Microhydro System Design

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MICRO- HYDRO SYSTEM DESIGN

2.1 Power from waterEnergy is generated from water since ancient time. In those days water wheels are normally used to

generate energy for grinding agricultural products. The efficiency for the production of energy inthose days were insignificants. Development has been done by many researchers in the generation of

energy from the water. According to the energy equation of Bernoulli, energy in the water is storedin terms of pressure energy, velocity energy and elevation energy as shown in the equation below,

Power (energy/sec) = pressure energy/sec + velocity energy/sec + elevation energy/sec

P = + + z (1.1)

When there is the difference between the energy of water, the difference in the energy can beefficiently converted into useable energy by using hydropower plant. The energy at the intake of

HPP will be high and the exist from the HPP will be low thus the energy from the water will be

obtain as follow.

Power (Energy/sec) = ( + + z)intake - ( + + z)exit (1.2)

There will be some loss of power during conversion from the available water energy by using

hydropower plant. Those loss are expressed in term of efficiencies. Finally the power that can be

generated by HPP is expressed as follow.

P = g H Q (1.3)

Where,P = electrical or mechanical power produced, W

= density of water, kg/m3g = acceleration due to gravity, m/s

2

H = elevation head of water, mQ = flow rate of water, m

3/s

= overall efficiency of MHP system

Thus, equation shows that, power generated by the water available depends upon the amount rate

(flow rate of water), elevation head (elevation difference between intake and exist of water),

gravitation force, density of water and efficiency of the HP system. Thus by using HP plant,

available water energy will be converted to the useful mechanical/ electrical energy as an output.

2.2 Classification of hydropower and end uses

Energy available in water will be converted into useful energy like mechanical or electrical energy byusing Hydropower plant. HPP can be classified according the generation of electricity, type of

storage, type of distribution grid system, type of load capacity etc. There are wide variety of HPP it

can be classified in different ways.


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According to the electricity generation HPP can be classified according to following table 1.1. there

may be few variation in power generation range according to the useful situation and norms of the

particular country.

Table 1.1 Classification of HPP according to the power generation capacity

Power generation capacity Type of hydropower plant

Less than 100 KW Micro hydropower plant100KW to 1000KW Mini hydropower plant

1MW to 10MW Small hydropower plant

10MW to 300MW Medium hydropower plant

300MW to above Large hydropower plant

According to the type of storage type HPP can be classified into storage type and run of the river

type. The storage type of HPP consists of dam to stop the flow of water in the river stream. Therewill be big reservoir behind the dam to store water. The reservoir stores rain water too. This type of

HPP supplies water continuously to the plant and there will be no flow variation during dry season.

These plant are generally costly, complex to design. This is generally used for small to larger HPP.For MHP storage type HPP is not used.

Run of the river type HPP does not stops the river stream but it diverts water into the water way ofHPP. There will not be any reservoir in these type of HPP. Flow of water in this type of HPP may

vary according to the seasons. These type of HPP is less costly and environmentally friendly. These

type of HPP is generally used for micro, mini and small HPP.

Forebay tank

Turbine

Pressure pipe leadingturbine (Penstock)

Diversion weir and intake

Canal for diverted water

Stream

Fig. 1.1(a) Run of the river type MHPP


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Storage reservoir

Dam with intake

Turbine enclosed

Fig. 1.1(b) Storage type MHPP

According to the distribution of grid system, HPP is classified into local grid and extensive grid. Inthe local grid system the electricity generated from HPP will be distributed for the small locality near

by the HPP. Highly sophisticated and costly electro mechanical and distribution systems are not

necessary for local grid system. The generated power from this HPP may not be high grade of

standard.

In extensive grid or national grid system electricity generated by various HPP will be loaded into a

one type of grid system. The load distribution from this type of HPP will be wide . In small countrylike Nepal, one National grid is used for all the different parts of the country. The electro mechanical

components for this type of the system are sophisticated and costly. The power generated should be

one of standard type. Generally larger HPP are made under this category.

According to the load capacity HPP is classified into base load plant and peak load plant.

base load plants are those which supply the base load of the distribution system. Such plants are

required to supply constant power when connected to the grid. This type of HPP generally, does notconsists of water storage reservoir system.

Peak load HPP are those which will supply power during peak load condition only. These HPP canalso be used for base load. This HPP generally consists of storage reservoir.

2.3 Main component of MHP plant

MHP plant is designed to generate electrical or mechanical power according to the demand of localcommunity. Main components of MHP plant are civil component, mechanical component,

electrical/electronic component. Civil components includes diversion, intake, de-sanding basins,

canal, fore-bay, spillway, penstock, power house, tailrace etc. These components are described inpreceding sub titles.


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Tailrace

River flow

Flood spillway

Intake

Wing walls

Sand trap

Regulating gates

Spillway drain

Channel crossing

Channel Forebay tank

Penstock support

Penstock

Anchor

Power house

Fig. 2.1 Components of MHP plant

Diversion structure is a structure designed to raise the water level in the stream in order to enable

water to be diverted off the river. The weir may be of natural or an artificial weir (temporary orpermanent construction). In MHP, generally temporary structures are built for this purpose. These

structures are in most cases simply consists of boulder/mud piling resembling the diversion practiced

in traditional watermills. In some cases gabion weirs are also used for diverting water.

2.3.1 IntakeThe receiving a flow from river in required quantity and that directing it towards the waterways of

a hydropower system with minimal structural interventions is called intake. It is the point from

where water flows from the river stream. Therefore intake is the beginning of the conveyance ofwater diverted for MHP Types of intake structure are chiefly distinguished by the method used to

divert water from the river. In micro hydropower, mainly two types of intake considered are side

intake and bottom intake. Trashracks are placed at the intake to prevent logs, boulders and other largewater-born objects from entering the waterway.

1.3.2 CanalThe headrace of a micro-hydropower scheme is a canal or a pipe that conveys water from the intake

to the fore-bay. In MHP sometimes pipes substitute canals. Many types of headrace canal made of

different materials and using different methods of construction are used in MHP schemes. The types

and the design depends on site condition (seepage, land slide, crossing) and availability of material

and manpower. The common types of canal used in MHP plant are earth canal, stone masonry in mudmortar canal, stone masonry in cement mortar canal, concrete canal, covered canals and pipes, Most

headrace pipes used in MHP are HDPE pipes. The length of headrace can be from a few meters toover a kilometer. Generally small slopes are preferred for designing canals. The slopes are med just

enough for the flow of water in the canal. Higher slope means higher velocity of water in canal. This

not only erode canal surface, it also lose water energy available.


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2.3.3 De-sanding basin

Generally rivers carry relatively high amount of sediments owing to high erosion activities taking

place in the hills and mountains. The high sediment density rivers are Koshi in Nepal and Huanghoriver in China. The sediment density fluctuates within the year. The sediment density is highest

during the high flow period. Sediments in river water have negative impacts for the MHP:

Sediments get deposited in the canal and fore-bay, which reduces carrying capacity of the canal. The

design canal capacity can be maintained only through frequent clearing, which is very expensive.Sediment among others consists of hard silica compounds. These compounds erode the penstock andturbine. This at the one hand increases the operating costs and at the other decreases efficiency of the

MHP.

The purpose of de-sanding basin is to trap sediments so that these do not enter the canal. The de-sanding basin is, as a rule, built at the head of the canal and it is regarded as a part of the head works.

The de-sanding basin is wide and long pool designed to settle the sediments carried by the diverted

water through reduction in the speed of water. Most de-sanding basins are designed to settle particles

above 0.20.3 mm. De-sanding basin is provided with a sediment flush in order to reduce the costassociated with its cleaning. During the rainy season daily flushing of the de-sanding basin may be

required.

Fig. 2.2 De-sanding basin

2.3.4 SpillwayExcess flow that enters into the intake during flood flow needs to be spilled as early as possible to

minimize foundation erosion, channel collapse in headrace canal. This is achieved by incorporating a

spillway close to the intake and easy access distance during flood condition. If the headrace canal is

long, numbers of spillway can be constructed in de-sanding basin and fore-bay. The excess flowsthat are discharged via a spillway should be safely diverted into the stream or nearby gully such that

they do not cause any erosion or damage to other struc tures. Sometimes, this may require the

construction of a canal to the natural water course. Locating spillways close to a gully will save thecost of canal construction.

Inlet Settling Outlet

Top view

Gate valve for flushing

Spillway drain


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Fig. 2.3 Spillway

2.3.5 Fore-bay

A fore-bay is located at the end of headrace. A fore-bay is a wide and deep pool from which thepenstock draws water. The purpose of the fore-bay is to avoid air trapping by the water entering the

penstock, as the entry of air through the penstock may cause cavitation, which is a type of erosion

created by the explosion of trapped air bubbles under the high pressure, of both penstock and turbine.

