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STEAM TURBINES
A BOOK OF INSTRUCTION
FOR THE ADJUSTMENT AND OPERATION OF
THE PRINCIPAL TYPES OF THIS
CLASS OF PRIME MOVERS
COMPILED AND WRITTEN
BY
HUBERT E. COLLINS
FIRST EDITION
Second Impression
McGRAW-HILL BOOK COMPANY, Inc.
239 WEST 39TH STREET, NEW YORK
6 BOUVERIE STREET, LONDON, E. C.
Copyright, 1909, by the Hill Publishing Company
All rights reserved
TRANSCRIBER'S NOTES
The authors of this book used the spellings "aline," "gage," and "hight"
for the conventional spellings "align," "gauge," and "height." As they
are used consistently and do not affect the sense, they have been left
unchanged. Obvious typos and misspellings that did not affect the sense
have been silently corrected. The following substantive typographical
errors have also been corrected: "being" to "bearing" (p. 68); "FIG. 50"
to "FIG. 56" (p. 91), and "Fig. 2" to "Fig. 73" (p. 159). Two other
likely errors have been left as transcriber queries: lead/load on p. 142
and beating/heating on p. 177.
Superscript numbers are indicated with carets: B^1. Subscript numbers
are indicated with curly braces: P{1} for P-sub-1.
INTRODUCTION
This issue of the Power Handbook attempts to give a compact manual for
the engineer who feels the need of acquainting himself with steam
turbines. To accomplish this within the limits of space allowed, it has
been necessary to confine the work to the description of a few standard
types, prepared with the assistance of the builders. Following this the
practical experience of successful engineers, gathered from the columns
of _Power_, is given. It is hoped that the book will prove of value to
all engineers handling turbines, whether of the described types or not.
Hubert E. Collins.
New York, April, 1909.
CONTENTS
CHAP. PAGE
I The Curtis Steam Turbine in Practice 1
II Setting the Valves of the Curtis Turbine 31
III Allis-Chalmers Steam Turbine 41
IV Westinghouse-Parsons Turbine 58
V Proper Method of Testing a Steam Turbine 112
VI Testing a Steam Turbine 137
VII Auxiliaries for Steam Turbines 154
VIII Trouble with Steam Turbine Auxiliaries 172
I. THE CURTIS STEAM TURBINE IN PRACTICE[1]
[1] Contributed to _Power_ by Fred L. Johnson.
"Of the making of books there is no end." This seems especially true of
steam-turbine books, but the book which really appeals to the operating
engineer, the man who may have a turbine unloaded, set up, put in
operation, and the builders' representative out of reach before the man
who is to operate it fully realizes that he has a new type of prime
mover on his hands, with which he has little or no acquaintance, has not
been written. There has been much published, both descriptive and
theoretical, about the turbine, but so far as the writer knows, there is
nothing in print that tells the man on the job about the details of the
turbine in plain language, and how to handle these details when they
need handling. The operating engineer does not care why the moving
buckets are made of a certain curvature, but he does care about the
distance between the moving bucket and the stationary one, and he wants
to know how to measure that distance, how to alter the clearance, if
necessary, to prevent rubbing. He doesn't care anything about the area
of the step-bearing, but he does want to know the way to get at the
bearing to take it down and put it up again, etc.
The lack of literature along this line is the writer's apology for what
follows. The Curtis 1500-kilowatt steam turbine will be taken first and
treated "from the ground up."
On entering a turbine plant on the ground floor, the attention is at
once attracted by a multiplicity of pumps, accumulators and piping.
These are called "auxiliaries" and will be passed for the present to be
taken up later, for though of standard types their use is comparatively
new in power-plant practice, and the engineer will find that more
interruptions of service will come from the auxiliaries than from the
turbine itself.
Builders' Foundation Plans Incomplete
It is impractical for the manufacturers to make complete foundation
drawings, as they are not familiar with the lay-out of pipes and the
relative position of other apparatus in the station. All that the
manufacturers' drawing is intended to do is to show the customer where
it will be necessary for him to locate his foundation bolts and opening
for access to the step-bearing.
[Illustration: FIG. 1]
Fig. 1 shows the builders' foundation drawing, with the addition of
several horizontal and radial tubes introduced to give passage for the
various pipes which must go to the middle of the foundation. Entering
through the sides of the masonry they do not block the passage, which
must be as free as possible when any work is to be done on the
step-bearing, or lower guide-bearing. Entering the passage in the
foundation, a large screw is seen passing up through a circular block
of cast iron with a 3/4-inch pipe passing through it. This is the
step-supporting screw. It supports the lower half of the step-bearing,
which in turn supports the entire revolving part of the machine. It is
used to hold the wheels at a proper hight in the casing, and adjust the
clearance between the moving and stationary buckets. The large block
which with its threaded bronze bushing forms the nut for the screw is
called the cover-plate, and is held to the base of the machine by eight
1-1/2-inch cap-screws. On the upper side are two dowel-pins which enter
the lower step and keep it from turning. (See Figs. 2 and 3.)
