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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 27 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 27 of 30)
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stage is much less than the limiting amount mentioned.

Diaphragm. As the steam passes through the turbine, it ex-
pands from one stage to the next and, of course, requires larger pas-
sage areas in each succeeding diaphragm. This is accomplished in
general by increasing the number of openings rather than by increasing
the size of them. The guide passages are arranged in groups, the
number in each group being increased, and, consequently, the width of
the group widened through successive stages until the openings finally
extend entirely around the disk. To provide larger passage areas to

*Page 19, Part I.




Fig. 52. Lower Half of Turbine Casing Showing Dia-
phragms in Place, and the Circumferential Grooves
for Holding Them. Western Electric Co.



397



7s



STEAM TURBINES




Fig. 53. Two-Fiece Stationary Diaphragms with
Distributing Vanes. Western Electric Co.



take care of still further expansion of the steam, the diameter of the
wheel and diaphragm must he increased, and, at the same time, the size

of the passage openings
enlarged. In condensing
turbines there are usually
three diameters of wheel.
In non-condensing tur-
bines, two is generally
sufficient, and sometimes
only one is used. The

no77 l p<5 nr HistriKntM-Q
HOZZlCS, OF distributers,

when only a portion of
the wheel is open to steam admission, are set to have an angular ad-
vance on the preceding group, this advance being proportioned to the

speed of the wheels, so
that the steam jet as it
leaves the revolving
vanes will strike the next
nozzle directly, avoiding
any shock of impact
against the solid wall of
the casing. Any kinetic
energy in the steam as it
leaves the revolving vanes
of one wheel is, therefore,
directly available for use
in the next stage. If the
steam were brought up
sharply against the solid
casing wall, this residual
energy would be lost to
useful work, and there
would result a further
loss due to eddying of the
steam in this particular

cell. Fig. 51 shows a group of diaphragms of various sizes for this
turbine, and the idea of increasing the extent of each succeeding
group of nozzles.




Fig. 54. Construction of Wheels of the
Rateau Turbine.



398



STEAM TURBINES



There is a distinct advantage in this partial admission at the
higher pressures, for, if the admission took place around the entire
periphery of the wheel, the height of vane would necessarily be so
small that the friction would be excessive. By using partial admis-
sion only, the vanes in these stages may be of much greater height
than otherwise, and a few high vanes afford the same passage area
for the steam that
a large number of
low vanes would
offer, with, conse-
quently, less fric-
tion.

Casing. Cir-
cumferential
grooves are turned
in the inside of the
casing to hold the
diaphragms i n
place, as shown in
Fig. 52. The larger
diaphragms are
usually made in
two pieces, as
shown in Fig. 53.
In all of them, the
sh a f t passes throu gh
collars of anti-fric-
tion metal with
clearances as small
as possible in order to prevent leakage from stage to stage. These
collars are frequently provided with a labyrinth packing which will
be explained later.

Vanes. The vanes are of crescent shape, similar in cross-section
to those of the DeLaval turbine. The earlier ones had no protection
at the outer ends, but the later ones have been provided with a shroud
ring to give additional stiffness. The vanes and the shroud ring
are generally made of nickel-steel as this is well adapted to resist




Fig.



Wheels of Kateau Turbine, Balancec
Holes in Disk.



399



80 STEAM TURBINES



Wheels. The rotating wheels are usually made of two plate-
steel disks. One is flanged at the outer edge to which the vanes are
riveted; the other, usually slightly conical, is riveted to the flange of
the first disk, and both are riveted to the cast-steel hub. Fig. 54
shows the construction of this type of disk. The conical built-up disk
makes a stronger wheel, but a flat disk is sometimes used. Some
wheels are turned out of solid steel increasing in section toward the
center to give greater strength. Each wheel is carefully balanced by
itself on knife edges by drilling holes in the disk, the latter clearly
showing in Fig. 55. When these wheels are assembled on the shaft




Fig. 56. Shaft and Wheels of High-Pressure Rateau Turbine.
Western Electric Co.

there is little likelihood that the complete rotor will be out of balance.
Fig. 56 shows the wheels assembled on the shaft.

By-Pass Valve. This turbine is provided with a by-pass valve
to carry overloads. This admits high-pressure steam into the inter-
mediate stages and, although not permitting a complete expansion of
such steam, it is an effective means of taking care of large overloads.

