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

Background and Motivation

The paper by Feng Yanrong, Li Cai, and Li Kuisheng, published in Journal of Hydraulic Engineering (2001, Vol. 20, No. 2, pp. 44-54), documents the reinforcement and renovation of the pressure steel pipes serving Unit 110 of the Fengman Hydropower Station. These pipes were manufactured and installed during the Japanese colonial period in Manchuria, meaning they had already been in service for over five decades by the time of the study. The paper is a rare and valuable engineering case study that addresses the challenge of maintaining and upgrading aging steel pipe infrastructure under severe operational constraints.

The motivation for this work was straightforward: the existing pipes exhibited widespread weld defects and material degradation that posed a credible risk of catastrophic brittle fracture. For a hydropower station, a pressure pipe rupture is not merely an equipment failure — it can result in massive flooding, loss of generation capacity, and potential loss of life. The decision to intervene was therefore driven by both technical necessity and operational risk management.

Assessment of Existing Pipe Condition

Weld Defects

The paper identifies three primary categories of weld defects in the original pressure pipes:

These defects are characteristic of the welding practices of the era. The original pipes were fabricated using manual arc welding (SMAW) with limited process control, inadequate preheating, and no non-destructive testing (NDT) protocols. The absence of proper shielding gas or flux coverage during welding led to extensive porosity, while insufficient heat input and poor technique resulted in incomplete penetration. Slag inclusions arose from inadequate slag removal between weld passes.

Material Degradation

Beyond weld defects, the base metal itself had suffered significant degradation:

• Low impact toughness — The base material exhibited poor Charpy V-notch impact values at service temperatures, indicating a susceptibility to brittle fracture.
• Poor crack arrest capability — The material's inability to arrest propagating cracks means that once a crack initiates, it can propagate rapidly through the entire pipe thickness.
• Hydrogen embrittlement susceptibility — The combination of high carbon equivalent and low toughness increases the risk of hydrogen-induced cracking, particularly in the heat-affected zone (HAZ) of any repair welds.

These material characteristics, combined with the weld defects, created a scenario in which a single defect could serve as a crack initiation site, and the low-toughness material could not arrest the resulting crack propagation. The result is a credible risk of full-thickness brittle fracture under normal operating pressure.

Reinforcement Methodology

Inner Liner Method

The primary reinforcement technique employed was the "inner liner method" (内衬管法), in which a new steel pipe is installed inside the existing pipe, effectively creating a double-wall pressure boundary. This method has several advantages:

1. No loss of operational capacity during installation — The inner liner can be installed while the existing pipe remains partially in service, minimizing downtime.
2. Redundancy — If the outer pipe (the original pipe) fails, the inner liner continues to contain the water, preventing a catastrophic release.
3. Reduced risk of brittle fracture — The inner liner, fabricated from modern high-toughness steel, does not have the same weld defects or material degradation as the original pipe.

The inner liner is typically fabricated from modern seamless or welded steel pipe with high Charpy impact energy values and full NDT coverage. The interface between the inner liner and the outer pipe is managed through controlled gap dimensions and, in some cases, grouting or mechanical locking devices.

Replacement of Exposed Pipe Sections

For pipe sections that are exposed (above ground or in accessible tunnels), the paper describes a complete replacement strategy. These sections were removed and replaced with new steel pipe fabricated to modern standards. The replacement procedure involved:

• Careful isolation and draining of the pipe section
• Cutting of the old pipe at designated locations using controlled thermal or mechanical methods
• Fabrication and installation of new pipe with modern welding procedures
• Full NDT of all new welds, including radiographic testing (RT) and ultrasonic testing (UT)
• Hydrostatic pressure testing of the new pipe section before returning it to service

Combined Approach

The overall strategy combined both methods: the inner liner method was applied to underground and inaccessible pipe sections, while the exposed pipe sections were completely replaced. This hybrid approach minimized operational disruption while ensuring that all pipe sections met modern safety standards.

Technical Analysis and Engineering Practice

Welding Quality in Modern Replacement

The replacement of exposed pipe sections provides a valuable contrast between the original welding practices and modern standards. The new pipe was fabricated using submerged arc welding (SAW) or gas metal arc welding (GMAW) with proper preheating, interpass temperature control, and post-weld heat treatment (PWHT) as required by the applicable code. All welds were subjected to full NDT coverage, including:

• 100% radiographic testing (RT) for butt welds
• 100% ultrasonic testing (UT) for butt welds and girth welds
• Magnetic particle testing (MT) for surface and near-surface defects

This level of quality assurance was entirely absent in the original fabrication, and the contrast underscores the importance of modern welding codes and NDT protocols.

Risk Management Framework

The decision to reinforce rather than replace all pipe sections can be understood through a risk management framework:

The inner liner method effectively converts a single-wall system with high failure probability into a double-wall system with negligible catastrophic failure probability. Even if the original pipe fails, the inner liner provides a second line of defense.

Key Questions and Reflections

Several important questions arise from this case study:

1. Long-term performance of the inner liner method — How does the inner liner method perform over a 30- to 50-year service life? Are there concerns about differential thermal expansion, corrosion at the interface, or loss of contact between the inner and outer pipes?
2. Applicability to other aging infrastructure — Many hydropower stations worldwide have aging pressure pipes with similar histories. Can the methods described here be generalized to other facilities, or are they specific to the Fengman site conditions?
3. Economic analysis — The paper does not provide a detailed cost-benefit analysis. A comparison of the cost of reinforcement versus complete replacement would be valuable for future decision-making.
4. Monitoring and inspection protocols — After reinforcement, what ongoing inspection and monitoring regime is required to ensure continued safety? The inner liner method reduces but does not eliminate the risk of the original pipe failing.

Study Insights and Implications

This case study is a powerful demonstration of the engineering challenges posed by aging infrastructure and the creative solutions that can be developed to address them. The inner liner method is particularly noteworthy because it achieves a significant improvement in safety without requiring a complete shutdown of the facility. This is a principle that can be applied broadly to aging infrastructure: rather than replacing everything, engineers can often achieve acceptable safety levels through targeted reinforcement that addresses the most critical failure modes.

The paper also serves as a cautionary tale about the long-term consequences of inadequate fabrication quality. The weld defects and material degradation identified in the original pipes were the result of fabrication practices that were acceptable — or at least tolerated — in the 1930s and 1940s. These defects did not manifest as failures for decades, but they created a latent risk that eventually required major intervention. This reinforces the importance of investing in fabrication quality and NDT at the time of construction: the cost of preventing defects is almost always lower than the cost of remediating them decades later.

The study also highlights the value of multidisciplinary collaboration. The reinforcement project required expertise in materials science (to assess the original pipe condition), welding engineering (to design and execute the repair welds), structural engineering (to design the inner liner system), and hydraulic engineering (to manage the operational aspects of the project). This level of collaboration is essential for successful infrastructure rehabilitation and should be a standard practice for similar projects.

In summary, the Fengman Hydropower Station pressure pipe reinforcement project represents a successful application of engineering judgment to a challenging real-world problem. The inner liner method, combined with selective replacement of exposed sections, provided a cost-effective and operationally feasible solution to a significant safety risk. The lessons learned from this project — regarding the assessment of aging infrastructure, the selection of reinforcement methods, and the importance of modern fabrication quality — are broadly applicable and should be considered by engineers managing similar challenges in other facilities.