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Title: Steam Turbines - A Book of Instruction for the Adjustment and Operation of - the Principal Types of this Class of Prime Movers
Author: Collins, Hubert E. (Hubert Edwin), 1872-1932
Language: English
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*** Start of this LibraryBlog Digital Book "Steam Turbines - A Book of Instruction for the Adjustment and Operation of - the Principal Types of this Class of Prime Movers" ***
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.
In Operation
In operation all that is necessary is to watch the steam pressure at the
"Throttle" and "Inlet" gages, to see that neither this pressure nor the
steam temperature varies much; to keep the vacuum constant, as well as
pressures on the water glands and those indicated by the "Relay Oil" and
"Bearing Oil" gages; to take care that the temperatures of the oil
flowing to and from the bearings does not exceed 135 degrees Fahr. (at
which temperature the hand can comfortably grasp the copper oil-return
pipes); to see that oil flows freely at all vents on the bearings, and
that the governor parts are periodically oiled. So far as the generator
is concerned, it is only essential to follow the practice common in all
electric power plant operation, which need not be reviewed here.
_Stopping the turbine_ is practically the reverse of starting, the
successive steps being as follows: starting the auxiliary oil pump,
freeing it of water and allowing it to run slowly; removing the load
gradually; breaking the vacuum when the load is almost zero, shutting
off the condenser injection and taking care that the steam exhausts
freely into the atmosphere; shutting off the gland water when the load
and vacuum are off; pulling the automatic stop to trip the valve and
shut off steam and, as the speed of the turbine decreases, speeding up
the auxiliary oil pump to maintain pressure on the bearings; then, when
the turbine has stopped, shutting down the auxiliary oil pump, turning
off the cooling water, opening the steam chest drains and slightly
oiling the oil inlet valve-stem. During these operations the chief
particulars to be heeded are: not to shut off the steam before starting
the auxiliary oil pump nor before the vacuum is broken, and not to shut
off the gland water with vacuum on the turbine. The automatic stop
should also remain unhooked until the turbine is about to be started up
again.
General
Water used in the glands of the turbine must be free from scale-forming
impurities and should be delivered at the turbine under a steady
pressure of not less than 15 pounds. The pressure in the glands will
vary from 4 to 10 pounds. This water may be warm. In the use of water
for the cooling coils and of oil for the lubricating system, nothing
more is required than ordinary good sense dictates. An absolutely pure
mineral oil must be supplied, of a non-foaming character, and it should
be kept free through filtering from any impurities.
The above refers particularly to Allis-Chalmers turbines of the type
ordinarily used for power service. For turbines built to be run
non-condensing, the part relating to vacuum does not, of course, apply.
IV. WESTINGHOUSE-PARSONS STEAM TURBINE
While the steam turbine is simple in design and construction and does
not require constant tinkering and adjustment of valve gears or taking
up of wear in the running parts, it is like any other piece of fine
machinery in that it should receive intelligent and careful attention
from the operator by inspection of the working parts that are not at all
times in plain view. Any piece of machinery, no matter how simple and
durable, if neglected or abused will in time come to grief, and the
higher the class of the machine the more is this true.
Any engineer who is capable of running and intelligently taking care of
a reciprocating engine can run and take care of a turbine, but if he is
to be anything more than a starter and stopper, it is necessary that he
should know what is inside of the casing, what must be done and avoided
to prevent derangement, and to keep the machine in continued and
efficient operation.
In the steam turbine the steam instead of being expanded against a
piston is made to expand against and to get up velocity in itself. The
jet of steam is then made to impinge against vanes or to react against
the moving orifice from which it issues, in either of which cases its
velocity and energy are more or less completely abstracted and
appropriated by the revolving member. The Parsons turbine utilizes a
combination of these two methods.
[Illustration: FIG. 34]
Fig. 34 is a sectional view of the standard Westinghouse-Parsons
single-flow turbine. A photograph of the rotor R R R is reproduced in
Fig. 35, while in Fig. 36 a section of the blading is shown upon a
larger scale. Between the rows of the blading upon the rotor extend
similar rows of stationary blades attached to the casing or stator. The
steam entering at A (Fig. 34), fills the circular space surrounding the
rotor and passes first through a row of stationary blades, 1 (Fig. 37),
expanding from the initial pressure P to the slightly lower pressure
P{1}, and attaining by that expansion a velocity with which it is
directed upon the moving blade 2. In passing through this row of blades
it is further expanded from pressure P{1} to P{2} and helps to push the
moving blades along by the reaction of the force with which it issues
therefrom. Impinging upon the second row of stationary blades 3, the
direction of flow is diverted so as to make it impinge at a favorable
angle upon the second row of revolving blades 4, and the action is
continued until the steam is expanded to the pressure of the condenser
or of the medium into which the turbine finally exhausts. As the
expansion proceeds, the passages are made larger by increasing the
length of the blades and the diameter of the drums upon which they are
carried in order to accommodate the increasing volume.
