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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 28 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 28 of 30)
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turbines of various sizes.

500 kilowatts approx. 1800 R. P. M.

1000 kilowatts approx. 1200 R. P. M.

2000 kilowatts approx. 900 R. P. M.

5000 kilowatts approx. 800 R. P. M.

9000 kilowatts approx. 750 R. P. M.

Marine Turbines approx. 250 R. P. M.

Casing. Fig. 75 shows the section of the casing of a four-stage
turbine. It is built of cast iron of four parts for sizes up to 3000 kilo-
watts, and six parts for 5000 kilowatts and larger sizes. This casing
holds the stationary reversing vanes and supports the diaphragm, the
details of which are clearly shown in Fig. 75. A represents the inlet
nozzle for the first stage, B the guides or intermediates, C the dia-
phragm separating the first and second stage, D the ledge on the
casing which supports the diaphragm, E the spider of the rotating
wheel, F the wheel plates, and G the distance piece at the outer rim
of the wheels.



*The General Electric Company builds a special 500-kilowatt, vertical
turbine having only two stages, with, consequently, three rows of revolving
vanes per stage.



420



STEAM TURBINES



101




Fig. 75. Section of Casing of Four-Stage, Curtis Vertical Turbine.



421



102



STEAM TURBINES



Nozzles. The nozzles are grouped together, not as in the Rateau
turbine, but in one single group for each stage, thus admitting a single
steam belt to a part of the wheel periphery only. The Rateau tur-
bine also admits steam
to only a portion of the
periphery but, in this tur-
bine, instead of one, there
are several groups of
nozzles arranged at equal
intervals around the pe-
riphery.

Fig. 76 shows a group of nozzles for the high-pressure end of
the turbine. The outlet is of rectangular section, slightly rounded
at the corners, which the makers claim gives better results with this
type of turbine than the elliptical outlet
used in the De Laval. The nozzles are
reamed out of bronze castings and
riveted to the casing.




Fig. 76. Group of Nozzles for High-Pressure End.





Fg.



Group of Buckets for Curtis Turbine.



Vanes. The vanes, usually called
buckets by the Curtis manufacturers,
are crescent-shaped in cross-section,
as is common for impulse turbines.
In the smaller sizes of turbines, the
buckets are cut by special machinery
directly on the periphery of the w r heel,
which is made of cast steel. In some

of the larger sizes, the buckets are cut on the circumference of a
ring, the inner diameter of which corresponds to the diameter of
the rotating wheel. This ring is then riveted onto the wheel disk.



Fig. 78. The Dovetailed Type of

Bucket in the General Electric

Curtis Turbine.



422



STEAM TURBINES



103



In the largest sizes the vanes are often milled from sections of cast
bronze, some two feet in length, containing fifteen to twenty-four
buckets each, Fig. 77, depending upon the size of the buckets,
these segments then being riveted to the wheel. A metal shroud




. .






Fig. 79.



Revolving Wheel for 2000-K. W., Curtis Steam Turbine.
General Electric Co.



ring covers the outer end of the buckets, and is securely riveted to
a projection on the bucket made for that purpose. Fig. 77 gives a
view of a section of such buckets ready to rivet to the wheel.

It is becoming a general practice with the General Electric Com-
pany to make their buckets separate, and fasten each in place on the
wheel or segment, by setting a dove-tailed piece at the lower end of the
bucket into a corresponding groove cut in the segment casting, as in



423



104 STEAM TURBINES




Fig. 78. These buckets are correctly spaced by means of small
distance pieces which are fitted alternately with the buckets.

Wheels. Fig. 79 shows the rotating wheels of a Curtis turbine,
with two sets of vanes so spaced that,as the wheel rotates, the one set of
vanes will be on each side of the guide vanes or intermediates. This is
equivalent to having two rotating wheels, but in construction is much
simpler. Each wheel, as shown, rotates in a chamber by itself. Fig.
79 and Fig. 80 show the old style of bucket segments which offer a
very rough surface. In the present style, Fig. 77, the segments have no
external webs and the riveting is flush. The wheels, except in the larger
sizes, are made of solid steel securely keyed to the shaft at the hub.
This hub varies in length according to the stage in which it revolves,

the low-pressure stages neces-
sarily being wider than the
high-pressure stages. The
wheel gradually tapers to-
ward its periphery, thus
maintaining a section of ap-
proximatelyuniform strength.

