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HYDROELECTRIC US ARMY CORPS USACERL REPORT ON HVOF
US ARMY CORPS
Of Engineers
Construction Engineering
Reseat Laboratories
USACERL Technical Report 97/1997
July 1997
Construction Productivity Advancement
Research (CPAR) Program
Cavitation-and Erosion-Resistant
Thermal Spray Coatings
by
Jeffrey H. Boy, Ashok Kumar,
Patrick March, Paul Willis, and Herbert Herman
Approved for public release: distribution is unlimited
A Corps/Industry Partnership To Advance
Construction Productivity and Reduce Costs
National Thermospray, Inc.
and
U.S. Army Corps of Engineers
Construction Productivity Advancement Research
(CPAR) Program
Cavitation-and Erosion-Resistant Thermal Spray Coatings
This is a reprint from the USACERL Technical Report 97/Draft,February 1997
Subject
Raccoon Mountain Demonstration Site
The field demonstration of cavitation resistant thermal spray coatings was conducted in September 1996 at the TVS's Raccoon Pumped-Storage Plant, Chattanooga, TN. The plant consists of four Francis pump/turbine units, each with a rated generating capacity of 392 MW at 1020 ft head. The pump/turbines are a reversible type with a vertical shaft manufactured by Allis Chalmers. The runner diameter is 16 ft,7in. The vane material is ASTM A296 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 SS.
Demonstration Material and Field Application Procedure
The HVOF coating systems applied in the field demonstration was Stellite 6 and Tribaloy T-400. Stellite 6 has 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 with out testing a significant number of additional samples. Based on previous research results which found that coatings of Tribaloy T-400 extended the life of the stainless steel substrate ( Baker 1994 ), as well as its greater availability, T-400 and Stellite 6 was selected.
The field demonstration was conducted by National Thermospray, Inc. 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 ( DJ ) HVOF system. This 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 when a coating is not being applied. Both the Metco and the Jet Kote HVOF systems are widely used in the thermal spray industry. Operating parameter have been established by the manufacturers of both HVOF systems for Stellite and Tribaloy alloys. When the coatings are applied the appropriate operating parameters for the specific HVOF system, the quality of the coating should be equivalent. 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 24. Test patches of approximately 1 foot square of both Stellite 6 and Tribaloy T-400 were successfully applied in the field by HVOF to the turbine vanes, cone, and drift tube liner.
Cost Analysis
Areas of Medium and severe cavitation will require the removal of damaged materials and weld repair of the areas with stainless steel. The thermal spray coating would be applied on top of the weld repair. The cost of cavitation repair using welding was determined for the Granite-Goose Hydroelectric Project, ( Ruzga 1993 ). Updating the cost to 1996 dollars, assuming a 4 percent per year cost increase, the total current cost of cavitation repair by welding would be about $561.00 dollars per sq ft. Using HVOF, the surface (whether weld repaired or as-found ) would be blasted with abrasive grit to remove corrosion products ant to smooth out the erosion and corrosion pits. The HVOF process would provide a 0.020 in. coating that follows the resulting surface profile of the abrasive blasted substrate with a surface finish of 300 microinches. The as-spray coating would be the finish surface, requiring no grinding or other additional work.
Cost analysis show that the cost of applying a 0.020 in. cavitation resistant Stellite 6 or Tribaloy T-400 coating to a hydroelectric turbine would be $187.00 dollars per sq ft, using the HVOF process. This estimate does not include any cost associated with repairing the damaged area and bringing it up to contour by fusion welding prior to thermal spraying. Its is anticipated that once a sprayed coating is applied, it will either prevent damage to the substrate or will be sacrificed under cavitation and erosion conditions, providing protection to the underlying base metal or weld repair material. Therefore, future repairs of the area, assuming an appropriate maintenance schedule is sustained, could be made by spraying HVOF coatings at $187.00 dollars per sq ft rather by weld repair to the substrate at three times the cost.
The cost of the HVOF Stellite 6 coating is estimated to be one-third the cost of weld repair. Based on the cavitation jet results, the required repair frequency for Stellite 6 HVOF coating would appear to be about four times that of welded stainless steel. However, there would be clear advantages to using HVOF Stellite 6 coatings because cavitation typically does not occur in isolation-it is usually associated with erosion and corrosion. The erosion resistance of Stellite 6 was superior to that of 304 stainless steel. The electrochemical potential difference of Stellite 6 coating relative to carbon steel was lower than the potential difference of stainless steel relative to carbon steel. Therefore, the overall protection of the substrate from the combined interaction of cavitation, erosion, and corrosion by HVOF Stellite 6 coating may justify the cost of repeated applications. The use of sacrificial Stellite 6 coatings would also eliminate the potential substrate damage caused by the considerable thermal stresses associated with repeated weld repair.
Conclusions
The thermal spray coatings deposited by the High Velocity Oxygen Fuel ( HVOF ) process and tested exhibited lower cavitation wear rates than the thermal spray coatings deposited by the plasma spray process, as determined by laboratory testing using the cavitation jet test apparatus.
