Copyright
American Technical Society.

Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. (page 26 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 26 of 30)
Font size
QR-code for this ebook


3000 revolutions per minute respectively.

This gearing, in the small-sized turbines, consists of a pinion and
a single gear, but in the larger-sized turbines there is a single pinion
with a gear on each side. This method has the advantage of dis-
tributing the load half on each gear, thus lowering the pressure on the
teeth and eliminating side pressure on the bearings of the flexible
shaft. One disadvantage in the large size, however, is that there are
two working shafts, each connected to a working unit. There is
also not only the large amount of friction in the gears but also the



*Lack of uniformity in the density of the steel might cause the center of
gravity of the wheel to be outside of its geometrical axis.

tSee Page 17, Part I.



375



58 STEAM TURBLVES



friction due to the double set of bearings throughout, and the losses
that attend the performance of work in two small units rather than
in one larger unit.

Nozzles. The wheel is enclosed in a casing in which the nozzles
are set, these nozzles being opened and closed by means of hand
valves. A detail of the nozzle and valve is shown in Fig. 34. A is
an annular space in the casing acting as a steam chest, C is the valve
which permits opening or closing of the nozzle, and B is the nozzle
itself. The nozzle is fitted into a taper hole in the casting and
drawn into place by a nut.




\

Fig. 34. Detail of Nozzle of DeLaval Turbine.

The design of the nozzle depends naturally upon the pressure
used, the degree of superheat, and the vacuum or back pressure.
The nozzles being easily removed, it is apparent that a turbine can
readily be altered to meet different conditions by inserting new nozzles.
A condensing turbine is often equipped \vith an extra set of nozzles
designed for non-condensing conditions, which may be used with
better economy in case the vacuum fails.

There are usually two to twenty-four nozzles in the casing, and
the power developed at any time is naturally proportional to the num-
ber of nozzles in operation. The clearance between the wheel and the
nozzle is about one-eighth of an inch. The clearance between the
tips of the blades and the casing is not a matter of importance, for
there is no tendency for steam leakage, the pressure in all parts of
the casing being practically the same as the back pressure. This
clearance, therefore, may be whatever practical conditions require.



376



STEAM TURBINES



59



Fig. 35 is the exterior view of the turbine and generator, showing
nozzles and valves set in the casing. By inserting nozzles in the
holes which are shown plugged in the figure, a greater power could
be obtained.




Vanes. The vanes are of the crescent shape common in impulse
turbines. They are made of drop-forged steel which resists erosion,
and have bulb shanks, as shown in Fig. 36, which are driven into



377



STEAM TURBINES



place. The outer ends of the vanes fit closely together, thus form-
ing a continuous ring which prevents any movement at the ends
of the vanes.

Steam at high velocities, especially if wet, is liable to cause
appreciable wear on the vanes, the wear being practically all on the
entrance side; but it is not very great, and tests of a 100-horse-power
turbine have shown that wear on the buckets could be as great as
one-sixteenth of an inch without increasing the steam consumption
more than 3%, according to the report of the manufacturers.

Wheel. At the very high speeds employed, centrifugal forces
are enormous ; hence, special high-grade nickel-steel must be used in
the manufacture of the rotating elements. This steel is said to be

high in carbon and to
possess a tensile strength
of approximately 135,000
Ibs. per sq. in. The wheel
is shown in cross-section
in Fig. 37 and is designed
to be of uniform strength
throughout, except that
just below the rim a nar-
row annular groove is
turned purposely to make
this section weak, for the
following reason :
Centrifugal force increases as the square of the speed, and, if the
safety devices fail to work, the rotating wheel must ultimately burst.
The reduced section near the periphery of this wheel makes the
stresses at this point approximately 50 % greater than elsewhere, and
yet, at normal speeds, this will be perfectly safe, as the factor of
safety is between four and five. Now, since the centrifugal force in-
creases directly as the square of the number of revolutions, the stresses
at the weakened point, when the speed is double, will be four times
as great, that is, about equal to the ultimate strength of the material.
The rim will therefore burst and fly into many small pieces, doing
but little damage, as the casing is made heavy enough to restrain
these fragments. When the rim flies off, the stresses in the main
portion of the wheel are thereby greatly reduced, and no further




Fig. 36. Method of Attaching Vanes in
DeLaval Turbine.



378




Ir



STEAM TURBINES



til



damage can ensue. Wheels without this weak section have burst
under test into a few large pieces which have possessed enough energy
to break through a 2-in. cast-iron casing.

