Inversin R. Allen Micro-hydropower Sourcebook

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Unlike most of the other components, the power Conduit

is not an essential part of every hydropower scheme.

Where the drop in elevation is concentrated near the

powerhouse and there is a significant horizonal distance

between the intake on the stream and this drop, a power

conduit is used to reduce the cost of the civil works.

Rather than using a costly penstock pipe for the entire

distance, a power conduit conveys the water from the

intake to a point as close to above the powerhouse a

possible. The design objective is to make the penstock

as steep as possible to reduce the diameter and length-

and therefore the cost-of the required penstock pipe.

The cost OL the conduit itself mubt then be added, but

the conduit frequently can be constructed at a fraction

of the cost

a penstock, especially if local materials

rather than imported steel, concrete, or plastic pipe can

be used.

To illustrate this point, consider a scheme that extends

approximately 1 km, a distance not uncommon for a

micro-hydropower scheme. Assume that this site, with

a gross head of 30 m, requires about 300 C/s to generate

50 kW (Fig. 5.10). One option is to convey the water

through a penstock from the intake to the turbine. This

would require 1000 m of 600 mm-diameter pipe. How-

ever, if there were a rapid drop near the powerhouse

which would require only 100 m of pipe to obtain approx-

imately the same head, a second option would be to

convey the water 1 km from the stream to the inlet of

that penstock pipe with as small a head loss as possible.

This is where a power conduit, perhaps in the form of a

canal, can play a

role.

With a drop of only 1 m over its

I km length from the intake to the penstock, the

required flow can b

conveyed with a canal cross-sec-

tion of about 0.2 m .

Then, only a 100 m length of

400 mm penstock pipe would be required to convey this

flow from the end of the canal to the turbine.

In summary, the first option requires 1000 m of 600 mm

penstock, whereas the second option requires 100 m of

400 mm penstock and the excavation of 1000 m of canal.

For a clearer impression of the cost implications,

assume that 400 mm steel pipe costs $60/m and

600 mm pipe costs $140/m. The costs for each option

would be:

first option-$140,000 for the penstock plus the cost

of excavating a trench for the 1000 m penstock if

required;

second option-$6000 for the penstock plus the cost

of excavating a 1000 m canal and a trench for the

100 m penstock if

required.

Assume further that the cost of the turbo-generating

equipment for this 50 kW plant is $BOO/kW or a total of

$40,000. The long penstock in the first option would add

$140,000, making it the major cost component of the

project. The price of the penstock proposed in the sec-

ond option, however, would contribute only marginally

to the cost of the project.

In the second option, the cost of the canal itself must be

considered. If the the canal is constructed as a self-

help project, only the cost of materials for any lining

deemed necessary

for a section of the canal which

requires special attention, such as a supported section,

would be incurred. If canal construction requires con-

tract labor or heavy equipment or if the terrain is diffi-

cult, larger costs could result, perhaps making the first

option more appropriate.

Although a power canal can considerably reduce costs,

the existence of a significant horizontal distance

between an intake on the stream and the powerhouse is

not sufficient reason to include a power canal or other

form of power conduit in the design of a hydropower

scheme. It is always necessary to compare the costs of

these two options--a long penstock

vs.

a conduit and

short penstock-and to consider the nature of the ter-

rain and the potential construction and maintenance

problems associated with each option.

The principal disadvantage of incorporating a power

canal when the topography is suitable is that, unlike a

penstock, a canal is usually open. Because it is more

exposed to the elements, it requires more upkeep. Fall-

ing trees or landslides might obstruct the flow, exces-

sive runoEf uphill of the canal might cause sections to

wash away, or sediment in the incoming water might

settle out along the canal. Although properly designing

a scheme can prevent many such problems, a penstock

pipe is less susceptible than a canal to these problems.

1 km

Fig. 5.10. These two approaches to developing the some site can lead to

constdembk differences in

cO3t.

70 Civil works

As mentioned, a low-pressure pipe can also serve as a

power conduit. This avoids some of the disadvantages

of using an open canal. By using a low-pressure conduit,

especially at medium- and high-head sites, a less costly

pipe can be used because a pipe wall thinner than that

used for the pressure penstock is necessary. A low-

pressure conduit generally will cost more than an earth

canal.

