CASE STUDY JHIMRUK, NEPAL

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1 SEDIMENT MANAGEMENT CASE STUDY JHIMRUK, NEPAL Key project features Name: Jhimruk Country: Nepal Category: modifying operating rule (focus or redistribute sediment); adaptive strategies Reservoir volume (original): n/a Installed capacity: 12 MW Date of commissioning: 1994 The Jhimruk hydropower plant was built and commissioned in 1994, and is located in the Himalayan region of Nepal. A desilting basin was designed to trap 90 per cent of particles greater than 0.2 mm, based on general design and experience in similar projects in Nepal. However, from the beginning this 12 MW run-of-river project has faced problems of abrasion in the hydromechanical equipment, due to the unexpectedly high content of silt particles finer than 0.2 mm. Settling basins, improved plant operation, and hard coating has reduced wear on the turbines. As a result, the loss of energy production has been minimised. Jhimruk hydropower plant, owned and operated by Butwal Power Company (BPC), is located in the Pyuthan district, in midwestern Nepal. The run-of-river project benefits from a 205 m net head caused by the water diversion from the Jhimruk river to the Madi river. Both rivers meet around 30 km downstream. International Hydropower Association Chancery House, St Nicholas Way, Sutton, London SM1 IJB, United Kingdom T: F: E: iha@ The project includes a weir in the Jhimruk river that diverts water to two parallel settling basins, before it is conveyed at a design discharge of 7.05 m3/s to the semi-underground powerhouse through a 1 km long headrace tunnel and a 250 m August 2017 long penstock. The water is finally discharged in the Madi river through a short tailrace channel. The diversion weir has a curvilinear shape with 205 m overflow length. The crest elevation is at 738 masl. Following the conventional design criteria of similar projects in Nepal the settling basins, of dimensions 42 m long, 5.5 m wide and 7 m deep, were designed to trap 90 per cent of particles larger than 0.2 mm. The powerhouse hosts 3 Francis turbines, each of 4 MW.

2 Hydrology and sediment The catchment tributary to the Jhimruk weir comprises 645 km 2, characterised by four months of rainy season (monsoon), lasting from June to September. The peak flow period is in July and August. The annual average precipitation is 1,610 mm, more than 80 per cent of which occurs during the monsoon, and the mean annual inflow is 851 Mm 3. The most significant sediment transport, of around 80 per cent, also occurs during the rainy season. In other Himalayan river basins, the annual sediment load can reach up to 100,000 tons. Sediment problems Sediment data for the Jhimruk river was not available during the planning and design phase of the project. Two desilting basins were therefore designed based on general sediment references for Himalayan rivers, and using conventional design criteria for hydropower plants in Nepal. The desilting basins were meant to trap 90 per cent of sediment particles larger than 0.2 mm in grain size. The first sediment handling issues occurred following the first five months of operation, after the monsoon. Damage to the hydromechanical components (turbine blades, guide vanes, facing plates and casing) was so severe that it became clear that exposure to abrasion through sandladen water was much higher than originally anticipated. Sediment abrasion caused by hard minerals is one of the biggest challenges facing hydropower projects in Nepal. It can lead to a reduction in efficiency and design life of turbines. The erosion suffered by the turbine wear and the guide vanes shown in figure 2 illustrates the high operation and maintenance costs at Jhimruk. Since commissioning in 1994, monitoring of suspended sediment at the plant has been carried out with a view to improving sediment management strategies. Aerial view of Jhimruk