On each side of the wheel are hubs extending into cylindrical
openings in the casing. These are known as safty bearings and
work with slight clearance under ordinary conditions. Should the
rim burst, the wheel would at once become unbalanced, and the result-
ing eccentricity of the center of gravity would cause the wheel and




Fig. 37. Method of Mounting Wheel of Small DeLaval Turbine.

shaft to rotate off-center, bringing a considerable pressure of the hub
against these safety bearings. These, acting as a brake, together
with the absence of further impelling forces due to the loss of rim and
buckets, will quickly bring the rotating wheel to a stop.

For small wheels, a bushing is fitted and shrunk to a short swelling
on the shaft, and in addition is pinned in place. The hub of the wheel
is bored to fit this bushing and it, together with the shaft, is drawn
into place by a nut, as shown in Fig. 37. The wheel may readily be
removed from the shaft by loosening the nut.



379



STEAM TURBINES



For large wheels, such a construction is not desirable, because a
wheel with a hole in the center is not nearly as strong as one without
such a hole, and, in the larger sizes of turbine, the strength of the
wheel is an exceedingly important factor. The hub, therefore, in such
a wheel is solid, but is recessed to fit the flanged end of the shaft, as
shown in Fig. 38. The recess is tapered one-half inch to the foot to
fit the shaft, which is securely bolted in place as shown.




Fig. 38. Method of Mounting Wheel of Large DeLaval Turbine.

The pitch circle of the vanes is about 4 in. in diameter for a
7-H. P. turbine making 30,000 R. P. M.; about 8f in. for a 30-H. P.
turbine making 20,000 R. P. M. ; about 19| in. for a 100-H. P. tur-
bine, making 13,000 R. P. M.; and 30 in. for a 300-H. P. turbine mak-
ing about 10,600 R, P. M. The rim of the wheel is drilled parallel
to the shaft, with cylindrical holes milled out, as shown in Fig. 36, to
hold the bulb shanks. This makes a strong construction, and the
vanes are easily replaced if necessary.



380



STEAM TURBINES



63



Shaft. ^Tien a body is rotating at high speed, it must be very
carefully balanced, by distributing the material sy metrically about
the center of rotation. If the center of gravity of the rotating mass
is not absolutely at the center of the shaft, a vibration more or less




serious will be set up, because a rotating body tends to rotate about
its own center of gravity instead of its geometrical center, thus caus-
ing a pressure alternately on one side or the other of the bearing.
For speeds of 3000 R. P. M., which is common in compound turbines,



04 STEAM TURBINES



the wheels can be balanced on knife edges, the wheel disks being
drilled at certain points until they become perfectly balanced. It
is reported that careful w r ork in this matter will ensure the center of
gravity of the wheel being within y 0*0 IT f an inch of the geometrical
center. Small as this error may be, it would be prohibitive at the
high rotative speeds used in the DeLaval turbine; hence the adoption
of the long, slender shaft on which the wheel is mounted. This bends
slightly, and allows the w^heel to rotate about its own center of
gravity without vibration. This feature is distinctive of the DeLaval
machine. The relatively small diameter of shaft is astonishing, being
but a little over \\ in. at its smallest section for the 300-H. P. turbine.

Gears. The speed-reducing gears are in the ratio of about ten
to one; i.e., if the turbine rotor has a speed of 30,000 11. P. M., the
larger gears have a speed of 3000 11. P. M. At the desired place, a
swelling on the shaft is provided, in which the pinion teeth are cut.
In the smaller sizes only one large gear is used, but in the larger
machines there are two large gears, one on each side of the pinion.
The teeth are cut spirally at MI angle of about 45, as shown in
Fig. 35, and have double sets of teeth at 90 to each other.

These reduction gears are fine examples of engineering and
mechanical skill, as only the best of work would stand up under the
high speeds of rotation. The shaft on which the pinion is cut is of
nickel-steel, but the gears are made of soft steel low in carbon. They
have a peripheral velocity of about 100 feet a second, and if kept
free from grit, will run for a long time with little or no wear. These
gears were originally made of bronze, but this has proved unsatis-
factory, as crystallization of the bronze developed, which resulted in
the fracture of the teeth after a few years' continuous use.

Fig. 39 illustrates the various working parts of the DeLaval steam
turbine. B is the rotating bladed wheel, A the long flexible shaft,
C the pinion cut on the shaft, // one set of reducing gears, and
M the flange for connection to the w r orking unit. Fig. 40 shows a
sectional view of a complete turbine and connections for a single
working shaft.

