Raabe J. Hydro power - the design, use, and function of hydromechanical, hydraulic, and electrical еquipment
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Fig.
3.3.7.
Spillway on the side-of the
main dam.
Fig.
3.3.8.
Pit type spillway
(morning glory).
Fig.
3.3.9.
Comparison between siphon a)
and
b) spillway with overflow weir on the crest of
a dam.
discharge increases finally propelled by the
head on the weir, nearly coinciding with thc !lead of the
plant in case of a river power plant. This is contrary to the weir at the crest of a dam or a gate, where
only the relatively small head on the crest
h,
does this (see Fig.
3.3.9).
When the head water level falls short of the inner crest of the syphon, the la~ter is aerated, thereby
cutting
suddcnly the flow. Thus the head water level is automatically controlled without any
sophisticated movable device, within the range of vertical distance,
h,,
of the ceiling of the intake and
the inner crest. This is of
the order of a few cm (see Fig.
3.3.9).
On the contrary the crest of an ordinary weir must be at a vertical distance
k,,
underneath the
maximum permissible
head water level, in which
h,,
is the head
on
the crest needed for spilling
catastrophic flood flow
Q,,,.
According to the known weir formulas
11.531
Q
is proportional
to
bh;A2,
b
being the width of the crest. Considcr a syphon of the same width and the satne cross
sectional area
bh,,
of the spilled flow lamina at the weil crest. This needs only
(h,,/~)''~
times the
crest length of
a
comrnon weir to discharge
Q,,,
H
being about the plant's head.
Hence a syphon is reasonable,
if
1)
the level of head water must be maintained accurately,
2)
this
level control shall be simple and automatic,
3)
the width
b
of the dam crest
is
limited,
4)
the catastrophic high water
flow
is very large.
87
3.3.2.4.
Weir
gatcs
as
311
adj1lstal)l~
tlood
discharging
device
1.
Catcs \vith tran.;latior~;~l nlotion: Tlicse arc sluice gatcs. I-or micro
power
plants md
small hc::~ls they con:;ist
of
a
frame
c;f
stccl c:lntilcvers
fill~~l
with
wooden
planks and
opcratcd
by
:I
rack.
For
larger plants they arc fabriciltc~l fro111
a
frit~ne\vork of stt'el
cantilzvers, covered by
a
stczl liner on thc side of hcad writer. They may be coniposcd of
simple
ar lnultiplc sluices.
The spillage may occur
I)
between the upper. and lower sluice,
2)
underneath the gate,
3)
above the upper fate usunlly either along a nozzle shagcd guide face or
a
rounded backed
weir.
The
procedure
3)
has
the advantage of spilling easily floating bodies like driftwood
and
leaves. For high water usually the gates arc lifted thus allowing the flow to pass tinder-
neath
:!le lowcs~ sate
[3.75
to
3.771.
The
gate
adjustment, eased by counterweights and guide ro!lers (see Fig.
3.3.10)
may be
effected
m:chanica!ly,
electrically,
hydraulically or pneumatically. When th? width of the
uatcr way is large or the hydraulic thrust excessivey the spillway is subdivided by abut-
ments into more openings.
Fig.
3.3.10.
Double hook
sate
(MAN).
Spillage of me-
dium discharge with
thc upper gatc; lowered. For cata-
stropilic high water both gates lifted
In
a gate
of
only one sluice the flow discharges usually underneath the gate. Sometimes
it is also convenient to discharge above the gate, not only for cleaning but also for
preventing excessive
ve!ocities along the walls of abutment. bottom, or the slaice, passibly
causing erosioti, cavitation and excessive vibration of the weir. However. to reduce the
cross section of the spilled flow, catastrophic flood flow is preferrably spilled underneath
the
lo\vest sate.
All tl~r possibilities are unified by a design wit11 various gates, e.g., double hook sluice gate (see
Fig.
3.3.10).
Here ttzc upper
gate
is used to retain the head water level in case of small spillage. For
large spiliage both
the
gntcs are encased and then liftcd together. The lifting of gates may
bc
induced
auto~ii;it~cillly
by
a
float.
w:lich actuates synchronized elecrro motors moving both ends of the gate
by
ch;!ins.
The
actuation of the motor then is neutralized by
a
feedback linkage between float and
g3
ts.
