Inversin R. Allen Micro-hydropower Sourcebook

In ordei not to exceed this pressure, the minimum area

“A” (m ) of the vent inlet is given by the following

equation:

A=

Q

400 cfi

(5.33)

where

Q = flow of air through inlet (m3/s) (see below)

C = coefficient of discharge through inlet

= 0.5 for ordinary air-inlet valves

0.7 for short air-inlet pipes (38)

To cover the wotse possible case, the maximum air flow

“Q” through a vent should equal the maximum flow of

water the turbine can accommodate. EXAMPLE 5.12

illustrates use of these equations. .

The vent pipe should be long enough so that its opening

to the atmosphere is above the high-water level in the

fotebay (see Fig. 5.21); otherwise, a high-water level in

the forebay will cause water to flow out the vent, possi-

bly inundating the area and undermining the forebay

or

penstock.

Powerhouses

In the first part of this chapter, it was noted that a

powerhouse is constructed to enclose and protect the

turbo-generating and associated equipment. This sec-

tion briefly reviews some factors which should be con-

sidered in designing such a structure.

A powerhouse needs a floor area only sufficient to

accommodate the equipment and permit easy access on

all sides for installation, maintenance, and repair. The

door to the powerhouse should be large enough to permit

passage of the largest piece of equipment. When large

and heavy turbo-generating machinery is to be used, the

building should be high enough to accommodate a hoist

or

other equipment which is used to assemble ot disas-

semble the machinery.

To handle heavy equipment at large plants, cranes which

travel on overhead rails supported by the powerhouse

superstructure are often used. For micro-hydropower

schemes, a hoist

or

b!ock and tackle, temporarily sup-

potted from a beam, is more commonly used for heavy

equipment. In these cases, the loads to be supported

govern the structural design of the powerhouse walls.

Rather than a beam, a temporary “A” frame can be

used. At a 100 kW scheme using two 50 kW turbo-

generating units in Kalam in Pakistan, rather than over-

designing the roof truss to support the occasional heavy

loads encountered when the equipment is moved, a hori-

zontal slot is incorporated in opposing stone-masonry

walls (Fig. 5.159). When equipment has to be lifted, one

ot more beams are supported within these slots and

shifted to the most appropriate position. The lower sur-

face of the slot is lined with a steel bar to distribute the

force of the beam over the wall.

Numerous materials have been used in the construction

150 Civil. works

of powerhouses. With larger schemes, steel and con-

crete ate common because their cost is insignificant

compared to other costs of a hydropower project. How-

ever, with small schemes in developing countries, not

only may the cost of cement and steel become mote

significant, but they ate often mote difficult to obtain,

and additional cost and effort are required to transport

them to remote sites.

Where cement is available, stone masonry (see Fig. 4.26)

provides a durable structure with less cement. So does

a powerhouse of concrete cinder blocks (Fig. 5.160).

Where appropriate skills exist, dry-rubble masonry-

stones without cement-can be used to make a fairly

robust structure requiting only local materials

(Fig. 5.161) as can stone using mud mortar (Fig. 5.162).

Rather than stone, timber can be used to make the

entire superstructure (Fig. 5.163) ot to make a frame

which is coveted by sheet metal (Fig. 5.164). Where

mud bricks ate used in local construction, these skills

and materials can be capitalized on to construct a

powerhouse. Even bush materials have been used

,&oof truss

Fig. 5.:59. Slots

in opposing

walls

of a powerhouse support

a beam used to support heavy machinery during its assem-

bly or disassembly.

Fig.

5.160. A

small powerhorcse

constructed of cinder

blocks.

Fig. 5.161. A powerhouse constructed

of

dry-rubble

masonry.

(Fig. 5.165). Although such a structure may protect the

equipment from inclement weather, it does not protect

it from possible tampering and should be used only in

areas where this does not pose a problem.

Locally available roofing materials can be used. Corru-

gated galvanized sheets are commonly used as roofing

Fig. 5.162. A powerhouse builrl

of stones

and earth,

with a

thatch

roof.

