0 :the water is precipitated (calcium carbonate tends to precipitate).
• If LSI < 0 :the water is corrosive (calcium carbonate on the surface of the equipment dissolves).

Parameters involved in calculating LSI

To calculate the saturated pH (pHs), the following parameters are necessary:

• Water temperature (°C)
• Actual pH
• Alkalinity in mg/L CaCO₃
• Calcium concentration (Ca²⁺) in mg/L CaCO₃
• TDS (Total Dissolved Solids)

These calculations are usually done with specialized software or standard tables.

Application of the Langelier Index

LSI is mostly used in slow-flowing systems (such as swimming pools, central heating systems or some boilers). In these systems, there is enough time for scale to form or dissolve.

However, in fast-flowing systems, LSI does not always give an accurate answer — and this is where the Ryznar Index comes in.

Ryznar Stability Index: An Improved Version of the LSI

Why was the Ryznar Index Created?

In the 1940s, a researcher named Riesnar discovered that in some fast-flowing industrial systems, water that was neutral according to the LSI still caused corrosion. The reason was the effect of the water flow on the process of forming a protective calcium carbonate layer.

The Riesnar Index formula:

RSI = 2 × pHs – pH(real)

• If RSI ≈ 6: the water is in equilibrium.
• If RSI > 6: the water is corrosive.
• If RSI < 6: the water is scaling.

Advantages of Riesnar Index

• More accurate in systems with flow rates higher than 0.6 m/s (equivalent to 2 ft/s).
• Based on experimental data from real systems.
• Best index for circulating water cooling systems.

Puckorius Scaling Index: A Solution for Alkaline Water

The Problem of High pH Water

In many industrial systems—especially those using alkaline anti-scalers or acid-free treatments—the water pH easily exceeds 8.5. In these cases, the actual alkalinity of the water does not match the pH, because the system is buffered.

In such cases, the LSI and RSI may give misleading results.

Solution: Puckorius Index

This index was introduced by Puckorius in the 1980s and is designed for waters with pH > 8.

The formula is as follows:

PSI = 2 × pHs – pHeq

where pHeq is an equivalent pH calculated based on the actual alkalinity (not the measured pH).

• If PSI > 6: The water is corrosive.
• If PSI < 6: The water is scummy.

Benefits of using PSI

• Reduce acid consumption in treatment systems
• Increase concentration cycle
• Save make-up water
• Better protection of pipes in cooling systems

Summary: Which index should we use?

The choice of the appropriate index depends on the type of system and water conditions:

In practice, a combination of these indices, along with regular water tests, is the best solution for smart water management.

How to implement these indices in practice?

1. Regular water sampling: Take samples from the inlet, outlet and return water cycle at least once a month.

2. Test key parameters: pH, alkalinity, calcium hardness, TDS, temperature.

3. Calculate indices: With specialized software or laboratory services.

4. Make decisions based on the results:

• If the water is scaling → Use chemical anti-scalants
• If the water is corrosive → Adjust pH or use anti-corrosion agents

5. Continuous monitoring: Continue testing even after treatment.

Frequently asked questions by users

Can the condition of the water be diagnosed only by pH?

No. pH is only one of the parameters. Without taking alkalinity, hardness and temperature into account, the behavior of the water cannot be predicted.

What is the difference between LSI and RSI?

LSI is based on thermodynamic equilibrium, while RSI is designed based on operational data and water flow.

Can these indices be used for drinking water?

Yes, but with caution. In drinking water, LSI is mostly used to prevent corrosion of lead or copper pipes.

Conclusion: Prevention is better than cure

Understanding the tendency of water to scale or corrosion is possible with just a few simple tests and calculating stability indices. This not only prevents expensive breakdowns, but also optimizes energy efficiency, equipment life and operating costs.

If you are in industry and use systems with circulating water, regular monitoring of the Langelier, Riesnar and Pocurius indices should be part of your maintenance program.

⭐⭐⭐ Take action before scale or corrosion becomes the deciding factor! ⭐⭐⭐

If you need detailed analysis of water samples, expert advice, or design of a chemical cleaning program for industrial equipment, you can use the services of well-equipped laboratories and industrial water specialists who will accurately determine the type and amount of chemicals needed based on the actual conditions of your system.

☎️ Free call and consultation with Abrizan specialists ☎️