The term cavitation refers to the formation and collapse of vapor bubbles or cavities in a fluid, generally due to localized reductions in the dynamic pressure. The collapse of vapor cavities can produce extremely high pressures that frequently damage adjacent surfaces and cause material loss. Cavitation is a major problem for the operation of hydraulic equipment such as hydroelectric turbines, valves and fittings, flow meters, hydrofoils, pumps, and ship propellers. Cavitation frequently contributes to high maintenance and repair costs; revenue lost due to downtime and cost of replacement power; decreased operating efficiencies; and reduction of equipment service life (March and Hubble 1996). The most commonly used method for cavitation repair is the fusion process (i.e., welding). This method involves removing material from the damaged areas and filling the space by welding. The most widely used filler materials are 308L or 309L stainless steel (Ruzga, Willis, and Kumar 1993). Extensive weld repair can introduce stresses in the area being repaired and can damage the component.
A preventive maintenance approach making use of cavitation-resistant coatings has the potential to substantially reduce the costs noted above and greatly reduce the need for welding-type repairs. The U.S. Army Construction Engineering Research Laboratories (USACERL) initiated a Cooperative Research and Development Agreement (CRDA) under the Corps of Engineers Construction Productivity Advancement Research (CPAR) Program to investigate the effectiveness of thermally sprayed alternative coatings in reducing cavitation and erosion. The CPAR-CRDA partner was the Thermal Spray Laboratory, State University of New York (SUNY), Stony Brook, NY.
The objective of this research was to demonstrate the effectiveness of innovative non-fusion thermal spray cavitation- and erosion-resistant coatings for hydroelectric and utility plant turbines and pumps. The research objective included the selection of special coating materials and development of detailed thermal spray processing techniques.
The approach was specified in the CPAR Research, Development, Commercialization Plan (RDCP) and consisted of the following six tasks:
Task 1: Preliminary Materials Evaluation. A list of candidate cavitation/erosion resistant coatings that could be thermally sprayed by high velocity oxyfuel (HVOF) and plasma spray was prepared by SUNY. The list consisted of three types of materials: Tribaloys (T-700, T-800, and T-400), Stellite (cobalt-based and nickel-based) and tungsten carbide. SUNY was to conduct preliminary laboratory screening using the ultrasonic vibratory horn to determine the optimum (HVOF and plasma) spraying parameters for up to 12 materials.
The results of these evaluations were to be used as a guide to determine the most effective means for cavitation/erosion-resistant coating repair using thermal spray. The technical and economical aspects of current repair/maintenance materials were to be studied for cost/performance comparison. The Corps of Engineers Hydroelectric Center (HDC) was to be utilized for technical assistance. USACERL was to conduct the economic analysis and select 6 materials/processing parameter for detailed laboratory evaluation.
Task 2: Thermal Spray Processing: Equipment, Materials, and Processes. The following activities were to be conducted by SUNY: equipment evaluation/ recommendations for each coating material; materials characterization including chemistry and particle size distribution of the powders; substrate surface preparation techniques; and spray process and initial optimization program.
Task 3: Deposit (Coating) Characterization, Bench Scale Tests, and Evaluation Using Statistical Process Control. USACERL was to contract the Tennessee Valley Authority (TVA) or an equivalent laboratory to test the 6 selected materials/processing systems using the cavitating jet method. SUNY was to conduct tests on the materials with special emphasis on metallography (porosity, raw material oxide content, and cracks). Mechanical properties of the deposited coating were to be determined including tensile adhesion strength, microhardness, and residual stress evaluation. Additional evaluations were to also include chemical and phase analysis.
Task 4: Field Demonstration: A field demonstration, using the best materials/ processing combination, was to be conducted on a Kaplan turbine at the Dalles Hydropower Dam, Portland District. Application procedures and materials properties were to be documented. The performance of the thermal spray coatings was to be evaluated relative to the performance of standard stainless steel weld repair.
Task 5: Commercialization/Technology Transfer. The Commercialization/Technology Transfer Plan was to be executed jointly by Flame Spray Industries, Inc., the Partner Participant, and SUNY through marketing, manufacture, distribution, and user support for the product. Flame Spray Industries, Inc. was to promote thermal spray for repair/maintenance of hydroelectric pumps and turbines. SUNY will present the research results at technical symposia and at trade shows.