HVOF coatings for erosion resistance of hydraulic turbines: Experience of Kaligandaki “A” Hydropower Plant Bhola Thapa Kathmandu University, PO Box 6250, Kathmandu Nepal
Piyush Upadhyay Kathmandu University PO Box 6250, Kathmandu Nepal
Ole G. Dahlhaug Norwegian University University of Science Science and Technology Trondheim Norway
Ranjil Basnet Larsen &Toubro: Eutectic Section Section Kathmandu Nepal
Madan Timsina Kaligandaki “A” Hydroelectric Plant Nepal Electricity Electricity Authority Nepal
Introduction Sand erosion causes operation and maintenance problem of hydraulic turbines. The climatic and geological conditions of Himalayan region are highly favorable for sedimentation. Erosion of turbine material depends on flow parameter, sand particle characteristics and properties of material. The underwater components of Pelton turbine such as nozzle, needle, buckets and Francis turbine such as runner, guide vanes are most affected by sand erosion. The erosion resistant hard surface coating of turbine is one of the methods of minimizing effect of erosion. Coatings such as Ceramic pastes, Ceramic paints and hard welding alloys were common practice in Nepalese hydropower industry. Recently hydropower industries are applying tungsten carbide based ceramic (86WC-10Co-4Cr) by High Velocity Oxygen Fuel (HVOF) process. The performance of such coatings depends both on spray parameters and powder properties. The The finding of erosion measurement measurement of similar coating coating on high velocity jet erosion test rig rig shows that the same powder sprayed by different coating companies has shown different erosion resistance. Kaligandaki “A” Hydropower Plant in Nepal has 3 units of 48 MW vertical shaft Francis turbine with head 115 m and operating at speed of 300 RPM. RPM. This project has desanding desanding facility, which has been designed designed to flush 100% sand particles above 200 microns. 2.8 million tons/year of fine sediment is expected to pass through turbines of Kaligandaki “A” Hydropower Plant with annual average sediment concentration 720 ppm. The typical problem in operation of inlet valve was observed in this power plant due to erosion damage of guide vanes and facing plates. Beside this operation problem, the runner, guide vanes, facing plates and wearing rings are damaged regularly.
The turbine components of Kaligandaki “A” Hydropower Plant are now coated with HVOF coating. When the plant was commissioned in 2002, runners (Material 13/4 CrNi) were HVOF coated but other turbine parts such as guide vanes, facing plates, wearing ring, and guide vane bushes were non-coated. Within one year of operation (running hour=7768), the non coated components were damaged by sand erosion. The coated area on runner inlet blade, band and crown sealing surfaces did not show erosion damage but remaining area was somewhat eroded. The runner and other components without coating were installed in unit#1, during overhauling in 2003. When the overhauling of the same unit was carried out after 3 years (running hour=14364) the non coated runner was heavily damaged with sand erosion especially on the sealing area, blade inlet and outlet surface. HVOF coated facing plates, bushes for guide vanes and runner have been installed with non coated guide vanes and wearing ring after overhauling of the same
unit#1 in 2006 and performance of this unit is being monitored. This paper also presents findings of comparison of HVOF coating used at Kaligandaki “A” Hydropower Plant with stainless steel using Rotating Disc Apparatus at Kathmandu University laboratory. With the growth of hydropower in Nepal, there will be more challenges for local engineers to select suitable type of coating materials to combat sand erosion, devise proper method to apply them effectively and asses the performance of newly developed coatings.
1. Background Nepal is rich in water resource because of Himalayan glacier, regular monsoon and topography. It has 1503 mm annual precipitation and 224 billion cubic meters annual runoff through 6000 large and small river/rivulets. Such a nature made Nepal rich in hydropower with potential of 83000 MW total and about 43000 MW economically feasible. Nepal has not been able to exploit even 2% of this capacity. Huge investment in hydropower project is major challenge for its development. On the other hand optimum operation of existing hydropower plant is another great challenge because of effect of sediment in operation and maintenance. Himalayan rivers contain large quantities of sediment with hard abrasive particles such as quartz.
