Copper Tube Condenser Cleaning & Corrosion Inhibition Technology

1. Introduction

Condensers are critical heat exchange components in thermal power plant turbine units, directly influencing unit vacuum and coal consumption rates. Copper tube condensers remain widely deployed—by the end of 2025, approximately 40% of sub-300MW units in China used brass tubes (HSn70-1, HA177-2) or copper-nickel tubes (BFe10-1-1, BFe30-1-1) as condenser heat exchange tubing. Compared to stainless steel, copper tubes offer superior thermal conductivity (approximately 100–120 W/(m·K), 6–8 times that of stainless steel) and better expansion joint sealing, but face significantly more complex corrosion and scaling challenges.

Ca²⁺ and Mg²⁺ in circulating cooling water deposit onto heat exchange surfaces forming carbonate scale, compounded by microbial slime and suspended solids, causing condenser terminal temperature difference (TTD) to rise and vacuum to decline year over year. According to DL/T 2025, chemical cleaning should be considered when TTD exceeds the design value by more than 3°C or vacuum drops by more than 2 kPa. However, copper alloys are sensitive to strong acids and oxidizing media—improper cleaning formulation selection can easily cause dezincification corrosion, stress corrosion cracking, or ammonia-induced perforation. This article systematically presents specialized cleaning technologies and corrosion inhibition systems for copper tube condensers.

2. Copper Tube Condenser Structure and Scaling Mechanisms

2.1 Typical Structure and Materials

Copper tube condensers consist of shell, tube sheets, tube bundles, and water chambers. Cooling water flows inside tubes (tube side), while turbine exhaust steam flows outside tubes (shell side). Common copper tube materials and applicable water conditions:

2.2 Scaling Mechanism Analysis

Copper tube condenser scaling is predominantly carbonate-based, with the reaction pathway: Ca(HCO₃)₂ in circulating water thermally decomposes to deposit CaCO₃ — Ca(HCO₃)₂ → CaCO₃↓ + CO₂↑ + H₂O. Additionally, the evaporative concentration effect in cooling towers continuously increases Ca²⁺ concentration. When the Langelier Saturation Index (LSI) > 0, scaling tendency exists. Microbial metabolism produces extracellular polymeric substances (EPS) that adhere to tube walls, forming bio-slime layers that further trap suspended particulates, constructing a three-layer composite fouling structure: inorganic scale – organic slime – suspended solids. This composite fouling has a thermal conductivity of only 0.5–1.5 W/(m·K), far below the copper tube substrate (~110 W/(m·K)), causing dramatic heat transfer efficiency decline.

3. Copper Tube Corrosion Risk Analysis

3.1 Dezincification During Acid Cleaning

Brass tubes (HSn70-1, HA177-2) contain approximately 22–30% Zn. In acidic media, Zn preferentially dissolves—dezincification corrosion. Reaction: Cu-Zn + 2H⁺ → Cu (porous) + Zn²⁺ + H₂↑. After dezincification, a loose, porous copper layer forms on the tube surface, dramatically reducing mechanical strength and potentially causing tube wall perforation. Therefore, efficient copper-specific inhibitors must be added to the cleaning solution.

3.2 Ammonia Corrosion (Stress Corrosion Cracking)

Dissolved oxygen and ammonia (from feedwater ammonia dosing for pH adjustment) in condensate form [Cu(NH₃)₄]²⁺ complex ions on the inner tube wall, inducing intergranular stress corrosion cracking (SCC) under residual stress (particularly concentrated in expansion joint zones). This is one of the most common leakage modes in copper tube condensers. Residual NH₄⁺ not thoroughly flushed after chemical cleaning may accelerate subsequent ammonia corrosion during operation.

3.3 Under-Deposit Corrosion (UDC)

Areas covered by scale form oxygen concentration cells—under-deposit areas become anodic zones (low O₂), exposed tube walls become cathodic zones (high O₂), accelerating localized corrosion beneath deposits. UDC in copper tube condensers typically manifests as pitting, with depths reaching 0.3–0.8 mm/year.

4. Specialized Chemical Cleaning Formulations for Copper Tube Condensers

4.1 Cleaning Agent Selection Principles

Copper tube condenser cleaning must follow the three principles: low concentration, low pH, and strong inhibition. Unlike stainless steel condensers that can use 5–8% HCl, copper tube cleaning must employ organic acids or Sulfamic Acid systems with low copper corrosion rates:

4.2 Copper Corrosion Inhibitor System

The most critical additive in copper tube condenser cleaning is copper-specific inhibitor, which forms a dense protective film on the copper surface to block acid–substrate contact. Industry-recognized high-efficiency copper inhibitors include:

BTA (Benzotriazole): Forms [Cu(I)-BTA]n polymer film on copper surfaces, with film thickness of approximately 5–50 nm. Maintains excellent inhibition efficiency even in pH 2–4 acidic media at addition rates of 0.05–0.2%.

MBT (Mercaptobenzothiazole): Forms monomolecular film on Cu surfaces through sulfur atom chemisorption. Better temperature tolerance than BTA (up to 70°C), addition rate 0.03–0.1%.

BTA+MBT Composite Inhibition: Extensive experimental and engineering practice demonstrates that BTA and MBT combined at a 2:1 ratio produce synergistic effects—BTA forms polymer film in low-coverage regions while MBT forms chemisorbed film at high-energy sites, complementary coverage raising inhibition efficiency above 99% and controlling copper corrosion rate below 0.2 g/m²·h. Recommended total composite inhibitor addition: 0.1–0.3%.

