International Journal of Emerging Technology and Advanced EngineeringWebsite: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 440 A Review of Desilting Basins Used in Small Hydropower Plants Gurdeep Singh 1 , Arun Kumar 2 1,2 M.Tech Student, Chief Scientific Officer, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand (India) -247667 Abstract - Small hydropower (SHP) is a renewable, efficient and eco friendly technology. Out of all the essential civil components, the desilting tank is one of the most vital part of SHP schemes, which ejects the sediment and foreign particles carried by water through the conductor system and protects the hydro mechanical equipments. With the passage of time, the desilting device has undergone modifications and updation as per the choice of developer or the geological condition and economic constrains. In this paper, attempt has been made to review the different types of desilting devices being deployed in SHP plants in the Himalayan region. Keywords-- Small Hydropower, Desilting basin, sediment, vortex settling basin. I. INTRODUCTION Small Hydropower (SHP) being most reliable and environmentally benign energy technology for electricity generation plays an important role in development of a region/nation. Major portion of hydropower potential in India lies in Himalayan region. The Himalayan rivers carry large amount of sediments (silt), which are harmful for the hydro mechanical equipment of SHP. One of the important components of SHP schemes is the desilting tank, which protects the hydro mechanical equipments from the harmful silt carried by the conducting system. Desilting tanks are used on water treatment plants and hydropower channels to remove objectionable sediment of a specified size and quantity [1].Though the problem of hydro-abrasion has emerged from „Alpine and Himalayas‟, what has added to it further is the recent „cost saving trend‟ towards smaller size faster machinery, designed to operate at higher heads. Such, damage is accentuated if the metallurgy of the runner blades is questionable and if the metal composition is less hard compared to sediment particles, when they are of quartz and feldspar. Abrasion of the runner caused by sediment laden water may in a very short operating period assume proportions seriously affecting the efficiency of the wheel and may even lead to eventual failure. Thus, during monsoon months, the Himalayan Rivers carry heavy sediment loads comprising boulders, gravel and sand as bed load and suspended load. Since coarser sediments cause excessive abrasion and aggravate cavitational affects on turbine parts, it is proposed that all the sediments coarser than 0.2 mm size be extracted from the water before it enters the headrace tunnel. To arrest the entry of larger size particles, 80 mm size trash-rack is provided at the power intake. For extracting smaller particles, from 80 mm to 0.2 mm, from the water entering the power tunnel, a sedimentation arrangement is provided[2]. When a canal receives sediment load in excess of its sediment transport capacity and effective measures are not taken for its control, the canal gets silted up. This results in a decrease in the discharge carrying capacity of the canal. In the case of power canals, that part of the sediment load, which is not extracted from the flow upstream of the power plant, passes through the turbines. The sharp edged silt/sand tends to damage the turbine runner blades/buckets due to abrasion, resulting in a decrease in the efficiency of the power plant. In India, it has been found in many cases that the turbines/pelton wheels have been considerably damaged after 2,000 to 3,000 h of operation because of the presence of sand in water. Turbines need to be repaired frequently causing shutdown of the units for considerable duration, thereby causing enormous loss of power and revenue. This review-paper focuses on the latest research and development desilting basin and the challenges of desilting basin are also summarized. The paper is organised as follows. Section II, of this paper describes classification of desilting devices. Section III, describes literature review of desilting devices. Section IV reviews the benefits and applications of desilting devices and finally conclusion has been concluded in section V. II. CLASSIFICATION OF DESILTING DEVICES Mainly two type of desilting tank used in SHP sites are:- (a)Settling basin (b)Vortex settling basin (a)Settling basin Settling basins are used on irrigation and hydropower channels to remove objectionable sediment of specified size and quantity. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 441 The channel is expanded into the basin by widening its width and lowering its floor through an expansion transition and restored back through a contraction transition at the end of the basin. Normally settling basins are constructed in compartments. However, single- chamber basins are not uncommon in the case of mini- and micro hydroelectric projects. The main aspect of their design to determine the dimensions, namely, length, breadth, and depth remains the same. (a) b) Fig 1.1 Definition Sketch: (a) Plan; (b) Section A-A[5] (b)Vortex settling basin This type of extractor makes use of vortex flow in a basin as the sediment removal device. A higher velocity flow is introduced tangentially into a cylindrical basin having an orifice at the centre of its bottom, which removes highly sediment concentrated flow. This along with tangential entry of flow causes combined (Rankin type) vortex conditions with free vortex forming near the orifice and forced vortex conditions forming in the outer region towards the periphery. Vortex flow causes a sediment concentration gradient across the vortex and a diffusive flux proportional but opposite to the centrifugal flux (Julian, 1986). 