For power plant boilers, especially supercritical units, the core of water quality testing lies in adhering to standards like DL/T 956-2017. It involves ultra-precise monitoring of trace parameters like silica, sodium, and cation conductivity in feedwater, steam, and boiler water. This requires a full-process scheme combining online chemical instruments with laboratory trace analysis to achieve "zero scaling, zero corrosion" extreme operating conditions.
For professionals in power plant chemistry, the saying "the boiler is the heart, water is its blood" holds profound meaning. An ion exceedance of a few micrograms per liter (μg/L) can trigger severe incidents like turbine salt deposition or hydrogen damage in boiler tubes within hundred-megawatt units. Unlike industrial boilers, power plant boiler water testing is the ultimate challenge of "trace amounts" and "precision."
Power plant boilers, particularly subcritical, supercritical, and ultra-supercritical units, operate under extremely high pressures and temperatures (e.g., ultra-supercritical units >25MPa, >600°C). Under these extreme conditions, any are greatly amplified in their:
Silica (SiO₂) Deposition:Silica compounds in steam can form glass-like hard scales on turbine blades, significantly reducing output and efficiency. According to research in the Thermal Power Generation journal, 0.5mm of scale accumulation in the high-pressure cylinder can lead to a 3%-5% efficiency drop, translating to massive power generation losses.
Acidic Corrosion: Trace anions like chloride (Cl⁻) and sulfate (SO₄²⁻) can destroy protective metal oxide layers, leading to stress corrosion cracking or pitting under high temperature and pressure. Cation Conductivity is the key comprehensive indicator for monitoring these harmful anions and must be controlled below 0.15 μS/cm.
Hydrogen Damage: The combined action of trace oxygen and acidic conditions can cause hydrogen atoms to penetrate boiler tube walls, leading to decarburization and embrittlement, drastically increasing the risk of tube failures.
Thus, the goal of power plant chemical supervision is no longer just "preventing thick scale" but pursuing "high-purity treatment" .
Beyond conventional parameters like pH and dissolved oxygen, the critical focus of power plant boiler water testing is the monitoring of trace impurities. This is primarily based on the Guide for Water and Steam Chemistry Supervision in Fossil Fuel Power Plants (DL/T 956-2017) and Water and Steam Quality for Fossil Fuel Power Generation Units and Steam Power Equipment (GB/T 12145-2016).
The table below outlines key trace parameters for high-pressure units:
| Sampling Point | Key Trace Parameter | Control Significance & Target (e.g., Supercritical Unit) | Primary Risk |
Feedwater | Cation Conductivity (25°C) | ≤0.15 μS/cm, the essential indicator for anion impurities. | System Corrosion |
Sodium (Na⁺) | ≤2 μg/L, a sensitive indicator for cation exchange resin leakage. | Superheater Salt Deposition | |
Silica (SiO₂) | Must ensure steam silica meets standards, typically ≤10 μg/L. | Turbine Scaling | |
Steam | Silica (SiO₂) | ≤10 μg/L, the critical limit for preventing silica scales in the turbine. | Blade Efficiency Loss |
Sodium (Na⁺) | ≤2 μg/L, directly reflects steam purity. | Superheater/Turbine Corrosion & Scaling | |
Condensate | Sodium (Na⁺) | ≤2 μg/L, the first line of defense for detecting condenser leakage. | Contamination of entire steam-water cycle |
Pro Tip: Cation Conductivity is measured after passing the water sample through a cation exchange column. This effectively eliminates the influence of additives like ammonia, purely reflecting the concentration of harmful anions. It is the gold standard indicator for detecting water quality abnormalities.
The power plant chemical monitoring system must achieve a three-level linkage: real-time monitoring, rapid diagnosis, and precise analysis.
This is the "neural network" installed throughout the main plant, providing 24/7 monitoring.
Online Silica Analyzer: Continuously monitors SiO₂ in feedwater and steam, typically with a range of 0-50 μg/L and accuracy of ±1 μg/L.
