INTAKE An Intake is a hydraulic structure that is located at the head-works for diverting desired amount of water to the canals or tunnels or pipes for power generation or any other required purposes. It is the structure containing an opening (orifice) or a pipe for pressure flow.
Requirements of an Intake • Safe release of floods • Low-loss entrances of the power flow • Preventing bed load intrusion into the canal • Deflecting and flushing of bed load accumulated in front of the weir • Protecting the canal fro floating debris and ice, if any, carried by the watercourse
General Layout of Typical Intake Structure Major Appurtenances • Inlet sill protects against bed load at bottom • The skimmer wall with splitter piers in the inlet section checks the passage of floating debris and sheet into the canal • Coarse rack withholds subsurface trash and ice from the canal • Settling basin followed after intake section collects silts carried into the canal • De-silting (or flushing) canal serves the purpose of removing silt deposited in the settling basin at the canal sill • Intake gate (or canal headgate) controls inflow into the canal
Factors to be considered for intake selection 1. 2. 3. 4. 5.
Topographical features of the area Type of project: run-of-river or storage type Content and nature of sediment in the river Construction planning Compatibility and integrity of intake wit other headwork components 6. Geotechnical aspects of banks, beds etc 7. Ratio of quantity of diverted water to the residual quantity of water at low discharge
Components of a typical intake
Intake of Modi HPP
Intake of Seti Hydropower
Classification of Intake structure • Based on the type of project 1. Run of river intake 2. Canal intake 3. Dam intake 4. Tower intake 5. Submerged intake 6. Shaft intake(in case of pump storage projects) • Based on the location in river 1. Side intake(lateral intake) 2. Frontal intake 3. Bottom Intake (Tyrolean intake)- opening under the bed of the river
Lateral intake
1. Retaining structure 2. Lateral intake a. forebay/ pond b. side weir c. intake sluice d. sand trap Qo=total flow QA=diverted flow
Elements of weir
Tyrolean/ bottom intake
Comparison between lateral and bottom intake
Planning of Intake Structure • Arrangement must be chosen that the diverted amount of water is ensured at any region of the river • The peak discharge must be safely evacuated from the river and intake structure without damaging them • Always tend to design the simple and moderately priced construction (easy operation and maintenance) • As far as possible diverted flow should free of suspended particles • Intakes are generally located at the outer bend of the river in order to minimize the sediment amount. In case of straight river section or the bend is slight(α<30ᴼ), groins are arranged in the side opposite to the intake to increase the bending effect. • The most favorable site for the intake structure is somewhat downstream of the apex of the bend because the spiral flow is strongest here causing most of the bed load to be transported towards the inner bank.
Flow pattern in river
Lateral intake without damming and repelling of bed load from intake by technical measures
Lateral intake without damming and repelling of bed load from intake by technical measures
Lateral intake with damming and repelling of bed load from intake by technical measures
Schematic potential arrangement of elements of an intake structure
Emergency gate and sliding sluice
Example of an intake structure and flushing canal for bed load removal(QA>0.5.Q0)
Intake structure for a small HPP with sand trap and bed load removal(flushing canal)
Intake structure with bed load removal(flushing canal ) and spillway(side weir)
Design of Intake • Fixing the invert level in every part (invert level of the water at the river and approach canal specifically ) • Discharge in intake should be 10-20% greater than the design discharge due to flushing requirements and tentative losses • Design intake structure such a way that the entrance velocity is neither too small nor large V = 1 – 3 m/sec
Losses in Intakes • Entrance Loss The entrance loss comprises: 1. The loss due to change in direction of flow. 2 ℎ𝐿1 =
𝑣2 2𝑔
− 𝐶
𝑣𝑓 2𝑔
where, v = velocity in canal
vf= velocity of flow in main stream C = constant depending on the angle of diversion and ranging between 0.8 to 0.4 2. The loss due to sudden contraction of area at the diversion 𝑣2 ℎ𝐿2 = 𝐾 2𝑔 where, K varies between 0.03 for rounded entry to about 1.3 for sharp entry.
Rack Loss 1.
