The Raccoon Mountain Demonstration Site
The field demonstration of HVOF thermal spray coatings was conducted in September 1996 at the TVA's Raccoon Mountain Pumped-Storage Plant, Chattanooga, TN. The plant consists of four Francis pump/turbine units (Figure 8), each with a rated generating capacity of 392 MW at 1020 ft head. The pump/turbines are a reversible Francis type with a vertical shaft, manufactured by Allis Chalmers. The runner diameter is 16 ft, 7 in. The original vane material was ASTM A 296 CA6NM, a grade of martensitic stainless steel. This material has relatively high strength and has a cavitation rate of 15 mg/h, which was the same as 308 stainless steel (see Table 1; Simoneau 1991).
Two materials are used by the TVA for weld repair depending on the degree of cavitation. HQ 914 is used in areas of high cavitation, such as the vanes; 308 stainless steel is used in areas of low cavitation, such as the cone. Since Hydroloy 914 has been used, repairs of severe cavitation are only necessary every 3 to 4 years. This is in contrast to TVA's earlier experience using 308 stainless steel weld alloy, in which repairs were necessary every year. Thermal spray coatings were applied during the field demonstration on top of weld-repaired areas of 308 stainless steel and areas of Hydroloy 914.
Demonstration Materials and Field Application Procedure
The HVOF coating systems applied in the field demonstration were Stellite® 6 and Tribaloy® T-400. Stellite had the highest cavitation resistance in both ultrasonic and cavitating jet testing (11.7 mg/h). NOREM® (16.9) and Tribaloy® T-400 (18.9 mg/h) had similar cavitation wear rates. The results could not be differentiated statistically without testing a significant number of additional samples. Based on previous research (Baker 1994), as well as its greater availability, Tribaloy® T-400 was selected. The chemical and particle size analyses of the materials used in the demonstration are shown in Table 23 and Table 24.
The field demonstration was conducted by a contractor8. The HVOF unit used in the demonstration is trailer-mounted and can apply a coating 250 ft from the trailer without any modifications. This setup controls contamination because the powder feed unit and spray materials are stored and secured in the trailer and only the gun is in the work area. HVOF system used in the field was a Metco Diamond Jet (D.J.) HVOF system (Figure 9 and 10). This HVOF system was made by a different manufacturer than the one used in the laboratory testing. In contrast to the system used in the laboratory, the D.J. system does not have a pilot system to permit idling when a coating is not being applied. Both the Metco and Jet Kote HVOF systems are widely used in the thermal spray industry. Operating parameters have been established by the manufacturers of both HVOF systems for Stellite and Tribaloy alloys. When the coatings are applied using the appropriate operating parameters for the specific HVOF system, the quality of the coating should be equivalent.
|
Table 24. Particle size analyses for materials used in the field demonstration. | |||
+ 53 micron |
53 microns < 44 microns |
-325 microns | |
Tribaloy® T-400 |
0 |
0 |
100 % |
Stellite® 6 |
0 |
1.99% |
98.0% |
Source: Karr et al. 1994.
Figure 8. Schematic of hydroelectric pump/turbine at Raccoon Mountain showing where HVOF coatings were applied.
Figure 9. Thermal spray powder feed and gas flow control systems mounted in a mobile field trailer.
Figure 10. HVOF gun used in the field demonstration.
Before application of the coating, the surface was grit blasted in accordance with SSPC 10 using virgin aluminum oxide grit. The pressure was at least 80 psi. Application of the coating was conducted within 4 hours after the grit blasting.
The spray parameters used during the field demonstration are listed in Table 25. Test patches of approximately 1 foot square of both the Stellite® 6 and Tribaloy® T-400 were successfully applied in the field by HVOF to the turbine vanes, cone, and draft tube liner. Weld repairs using 308 stainless steel were made to the cone and areas of mild cavitation, and repairs to the vanes and areas of severe cavitation using Hydroloy® 914 were conducted during the same outage as when the thermal spray coatings were applied. HVOF coatings were applied over these weld-repaired areas. Thickness measurements of coatings applied over weld-repaired with stainless steel could not be obtained using magnetic thickness gauges. The coating thickness was estimated from the weight of the materials applied. The thickness was also measured using a micrometer on steel test panels that were sprayed at the same time as the turbine components. The average thicknesses of the Stellite® 6 and Tribaloy® T-400 test panels were 0.025 in. and 0.020 in., respectively.
