Circulating Water Quality Stability Assessment: Indices, Testing Methods, and Applications

Circulating water quality stability assessment is a systematic evaluation used to determine whether recirculating cooling water is chemically balanced, scaling, or corrosive within industrial cooling systems. By measuring parameters such as pH, alkalinity, calcium hardness, and total dissolved solids, and by calculating indices such as the Langelier Saturation Index, engineers can optimize water treatment programs, prevent scale deposition and corrosion damage, and extend equipment service life. This article provides a comprehensive technical overview of sample types, testing items, analytical methods, applicable standards, and practical applications of circulating water stability evaluation.

Technical Overview

Circulating water systems, whether open recirculating cooling towers or closed chilled-water loops, continuously concentrate dissolved minerals as water evaporates and is reused. As the cycles of concentration increase, the water may become supersaturated with respect to calcium carbonate and deposit scale on heat transfer surfaces, or it may retain excess carbon dioxide and become aggressive toward metal surfaces and cementitious materials. A water that neither deposits an objectionable mineral layer nor attacks system materials is described as "stable."

The concept of stability is rooted in the carbonate equilibrium system, in which carbon dioxide, bicarbonate, carbonate, and calcium ions coexist in a temperature-dependent balance. When water is heated inside heat exchangers or condensers, bicarbonates decompose and calcium carbonate precipitates, forming insulating scale that reduces heat transfer efficiency and increases energy consumption. Conversely, water deficient in carbonate alkalinity dissolves protective films and promotes general and localized corrosion. Circulating water stability assessment therefore evaluates the position of the water relative to calcium carbonate saturation and quantifies its scaling or corrosive tendency.

The evaluation framework is guided by recognized standards and codes, including GB/T 50050 (the design code for industrial recirculating cooling water treatment) for control targets and monitoring requirements, together with ISO and ASTM methods for the underlying chemical analyses. In practice, the assessment combines physicochemical testing, stability index calculation, calcium carbonate deposition testing, corrosion coupon exposure, and, where required, dynamic simulation testing performed by accredited third-party laboratories.

• Scaling water: supersaturated with calcium carbonate, tending to deposit scale on hot surfaces.
• Corrosive water: undersaturated with calcium carbonate, tending to dissolve protective films and corrode metals.
• Stable water: at or near calcium carbonate equilibrium, with neither pronounced deposition nor dissolution.

Testing Samples

Open Recirculating Cooling Water

Samples are collected from the cooling tower basin or cold water basin, the hot water return line before distribution over the fill, and the pump suction side. These locations reflect the warmest and most concentrated water in the loop, where scaling and corrosive tendencies are most pronounced. Sampling points should follow the monitoring layout recommended by GB/T 50050 to ensure representative conditions.

Closed Circulating Water Systems

Closed loops for chilled water, hot water, or process cooling are sampled at the expansion tank return, system drain, and circulation pump discharge. Because closed systems experience minimal evaporative concentration, the assessment focuses on corrosion tendency, dissolved gases, and inhibitor reserve rather than on concentration-driven scaling.

Makeup Water

Makeup water chemistry defines the baseline of the cycle. Sampling the makeup source allows calculation of the cycles of concentration by comparing chloride, conductivity, or silica levels between makeup and circulating water, and it supports prediction of how the system water will evolve during operation.

Heat Exchanger and Condenser Local Samples

Samples taken directly at heat exchanger inlets and outlets, and drainage from low points or dead legs, capture localized conditions that bulk samples may mask. Deposits, sludge, or corrosion products recovered from tube sheets and tower fill can also be submitted as deposit samples for characterization.

Sample Handling and Preservation

Careless handling can shift the carbonate balance, for example through carbon dioxide outgassing that artificially raises pH and induces calcium carbonate precipitation in the bottle. Bottles should be filled completely with minimal headspace, sealed promptly, and analyzed as soon as possible. Preservation and transport follow ISO 5667-3 guidance, and temperature at the time of sampling should always be recorded because saturation calculations are temperature dependent.

