REview of Desilting Basin

8/10/2019 REview of Desilting Basin

1/5

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)

440

A Review of Desilting Basins Used in Small HydropowerPlants

Gurdeep Singh1, Arun Kumar

2

1,2M.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 undergonemodifications 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 aregion/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 problemof 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 hardcompared 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. Forextracting 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 notextracted 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, ithas been found in many cases that the turbines/peltonwheels 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 applicationsof 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.


2/5



441

The channel is expanded into the basin by widening its

width and lowering its floor through an expansiontransition 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 centrifugalflux (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 bywidening 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 usinga spreadsheet and yields similar results to Camps (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 anumber 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 thediffusion-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* wasshear velocity in the settling basin and wfall velocity of

the sediment in clear water.


3/5



442

Daniel Develay et al.[6] have been designed

underground desilting basins on the basis of a theoreticalapproach 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 adesilting 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

30oto 10

o.

S.K Sharma[2]A detailed qualitative understanding

has to be developed to deal with sedimentation problemin 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 investigationshave 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 fordetermination 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


4/5



443

M. Athar et al.[16]In this paper an attempt has been

made to study the distribution of suspended sedimentconcentration 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,

Q0/Qc, is 10-25%. Here Q0is the flushing discharge andQc is the inlet canal discharge. Trapping efficiency,P, of

tunnel-type sediment extractors is about 40%, while

QD/QC 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

Q0/Qc is 0.5-3%. Conventional settling basins suffer

from two main disadvantages: (1) Requirement of largedimensions; 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 basinto 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 literaturereview 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 largedimensions 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 beforeits 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 moreprominent. Vortex settling basin can mitigate Operation

and Maintenance problemsface 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,


5/5



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 RiversHydroVision 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 onHydraulics 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. VenkateshaDesilting 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 andDesign of Settling Basinpp 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 ChamberTypeSediment 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 forExclusion 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 RemovalEfficiency 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 extractorJournal 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 basinswith 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.

Documents

REview of Desilting Basin