DESIGN OF ELECTRIC WATER-POWER STATIONS.
[Illustration: FIG. 25.--Cross Section of Columbus, Ga., Power-station.]
Water-wheels must be located at some elevation between that of head- and
tail-water. With horizontal shafts and direct-connected wheels and
generators the main floor of the station is brought below the level of
the wheel centres. This is much the most general type of construction,
and was followed in the Massena, Sault Ste. Marie, Cañon Ferry, Colgate,
Electra, Santa Ana, and many other well-known water-power stations. If
horizontal shafts are employed for wheels and generators with belt or
rope connections between them the floor of the generator room may be
elevated a number of feet above the wheels. This difference of elevation
is usually provided for either by upper and lower parts of the same
room, or by separate rooms one above the other and a floor between them.
A two-story construction of this latter sort was frequently adopted in
the older water-power stations, and good examples of it may be seen in
connection with the electrical supply system at Burlington, Vt., and the
Indian Orchard station in the Springfield, Mass., system. Vertical wheel
shafts make the elevation of the main or generator floor of a station
independent of that of the wheels, and thus give the highest degree of
security against high water. After the vertical wheel shaft reaches the
generator room, it may be geared to a horizontal shaft that has one or
more dynamos directly mounted on it, or drives dynamos through belts or
ropes. Belt-driving in this way, from horizontal shafts connected by
bevel gears with vertical wheel shafts, is not uncommon in the older
class of water-power stations. Generators mounted singly or in pairs on
horizontal shafts that are driven by gearing on vertical wheel shafts
have been adopted at the Lachine Rapids and South Bend plants, and it
seems to offer a desirable method of connection in cases where vertical
wheels are necessary and the cost of generators must be kept at a low
figure. With this method of driving the generators can be designed for
any economical speed and step bearings avoided.
[Illustration: FIG. 26.--Cross Section of Combined Steam- and
Water-power Station at Richmond, Va.]
[Illustration: FIG. 27.--Cross Section of Wheel House at Buchanan,
Mich.]
The most desirable method of driving generators with vertical wheels,
where the expense is not too great, is the direct mounting of each
generator on the upper end of a wheel shaft (see cut). This method of
connection not only requires a special type of generator, but may put
serious limits on its speed. In general, the peripheral speed of a
pressure turbine should be about 75 per cent of the theoretical velocity
of water issuing under a head equal to that at which the wheel operates,
in order to give the best efficiency. The rotative speeds of turbines,
operating under any given head, should thus increase as their capacities
and diameters decrease. Because of these principles it is the common
practice, with horizontal wheels, to mount two or more on each shaft to
which a generator is direct-connected in order to obtain a greater speed
of rotation than could be obtained with a single wheel of their combined
power. Thus, at Sault Ste. Marie the horizontal shaft on which each
400-kilowatt generator is mounted is driven at 180 revolutions per
minute by four turbines under a head of about 20 feet. At Massena the
head of water is 50 feet, and each 5,000 horse-power generator is driven
at 150 revolutions per minute by six turbines on a horizontal shaft.
Vertical turbines are sometimes mounted singly on their shafts, as was
done in the hydroelectric plant at Oregon City on the Willamette River,
and this practice gives speeds that are too low for direct-connected
dynamos of moderate cost, unless the head of water is unusually great.
At the Oregon City plant the head of water is only 40 feet, and yet a
single 42-inch turbine was mounted on the vertical shaft that drives
each generator.
[Illustration: FIG. 28.--Longitudinal Section of Buchanan, Mich.,
Power-house.]
The most notable examples of direct-connected generators and vertical
turbines is that at Niagara Falls, where twenty-one generators of 5,000
horse-power each are mounted at the tops of as many vertical wheel
shafts in two of the four stations. Each vertical shaft in the Niagara
stations is driven at 250 revolutions per minute by a pair of turbines,
one above the other. The maximum head between the water in the Niagara
canal and that in the tunnel which forms the tail-race is 161 feet. On
ten shafts the centres of the wheel cases are 136 feet below the level
of water in the canal, and no draft tubes are used.
[Illustration: FIG. 29.--Section of Power-house No. 2 at Niagara Falls.]
The eleven pairs of wheels at the second Niagara power-house have their
centre line 128.25 feet below the canal level and a draft tube for each
pair of wheels extends to a point below the tail-water level. It is
entirely practicable to use more than a single pair of turbines on the
same vertical shaft, as is shown at the Hagneck station on the Jura,
in Switzerland, where the head of water is about twenty-one feet and
four turbines are mounted on each vertical shaft. The combined capacity
of these four wheels on each shaft is 1,500 horse-power and its speed is
100 revolutions per minute. At the top of each shaft an 8,000-volt
generator, with external, revolving magnet frame, is mounted. The use of
four wheels per vertical shaft presents no great difficulty and should
be resorted to more frequently in the future.
