The word turbine is derived from the Latin, “_turbo_”—that which spins
or whirls around—a whirlwind.
[Illustration: FIG. 117.]
The _turbine_ is a horizontal water wheel, and is similar to the
_hydraulic tourniquet_ or reaction wheel shown in Fig. 117. This
consists of a glass vessel, M, containing water and capable of moving
about its vertical axis. At the lower part there is a tube, C, bent
horizontally in opposite direction at the two ends. If the vessel were
full of water and the tubes closed, the pressure of the sides of C
would balance each other, being equal and acting in contrary direction:
but, being open, the water runs out and the pressure is not exerted on
the open part but only on the opposite side, as shown in the figure A.
And this pressure, not being neutralized by an opposite pressure,
imparts a rotary motion in the direction of the arrow, the velocity
of which increases with the height of the liquid and the size of the
aperture. This description and the illustration gives an idea of the
crude _reaction wheel_ invented by Barker about 1740; again a turbine
is simply a centrifugal pump reversed, but the turbine is usually
furnished with curved _guide vanes_ to guide the water as it enters the
wheel.
NOTE.—The _steam turbine_ has come into common use and competes in
its economical performance with the simpler and less economical
types of the steam engine; it is impelled by steam jets, the steam
impinging upon vanes or buckets on the circumference of a rotating
disc or cylinder.
_In those turbines which are without guide blades_—i. e., which have a
high fall—the discharged water still possesses a great velocity and the
wheel is thereby deprived of a considerable part of mechanical power.
This loss can, however, be obviated or lessened by using the energy of
this discharged water to drive a second wheel.
A construction of this sort has been carried out by Ober-Bergrath
Althaus in the tanning mill at Vallendar near Ehrenbreitstein. The
essential parts of the arrangement can be seen in Fig. 118. A E A is
an ordinary reaction wheel with four curved revolving pipes and a fall
of 124 ft., and B B is a larger wheel with floats which is set in
rotation by the water issuing from A. A. _Since the two wheels turn in
opposite directions, they must be connected together by a special form
of wheel-work._ The outer wheel affords the additional advantage of
serving at the same time as a fly-wheel, thereby giving a more uniform
rate of motion to the whole machinery.
[Illustration: FIG. 118.]
Turbines are variously constructed, but all have curved floats or
buckets against which the water acts by its impulse or reaction in
flowing either _outward_ from a 1 central chamber, 2 _inward_ from an
external casing, 3 _from above downward_, and, 4 from _below_; these
constructions are either divided into _outward, vertical or central
discharge wheels_.
Turbines may also be divided into _reaction turbines_, or those
actuated substantially by the water passing through them (their buckets
moving in a direction opposite to that of the flow); _impulse turbines_
or those principally driven by impact against their blades or buckets
(the buckets moving with the flow); and _combined reaction and impulse
wheels_ which include the best modern types of turbines. In reaction
turbines the wheel passages are designed to be always full and
therefore the water under pressure; in the impulse turbine the passages
are not usually full.
[Illustration: FIG. 119.]
Turbines in which the water flows in a direction parallel to the axis
are called _parallel flow turbines_—or _journal turbines_.
The _turbine-dynamometer_ is a device used for measurings or testing
the power delivered by turbines (whence its name).
_Fourneyron’s Turbine._ This is, in its latest form, when properly
constructed, the nearest perfect of the horizontal water-wheels. It
revolves either in the air or under water, and may be either high or
low pressure. For the low-pressure wheel, the water enters the flume
from the open reservoir, with free surface, as in Fig. 119. For high
pressure, the reservoir is boxed up and the water brought in at the
side through a pipe, as shown in Fig. 124, page 136. The first is for
low and the second for high falls.
NOTE.—The early history of the turbine is one of considerable
interest _especially in view of the development of the steam, from
the water turbine_.
“M. Fourneyron, who began his experiments in 1823, erected his
first turbine in 1827, at Pont sur l’Ognon, in France. The result
far exceeded his expectations, but he had much prejudice to contend
with, and it was not until 1834 that he constructed another, in
Franche Comté at the iron-works of M. Caron, to blow a furnace. It
was of 7 or 8 horse-power, and worked at times with a fall of only 9
inches. Its performance was so satisfactory that the same proprietor
had afterwards another of 50 horse-power erected, to replace 2
water-wheels, which together, were equal to 30 horse-power. The fall
of water was 4 feet 3 inches, and the useful effect, varied with the
head and the immersion of the turbine, 65 to 80 per cent. Several
others were now erected: 2 for falls of seven feet; 1 at Inval,
near Gisors, for a fall of 6 feet 6 inches, the power being nearly
40-horse, on the river Epté, expending 35 cubic feet of water per
second, the useful effect being 71 per cent. of the force employed.”