Cc?ntmry
to
tlic rotary gates
of
the sector and segmental type the streamwise extensio~l
of
sluice gate
is small
a~ld
its height rather unlimited. Contrary to the below mentioned rotary gates and the roller
\\sir the
plate-like
structure
or the sluice gate yie!ds
a
rather high weight to load ratio. Hence these
gates are predestined for flood control in low head run-of-river plants, where the width of river
provides s~ificicnt place, e.g. Kuibisheff, Volga,
USSR,
Q,,,
=
64000
m3/s,
38
opcnings of
29
n~
width.
11. Gates with rotary motion: By rotating about an axis, friction and wear is lower
than
in sliding gates. This feature also limits thc height of the individual gate and its stroke and
makes the streamwise length of the gate
largcr than in case
1.
Moreover a nlultiple
arrangement of gates as in case
I
is rarely found. The cylindrical shell type of these weirs
yields a lower weight to load ratio as type
I.
-
Fishbelly flaps: These are mainly used as an attachenient to other weirs and then
excIusively applied to head water level regulation (see Figs. 3.3.11 and 3.3.12
b,
[3.76]).
-
Segmental barrage type tainter: Here (see ~ig. 3.3.12 a and
b)
a
water-loaded cylindri-
cal surface eliminates the torque of the hydraulic thrust and usually also that of the
weight
about its fulcrum. This fits the gate also for medium heads. The flow is underneath the
gate and the torque
due to the gate's weight is usually hydraulically balanced by shifting
the segment's centre away from its fulcrum (Fig.
3.3.12 b). Operation by one or two
oilhydraulic servomotors, whose swiveling cylinders are supported by a cross beam
downstream.
-
Submersible tainter gate dam (see Fig. 3.3.13 a). Its wetted surface consists
of
a
com-
plete watertight sector, which is turned (and thus submerges) into a suitable pit, water
tight along the sliding face of gate. The flow is discharged above the upper edge. Instead
of only the
t1s.o fulcrums of the tainter type, it is supported by many of them. This enables
a larger clear width. Hence this weir is predestined for large flow in low head run-of-river
pla~lts.
Here in the floating type (see Fig.
3.3.13
b) not only the torque due to water load disappcars. but also
that owing to weight is compensated by hydrostatic
uplifr of the floating sector (so called drum
barrage dam).
The regulation may be effected automatically by a float on the head water level. Its linkage actuates
simultaneously two valves that connect or disconnect
respectivcl~ the head water with the pit and
the pit with
the tailwater, stabilized by a feedback from the gate.
-
Bear trap dam: This gate consists of two rotary parts (see Fig. 3.3.14). One first part
downstream rests on
water by means of a float. The flow is discharged above its crest.
Fig.
3.3.11.
Schematic view of fishbelly
flap and stilling basin under spillage a)
scour past stilling basin favoured by
ford at the
downhtrearn end b) scour
past stilling basin diminished by in-
dentation of
a
ford (From
MAN).
I--ig.
5.3.12.
a)
Ssg~nental barrage of thc taintcr typc. 1nclinat;on of the seglnent townrds the horizon-
I,%!
i;!b:ilitater thc ba!:tnce of weight-induced torque aboui the fulcruni and also by that of the
h~clrostatic
lift.
Thc
pa!c
is mo;rcd cithcr by synchronized cllaiil drive on both sidcs or by a swivelling
oil operated servomotor in the nicdian plane. b) Tainter gate wit11 lishbelly
flap
on crest of abutment
po\\.i.i
plant Pcrach. Inn, LVcst Gcrrnany (owner innkraftwcrke Tijging). Buil:
by
Nocll, Wiirzburg.
C!c.ar
ividth
1.
18
nl.
hcieht
of closure
9,7
ni with
2,9
rn
for
the
flap.
Tclintcr gate opcrntcd
by
s\,\i\c.lling oil h>drnulic servomotor
on
b~th sides. Additional servomotor for
!lap.
(Drawing
cour-
tzs!. Xoell! \Viirzburg).
Fig.
33.13.
a)
Submcrgibie taintcr gate
dam,
in the lowered position
b)
Floating
type
of a) (drum
bnrragc
dilm)
in the r'lev3ted position (From
MAN).
Fig.
3.3.14.
Bcar trap dam. Auto-
m3iic control by float, actuating a
val\~r, from upstream.
Fig.
3.3.15.
Rollcr weir in extreme positions, specifically light
construction
(After MAN).
To enable control, by watering or
dewatering respectively, the pit of this bcdy, and the pit's upper
end are made watertight by a second rotary gate upstream and sealed against the first in addition
to the seals at the walls. A hook at this gate limits
the stroke. Automatic control by float-actuated
valves.
-
Roller weirs: They comprise a tight steel cylinder (see Fig.
