Foreword
Within industrial sectors such as steel mill waste heat power generation and thermal power plants, the steam-water system functions akin to the human circulatory system. The quality of its water directly impacts equipment longevity, operational efficiency, and safety and environmental standards. Yet, within high-temperature, high-pressure environments, even slight fluctuations in dissolved oxygen, chloride ions, or hardness levels within the steam-water mixture can precipitate catastrophic consequences such as boiler tube ruptures or turbine scaling. Steam-water sampling and monitoring serve as the ‘eyes’ for discerning system health. By providing real-time analysis of steam and water quality, it equips operational personnel with scientific decision-making support, thereby forming the ‘first line of defence’ in ensuring the safe, economical, and environmentally sound operation of industrial thermal systems.
1 Significance of Steam-Water Sampling Monitoring
01 Technical Aspect: Ensuring System Stability
Steam-water sampling monitoring precisely suppresses metal corrosion and scale deposition by continuously tracking core indicators such as pH, dissolved oxygen, and hardness, thereby guaranteeing thermal equipment operates under stable conditions. Simultaneously, monitoring impurity levels in steam optimises steam quality and rapidly locates system leaks, preventing faults from escalating and impacting overall efficiency.
02 Economic Aspect: Reducing Operational Costs
By preventing equipment failures caused by corrosion and scaling, steam-water monitoring minimises losses from unplanned shutdowns, averting direct economic losses ranging from millions to tens of millions of pounds. Furthermore, optimising water treatment processes and fuel utilisation significantly reduces chemical and energy costs while extending the service life of critical equipment such as boilers and steam turbines, thereby enhancing long-term economic benefits.
03 Safety Aspect: Preventing Major Accidents
Abnormal steam-water quality may trigger catastrophic incidents such as boiler tube ruptures, hydrogen embrittlement, and stress corrosion cracking. Real-time monitoring provides advanced risk warnings, reducing accident probability. Simultaneously, by averting hazards like high-pressure steam leaks, the monitoring system effectively safeguards personnel safety, fortifying the security defences of industrial operations.
04 Environmental Compliance: Meeting Emission Requirements
Steam-water monitoring optimises zero-liquid discharge processes, minimises pollutant accumulation in brine, ensures wastewater reuse rates, and reduces aquatic pollution risks. Furthermore, enhancing unit thermal efficiency lowers carbon emissions per unit of electricity generated, supporting enterprises in achieving energy conservation and emission reduction targets while advancing green, sustainable development.
05 Industry Compliance: Meeting Standards and Regulatory Requirements
Steam and water quality must strictly comply with standards (such as boiler water quality and steam water quality). Monitoring data provides enterprises with evidence of compliance.
2 Interpretation of Monitoring Indicators
- Corrosion Control: Preventing Equipment Lifespan Degradation
(1) pH Value: Adjusting steam-water acidity to inhibit metal corrosion
Low pH (acidic) accelerates carbon steel corrosion, generating Fe²⁺/Fe³⁺ and causing boiler tube wall thinning.
High pH (alkaline) may induce caustic embrittlement (particularly under high temperature and pressure), disrupting the metal crystal lattice.
(2) Dissolved Oxygen: Eliminating Oxidative Corrosion Risks
Dissolved oxygen (DO) reacts with metals to form iron oxides (Fe₂O₃/Fe₃O₄), causing pitting or ulceration corrosion.
(3) Chloride Ions (Cl⁻): Preventing Stress Corrosion Cracking (SCC)
Cl⁻ ions possess small radii and high penetrability, readily adsorbing onto crack tips on metal surfaces to accelerate crack propagation.
Stainless steel may exhibit chloride embrittlement in Cl⁻-containing environments (e.g., condenser tubes).Scale Inhibition: Maintaining Thermal
- Efficiency Stability
(1) Hardness (Ca²⁺/Mg²⁺): Preventing Calcium-Magnesium Salt Deposition
Hardness ions combine with CO₃²⁻/SO₄²⁻ at elevated temperatures to form scale deposits (e.g., CaCO₃, MgSiO₃).
Scale thermal conductivity is merely 1/50–1/100 that of steel, reducing boiler heat transfer efficiency by 10–20%.
(2) Silicate (SiO₂): Prevents deposition in steam flow paths
Silicic acid polymerises into silica gel at high temperatures, adhering to turbine blades or valve surfaces to form hard deposits.
These deposits reduce turbine efficiency by 3%-5% and are difficult to remove completely through chemical cleaning.
- Water Quality Compliance: Meeting Environmental and Energy Efficiency Requirements
(1) Conductivity: Comprehensive Indicator of Total Ion Impurities
Conductivity is directly proportional to dissolved salt concentration in water, serving as a rapid indicator of water purity.
Elevated conductivity (e.g., >1μS/cm) may indicate excessive Cl⁻/SO₄²⁻ levels, necessitating further analysis of specific ions.
Boiler feedwater conductivity must be <0.3μS/cm (at 25°C) to minimise impurity carryover in steam.
Condensate conductivity must be <0.3μS/cm to prevent corrosion in the cold-end system.
(2) Phosphate (PO₄³⁻): Maintaining phosphate treatment efficacy in boiler water
Phosphate reacts with Ca²⁺/Mg²⁺ to form loose slag (e.g., Ca₃(PO₄)₂), which is removed via blowdown.
Excessively low phosphate concentrations (<5mg/L) fail to provide effective scale inhibition, while excessively high levels (>50mg/L) may induce phosphate masking.
- Process Optimisation: Enhancing Operational Economy
(1) Sodium Ions (Na⁺): Monitoring Steam Quality and Condenser Leakage
Na⁺ in steam primarily originates from condenser leakage (cooling water ingress into the steam system).
Elevated Na⁺ concentrations (>5μg/L) indicate condenser tube corrosion or seal failure, requiring immediate investigation.
(2) Total Iron (Fe): Assessing System Corrosion Rate
Total iron encompasses Fe²⁺/Fe³⁺ and suspended iron oxides, reflecting metal corrosion severity.
Feedwater total iron must remain below 5μg/L; otherwise, corrosion products will deposit within the turbine low-pressure cylinder, diminishing efficiency.