Online Sodium Analyzer: Employs advanced ion-selective electrode technology, measuring ranges like 0.1-1000 μg/L. Crucial for monitoring condenser leakage and condensate polishing system performance.
Online Cation Conductivity Analyzer: The core of the core, requiring automatic cation exchange column regeneration and temperature compensation.
Online pH Analyzer & Dissolved Oxygen Analyzer: While measuring conventional parameters, these demand extremely high sensor stability and accuracy.
This is the "forensic lab" for chemical supervision, used for calibrating online instruments and conducting deeper analysis.
Ion Chromatograph (IC): Used for simultaneous precise analysis of various anions (Cl⁻, SO₄²⁻, NO₂⁻, etc.) down to μg/L levels.
Atomic Absorption Spectrometer (AAS) or Inductively Coupled Plasma Spectrometer (ICP-OES/ICP-MS): Used for analyzing metal corrosion products like iron and copper. ICP-MS can detect down to ng/L (ppt) levels, aiding in root cause analysis of failures.
High-Precision Benchtop pH/Conductivity Meters: Used for periodic calibration of online instruments, ensuring data traceability and accuracy.
Background: A 600MW supercritical power plant found slight silicate deposits on HP turbine blades during an overhaul. Minor fluctuations in steam cation conductivity were occasionally observed during operation.
Diagnosis & Solution:
1. In-Depth Investigation: The chemistry team first used an Ion Chromatograph to perform a full scan of water samples from various stages. They discovered periodic, minor spikes (~3-5 μg/L) in chloride after the condensate polishing plant, though still within limits. The online sodium analyzer data remained stable.
2. Precise Identification: Suspicion fell on slight resin degradation or imperfect regeneration in the condensate polisher mixed beds, leading to leakage of trace organics or colloidal silica. These substances can decompose under high temperature, producing anions that cause cation conductivity fluctuations.
3. Upgraded Measures:
Optimized the regeneration process for the condensate polisher and enhanced resin separation checks.
Added an Online Total Organic Carbon (TOC) Analyzer after the polisher to monitor trace organics.
Increased the frequency of Ion Chromatograph use for trend analysis.
Results:After three months of adjustment and monitoring, the steam cation conductivity curve stabilized significantly. During the next inspection, the turbine blades were found clean and bright. The estimated annual benefit from improved efficiency and extended maintenance intervals exceeded one million RMB.
Referring to tender documents for large power plant chemical instruments, a qualified instrument list should possess the following characteristics:
Online Instruments (Requiring High Reliability & Precision):
Online Silica Analyzer (Range: 0-50/200 μg/L, Accuracy: ±1% FS)
Online Sodium Analyzer (Range: 0.1-1000 μg/L, Accuracy: ±5% of reading)
Online Cation Conductivity Analyzer (with automatic regenerating cation exchange column)
High-Precision Online pH/Dissolved Oxygen Analyzers
Laboratory Analysis (Requiring High Sensitivity & Versatility):
Ion Chromatograph (Essential) for precise anion/cation analysis.
Atomic Absorption Spectrometer or superior ICP-OES for metal element analysis.
UV-Vis Spectrophotometer for auxiliary analysis of specific parameters like phosphate or hydrazine.
Selection Advice: Instrument selection for power plants must heavily prioritize long-term stability, data accuracy, after-sales technical support, and ease of integration into plant-wide SIS or MIS systems. Avoid opting for less stable equipment based solely on low cost, as the losses from unplanned outages caused by such equipment far exceed their initial price.
For modern, large-capacity power plants, chemical supervision has evolved from a supporting role to a core discipline on par with mechanical, boiler, and electrical disciplines. Investing in a comprehensive and accurate water/steam quality monitoring system is not only about complying with national standards but is a strategic move to ensure the safe, environmentally compliant, and economical operation of the unit, thereby enhancing the plant's core competitiveness.
Next Steps: Evaluate the completeness of your plant's existing chemical monitoring system, particularly identifying any blind spots in trace parameter monitoring. If you require more detailed instrument technical parameters or scheme consultation, please feel free to contact our expert team.