[Illustration: FIG. 2]
[Illustration: FIG. 3]
The step-blocks are very common-looking chunks of cast iron, as will be
seen by reference to Fig. 4. The block with straight sides (the lower
one in the illustration) has the two dowel holes to match the pins
spoken of, with a hole through the center threaded for 3/4-inch pipe.
The step-lubricant is forced up through this hole and out between the
raised edges in a film, floating the rotating parts of the machine on a
frictionless disk of oil or water. The upper step-block has two
dowel-pins, also a key which fits into a slot across the bottom end of
the shaft.
[Illustration: FIG. 4]
The upper side of the top block is counterbored to fit the end of the
shaft. The counterbore centers the block. The dowel-pins steer the key
into the key-way across the end of the shaft, and the key compels the
block to turn with the shaft. There is also a threaded hole in the under
side of the top block. This is for the introduction of a screw which
is used to pull the top block off the end of the shaft. If taken off at
all it must be pulled, for the dowel-pins, key and counterbore are close
fits. Two long bolts with threads the whole length are used if it
becomes necessary to take down the step or other parts of the bottom of
the machine. Two of the bolts holding the cover-plate in place are
removed, these long bolts put in their places and the nuts screwed up
against the plate to hold it while the remaining bolts are removed.
How to Lower Step-Bearings to Examine Them
Now, suppose it is intended to take down the step-bearings for
examination. The first thing to do is to provide some way of holding the
shaft up in its place while we take its regular support from under it.
In some machines, inside the base, there is what is called a "jacking
ring." It is simply a loose collar on the shaft, which covers the holes
into which four plugs are screwed. These are taken out and in their
places are put four hexagonal-headed screws provided for the purpose,
which are screwed up. This brings the ring against a shoulder on the
shaft and then the cover-plate and step may be taken down.
While all the machines have the same general appearance, there are some
differences in detail which may be interesting. One difference is due to
the sub-base which is used with the oil-lubricated step-bearings. This
style of machine has the jacking ring spoken of, while others have
neither sub-base nor jacking ring, and when necessary to take down the
step a different arrangement is used.
[Illustration: FIG. 5]
A piece of iron that looks like a big horseshoe (Fig. 5) is used to hold
the shaft up. The flange that covers the entrance to the exhaust base is
taken off and a man goes in with the horseshoe-shaped shim and an
electric light. Other men take a long-handled wrench and turn up the
step-screw until the man inside the base can push the horseshoe shim
between the shoulder on the shaft and the guide-bearing casing. The men
on the wrench then back off and the horseshoe shim supports the weight
of the machine. When the shim is in place, or the jacking ring set up,
whichever the case may be, the cover-plate bolts may be taken out, the
nuts on the long screws holding the cover in place.
The 3/4-inch pipe which passes up through the step-screw is taken down
and, by means of the nuts on the long screws, the cover-plate is lowered
about 2 inches. Then through the hole in the step-screw a 3/4-inch rod
with threads on both ends is passed and screwed into the top step; then
the cover-plate is blocked so it cannot rise and, with a nut on the
lower end of the 3/4-inch rod, the top step is pulled down as far as it
will come. The cover-plate is let down by means of the two nuts, and the
top step-block follows. When it is lowered to a convenient hight it can
be examined, and the lower end of the shaft and guide-bearing will be
exposed to view.
[Illustration: FIG. 6]
The lower guide-bearing (Fig. 6) is simply a sleeve flanged at one end,
babbitted on the inside, and slightly tapered on the outside where it
fits into the base. The flange is held securely in the base by eight
3/4-inch cap-screws. Between the cap-screw holes are eight holes tapped
to 3/4-inch, and when it is desired to take the bearing down the
cap-screws are taken out of the base and screwed into the threaded holes
and used as jacks to force the guide-bearing downward. Some provision
should be made to prevent the bearing from coming down "on the run," for
being a taper fit it has only to be moved about one-half inch to be
free. Two bolts, about 8 inches long, screwed into the holes that the
cap-screws are taken from, answer nicely, as a drop that distance will
not do any harm, and the bearing can be lowered by hand, although it
weighs about 200 pounds.
The lower end of the shaft is covered by a removable bushing which is
easily inspected after the guide-bearing has been taken down. If it is
necessary to take off this bushing it is easily done by screwing four
5/8-inch bolts, each about 2 feet long, into the tapped holes in the
lower end of the bushing, and then pulling it off with a jack. (See Fig.
7.)
Each pipe that enters the passage in the foundation should be connected
by two unions, one as close to the machine as possible and the other
close to the foundation. This allows the taking down of all piping in
the passage completely and quickly without disturbing either threads or
lengths.
[Illustration: FIG. 7]
Studying the Blueprints
Fig. 8 shows an elevation and part-sectional view of a 1500-kilowatt
Curtis steam turbine. If one should go into the exhaust base of one of
these turbines, all that could be seen would be the under side of the
lower or fourth-stage wheel, with a few threaded holes for the
balancing plugs which are sometimes used. The internal arrangement is
clearly indicated by the illustration, Fig. 8. It will be noticed that
each of the four wheels has an upper and a lower row of buckets and that
there is a set of stationary buckets for each wheel between the two rows
of moving buckets. These stationary buckets are called intermediates,
and are counterparts of the moving buckets. Their sole office is to
redirect the steam which has passed through the upper buckets into the
lower ones at the proper angle.