Fig. 80. Revolving Buckets for Curtis Steam "Wheels for the largest sizes of
Turbine. General Electric Co.

turbine are built up of a

cast-steel hub, or spider, to which are riveted steel disks, one on
each side, and between them is riveted a cast-steel distance ring.
The vanes are riveted in segments onto the outer edges of the
disks, as is clearly shown in Fig. 73 and Fig. 74.

A channel is turned in the wheel disk near the rim to receive
the bucket segments. By means of this channel, therefore, the
rivet, which passes through the wheel disk and the bucket seg-
ments, is relieved of a portion of the strain produced by the centrif-
ugal forces. Fig. 81 shows the rotor of the four-stage turbine, with
wheels assembled on the shaft. The separate sets of vanes, one on
each side of the wheel, and the channel in the rim may be clearly
seen.

Guides. The guide vanes, or intermediates, are necessarily in
one group of sufficient extent to catch the steam belt as it issues from
the rotating wheels. The group naturally extends at first over but
a small arc of the circumference, this arc increasing in extent in each
stage until both nozzles and intermediates entirely surround wheel and



424



STEAM TURBINES



105



casing. The guide vanes are in cross-section like the revolving vanes,
but are set in a reverse position. They are set with a certain angular
advance from the nozzles, depending upon the sneed of rotation, so




that the steam leaving the vanes will strike full upon them. Like
the vanes, they have to provide a successively increasing passage
area to allow for the lower velocity of the steam, if more than one set
of guides is used per stage.



425



106 STEAM TURBINES



Diaphragm. The style of diaphragm is best shown in Fig. 74
and Fig. 75, which also clearly show the construction of the large
wheels. The diaphragm is an iron casting steel in the higher stages,
because there the drop in pressure is greatest and consequently there
is a greater load on the diaphragm slightly dished in shape toward
the high-pressure side to give greater strength. It is provided with
bronze bushings where the shaft passes through, these bushings being
fitted with slight clearance to prevent leakage. The ends of the casing
are packed with carbon packing to prevent leakage of steam into the
air at the high-pressure end, and leakage of air into the turbine at the
low-pressure end, which leakage would have a detrimental effect upon
the vacuum.




Fig. 82. Bearing Surfaces in Step Bearing of Curtis Turbine.
General Electric Co.

Bearings. All sizes of the Curtis turbine of 500 kilowatts and
over are of the vertical type with but one working bearing, located
at the lower end of the shaft. This bearing consists of a short cylin-
drical block of cast iron fitted with two dowel-pins and a key, as
shown in Fig. 82, and a corresponding cast-iron block having a hole
through its center, into which is threaded a pipe for supplying some
form of lubricant, either water or oil.

Fig. 83 shows the bearing assembled on the lower end of the
shaft. The upper bearing with dowel-pins and key fit into corre-
sponding dowel holes and key-way in the bottom of the shaft, and
rotate with it. When the oil is supplied to the bearing, which is,
of course, under a high pressure, it fills the central circular space
between the blocks and forces them slightly apart. The oil then
escapes between the annular edges of these two blocks and is col-



426



STEAM TURBINES



107



lected into a drain and returned to the original supply. If water is
used for a lubricant, it is allowed to flow up into the base of the turbine
and mingle with the exhaust steam on its way to the condenser. The
pressures maintained by the lubricating pump in practice vary from
180 to 450 pounds per square inch. It is thus seen that the two
bearing blocks do not
come into actual contact,
but that the weight of the
turbine is supported upon
a film of lubricant.
Should the lubricating
pump fail in its supply,
no more serious damage
would occur than the
abrasion of the step bear-
ing, and a new one could
readily be inserted, as the
figure will show.

Riedler=Stumpf Tur=
bine. The turbine de-
veloped by Professors
Riedler and Stumpf for
the larger powers neces-
sitating lower speeds was
provided with two to four
pressure stages, with two
velocity steps per stage,
and this turbine expand-
ed the steam on the same
principle as the Curtis, but the details of construction and general
arrangement were entirely different. In the two-stage, and even in
the four-stage, turbine, the overhung type of wheel developed in the
single-stage turbine was adhered to, the generator being between
the two turbine \vheels with a bearing at each end. Fig. 84 shows
this arrangement in a 5000-kilowatt, two-stage turbine revolving at
750 R. P. M., with two velocity steps to each stage. In this particular
turbine, the steam is returned through U-shaped guide passages to a
second set of buckets on the same wheel, these buckets being larger




O/7 Suppfy.
Fig. 83. Step Bearing of Curtis Turbine.



