Lec7. Design Of Forebay-1.pdf

topic is linked with hydropower subject

1. 1

2. Design of Forebay
PRESENTED BY HADIQA QADIR
2K22-MS-HIE-02
CIVIL ENGINEERING DEPARTMENT (CED), UCE&T, BZU, MULTAN
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3. OUTLINE Of The Topic
Components Of Hydropower
Introduction Of Forebay
Function Of Forebay
Components Of Forebay
Hydraulic and Hydrological design aspects
Design Flowchart
Design Guidelines
Design Steps
Design Parameters
Design Case I
Design Case II
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4. Components of Hydro Power System
The scheme in which water is supplied to hydro power system has the following components.
Main parts:
1.Head work
2.Intake structure
3.Head race (canal)
4.Forebay
5.Head pond
6.Penstock
7.Power house
8.Tail race
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5. Introduction of Forebay
Forebay is a structure like a small reservoir located at the end
of the water passage from the reservoir and before the water is
fed to penstock.
We can define it as,
Animpoundmentimmediatelyupstreamofadiversiondam
orhydroelectricplantintake,wherewateristemporarily
storedbeforegoingintopenstock.
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6. Introduction of Forebay
A forebay is required in the case of run-of river plants at the upstream of
diversion work.
When canal leads water to the turbines the section of the canal in front of
turbines is enlarged to create forebay.
The reservoir acts as forebay when penstock takes water directly from it.
Some projects, such as those associated with a large dam having a deep
power intake, may have no specifically designed forebay. Again, other
projects may need more than one forebay; for example, a forebay at the
entrance to the headrace canal and a second forebay at the power intake at
the downstream end of the headrace canal
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7. Introduction of Forebay
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8. Introduction of Forebay
Theforebaytemporarilystoreswaterforsupplyingthesametothe
turbines.
Thestorageofwaterinforebayisdecidedbasedonrequiredwater
demandinthatarea.Thisisalsousedwhentheloadrequirement
inintakeisless.
Itformsthetransitionbetweenthereservoirorconveyancecanal
andthepowerintakeandassuchisdesignedtofacilitatethe
necessaryentryflowconditionsatthepowerintake.
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9. Components of Forebay
Entrance Bay or Basin/Tank Body
Escape weir/spillway
Fine Trash rack
Flushing gate
Water level control system
Penstock Inlet
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spillway

10. Flushing Gate
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11. Components of Forebay
Escape weir/spillway
Spillwayisconstructedtoactasasafetyvalve.Itdischargethe
overflowwatertothed/ssidewhenforebayisfull.Thepreferred
locationfortheescapeweirisintherimoftheforebaytank.Wherethis
isnotpracticalfortopographicreasonstheescapeweirshouldbe
locatedatthenearestsuitablesiteupstreamoftheforebaytank.
Asimpleoverflowweirisrecommendedwithadesignheadthatcanbe
containedwithinthenormalcanalfreeboard.Weirdischargeshouldbe
routedtowardsanaturalwatercourseofadequatecapacityoraditch
providedthatissuitablyprotectedagainsterosion.
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12. Components of Forebay
Fine Trash rack
Thefine trashracks are used to protect the turbine from small debris. They
can also, in some cases, prevent fish or eel to pass through it.
Flushing Gate
Theaccumulateddebrishastobeflushedoutfromtimetotime.Itisused
tofacilitateflushoutofanysedimentordebristhatmightsettleinthe
bottomoftheforebaytankandcanbedrawnintothepenstock.Thiscan
bedonebyopeningtheflushinggateorvalve.Inthismannerthematerial
isallowedtoreturntotheriverbed.
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13. Components of Forebay
Waterlevelcontrolsystem:
Awaterlevelcontrolsystemrequiresthatrealtimewaterlevel
measurementsintheforebaytankandtailracecanalbetransmitted
totheturbinegovernor.Inthewaterlevelcontrolmodethegovernor
willestimatetheinflowtotheforebaytankandadjustthewicket
gatestocorrectfordifferencebetweenturbineandcanalflowssoas
tomaintainforebaytanklevelswithinaprescribedrange.Afloat
typewaterlevelgaugewithelectronicdatatransmitterisusedfor
thispurpose.
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Wicket gateis are the series of adjustable
vanes used to guide water in turbines.