Because power canals are the most commonly used form

of power conduit,

Power

conduits (p. 96) deals primarily

with their design and construction; however, some

information on the use of pipes as power conduits is also

included.

Depending on the length, slope, and dimensions of the

power canal, spillways (p. 159) might be incorporated at

appropriate intervals. When they are used, provisions

must be made to ensure that any overflow leaving via

the spillways will not undermine the canal or other

components. Spillways are necessary if there is any

chance that a canal might overflow if excess water

enters the canal, although proper design of the intake

should minimize this possibility. Spillways are also

necessary if dirt from landslides or other debris entering

a canal could restrict the flow, causing water to back up

until the canal overflows.

If a canal is built on a hillside where runoff along with

debris and sediment can enter the canal, provision may

have to be made for proper drain= (p. 108) around the

___

canal. If significant runoff can still enter a canal, a

Fig. 5.11. A sheet-metal

flume

supported by

trestle con-

structed of angle iron.

forebay of sufficient size will be needed to serve as a

settling area before the water enters the penstock.

When the power canal must cross difficult terrain or an

obstruction such as a ravine, streambed, topographical

depression, or road, any one of several approaches might

be adopted:

e The slope

the canal can be maintained and the

canal supported above this obstruction in a flume

(p. 112) constructed of wood, steel, or concrete

(Fig. 5.11).

o The slope of the canal can be maintained, with a

structure to reroute that obstruction over the canal

(Fig. 5.12) which would then flow in a culvert

(p. 116).

Fig. 5.12. A power

canal

passes through a culvert under a

ravine which collects considerable

runoff during

the mon-

soon Two walls visible above the culvert keep this flow

within its banks.

e An inverted siphon (p. 115) can be used to lead the

flow under the obstruction (Fig. 5.13). This is a

closed conduit which permits the water in the canal

to drop in elevation until it passes the obstruction

and then to rise again on the other side. Under cer-

tain circumstances, this might be the only option.

If so, it must be designed carefully because sediment

can accumulate in the siphon and obstruct the flow.

Adequate provision should be made at the design

stage to permit removal of this sediment.

Although a power canal is usually designed to minimize

head loss, drops along its length might sometimes be

necessary to avoid obstacles in the terrain. The energy

gained by the falling water must be dissipated in a con-

trolled manner; otherwise, it might erode part of the

canal. Properly designed drop structures (p. 1 i0) are

used for this purpose (Fig. 5.14).

When a power conduit runs near a stream, it is neces-

Civil works 71

Fig. 5.13. An inverted siphon conveys water

across the

main Kathmandu-Pokham road in Nepal to irrigate fields

and power a traditional water mill.

sary to ensure that the stream cannot undermine this

conduit (Figs. 5.15 and 5.200). It is also important to be

aware of springs which may either undermine power

conduits or cause landslides above them (see Fig. 5.30).

Forebap

The forebay is a basin located just before the entrance

to the penstock. Possible designs range from a simple

excavated 6:~ or pond to a structure of reinforced

concrete. Its size may vary depending on the quality of

the water being conveyed to the penstock and whether

Fig. 5.14. i’wo stone-masonry drops along a power canal.

Fig. 5.15. Gabions are used to protect the foundation of a

canal conveying

water

for power genemtion and irrigation.

it is to serve for storage. In some cases, it is virtually

nonexistent.

To be most cost-effective, the forebay

must be of a size adequate to fulfill its function, neither

significantly larger nor smaller.

The forebay can serve several purposes:

o It can serve as a final settling basin where any

waterborne debris which either passed through the

intake or was swept into the canal can be removed

before the water passes on to the turbine. In this

case, the forebay must be large enough to reduce

flow velocities sufficiently for settling to occur (see

Settling

basins, p. 1661 and to accommodate the sed-

iment which accumulates between cleanings.

o The forebay can also provide storage. Because a

significant volume of stored water would be required

to supply a turbine for several hours ea.ch day, water

is more frequently stored behind a dam in the

streambed itself. However, if the topography, geo-

logy, or land-use in an area makes storage within the

streambed difficult, a forebay might have to serve

that purpose. Storage in the streambed requires a

dam of sufficient height. On the other hand, forebay

storage usually requires excavation of a significant

volume of ground which then has to be sealed to pre-

vent water seepage which can undermine the struc-

ture. A forebay serving as storage contains a large

volume of water and is usually located just above the

powerhouse, at the top of a steep drop; therefore, it

must be carefully designed and constructed.