Bearings. The bearings are of the plain ring-oiling type, usually
provided with water jackets. The shaft not being unduly long, there
is little danger of whipping, and the speeds of rotation not being very
high, special precautions are not necessary. Sometimes the turbines
are supported by three bearings, as shown in Fig. 57, the high and
intermediate stages being separated from the low by a third bearing.

Fig. 58 shows in section the turbine as built at Hawthorne,
111., by the Western Electric Co., which is licensed to build the Rateau



400




401



82



STEAM TURBINES



turbines. Professor Rateau claims for a 1500-H. P. turbine at 1500
revolutions per minute only H% leakage and bearing loss, and a
2 2% l ss m friction of the wheels against the steam.




Zoelly Turbine. The Zoelly turbine has been developed rather
extensively abroad, and is being manufactured largely through a
syndicate of builders including some American firms. It is a turbine
essentially of the Rateau type, that is, a multi-pressure-stage turbine,



402




403



84



STEAM TURBINES



but it has fewer stages, usually not over ten for condensing and five
for non-condensing, and it differs from the Rateau materially in detail.
The blades are very much longer, sometimes being as much as half the
diameter of the rotating wheel and, because of the fewer stages, many
essential details are different. The turbine is sometimes divided into
two parts when built condensing, the high pressure being separated

from the low sufficiently for a third
bearing to be placed between, as
shown in Fig. 59.

Vanes, There being fewer
stages than in the llateau, the
steam velocities must be much
greater, and, consequently, if the
rotative speeds are to be the same,
the diameter of the wheels must be
greater. Exceedingly long vanes are
used, which permit of a relatively
small wheel disk; the centrifugal
stresses in its rim, therefore, will
not be materially greater than in
other turbines. Again, the vanes
are few in number compared with
other turbines and are made as
light as possible, tapering at the
outer end in order to further reduce
the centrifugal forces. The expan-
sion from stage to stage is not
enough to require diverging expan-
sion nozzles except, perhaps, in the

high-pressure end, but the expansion is, of course, very much greater
than in the Rateau turbine. In the latter^turbine, the roots of the vanes
are cut off parallel with the shaft, but in the Zoelly they are cut on a
slope, giving a larger outlet than inlet to the vane. It might appear
that this was done to permit of steam expansion in the vanes, but this
is not so. Expansion is complete in the nozzles just as in the Rateau
turbine, but, there being a much greater expansion from stage to
stage, it follows that the area of the steam passages through each
successive diaphragm must be greatly increased; this difference is




Fig. 60.



Section C-C

Detail of Wheel and Vaiies
of Zoelly Turbine.



404




405



86



STEAM TURBINES



made up by increasing the depth of the openings in the succeeding
diaphragm. Therefore, to permit a free passage of steam from an
opening having one depth to an opening having a greater depth, and
to prevent the formation of eddies, it is necessary to slope the root of
the vanes. The vanes of the turbines being very much longer than in

the Rateau, it is quite
feasible to increase the
depth of the steam pas-
sages through the dia-
phragms.

Wheel. The vanes are
set in slots cut in the rim
of the wheel, and are
secured by a clamp ring
securely riveted to the
main portion of the wheel,
as shown in Fig. 60. It
is probable that the exces-
sively long vanes produce
a considerably greater
friction loss, revolving, as
they do, in the steam-
filled cells of the casing;
but as there are compar-
atively few cells and,
therefore, comparatively
few revolving wheels, it
is probable that this fric-
tion may not be any
greater than would be
expected in the Rateau
turbine. Economies, as shown by test, do not appear to be essen-
tially different from those of other first-class turbines. Zoelly
turbines have been built as shown in Fig. 61. The typical style of
long vane prevails, but, in the high-pressure stages, the vane is of
the double-U-shaped cross-section, a detail of which is shown in
Fig. 62. The steam jet necessarily impinges upon these vanes tan-
gen tially, instead of from the side.




Fig. 62. Details of Construction of the Double-U-
Shaped Vanes of the Zoelly Turbine.