[Illustration: FIG. 120.]
_The Leffel-Samson turbine_ wheel is shown in the engraving, Fig.
120, page 129, where _N.N._ represents the bottom casting of the case
flanged to support the wheel by resting upon the bottom of the penstock.
The draft tube _I_ is of conical shape as represented at _J. J._ to
reduce the friction of discharge water which after performing its work
in the wheel escapes at the bottom of the draft tube. _This tube must
always project into the tail water at least two or three inches._ The
gates _H.H.H._ are pivoted at the center so that they are balanced and
are opened and closed with the least possible friction. These gates are
operated by rods _L. L._, connecting with a rack and pinion which are
manipulated by the operator as occasion requires, by an extension shaft
from the coupling _K_, having a hand-wheel on top.
[Illustration: FIG. 121.]
Power is transmitted from the wheel shaft _F_ to the gears and pulleys
connected with the coupling above; the manner of setting turbine wheels
in openstocks is shown a few pages further on.
From the construction of gates and guides upon turbine wheels one may
readily see the absolute necessity of carefully guarding the flume
against the admission of sticks and other solid materials that might
wreck the wheel or jam the gates so that they could not be operated;
this is best accomplished by placing a water rack in front of the head
gate at the entrance to the flume.
These water racks are best made of flat bars of wrought iron placed
edgewise in a vertical position, or what is better, let the top incline
say one foot or two (depending upon the size of flume) towards the
head gates. When placed in an inclined position it is very much easier
to clean the rack from drift wood and the like than when placed in a
vertical position as by means of a hoe or scraper these obstructions
may be hauled up over the top of the inclined rack.
The racks should be made very strong and substantial to guard against
being broken by ice in the winter, for should the rack give way at any
inopportune time the admission of sticks and other rubbish might wreck
the wheel.
The “runner” which is the revolving part, as shown in Fig. 121, is
composed of two separate and distinct types of wheels, and has two
diameters, as shown. Each wheel or set of buckets receives its separate
quantity of water from one and the same set of guides but each set acts
only once and independently upon the water used, hence the water does
not act twice upon the combined wheel as might be supposed, as in the
compound steam engine.
The upper wheel _G_ receives the water as shown by the arrows at _A_,
and has _a central and downward discharge_, while the lower wheel _C_
receives the water as shown by the arrows at _B_ and has an _inward,
downward and outward_ discharge as shown by the arrows at _D_.
These two sets of buckets need to be exceedingly strong. The lower
set _B_ are made of heavy flanged steel plate and are cast into their
places by being placed in the sand mould, and the cast iron flows
around them forming the heavy ring _C_, surrounding the outer and lower
edges. This ring is a part of the diaphragm which separates the two
wheels. The upper edge of the ring _C_ is beveled to form a neat joint
which prevents any unnecessary loss of water.
This runner is balanced and secured to a hammered iron or steel
shaft _F_. It is supported usually by a step of the best specially
selected hard wood thoroughly soaked in oil for months before use. The
lower end of the shaft is dished out at _E_, forming a true arc of a
circle—concave—while the wooden step is made spherical—convex—to fit
into the end of the shaft. The step is formed in this way so that no
sand can lodge between the bearing surfaces, and cut them out. The
resident oil in the wood combined with the water make a most durable
means of lubrication, and these steps last for many months where the
water is clear.
[Illustration: FIG. 122.]
To get at the exact quantity of water consumed by a turbine wheel,
one cannot make an accurate calculation from the openings through the
wheels but the water is measured after it has passed through the
wheel, as it flows away into the tail race. Any slight variation in
the form of buckets or admission apertures will make an appreciable
variation in the quantity of water discharged by a turbine wheel.
These wheels are made either for vertical or horizontal shafts and are
also made single or double. The engraving, Fig. 122, shows a _Hercules
turbine within the case and gate ready to set in the penstock_.