[Illustration: FIG. 8. ELEVATION AND PART-SECTIONAL VIEW OF A
1500-KILOWATT CURTIS TURBINE]
The wheels are kept the proper distance apart by the length of hub, and
all are held together by the large nut on the shaft above the upper
wheel. Each wheel is in a separate chamber formed by the diaphragms
which rest on ledges on the inside of the wheel-case, their weight and
steam pressure on the upper side holding them firmly in place and making
a steam-tight joint where they rest. At the center, where the hubs pass
through them, there is provided a self-centering packing ring (Fig. 9),
which is free to move sidewise, but is prevented from turning, by
suitable lugs. This packing is a close running fit on the hubs of the
wheel and is provided with grooves (plainly shown in Fig. 9) which break
up and diminish the leakage of steam around each hub from one stage to
the next lower. Each diaphragm, with the exception of the top one,
carries the expanding nozzles for the wheel immediately below.
[Illustration: FIG. 9]
The expanding nozzles and moving buckets constantly increase in size and
number from the top toward the bottom. This is because the steam volume
increases progressively from the admission to the exhaust and the entire
expansion is carried out in the separate sets of nozzles, very much as
if it were one continuous nozzle; but with this difference, not all of
the energy is taken out of the steam in any one set of nozzles. The idea
is to keep the velocity of the steam in each stage as nearly constant as
possible. The nozzles in the diaphragms and the intermediates do not,
except in the lowest stage, take up the entire circumference, but are
proportioned to the progressive expansion of steam as it descends toward
the condenser.
Clearance
While the machine is running it is imperative that there be no rubbing
contact between the revolving and stationary parts, and this is provided
for by the clearance between the rows of moving buckets and the
intermediates. Into each stage of the machine a 2-inch pipe hole is
drilled and tapped. Sometimes this opening is made directly opposite a
row of moving buckets as in Fig. 10, and sometimes it is made opposite
the intermediate. When opposite a row of buckets, it will allow one to
see the amount of clearance between the buckets and the intermediates,
and between the buckets and the nozzles. When drilled opposite the
intermediates, the clearance is shown top and bottom between the buckets
and intermediates. (See Fig. 11.) This clearance is not the same in all
stages, but is greatest in the fourth stage and least in the first. The
clearances in each stage are nearly as follows: First stage, 0.060 to
0.080; second stage, 0.080 to 0.100; third stage, 0.080 to 0.100; fourth
stage, 0.080 to 0.200. These clearances are measured by what are called
clearance gages, which are simply taper slips of steel about 1/2-inch
wide accurately ground and graduated, like a jeweler's ring gage, by
marks about 1/2-inch apart; the difference in thickness of the gage is
one-thousandth of an inch from one mark to the next.
[Illustration: FIG. 10]
[Illustration: FIG. 11]
To determine whether the clearance is right, one of the 2-inch plugs is
taken out and some marking material, such as red lead or anything that
would be used on a surface plate or bearing to mark the high spots is
rubbed on the taper gage, and it is pushed into the gap between the
buckets and intermediates as far as it will go, and then pulled out, the
marking on the gage showing just how far in it went, and the nearest
mark giving in thousandths of an inch the clearance. This is noted, the
marking spread again, and the gage tried on the other side, the
difference on the gage showing whether the wheel is high or low.
Whichever may be the case the hight is corrected by the step-bearing
screw. The wheels should be placed as nearly in the middle of the
clearance space as possible. By some operators the clearance is adjusted
while running, in the following manner: With the machine running at full
speed the step-bearing screw is turned until the wheels are felt or
heard to rub lightly. The screw is marked and then turned in the
opposite direction until the wheel rubs again. Another mark is made on
the screw and it is then turned back midway between the two marks.
Either method is safe if practiced by a skilful engineer. In measuring
the clearance by the first method, the gage should be used with care, as
it is possible by using too much pressure to swing the buckets and get
readings which could be misleading. To an inexperienced man the taper
gages would seem preferable. In the hands of a man who knows what he is
doing and how to do it, a tapered pine stick will give as satisfactory
results as the most elaborate set of hardened and ground clearance
gages.
Referring back to Fig. 11, at A is shown one of the peep-holes opposite
the intermediate in the third stage wheel for the inspection of
clearance. The taper clearance gage is inserted through this hole both
above and below the intermediate, and the distance which it enters
registers the clearance on that side. This sketch also shows plainly how
the shrouding on the buckets and the intermediates extends beyond the
sharp edges of the buckets, protecting them from damage in case of
slight rubbing. In a very few cases wheels have been known to warp to
such an extent from causes that were not discovered until too late, that
adjustment would not stop the rubbing. In such cases the shrouding has
been turned or faced off by a cutting-off tool used through the
peep-hole.
Carbon Packing Used
Where the shaft passes through the upper head of the wheel-case some
provision must be made to prevent steam from the first stage escaping.