The turbines of Marsyangdi hydropower project, Jhimruk hydropower project, Khimti hydropower project and others are heavily damaged by sand erosion. Such damage decreases life of turbine, reduces efficiency of the system and creates difficulty in operation management of hydropower plants. Erosion is progressive material loss caused by repetitive impacts of sand against the turbine surface. The factors affecting erosion are as follows: 1. 2. 3.
Factors associated with operating conditions – velocity, acceleration, impingement angle, concentration Factors associated with eroding particles - size, shape, hardness and minerals Factors associated with substrate (base material) - chemistry, material property and surface morphology
The nozzle, needle and buckets are most affected components of Pelton turbine. Runner, guide vanes, facing plates and labyrinth seals are most affected components of Francis turbine. Some hydropower projects have experience of application of erosion resistance material. Marsyangdi hydropower project initiated erosion resistant welding and coatings to increase turbine life. Erosion resistant coating materials ceramic pastes, ceramic paint and flame spray of ceramic powder were used to coat the turbine blades in Marsyangdi and Jhimruk hydropower plants. These ceramic paste and paint is silicon steel alloy blended within high molecular weight reactive polymers and oligomers. These materials are applied in cold condition. The ceramic powder used was chromium based, oxygen stabilized ceramic compound in powder form with specific grain size distribution. Even though erosion rate is retarded in Marsyangdi hydropower project, these coatings have not lasted more than 1 season in Jhimruk hydropower plant.
2. Ceramic Coating Stainless steel is most common turbine material because of wide range of toughness and hardness to suit various operating conditions. Both austenitic and martensitic steel with different Cr and Ni contents are used in turbine industry. But even such hard steels are eroded by harder particles like quartz. Ceramic and cermets are hard material to resist erosion but they have poor toughness. WC coatings with Co, Ni, Cr, Mo as binder are widely used in turbine industry. Carbides provide good erosion resistance. Ceramic coatings are applied by thermal spray process and their quality depends on powder properties and spray process. Tungsten carbide grains with a cobalt as binder is widely used in turbine industry. Corrosion resistance of such matrix is improved by adding Cr to the binder. Large volume fraction of finely distributed tungsten mono carbide (WC) particles provides optimum wear properties (Berget, 1998). During spraying, the decarburization of WC can occur at high temperature forming W2C, which is
harder than WC but more brittle. This decarburization also detaches C from WC and liberated C oxidizes to form CO or CO2 gas. The diffusion of CO in to matrix causes poor coating quality. The decarburization reaction depends on time, temperature, powder and spray parameters. If the carbides are very small compared to abrasive particles, the hardness of overall metal matrix will increase. On the other hand, if the size of carbide is large, hardness of carbide itself is effective in combating erosion. Ceramic coatings are applied by thermal spray process. The coating material in the form of powder or wire are melted by heat source and sprayed in to the base material. There are several thermal spraying processes which in general consist of gun (for melting of particle and accelerating towards substrate), energy control unit and coating material feeding mechanism. The accelerated molten coating material strike substrates, flattened in the form of thin platelets and bind together with the substrate. The layers of flattened particles form protective coating. The quality of thermal spray broadly depends on coating material property (chemical composition, melting point, morphology, particle size distribution, shape) and spraying process control (flame temperature, gas pressure, powder flow rate, geometry of nozzle, spray distance, surface preparation of the substrate). The results of erosion test on tungsten carbide ceramic metallic coatings 86WC-10Co-4Cr and 86WC-6Co-8Cr in high velocity jet erosion test rig and other laboratory tests have shown better performance of 86WC-6Co-8Cr. The better result of this coating is due to finely distributed uniform small WC particles (Thapa, 2004 and Berget 2003). Similarly same coatings applied by different coating companies have also significant difference in their performance. Five other ceramic-metallic coatings (WC-Co-Cr coatings with different microstructures and carbide size) have also shown difference in performance both in laboratory and in field. But all these coatings have shown better erosion resistance than stainless steels (Gray et al, 2004).
