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Recirculating Cooling Water Chemistry Analysis
ASTM D3739 LSI, Ryznar Stability & Puckorius Scaling Index Architecture
Water Balance & Saturation Index Diagnostics
Langelier Index (LSI, Basin)
--
-- status
Hot Tube Skin LSI (Worst-Case)
--
-- exchanger risk
Ryznar Index (RSI)
--
-- scale vs corr
Larson-Skold Pitting Index
--
-- pitting risk
Saturation pH (pH_s)
--
Equilibrium calcite pH
Puckorius Scaling Index (PSI)
--
-- buffered index
Gypsum (CaSO4) Product
--
-- limit: 5.0e6
Interactive Water Chemistry Index & Scaling Regime Map
CTI Water Quality Criteria & Scale/Corrosion Mapping
Water Quality Metric
Calculated Value
Recommended CTI Operating Range
System Tendency
Corrective Operational Action
Mathematical Formulations & Chemical Equilibrium Derivations
The precipitation kinetics of calcium carbonate in industrial recirculating cooling water are governed by the carbonate-bicarbonate thermodynamic equilibrium per ASTM D3739 and CTI standard guidelines.
1. Saturation pH (pH_s) Derivation:
pH_s = (9.30 + A + B) - (C + D)
Where:
A = [ log10(TDS) - 1.0 ] / 10.0
B = -13.12 * log10( T_celsius + 273.15 ) + 34.55
C = log10( Calcium_Hardness_ppm ) - 0.40
D = log10( Total_M_Alkalinity_ppm )
2. Langelier Saturation Index (LSI):
LSI = Measured_pH - pH_s
Interpretation:
LSI > +0.50 : Scale forming (calcium carbonate supersaturation)
0.0 to +0.50: Slightly scale forming (balanced protective film)
< 0.00 : Corrosive (calcium carbonate undersaturation)
3. Ryznar Stability Index (RSI):
RSI = 2.0 * pH_s - Measured_pH
Interpretation:
RSI < 5.5 : Severe scaling
5.5 to 6.2 : Light scaling
6.2 to 6.8 : Balanced / non-aggressive
6.8 to 8.5 : Corrosive
RSI > 8.5 : Extremely aggressive corrosion
4. Puckorius Scaling Index (PSI):
pH_eq = 1.465 * log10( Total_M_Alkalinity_ppm ) + 4.54
PSI = 2.0 * pH_s - pH_eq
5. Larson-Skold Pitting Index (LI):
epm_Cl = ppm_Cl / 35.45
epm_SO4 = ppm_SO4 / 48.03
epm_Alk = ppm_Alk / 50.04
LI = ( epm_Cl + epm_SO4 ) / epm_Alk
LI > 1.2 indicates severe localized pitting risk on mild steel.
Because calcite precipitation rate scales exponentially with surface temperature, calculating LSI only at the basin temperature creates a false sense of security. Hot heat exchanger tube wall temperatures must always be evaluated as the governing precipitation site.
1. Evaluating LSI Only at Basin Temperature
A cooling tower cold-water basin operating at 85°F (29°C) may show an ideal balanced LSI of +0.2. However, inside steam condensers or refining process exchangers, the metal tube skin temperature frequently reaches 135°F to 160°F. Due to retrograde solubility, the skin LSI spikes to +1.4, causing rapid calcite crystallization directly onto hot tube walls that cuts heat transfer efficiency by 40% within weeks.
To eliminate scale, operators frequently overdose sulfuric acid ($H_2SO_4$) to depress pH below 6.8. While this keeps calcium completely soluble, it strips the protective bicarbonate buffer and dumps heavy sulfate loads into the water. The Larson-Skold index surges past 2.5, creating an aggressive electrolyte that initiates catastrophic galvanic pitting and perforates carbon steel exchanger tubes.
3. Exceeding the Gypsum ($CaSO_4$) Solubility Limit ($>5.0 imes 10^6$)
When feeding sulfuric acid into high-calcium water, the calcium sulfate ion product $[Ca] imes [SO_4]$ increases linearly with cycles of concentration. If this product exceeds $5.0 imes 10^6$, insoluble gypsum ($CaSO_4 cdot 2H_2O$) precipitates. Unlike calcium carbonate, which dissolves readily during citric or sulfamic acid cleanings, gypsum scale is virtually impervious to chemical cleaning and requires destructive mechanical reaming.