This is provided for by carbon packing (Fig. 12), which consists of
blocks of carbon in sets in a packing case bolted to the top head of the
wheel-case. There are three sets of these blocks, and each set is made
of two rings of three segments each. One ring of segments breaks joints
with its mate in the case, and each set is separated from the others by
a flange in the case in which it is held. In some cases the packing is
kept from turning by means of a link, one end of which is fastened to
the case and the other to the packing holder. Sometimes light springs
are used to hold the packing against the shaft and in some the pressure
of steam in the case does this. There is a pipe, also shown in Fig. 12,
leading from the main line to the packing case, the pressure in the pipe
being reduced. The space between the two upper sets of rings is drained
to the third stage by means of a three-way cock, which keeps the balance
between the atmosphere and packing-case pressure. The carbon rings are
fitted to the shaft with a slight clearance to start with, and very soon
get a smooth finish, which is not only practically steam-tight but
frictionless.
[Illustration: FIG. 12]
The carbon ring shown in Fig. 12 is the older design. The segments are
held against the flat bearing surface of the case by spiral springs set
in brass ferrules. The circle is held together by a bronze strap screwed
and drawn together at the ends by springs. Still other springs press
the straps against the surface upon which the carbon bears, cutting off
leaks through joints and across horizontal surfaces of the carbon. The
whole ring is prevented from turning by a connecting-rod which engages a
pin in the hole, like those provided for the springs.
[Illustration: FIG. 13]
[Illustration: FIG. 14]
[Illustration: FIG. 15]
[Illustration: FIG. 16]
The Safety-stop
There are several designs of safety-stop or speed-limit devices used
with these turbines, the simplest being of the ring type shown in Fig.
13. This consists of a flat ring placed around the shaft between the
turbine and generator. The ring-type emergencies are now all adjusted so
that they normally run concentric with the shaft, but weighted so that
the center of gravity is slightly displaced from the center. The
centrifugal strain due to this is balanced by helical springs. But when
the speed increases the centrifugal force moves the ring into an
eccentric position, when it strikes a trigger and releases a weight
which, falling, closes the throttle and shuts off the steam supply. The
basic principle upon which all these stops are designed is the same--the
centrifugal force of a weight balanced by a spring at normal speed.
Figs. 14, 15, and 16 show three other types.
The Mechanical Valve-Gear
Fig. 17 shows plainly the operation of the mechanical valve-gear. The
valves are located in the steam chests, which are bolted to the top of
the casing directly over the first sets of expansion nozzles. The
chests, two in number, are on opposite sides of the machine. The
valve-stems extend upward through ordinary stuffing-boxes, and are
attached to the notched cross-heads by means of a threaded end which is
prevented from screwing in or out by a compression nut on the lower end
of the cross-head. Each cross-head is actuated by a pair of
reciprocating pawls, or dogs (shown more plainly in the enlarged view,
Fig. 18), one of which opens the valve and the other closes it. The
several pairs of pawls are hung on a common shaft which receives a
rocking motion from a crank driven from a worm and worm-wheel by the
turbine shaft. The cross-heads have notches milled in the side in
which the pawls engage to open or close the valve, this engagement
being determined by what are called shield-plates, A (Fig. 18), which
are controlled by the governor. These plates are set, one a little
ahead of the other, to obtain successive opening or closing of the
valves. When more steam is required the shield plate allows the proper
pawl to fall into its notch in the cross-head and lift the valve from
its seat. If less steam is wanted the shield-plate rises and allows the
lower pawl to close the valve on the down stroke.
[Illustration: FIG. 17]
[Illustration: FIG. 18]
The valves, as can easily be seen, are very simple affairs, the steam
pressure in the steam chest holding the valve either open or shut until
it is moved by the pawl on the rock-shaft. The amount of travel on the
rock-shaft is fixed by the design, but the proportionate travel above
and below the horizontal is controlled by the length of the
connecting-rods from the crank to the rock-shaft. There are besides the
mechanical valve-gear the electric and hydraulic, but these will be left
for a future article.
The Governor
The speed of the machine is controlled by the automatic opening and
closing of the admission valves under the control of a governor (Fig.
19), of the spring-weighted type attached directly to the top end of the
turbine shaft. The action of the governor depends on the balance of
force exerted by the spring, and the centrifugal effort of the
rectangular-shaped weights at the lower end; the moving weights acting
through the knife-edge suspension tend to pull down the lever against
the resistance of the heavy helical spring. The governor is provided
with an auxiliary spring on the outside of the governor dome for varying
the speed while synchronizing. The tension of the auxiliary spring is
regulated by a small motor wired to the switchboard. This spring should
be used only to correct slight changes in speed. Any marked change
should be corrected by the use of the large hexagonal nut in the upper
plate of the governor frame. This nut is screwed down to increase the
speed, and upward to decrease it.
[Illustration: FIG. 19]
The Stage Valves
Fig. 20 represents one of the several designs of stage valve, sometimes
called the overload valve, the office of which is to prevent too high
pressure in the first stage in case of a sudden overload, and to
transfer a part of the steam to a special set of expanding nozzles over
the second-stage wheel. This valve is balanced by a spring of adjustable
tension, and is, or can be, set to open and close within a very small
predetermined range of first-stage pressure. The valve is _intended_ to
open and close instantly, and to supply or cut off steam from the second
stage, without affecting the speed regulation or economy of operation.
