The global surge in new electricity generation plants and refits over the next two decades presents significant opportunities for the fabrication sector. Innovations in welding technology are critical to meeting the high standards required, particularly in constructing safety-critical components. As the power industry gears up for this challenge, traditional and advanced welding techniques are being pushed to their limits to ensure the reliability and safety of new and existing power plants.
The planned expansion includes over 300 proposed nuclear reactors worldwide, with significant investments in China, the USA, and Russia. With its delayed nuclear power program, India is set to see a revival with Electricite de France (EDF) agreeing to build six nuclear plants in the country, including the massive Jaitapur project, expected to be the world’s largest atomic contract. Alongside nuclear energy, fossil fuel-powered generators remain integral to global energy strategies, with an estimated 1000 GW of new coal-fired power stations projected over the next 20 years.
The Welding Challenges in Power Plant Construction
Fabricating steel pipes and tubes for these projects presents significant welding challenges. Stainless steels are extensively used in nuclear power plants for their corrosion resistance, with applications in reactor cores, secondary parts, and waste storage containers. The high pressures and temperatures in steam generation circuits necessitate using creep-resistant steels like chromium/molybdenum/vanadium alloys, which offer improved oxidation and corrosion resistance and high strength. However, these materials are prone to welding-related issues such as cracking, especially in high-pressure steam pipes.
Pre-heating and post-weld heat treatment are crucial to mitigate the risk of cracking, particularly in materials like SA 213 T91 or SA 335 P91, ferritic alloy steels used in high-temperature steam-generating plants. These processes reduce the sensitivity to cracking, with temperatures typically around 200°C (392°F), though they can be higher for certain materials. Adhering to strict welding procedures is vital to prevent catastrophic failures in service.
Advanced Welding Techniques
For the construction of these power plants, Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW) are preferred because they protect the exposed upper fusion zone during welding. However, the root weld, or under the bead, requires additional protection through inert gas purging to prevent oxidation and maintain the weld’s integrity.
High-Temperature Purging Systems:
Meeting the demands of inert gas purging at high temperatures, especially when pre- and post-heating are involved, necessitates specialised purge systems. HotPurge® systems, designed by Huntingdon Fusion Techniques HFT®, are engineered to withstand temperatures up to 300°C (572°F) for extended periods, ensuring reliable gas sealing and reducing purge times and gas costs (Fig 1).
Welding Stainless Steels: The Challenge of Corrosion
Stainless steels owe their corrosion resistance to a thin layer of chromium oxide on the surface. However, when exposed to high temperatures during welding, this layer can thicken and become unstable, leading to various forms of corrosion, including crevice corrosion, pitting, stress corrosion cracking, and microbiologically induced corrosion (MIC).
Preventing Corrosion:
Heat tinting, a visible discolouration that occurs when the protective oxide layer is compromised, can significantly reduce corrosion resistance. Removing heat tints before exposing the welded equipment to aggressive environments is crucial. While bright annealing or acid pickling can remove light discolouration, heavier deposits may require more intensive methods like grinding and polishing. Preventing heat tint during welding through effective inert gas purging is often more practical and cost-effective.
Weld Purging Techniques and Equipment
Over the past decade, significant advancements have been made in weld purging technology, with robust, multi-use systems now available for various applications. QuickPurge® systems are among the most effective, using connected inflatable dams to control gas flow and pressure, ensuring thorough purging with minimal operator intervention (Fig 6). These systems are particularly suited to the oil and gas industries, where preventing contamination is critical.
Monitoring and Controlling Oxygen Levels:
Even low oxygen concentrations in weld gases can cause discolouration, loss of corrosion resistance, and reduced mechanical strength. To maintain control over the oxygen content in purge gases, specialised instruments like the PurgEye® Weld Purge Monitors® are used. These monitors can measure oxygen levels to 10 ppm, ensuring the conditions for high-quality welds. The PurgEye® range also includes advanced features like PurgeNet™, which allows for connection to automated welding machines, further enhancing process control (Fig 8).
Conclusion
The power industry faces significant challenges in meeting the demand for new and upgraded power plants while maintaining the highest safety and quality standards. Advanced welding techniques, supported by specialised equipment like HotPurge® systems, QuickPurge® systems, and PurgEye® Weld Purge Monitors®, are essential to overcoming these challenges. By ensuring the integrity of welds in critical applications, these technologies play a crucial role in the future of global power generation.
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