It has air vet for the release of air. The water level at the fore-bay determines the operationalhead of the micro-hydro scheme.

A small overflow is to be maintained from the fore-bay in order to avoid fluctuation of its level andconsequently the possible entry of air to the penstock. At the fore-bay to spill the entire design flow

in case of sudden valve closure at the powerhouse Such overflow may continue for long time if the

canal intake is not closed.

Fig. 2.4 A fore-bay

Sediments get settled down in the fore-bay, as the speed of water is much slower in the fore-bay

compared to that in the headrace. Therefore a sediment flush system is provided in the fore-bay. A

spillway is to be provided from the fore-bay for safe passage of sediment flush and overflow water tothe river. At the outlet of the fore-bay, which is inlet of the penstock a trash rack is provided to

SpillwayAir vent

Fine trashrack

Gate

Compact earth

Penstock1

3

hs

300mm

minimum

Design Flow

Flood flow

hflood

h

sp

Cross section Longitudinal section


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prevent the floating debris from entering the penstock. It has provided with a valve to regulate water

flow into the penstock pipe.

2.3.6 PenstockA penstock is a close conduct pipe that conveys the flow from the fore-bay to the turbine. Penstock is

made of steel or HDPE, and rarely of timber. Recently PVC penstock has also been introduced. IfHDPE penstock is prevalent at lower heads, steel penstock is prevalent at higher heads. The MHP

head varies from a few meters to over hundred meters. Ghandruk MHP of Nepal, has a head of 220m, which is the highest in Nepal among MHP. Mild steel and HDPE pipes are the most commonmaterials used for the penstock in MHP schemes. HDPE pipes are usually economical for low heads

and flows and are easy to join and repair.

The conversion of potential energy of water into kinetic energy takes place in the penstock. Thetypical velocity of water in the penstock is around 3 m/sec. In order to reduce the head loss in

penstock it is desirable to make the penstock short and less bends. For this purpose penstock is

located in a steep slope, which is very often over 45 too. Above ground penstock pipes aresubjected to expansion or contraction in length as a result of changes in the ambient temperature. Asliding type of expansion joint, is commonly used in MHP schemes. It can be placed between two

consecutive pipe lengths and can either be welded or bolted to the pipes.

Anchor blocks are used to holds the penstock to restrain the pipe movement in all directions. It is amass of concrete fixed into the ground. Support piers are short columns that are placed between

anchor blocks along straight sections of exposed penstock pipe. Support piers prevent the pipe from

sagging and becoming over stressed.

Valve

Power house

Pipe joint

Anchor block

Side block

Vent pipe

Expansion jointPenstock gate

Fig 2.5 Components of penstock assembly

2.3.7 PowerhouseThe powerhouse accommodates electro-mechanical equipment such as the turbine, generator, agro-

processing units and control panels. Conversion of mechanical energy of water into electrical energy

takes place in the powerhouse. The main function of the powerhouse is to protect the electro-


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mechanical units from rain and other weather effects as well as possible mishandling by un-

authorised person.

2.3.8 Tailrace

The tailrace is the final civil structure that conveys the design flow from the turbine (after powergeneration) back into the stream, generally the same stream from which the water was initially

withdrawn. Similar to the headrace, open channel or pipes can be used for the tailrace section.

2.4 Turbine

2.4.1 Introduction

A hydraulic turbine is a prime mover that uses the energy of flowing water and converts it into themechanical energy (in the form of rotation of the runner). Science ancient time turbines are used

under the name of water wheels, made out of wood. The water wheels have very low efficiency and

short life.

There are different types and sizes of turbine available but the particular type and size for the

particular site is determined by,

Designed head and discharge at which the turbine is to operate, Availability and cost of the turbine Availability of skill man power after sales services and cost etc.

Particular speed of each turbine rotor at which it performs best is called its optimum speed. Theturbine needs to be operated at this speed at all loading conditions to get the maximum output.

2.4.2 Types of turbine

Principally, according to the working of turbine it can be categorized into two types, as impulse

turbines, and reaction turbines. Under these two main categories there comes many types of impulseturbines which can be selected for given site.

Impulse Turbine.There are three types of impulse turbines known as Pelton turbine, Turgo turbine and Cross flow

turbine. In these turbines the rotor rotates freely in atmospheric pressure. The rotor is never be

submerged in water of the tail race. It is kept above the tail race water level and the nozzles of theseturbines are free jet type. In this turbine pressure energy in water is converted into kinetic energy

when water passed through nozzle. Free high velocity water jet impinge on the bucket mounted on

the periphery of the runner. Impulse force on the bucket rotates the runner and shaft of turbine.

Reaction turbineIn reaction turbine s rotor remains immersed in water all the time and water acting on wheel is under

pressure which is greater than atmospheric pressure. Draft tube is an integral part of the reaction

turbine fitted at outlet. It runs by the reaction force of the exiting fluid. Potential energy and kineticenergy of the fluid come to stationary part of turbine blades and partly changes potential energy and

kinetic energy. Moving part (runner) utilize both potential energy and kinetic energy of water.


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2.5 Design Parameter:

2.5.1 Hydrology and site survey

Preparation for site surveyMHP plants are designed to produce electrical and mechanical power from water power. The power

is generated according to the demand of the local community. It is necessary to carry out survey to

collect information of power demand and willingness to pay for that. The sight survey should be

done for the MHPP potential to the demanded generation. Proper performance of the survey leads to

the success of the whole MHP schemes.

Adequate and accurate survey work is essential if the MHP project is implemented is to besuccessful. Survey should included both technical and socio economical issues of the project.

Demand survey is mainly concerned with the counting of households and other potential consumers

(shops. lodges, offices, temples, schools, industry), who are ready to commit themselves to receivepower and pay for it, and with calculating the total demand for power. The survey can be conducted

before the comprehensive meeting or even before the reconnaissance of survey.

Appropriate time for survey should be selected. It is prepared to perform survey during dry periodwith not much rain or cold or hot seasons or according of the specific site conditions. Documented

information about hydrology, geology, social structure of the selected site could be obtained fromdifferent sources previous to the survey. It is very useful to acquire to collect topographical maps for

the project area. It is better to allocate experienced survey persons with helpers from the localcommunity. Generally the method to be used for survey should be done beforehand and list of

equipments for that should be carried or transported to the project site.

Map study of siteThe objective of field survey for MHP project is to obtain necessary data and information of the

identified hydropower site and the electricity supply area to carry out the technical feasibility and

financial viability of the project.

Maps help to develop ideas and methods for the technical survey. It helps to design and locate waterintake, water way, fore-bay, power house and transmission system. Accurately design and locate all

the components of MHP plant will be formed after different stages of survey. Maps, chart or data forclimatic condition, ground condition, plantation, government policies are important tools for the

success of the project.

Meteorological data analysisPower generated from energy plants mostly depends on flow conditions of the river streams. River

stream condition is affected by the meteorological condition of the site. Data obtained in the survey

is co-related with the previously existing data to develop data bank. More the wider data on the data

bank more will be reliability of the meteorological out put data. It is important to get meteorological

information of many years as possible.

2.5.1.1 Site surveySite survey includes flow measurement (preferably during dry seasons), determination of head

needed to generate required rated power, land survey (including slopes and distances), location of

different civil components so that rated power could e generated.


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Head measurementThe head in MHPP is determined by the location of he powerhouse and the fore-bay, which also

determine the route of the canal and intake. The surveyor should first calculate the value of grosshead required from the simplified power equation,

P = g H Q (1.1)

Where,P = minimum present power demand plus losses, W

= density of water, kg/m3g = acceleration due to gravity, m/s

2

H = elevation head of water, m

Q = flow rate of water, m3/s

= overall efficiency of MHP system

The surveyor then starts by tentatively selecting a suitable site for the powerhouse and the fore-bay

and measuring the height and distance between the two. The process of 'determining the head'

involves a lot of surveying including measurement of distances (both horizontal and along a slope),

heights, and angles and bends. At the same time geological and other conditions of the selectedlocations must also be examined and evaluated to ensure that they are fit for constructing such

structures and that no natural or human/animal damage will result

Once suitable locations have been selected for all the civil structures, the available head and the

distances between these structures should be measured. There are different method for head

measurement. The head can be measured using one or two of the following methods. Some commontypes generally used for MHP plant are as follows:

(i) Abney level (Clinometers)

Hand-held Abney level (sighting meters) measures angle of inclination of a slope. Since the method

demands that the linear distance along the slope is recorded, it can have the advantage of doubling asa measure of the length of penstock pipe too.

Height is calculated as,

H = L sine (1.2)

Where,

H = height, mL = linear distance, m

= angle of inclination, degree

Other equipment needed are a measuring tape, two graded rods, marking pins pegs etc.