If any leaking occurs past the valve it is taken care of by a drip-pipe
to the third stage.
[Illustration: FIG. 20]
The steam which passes through the automatic stage valves and is
admitted to the extra set of nozzles above the second-stage wheel acts
upon this wheel just the same as the steam which passes through the
regular second-stage nozzles; i.e., all the steam which goes through the
machine tends to hasten its speed, or, more accurately, does work and
_maintains_ the speed of the machine.
II. SETTING THE VALVES OF THE CURTIS TURBINE[2]
[2] Contributed to _Power_ by F. L. Johnson.
Under some conditions of service the stage valve in the Curtis turbine
will not do what it is designed to do. It is usually attached to the
machine in such manner that it will operate with, or a little behind, in
the matter of time, the sixth valve. The machine is intended to carry
full load with only the first bank of five valves in operation, with
proper steam pressure and vacuum. If the steam pressure is under 150
pounds, or the vacuum is less than 28 inches, the sixth valve may
operate at or near full load, and also open the stage valve and allow
steam to pass to the second-stage nozzles at a much higher rate of speed
than the steam which has already done some work in the first-stage
wheel. The tendency is to accelerate unduly the speed of the machine.
This is corrected by the governor, but the correction is usually carried
too far and the machine slows down. With the stage valve in operation,
at a critical point the regulation is uncertain and irregular, and its
use has to be abandoned. The excess first-stage pressure will then be
taken care of by the relief valve, which is an ordinary spring safety
valve (not pop) which allows the steam to blow into the atmosphere.
The mechanical valve-gear does not often get out of order, but sometimes
the unexpected happens. The shop man may not have properly set up the
nuts on the valve-stems; or may have fitted the distance bushings
between the shield plates too closely; the superheat of the steam may
distort the steam chest slightly and produce friction that will
interfere with the regulation. If any of the valve-stems should become
loose in the cross-heads they may screw themselves either in or out. If
screwed out too far, the valve-stem becomes too long and the pawl in
descending will, after the valve is seated, continue downward until it
has broken something. If screwed in, the cross-head will be too low for
the upper pawl to engage and the valve will not be opened. This second
condition is not dangerous, but should be corrected. The valve-stems
should be made the right length, and all check-nuts set up firmly. If
for any purpose it becomes necessary to "set the valves" on a
1500-kilowatt mechanical gear, the operator should proceed in the
following manner.
Setting the Valves of a 1500-Kilowatt Curtis Turbine
We will consider what is known as the "mechanical" valve-gear, with two
sets of valves, one set of five valves being located on each side of the
machine.
[Illustration: FIG. 21]
In setting the valves we should first "throw out" all pawls to avoid
breakage in case the rods are not already of proper length, holding the
pawls out by slipping the ends of the pawl springs over the points of
the pawls, as seen in Fig. 21. Then turn the machine slowly by hand
until the pawls on one set of valves are at their highest point of
travel, then with the valves wide open adjust the drive-rods, i.e., the
rods extending from the crank to the rock-shaft, so that there is 1/32
of an inch clearance (shown dotted in Fig. 17, Chap. I) at the point of
opening of the pawls when they are "in." (See Fig. 22.) Then set up the
check-nuts on the drive-rod. Turn the machine slowly, until the pawls
are at their lowest point of travel. Then, with the valves closed,
adjust each _valve-stem_ to give 1/32 of an inch clearance at the point
of closing of the pawls when they are "in," securely locking the
check-nut as each valve is set. Repeat this operation on the other side
of the machine and we are ready to adjust the governor-rods. (Valves
cannot be set on both sides of the machine at the same time, as the
pawls will not be in the same relative position, due to the angularity
of the drive-rods.)
[Illustration: FIG. 22]
Next, with the turbine running, and the synchronizing spring in
mid-position, adjust the governor-rods so that the turbine will run at
the normal speed of 900 revolutions per minute when working on the fifth
valve, and carrying full load. The governor-rods for the other side of
the turbine (controlling valves Nos. 6 to 10) should be so adjusted that
the speed change between the fifth and sixth valves will not be more
than three or four revolutions per minute.
The valves of these turbines are all set during the shop test and the
rods trammed with an 8-inch tram. Governors are adjusted for a speed
range of 2 per cent. between no load and full load (1500 kilowatt), or 4
per cent. between the mean speeds of the first and tenth valves (no load
to full overload capacity).
The rods which connect the governor with the valve-gear have ordinary
brass ends or heads and are adjusted by right-and-left threads and
secured by lock-nuts. They are free fits on the pins which pass through
the heads, and no friction is likely to occur which will interfere with
the regulation, but too close work on the shield-plate bushings, or a
slight warping of the steam chest, will often produce friction which
will seriously impair the regulation. If it is noticed that the
shield-plate shaft has any tendency to oscillate in unison with the
rock-shaft which carries the pawls, it is a sure indication that the
shield-plates are not as free as they should be, and should be attended
to. The governor-rod should be disconnected, the pawls thrown out and
the pawl strings hooked over the ends.
