1. Introduction
China's total installed power generation capacity reached 3.35 billion kW by end of 2025, with thermal power accounting for over 42% at approximately 1.43 billion kW. Condensers and boilers, as the core heat exchange equipment in thermal power units, directly influence generation efficiency, coal consumption rate, and operational safety. For every 1 kPa drop in condenser vacuum, the unit heat rate increases by approximately 1.2%~1.5%. For a 300 MW unit operating 7,000 hours annually, this translates to an additional 800~1,200 tons of standard coal consumption per year. When boiler heating surfaces accumulate 1 mm of scale, thermal efficiency drops 2%~3%, and tube wall temperature rises 40~60°C, potentially causing tube rupture. The challenge lies in effective cleaning without damaging thousands of thin-walled condenser tubes (0.5~1.0 mm thickness) or boiler pressure components, while complex mixed scale (CaCO₃, CaSO₄, SiO₂, Fe₃O₄) often requires tailored cleaning formulations. With typical overhaul windows of only 25~45 days, cleaning procedures must be both effective and time-efficient.
2. Scale Formation Mechanisms and Impacts
2.1 Condenser Scale Formation
Scale in condenser cooling water systems falls into three categories. Carbonate scale (CaCO₃) forms when cooling water is concentrated through evaporation in cooling towers, raising Ca²⁺ and HCO₃⁻ concentrations until the Langelier Saturation Index (LSI) exceeds 0 and CaCO₃ precipitates on tube walls. Microbial slime results from algae and bacteria entering the system with makeup water and forming biofilms on tube surfaces. Corrosion product deposits arise from oxygen corrosion of carbon steel tubes, producing Fe₂O₃·nH₂O rust layers.
Scale has a dual impact: it reduces the heat transfer coefficient (CaCO₃ scale conductivity is only 0.6~2.2 W/(m·K) versus 398 W/(m·K) for copper tubes) and narrows the effective flow cross-section. Field data from a 300 MW unit shows that 0.3 mm of scale on stainless steel condenser tubes reduced the cleanliness factor from 0.85 to 0.68, increasing terminal temperature difference by 3.2°C and decreasing vacuum by 1.8 kPa.
2.2 Boiler Heating Surface Scale
Boiler scaling involves coupled thermodynamics, fluid dynamics, and chemical kinetics. Even when feedwater Ca²⁺/Mg²⁺ concentrations are as low as 0.5~1.0 mg/L, the extreme concentration factor of 10⁴~10⁵ in high-temperature zones (900~1,100°C) causes Ca(HCO₃)₂ to decompose into CaCO₃↓ + H₂O + CO₂↑. In high heat flux zones, CaSO₄ precipitates as dense hard scale with thermal conductivity of only 0.4~0.9 W/(m·K), even harder to remove than CaCO₃. Silicate scale (SiO₂/Ca²⁺/Mg²⁺ composite) has conductivity as low as 0.2~0.5 W/(m·K) and typically requires higher acid concentrations for effective dissolution.
Scale poses serious safety risks. With 1.0 mm scale on 20G carbon steel tubes, wall temperature rises from 450°C to 510~540°C, reducing tensile strength by approximately 25% and exceeding the material's allowable service temperature. Combined with under-scale acidic corrosion (Cl⁻ concentration causing local pH drop to 3~4), hydrogen damage and caustic embrittlement risks increase substantially. Statistics indicate that approximately 40% of boiler tube rupture incidents in Chinese power plants are directly linked to scale or corrosion product deposits.
3. Cleaning Program Design
3.1 Condenser Chemical Cleaning Formulations
| Target | Tube Material | Primary Agent | Conc. (%) | Corrosion Inhibitor |
|---|---|---|---|---|
| Water Side | 316L Stainless Steel | Sulfamic Acid | 5~8 | BTA 0.1% + Urotropine 0.2% |
| Water Side | Copper (HSn70-1) | Sulfamic Acid | 3~5 | BTA 0.15% + Na₂MoO₄ 0.1% |
| Steam Side | Copper/Stainless | NaOH + Na₃PO₄ | 2+1 | Surfactant 0.05% |
3.2 Boiler Chemical Cleaning Formulations
| Type | Scale Type | Cleaning System | Temp (°C) | Flow (m/s) |
|---|---|---|---|---|
| New Boiler | Mill Scale + Grease | HCl 3~5% + Alkaline NaOH 2% | 55±5 / 85±5 | 0.3~0.5 |
| Operating (Carbonate) | Primarily CaCO₃ | HCl 4~6% + NH₄HF₂ 0.5% | 50~60 | 0.3~0.5 |
| Operating (Silicate) | CaSiO₃+CaSO₄ Composite | HCl 5~7% + NH₄HF₂ 1~1.5% | 55~65 | 0.3~0.5 |
| Operating (Iron Oxide) | Fe₃O₄+Fe₂O₃ | Citric Acid 3% + EDTA 5% | 90~95 | 0.3~0.5 |
3.3 Corrosion Inhibitor Selection
For copper tube condensers, BTA (Benzotriazole) is recommended — it forms a dense [Cu-BTA]n complex film on tube surfaces with over 99% coverage, stable at pH 3~5. Stainless steel systems use Urotropine + Na₂MoO₄ combination: the former provides physical adsorption barrier protection (75%~85% inhibition efficiency), while the latter promotes passive film self-repair (0.1% Na₂MoO₄ raises stainless steel pitting potential by 300~400 mV). For boiler carbon steel HCl cleaning, Lan-826 multi-purpose inhibitor or acetylenic alcohol-based formulations should be used, maintaining corrosion rates below 1.0 g/(m²·h). For EDTA or Citric Acid boiler cleaning, use MBT (2-Mercaptobenzothiazole) + Urotropine combination. Citric Acid dissolves iron oxide scale through dual chelation-reduction: C₆H₈O₇ reacts with Fe₃O₄ under acidic conditions to form soluble ferric citrate complexes while reducing Fe³⁺ to Fe²⁺ for accelerated dissolution. EDTA is preferred for supercritical and ultra-supercritical boiler pre-commissioning cleaning — it can complete descaling and passivation in a single step under alkaline conditions (pH 8.5~9.5), reducing total cleaning time by approximately 30%.