Bearing. On the governor end of the shaft is a spherically-
seated, self-aligning bearing, provided to give greater flexibility to
the shaft, and to take up the thrust due to the action of the steam on
the blades. The latter is a relatively unimportant matter, for the



384




88*



STEAM TURBINES



steam is completely expanded in the nozzle, and the pressure is,
therefore, practically the same on both sides of the vanes. This
bearing is held in place by a spring and cap, shown at A, Fig. 40.

In the high-speed bearings, ring oiling is not satisfactory, and
the oil is supplied to shallow spiral grooves. The oil filters through
wicking in a sight-feed lubricator and drips upon these bearings.
On the gear shafts, the bearings are provided with the ordinary ring
oiling devices. All bearings are carefully lined with white metal.

Riedler=Stumpf Turbine. The first turbine developed by Pro-
fessors Riedler and Stumpf was of the single-stage type, both pressure




Fig. 41. Milled Buckets in Riedler-Stumpf Turbine.

and velocity, like the DeLaval, but with this radical difference a
wheel about ten times as large in diameter as the DeLaval wheel was
used and, therefore, the same peripheral speed was obtained with
about one-tenth as many revolutions. The reduction which DeLaval
accomplished by means of gears, Riedler and Stumpf accomplished by
increasing the diameter of the wheel. By this reduction in the number
of revolutions, the error in balance, which, it is claimed, could be
brought to less than 6 J 7 millimeter, was rendered insignificant.

Their wheels were said to be made of 10% nickel-steel with
135,000 Ibs. tensile strength, and were G| to 9 ft. in diameter, revolving
about 3000 R. P. M. for machines of 2000 to 3000 H. P. Their
single-stage turbine did not meet with general favor, and was usually
compounded either by pressure stages or velocity steps, but a de-
scription of it will nevertheless be valuable.



STEAM TURBINES



'.7



Instead of using vanes of the DeLaval type, U-shaped buckets
were milled in the face of the solid wheel, overlapping one another
as shown in Fig. 41. The steam jet impinged upon the buckets not
on the side of the wheel, as in the DeLaval type, but directly upon




Fig. 42. 20-JI. P. Kiedler-Stumpf Turbine and Direct-Connected Generator.

the face of the wheel thus permitting a more nearly complete reversal
of the steam jet and, other things being equal, a higher efficiency. It
will be recollected that if the jet is delivered to the vanes at the side,
and at entrance and exit makes an angle w r ith the plane of rotation,
the velocity of the jet* is divided into two components. The velocity



'Page 17, Part I.



387



68



STEAM TUR BINES



of whirl, which is equal to V cos a, a, usually being 20 to 35, is
the only component that produces a rotative effort.

The nozzles were made with a square instead of an elliptical
section at the outlet, and were arranged at regular intervals about
the casing, as in the DeLaval turbine. With a given size of wheel,
the power was increased by increasing the number of these nozzles
until steam injection took effect upon the entire periphery of the wheel.

There being but one rotating wheel, it overhung the shaft bear-
ing, thus passing through the casing on one side only, requiring but
one stuffing box, and, therefore, giving
a comparatively small bearing loss. A
20~H. P. turbine of this type with a
direct-connected dynajno, is shown in
Fig. 42. Fig. 43 shows details of the
wheel. This wheel is fitted with double
buckets, which were generally used on
the large sizes. A 5000-K. W. turbine
of this type would require a wheel 20
feet in diameter, admitting steam to
the whole periphery, and making 1500
revolutions per minute. More details
of the Riedler-Stumpf turbines will be
described in connection with the com-
pound turbine.




Fig. 43. Detail of Wheel of Riedler-Stumpf Turbine.



COMPOUND IMPULSE TURBINES WITH VELOCITY STEPS

It has been shown that steam may be fully expanded to the back
pressure in a single nozzle, and the kinetic energy absorbed by passing
the jet through several sets of revolving wheels or vanes in succession,
each taking out part of the velocity. To employ velocity steps, some
sort of reversing device must be arranged to bring the steam back,



STEAM TURBINES



60



either onto another bucket of the same wheel or onto a bucket of an
adjoining wheel attached to the same shaft. The former method was
adopted in the Riedler-Stumpf turbine, the latter in the Curtis. In
either case, a simple and compact turbine is the result; but the





Fij?. 44. Double Buckets in Riedler-Stumpf Turbine.

type has disadvantages already enumerated in Part I, which
limit the number of velocity steps that can be economically
used, to three or four at the outside. Since the work derived
from the fifth action of the steam would theoretically be only
I of that derived from the first action,* and might easily be



*Page 31, Part I.