[Illustration: FIG. 30.--Interior of Power-house, Buchanan, Mich.]
For horizontal, direct-connected turbine wheels and generators the
nearly uniform practice is to locate the generators in a single row from
one end of a station to the other, and this brings the turbines into a
parallel row. On this plan the shaft of each connected generator and its
group of turbines sets at right angles to the longer sides of a station
and approximately parallel with the direction in which water flows to
the wheels. The typical water-power station with direct-connected units
is thus a rather long, narrow building into which water enters on one
side through penstocks and leaves on the other through tail-races. Such
stations usually set with one of the longer sides parallel to the river
into which the tail-water passes and between this river and the canal or
pipe line. At Massena the electric station occupies the position of a
dam between the end of the power canal and the Grass River, being about
150 feet wide and 550 feet long. Canal water entering this station
passes through its wheels to the river under a head of about 50 feet. A
similar construction was followed at Sault Ste. Marie, where the
power-station separates the end of the canal from the St. Mary’s River.
This station is 100 feet wide, 1,368 feet long, and is to contain 80
sets of horizontal wheels, each set being connected to its own
generator, and through these wheels the canal water passes under a head
of approximately 20 feet. Ten generators are placed in line at the Cañon
Ferry station which is 225 by 50 feet inside, and each generator is
driven by a pair of horizontal wheels under a head of 30 feet. This
station sets between a short canal and the Missouri River, near one end
of the dam. Passing from water-heads of less than 50 to those of several
hundred or even more than 1,000 feet, the general type of station
building remains about the same, but there is an important change in the
arrangement of direct-connected wheels and generators. With these high
heads of water, wheels of the impulse type, to which the water is
supplied in the form of jets from nozzles, are employed. These jets pass
to the wheels in planes at right angles to their shafts, instead of
flowing in lines parallel to these shafts like water to pressure
turbines. The shafts of impulse wheels and their direct-connected
generators are consequently arranged parallel with the longer instead
of the shorter sides of their stations. This plan results in long,
narrow stations with water entering at one and leaving at the other of
the longer sides, just as in the case of direct-connected turbines under
moderate heads. Stations with direct-connected impulse wheels are even
longer for a given number and capacity of units than are stations with
pressure turbines. Colgate power-house, on the North Yuba River,
contains seven generators, each direct-connected to an impulse wheel and
shafts all parallel to its longer sides. This station is 275 feet long
by 40 feet wide, and the water which enters one side by five iron pipes,
30 inches each in diameter, under a head of about 700 feet, is
discharged from the other side into the river.
[Illustration: FIG. 31.--Plan of Generating Station near Cedar Lake for
City of Seattle, Wash.]
[Illustration: FIG. 32.--Foundation of Power-station at Spier Falls.]
[Illustration: FIG. 33.--Plan of Power-station at Great Falls.]
At Electra station on the Mokelumne River five pairs of impulse wheels
are direct-connected to five generators, each unit having its shaft
diagonal with the walls of the building, and pipes deliver water to the
wheels under a head of 1,450 feet. The ground plan of the generator room
at this plant is 40 by 208 feet. The power-station on Santa Ana River,
whence energy is transmitted 83 miles to Los Angeles, measures 127 feet
long and 36 feet wide inside, and contains four generating units in
line, each of which consists of a direct-connected dynamo and impulse
wheel, with shafts parallel to the longer sides of the station. Jets
driving the wheels in this station are delivered under a head of 728
feet minus the loss by friction in a penstock 2,210 feet long.
[Illustration: FIG. 34.--Power-house at Red Bridge on Chicopee River.]
Both of the first Niagara plants, with vertical wheels far below the
stations in the pits, are long and narrow and have their generators in a
single row. The later of these two stations has a ground area of
approximately 72 by 496 feet outside, and contains eleven generators all
in line. From these examples it may be seen that the prevailing type of
electric water-power station, whether designed for horizontal or
vertical wheels of either the pressure or impulse type, is wide enough
for only a single row of generators and wheels, and has sufficient
length to accommodate the required number of units.
A few modern stations that depart from this general plan will be found,
as that at Great Falls, on the Presumpscot River, whence electrical
supply for Portland, Me., is drawn. This station sets about forty feet
in front of the forebay end of the dam, and two penstocks enter the rear
wall, while the other two enter one each through two of the remaining
opposite sides. Of the four generators, with their direct-connected
wheels, two are arranged with parallel shafts, while the other two have
their shafts in line and at right angles to the lines of the former two.