Civil works 151

Fig. 5.163. This powerhouse

has

a wooden superstructure

and a reinforced-concrete

machine room. A wooden frame

and deck dam in the background creates the

head for

power

generation.

material because of their low weight, ease of transport,

and durability. However, such a

roof

provides a tempt-

ing target for stones thrown by children and others

intrigued by the noises produced. The number of stones

on several powerhouse roofs in Burundi bears witness to

this fact (Fig. 5.166). Unfortunately, much of the avail-

able roofing is thin and easily pierced. If corrugated

galvanized roofing is used, it might be advisable to sus-

pend a second ceiling below the first, at least over the

area where the generator and other electrical equip-

Fig. Zi.164. This powerhouse is constructed of flat galva-

nired steel

sheets over a

timber frame. Although

no per-

manent windows have been incorpomted, shutters con-

structed in the same manner as the walls can be opened to

provide for

light and

ventilation during visits to the power-

house.

ment are located, to prevent rain leaking in from falling

on this equipment.

Whereas the powerhouse floor may be of dirt or other

material, the foundation for the turbo-generating

equipment is generally made from a good-quality con-

crete mix. This serves as a slab (Fig. 5.167) for mount-

ing the equipment as well as a mass to dampen any

vibrations during operation of machinery. The concrete

slab should be poured on rock, original subsoil-soil with

the upper layer of topsoil and organic matter removed--

or we!l-compacted fill. (See

Excavation,

p. 104, for

more detail on how a well-compacted foundation is pre-

pared.) If concrete is also used for the powerhouse

floor, the equipment foundation is sometimes poured

separately; expansion joints between tbe powerhouse

floor and equipment-mounting slab are used to reduce

vibrations transmitted to the remainder of the building.

Reinforcing steel is often used to maintain the integrity

of the concrete base. If the slab is not reinforced ade-

quately, it may crack from differential settling and

cause misalignment of the turbo-generating equipment.

Using a packaged turbo-generating unit, with all compo-

nents mounted on a single rigid steel frame, has two

advantages:

m the frame spreads the weight of the turbo-generat-

ing equipment more uniformly over the concrete

slab, somewhat reducing the possibility of cracking,

and

e this frame can be embedded in a stone-masonry

rather than in a reinforced-concrete pad, reducing

Fig. 5.165. This powerhouse design permits use

of

indige-

nous skills and materials--bamboo and thatch.

152 Civil works

Fig. 5. f66.

Stones

thrown on galvanized sheet roofing

can

pierce it. At this

site,

minwater leaking through such a

hole

fell on the

genemtor, causing a short and putting the

plant

out of

commission until five

years

later

when the ris-

ing

price

of diesel fuel

forced the site owners

to overhaul

the genemtor.

Fig. 5.167. A concrete slab derves as the powerhouse floor

on which a pa&aged turbo-genemting unit will be

mounted. The spent water from

the

Pelton turbine

will

drop into

the

discharge pit,

or afterbay,

and be channeled

into the tailmce.

the quantity of cement .vhich may have to be trans-

ported to the site.

Below the powerhouse is located a structure-sometimes

referred to as the afterbay-which conveys the water

emerging from the turbine to the tailrace (Fig. 5.168).

With smaller schemes, this could simply be a pipe; for

larger micro-hydropower schemes, it is an opening under

the powerhouse usually built of concrete or stone

masonry. Although the afterbay for the turbine with a

draft tube can be large if it is a chamber located

directly below the powerhouse, as shown in this

Fig. 5.168, Figs. 10.25-10.27 illustrate an approach to

reducing the amount of concrete required in the case.

To guarantee proper operation, the exit from a draft

tube must be submerged. A properly designed tailrace

can ensure this; for a flow equal to or greater than min-

imum usable flow, the tailrace is sized so that the water

backs up and submerges the opening of the draft tube.

.& more common approach is simply to incorporate a

weir at the end of the afterbay just high enough to raise

the water level for proper submergence (Fig. 5.168a).

penstock

(a)

tailrace

turbine

i

tailrace

/

Fig. 5.168.

Water emerging

from a turbine

(aI with a draft

tube and (b) without

a draft tube enters the after&y

before flowing

into

the

tailmce.

Civil works 153

Several other features should be considered in the

design of a powerhouse:

o High ambient temperatures are found in many coun-

tries. In addition, the heat produced by losses in a

generator also increases the temperature in a

powerhouse. Because high temperatures reduce

generator life, adequate ventilation is required. For

micro-hydropower plants, louvers and roof vents

provide adequate ventilation.

a For reasons of safety, the powerhouse floor should

be kept dry. Adequate floor drainage should be

incorporated to remove any water resulting from

leaks, condensation, rain, etc.

e To minimize overhead obstructions, power and con-

trol cables should be placed in conduits in floor

trenches.

m Before the power leaves the powerhouse, a tap

should be included to provide power for lighting-

indoors as well as possibly outdoors-and electric

outlets or power points.