However, a forebay generally provides only enough

storage to cope with water demands created by a

sudden increase in loading on the turbine. This is a

transient condition because, after flow in the power

canal has re-established itself, it should convey the

increased flow necessary to continue meeting the

turbine’s increased demand. Where streamflow is

sufficient, it might be more appropriate to permit

the maximum flow that might be required by the

turbine into the forebay and to allow flows not

72 Civil works

required by the turbine at times of reduced load Lo

leave by the spillway. ‘Where a load-controlling

device is used with the turbine (see Iamb

controller,

p. 204), penstock flow remains constant, thus elimi-

nating the need for storage to compensate for tran-

sient flow variations.

Several important components are usually included in

the forebay. These include a spillway, a scouring gate,

a gate to the penstock, a trashrack, and possibly a vent.

On occasion, the flow entering the forebay may exceed

the flow leaving via the penstock, such as when a gate

at the intake has been set improperly, when the valve to

the turbine has been closed, or during heavy rains when

excess flows enter the canal from the stream or from

runoff uphill of the canal. For this reason, it is advisa-

ble to include a spillway (p. 159) in one wall of the fore-

bay. The water passing over the spillway must be

diverted properly to prevent erosion that might under-

mine the forebay, penstock, or powerhouse (Fig. 5.16).

The dimensions and slope of a spillway ca.aal must be

adequate to convey safely the maximum flow which

might ever enter the forebay. A natural rock ravine

might serve that purpose. If a gate is located at the

entrance to the forebay to prevent water from entering

during its repair or cleaning out, the canai wall just

before this gate must also incorporate a spillway large

enough to ptarmit any water descending the canal to be

diverted safely.

Fig. 5.16. Excess water is led around the powerhouse back

into the tailmce canal.

Stones

embedded in the concrete

chute dissipate the

energy of the descendfng water.

Incoming water can carry sizable quantities of floating

debris. A trashrack (p. 162) is often included at the

inlet to the penstock to prevent this debris from enter-

ing the penstock and turbine. Skimmers (p. 165) are also

used occasionally (Fig. 5.17). If excess water is suffi-

cient, the spillway can be located to remove most of the

floating debris automatically.

A gate or valve (p. 154) should he incorporated to drain

the forebay so that any sediment which has entered and

settled can be removed easily. Draining is also required

-hen the forebay is being repaired. The flow through

the drain can be led away in the same canal that

removes the overflow from the spillway(s) at the fore-

bay. When the penstock must be emptied for repairs, a

valve might be incorporated at the beginning of the pen-

stock. However, because such a valve would be the

same size as the penstock, it can be costly. In addition,

it would only be used infrequently. A less costly

approach is to ensure no water enters the forebay, by

closing a gate either at the intake to the scheme or just

before the forebay (if a spillway is located just

upstream of that gate). To ensure that water which

might unexpectedly enter the forebay does not cause

erosion along the penstock under repair or flood the

powerhouse, a gate is often placed just before the inlet

to the penstock. This gate is usually a stoplog because

it is used infrequently and is inexpensive.

Fig. 5.17. Although the tmshrack is oriented upstream, a

wooden pole secured to the walls serves

as a skimmer. It

prevents

some

of the floating debris

from

getting

caught

up in the tmshmckand, being inclined to the

flow, encour-

ages the stream gmdualfy to sweep this debris down-

St.-earn.

Civil works 73

As is described in Penstock? an air vent (p. 76) is often

used as a safety precaution against collapse of the pen-

stock pipe. One design option would be to incorporate

this pipe in the wall of the forebay just below the inlet

to the penstock (see Fig. 5.109).

If a second penstock and turbine may be added at some

later date, its installation would be facilitated by incor-

porating a wooden plug in the concrete forebay wall,

possibly with a steel plate on the inside to give the

required strength. When the second penstnck is to be

installed, piercing the forebay wall will prove much

easier.

