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BrowseUploadSign inJoinBooksAudiobooksComicsSheet MusicWelcome to Scribd! Start your free trial and access books, documents and more.Find out moreDRAFT FEB 07, 2008STANDARDS / MANUALS / GUIDELINES FOR SMALL HYDRO DEVELOPMENT
1. head works and intake. feeder canal.
AHEC/MNRE/SHP Standards/ Civil Works . tailrace canal below the turbine and related ancillary works.
1. desilter (if required). forebay tank. To decant relatively clean surface water into the intake. Sufficient volume must be provided to support these functions. etc). To safely discharge the design flood without causing unacceptable upstream flooding. There are three types of head works that are widely used on mini and small hydro projects. Control of sediment. penstock and surge tank (if required) up to the entry of the turbine.1. This reservoir may be used to provide daily pondage in support of peaking operation or to provide the control volume necessary for turbine operation in the water level control mode.1
Head Works with Lateral Intakes (Small Hydro) Head works with lateral intakes are typically applied on rivers transporting significant amounts of sediment as bed load and in suspension. This system includes.
1. This latter case would apply where the penstock draws its water directly from the head pond. pipelines. The functions of the head works are: Diversion of the required project flow from the river into the water conductor system. power canal or alternative conveyance structures (culverts. diversion dam. Flood handling. tunnels. To arrest floating debris at intake trashracks for removal by manual raking.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 1 . The functional objectives are: To divert bed-load away from the intake and flush downstream of the dam (the bed load flushing system should be operable in both continuous and intermittent modes).
GUIDELINES FOR HYDRAULIC DESIGN OF SMALL HYDRO PLANTS This section provides standards and guidelines on the design of the water conductor system. as below: Lateral intake head works Trench intake head works Reservoir / canal intakes Each type will be discussed in turn. Typically a head pond reservoir is formed upstream of the head works.1
HYDRAULIC DESIGN OF HEAD WORKS In general head works are composed of three structural components. intake and bed load sluice.
1. Ideal site conditions are rare.3 Site Selection: Selection of the head works site is a practical decision which involves weighing of several factors including hydraulic desiderata (Section 2. mini hydro) (data on suspended sediment loads) . 1.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 2 . rating curves at diversion dam) Topographic mapping of the site including river bathymetry covering all head works structure sites. foundation conditions.1.1.H-Q Curves (W. The following guidelines assume head works are located on a straight reach of a river. For important projects or unusual sites hydraulic model studies are recommended.1/1. thus design will require compromises between hydraulic requirements and constraints of site geology.2.4 Determination of Key Elevations:
AHEC/MNRE/SHP Standards/ Civil Works .Qp (plant flow) .3. Typical layouts are shown in Figures 2.0). head optimization. Satisfactory foundation conditions. Given the importance of intake design to the overall performance of the plant it is recommended that an experienced hydraulic engineer be consulted during studies on head works layout. Site geology report.L. The intake should be located at the head of a steeper section of the river.Cw .2. The following data are required for design: Site hydrology report as stipulated in Section 1.Q100 (design flood flow. accessibility and constructability factors.The following site features promote favourable hydraulic conditions and should be considered during site selection: The intake should be located on the outside of a river bend (towards the end of the bend) to benefit from the spiral current in the river that moves clean surface water towards the intake and bed load away from the intake towards the centre of the river.Q10 (design flood flow. small hydro) .3 of this Standard giving: . 1. A step by step design approach is recommended and design parameters are suggested for guidance in design and layout studies.2. accessibility etc.2 Data Required for design.1 to 2.1. This will promote removal of material flushed through the dam which may otherwise accumulate downstream of the flushing channel and impair its function.
say 0.5 m (= 1.77 m. Deflection of the main current by the flushing channel sill and divider wall has a further beneficial effect of establishing (to
AHEC/MNRE/SHP Standards/ Civil Works .28 m) NOL = Z0 + ye + yd + H NOL = 97.S (= 0.1. The orientation of the intake face depends on river bank topography.08 m. say 7. When this angle becomes too large the intake will attract excessive amounts of sediment and floating debris.792. for straight river reaches the recommended values for tilt vary from 10o to 30o depending on the author.0 m3/s (= 0.5 yo or 1.28 + 1.2 A0 = Q ÷ V0 (= 12.38m.5 Q0.7m) Crest of weir or head pond NOL = 101.0 m) ye = greater of 0. Intake hydraulics are enhanced if the intake face is slightly tilted into the flow.5 m2) A0 (= 1. 1. This is usually achieved by releasing major flows via large spillway gate/s in mid-river.5 + 1.80 + 0.80 m/s) Determine V0 = 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 3 . It is also recommended that the main river current be maintained in mid stream beyond the intake and bed load flushing structures.80 m) H= 4 Assume L = 4H (= 7. say 101.80 (=101.50 m) Sill = NOL – H (= 99.80m) yd = L.5 m Height of weir = 4.For the illustrative example: Qd = 10.0 m These initial key elevations are preliminary and may have to be adjusted later as the design evolves. say 1.5 Head Works Layout The entry to the intake should be aligned with the river bank to provide smooth approach conditions and minimize the occurrence of undesirable swirl. A guide wall acting as a transition between the river bank and the structure will usually be required.
1) the axis of the intake = 105° & Qf = 2. Weir..Lw.0 m ∴ W = 4.5W) Yo = normal flow depth as shown in 2. Assume flushing flow equal to twice project flow then estimate the width and height of the flushing gate from orifice formula.0 = 0. Assume gate W/H ratio = 1:2 H = 4.8 m (say 3.5 W2 ∴ W = 2. Divider wall should be completely “cover” 80% to 100% of the intake.. the following design data are assumed (see Figure2.S1. Select this water level as the MFL..20 m.70 .: Qf = 0.6 ×0. This provides a flood surcharge (S) of 1. W.6 Flood Handling.sill on slab at river bottom.2.. (MFL .80 .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 4 .5 Qweir = Cw.2): Design flood.0 m
AHEC/MNRE/SHP Standards/ Civil Works .2.0 = 20m3/s ∴ 20. Cw = 1. This effect is more pronounced when spillway flow is much greater than flow entering the intake and flushing channel (Nigam pages 357 – 365).0 m.50 (= 103.0×10.2.1/2.6 ×0.. 1.1. For small hydro a simple overflow diversion weir would be the preferred option if flood surcharge would not cause unacceptable upstream flooding.0m) Qgate = Cw. In the sample design (Fig.1.8 (say 5..0m) and H = 1.ZS)1. = NOL + 1.5 Capacity check for MFL = 103.5 m.some degree) curved flow paths away from the intake.2. it is recommended that the following parameters be used for layout: Axis of intake should be between 105° to axis of river (tilt = 15o).. Cw = 1.0 Sill should be straight and perpendicular to the flow direction. For development of the head work plan. For purpose of illustration. A spur on the bank opposite the head regulator may enhance the curvilinear flow but its design cannot be done reliably without a hydraulic model study. MFL and Number of Gates.-ogee profile.1.. Q100 = 175 m3/s A review of reservoir topography indicated that over bank flooding would occur if the flood water level exceeded 103. Assume weir coefficients as below: Gate.5W2 Where: Qf = flushing flow W = gate width H = gate height (= 0.0 m) MFL.
3. of Length of Overflow QG Gates Section (m) (m3/s) 0 35.3 of this Standard.3 of this Standard. Plains Rivers: Stability of structures founded on alluvial foundations typical of plains rivers.20 0.8 68.0 and shall be reduced by the following safety factors: Types of foundation Shingles / cobbles Coarse sand Fine sand Safety factor 5 6 7 Allowable Exit Gradient 0. Stability and stress design should be done in accordance with Section 2. Chapter 9.0 1 29.6
QT (m3/s) 82. QW.Section 9.5s-1) QG.6 Therefore one gate is sufficient.8 178. 1983) explain determination of uplift pressure distributions and exit gradients. Further details on structural aspects of design are given in Section 2.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 5 . weir and total flows
QW (m3/s) 82. Design of diversion weirs and barrages on permeable foundation should follow IS 6966 (Part 1).0 109. Where: MFL = Maximum flood level (m) NOL = Normal operating level (m) S = flood surcharge above NOL (m) W = width of gate (m) H = height of gate (m) ZS = elevation of gate sill (m) Cw = weir coefficient (m0. 1. Sample calculations in Chapter 12 of “Fundamentals of Irrigation Engineering” (Bharat Singh.8 (USBR 1987).5 m or greater use of an ogee profile should be considered. QT = gate.143
Also barrages or weirs on plains rivers will normally require stilling basins to dissipate the energy from the fall across the dam before the water can be returned safely to the river.0 0.3.2 >175
The flow capacity of the sediment flushing gate may also be included in calculating flood handling capacity.
AHEC/MNRE/SHP Standards/ Civil Works . is governed by the magnitude of the exit gradient.7 Diversion Dam and Spillway Hydraulic Design of Weir: For weir having design heads of 0.1. A suitable methodology is provided in “Design of Small Dams”.167 0. The critical gradient is approximately 1.No.
For initial design a width to height ratio of 2:1 for the flushing gate is suggested. Also the beds of mountain rivers are often boulder paved and are much more resistance to erosion than plains rivers.44 6 / 7 q Where: io = critical scouring velocity d = sediment size q = flow per unit width (m3/s per m) Verify that flow through pocket in continuous flushing mode (Qs = 3Qs) will be sub critical.1. For low structures height less than 2. if not lower entrance sill elevation further.50 io d 9/7 i0 = 0. Determine slope of channel to provide the required scouring velocity. Establish entrance sill elevation and channel slope assuming an intermittent flushing mode (intake closed) with Qs = 2Qp. for this calculation). Stability and stress design shall be in accordance with requirements of Section 2.5m below NOL. In case of diversion weir without gates assume sediment accumulation to be level with the weir crest.Mountain Rivers: Bedrock is usually found at relatively shallow depths in mountain rivers permitting head works structures to be founded on rock. 1. (Assume continuous flushing with 3×Qp entering the pocket. The head works structures would be designed as gravity structures with enough mass to resist flotation.3. Increase the above theoretical gate height by 0.20) and a reservoir operating level 0.9 Intake/Head Regulator:
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 6 . using the following formula which incorporates a safety factor of 1. Determine height of gate and gate opening based on depth of flow at gate location and corresponding gate width.25 m to ensure unrestricted open channel flow through the gate for intermittent flushing mode and a flushing flow of 2 Qp. Therefore there may be no need for a stilling basin.0 m anchors into sound bedrock may be used as the prime stabilization element in dam design. The engineer may consider impact blocks on the downstream apron or simply provide an angled lip at the downstream end of the apron to “flip” the flow away from the downstream end of the apron.3 of this Standard.5: i = 1. A cut-off wall to bed rock of suitable depth should also be provided for added protection against undermining by scour. critical flow at the sill. supercritical flow downstream (FN ≥1. 1.8 Sediment Flushing Channel The following approach is recommended for design of the flushing channel: Select flushing channel flow capacity (Qf) = 2×Qp Estimate maximum size of sediment entering the pocket from site data or from transport capacity of approaching flow and velocity.1.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 7 .5 m/s per linear m.20 Ve = 0. 2g ⎝b⎠ Where: Kf = head loss factor (= 2. If sediment loads are very high consider installing a vortex silt ejector at the downstream end of the transition. Bharat Nigam. V should not exceed 1.. wood on other floating objects. Friction losses can be omitted as they are negligible: V 22 Calculate form losses as: H L = 0. branches.1. Trashrack detailed design should be in accordance with IS 11388.5 Q p m / s For trashracks that are manually cleaned.42 assuming rectangular bars) T = thickness of bars (mm) B = clear bar spacing (mm) β = angle of inclination to horizontal (degrees) V = approach velocity (m/s)
1. • Height of sill above floor of flushing channel (ye) = greater of 1. • Provide coarse trashracks to guard entry to the head gate.. A clear spacing of 150 mm between bars is recommended. Calculate trashrack losses as:
V2 ⎛t⎞ H L = K f ⎜ ⎟ . Check that the flow velocity in the transition is adequate to prevent deposition in the transition area.3 2g Where: V2 = velocity at downstream end of contraction.In intake provides a transition between the river and the feeder canal.pages 357 to 365 Nem Chand & Bros.Guidelines Recommendations for Design of Trashracks for Intakes Design of Small Dams Fundamentals of Irrigation Engineering Nem Chand & Bros. • The invert of the feeder canal shall be determined taking into consideration head losses through the trashrack and form losses through the structure.0 m/s. Convergence of side walls 2. The main design objectives are to exclude bed-load and floating debris and to minimize head losses.5:1 with rate of increase in velocity not exceeding 0.Roorkee (1985)
AHEC/MNRE/SHP Standards/ Civil Works . P.10 References on Lateral Intakes and Diversion Weirs. The following parameters are recommended: Approach velocity at intake entrance (on gross area) 0. • The floor of the transition should be sloped down as required to join the invert of the feeder canal.-Roorkee (1983) Handbook of Hydroelectric Engineering (Second edition) …. IS Standards Cited: IS 6966 (Part 1) IS 11388 USBR (1987) Singh.
Hydraulic Design of Barrages and Weirs .S.Sinβ .5m or 50% flow depth. The trashrack would be designed to step floating debris such as trees.
0 QP) Spillway design flow (SDF) = Q10 Where: Q10 = flood peak flow with ten year return period.A.2. M. Hydro ’88/3rd International Conference on Small Hydro. A typical layout is shown in Figure 2.0 m Assumed flushing gate W/H = 2. Razvan.1.
Guidelines for the Design of Intake Structures for Small Hydro Schemes.2. A. Cancun – Mexico. 2 gY1 Y1 = HD for design condition Where: W = width of gate (m) H = height of gate (m) Yi = upstream depth (m)
AHEC/MNRE/SHP Standards/ Civil Works . determine H from orifice equation.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 8 .2. as below: Q f = 0.20 QT) m3/s QF = gravel flushing flow (= 2. IAHR Monograph. This challenge is addressed by adoption of less rigorous standards and the application of simplified designs adapted to the skills available in remote areas.
SEMI PERMANENT HEADWORKS (MINI HYDRO) For mini hydro projects the need to minimize capital cost of the head works is of prime importance.1 Design Parameters Hydraulic design should be based on the following design criteria: Plant flow (Qp) = QT + QD Where: QT = total turbine flow (m3/s) QD = desilter flushing flow (= 0.3. E.53× 2 H 2 . Balkema – Rotterdam (1992) River Intakes and Diversion Dams Elsevier.
1. 1. Mobile Barrages and Intakes on Sediment Transporting Rivers.2. Amsterdam (1988)
1.2 Layout Intake approach velocity = 1.1.20 QT) m3/s QFC = feeder canal flow (= 1.0 m/s Regulator gate W/H = 2 Flushing channel depth (HD) = 2H + W/3 Flushing channel minimum width = 1.11 Other References: Bucher and Krumdieck Bouvard. This issue poses the greatest challenge where the head works have to be constructed on alluvial foundations.
A characteristic of trench intakes is that they have minimum impact on river levels.HD
Select the next largest manufactures standard gate size above the calculated dimensions. the stability of the weir cross-section design should be checked for flotation. The spacing between racks is selected to prevent entry of bed load into the trench. for flatter slopes diversion weirs should be considered.5 m and the rise over run ratio should not less than 1/3. 1.3. The trench intake should be located in the main river channel and be of sufficient width to collect the design project flow including all flushing flows. when referring to trench intakes on • mountainous streams. Trench weir. over turning and sliding in accordance with Section 2.3 Weir Determine weir height to suit intake gate and flushing gate dimensions. Trench intakes are applied in situations where traditional headwork designs would be excessively expensive or result in objectionable rises in river levels. The following terms are sometimes used in referring to trench intake designs. • Tyrolean or Caucasian intakes. 1.3 TRENCH INTAKES Trench intakes are intake structures located in the river bed that draw off flow through racks into a trench which conveys the flow into the project water conductor system.1/4.3. There are two quite different applications: on wide rivers and on mountainous streams.1 of this Standard.2. For weirs founded on permeable foundations the necessary structure length to control failure by piping should be determined in accordance with Section 2. Features:
AHEC/MNRE/SHP Standards/ Civil Works . but the basic equations are the same for both types.2.2.1. cut off walls will be required into each bank to prevent the river from bypassing the structure. A stepped arrangement is recommended for the downstream face of the weir to dissipate hydraulic energy.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 9 . as shown in Figure 2. The height of the steps should not exceed 0. Trench weirs function best on weirs with slopes greater than 4%-5%. If the length of the trench is less than the width of the river. when the trench is installed in a raised embankment.
0 QT • Total design flow = 1. categorized as “sand starved” rivers. have low suspended sediment loads and may not require a desilter.3.
AHEC/MNRE/SHP Standards/ Civil Works . in such cases the requirement for the desilter flushing flow can be omitted. • Design Flows: The following design flows are recommended: Bedload flushing flow (from collector box) = 0.1.4 QT
Some mountainous rivers.2
Design Parameters The following design parameters are suggested for the dimensioning of trench weirs.2 QT • Turbine flow = 1.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 10 .2 QT • Desilter flushing flow = 0.
A slope across the rack should be provided to avoid accumulation of bed load on the racks.0.0 of this Standard. which implies.•
Dimensional Layout The following factors should be considered in determining the principal dimensions: length. Hydraulic design is based on the following assumptions: • Constant specific energy across racks.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 11 . breadth and depth of a trench weir: Minimum width (B)= 1. Trashrack bars longer than about 2. Assume 30% blockage.0 cm is recommended for design.50m.2.