406



STEAM TURBINES



87



Hamilton=Holzwarth Turbine. The Hamilton-Holzwarth steam
turbine is another turbine of the Ilateau type, the chief difference being
that, instead of having the admission guides arranged in groups, the
admission of steam in the high-pressure end takes place around the
entire circumference of the diaphragm. The wheels of this turbine,
therefore, would be smaller at the high-pressure end than would be
the case with the Rateau turbine, and the vanes would be of appreciably




Fig. 63. Details of Wheel of Hamilton-Holzwarth Turbine.

less depth. Theoretically, the diameter of each succeeding wheel should
increase as the steam expands but, for simplicity of manufacture, it
is better to keep a number of wheels of the same diameter, increas-
ing the length of blades to give larger passage areas. When the point
is reached where this is no longer practical, the diameter of the wheel
may be increased considerably, and the depth of the blade reduced.
There are approximately the same number of stages in the
Hamilton-Holzwarth as in the Rateau turbine, and the speeds of
revolution are not materially different. The running wheel shown in



407



88 STEAM TURBINES



Fig. 63 consists of a steel disk riveted to each side of the steel hub, or
spider, which is keyed to the shaft. The outer edges of the disk are
flanged outward, leaving a space to take the shank of the vane. The
vanes are of drop-forged steel of the usual crescent-shaped cross-
section. The shank of the vane fitted between two flanged disks, is
riveted in place. Since the steam is admitted around the whole
circumference of the diaphragm, vanes can be used better than
nozzles to give the necessary expansion, and direct the steam upon
the running wheels. Vanes could be used in the Rateau dia-
phragm, even with partial admission, but in such a case it has
seemed simpler to drill openings through the diaphragm.

For small pressure drops, the vanes may be parallel, top and
bottom, but, at the low-pressure end of the turbine where the
volumes increase rapidly, they are usually deeper at the outlet than
at the inlet, thus forming an easy passage of the steam to the next
larger set of vanes.

A diaphragm, Fig. 64, separating the various cells, consists of a
cast-iron disk bored loosely to fit the shaft. This disk has a groove C
into which the shanks of the guide vanes are set, and a rivet holds
them in place. A steel band is then shrunk over the ends of these
guide vanes and this band projects into grooves in the casing to hold
the diaphragm in place.

The Hamilton-Holzwarth turbine is manufactured by the Hooven,
Owens, Rentschler Co., of Hamilton, O. Few machines, however,
have been actually installed.

The Wilkinson Turbine. James Wilkinson has been developing
a turbine of the multi-cellular impulse type with comparatively few
stages, that is noteworthy because of the packing employed to prevent
leakage of steam from stage to stage. It must be borne in mind that in
a turbine of the Rateau type there are so many stages that there is
comparatvely little difference in pressure between one stage and the
next, and, consequently, there is little tendency for steam to leak
through between the shaft and the bushing in the diaphragm. As the
number of stages is reduced, the difference in pressure is increased,
and it then becomes essential that some sort of packing should be pro-
vided; otherwise the leakage is likely to be excessive. In practically
all turbines, to prevent leakage of steam from the high-pressure end
into the air, a labyrinth packing is used, consisting of a series of



408



STEAM TURBINES



grooves into which metal spring rings are fitted with slight clear-
ances. These rings are not very different from the spring rings
employed to pack the piston of the reciprocating engine, except that
there are usually very many more of them. For steam to leak out,
it must follow a tortuous course between the rings and sides and




h



Fig. 64. Details of Diaphragm of Hamilton-Hoi zwarth Turbine.

bottom of the grooves, greatly expanding and condensing as it leaks
through. The volume at the outer end is so large, due to this
expansion, that the leakage becomes very slow. Condensation from
such a packing is usually caught by a drip and taken to the hot well.
The Wilkinson idea is to groove passages along the shaft between
certain rings of the labyrinth packing and the diaphragm bushings,
as shown in Fig. 65. These grooves permit steam to pass to the



409



90



STEAM TURBINES



diaphragm bushing from a groove in the laybrinth packing, which
is at slightly higher pressure than that in the cell at either side of the
diaphragm bushing. Wet steam will, therefore, leak from the laby-
rinth packing to the diaphragm bushing and, as the pressure of the
steam in the cells is less than the pressure of that which enters the
bushing, this w r et steam will expand, thereby vaporizing a portion
of its moisture, and will then leak through into the cells of the turbine
and have a possible chance of doing some useful work in the lower
stages. The leakage from the labyrinth packing will be somewhat
augmented by this means, but the leakage from stage to stage will be
practically eliminated.




Fig. 65. Grooved Steam Passages and Labyrinth Packing of Wilkinson Turbine.

The Wilkinson turbine has a unique governing device, but is
essentially of the Rateau type in principle, differing only in detail.