[Illustration: FIG. 35]
[Illustration: FIG. 36]
[Illustration: FIG. 37]
It is not necessary that the blades shall run close together, and the
axial clearance, that is the space lengthwise of the turbine between the
revolving and the stationary blades, varies from 1/8 to 1/2 inch; but in
order that there may not be excessive leakage over the tops of the
blades, as shown, very much exaggerated, in Fig. 38, the radial
clearance, that is, the clearance between the tops of the moving blades
and the casing, and between the ends of the stationary blades and the
shell of the rotor, must be kept down to the lowest practical amount,
and varies, according to the size of the machine and length of blade,
from about 0.025 to 0.125 of an inch.
[Illustration: FIG. 38]
In the passage A (Fig. 34) exists the initial pressure; in the passage B
the pressure after the steam has passed the first section or diameter of
the rotor; in the passage C after it has passed the second section. The
pressure acting upon the exposed faces of the rows of vanes would crowd
the rotor to the left. They are therefore balanced by pistons or
"dummies" P P P revolving with the shaft and exposing in the annular
spaces B^1 and C^1 the same areas as those of the blade sections which
they are designed to balance. The same pressure is maintained in B^1 as
in B, and in C^1 as in C by connecting them with equalizing pipes E E.
The third equalizing pipe connects the back or right-hand side of the
largest dummy with the exhaust passage so that the same pressure exists
upon it as exists upon the exhaust end of the rotor. These dummy pistons
are shown at the near end of the rotor in Fig. 35. They are grooved so
as to form a labyrinth packing, the face of the casing against which
they run being grooved and brass strips inserted, as shown in Fig. 39.
The dummy pistons prevent leakage from A, B^1 and C^1 to the condenser,
and must, of course, run as closely as practicable to the rings in the
casing, the actual clearance being from about 0.005 to 0.015 of an inch,
again depending on the size of the machine.
[Illustration: FIG. 39]
The axial adjustment is controlled by the device shown at T in Fig. 34
and on a larger scale in Fig. 40. The thrust bearing consists of two
parts, T{1} T{2}. Each consists of a cast-iron body in which are placed
brass collars. These collars fit into grooves C, turned in the shaft as
shown. The halves of the block are brought into position by means of
screws S{1} S{2} acting on levers L{1} L{2} and mounted in the bearing
pedestal and cover. The screws are provided with graduated heads which
permit the respective halves of the thrust bearing to be set within one
one-thousandth of an inch.
[Illustration: FIG. 40]
The upper screw S{2} is set so that when the rotor exerts a light
pressure against it through the thrust block and lever the grooves in
the balance pistons are just unable to come in contact with the dummy
strips in the cylinder. The lower screw S{1} is then adjusted to permit
about 0.008 to 0.010 of an inch freedom for the collar between the
grooves of the thrust bearing.
These bearings are carefully adjusted before the machine leaves the
shop, and to prevent either accidental or unauthorized changes of their
adjustment the adjusting screw heads are locked by the method shown in
Fig. 40. The screw cannot be revolved without sliding back the latch
L{3}. To do this the pin P{4} must be withdrawn, for which purpose the
bearing cover must be removed.
In general this adjustment should not be changed except when there has
been some wear of the collars in the thrust bearing; nevertheless, it is
a wise precaution to go over the adjustment at intervals. The method of
doing this is as follows: The machine should have been in operation for
some time so as to be well and evenly heated and should be run at a
reduced speed, say 10 per cent. of the normal, during the actual
operation of making the adjustment. Adjust the upper screw which, if
tightened, would push the spindle away from the thrust bearing toward
the exhaust. Find a position for this so that when the other screw is
tightened the balance pistons can just be heard to touch, and so the
least change of position inward of the upper screw will cause the
contact to cease. To hear if the balance pistons are touching, a short
piece of hardwood should be placed against the cylinder casing near the
balance piston. If the ear is applied to the other end of the piece of
wood the contact of the balance pistons can be very easily detected. The
lower screw should then be loosened and the upper screw advanced from
five to fifteen one-thousandths, according to the machine, at which
position the latter may be considered to be set. The lower screw should
then be advanced until the under half of the thrust bearing pushes the
rotor against the other half of the thrust bearing, and from this
position it should be pushed back ten or more one-thousandths, to give
freedom for the rotor between the thrusts, and locked. A certain amount
of care should be exercised in setting the dummies, to avoid straining
the parts and thus obtain a false setting.
The object in view is to have the grooves of the balance pistons running
as close as possible to the collars in the cylinder, but without danger
of their coming in actual contact, and to allow as little freedom as
possible in the thrust bearing itself, but enough to be sure that it
will not heat. The turbine rotor itself has scarcely any end thrust, so
that all the thrust bearing has to do is to maintain the
above-prescribed adjustment.