389



70



STEAM TURBINES



consumed in additional friction, it is customary to allow the
steam to act no more than four times. Single-stage turbines are
not considered practicable in sizes above 200 H. P. or 300 H. P., it

being more economical in such
cases to reduce the steam velocity
by using another pressure stage.
Curtis Turbine. The earlier
forms of Curtis turbine were of
the single-pressure stage type with
several velocity steps, and the
smaller turbines now made by
the General Electric Co. are after
this pattern. 35-K. \V. sizes and
smaller have a single-pressure
stage with three velocity steps,
that is, three sets of rotating vanes
with two intermediate sets of
guide vanes. The details of con-
struction are in all ways similar
to those of the ordinary form of
Curtis turbine, which is com-
pounded both by pressure and
velocity and will be described
under that heading.

R ied ler = Stumpf . Large
powers of the simple impulse
type required either abnormally
large wheels or too high speeds
of rotation and it was, therefore,
frequently more convenient to ex-
tract the velocity from the steam

jet in two steps. For powers larger than could be dealt with in the
single-stage type, the steam passed successively through buckets of
the same wheel, and for still larger powers, pressure stages were em-
ployed as well as the velocity steps. The compound velocity turbines
developed by Professors Iliedler and Stumpf had wheels and buckets
of the general type described in connection with their simple im-
pulse turbine. The device employed to reverse the direction of




Fig. 45. Double Guide Vanes.



390



STEAM TURBINES



71



the steam and bring it back again to other buckets on the same
wheel was clearly described on Page 9, Part I, to which the student
is referred.

In one type of their turbine, the buckets were double, a small
bucket on one side of the wheel being for initial admission, and, since
part of the steam velocity was abstracted, it was necessary that, as
the steam returned, it should enter a larger bucket which was pro-
vided on the other side of the wheel, as shown in Fig. 44 and Fig. 45.

Another device of Riedler and Stumpf for reducing speeds
of rotation was the employment of counter-running wheels. The




Fig. 46. Rotor and Shaft of Terry Single-Stage Turbine.
Terry Steam Turbine Co.

guide vanes were buckets cut on a wheel which, instead of being
stationary, was free to revolve in a contrary direction. Thus, the
absolute velocity of each wheel would be half the relative velocity of
the two wheels. In a turbine of this type, besides the obvious
objection of rotation in two directions, the wheel of initial admission
would do more work than the counter-running wheel, because
the work absorbed would be in proportion to the difference of the
squares of the steam velocities at entrance and exit, and the higher
velocities would naturally exist in the first wheel.*

Terry Turbine. The turbine developed and now built by the
Terry Steam Turbine Co., of Hartford, Conn., is, in sizes up to 200-

*Page 31, Part I.



391



72



STEAM TURBINES



H. P., of the single-stage, compound-velocity type. The buckets are
U-shaped, milled in the face of the wheel, overlapping one another
something like the single bucket arrangement of the lliedler-Stumpf




Fig. 47. 110-11. P. Terry Steam Turbine, Opened.

machines. Fig. 46 shows the rotor of a Terry turbine. The steam
is expanded in the nozzles to within about one pound of back pres-
sure. As it leaves the nozzles, it impinges upon one side of the bucket,
reversing through 180. As it leaves the first bucket, it enters a



302



STEAM TURBINES




Fig. 48. Wheel of Sturtevant Turbine. The Bucketc are Milled from
the Side of the Wheel.



similar bucket attached to the casing, which reverses its direction
through 180 and causes it again to impinge upon another bucket of



STEAM TURBINES



the wheel, and so on until the velocity is all absorbed. The reverse
buckets are arranged in groups (usually of four), one group for each
nozzle, the steam being returned to the wheel as many times as there
are reverse buckets in each group. Fig. 47 clearly shows these buckets
on the inside of the lifted casing. A crescent-shaped hole may be
seen cut in the bottom of each reverse bucket. These holes release

a part of the expanded
steam and thus reduce
the volume in propor-
tion to the lessened
velocity, as otherwise
there ought to be suc-
cessively larger pass-
age areas.