Up to the year 1876 this make of wheel tested at the flume of the
Holyoke Water Works showed the highest efficiency at all stages of
gate, namely 87 per cent. (page 97, Emerson’s tests).
The design of case will naturally lead the reader to conclude that this
wheel has, 1, an inward, 2, downward and, 3, an outward discharge which
is correct. _The gate is simply a curb_ or hollow cylinder which forms
a sleeve outside the case and is raised and lowered by the gearing and
rack shewn in the engraving. As this sleeve rises it gradually uncovers
the openings shown which admits water into the wheel.
_Horizontal Turbine_—The turbine of 10,500 horse power installed in the
Shawinigan plant, Canada, see IV Pt. 2. is of _the horizontal type_,
the water entering at, A, the lowest part of the turbine and flows
around and fills the outer special tube, passes through an annular
gate, flows radially through the wheel thence out through two draft
tubes, B, one on each side. The weight of the water wheel is 182 tons,
the shaft weighing 10 tons and the bronze runner 5 tons. It is 30 feet
from base to top and 32 feet 2-1/2 inches wide over all. The shaft, C,
is of solid forged steel, 22 inches diameter in the middle, tapering
down to 10 inches diameter on one end and 16 inches diameter on the
other, the distance between bearings being 27 feet. The intake is 10
feet in diameter and the quantity of water going through the turbine
when developing full power is 395,000 gallons a minute. The speed of
the wheel is 180 revolutions per minute with a head of water acting on
the turbine of 125 to 135 feet.
Fig 123 is designed to show _The Setting of a turbine wheel in a wooden
penstock_.
[Illustration: FIG. 123.]
The principal and most essential dimensions necessary to be considered
in setting turbines are indicated by letters, each size having its own
particular dimensions.
_In setting the wheel_ in the ordinary penstock it is necessary in the
first place, to have the floor exactly level, and it is generally more
convenient to lay down a ring of soft wood around the hole in the
floor, as it will be much easier to dress off with a plane than the
plank floor. The floor should be supported by posts under the timbers
around the hole, so that there will be no settling of the floor after
being once made level. As the flange on which the wheel rests is turned
true, the wheel will, when placed on this level floor, stand in the
exact position required, _i. e._, the shaft will be exactly vertical.
If the wheel is a large one and was taken apart for shipment, the draft
tube is first erected in position, then the wheel is placed on its
step, the other parts being put on in their order. The step and other
bearings are adjusted before leaving the shop, but it will sometimes
happen that they will in some way get shifted, and as the wheel is
being put together, they should be inspected and readjusted, if
necessary. The only change that can occur in the step is its vertical
adjustment, which is regulated by screws. When the right height is
found, the broad flange around the lower part of the wheel should stand
about one-sixteenth of an inch below the under side of the base of the
guide rim where it rests upon the draft tube. The adjustable bearing
on the top of the cover plate should be fitted up closely around the
shaft, but not screwed so tightly as to bind it.
All these wheels above fifteen inches in diameter are provided with
chains and weights to counterbalance the weight of the gate, so that it
will move easily. It is best, when it can be done without much trouble,
to carry the weights outside of the flume, but they can be used inside
where the height is sufficient, although it will require a little more
weight to be as effective. When the wheel is not likely to be started
up at once, it is a good plan, when putting it together, to smear the
step and the shaft at the bearing with tallow, as a protection against
rust while it remains idle.
It is sometimes necessary to use a draft tube longer than is ordinarily
attached to the wheel. If properly constructed and applied there will
be no sensible loss of power, but it must be air tight, and when of
considerable length it is better enlarged gradually toward the lower
end, especially in cases where it may be necessary to carry this tube
near the pit bottom.
_Iron cases for Turbines._ Fig. 124 shows the setting of a Hercules
wheel within an iron case.
[Illustration: FIG 124.]
Although the expense of iron is as a rule considerably greater than
wood, the results obtained by the use of iron cases and penstocks
are much better than could be possible with wood, on account of their
durability and freedom from leakage.
It is generally conceded that there is a great risk of the step
becoming heated and burning out when placed in a draft tube above the
tail-water, and a jet of water is required to counteract this tendency
to overheat. As all such fixtures are liable to derangement and often
fail to operate, we are in favor of setting the wheel with the step
immersed, whenever it can be done without too great expense.