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H1

H2

H3

H4

L1

L2

L3

L4

Fig. 1.1 Measuring head by Abney level (Clinometers)

ii) Water filled tubes

This method is useful for low head sites, since it is cheap and reasonably accurate. It consists of atransparent plastic pipe (diameter between 4 and 10 mm is convenient) of both ends open. In this

pipe the water is filled. With the help of water level the height is measured. Other equipment needed

area transparent pipe (20m long and 4 to 10mm diameter), two graded rods, measuring tape markingpins pegs etc.

Y1

Y2

Y3

Y4

Y5

B1

A2

X

Y


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H1

H2

H3

H4

H5

X

X

Bubble

error

Fig. 1.2 Measuring head by water filled tubes

(iii) Altimeter

New digital altimeters are easier to use and increasingly safe in inexperienced hands for initial andrough measurements, specially for high heads. The method of measurement with an altimeter simply

involves taking the readings wherever needed, say at the site of the turbine, the fore-bay, and the

intake, and using the differences in readings to calculate the head, gradient, and other desiredquantities. The principle of the altimeter is that it measures the atmospheric pressure. Atmospheric

pressure gives elevation of the corresponding position. The readings of altimeter are affected by

changes of temperature and humidity.

Fig. 1.3 Measuring head by Altimeter


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(iv) Other methods

There are many other methods that can be used to measure the head. Professional surveyors would

have no problem using them. For example, a simple plank to which a spirit level has been attachedcan be used together with two graded rods to measure height differences between two positions.

Similarly, more accurate and expensive levels and theodolites can be used, but these require

considerable practice and skill to master. Consequently, the Abney level and water-filled tubemethods, which are fairly accurate, cheap, and easy to use, are probably the best methods for MHP

schemes with heads of less than 100m.

2.5.1.2 Flow measurementAmount of water flow in one of the prime factor for generation of power using MHPP. Flow

measurement method for specific size and location of MHPP depends mainly upon the volume rate of

flow and condition of turbulancy. There are different method for flow measurement. Some commontypes generally used for MHP plant are as follows:

Table 1.1 Different flow meter and their application:

Different flow meter Field of application

1 ) The bucket method For flow up to 20 l/s

2) The velocity-area method using

a) a flow meter

b) a float

For larger flow ( Q 20 l/s) with a depth of atleast 10 cm at deepest point

For larger or smaller flow with turbulence

3) The weir method For larger flow ( Q 20 l/s) rectangular weirand smaller flow triangular (vee-notch) weir

4) The salt dilution method For smaller flow stream

(i) Bucket methodThis is a very simple and accurate method if the flow is relatively small (Q < 2 0 l/s). A bucket or

other container of known size is used as to measure (Figure 1.4). All the water in the stream is

diverted into the bucket container through a pipe or a trough and the time taken to fill the container is

measured The flow, Q, is given by:

Q(l/s) = volume of container in liters / numbers of seconds to fill it (1.3)


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Fig. 1.4 Measuring flow by bucket method

(ii) Velocity area method

(a) Using a flow meter

This method is quite useful and reasonably accurate if a proper flow measuring instrument is

available. The basic technique is illustrated in Figure 1.5. A suitable point is selected carefully along

the stream; the cross-sectional area at this point is divided into different sections: the width, depth,and profile are used to calculate the area of each section, and the average velocity of each section is

measured by a current meter (flow meter) held at its centre. The average flow is calculated using the

general formula for flow, Q:

Q= Ai vi (1.4)

L

d1

d2

d3

d4

V1V2

V3

V4

Fig. 1.5 Measuring flow by velocity area method (using a flow meter)


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(b) Using a float

This method is similar to the above, but the flow is measured using a small floating object rather thana flow meter. The object chosen should float partially submerged in the water and can be a piece of

light wood or a more elaborate, specially constructed float. The float is placed at the centre of the

stream and the time taken for it to travel a certain distance (or the distance covered in a certain time)is measured. The surface velocity (vS) of the water at the centre of the stream is given by:

vS (m/s) = Distance travelled by float (m) / Time taken(s) (1.5)

(iii) Weir method

Many types of weir can be used to measure the flow in streams. The method of measuring by two

different types of weir, and the equations used to calculate the flow Q, are shown in Figure 1.6. Themost convenient weir is the rectangular type, mainly because it can be constructed from wood on site

if an amateur carpenter is available. If the weir has been made properly, the flow measurement can be

accurate within 5 percentwhich is acceptable for MHP schemes.

.

h

L'>2h L'>2hL>3hL'>2h

L'>2h

90

h

L">4h

L>2h

h

v


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readings will rise, reach a peak, and fall back to the base level, over a period of time. Usually two

people are needed to take and record the readings.

The readings are taken continuously until the conductivity values have returned to normal (which

means that all the salt water has passed the probe). A graph of change in conductivity with time is

plotted, and the area under the curves calculated (Figure 1.7). If graph paper is used, the area can becalculated easily by counting the squares. The temperature of the stream should also be measured.

The flow, Q, is then calculated by using the following equation:

Q (m3/s)= mass of salt in (kg) / [conversion factor (kg/m3/ohm-1) area under the curve (ohm-1s)]

(1.6)

The conversion factor, k, depends on the temperature, and its value is given in the manual for theconductivity meter.

(v) Propeller device method

Often called current meters, consist of a shaft with a propeller or cups connected to the end. Propelleris free to rotate and the speed of rotation is of course related to stream velocity. A simple mechanical

counter records the number of revolutions of a propeller placed at a desired depth. Main principle isthat current meters will be supplied with a formula relating rotational speed to the speed of thestream. A simple propeller meter can be constructed & calibrated. Generally these devices are used

to measure velocities from 0.2 to 5 m/s with a probable error of approx. 2 percent.

2.6 Layout design of civil components of MHP system

Fig.2.1 Typical layout of micro-hydropower system

After receiving survey information like location, size, materials, head, flow and other parametersleads to design of different civil components. Civil components consist of intake, weir, headrace

canal, settling basin, spillways, fore-bay, penstock, anchor blocks, support piers, expansion joints,

powerhouse etc. The ideal layout of a scheme depends on appropriate site selection. Design andexistence of the components are specific to the selected site.

730

720

710

700

700

710

720

730

Direction of flow

Weir

Contour line

Canal with small slopeForebay tank

Penstock pipe

Spillway

Power house


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Canal

Canal

Direction of flow

Diversion dam

2.6.1 Intake and weir

Weir and intake structure helps in regulating and controlling the water flowing (at fairly constant

rate) in headrace during high river flow and low river flow conditions. The function of the weir is tomaintain a permanent water level above the intake mouth during both high and low flow seasons. A

weir should be constructed to raise the water level in the river upstream if the adequate river flow

cannot be diverted naturally into the intake during the low flow period.The weir may be of natural or an artificial weir (temporary or permanent construction). There are

two common types of temporary weir, weir across the whole or weir across the part width of riverstream. The length of the weir across the river should be kept to a minimum. The part width weircan further be extended if more river flow needs to be diverted.

Other important design parameter being the height of weir. For both permanent and temporary

weirs, the height should be kept as low as possible but enough to divert the required flow. In order

to determine the height of a temporary or permanent weir the river depth/level during the dryseason must be known together with the upper height of the orifice of the intake mouth.

Fig. 2.2 Natural and temporary weir with side intake

The intake height should be such that the water level rises above the upper edge of the orifice.

The height of temporary weirs may have to be increased or decreased during the operation of theplant. The weir height should be as low as possible. This makes the structure more stable, less

susceptible to flood damage and also minimises sediment deposition. Weir should be designed with

gradual slopes so that boulders can roll over the weir and also discourages sediment depositionupstream of the weir.

Overview of intakeThe receiving a flow from river in required quantity and that directing it towards the waterways ofa hydropower system with minimal structural interventions is called intake. The intake should beso designed that the head loss is minimal and the entry of excessive flow as well as bed load and

other floating debris are minimised during flood and high-flow season. Design of intake should

be simple, less expensive and stable.


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The location of an intake structure must be so chosen that the largest possible portion of the bed

load remains in the river and is not diverted into the headrace. It is desirable to locate the intake

behind or under large, permanently placed boulders or rock. This limits the water that can enterthe intake, and deflect flood flows and river borne debris away. Advantage can also be taken of

stable banks and rock outcrops. If we have to design intake in river bend, outer bend is preferable

as it limits sediment deposition and to ensure flow availability during the dry season. In straightsections the location of the intake is governed by factors such as bank stability and headrace

alignment.

Fig. 2.3 Suitable and unsuitable locations for an intake

Types of intake structure are chiefly distinguished by the method used to divert water from the

river. In micro hydropower, mainly two types of intake considered are side intake and bottom

intake.