427



108



STEAM TURBINES



than the first because of the lower velocity. Fig. 85 shows a 500-
kilowatt, four-stage turbine of the same type. These have been
built both vertical and horizontal, the vertical arrangement resembling
externally the Curtis turbine.




Fig. 84. Overhung Wheels of the Riedler-Stumpf Turbine.

Fig. 86 shows a vertical four-stage, two-step turbine of 750
R. P. M. developing the same power as that shown in Fig. 85.

The Riedler-Stumpf turbine was formerly manufactured by the
Allgemeinie Elcktricitats Gescllschaft of Berlin, but, as this company
is now licensed to manufacture under Curtis patents, it does not
appear that the manufacture of the former type is being actively
carried on at the present time.

Terry Turbine. In sizes developing over 300 H. P., the Terry
Steam Turbine Co. are compelled to resort to the two-stage type.



428





429




.Fig. 86. Vertical, Four-Stage, 500-K. W., Riedler-Stumpf Turbine.



430



STEAM TURBINES



111



At present, they are not building turbines of over 600 H. P. but, in
very large sizes, more pressure stages would be required. Their
compound-pressure turbines are not essentially different from the
single-stage in detail of construction. Fig. 87 shows a section of
this turbine.




Fig. 87. Section Through Two-Stage, Condensing,
Terry Steam Turbine.

REACTION TURBINES

In this type of turbine, the steam expansion takes place, not
alone in the nozzles and guide passages, as in all the types of turbine
previously described, but in the revolving vanes as well, approxi-
mately half the expansion taking place in each. In this type of tur-
bine, the difference in pressure on the two sides of the vanes brings
about a leakage of steam over the tips of both the rotating vanes and
guide vanes. That this leakage may not be unduly large, the drop
in pressure is made small from stage to stage. The leakage at the
high-pressure end gradually expanding does some work in the suc-
ceeding stages and becomes relatively small at the low-pressure end.
Clearances, however, in this type of turbine are all-important and,
other things being equal, that reaction turbine showing the best
economy will be the one with the smallest radial clearance. To make



431



112 STEAM TURBINES



the leakage in the high-pressure stages as small as possible, the vanes
should be made as long as convenient, about five per cent of the
diameter of the rotor being considered a minimum.

In turbines of the Rateau type, there is a diaphragm separating
the different stages. This extends close to the shaft, permitting
leakage only through the small annular space between shaft and
bushing, a comparatively unimportant matter, as these clearances
may be made very small. Besides this, carbon or labyrinth pack-
ing might be provided, which would make the leakage from stage to
stage almost negligible. Moreover, there is no tendency to leak
over the tips of the running vanes, because the pressure is the same
on both sides of them.

In the reaction type of turbine, however, there is no diaphragm
from stage to stage, and, instead of each rotating wheel being a
separate element, it is customary to fasten the vanes in rows to the
rotating drum or cylinder. In this style of machine, one set of guide
vanes and one set of revolving vanes constitute a stage, and it can
readily be seen that the opportunity for leakage between the tips of
the stationary vanes and the drum of the rotor is very much greater
than in the Rateau type, the annular space in the two types being
proportional to the diameters of shafting and drum. The tendency
to leak over the tips of the running vanes is even greater, because of
a greater diameter. However, friction losses, which are approxi-
mately proportional to the square of the steam velocity with reference
to the vanes, will be very much less in the reaction type of turbine
than in the impulse type, because the steam velocities are compara-
tively low.

Parsons Turbine. The Honorable Charles A. Parsons, of Eng-
land, is responsible for the successful development of the reaction
type of turbine. His first turbine, made in 1884, was of 10 H. P.,
18,000 R. P. M., and when running non-condensing with 92 Ibs. of
steam pressure by the gauge, it is claimed that but 25 Ibs. of steam per
brake horse-power per hour were consumed. In 1888, a 50-H. P.
turbine at 7000 R. P. M. was constructed, and soon after a 200-H. P.
at 4000 R. P. M. showed good economy. It must be remembered
that the chief problem of the turbine designer has been to reduce
rotative speeds without material sacrifice in economy.

