Replacement of Brass Tubes with Stainless Steel Tubes in Power Plant Condensers

Literature Overview

This engineering case study by Liang Lei, Zhou Guoding, Ni Peng, Zhang Dengyun, Sun Zhilin, Sun Jun, and Wang Yihao, published in Turbine Technology (2004), documents the successful replacement of HSn70-1 brass tubes with 304 stainless steel tubes in the condenser of Unit 1 at Tongling Power Plant. Funded by the Shanghai Municipal Education Commission Key Discipline Foundation, this paper provides practical engineering experience in condenser tube material selection and replacement.

Core Technical Content

Condenser tubes in thermal power plants are subjected to aggressive operating environments involving cooling water with varying chemical compositions, temperature cycling, and continuous mechanical vibration. The original HSn70-1 brass tubes (a tin bronze alloy containing approximately 70% copper, 29% zinc, and 1% tin) experienced corrosion issues that necessitated material replacement.

The replacement with 304 stainless steel tubes (UNS S30400, containing approximately 18% chromium, 8% nickel, and 2% maximum carbon) was preceded by comprehensive investigation, testing, and research to ensure technical feasibility and economic viability.

Material Property Comparison

The replacement achieved better corrosion performance, improved mechanical properties, and comparable overall heat transfer performance, demonstrating the technical viability of stainless steel tubes for condenser applications.

Technical Points Interpretation

The decision to replace brass tubes with stainless steel tubes involved careful consideration of multiple factors:

Corrosion Mechanisms:

Brass tubes in condenser service are susceptible to:

• Dezincification in chloride-containing cooling water
• Erosion-corrosion at high flow velocities
• Intergranular corrosion under certain environmental conditions
• Pitting corrosion in the presence of chlorides and oxygen

304 stainless steel tubes offer superior resistance to these corrosion mechanisms through the formation of a passive chromium oxide film. However, they require careful attention to:

• Welding procedures to prevent sensitization and intergranular corrosion
• Surface finish quality to maintain passive film integrity
• Avoidance of crevice configurations where differential aeration can initiate localized corrosion

Heat Transfer Performance:

The significantly lower thermal conductivity of 304 stainless steel compared to brass raises concerns about heat transfer efficiency. However, the study demonstrates that overall heat transfer performance is maintained through:

• Design optimization of tube geometry and layout
• Consideration of fouling resistance factors
• Recognition that tube material thermal conductivity is not the sole determinant of overall heat transfer coefficient
• The relatively thin tube walls minimize the thermal resistance contribution of the tube material itself

Welding and Fabrication Considerations

The fabrication and installation of stainless steel condenser tubes require specialized welding procedures:

• GTAW (Gas Tungsten Arc Welding) is the preferred process for tube-to-tubesheet welds
• Back purging with inert gas is essential to prevent chromium oxide formation on the weld root
• Pre-heat is generally not required for 304 stainless steel but interpass temperature should be controlled below 150°C
• Post-weld passivation treatment is recommended to restore the passive film
• Weld inspection using penetrant testing (PT) or dye penetrant methods is standard practice

Integration with Engineering Practice

This case study provides valuable practical experience for engineers involved in power plant maintenance and modification projects. The systematic approach of investigation, testing, and research before implementation is exemplary engineering practice.

Key lessons from this replacement project include:

1. Material Selection: The selection of 304 stainless steel over alternative materials (such as 316L or duplex stainless steels) was based on the specific operating environment and economic considerations. The cooling water chemistry at Tongling Power Plant did not require the enhanced pitting resistance of 316L.
2. Performance Verification: The successful replacement was confirmed through operational monitoring of heat transfer performance, demonstrating that theoretical concerns about reduced thermal conductivity were not validated in practice.
3. Economic Analysis: While 304 stainless steel tubes have higher initial cost than brass tubes, the extended service life and reduced maintenance requirements provide better lifecycle economics.
4. Installation Quality: The success of the replacement depended heavily on proper installation procedures, including careful handling to avoid surface damage, proper welding techniques, and thorough inspection of all welds.

Key Questions and Reflections

An important question arising from this case study is the long-term performance of 304 stainless steel tubes in condenser service. While the initial performance is satisfactory, long-term exposure to cooling water containing chlorides, sulfates, and organic compounds could potentially lead to:

• Stress corrosion cracking if residual stresses from welding are not properly relieved
• Pitting corrosion in localized areas of high chloride concentration
• Crevice corrosion at tube-to-tubesheet joints if proper sealing is not maintained

The study does not provide long-term performance data, which would be valuable for establishing the expected service life of stainless steel condenser tubes. Future monitoring and periodic inspection programs should be implemented to track the condition of the replaced tubes.

Another consideration is the welding quality at tube-to-tubesheet joints. These welds are critical for both structural integrity and sealing. Any defects at these joints can lead to:

• Coolant leakage into the steam side
• Loss of vacuum in the condenser
• Potential safety hazards if leakage is not detected

Study Insights and Implications

This engineering case study demonstrates the practical feasibility of replacing brass condenser tubes with 304 stainless steel tubes in thermal power plant applications. The comprehensive approach of investigation, testing, and research before implementation provides a model for similar engineering modifications.

For steel pipe engineers, this study highlights several important considerations:

• Material selection for specific service environments requires careful evaluation of corrosion mechanisms, mechanical requirements, and economic factors
• The thermal conductivity disadvantage of stainless steel can be mitigated through design optimization
• Welding procedures for stainless steel require specialized techniques and rigorous quality control
• Long-term performance monitoring is essential for validating material selection decisions

The success of this replacement project has implications for the broader industry, suggesting that stainless steel tubes can be a viable alternative to traditional copper alloys in condenser applications. This opens possibilities for material standardization, reduced inventory complexity, and improved supply chain management for power plant maintenance operations. The study also underscores the importance of engineering judgment in material selection, where the optimal choice depends on the specific operating conditions, economic constraints, and maintenance philosophy of each facility.