Penstock

The penstock is 3 pipe that conveys water, under pres-

sure, to the turbine. Except for some schemes with

very low head--such as open-flume Francis and propeller

turbines, in-stream turbines, and traditional water-

wheels-it is an essential part of any micro-hydropower

plant.

A penstock pipe can be installed either above

below

ground. Flexible, small-diameter penstocks used for

plants with very low outputs are sometimes draped over

the terrain down the hillside; however, larger-diameter

penstocks installed above the ground must be secured

properly to prevent movement which could damage the

pipe. Although this increases the cost of pcnstock con-

struction and maintenance, the alternative-excavation

for a buried pipe--might he no less costly. The installa-

tion of a penstock above ground increases its exposure

to the elements, but its accessibility also facilitates

inspection, maintenance, or repairs. Burying d penstock

may involve considerable time and expense, but it pro-

tects the pipe from the elements and from landslides,

falling rocks, brush fires, and tampering. In addition,

the compacted soil firmly secures the penstock, provid-

ing adequate anchorage for most small-diameter pipes.

A penstock above ground is subject to greater tempera-

ture variations. If the turbine is not functioning contin-

uously, these temperature variations can be pronounced,

because the moderating effect of flowing water with

fairly constant temperature is not felt. Variations in

temperature result in thermal expansion, which in turn

causes stresses in the pipe. Thermal expansion and con-

traction are greatest ior a penstock which is likely to

remain empty during construction or repair, and provi-

sion must be made to accommodate these; either the

pipe must be designed to have sufficient structural

rigidity with the help of anchors and supports or expan-

sion joints (p. 136) should be incorporated. These joints

also help protect the pipe against earth tremors and are

sometimes used to accommodate a slight misalignment

of two pipe seciiilns.

For above-ground steel penstocks, a common type of

expansion joint consists of two concentric portions of

the penstock which permit relative motion with respect

to each other, with a gland to prevent leakage between

them (Fig. 5.18). Polyvinyl chloride (PVC) pipe is com-

monly buried, eliminating the need for expansion joints.

74 Civil works

Fig.

5.18.

An expansion

joint

along a

penstock Although

this plant was

not yet opemtional, leaking had already

caused

potentially troublesome erosion

However, joints for PVC pressure pipe are often fitted

with gaskets which permit some relative motion

between adjacent sections. Although thermal expansion

might he insignificant for buried pipe, ihe joints protect

the pipe from vibration and earth movement and permit

it to settle somewhat under loads. Cement pipes are

also available with such joints. Long lengths of poly-

ethylene pipe coupled by rigid joints are sometimes laid

above ground, but this pipe is flexible and can accom-

modate changes in length caused by thermal expansion.

Rigid penstock pipes frequently require support piers,

anchors, or thrustblocks to resist forces which can dis-

place the pipe. Supports piers (p. 138) are used along

straight runs of exposed pipe, primarily to prevent the

pipe from sagging and becoming overstressed. They

might also have to resist the longitudinal forces result-

ing from temperature-induced movemeLt of the pipe

over the support. The pipe usually lies in a saddle on a

reinforced-concrete support pier and should be free to

move longitudinally, to accommodate small pipe move-

ments without abrading or cutting the pipe material

(Fig. 5.19). The spacing of the support piers is deter-

mined by the maximum unsupported span associated

with the specific penstock pipe material and size. For

buried pipe, the bed of soil on which it lies supports it,

and separate supports are not generally required.

Significant forces can be concentrated at bends along a

rigid, exposed penstock. These bends can be in a verti-

cal or horizontal plane or a combination of both. The

largest force is usually caused by the hydrostatic pres-

sure within the pipe--the pressure of water which tends

to cause the penstock to crawl or the joints to separate.

Depending on the alignment and design of the penstock,

other forces also contribute to a varying extent, such as

those caused by thermal expansion of the pipe, the

weight of the upstream portion of pipe pushing downhill

against the bend, and reductions in pipe diameter.

