Use of a hard coating on the runners and related components at the 1,500 MW Nathpa Jhakri plant in India significantly decreased damage as a result of hydroabrasive erosion. The coated units have operated successfully through the monsoon season with minimal damage.
By Kirsten Winkler, Roger Dekumbis, and Ajay Uppal
The 1,500 MW Nathpa Jhakri hydro project has experienced problems related to hydroabrasive erosion of the turbines and mechanical components since it began operating in 2003. At first, owner Satluj Jal Vidyut Nigam Ltd. (SJVNL) had to perform significant welding repairs after every monsoon season. Given the high cost of these repairs — about €500,000 (US$676,000) — and the negative effect on unit efficiency, SJVNL searched for alternative solutions. After some experiments with different protection systems, SJVNL decided to have the turbines coated with SXHTM70, a high-velocity oxygen-fueled (HVOF) tungsten carbide coating. In 2008, this work was completed for the first two units, which operated successfully through the 2009 monsoon season with only minor damage.
Erosion problems at Nathpa Jhakri
The Nathpa Jhakri hydro plant is on the Satluj River in India's Himachal Pradesh State in the Western Himalayas. The plant contains Francis turbines, each with a 225 MW rated load.
Problems arising from hydroabrasive erosion are common in the Himalayas for several reasons. Firstly, because the Himalayas are a relatively young geological formation, they are still undergoing changes, including erosion. This results in a high particle load in rivers. Secondly, the rocks in the catchment area of these rivers include high fractions of quartz, feldspar, and tourmaline, all of which are hard materials. Thirdly, most hydro plants in the Himalayas do not have an extensive reservoir in which substantial sedimentation can take place.
For run-of-river plants, expensive measures (such as desanders) must be put in place. Even plants with desanders experience an extensive problem with hydroabrasive erosion, mainly in the distributor and the turbine because the high relative velocity of the water in these areas enhances the aggressiveness of the particles.
Nathpa Jhakri has experienced problems related to hydroabrasive erosion since the plant began operating in 2003, despite the fact that the world's largest underground desilting chambers were included in the design of the plant to reduce the particle concentration of the water passing through the six turbines. These desanders are 525 meters by 16.3 meters by 27.5 meters. The efficiency of a desander is dependent on the grain size distribution. Using measurements taken in July and August 2004, SJVNL calculated that the efficiency of the four desanders at Nathpa Jhakri exceeded 50% on average. Even so, despite a significant reduction in the particle load, erosion damage still was severe.
This damage decreased the efficiency of the power plant by more than 5% and impaired secure operation of the plant because there was no possibility of shutting down the units by closing the guide vanes. Damage was so severe that an overhaul was necessary after every monsoon season. Furthermore, the high amount of welding performed, with necessary heat treatment, resulted in hydraulic geometry changes, which have an effect on unit efficiency.
To determine the best way to deal with this problem, SJVNL is performing extensive measurements regarding the particle load in the water passing through the facility. These measurements are taken regularly over the course of a day, mainly during the monsoon season.
In addition to the particle concentration, it is important to know the mineral composition, size distribution, and shape of the particles that pass through the facility over the entire year. The fractions of minerals with a Mohs hardness higher than 5 were determined, see Table 1, showing that about 75% of all particles have a Mohs hardness above that of the martensitic steel base material of the turbine runners.
The particle shape is mainly angular or only sub-rounded, which increases the aggressiveness of the particles. Figure 1, right, shows the particle concentration over the course of a year in 2003, as well as the discharge of the river. The distribution over the year is typical for this region. During the monsoon, soil erosion in the catchment area increases, and about 99% of the entire amount of particles are transported during four to five months (May to September). But particle loads fluctuate significantly over the years and can vary between the lowest and highest yearly average by a factor of three.
SJVNL took several different measures to deal with erosion. These include testing abrasion-resistant polymer-based coatings, using plasma nitride on the underwater parts, and performing manual hard coating.
Another measure, mentioned above, was to optimize the operation of the desanders. The goal was to determine when the particle concentration was increasing and thus allow the plant to be shut down to prevent a particle concentration above a certain limit passing through the turbines.
To minimize downtime at Nathpa Jhakri, this limit for shutting down the units had to be as high as possible. Thus, SJVNL considered using a protective coating on all components of the turbine that were in contact with water to extend the amount of time during which the turbines could operate in an abrasive environment and minimize the damages. SJVNL contracted Andritz Hydro in 2006 to perform this work.