The blades are so gaged that at all loads the rotor has a very light but
positive thrust toward the running face of the dummy strips, thus
maintaining the proper clearance at the dummies as determined by the
setting of the proper screw adjustment.
Main Bearings
The bearings which support the rotor are shown at F F in Fig. 34 and in
detail in Fig. 41. The bearing proper consists of a brass tube B with
proper oil grooves. It has a dowel arm L which fits into a corresponding
recess in the bearing cover and which prevents the bearing from turning.
On this tube are three concentric tubes, C D E, each fitting over the
other with some clearance so that the shaft is free to move slightly in
any direction. These tubes are held in place by the nut F, and this nut,
in turn, is held by the small set-screw G. The bearing with the
surrounding tubes is placed inside of the cast-iron shell A, which rests
in the bearing pedestal on the block and liner H. The packing ring M
prevents the leakage of oil past the bearing. Oil enters the chamber at
one end of the bearing at the top and passes through the oil grooves,
lubricating the journal, and then out into the reservoir under the
bearing. The oil also fills the clearance between the tubes and forms a
cushion, which dampens any tendency to vibration.
[Illustration: FIG. 41]
The bearings, being supported by the blocks or "pads" H, are
self-alining. Under these pads are liners 5, 10, 20, and 50 thousandths
in thickness. By means of these liners the rotor may be set in its
proper running position relative to the stator. This operation is quite
simple. Remove the liners from under one bearing pad and place them
under the opposite pad until a blade touch is obtained by turning the
rotor over by hand. After a touch has been obtained on the top, bottom,
and both sides, the total radial blade clearance will be known to equal
the thickness of the liners transferred. The position of the rotor is
then so adjusted that the radial blade clearance is equalized when the
turbine is at operating temperature.
On turbines running at 1800 revolutions per minute or under, a split
babbitted bearing is used, as shown in Figs. 42a and 42b. These bearings
are self-alining and have the same liner adjustment as the
concentric-sleeve bearings just described. Oil is supplied through a
hole D in the lower liner pad, and is carried to the oil groove F
through the tubes E E. The oil flows from the middle of this bearing to
both ends instead of from one end to the other, as in the other type.
[Illustration: FIG. 42A]
[Illustration: FIG. 42B]
Packing Glands
Where the shaft passes through the casing at either end it issues from a
chamber in which there exists a vacuum. It is necessary to pack the
shaft at these points, therefore, against the atmospheric pressure, and
this is done by means of a water-gland packing W W (Fig. 34). Upon the
shaft in Fig. 35, just in front of the dummy pistons, will be seen a
runner of this packing gland, which runner is shown upon a larger scale
and from a different direction in Fig. 43. To get into the casing the
air would have to enter the guard at A (Fig. 44), pass over the
projecting rings B, the function of which is to throw off any water
which may be creeping along the shaft by centrifugal force into the
surrounding space C, whence it escapes by the drip pipe D, hence over
the five rings of the labyrinth packing E and thence over the top of the
revolving blade wheel, it being apparent from Fig. 43 that there is no
way for the air to pass by without going up over the top of the blades;
but water is admitted to the centrally grooved space through the pipe
shown, and is revolved with the wheel at such velocity that the pressure
due to centrifugal force exceeds that of the atmosphere, so that it is
impossible for the air to force the water aside and leak in over the
tips of the blades, while the action of the runner in throwing the water
out would relieve the pressure at the shafts and avoid the tendency of
the water to leak outward through the labyrinth packing either into
the vacuum or the atmosphere.
[Illustration: FIG. 43]
[Illustration: FIG. 44]
The water should come to the glands under a head of about 10 feet, or a
pressure of about 5 pounds, and be connected in such a way that this
pressure may be uninterruptedly maintained. Its temperature must be
lower than the temperature due to the vacuum within the turbine, or it
will evaporate readily and find its way into the turbine in the form of
steam.
[Illustration: FIG. 45]
In any case a small amount of the steaming water will pass by the gland
collars into the turbine, so that if the condensed steam is to be
returned to the boilers the water used in the glands must be of such
character that it may be safely used for feed water. But whether the
water so used is to be returned to the boilers or not it should never
contain an excessive amount of lime or solid matter, as a certain amount
of evaporation is continually going on in the glands which will result
in the deposit of scale and require frequent taking apart for cleaning.
[Illustration: FIG. 46]
When there is an ample supply of good, clean water the glands may be
packed as in Fig. 45, the standpipe supplying the necessary head and the
supply valve being opened sufficiently to maintain a small stream at the
overflow. When water is expensive and the overflow must be avoided, a
small float may be used as in Fig. 46, the ordinary tank used by
plumbers for closets, etc., serving the purpose admirably.
When the same water that is supplied to the glands is used for the
oil-cooling coils, which will be described in detail later, the coils
may be attached to either of the above arrangements as shown in Fig. 47.