AHEC/MNRE/SHP Standards/ Civil Works . that river flow is equal to plant flow for the design condition. men and equipment. • Racks
The clear spacing between bars should be selected to prevent entry of bed-load particles that are too large to be conveniently handled by the flushing system. The set of equations proposed is based on the method given by Lauterjung et al (1989). Generally designs are based on excluding particles greater than medium gravel size from (2 cm to 4 cm).2. The next step in hydraulic design is to determine the minimum trench breadth (B) that will capture the required design flow.
Spacing between racks is designed to prevent the entry of bedload but must also be strong enough to support superimposed loads from bedload accumulation. An appropriate contraction coefficient should be selected as explained in the following sub-section.25 m (to facilitate manual cleaning) • Length should be compatible with river cross section.2. This issue is discussed further in Subsection 2. Maximum width (B) ≅ 2. • Effective head on screen is equal to base pressure (depth) • Approach velocity is subcritical with a critical section at the entry to the structure as shown in figure 2. Invert of collector box should be kept a high as possible.1/5.3. Slopes normally used vary from 0° to 20°. It is • recommended that the trench be located across main river channel. 1. Bar structural dimension shall be designed in accordance with Section 2. The design approach assumes complete capture of river flow.3 / 2.3 Hydraulic Design of Trench Intake The first step in hydraulic design is to decide the width of the trench intake bearing in mind the flow capacity required and the bathymetry of the river bed. A clear opening of 3.50 m • may require support as slenderness ratios become excessive. Rectangular bars are recommended.3/1.
“C” can be calculated from the following formula (as reported by Raudkivi) Rectangular bars:
⎛e⎞ C = 0..15< < 0.879 α k Then calculate the breadth of the collector trench from the following equations (2) to (4) 1.852 10° 0.812 12° 0..980 16° 0..50 q .. This formula is valid for 3.5>
h e >0.....66 ⎜ ⎟ ⎝m⎠
−0.1/1 Values of k as a Function of Rack Slope (α ) 2° 0.944 20° 0..2. the required breadth (B) can be determined as below:
AHEC/MNRE/SHP Standards/ Civil Works ..(1) (m) (m) (-)
k is a function of inclination of the rack and can be determined from the following table: Table: 2.800
α k = = = = 0° 1.E 2 C.....cosα 3/2 .837 8° 0..30 m m
Finally.000 14° 0... H0 3
.961 18° 0.....13
..894 26° 0.3 (30%) blockage.(3)
Assume h = 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 12 ..910 24° 0.865 4° 0..•
First calculate y1: 2 y 1 = k..(2) L= E1. (m) q = unit flow entering intake (m3/s per m) e = clear distance between bars (cm or m) m = c/c spacing of bars (cm or m) Assume E1 = 0...⎜ ⎟ ⎝h⎠
0..851 6° 0... 2gy 1 Where: L = sloped length across collector trench (m) E1 = blockage factor E2 = Effective screen area = e/m C = contraction coefficient α = slope of rack in degrees y1 = flow depth upstream from Equation 1...5 y1.927 22° 0.2 and 0.16
⎛m⎞ ....
The deck of the collector box should be located above the design flood level to provide safe access to operate gates. A velocity of at least 3. A suitable starting point would be to assume critical flow depth at the exit of the trench. the latter should be lowered to the elevation of the trench outlet or below..6
AHEC/MNRE/SHP Standards/ Civil Works . In most cases the profile will be sub critical with control from the downstream (exit) end. as required.
1..3. If the trench outlet invert is below the flushing pipe invert. The minimum depth of the trench at the upstream and is normally between 1. while the project flow is withdrawn via the intake.0m to 1..0m above the river bed level to provide energy to keep the outlet area free from accumulation of bed load that could block the pipeline.Chow (1959).0 m/s should be provided. Flushing Pipe The flushing pipe should be designed to provide a high enough velocity to entrain bed-load captured by the weir.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 13 . If possible. The bottom of the collection box must be designed to provide adequate submergence for the flushing pipe and intake to suppress undesirable vortices.3. an intake to the water conductor system and a flushing pipe. T.B = L cos α
1.3 m.-(4)
Hydraulic Design of Collector Trench Normally a sufficient slope on the invert of the trench is provided to ensure efficient flushing of bed-load particles that would otherwise accumulate on the invert of the trench. The flushing pipe must be design with the capacity to flush the bed-load sediment entering from the trench. If the flushing pipe invert is below the outlet of the trench.3.. A step-by-step procedure for calculating the flow profile that is applicable to this problem can be found in Example 124.5 Collector Box The trench terminates in a collector box.4
.5 m. page 342-345 of “Open-Channel Hydraulics” by Ven. The collection box has two outlets. the outlet end of the pipe should be located a minimum of 1. the Engineer should consider steepening the trench invert.... The flushing pipe should be lower than the intake and the flushing pipe sized to handle the discharge of bed load.66 d 9 / 7 6/7 qo
Where: d = sediment size (m) qo = flow per unit width (Q/B) at outlet of trench (m3/s per m) Ss = design slope of trench invert. For final design the flow profile should be computed for the design slope and the trench bottom profile confirmed or adjusted..
1.... based on water depth plus a freeboard of 0.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 14 .Channel Hydraulics Publisher: McGraw-Hill Book Company.Germany IAHR (1993): Diverted Water. Chow (1959): Open. New York. By: Arved J.
1.4 RESERVOIR.1. New York.3. However.7
CBIP. (2001): Manual on Planning and Design of Small Hydroelectric Scheme Lauterjung et al (1989): Planning of Intake Structures Freidrich Vieweg and Sohn. Raudkivi Publisher: Taylor & Francis. 1995). canal and penstock intakes are all based on the same principles. CANAL AND PENSTOCK INTAKES The designs of reservoir. there are significant variations depending on whether an intake is at the forebay reservoir of a run-of-river plant or at storage reservoir with large draw down or is for a power tunnel. The features common to all designs are shown in the following sketch:
AHEC/MNRE/SHP Standards/ Civil Works . Braunswchweig . etc. Examples of a variety of layouts can be fond in IS 9761 Hydropower Intakes – Criteria for Hydraulic Design or Guidelines for Design of Intakes for Hydropower Plants (ASCE.
For normal approach flow the submergence can be determined from the following formulae: S Where: S D V = submergence to the roof of the gate section (m) = diameter of penstock and height of gate (m) = velocity at gate for design flow.4.
1. • To minimize hydraulic losses.0m/s to facilitate manual raking.The objectives of good design are: To prevent entry of floating debris. Trashracks may be designed in panels that can be lowered into place in grooves provided in the intake walls or permanently attacked to anchors in the intake face.725VD0. IS 11388 Recommendations for Design of Trashracks for Intakes should be consulted for information about spacing between trashracks bars. structural design and vibration problems. The bellmouth type intake is preferred when ever the additional costs are
AHEC/MNRE/SHP Standards/ Civil Works ..4. • To avoid formation of air entraining vortices. (m/s) = 0. For a bellmouth intake the transition section is formed with quadrants of ellipses as shown in the following sketch. The spacing between bars is determined as a function of the spacing between turbine runner blades.2.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 15 .5
A recent paper by Raghavan and Ramachandran discusses the merits of various formulae for determining submergence (S).1/5 of this Standard. If flow approaches at a significant angle (greater than 45o) from axial these will be significant risk of vortex problems. In such a situation an experienced hydraulic engineer should be consulted and for important projects hydraulic model studies may be required. •
1. see Section 2.2 Control of Vortices
First of all the direction of approach velocity should be axial with respect the intake if at all possible.4. The trashracks should to sloped at 14° from the vertical (4V:1H) to facilitate raking.3 Minimization of Head losses
Head losses are minimized by providing a streamlined transition between the entry section and gate section. For small hydro plants the trashrack overall size is determined based on an approach velocity of 0. Also.75 m/s to 1.
1.1 Control of floating debris
To prevent the entry of debris a trashrack is placed at the entry to the intake. Minimum losses will be produced when a streamlined bellmouth intake is used.
This permits some reduction in the cost of gates without a significant sacrifice in hydraulic efficiency. There is a second transition between the gate and penstock.785 (D): 1. mainly mini hydropower stations.economically justified.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 16 . Details on the geometry of both types are given • Bellmouth Intake Geometry
Geometries for typical run-of-river intakes are shown below: A gate width to height of 0.785D the flow velocity at the gate will be equal to the velocity in the penstock so no further flow acceleration is produced in this section. For a gate having H = D and W= 0. A length for this transition of 1. For smaller.00 (H) with H = D is recommended. simpler designs are often optimal as the cost of construction of curved concrete surfaces may not be offset by the value of reduction in head losses.0x D should be satisfactory. rectangular to circular.
The following design rules are recommended: Air vent area should the greater of the following values AV = 0.The head loss co-efficient for this arrangement in Ki =0.20 Ap or Q AV = T 25.4.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 17 . • Simplified layout (Mini-Hydro): For smaller/mini hydro projects intake design can be simplified by forming the transition in plane surfaces as shown below: The head loss for this design (Ki) = 0. AIR VENT
An air vent should be placed downstream of the head gate to facilitate air exchange between atmosphere and the penstock for the following conditions: • Penstock filling when air will be expelled from the penstock as water enters. • Penstock draining when air will enter the penstock to occupy the space previously filled by water.10 Details for layout of bell mouth transitions connecting to a sloping penstock are given in IS9761. The air vent (pipe) must have an adequate cross section area to effectively handle these exchanges of air.
1.19V2/2g.4.0
IS 9761: Hydropower Intakes – Criteria for Hydraulic Design
Guidelines for Design of Intakes for Hydroelectric Plants ASCE. Layman’s Guidebook European Small Hydro Association Brussels. Gordon Water Power & Dam Construction April 1970
1.K. The recommended practice is to control filling rate via the head gate. New York (1995) • • Validating the Design of an Intake Structure : By Narasimham Raghavan and M.4.1
TRASHRACKS AND SAFETY RACKS Trashracks: Trashracks at penstock intakes for small hydro plants should be sloped at 4 V: 1H to facilitate manual raking and the approach velocity to the trashracks limited to 1.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 18 . The head gate should not be opened more than 50 mm until the penstock is completely full. Vortices at Intakes By J.L. Ramachandran.0 m/s or less. Support beams should be alignment with the flow direction to minimize
AHEC/MNRE/SHP Standards/ Civil Works .5 PENSTOCK FILLING
A penstock should be filled slowly to avoid excessive and dangerous “blowback”.5.Where:
The air vent should exhaust to a safe location unoccupied by power company employees on the general public. 1.6 REFERENCES ON PENSTOCK INTAKES:
1. Belgium (June 1998) Available on the internet.4. HRW – September 2007.4.7
Indian Standard Cited. Use of rectangular bars is normal practice for SHP’s. (This is sometime referred to as “cracking” the gate.5.)
A clear spacing of 200 mm between bars is recommended.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 19 .2 Safety Racks: Safety racks are required at tunnel and inverted siphon entries to prevent animals or people who may have fallen into the canal from being pulled into these submerged water ways. Detailed trashrack design should be done in accordance with IS 11388.
1.5. Other aspects of design should be in accordance with IS 11388.3
IS11388 – “Recommendations for Design of Trashracks for Intakes”.hydraulic losses. DRAWINGS:
AHEC/MNRE/SHP Standards/ Civil Works .5. ASCE (1995) --“Guidelines for Design of Intakes for Hydroelectric Plants”. References on Trashracks
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 20 .
Several typical cross section designs are shown below:
2. CANALS Canals for small hydro plants are typically constructed in masonry or reinforced concrete.2. inverted siphons and pipelines connecting the head works with the forebay tank.1.1 2.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 21 .
HYDRAULIC DESIGN OF WATERWAYS The waterways or water conduction system is the system of canals.1
AHEC/MNRE/SHP Standards/ Civil Works . This Section provides guidelines and norms for the hydraulic design of these structures. aqueducts. tunnels.
if required.1 Feeder canal hydraulic design shall be based on the following criteria: Design flow (Qd) = Turbine flow (QT) + Desilter flushing flow (QF).1.
AHEC/MNRE/SHP Standards/ Civil Works .Power canal to connect the desilter to the Forebay tank.Feeder canal to connect the head regulator (intake) to the desilter .Lined canals in earth.1. should be designed in accordance with Indian Standard: IS 10430.2.
2.2 Feeder Canals
2. A further division of canal types is based on function: .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 22 .
2.2 Scouring velocity: A sufficiently high velocity must be provided to prevent deposition of sediment within the canal. The selected design would be based on the highest of Vs or Voptimum. Masonry construction would normally be preferred for canals with widths (W) less than 2.Service life in years (n) .2.Plant load factor . The economic parameters for this analysis should be chosen in consultation with the appropriate regional. as explained in the previous section.Escalation rate(e) .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 23 . O&M and head losses (as capitalized value).3 Power Canals: Power canal design shall be based on the following criteria a) Design flow = total turbine flow (QT) b) Power canal design should be based on optimization of dimensions. S C/ 2 n Where: Sc = Scouring slope d = Target sediment size (m) q = Flow per unit width (Q/W) (m/s/m) R = hydraulic radius (m) Vs = scouring velocity (m/s) n = Manning’s roughness coefficient 2.0 m (flow area =
AHEC/MNRE/SHP Standards/ Civil Works . Also see Section 1. R 2 / 3 .015 q 1 1 ∴ VS = .3 Optimization: The optimum cross section dimensions.0 m3/s optimal geometric design dimensions for Type 1 (masonry construction) can be estimated by assuming a longitudinal slope of 0. This (scouring) velocity can be determined from the following formulae: d 9/7 S C = 0.2. A minimum of 0.4 Freeboard: A freeboard allowance above the steady state design water level is required to contain water safely within the canal in event of power outages or floods.5 m is recommended.
2.1. state or central power authorities these parameters include: .2. slope and velocity should be determined by economic analysis so as to minimize the total life time costs of capital.2.1.Discount rate (i) .Value of energy losses (Rs/kWh). For mini-hydro plants Q < 2. slope and velocity.Annual O+M for canal (% of capital cost) .018.7 of this Standard.1.66 6 / 7 n = 0.004 and a Manning’s n value of 0.1.
For larger canals with flow areas greater than 2. The adequacy of the above minimum freeboard should be verified for the following conditions: • Maximum flow in the power canal co-incident with sudden outage of the plant.0 m2). This tank is normally equipped with an escape weir to discharge surplus water or an escape weir is provided near to the forebay tank.2 AQUEDUCTS
Aqueducts are typically required where feeder or power canals pass over a gully or side stream valley. For longer aqueducts design would be based on economic analysis subject to the proviso that flow remains sub critical with NF ≤ 0.0m2.05 QT) and above rated operation (+ 0.8 in the flume sections.02 to +0.2. The waterway in most SHP’s terminates in a Forebay tank.1QT).50 m is recommended. a Type 3. box culvert design would be preferred – based on economic analysis.1. giving above static water levels at the downstream end.4 Rejection Surge
Designs which do not incorporate downstream escape weirs would be subject to the occurrence of a rejection surge in the canal on sudden turbine shutdown. For mini-hydro plants a minimum freeboard of 0.
2.25 m after taking these factors into consideration.
AHEC/MNRE/SHP Standards/ Civil Works . c) Freeboard: A freeboard allowance above the steady state design level is required to contain water safety within the canal in event of power outages. The following sketch shows the principal dimension of aqueduct entry and exit transitions and flume section. • Characteristics of head regulator flow control. The freeboard allowance may be reduced to 0. If the length of the aqueduct is relatively short the same channel dimensions as for the canal can be retained and there would be no change in hydraulic design. reducing to the static level at the upstream (entry) end of the water way. Methods for evaluating water level changes due to a rejection surge are explained in Section 2.2 / 7.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 24 . The maximum water level occurring in the forebay tank can be determined from the weir equation governing flow in the escape weir.
2.2. • Design flow plus margins for leakage losses (+0.0 of this Standard.
V2 V2 Z 1 + D + 1 = Z 2 + d + 2 + hL 2g 2g and
b⎞V ⎛ hL = 0.The changes in invert elevation across the entry and exit structures can be calculated by Bernouli’s equation as below: • Entry transition – consider cross – section (1) and (2). A Manning’s n = 0.018 is suggested for concrete channels. since all geometrical parameters are known.10 ⎜1 − ⎟. Exit transition – consider cross section (3) and (4): V2 V2 Z 3 + d + 3 = Z 4 + D + 4 + hL 2g 2g
⎛ Vn ⎞ ( S ) = ⎜ 2 / 3 ⎟ . ⎝R ⎠ Some designers increase this slope by 10% to provide a margin of safety on flow capacity of the flume.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 25 . 2 ⎝ B ⎠ 2g Z2 can be determined from the above equations.
3. of this Standard.and
b⎞V ⎛ hL = 0. Invert elevations are determined by accounting for head losses from entry to exit of the structure using Bernouli’s equation.8. The head loss coefficients for mitre bends can be determined from USACE HDC 228. • Syphon barrels: The syphon barrel dimensions are normally determined by optimization ⎛ V ⎞ ⎟ does not studies.1 INVERTED SYPHONS
Inverted syphons are used where it is more economical to route the waterway underneath an obstacle.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 26 . 2. Follow the guidelines given in Section 2. using mean flow width (B) = A/D.