14. Functions of Forebay
Provideavolumeofstoredwatertopermitwaterlevelcontrolofturbineoperation.
Reducestheentryofairintopenstockpipe,whichinturncouldcausecavitation(explosionofthetrapped
airbubblesunderhighpressure)
Flowadjustment:theforebaytankandescapeweirfacilitatetheadjustmentofturbinedischargedueto
systemloadchangesbydivertingsurplusflowovertheescapeweirbackintotheriver.
Waterlevelcontrol:Forsmallhydroplantsconnectedtothegriditisconvenienttomatchturbineoutputto
availableflow,therebymaximizinguseofavailablewater.
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15. Hydraulic and Hydrological design
aspects
Fish exclusion
Flow characteristics including flow patterns and velocity distribution in the forebay, and particularly at the
approach to the power plant intake.
The submergence required at the intake and any depth requirements at sluiceways
Minimization of hydraulic losses
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16. Flow Chart to design forebay
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Submergenceheadistheminimumlevelof
waterrequiredabovethepenstockpipeto
preventtheentryofairintopenstockpipes
Retentiontimeisthetimeduringwhich
turbinewillbeshutdownandwaterwillbe
storedinforebay.Itistakenas3-4minutes.

17. PLAN & SECTION OF FOREBAY
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18. Design Guide Lines of Forebay
Thedesigndischargeforpowergenerationislinkedtotheturbineandthediameterofthepipeandthe
velocityoftheflowinthepenstocklinkedtothepenstockdesign.
Setthewidthoftheforebay.Asathumbrule,tostartthedesign,itcanbeassumedthatthelengthofthe
forebaywillbe2to2.5timesthissize.Howeveraccordingtotherequiredvolumeofwaterabovethe
penstockandtomeetthesiteconditionsthedesignermayhavetochangethisratio.
Settheclearanceofthepenstockfromthebottomoftheforebay.Thisistoavoidthatparticlesand
sedimentssettledintheforebaygetinthepenstock.Theminimumclearanceis0.30m.While,0.50to1m
isacommonandreasonablevaluetouse.
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19. Design Guide Lines of Forebay
Forthedesignoftheforebayitisconsideredthatitshouldbeavailable(tocopewiththeflowvariationsin
theturbineduringnormaloperationconditions)abuffervolumeequivalentto15secondsofsupplyatthe
designflow.Thisisalsoautomaticallycalculatedaswellasitscorrespondingdepthofwater.
Setthedischargercoefficientofthespillway.Thisdependsontheshapeofthecrestandspillway.
Setthedepthofwateroverthecrestofthespillway.Haveinmindthatthehigherthewaterdepththe
smallerwillbethecrestlengthofthespillway.Thespillwayisdesignedtospillallthedesignflowin
casethepowerhouseisshutdownandnoflowisgoingthroughthepenstock.
Setthefreeboardforthespillway.Thisisasafetymarginincasehigherflowsthanthedesignflowarrive
totheforebay.Consideravaluethatishalfthewaterdepthovertheweircrest.Thiswillincrease,if
necessary,in50%ofthedischargecapacityofthespillway
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20. Design Guide Lines of Forebay
Thelengthofthespillwayisautomaticallycalculatedusingthepreviousinputdataandtheweirequation.
Ithastobenotedthatthisvaluemustbesmallerthanthelengthoftheforebay.
Notethatthecrestofthespillwaywillbeplaced0.05mabovetheNormalWaterLevel(NWL)sothat
smallchangesinthefloworfluctuationsorturbulenceinthewatersurface(likewind)donotcausean
immediatespilling.
Thespillwayshouldbesizedsuchthatitcanreleasetheentiredesignflowwhenrequired.Thisisbecause
iftheturbinevalveisclosedduringemergencies,theentiredesignflowwillhavetobespilledfromthe
forebayuntiltheoperatorreachestheintakeorothercontrolstructuresupstreamoftheforebay.
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21. Design Guide Lines of Forebay
Thetrashrackattheforebayshouldbeplacedat1:3slopeforbothefficienthydraulic
performanceandeaseofcleaning.
Tominimizeheadlessandblockage,therecommendedvelocitythroughthetrashrackis0.6m/s.
butamaximumof1m/scouldbeused.
Settheangleofthetransitionwallsattheentranceofthepenstock.Thistransitionmustbe
smoothsomildanglesarerecommended(around20%).Howevertofitsiteconditionsthevalues
canbeadjusted.
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22. Design Steps of Forebay
Calculate
discharge
through
forebay
Assume
detention
time if not
given
Calculate
volume of
forebay
Assume
free
board
and
settling
Height
Calculate
submerge
nce head
above
penstock
Calculate
total
depth of
forebay
Calculate
width of
forebay
for normal
and worst
case
Calculate
length of
forebay
for total
head
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23. FOREBAY DESIGN PARAMETERS
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Design Discharge = Qd
Forebay Discharge= Qf = 2(Qd)
Volume Of Forebay = V = Qf x t x 60
Detention time = t (3-4 minutes)
Limiting Velocity In Forebay = Vf (0.2-0.6 m/s)
Submergence Head = Hs, Hs >1.5
????????????
2
2�
Or 0.5 Vp ????????????
0.5
Diameter of Penstock = Dp or ??????
Total Head/ Depth Of Forebay = H= H-drawdown + Hs + dia of penstock
+ Freeboard + minimum bottom height H
Total Head in worst condition = Hw = Hs + dia of penstock
H-drawdown/H-downsurge = Vp
??????????????????
�??????�
, but we will take it 0 in our design examples

