ISH Journal of Hydraulic Engineering ISSN: 0971-5010 (Print) 2164-3040 (Online) Journal homepage: http://www.tandfonline.com/loi/tish20 Performance evaluation of desilting basins of small hydropower projects Gurdeep Singh & Arun Kumar To cite this article: Gurdeep Singh & Arun Kumar (2016): Performance evaluation of desilting basins of small hydropower projects, ISH Journal of Hydraulic Engineering, DOI: 10.1080/09715010.2015.1094750 To link to this article: http://dx.doi.org/10.1080/09715010.2015.1094750 Published online: 05 Jan 2016. Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tish20 Download by: [gurdeep singh] Date: 06 January 2016, At: 18:54 ISH Journal of HydraulIc EngInEErIng, 2016 http://dx.doi.org/10.1080/09715010.2015.1094750 Performance evaluation of desilting basins of small hydropower projects Gurdeep Singh and Arun Kumar Downloaded by [gurdeep singh] at 18:54 06 January 2016 alternate Hydro Energy centre, Indian Institute of Technology roorkee, roorkee, India ABSTRACT ARTICLE HISTORY There are many silt removing devices/methods used in small hydropower projects. Selection of these devices/methods depends upon the size and quantity of incoming sediments, land utilization for construction, size of sediment to be removed, and sequent of sediment lushing. The settling basin is the most widely used desilting device for small hydropower project (SHP). Vortex sediment settling basin developed recently is other cost-efective and eicient desilting device. This paper presents performance evaluation of settling basins and vortex sediment desilting devices constructed at SHP stations through ield investigations at number of SHP stations in almost similar type of climatic and physiographic conditions. Silt removal eiciency has been compared using diferent methods for computing eiciency provided by diferent investigators. For SHP stations, eicient and economic desilting basin has been also suggested. received 26 november 2014 accepted 13 September 2015 KEYWORDS Vortex sediment settling basin; SHP; performance Introduction Settling basin Small hydropower (SHP) stations in mountainous streams are generally run-of-river types, where the catchments are steep and vulnerable to high soil erosion. Heavy rains, especially in tropics and monsoon regions produce large sediment yield from these catchments and the streams experience high sediment concentrations during seasonal loods. he desilting basin constructed in series with the headrace channel play an important role for hydropower stations. he design dimensions of the desilting basin are the major factor for their sand trapping eiciency. he sediment, not only cause erosion on turbine, capital cost of replacement, but also leads to loss of generation (due to reduced eiciency). hough it may not be possible to remove all sediments, even sediments should be removed to the extent to maintain hydraulic carrying capacity of the waterway and to prevent the abrasion of turbines. he desilting devices allow suspended particles to settle down and deposit on the bottom of the basin by reducing the velocity of the water low. Deposited sediments are removed, by lushing or by dredging manually or mechanically. In recent years, desilting basins based on diferent methods for the removal of sediments have been developed and are in use. hese desilting devices are:- Settling basins are used on irrigation and hydropower channels to remove sediment of speciied size and quantity. he channel is expanded into the basin by widening its width and lowering its loor (increasing depth) through an expansion transition and restored back through a contraction transition at the end of the basin (Athar et al. 2002).he deposited sediments are removed through one or more duct(s) unit continuous or intermittent generally based lushing or manually. he schematic sketch of Settling basin is shown in Figure 1(a) and (b). Where B1 – width of the channel; B2 – width of the settling basin; D – depth of low in the settling basin; D1 – depth of low in the channel; L – length of the settling basin; Qsi – Inlet Discharge; Qse – Outlet Discharge; U1 – Flow Velocity in channel; U – Flow Velocity in desilting basin. (a) (b) (c) (d) Vortex tubes, Desilting basin, Tunnel extractor, Hydro cyclone. he desilting basin is the most commonly and preferred desilting devices used in SHP stations. here are two types of desilting basins: (i) Settling basin and (ii) Vortex sediment settling basin. CONTACT gurdeep Singh © 2016 Indian Society for Hydraulics gurdeep5206@gmail.com, gsj5206@yahoo.com Vortex sediment settling basin The vortex chamber-type sediment basin is a fluidic device which is used to extract bed load and suspended load sediment from the diverted water by the vortices of flow (Paul et al. 1991). A higher velocity flow is introduced tangentially into a circular basin having an orifice at the center of its bottom, which removes highly concentrated sediment flow. The secondary flow resulting from this phenomenon causes the fluid layers near the basin floor to move towards the outlet orifice at the centre. The sediment particles present in the flow move along a helicoidal path towards the orifice, thereby obtaining a long settling length compared to the basin dimensions (Paul et al. 1991). The sediment reaching the centre can be flushed out through the orifice outlet channel/pipe. The schematic diagram of vortex settling basin is shown in Figure 2(a) and (b). 2 G. SinGH And A. KumAr (a) (b) Figure 1. (a) Plan (garde et al. 1990); (b) section a-a (garde et al. 1990). (b) (a) Downloaded by [gurdeep singh] at 18:54 06 January 2016 Figure 2. (a) Plan (Paul et al. 1991); (b) section X-X (Paul et al. 1991). Table 1. Vortex settling basin eiciency relations (athar et al. 2002). Investigator curie et al. (1979) Mashauri (1986) Paul et al. (1991) relationship � ( athar et al. (2002) �0.88 ⎞ ⎛ do0.11 𝛾s 𝛾 ⎟ 𝜂O = 1.74 + ln ⎜ Q0.58f ⎟ ⎜ ⎠ ⎝ ( ) ( ) ) ( + (1.71 × 10−2 dd − 5.93 × 10−4 d dk 𝜂O = 0.835 − 0.0292 k1 o o 1 ( ) 𝜔o 𝜂O = 73.4 + 8 Log W 𝜔o ) 𝜂O = 98 + 0.92 Log W [ ] ( )0.25 ( )0.35 ( )0.15 ( 2 )0.11 Qo 𝜔o ds Zh Qw 𝜂O = ko Q h 𝜈 gR3 h2 i p ( )0.04 ( )1.27 𝜔 Qo 𝜂O = 2.16 Vo Q t 𝜂O = ( 𝜔o Vto t )0.0045 ( )0.01 Qo Qi T P Where h1 – height of diaphragm from bed of inlet canal; h2 – basin depth at its periphery from inlet canal bed; h – low depth in inlet canal, full-supply depth; h0 – depth of low over oriice; hp – water depth at basin periphery; d0 – lushing pipe diameter; d – basin diameter; d – basin diameter; Qc – inlet-canal discharge, full-supply discharge; Qcc – discharge entering basin; Qs – Overlow Discharge; Q0 – Flushing Discharge; S – bed slope (S horizontal to one vertical); Sc – radial slope of basin loor (Sc horizontal to one vertical). developed relationships for sediment removal eiciency assessment are given in Table 1. he SHP plant owners design these settling basins based on limited data, as for SHP stations installed on small streams data available are very poor and limited. Many desilting basins get choked with deposited silt during heavy rainfall or lash Vortex sediment settling basin has the following advantages:(a) Land area required for sedimentation is less. (b) Require lower lushing water. (c) Cost of construction is much low compared to settling basin. (d) Hydraulic eiciency as compared to settling basin is high. (e) In vortex settling basin, water may be diverted directly to power channel during lean season when water is almost free of sediments. his is important in cold region, where settling basin freezes due to lower velocity and causes the shutting down of plants. However, design of vortex sediment settling basin needs to be supported with model study for performance assessment before execution at site. Paul et al. (1991) and Ather et al. (2002) Figure 3. camp and dobbins’ relation for eiciency of settling basin (camp 1946). iSH JournAl oF HydrAulic EnGinEErinG 3 quite common). he design parameters used at each SHP station under study was collected including the design details by contacting plant owners and visiting the SHP stations. Samples of sediment were also collected from head works (natural stream) as well as at silt lushing outlets of several SHP stations. For settling basin, the eiciency of basins computed using Camp Dobbins curve, as well as Garde method were compared with eiciency observed at each desilting basin. he eiciency observed at site was by comparing grain size distribution (GSD) curve at inlet and lushing outlet. Similarly, for vortex settling basin the eiciency of basins was computed using Paul and Ather method, and was compared with the eiciency of desilting basin observed at site. Evaluation of Settling basin eiciency Eiciency computation using Camp-Dobbins method Camp-Dobbins curve used for silt removal eiciency is given in Figure 3. Where L is the length of the basin, D is the depth of the basin, w and u are settling velocity of particle and low velocity in basin, respectively, n is the regosity cofecient, and g is the acceleration due to gravity 9.81 m/s2. Using basin dimension, low, and sediment characterstics, the eiciency is assumed. Downloaded by [gurdeep singh] at 18:54 06 January 2016 Figure 4. Variation of k and η0 with w/u* (garde et al. 1990). loods. heir eiciency gets reduced and did not perform well. here are not many studies reported on performance evaluation of desilting basins on SHP stations. hus, the performance of these basins is evaluated, proper design is carried out, and corrective measures are undertaken so that sediment efects on SHP stations can be minimized. Eiciency computation using Grade method An inherent limitation of the Camp-Dobbins relationship is that it gives an eiciency of nearly 100% if the settling basin is very long, even if it has not been widened or deepened, to give a signiicant decrease in velocity (Cecen and Akmandor 1973). Grade designed a new relationship for assuming the sediment removal eiciency of settling basin (Garde et al. 1990), and is as follows: Methodology he performance evaluation of desilting basins has been carried out by studying a number of desilting basins constructed recently at SHP stations in the state of Himanchal Pradesh (a Himalayan state where sediment problem for power stations is 𝜂 = 𝜂0 (1 − e −kL D (1) ) Table 2(a). Settling basin features (Singh and Kumar 2013). name of name of Sr. no station stream 1 Tarila Tareila nallah 2 Tarila-ii Tareila nallah 3 Baner-iii Baner khad 4 Iku-ii Iku khad 5 upper Khauli khauli Khad 6 drinidhar Brahl Khad 7 aleo allain manali stream 8 Baragoan Sanjoin nallah 9 Sarbari Sarbari khad 10 Jirah Jirah nallah 11 Toss Toss nallah 12 Brahma- Bharamganga gana 13 gurahan gurahan Khud 14 Patkari Bakhli khad inlet channel Head discharge width (m) (cumec) (m) 184 6.00 2.0 installed capacity (mW) 2 × 2.5 Type of turbine francis 2 × 2.5 francis 133 6.10 2 × 2.5 Pelton 302 2 × 2.5 2 × 2.5 Pelton Pelton 2 × 2.5 desilting basin dimensions (m) u/S transition 10.4 depth 4.5 d/S transition 7.8 no. of outlet 1 Flushing conduit (mm) 600Ø length 54.7 Width 7.5 2.0 5.0 36.0 8.5 3.1 5.0 1 600Ø 2.70 1.8 7.5 30.0 5.0 2.5 1 500 × 500 362 430 3.96 2.34 1.8 1.8 7.7 8.0 52.0 51.5 5.0 4.5 2.5 4.1 4.0 4.0 1 1 500 × 500 500 × 500 Pelton 249 3.10 1.8 10.5 45.0 5.0 2.5 5.0 1 500 × 500 2 × 1.5 Pelton 290 1.60 1Ø – 25.0 5.0 2.0 – 5 300Ø 1 × 1.9 francis 170 4.25 2.3 18.0 62.0 6.0 3.0 – 1 1000Ø 2 × 2.25 Pelton 202 4.50 n.a – 43.9 9.4 4.1 – 4 300Ø 2×2 Pelton 348 1.31 1.2Ø 9.5 30.0 5.0 6.0 – 3 800Ø 2×5 Pelton 186 4.32 n.a – 45.0 7.0 3.0 – n.a n.a 2 × 2.5 Pelton 230 3.15 1.6Ø 20.6 40.0 7.0 3.0 6.0 1 500Ø 1 × 1.5 Pelton 216 1.10 1.2 4.5 40.0 3.0 1.0 4.5 1 300Ø 2×8 Pelton 375 5.83 3.5 10.0 54.0 7.0 3.5 – 3 500Ø 4 G. SinGH And A. KumAr Table 2(b). Vortex settling basin features (Singh and Kumar 