2. with the proviso that the Froude Number ⎜ N F = ⎜ gd ⎟ ⎝ ⎠ exceed 0. The inverted syphon is made up of the following components: • Entry structure • Syphon barrels • Exit structure • Entry Structure: Hydraulic design of the entry structure is similar to the design of reservoir.20 ⎜1 − ⎟.2.
The same basic geometry can be adapted for transition between trapezoidal canals sections and rectangular flume section.2/2. 3 ⎝ B ⎠ 2g Z4 can be determined from the above equations. since all geometrical parameters are known. For reinforced concrete channels a Manning’s “n” value of 0.018 is recommended. canal and penstock intakes.2.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 28 .Exit structure: The exit structure is designed as a diverging transition to minimize head losses.2/2 of this Standard. The following sketches show the layout of a typical inverted siphon.
AHEC/MNRE/SHP Standards/ Civil Works .2. the design is similar to the outlet transition from flume to canal as discussed in Subsection 2.
since relatively large diameter pipe possesses significant inherent structural strength.
Manning’s “n” Values for selected Pipe Materials Material Welded Steel Polyethylene (HDPE) Poly Vinyl Chloride (PVC) Asbestos Cement Cast iron Ductile iron Precast concrete pipe Manning’s “n” 0. and offer alternatives to inverted syphons of reinforced concrete construction.2
Reference on Aqueducts and Inverted Syphons “Hydraulic Structures” By C.D. plastic and steel pipes are suitable depending on site conditions and economics. The pipe profile should be chosen so that pressure is positive through out. Otherwise.015 0. as necessary). inverted syphon or aqueduct.2. low pressure pipeline land pipeline material). depends on economic and constructability considerations.009 0.013(2)
Note: (1) From Table 5. Steel pipe (with stiffening rings. aqueducts or inverted syphons. in the context of a given SHP.011 0. hydraulic design for low pressure pipelines is similar to the requirements for inverted syphons.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 29 .4. Steel pipe is often an attractive alternative in place of concrete aqueducts in the form of pipe bridges. Chow – Open Channel Hydraulics
AHEC/MNRE/SHP Standards/ Civil Works .012 0.
Low pressure pipelines may be employed as an alternative to pressurized box culverts.014 0.009 0. Concrete. Smith University of Saskatchewan Saskatoon (SK) Canada LOW PRESSURE PIPELINES
2. If there is a high point in the line that could trap air on filling an air bleeder valve should be provided. concrete and plastic pipe also have significant resistance against external pressure.4 Layman’s Guide Book – ESHA (2) From Ven T. if buried.3. The choice of type of design. Generally pressurized flow is preferred.
b) Rock surfaces are sound and not vulnerable to erosion (or erodible zones are suitably protected.5. 2.
Controlled perimeter blasting is recommended in order to minimize over break and produce sound rock surfaces. Tunnels for SHP are generally of two types.
2.5 to 2. this construction approach tends to produce relatively uniform surfaces and minimizes the hydraulic roughness of the completed tunnel surfaces.1
Tunnels often provide an appropriate solution for water conveyance in mountainous areas.0 m/s on the mean
AHEC/MNRE/SHP Standards/ Civil Works .5. Design velocities of 1.2 Unlined tunnels: Unlined water tunnels can be used in areas of favourable geology where the following criteria are satisfied: a) Rock mass is adequately water tight.2. Additionally.5.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 30 . c) The static water pressure does not exceed the magnitude of the minor field rock stress. • Unlined tunnels • Concrete lined tunnels On SHP tunnels are usually used as part of the water ways system and not subject to high pressures.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 31 . CULVERTS AND CROSS-DRAINAGE WORKS
Small hydro projects constructed in hilly areas usually include a lengthy power canal routed along a hillside contour. The flow in these chains must be periodically discharged or the drain capacity will be exceeded.
2. For the purpose of this standard. 2. These culverts would normally be located where gullies or streams cross the canal alignment. The capacity of canal ditches should be decided taking into consideration the average distance between culverts. the designer should consult an experienced geotechnical engineer or engineering geologist. 2. Norway.4 High Pressure Tunnels Design of high pressure tunnels is not covered in this standard. Other References: “ Norwegian Hydropower Tunnelling” (Third volume of collected papers) Norwegian Tunneling Society Trondheim. if required. Lateral inflows from streams and gullies intercepted by SHP canals often transport large sediments loads which must be prevented from entering the canal. www.5. Flow from these drains is usually evacuated via culverts passing underneath the canal.3 Lined Tunnels Where geological are unfavourable it is often necessary to provide concrete linings for support of rock surfaces. For high pressure design.6. In the rare cases when distance between culverts is excessive. The first line of defense is the canal upstream ditch which intercepts local lateral runoff.5. high pressure design is defined as tunnels subject to water pressures in excess of 10m relative to the crown of the tunnels. IS4880: Parts 1-7 give comprehensive guidelines on the design of lined tunnels. It is normal practice to provide a 100mm thick reinforced concrete pavement over leveled and compacted tunnel muck in the invent of the tunnel. consideration should be given to diverting
AHEC/MNRE/SHP Standards/ Civil Works . IS 4880: Part 3 provides additional guidance on the hydraulic design of tunnels and on the selection of appropriate Manning’s “n” values. 2.tunnel.5 Reference on Tunnels IS Standards: IS 4880 “Code of Practice for the Design of Tunnels Conveying Water”.no Notably: Development of Unlined Pressure Shafts and Tunnels in Norway.5. by Einar Broch.cross section area give optimal cross section design.
the procedures given in Section 1. Army Corps of Engineers (1984) www.S. 1 in 10 year flood (Q10) • For small hydro projects.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 32 .Corrugated Steel Pipe Institute www.ditch flows across the canal in flumes or half round pipes to discharge over the downhill side of the canal at suitable locations.ca
2. The rejection surge will typically cause the downstream water level to rise above static level and may control the design of canal freeboard.7.usace.1
Power canals that are not provided with escape weirs near their downstream end will be subject to canal surges on rapid load rejections or load additions.4 should be applied.mil/publications/eng-manuals Manufacturer’s guides. Engineering Manual EM 1110-3-136 U. Otherwise design flows should be estimated from field measurements of cross section area and longitudinal slope at representative cross section of the gully or side stream.org .American Concrete Pipe Association www.7 2. • For mini hydro projects. Detailed hydraulic design should be based on information from reliable texts or design guidelines – such as: • “Design of Small Bridges and Culverts” Goverdhanlal • “Engineering and Design – Drainage and Erosion Control”. A survivable design approach is further recommended with canal walls strengthened to allow local over topping without damage to the canal integrity when floods exceed the design flood values.army.cspi. notably: . The following formulae taken from IS 7916: 1992 can be used to estimate the magnitude of canal surges.concrete-pipe. 1 in 25 year flood (Q25) Where it is practical to extract the necessary basin parameters. For load additions there is a risk that the level will fall to critical at the downstream end and restrict the rate at which load can be taken on by the unit. It is recommended that culverts design be based on the following hydrological criteria. Culverts are usually required where the canal route crosses gullies or streams.
AHEC/MNRE/SHP Standards/ Civil Works . Culverts at these points provide for flow separation between lateral inflows and canal inflows and often present the most economical solution for crossing small but steep valley locations.
2.= YS .2 Canal Surges on Complex Waterways: For waterway systems comprising several different water conductor types. Canada (2002).L. In such cases a more detailed type of analysis will be required.7.Maximum surge height in a power channel due to load rejection may be calculated from the empirical formulae given below: For abrupt closure hmax = K 2 + 2 Kh For gradual closure within the period required for the first wave to travel twice the length of the channel: K hmax = + V .hmax
Where: Yo YS = steady state downstream water level = static downstream water level. V = mean velocity of flow. National Weather Service FLDWAV computer program can be used to solved for the transient flow conditions in such cases (Helwig.
AHEC/MNRE/SHP Standards/ Civil Works . 2. and area of cross sec tion h = effective depth = top width • Maximum water level resulting from a rejection surge at the downstream of a canal: Maximum W.S.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 33 .
The maximum water level profile can be approximated by a straight line joining the maximum downstream water level to the reservoir level. the above equations are not applicable. Helwig Canadian Society for Civil Engineering Proceedings.L. Other References “Application of FLDWAV(Floodwave) Computer Model to Solve for Power Canal Rejection Wave for Simple and Complex Cases”. Annual Conference Montreal. P. K = V2/2g = velocity head. The U. h / g 2 Where: hmax = maximum surge wave height.3 References IS Standards cited: IS 7916: 1992 “Open Channel – Code of Practice”. 2002).7. = Yo + hmax • Minimum water level resulting from by a start up surge at the downstream end of a canal: Minimum W.
turbine mechanical and hydraulic design.
The adoption of 0. The severity of damage to equipment is a function of several variables. the following limits are suggested by Naidu (2004): Table 2.20 m regardless of head. Low and Medium Head Turbines ≤ 150 m High Head Turbines > 150 m
3.3.20 mm is not practical. Accordingly the design parameters for desilter design should be made in consultation with the mechanical designers and turbine manufacturer.
AHEC/MNRE/SHP Standards/ Civil Works . can be harmful to turbine components.1.
The control of turbine wear problems due to silt erosion requires a comprehensive design approach in which sediment properties.3/1. particle shape. notably: sediment size. Nozaki (1985) suggests a size range of between 0. sediment hardness.2 Removal Size There are also considerable divergences of opinion on the selection of design size for sediment removal. especially particles of hard materials such as quartz.1 Need The first design decision is to determine whether the sediment load in the river of interest is sufficiently high to merit construction of a desilter.20 mm is the design (target) sediment size is recommended for Indian SHP designs.
3.2. material selection and features to facilitate equipment maintenance are all considered (Naidu. There is little guidance available on this topic.6 mm for plant heads ranging from 100 m to 300 m. 3. however.1
HYDRAULIC DESIGN OF DESILTERS BACKGROUND Sediment transported in the flow.3 mm to 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 34 . 2004). Where the risk of damage is judged to be high a settling basin (or desilter) should be constructed in the plant waterway to remove particles.0 Concentration Parameter Head Maximum allowable sediment concentration Suggested Maximum Allowable Sediment
versus Plant Head. sediment concentration and plant head. Indian practice is to design for a particles size of 0.1. greater than a selected target size. Some authors suggest that removal of particles smaller than 0.
1. Some authors suggest that the vertical variation of sediment concentration and variations horizontally across the river be measured.3. this is only true on the average. in a statistical sense. Therefore.2. On glacier fed rivers where diurnal flow variations may exist. on fast flowing rivers inherent turbulence should ensure uniform mixing and sampling at one representative point should be sufficient. A five year long sediment collecting program would be ideal. the schedule of sampling should be adjusted to take this phenomenon into account and the scheduled sampling times be adjusted to coincide with the hour of peak daily flow with another sample taken about twelve hours later.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 35 . that the peak sediment event of a year may be associated with a unique upstream event such as a major landslide into the river.3
There are two basic types of desilters: Continuous flushing type Intermittent flushing type Guidelines for design of both types are given in this section. it would also be desirable to design the sediment measurement program to provide more detailed information about such events. The sampling program should extend through the entire rainy season and should comprise at least two readings daily. DESIGN CONSIDERATIONS
3. The data collected in a sediment sampling program should include: • Mean daily concentration of suspended sediment (average of two readings twelve hours apart) • Water temperature • Flow (from a related flow gauging program) The following additional information can then be derived from collected samples.1 Data Requirements (Small Hydro Plants) It is recommended that a program of suspended sediment sampling be initiated near the intake site from an early stage during site investigations to ensure that sufficient data is available for design. Such events often account for a disproportionately large proportion of the annual sediment flow. However.
While it is often assumed that sediment load is directly related to flow.
AHEC/MNRE/SHP Standards/ Civil Works . Less than one monsoon season of data is considered unsatisfactory. basically to increase the sampling frequency to one sample per 1 or 2 hours at these times.
3.2. In fact it is quite likely.
Fe1. The following regional formula by Garde and Kothyari (1985) can be used to support engineering decision making.2 Data Requirements (Mini Hydro Plants)
On mini hydro projects where resources and time may not be available to undertake a comprehensive sampling program.
AHEC/MNRE/SHP Standards/ Civil Works .6. It is likewise recommended that experiments be made on selected ranges of particles sizes to determine settling velocities. Total (design) flow: QT = QP + QF = 1. Where QP is plant flow capacity in (m3/s).⎜ max ⎟ ⎝ P ⎠ Where Vs = mean sediment load in (tonnes/km2/year) s = average slope (m/m) Dd = drainage density. as total length of streams divided by catchment area (km/km2) P = mean annual precipitation (cm) Pmax = average precipitation for wettest month (cm) Fe = ground cover factor.3 Design Criteria
The principle design criteria are: 1. 0.10 AW ] ∑ Ai AA = arable land area = grass land area (all in km2) AG AF = forested area AW = waste land area (bare rock)
3. Flushing flow: QF = 0. supplemented by observations on site and local information.
It is also recommended that a petrographic analysis be carried out to identify the component minerals of the sediment mix.2 QP is recommended 3.19 ⎛P ⎞ 0 Vs = 530.7. The target size for removal (d): d = 0.S0. selection of design parameters will depend to a great extent on engineering judgment.20 mm is recommended 2. as below: 1 Fe = [0.30 AF + 0.2. A further discussion on the subject of sediment sampling is given in Avery (1989) The characteristics of the sediment on a given river as obtained from a data collection program will assist in selection of appropriate design criteria.10 .80 AA + 0.2 QP.
3.2.0 P0.• • •
A sediment rating curve (sediment concentration versus flow – where possible) Particle size gradation curve on combined sample Specific gravity of particles.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 36 .60 AG + 0.25 Dd .
4 Siting The following factors control site selection 1. A site along the water way of appropriate size and relatively level with respect to cross section topography 2.50 m is recommended. For preliminary layout a reference river level corresponding to the mean annual flood and minimum flushing head of 1. A design with vanes may also be considered.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 37 .3.
3. Alternatively. A site high enough above river level to provide adequate head for flushing.1
AHEC/MNRE/SHP Standards/ Civil Works . a location as close to the head works is normally preferred. transition structure with walls diverging at a rate of 6:1 is recommended.2. Sometimes it is convenient to locate the desilting basin at the downstream end of the waterway system where the desilter can also provide the functions of a forebay tank. as shown in Figure. For efficient functioning of the settling tank the velocity should be as uniform as possible without short circuits or localized high velocity areas.3. upstream of the penstock intake. introducing flow into the settling section via a distribution weir or diffuser wall is preferred. Where possible. site topography permitting. However. In principle a desilting tank can be located anywhere along the water conductor system. 3.3 Hydraulic Design A desilter is made up of the following elements: • Inlet section Settling tank • Outlet section • • Flushing system Inlet Section The purpose of the inlet section is to reduce flow velocity from the relatively high speed of the feeder canal to the low speed of the settling tank.
2.36 for d > 1.0 mm > d ≥ 0.10 mm = 0.10 mm
b) Select fall velocity (wo) for d from Figure 2.3 2 (VF −VT2 ) hL = 2g Where: VF = velocity in feeder canal (m/s) VT = velocity in settling basin (m/s)
3.51 for d < 0.Hydraulic losses in the inlet transition can be estimated as: 0.2 Settling Section The fundamental design objective is to remove all particles equal or greater than the chosen target removal size (d).0 mm = 0.44 for 1. The methodology recommended follows the approach given by Mosonyi:
a) Flow velocity in the tank should not entrain material that has settled out to the bottom of the tank Thus Where.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 38 .3.3.
AHEC/MNRE/SHP Standards/ Civil Works . assuming an appropriate water temperature. U ≤a U d a
d = velocity through tank (m/s) = target sediment size in (mm) = 0.2.
ω 0 − 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 39 . whence: D = Q/BU (m) d) Adjust for effects of turbulence ω = ω0 − ω ' ( m / s )
D ω0 − α U L = U.c) Assume width of basin (B) and calculate depth (D) from the equation of continuity.132U / D
3.3 Outlet Section The outlet section provides a transition between the settling tank and power canal. Flushing flow is withdrawn from the bottom of the hopper and controlled by a manually operated valve.2. The recommended flushing flow is 1. Determination of this volume should be based on the incoming sediment load. Figure 2.3. The usual design procedure is to assume equal flow through each hopper.3.20QP and the flushing gates should be large enough so as not the throttle this flow. Flushing system – Intermittent Flushing Type The same laws govern the design of intermittent flushing desilters. Hydraulic losses can be estimated as: ⎛V 2 V 2 ⎞ h L = 0 .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 40 . but the ⎝B⎠ ⎛L⎞ minimum ⎜ ⎟ ratio should not be less than 4.2.3.2 ⎜ P − T ⎟ ⎜ 2g 2g ⎟ ⎝ ⎠ = velocity in power canal (m/s) Where: VP VT = velocity in settling basin (m/s) 3.25 are should be applied.3. settling basin ⎛L⎞ and outlet section.2.65 and a bulking factor of ×1. A length to width ratio ⎜ ⎟ of 8 to 10 is preferable.5
3. The flushing system may be designed for either pressurized or non pressurized flow. one for each row of hoppers.3.4. ⎝B⎠
3.2. Where head is available the non pressurized flow design is to be preferred since water passages can be made larger and therefore are easier to maintain.4 Flushing system – Continuous Flushing Type The recommended flushing system comprises a series of hoppers built into the base of the settling tank with side slopes of 1:1 leading to a central outlet at the bottom of the hopper. thus the main basin dimensions can be obtained using the same procedures as outlined in Subsection 2. In converting sediment flows in mass terms to volumes a relative density of 2. A transition with walls converging at 2:1 will be satisfactory.3 shows a typical design (at end of text).Vary the value of B to optimize the layout including: inlet section. It is recommended that this volume be computed from the mean maximum monthly sediment load as measured or from comparable data from another plant operating in similar conditions with respect to sediment and water flows. References:
3.0. Trap efficiency can be calculated using Camp’s Sediment Removal Function as given in Figure 2. trap efficiency and frequency of flushing.6
AHEC/MNRE/SHP Standards/ Civil Works . In place of hoppers used in a continuous flushing desilter a sufficient storage volume must be provided.3/3.
of 2nd International Workshop on Alluvial River Problems.Volume 2A: High – Head Power Plants (Pages: 18-26).J. By E.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 41 .7 Drawings
Drawings are shown on the following pages. Kothyari Proc. Garde and U. 2004 Estimation of Repair Cycle of Turbine due to Abrasion by Suspended Sand and Determination of Desilting Basin Capacity By Tsugo Nozaki – Electric Power Civil Engineering (Japan) Volume 218 pp 143-152 – January 1989. Roorkee. Hungary (1991) Sediment Control at Intakes – A Design Guide Edited by P. Mosonyi Akadémiai Kiadó Budapest. (Original in Japanese)
3. Naidu Published by the National Power Training Institute. India (1985) Silt Erosion Problems in Hydro Power Stations and their Possible Solutions By B.K. England (1989) Sediment Erosion from Indian Catchments By R.