Side intakeSide intake is designed as an extension of the headrace canal capable of conveying the designflow and extent it to the side of the river bank. Side intakes are most commonly used in MHP

schemes since they are simple and less expensive than other types and most suitable for run-of-the-

river type plants. They are easy to build, operate and maintain. Side intake could be vulnerable to

flood so it normally includes an orifice downstream of the trash rack at the river bank, throughwhich water is initially drawn in to the headrace to limit excessive flows during floods and to

minimise the amount of sediment.

Large

boulders

Intake Prepared

by large boulders

Canal

Suitable location

for intake

Unsuitable location

for intake

Flow

Flow


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Fig. 2.4 Side intake with temporary diversion

i) Design of orifice for side intakeA side intake normally includes an orifice downstream of the trashrack at the riverbank, through

which water is initially drawn into the headrace. It allows the design flow to enter into the

headrace during normal conditions but limits excess flows during floods. It should be sized such

that it is submerged at the time of design flow during the low flow season, and it will also limitexcess flows during floods. It is economic and feasible to construct such an orifice for a MHP

intake. Sometimes, the side intake is just a continuation of the headrace canal up to the riverbankexcess flow cannot be controlled during floods in such design.

1. The discharge through an orifice when submerged is given as

..(2.1)

Fig. 2.5 Sections through a weir and a submerged orifice

where,

Q = discharge through the orifice in m3/sV = velocity through the orifice in m/s

A = area of orifice in m2

hr -hh= difference between the river and the headrace canal water levelsC = coefficient of discharge of the orifice

2. For a sharp edged and roughly finished, fully submerged concrete or masonry orificestructure value of C can be as low as 0.6 and for a carefully finished and smooth opening it

Weir

Side intake

River

Flow direction

A section through weir A section through submerged

orifice

Orifice

Datum

River flow level

BH

HrHh

Hr-Hh


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can be up to 0.8. The value of C decreases with the amount of turbulence induced by the

intake.

3. The value of (hrhh) will vary according to the discharge in the river since a higher water

level in the river will produce a greater head at the orifice.

4. The maximum velocity for a well constructed concrete/ masonry orifice is 3 m/s. If the velocity

exceeds this value, the orifice surface will be scoured. For micro-hydro, the recommendedvelocity through the orifice during normal flow is 1.0 - 1.5 m/s. However, if the orifice isdirectly at the river (without a trashrack) the velocity should be less than 1.0 m/s to avoid

drawing bed load into the intake.

5. If a weir is placed across the river, the flood level may be somewhat higher thanbefore since the weir raises the water level. For temporary weirs this is not a p roblem

since they normally get washed away during high flow condition. If a permanent weir is used,

allowances should be made for this when calculating hr as by adding the weir height above the

measured food level.

Example: Design of orifice for side intakeDesign a suitable size of an orifice for a design flow of 250 l/s. The normal water level in the river is0.8 m above the bed level. The design flood level is about 0.6 m above the normal water level. What

is the discharge through the orifice during such a flood?

1. Given:

Design flow, Q = 0.250 m3/s

Normal water level in the river, hr= 0.8 m

Design flood level, hf=0.8 m + 0.6 m = 1.4 m

2. Let velocity through the ori fice, V = 1.2 m/s

(since for MHP the recommended velocity through the orifice during normal flow is 1.0 - 1.5m/s.)

Area of orifice, (A) = Q/v= (0.250 m3/) / (1.2 m/s) = 0.21 m2

3. A = Orifice height (H) Width of orifice (B)Let Orifice height (H) = 0.2 m (consider)

Width of orifice (B) = A / H = (0.21 m2) / (0.2 m) = 1.05 m4. Let bottom of orifice 0.2 m above the river bed level

(This value is normally taken for MHP this will minimise the bed load. Also, set the datum at

the river bed level.)

5. Let water level at headrace canal, hh = 0.5 m with respect to the datum as shown in Fig.2.6 (i.e.

100 mm above the upper edge of orifice to ensure submerged condition. Later the headrace canalwill have to be designed accordingly.)

6.

Let C = 0.6 (for roughly finished masonry orifice)

= 310 l/s

Q required = 250 l/s

7. Therefore orifice design is OK. Since the designed orifice can deliver 310 l/s


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8. Discharge through the orifice during flood flow:

Let C = 0.6 (for roughly finished masonry orifice)

= 530 l/s

Q flood = 530 1/s

2.6.2 Trashracks

Trashracks are placed at the intake to prevent logs, boulders and other large water-born objects fromentering the waterway. It is also placed at fore-bay to prevent leaves, twigs and branches from

entering the penstock. The trashrack at the intake is also known as "coarse trashrack" since the bar

spacing is wider here compared to the trashrack at the fore-bay. The spacing, strength, type (flats or

angles) depends on particle size of the sediments carried by the river flow (i.e. bed load), type ofintake and other provision for a settling basin in the canal system. The trashrack for intakes can be

manufactured from flat steel, angles, tees or round bars welded together at fixed intervals. It is also

important to place the trashrack such that the bars are along the direction of flow, this minimises the

risk of clogging.

Trashracks for side intakes are coarse trashrack not designed to exclude gravel and sediment. The

size of the trashrack should be such that the water velocity is approximately 0.6 m/s (a lower velocityis uneconomic, whereas a high velocity tends to attract bed load and debris, and results in increased

head loss). Since boulders can frequently impact the coarse trashrack, it needs to be robust, i.e. thick

steel sections should be used. Depending on the length and width of the opening, nature of thesediment load and the required flow, a clear spacing of 50 mm to 200 mm can be used.

Shape of trashrack of bottom intake is also very important, since this affects the chances of clogging.

Round bars, for example, are more prone to clogging, because the opening in the middle is smallerthan on the top. The section chosen must be strong enough to withstand impact by any bed load

moving during floods. The recommended clear spacing between these flats, angles or bars is 6 to 15

mm and a commonly used spacing is 12 mm. The reason why these bars are closer than those of the

side intake trashrack is that gravel also needs to be excluded from the bottom intake. If the openingsare too narrow, there is a high chance of clogging necessitating frequent cleaning of the trashrack.

One of the drawbacks of the bottom intake is the clogging of trashrack by pebbles and dry leaves.

Especially during the dry season, the river may carry a lot of leaves, which become trapped in thetrashrack and reduce the flow through it. Therefore the trashrack needs to be cleaned periodically

during the dry season. During monsoon, this is not a problem; the river flow sweeps the gravel and

leaves before they can clog the trashrack.

Fig. 2.9 Trashrack

Side intakeDirect intake


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2.6.3 Headrace canal

The headrace of a micro-hydropower scheme is a canal or a pipe that conveys water from the intake

to the fore-bay. The headrace alignment is usually make gently sloping ground and the flow is causedby gravity. Since canals are generally less expensive than pipes, they are used more often for

headraces in micro-hydro schemes. The general rule is to use canals as often as possible and to use

pipes only for the difficult stretch of the headrace alignment, such as to negotiate cliffs or unstableareas. A headrace pipe is generally not subjected to significant hydraulic pressure. They are designed

to keep seepage, friction and erosion to a minimum. The velocity in the initial headrace length needsto be high enough to carry gravel and sediment up to the gravel trap and settling basin respectively.However, for headrace alignments on stable ground where seepage is not likely to cause instability,

earth canals are the most economic option.

Many types of headrace canal made of different materials and using different methods of constructionare used in MHP schemes. The types and the design depends on site condition (seepage, land slide,

crossing) and availability of material and manpower. The types of canal and methods of designing the

various components are described in the following sections

Earth canal

These are constructed by simply excavating the ground to the required canal shape. Such canals areused on stable and gently sloping ground where seepage is not likely to cause instability such aslandslides, earth canals are the most economic option. Compaction of the earth and planting

vegetation on the canal banks will increase stability and reduce seepage.

Stone masonry in mud mortar canalThere will be less seepage from this type of canal than from an earthen canal, but the

construction will require more labor, materials, and funds. These canals should be used where a

small amount of seepage will not cause slope instability, or where flow is limited, that is there isno extra flow that can be diverted into the canal to compensate for seepage. For similar flows,

the cross section of this type of canal can be smaller than the earth canal because a higher

velocity is acceptable without causing erosion.

Stone masonry in cement mortar canalIn this type seepage is minimal but more expensive in comparison with earthen or stone-mud canals.

A stone masonry in cement mortar canal should be used at locations where the soil type is porous(leading to losses of unacceptable amounts of flow) and seepage is likely to cause landslides.

Concrete canalMost micro-hydro schemes do not have headrace canals constructed of concrete since they are

very expensive. There is virtually no seepage through such canals. Sometimes, reinforced

concrete canals are used for short crossings. Generally, HDPE headrace pipes are more economic

than concrete canals.