The plates should then be rocked up and down by hand and the friction at
different points noted. The horizontal rod at the back of the valve-gear
may be loosened and the amount of end play of each individual
shield-plate noticed and compared with the bushings on the horizontal
rod at the back which binds the shield-plates together. If the plates
separately are found to be perfectly free they may be each one pushed
hard over to the right or left and wedged; then each bushing tried in
the space between the tail-pieces of the plates. It will probably be
found that the bushings are not of the right length, due to the
alteration of the form of the steam chest by heat. It will generally be
found also that the bushings are too short, and that the length can be
corrected by very thin washers of sheet metal. It has been found in some
instances that the thin bands coming with sectional pipe covering were
of the right thickness.
After the length of the bushings is corrected the shield-plates may be
assembled, made fast and tested by rocking them up and down, searching
for signs of sticking. If none occurs, the work has been correctly done,
and there will be no trouble from poor regulation due to friction of the
shield-plates.
The Baffler
The water which goes to the step-bearing passes through a baffler, the
latest type of which is shown by Fig. 23. It is a device for
restricting the flow of water or oil to the step- and guide-bearing. The
amount of water necessary to float the machine and lubricate the
guide-bearing having been determined by calculation and experiment, the
plug is set at that point which will give the desired flow. The plug is
a square-threaded worm, the length of which and the distance which it
enters the barrel of the baffler determining the amount of flow. The
greater the number of turns which the water must pass through in the
worm the less will flow against the step-pressure.
[Illustration: FIG. 23]
The engineers who have settled upon the flow and the pressure decided
that a flow of from 4-1/2 to 5-1/2 gallons per minute and a
step-pressure of from 425 to 450 pounds is correct. These factors are so
dependent upon each other and upon the conditions of the step-bearing
itself that they are sometimes difficult to realize in every-day work;
nor is it necessary. If the machine turns freely with a lower pressure
than that prescribed by the engineers, there is no reason for raising
this pressure; and there is only one way of doing it without reducing
the area of the step-bearing, and that is by obstructing the flow of
water in the step-bearing itself.
A very common method used is that of grinding. The machine is run at
about one-third speed and the step-water shut off for 15 or 20 seconds.
This causes grooves and ridges on the faces of the step-bearing blocks,
due to their grinding on each other, which obstruct the flow of water
between the faces and thus raises the pressure. It seems a brutal way of
getting a scientific result, if the result desired can be called
scientific. The grooving and cutting of the step-blocks will not do any
harm, and in fact they will aid in keeping the revolving parts of the
machine turning about its mechanical center.
The operating engineer will be very slow to see the utility of the
baffler, and when he learns, as he will sometime, that the turbine will
operate equally well with a plug out as with it in the baffler, he will
be inclined to remove the baffler. It is true that with one machine
operating on its own pump it is possible to run without the baffler,
and it is also possible that in some particular case two machines having
identical step-bearing pressures might be so operated. The baffler,
however, serves a very important function, as described more fully as
follows: It tends to steady the flow from the pump, to maintain a
constant oil film as the pressure varies with the load, and when several
machines are operating on the same step-bearing system it is the only
means which fixes the flow to the different machines and prevents one
machine from robbing the others. Therefore, even if an engineer felt
inclined to remove the baffler he would be most liable to regret taking
such a step.
If the water supply should fail from any cause and the step-bearing
blocks rub together, no great amount of damage will result. The machine
will stop if operated long under these conditions, for if steam pressure
is maintained the machine will continue in operation until the buckets
come into contact, and if the step-blocks are not welded together the
machine may be started as soon as the water is obtained. If vibration
occurs it will probably be due to the rough treatment of the
step-blocks, and may be cured by homeopathic repeat-doses of grinding,
say about 15 seconds each. If the step-blocks are welded a new pair
should be substituted and the damaged ones refaced.
Some few experimental steps of spherical form, called "saucer" steps,
have been installed with success (see Fig. 24). They seem to aid the
lower guide-bearing in keeping the machine rotating about the mechanical
center and reduce the wear on the guide-bearing. In some instances,
too, cast-iron bushings have been substituted for bronze, with marked
success. There seems to be much less wear between cast-iron and babbitt
metal than between bronze and babbitt metal. The matter is really worth
a thorough investigation.
[Illustration: FIG. 24]
III. ALLIS-CHALMERS COMPANY STEAM TURBINE
In Fig. 25 may be seen the interior construction of the steam turbine
built by Allis-Chalmers Co., of Milwaukee, Wis., which is, in general,
the same as the well-known Parsons type. This is a plan view showing the
rotor resting in position in the lower half of its casing.
[Illustration: FIG. 25]
Fig. 26 is a longitudinal cross-section cut of rotor and both lower and
upper casing. Referring to Fig. 26 the steam comes in from the
steam-pipe at C and passes through the main throttle or regulating valve
D, which is a balanced valve operated by the governor. Steam enters the
cylinder through the passage E.
