HVOF COATINGS TO COMBAT HYDRO ABRASIVE EROSION

Transcription

1 HVOF COATINGS TO COMBAT HYDRO ABRASIVE EROSION Dr.D.A.Karandikar Head-coating plant, ndritz Hydro Pvt Ltd, Village Prithla, Dist-Palwal Haryana , India ABSTRACT Hydro abrasive erosion is a common damaging mechanism in Hydropower plants situated on the rivers carrying excessive hard particles such as quartz along with other minerals. These hard particles with high velocity, damage the most common stainless steel being used for manufacturing the underwater components of hydropower plant. Hydro abrasive erosion is a complex phenomenon and is the result of abrasion and cavitation damage mechanism and hence the solution to avoid such damage has to be a unique and addressing to both of them. Abrasion is normally caused by movement of hard particles at high speeds whereas cavitation is because of disturbances in the water flow.abrasion can be minimized with choosing a material harder than the particles hardness, and cavitation could be minimized by selecting appropriate design parameters to ensure smooth water flow. From both these damaging mechanisms mostly abrasion starts at the early stage resulting in damaging the profile and then cavitation takes over. Hence the efforts have been made to avoid or delay the abrasion with either using hard material or material with modified surface to match the hardness to that of quartz particles. HVOF is high velocity oxy fuel spray technique which provides such harder surfaces on stainless steel components. This paper discusses the damaging mechanism, solutions to avoid or delay them alonwith ANDRTZ long experience in this field. Also some typical coating applications of underwater components pertaining to hydropower plants of Indian origin are also discussed with their performance. INTRODUCTION Hydro abrasive erosion is common damaging mechanism in hydropower plants situated on the rivers with water containing hard particles such as quartz and mica along with other minerals such feldspar, muscovite etc. These particles in appreciable amount travelling at high speeds strike the surfaces of underwater parts causing abrasion removal of material. These damages then cause the disturbance in water flow resulting in cavitation. Abrasion can be minimized by selecting appropriate stainless steel material for manufacturing the underwater parts, removing the particles from turbined water through desilting chamber and modifying their surfaces to resist the attack of such hard particles like quartz.andritz, since last four decades are providing solutions for hydro abrasive erosion worldwide with their expertise in modeling, cavitation free designing and modifying the surfaces with appropriate coatings through various techniques. The computational fluid dynamics technique is being used to predict the damages at the design stage itself and modify the water path accordingly to minimize the velocity and also abrasion. Even coating can be designed to suit the areas with more abrasion as detected in this study. The channels and gap between the channels are chosen in such a manner to 171

2 ICHPSD-2015 facilitate the movement of coating tool. Various coating materials and techniques were tried in the past to conclude that the HVOF (high velocity oxy fuel spraying) Fig 1, with tungsten carbide as material providing the optimum hydro abrasive erosion resistance. Recently under the umbrella of HVOF, many of its variants are also available, such as gas /liquid fuel equipment s with the aim of getting optimum particle velocity and temperature to result in best properties Fig 2. Under tungsten carbide material also various combinations are available depending on manufacturing techniques such as agglomerated, sintered and crushed with carbide particle size of submicron level(nano powders) Fig 3.Theses coatings have some roughness and can be ground by diamond wheel as per the required surface finish (Mainly on injector components in pelton and guide vanes in Francis turbines ).Recently some more coating techniques have been surfaced to provide still better properties such as WARM SPRAYING,HIGH VELOCITY AIR FUEL SPRAYING but its actual use in hydropower industry in INDIA is yet to be ascertained. TECHNIQUE SELECTION Fig. 1 Thermal spray technology family 172

3 HVOF Spray 173

4 ICHPSD-2015 Material selection Fig 3: Material selection 174

5 SXH70/7X/80/8X: a brief description A unique coating proprietary to ANDRITZ VA TECH Type: Tungusten-carbide Density: kg/m3 (as sprayed) Chemical composition: Proprietary composition for highest silt abrasion resistance. Micro hardness: HV0.3 (Vickers Diamond Pyramid Hardness, DPH 0.3 kg load). Adhesive bond strength: MPa (tensile test in normal direction to surface). Corrosion resistant in turbine waters, even if waste water or sea water Not resistant to cavitation: avoid use on bad hydraulic designs avoid bad flow conditions (Pelton: general beam quality, obstacles in injector, displacement between injector-housing and seat-ring) HYDRO ABRASIVE EROSION Hydro abrasive erosion is a complex damaging mechanism involving abrasive wear alone or in combination with cavitation phenomenon. Abrasive wear is caused due to rubbing of hard particles present in water and moving with high speeds. The extent of wear shall depend on amount of hard particles in water, particle size of the particles and shape of particles.himalyan rives mainly contains the particles hard particles such as Quartz, feldspar, muscovite and mica. Among all these Quartz is more detrimental and causes severe damages on underwater parts of hydroturbine.its hardness on Mohs scale range from 7 to 8 and hence when it moves with a velocity of meters/sec causes heavy wear on 13/4 steel components.a typical particles analysis is shown below Fig 4.some of the photos of damaged components are shown in Fig 5. Fig. 4: Particle analysis 175