Covered canals and pipes

Where stones and other debris are likely to fall from above the headrace route, the canal can either becovered or pipes may be used. Flat stones are an economical way of covering canals; an expensive

alternative is to use reinforced concrete slabs. Buried pipes made, for example, from HDPE also offer

protection from falling debris. Another advantage of HDPE pipes is that they are flexible and canadjust to a certain amount of ground movement. Pipes should be used at crossings where the ground

is unstable and/or steep and at other locations where open canals are not possible.


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Either an open channel or low-pressure pipes (or a combination of these) should be used as the

headrace. Flow velocity is able to carry gravel and sediment and not to cause erosion to channel wall

and base. It should be unlined or stone masonry in 1:4 cement mortar or reinforced concrete is better.

Sometimes the headrace or the penstock alignment may need to cross gullies and small streams.

Crossings are such structures that convey the flow over streams, gullies or across unstable terrainsubject to landslides and erosion.

Pipes may be required along the headrace alignment where slopes are unstable and where landslidesmay occur. The use of flexible pipes is when the entire hillside is slowly sliding (i.e. mass movement

is occurring) and part of the headrace alignment needs to traverse it. HDPE pipes are often used to

address the above problems. These pipes are flexible enough to accommodate some ground movement

and can be joined by heat welding.

Design criteria of the headrace canalThe canal dimensions and cross-section are governed by the following criteria.

- Capacity- Velocity- Slope of the side- Head loss and seepage- Stability- Economics- Sediment deposition in canal

Table 2.2 Recommended side slopes and maximum headrace canals velocities

Canal material

Side slope

(N = h/v)

Maximum recommended

velocity for canals (V)

less than 0.3m depthless than 1 mdepth

Sandy loam 1.5 to 2 0 0. 4 0 7

Loam 1.0 to 1.5 0. 5 0 8

Clay loam 1.25 0.6 0.9

Clay 1.0 0.8 1.0

Stone masonry with mud mortar 0.5 to 1.0 1.0 1.0

Stone masonry with cement mortar 0 to 1.5 1.5 1.5

Concrete 0 to I.5 2.0 3.0


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Table 2.3 Roughness Coefficients for Different Canals

Canal type Description Roughness

coefficient 'n'

Earthen

canals

Clay, with stones and sand, after ageing0.020

Gravelly or sandy loam, maintained with

minimum vegetation 0.030Lined with coarse stones, maintained with

minimum vegetation0.040

Rock canals Medium coarse rock muck 0.037

Rock muck from careful blasting 0.045

Very coarse rock muck, large irregularities 0.059

Rubble masonry with mud mortar 0.025

Masonry

canals

Brickwork, bricks, and/or clinker with well-

pointed cement mortar0.015

Normal masonry with cement mortar0.017

Coarse rubble masonry and coarsely hewnstones with cement mortar

0.020

Concrete

canals

Smooth cement finish0.010

Concrete for which wood formwork was

used, un-plastered

0.015

Tamped concrete with smooth surface 0.016

Coarse concrete lining 0.018

Irregular concrete surface 0.020

Design for headrace canal1. Decide the canal type according to the site conditions and stability.

2. Choose a suitable velocity (V) for the type of canal selected by referring to Table 2.2. and findthe roughness coefficient (n) from the same Table 2.2. Note that unacceptable head loss may

result if chosen velocities are close to maximum velocity.

3. Calculate cross-sectional area (A) from the equation A = Q / V (2.6)Where Q is the design flow

4. Using Table 2.2, decide on the side slope (N). Note that N is the ratio of the horizontal length

divided by the vertical height of the side wall (i.e. N = h/v as shown in Figure 2.10.

5. Calculate the optimum canal height (H), canal bed width (B), and the canal top width (T) usingthe following equations:

= 2 (1+N2)-2N (2.7)H = A / ( + N) (2.8)B = H (2.9)T=B+(2HN) (2.10)


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B

H

T

v

h

Fig. 2.10 Headrace canal with trapezoidal cross-section

is the factor used to optimise the canal shape, for a rectangular canal N= 0 and =2, H = (A / 2), T = B = 2 H

If an optimum canal shape is not possible due to site specific conditions (such as narrow width along

a cliff) then either the width or the height should be selected to suit the site conditions. Then the other

dimension can be calculated.

6. To ensure stable and uniform flow in a long canal, the velocity must be less than 80% of the

"critical velocity, Vc.

Vc = (Ag / T) (2.11)For a rectangular canal Vc= (Hg ) (2.12)If the canal velocity is greater than 0.8V then repeat calculations with lower velocity.

7. Calculate the wetted perimeter (P) using the following equation:

P= B + 2 hr(1+N2) (2.13)For rectangular canal, P=B+2H (2.14)

8. Calculate the hydraulic radius (R) as follows: R = A/P (2.15)

9. The slope (S) can now be found from Manning's equation:

S = [n V / R0.667]8 (2.16)Now all dimensions required for the construction of the canal are known.


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10. Calculate the head loss in canal by: Head loss = L S (2.17)where L = length of the canal section. If the slope of the canal varies along different sections,calculate the head loss for each section and add them up. If the loss is too high, or if the actualground slope differs from the calculated canal slope, repeat the calculations using different

velocities. Again Manning's equation ca be rewrite for H,

Q = [(BH + NH2)

5/3 S ] / [n { B+2H (1+N2)}2/3] (2.18)

12. Allow a freeboard of about 300mm for Q 500 l/s and 400m for 500 l/s Q 1000 l/s. Freeboard allows for uncertainties in the design (e.g. the value of `n' may differ by 5% to 10% from

estimate), water level being above the design level due to obstruction in the canal or duringemergencies and deterioration of the canal embankment.

13. Calculate the size of the largest particle that will be transported in the canal:

d = 11 RS (2.19)If this is less than the possible size in the canal, repeat the design using a higher velocity.

14. Check that possible flood flow in canal can be accommodated without using more than 50% of

the freeboard.

15. Find the total head loss. If this is too high or too small, repeat the calculations with a differentvelocity. Consider using different types of canal keeping the overall cost in mind.

16. Avoid a canal width of less than 300 mm as narrow canals can be easily blocked. Also for

stone masonry canals, smaller sizes are difficult to construct.

Example: Design of a headrace canal

Design a headrace canal to convey a flow of 285 1/s. Site conditions indicate that the canal would be

stable if stone masonry in mud mortar is used. The expected flow through the intake during a 20-

year return flood is about 480 1/s.1. Canal type: stone masonry in mud mortar Q = 0.285 m

3/s

From Table 2.3 Roughness coefficient n = 0.035From Table 2.2, for gravelly earth, select side slope, N= 0.5, (lh/2v) and V = 1.0 m/s

Cross sectional area, A = 0.285/1.0 = 0.285 m2

=2(1+N2) -2N=2 (1+0.52) -20.5 = 1.236

2. Calculate the water depth in the canal H

H = [A / ( + N)]H = [0.285 / (1.236+0.5)]H = 0.405 m

3. Calculate the bed width, B

B=H B = 0.405 1.236B = 0.50m

4. Calculate the top width up to the design water levelT = B + (2HN)

T = 0.50 + (2 0.405 0.5)


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T = 0.905 m5. Check if V < 0.8 Vc

Vc= [Ag /T ] = [0.285 9.8/ 0.905]VC = 1.76 m/s

0.8Vc =1.41m V=1.0 m/sHence the design is OK.

6. Calculate the wetted perimeter, P

P=B+2H(1+N2)P=0.5+2 0.405(1+0.52)P = 1.406 m

7. Calculate the hydraulic radius, R

R = A/P = 0.285 / 1.406

R = 0.203 m8. Calculate the required canal bed slope, S

S = [nV / R0.667

]2= [0.03510.2030.667]2

S = 0.0103 or 1:97 (i.e. 1 m of drop in 97 m of horizontal canal length)

Finally allow 300 mm of freeboard. The canal dimensions can be seen in Figure

9. Check the flow depth for maximum flood flow in the canal

Q = [(BH + NH2)

5/3 0.0103 ] / [0.035 { B+ 2H (1+N2)}2/3]

0.480= [(0.5 H + 0.5 H2) 5/3 (1 / 975] / [0.02 { 0.5 +2H (1+0.52)}2/3]

By trial and error method, the above equation is balanced when H = 0.55 m. Therefore, the flood flow

occupies 50% of the freeboard (the maximum allowed, as discussed earlier) and the head on thespillway (h

overtop)will be 100 mm.

11. Check the size of particle that will settle in the canal at a velocity of 1.0 m/s.D=11RS = 11 x 0.203 x 0.0103 = 23 mm

12. The particles larger than 23 mm would settle in this headrace canal. Therefore, to avoiddeposition upstream of the settling basin, the gravel trap must be designed to remove all

particles greater than 23 mm.