The station containing these generating sets has a floor area of 55 by
67.5 feet.
[Illustration: FIG. 35.--Plan and Elevation of Red Bridge Station on the
Chicopee River.]
Modern electric stations driven by water-power are usually but one story
in height and are clear inside from floor to roof, save for cranes and
roof trusses. This construction may be seen in the Niagara, Spier Falls,
Cañon Ferry, Colgate, Electra, Santa Ana River and many other notable
plants. In spite of this one-story style of construction, the electric
stations reach fair elevations because of the necessity for head room to
operate cranes in placing and removing generators. At Garvin’s Falls, on
the Merrimac River, the electric station contains generators of 650
kilowatts each and the distance from floor to the lower cords of roof
trusses is 27 feet. In the station at Red Bridge, on the Chicopee River,
where generators are of 1,000 kilowatts capacity each, the distance
between floor and the under side of roof beams is 30.66 feet. Between
the floor and roof trusses at the Birchem Bend station, on the river
last named, the distance is 26.25 feet, but each generator is rated at
only 400 kilowatts. In the Cañon Ferry plant, with its generators of 750
kilowatts each, the distance from floor to roof trusses is 28 feet. At
the plant on Santa Ana River, the 750-kilowatt generators, being
connected to impulse-wheels, operate at 300 revolutions per minute, have
relatively small diameters and are mounted over pits in the floor so
that their shaft centres are only about two feet above it. By these
means the distance from floor to roof trusses was reduced to 18.25 feet.
All these examples of elevations between floors and roof supports are
for stations with direct-connected generators and horizontal wheels. In
the new Niagara station, where generators of 3,750 kilowatts each are
mounted on vertical wheel shafts that rise from the floor, the distance
between the floor and roof trusses is 39.5 feet.
Electric stations driven by water-power are now constructed almost
entirely of materials that will not burn--that is, stone, brick, tile,
concrete, cement, iron, and steel. Stone masonry laid with cement mortar
or concrete masonry is very generally employed for all those parts of
the foundations that come in contact with the tail-water. For
sub-foundations bedrock is very desirable, but where this cannot be
reached piles are driven closely and their tops covered with several
feet of cement concrete as a bedding for the stone foundation. Where
stone is plenty or bricks hard to obtain, the entire walls of a
water-power station are frequently laid entirely with stone in concrete
mortar. If bricks can readily be had they are more commonly used than
stone for station walls above the foundations. Concrete formed into a
monolithic mass is a favorite type of construction for the foundations,
walls and floors of water-power plants in Southern California. Cement
and concrete are much used for station floors in all parts of the
country, and these floors are supported by masonry arches in cases where
the tail-water flows underneath the station after leaving the wheels.
Station roofs are usually supported by steel trusses or I-beams, and
slate and iron are favorite roof materials. With iron roof-plates an
interior lining of wood, asbestos, or some other poor conductor of heat
is much used to prevent the condensation of water on the under side of
the roof in cold weather. Walls of water-power stations are usually
given sufficient thickness of masonry to support all loads that come
upon them without the aid of steel columns. In some cases where cranes
do not extend entirely across their stations, one end of each crane is
supported by one of the station walls and the other end by a row of
iron or steel columns rising from the floor. Where the generator-room of
a station has its floor level below high-water mark especial care should
be taken to make the walls water-proof to an elevation above this mark.
As the travelling-crane and the loads which it carries in erecting
wheels and generators form a large part of the weight on the station
walls, these walls are often reduced as much as one-half in thickness at
the level of the crane, thus forming benches on which the ends of the
cranes rest.
[Illustration: FIG. 36.--Steel Penstocks at Chamblay Power-house.]
The Garvin’s Falls station, on the Merrimac River, rests on arches of
stone masonry through which the tail-water passes, and the brick walls
are water-proofed to an elevation eight feet above the floor. At twenty
feet above the floor the twenty-four-inch brick walls on the two longer
sides are reduced to eight inches in thickness, thus forming benches
each sixteen inches wide on which the crane travels. Arches of stone
masonry support the twenty-four-inch brick walls of the station at Red
Bridge, on the Chicopee River, and these walls on the two longer sides
decrease in thickness to twelve inches at an elevation of twenty-one
feet above the floor, thus forming benches twelve inches wide for the
ends of the crane.
One concrete wall of the Santa Ana station is 2.5 feet thick to a
distance of 13.5 feet above the floor, and then shrinks to a thickness
of 1.5 feet, corresponding to that of the opposite wall, thus forming a
bench twelve inches wide for one end of the crane. The other end of the
crane in this case is supported by an I-beam on a row of iron columns.
