Reinforcement and Renovation of Legacy Pressure Steel Pipes at Fengman Hydropower Station

Literature Overview

This paper published in Journal of Hydroelectric Engineering (2001, Vol. 20, No. 2, pp. 44-54) by Feng Yanrong, Li Cai from Fengman Power Plant, and Li Kuisheng from Fengman Hydroelectric Technical School documents the comprehensive reinforcement and renovation program undertaken for the pressure steel pipes of the Fengman Hydropower Station in China. The pressure pipes in question were manufactured and installed during the Japanese occupation period (1930s-1940s), and by the time of the renovation, they had been in service for approximately 50-60 years. The study provides valuable lessons for engineers dealing with the rehabilitation of aging pressure pipe infrastructure in hydropower and other hydraulic applications.

Background and Condition Assessment

The pressure steel pipes at Fengman Hydropower Station Unit No. 110 and Unit No. 2 were constructed during a period of limited manufacturing and welding technology. The condition assessment revealed the following critical defects:

The combination of these defects created a situation where the pressure pipes were susceptible to brittle fracture. In hydraulic systems, brittle fracture of a pressure pipe can lead to catastrophic failure with significant consequences for personnel safety, equipment damage, and environmental impact.

Root Cause Analysis

The defects can be attributed to several factors:

1. Limited welding technology. During the 1930s-1940s, arc welding was in its early stages of development. Welding procedures were not optimized, welding consumables were of limited quality, and welding operator training was minimal.
2. Inadequate quality control. Non-destructive testing methods were either unavailable or not systematically applied. Weld quality was assessed primarily through visual inspection, which cannot detect internal defects such as porosity, slag inclusions, or lack of fusion.
3. Base metal quality. The steel used for the pressure pipes may not have met modern specifications for impact toughness and fracture resistance. The chemical composition and metallurgical quality of the steel were likely inferior to contemporary standards.
4. Ageing and fatigue. Sixty years of cyclic pressure loading, combined with potential corrosion and material degradation, would have further reduced the fracture resistance of the pressure pipes.

Reinforcement Methodology

The authors describe a combined approach to reinforce the aging pressure pipes:

Inner Liner Pipe Method

The primary reinforcement method involved installing an inner liner pipe within the existing pressure pipe. This method provides the following benefits:

• Restores structural integrity by providing a new, defect-free pressure boundary
• Reduces stress concentration by distributing the hoop stress over a larger effective cross-section
• Prevents crack propagation from existing defects in the outer pipe by isolating the defect from the pressure load
• Extends service life without requiring complete replacement of the existing infrastructure

The inner liner pipe is typically a smaller-diameter pipe that fits within the existing pipe with a controlled clearance. The gap between the liner and the outer pipe can be filled with grout or left as an annular space for thermal expansion accommodation.

Replacement of Exposed Pipe Sections

For sections of the pressure pipe that are exposed and accessible, the authors replaced the original pipe sections entirely. This approach is appropriate for sections where:

• The existing pipe is severely corroded or damaged
• The section is readily accessible for removal and replacement
• The reinforcement requirements exceed the capability of the inner liner method

The replacement sections are fabricated from modern steel with verified mechanical properties and welded using qualified welding procedures with full non-destructive testing coverage.

Engineering Practice Lessons

This case study provides several important lessons for engineers dealing with aging pressure pipe infrastructure:

1. Condition assessment is the foundation of any reinforcement program. A thorough inspection and testing program must be conducted before selecting reinforcement methods. The condition assessment should include visual inspection, non-destructive testing (ultrasonic, radiographic, magnetic particle), mechanical property testing, and corrosion assessment.
2. Reinforcement methods should be selected based on defect severity and location. Not all defects require the same level of intervention. Minor surface defects can be addressed by local repair, while widespread internal defects may require inner liner installation or complete replacement.
3. The inner liner method is a cost-effective solution for extensive internal defects. When the defects are primarily internal and the outer pipe retains sufficient structural integrity, the inner liner method provides a practical and economical solution. It avoids the need for complete pipe replacement, which would require significant excavation and disruption of the facility.
4. Quality assurance during reinforcement is critical. The reinforcement work must be executed to the same quality standards as new construction. Welding procedures must be qualified, non-destructive testing must be performed, and mechanical property verification must be conducted on the reinforcement materials.
5. Long-term monitoring should be implemented after reinforcement. The reinforced pressure pipes should be incorporated into a regular inspection and monitoring program to detect any new defects or degradation that may develop over time.

Risk Assessment and Failure Prevention

The original paper emphasizes the risk of brittle fracture as the primary concern driving the reinforcement program. Brittle fracture is a sudden, catastrophic failure mode that can occur without significant prior deformation. The risk of brittle fracture is influenced by:

• Material toughness at the operating temperature
• Stress state (particularly the presence of stress concentrators such as weld defects)
• Strain rate (higher strain rates reduce the apparent fracture toughness)
• Plate thickness (thicker sections are more susceptible to brittle fracture due to triaxial stress states)

The reinforcement program addressed these risk factors by providing new, high-toughness pressure boundaries (inner liner pipes) and replacing severely degraded sections with modern materials. The inner liner pipe effectively shields the original pipe from the pressure load, reducing the stress state in the original pipe to a residual stress condition that is well below the fracture threshold.

Key Reflections

This case study is a compelling example of engineering judgment applied to a legacy infrastructure problem. The authors faced a situation where complete replacement of the pressure pipes was not practical due to the complexity of the installation, the integration with the powerhouse structure, and the cost implications. The inner liner method provided a pragmatic solution that addressed the fundamental safety concern while preserving the existing infrastructure.

The study also highlights the importance of understanding the failure modes of legacy systems. The defects identified in the Fengman pressure pipes—porosity, slag inclusions, lack of fusion—are common in older welded structures. These defects may be acceptable for static loading but become critical under dynamic or cyclic loading conditions, particularly when combined with low-temperature service or high plate thickness.

For engineers working on similar projects, the study provides a framework for evaluating reinforcement options. The decision between inner liner installation, local repair, and complete replacement should be based on a systematic assessment of defect severity, accessibility, structural requirements, and economic considerations. The inner liner method is particularly attractive for large-diameter pressure pipes where complete replacement would be prohibitively expensive and disruptive.

The paper also serves as a reminder that the quality of construction during the original installation has long-term consequences. The welding defects that were present from the 1930s installation continued to pose a safety risk for decades, ultimately requiring a comprehensive reinforcement program. This underscores the importance of rigorous quality control during initial construction, even when the immediate economic pressure to reduce costs is significant.