Fig. 2.11 Proposed internal canal dimensions

0.50 m

0.40m

0.9 m

2

1

0.30m


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2.6.4 Spillway

Excess flow that enters into the intake during flood flow needs to be spilled as early as possible to

minimize foundation erosion, channel collapse in headrace canal. This is achieved by incorporating aspillway close to the intake and easy access distance during flood condition. If the headrace canal is

long, numbers of spillway can be constructed so that the entire design flow can be diverted if the

canal is blocked as a result of falling debris or landslides. It is also constructed in silt basin and fore-bay. At the fore-bay to spill the entire design flow in case of sudden valve closure at the powerhouse

as may occur during emergencies. Here it consists of a means of canal emptying combined withcontrol gates. The excess flows that are discharged via a spillway should be safely diverted into thestream or nearby gully such that they do not cause any erosion or damage to other structures.

Sometimes, this may require the construction of a canal to the natural water course. Locating

spillways close to a gully will save the cost of canal construction.

Fig. 2.12 A spillway

Design of spillway1. The sizing of the spillway is based on

Lspillway = (QfloodQdesign) / Cw (hfloodhsp)1.5 (2..20)where,Lspillway, is length of the spillway in m Qflood is the flood flow that enters the intake in m

3/s

Qdesign is the design flow in the headrace canal in m3/s

hflood is the height of the flood level in the canal in m

hsp is the height of the spillway crest from the canal bed in m

hovertop = hfloodhsp is head overtop

2. Cw = a coefficient (similar to weir coefficient) which aries according to the spillway profile.Cw for different weir profiles. Choose a spillway profile and determine C w . For MHP, a

broad, round edged profile (Cw = 1.6) is suitable since it is easy to construct.

3. Calculate the flow through the intake during floods . The spillway should be sized such that

the entire flood flow can be diverted away from the canal. This is because the micro-hydro

system could be closed during flood or there could be an obstruction in the canal.

Design Flow

Flood flow

hfloo

d

hsp

Cross section Longitudinal section


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4. The design procedure involves first calculating the maximum height of the water level in the

canal during a flood (hflood). Then the height of the spillway crest (hsp) is set such that it is about

50 mm higher than the design water level. This ensues that part of the design flow is not spelled,which would decrease the power output.

5. A coefficient for a road crested weir of a spillway with round edges and easy to construct isconsidered as 1.6 for MHP.

6. Spillway crest level should be 0.05 m above normal canal water level. No more than 50% ofthe freeboard should be used. Therefore, with a generally used freeboard of 300 mm, the

available hovertop is 0.5 x 0.30 - 0.05 = 0.10 m. The required length can then be calculated for

the chosen hovertop and flood flow.

7. Where there is no pounding immediately downstream, such as in the headrace canal, the spillwaylength calculated above equation should be multiplied by 2. This accounts for the gradual

decrease in head over the spillway, until the required level is reached at the downstream end of

the spillway. In this case only the excess flow (Qflood - Qdesign) should be used. Note that in such

cases, locating the spillway immediately upstream of an orifice will increase the flow through theweir.

Example: Spillway designAstone masonry in mud mortar headrace canal convey a design flow of 285 1/s. The expected flow

through the intake during a 20-year return flood is about 480 1/s. Design an adequate spillway.

Let head overtop is 100 mm.

Note that two cases need to be checked as follows:

1. The spillway must be able to convey the entire flood flow of 4801/s in case the headrace canal

downstream gets obstructed (pounding case).

2. The spillway should be able to spill the excess flow (480 l/s - 2851/s) when there is no obstruction

downstream. The calculated maximum spillway length should be used in the design.

Case 1:

1. Choose a broad crested weir with round edges profile, so Cw = 1.6

Qspiilway = 4801/sQdesign = 0 1/s

hovertop = 100 mm calculated earlier.

Now calculate the length of the spillway,Lspillway = (QfloodQdesign) / Cw (hfloodhsp)1.5

= (0.480-0) / 1.6 ( 0.1)1.5

= 9.5 m


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Case 2:

Qspillway = 4801/s

Qdesign = 285 1/shovertop = 100 mm calculated earlier.

Now calculate the length of the spillway,

Lspillway = 2 (QfloodQdesign) / Cw (hfloodhsp)1.5= 2 (0.480-285) / 1.6 ( 0.1)1.5

= 7.7 m

Therefore a spillway length of 9.5 m is required for the above canal (which solve both Case 1 and

case 2).

2.6.5 Settling basinsMost rivers carry a substantial quantity of sediment of different sizes (in the form of gravel,

sand or finer material) depending on the river characteristics, geology of the catchments area

and the discharge. Also steeper rivers of some types carry cobbles and even move largeboulders during annual floods. Intakes are located and designed to prevent boulders and

cobbles from entering into the system but sediments like gravel, sand or finer material cannotbe entirely eliminated. Large particles can block the headrace and reduce its capacity.Suspended sediment can cause severe wear on the turbine runner, seals and bearings, since the

flow velocity at runner is high.

A settling basin is to settle the suspended particles present in the diverted river flow. The basic

principle of settling basin is the greater the basin surface area and the lower the through velocity

the smaller the particles that can settle. Settling basin can further divided into sand trap

(commonly known as settling basin) which settle sediment size less than 0.3 mm and gravel trapfor larger sediments. To reduce costs, one settling basin (sand trap) should be combined with the

fore-bay or combined with the gravel trap but with adequate size, if possible according to site

conditions. If flood or excess flows can reach the settling basin, such as when it is combined withthe gravel trap, a spillway should be incorporated and sized adequately. A trashrack could also be

an additive in settling basin.

Settling basin should be located at a safe place but as close to the intake as possible. The settlingcapacity should be large enough to reduce the velocity sufficiently to settle the sediments in the basin.

It should be easy to flush the deposited silt. The basin should have a sufficient volume to storage

capacity the settled particles until they are flushed (a flushing frequency of twice a day, i.e. 12 hoursfor wet and dry season). It should be possible to lead the discharge and sediments flushed from the

basin safely into the river or a nearby gully without causing erosion or damage to other struc -

tures. Sharp bends should be avoided just before or within the basin since they cause turbulent

flows which prevent the settling of particles.

Components of settling basin

Inlet zoneThis is the initial zone where the transition from the headrace to the settling basin occurs andthere is a gradual expansion in the basin width. Gradual expansion of the inlet channel about 1:5

(1 = 11) as shown in Figure 2.13. This will allow an even flow distribution at the beginning ofthe settling zone. The vertical expansion ratio can be higher at about 1:2 (1= 27).


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Fig.2.13 A settling basin (side and top view)

Settling zoneParticles are settled, stored and flushed in this zone. The length of this zone is longer than the inlet

or the outlet zones. A range of 4 to 10 is recommended for the ratio of the length to width (L/B).Provision for flushing the stored sediment should be at the end of the settling basin. A floor slope of

1:2 to 1:5 in the settling zone facilitates flushing.

Outlet zoneThis forms the transition from the settling zone to the headrace. The transition can be more abrupt

than the inlet expansion (i.e., horizontally 1:2 or2 = 26.50 and vertically 1:1 as shown in Figure2.13. Note that if the settling basin is combined with the fore-bay, then this zone is not necessary.

Flushing arrangementThere are various ways of removing the stored sediment from the settling basin. An appropriate

method for micro-hydro settling basins is the "hallow vertical flush pipe". In this system it can spillsome excess flow such as during floods when the water level inthe basin is above the normal level.

Ystorage

Y

1

2

1

1L

Inlet zone Settling zone Outlet zoneFreeboard

Side view

1 2

1

5

1

2


Top view

1 2


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However, in practice, a larger basin area is required because of the turbulence of the water in the

basin; imperfect flow distribution at the entrance; and the need to converge (sometimes curve) theflow towards the exit.

Design of settling basin1. Choose a suitable basin width. W, two to five times the width of the headrace canal, depending

upon the available width at the site (the larger the better).

2. Calculate the settling length (Lsettling) using the following equation:

Lsettling = 2Q / (W Vvertical) (2.21)where,

Q = design flow in m3/s.

Vvertical = fall velocity, taken as 0.03 m/s for the value for 0.3mm particles.

Normally, the length of the settling basin should be four to 10 times the width.

3. Calculate the expected silt load, Sload in the basin using the following equation:

Sload = Q T C (2.22)where,Sload = silt load in kg stored in the basin

Q = discharge in m3/s

T = silt emptying frequency in seconds Use 12 hours = 12 x 60 x 60 = 43,200 seconds

C = silt concentration of the incoming flow in kg/m3, use 0.5 kg/m

3in the absence of actual silt

concentration data.4. Now calculate the volume of the silt load using the following equation:

VOsilt = Sload / (Sdensity Pfactor) (2.23)where,

VOsilt = volume of silt stored in the basin in m3.

Sdensity = density of silt, use the value 2.600kg/m3

unless other reliable data are available

Pfactor= packing factor of sediments submerged in water = 0.5 (50%).