Parsons turbines are manufactured in the United States by the



432







433



114 STEAM TURBINES



Westinghouse Machine Co., of Pittsburg, and by the Allis-Chalmers
Co., of Milwaukee, Wis., the former acquiring tne right to manu-
facture in 1885, and putting the first turbine on the market three
years later. For marine purposes, licenses to build Parsons turbines
have been issued to several firms.

The essential features of the Parsons turbine are clearly illus-
trated by the cross-section shown in Fig. 88. Steam enters at E and,
in passing through the annular space between the cylinder walls and
rotating elements, gradually expands in volume until it exhausts at
G. The rotor is usually built in three different diameters to facilitate
mechanical construction and to avoid excessively small and exces-
sively large vanes. It is thus possible to use a large number of vanes
of the same size. When the length of vane would otherwise become
too great, the same passage area may be provided by shortening the
vane and increasing the diameter of the drum. While, theoretically,
the passage area of the vanes should gradually increase, it is found in
practice that without any detrimental effect in economy several rows
of vanes may be made of the same height, and thus the areas will
increase step by step instead of in a gradual curve.

Since the pressure of steam is greater on the steam side of the
vanes than on the exhaust side, there will result an end thrust which
must in some way be balanced. This thrust, due to the static pres-
sure of the steam, is augmented by the thrust on the vanes due to the
impact and reaction of the steam in passing through them. To
balance these thrusts, the balancing pistons shown at L, M, and N,
Fig. 88, are provided. These are connected to the steam and exhaust
spaces by the passages 0, P, and Q, so that the pressure can be read-
ily balanced. It is, of course, necessary to provide some sort of
thrust block to meet the requirements of varying conditions, but it
need not be large, and it serves in general as an adjustment bearing
to keep the rotor in correct alignment, and is so arranged that the
longitudinal position of the rotor may be slightly changed if desired.
A mark on this block shows when the rotor is in its correct position.

Casing. The casing is made with diameters to accommodate the
various sizes of drum and blading on the rotor. On the inside of
the casing are the rings of fixed vanes or guides which fit between the
rings of rotating vanes on the drum. The casing is divided hori-
zontally, as shown in Fig. 89, so that by lifting the cover all working



434



116



STEAM TURBINES



O 4 : ../ 9



parts are exposed. In very large turbines the cover slides over four
graduated guides, one at each corner, so that in lifting it the engineer
can readily see that it is kept always horizontal while being moved,
without danger of binding or injuring the blading.

Vanes. The vanes
or blades, as they are
most often called, are
formed of special cold-
drawn bronze or steel
drawn out into strips
and cut into proper
lengths. Annular
rings are turned on the
outer side of the rotor
and the inside of the
casing, and in the
grooves thus formed
the blades are fitted
and properly spaced
by means of distance
pieces, all carefully
caulked in place. The
entrance and exit
angles of the vanes of
the turbine not being
equal, it would be an
exceedingly difficult
matter to properly set
the blades were it not
for these distance

pieces which insure not only the correct spacing but that the blade
shall be set at the correct angle. To stiffen the outer ends of the
blades and to maintain a uniform spacing, a wire lacing is threaded
through openings near their outer edge, tw r isted between the adja-
cent blades, and soldered into position.

The Westinghouse Machine Co. use a cold-drawn wire lacing, or
lashing of comma-shaped cross-section. This is threaded through
similarly-shaped holes near the tips of the blades, and then the tail





Fig. 90. Blade- Lashing of Westinghouse-
Parsons Turbine.



436




437




438



STEAM TURBINES



119



of the comma is bent over between the blades, holding them firmly
together, as shown in Fig. 90.

Rotor. The rotor, as before noted, consists of a drum of three
diameters. This drum is usuallyjbuilt up of a hollow, steel casting
fixed to some form of spider. The solid rotor would be prohibitively
heavy. Fig. 91 shows the Westinghouse-Parsons rotor with balance
pistons. Fig. 92 shows this turbine in cross-section.



(o(|e e|)o)






Fig. 93. Bearings of Westinghouse-Parsons Turbine.



Bearing. The rotor being long and heavy, some sort of flexible
or self-aligning bearing is desirable, if not absolutely necessary.
Flexibility is provided in the DeLaval turbine by means of the long,
slender shaft, and in earlier forms of Parsons turbine by a nest of
loosely-fitting concentric bronze bushings, with sufficient clearance to
permit a continuous oil film between each two bushings. This was
intended to form a cushion, permitting a certain amount of vibra-
tion in the shaft, yet restraining it within very narrow limits.