3. Kaligandaki “A” Hydropower Plant The largest hydropower project in Nepal, Kaligandaki “A” Hydropower Plant with installed capacity of 144 MW is run off the river scheme, and generates about 842 GWh of electrical energy per annum. This project is located at Syangja District, approximately 180 km west of Kathmandu. The diversion dam is at 500 meters downstream of the confluence of Kaligandaki and Andhikhola River. The main component of project comprises 100 meters long and 43 meters high concrete gravity diversion dam. It has 6 km tunnel of 7.4 meter diameter and a surface powerhouse. This project has 7618 Km 2 catchments area in the Himalayan and hilly region. The 90% dependable flow is 48 m 3/s whereas average annual flow is 288 m 3/s because of monsoon climate. The local climatic, tectonic and geological conditions of the Himalayan region and human activities in the area are highly favorable for soil erosion, which causes heavy sediments in River with large quantities of hard abrasive particles. It has daily peaking reservoir of volume 7,700,000 m3. In spite of this reservoir, large amount of sediment is passing through waterways towards power plant. There are two open surface desander basins of 40 m X 88 m to give flow rate of 0.2 m/s. There are 7.4 m X 18 m collector channel at bottom with intermittent flushing facility. These desanding facility is designed to trap 100% sediments above 200 mm, and 95% up to 125 mm particles, but only 55% of fine sediments of size 63-125 mm. Out of 65 million ton/year sediment flux only 5.9 million ton/year reach to desander basin. The 2.8 million ton/year fine sediment can not be trapped and hence pass through turbine giving yearly average sediment concentration 720 ppm. This project has three units (48 MW each) of vertical shaft Francis turbine operating at net head of 115 meters with rated discharge of 141m3/s. The turbines run at speed of 300 RPM. Turbines are manufactured by Toshiba, Japan with 13/4 Cr-Ni stainless steel. 3.1 Turbine overhauling experience
The second overhauling of turbine unit 1 was done in September 2006. The running hour of this unit from commissioning (April 2002) is 22,082 hrs (6 flood seasons) but the running hour between 1 st overhauling (Sept 2003) and 2nd overhauling (Sept 2006) is 14,314 hrs (3 flood seasons).
All the major turbine components were heavily damaged due to sand erosion. Some damaged has appearance of combined effect of sand erosion and cavitation as shown in figure 1 (a). The extent of damage in facing plates and guide vanes are very high figure 1 (b). The gaps between facing plate and guide vane and in between guide vanes became so large that the pressure on spiral casing could not be maintained because of water leakage, which resulted non opening and non closing of the inlet valve. Combined Effect
Sand Erosion
Fig .1 (a). Turbine Runner with sand erosion damage and combined effect of sand erosion and cavitation in inset
Fig .1 (b). Sand erosion damage in guide vane
HVOF coating is done at Kaligandaki “A” Hydropower Plant with the help of Larsen & Toubro, Eutectic Division. The surface is prepared by Alumina grit blasting before HVOF coating. The agglomerated & sintered ceramic coating powder TAFA 1350VM WC 86% Co 10% Cr 4% is applied with HVOF spray equipment ST-4000 HVOF coating equipment from Praxair. The CoCr matrix shows higher corrosion and erosion resistance then merely Co matrix materials, which is more favorable in wet corrosive environments. This coating is dense and smooth with fine microstructure and high bond strengths. It can be applied up to 2 mm thickness with surface finish up to 130 µ-in Ra in as sprayed condition. The microstructure has porosity less than 1% and hardness up to 68 R C. The typical powder shape and microstructure of coating is shown in figure 2.
Fig 2. Microstructure of TAFA 1350VM 86WC-10Co-4Cr coating in magnification 200X in as sprayed condition and particle shape in inset.