4.3 Recommended Cleaning Formulation

Cleaning solution pH should be controlled at 2.0–3.0, temperature at 40–55°C. HCl is strictly prohibited for copper tube condenser acid cleaning—Cl⁻ induces pitting and stress corrosion in copper tubes and accelerates localized corrosion at BTA film defects. Strong oxidizing Fe³⁺ additives (such as FeCl₃) must also be avoided—the redox reaction Fe³⁺ + Cu → Fe²⁺ + Cu²⁺ causes severe copper tube corrosion.

5. Cleaning Process Flow

5.1 Shutdown Cleaning Process

1. Isolation and Drainage: Close condenser inlet/outlet valves, drain residual cooling water from water chambers and tube bundles.
2. Scale Sample Analysis: Collect samples from tube sheets and inner tube walls. Determine scale composition (CaCO₃, Mg(OH)₂, SiO₂, Fe₂O₃ content) via XRF or chemical titration to fine-tune acid concentration and assess whether HF is needed for silica removal.
3. Pre-Flush: Flush tube bundles with 0.5–1.0 MPa industrial water to remove loose silt and surface fouling.
4. Alkaline Degreasing (if oil present): NaOH 1% + Na₂CO₃ 0.5% + Surfactant 0.05%, circulate at 50°C for 2–4 h.
5. Acid Cleaning: Prepare cleaning solution per recommended formulation, circulate at 0.15–0.3 m/s. Sample every 30 min to monitor acid concentration, Fe³⁺, and Cu²⁺ levels. Endpoint is reached when acid concentration change <0.02% in two consecutive measurements and Fe³⁺/Cu²⁺ concentrations stabilize. Typical duration: 4–8 h.
6. Water Flush: Drain acid solution after cleaning; high-volume industrial water flush until effluent pH ≥ 6 and total iron <5 mg/L.
7. Passivation: Na₂CO₃ 1% + NaNO₂ 0.5%, pH 9–10, circulate at 50°C for 4 h to form alkaline passivation film on copper surfaces.
8. Final Flush and Acceptance: Flush until effluent is clear and conductivity approaches inlet water values. Randomly inspect 5% of tube openings via borescope—visual tube wall cleanliness should meet DL/T 2025 Grade I–II standards.

5.2 Online Cleaning Option

For units that cannot schedule shutdowns, EDTA online cleaning can be employed: utilizing the single-side isolation operating window, circulate EDTA 5–8% (pH 5.5–6.5, BTA 0.1%) in the isolated side, leveraging EDTA's Ca²⁺ complexation capacity (stability constant lgK = 10.7) to gently dissolve carbonate scale. Online cleaning cycle is approximately 24–48 h, with copper corrosion rate controllable below 0.05 g/m²·h. Compared to shutdown acid cleaning, online cleaning achieves 60–80% scale removal (vs. 90–95% for shutdown cleaning), but the advantage is no impact on unit output.

6. Engineering Case Study

Project Background: A power plant with 2×135 MW units, condenser model N-8500, HSn70-1 copper tubes, 5,000 tubes (Φ25×1 mm), once-through fresh water cooling. After 10 years of operation, condenser TTD increased from the design 4°C to 12°C, vacuum deteriorated from -95 kPa to -88 kPa, affecting coal consumption by approximately 6 g/kWh.

Scale Analysis Results: CaCO₃ 82%, Mg(OH)₂ 8%, SiO₂ 3%, organics 7%. Average scale thickness 1.2 mm (maximum 2.5 mm).

Cleaning Program: Sulfamic Acid 4% + Citric Acid 2% + BTA 0.15% + MBT 0.08% formulation. Cleaning temperature 50±3°C, circulation velocity 0.2 m/s, total circulation time 6 h. HSn70-1 corrosion coupons (C1–C3) suspended during process for post-cleaning gravimetric corrosion rate calculation.

Cleaning Results:

Post-cleaning, unit vacuum recovered to near-design values, translating to annual savings of approximately 1,200 tonnes of standard coal equivalent. Borescope inspection confirmed tube walls displayed original brass color with no dezincification or pitting—the BTA+MBT composite inhibition system effectively protected the copper tube substrate.

7. Summary and Recommendations

The key to successful copper tube condenser chemical cleaning lies in "selecting the right acid and controlling corrosion"—replacing HCl with Sulfamic Acid or Citric Acid systems, and replacing single inhibitors with BTA+MBT composite inhibitors, represents a safe and efficient approach validated through extensive engineering practice. Cu²⁺ concentration in the cleaning solution must be strictly monitored during the process—if Cu²⁺ exceeds 100 mg/L, cleaning should be stopped immediately for investigation, as this typically indicates inhibitor failure or localized overheating.

Preventive maintenance is equally critical. Power plants are advised to establish online condenser monitoring systems: record TTD and vacuum trends monthly, sample circulating water quarterly for hardness and Cl⁻ analysis. When TTD rises more than 3°C above design, cleaning assessment should be initiated—avoid waiting until severe scaling necessitates "rescue cleaning." Additionally, post-cleaning passivation must never be omitted—Na₂CO₃ + NaNO₂ passivation for ≥4 h, with copper inhibitor dosing maintained for 72 h after unit restart to preserve passivation film integrity. From a lifecycle cost perspective, regular preventive cleaning (every 2–3 years) is far superior to deep cleaning after severe condenser performance degradation—the former maintains sustained high-efficiency operation and significantly reduces the risk of premature copper tube replacement due to localized under-deposit corrosion.