It is not uncommon to locate horizontal turbines in a room separate from
that occupied by the generators to which they are direct-connected, in
order to protect the latter from water in the event of a break in
penstocks or wheel cases. In cases of this sort the shafts connecting
wheels and generators pass through the wall between them. The horizontal
turbines may be located at the bottom of a canal whose water presses
against the wall through which the wheel shafts pass, or they may be
contained in iron cases at the ends of penstocks. In this latter case an
extension of the station is often provided for a wheel room to contain
these cases. Such wheel rooms are long, narrow, low-roofed and parallel
to the generator rooms of their stations. The floors of these wheel
rooms are at nearly the same levels as the floors of generator rooms,
but elevations of their roofs above the floors are much less than like
elevations in the main parts of the stations. The Garvin’s Falls, Red
Bridge, and Apple River stations have wheel rooms of the type just
described. With impulse-wheels to which water passes in planes at right
angles to their shafts it is desirable, in order to avoid changes in the
direction of water pipes, that direct-connected wheels and generators
occupy the same room, and this is the arrangement at the Colgate,
Electra, Santa Ana, Mill Creek, and many other power-houses using such
equipments. The area of a wheel room may frequently be reduced at
stations operating direct-connected horizontal-pressure turbines under
low heads by placing the wheels at the bottom of the canal which has one
side of the station or generator room for a retaining wall. This plan
was adopted at the Birchem Bend plant with a head of fourteen feet, and
at the Sault Ste. Marie station where the head of water is about twenty
feet. Vertical wheels direct-connected to generators must be directly
underneath the main room of their station, and may be in a canal over
which the station is built, in a wheel room that forms its lower part,
or in a wheel pit and supplied with water through penstocks, as at the
Niagara Falls plants.
Step-up transformers developing very high voltages are not an element of
safety in a generator room, and the better practice is to locate them in
a separate apartment by themselves, if not in a separate building. For
the Niagara Falls plant, the transformers that deliver three-phase
current at 22,000 volts are located in a building across the canal from
the generating plant. At Cañon Ferry the transformers operating at
50,000 volts, three-phase, are located in a steel and iron addition to
the power-house. Transformers at Electra station, which are intended to
work ultimately at 60,000 volts, are located in an extension of the main
building and are separated from the generator-room by a wall. At the
Santa Ana plant the 33,000-volt transformers are grouped in one corner
of the generator room, but no partition separates their space from the
remainder of the room. In the Colgate plant the transformers, working at
40,000 volts, are spaced along one of the longer sides of the station
opposite to and only a few feet from the row of generators. One end of
the main room in the Apple River plant is devoted exclusively to the
25,000-volt transformers, and there is a distance of about twenty-seven
feet between them and the nearest generator. The highest degree of
safety for transformers at these great voltages seems to require that
they be located in a separate room where the floor, walls, and roof are
made entirely of incombustible material.
[Illustration: FIG. 37.--One of the Turbine Wheels at Spier Falls on the
Hudson River.]
Water supplied to horizontal turbine wheels under moderate heads usually
enters the station by penstocks on one side and leaves it by the
tail-race on the other, but this is not true in every case. At the
Birchem Bend plant, the canal in which the wheels are located being
between the station and the river, water never enters or passes under
the station, which has a continuous foundation. So again at the Apple
River plant the single supply pipe, twelve feet in diameter and
delivering water under a head of eighty-two feet, lies parallel with the
greater length of the station and between it and the river. Short
penstocks pass from this supply pipe into the wheel section of the
power-house, and the water after passing through the wheels flows out to
the river between the masonry piers that support the twelve-foot pipe.
The generator section of this station has thus no water flowing under
it. An interesting distinction may be noted between the conditions as to
the tail-water about the foundations of stations working under low and
those under great water heads. In cases of the former sort the volumes
of water are relatively great and the foundations of stations are
usually submerged, and much reduced in area to make room for the
tail-races. Thus, the foundations of the station at Red Bridge, where
there is 49 feet head, have nearly all of their footings under water,
and of a total length of 145 feet at the top of these foundations the
six tail-races underneath cut out 92 feet. These tail-races extend
underneath both the wheel and generator rooms.
Where power is derived from water delivered under great head from pipe
nozzles to impulse-wheels, stations are usually well above the water
levels of streams into which they discharge, and passages for tail-water
underneath the station shrink to small tunnels through their
foundations. Seven of these tunnels have a total width of less than 25
feet at the Santa Ana River station, which is 127 feet long, and where
the head of water is 728 feet. At the Colgate plant, with its head of
700 feet, the water, at times of light load, instead of flowing out of
its passages underneath the station, shoots from the pipe nozzles clear
across the North Yuba River on the bank of which the station stands.