Anchors (p. 140) are incorporated at bends in the pen-

stock either to provide the weight necessary to counter-

act the resultant of all these forces or simply to trans-

force which may have to be held in check arises from

hydrostatic pressures acting at bends along the pen-

stock; wit.h an exposed pipe, this is also one of the prin-

cipal forces an anchor must resist. For pipes with

welded or mechanically joined sections, soil reaction

forces may be adequate to prevent movement of the

pipe. For loosely coupled pipe, such as PVC pipe with

push-together joints where an elastometric ring that fits

in a groove of a pipe section’s*bell end provides a water-

tight seal, anchoring may be necessary. If a bend is

concave downward so that the thrust acting at the bend

acts upward, an anchor is used to counter this force by

means of its weight. Where the pipe is laid in a trench

and bends are either in a horizontal plane or concave

upward, thrustblocks (p. 1441 are used to tr.ansfer the

thrust to the undisturbed soil. These do not rely on

their mass to provide the necessary restraining force.

Gates or valves (p. 1541 can be incorporated at either

end of the penstock to control the flow of water. A

turbine isolation valve is often included at the bottom

of the penstock. It is used to turn the flow to the tur-

bine on or off and is generally not used to regulate it. A

valve supplied as part of the turbine generally provides

this regulation, if required. Even though this latter

valve can also turn the flow to the turbine on or off, a

shutoff valve is often included to perform this function

it the turbine or the Flow-regulating valve needs repair.

A lower-cost, although possibly less convenient, option

to stop the flow of water down the penstock is to avoid

this shutoff valve and simply to open a gate at the fore-

bay to drain it and prevent water from entering the pen-

stock. The water remaining in the penstock can be

drained through the turbine before repairs are Imder-

taken. Another option would be to close a stoplog gate

located at the inlet to t’le penstock as part of the fore-

bay (see Forehay, p. 72). Rather than a stoplog gate, a

butterf!y or gate valve is sometimes incorporated at the

beginning of the penstock pipe itseif. In this case, an

air vent (p. 76) must be incorporated to permit the

water in the penstock to drain without potential damage

to the pipe. However, since the pipe diameter is proba-

bly reduced just before entering the turbine, a valve at

the beginning of a penstock would probably be larger

and costlier than one located just before the turbine.

Therefore, if a valve rather than a gate is used, it is

usually placed just before the turbine.

Fig. 5.19. Metal stmps are used to secure the penstock

a support.

mit it safely to the ground (Fig. 5.20). Even along a

straight section of pipe down a steep slope, anchors may

be required at intervals to prevent the pipe from sliding

downhill because of its weight. Unlike support piers,

ihr

anchor holds the pipe secureiy. An anchor biock is

usually constructed of concrete and held together and

around the pipe with hoop reinforcement. A collar is

sometimes affixed to the pipe to allow it to be keyed

firmly into the mass of concrete.

When the pipe is buried, properly compacted backfill

generally serves the same function as anchors and sup-

port piers used with exposed pipe. Unless the penstock

descends a steep slope, friction between the pipe and

soil provides sufficient force to counteract the weight

of the pipe pulling it downhi!l. Also, forces caused by

thermally induced stresses are small because the pipe is

shielded from large temperature variations. The one

Fig. 5.20.

An anchor at

a bend and

bifurcation

at the base

the

penstock

In the operation of a hydropower plant, the flow velo-

city in a penstock can change suddenly when ‘the plant

operator or mechanical governor rapidly closes the

shutoff or flow-regulation valve located just before the

turbine. This sudden closure can create momentary

pressure peaks or water hammer in the penstock which

could damage the pipe. This pressure rise is determined

by the rate of valve ciosure, the initial velocity of flow

in the penstock, and the elastic properties and physical

dimensions of the pipe (see .APPENDIX F, p- 274).

One way of protecting against damage caused by water

hammer is to ensure that the penstock is thick enough

to accommodate these higher transient pressures. This

is the common approach used in the design micro-hydro-

power schemes. Water hammer pressures can also be

reduced by increasing the area of the penstock, which

Civil works 75

decreases the velocity of the water descending the pipe.