[Illustration: FIG. 47]
When the only available supply of pure water is that for the boiler
feed, and the condensed steam is pumped directly back to the boiler, as
shown in Fig. 48, the delivery from the condensed-water pumps may be
carried to an elevation 10 feet above the axis of the glands, where a
tank should be provided of sufficient capacity that the water may have
time to cool considerably before being used. In most of these cases, if
so desired, the oil-cooling water may come from the circulating pumps of
the condenser, provided there is sufficient pressure to produce
circulation, as is also shown in Fig. 48.
[Illustration: FIG. 48]
When the turbine is required to exhaust against a back pressure of one
or two pounds a slightly different arrangement of piping must be made.
The water in this case must be allowed to circulate through the glands
in order to keep the temperature below 212 degrees Fahrenheit. If this
is not done the water in the glands will absorb heat from the main
castings of the machine and will evaporate. This evaporation will make
the glands appear as though they were leaking badly. In reality it is
nothing more than the water in the glands boiling, but it is
nevertheless equally objectionable. This may be overcome by the
arrangement shown in Fig. 49, where two connections and valves are
furnished at M and N, which drain away to any suitable tank or sewer.
These valves are open just enough to keep sufficient circulation so that
there is no evaporation going on, which is evidenced by steam coming out
as though the glands were leaking. These circulating valves may be used
with any of the arrangements above described.
[Illustration: FIG. 49]
The Governor
On the right-hand end of the main shaft in Fig. 34 there will be seen a
worm gear driving the governor. This is shown on a larger scale at A
(Fig. 50). At the left of the worm gear is a bevel gear driving the
spindle D of the governor, and at the right an eccentric which gives a
vibratory motion to the lever F. The crank C upon the end of the shaft
operates the oil pump. The speed of the turbine is controlled by
admitting the steam in puffs of greater or less duration according to
the load. The lever F, having its fulcrum in the collar surrounding the
shaft, operates with each vibration of the eccentric the pilot valve.
The valve is explained in detail later.
[Illustration: FIG. 50]
This form of governor has been superseded by an improved type, but so
many have been made that it will be well to describe its construction
and adjustment. The two balls W W (Fig. 50) are mounted on the ends of
bell cranks N, which rest on knife edges. The other end of the bell
cranks carry rollers upon which rest a plate P, which serves as a
support for the governor spring S. They are also attached by links to a
yoke and sleeve E which acts as a fulcrum for the lever F. The governor
is regulated by means of the spring S resting on the plate P and
compressed by a large nut G on the upper end of the governor spindle,
which nut turns on a threaded quill J, held in place by the nut H on the
end of the governor spindle and is held tight by the lock-nut K. To
change the compression of the spring and thereby the speed of the
turbine the lock-nut must first be loosened and the hand-nut raised to
lower the speed or lowered to raise the speed as the case may be. This
operation may be accomplished while the machine is either running or at
rest.
The plate P rests upon ball bearings so that by simply bringing pressure
to bear upon the hand-wheel, which is a part of the quill J, the spring
and lock-nut may be held at rest and adjusted while the rest of the
turbine remains unaffected. Another lever is mounted upon the yoke E on
the pin shown at I, the other end of which is fastened to the piston of
a dash-pot so as to dampen the governor against vibration. Under the
yoke E will be noticed a small trigger M which is used to hold the
governor in the full-load position when the turbine is at rest.
The throwing out of the weights elevates the sleeve E, carrying with it
the collar C, which is spanned by the lever F upon the shaft H. The
later turbines are provided with an improved form of governor operating
on the same principle, but embodying several important features. First,
the spindle sleeve is integral with the governor yoke, and the whole
rotates about a vertical stationary spindle, so that two motions are
encountered--a rotary motion and an up and down motion, according to the
position taken by the governor. This spiral motion almost entirely
eliminates the effect of friction of rest, and thereby enhances the
sensitiveness of the governor. Second, the governor weights move outward
on a parallel motion opposed directly by spring thrust, thus relieving
the fulcrum entirely of spring thrust. Third, the lay shaft driving the
governor oil pump and reciprocator is located underneath the main
turbine shaft, so that the rotor may be readily removed without in the
least disturbing the governor adjustment.
The Valve-Gear
The valve-gear is shown in section in Fig. 51, the main admission being
shown at V{1} at the right, and the secondary V{2} at the left of the
steam inlet. The pilot valve F receives a constant reciprocating motion
from the eccentric upon the layshaft of the turbine through the lever F
(Fig. 50). These reciprocations run from 150 to 180 per minute. The
space beneath the piston C is in communication with the large steam
chest, where exists the initial pressure through the port A; the
admission of steam to the piston C being controlled by a needle valve
B. The pilot valve connects the port E, leading from the space beneath
the piston to an exhaust port I.
