In the context of thermal power plants, cogeneration, and industrial processes operating with boilers and steam turbines, water and steam quality is one of the fundamental pillars for ensuring operational reliability, energy efficiency, and... Asset integrity. Among all the analytical parameters used in the water-steam cycle, conductivity, in its different forms of measurement, plays a central role, being considered by international standards as a key parameter.
The VGB and EPRI standards, widely adopted as global technical references, clearly position conductivity as the primary tool for early contamination detection, supporting operational decision-making, and preventing severe corrosion and deposition mechanisms.
This article presents, in a technical and applied manner, the importance of specific conductivity, cationic conductivity, and degassed cationic conductivity (DAC) measurements in the water-steam cycle, their fundamentals, practical applications, and direct impacts on the safety and operating costs of plants.
Fundamentals of conductivity in the water-steam cycle
The electrical conductivity of water is an indirect measure of the concentration of dissolved ionic species. In high-purity systems, such as modern water-steam cycles, small amounts of ionic contaminants are sufficient to cause significant variations in conductivity.
For this reason, conductivity stands out as a global indicator of purity, capable of quickly detecting the presence of salts, acids, bases, and chemical decomposition products.
The main measurement methods applied to the cycle are:
- Specific conductivity: measures the total conductivity of the sample, including all ions present.
- Cationic (or acid) conductivity: measures conductivity after the removal of alkaline cations using an ion exchange resin, revealing strong anionic contaminants such as chlorides, sulfates, and nitrates.
Conductivity as a key parameter
The VGB and EPRI guidelines classify conductivity as a key parameter, meaning a parameter that must be monitored continuously and online, with fast response times and high reliability.
The technical rationale behind this classification is based on three main factors:
- Immediate response to contamination: conductivity reacts instantly to external inputs, such as condenser leaks, condensate polisher failures, or makeup water contamination.
- Comprehensive risk coverage: a single analyzer is capable of indicating various types of chemical failures.
- Basis for Action Level systems: conductivity supports the philosophy of action levels (AL1, AL2, and AL3), allowing for corrections before irreversible damage occurs.
Importance of specific conductivity
Specific conductivity is widely used in virtually all aspects of the water-steam cycle, including:
- Feed water
- Boiler water
- Saturated vapor
- Superheated steam
- Condensed
Main technical functions
- Comprehensive assessment of water purity
- Monitoring of chemical regimes (AVT, OT, PT, CT)
- Blowdown control in drum boilers
- Detection of operational variations during startups and transients.
In feedwater and steam, high specific conductivity values indicate a direct risk of deposition, corrosion under deposits, acid corrosion, and contaminant transport to the turbine.
In boiler water, specific conductivity is essential for chemical balance, preventing both excessive salt concentration and unnecessary losses due to excessive purging.
Importance of cationic conductivity
Cationic conductivity is considered by VGB and EPRI standards to be the most sensitive measurement for detecting anionic contamination, especially the most aggressive types.
Contaminants detected with high efficiency.
- Chlorides (Cl⁻)
- Sulfates (SO₄²⁻)
- Nitrates (NO₃⁻)
- Organic and inorganic acids
- Degradation products of ion exchange resins
Operational relevance
Even when specific conductivity appears to remain within acceptable limits, cationic conductivity can reveal incipient faults, allowing corrective action before contaminants reach critical regions such as:
- Superheaters
- Control valves
- Steam turbines
- Zones of first condensation
This characteristic makes cationic conductivity an indispensable tool for turbine protection and for extending the service life of cycle components.
Degassed cation conductivity (DAC)
Degassed acid conductivity (DAC) represents a fundamental evolution in the concept of chemical monitoring of the water-steam cycle, and is strongly recommended by the most modern VGB and EPRI guidelines for high-reliability power plants.
Measurement principle
After the cation exchange step, the sample undergoes a process to remove dissolved gases, mainly CO₂, which is the primary cause of high and unrepresentative readings in conventional cation conductivity tests.
With the elimination of CO₂, the DAC measurement now exclusively represents the contribution of strong acids, such as chlorides and sulfates — precisely the most critical contaminants for the water-steam cycle.
Technical importance of DAC
DAC measurement offers significant analytical gains:
- Elimination of false alarms associated with the presence of CO₂
- Clear separation between actual contamination and normal operational variations.
- Greater sensitivity for detecting salt ingress.
- Better correlation with real corrosion risks
These characteristics make DAC particularly valuable in cycles with:
- AVT(O) and AVT(R) regimes
- Operation with high variable loads
- Frequent departures and stops
- Intensive use of amines and organic conditioners.
Typical applications of DAC in the cycle
Degassed cationic conductivity is especially suitable for:
- Condensed after polishing.
- Superheated steam
- Among other points
In these areas, DAC allows for a much more precise assessment of chemical purity, reducing uncertainties and increasing operational safety.
DAC as support for action levels (VGB)
VGB emphasizes that operational decisions should be based on parameters that reflect real risks to the equipment. In this context, DAC proves ideal for supporting the AL1, AL2, and AL3 concepts, avoiding unnecessary actions and focusing only on events that effectively threaten the integrity of the cycle.
Direct impact on asset integrity.
Failures in conductivity control are directly associated with severe degradation mechanisms, such as:
- Corrosion under deposits
- Acid corrosion in condensation zones
- Stress Corrosion Cracking (SCC)
- Deposition of salts and silica on turbine blades
- Loss of thermal efficiency
Technical literature and operational experience demonstrate that contamination events that are not detected early can result in catastrophic damage, with significant financial and operational impacts.
Specific field case – DAC avoiding false alarm due to CO₂
In a thermal power plant operating in AVT(O) mode, with frequent starts and rapid load variations, recurring alarms of high cationic conductivity in the feedwater were observed during start-up periods.
Initial situation
- Specific conductivity within regulatory limits
- Cationic conductivity exhibiting intermittent peaks.
- Recurring operational actions: load reduction, condenser and polisher inspection.
Technical diagnosis
Detailed analysis indicated that the peaks were associated with the temporary influx of CO₂ during the warming and displacement phases of dissolved gases, without any actual influx of aggressive anionic contaminants.
In other words, these were false alarms, leading to conservative decisions and a loss of unit availability.
DAC Implementation
With the installation of degassed cation conductivity (DAC) at the feedwater point:
- The effects of CO₂ were eliminated from the measurement.
- Reading began to reflect exclusively strong acids.
- The spurious alarms have been completely eliminated.
Results obtained
- Significant reduction in unnecessary operational interventions.
- Increased operational confidence in analytical data.
- Direct improvement in unit availability.
- More precise application of VGB Action Levels
Case conclusion
This case demonstrates that DAC not only improves measurement quality but also raises the operational maturity level of the water-steam cycle, transforming online analysis into a reliable and strategic decision-making tool.
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