24. Design Case-1
Design a forebay with Design discharge of 2mᶟ/s, flow is carried into a penstock of diameter 1.5m. Limiting velocity is
0.3m/s.
Solution
Step1: Discharge through the forebay = Qf =2(Qd) = 2x2=4 mᶟ/s
Assume detention time, t = 3 min
Step 2: Volume of forebay = V = Qf * t*60 = 4*3*60 =720 mᶟ
Step 3:Determination of Total Head/depth of forebay = H-downsurge + Hs + Diameter of Penstock + Freeboard +
minimum bottom height
take Free board = 1m Settling height/Bottom height = 1m,
Diameter of Penstock = 1.5m (given)
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25. Solution
For Submergence Head, Hs
Hs >1.5
????????????
�
�??????
Or 0.5 Vp ????????????
�.??????
(Select Greater Value)
Vp = Velocity In Penstock,
We Know that Q=A*V ; Area Of Penstock = (????????????
2
)/4 = ??????*1.5
2
/4 = 1.766m²
Vp = Qd/Ap = 2/1.766 =1.1325 m/s
Then, Hs is
=(1.5 *1.13^2)/(2*9.81)=0.0977m
Or
= 0.5*(1.13)*(1.5)^0.5=0.69m
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26. Solution
Taking Greater Value,
Hs= 0.69m ≈ 0.7m
Also, assume H-downsurge = 0
Total Head/depth of forebay = H-downsurge + Hs + Diameter of Penstock + Freeboard + minimum bottom
H= 0+0.7+1.5+1+1 = 4.2m
Step 4: Determination of Forebay Width
For normal case B= Qf/(H*Vf) = 4/(4.2*0.3) =3.175m
Also assumed Vf = 0.3 m/s
For worst case
Total Head against worst cond= Hw = Hs + diameter of Penstoke = 0.7+1.5 = 2.2m
B՛= Qf/(Hw*Vf) = 4/(2.2*0.3) = 6.06 m (Select greater value of B)
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27. Solution
Step 5 Determination of Length of forebay against worst case
L= volume/(Bw*H) = 720/(6.06*2.2) = 55m
Dimensions of forebay = (55m*6.06m*4.2m)
Step 6 Determination of Spillway Length (Ls)
As we know that discharge eq for spillway is
Qf = Cd*L*(????????????)
�.??????
Take head over crest of spillway, H-spillway = 0.5m, Cd = 1.7
So, Ls = Qf/(Cd*(????????????)
�.??????
) = 4/(1.7*0.5^1.5) = 6.65m ≈ 7m
L-spillway < L-forebay
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28. Solution
Step 7: Design Check
Check 1 Since, L-spillway < L-forebay (OK)
Check 2: Let’s check for limiting velocity, for which forebayand
settling depth are not considered,
Vf = Qf/(Bw*Hw) = 4/(6.06*2.2) = 0.3 m/s (OK)
Note:Here“V”whichwecalculatedistheaveragehorizontalflow
velocityofthewaterinsidetheforebayafterthewaterentersinthe
forebay.Itisthevelocityofwateralongthelengthofforebay.
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29. Design Case-2
Design a forebay with turbine discharge 12 m3/s. Water conveyed from forebay to powerhouse by 2