4. Chlorine Biocide Degradation of Polyphosphonate Inhibitors
Modern alkaline cooling programs rely on organic phosphonate polymers (such as PBTC or HEDP) to stabilize supersaturated calcite up to LSI +2.0. If plant technicians overfeed sodium hypochlorite (chlorine bleach) without monitoring free residual halogen, the chlorine oxidizes the phosphonates into simple orthophosphates. The orthophosphate immediately reacts with calcium to form insoluble calcium phosphate sludge, causing instantaneous system fouling.
5. Ignoring Chloride Accumulation on Stainless Steel Exchangers
When cycling up cooling water to save blowdown, chloride concentrations ($Cl^-$) concentrate proportionally. Type 304 and 316 austenitic stainless steel tubes suffer rapid chloride stress corrosion cracking (CSCC) and severe crevice pitting when chloride levels exceed 200 ppm (for 304) or 500 ppm (for 316) at temperatures above 130°F (54°C). High-alloy duplex stainless or titanium must be specified if high-chloride cycles are maintained.
Frequently Asked Questions
What is the fundamental difference between the Langelier (LSI) and Ryznar (RSI) indices?+
The Langelier Saturation Index (LSI = pH - pHs) is an equilibrium indicator predicting whether water has a thermodynamic driving force to precipitate or dissolve calcium carbonate (CaCO3). However, LSI does not indicate the quantity or tenacity of scale that will form. The Ryznar Stability Index (RSI = 2*pHs - pH) is an empirical correlation developed on real-world industrial heat exchangers that accounts for film temperature dynamics and distinguishes between mild protective film formation and heavy, heat-transfer-choking scale deposition or aggressive pitting.
Why does calcium carbonate (CaCO3) precipitate faster on hot heat exchanger tubes than in the cooling tower basin?+
Unlike most chemical compounds which become more soluble at higher temperatures, calcium carbonate exhibits retrograde (inverse) solubility. As water temperature rises, CaCO3 becomes significantly less soluble and the saturation pH (pHs) decreases. Consequently, cooling water that appears perfectly balanced and non-scaling in an 85 deg F (29 deg C) tower cold-water basin can be intensely supersaturated (LSI > +1.5) at the 140 deg F (60 deg C) boundary layer film of a condenser tube wall, precipitating insulating rock-hard calcite scale.
What is the Puckorius Scaling Index (PSI) and when should it be used?+
The Puckorius Scaling Index (PSI = 2*pHs - pHeq) modifies the Ryznar index by replacing actual pH with an equilibrium pH (pHeq) based solely on total alkalinity: pHeq = 1.465 * log10(Alkalinity) + 4.54. Water with high buffering capacity can resist pH swings while precipitating scale. PSI provides a more reliable assessment of scaling potential in high-alkalinity or recirculating cooling systems where organic phosphonate scale inhibitors or acid dosing distort raw pH measurements.
What is the Larson-Skold Index (LI) and why is it critical for piping corrosion?+
The Larson-Skold Index evaluates the ratio of aggressive corrosive anions (Chloride Cl- and Sulfate SO4--) to passivating buffering anions (Bicarbonate HCO3- and Carbonate CO3--): LI = (epm Cl- + epm SO4--) / (epm Alkalinity). In cooling towers where sulfuric acid is dosed to control scale or cycles of concentration are elevated, sulfate and chloride ions break down the passive oxide film on carbon steel. An LI > 1.2 indicates a severe risk of localized pitting and crevice corrosion under heat exchanger deposits.
What is the calcium sulfate (gypsum) solubility limit in industrial cooling water?+
When sulfuric acid (H2SO4) is fed to neutralize alkalinity, sulfate ions accumulate rapidly. If the product of calcium hardness (as ppm CaCO3) and sulfate (as ppm SO4) exceeds 5,000,000 (i.e. [Ca as CaCO3] * [SO4] > 5.0e6), calcium sulfate (gypsum, CaSO4.2H2O) will precipitate. Unlike calcium carbonate, which can be dissolved by on-line acid cleaning, gypsum scale is practically insoluble in mineral acids and requires mechanical tube drilling or hydro-blasting to remove.