5. Calculate the average collection depth required, DcollectionThe settling zone should have the capacity to store the calculated value of VOsilt. This storage

space is achieved by increasing the depth of the basin for the area calculated earlier.

Dcollection = VOsilt / (Lsettling W) (2.24)6. A tapered entry ensures that the incoming flow is evenly distributed in the basin. The entry

length should have a slope of 1:4. The exit length can be shorter, with a slope of up to 1:2. Note

that no exit length is required if the settling basin is combined with the fore-bay.

Example: Settling basin

Design a settling basin such that canal depth is 0.5m, gross flow rate is 132 l/s and the particles largerthan 0.3 mm are not allowed. The possible emptying frequency (T hours) during most of the year,

when the carrying load (S) is 0.5 kg/m3, be twice daily.

1. Calculate the settling length (Lsettling)

Qgross = 0.132 m3/s

Let W =2 m; Vvertical = 0.03 m/s (from table)

Lsettling = 2Q / (W Vvertical) = (2 0.132) / (2 0.03) = 2.2 m


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2. Calculate the expected silt load

T = silt emptying frequency in seconds = 12 hours = 12 x 60 x 60 = 43,200 seconds

Sload = Q T C = 0.132 43200 0.5 = 2851 kg3. Now calculate the volume of the silt load

Let, Sdensity = density of silt, use the value 2600 kg/m3

Pfactor= packing factor of sediments submerged in water = 0.5 (50%).

VOsilt = Sload / (Sdensity Pfactor)= 2851/ (2600 0.5) = 2.2 m34. Calculate the average collection depth required, Dcollection

Dcollection = VOsilt / (Lsettling W) = 2.2 / (2.2 2) = 0.5 m

5. In practice, some extra volume is available to tapering which allows a safety factor in this design.

6. Note that Dsettling is equal to the channel depth. In order to avoid turbulence tapered entrance and

exit lengths are needed. The design rule for these is to make them each equivalent in length to onebasin width i.e. 2 m.

Fig. 2.14 Designed dimension of settling basin

2.6.6 Fore-bayA fore-bay is a tank located at the end of the headrace and the beginning of the penstock pipe. It is a

structure that allows for the transition from open channel to pressure flow conditions. The water

level at the fore-bay determines the operational head of the micro-hydro scheme. The functionof the fore-bay is to provide adequate submergence for the penstock mouth so that the transition

from an open channel to pressure flow in a pipe can occur smoothly. Here water slows down

0.5mY

1

2

11

L

2 m 2.2 m 2 m

1

5

1

2


2m


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34

for shot time and escape entrapped air and settle silt. It controls the flow into penstock and ensure

smooth and without turbulence. It releases the surge pressure as the wave travels out of the penstock

pipe. It can also serve as a secondary/final settling basin and trap some particles that enter theheadrace downstream of the settling basin. In short headrace fore-bay combines with settling basin.

If the length of the headrace canal between the settling basin and the fore-bay is long, then

sediment can enter the canal, fore-bay should also be designed to serve as a secondary settlingbasin. A spillway should be incorporated with it sudden shut down of penstock. Although very rare

in micro-hydro schemes, the fore-bay can also provide water storage for use during peak powerdemand period.

Design of fore-bay

Structurally, the forebay tank is similar to the settling basin except that the outlet transition isreplaced by a trashrack and the entrance into the penstock pipe.

1. Submergence head

The position of the submergence head (depth of water above the crown of the penstock pipe) isshown in figure 2.15. If the head is too small, the pipe will draw in air and the flow in the

penstock will fluctuate. The minimum submergence head required for the penstock pipe can becalculated as follows.

hs 1.5 V2/2g (2.25)

where, V = velocity in the penstock.

2. Storage depth

Storage depth below the pipe invert should be allowed for. A depth of 300 mm or equal tothe pipe diameter, whichever is larger is recommended for this purpose.

3. Structure and size

Its shape is similar to settling basin with outlet zone. The minimum size of the fore-bay shouldbe such that a person can get in and be able to clean it, occasionally, at least during the annual

maintenance period. The minimum clear width required for this is 1 m. If possible, the fore-bay should also be sized such that 15 seconds of the design flow can be safely stored in thetank above the minimum pipe submergence level. This is more important if the scheme consists

of a headrace pipe instead of a canal. There can be small transient surges in the headrace pipe

which result in uneven flow. The 15 second storage capacity helps to balance such uneven flows.

4. A gate valve

A gate at the entrance of the penstock will make maintenance work on the turbine easier. The

gate can be closed and the penstock emptied so that work can be carried out on the turbine.Rapid closure of the gate, however, could create negative pressure (i.e., a vacuum) inside the

pipe and even cause it to collapse.

5. An air ventAn air vent should be placed as shown in figure 2.15, prevent such a situation. Air can then be

drawn from the air vent pipe into the penstock

Diameter of air vent, dairvent is given as,

dairvent = Q [(F/E )(D / teffective)3] (2.26)

where,dairvent= internal diameter of air vent in mm


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Q = maximum flow of water through turbine l/s

E = youngs modulus for the penstock N/mm2

D = penstock diameter mmteffective = effective penstock wall thickness at upper end mm

F = safety factor, 5 for buried and 10 for exposed pipe

Fig. 2.15 A fore-bay

6. TrashrackThe trashrack at the fore-bay should be placed at a slope of 1:3 both for efficient hydraulic

perform and ease of cleaning (by raking, for example). To minimise head loss and blockage,

the recommended velocity through the trashrack. 0.6 m/s, but a maximum of 1 m/s could beused. The spacing between the trashrack bars should be about half the nozzle diameter for

Pelton turbines and half the pacing between blades for crossflow turbines. This prevents the

turbines from being obstructed by sediments and minimises the chances of surge. Cleaning of

the trashrack can be minimised by fixing it such that it is submerged during the design flow orsome additional flow (than the design flow) will be constantly required.

7. SpillwayAs discussed earlier, a spillway should also be incorporated at the fore-bay. The spillway should

be sized such that it can release entire design flow when the turbine valve is closed during

emergencies.

2.6.7 Penstock

A penstock is a close conduct pipe that conveys the flow from the fore-bay to the turbine. The penstockpipe starts where the ground profile is steep. Small portion of water energy is used to convey water

from intake to fore-bay. The penstock alignment should be chosen steep such all the remaining water

energy in the form of elevation is converted into pressure and velocity energy in penstock. Thepenstock pipe conveys water under high.

SpillwayAir vent

Fine trashrack

Gate

Compact earth

Penstock1

3

hs

300mm

minimum


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36

Penstock constitutes major expense in the total micro-hydro construction cost. Therefore it is

worthwhile optimising penstock design. This involves a careful choice of pipe material, an economi-

cal diameter such that the head loss is within acceptable limits and wall thickness so the pipe is safefor the designed head and strong enough to withstand any high pressure (surge) that may result

from sudden blockage of the flow.

An ideal ground slope for the penstock alignment is between 1:1 and 1:2 (V:H). The flatter the ground

slope the less economic is the penstock since a longer pipe length is required for a lower head. Butsteeper slop is un favorable due to difficulty in construction. The number of bends (horizontal andvertical) should be kept to a minimum so that the number of anchor blocks and head loss can be

minimised. Since the penstock alignment is on steep ground slopes and the pipe is under pressure, it is

important for the alignment to be on stable ground. For an exposed (i.e. above-ground) penstock

alignment, a clear cover of 300 mm between the pipe and the ground should be provided to facilitatemaintenance and to minimise corrosion. Buried penstock pipe will have better protection

but it is complicate for maintenance.

Design of the penstock pipe

1.MaterialMild steel and HDPE pipes are the most common materials used for the penstock in MHP schemes.HDPE pipes are usually economical for low heads and flows and are easy to join and repair. Theyare light and flexible enough to accommodate small angle bends or radial expansions resulting

from pressure surges. The disadvantage is that these pipes can degrade if exposed to ultra-violet

rays (sunlight) and temperature variations and hence these pipes need to be buried.

Table 2.4 Penstock pipe material

(Note: more the numbers * more will be the favorable condition)

S.N. Material Friction loss Weight Corrosion Cost Jointing Pressure

1 Mild steel *** *** *** **** **** *****

2 uPVC ***** ***** **** **** **** ****

3 Concrete * * ***** *** *** *

4 Ductile

Iron

**** * **** ** ***** ****

2. Pipe diameter

A pipe diameter is designed such that the velocity, V, is between 2.5 m/s and 3.5 m/s. In

general, velocity lower than 2.5 m/s results in an uneconomically large diameter. Similarly, ifthe velocity exceeds 3.5 m/s, the head loss can be excessive and hence uneconomical in the

long run due to loss in power output. Furthermore, higher velocities in the penstock will

result in high surge pressure as will be discussed later.


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dpipe = [ 4Q/V] (2.26)wheredpipe is the inside pipe diameter in mQ is the design flow in m

3/s.