[Illustration: FIG. 51]
When the pilot valve is closed, the pressures can accumulate beneath the
piston C and raise the main admission valve from its seat. When the
pilot valve opens, the pressure beneath the piston is relieved and it is
seated by the helical spring above. If the fulcrum E (Fig. 50) of the
lever F were fixed the admission would be of an equal and fixed
duration. But if the governor raises the fulcrum E, the pilot valve F
(Fig. 51) will be lowered, changing the relations of the openings with
the working edges of the ports.
The seating of the main admission valve is cushioned by the dashpot, the
piston of which is shown in section at G (Fig. 51). The valve may be
opened by hand by means of the lever K, to see if it is perfectly free.
The secondary valve is somewhat different in its action. Steam is
admitted to both sides of its actuating piston through the needle valves
M M, and the chamber from which this steam is taken is connected with
the under side of the main admission valve, so that no steam can reach
the actuating piston of the secondary valve until it has passed through
the primary valve. When the pilot valve is closed, the pressures
equalize above and below the piston N and the valve remains upon its
seat. When the load upon the turbine exceeds its rated capacity, the
pilot valve moves upward so as to connect the space above the piston
with the exhaust L, relieving the pressure upon the upper side and
allowing the greater pressure below to force the valve open, which
admits steam to the secondary stage of the turbine.
It would do no good to admit more steam to the first stage, for at the
rated capacity that stage is taking all the steam for which the blade
area will afford a passage. The port connecting the upper side of the
piston N with the exhaust may be permanently closed by means of the hand
valve Q, to be found on the side of the secondary pilot valve chest,
thus cutting the secondary valve entirely out of action. No dashpot is
necessary on this valve, the compression of the steam in the chamber W
by the fall of the piston being sufficient to avoid shock.
The timing of the secondary valve is adjusted by raising or lowering the
pilot valve by means of the adjustment provided. It should open soon
enough so that there will not be an appreciable drop in speed before the
valve comes into play. The economy of the machine will be impaired if
the valve is allowed to open too soon.
Safety Stop Governor
This device is mounted on the governor end of the turbine shaft, as
shown in Figs. 52 and 53. When the speed reaches a predetermined limit,
the plunger A, having its center of gravity slightly displaced from the
center of rotation of the shaft, is thrown radially outward and strikes
the lever B. It will easily be understood that when the plunger starts
outward, the resistance of spring C is rapidly overcome, since the
centrifugal force increases as the square of the radius, or in this case
the eccentricity of the center of gravity relative to the center of
rotation. Hence, the lever is struck a sharp blow. This releases the
trip E on the outside of the governor casing, and so opens the steam
valve F, which releases steam from beneath the actuating piston of a
quick-closing throttle valve, located in the steam line. Thus, within a
period of usually less than one second, the steam is entirely shut off
from the turbine when the speed has exceeded 7 or 8 per cent of the
normal.
[Illustration: FIG. 52]
[Illustration: FIG. 53]
The Oiling System
Mounted on the end of the bedplate is the oil pump, operated from the
main shaft of the turbine as previously stated. This may be of the
plunger type shown in Fig. 54, or upon the latest turbine, the rotary
type shown in Fig. 55. Around the bedplate are located the oil-cooling
coils, the oil strainer, the oil reservoir and the oil pipings to the
bearing.
[Illustration: FIG. 54]
The oil reservoir, cooler, and piping are all outside the machine and
easily accessible for cleaning. Usually a corrugated-steel floor plate
covers all this apparatus, so that it will not be unsightly and
accumulate dirt, particularly when the turbine is installed, so that all
this apparatus is below the floor level; i.e., when the top of the
bedplate comes flush with the floor line. In cases where the turbine is
set higher, a casing is usually built around this material so that it
can be easily removed, and forms a platform alongside the machine.
[Illustration: FIG. 55]
The oil cooler, shown in Fig. 56, is of the counter-current type, the
water entering at A and leaving at B, oil entering at C (opening not
shown) and leaving at D. The coils are of seamless drawn copper, and
attached to the cover by coupling the nut. The water manifold F is
divided into compartments by transverse ribs, each compartment
connecting the inlet of each coil with the outlet of the preceding coil,
thus placing all coils in series. These coils are removable in one piece
with the coverplate without disturbing the rest of the oil piping.
[Illustration: FIG. 56]
Blading
[Illustration: FIG. 57]
The blades are drawn from a rod consisting of a steel core coated with
copper so intimately connected with the other metal that when the bar is
drawn to the section required for the blading, the exterior coating
drawn with the rest of the bar forms a covering of uniform thickness as
shown in Fig. 57. The bar after being drawn through the correct section
is cut into suitable lengths punched as at A (Fig. 58), near the top of
the blade, and has a groove shown at B (Fig. 59), near the root, stamped
in its concave face, while the blade is being cut to length and punched.
The blades are then set into grooves cut into the rotor drum or the
concave surface of the casing, and spacing or packing pieces C (Fig. 59)
placed between them. These spacing pieces are of soft iron and of the
form which is desired that the passage between the blades shall take.