AHEC/MNRE/SHP Standards/ Civil Works . Avery BHRA – Fluids Engineering Centre Cranfield. Faridabad (Haryana).C.3.S.
798 m3/s.QP = 0. QF = 0.00 m3/s & d = 0.3: TYPICAL HOPPER TYPE DESILTING TANK
AHEC/MNRE/SHP Standards/ Civil Works .25 mm
FIGURE 3.202 m3/s. QT = 1.3.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 42 .
AHEC/MNRE/SHP Standards/ Civil Works .3.5 & .Figure 3.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 43 .6 shows a typical design for a flow of 15 m3/s per chamber.
Function These are two main functions: • Provide for adjustment of turbine discharge according to load demand. For mini hydro plants equipped with load controllers.
4. Water level control: For small hydro plants connected to the grid it is convenient to match turbine output to available flow.1/1). These plants always operate in a “water wasting” mode so that the forebay tank water level is always maintained above the escape weir crest elevation.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 45 .4.
4.2 Design Criteria . thereby maximizing use of available water. However. Normally in this mode of operation requires that canal flow be greater than plant demand flow. there is no feed back to the turbine.Tank
HYDRAULIC DESIGN OF FOREBAY TANK General A forebay tank is normally located at the downstream end of the water conductor system and provides a transition between the power canal and penstock. This requires that water levels be measured in the forebay tank and tailrace and transmitted in real time to the turbine governor which adjusts turbine output (and flow) so as to keep forebay water levels within a prescribed water level range. The forebay tank acts as a buffer to adjust for errors in turbine setting and actual inflow into the forebay tank. It is usually located on a ridge on a firm foundation respecting topographical and geological conditions.2. during low flow periods when plant flow capacity (Qp) may be greater than river flow (Q) it would be necessary to adjust the ballast load to limit plant flow to about 90% of river flow in order to avoid draining the forebay tank. This is achieved by means of a water level control system whereby the turbine load is adjusted to equalize available flow in the power canal with turbine flow. This section does not deal with designs having short canals where flow is controlled by the turbine (for this case the reader is referred to Sub-Section 2. • Provide a volume of stored water to permit water level control of turbine operation. Upstream from the forebay tank the waterway is characteristically open channel flow whereas downstream penstock flow is under pressure. The forebay design addressed in this sub-section is typical of designs associated with long canals where flow is controlled by the head gate and flow surplus to turbine demand is discharged over an escape weir back into the river. This is not a problem during periods of high flow when river flow is much greater than plant demand.
Flow adjustment: the forebay tank and escape weir facilitate the adjustment of turbine discharge due to system load changes by diverting surplus flow over the escape weir back into the river. thus cost of water level gauges and data transmission systems is avoided.
Weir discharge should be routed towards a natural water course of adequate capacity or a ditch provided that is suitably protected against erosion. Water Level Control A water level control system requires that real time water level measurements in the forebay tank and tailrace canal be transmitted to the turbine governor.0 m below the crest of the escape weir is recommended. As practical. In the water level control mode the governor will estimate the inflow to the forebay tank and adjust the wicket gates to correct for difference between turbine and canal flows so as to maintain forebay tank levels within a prescribed range. For digital governors the control volume can be further reduced.5
AHEC/MNRE/SHP Standards/ Civil Works . Normally a volume of Qp×120 m3 (or two minutes at (Qp= maximum plant flow) will be satisfactory for mechanical governors.4 Flushing Gate A flushing gate is recommended by some designers to facilitate removal of any sediment or debris that might settle in the bottom of the forebay tank and be drawn in to the penstock. c) The depth of the tank should be chosen so as to provide adequate submergence for the penstock intake in accordance with Sub-Section 2.1/4 of this standard.The following hydraulic design criteria are recommended: a) The live storage volume of the forebay tank should be determined according to the response characteristics of the turbine governors.0 m to 2. Where this is not practical for topographic reasons the escape weir should be located at the nearest suitable site upstream of the forebay tank.
4.3 Escape Weir The preferred location for the escape weir is in the rim of the forebay tank. In this case the engineer should contact the turbine manufacturer to define the control parameters in order to calculate the control volume needed.2. the cross section areas of the forebay tank should be designed to avoid abrupt changes in direction which could cause undesirable vortex formation. A float type water level gauge with electronic data transmitter is recommended.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 46 . A linear alignment of power canal and penstock intake is preferred.
A simple overflow weir is recommended with a design head that can be contained within the normal canal freeboard. The features of a float gauge are shown below:
4. b) A live storage drawdown of 1. For this case the effects of hydraulic transients in the power canal section between the forebay tank and escape weir should be checked to assess their impact on water level control.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 47 . Main Report: Manuals www.The precision should be +/.6 References
IS Standards cited: IS 9116(2002) “Water stage Recorder (Float type) – Specification”. For additional information the reader is referred to IS 9116 (2002) “Water Stage Recorder (Flow Type) Specification”. For mini hydro plants it is recommended that a staff gauge be attached to the wall of the forebay to facilitate estimation of canal flow prior to start up of the turbines.jica.go.
4.3 mm or better. Other References: “The Study on Introduction of Renewable Energies in Rural Areas in Myanmar” – Volume 4.lvzopac. Staff gauges or float wells should be located in areas of relatively quiet water to minimize risk of errors due to water level fluctuations.jp
turbine speed deviations and turbine/ generator runaway speeds must be kept within appropriate limits. (See Appendix 5 to this sub. • A surge tank may be required where L/H > 4 to 8 ΣLi V i Or when • > 7 to 13 ( SI units ) H • A surge tank should be provided if the maximum speed rise following rejection of the maximum turbine output cannot be reduced to less than 45 % of the rated speed by other practical methods. There are
AHEC/MNRE/SHP Standards/ Civil Works . L = length of penstock or section of penstock (m) and V = flow velocity (m/s). Since these factors also impact the electrical system (grid) they must be controlled so that frequency and voltage deviations are maintained within strict limits.section).2 Methods For Control Of Hydraulic Transients
The use of a surge tank provides the most effective and reliable method for dealing with hydraulic transients. Sub-section 2.
The above criteria apply in particular to isolated plants where the capacity of the unit contributes more than 40% of the system capacity.2. This situation should be checked for both high and low reservoir levels. The following criteria may be used to judge whether a surge tank or an alternative device is required for control of frequency and/or waterhammer.1
CONTROL OF HYDRAULIC TRANSIENTS BACKGROUND The design of pressurized conduits must take into account the transient behaviour of the conduit / turbine system / power system. Where plant characteristics are favourable no special means are required for dealing with the above problems. USBR Engineering Monograph 20 provides an approximate method for estimating speed rise. Less stringent criteria may be considered for units connected to a large system where their role in frequency regulation is less important. The speed rise should be computed assuming one unit to be operating alone if there is more than one unit on the penstock.6 of this standard provides guidelines on calculating water hammer pressure in penstocks. such as increasing the generator inertia or penstock diameter or decreasing the effective wicketgate closing time. These phenomena are interrelated in such a way that reducing the rate of wicket gate closing to control water hammer may result in excessive speed (frequency) deviations or unacceptably high turbine/ generator runaway speeds. Water hammer pressures. where situations are unfavourable some method of controlling water hammer and its related effects is required.
5. Note: H = gross head (m). Conversely. rapid adjustment of the wicket gate to minimize speed (frequency) deviations may result in unacceptably high water hammer pressures.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 48 . but it is also the most expensive. • Experience shows that a turbine / generator will function satisfactorily if the pressure rise at the scroll case does not exceed 50% of gross turbine head for full load rejection.5 5. However.
The designer should weigh the advantages and disadvantages of each alternative before making a final choice as to which solution is best suited for a given project. The bypass valve is designed with a linkage to the turbine operating ring in such a way that the bypass valve opens synchronously as the wicket gates close.2. Their paper provides an empirical method for assessing the feasibility of adding generator inertia for control of waterhammer and frequency regulation. can be used to divert flow past passed the turbine. until the bypass valve is fully opened and a slower rate governed by closure of the bypass valve to the new operating position. This type of bypass valve is sometimes referred to as a synchronous bypass valve as it operates in unison with the turbine wicket gates.2 Addition of Machine Inertia
Speed rise of the generator can be reduced by addition of inertia to a generator turbine unit. They also quote a rule of thumb stating that the cost of a generator would increase by 1% for each 4% of inertia added.3 (next page) shows a schematic design of a turbine bypass valve system. This approach provides good responses for loss off load situations. A bypass valve having a capacity of 33% .0 times standard inertia.2.
Figure 5. For vertical axis generators the cost of increases in crane capacity and load bearing strength of the powerhouse structure must also be considered.several lower cost approaches that can be applied for controlling waterhammer pressure rises and related generator speed deviations.
5.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 49 . The features of these alternatives are described below:
5.2. Some approaches will provide protection against pressure rises but give little support for speed regulation. but load addition characteristics are less satisfactory as the turbine would only be able to take on load at a reduced (slow) rate. Some alternatives are more reliable than others.3 Bypass Valve A bypass valve. as the name implies. This is easily achieved by the addition of a flywheel for horizontal axis machines or by the addition of mass to the generator rotor for vertical axis machines.5 to 3. According to Gordon and Whitman (1985) inertia of a vertical axis machine can be readily increased up to 2.60% of turbine flow capacity is usually satisfactory.2.1 Increasing Conduit Flow Areas
Increasing the diameter (and flow area) of the penstock will improve control of hydraulic transients but this approach is rarely economic due to the increases in penstock cost. This allows the turbine to be closed quickly while diverting flow through the bypass valve thereby avoiding excessive waterhammer and generator speed rises. In effect the turbine wicket gate closure curve has two portions an initial fast closure rate. A copy of this paper is provided in the appendix to this-sub section. Savings in capital costs may be offset by increases in maintenance.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 50 .2.Figure 5.3: Schematic of Turbine Bypass Valve System.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 51 .4
Deflector Arm (Pelton Turbine) Pelton turbines are usually provided with jet deflector arms. In this manner the hydraulic load on the runner is diverted and the generator speed rise is thereby limited. In contrast to bypass (synchronous) valves PRV are activated by pressure rise. The effect is similar to the behaviour of a bypass valve. When surge level exceeds this elevation water starts to spill from the stand pipe.
5.2.2. translated from Russian. thus measures for erosion protection and drainage must also be provided. spray control and drainage are also required.
5.2. Limited information is available on the design of safety membranes.6 Stand Pipe (Mini Hydro)
Pressure relief can also be provided by a stand pipe.2.5 Pressure Relief Valve (PRV)
Pressure relief valves are sometimes employed to control excessive pressure rises due to waterhammer. which is essentially the riser pipe for a surge tank without the tank. As releases may be at high pressure facilities for pressure dissipation.5. Safety membranes. When pressure on the membrane rises to a prescribed (design) level the membrane will burst to suppress further pressure rise. A paper by Kovalskii and Fedotov (1965). the needle valves can be closed slowly limiting waterhammer pressure rise. It is reported that this device is widely used in China and gives reliable service. A load controller switches the generator output the between the system load and a ballast load to equate the total load (system plus ballast loads)
AHEC/MNRE/SHP Standards/ Civil Works . meanwhile.2. gives a method for design.
5. However. the rate of load addition would be relatively slow. This type of governor is called a “two part” governor since the governor must control both needle valve positions and deflector arm operation. usually made of sheet aluminium and mounted on the penstock near of the power house are controlled points of weakness. (This approach is usually applied to mini-hydro plants). When a jet deflector arm is deployed it introduces a deflector bucket into the Pelton jet diverting the jet away from the runner. The stand pipe top should be set at a height somewhat above the static water level. such valves should be designed to open at a specified pressure and then re-close at a specified lower pressure.7 Safety Membrane (Mini – Hydro)
For mini-hydro plants protection against excessive waterhammer pressures can be provided by safety membranes. as for a bypass valve.8 Load Control Governors (Mini -Hydro):
Load control governors provide an effective means for control of frequency and waterhammer.
5. The operation of the deflector arms are be controlled by the turbine governor. The height of the tank should be selected to accommodate small load rejection without spilling compatible with system operating requirements.
The action of the simple surge tank is sluggish and requires the greatest volume.2. the role of the planned power plant should be carefully assessed through discussions with the power system operator and the most cost effective solution selected. • To improve the regulating characteristics of a hydraulic turbine. In such a situations use of a surge tank may be required.
5.to the output of the turbine / generator unit. differential and orifice types. Modern designs usually employ either the restricted orifice. the simple surge tank and two types of throttled surge tanks. With a surge tank.
There are three common types of surge tank used in hydropower plant design.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 52 . It is the most expensive and seldom adopted in preference to the other types. On larger interconnected systems where the contribution to frequency control of a given unit is less important one of the other less costly alternatives (2.3 5. The main reason for considering construction of a surge tank is where a plant operates in an isolated system where both frequency and waterhammer control must be provide by the proposed hydro plant.1 to 2. the length of water column initially accelerated (or decelerated) is limited to the portion of conduit downstream of the surge tank junction to the powerhouse which is typically much shorter than the full length from intake to powerhouse. thus limiting the play of waterhammer to the section between surge tank and powerhouse rather than between reservoir (intake) and powerhouse.7) is likely to be satisfactory. while during load acceptance water can initially be drawn from this storage. This approach is usually suitable for mini-hydro plants but becomes less attractive for plants greater than about 1000 kW due to the expense of the ballast load. This permits water in the upstream conduit to be accelerated without excessive drop in pressure in the penstock supplying the turbine. It is the most expensive of the alternatives given in this section. but the most effective and reliable. • A surge tank provides storage for excess water on load rejection.2. Therefore. In effect the turbine / generator unit operates at its hydraulic (flow) capacity at all times and load changes are made without adjusting turbine flow or provoking waterhammer.5/2.1 HYDRAULIC DESIGN OF SURGE TANKS Background The main functions of a surge tank are: • To reduce the magnitude of waterhammer pressures at the turbine by reflecting incident waterhammer waves at the surge tank.
5. or differential type. Details on the hydraulic design of surge tanks are given in the following subsection. The latter is a compromise between the simple and restricted orifice types.3. It
AHEC/MNRE/SHP Standards/ Civil Works .9 Surge Tank
A surge tank provides a reliable solution that controls excessive waterhammer pressure rises and provides good speed regulation characteristics as well.
However the differential surge tank is more expensive than the orifice type by the cost of an internal riser. but does not provide formulae for computing minimum surge levels (this omission is addressed in Sub-Section 2. Alternatively. Army Corp. IS 7396 provides a comprehensive methodology for the dimensioning and hydraulic design of simple.6
AHEC/MNRE/SHP Standards/ Civil Works .1 of this Standard. the function of the combined turbine penstock surge tank system can be investigated using a simulation program such as WHAMO.0 1. For a long conduit the length affected by waterhammer will be longer for an orifice type than for a differential type of surge tank. Restricted Orifice and Differential Surge Tanks” provides detailed advice on the hydraulic design of surge tanks. The standard provides formulae for preliminary design. IS 7396 also recommends that the selected design be verified by detailed numerical calculations. IS 7396 recommends the following factors of safety be applied to Thoma’s area Ath and Thoma’s adjusted area. The extent is mainly a function the ratio of orifice area to conduit area: where this ratio is large is favours the orifice type and where this ratio is small it favours the differential type.