The total of approximately 42 square meter surface is coated with HVOF in different turbine components to give coating thickness from 300-500 microns. The main activities during second overhauling are as given: Runner The runner in unit 1 is replaced by the spare used runner from unit 3. This runner was supplied by Toshiba with coating in part of blade, lower and upper wearing ring during commissioning itself. This runner was removed from unit 3, during overhauling on September, 2005. This runner has run for 16,027 hours in unit 3. There was erosion damage only in non-coated region while coated region was protected. This runner is welded in damaged area,
grinded, surface polished by grit blasting and finally coated with HVOF at site in remaining recommended areas in 120mmX140mm area at upper side of blade as shown in figure 3 (a). There is also coating in joint between blade and skirt in inlet and outlet. The suction side at blade outlet is coated in the area as shown in figure 3 (b). Two of the originally coated runner is further coated in remaining are as above. Two uncoated runners, which were severely damaged were repaired and coated in all erosion prone area such as wearing rings, blade inlet and outlet as shown in figure 3 (c). The thickness of coating in band is 500 micron whereas thickness in blade is 300 micron. Among these four runners, one runner is already installed and three coated runners are available as spare.
Present coated area 120mmX140mm
550mm
Original coated area
600mm
Fig 3 (a). Recoating in selected area at blade inlet
600mm X 200mm
430mm
Fig 3 (b). Recoating in outlet area of the blade
Fig 3 (c). New coating in repaired turbine in wearing ring, blade inlet and outlet
Guide Vanes The guide vanes removed from unit 2 in 2005 were weld repaired and installed in unit 1. This set of 20 numbers could not be coated by HVOF due to pressure in generation because of scarcity of electricity in grid. But 40 nos. of guide vanes have been coated with HVOF, in total ring area, and 150 mm from both top and bottom in the suction side with high erosion. The top and bottom faces of guide vanes are also coated 300 microns thickness (figure 4 a). These 40 coated guide vanes are in spare. Facing plate Both of the facing plates were made of SUS 410 stainless steel. Since they were not coated, they were heavily damaged by sand erosion (Figure 4 b). These facing plates were replaced by new HVOF coated spare, supplied by the manufacturer in base material is 13/4 Cr-Ni Stainless Steel Bush for guide vane shaft The top ring of guide vane bushes was also damaged due to erosion and abrasion. Hence both upper and lower bush is replaced with new HVOF coated spare, supplied by Toshiba, under the contract of rehabilitation works.
Fig 4 (a). Coating in guide vane
Fig 4 (b). Damage of facing late
Fig 4 (c). HVOF coating in top ring of guide vane bush
All other parts, which were not replaced, like head cover, bottom ring, spiral casing, stay vanes, turbine guide bearing are repaired. Wear resistant ceramic coating in the form of paint and putty was applied in stay vanes and bottom ring.
4. Laboratory Testing Cavitation and sand erosion are operational problem of hydraulic turbine. A perfect hydraulic design of turbine components, in principle, should yield cavitation-free geometry. But when the surface integrity changes due to sand erosion; evidence of cavitation can be found even in cavitation-free design. The reason is that both the phenomena are more likely to occur in high water velocity zones. Combined effect of sand erosion and cavitation can be observed in high velocity regions. One of the regions in Francis turbine, where erosion caused by combined effect of cavitation and sand erosion has been observed is the inlet region of runner blade of turbine of Kaligandaki “A” Hydropower Plant. This cavitation erosion is caused by collapse of bubbles generated at the guide vane shaft in the gaps between the vane and face plate. Rotating Disc Apparatus (RDA) is developed at Kathmandu University to study the sand erosion, cavitation and their combined effect. It consists of a rotating disc with four cavitation inducers and driven by a 7.5 kW motor at 2880 rpm. The motion of submerged body can be simulated up to 39 m/s velocity. HVOF ceramic coating applied at turbine of Kaligandaki “A” Hydropower Plant is compared with stainless steel using RDA. The disc is made up of Stainless Steel SS316 and only half of this disc is coated with tungsten carbide. The erosion damage in stainless steel and HVOF coating are compared and analyzed based on the area and pattern of erosion.