Other components

Gates and valves

Gates and valves control the flow through water con-

veyance structures. Numerous types are in use. For

micro-hydropower schemes, this discussion is limited to

stoplogs and sliding gates and butterfly and gate valves.

Gates are generally used for control of flow through

--

open passageways or for other low-pressure applica-

tions. Sliding gates--also referred to as vertical lift

gates or sluice gates-are used where there is a :red to

adjust fairly frequently the flow through an openi;++

Stoplogs are not usually used to adjust the flow, but to

stop it completely from time to time, such as when

maintenance or repair work is needed downstream.

Valves are used to control the flow through a pipe.

For micro-hydropower applications, they control flow

through Ihe penstock, usually at the base of the pen-

stock-as a turbine isolation valve to permit uncoupling

of the turbine and the penstock--and occasionally at the

inlet to the penstock. Gate valves are often more read-

ily available, but butterfly valves are increasing in

popularity.

Stoplogs. The simplest type of gate is a stoplog, a ver-

tical wall of loose, horizontally placed timbers spanning

an opening and supported at each end in grooves

usually set in a concrete or stone-masonry structure

(Fig. 5.169). These timbers are usually manipulated

manually. To facilitate installing and removing stop-

logs,

a narrow platform spanning the opening is usually

included just downstream of the grooves. For larger

gates, the timhers can be difficult to remove and

install, either because of unbalanced head-water on

only one side-forcing the timber against the opposite

154 Civil works

Fig. 5.169. The stoplog at this site regulates the height

of

water

behind

a

stone-masonry weir.

side of the groove or because of their buoyancy. Some-

times steel or steel and timber “logs” are used. Stoplogs

have another problem. Because they are used infre-

quently, logs stored near the opening can be appro-

priated

for

other uses and therefore not be available

when they are needed.

In their simplest form, the stoplog

grooves

are vertical

rectangular notches formed in concrete (Fig. 5.170a).

To reinforce their edge, they can be lined with angle

iron

or

steel channel (Figs. 5.170b and 5.17Oc, respec-

tively). Using a steel guide also facilitates insertion and

removal of the timbers by reducing friction and permits

a more watertight seal than concrete.

To stop flow through a stoplog gate completely, such as

when repairs are needed downstream, inserting the tim-

bers is rarely sufficient. Rather, to accomplish this,

two parallel stoplogs separated by lo-20 cm could be

included in a design, so that clay can be temporarily

packed between the two to provide a seal when needed.

It may also he possible to simplify construction slightly

by designing trashrack grooves to accept stoplog tim-

bers. When flow has to be stopped, the trashrack can be

withdrawn and replaced by these timbers.

(a)

(b)

Fig. 5.170. Stoplog grooves.

Another related, but not common, means of flow control

is the use of needles. These are vertical or slightly

inclined timbers, supported on top by a beam and on the

bottom by a sill on the passageway floor (Fig. 5.171).

The needles are place6 irne by one. Each is extended

horizontally upstream of the platform, letting the water

current pull one end down until it strikes the floor of

the channel near the sill. It is then slightly withdrawn

until the end is in contact with the sill, and rolled side-

ways against the needles already installed. When

required, the needles are removed one by one. Each

needle should have a hole at its top to accommodate a

rope, which can then hold the needle if it gets away dur-

ing handling.

Fig.

S.172. This gate has

features common to both needles

and a

stoplog gate.

Ship-lap joints (see Fig. 5.96) are used

between adjacent

timbers.

Figure 5.172 has features common to both needles and

stoplog gates; vertical timbers are used, but so are stop-

log grooves on the sides of the opening. In addition,

horizontal steel

rods

span the opening to restrain the

timbers. This design makes the timbers more accessible

thau stoplogs, because one end is always above water.

Figure 5.119 shows one such gate on the same canal,

with the timbers Temoved. Because the opening at this

site is tot deep, it would aot have been feasible to

insert horizontal stoplogs manually. Another altema-

tive would have been to use a sliding gate, at the

expense of simplicity.

. .

SlldlDgga tes.

Unlike the stoplog gate, the sliding gate

% composed of a single sliding member which, except

for simple designs, c&not becompletely withdrawn and

misplaced easily. There are numerous design options.