Kerr Turbine. The turbine built and developed by the Kerr
Steam Turbine Company, of Wellsville, N. Y., is of the multi-stage
type and differs from the Rateau in three important particulars.
There are comparatively few stages; the vanes and buckets of the
rotating wheels are of the double cup-shape sort after the style of
buckets of the Pelton w r ater wheel; and the steam is directed onto the
wheel through nozzles instead of guide passages, striking the w r heel
tangentially in the plane of rotation instead of at the side. Otherwise
the difference between this turbine and others of the multi-stage
type is one of detail of construction.

The turbines are built in standard sizes with wheels 12, 18, 24,
and 36 inches in diameter, with one to eight stages, developing up to



410



STEAM TURBINES



600 H. P. The power developed in a single wheel of given diameter
will naturally depend upon the number of nozzles in action.

The buckets are made of drop forgings and are reamed out with
a special reamer, each cup being, in cross-section, a surface of revolu-
tion. Fig. 66 illustrates this type of bucket.

The wheel is a solid disk and
the buckets may be attached by
riveting, as shown in Fig. 66, or
by dove-tailing, as shown in the
same figure, the latter being the
preferred and more usual form
of construction. Each wheel is
carefully balanced and then as-
sembled on the shaft, as shown
in Fig. 67.

The nozzles are of machine
steel, screwed into a steel nozzle
body and this latter securely riv-
eted to the diaphragm casing.
Fig. 68 shows the nozzles, wheel, and diaphragm.

The shell is cast in sections, one for each stage. These are set
between two end castings, turned, and bored. Tongue-and-groove
joints on these castings insure correct alignment, and fibrous packing




Fig. 66. Buckets of Kerr Turbine.




Fig. 67. Kerr Turbine Wheels Assembled on Shaft.

in the grooves, in addition to metal contact of the surfaces, insures
steam tightness. A casing built in this manner possesses two dis-
tinct advantages over a solid casing. There is no probability of crack-
ing due to rapid temperature changes, and the size of turbine may be



411



92



STEAM TURBINES



increased by the addition of more units. The end castings carry
the weight of the turbine, and have supports bolted to the bed plate.
The shaft, where it passes through the diaphragm, is fitted to a
bronze bushing with a few thousandths of an inch clearance. This
bushing seats on the metal surface of the diaphragm with latitude
for slight side motion. It is kept to its seat by the steam pressure, but
can move sideways to accommodate any whipping motion of the
shaft. Fig. 69 shows the sectional view of the Kerr turbine.




Fig .68. Nozzles, Wheel, and Diaphragm of Kerr Turbine.

COMPOUND IMPULSE TURBINE WITH PRESSURE STAGES
AND VELOCITY STEPS

One of the simplest and most effective ways of compounding
turbines is by both pressure stages and velocity steps. The turbine
shell is divided by diaphragms into a number of different cells, seldom
more than five, except for marine work, where more are necessarily
employed. There being comparatively few stages, it will, in general,



412



STEAM TURBINES



93




413



94 STEAM TURBINES

be necessary to employ diverging nozzles so proportioned that the
steam may be completely expanded within the confines of these
nozzles, from the initial pressure to the pressure in the chamber into
which the stream is discharging. It will be remembered that multi-
stage turbines of the Rateau type do not require expanding nozzles
because of the relatively small drop in pressure from stage to stage.

Turbines of the type now being described differ from the
Rateau type in another most important particular. Each cell, or
chamber, of this type of turbine contains two or more sets of rotating
vanes, while turbines of the Rateau type have but one wheel and
one set of vanes in each chamber. The steam leaving the nozzles
impinges upon the first set of running vanes, and, as the steam
leaves these vanes, it flows into a set of guides of some sort, and is
returned to other vanes of the same set, or to a different set of
vanes on the same wheel, or to the vanes of a separate wheel, as
the case may be. The steam may pass through from one to three
sets of redirecting guides, and may impinge upon two to four sets
of rotating elements. It is immaterial, so far as the principle of
the action is concerned, whether the steam acts successively upon
a number of rotating wheels or whether it is returned again and
again into different vanes of the same wheel. If the latter form
of construction is adopted, the turbine will necessarily be more
compact and the rotating shaft will be shorter.