Turning in the direction of the bearing A, it passes through alternate
stationary and revolving rows of blades, finally emerging at F and
going out by way of G to the condenser or to atmosphere. H, J, and K
represent three stages of blading. L, M, and Z are the balance pistons
which counterbalance the thrust on the stages H, J, and K. O and Q are
equalizing pipes, and for the low-pressure balance piston similar
provision is made by means of passages (not shown) through the body of
the spindle.
[Illustration: FIG. 26]
R indicates a small adjustable collar placed inside the housing of the
main bearing B to hold the spindle in a position where there will be
such a clearance between the rings of the balance pistons and those of
the cylinder as to reduce the leakage of steam to a minimum and at the
same time prevent actual contact under varying temperature.
At S and T are glands which provide a water seal against the inleakage
of air and the outleakage of steam. U represents the flexible coupling
to the generator. V is the overload or by-pass valve used for admitting
steam to intermediate stage of the turbine. W is the supplementary
cylinder to contain the low-pressure balance piston. X and Y are
reference letters used in text of this chapter to refer to equalizing of
steam pressure on the low-pressure stage of the turbine. The first point
to study in this construction is the arrangement of "dummies" L, M, and
Z. These dummy rings serve as baffles to prevent steam leakage past the
pistons, and their contact at high velocity means not only their own
destruction, but also damage to or the wrecking of surrounding parts. A
simple but effective method of eliminating this difficulty is found in
the arrangement illustrated in this figure. The two smaller balance
pistons, L and M, are allowed to remain on the high-pressure end; but
the largest piston, Z, is placed upon the low-pressure end of the rotor
immediately behind the last ring of blades, and working inside of the
supplementary cylinder W. Being backed up by the body of the spindle,
there is ample stiffness to prevent warping. This balance piston, which
may also be plainly seen in Fig. 25, receives its steam pressure from
the same point as the piston M, but the steam pressure, equalized with
that on the third stage of the blading, X, is through holes in the webs
of the blade-carrying rings. Entrance to these holes is through the
small annular opening in the rotor, visible in Fig. 25 between the
second and third barrels. As, in consequence of varying temperatures,
there is an appreciable difference in the endwise expansion of the
spindle and cylinder, the baffling rings in the low-pressure balance
piston are so made as to allow for this difference. The high-pressure
end of the spindle being held by the collar bearing, the difference in
expansion manifests itself at the low-pressure end. The labyrinth
packing of the high-pressure and intermediate pistons has a small axial
and large radial clearance, whereas the labyrinth packing of the piston
Z has, vice versa, a small radial and large axial clearance. Elimination
of causes of trouble with the low-pressure balance piston not only makes
it possible to reduce the diameter of the cylinder, and prevent
distortion, but enables the entire spindle to be run with sufficiently
small clearance to obviate any excessive leakage of steam.
Detail of Blade Construction
In this construction the blades are cut from drawn stock, so that at its
root it is of angular dovetail shape, while at its tip there is a
projection. To hold the roots of the blades firmly, a foundation ring is
provided, as shown at A in Fig. 27. This foundation ring is first formed
to a circle of the proper diameter, and then slots are cut in it. These
slots are accurately spaced and inclined to give the right pitch and
angle to the blades (Fig. 28), and are of dovetail shape to receive the
roots of the blades. The tips of the blades are substantially bound
together and protected by means of a channel-shaped shroud ring,
illustrated in Fig. 31 and at B in Fig. 27. Fig. 31 shows the cylinder
blading separate, and Fig. 27 shows both with the shrouding. In these,
holes are punched to receive the projections on the tips of the
blades, which are rivetted over pneumatically.
[Illustration: FIG. 27]
The foundation rings themselves are of dovetail shape in cross-section,
and, after receiving the roots of the blades, are inserted in dovetailed
grooves in the cylinder and rotor, where they are firmly held in place
by keypieces, as may be seen at C in Fig. 27. Each keypiece, when driven
in place, is upset into an undercut groove, indicated by D in Fig. 27,
thereby positively locking the whole structure together. Each separate
blade is firmly secured by the dovetail shape of the root, which is held
between the corresponding dovetailed slot in the foundation ring and the
undercut side of the groove.
[Illustration: FIG. 28]
Fig. 29, from a photograph of blading fitted in a turbine, illustrates
the construction, besides showing the uniform spacing and angles of the
blades.
[Illustration: FIG. 29]
The obviously thin flanges of the shroud rings are purposely made in
that way, so that, in case of accidental contact between revolving and
stationary parts, they will wear away enough to prevent the blades
from being ripped out. This protection, however, is such that to rip
them out a whole half ring of blades must be sheared off at the roots.
The strength of the blading, therefore, depends not upon the strength of
an individual blade, but upon the combined shearing strength of an
entire ring of blades.
[Illustration: FIG. 30]
The blading is made up and inserted in half rings, and Fig. 30 shows two
rings of different sizes ready to be put in place. Fig. 31 shows a
number of rows of blading inserted in the cylinder of an Allis-Chalmers
steam turbine, and Fig. 32 gives view of blading in the same turbine
after nearly three years' running.