Testing Items

A complete stability assessment typically includes the following groups of parameters:

• pH and temperature, required inputs for saturation pH calculation.
• Total alkalinity (M alkalinity) and phenolphthalein alkalinity (P alkalinity), expressed as CaCO3.
• Total hardness, calcium hardness, and magnesium hardness, expressed as CaCO3.
• Chloride and sulfate, which drive corrosivity indices and cycles-of-concentration calculations.
• Electrical conductivity and total dissolved solids (TDS).
• Silica, iron, aluminum, zinc, and copper, which participate in deposition or reflect corrosion activity.
• Turbidity and suspended solids, indicators of fouling potential and sidestream filtration performance.
• Free residual chlorine and dissolved oxygen, relevant to corrosion of copper alloys and steel.
• Calculated indices: Langelier Saturation Index (LSI), Ryznar Stability Index (RSI), Puckorius Scaling Index (PSI), and Larson-Skold Index.
• Calcium carbonate deposition tendency measured by the marble or deposition test.
• Corrosion rate of carbon steel, copper alloy, and stainless steel coupons in mm/a or mils per year (mpy).
• Deposition rate and fouling resistance development from dynamic simulation testing.
• Cycles of concentration derived from chloride, conductivity, or silica ratios.

Testing Methods

Physicochemical Analysis

Core parameters are measured using standardized methods: pH by ISO 10523 or ASTM D1293, alkalinity by ISO 9963 or ASTM D1067, calcium and magnesium by ISO 6058 and ISO 6059 or ASTM D511, hardness by ASTM D1126, chloride by ISO 9297 or ion chromatography per ASTM D4327, conductivity by ASTM D1125, and dissolved oxygen by ISO 5813. For industrial recirculating cooling water specifically, GB/T 15451 (alkalinity), GB/T 15452 (calcium and magnesium by EDTA titration), and GB/T 15453 (chloride) provide the customary analytical basis.

Calculation of Stability Indices

The Langelier Saturation Index expresses the difference between the actual pH and the saturation pH (pHs) at which the water is in equilibrium with solid calcium carbonate: LSI = pH − pHs. The saturation pH is computed from the total dissolved solids, temperature, calcium hardness, and total alkalinity using the conventional relationship pHs = (9.3 + A + B) − (C + D), where A = (log TDS − 1)/10, B = −13.12 log(T + 273) + 34.55 with T in degrees Celsius, C = log(calcium as CaCO3) − 0.4, and D = log(alkalinity as CaCO3). An LSI near zero indicates stable water, a positive value indicates a scaling tendency, and a negative value indicates a corrosive tendency.

• Ryznar Stability Index: RSI = 2pHs − pH; values below 6 indicate scaling, 6 to 7 indicates approximately stable water, and above 7 indicates scale dissolution and corrosivity.
• Puckorius Scaling Index: PSI = 2pHs − pHeq, using an equilibrium pH derived from buffering capacity; it corrects for systems where the measured pH does not reflect the carbonate reserve.
• Larson-Skold Index: the ratio of chloride plus sulfate to alkalinity in milliequivalents per liter; values below 0.5 indicate low corrosivity toward steel, while values above 1.0 indicate a high risk of localized corrosion.

Calcium Carbonate Deposition Test

The deposition test, performed in accordance with GB/T 16632, exposes the water sample to controlled temperature and calcium carbonate supersaturation conditions, after which the soluble calcium retained in solution is measured. The difference between initial and final calcium concentrations quantifies the deposition tendency and, when applied to treated water, the scale inhibition efficiency of the treatment program. The classical marble test, in which pH and alkalinity are compared before and after contact with calcium carbonate chips, provides a simple supplementary check of stability.