IS 7396 “Criteria for Hydraulic Design Surge Tanks” Part 1: Simple. It is recommended that the designer follow this standard. which reads: The assumed area of the orifice should be so altered that the values of waterhammer pressure and pressure due to upsurge are nearly the same. customary units). The necessary formulae for such calculations are also given.S. as explained in Sub-section 2. typically the coefficient for inflow is higher than for outflow.5/5 Design Aids).Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 53 .decelerates flow less abruptly than the orifice type and transmits less waterhammer upstream.S.3.5. IS 7396 recommends surge tank design should be based on “balanced design” – as recommended in Article 5.6/7.1). Some additional comments and design suggestions are added for the designer’s consideration.2.3. (The main inconveniences of this program are that detailed turbine characteristics must be known and all data and results are given in U. including recommendations on design conditions (Clause 5. restricted orifice and differential surge tanks. Orifice design may incorporate geometry giving different head loss coefficients for inflow and outflow. of Engineers (USACE) and currently available over the internet. In such a situation the additional cost of a differential surge tank may be offset by savings in conduit steel. As: Types of Surge Tank Simple Restricted orifice or differential Factor of Safety 2.2. developed by the U. including tank diameters and maximum surge levels. The choice depends on the extent of upstream conduit affected by waterhammer in a given case. This issue is discussed further with reference to penstock design.
S.S. Selection of paints.I. Authors such as Chaudhry recommend factors of safety of 1.0 MW) consideration should be given to more detailed computer simulation analyses to verify the feasibility of reducing the safety factors prescribed in IS 7396. governors.0 MW to 25.With modern digital electronic governors. For mini hydro plants (P ≤ 1000 kW) it is recommended that detailed numerical analysis be omitted and surge tank design based on graphical methods of Parmakian for simple and restricted orifice surge tanks and Creager and Justin for differential surge tanks. respectively.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 54 .H.R.D.5 and 1.
5.3. especially where significant cost savings can be realized. U. For larger projects (1.B. the tank shall also be protected against internal corrosion by painting. Gordon and D. notably P. 5.L. especially in the case of multi-unit plants where sequential load acceptance may produce a smaller down surge than the down surge produced by oscillation subsequent to the maximum surge on sudden load rejection. Other References:
Selecting Hydraulic Reaction Turbines Engineering Monograph 20 U. steel surface preparation and applicable shall be in accordance with the applicable norms. the adjustments to flow can be made more precisely and smaller safety factors can be employed without risk of stability problems. Army Corps of Engineers Generator Inertia for Isolated Hydropower Systems J.3 REFERENCES IS Standards cited: IS 7396 (Part 1): Criteria for Hydraulic Design of Surge Tanks. Water Hammer and Mass Oscillation (WHAMO) – Computer Program USACERL ADP Report 98/129 Construction Engineering Research Laboratories.25 for simple and throttled surge tanks. (Some further work is required to confirm this recommendation. Comments requested).2 Other considerations Structural design of surge tanks shall comply with applicable IS structural steel and concrete standards. Whitman Canadian Journal of Civil Engineering
• Johnson’s charts for estimating maximum up and down surges for differential surge tanks.M. Parmakian.S. no.Koval’skii and V. New York (ca..Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 55 .Chaudhry. 1950).
5.Volume 12. • Paper by Gordon and Whitman on determination of generator inertia.P. New York (1978). Number 4 – 1985
Applied Hydraulic Transients H. The following information is provided • Formulae for determining loss coefficients of conical orifices • Chapter 17 on design of restricted orifice surge tanks and design charts from Water Hammer Analysis by J. Hydroelectric Handbook Creager and Justin McGraw Hill. Table 24
AHEC/MNRE/SHP Standards/ Civil Works .May 1965).Fedotov (translated from Khimicheskoe I Neftyanoe Mashinostroenie. Bursting Safety Membranes B. DESIGN AIDS
Selected information is provided in the following appendices to supplement information provided in IS 7396 (Part 1). Included are methods for calculating minimum surge levels. 6 . • USBR Engineering Monograph 20.4. Van Nostrand – Reinhold Co.
5 α − α ) (1 − φ ) ⎦ ' 2 H LO = K .5α − α ) (1 − φ ) ⎦ H LO = K .4.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 56 .1
⎡ ⎤ 1 K =⎢ −ψ ⎥ 3/ 2 2 ⎣1 − (1.2
⎡ ⎤ 1 K =⎢ −ψ ' ⎥ ' '3/ 2 '2 ⎣1 − (1.4.5.V / 2 g
AHEC/MNRE/SHP Standards/ Civil Works .V 2 / 2 g
possibly with increasing amplitude. Operation
A surge tank is often used at a power or pumping plant to control the pressure changes resulting from rapid changes in the flow. Similarly. a surge tank can also be used to effectively control the pressure changes in the discharge line resulting from the shutdown or starting up of a pump. In order to accomplish its mission most effectively. following the sudden shutdown of a pump. In the event the area of the tank is too small. the farther the surge tank is away from the plant the less effective it will be. (d) The bottom of the surge tank should be low enough that during its operation the tank will not drain and admit air into the turbine penstock or pump discharge line. This problem of surge tank instability is outside the scope of this treatment. the surge tank must have sufficient cross-sectional area to prevent unstable action. Such a conversion of energy reduces the rate of change of flow and the waterhammer in the penstock between the forebay and surge tank. energy is provided by the surge tank for the immediate demand of the turbine. a load change on the turbine will cause continuous oscillations of the water level in the surge tank.
Waterhammer Analysis – by J. Stucky). upon an opening movement of the turbine gates.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 57 . where from other considerations it is necessary to place the surge tank at a considerable distance from the power or pumping plant. Upon starting a pump. the cross-sectional area of a surge tank at a power plant should be large enough that the magnitude of the surges will be small during normal load changes on the turbine. (c) The surge tank should be of sufficient height to prevent overflow for all conditions of operation unless an overflow spillway is provided. For example. the surge tank provides energy to reduce the rate of change of flow and the waterhammer in the discharge line. when the turbine gates are closed at a power plant which is supplied by a long penstock. turbine speed regulation will be difficult or impossible. For example. This action reduces the waterhammer effects in the long penstock and assists the turbine to pick up its increased load more rapidly. At a pumping plant with a long discharge line. At such installations the
AHEC/MNRE/SHP Standards/ Civil Works . most of the initial flow from the pump enters the surge tank and this action reduces the waterhammer effects in the long discharge line.
67. On high-head plants. (b) The surge tank should be located as close to the power or pumping plant as possible.(Source: Chambres d’ Equilibre by A. Otherwise. Parmakian CHAPTER XVII
(For determination of maximum up and down surges for restricted orifice surge tanks). In addition. the water surface in the surge tank rises slowly above the original running level as the kinetic energy of the rejected flow is converted into potential energy. the surge tank dimensions and location are based on the following considerations: (a) At a power plant where the turbine output is controlled by a governor.
the rigid water column theory of waterhammer is utilized since the effect on the upsurge of the stretching of the pipe walls and the compressibility of the water due to an increase in pressure is negligible. that is. the flow of water into the surge tank is the same as that out of the penstock.0. the hydraulic losses and the velocity head in the pipe line are initially neglected. It is desired to find the maximum upsurge in the surge tank and the time at which this upsurge occurs. Then
AHEC/MNRE/SHP Standards/ Civil Works . Prior to the gate closure.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 58 . F dS = AV1 dt (61)
The simultaneous solution of Equations (60) and (61) is performed with the following boundary conditions: When t = 0. Upon gate closure the unbalanced force acting on this water column is wAS. From Newton's second law of motion the deceleration of the water column in the penstock is dV gS − 1 = (60) dt L
From the condition of continuity of flow following complete gate closure.waterhammer effects in the length of pipe between the plant and the surge tank should be investigated by the methods described in either Chapter XIV or Chapter XV. S = 0 and dS/dt = Qo /F. In order to present the phenomena in its most
Consider the simple surge tank installation shown in Figure 76 where the initial flow through the control gate is cut off rapidly. Moreover. the mass of water which is moving in the penstock is LAw/g.
The head tending to accelerate the water in the pipe line in the direction of the positive velocity V1 is H a = − S ± C1V12 ± c 2V22 (65)
where the signs of the last two terms depend on the direction of V1 and V2. The magnitude of the surge in the tank with the friction effects included will now be determined.8 feet and the time required to reach this upsurge is 30.
AHEC/MNRE/SHP Standards/ Civil Works .3 seconds. (62) 69. Analysis including hydraulic losses and throttling Consider the surge tank system shown in Figure 77 where the positive directions of flow and surge are designated.from which S max =
For the installation shown in Figure 76 the maximum upsurge in the surge tank above the static level due to the gate closure is computed to be 51.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 59 .
Case 1 for turbine shutdown reduces to the following differential equation: 2 Q2 ⎞ Qg dS Ag ⎛ A ⎞ ⎛ dS ⎞ d 2 S Fg ⎛ ⎜ c1 + c2 2 ⎟ ⎜ ⎟ + 2c1 ⎜ s + c1 2 ⎟ = 0 + + AL dt FL ⎜ AL ⎜ A ⎟ A2 ⎟ ⎝ dt ⎠ dt 2 ⎝ ⎠ ⎠ ⎝ (70) The substitutions b=±
AHEC/MNRE/SHP Standards/ Civil Works . V2 < 0. H a = − S − c1V12 + c 2V22 (65B) (c) Case 3 (down surge caused by pump shutdown). For example. V1 > 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 60 . Then c 2 is a constant such that c 2V22 = H f 2 and represents the throttling loss for the flow into or out of the surge tank. (65C ) H a = − S + c1V12 + c 2V22 (d) Case 4 (upsurge caused by starting pump). V2 > 0. (65 A) H a = − S − c1V22 − c 2V22 (b) Case 2 (down surge caused by starting up turbine). V2 < 0. and t1 is a function of t. By suitable changes in variable this equation reduces to the following form:
In this equation H f1 c1Q Fg Fg =± A AL Q ( L / A) Now S2 is a function of S. The following tabulation gives Ha for four possible cases: (a) Case 1 (upsurge caused by turbine shutdown). V1 > 0. pipe line friction loss. and velocity head in the pipe line. Then from Newton's second law of motion dV1 g = Ha (66) dt L For continuity of flow AV − Q (67) V2 = 1 A2 dS AV − Q = and (68) dt F By substituting Equation (65) into (66) and using (67) and (68) to eliminate V1 and V2 a differential equation is obtained in S and t. V1 < 0. V2 > 0. V1 < 0.In this equation c1 is a constant such that c1 v12 = H f 1 and represents the sum of the entrance loss. (65D) H a = − S + c1V12 − c 2V22
The mass of fluid in the pipe line being accelerated is ω AL / g and its acceleration at any time is dV1/dt.
Q2 S = S1 − c1 2 . Ag reduce Equation (70) to one form of Equation (69).
See Reference 19. The solutions2 of Equation (69) for the four special cases of turbine and pump operation are given in Figures 78 and 79.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 61 .
AHEC/MNRE/SHP Standards/ Civil Works . A ALS 2 S1 = 2 2 Fg [c1 + c2 ( A 2 / A2 )]
FL t1 .
based on theoretical analysis. which is now becoming more important as customers install computers. Introduction
AHEC/MNRE/SHP Standards/ Civil Works . 1985 Revised manuscript accepted August 7. an empirical equation is developed for the generator inertia as a function of the aforementioned parameters. Montreal. type of load. and relief valve operation – are all discussed. speed regulation. L. on which data from hydro projects is plotted. This paper presents a methodology for determining hydroelectric generator inertia. P. The parameters that affect generator inertia-system size. coupled with a review of data from over 50 hydroelectric projects with units having capacities between 2 and 300 MW. 1985 Abstract: Speed regulation of hydroelectric power plants of isolated systems is a complex subject.Generator inertia for isolated hydropower systems J.Q. GORDON AND D. and advanced satellite dish electronic equipment in such systems.. Box 6088. allowable frequency variation. hydro design. Key words: hydroelectric power. Canada H3C 3Z8 Received 1anuary 17.O. A chart combining these parameters is developed. WHITMAN Monenco Consultants Limited. H. Station A. From an analysis of the plotted data. P. water column start time. turbine and governor.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 62 Appendix 3:
. governor time. stereophonic equipment. generator inertia.
the power-systems engineer will determine the allowable frequency deviation. and civil engineers: electrical through the generator and controls. with consequent higher waterhammer. Its scope affects the work of electrical. First. the town power generation was by large. mechanical through the turbine. at a value of 0. often referred to as "standard inertia" (Westinghouse 1959).98. and has control over the major parameters that influence selection of generator inertia. However. and all except one. On large systems such faults due to storms are an infrequent occurrence. were commissioned. and powerhouse crane. If a storm should interrupt incoming power on a transmission line. and support of the powerhouse crane. ranging in size from 615000 kV.A down to 300 kV' A. mechanical. it is the civil engineer to whom this paper is directed.25 N s−1. locating the surge tank closer to the turbine. hence determining the optimum configuration will require a great deal of study. This is when inertia becomes valuable. the generator usually has a minimum inertia. since the large inertia of the interconnected system keeps frequency deviations within a fraction of 1 Hz (Schleif 1971). the local hospital converted a water heating
AHEC/MNRE/SHP Standards/ Civil Works . were found to have inertias equal to or higher than the minimum indicated by [1]. When connected to such systems. slow-speed diesels. installed to provide power for an adjacent town. which has a value of [1] GD2 = 310 000 (MVA) 1. All of these alternatives add cost. and then the civil engineer. using larger water passages to slow down the water velocity. using faster governor times. with larger inertia reducing the magnitude of the frequency excursion. Previously. The options available to the designer for improving frequency regulation include use of a surge tank.875
This formula was used to check the inertia ratio J of over 120 generators.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 63 . there is no greater interdisciplinary problem than that of selecting the required generator inertia. civil through sizing of the water passages. which will result in a rapid load-on at the remaining power plants on the system. The inertia requirements for hydropower generators have received very little attention. mainly because it is the civil engineer who is most directly affected. This simple fact was brought to the authors attention by an incident that occurred shortly after two 5. layout of the powerhouse. To take advantage of the new hydropower source. However. will have to develop a layout and equipment configuration that will meet the frequency requirements. hence the cost of adding extra inertia usually cannot be justified.6 MW Kaplan units at a power plant in north western Canada. It is only when there is a disturbance to the system that inertia comes into use. the sudden loss of generation will cause a major frequency drop. and adding inertia to the generator. due to the fact that on a large interconnected electric power system the governor is rarely needed to counter a frequency deviation. governor. using a relief valve on the turbine.During design of a hydropower project. and the amount of inertia must be carefully assessed in order to avoid excessive frequency deviations. on smaller systems normal changes in load often become a significant proportion of the total system capacity. with some help from the turbine-generator engineer.
and hence the extent of the temporary frequency deviation. The cost of inertia The inertia of a generator can be increased up to about 2. It has been applied with success for over 20 years to generators powered by reaction turbines. sufficient to keep the frequency deviation within about 1 or 2 Hz. A general rule of thumb states that the cost of a generator increases by 1% for every 4% increase in inertia. The methodology indicated that the system was not stable when subjected to a major load change. It was not until 1968. or a combination of both (Gordon 1978). In
AHEC/MNRE/SHP Standards/ Civil Works . With vertical shaft units.generator unit. such as that caused by loss of generation at one of the plants due to a fault. it was initially believed that the approach was not correct. inertia is added to the generator rotor by either increasing the diameter.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 64 . On loss of generation the whole system shuts down because under frequency relays trip out at substations. Between 1/2 and 2 h was usually required to reconnect the system. both of which have a very important bearing on the reaction of the turbine .5–3. so that the load was added in 1 MW steps with a time delay between steps.boiler from oil to electricity. Another solution would have been to install generators with a higher inertia. isolated power system in the high Andean mountains of South America. For impulse units. More recently. capacity of the generator. This conclusion was reached after applying the methodology to a small. This solution would have required implementation during construction. due to the different governing mode on load rejection at an impulse unit. There is no allowance in this formula for such factors as the governor time and the magnitude of the load change. the incident does serve to illustrate the type of problems that can arise in isolated systems when the size of the load application relative to the generator capacity is not taken into account. resulting in a temporary blackout. and the power plant automatic under frequency relays initiated breaker opening to disconnect the power plant source from the town. where several impulse unit power plants supply a city and a few small industrial loads. The problem was solved by changing the boiler controls. when one of the authors visited the area and enquired as to what happens when one of the power plants drops off the system. Whenever the 4 MW boiler started up. The methodology developed in this paper will enable to the designer of a hydro power plant to determine the minimum requirements for generator inertia. and will also permit comparison of the selected inertia with that at other hydro plants with similar operating criteria. the analysis has also been used for impulse units. thus avoiding the cost of excessive inertia. the sudden load application caused the frequency to drop. or the weight. For this development. However.0 times standard inertia. and water column start time only. that the system operating problems became apparent. the inertia of the two generators was based on an approach outlined by NEMA (1958). the system appeared to be operating correctly. and would have been too costly. However. as follows: [2] Tml + Tm2 > 100Tw(MW)-1 with the size of the unit varying from a maximum of 50 MW to a minimum of 20 MW. thus avoiding tripping of the under frequency relays. the authors realized that a more comprehensive approach was required. From this incident. and therefore developed a preliminary version of the analysis outlined in this paper. using a formula for unit inertia with functions for unit speed.