Performing the work
Andritz Hydro began working to solve this problem in June 2006. To reduce abrasive erosion on the guide vanes, Andritz Hydro coated them with SXH70. SXH70, developed by Andritz Hydro, is a tungsten carbide-cobalt chromium coating with a hardness of 900-1,200 HV0.3 (Vickers hardness under 300 grammes of load) and an adhesive bond strength of at least 70 MPa. This coating was subsequently applied on the guide vanes using the HVOF process in 2006 and 2007. HVOF is a thermal spray technology in which a powder is fed into a spray gun. There, a fuel (hydrogen for SXH70) is burned with oxygen, and the heated and softened powder is ejected together with the gases at a supersonic velocity.
Abrasive particles in the water resulted in significant damage to the runners (left) and guide vanes (right) of the four units in the 1,500 MW Nathpa Jhakri facility.
Worldwide, numerous suppliers offer hard coatings, mainly applied by the HVOF process but using different powders. While the choice of chemical compositions is quite limited, the manufacturing process of the powder has a significant influence on the powder parameters and therefore the coating properties. Another major influence on the coating quality is the coating process itself, as small variations from the optimum process parameters can result in a significant reduction in quality, and the process has to be fine-tuned for different applications to achieve the best results.
Another measure by SJVNL was to order new runners that would be fully hard coated with SXH70. A new design of the runners was necessary because the original runner, a splitter blade design with 15 full and 15 half blades, did not provide enough space to fully coat the original runners. Therefore, the design had to be changed to achieve wider channels by having only 13 blades.
However, even this change was not sufficient with the state-of-the-art in coating technology at that time to achieve a full coating of the water channels. Thus, Andritz Hydro undertook a project to develop and invest in coating equipment so that the water passages of the newly designed runners could be coated using a robot. Use of a robot was the only method possible to coat this amount of area with a high quality and homogeneous layer thickness.
The new runners for the 1,500 MW Nathpa Jhakri facility were hard coated to minimize damage due to erosive abrasion. A robot was used to apply the homogenous layer.
The new runners were manufactured in the Andritz Hydro facility in Ravensburg, in southern Germany. The coating operation was performed in Kriens, Switzerland, a facility which contains two coating cabins that can be used for Francis runners of up to 6 meters in diameter. Development and application of the anti-abrasive coating was completed in 2007 and 2008.
Despite the fact that the new design has fewer blades and slightly increased surface roughness due to the hard coating, no efficiency loss was observed compared with the uncoated former runners. The loss in efficiency that was expected was successfully compensated for with the new hydraulic design.
The Nathpa Jhakri runners are the first runners worldwide that were fully hard coated using a robot. The two new runners were put into operation during the low flood season of 2008/2009. Two additional runners were installed at the beginning of 2010.
After each monsoon season, the turbines at the Nathpa Jhakri plant are opened to determine the amount of abrasive erosion and to check for eventual repairs. After the end of the 2008 monsoon season, when the SXH70-coated guide vanes were in operation for one monsoon season, the turbine was inspected for the yearly overhaul. The overall state of the guide vanes was good, and no damage could be seen on most of the guide vanes. Some guide vanes had nonetheless suffered some small damages. However, all the guide vanes were installed again without repairs and were in operation throughout the 2009 monsoon season, together with the first two fully coated runners.
The minimal damage to the coated runners after one year of operation consisted of damage to the entrance edges (left) and at the transition of the blades with the band at the outlet edge on the pressure side (right). These damages can be repaired easily.
After one monsoon season, the fully coated runners showed two types of damage. At some entrance edges, damage was observed that is most likely due to larger objects going through the turbine. In addition, at the transition of the blades with the band at the outlet edge on the pressure side, small damaged areas were observed. These have nearly no effect on efficiency and can be repaired easily.
SJVNL will continue to open units at Nathpa Jhakri and gauge the performance of the coating over a period of time before deciding on future plans. However, the utility already has decided to install only hard-coated underwater parts in all six units at its 412 MW Rampur project, which is being planned for development.
In addition, Andritz Hydro supplied eight fully coated runners and components for the 1,200 MW Karcham Wangtoo project in India. This project is being developed by Jaiprakash Hydro Power Ltd and these units were coated between 2009 and 2011.
Kirsten Winkler is head of Andritz Hydro AG's Centre of Competence for Coating and Roger Dekumbis is former head of this center. Ajay Uppal is assistant general manager of the maintenance department with Satluj Jal Vidyut Nigam Ltd., which owns the Nathpa Jhakri project.