There are usually
four nozzles to a tur-
bine, each being con-
trolled by a hand valve
so that the power may
readily be regulated.
The main bearings are
of the ring-oiling type.
As the weight of the
rotor is comparatively
small and the speed of
revolution 1250 R. P.
M. for 200 to 300 H.
P., large sizes offer
no practical difficulty.

Sturtevant Turbine. The B. F. Sturtevant Company, of Hyde
Park, Mass., is building a turbine in small sizes to drive electric
generators and blowers. In sizes of 100 H. P., or less, these turbines
have a single-pressure stage, using the steam over and over again on
the wheel in much the same manner as is done in the Terry turbine.
Powers of 200 H. P., or over, would be built with two or more pressure
stages. The wheel is a single forging of open-hearth steel. The
buckets, which are the U-shape type, are cut from the solid rim by a
milling machine.




Fig. 49. Sturtevant Turbine Showing Reverse
Guides in Casing.



STEAM TURBINES



75



The earlier turbines have buckets cut on the side of the wheel,
as shown in Fig. 48. Steam entering the outer edge of these buckets,
passes through the buckets into stationary reverse guides in the casing
shown in Fig. 49. At A are inserted the nozzles, which are of the
ordinary expansion type with elliptical openings. The guides are
of two types; about four are U-shape like the buckets and return
the steam to the wheel again, returning it as many times as there are
return buckets; the others, shown at C, are cut open at the inner
edge in such a manner that the
steam, instead of returning to
the wheel, is exhausted into the
middle of the casing and there
allowed to pass out. To
avoid a troublesome end
thrust in this machine, buck-
ets are cut on both sides of
the wheel, thus equalizing the
side pressure.

The later machines built
by the Sturtevant Co. have the
buckets on the outer rim
much like those shown in the
illustrations of the Terry tur-
bine, but the action of the steam is not different from that in the
earlier type. Fig. 50 shows this style of turbine.

COMPOUND IMPULSE TURBINES, PRESSURE STAGES

In the discussion of compound turbines in Part I, it was shown
that the available head could be divided into several stages, thus
making the steam velocity from stage to stage relatively small, and
permitting smaller speeds of revolution. Turbines of this type are,
in principle, like a number of DeLaval wheels on the same shaft.
They consist essentially of a casing which supports a number of
diaphragms, dividing the interior into separate cells, in each of which
a single impulse wheel containing the vanes is free to revolve. Each
stage or element comprises a rotary wheel and a set of nozzles, or
distributing vanes, which guide the steam from one chamber to the
next and direct it at the proper angle onto the vanes of the wheel in




Sturtevant Turbine with Buckets
Cut on Outer Rim.



395



76



STEAM TURBINES



the following chamber. These passages may or may not be of the
diverging type, depending upon the drop of pressure from stage to
stage. In all machines of this type the drop in pressure is so arranged
that an equal number of heat units will be given up per stage, which,
as will be remembered, does not correspond by any means to an equal
drop in pressure.




Fig. 51. Group of Diaphragms of Rateau Turbine.

In such a turbine as this, a foreign substance is not likely
to injure more than one wheel, for it cannot pass the diaphragm
separating the different chambers except through the nozzles,
and, as there are many stages to such a turbine as this, the machine
might run fairly well, even if one or two wheels were removed. It
would, of course, give less power and poorer steam economy. The
clearance between the nozzle and vanes should be small to prevent
mingling of the steam jet with the stagnant steam in the casing, but



396



STEAM TURBINES



77



the clearance over the ends of the vanes is of little consequence,
especially if a shrouding is used, for there is no tendency for steam
to leak by the vanes, the pressure being constant throughout the
chamber.

Rateau Turbine. The turbine using pressure stages only is
best exemplified by the Rateau turbine, designed and developed by
Professor Rateau, of Paris. His turbine is a horizontal, multi-stage,
impulse machine and consists of sometimes as many as forty pressure
stages, but usually less. Those built by the Western Electric Co. have
from four to eighteen,
depending upon the
power and range of
steam pressures. The
large number of stages
employed in these tur-
bines means but little
drop in pressure from
stage to stage. Hence,
the DeLaval type of
expanding nozzle is
not needed, it having
been shown in the dis-
cussion of nozzles*
that, if the final press-
ure is more than about 58% of the initial pressure, a parallel-sided
passage or a slightly converging nozzle is sufficient to permit the
proper expansion of the steam, and to secure the maximum available
energy. In the Rateau turbine, the drop in pressure from stage to



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 26 of 30)