Figure 5.173 shows a small gate formed of unreinforced

steel plate which includes a handle to lift it. It rides

Fig. 5.173. A small sliding gate.

within steel grooves set in concrete. Inserting an

object, even a twig or small stone, in any one of sever?.1

slots holds the gate in an open position. Another simple

gate is shown in Fig. 5.174.

Figure 5.175 shows a larger gate built from a reinforced

steel plate. Because substantially larger forces are

required to raise this gate, a handwheel-activated hoist

is used. Constructing a sliding gate of wooden timbers

can reduce its cost and weight (Fig. 5.176).

Civil works 155

Fig.

5.174.

This simple sliding gate is used when the set-

tling basin is to be dmined.

Some form of simple machine is used to lift larger

gates. One is the screw, as illustrated in the previous

figures. If a handwheel is not readily available, a sub-

stitute can be improvised by welding a length of steel

bar to an appropriate nut (Fig. 5.177). Figure 5.178

illustrates how a car jack was used for the same pur-

pose.

Fig. 5.175.

Steel

sliding

gate

operated by a handwheel.

156 Civil works

Fig. 5.176.

Sliding gates

formed

of timbers.

Fig. 5.177. An improvised “handwheel” on a sliding gate

at

the intake is locked to prevent tampering. The gate is

closed by gravity.

Another simple machine is the lever. Figure 5.179 illus-

trates two applications of the lever principle for acti-

vating sliding gates. Option (a) is a triangular member

pivoting about one of its vertices. The travel is rela-

tively small; therefore, use of this option is limited to

small gates, such as those controlling flow through a

scouring outlet, Option (b) is a rudimentary design. The

stem extending above the gate is perforated and a pole

OP

length of steel pipe, pivoting off a timber spanning

the outlet channel, is used to jack up the gate. A pin

Fig. 5.178. A

hoist to raise the

gate

which controls flow

at

the Intake to the power canal has been fabricated from a

car

jack.

ing

pole

outlet

inserted

into the gate prevents it from falling as the

pole is removed from one hole and placed in the next. If

the gate is partially submerged, forces caused by fric-

tion of the gate against the grooves may be sufficient to

prevent the gate from dropping while the pole is with-

drawn.

The wheel and axle is another simple machine which can

be used to raise a gate (Fig. 5.180). This design relies

on the weight of the gate to lower itself. The gate can

be kept in a raised position by a ratchet on the shaft or

simply by a pin through the vertical steel channel which

prevents the handle from turning.

When designing a gate-hoisting mechanism, the force to

operate the gate must first be determined. In addition

to the weight of the gate itself, the friction force bet-

ween the gate and the channel in which it rides must be

overcome, This can be a significant force. Its magni-

tude, which depends on the materials in contact and the

condition of their surfaces, can be approximated by the

following equation:

F = 1000fiAh

(5.34)

where

F = force required to overcome friction (kgf)

Fig. 5.180.

The wheel-and-axle

principle used to raise a

gate.

Civil works 157

= coefficient of static friction. (see Table 5.8)

i = submerged area of gate

(m2)

h = distance from water surface to center of A (m)

The valves of “,e” given in Table 5.8 are fOF smooth-fin-

ished suFfaces. Ferrous materials subject to rusting

may develop excessive friction from pitting and the

accumulation of rust when they are exposed to succes-

sive wetting and drying. If a gate is closed for long

periods of time, the valves of “p* for metal on metal

should be incteased by 50% (38). JIven though steel on

steel should not be used for normally closed gates,

because they are likely to rust closed, steel is still

widely used at micro-hydropower sites because of its

strength, availability, and low cost. Using wood for at

least one of the surfaces avoids these problems.

Surface

coefficient

of friction

steel on bronze

0.45

Steel on steel

0.60

Wood on metal

1.0

Wood on wood 1.1

Rubber on wood

1.1

The actual force encountered in raising a gate is the

sum of the friction force described above and the

weight of the submerged gate. If a timber gate is used,

the buoyancy of the submerged timbers will offset the

weight of the gate to some degree.

A sliding gate used with miCFO-hydFOpOWeF schemes

usually does not have to be Watertight. It can simply

rest directly against the sides of the groove and channel

bottom. Provision to drain away any leakage flow usu-

ally can be made if necessary; however, a number of

approaches for providing a good seal around a sliding

gate exist. The simplest bottom seal is rigid-metal on

metal OF on wood (Figs. 5.181a and 5.181b). Metal on

metal seals usually require more accurate workmanship.