3.3.1
5)
whose front disks are driven
by
a gear wheel at both ends. These engage with an inclined rack. This mechanism provides for the
lift
of the weir by means of a cable wrapped around it and pulled by a motor. The
discharge
is
underneath.
A
shield on the upstream front of weir provides for water tightness of lowered gate. This
type is deemed to be especially frost resistant.
3.3.2.5.
Dynamic behaviour of spillways
Any spillage is usually
connected
with high overcritical velocities in the lamina spilled
and then with the conversion of them into subcritical speed. The first may be connected
with high wear and tear
at
the gatc walls
in
the case of
a
bottom outlet or at least on the
bottom
of
the stilling basin. This is in connection with silt transport.
If
passing a wall projection, which is too sharply curved or sharply edged towering in the through-
flow section of a bottom outlet, the spillcd flow may cavitate and cause pitting. Spilled flow requires
attention to its hydro elastic effects
(Naudascher
[3.78
to
3.801).
For its low velocity at crest and the
respective low
inertiarorces, a nappe tends to start flutter there
(3.811.
This may be damped by
ventilation, and excited by hydro-elastic response of elastic guide walls but mainly of air pockets
formed by the bottom impact of the nappe.
Often streamwisely orientated
vortices moving across the back of weir during spillagc may amplify
tllesc effects. To prevent this cross motion the back of the weir is usually equipped with guide plates
in
the streamwise direction
[3.82].
Also other kinds of vibrations may occur at gates
[3.83].
3.3.3.
'The
stilli~jg
basin
The flow spilled, flows ngainst tlie stilling basin with a n~i?.;or resonarit spccd duc to thc
depth of the tai1v;nter
[3.1S].
Then in tlic stilling b;~sin the flow n_caiil is convcrtcd into
subcritical flow with rcspect to channel tleplh via
a
1iydr:lulic jump. This conversion is
connected
with tlie dissipation of hcad at thc spillway into heat.
Without a properly
dcsigned stilling basin the foundations of the spillway and its neigh-
bouring structures would be undcrrnincd by erosion in consequence of shock and wall
shear stress due to this energy conversion
[3.84].
Hence scour has to be prevented.
The conversion of kinetic energy into heat and dissipation results essc~itially from shear flow layers
due
to
vortices. In order not to damage the walis too much, vortices are iirtificially created by means
of prisms of concrete, so called Rehbock teeth on
thc
stillifig basin's bottom (see Fig.
3.3.1
1).
The axes of these vortices should be mainly ii~clined into thc
free
flow
of a stilling basin, that is
sufficiently deep. Thus
the dissipation expands into
a
large volume of water.
In low head pl~nts with by pass-outlets around the sets or spillways above the sets
(submersible power plant) the design of the stilling basin should be such as to favour the
forrnation
of
a hydraulic jump downstream of the turbine's exit, thus enlarging the head
by
the hydraulic jump's height
[1.53]
when spillage is occurring.
Ski jump spilli\,iys (see Fig.
3.3.5)
may save the expenditure of the stilling basin
[3.86].
Here the
kinetic energy of the
jetlike flow lamina spilled is dissipated in the tailwater suficieritly far away from
the plant's foundation.
If
the ski jump spillway orients
the
individual jets somewhat against each
other,
the spilled flo;s lamina dissipates
a
portion of its energy by this interaction before
it
reaches
the taiiwater level.
Furthcr references on spillway design see
[3.85
to
3.901.
3.3.4.
Valves
I.
General remarks: Valves may serve different purposes.
In
general
a
valve located
upstream
of
the entrance
of
the turbine should interrupt the flow and stop the turbine,
thus permitting inspection. Hecce
a
valve usually is used for higher pressures than a gate.
Therefore the cross section blocked by a valve is s~aller than that closed by a weir gate.
To save rather expensive valves at the inlet of large high head turbines, these highly
locldcd valves have been omitted in favour of a low loaded gate at the inlet of the penstock
in
Churchill Falls, see also
2.4.
Another purpose of a valve may be that of a locking or
a
regulated closi~g apparatns of
a bottom outlet assisting a spillway or substituting for it.
~urther valves are found in
FTs
as by-pass outlet against water hammer-induced pressure surges.
In
both the last cases
the valve serves as an energy dissipator (in the first cases assisted by a stilling basin or
at
least the tail water)
[3.76, 3.911.
11.
1Vedge valves: Advantages: Short and simple, for the case of a design with wetted
spindle, also small height, unlimited pressure, no loss under full load.-Disadvantages: No
part load opening. closure may provoke vibration by stall and cavitation, tightness needs
careful finish, a cheap and
we11 accessible- drive via a dry spicdle of carbon steel needs a
spacious casing.