If the penstock is unusually long, however, either of

these ilpproaches may increase the cost of the penstock

pipe considerably. To reduce cost, a design

for

a hydra-

power scheme which minimizes penstock length-possi-

bly using a power canal to convey the water most of the

horizontal distance from the intake to the powerhouse--

should be sought. Pressures can also be reduced by

designing the system to prevent rapid closure of any

valves preceding the turbine.

With large hydropower schemes, long power tunnels or

penstocks are often necessary and the preceding options

are not practical. In these cases, a surge tank can be

used to protect the low-pressure conduit section above

the tank from high internal pressures caused by rapid

valve closure. This tank is a storage reservoir placed

along the pipeline as close to the powerhouse as possible

to relieve the pressure peaks rising up the penstock

before they continue through the upper portion of the

conduit. The surge tank therefore reduces upstream

pressure fluctuations and permits the use of lower-cost,

low-pressure conduit. Nevertheless, the penstock sec-

tion between the surge tank and turbine still has to be

designed to resist high transient pressures. Because

surge tanks are rarely used at micro-hydropower instal-

lations, this publication does

not

cover their theory and

design.

When the pipe section between the surge tank or inlet to

the penstock and the turbine is long and pressure rise

caused by rapid closure of the shutoff valve can be too

large, a pressure relief valve can be installed imme-

diately above the shutoff valve. With micro-hydropower

schemes, the relief valve is often activated by the high

pressure peak and permits the water in the penstock to

continue flowing by bypassing the turbine and shutoff

valve, usually discharging it into the air.

At times, air has to be permitted to enter or leave the

penstock at specific points along its length. This might

be necessary at three points along a penstock: near the

inlet, at intermediate high points along a penstock, and

between a gently sloping section and a steep drop to the

powerhouse.

If a trash pile-up or a valve closure can significantly

restrict or seal the inlet to the penstock, a means of

letting air into the penstock just below the inlet

must be included; otherwise, as the penstock emp-

ties, the pressure within will fall below atmospheric

and the pipe may collapse. To prevent this possibil-

ity, a simple air vent-a pipe section open to the

atmosphere (Fig. 5.21)--is located at the upper end

of the penstock, possibly in the forebay wall itself.

If a scheme has been designed with no valve or gate

at the inlet, as small schemes often are, and so that

a clogged trashrack cannot completely seal the inlet,

inlet, no air vent is needed.

Including a valve at the inlet does not necessarily

require a vent, because proper plant operation can

avoid low penstock pressures. However, a novice

operator may think that closing such a valve (possi-

bly in an emergency) can shut down the plant safely;

76 Civil works

water level during

normal operation

Fig. 5.21. An

air

vent is fncorpomtea at the upper

end of

the penstock

prevent possible collapse of

the

pipe

if the

intake value is closed during

the

plant’s opemtion.

with anything but low-head schemes, this is not the

case unless the penstock includes an air vent. Or the

operator simply may forget that the inlet valve has

been closed and open the turbine isolation valve at

the lower end of the penstock to start the turbine.

This could also cause the pipe to collapse if the pen-

stock is full. For these

reasons,

if a valve has been

included at the inlet to the penstock, it is advisable

to incorporate an air vent just downstream.

a* a valve is in&tided at the inlet to the pensto&, it

may be necessary to fill the penstock to equalize

pressures across this

valve before

it physically can

be opened. This is done by allowing flow into the

penstock through a small bypass valve. An air vent

is then necessary just downstream of the valve to

permit

air

trapped there to escape during the filling

operation.

If the penstock has intermediate high points along its

length, any air inside accumulates at these points,

restricting flow through the penstock. An air-

release valve must therefore be included ateach

peak to remove this air. Even if no air enters the

penstock during normal operation, such a valve is

required at these points to release air while the pen-

stock is being filled. This may be an intricate valve,

shown figuratively in Fig. 5.22. As air which gathers

at the peak of the pipe enters the valve chamber,

the float descends, opening the valve and automati-

cally releasing the air. The ensuing rise in water

level closes the valve again. A small, manually

operated gate valve or faucet also can be used.

Although it is more costly, a proper air-release valve

can also serve as an air vent or air-inlet valve to

admit air into the penstock automatically when it is

being emptied.