Wood provides a good contact surface if it is not

allowed to dry out. Flexible seals provide greater

watertightness and can be used as both bottom seals

(Figs. 5.181c-5.181e) and side seals (Figs. 5.181f-

5.181g).

An effective seal can be constructed of ordinary 25 mm

garden hose (Fig. 5.1814 C38). To hold it in place, holes

are cut at about 15 cm ir: tervals along the upper part of

the hose to accept the eyes of the eyebolts placed along

the recess. Once the hose has been placed over the

eyebolts, a 6 mm rod is nm tbrbugh the hose and eye-

bolts, and the eyebolts are tightened to draw the upper

part of the hose tightly against the top of the recess.

A cylinder gate is a special type of sliding gate which

overcomes one of the major problems of such a gate-

the significant force required to lift it. A cylinder gate

is basically a section of pipe which fits into a vertical

outlet hole (Fig. 5.182). Because horizontal forces act-

ing on the pipe from opposing directions cancel each

other out and because no vertical forces caused by

water pressure act on the pipe, the only

force

which the

gate operator must overcome is the weight

of the

pipe

itself. Because of the low pressures encountered, non-

pressure-rated, thin-walled pipe can be used. To sup-

port it in a vertical position, the pipe passes through a

loosely-fitting ring anchored to the walls of the water

conveyance structure. To hold open the gate, its han-

dles can temporarily rest on boards spanning the oppos:

ing walls (see Fig. 5.202). It is also possible in some

cases to eliminate the supporting

ring

and instead to

insert the end of the cylinder gate a short distance into

the outlet opening. In this case, a close fit should be

avoided as the gate may jam and be difficult to lift.

Gate valta. The gate valve is basically a circular slid-

ing gate across a pipeline, surrounded with a body to

contain the gate when the valve is open and the pipe is

under pressure (Fig. 5.183). When the valve is closed,

water pressure forces the gate against the seat. At high

operating pressures or with large-diameter valves, a

significant force is required to overcome the resulting

friction forces. FOF this reason, when a large gate valve

is placed at the high-pressure end of a penstock, a small

bypass gate valve is placed in parallel to bleed water

from the high- to low-pressure end. If the low-pressure

end is not open to atmospheric pressure, pressure there

1 (a) lb)

(cl

Id)

(4

Fig. 5.181.

Types

of

seals

for

sliding gates.

158 Civil works

cylinder

gate

\

loosely

fitting

supporting

drain /

pipe

‘ig. 5.182. A cylfnder gate permits quick and easy opem-

1 I

rlon.

Fig. 5.183.

A

gate wk.

will build up, eventually equaling the hi&: oressure.

Then, when there is little or no pressure differential

across the main valve, it can be opened easily.

Butte valves. The butterfly valve is essentially an

extension of the pipe within which a lens-shaped disk,

mounted on a shaft, is placed (Fig. 5.184). This type of

valve can be operated with little force, because the

upstream pressure on each half of the disk is essentially

balanced. Because of its design, a butterfly valve can

be closed quickly. Although this may be useful if the

valve is used to shut down the turbine when turbine run-

away conditions exist, it must not be closed so fast that

an unacceptable pressure rise or water hammer in the

penstock is created.

ig.

5.184. Lever-

and geapopemted butterfly wives.

spillways might be incorporated: in a dam, intake area,

or forebay,

or

along a power canal.

If a dam is included in a scheme, the spillway portion

along its crest must be designed to permit the maximum

flood flow expected in the river to overflow safely.

Clearly, the shorter the spillway, the higher the water

level behind the dam will rise. A higher water level

behind the dam means that the portion of the dam’s

crest not used as a spillway must be higher to avoid

uncontrolled overflow (and the dam must therefore be

larger and costlier).

The intake structure must also be protected from high

flows while it continues normal operation. If the intake

is an open structure and too much water enters, either

its walls must be high enough to prevent them from

overflowing unexpectedly, or one or more spillways of

adequate length must be incorporated in the intake area

through which this water can flow back into the river

downstream of the dam or diversion structure. If the

intake leads into a power canal, spillways are essential

Civil works 159

Inversin R. Allen Micro-hydropower Sourcebook

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