Effect of concentration and size of sediments on hydro-abrasive erosion of Pelton turbine
Introduction
Most of the streams in geologically young mountains, like the Andes and the Himalaya, contain high sediment concentration during snow melt and rainy seasons causing hydro-abrasive erosion of hydraulic turbines and components coming in contact with sediment particles [[1], [2], [3]]. The hydro-abrasive erosion poses challenges for smooth and efficient operation of existing hydropower plants (HPP) as well as for the development of new hydropower plants in these regions. Recently, erosion is getting more attention because of generation loss of much needed renewable energy, experienced maintenance and high efficiency requirement. Erosion changes the profile of the turbine blades/buckets, which plays major role on efficiency of the turbine, finally leading to power generation loss [3,4]. On the one hand run-of-river plants face high erosion as less or no storage for sediment settling is available and on the other hand even small sediment particles cause acute hydro-abrasive problems in Pelton turbines due to high head [5,6]. Though researchers have identified parameters involved in erosion of Pelton turbines like sediment concentration, size, material composition, flow velocity; the quantitative information of the influence of these parameters is not fully known [3,5,7].
Hydro-abrasive erosion models are useful for designing turbine components, sediment settling basins and optimization of hydropower plant operation in rivers with high sediment contents [5,7,8]. Most often, individual particle dynamics are considered for developing erosion models [5]. Empirical and statistical relations for hydro-abrasive erosion have been developed from experimental as well as field measurements [8,9]. However, only limited erosion models in hydraulic machineries have been validated for their reliability [3]. Since erosion studies are moving towards numerical modelling and simulations, the importance of erosion models has increased recently [10].
Several researchers attempted to simulate hydro-abrasive erosion using slurry pot tester [11,12] and jet type erosion tester [10,13]. The slurry pot testers do not simulate erosion conditions of Pelton turbines due to high concentration of erodent and continuous contact of sediment with specimen. The static specimen in the jet type erosion tester lacks the effect of forces like centrifugal force and Coriolis force etc., which are present in rotating frame only. Moreover, factors like the Coanda effect on the backside of buckets [14] and secondary erosion (cavitation) [15] are not observed in such set-ups, but are encountered in the prototype hydropower plants.
Based on literature survey, it is found that the erosion tests give widely varying results depending on the features of different test rigs, flow velocity, impact angle, composition, concentration and size of particles [3,8]. Additionally, the existing data from experiments is difficult to be transferred to the prototype due to the fact that most of the earlier and existing test rigs do not simulate the conditions of the prototype plant. Hence, a test method resembling the conditions in a prototype plant as closely as possible is required for investigations and analysing the impact of erosion in Pelton turbine.
To include the effects of dynamic conditions, some researchers [16,17] used model impulse turbines as test rig for hydro-abrasive erosion. However, these studies involved high sediment concentration and soft material like brass specimens during experiments to get reasonable amount of erosion in less time. These conditions differ from prototype plant operation where the plant is shut down at concentrations higher than 3000 ppm to reduce hydro-abrasive erosion [18] and soft metal is not used for plants having sediment laden water. Moreover, a hard material like martensitic steel is actually used for turbines. Brekke [19] explained that different materials in similar flow conditions exhibit different erosion behaviour (also in Ref. [8]). Hence, erosion results from brass samples may not be applied to estimate the erosion of hydro turbine steel used in prototype. Moreover, attempts have not been made under the reported studies to correlate the erosion of brass and steel, being the actual turbine material, in the experimental set-up to make the outcome applicable. To obtain results applicable for prototype plants, turbine materials like 13Cr-4Ni, 16Cr-5Ni martensitic steel etc. are needed to be studied.
The relative wear resistance of different materials may vary under varying test conditions. Therefore, the wear resistance index of the materials, obtained under different tests, may vary. In the present study, a test method resembling to the prototype conditions as closely as possible is developed and used for analysing the impact of erosion. For the purpose, a 1:8 geometrically scaled down model of Pelton buckets from a HPP severely affected by hydro-abrasive erosion was selected in laboratory investigation. The model Pelton turbine was fabricated with detachable buckets made of 6 different materials and tested with sediment particles obtained from the same HPP. Four major parameters, specifically SSC, size, erosion velocity and duration of operation, were varied during tests in laboratory. Surface roughness of the various buckets was also analysed.
Section snippets
Methodology
The details of the set-up, parameters considered, procedure adopted, measurement methodology and data reduction methods for the study are presented in this section.
Results and discussions
In this section, the effect of various parameters such as SSC, PSD, erosion duration, erosion velocity and roughness of buckets on hydro-abrasive erosion are presented. Further, a correlation of normalised erosion with these parameters is developed for turbine designers to predict erosion of Pelton turbine in sediment laden flow conditions. The major eroded portions are splitter, cut-out and outer zone of curved portion of buckets similar to the patterns reported in literature [6,10,16]. The
Uncertainty analysis
The uncertainty in this research work is analysed using formulae given by Klein and McClintok [26], which estimates the uncertainty as per eqn. (8) to eqn. (10).where.
= n number of basic independent variables,
δY = absolute uncertainty in measurement of Y, which is a function of basic independent variables
= possible error in
Conclusions
In this study, the hydro-abrasive erosion in Pelton buckets has been analysed using an experimental approach. The experiments were performed on 1:8 scaled down model of Pelton buckets of an erosion affected prototype plant located in Indian Himalayas. Six types of materials were tested in similar hydraulic conditions and it is found that the WC-Co-Cr coating applied through HVOF process performed best against hydro-abrasive erosion. The erosion model developed from this study showed good
Acknowledgements
The financial support received from Ministry of Human Resource Development (MHRD), Government of India in the form of a PhD scholarship is gratefully acknowledged.
References (26)
- et al.
Analyzing hydro abrasive erosion in Kaplan turbine: a case study from India
J. Hydrodyn. Ser. B. - J.– Elsevier
(2016) - et al.
A review on silt erosion in hydro turbines
Renew. Sustain. Energy Rev.
(2008) - et al.
Hydro-abrasive erosion in Pelton buckets: classification and field study
Wear
(2017) - et al.
Empirical modelling of sediment erosion in Francis turbines
Energy
(2012) - et al.
Particle size effects on the slurry erosion of aluminium alloy (AA 6063)
Wear
(2009) - et al.
Zirconium based bulk metallic glass - better resistance to slurry erosion compared to hydroturbine steel
Wear
(2013) - et al.
Effect of size and concentration of silt particles on erosion of Pelton turbine buckets
Energy
(2009) - et al.
Effect of the material surface hardness on the erosion of AISI316
Wear
(2005) - et al.
Roughness parameters
J. Mater. Process. Technol.
(2002) - et al.
Sediment monitoring for hydro-abrasive erosion: a field study from Himalayas, India
Int. J. Fluid Mach. Syst.
(2017)
Erosion of Pelton buckets and changes in turbine efficiency measured in the HPP Fieschertal
Hydro-abrasive erosion: problems and solutions
Cited by (54)
Effective monitoring of Pelton turbine based hydropower plants using data-driven approach
2023, International Journal of Electrical Power and Energy SystemsResearch on synergistic erosion by cavitation and sediment: A review
2023, Ultrasonics SonochemistryData-driven internet of things and cloud computing enabled hydropower plant monitoring system
2022, Sustainable Computing: Informatics and Systems