2013). Sr. no 15 name of station gaj 16 Khauli 17 Bhuri singh name of stream gaj & leond Khauli Khad Sal nallah discharge (cumec) 6.93 inlet channel width (m) 3.4 diameter 17 475 3.19 2.4 13.72 5.14 3.4 installed capacity (mW) 3 × 3.5 Type of turbine Pelton Head (m) 213 2×6 Pelton 0.45 francis where η0 is the limiting eiciency obtained for a given w/u* at large values of L/D and k is a coeicient. he values of k and η0 are determined using the mean curves and their variation with w/u*is shown in Figure 4. Where u* is the shear velocity in the settling basin, w is the settling velocity in settling basin, and L and D are length and depth of basin, respectively. Downloaded by [gurdeep singh] at 18:54 06 January 2016 Evaluation of vortex settling basin eiciency Eiciency of vortex type settling basin is evaluated using relation given by diferent investigators and is described as follows: Computation of eiciency by Paul method he Eiciency computation relation given by Paul’s relation (Paul et al. 1991): ( ) Vs P = 98 + 0.92 log W (2) Figure 5. SHP stations studied in Himachal Pradesh, India (Singh and Kumar 2013). desilting basin dimensions (m) depth 2.05 Slope (1V:H) 01:10 no. of outlet 1 Flushing conduit (mm) 600Ø 12 2.00 01:10 1 450Ø 17 2.15 01:10 1 600Ø where P is the efficiency in (%), D t is the diameter of the tank, S c is the slope adopted at all project sites (10H:1V), V s is the settling velocity of the particle, W is the vertically upward velocity (W = (4Q s/πd 2), Q s is overflow discharge (Q i–Q 0), Q i is inlet discharge, and Q 0 is flushing discharge. Computation of eiciency by Ather method he eiciency computation method using M. Ather relation (Athar et al. 2002) is as follows:)] [( ) ( ) ( ) ( Zh Qw2 Qu 𝜔d , , , 𝜂0 = f (3) Qi hp 𝜈 gRT3 hp2 where Qi is discharge in the inlet channel, Qu is discharge flushed out through the under flow, ω is fall velocity of sediment, d is the sediment size, hp is the depth of flow at periphery of the chamber, Zh is the elevation difference between inlet and outlet channel beds at their junctions with iSH JournAl oF HydrAulic EnGinEErinG Tareila SHP 60% Efficiency from G.S.D Curve 40% 20% 0% 0.5 1 Particle size (mm) Efficiency in (%) 100% 80% 60% Efficiency from G.S.D curve 40% 20% 0% 0.5 1 Downloaded by [gurdeep singh] at 18:54 06 January 2016 Particle size (mm) Efficiency from Camp dobbins curve Efficiency from G.S.D curve Efficiency in (%) 80% 60% 40% 20% 0% 0.5 1 Efficieny from Garde et al method Aleo Manali SHP Iku-II SHP 80% 60% Efficiency in (%) 60% 40% 20% 0% 0 0.5 1 Efficiency from G.S.D curve 40% 20% 0% 0 Efficieny from Garde et al method Particle size (mm) 0.5 1 Particle size (mm) Efficieny from Garde et al method DrinidharSHP Efficiency from camp dobbins curve Efficiency from G.S.D curve 100% 80% 60% 40% 20% 1 Efficieny From Garde et al method 1 Efficiency from camp dobbins curve Efficiency from G.S.D curve Efficiency from Garde et al method 0% 0 0.5 Particle size (mm) Baragran SHP 100% Efficiency in (%) Efficiency from camp dobbins curve Efficiency from G.S.D curve 80% Efficiency from camp dobbins curve 100% (h) 100% 1 Efficieny from Garde et al method Particle size (mm) Particle size (mm) (g) 0.5 0 (f) 100% 0 20% 0% Efficiency From Garde et al method Upper Khauli SHP (e) Efficiency from G.S.D curve 40% (d) Efficiency from Camp dobbins curve 0 60% Efficiency from Garde et al method Baner-III SHP (c) Efficiency from Camp dobbins curve 80% Efficiency in (%) 80% 100% Efficiency in (%) Efficiency in (%) 100% 0 Tareila-II SHP (b) Efficiency from Camp doddins curve Efficiency in (%) (a) 5 80% 60% 40% 20% 0% 0 0.5 Particle size (mm) Figure 6. (a)–(n) Eiciency vs. particle size (mm) of settling basin at diferent SHP station (Singh and Kumar 2013). the vortex chamber, RT is the radius of the vortex chamber, g is the gravitational acceleration, v is the kinematic viscosity, and k is a coefficient. Evaluation of eiciency of