V is average velocity inside pipe in m/s

3. Head loss

To calculate the head loss in the pipe length is given as,

Total head loss = major head loss + minor head losses (2.27)major head loss hf= fLV

2/2g dpipe (2.28)

minor head losses,hminor= v2

(Kentrance + Kbend + Kcontraction + Kvalve)/2g (2.29)

where,F = friction factor for pipe material, dimension less

L = length of pipe in m

V = average velocity inside pipe, m/s

dpipe = the inside pipe diameter, mKs = coefficients for pipe shape geometry, dimension less

4. Pipe thickness

The thickness of the pipe depends on the pipe diameter, the material, and the type of turbineselected. The surge effect is different for different types of turbine and hence the pipe thickness

can differ even when the design flow, static head, and pipe materials are similar. If the pipe is

strong enough to withstand the initial surge effect, the pressure will ultimately dissipatethrough friction losses in the water and pipe wall as well as through the fore-bay.

The calculation of the minimum wall thickness of the penstock for Pelton turbine is as follows:

(a) To calculate the surge head. Calculate the pressure wave velocity, a

a = 1400 / [1 + {2.1 x 109 d / (E t)} ] (2.30)where,E = the value of Young's Modulus for mild steel is 210 x 10

9N/m

2and for HDPE is 0.2 to 0.8

x 109

N/m2

d = is the pipe diameter in mm

t = the wall thickness in mm

(b) Calculate velocity V in the penstock,

V=4Q/d2

(2.31)(c) Calculate the surge head (hsurge),

hsurge= aV/ ng (2.32)where,n = the total no. of nozzles in the turbine(s)

(d) Calculate the total head

htotal = hgross + hsurg (2.33)

(e) As a precaution, calculate the critical time, T, from the following equation:

Tc = (2L)/a (2.34)


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Tc = the critical time in seconds,

where,

L = the length of penstock in m,a = the wave velocity calculated earlier.

If the turbine valve closure time, T, is less than Tc, then the surge pressure wave is significantlyhigh. Similarly, the longer T is compared to Tc, the lower the surge effect. Note that this

calculation is based on the assumption that the penstock diameter, material and wall thicknessare uniform. If any of these parameters vary, then separate calculations should be done foreach section. Closure time of at least twice the critical time (i.e., T > 2Tc) is recommended.

When the operator closes or opens the valve, timing should be such that there is no observable

change in the pressure gauge reading if installed upstream of the valve.

(d) Once the surge head has been determined, the nominal wall thickness (t) can be calculated. If

the pipe is made of mild steel, it will be subject to corrosion and welding or rolling defects.

Thus the effective thickness, teffective,will be less than the original thickness. t. For mild steel,

assume an initial thickness, t and calculate teffective,, using the following guidelines

a) If the pipes are joined by welding divide the initial thickness by 1.1.b) If the pipe is prepared by rolling flat sheets, divide the initial thickness by 1.2.c) Since mild steel pipe is subject to corrosion, subtract one mm for every 10 years of plant life.

For example, the effective thickness of a four mm thick flat rolled and welded mild steel pipedesigned for a 10-year life is

teffectiv = 4 / [(1.2 1.2)1] = 2.03 mm (2.35)

Note that this does not apply to HDPE pipes where the effective thickness is the same as the originalthickness of the pipe.

5. Calculate the safety factor (SF)

SF = (teffectiv S) / (5 htotal 103 d ) (2.36)

where,S = the ultimate tensile strength of the pipe material in N/m

2. For mild steel S is usually taken as

350 x 106

N/m2. For HDPE the value is between 6 and 9 x 10

6N/m

2

d = the internal diameter of the pipe, m

For mild steel or PVC pipes, if SF < 3.5, reject this penstock option and repeat the calculation for

a greater thickness. For HDEP pipe SF 1.5 is acceptable.In order to provide an adequate factor of safety against buckling, the minimum pipe wallthickness is given by:

teffectiv d[ F P / 2E]0.33

(2.37)

where,

teffectivee = the effective pipe wall thickness, mmd = the pipe internal diameter, mm

F = factor of safety against buckling (2 for buried penstock and 4 for exposed penstock)


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P = the negative pressure, N/mmZ

(10 m head = 0.1 N/mmZ)

E = Young's modulus for the pipe material, N/mmZ (from Table 6.2).

Example: Design of penstock pipe diameterDesign a penstock pipe diameter and wall thickness.. Data available are:

Q = 4501/s and hgoss = 180m

Ten vertical bends, = 69, 23, 26, 37, 40, 2, 3, 12, 8 & 3Penstock material: mild steel, flat rolled and site welded, 550 m long.High quality steel plates were bought and tested for tensile strength at the laboratory. Minimum

tensile strength, S = 400 N/mm2 was ensured through the tests.Turbine type: 3 Pelton turbines with 2 nozzles in each turbine, therefore n = 3 x 2 = 6.

1. Pipe diameter calculationSince the pipe is long set, V= 2.5 m/s to minimise head loss.

dpipe = [ 4Q/V] = [ 4 0.450 / 2.5] = 0.479 m

2. Total head loss

To calculate the head loss in the pipe length is given as,

Total head loss = major head loss + minor head lossesf =0 .0014 (from Moody Chart)

Major head loss hf= (fLV2)/ (2g dpipe ) =[0.0014 550 2.5

2]/ [2 0.47 g] =5.13 m

Minor head losses,hminor= V2

(Kentrance + Kbend + Kcontraction + Kvalve)/2g

From Tables of pipe loss (source: if any book of pipe loss)

Kentrance = 0.2Kcontraction = 0 (not available in this case)

Kvalve = 0 (not available in this case)

Kbend = 0.34 for = 69Kbend = 0.11 for = 23Kbend = 0.13 for = 26Kbend = 0.18 for = 37Kbend = 0.20 for = 40Kbend = 0.02 for = 2Kbend = 0.02 for = 3Kbend = 0.06 for = 12Kbend = 0.04 for = 8Kbend = 0.02for = 3Minor head losses, hminor= [V

2/2g] (Kentrance + Kbend s+ Kcontraction + Kvalve)

= [2.52

/2g] ( 0.2+0.34+0.11+0.13+0.18+0.20+0.02+0.02+0.06+0.04+0.02)

= 0.42 mTotal head loss = 5.13 m + 0.42 m = 5.52 m

head loss = 5.52 / 180 =3.1 5

Therefore, the diameter can be made little smaller. The adopted diameter of penstock is 450 mm,

which gives 4.1% head loss, through a repeat of the above calculations.

3. Pipe thickness

First calculate the thickness required at the downstream end of the penstock


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40

(hstatic = hgross = 180 m and d = 450 mm. Try for t = 6 mm E= 210 109N/m

2)

a = 1400 / [1 + (2.1 109 d / E t) ]a =1058 m/s

V = 4Q / d2 = 4 0.450 / 0.4502 = 2.83 m/shsurge= a V / (n g) = 1058 2.83 / 6 9.8 = 51 mhtotal =hgross + hsurge= 180 + 51 = 231 m

teffective = [6/ 1.1 1.2] -1.0 = 3.55 mm

3. Calculate the safety factor (SF) using the following equationSF = (teffectiv S) / (5 htotal 10

3 d )= 3.55 400 106 / 5 231 103 450 = 2.742.5 SF(2.74) 3.5Hence the design will be acceptable for the well trained technician if not redesign for less safetyfactor.

2.6.8 Anchor blocksAn anchor block is a mass of concrete fixed into the ground that holds the penstock to restrain the

pipe movement in all directions. Anchor blocks should be placed at all sharp horizontal and vertical

bends, since there are forces at such bends which will tend to move the pipe out of alignment. Anchorblocks are also required to resist forces in long straight sections of penstock.

Design criteria of anchor blocksFor micro-hydro schemes with a gross head less than 60 m and an installed power capacity less than

or equal to 20 kW, following guidelines is used to design anchor blocks.

1. It is constructed of concrete which is 1:3:6 with about 40 per cent plums (boulders) placed evenlyaround the block as shown in Figure 2.16. The boulders add weight to the block and therefore

increase stability while reducing the volume of cement required.

Penstock

Metal tagReinforced bar

3:6 PCC with 40% pulms

Fig. 2.16 An anchor block

Bend angle ( - )


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2. For straight sections, locate one anchor block every 30m along the length of the penstock. Use

one m3

of plum concrete for a pipe diameter of 300mm. If the pipe diameter is more or less, say200mm, then adjust the amount proportionately. (200/ 300) x 1 m

3=0.67m

3

3. Always provide an anchor block at the bends in the penstock keeping a maximum distance of30m between two blocks. For bends less than 45, use double the concrete volume required for a

straight section. For example, if the pipe diameter is 350mm and the bend is 20, then use(3

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Microhydro System Design