This case consists of two cast iron heads with boiler iron sides and is
provided with a cover, so that the wheel may be taken out entire. This
cover is fitted with stuffing boxes for both wheel and gate shafts,
and a manhole affords easy access to the wheel. The bearing surfaces
of the heads are nicely turned, insuring tight joints, and all holes
for rivets are accurately spaced and drilled. The heads of the larger
cases are made to clamp, the two halves being planed together; the
cases are fitted with mouthpieces having cast iron flanges for feeder
connections, to secure by bolts to either iron or wooden feeders.
Where two wheels of the same or different diameters are to be used,
corresponding cases connected in the centre with one common feeder
connection, or are placed in cases provided with separate feeders. In
connection with these cases an _iron draft tube of any desired length
may be used_. The wheel is usually fitted to the case before leaving
the works, and in erecting the smaller wheels, all that remains to be
done is to set the case on the foundations provided for it and make
the necessary connections as stated in explanation previously made for
Figs. 122 and 123.
The general arrangements required for the proper erection of turbines
are well understood by competent millwrights and do not in ordinary
cases present any serious difficulties. It may be of interest to many,
and to the advantage of some who may consider the use of water power,
if a few general remarks on this subject are added.
In practice there is almost always a little loss of head due to the
velocity with which the water passes through the channels leading to
and away from the wheel, and it should be the aim in constructing
flumes to bring the loss to a minimum. When the size of the wheel and
the quantity of water to be used have been determined, 1, the size of
the conduit for carrying the water to the wheel, 2, the width and depth
of the wheel-pit and tail-race, and 3, the dimensions and location of
the flume for the wheel are to be considered and properly arranged.
All of these should be of such dimensions as to insure the flow of
water through them at a moderate velocity, and with as little change of
direction as may be practicable.
The larger the pipe or canal the better, but there must be a limit in
practice, and it may be laid down as a general rule that a velocity
of _three feet per second_ is good practice in short tubes of uniform
section of not more than fifty feet in length; but _the velocity should
be reduced as the distance increases_, until in a length of 200 feet,
it should not exceed two feet per second.
The same rule applies to the tail-race, except that the velocity should
be somewhat lower in ditches cut through rock or earth and having the
naturally resulting roughness of sides and bottom.
The width and depth of the pit below the wheel may, for a given wheel,
vary somewhat as the water discharged into it is greater or less;
therefore, the dimensions should increase with a greater head for the
same wheel. The following is an approximate rule for the dimensions
of the pit, say for a head of twenty feet: width of pit equal to four
times the diameter of wheel, depth below the level of tail-water
one and a half times the diameter of wheel. The flume for the wheel
should be about three times the diameter of the wheel in its width or
diameter, and if it is decked over at the top it should be high enough
inside to clear the coupling on the wheel shaft.
_The Watertight Turbine_ is a special machine designed to keep the case
tight by the pressure of the water against the gates at the sides, no
matter how much these gates wear.
Fig. 125 shows the plan of chutes, gate-seats, gates, and buckets of
the wheel. Part of the gates are shown open, and others are closed. The
gates make a quarter of a turn in opening, and the same in shutting,
and to open all the gates, the gate wheel makes half a revolution. The
upper half of case shows the gate-wheel and pinion for operating its
parts.
[Illustration: FIG. 125.]
[Illustration]
The small illustration is a perspective view of the gate and gate
segment used on the watertight turbine. The part cut away forms part of
the chute when the gate is open as shown in the lower left-hand side
of the figure. The sharp edge of the gate cuts off sticks and rubbish
which are liable to get in the wheel, which is an obvious advantage.
Another desirable feature claimed for this wheel is the plan of
operating the gates in opposite pairs; by this means 2, 4, 6, 8 or 10
full gates may be opened at will, according to the power required.
[Illustration: THE NIAGARA FALLS TURBINE.—FIG. 126.]
_Turbines at Niagara Falls._ A number of turbines installed at Niagara
Falls, N. Y., are here briefly described; they are about 5,000
horse-power each; a canal leads water from the river to the wheel
pit. The water is carried down the pit through steel penstocks to the
turbines, which are placed 136 feet below the water level in the canal.
After passing through the wheels the waste water is conveyed to the
river below by a tunnel 7,000 feet long. The “plan” Fig. 126 shows a
cross-section of the wheel pit, with an end view of a penstock, wheel
case and shaft. Fig. 126 exhibits part of a vertical section of the
wheel pit _and a side view_ of this penstock, with the enclosing case
and shaft of the turbine.
