New Techniques Demonstrate Significant Reductions in Cavitation, Critical for Extending the Service Life of Hydropower Components

In 2019, PNNL’s Solid-State Processing (SSP) project completed a series of laboratory tests and successfully identified a number of techniques with potential to reduce cavitation erosion, a critical step toward enhancing the performance and service life of new and repaired hydropower components. The recipient of a 2017 R&D 100 Award—which recognizes the top 100 most innovative technological breakthroughs in the world—PNNL’s SSP work presents a novel approach for producing a wide range of high-performing materials with efficient manufacturing methods to lower operation and maintenance costs, reduce the duration and frequency of outages, and extend the life of hydropower components. Testing results have thus far proved promising, with SSP outperforming more conventional techniques of component repair following a series of experiments.

Cavitation is a phenomenon affecting many hydropower metal components where vapor bubbles form and collapse as a result of rapid pressure changes in the water. When the vapor bubbles collapse, they generate small but powerful shock waves that create “pits,” or small cavities on metal surfaces. As the number and size of pits increases, wear rates and intensity of cavitation accelerate. Cavitation damage is usually the costliest maintenance item for hydropower facilities because of unexpected shutdowns and unplanned maintenance. Mitigation of material loss resulting from cavitation is key to the cost-effective long-term operation of hydraulic machines.

Although a variety of techniques are used for repairing cavitation, the most common is arc-welding filler material onto a component to replace eroded material. One of the major drawbacks of this technique is that arc welding can degrade the microstructure of the component and original metal, causing high heat input and melting, which can result in additional costs. Unlike arc welding, SSP technologies can produce repairs that match or exceed the performance of the parent material of existing turbines. In addition, they also have the potential to produce new components with improved mechanical properties and cavitation resistance, a differentiating quality that can aid in long-term maintenance strategies.

To demonstrate the feasibility of SSP to reduce cavitation damage in hydropower components, PNNL performed a series of tests by applying two promising SSP techniques—cold spray and friction stir processing—to base metal samples commonly used in hydropower components. Cavitation testing was successfully performed on the samples, and the results were benchmarked against commercially available, unprocessed materials and arc-welded counterparts. The results demonstrated SSP can lead to significant improvements in cavitation erosion resistance compared to that of the unprocessed base metal and conventionally repaired counterpart. This improved performance provides a stepping stone for further adoption of SSP to enhance U.S. hydropower infrastructure.

Commencing in 2020, the next stage of the project focused on advancing cold-sprayed technologies, which were identified as the best method for on-site cavitation repair. Cold spray is a metal powder deposition process in which material coatings are built up by supersonic impingement of metal powders upon a base metal. Innovations in tooling equipment have made it possible to execute cold-spray repairs in areas with clearances as small as 1.5 inches, making this technology ideal for performing repairs in tight spaces, which are characteristic of typical hydropower plants. PNNL is currently optimizing the chemistry for hydropower applications, establishing processes and parameters for portable equipment, and exploring the feasibility of combining cold-sprayed technologies with robotic repair in anticipation of a forthcoming field demonstration. This technology is expected to dramatically improve the service life of hydropower components, resulting in maintenance cost savings, improved plant reliability, and reduced frequency of outages—all important for grid reliability and resilience.

For additional information, contact TJ Heibel.