3 From 1994 to 1997, the monitoring programme recorded the suspended sediment concentration during the monsoon in Jhimruk river. The data showed mean values ranging from about 2,000 to 6,000 ppm, with maximum values ranging from about 20,000 ppm up to 60,000 ppm during peak flow. The runners were so damaged after each monsoon that the turbines needed to be repaired on an annual basis. In 2003, Hydro Lab in Nepal launched a research project on optimum sediment handling strategies in run-of-river projects, featuring Jhimruk as its initial case. Particle size distribution and mineralogical studies were performed to complete the suspended sediment profile. Both are critical parameters for defining the potential abrasion in the turbines and for identifying the best sediment management strategies and the most appropriate hydromechanical equipment. In 1996, a sample from the desilting basins showed that 90 per cent of the particles entering the turbines was finer than 0.1 mm in grain size, and that the mean diameter D50 was mm. A more recent study in 2012 compared samples from both locations at the headworks above the weir and at the tailrace of the power plant. As shown in figure 4, the study found that 80 per cent of sediment at the headworks had a grain size range of between 0.1 and 0.2 mm, and about 95 per cent of the sediment at the outlet was within this size range. Both studies demonstrated that the settling basins filter sediment particles larger than 0.2 mm, and that the critical size of sediment particles reaching the turbines is of 0.1 to 0.2 mm. Civil structures such as a desilting basin to trap sediment particles finer than 0.2 mm are extremely costly, and therefore the exclusion of particles smaller than 0.1 mm is economically unfeasible. The mineralogical studies found that about 80 per cent of the sediment is composed of two hard minerals: quartz and feldspar, whose hardness in the Mohs scale is 7 and 6 respectively. These minerals will wear the uncoated turbines and their components away, as their hardness does not normally exceed a 5 in the Mohs scale. The high percentage of hard minerals, combined with the large proportion of sediment particles finer than 0.2 mm that are not being trapped in the settling basins, are causing abrasion of the turbines. As a result, the Jhimruk power plant is normally operated at a lower capacity than is installed, including temporal plant shutdown. Sediment management strategies Two settling basins were designed to trap sediment particles greater than 0.2 mm in grain size. Despite the proven efficiency to exclude these particles, the settling basins did not prevent severe damage to the turbines. In 2003, Hydro Lab in Nepal launched a research project on optimum sediment handling strategies in run-of-river projects, featuring Jhimruk as its initial case. Measures were researched to reduce the costs of operation and maintenance at the power plant. These include measures such as: the exclusion of fine sediments through additional settling basins; reduction of sediment exposure of the turbines through an improved operational plant rule; and an increase the life of the turbines. A scale model of two additional settling basins was built in order to study the hydraulic feasibility of increasing the trapping efficiency of finer particles. The research achieved improved flow distribution and a favourable pattern for the settling basins to trap more sediments. The plant s operation strategy links the generation of the turbines to the silt particle concentration in the suspended sediment. As presented in figure 7 and figure 8, if the concentration is below 1,500 ppm, the power plant will operate at full capacity. However, if the concentration is above 3,000 ppm, the power plant will shut down completely. Between 1,500 and 3,000 ppm, the three units are shut down one by one as the concentration rises. The enhanced maintenance programme proposes changes in maintenance periods and hard coating of the hydromechanical equipment in order to increase turbine efficiency. Just before and after the monsoon, maintenance should be applied to slow the decrease in efficiency. However, outside of the rainy season, the efficiency of the power plant should be higher in order to produce extra generation. The periods of the maintenance programme are shown in figure 9. Ceramic spray coating (R-type), coating of the guide vanes, and the spare runner sets are suggestions that have been explored to reduce wear on the turbines and therefore optimise energy generation. However, ceramic coating has not proven to be satisfactory. Instead, another type of coating which was used on the guide vanes and applied to one turbine withstood the abrasion. Three sets of turbine parts have been kept to reduce the annual maintenance time to a few hours. Graphs and figures >

4 Graphs and figures Figure 1: Jhimruk hydropower project: diversion headworks and power plant site operated by Butwal Power Company (source: Google Earth) Figure 2: damage of turbine at Jhimruk Graphs and figures cont. >

5 Figure 3: sediment concentrations in Jhimruk river (source: Biskwakarma, 1998) C min C mean C max Cmin C mean Cmax C min C mean C max C min C mean C max June ,797 12, ,301 20, July 91 3,685 23, ,077 42, ,908 57, ,675 47,602 August 102 2,316 18, ,536 31, ,904 16, ,882 27,693 September , ,064 31, ,739 28, ,139 October ,646 9, Figure 4: particle size distribution in Jhimruk river (source: Neopane, 2012) Graphs and figures cont. >

6 Figure 5: desilting basins at Jhimruk headworks Figure 6: scale model of additional desilting basins Graphs and figures cont. >

7 Figure 7: plant operation strategy for Jhimruk power plant Silt concentration (ppm) Below 1,500 Between 1,500 and 3,000 Above 3,000 Generation (MW) Full generation Reduced generations 1,500 to 2,000-3 units 2,000 to 2,500-2 units 2,500 to 3,000-1 unit Complete shutdown Figure 8: plant operation strategy for Jhimruk power plant Graphs and figures cont. >

8 Figure 9: change in maintenance period for Jhimruk power plant This is part of a series of sediment management case studies collated by International Hydropower Association with support from the South Asia Water Initiative (SAWI), trust funds to the World Bank.