5. Results and Discussion The river sand of sieve size ranging from 90-212 micron is inserted in to the RDA, which is filled with water. After certain interval, the appearance of wavy scale shape erosion is seen and later it is developed in the form of horseshoe vortex. The occurrence of horse-shoe shape of wear pattern is not caused by sand erosion alone. The groove on disc surface is due to horse shoe vortex created by cylindrical pier in the flow field. It is a familiar phenomena occurring in the facing plate around guide vane shaft as seen in figure 4 (b) of Kaligandaki “A” Hydropower Plant. Cavitation bubbles are trapped in the core of the vortex and are carried away with the water to collapse somewhere downstream in high pressure region. This vortex initiates and carries cavitating bubbles along with swirling sand particles.
Fig 5: Progressive erosion pattern with waves generated by sands erosion. Pictures taken on progression of test in 45 Min, 90 Min, 450 Min and 540 Min.
Figure 5 shows top view of the progressive horse shoe erosion pattern being developed near cavitation inducer at the uncoated region. As the run time elapses the wavy pattern deepens and develops in to sharp edged groove as shown in the last two photos. As the test duration increases the erosion groove increases in depth rather than propagating further. Figure 6 shows the damage pattern in coated region. The ceramic coating prevents the formation and propagation of horse shoe shape groove. But the tunneling beneath the coating initiated at weak coating region in the front side of inducer. The coating removal process may aggravate after initial weakening. The thickness of the coating in region affected by horseshoe vortex is reduced to nearly half of original coating thickness.
Fig 6. Progressive erosion pattern in coated region of test disc with a total test duration of 4.5 Hours. Pictures taken on progression of test duration as 30 Min, 180 Min, 270Min, 310 Min, 400Min and 490Min
6. Conclusion Ceramic coatings applied with HVOF in hydraulic turbine in sediment laden river reduce sand erosion. There is a possibility of combined effect of sand erosion and cavitation in the hydraulic turbine components. Even in cavitation free geometry, surface roughness due to sand erosion at high velocity region may initiate cavitation erosion. In case of chipping, scooping or removal of ceramic coating from some portion, there could be combined effect of sand erosion and cavitation. The effect of sand erosion and cavitation can be simulated in laboratory using Rotating Disc Apparatus for accelerated test and it can be used to rank different turbine and coating material.
The tungsten carbide ceramic coating applied at turbines of Kaligandaki “A” Hydropower Plant has shown better erosion resistance than turbine steel both in field and laboratory. References Berget J., Influence of power and spray parameters on erosion and corrosion properties of HVOF sprayed WC-Co-Cr Coatings, Dr. Ing. Thesis, NTNU, Trondheim, 1998 Thapa B. Sand Erosion in Hydraulic Machinery, Doctoral theses, NTNU, Trondheim, 2004 Gray D., Aunemo H. and Rommetveit O. , “Recent developments in silt erosion coatings”, Proceeding, IAHR symposium on hydraulic machinery and systems, Stockholm, 2004
The Authors Bhola Thapa is PhD from Norwegian University of Science and Technology. His research area is Sand Erosion in Hydraulic Machinery. He has done ME and BE in Mechanical Engineering from India. Currently he is Associate Professor at Department of Mechanical Engineering and Acting Dean of School of Engineering at Kathmandu University. Email:
[email protected] Piyush Upadhyay is a Mechanical Engineer. He has done BE from India. Currently he is Research Fellow at Kathmandu University, working on Energy Research Project under the support of Happy House Foundation, Switzerland. Ole G. Dahlhaug is PhD from Norwegian University of Science and Technology. He is a Professor at Waterpower Laboratory at NTNU. He has been actively working in Nepalese hydropower plants in research of sand erosion of turbine components and efficiency measurements of hydropower plants.
Ranjil Basnet is a Mechanical Engineer. He has done BE from Kathmandu University in 200 4. He is working with Larsen & Toubro Ltd. (Eutectic Section) from last 3 years. He was involved in supervision of HVOF Coating works of Turbine Components of Kaligandaki “A” Hydropower Plant. Madan Timsina is a Mechanical Engineer. He has done BE from India and ME from Kathmandu University. He has been working at Nepal Electricity Authority since 1997. He has worked at E/M design division, Modikhola hydropower project and currently he is working at Kali Gandaki ‘A’ hydropower plant as Assistant Manager.