[Illustration: FIG. 31]
[Illustration: FIG. 32]
The Governor
Next in importance to the difference in blading and balance piston
construction, is the governing mechanism used with these machines. This
follows the well-known Hartung type, which has been brought into
prominence, heretofore largely in connection with hydraulic turbines;
and the governor, driven directly from the turbine shaft by means of cut
gears working in an oil bath, is required to operate the small, balanced
oil relay-valve only, while the two steam valves, main and by-pass (or
overload), are controlled by an oil pressure of about 20 pounds per
square inch, acting upon a piston of suitable size. In view of the fact
that a turbine by-pass valve opens only when the unit is required to
develop overload, or the vacuum fails, a good feature of this governing
mechanism is that the valve referred to can be kept constantly in
motion, thereby preventing sticking in an emergency, even though it be
actually called into action only at long intervals. Another feature of
importance is that the oil supply to the bearings, as well as that to
the governor, can be interconnected so that the governor will
automatically shut off the steam if the oil supply fails and endangers
the bearings. This mechanism is also so proportioned that, while
responding quickly to variations in load, its sensitiveness is kept
within such bounds as to secure the best results in the parallel
operation of alternators. The governor can be adjusted for speed while
the turbine is in operation, thereby facilitating the synchronizing of
alternators and dividing the load as may be desired.
In order to provide for any possible accidental derangement of the main
governing mechanism, an entirely separate safety or over-speed governor
is furnished. This governor is driven directly by the turbine shaft
without the intervention of gearing, and is so arranged and adjusted
that, if the turbine should reach a predetermined speed above that for
which the main governor is set, the safety governor will come into
action and trip a valve which entirely shuts off the steam supply,
bringing the turbine to a stop.
Lubrication
Lubrication of the four bearings, which are of the self-adjusting, ball
and socket pattern, is effected by supplying an abundance of oil to the
middle of each bearing and allowing it to flow out at the ends. The oil
is passed through a tubular cooler, having water circulation, and pumped
back to the bearings. Fig. 33 shows the entire arrangement graphically
and much more clearly than can be explained in words. The oil is
circulated by a pump directly operated from the turbine, except where
the power-house is provided with a central oiling system. Particular
stress is laid by the builders upon the fact that it is not necessary to
supply the bearings with oil under pressure, but only at a head
sufficient to enable it to run to and through the bearings; this head
never exceeding a few feet. With each turbine is installed a separate
direct-acting steam pump for circulating oil for starting up. This will
be referred to again under the head of operating.
[Illustration: FIG. 33]
Generator
The turbo-generator, which constitutes the electrical end of this unit,
is totally enclosed to provide for noiseless operation, and forced
ventilation is secured by means of a small fan carried by the shaft on
each end of the rotor. The air is taken in at the ends of the generator,
passes through the fans and is discharged over the end connections of
the armature coils into the bottom of the machine, whence it passes
through the ventilating ducts of the core to an opening at the top. The
field core is, according to size, built up either of steel disks, each
in one piece, or of steel forgings, so as to give high magnetic
permeability and great strength. The coils are placed in radial slots,
thereby avoiding side pressure on the slot insulation and the complex
stresses resulting from centrifugal force, which, in these rotors, acts
normal to the flat surface of the strip windings.
Operation
As practically no adjustments are necessary when these units are in
operation, the greater part of the attention required by them is
involved in starting up and shutting down, which may be described in
detail as follows:
_To Start Up_
First, the auxiliary oil pump is set going, and this is speeded up until
the oil pressure shows a hight sufficient to lift the inlet valve and
oil is flowing steadily at the vents on all bearings. The oil pressure
then shows about 20 to 25 pounds on the "Relay Oil" gage, and 2 to 4
pounds on the "Bearing Oil" gage. Next the throttle is opened, without
admitting sufficient steam to the turbine to cause the spindle to turn,
and it is seen that the steam exhausts freely into the atmosphere, also
that the high-pressure end of the turbine expands freely in its guides.
Water having been allowed to blow out through the steam-chest drains,
the drains are closed and steam is permitted to continue flowing through
the turbine not less than a half an hour (unless the turbine is warm to
start with, when this period may be reduced) still without turning the
spindle. After this it is advisable to shut off steam and let the
turbine stand ten minutes, so as to warm thoroughly, during which time
the governor parts may be oiled and any air which may have accumulated
in the oil cylinder above the inlet valve blown off. Then the throttle
should be opened sufficiently to start the turbine spindle to revolving
very slowly and the machine allowed to run in this way for five
minutes.
Successive operations may be mentioned briefly as admitting water to the
oil cooler; bringing the turbine up to speed, at the same time slowing
down the auxiliary oil pump and watching that the oil pressures are kept
up by the rotary oil pump on the turbine; turning the water on to the
glands very gradually and, before putting on vacuum, making sure that
there is just enough water to seal these glands properly; and starting
the vacuum gradually just before putting on the load. These conditions
having been complied with, the operator next turns his attention to the
generator, putting on the field current, synchronizing carefully and
building up the load on the unit gradually.
The principal precautions to be observed are not to start without
warming up properly, to make sure that oil is flowing freely through the
bearings, that vacuum is not put on until the water glands seal, and to
avoid running on vacuum without load on the turbine.