11
I.
Ring (needle) valve: Designed either with
a
drive inside the hub or outside with a rod
cros3ing
a
bend (see Fig.
3.3.16).
Advantages: Partial opening possible, high head, small
Ring (necdlc) valve in tcnd
\\
ith
ivc.
adjusting power. Disadvantages: Expensive, demands large space. also srnall loss durinp
full opening, bad vibrations and cavitation possible under small openings, limited in size
[3.92].
To
save expenses the FTs of La Grande
2
(Canada) have becn equipped with spindle-operated rins
valves or cylindrical gates between stay vanes and guide vanes, as a watertight :hut down debice
[3.93].
It may be hinted that the joint between stay ring and head cover uscally in
a
FT
has to
transmit large
forccs and moments, which need an absolutely tight seal close to the guide vane
channel. The above
arrangement
spreads this joint and loads it in the critical case of sudden shut
down with control-induced forces
[3.83].
The coriical valve may be considercd as a related design, diffusing the flow instead of
contracting it
[3.94], especially used as by-pass outlet of FTs in North America and
known there under the designation Howell-Bunger valve.
IV.
Spherical valve: (see Fig. 3.3.17). Occasionally equipped with a locking apparatus for
revision (see Fig.
3.3.17~). Advantages: No losses in the opened position, smal! openi~ig
power,
if
equipped with
a
filling device for the pipe between the valve and the turbine's
control mechanism (needle valve, gate), then closed; suitable for highest head
[3.95; 3.961.
Disadvantages: Expensive, somcwhat space consun~ing, large adjusting power for shut
down under full load flow,
then also cavitating and badly vibrating, no part load
operation,
limited size.
V.
Butterfly valve: (see Fig. 3.3.18). ~dvanta~es: Cheap and space saving for heads up to
400m,
losses under
full
opening can be reduced by the biplane disk design. Disadvan-
tages: Part load operation is not steady and may lead to cavitation.
Fig.
3.3.17.
Spherical valve a) closed b) opened c) with additional revision seal. In a recent desiga
the short stroke piston on the rotor is replaced by a prossure loaded elastic ring plate, clamped
at
its
tip diameter (Drawing courtesy Sulzer Escher Wyss).
Fig. 3.3.18. Aut~crfly v;llvc
io
wcltlcd
and
cast
tlcsigli
for
tlic pl;i~it :\I I.:lvcy, K!ionc;
tipdin~n-
ctcr
5.1
m.
rn:~silnu!!l prcssurc
head
45
rn,
Q
=
65 n~.'is (Ilratving courtesy Sul~cr Ilsclicr
W
y
ss).
The ciynarnic hehavioi~r cf butterfly valves also undcr cavitation was described by
Grrir~
and
Oster\\,cilde,r
[3.9?
to
3.1031. For any case, the dynamic bchaviour of valves, their cavitation,'their
cldjustment forces versus stroke havc been carefully investigated [3.103; 3.1041, also under natural
pressure heads
and
under penstock failure [3.105 to 3.1091.
3.4.
The
power
house
and
its
equipment.
3.4.1.
The constituent
e!zments
of
a
power
hcuse
3.4.1
.l.
General
remarks
The
main purpose
oT
thc power house design is the proper location of the electrical and
hydro mechanical equipment of plant.
in
installatio~ls with vertical shafts (which predominate in the head range up to about
1300
nl, except the majority of pumped storage plants with ternary sets) the structure
cf
the power house can be divided vertically illto three zones fro111 the bottom to the top.
They are:
1)
The zone of the draft tube and tailrace tunnel,
2)
the zone of turbines, their
pit and penstock,
3)
the zone of generator and building construciio~~.
In
installations with
a
horizontal shaft, zones
2)
and
3)
are at the same level. The zone
1)
along with the foundation
of
machinery and its possible draft tubes dominate the
"substructure". and zone
2j
and
3)
the "superstructure". In plants with vertical shaft, large
submergence,
and
ternary sets with one thrust bearing underneath, also zone
2)
may be
included in zone
1).
Even if the vertical nrrangernent is common prsctice with large sized sets, a comparison
\\.ill be made betwzen some features of a power house due to vertical sets and horizontal
sets
Lvith respect
to
floor required, rectilinearity of flow passage, ease of assembly, revision
and dismilntlin_e.