settling/vortex settling basin observed data Eiciency of settling/vortex settling basin at project sites is evaluated by comparing the GSD curve at inlet and lushing outlet of the basin. Eiciency at project sites (Singh and Kumar 2013) is computed as follows:- 𝜂site = % of particular dia of particle at flushing outlet (4) % of particular dia of particle at inlet SHP stations for study Diferent SHP project stations of medium and high head with diferent designed discharge, diferent installed capacity in Himalayan region located mainly in Himachal Pradesh, India were selected and visited. Features of Settling basin and Vortex sediment settling basins are given in Table 2(a), and (b), Figure 3 shows the locations of SHP stations in Himachal Pradesh, India (Singh and Kumar 2013) (Figure 5). G. SinGH And A. KumAr Sarbari SHP (i) 100% Efficiency in (%) Jirah SHP (j) Efficiency from camp dobbins curve 80% Efficiency from G.S.D curve 60% 40% Efficieny from Garde et al method 20% Efficiency from camp dobbins method 100% 80% Efficiency in (%) 6 Efficiency from G.S.D curve 60% 40% 0% 0% 0 0.5 1 0 Particle size (mm) (k) (l) Tosh SHP Efficiency from camp dobbins curve 40% 20% Efficieny from Garde et al method 0% 0.5 1 Particle size (mm) 60% Efficiency from Garde et al method 40% 20% 0 0.5 1 Particle size (mm) Patikari SHP (n) 100% 100% Efficeincy from camp dobbins curve 80% 60% Efficiency in (%) Efficiency in (%) Efficiency from G.S.D curve 80% 0% Gurhan SHP Efficiency from G.S.D curve 40% 20% 0.5 1 Efficency from camp dobbins curve 80% 60% Efficiency from G.S.D curve 40% Efficiency from Garde et al method 20% Efficieny from Garde et al method 0% 0 Efficiency from camp dobbins curve 100% Efficiency from G.S.D curve 60% (m) 1 Bramganga SHP Efficiency in (%) Efficiency in (%) 80% 0 0.5 Particle size (mm) 100% 0% 0 0.5 1 Particle size (mm) Particle size (mm) Figure 6. (continued) Bhuri singh SHP (a) Efficiency from Ather Relation 60% 40% Efficiency from G.S.D curve 20% 0% 0.2 0.4 0.6 0.8 Efficiency from Paul relation 100% 80% Efficiency in (%) 80% 0 Gaj SHP (b) Efficiency From Paul RElation 100% Efficiency in (%) Efficiency from Ather Relation 60% 40% Efficiency from G.S.D curve 20% 0% 1 0 Particle dia (mm) 0.2 0.4 0.6 0.8 1 Particle dia (mm) Khauli SHP (c) Efficiency from Paul relation 100% Efficiency in (%) Downloaded by [gurdeep singh] at 18:54 06 January 2016 Efficiency from Garde et al method 20% 80% 60% Efficiency from Ather Relation 40% 20% 0% 0 0.2 0.4 0.6 0.8 1 Efficiency from G.S.D method Particle dia (mm) Figure 7. (a)–(c) Eiciency vs. sediment size (mm) of vortex sediment settling basin at diferent SHP station (Singh and Kumar 2013). iSH JournAl oF HydrAulic EnGinEErinG Results and discussion Comparative eiciency of settling basins he eiciency of settling basin for each SHP station computed using diferent methods described above is shown as Figure 6(a)–(n). From above, it is observed that actual eiciency of most of the settling basin is less than the eiciency computed using diferent relationships. Comparative eiciency of vortex sediment settling basins he eiciency of vortex sediment settling basins computed using diferent methods described above is shown in Figure 7(a)–(c). Downloaded by [gurdeep singh] at 18:54 06 January 2016 Conclusions Performance evaluation of desilting basins was carried out for a number of desilting basins in various locations of Himachal Pradesh, India. Following conclusions may be drawn for both types of desilting basin respectively. Settling basin (a) hough dimensions of the settling basin under study were suicient to trap the particles less than 0.2 mm (mostly recommended minimum size for removal), observed eiciency of settling basin was found to be less and may be attributed to:(i) Turbulence in water leading sediment in suspension. (ii) Transitions were not designed and executed properly. (iii) Inadequate sediment lushing arrangements. (b) hese settling