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. The sediment reaching the centre can be flushed out through the orifice outlet channel/pipeAs compared to the conventional type of settling basins and tunnel type sediment extractors, the vortex chamber type of sediment extractor has the advantage of smaller dimensions and low flushing discharge for obtaining a certain efficiency of sediment removal. Vortex-Settling Basin [10] III. LITERATURE REVIEW Nandana Vittal et al.[1] settling basin were formed by widening the approach channel and lowering its floor through an expansion transition, so as to reduce the mean velocity of flow into the basin. However, various combinations of width, depth, and length of the basin are possible to achieve desired removal efficiency in a given situation. Taking the cost of the straight and prismatic portion of the basin as the criterion, equations have been developed for its best width, depth, and length. R.H.A. Janssen [3] A numerical model for computing efficiency of sediment basins was presented and was compared to these methods. The model was solved using a spreadsheet and yields similar results to Camp‟s (1946) detailed analytical approach. The comparison indicates that when basins are sized using ideal settling theory with typical turbulence factors, up to 15% of the target sediment particles may not be removed. B. M. Sumer[4]The authors examined the results of a number of settling tests in a rectangular flume in order to derive a relationship for the efficiency (removal ratio) of settling basins, based on a dimensional analysis. The relation found by the authors qualitatively confirms solutions obtained by numerical simulations with the diffusion-advection equation. R. J. Garde et al.[5] Experiments have been carried out in the laboratory concerning the efficiency of settling basins. The data indicate that the existing methods of their design were not satisfactory. Analysis of all the available data has led to a new relationship for the efficiency. The parameters L/D and w/u, were found to govern the efficiency. where L was length of the settling basin, D was depth of flow in the settling basin, u * was shear velocity in the settling basin and w fall velocity of the sediment in clear water. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 442 Daniel Develay et al.[6] have been designed underground desilting basins on the basis of a theoretical approach further checked and developed using hydraulic model tests. The latter showed that with a design discharge of 123 m3/s a 240-m-Iong basin equipped with a 48-m diffuser and having a cross-sectional area of 170 m2 can satisfactorily meet the requirement. Keh-Cbia Yeb et al[7]developed a two-dimensional numerical model as well as the optimal regression equations for the determining settling basin dimension, and then simulated and compared the deposition efficiency of the selected settling basins. S. B. Weerakoon et al[8]presented a series of laboratory experiments carried out to investigate the effect of the entrance zone on the sand trapping efficiency of the desilting tanks using a scale model of a desilting tank with varying entrance expansion angles. The sand trapping efficiency was found to vary from 50% to 85% with the reduction of expansion angle from 30 o to 10 o . S.K Sharma[2]A detailed qualitative understanding has to be developed to deal with sedimentation problem in the Himalayan region. To start with, philosophy of abrasion index was proposed in this paper. Apart from design aspects, emphasis was ought for maintenance and monitoring. K. G. Ranga Raju et al.[9]Experimental investigations have been carried out on the sediment removal efficiency of settling basins. Laboratory data on removal efficiency from the present and earlier studies were first used for checking the accuracy of the existing empirical and analytical methods for determination of the sediment removal efficiency of settling basins T. C. Paul,et al[10].showed that the circular basin should have diameter equal to five times (as compared with six times stipulated in American practice) the bed width of inlet canal. The distinctive features of the proposed design were formation of free vortex in the outer region and flow in the basin traversing a relatively longer path before reaching the overflow weir. Mohammad Athar et al[11]Data from laboratories and field have been analysed for validation of the existing relationships on its sediment removal efficiency. Since the existing relations were not found to produce satisfactory results, a new relationship was developed for determination of the efficiency. Mohammad Athar et al [12] Experimental results on sediment removal efficiency of vortex chamber type sediment extractors were reported. A geometric configuration of the extractor is identified that is able to remove even the fine sediment (0.055<d<0.22 mm) from flow with high efficiency. Since the existing relations were not found to produce satisfactory results, a new relationship is developed for determination of the sediment removal efficiency of the vortex chamber type sediment extractors. Alired D. Mashauri [13] discussed the hydraulic performance of vortex-type settling basins both, with horizontal and sloping floor in the sediment removal problem for water treatment intakes, hydropower plants and irrigation schemes. Niknia, Naser et al.