Vertical sets have the smallest floor space requirement. This may be important in narrow
vi~lleys and densely populated areas
but
also if only to
a
lesser degree in underground
stations as then the expensive roof of the gallery is shortest. Any vertical plant with
reaction turbines requires a bend in the draft tube,
a
source of additional loss. In high
head plants also the admission duct shows two bends in series in different planes, a cause
for secondary flow.
During assembly the alignment of rotor and stator with its small gap clearance is eased
because of the absence of
shaft deflection. However, dismantling the lowest parts may be
more difficult. Servicing becomes more difficult especially in peak load or pumped
storage
plants, often changing their modes. The axial thrust misplaces rotor vs stator.
The horizontal set requires more floor space. It guarantees the smallest deflection of flow
especially in tubular turbines and in plants fed by one penstock, that is in the same plane
as the shaft. The assembly is rendered difficult
by
shaft deflection especially caused by the
heavy alternator rotor of large high head machines. To keep the clearance of turbine
labyrinths,
e.g. shin plates are required. Servicing on one floor and accessibility of all parts
(e.g. dismantling) is eased.
3.4.1.2.
The superstructure
of
the power house
The superstructure consists mainly of the machinery (the sets, the main crane, the repair
shop, the erection bay, the control room, the
offices, sometimes as in underground
stations also the transformers, the high voltage switch plant with high capacity circuit
breaker, disconnector). Usually the latter equipment is installed
in
an open air switch
yard.
I. Impulse turbines,
PT:
1)
With horizontal shaft, with up to two wheels, overhung, on
both sides of the alternator (Fig. 3.4.1)
with 1 nozzle pipe, curved and passzd by the needle
rod or straight for the highest head, with 2 nozzle pipes in the "tongs design" (Fig.
3.4.2)
[3.110]
with needle servomotor inside or outside the nozzle pipe. 2) With vertical shaft,
up to
6
nozzles, with spiral distributor (see Fig. 3.4.3), or other arrangement of manifolds,
e.g. the "pine-tree-design" (see Fig. 3.4.4).
A
price comparison shows the advantage of a supply by multiple branches. This solution, in spite
of its apparent complexity, results in considerable savings for the penstock equipment, when at the
same time the single spherical valve for each turbine is replaced by one for two sets. First
oi all,
according to
Vevey-Chsrmilles, a large bus-pipe distributor can be avoided. Then the reduction of
Fig.
3.4.1-
Elevation of horizontal shaft, 2-wheel
PT
of Biasca, Ticino, Switzerland, overhung design,
1
nozzle
per
wheel,
H
=
707
m,
P
=
4
-
79
MW (Drawing Courtesy Sulzer Escher Wyss).
95
Fig.
3.4.2.
Cross scction
of
iori.11 shaft,
2-nozzle,
2-wliccl
IT
;i~
Kops, Austria
(owncr Vorarlbcrg Illwcrke,
13rcgcii7). It
=
780
in.
n
=
500
rpni,
P
=
3
.76,6
M
W,
run-
ilcr wcight
7,7
tons. r\clmission
pipcs in thc so-callcd tongs
de-
sign. I,~IK~c spacc requircmcnt
for rcduccd
curvature
of pipcs
(Drawing
courtesy Voith).
Fig.
3.4.3.
Spiral distributor of 6-nozzle
PT
for Rcstitticion, Peru in I-lydroart Works, Milan, Italy.
H
=
255
in;
Q
=
32
m3;'s;
P
=
75
MW;
n
=
200 rpm. Integrally cast whell of 13/4 Cr/Ni stcel;
D
=
4190
mm,
33
tons. bucket width 1000 inm;
20
buckets. Distributor inlct diameter 2400 mm.
Sote guide rib in nozzle pipe inlet. (Photograph courtesy Hydronrt, Milan, Italy).
the pipe diameters occurs much sooner than is the case with
a
conventional spiral, thus rcducing the
dimensions of the branch pipes as well as the thickness of
stcel plate and the amount of welding
[3.111].
According to their high head, PTs are usualIy located in underground stations (see
Fig.
3.4.9,
or in
an
open house on the base
of
a
slope (see
Fig.
3.4.6). For horizontal sets,
the
polver house volume is smallest (see Fig. 3.4.5). Vertical sets have the smallest floor
space
requiremeni [3.112].
To avoid flow disturbance, the needle servomotor is housed within the no7zle pipe (see
Fig.
3.4.7)
us~tally equipped with coil springs.to balance the hydraulic force on the needle.
To
protect the environnlent from the noise of the jet if cut by the buckets, the tail water
tunnel needs a "ducking wall" as
a
short immersion ceiling between the wheel chamber
and the surroundings.