A circulating water system is the “lifeline” of industrial operations, quietly supporting the stable performance of heat exchangers, pipelines, and production units.However, beneath seemingly normal operation, scaling and corrosion often develop silently - reducing heat transfer efficiency, increasing energy consumption, shortening equipment lifespan, and even causing unexpected shutdowns.

The good news is: scaling and corrosion never occur without warning. Early signals are already reflected in subtle changes in water quality parameters. By continuously monitoring and analyzing key water quality indices, operators canpredict risks in advance, issue early warnings, and take proactive control, addressing potential problems before they escalate. Under standing the following core water quality indices is a critical step in building an effective early-warning system for scaling and corrosion in circulating water systems.

01. Hardness: The First Sentinel of Scaling Risk

In circulating water systems, hardness is one of the most critical indicators for assessing scaling potential. It represents the total concentration of calcium and magnesium ions in water and is generally classified into: 

Temporary hardness (carbonate hardness)

Permanent hardness (non-carbonate hardness)

Among them, temporary hardness is the primary driving force behind scaling formation. When circulating water passes through high-temperature equipment such as heat exchangers, bicarbonates decompose under heat, forming sparingly soluble precipitates such as calcium carbonate and magnesium carbonate. These deposits gradually adhere to pipe walls and heat-transfer surfaces, reducing thermal efficiency and increasing operational energy consumption.

Key Assessment Insight
Tolerance to hardness varies across industries and operating conditions. As a general guideline, the total hardness of industrial circulating cooling water is recommended to be controlled at≤800 mg/L (as CaCO₃). In practice, this threshold should be dynamically optimized based on system concentration ratio, operating temperature, and makeup water quality.

02. pH Value: The Critical Benchmark Balancing Scaling and Corrosion

Among all water quality parameters in circulating water systems, the pH value stands out as one of the most influential control indicators.It reflects the acid - base condition of water and plays a decisive role in the development and rate of both scaling and corrosion, making it a central lever in water quality management.

Key Assessment Insights | Under Normal Operating Conditions

pH＜7 (Acidic conditions)
Water becomes highly corrosive. Metal surfaces are exposed to aggressive environments, significantly accelerating corrosion of pipes and heat-exchange equipment.

pH＞8.5 (Alkaline conditions)
The stability of calcium and magnesium ions decreases, promoting precipitation and deposit formation, which sharply increases scaling potential and impairs heat transfer performance.

Under typical operating conditions, the recommended pH range for circulating cooling water is 7.5~8.5.This “optimal window” helps mitigate both corrosion and scaling risks, supporting stable, efficient, and reliable system operation.

03. Dissolved Oxygen: The Invisible Blade Behind Corrosion

In circulating water systems, dissolved oxygen (DO) is often invisible -yet it is one of the most critical drivers of metal corrosion. Under normal temperature and flowing conditions, dissolved oxygen continuously participates in oxidation reactions on metal surfaces, forming porous rust layers. Over time, repeated cycles of rust formation and detachment gradually thin pipe walls and lead to pits of varying depth, posing long-term risks to system integrity. What deserves special attention: uneven distribution of dissolved oxygen can trigger localized corrosion, which is far more destructive than uniform corrosion and a common cause of premature equipment failure.

Key Assessment Insights

Dissolved oxygen concentration shows a strong positive correlation with corrosion risk - the higher the DO level, the greater the corrosion driving force.

Closed circulating water systems: have limited contact with air, typically resulting in lower DO levels and more manageable corrosion risk.

Open circulating systems (e.g. cooling towers): allow extensive air–water interaction, leading to consistently higher DO levels, and a greater need for corrosion control.

Under normal operating conditions, a dissolved oxygen range of 6~10 mg/L in circulating water is generally acceptable, and does not require aggressive reduction. Effective management lies in system-specific assessment, trend monitoring, and targeted control strategies, rather than indiscriminate minimization.

04. Cycles of Concentration: A Comprehensive Reflection of System Water Quality

Among the many operating parameters of circulating water systems, cycles of concentration (COC) is one of the most representative composite indicators.

It is defined as: Ion concentration in circulating water ÷ Ion concentration in makeup water

This simple ratio acts as a mirror of system operation, clearly reflecting blowdown rate, makeup water balance, and overall water concentration level, making it a key reference for evaluating system performance and stability.

Key Assessment Insights

Excessively high COC: indicates continuous accumulation of hardness, salts, and other impurities, significantly increasing the risks of both scaling and corrosion, and threatening system reliability and heat transfer efficiency.

Overly low COC: reduces water quality risks but leads to unnecessary consumption of water and treatment chemicals, driving up operating costs.

For typical open circulating cooling water systems, are commended COC range is 3~5, which helps achieve a balance between operational safety and economic efficiency. If monitoring shows a persistently elevated COC, timely actions such asincreasing blowdown and supplementing fresh makeup water are essential to prevent gradual risk accumulation.

05. Conductivity: A Practical Gauge of Water Quality Purity

In circulating water system management, conductivity is one of the most responsive and intuitive water quality indicators. It directly reflects the total ionic content of water, commonly expressed as total dissolved solids(TDS). The higher the ion concentration, the higher the conductivity value. For this reason, conductivity is closely correlated with cycles of concentration, making it a convenient indicator for quickly assessing water concentration levels.

More importantly, conductivity often shows a positive correlation with corrosion rate -higher ion concentrations increase water conductivity, facilitating corrosion currents and accelerating metal corrosion processes.

Key Assessment Insights

Trends in conductivity closely mirror changes in concentration cycles.

A continuous rise in conductivity:indicates ongoing accumulation of salts in the system, accompanied by increasing risks of both scaling and corrosion.

Under typical operating conditions, there commended conductivity range for industrial circulating cooling water is1000~2000 μS/cm at 25°C. Actual control limits should be adapted to make up water quality, system materials, and specific operating conditions. With real-time online conductivity monitoring, operators can quickly assess water quality status and make timely, data-driven decisions for blowdown, makeup water control, and chemical dosing.

Conclusion | Safeguarding the Stable Operation of Circulating Water Systems

Scaling and corrosion in circulating water systems may appear sudden, but in reality, they are always preceded by clear signals.Hardness, pH value, dissolved oxygen, cycles of concentration, and conductivity are the five core water quality indicators that unlock insight into system performance and enable early prediction of scaling and corrosion risks.

Only through regular and systematic monitoring, combined with trend-based data interpretation an dope rating - condition - specific control strategies, can risks be effectively managed in advance -reducing the likelihood of scaling and corrosion, extending equipment service life, and ensuring efficient, stable, and reliable system operation over the long term.

Tips: Circulating water systems vary significantly across industries -such as power generation, chemical processing, and refrigeration-resulting in different water quality control requirements. It is recommended to refer to relevant industry standards and equipment manufacturer guidelines and develop customized monitoring and control programs tailored to the specific characteristics of each system.