The groove made upon the inner face of the blade is sufficiently near to
the root to be covered by this spacing piece. When the groove has been
filled the soft-iron pieces are calked or spread so as to hold the
blades firmly in place. A wire of comma section, as shown at A (Fig.
59), is then strung through the punches near the outer ends of the
blades and upset or turned over as shown at the right in Fig. 58. This
upsetting is done by a tool which shears the tail of the comma at the
proper width between the blades. The bent-down portion on either side
of the blade holds it rigidly in position and the portion retained
within the width of the blade would retain the blade in its radial
position should it become loosened or broken off at the root. This comma
lashing, as it is called, takes up a small proportion only of the blade
length or projection and makes a job which is surprisingly stiff and
rigid, and yet which yields in case of serious disturbance rather than
to maintain a contact which would result in its own fusing or the
destruction of some more important member.
[Illustration: FIG. 58]
[Illustration: FIG. 59]
Starting Up the Turbine
When starting up the turbine for the first time, or after any extended
period of idleness, special care must be taken to see that everything is
in good condition and that all parts of the machine are clean and free
from injury. The oil piping should be thoroughly inspected and cleaned
out if there is any accumulation of dirt. The oil reservoirs must be
very carefully wiped out and minutely examined for the presence of any
grit. (Avoid using cotton waste for this, as a considerable quantity of
lint is almost sure to be left behind and this will clog up the oil
passages in the bearings and strainer.)
The pilot valves should be removed from the barrel and wiped off, and
the barrels themselves cleaned out by pushing a soft cloth through them
with a piece of wood. In no case should any metal be used.
If the turbine has been in a place where there was dirt or where there
has been much dust blowing around, the bearings should be removed from
the spindle and taken apart and thoroughly cleaned. With care this can
be done without removing the spindle from the cylinder, by taking off
the bearing covers and very carefully lifting the weight of the spindle
off the bearings, then sliding back the bearings. It is best to lift the
spindle by means of jacks and a rope sling, as, if a crane is used,
there is great danger of lifting the spindle too high and thereby
straining it or injuring the blades. After all the parts have been
carefully gone over and cleaned, the oil for the bearing lubrication
should be put into the reservoirs by pouring it into the governor gear
case G (Fig. 34). Enough oil should be put in so that when the governor,
gear case, and all the bearing-supply pipes are full, the supply to the
oil pump is well covered.
Special care should be taken so that no grit gets into the oil when
pouring it into the machine. Considerable trouble may be saved in this
respect by pouring the oil through cloth.
A very careful inspection of the steam piping is necessary before the
turbine is run. If possible it should be blown out by steam from the
boilers before it is finally connected to the turbine. Considerable
annoyance may result by neglecting this precaution, from particles of
scale, red lead, gasket, etc., out of the steam pipe, closing up the
passages of the guide blades.
When starting up, always begin to revolve the spindle without vacuum
being on the turbine. After the spindle is turning slowly, bring the
vacuum up. The reason for this is, that when the turbine is standing
still, the glands do not pack and air in considerable quantity will rush
through the glands and down through the exhaust pipe. This sometimes has
the effect of unequal cooling. In case the turbine is used in
conjunction with its own separate condenser, the circulating pump may be
started up, then the turbine revolved, and afterward the air pump put in
operation; then, last, put the turbine up to speed. In cases, however,
where the turbine exhausts into the same condenser with other machinery
and the condenser is therefore already in operation, the valve between
the turbine and the condenser system should be kept closed until after
the turbine is revolved, the turbine in the meantime exhausting through
the relief valve to atmosphere.
Care must always be taken to see that the turbine is properly warmed up
before being caused to revolve, but in cases where high superheat is
employed always revolve the turbine just as soon as it is moderately
hot, and before it has time to become exposed to superheat.
In the case of highly superheated steam, it is not undesirable to
provide a connection in the steam line by means of which the turbine may
be started up with saturated steam and the superheat gradually applied
after the shaft has been permitted to revolve.
For warming up, it is usual practice to set the governor on the trigger
(see Fig. 50) and open the throttle valve to allow the entrance of a
small amount of steam.
It is always well to let the turbine operate at a reduced speed for a
time, until there is assurance that the condenser and auxiliaries are in
proper working order, that the oil pump is working properly, and that
there is no sticking in the governor or the valve gear.
After the turbine is up to speed and on the governor, it is well to
count the speed by counting the strokes of the pump rod, as it is
possible that the adjustment of the governor may have become changed
while the machine has been idle. It is well at this time, while there is
no load on the turbine, to be sure that the governor controls the
machine with the throttle wide open. It might be that the main poppet
valve has sustained some injury not evident on inspection, or was
leaking badly. Should there be some such defect, steps should be taken
to regrind the valve to its seat at the first opportunity.
On the larger machines an auxiliary oil pump is always furnished. This
should be used before starting up, so as to establish the oil
circulation before the turbine is revolved. After the turbine has
reached speed, and the main oil pump is found to be working properly, it
should be possible to take this pump out of service, and start it again
only when the turbine is about to be shut down.