[Illustration: TOP VIEW OR PLAN OF FIG. 126.]
This turbine has a rock-surface wheel pit, but this surface is
protected by a brick lining having a thickness of about 15 inches.
The width of the wheel pit is 20 feet at the top and 16 feet at the
bottom, and the cylindrical penstock is 7-1/2 feet in diameter. The
shaft of the turbine is a steel tube 38 inches in diameter, built in
three sections, and connected by short solid steel shafts 11 inches
in diameter, which revolve in bearings. On the top of each shaft is a
dynamo for generating the electric power.
In Fig. 126 is shown a vertical section of the lower part of the
penstock, shaft, and twin wheels. The water fills the casing around the
shaft, passes both upward and downward to the guide passages, through
which it enters the two wheels, causes them to revolve, and then drops
down to the tail race at the entrance to the tunnel, which carries it
away to the river. The gate for regulating the supply is seen upon the
outside of these wheels, both at the top and bottom, Fig. 126.
Fig. 128 gives a larger vertical section of the lower wheel with the
guides, shaft, and connecting members. The guide passages, and the
wheel passages, are triple as shown so that the latter may be filled
not only at full gate, but also when it is one-third or two-thirds
open, thus avoiding the loss of energy due to sudden enlargement of the
flowing stream. The two horizontal partitions in the wheel are also
advantageous in strengthening it. The inner radius of the wheel is
31-1/2 inches and the outer radius is 37-1/2 inches, while the depth is
about 12 inches. In this figure the gate is represented closed and to
open, it moves downward uncovering the guide passages as shown in Fig.
126, the position it occupies loaded.
In Fig. 127 is shown a half-plan of one of the wheels, in a part of
which are seen the _guides and vanes_, there being 36 of the former and
32 of the latter. Although the water on leaving the wheel is discharged
into the air, the very small annular space between the guides and
vanes, together with the decreasing area between the vanes from the
entrance to the exit orifices, _ensures that the wheels move like
reaction turbines_ for the three positions of the gates correspond to
the three horizontal stages or openings through the guides as shown in
Fig. 128, _i. e._, three stages of gate.
NOTE.—A test of one of these wheels, made in 1895, developed 5,498
electrical horse-power, generated by an expenditure of 447·2 cubic
feet of water per second under a head of 135·1 feet. The efficiency
of the dynamo being 97 per cent., the efficiency of the wheel and
approaches was 82-1/2 per cent.
[Illustration: FIG. 127.]
The average discharge through one of these twin turbines is about 430
cubic feet per second, and _the theoretic power_ due to this discharge
is 6,645 horse-power. Hence if 5,000 horse-power be utilized _the
efficiency_ is 75.2 per cent. Under this discharge the mean velocity
of water in the penstock is nearly 10 feet per second, but the loss of
head due to friction in the penstock will be but a small fraction of a
foot. The pressure-head in the wheel case is then practically that due
to the actual static head, or closely 141-1/2 feet upon the lower and
130 feet upon the upper wheel.
The absolute velocity of the water when entering the wheel is about 66
feet per second, so that the pressure-head in the guide passages of the
upper wheel is nearly 66 feet. The mean absolute velocity of the water
when leaving the wheels is about 19 feet per second, so that the loss
due to this is only about 4 per cent. of the total head.
NOTE.—The above description refers to the ten turbines in wheel
pit No. 1. The illustrations are those of the wheels called units
1, 2, and 3 which were installed in 1894 and 1895. Units 4 to 10,
inclusive, installed in 1898-1900, are of the same type except that
both the penstock and wheel case have cast-iron ribs on their sides
which rest on massive castings built into the masonry of the side
walls. This arrangement dispenses with the supporting girders shown
in Fig. 126 and gives much greater rigidity to both penstocks and
wheels.
[Illustration: FIG. 128.—ENLARGED VERTICAL SECTION OF LOWER WHEEL,
SHOWING GATES.]
The weight of the dynamo, shaft, and turbine is balanced, when the
wheels are in motion, by the upward pressure of the water in the
wheel case on a piston placed above the upper wheel. The upper disc
containing the guides is, for this purpose, perforated, so that the
water pressure can be equalized.