in foot pound units. In practice. it is probable that a 4% increase in inertia will add a cost equal to about 2% of the generator cost. as weight times radius of gyration squared. For small horizontal units.231× 10-6 (WR2) N s2 (KVA)-1 in foot pound units. the extra cost of the powerhouse superstructure. which has a value of [4] H = 0. A is equal to turbine capacity in kW. For economy. In this case the inertia value is for the entire rotating mass. In American units the inertia is termed WR2. based on the following equation: [9] N2 N s−1 = 0. it will be seen that [8] Tm = 2H when generator rating in kV.addition. as weight times diameter of gyration squared. In metric units. The mechanical start-up time is measured in seconds and represents the theoretical time required for the unit to reach synchronous speed when accelerated by a force equal to the full load output of the turbine. and perhaps the substructure required for the larger. Use of generator inertia constant The generator inertia constant can be used to quickly calculate an approximate value for the unit speed deviation for sudden pulse load changes (Moore 1960). since manufacturers work with different frame sizes. Another measure of inertia is known as the unit mechanical start-up time Tm(USBR 1954). including turbine runner and any flywheel. Alternatively it can be expressed in tonne metre units as [5] H = 1. in kilowatt seconds per kilovolt ampere. By comparing [5] with [7]. inertia is termed GD2in tonne metre units. However. By the time all costs are included. it is therefore essential to keep generator inertia to an absolute minimum.5 (KW) tH-1 (KVA)-1
AHEC/MNRE/SHP Standards/ Civil Works . It usually has a value ranging between 1 and 4. The start-up time is given by the following equations: [6] Tm = 0. these step increment costs will occur at different points. must also be taken into account. There will then be a step incremental cost for the larger frame size for the next increment of inertia. the cost of extra inertia is small provided the additional inertia can be fitted into the same generator frame size. heavier generator. thus smoothing out the cost increments in a competitive bidding situation.37 × 10-3 (GD2) N s2 (KV A)-1 H is the inertia constant. the common measure of inertia is the H factor (Hovey 1960).Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 65 . but space requirements are significantly larger than that I for a comparable vertical-shaft unit. formulae for inertia are given in both metric and American units.m2) For a generator. assuming that there is no reaction from the turbine governor. and neglecting the inertia of the turbine runner. extra inertia is usually added with flywheels and the cost increment is lower. for a particular manufacturer. Measures of inertia For the convenience of readers. The relationships between them is [3] WR2 (Ib-ft2) = 5932GD2 (t.621× 10-6 (WR2) N s2 (HP)-1 [7] Tm = 2.74 × 10-3 (GD2) N s2 (KW)-1 The inertia constant and the unit start-up time are obviously related.
the value used for time t should be equal to about one half of the governor response time required for the load change. Another factor is the number of generators connected to a system. (4) the type of turbine. assume a generator rated at 40000 kVA with an H value of 2. This conclusion can be reached by using [9] to determine the approximate frequency deviation. some attention has to be given to unit inertia. In the above example. for small systems with a total installed capacity of about 15 or 20 times the magnitude of the load change. If the system has only a few generators. stereo systems. if there had been five generators. nowadays. the frequency deviation would reduce to 1% or 0. and microwave equipment. and a pulse load of 5000 kW applied for 2 s.5 ×5000 × 2× 2. Each of these factors is discussed as follows: The size of the system – As mentioned previously. (5) the type of governor. so that the addition of inertia for frequency regulation can be neglected except in the case of system fragmentation. The speed deviation will then be N2 N s−1 = 0.6 Hz. However.05 In a 60-cycle system. this would mean a speed deviation of 60×0. These are (1) the size of the system. Another method of calculating the speed deviation.5-1 × 40000-1 = 0.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 66 . As mentioned previously.05 = 3 Hz. and (8) the relief valve operation. the largest frequency deviation will probably be caused by dropping the largest fully loaded generator. which allows for governor action. Factors affecting inertia selection There are eight basic factors that must be taken into account when determining the amount of inertia in the generator. frequency excursions of up to two or three cycles were acceptable. (3) the type of load. has been published (Gordon and Smith 1961). both of which will reduce the magnitude of the speed deviation. and nowadays there are several computer programs available that take into account the action of modern electronic governors. for the unit on the system used to control frequency. television. and the size of the largest generator on the system. large systems have excellent frequency control. leaving the other generators to cope with a large sudden increase in load. the total kilowatt seconds of flywheel effect are simply added together. However.5. (7) the governor time. or (2) the rotating inertia of the connected load. particularly to high-speed
AHEC/MNRE/SHP Standards/ Civil Works .For example. (2) the allowable frequency excursion. If there are several generators on the system. The allowable frequency excursion – Before the advent of electronic computers. (6) the water start time. and then using judgement to determine whether further investigation is necessary. frequency excursions of more than one-half cycle can cause problems. this method of calculating speed deviations is approximate since it does not allow for (1) the action of the governor. In [9].
the frequency excursion on load rejection can be kept within an extremely small value by rapid action of the jet deflectors. it cannot be determined with any accuracy. Hence an impulse unit will have a better response to pulse load changes than a reaction unit. the governor controls flow of water through the unit. which require an almost constant frequency if breaks in the paper roll are to be avoided. or electric-powered shovels in an open pit mine. which is then removed from the system a few moments later. several can commence excavation of the ore body at the same instant. however. The type of turbine – With a reaction turbine. rate of change of speed. [9] can be used to determine whether frequency excursions are likely to be within tolerable limits. very large sudden load changes can be expected. The type of governor – Currently it is possible to purchase either mechanical governors. allow use of longer water start-up times (Howe 1981). Ina Kaplan unit the blades are also moved. or a deep underground mine with a high-powered shaft hoist. Large shaft hoists usually have a large power demand on starting and acceleration. and can be neglected in an initial appraisal. With shovels. but response to a large load increase will be about the same as that with a reaction unit. with the motors demanding full stalling torque. and have more adjustments. sudden load changes will be small and inertia can be kept to a minimum. On the other hand. if the load consists of large electric arc furnaces. which measure speed and speed deviation (two elements). These varying and pulsating types of load to not contribute towards system stability. This is usually about 10.25% of the connected generator inertia. followed later by generation of power on braking to decelerate and stop. permitting a better matching of the governor
AHEC/MNRE/SHP Standards/ Civil Works . Another factor is the rotating inertia of the load. in an impulse unit. by means of the wicket gates.paper machines. However.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 67 . but at such a slow rate relative to the wicket gates that their effect can be neglected. and hence power. Again. and will require a detailed examination of unit inertia. resulting in a more conservative answer. or electronic governors. which measure speed. Electronic governors are more precise. The type of load – If the load consists of a town with only small industrial establishments. and speed deviation (three elements).
The total time is the full stroke time including cushioning. Load-on speed deviation The equation that has been developed for speed deviation during a part load change is
AHEC/MNRE/SHP Standards/ Civil Works .response to the nature of the load change. frequency excursion. Also. this results in a large loss of water and hence is rarely cost-effective. the smaller will be the frequency deviation. and larger frequency deviations. There are two measures for the governor time. resulting in a more sluggish response of the governor. or surge tank. If the unit responds well to load acceptance. namely. to the reservoir. However. Usually. the total time is equal to the affective time plus a few seconds. but is usually included in a governor stability analysis.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 68 . Furthermore. the chart could then be used to compare the relative performance of units on different systems. type of load. as demonstrated by Ransford (1983). They can be used to limit frequency deviations if operated in a water-wasting mode. the response of the unit to a large load-on condition becomes the prime criteria in assessing unit performance. the response of a three-element governor to a large load change is very similar to that of a two-element governor. − The relief valve option can be discarded since its use is not recommended for speed regulation. The governor time – The response time of the governor is of prime importance. The problem now becomes one of developing an analysis that takes into account all of the remaining factors.7 times the water start time Tw for a maximum water hammer in the region of 50% to about 10 times Tw for a water hammer of about 10%. Tg. However. so does the governor time. maintenance costs for the relief valve will be excessive. The effective time is the time taken to move the wicket gates or needle valves through a full stroke with no cushioning at the ends of the stroke. If the results of such an analysis are plotted. It can be calculated from the following equation: [10] Tw = ( (Σ LV ) g-lh-1 where Σ LV is the sum of the length times velocity for the water conduit upstream of the turbine. the response to load rejection will be equal or better. water start time. − Governor type can be discarded since response to large load changes is similar.) As the water start time increases. since the faster the movement of the governor. The relief valve operation – Relief valves are usually added to a turbine to limit water hammer on long conduits during load rejection. and the governor time. On this basis. the effective time Te.
For an isolated system. the LV of the draft tube is not included. and the total time. The water start time – This is the theoretical time required to accelerate the water column to the velocity at full turbine load. the effective time Te varies from a minimum of 2. hence relief valves are not recommended for limiting speed deviations. (Note that in this particular analysis. several of the factors that affect unit performance can be neglected for the following reasons: − Turbine type can be discarded since response to load on is similar for impulse and reaction units. In this mode the valve operation is synchronized with the wicket gate movement so that when the wicket gates open the valve closes and vice versa. system size.
as the water hammer ratio Tw / Te increases. electronic. are energy producers. two element or three element) and the magnitude of the load change. with the units divided into four categories: . three lines can be drawn in Fig.System units. Based on the distribution of these units. . the authors have found that the ratio can be approximated. and for the related inverse Tm / Tg. improved speed regulation can be expected from units that plot on the lower right of the chart. speed deviation will decrease. all connected to a utility power grid. .Isolated units. as this ratio increases. and to have a higher ratio correspond to a higher inertia and therefore a more stable system. An examination of [11] will indicate that for the same load change (1) as hw increases.The ratio of T/Tm depends to a great extent on the type of governor (mechanical. in other words. Note that both of these ratios are non dimensional. most of which provide power to small mining operations or towns in northern Canada. and a negative water hammer of 50% will be reached with a Tw / Te ratio of only 0.5 ]
For a defined load-on this equation indicates that the speed deviation will become a function of two parameters: . designed to provide frequency control to the interconnected system. wherein it will be noted that a positive water hammer of 50% will occur when the Tw/Te ratio reaches 0. to separate the chart into four distinct areas:
AHEC/MNRE/SHP Standards/ Civil Works . 1. all of which have very low inertia – governor time ratios.
A chart can now be developed (Fig. (2) as T / Tm increases. The characteristics of over 50 hydroelectric developments have been plotted in Fig. so does the speed deviation.Base load system units. The ratio has been inverted to Tm / Tg for convenience. A chart showing the part load response rate of a typical mechanical governor has been published elsewhere (Gordon and Smith 1961). In order to simplify the problem.36.[11]
2 − N 2 N s−2 = 1 − TTm 1 [2 P2 − ( P1 + P2 )(1 − hW ) 1. speed deviation increases. with the longer time so obtained used to allow for the slower rate of response of a governor to part load changes. The water hammer ratio hw is a function of both the water column start-up time Tw and the effective governor time Te.41.Isolated units providing power to mining operations where large electric-powered shovels or large shaft hoists are used. . for comparison purposes. The Allievi water hammer charts can be used to develop this relationship as outlined by Brown (1958). 1) in which the water hammer ratio Tw / Te is plotted as the abscissa and the inertia per unit time ratio Tm / Tg is plotted as the ordinate. 1. speed deviation increases. Accordingly. and are not designed to provide any frequency control to the power system.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 69 . by using Tg / Tm where Tg is the total governor stroke including cushioning.
and 20% respectively.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 70 .6(1 + 1. then a value for J can be obtained by dividing [13] by [1].82 < k < 1. As an example. The lines between areas A–B. and assuming that MYA = 1. The relationship between Tm. and has the following values:
k < 0. the effective governor time will be about 4. 25%.1 × 4. The units would have to be equipped with relief valves operating in the water-wasting mode and fast governor times to assist in frequency regulation. and Te can now be defined in one equation as follows: [12] Tm= kTg(1+ TwTe-1) with k being an inertia factor that depends on the size of the system and the nature of the load.55 0. Tg. Area D – Units in this area can be expected to provide good to acceptable frequency regulation on isolated systems with large load changes.55 < k < 0. Area B – Units in this area can be expected to assist with frequency regulation on large systems only.
The three lines that separate these areas are based on using [11] to determine a theoretical speed drop for a large load-on.6 s.125 Tg (1+ TwTe−1 )
Equation [14] can now be used to determine how much extra inertia will be required in an isolated system to provide reliable frequency control. B–C.1 s. using the procedure developed by Gordon and Smith (1961). with a penstock layout that has a water start time of 1.10 < k (Area A) No frequency regulation possible (Area B) Frequency regulation on large systems only (Area C) Frequency regulation on small systems with small load changes (Area D) Frequency regulation on small systems with large load changes Equations [7] and [12] can now be combined to produce an equation for generator inertia as follows: [13] GD2 = 3.65 × 105k (MW) Tg (1 + Tw Te−1 ) N s−2 If the ratio of generator inertia to normal inertia is defined as J. deteriorating to barely acceptable speed regulation as load changes increase. A value for J can then be calculated as J = k ×20-025×150-0125 × 5. even on large systems. Area C – Units in this area can be expected to provide good frequency regulation on isolated systems with small load changes. to obtain [14] J = k (MW) -0. assume a 20 MW unit operating at 150 rpm providing power to a large mining operation. assuming an instantaneous 50% load increase.Area A – Units in this area will not be able to provide any frequency control.10 1.82 0.0 s and total governor time will be 5. Tp.25 N s−0.0-1)
AHEC/MNRE/SHP Standards/ Civil Works . and C–D correspond to theoretical frequency drops of 40%.14 MW.
k must have a minimum value of 1. 25-26.14. 1981. L. connected to the provincial grid. Allis-Chalmers Electrical Review. HOWE. If the transmission line is longer. Vol. 25(3). (2) Mayo in the Yukon. 5. a word of caution. 1978. and to indicate the types of development likely to be found in each area. 1958. to illustrate use of the chart. which gives a minimum value for J = 2. M. C. Predicting the stability of regulation. J. pp.W. L. This analysis has assumed that the length of any transmission line between the generators and the load is not excessive. BROWN. 30(1l). and SMITH. Finally. Blackie & Son Ltd. London.. MOORE. a not unreasonable figure. 11. pp. A 136MW two-unit impulse turbined power development operating under 381 m head on a 2. J. G. each of 110 MW. pp. 200.
AHEC/MNRE/SHP Standards/ Civil Works .0.4 MW power plant supplying an isolated gold mining operation. 1960. J. 33(7). 44(10). A 6.9 km tunnel with no surge tank. the Kainji development was the main source of power to the national grid. Hydro-electric engineering practice. If in doubt. or only normal inertia would be needed to assist in frequency control.12 installation.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 71 .3 MW unit operating at the end of a long penstock. Hence the unit must have at least 100% extra inertia in the generator. 1958. p. 32-35. National Electrical Manufacturers Association. Water Power and Dam Construction.For large load changes on a small system. Area A – Maggotty in Jamaica. J. 3-10. connected to the large Hydro-Quebec grid. a more detailed analysis will be required. Large 12unit. Typical examples Six typical power plants are identified in Fig. Engineering Journal.. At time of unit 11. NEMA. HT4-1958. A 19 MW isolated hydro development providing power to an open pit mining operation at Pine Point. A relief valve provides water hammer control.3 in Quebec. HOVEY. New York. Engineering Journal. which experiences major load changes. GORDON. Area D – Taltson in the Northwest Territories. or where the length in kilometres does not exceed about 12–15 times the power plant capacity in megawatts.R. Fig. 1. England. part of the James Bay complex. 1-6. 43(11).55. Determination of WR2 for hydraulic turbine generator units. Water Power and Dam Construction. GORDON. NY.J. a more detailed analysis will be necessary using a computer program to simulate action of the governor and water conduit during a load change. Estimating hydro powerhouse crane capacity. C. k = 0. Area C – (1) Kainji units 11and 12 in Nigeria. (2) La Grande No. 1960. Conclusions Figure 1 along with [13] and [14] can be used to determine whether generator inertial will be adequate. Publication No. L. 14-17.0. A small two-unit 4. pp. Area B – (l) Cat Arm in Newfoundland. Optimum adjustment of governors in hydro generating stations. For the same development on a large hydro system. WR2 versus rotor loss. and J = 1. based on the requirements of the load and the size of the connected system.1. 2304 MW power plant. pp. 1961. Speed regulation for hydraulic turbines.
ERC. 1971.RANSFORD. expressed as a fraction of full load output Final turbine output. in kilovolt amperes Generator capacity. in megawatts Synchronous speed. in kilowatts The sum of water passage length times water velocity in that length. Governor characteristics for large hydraulic turbines. Bureau of Reclamation. D. F. SCHLEIF. in revolutions per minute Speed at end of load change. pp. Selecting hydraulic reaction turbines. Publication No. 1959. depends on system and load Generator rating. R. P. Publication REC. in kilowatt seconds per kilovolt ampere Turbine‐rated horsepower Generator inertia expressed as a fraction of normal GD2 Inertia factor.34. in megavolt amperes Generator capacity. in metres Water hammer head. based on diameter of rotating mass Turbine rated head. in tonne square metres.D. LG2-1959. United States Department of the Interior. Engineering Monograph No. Water Power and Dam Construction. Bureau of Reclamation. expressed as a fraction of full load output Time duration of pulse load. in seconds Governor time required for a part load change. Pittsburgh. WESTINGHOUSE. in square metres per second Generator rating. in metres per second squared GD2 h hw H HP J k KVA KW Generator inertia.I. regulation revisited. 1983. G. 1954. List of symbols g Acceleration due to gravity. expressed as a fraction of h Generator inertia constant. PA. 20. in revolutions per minute Initial turbine output.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 72 . USBR. 31. United States Department of the Interior. Normal rotor flywheel effect for standard ratings of large vertical hydraulic turbine driven synchronous generators. 35(1).71-14. in seconds ∑ LV MVA MW Ns N2 P1 P2 t T AHEC/MNRE/SHP Standards/ Civil Works .