The more velocity steps the turbine has per stage, the less number
of stages will be necessary, but, in general, it is found more economical
to increase the pressure stages than to increase the velocity steps. It
must be remembered that in this type of turbine the steam is com-
pletely expanded within the nozzle, and that the temperature and
pressure of this expanded steam are the same within the confines of
any particular cell. As the steam passes through successive rotating
vanes, it gradually loses velocity and, consequently, the succeeding
passages must be made larger and larger, in order that the same
volume of steam may pass through at the lower velocity in a given
time. In other words, the passage area must increase in proportion to
the reduction in velocity. If this point is not clearly borne in mind
when looking at the vanes of a turbine of this type, one might think
that the increased size of passages were to provide for increased
expansion of the steam. In the Terry steam turbine, which is of this



414



STEAM TURBINES



95



type, a portion of the steam is allowed to escape as it passes through
successive buckets, so that the volume is gradually reduced as the
velocity decreases.

Curtis Turbine. Undoubtedly the best-known turbine of this
type is the Curtis, which has been developed by the General Electric
Co., and is being built at their works at Schenectady, N. Y., and



in in




fSoxz./*



flfo\s/r->r J3/oc/^Ji



:i

mMMS



:(((<(((<((((((((<(((




* * *




Fig. 70. Diagram of Nozzles and Buckets in Curtis Steam Turbine.

Lynn, Mass. Rights to build the Curtis turbine for marine propul-
sion are controlled by the Fore River Ship-Building Co., of Quincy,
Mass.

For ordinary purposes up to 9000 kilowatts, the Curtis turbine
does not have over four stages, with two velocity wheels per stage,
except in the larger sizes of turbine, when a fifth cell is provided,
which contains a single rotating wheel without redirecting guides,



415



96



STEAM TURBINES



for the purpose of abstracting any residual velocity there may be in
the steam after passing the fourth stage. The marine turbine of this
type installed in the U. S. Cruiser Salem had seven pressure stages




with four velocity steps in the first stage, and three in each of the others.

Fig. 70 shows in diagrammatic form the principle of the steam

action in the Curtis turbine. Steam passes from the steam chest



416



STEAM TURBINES



IT



through the nozzles, each of which may be closed by a valve operated
from the governor. The number open at any one time depends
upon the load. There are enough valves and nozzles to take a large
overload without the use of the by-pass which would admit high
pressure steam into the lower stages. Such a by-pass is, however,
usually provided. It works automatically and admits high-pressure
steam to a set'of auxiliary nozzles fitted into the second stage of the
turbine, thus increasing the power with but a slight sacrifice in steam
expansion and consequent economy.




Fig. 72.



100-K. W. Curtis Turbine and Direct-Current Generator Partly Assembled.
General Electric Co.



The nozzles are designed with a view to producing such a drop
in pressure from stage to stage that equal amounts of work are done
in each stage. This does not correspond to an equal drop in pressure
by any means, for there are more heat units in a given drop of pressure
in the lower than in the higher ranges. With many stages in a Curtis
turbine, the low-pressure diaphragms might be fitted with plain
cylindrical nozzles.

Steam enters the first row of rotating vanes* from the nozzles,



*The rotating vanes are called buckets by the General Electric
Company.



417




Fig. 73. Vertical Curtis Turbine of 9000-K. W. Capacity.



418




419



100 STEAM TURBINES



is deflected from these vanes to the guides or intermediates, as they are
technically called by the General Electric Company, and is re-
directed to the next row of moving vanes, and so on, passing from the
last row directly into the next set of nozzles, and again to the next
stage as before.

As has already been mentioned, the number of pressure stages
and velocity steps in the Curtis turbine vary with the size of the unit.
In sizes from 75 to 300 kilowatts, there are two stages and three
velocity steps, as shown in the diagram, Fig. 70. The 500-kilowatt*
to 3000-kilowatt sizes are four-stage with two velocity steps per stage,
while those of over 3000 kilowatts are five-stage with but a single
wheel in the fifth chamber and, of course, no redirecting buckets.
The turbines, in sizes up to 300 kilowatts, are generally of the hori-
zontal type; the larger sizes are vertical. Fig. 71 shows a 300-kilowatt
horizontal turbine, Fig. 72, a 100-kilowatt turbine, partly assembled,
and Fig. 73, a vertical turbine of 9000-kilowatt capacity. A sec-
tional view of the turbine as fitted in the U. S. S. Salem is shown in
Fig. 74.

It may be interesting to note the speeds of rotation of Curtis



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 27 of 30)