If possible, the load should be thrown on gradually to obviate a sudden,
heavy demand upon the boiler, with its sometimes attendant priming and
rush of water into the steam pipe, which is very apt to take place if
the load is thrown on too suddenly. A slug of water will have the effect
of slowing down the turbine to a considerable extent, causing some
annoyance. There is not likely to be the danger of the damage that is
almost sure to occur in the reciprocating engine, but at the same time
it is well to avoid this as much as possible. A slug of water is
obviously more dangerous when superheated steam is being employed, owing
to the extreme temperature changes possible.
Running
While the turbine is running, it should have a certain amount of careful
attention. This, of course, does not mean that the engineer must stand
over it every minute of the day, but he must frequently inspect such
parts as the lubricators, the oiling system, the water supply to the
glands and the oil-cooling coil, the pilot valve, etc. He must see that
the oil is up in the reservoir and showing in the gage glass provided
for that purpose, and that the oil is flowing freely through the
bearings, by opening the pet cocks in the top of the bearing covers. An
ample supply of oil should always be in the machine to keep the suction
in the tank covered.
Care must be taken that the pump does not draw too much air. This can
usually be discovered by the bubbling up of the air in the governor
case, when more oil should be added.
It is well to note from time to time the temperature of the bearings,
but no alarm need be occasioned because they feel warm to the touch; in
fact, a bearing is all right as long as the hand can be borne upon it
even momentarily. The oil coming from the bearings should be preferably
about 120 degrees Fahrenheit and never exceed 160 degrees.
It should generally be seen that the oil-cooling coil is effective in
keeping the oil cool. Sometimes the cooling water deposits mud on the
cooling surface, as well as the oil depositing a vaseline-like
substance, which interferes with the cooling effect. The bearing may
become unduly heated because of this, when the coil should be taken out
at the first opportunity and cleaned on the outside and blown out by
steam on the inside, if this latter is possible. If this does not reduce
the temperature, either the oil has been in use too long without being
filtered, or the quality of the oil is not good.
Should a bearing give trouble, the first symptom will be burning oil
which will smoke and give off dense white fumes which can be very
readily seen and smelled. However, trouble with the bearings is one of
the most unlikely things to be encountered, and, if it occurs, it is due
to some radical cause, such as the bearings being pinched by their caps,
or grit and foreign matter being allowed to get into the oil.
If a bearing gets hot, be assured that there is some very radical cause
for it which should be immediately discovered and removed. Never, under
any circumstances, imagine that you can nurse a bearing, that has
heated, into good behavior. Turbine bearings are either all right or all
wrong. There are no halfway measures.
The oil strainer should also be occasionally taken apart and thoroughly
cleaned, which operation may be performed, if necessary, while the
turbine is in operation. The screens should be cleaned by being removed
from their case and thoroughly blown out with steam. In the case of a
new machine, this may have to be done every two or three hours. In
course of time, this need only be repeated perhaps once a week. The
amount of dirt found will be an indication of the frequency with which
this cleaning is necessary.
The proper water pressure, about five pounds per square inch, must be
maintained at the glands. Any failure of this will mean that there is
some big leak in the piping, or that the water is not flowing properly.
The pilot valve must be working freely, causing but little kick on the
governor, and should be lubricated from time to time with good oil.
Should it become necessary, while operating, to shut down the condenser
and change over to non-condensing operation, particular care should be
observed that the change is not made too suddenly to non-condensing, as
all the low-pressure sections of the turbine must be raised to a much
higher temperature. While this may not cause an accident, it is well to
avoid the stresses which necessarily result from the sudden change of
temperature. The same reasons, of course, do not hold good in changing
from non-condensing to condensing.
Shutting Down
When shutting down the turbine the load may be taken off before closing
the throttle; or, as in the case of a generator operating on an
independent load, the throttle may be closed first, allowing the load to
act as a brake, bringing the turbine to rest quickly. In most cases,
however, the former method will have to be used, as the turbine
generally will have been operating in parallel with one or more other
generators. When this is the case, partially close the throttle just
before the load is to be thrown off, and if the turbine is to run
without load for some time, shut off the steam almost entirely in order
to prevent any chance of the turbine running away. There is no danger of
this unless the main valve has been damaged by the water when wet steam
has been used, or held open by some foreign substance, when, in either
case, there may be sufficient leakage to run the turbine above speed,
while running light. At the same time, danger is well guarded against by
the automatic stop valve, but it is always well to avoid a possible
danger. As soon as the throttle is shut, stop the condenser, or, in the
case where one condenser is used for two or more turbines, close the
valve between the turbine and the condenser. Also open the drains from
the steam strainer, etc. This will considerably reduce the time the
turbine requires to come to rest. Still more time may be saved by
leaving the field current on the generator.