Corrosion Coupon and Electrochemical Testing

Corrosion tendency is quantified by exposing pre-weighed metal coupons of carbon steel, copper alloy, or stainless steel to the circulating water under standardized flow and temperature conditions, following the rotating coupon method of GB/T 18175 or field rack exposure in the actual system. Mass loss after a defined period yields the corrosion rate; typical control targets for carbon steel in well-managed open cooling systems are below 0.075 to 0.1 mm/a, with copper alloys controlled to far lower values. Linear polarization resistance probes can supplement coupons with continuous, real-time corrosivity data.

Dynamic Simulation Testing

For critical systems or new treatment programs, dynamic simulation rigs reproduce a scaled-down cooling circuit including a heat exchanger tube, and monitor the development of fouling resistance and the quantity and composition of deposits over an extended run. This integrates scaling, corrosion, and biological fouling behavior under realistic thermal-hydraulic conditions, aligned with the evaluation principles of GB/T 50050.

Applications

Stability assessment results directly support decisions on chemical dosing, blowdown control, and cycles of concentration, translating into measurable improvements in heat transfer efficiency, water consumption, and equipment life. Typical application sectors include:

• Thermal power plants, for condenser cooling water and auxiliary cooling loops.
• Petrochemical refineries and gas processing plants, where exchanger fouling carries high energy penalties.
• Chemical and fertilizer plants with extensive cooling networks.
• Steel and non-ferrous metallurgy, including furnace cooling and continuous casting water systems.
• HVAC systems, district cooling networks, and data center cooling infrastructure.
• Food, beverage, and pharmaceutical utilities where closed-loop corrosion control is critical.
• Reclaimed water and reuse projects, where stability must be re-verified after changing the makeup source.

Based on the findings, treatment programs can be rebalanced: scale inhibitor or acid feed adjusted when the LSI or RSI indicates a deposition risk, alkalinity and calcium adjusted or film-forming inhibitors reinforced when indices and coupon rates reveal corrosivity, and blowdown rates optimized to operate at the highest stable cycles of concentration consistent with the acceptance criteria.

FAQ

What makes circulating water "stable"?

Stable water is at, or very near, calcium carbonate saturation. It deposits neither a visible mineral film on heat transfer surfaces nor dissolves protective carbonate films from piping, so both scaling and corrosion remain within acceptable control limits.

How are the LSI and RSI results interpreted?

An LSI within roughly −0.5 to +0.5 is commonly regarded as stable, a positive LSI signals a scaling tendency, and a negative LSI signals a corrosive tendency. For the RSI, values between 6 and 7 indicate near-balanced water, below 6 indicates scaling, and above 7 indicates corrosivity. Because each index has known limitations, good practice compares several indices alongside coupon and deposition test data.

How often should stability be assessed?

Key parameters such as pH, conductivity, alkalinity, and calcium hardness should be monitored daily or continuously by the plant, while a comprehensive stability assessment including indices, deposition testing, and coupon evaluation is typically performed monthly or quarterly, and always after changes in makeup water source, treatment chemistry, or cycles of concentration.

What samples and volumes are required?

Representative samples of circulating water, makeup water, and, where relevant, closed-loop water are needed, generally one to two liters per location in clean, completely filled polyethylene bottles with minimal headspace. Sampling temperature must be recorded, and rapid analysis is essential to preserve the carbonate balance.

Which standards govern the assessment?

GB/T 50050 defines control targets and monitoring principles for industrial recirculating cooling water, GB/T 16632 and GB/T 18175 cover deposition and corrosion coupon testing, and the underlying chemical analyses are performed per ISO, ASTM, and GB/T methods. These evaluations can be carried out in-house or by accredited third-party laboratories.

How do results translate into treatment actions?

Scaling indicators lead to increased scale inhibitor dosing, pH adjustment, partial acidification of alkalinity, or reduced cycles of concentration; corrosive indicators lead to alkalinity build-up, calcium supplementation, or enhanced corrosion inhibitor feed. Verified results should be confirmed with follow-up testing after every program change until the system returns to the stable operating window.