penstock of 2m diameter each. The retention time is 3 minutes and limiting velocity is 0.2m/s.
Solution
Qd = 12 mᶟ/s
Diameter of penstock = 2m
Discharge in each penstock = qd= 12/2= 6mᶟ/s
Discharge for forebay = 2* Qd = 2*12 = 24mᶟ/s
Given retention time, t = 3 min
Area of penstock = Ap = (pi*2^2)/4 = 3.1415 m²
Velocity in penstock, Vp = qd/ap = 6/3.1415= 1.91m/s
Given Limiting velocity (Vf) = 0.2m/s
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30. Solution
Volume of forebay (V) = Qf *t*60
= 24*3*60 = 4320 mᶟ
For height (H)
take Free board = 1m Settling height = 1m,
And submergence head, Hs
Hs >1.5
????????????
2
2&#3627408468;
Or 0.5 Vp ????????????
0.5
(Select Greater Value)
Then, Hs is =(1.5 *1.91^2)/(2*9.81)=0.278m Or = 0.5*(1.91)*(2)^0.5=1.47m
Take greater value, Hs = 1.47m
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31. Solution
Total head/depth of forebay = H-downsurge + Hs + diameter of penstock + freeboard +minimum bottom
height
H = 0+1.47+2+1+1 = 5.47m
For Width,
For normal case, B = Qf/(H*V) = 24/(5.47*0.2) = 21.937m
For worst case,
Total head against worst case= Hw = Hs + diameter of penstoke = 1.47+2 = 3.47m
B՛= Qf/(H*V) =24/(3.47*0.2) = 34.5 m
For length
L= volume/(Bw*H) = 4320/(34.5*3.47) = 36m
Dimensions of forebay = (36m*34.5m*5.47m)
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32. Solution
For Length of spillway (Ls),
Qf = Cd*L*????????????
&#3627409359;.??????
Take head over crest for spillway= Hc= 0.5m
Coefficient of discharge = Cd = 1.7
Then, Ls = Qf/(Cd*????????????
&#3627409359;.??????
) = 24/(1.7*0.5^1.5) = 9.98m ≈ 10m
Since, L-spillway < L-forebay (OK)
Let’s check for limiting velocity, for which forebayand settling depth are not considered,
V = Qf/(Bw*Hw) = 24/(34.5*3.47) = 0.21 m/s > 0.2m/s (OK)
Note: Here “V” which we calculated is the average horizontal flow velocity of the water inside the forebayafter the
water enters in the forebay. It is the velocity of water along the length of forebay.
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33. Air Vent Pipe at inlet of penstock

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34. References
BPC Hydroconsult. (2012). Civil Works Guidelines for Micro-Hydropower in Nepal.
Chow, V. T. (1959). Open-Channel Hydraulics. McGraw-Hill.
Micro/Mini Hydropower Design Aspects, Micro/Mini Hydropower Design Aspects -Pakistan
Poverty ...www.ppaf.org.pk › hre› publications › vol 11 mhpdesign-final-05-12-13, Carlos Martins,
AjoyKarki, Ulrich Frings, 2013.
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