440



STEAM TURBINES 121



It has been found by experience that this so-called flexible
bearing is not necessary at the rotative speeds employed in Parsons
turbines, and the Westinghouse Machine Co. have now adopted the
style of bearing shown in Fig. 93. Turbines with short rotating
shafts would not require a special form of bearing.

Allis=ChaImers Turbine. The turbine built by the Allis-Chalmers
Co. of Milwaukee, is of the Parsons type with a few special modifi-
cations. It is claimed by its manufacturers that, in the large turbines,
trouble is experienced with the largest or low-pressure balancing
pistons. These being of comparatively large diameter, it is found
difficult to so construct them that they will not be unduly distorted
under pressure and repeated heatings and coolings. Distortion, of
course; renders it impossible to run the packing rings on this piston
with a sufficiently small clearance to prevent excessive leakage of
steam. To overcome this difficulty, they have adopted the device
shown in Fig. 94, which differs from the ordinary Parsons type shown
in Fig. 88, by having the low-pressure balance piston placed at the
low-pressure end of the turbine instead of at the high-pressure end,
and making it of considerable less diameter. The ordinary Parsons
piston has an effective area for balancing pressure equal to the dif-
ference between its area and the area of the next smaller piston; but,
by putting this piston at the other end of the turbine, the whole
circular area can be utilized and the same area may therefore be
obtained with a much smaller diameter. This balancing piston is
shown in Z, Fig. 94. It works inside of a supplementary cylinder W.
In this construction, the equalizing pipe P, shown in Fig. 88, is
omitted. The pressure on the balance piston Z is obtained by
passages permitting communication between the steam spaces X and
Y, but which are not shown on the figure. The balancing piston Z
is not only much smaller than the piston in the ordinary type, but
is stiffer, being backed up by the main body of the spindle.

In all reaction turbines, expansion of the casing is a trouble-
some feature; as this is due to the varying temperatures, there is an
appreciable difference in the endwise expansion of the spindle and the
casing. The high-pressure end of the spindle is held by a collar
bearing, and the difference in expansion is taken up at the low-
pressure end. The labyrinth packing employed at the high-pressure
end has small axial and much radial clearance, while the labyrinth



441



122



STEAM TURBINES



packing of the balance piston at the low-pressure end may have small
radial but must have large axial clearance to provide for the difference
in expansion. The Allis-Chalmers Company claim that this type
of construction permits smaller working clearances in high-pressure
and intermediate pistons.




Spindle.




Cy/irrder.



Fig. 95. Method of Fastening Blades of
Allis-Chalmers Turbine.

Bidding. The general shape of vanes of the Allis-Chalmers
turbine does not differ from that of other reaction turbines, but the
method of securing the blade to the casing and to the rotor is different
from that adopted in the ordinary Parsons type. Each blade is so
formed that, at its root, it is of an angular, dove-tail shape and has a
small projection at its tip. To hold the roots of the blade firmly,







500 Kw. CURTIS STEAM TURBINE.
Direct Connected to Continuous Current Generator.



STEAM TURBINES 123



there is a foundation ring A, Fig. 95, which, after being formed to a
circle of the proper diameter, has slots cut in it by a special milling
machine, these slots being so shaped as to receive the roots of the
blade. They are at the same time accurately spaced, and so cut as
to give the required angle to the blades. To protect the tips of the
blades and to bind them together in a substantial manner, a channel-
shaped shroud ring B, Fig. 95, is fitted. The small projections on
the tips of the blade fit through holes cut in this shroud ring and are
riveted over. These rings, A and B, cover half the circumference and
the blades are assembled on these rings, as shown in Fig. 96, before
being put onto the rotor or casing. These foundation rings are of
dove-tail shape in cross-section and are inserted into corresponding
grooves in the turbine casing and spindle, in which they are firmly
held by key pieces. These key pieces are driven into place and upset,
so as to fill a small undercut, shown at C in Fig. 95, thus securely
locking them into place. This construction is applied to all blades
of whatever length.

The flanges of the channel-shaped, shroud rings are made thin,
so that in case of contact with the casing from any accidental cause,
no dangerous results are likely to follow, the accidental touch merely
causing a slight wearing away of the flanges of the shroud without
excessive heating. It is claimed that this shape of shroud ring: acts

r t>

in a measure like a labyrinth packing, retarding appreciably the leak-
age of steam. Fig. 97 shows a number of sets of blades as assembled.

GOVERNING



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 28 of 30)