[14]It was found that the trap efficiency of the vortex settling chamber was about 88% for the median particle size of 0.35mm used in this study. To understand the flow structure inside the model under the deflector with clear water flow, the flow velocity in three directions was measured using ADV. Nguyen Quang Truong [15]Based on the models of vortex chamber from the earlier investigations, a deep- depth type vortex chamber was developed and studied in this paper.The experimental result also indicated that the values of η are considerably stable and reach the maximum value for the case of three deflectors, so this design was recommended for application. It can be concluded that the new design of deep type vortex chamber presented in this paper is feasible for removing very fine sediment particles. The experimental results also indicated that the values of η are considerably stable and reach the maximum value for the case of three deflectors, so this design was recommended for application. It can be concluded that the new design of deep type vortex chamber presented in this paper was feasible for removing very fine sediment particles Salakhov (1975) firstly started to study the vortex chamber by investigating a circulation chamber with tangential inlet and spilling weir outlet at the periphery. His design was applied to clarify water used for hydropower plant with rivers in mountains having steep slopes and high sediment concentration. Then, Mashauri (1986) improved some features based on the earlier designs. One of the most significant improvements was a horizontal divider or a deflector above the inlet port which acts as a flow separator to separate the flow so that the sediment removal efficiency is larger. Then, the study on sediment removal efficiency was also continued to be conducted with different designs of vortex chamber (Paul et al. 1991, Athar et al. 2002 and Keshavarzi and Gheisi 2006). shows a schematic diagram of a vortex chamber developed and used by Paul et al. (1991). However, previous studies were limited in a shallow-depth type vortex chamber that haves its chamber‟s height (H) smaller than its chamber‟s diameter (D). Hence, to improve the sediment removal efficiency of finer particle, we need to raise the depth of vortex chamber so that the residence time of sediment increases in the chamber. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 443 M. Athar et al.[16]In this paper an attempt has been made to study the distribution of suspended sediment concentration within the chamber of vortex type sediment Extractor. A satisfactory agreement was found to exist between the observed values of sediment concentration and its values computed using the method proposed. IV. DEVELOPMENT AND THE CHALLENGES OF DESILTING DEVICES One of the major problems confronting hydraulic engineers is the control of sediment entering irrigation and power canals. Methods of sediment control have been described by Huffered et al. (1975). To remove the sediment that has entered a canal, vortex tubes, tunnel- type sediment extractors, and settling basins are often used. Vortex-tube installations are very rare, presumably because of the no availability of a dependable design method. Vortex tubes are not so efficient in extracting suspended sediment, though the water abstraction ratio, Q 0 /Qc, is 10-25%. Here Q 0 is the flushing discharge and Qc is the inlet canal discharge. Trapping efficiency, P, of tunnel-type sediment extractors is about 40%, while Q D /Q C is 15-25%.. Settling basins perform reliably as long as the suspended sediment is larger than 0.06 mm. Velocity in the basin ranges from 0.08-0.45 m/s, while Q 0 /Qc is 0.5-3%. Conventional settling basins suffer from two main disadvantages: (1) Requirement of large dimensions; and (2) long residence time, t. A vortex- settling basin (VSB) is a fluidic device that uses only the vortices of the flow to extract the bed and suspended loads in the inlet canal. Principal features of VSB designs after Salakhov (1975), Cecen and Bayazit (1975), Ogihara and Sakaguchi (1984), and Mashauri (1986).The size of a VSB is very small, compared with conventional settling basins treating the same volume of water and sediment load (Cecen and Akmandor 1973). Thus the cost of construction of a VSB is just a fraction of the cost required for the construction of a classical settling basin to extract comparable particles (Mashauri 1986). The VSB structure holds promise as an economical, efficient, and water-conserving alternative to the other available sediment-extraction devices. Investigators have carried out a detailed investigation on the performance of vortex type sediment extractors of various configurations, with the object of determining their removal efficiency basin. Trap efficiency relationship of vortex settling