If a gently sloping portion of the penstock is fol-

lowed by a steep drop to the powerhouse, an air-inlet

or air-admission valve may be required at the bend.

If the flow into the turbine increases rapidly, water

in the steeper portion of the penstock may acceler-

ate faster than water in the upper part. This can

cause the column of water in the penstock to sepa-

rate at the change in grade, subjecting this part of

the pipe to a pressure low enough to collapse it. An

air-inlet valve allows air in to relieve this transient

Fig. 5.22. A figurative depiction of

an air-release valve at

a high point

along

a penstock

effect but prevents water from escaping. It is sim-

ply a check valve

set on top

of the pipe, normally

held shut by the penstock pressure aided by a light

spring. Even when a penstock includes a gradually

sloping section followed by a. steep drop, this

valve

can be omitted provided the system design prevents

turbine flow from increasing suddenly.

To function properly, an air-inlet valve or vent must be

adequately sized. If the opening is too small, air will

enter too slowly and decreasing pressure within the pen-

stock may still cause the pipe to collapse. (See Air

valves, p. 147, to determine the appropriate size for this

opening. j

If a penstock crosses undulating terrain, a blowoff

should be installed at each low point alongsgth to

drain the penstock or to remove any sediment which has

accumulated at these depressions. A blowoff usually

consists of a short pipe connected beneath the penstock

at these low points. This pipe is generally fitted with a

gate valve and is long enough to lead the water away

from the penstock. Sometimes a valve is also placed

just uphill of the turbine isolation valve for the same

purpose.

-Access ports also may be incorporated at the end of a

penstockzhat any large objects which have unexpect-

edly entered the penstock can be removed by hand with-

out removing the turbine or nozzle. At times, a bifur-

cation or “Y” is placed just before the turbine, with an

access port at the end of the lower branch of the ‘Y”.

Powerhouse

The powerhouse protects the turbine, generator, and

other electrical and mechanical equipment. Its size and

configuration depend on the functions it

serves. Con-

ventionally, it is large enough to include not only this

equipment but also a workshop, office, and sanitary

facilities. This is not

generally

the case for micro-

hydroelectric schemes, except for those built by institu-

tions in the habit of designing and implementing larger

schemes (Fig. 5.23). If project size and cost are to be

minimized, a powerhouse should be sized to house only

the turbo-generating equipment, with sufficient space

on all sides to permit easy access for installation, oper-

ation, maintenance, and repair. The only exception is a

plant designed to accommodate mechanically driven

agro-processing equipment or other machinery. In this

case, additional space is required for the equipment

operators and customers bringing their crops to be pro-

cessed.

Fig. 5.23.

This pou;erhouse, which

is expected to include a

second

50 kW turbo-genemtlng unit

in the left foreground,

IS significantly oversized for the function it swvea.

With small micro-hydroelectric powerplants, it is

also

possible simply to cover the generator and other compo-

nents sensitive to the elements (Fig. 5.24). Although

this “powerhouse” costs virtually nothing, this design

makes it difficult to repair the equipment during incle-

ment weather. If a unit is properly designed, however,

Fig. 5.24. At this site which

has

been

opemtion since

1975, a simple metal box hinged on one side and a belt

guard protect the turbo-generating

equipment from the

elements. See Fig. 6.9 for a view

of the unit

with the

cover removed.

Civil works 77

access to the turbo-generating equipment is not fre-

quently required.

With some low-head axial-flow turbines in the bulb

con-

figuration, both the turbine and generator are located in

the water passageway (see Fig. 6.12b); only cables car-

rying the power emerge. In this case, neither power-

house nor simple cover is required. Any switchgear near

the turbo-generating equipment can be placed in a small

weatherproof enclosure.

The location of the powerhouse is as important as is its

design. For a discussion of this subject, see LOCATING

THE F’OWERHOUSE (p. 56).

head, more care is necessary because head losses can be

more significant. Flow in a tailrace canill is governed

by the same law governing flow in power canals (see

Determining canal dimensions and slope, p. 96).

DESIGN AND CONSTEXJCTION DETAaS

Dams and weks

This section emphasizes <J&s, because a majority of

micro-hydropower schemes in developing c,Iuntries only

need water to be diverted into the intake. However,

because there ix sometimes also a need for water stor-

age or for increasing head, several types of dams are

also reviewed.