6 ICHPSD-2015 Fig 5: Damaged components POSSIBLE SOLUTIONS TO AVOID THE DAMAGES 176

7 ANDRITZ EXPERIENCE ANDRITZ is providing solution to combat the hydro abrasive erosion since last four decades.in 1980 s then SULZER HYDRO started an extensive research programme to investigate the hydro abrasive erosion and the solutions.first they build the test rig to simulate the water flow conditions such as particle concentration and velocity. The pure quartz is used as particles and the extreme velocities as experienced in actual power plants were simulated in the test rig. Different materials were tested for its abrasion resistance factor to get to know the best suitable material in comparison with reference material of 13/4 stainless steel. Various parameters such as velocity, particle concentration impingement angle, coatings of different material were used in this study. It was noted that the coating from tungsten carbide material composition designated as SXH 70 have been found with optimum abrasion resistance factor and became the then SULZER HYDRO (ANDRITZ) proprietary coating. Since 1995 with the development of gas fuelled HVOF spraying technique is employed to provide the coating on hydro turbine components. Since then ANDRITZ successfully provided the best solution to combat hydro abrasive erosion globally in areas where the river water has high and hard particle content. In the meantime ANDRITZ also developed expertise in predicting a wear index (pattern) in some hydropower plant based on some typical information such as concentration of particles, mineralogical composition of particles, size and shape of particles, concentration of particles with respect to time. This model empirically compares the extent of wear with respect to reference power plant in Switzerland and helps in framing the guarantees and warrantees for the ANDRITZ coatings. In 2008 ANDRITZ started their coating activities in India to cater to the needs of Indian power plants. In 2011 ANDRITZ developed a technique to stabilize the splitter of pelton runners by introducing SXH 7X coating. In 2013 ANDRITZ introduced a premium quality coating SXH 80/8X to replace SXH70/7X have been introduced with still further better wear resistance than SXH 70/7X. ANDRITZFirst provided the SXH 70 coatings in India for under water parts in Baglihar STAGE I and Nathpa Jhakri project Subsequently other projects such as, KARCHAM WANGTOO, MALANA, CHAMERA, TALA, TEESTA, KALIGANDAKI, ALLAN DUHAGAN, BHILANGANA, CHINANI, BARAGAON etc. have been provided coatings from Indian operation. The performance of ANDRITZ coatings has been found to be satisfactory as per the feedback from our esteem customers. Baglihar power plant stage 1:-This power plant is in two stage,1 st stage is 3 x 150 MW.The Francis runners 3 no s of diameter 3500 mm is fully coated along with guide vanes which are also fully coated. This was the first fully coated Francis runner in India. Last year we (ANDRITZ INDIA) coated the guide vanes and labyrinth rings for 2 nd stage of Baglihar which is under commissioning. Karcham wangtoo:--runners, guide vanes, labyrinths, wearing rings were all coated. Teesta urja- This is 6 x 200 MW power plant and needles, runners, seat rings and seal rings have been coated. 177

8 ICHPSD-2015 Nathpa Jhakri power plant: - ANDRITZ supplied 2 sets of guide vanes and four modified design runners to NJ. The Computational fluid dynamic studies along with wear analysis were done to frame the guarantee statements. Malana power plant: -- Malana power plant is a pelton machine and ANDRITZ supplied Micro guessrunners duly coated 2 no s. Chinani and Baragaon projects: Both projects were under CH segment and these were the minimum diameter pelton runners coated fully by robot. Some of the photographs of components coated are shown below. 178

9 CONCLUSIONS 2014 SXH80/8X HVOF ) HVOF coatings have been found to protect the underwater parts of hydro turbine to a sufficient extent. 2) ANDRITZ coatings have been proved to combat hydro abrasive erosion in hydro power plants. 3) More studies are still needed to find newer coatings and techniques to apply them for extending the life of such components further. REFERENCES 1) Andritz internal literature 2) Andritz sales kit 3) Dr.winkler, Dr.D.A.Karandikar- Innovative solutions for turbine erosion presented at workshop on regional cooperation on sediment management in hydropower projects