Kirschmer’s Formula 4
• ℎ𝐿𝑟 = 𝐾𝑟
𝑡 3 𝑏
∗
𝑣𝑏 2𝑔
sin 𝜑
Where, h Lr= rack loss Kr = a factor depending on the cross-section of bars t = thickness of rack-bars b = spacing between bars vb= velocity of flow in front of bars/rack φ = angle of bars with the horizontal The factor Kr has values as shown in table 12.1 ( 2ref. Dandekar Pg 273) 𝑣 • h Lr = 𝐾𝑡 2𝑔 and Kt= 1.45 -0.45R –R2 Where Kt = loss coefficient V = velocity through contracted opening R = ratio of net area through trash rack bars to gross area of the racks and supports Thus, for a typical value of R=0.65, the value of Kt= 0.74
Hydraulic Design of Intakes 𝒅𝒆𝒔𝒊𝒈𝒏𝒆𝒅 𝑸𝒐𝒓𝒊𝒇𝒊𝒄𝒆 = 𝑨 ∗ 𝑽𝒐𝒓𝒊𝒇𝒊𝒄𝒆 𝑽𝒐𝒓𝒊𝒇𝒊𝒄𝒆 = 𝑪 𝟐𝒈𝒉𝒐
C =coefficient of Discharge = Ci + Ct Where, Ci= inlet contraction coefficient Ct= coefficient for turbulence ho=hNWL-hcanal shape
Ci
Bell mouth
0.03-0.05
Slightly rounded 0.12-0.25 Sharp cornered
0.5
Cone angle(α) Ct 300 0.002 45ᴼ 60ᴼ
0.04 0.07
Sketch for hydraulic design of orifice
Numerical Example The invert level of the different components of intake are as follows: • River Bed Level = 2000m • NWL at river = 2001m • FWL/HFL = 2001.8 m • Sill level= 2000.3 m • Lower edge of orifice opening= 2000.2 m • Water level at canal(hcanal)=2000.6 m Ci = 0.6 for roughly (sharp) finished masonry Orifice(assume Ct=0 ) Qdesign = 0.5 m3/s V = 1.5 m/s Design an orifice for the intake with suitable dimensions.
Cont… Q orifice= Q design x 1.1 = 0.55 m3/sec Now, Qorifice = A x 1.5 [v=1.5 m/s] A = 0.367 m2 Take height of orifice opening as 20cm(hence upper edge of the orifice has invert level 2000.3 m) A=BxH L = 0.367/0.2 = 1.84 m Now, check the capacity of orifice ho= NWL – 2000.6 = 2001-2000.6 = 0.4 m Q𝒐𝒓𝒊𝒇𝒊𝒄𝒆 = 𝑪𝑨 𝟐𝒈𝒉𝒐 =0.6*0.2*1.84*√(2*9.81*0.4) =0.62 m3/s, OK
Trashrack Design for Side intake • Assume clear spacing = 50 mm to 200mm (Depending upon Length and width) • Design trashrack so the approach velocity (V0) lies between 0.6 m/sec to 1.5 m/sec Where, S = Total area of the submerged part of the screen Q = Rated flow V. = Approach velocity t= Bar width a= space between bars K1 = coefficient related to the partial clogging of the sreen (no automatic racker 0.2 – 0.3, automatic racker 0.4 – 0.85)
Losses through Trashracks
Where, hr = loss of head through rack, m t = thickness of rack bars, mm a = clear spacing between rack bars, mm v= velocity of flow through the trash rack, m/s a = angle of bar inclination to the horizontal (maximum 900 )
Assignment • Design side intake with coarse trash rack for a project in which river bed level is 3315.0m amsl, weir crest level (normal water level) is fixed to 3317.5 m amsl. From hydrological analysis and in for proposed headworks design condition (for given undersluice bed level and opening, weir length and height), the highest flood level in 100 years return period is 3320.83m amsl and flood level in 20 years return period is 3319.55m amsl. The canal water level is fixed as 3317.3m amsl. The turbine discharge of a project is 1.45m3/sec. Assume other suitable data for design.
References • Planning of intake structure, Helmut Lauterjung/ Gangolf Schmidt • Fundamentals of Hydropower engineering , Er. Sanjeev Baral