Tailrace

The tailrace is usually a short, open canal which leads

the water from the powerhouse back into a stream-

generally the stream from which the water came

(Fig. 5.25). It is a component of every scheme except

low-head plants where the water emerges from a draft

tube directly into the stream.

Fig. 5.25. This tailmce will carry

water from the power-

house back to the stream. The short penstock can be seen

behind the powerhouse (also see Fig. 5.1211.

Like a power canal, a tailrace is also a canal, but much

more effort is spent on the design and construction of

the former. Power canals are usualiy long and must

traverse sloping hillsides and ravines while keeping head

losses to a minimum, maintaining water quality, and

guarding against excess water. A tailrace, on the other

hand, is usually very short and located near a stream.

Often it is simply a ditch which is designed to ensure

that erosion will not undermine the powerhouse. If a

tailrace is long and/or a plant is operating under a low

Where water resources are adequate, the simplest

and least costly diversiorl structure is a weir of loose

boulders extending across part or all of the stream

(Fig. 5.26). This is clearly a temporary structure, but

that is one of its virtues. Large flows, common in the

tropics, sweep the structure away, permitting boulders,

tree trunks and branches, sediment, and other debris to

pass the intake unhindered. There is no structure in the

stream which restrains debris and sediment and has to

be cleared out manually after the flows have subsided.

An.d a weir is not needed during periods of high ilows,

because the level of the stream is already sufficient for

adequate water to enter the intake. The only disadvan-

tage of this approach is that it requires a manual effort

to rebuild the weir when flows recede. This is a minor

effort, however, and many developing countries have no

shortage of labor. Such temporary weirs are found in all

countries where irrigation has been practiced tradition-

ally and are used at virtually all of the hundreds of

modern micro-hydropower plants in Nepal and Pakistan.

It is interesting to rrotc that conventional designs for

small-hydropower plants often include a “permanent”

weir across a stream.

Not infrequently, however, these

are eventually damaged or destroyed and, in the end, a

temporary weir rrf loose boulders is still cotnmonly

adopted as a permanent solution (Fig. 5.27). Use of a

temporary diversion weir is not limited to small

Fig. 5.26. A simple weir of loose

stones

diverts water to a

smal: turbine-driven mill.

78 civil works

Fig. 5.27. Though the weir was ruptured

by flood flows and

has

never

been rebuilt, the plant run by Electroperu con-

tinues

to operate using flows diverted into the intake

means

of a temporary

weir

of boulders slightly upstream.

schemes; Fig. 5.28 illustrates its use at a 1000 kW plant

to divert dry-season flows toward a side intake. The

dam was incorporated solely to increase available head

by several meters. It neither permits storage of water

nor ensures that adequate water is diverted for power

generation. A temporary weir serves this latter pur-

pose.

Where a streambed is likely to lower over time, leaving

the intake high and dry, the previous approach has been

modified to include a permanent sill across the stream-

bed (Fig. 5.29). At some sites (Fig. 5.301, a sill con-

structed of box gabions and mattresses--stone-filled,

wire baskets-is used to stabilize the area around the

intake. At others, gabions are keyed into the streambed

ad overlaid with reinforced concrete (Fig. 5.31). In

addition to these permanent sills, a simple weir of loose

Fig. 5.28. A temporary diversion weir across the stream is

used to divert

water toward an

underground intake at the

left. This dum is used only to increase

head; it does not

form a reservoir to store water.

intake to

weir

Fig. 5.29. A weir used

for

diversion purposes.

stones is frequently constructed across the stream near

the crest of the sill to divert flow toward the intake

during low flows.

The sill in Fig. 5.32 is a variation of the concrete sill

just mentioned and was designed to overcome the need

to build a temporary stone weir during low-flow periods.

Fig. 5.30. Box

gabions and mattresses serve as a sill

across

the stream. A small wall of stones near the crest diverts

the low flows found in the dry season. The covered portion

of canal and masonry retaining wall were built as an after-

thought, after a spring just uphill

this area caused a

small landslide over the canal.

Civil works 79