Figure 11. Severe cavitation at a vane/crown intersection on a Francis pump/turbine at the Raccoon Mountain plant.
Severe cavitation damage at a runner crown/vane intersection of the turbine is shown in Figure 11. The condition of the turbine cone is shown in Figure 12. The area coated included an area near the base of the cone which had been repaired by welding 308 stainless steel and an area above this that was carbon steel. The weld repair was done by TVA personnel prior to start of the demonstration. The same area of the cone is shown in Figure 13 after the Stellite® 6 was successfully applied by HVOF thermal spray process. Tribaloy® T-400 and Stellite® 6 were also successfully applied to the draft tube liner and turbine vanes by HVOF. The surface of the turbine vanes had been weld-repaired with Hydroloy® 914 while the draft tube liner was the original carbon steel. Problems reported by Lontz (1992) with the field application of HVOF coatings were not experienced during this field demonstration.
Figure 12. Cavitation and corrosion on the cone of a Francis pump/turbine at the Raccoon Mountain plant.
Figure 13. Stellite® 6 applied by HVOF to the cone of a Francis pump/turbine at the Raccoon Mountain plant.
Test Samples From the Field Demonstration
During the field demonstration, the technician grit blasted and thermally sprayed 1/8 in. thick steel test panels. These panels were taken back to the laboratory where metallographic samples were prepared. The optical micrographs of the Stellite® 6 and T-400 are shown in Figures 14 and 15. The micrographs show good bonding at the interface between the substrate and the coating and very little porosity in the coating near the substrate (see Figure 14).
Both the Stellite® 6 and Tribaloy® T-400 coatings showed decreasing porosity in the coating from the coating surface to the coating/substrate interface. Greater porosity was observed in the Tribaloy® T-400 micrographs as compared to the Stellite® 6 (see Figure 15). This graduated porosity may decrease the cavitation resistance of the coating compared to a uniformly dense coating. Interconnected porosity could conceivably provide a continuous path through which water may reach the substrate. The presence of water at the coating/substrate interface may cause corrosion. However, the porosity in the sample field coatings was low near the substrate, and no continuous path was evident.
Initial Observations and Results
The demonstration at the TVA's Raccoon Mountain plant successfully showed the field applicability of the HVOF thermal spray process to deposit a good-quality coating inside a hydroelectric turbine. Visual observations of the turbine after 3 months of operation found the demonstration coatings to be intact and in good condition.
An additional inspection was conducted on 3 March 1997, 6 months after the field application of the coatings by the HVOF process. This interval represents 995 hours of the unit generating power and 1180 hours of the unit operating as a pump. At the time of this second inspection both the Stellite 6â and Tribaloyâ T-400 coatings were intact and in good condition on the cone and throat ring of the hydroelectric turbine. These coatings did have some areas where rust from adjacent carbon steel stained the thermal sprayed coatings. In addition, these coatings had some areas where rust from the carbon steel substrate bled through the coating. There was no corrosion product bleed-through of the coatings applied over the stainless steel weld repair. A small portion of the Stellite 6â coating applied to the coated area on the vane showed an angular wear pattern. In all areas, the coating was still adhering well and showed no signs of separating from the steel. A photograph of the HVOF sprayed coatings on the vane is shown in Figure 16. The vanes are subjected to more aggressive cavitation attack than the cone or the throat ring. Based on the good condition of the coatings in service, the coatings would be expected to continue to provide protection of the substrate. The coating conditions will continue to be monitored when outages and access permit. Such long-term monitoring of the thermal spray coating performance in the field must necessarily extend beyond the duration of this CPAR project.
Figure 14. Micrograph of HVOF Stellite® 6 coating (20 mil) steel test panel showing decreasing porosity at the coating/substrate interface.
Figure 15. Micrograph of HVOF Tribaloy® T-400 coating (20 mil) on steel test panel showing good anchor profile at the coating/substrate interface.
Stellite 6
Tribaloy T-400
Stellite 6
Figure 16. HVOF coatings applied to the vane of a Francis pump/turbine at the Raccoon Mountain plant after 2175 hours of operations.
8 National Thermal Spray, Cypress, TX