basin proposed by various investigators given in table no.1 Table 1 Previously published relationships V. CONCLUSIONS AND DISCUSSION The main objective of this review paper is to give an overview in the development of desilting basin. Classification of desilting basin, development and the challenges of desilting devices and detailed literature review have been presented. The vortex chamber mainly composes of a cylindrical hopper, a bottom cone and a tangential inlet. This type of sediment extractor has overcome the disadvantages of conventional settling basins, i.e. the requirement of large dimensions and long residence time. The size of a vortex settling chamber is small, as compared with conventional settling basins treating the same volume of water and sediment. The problem associated with vortex settling basin is that physical model studies has to be carried out before its implementation at site. It is concluded that efficiency of vortex chamber is better than simple settling basin for same discharge. It has been suggested that vortex settling basin should be integral part of water conductor system carries the diverted discharge used where sediment problem is more prominent. Vortex settling basin can mitigate “Operation and Maintenance” problems face by Power Stations such as; 1. Damage to runner vanes of the turbines, 2. Wear of penstock, 3. Frequent choking of strainers, 4. Choking and puncturing of coolers tubes, 5. Damage to cooling water pumps, valves etc, 6. Frequent damage of turbine shaft seal, 7. Damage to drainage and dewatering system besides siltation of sumps, 8. Higher leakage through runner labyrinths resulting in high top cover pressure, International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 444 9. Damage to guide vane bushes and their cup seals, 10. Damage to seals of intake valve and main inlet valve, 11. Seating/Sealing problems in hydro-mechanical gates (intake as-well-as draft tube Gates). REFERENCES [1 ] Nandana Vittal and Mavendra Singh Raghav “Design Of Single- Chamber Settling Basins” Journal Of Hydraulic Engineering / May 1997/ pp 469-471. [2 ] S.K Sharma “Sediment Management in the Himalayan Rivers” HydroVision 2006 - Copyright HCI Publications, 2006 - www.hcipub.com pp 1-12. [3 ] R.H.A. Janssen “Analysis and Design of Sediment Basins” The Institution of Engineers, Australia 8th National Conference on Hydraulics in Water Engineering ANA Hotel Gold Coast, Australia 13-16 July 2004. [4 ] B. M. Sumer “Design Of Settling Basins” Journal of Hydraulic Research, 29:1, 136-143. (1991). [5 ] R. J. Garde , K. G. Ranga Raju and A. W. R. Sujudi “Design of settling basins” Journal of Hydraulic Research, vol 28:1, 81-91 (1990). [6 ] Daniel Develay, Jean Binquet, Divatia and C. R. Venkatesha “Desilting Basin System Of The Dul Hastihydroelectric Project” Journal Of Hydraulic Engineering october 1996 pp 565-572. [7 ] Keh-Cbia Yeb and En-Tian Lin “Efficiency Simulation and Design of Settling Basin”pp 655-666. [8 ] S. B. Weerakoon and U. S. Rathnayake “Effect of the Entrance Zone on the Trapping Efficiency of Desilting Tanks in Run-of- River Hydropower Plants” International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007 pp 1-6. [9 ] K. G. Ranga Raju, U. C. Kothyari, Somya Srivastav, and Manish Saxena “Sediment Removal Efficiency Of Settling Basins” Journal Of Irrigation And Drainage Engineering / September/October pp 308-314. [10 ] T. C. Paul,S. K. Sayal, V. S. Sakhuja, and G. S. Dhillon “Vortex- Settling Basin Design Considerations” J. Hydraul. Eng. 1991.117:172-189. [11 ] Mohammad Athar M.ISH , U. C. Kothyari and R. J. Garde “Studies On Vortex Chamber Type Sediment Extractor” ISH Journal of Hydraulic Engineering, vol 8:, 1-16 (2002). [12 ] Mohammad Athar, Umesh C. Kothyari, and Ramchandra J. Garde “Sediment Removal Efficiency of Vortex Chamber Type”Sediment Extractor” J. Hydraul. Eng. 2002.128:1051-1059. [13 ] Alired D. Mashauri “Removal Of Sediment Particlesby Vortex Basin” Aqua Fennica 13: 27-33.(1983). [14 ] Niknia, Naser, Keshavarzi, Ali-Reza, Hosseinipour, E. Zia “Improvement the Trap Efficiency of Vortex Chamber for Exclusion of Suspended Sediment in Diverted Water” World Environmental and Water Resources Congress 2011,Bearing Knowledge for Sustainability ASCE 2011 pp 4124-4134. [15 ] Nguyen Quang Truong “Effect Of Deflectors On Removal Efficiency of A Deep- Depth Vortex Chamber Sediment Extractor” HCMUT – 26-28/10/2011 pp 1-6. [16 ] M. Athar, U.C. Kothyari & R.J. Garde “Distribution of sediment concentration in the vortex chamber type sediment extractor” Journal of Hydraulic Research, 41:4, 427-438 (2003). [17 ] Cecen, K. (1977). "Hydrauliccriteria of settling basins for water treatment, hydropower and irrigation." Proc. 17th Congress of the Int. Assoc, of Hydr. Res., Baden-Baden, West Germany, 275-294 [18 ] Cecen, K., and Akmandor, N. (1973). "Circular settling basins with horizontal floor."MAG Report No 183, TETAK, Ankara, Turkey. [19 ] Salakhov, F. S. (1975). "Rotational designs and methods of hydraulic calculation of load-controlling water intake structures for mountain rivers." Proc. of Ninth Congress of the ICID, Moscow, Soviet Union, 151-161. [20 ] Sullivan, R. H. (1972). "The swirl concentrator as a combined sewer over-flow regulatory facility." Report No: EPA-R2-72-008, U.S. Environmental Protection Agency,Washington, D.