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 73 . in seconds Generator inertia. in seconds Total governor time.Te Tg Tm Tw WR2 Effective governor time. based on radius of rotating mass AHEC/MNRE/SHP Standards/ Civil Works . in seconds Start‐up time of column. in seconds Start‐up time of water column. in pound square feet.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 74 .Appendix 4: Johnson’s Charts for Estimating Maximum Up and Down Surges for Differentials Surge Tanks.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 75 .
whence S ↓ = KdC 2 (V22 −V32 ) .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 76 . Where: ht = total hydraulic losses from intake to surge tank “T” V2 = average initial flow in u/s pipeline V3 = average final flow in u/s pipeline
APPENDIX 5: Table 24 (USBR Monograph 20) for estimation of generator runaway speed. W.L.2 g A3 ht and 100 Nd 2 = 100V2 .
AHEC/MNRE/SHP Standards/ Civil Works .L. in tank. to get min.C 2 2 V2 A1 L ∆ V2 as a percent of V2 gives the value of Kd from Graph 2. Subtract S ↓ from steady state W.
A step by step methodology is proposed. These economic calculations should be in accordance with Sub-section 1. • Assessment of the adequacy of machine rotating inertia. • When pressure rise at the scroll case on full load rejection < 50%. a method for determining the design pressures in a pressure conduit incorporating a surge tank will also be given. Experience shows that pipeline / turbine systems meeting the following conditions do not require additional protective devices: • Where L/H < 5 to 8.5/6.6 6. In this analysis the optimum diameter should be determined as the diameter for which the capital cost of the penstock plus capitalized value of hydraulic losses would be a minimum.55 ⎜ ⎜ 2 gH ⎟ ⎟ ⎝ ⎠ 3 Where: Q = flow (m /s) H = rated net head on turbine (m)
The reliability of these results will be enhanced if Sarkaria’s formula is first calibrated against recent and comparable designs. • Determination of waterhammer pressure extremes. additional machine inertia and if other protective devices are required. as below: • Preliminary design and optimization.0 of this Standard).
(Taken from Sub-section 2.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 77 .7 of this standard.
6. • Confirmation of acceptable effective wicket gate closing time.2 PRELIMINARY DESIGN AND OPTIMIZATION: For mini-hydro and small hydro plants the optimum penstock diameter can be determined using Sarkaria’s (1958) formula: ⎛ Q2 ⎞4 D = 3.1
HYDRAULIC DESIGN OF PENSTOCKS BACKGROUND The design of penstocks must take into account the related issues of waterhammer and speed control. While the focus of this sub-section is on the design of simple penstocks.2. or ΣLiVi • Where < 7 to 13 Hn • Where maximum speed rise of the generator on full load reject < 45%. For larger SHP it is recommended that a more detailed optimization analysis be undertaken.
3 to 0. 140 340 Poisson’s Ratio µ 0.75 n. The following table gives Manning’s values and other material properties for various pipe materials. cost.5 (use 0.3 Manning’s n
0.4) H0
Note that Z2 used in Allievi’s method is the same as (H0 + h)/H0.3) H0 c) For low head plants with reaction turbines (Kaplan.015
At this stage of design it is opportune to consider the use of pipes of various materials.2 to 0.
Table .Where water carries a significant sediment load steel pipes are preferred. steel or ductile iron pipes are preferred.009 0.a. n. taking into consideration.5 16. For high heads.a.3/9).2.a.a.3 0.014 0. For preliminary designs the following normal water hammer pressure rises may be assumed. if they are available in the required sizes. n.15) H0 b) For medium head plants with reaction turbines (Francis) h = 0.a.
AHEC/MNRE/SHP Standards/ Civil Works . because the internal and external corrosion protection layers do not decrease with the wall thickness and because there is a minimum wall thickness for pipe handling. Fixed Propeller) h = 0.106 12 140 54 8. Nevertheless spiral machine-welded steel pipes should be considered.109 206 0.009 0. constructability and service aspects.012 0.106 400 5 13 n. as a ratio of max head (h) divided by static head (H0): a) For high head plants with impulse turbines (Pelton) h = 0. due to their lower price.7 Coefficient of linear expansion (a) (m/m °C) . In such cases Mosonyi also recommends that water velocities should not exceed 3-5 m/s.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 78 .5 (use 0. but at medium and low heads steel becomes less competitive. fabricated welded steel is probably is best option.6. Today there is a wide choice of materials for penstocks.25 (use 0.1: Materials used in pressure pipes
Material Young’s modulus of elasticity (E) (N/m2) .55 2.3 n. 78.10 to 0.011 0.1 10 11 Ultimate tensile strength (N/m2) . For the larger heads and diameters. d) Finally a preliminary pressure design must be performed to determine penstock shell thickness (see Section 2. 0.
may be not available in sizes over 300 mm diameter. friction factor of and loss coefficients for trashrack and other form losses can be found in Section 2. The minimum radius of curvature of a PVC pipe is relatively large – 100 times the pipe diameter – and its coefficient of thermal expansion is five times higher that for steel. or final steady state velocity for valve opening (m/s) T = effective opening time of the valve a wicket gates (s) L = effective length of conduit (m) For conduits with variable diameters:
AHEC/MNRE/SHP Standards/ Civil Works . cement-asbestos. Appropriate values for Manning’s n.3 ESTIMATION OF WATERHAMMER PRESSURE RISES / DROPS Allievi has developed a graphical method which can be used to determine waterhammer pressure changes. bolted on site plain spun or pre-stressed concrete. PVC pipes are easy to install because of the spigot and socket joints provided with “O” ring gaskets.4 m diameter can be used up to a maximum head of 200 meters – because it is often cheaper. which eliminates field welding or with welded-on flanges. lighter and more easily handled than steel and does not need protection against corrosion. His graphs give pressure rise or drop ratios (Z2) as a function of two parameters ρ and θ . They are also rather brittle and unsuited to rocky ground. PVC or polyethylene (PE) plastic pipes. there is a choice between manufactured steel pipe. requiring a special machine. His charts are based on linear gate operation. Due to their low resistance to UV radiation they cannot be used on the surface unless painted coated or wrapped. PE pipes can withstand pipeline freeze-up without damage.1/4 and 5 and Section 2. but for the time being.2. while Appendix 1 gives formulae for calculation of waterhammer wave speeds.For smaller diameters. as defined below: aV0 Pipeline parameter : ρ= 2gH 0 aT Valve operation parameter: θ = 2L Where: a = waterhammer wave velocity (m/s) V0 = initial water velocity for valve closure.2. special factory fittings are required – PE pipe floats on water and can be dragged by cable in long sections but must be joined in the field by fusion welding. supplied with spigot and socket joints and rubber “O” gaskets. ductile iron spigot and socket pipes with gaskets.2 /5. Pipes of PE – high molecular weight polyethylene – can be laid on top of the ground and can accommodate bends of 20-40 times the pipe diameter – for sharper bends.
6.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 79 . Plastic pipe is a very attractive solution for medium for medium heads – a PVC pipe of 0. PVC pipes are usually installed underground with a minimum cover of one meter. glass-reinforced plastic (GRP). that is the effective area of valve or wicket gate varies uniformly with time.
aV0 2gH 0
H0 = initial stead state head Hs = static head Σhl = head losses Knowing ρ .(T75 – T25).2. see Figure 2.0.ΣLi Ai ΣLi Q V0 = and Ae For conduits with varying diameter / thickness ratios: ΣL a a= i i ΣLi Formulae for computation of waterhammer wave velocity (a) are given in Appendix 1. Z 2 find θ from the appropriate Allievi chart.3.
Assume Z2.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 80 .1. Effective area ( Ae ) = Effective opening or closing time for wickets gates is usually taken as 2.1 Determination of Waterhammer Pressure Rise Due to Valve / Wicket Gate Closure (Normal Operations): In this analysis forebay maximum operating should be assumed.
6. Allievi’s charts for determination of waterhammer pressure rises are given in Appendix 2 of this section.6 / 2. per sub-section 2.2. 2 Lθ Whence Te = a (Allievi’s charts are given in Appendix 2 of this Sub-section)
AHEC/MNRE/SHP Standards/ Civil Works .6.
aT 2L Knowing ρ and θ find Z2 from the appropriate Allievi chart. Minimum head at valve (H) = H0Z2 Note that governor opening and closing times can be set at different values, if required, to avoid vacuum conditions in the line, as explained in Sub-section 2.2.6/5.
PARAMETERS FOR FINAL DESIGN: Water hammer pressure extremes shall be determined for normal and emergency operating conditions. Normal Conditions: Governor and needle valves / wicket gates operating as designed. For maximum waterhammer pressure rises full load rejection shall be assumed coincident with maximum forebay water levels. For maximum pressure drops partial or full load addition under governor control is to be assumed, consistent with electrical system characteristics. Minimum forebay operating W.L. is to be assumed for these computations. Emergency Conditions: Emergency waterhammer is produced under the following conditions: Load rejection Governor cushioning stroke inoperative 2L Part gate closure in seconds at maximum rate of gate movement. a The waterhammer pressure rise (h) can be calculated by Michaud’s formula:
AHEC/MNRE/SHP Standards/ Civil Works - Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 81 Figure 2.2.6.2 (a) shows the results of the analyses for simple penstocks. For pipelines with surge tanks the surge and water hammer pressures are combined as shown in Figure 2.2.6.2 (b). This approach is approximate but on the conservative side. Diagrams for both normal and emergency water hammer have to be developed. It should be emphasized that waterhammer excess pressure vary linearly with respect to the length of the pipelines. In cases where effective velocities and wave velocities have been used in Allievi’s procedure, the result must be adjusted to give the correct, distribution of water hammer pressure. For larger projects (>10 MW) a more detailed waterhammer should be considered, especially where surge tanks or other pressure control devices are incorporated penstock/pipeline. Use of a simulation model such as WHAMO is recommended. Alternatively, traditional graphical or numerical methods can be used as explained in reputed authors such as Parmakian and Chaudhry.
AHEC/MNRE/SHP Standards/ Civil Works - Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 82 AHEC/MNRE/SHP Standards/ Civil Works - Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 83 5.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 84 . Generally the radius of curvature of a bend (R) = 3 to 5 times (d) the diameter of the penstock.5 LAYOUT 6.1 Route Selection The following points should be considered in choosing the layout and routing of a penstock.6. • Sharp bends should be avoided.
AHEC/MNRE/SHP Standards/ Civil Works . • The shortest practical route is preferred.
Depending of the size of the plants and the number of units proposed. Where this is not possible care must be taken design an effective drainage system to divert surface runoff away from the penstock and powerhouse.3 Buried versus surface design:
Surface Penstock: Advantages: • Easily accessible for inspection. single in the upper stretch and branching in the lower stretch.
AHEC/MNRE/SHP Standards/ Civil Works . Disadvantages: • Prone to rusting and corrosion being exposed.5. Choose an alignment that will ensure that penstock is always under positive pressure.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 85 . A horizontal section of 5 times the diameter of the penstock should be provided upstream of the scroll case entry – to ensure uniform distribution of flow velocity to the turbine. • Installation often less expensive • Easily accessible for maintenance and repairs • Safety against sliding may be ensured by properly designed anchorages. The following points should be addressed:
Number of Penstocks • Decided on the basis economic analysis of the merits and demerits and of different feasible alternatives.• •
A route following a ridge-line is preferred to avoid drainage problems. Vulnerable points are typically as “knees in the vertical alignments. • Short penstock – one to each unit. The decision number of penstocks is based on consideration of economics and practicality. • Long penstock (high head penstock). • A single big size penstock with manifold distributor to each unit . 6.
6.less costly when compared to multiple penstocks but hydraulic losses at the manifold may be significant • Civil works and number of accessories increase as numbers of penstocks increase.5. The minimum pressure gradient line should be at least one (1) penstock diameter above the elevation of the bend with reference to the top of the bend.
Buried Penstocks: Advantages: • Protection against effect of temperature. Susceptible to damage by landslides and rockfalls.• • • • •
Repeated painting of outer surface is needed.
The waterhammer wave initiated by action of the wicket gates is not perfectly reflected at the surge tank but a components is transmitted into the power tunnel upstream of the surge tank “tee” If the power tunnel is relatively long this will result in a waterhammer over pressure extending some distance upstream to a critical point C as shown in Figure 2. earthquake shocks.
AHEC/MNRE/SHP Standards/ Civil Works . Expansion joints necessary. 6. • Less visual impact. Chances of water conveyed being frozen in severe cold climates. • Maintenance and repairs difficult. • Installation costly. • No expansion joints are needed • Continuous support helps in reducing steel plates thickness Disadvantages: • Less accessible for inspection – difficult to locate leaks.2.6 UNUSUAL CIRCUMSTANCES
6. • Tendency of sliding of pipes on steep slopes.1 Penstock / Surge Tank Layouts with Long power Tunnels.3. • Protection against animals.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 86 . Supporting and anchoring on steep hill slope is difficult and costly.6. • Protection against freezing of water. • Need special coating against the corrosive action of ground water.
For restricted orifice surge tanks: t ≈ Tc (Where Tc is wicket gate closure time) For differential surge tanks: Y R t' t≈ r + Vz A 2 Where: Yr = max. surge tank and turbine is recommended using WHAMO or an equivalent computer program. above static R = area of internal uses (assume 0.2 Choking For low specific speed Francis turbines (NS<270) located on long penstocks there is a risk that maximum waterhammer could be caused by choking of flow by the turbine runner under runaway conditions. a detailed analysis of conduits.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 87 .The extents of this zone can be estimated by estimated as l * =
ta . If a preliminary assessment indicates the presence of persisting water hammer in the power tunnel.9A) A = area of power tunnel = initial velocity in power tunnel V2 t' = Tc ) This phenomenon does not affect simple surge tanks. rise in surge tank W.
6.6. This problem is a characteristic of low specific speed Francis turbines where turbine flow decreases as runaway speed increases (unlike higher specific speed Francis runners where turbine flow increases with
For low inertia machines full runaway can be reached in a few seconds (similar or less than the normal wicket gate closure time).runaway speed). In such cases the maximum waterhammer produced by “choking” could be greater than from normal wicket gate operation.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 88 . the calculation begins with the turbine NS value. The turbine and
AHEC/MNRE/SHP Standards/ Civil Works . If the above calculation indicates the likelihood of a problem due to the effects of turbine overspeed then the turbine manufacturer should be consulted and a detailed analysis of the power tunnel / surge tank / penstock / turbine system using WHAMO (or an equivalent computer program) should be undertaken in collaboration with the turbine manufacturer. Knowing the TW/TM value (the relative water and turbine inertia time constants) and selecting the relative wicket gate closure time TC/TE the relative maximum waterhammer pressure (∆HM/HO) can be determined. The objective of theses calculations would be to verify the severity of the problem and to evaluate corrective measures. The variables shown in this chart are defined below: (m3/s) • QRw = flow at full runaway • Qo = turbine rated discharge (m3/s) • TW = water starting time (s) Tm = mechanical staring time (s) • • TC = effective wicket gate closure time (s) • TE = time for one round trip for first elastic wave (s) • HO = gross head (m) • ∆HM = waterhammer pressure increase (m)
As indicated in the figure (follow the blue arrows). Moving horizontally the NS dashed line is reached and QRw/Qo can be found. This graph gives an approximate prediction of the maximum waterhammer pressure due sudden load rejection in Francis units of a small power plant. This phenomenon has been investigated by Ramos who produced the following chart from which the effects of turbine overspeed on waterhammer can be estimated.
Guidelines for Design of Small Hydropower Plants H. North Ireland (2000) Water Hammer and Mass Oscillation (WHAMO) – Computer Program USACERL ADP Report 98/129 Construction Engineering Research Laboratories. Army Corps of Engineers
6.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 89 . of Development.8. ⎥. new York (1978).. Belfast.7 References Water Power Development Volume 2A: High Head Power Plants E.S.Chaudhry Van Nostrand Rienhold Co.M.c1 ⎣E e ⎦ Where: a = wave celerity (m/s) K = modulus of deformation of water (GPa) ρ = mass density of water (kg/m3) E = young’s modulus pipe shell (GPa) D. Parmakian Dover Publications. Mosonyi Akadémiai Kiadó Budapest. Ramos Western regional Energy Agency & Dept.8 6.1 APPENDICES: Appendix 1: Formulae for Determination of Waterhammer Wave Celerity General Equation for wave celerity (a) k/ρ a = ⎡ k D⎤ 1 + ⎢ . U.
6. New York (1963) Applied Hydraulic Transients H. e = pipe diameter & thickness (mm) C1= factor for pipe restraint µ = Poisson’s ratio
AHEC/MNRE/SHP Standards/ Civil Works . Hungary (1991)
Waterhammer Analysis J.generator manufacturers should also be required to guarantee unit inertia (WD2) and TC (effective wicket gate closure time) at the design stage.
(1 + µ ) + D D+e • Pipe with expansion joints throughout its length D 2e C1 = (1 + µ ) + D D+e Circular tunnel: • Pipe anchored at upstream end only C1 = 1 −
Selected Material properties are listed in Table 2.2. ⎜1 − ⎟ C1 = (1 + µ ) + D D+e ⎝ 2 ⎠ • Pipe anchored against longitudinal movement D 1− µ 2 2e C1 = .6/1
Fluid Transients By Streeter and Wylie.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 90 .
AHEC/MNRE/SHP Standards/ Civil Works .⎛D ⎞ Restraint conditions for thin walled elastic pipes ⎜ ≥ 100 ⎟ ⎝e ⎠ 2 • Pipe anchored against longitudinal movements C1 = 1 − µ 2 • Pipe with expansion joints throughout its length C1 = 1 ⎛D ⎞ Restraint conditions for thick walled elastic pipes ⎜ <100 ⎟ ⎝e ⎠ • Pipe anchored at upper end only 2e D ⎛ µ⎞ .