4. Online vs. Offline Cleaning Comparison
| Parameter | Condenser Online | Condenser Offline | Boiler Offline |
|---|---|---|---|
| Application | Unit online; scale <0.3mm; vacuum loss <1.5kPa | Overhaul or fault outage; scale >0.3mm; vacuum loss >1.5kPa | Scheduled overhaul or post-tube-rupture |
| Medium | Sponge ball system or low-conc. Sulfamic Acid 2~3% bypass | Sulfamic Acid 5~8% circulation or HP water jetting (500~700 bar) | HCl/Citric Acid/EDTA circulation |
| Frequency | Balls: 2~4x daily; Chemical: every 3~6 months | Every 1~2 years | New: pre-commissioning; Operating: every 3~5 years or scale >0.5mm |
| Duration | No generation loss | 3~7 days | 5~10 days (acid wash + rinse + passivation) |
| Result | Maintain cleanliness ≥0.80 | Cleanliness restored to 0.90~0.95 | Scale removal ≥95%; uniform passive film |
5. Case Studies
Case 1: 600 MW Subcritical Unit Condenser Cleaning
This condenser was a dual-pass surface type with 26,800 HSn70-1 copper tubes (Φ25×1.0 mm). Pre-cleaning vacuum was 91.2 kPa (design: 95.5 kPa), terminal temperature difference 9.8°C (design: ≤5°C), and water-side pressure differential 0.12 MPa (normal: 0.06 MPa). Inspection revealed substantial CaCO₃ hard scale averaging 0.4~0.7 mm, with up to 40% of tube openings blocked.
Cleaning approach: 5% Sulfamic Acid + 0.15% BTA + 0.2% Urotropine at 45±5°C, flow velocity 0.15~0.25 m/s, alternating A/B sides. Acid concentration and Cu²⁺ levels were sampled every 30 minutes; acid was replenished when concentration dropped to 40% of initial. Total cleaning time: 12 hours (6h per side). Final scale removal rate 98%, copper tube corrosion rate 0.08 g/(m²·h), stainless steel <0.01 g/(m²·h). Post-cleaning vacuum recovered to 95.1 kPa, terminal difference dropped to 5.3°C, unit heat rate improved ~1.8%, saving ~2,000 tons of standard coal annually.
Case 2: 670 t/h Pulverized Coal Boiler Chemical Cleaning
Boiler parameters: 670 t/h evaporation, 540°C/9.81 MPa main steam, water wall material 20G carbon steel. Last cleaned over 5 years prior. Inspection showed water wall scale density of 450 g/m² (standard: ≤250 g/m²). Scale analysis: CaO 38%, MgO 12%, Fe₂O₃ 28%, SiO₂ 15%, SO₃ 5%. HCl 5% + NH₄HF₂ 0.8% + Lan-826 inhibitor 0.3% system was used at 55±5°C, 0.35~0.45 m/s. After 6 hours of acid washing, Fe³⁺ concentration rose above 420 mg/L; Na₂SO₃ was promptly added to control below 200 mg/L. Process: acid wash → water rinse → 0.2% Citric Acid rinse → NaNO₂ 0.5% + Na₃PO₄ 0.3% passivation (pH 9.5~10.5, 60°C×6h). Post-passivation inspection showed uniform steel-gray Fe₃O₄ film; copper sulfate spot test >60s passed. Scale removal 97.5%, corrosion rate 0.82 g/(m²·h) (well below the 8 g/(m²·h) standard). Post-cleaning boiler efficiency improved from 87.3% to 90.1%.
6. Summary and Recommendations
Power plant condenser and boiler cleaning programs must integrate equipment material, scale composition, operational conditions, and environmental requirements. For routine condenser maintenance, sponge ball cleaning system availability should exceed 98%, combined with biocide dosing in circulating water and semi-annual full tube inspections. Copper tube condensers must use copper-specific inhibitors (BTA) — HCl must never be used directly on copper tubes due to rapid dissolution and dezincification risk. Boiler chemical cleaning must comply with DL/T 794-2012 standard. All new boilers require pre-commissioning chemical cleaning to remove mill scale. For condensers with established scale, the combined approach of high-pressure water jetting (500~700 bar) followed by chemical cleaning yields the best results — water jetting removes bulk hard scale at tube openings, while chemical cleaning addresses thin scale and microbial slime deep inside tubes, achieving >98% overall scale removal. Selecting a qualified power industry cleaning contractor is critical — the contractor should hold power engineering construction qualifications, employ certified cleaning technicians, and maintain proven inhibitor programs for copper, stainless steel, and carbon steel multi-material systems.