[Illustration: FIG. 38.--Power Plant of Ludlow Manufacturing Company.]
In a comparison of floor areas per kilowatt of main generator capacities
in electric stations using water- and those using steam-power, the
matter of space for transformers may be entirely omitted, because the
extent of this space is independent of the type or location of
water-wheels, or the difference of water and steam as motive powers.
Where water-wheels and their connected generators occupy separate rooms,
as is often the case with turbines under low pressures, the wheel room
has a little less length, and is generally narrower than the generator
room. Thus, at the Red Bridge station the generator room is 141 feet
long and the wheel room about 127 feet, while the former is 33.33 feet
and the latter 24 feet wide. So again at Apple River Falls the generator
room is 140 by 30 feet and the wheel room 106 by 22 feet, the generator
room in this case containing also transformers. It follows that if
wheels can be located outside of the station, as in a canal, quite a
reduction in its total floor area can be made, which may easily range
from 20 to 40 per cent. The kilowatt capacity per square foot of floor
area in both wheel and generator rooms combined tends to increase with
the individual capacity of the generating units. Generators on vertical
shafts seem to require about as much floor space per unit of capacity as
do generators on horizontal shafts. In the Red Bridge station the total
capacity is 4,800 kilowatts of main generators in six horizontal units,
and the area of the generator room alone is 0.96 square foot per
kilowatt of this capacity. The second station with vertical units at
Niagara Falls has a capacity of 41,250 kilowatts in eleven generators on
vertical shafts, and its floor area amounts to 0.86 square foot per
kilowatt; narrow impulse-wheels of large diameter tend to economy of
floor space, as in Electra station, where the room containing wheels and
generators has an area of only 0.83 square foot per unit of its 10,000
kilowatts capacity. At the Colgate plant, where the total rating of
generators is 11,250 kilowatts, the floor area under wheels and
generators is almost exactly one square foot per kilowatt. The Santa Ana
station, with a total capacity of 3,000 kilowatts, has 1.52 square feet
of floor area for each unit of capacity. This last figure may be
compared with the 1.72 square feet per kilowatt of generator rating for
the 4,800-kilowatt station at Red Bridge and the 1.75 square feet per
unit of capacity in the 800-kilowatt plant at Birchem Bend.
[Illustration: FIG. 39.--Power-house on Payette River, Idaho.]
All types of water-power stations with direct-connected wheels and
generators have much smaller floor areas per unit capacity than do
steam-power stations with direct-connected horizontal units. Thus, the
modern steam-driven station at Portsmouth, N. H., has a plan area in
engine- and boiler-rooms of 16,871 square feet, and its total capacity
in four direct-connected units is 4,400 kilowatts, so that the area
amounts to 3.82 square feet per kilowatt rating of its generators. Of
this area about 46 per cent is in the boiler-room.
FLOOR DIMENSIONS FOR DIRECT-CONNECTED, HORIZONTAL WATER-WHEELS AND
GENERATORS AT ELECTRIC STATIONS.
+-----------------+----------+------------+-----------+---------+
| | | | Number | Total |
| Station. |Feet Long.| Feet Wide. | of |Kilowatt |
| | | |Generators.|Capacity.|
+-----------------+----------+------------+-----------+---------+
|[A]Niagara, No. 2| 496 | 72 | 11 | 41,250 |
|Sault Ste. Marie | 1,368 | 100 | 80 | 32,000 |
|Colgate | 275 | 40 | 7 | 11,250 |
|Electra | 208 | 40 | 5 | 10,000 |
|Cañon Ferry | 225 | 50 | 10 | 7,500 |
|Red Bridge | 141 | 57 | 6 | 4,800 |
|Apple River | { 140 | 30 } | 4 | 3,000 |
| | { 106 | 22 } | | |
|Santa Ana River | 127 | 36 | 4 | 3,000 |
|Great Falls | 67.5 | 55 | 4 | 2,000 |
|Garvin’s Falls | { 62 | 30 } | 2 | 1,300 |
| | { 50 | 23 } | | |
|Birchem Bend | 56.6 | 26.7 | 2 | 800 |
|Portsmouth |{ 14.4 | 119.66 }| | |
| (steam-driven)|{inside, but minus 360}| 5 | 4,400 |
| |{ square feet. }| | |
+-----------------+----------+------------+-----------+---------+
[A] Vertical wheel shafts.
Some of these dimensions apply to the inside and some to the outside
of stations. Some small projections are not included.