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 91 .8.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 92 .
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 93 .AHEC/MNRE/SHP Standards/ Civil Works .
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 94 .
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 95 .
The choice and of cross section locations should be representative of the river channel. It is normally more practical to carry out a cross-section survey of the river downstream of the powerhouse. taking into account: access. Ideally a relationships established by flow and water level measurement is preferred. space requirements. accordingly. This data is needed to establish the tailrace head-discharge curve.4 7. from which turbine runner centre line and main floor elevations can be determined.4. as outlined in most standard text on open channel hydraulics.
7. also the water level at each cross-section should be recorded during the survey. Cross-sections should be extended above the visible high water level to accommodate at least the 1 in 100 year flood. • Determine appropriate canal lining and/or erosion protection DATA REQUIREMENTS It is of utmost importance that sufficient data be collected to establish a reliable head-discharge relationship in the river opposite the outfall of the tailrace canal. Suitable values for Manning’s “n” coefficient are given in Appendix 1 to this section. This will help in keeping the tailrace channel clear of bed load deposits that could cause back water effects at the powerhouse or require expensive maintenance dredging to control waters levels. However. with one at the time of river survey.3
AHEC/MNRE/SHP Standards/ Civil Works . LAYOUT The powerhouse–tailrace setting is usually determined by practical engineering judgment. This will require backwater computation. Where site conditions are suitable it is recommended that the tailrace canal be oriented to discharge at an angle of 30°-45° to the centre line of the receiving river. foundation conditions and the like. If possible the reach surveyed should start above a control section. river flow at most locations along a river will be close to the normal depth of flow. Such a site specific measurement program is usually impractical.2
7. • Establish optimal canal layout and cross-section dimensions. The main objectives of hydraulic design of the tailrace channel are: • Determine the head-discharge relationship at the powerhouse.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 96 .1
TAILRACE CANAL BACKGROUND After passing though the turbine flow is returned to the river via the tailrace canal. HYDRAULIC DESIGN Head discharge Curve: Determine the head-discharge curve for the river at the tailrace outfall and at the plant. Several flow measurements should be made for use in estimating Manning’s “n”.7 7. the initial section for backwater calculation should be located at a uniform section of the river and the length of reach sufficient that errors in estimating the starting water level at this section will be sufficiently attenuated before reaching the location of the tailrace outfall.
0 m/s with the design flow (QP).Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 97 . Reference Open Channel Hydraulics Ven T. Chow pp.4. 168).2. slope and erosion protection for earthen canals should be done using the “Tractive Force Method”. 7.The following water levels are of particular interest: a) TWL for minimum plant / river flow combination to establish the turbine centre line elevation.5 – 2.3
7. (Ven T. b) TWL corresponding to the 1 in 100 year flood (Q100) to establish plant main floor elevation.2 Design of Tailrace Canal The design of the tailrace canals differs from power canal design in that water tightness is of less importance. The design should be based on the maximum tailrace flow (Qp). as recommended in Section 2.4
AHEC/MNRE/SHP Standards/ Civil Works .4. For short tailrace canals (L ≤ 50m) detailed optimization analysis may be omitted and design based on a velocity of 1.4.2/2 for power canals. The dimensions of the canal should be based on economic optimization. accordingly an earthen canal with rock/riprap lining is often satisfactory. but the erosion protection should be extended up to the 1 in 100 year level. Erosion Protection Detailed design of the canal section. Chow Mc Graw Hill Book Company New York (1959)
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 98 .5
Appendix: Manning’s “n” values.7.
AHEC/MNRE/SHP Standards/ Civil Works .Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 99 .
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 100 .AHEC/MNRE/SHP Standards/ Civil Works .
TEMPORARY RIVER DIVERSION DURING CONSTRUCTION BACKGROUND Temporary river diversion is normally required to facilitate construction of dams and other works located in the river bed.8 8. Accordingly.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 101 . The climate in most of India is characterized by two distinct seasons. • Magnitude and duration of floods during construction period. it would be advantageous to schedule construction of in-river structures and related temporary diversion works for the dry season. These risks are mainly attributed to site hydrology and the frequency of occurrence of large floods. Depending on the magnitude of river flows the design and construction of diversion works can be difficult and expensive.
8. The dry season provides the best conditions for construction of in-river works as the flows to be handled are much smaller than during the wet season. • Downstream damages. Construction of head works and related temporary river diversion works are weather dependent and constitute a key activity in any project construction schedule. which must be controlled by competent design and attention to quality control of construction. Of course there are other risks related to design and construction of the cofferdams and other water diversion structures. • Cost of delays. The following factors influence the design of temporary river diversion works: • Duration of construction of in-river structures. Typically. The design engineer needs to be aware of the importance of this activity and to ensure that his estimate includes an adequate allowance for the costs of temporary river diversion and that his construction plan allows for the challenges of in-river construction. or at least to advance the work to a stage where the incomplete works are safe from wet season floods. a wet season with high flows. • Vulnerability to overtopping (concrete dams versus embankment dams). diversion works comprise.2
SELECTION OF DIVERSION FLOOD: The design of temporary river diversion works involves evaluation of risk versus cost of diversion works.
AHEC/MNRE/SHP Standards/ Civil Works . On large projects the design flood is sometimes determined from cost – benefit analysis of the issue. it is more common to apply design criteria based on precedents. However. and a dry season with low flows. but costs vary greatly depending on site features and hydrology. For SHP’s it may be possible to complete all vulnerable works within a single dry season. Risk could include: • Damage to the works. • Stream flow characteristics. • (Sometimes) dangers to public health and safety. 20% to 25% of head works capital costs.
8. plastic sheeting or a barrier of sand-cement bags.3.For SHP the following criteria are recommended: • For concrete or gabion dams that can resist limited overtopping . gabion dams or crib dams could be built in running water and the impermeable element.0 m/s). especially if flow velocities were high (>1. otherwise annual floods should be used. water could be diverted via flumes.the 1 in 20 year flood (Q20). Impermeable barriers could be timber planking.1 In Situ: For very small rivers construction of the head works may be possible without diversion. flood frequencies should be computed for dry season floods.3 METHODS OF CONSTRUCTION
8.3. Where the works can reasonably be completed within a single dry season.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 102 .
AHEC/MNRE/SHP Standards/ Civil Works .2 Simple Diversion techniques For small to medium sized rivers. Flow depths much greater than this would be too dangerous. culverts or ditches as shown in the following photographs from the Design of Small Dams (USBR – 1987).the 1 in 10 year flood (Q10) • For embankment dams that would be destroyed or severely damaged . stone barriers.5 m or “knee depth”. In such cases. added later. 8. The in-situ work assumes water depths generally less than 0.
AHEC/MNRE/SHP Standards/ Civil Works .Figure 11-1: Diversion by flume.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 103 .
8. portion of the river bed is un-watered behind cofferdams to permit construction of the spillway.3.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 104 . There are many variation of this approach depending on site topography and features of the head works.3 Staged Diversion On large rivers a staged diversion approach is sometimes employed.
AHEC/MNRE/SHP Standards/ Civil Works . For example: in Stage I.
Continued on next page. While in Stage II the dam and / or powerhouse is constructed while diverting water though the spillway.
Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 105 . 8.4
Diversion by tunnel In confined.4 8. the
AHEC/MNRE/SHP Standards/ Civil Works .3.5 Cofferdams A cofferdam is a temporary dam or barrier to divert a stream or enclose an area during construction. For small cofferdams polyethylene sheets can also be used. narrow valleys diversion via tunnel is often the preferred approach. If large size rocks are used for the embankment an intermediate filter zone may be required.3.8.1 RESPONSIBILITIES Contractor’s Responsibilities: It is general practice to require the contractor to assume responsibility for the diversion of the stream during the construction of the dame and appurtenant structures. cofferdams may be composed of crib or sheet pile cells.
8.4. This requirement should be defined by appropriate paragraphs in the specifications that describe the contractor’s responsibilities and define the provisions incorporated in the design to facilitate construction. Usually. Alternatively. The most common type of cofferdam is a rock embankment or berm built across a river by end dumping with an upstream zone of impermeable fill for water proofing (also placed by dumping).
Another point to consider is that the orderly sequence of constructing various stages of the entire project often depends on the use of a particular diversion scheme.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 106 .4. because the loss of life and property damage might be heavy if a cofferdam were to fail.specifications should not prescribe the capacity of the diversion works or the details of the diversion method to be used.S. One reason for this is that contractors tend to increase bid prices for river diversion if the specifications contain many restrictions and there is a large amount of risk involved. the specifications usually require that the contractor’s diversion plan be subject to the owner’s approval.5 REFERENCE Design of Small Dams U. 8. Where a dam is to be constructed in a narrow gorge. it may prove economical for the owner to assume the responsibility for the diversion plan.2 Designer’s Responsibilities For difficult diversion situations. Bureau of Reclamation Denver. 8. In addition. Colorado (1987)
AHEC/MNRE/SHP Standards/ Civil Works . but hydrographs prepared from available stream-flow records should be included. a definite scheme of cofferdams and tunnels might be specified.
For use of people living beside the river in the impacted area (between diversion dam and powerhouse).1
FISHWAY Background Fishways are required on rivers where one or several important fish species need to migrate upstream as part of their life cycle requirements. 2. For meeting the biological needs of biota living in the river reach between diversion dam and powerhouse.2
9. During high flow periods excess flows released at the head works of run-of-river plants will usually suffice to meet reserve flow requirements. whereas. On the other hand during low flow periods where plant demand is equal on greater than inflow.1 9. While such releases may be provided by opening a spillway gate or sediment flushing gate – it is recommended that a pipe sized for this purpose with a control valve be installed. In this case the hydropower plant developer should investigate the relative economics of providing piped water to the affected households.1. Scope The guidelines cover the most common types of mitigation works: • Supply of reserve flows • Fishways • Compensation channels RESERVE / RIPARIAN FLOW RELEASES The responsible authority may require minimum flows be maintained in the river channel downstream of a diversion dam. The extent of mitigation works will vary greatly between projects as a function of the setting and features of a given project.
9.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 107 . reserve flows must be provided by releases from the reservoir. On some projects no mitigation works may be needed. The main types of fishways are: • Vertical slot fishways
AHEC/MNRE/SHP Standards/ Civil Works .9 9.3 9. on others expensive works may be required.1. in this case the requirements may vary from month to month. These minimum flows could be required for the following reasons: 1. A fishway provides a means for fish to bypass a diversion dam which in other circumstances would be a barrier to fish migration.3.2
temperature (or other) that would control upstream migration. The following guidelines describe its features and recommend appropriate design parameters.• •
The vertical slot type is recommended as its function is relatively stable over a wide range water level variation. 4. 9. will also use the fishway.3. estimates can be obtained from the following graphs:
(From Katopodis – 1992).
AHEC/MNRE/SHP Standards/ Civil Works . 2. If this is not known for the target the target species. mean size (length) and range of sizes (at 5% and 95% exceedence limits). Rate of Migration: mean and peak daily rate of migration in numbers per hour. other fish having similar or better swimming abilities. additionally its construction is relatively simple. Swimming ability: burst swimming speed. 3.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 108 . Period of upstream migration and any thresholds of flow.2 Biological Design Criteria Biological design criteria must be defined in consultation with an experienced fisheries biologist as below: 1. Target specie. While design is based on a specific target species.
At this velocity the drop in elevation between pools is limited to about 0. Creation of such a velocity condition is called “attraction water”. so if the entrance to the ladder is located in a dead area the fish may not find it. Note: Subcarangiform fish have torpedo shaped bodies and are strong swimmers. as its is intended to attract the fish. The velocity should be about 1. A typical velocity is 2. since this drop is converted to velocity
AHEC/MNRE/SHP Standards/ Civil Works . examples – trout and salmon.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 109 . otherwise the fish may be carried back downstream. 3.3m. Maximum velocities in the fishway should not exceed the burst speed (or darting speed) for the fish. This is the speed that the fish can swim for a few second and is in the order of 8 to 12 body lengths per second.0 m/s. 9. 2.3 Principles of Design Fishways are usually planned according to the following principles: 1. In their upstream migration fish use the current as a direction guide. The upstream exit for fish from the fishway should be in a quiet area well away from the overflow section or sluiceway.3.5 m/s. The layout should be designed so that there is a significant velocity in the area approaching the fishway.Note: Anguilliform fish have long slender bodies and are weak to moderate swimmers
• Bathymetry at inlet and outlet zones.7/m3s. The volume of the fishway should provide from 0.
5. • Fish swimming characteristics (Vb).3. Establish Biological Criteria: The following biological design criteria must be decided: • Target specie or species and sizes. This velocity is the basis for selecting the fishway discharge from Q = VA. • Migration rate (for peak week) fish per hour (Na). Base design of fishway on the lesser of 0. Average velocities in the fishway should be about 0. Items 1 to 5 from Smith (1995). 9.5 m/s is a velocity that the fish can comfortably swim against for a short duration.4.10 MSF or 5. 9.5 to 3.3. lower velocities would increase the length and cost of the structure. first determine the mean river flow for the season of upstream migration (MSF).12 m3 of water per fish. in which A is the cross sectional area of the ladder.3.7 m3/s (Qd) 9. up to 0.4 Design Procedure: A stepwise design procedure is suggested. but may rest about 4 hours each day. • Vertical rate of ascent through the fishway (Va).4. given sufficient space to maneuver.4 Estimate Head Difference (∆H) Between Pools and Number of Pools Let Vs = 0. whereas.6m. It has been observed that 2. Biologists may estimate migration rate from fish tagging data or by use of counting fences.9 m and at the exit 0. It has been observed that. as outlined below: 9.30 to 0. 9.
7. Minimum depth of water opposite the entry to fishway: 0. depending on the size of the fish.1.flow relationships for fishway inlet and outlet zones.2. This relatively low average velocity has been found necessary to allow rest stops for the fish while going up the ladder.
6.4.5 m/h. Higher velocities tend to discourage some fish from using the fishway. On short fishways higher drops have sometimes been used.6m to 0. Collect site data The following data are required: • Daily flow data.
Given data on the peak migration rate and the rate of climb. • Head .6m. Fish are overly cautious when proceeding in unfamiliar channels and the average rate of climb is surprisingly low often only 2.45 m/s.4. • Period (season) of upstream migration. Fish do not feed during upstream migration.3.3. the fish will not injure themselves even on the sharpest corners or baffles.06 to 0.3 Determination of Design Flow (Qd) Calculate mean river flow for migration season and determine fishway design flow as the lesser of 0. a suitable fishway can be designed with no other information except the foregoing seven principles.9 Vb
AHEC/MNRE/SHP Standards/ Civil Works .10 MSF or 5.
head between pools.4. For fishway design flow.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 110 .
(From Katopodis – 1992). ∆h typically S = 1 (V ) : 8 to 10 ( H ) Slope ( s ) = Lp Mean pool depth (Dm) > ( D2 + SL P .0.5) or 0.6m − − − − − −(7) Volume of pool (V p' = W p . L p .D2 ) Check the following: • Volume of pool greater of Vp or V'p. •
AHEC/MNRE/SHP Standards/ Civil Works - Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 112 (From Katopodis – 1992)
AHEC/MNRE/SHP Standards/ Civil Works - Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 113 University of Saskatchewan Printing Services. M. These channels are called “compensation channels” and they are designed to mimic real rivers with pools and riffles and a variety of substrate as would be found in nature. Oregon U. Hydropower Plants (Chapter 7).A. (1995). ASCE (1995).
OTHER REFERENCES Bell. Saskatoon. Design of Hydraulic Structures (Chapter 5).4. guidance of downstream migrants away from plant intakes is a problem. (SK) – Canada Introduction to Fishway Design Freshwater. C.8 REFERENCES CITED Smith. C. Penche. of Fisheries and Oceans. North Pacific Division Portland. (1961)
Katopodis. (1973) Fisheries Handbook of Engineering Requirements and Biological Criteria.Guidelines For Hydraulic Design Of Small Hydro Plants /Feb 2008 114 . (1992)
ASCE Committee on Hydropower Guidelines for Design of Intakes Intakes (1995). Brussels – Belgium. USACE. European Small Hydropower Association. Institute Dept. C. R3T. 9.D.7 COMPENSATION CHANNELS Artificial channels are sometimes constructed to compensate for lost habitat. Ottawa
Clay.S. of fisheries and Oceans Winnepeg. Often a compensation channel can be constructed in the power plant tailrace channels.
Layman’s Guidebook (Chapter 7). A fisheries biologist should be consulted to advise biological requirements. (1988). ESHA.3. 9. Approaches to dealing with this problem are discussed in ASCE Guidelines for Design of Intakes and ESHA’s Layman’s Guidebook. Exceptionally.4. H. C. Dept. A discussion of design of artificial river habitat is given by Newbury and Gaboury (1994).with run-of-river plants. Manitoba Canada .3.C. Design of Fishways and Other Fish Facilities.
Guidelines for Hydraulic Design of Small Hydro Plants by Sarah Balangalibun318 viewsEmbedDownloadRead on Scribd mobile: iPhone, iPad and Android.Copyright: Attribution Non-Commercial (BY-NC)List price: $0.00Download as PDF, TXT or read online from ScribdFlag for inappropriate contentMore informationShow less
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