basins mostly were designed for continuous lushing specially during monsoon which mostly time was not observed resulting in choking the lushing ducts and decreasing eiciency of the basins. Adequate design and execution of transitions as well as proper arrangement operation of silt removal of silt removal arrangements be ensured for eicient operation of settling basin (Singh and Kumar 2013). Vortex settling basin he observed eiciency of vortex settling basin at site is close to designed eiciency computed using diferent methods for all particles. Further, these basins required less volume of water to remove the sediment and prove to be economical and eicient desilting device, especially for the removal of particle size 0.1–0.2 mm (Singh and Kumar 2013). In settling basin, the length of the basin should be such that the particle will settle on covering that much distance, whereas in Vortex settling basin, the diameter of basin is kept that much so that sediment particles present in the low would cover a 7 helicoidal path towards the oriice, thereby obtaining a long settling length compared to the basin dimensions. By keeping discharge (Q), Sediment concentration (ppm) low velocity (v), settling velocity (w) constant for a given type of sediment particle the length of Basin (in case of settling basin) and diameter of basin (in case of vortex settling basin) will govern the eiciency of desilting basin. A comparative study by Badrinath and Bhardwaj “Vortex Desilting Tanks for Sediment Exclusion For small Hydro Projects” proceedings of design of hydraulic structures Dept. of civil egg. UOR, Roorkee pp. 283–291 (1994) concluded that vortex settling tanks are highly eicient, 55% cheaper and lesser land. As criteria for designing the most of desilting basin (under study) designed to remove 0.25 mm particle completely, but from the eiciency curves it has been shown that the most of desilting basins do not perform with 100% eiciency removal of 0.25 mm particle but removal eiciency of Vortex settling basin was found higher that settling basin. To achieve more accurate results, model studies should be carried out for a project having same parameters for both types of desilting Basin. hus, the comparative study of settling basin and vortex settling basin shows that vortex settling basin is eicient and economical desilting device as compared to settling basin for small hydropower stations. Acknowledgment he author express the gratitude to SHP plant owners for providing the details of their SHP stations and allowing the access to their SHP station for ield observation and investigations. he corresponding author is thankful to IITR administration and MHRD for providing assistantship for carrying out the study as Master of Technology dissertation work. Disclosure statement No potential conlict of interest was reported by the authors. References Athar, M.ISH, Kothyari, U.C., and Garde, R.J. (2002) “Studies on vortex chamber type sediment extractor.” ISH J. Hydraul. Eng., 8, 1–16. Camp, T.R. (1946). “Sedimentation and the design of settling tanks.” Trans. ASCE, 111, 895–936. Cecen, K., and Akmandor, N. (1973). “Circular settling basins with horizontal loor.” MAG Report No 183, TETAK, Ankara. Curi, K.V., Esen, I. I., and Velioglu, S.G. (1979). “Vortex type solid liquid separator.” Prog. Water Technol., 7(2), 183–190. Garde, R.J., Ranga Raju, K.G., and Sujudi, A.W.R. (1990). “Design of settling basins.” J. Hydraul. Res., 28(1), 81–91. Mashauri, D.A. (1986). “Modelling of a vortex settling basin for primary clarification of water.” PhD thesis, Tampere Univ. of Technology, Tampere, Finland. Paul, T.C., Sayal, S.K., Sakhuja V.S, and Dhillon G.S. (1991). “Vortexsettling basin design considerations.” J. Hydraulic. Eng., 117, 172–189. Singh, G., and Kumar, A. (2013). “Performance evaluation of desilting devices for SHP sites.” M.Tech dissertation, Alternate hydro energy center (AHEC), IIT Roorkee.