Of the 21 thermal spray coatings tested in the laboratory using the cavitation jet apparatus, the lowest cavitation rate was for Stellite 6 as applied by the HVOF thermal spray process. The cavitation rate of Stellite 6 was 11.7 mg/h, while the corresponding cavitation rate for 308 stainless steel weld metal was 3.2 mg/h.
The field applicability of Stellite 6 thermal spray coatings deposited by the HVOF process was successfully demonstrated on a hydroelectric pump/turbine at the TVA's Raccoon Mountain plant near Chattanooga, TN. Thermal spray coatings were applied to stainless steel weld-repaired substrates and carbon steel substrates.
The cavitation rates of advanced weld metal overlays, such as NOREM, D-cav, CaviTec, and Hydrology 914, ranged from 1.0 to 2.6 mg/h, which were lower than the corresponding cavitation rate for standard 308 stainless steel weld metal ( 3.2 mg/h ).
In slurry erosion wear testing, the volume loss for Stellite 6 coatings deposited by the HVOF process was 5.33 cm3/h, less than half the volume loss of 11.17 cm3/h for 304 stainless steel. The corresponding loss for ASTM A572 carbon steel was 19.70 cm3/h.
The electrical potential differences between Stellite 6 coated specimens and both ASTM A572 and A36 carbon steel in tap water were 0.25 volts, half the potential difference between 304 stainless steel and mild steel (i.e.,0.50 volts).
Stellite 6 coatings deposited by the HVOF process over surfaces having dissimilar metals (i.e., stainless steel weld repair adjacent to the mild steel base metal) will mitigate the corrosion activity at the dissimilar metal boundary because of its superior corrosion resistance as compared to the carbon steel substrate material.
The cost of applying Stellite 6 coating to a hydroelectric turbine in the field, after the damaged surface was weld repaired, was determined to be $187.00 dollars per sq ft. The sprayed coating will either prevent damage to the substrate under erosion conditions. In either case, the material will protect the underlying base metal of previous weld repair material. Assuming an appropriate maintenance schedule, subsequent repairs using HVOF-applied coatings should cost $187.00 per sq ft, as opposed to weld repair of the substrate at three times the cost. The overall protection of the substrate from the combined interaction of cavitation, erosion, and corrosion may justify the cost of repeated applications of HVOF-sprayed Stellite 6 coatings.
Applying Stellite 6 to a Rebuild Turbine Unit using HVOF Process 1997 &1998
Recommendations
Stellite 6 deposited by the HVOF process should be considered for the repair and mitigation of damage resulting from mild cavitation and erosion. Stellite 6 should also be considered for the mitigation of corrosion associated with contact between dissimilar metals.
Stellite 6 coatings deposited by HVOF process should be considered for application to turbine throat rings and drift tube liners in order to prevent cavitation, corrosion and erosion damage to the carbon steel substrate and to avoid the thermal stresses associated with fusion welding of the stainless steel. This application will also minimize galvanic corrosion caused by the potential difference between the carbon steel and conventional stainless steel weld materials. However, if the interface corrosion between the stainless steel and mild carbon steel is shifted to the Stellite 6 / carbon steel interface, then complete coverage by the thermal spray coating may be required.
Technology Transfer and Commercialization
The goal of the CPAR-CRDA is to develop and market thermal spray powder materials and the associated HVOF processing technology for use in cavitation and erosion damage on hydraulic pumps and turbines. SUNY (the CPAR partner Flame Spray Industries (the partner participant) have extensive experience in the application of thermal spray coatings by HVOF process. The participants are concentrating on thermal spray application of nickel and cobalt superalloys in field applications for the energy and petrochemical industries. The participants have developed onsite equipment to control the spray parameters and monitor the operation.
Flame Spray Industries has begun a marketing initiative to promote cavitation and erosion resistant coatings in the hydroelectric and electric generation markets. The marketing plan being pursued is to market to the TVA and USACE facilities for onsite repair of hydroelectric turbines and pumps. The specific components targeted for rebuild and protection coatings with the Stellite 6 are hydroelectric turbine drift tube liners and pumps. Other potential coating applications for erosion and corrosion prevention include commercial pump components, toroidal rings of the cooling components of nuclear plants, and the water boxes of heat exchangers.
Flame Spray Industries will market the process through National Thermospray, Inc., Cypress, Texas and other companies with experience applying thermal spray coatings in the field. National Thermospray, Inc., has extensive experience in applying nickel and cobalt superalloys in the field for applications to the petrochemical industry. The majority of their current work is in confined spaces. The company is capable of preparing and applying coatings to interior surfaces 5 to 20 ft in diameter with thermal spray equipment (robotics) that can be passed through a 20 in. hatch. As part of their business plan National Thermospray has acquired additional equipment to increase their field application capabilities. The process can be obtained by contacting Flame Spray Industries, Fort Washington, NY, or National Thermospray, Inc., Cypress,Texas.
First applied coating in September 1996
UPDATE AS OF APRIL 1998 DRIFT TUBE WALL
STILL INTACT
First applied coating in September 1996
UPDATE NOSE CONE AS OF APRIL 2000
Link to Full Report
http://www.cecer.army.mil/techreports/boycavit/boycavit.jef.htm#TopOfPage
Stellite® and Tribaloy® are registered trademarks of Deloro Stellite Co.Inc.
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