Care should be taken, when the vacuum falls and the turbine slows down,
to see that the water is shut off from the glands for fear it may leak
out to such an extent as to let the water into the bearings and impair
the lubricating qualities of the oil.
Inspection
At regular intervals thorough inspection should be made of all parts of
the turbine. As often as it appears necessary from the temperature of
the oil, depending on the quality of the oil and the use of the turbine,
remove the oil-cooling coil and clean it both on the inside and outside
as previously directed; also clean out the chamber in which it is kept.
Put in a fresh supply of oil. This need not necessarily be new, but may
be oil that has been in use before but has been filtered. We recommend
that an oil filter be kept for this purpose. Entirely new oil need only
be put into the turbine when the old oil shows marked deterioration.
With a first-class oil this will probably be a very infrequent
necessity, as some new oil has to be put in from time to time to make up
the losses from leakage and waste.
Clean out the oil strainer, blowing steam through the wire gauze to
remove any accumulation of dirt. Every six months to a year take off the
bearing covers, remove the bearings, and take them apart and clean out
thoroughly. Even the best oil will deposit more or less solid matter
upon hot surfaces in time, which will tend to prevent the free
circulation of the oil through the bearings and effectively stop the
cushioning effect on the bearings. Take apart the main and secondary
valves and clean thoroughly, seeing that all parts are in good working
order. Clean and inspect the governor and the valve-gear, wiping out any
accumulation of oil and dirt that may appear. Be sure to clean out the
drains from the glands so that any water that may pass out of them will
run off freely and will not get into the bearings.
At the end of the first three months, and after that about once a year,
take off the cylinder cover and remove the spindle. When the turbine is
first started up, there is very apt to be considerable foreign matter
come over in the steam, such as balls of red lead or small pieces of
gasket too small to be stopped by the strainer. These get into the guide
blades in the cylinder and quite effectively stop them up. Therefore,
the blades should be gone over very carefully, and any such additional
accumulation removed. Examine the glands and equilibrium ports for any
dirt or broken parts. Particularly examine the glands for any deposit of
scale. All the scale should be chipped off the gland parts, as, besides
preventing the glands from properly packing, this accumulation will
cause mechanical contact and perhaps cause vibration of the machine due
to lack of freedom of the parts. The amount of scale found after the
first few inspections will be an indication of how frequently the
cleaning should be done. As is discussed later, any water that is
unsuitable for boiler feed should not be used in the glands.
In reassembling the spindle and cover, very great care must be taken
that no blades are damaged and that nothing gets into the blades. Nearly
all the damage that has been done to blades has resulted from
carelessness in this respect; in fact, it is impossible to be too
careful. Particular care is also to be taken in assembling all the
parts and in handling them, as slight injury may cause serious trouble.
In no case should a damaged part be put back until the injury has been
repaired.
If for any reason damaged blades cannot be repaired at the time, they
can be easily removed and the turbine run again without them until it is
convenient to put in new ones; in fact, machines have been run at full
load with only three-quarters of the total number of blades. In such an
event remove the corresponding stationary blades as well as the moving
blades, so as not to disturb the balance of the end thrust.
Conditions Conducive to Successful Operation
In the operation of the turbine and the conditions of the steam, both
live and exhaust play a very important part. It has been found by
expensive experimenting that moisture in the steam has a very decided
effect on the economy of operation; or considerably more so than in the
case of the reciprocating engine. In the latter engine, 2 per cent. of
moisture will mean very close to 2 per cent. increase in the amount of
water supplied to the engine for a given power. On the other hand, in
the turbine 2 per cent. moisture will cause an addition of more nearly 4
per cent. It is therefore readily seen that the drier the entering
steam, the better will be the appearance of the coal bill.
By judicious use of first-class separators in connection with a suitable
draining system, such as the Holly system which returns the moisture
separated from the steam, back to the boilers, a high degree of quality
may be obtained at the turbine with practically no extra expense during
operation. Frequent attention should be given the separators and traps
to insure their proper operation. The quality of the steam may be
determined from time to time by the use of a throttling calorimeter. Dry
steam, to a great extent, depends upon the good and judicious design of
steam piping.
Superheated steam is of great value where it can be produced
economically, as even a slight degree insures the benefits to be derived
from the use of dry steam. The higher superheats have been found to
increase the economy to a considerable extent.
When superheat of a high degree (100 degrees Fahrenheit or above) is
used special care must be exercised to prevent a sudden rise of the
superheat of any amount. The greatest source of trouble in this respect
is when a sudden demand is made for a large increase in the amount of
steam used by the engine, as when the turbine is started up and the
superheater has been in operation for some time before, the full load is
suddenly thrown on. It will be readily seen that with the turbine
running light and the superheater operating, there is a very small
amount of steam passing through; in fact, practically none, and this may
become very highly heated in the superheater, but loses nearly all its
superheat in passing slowly to the turbine; then, when a sudden demand
is made, this very high temperature steam is drawn into the turbine.
This may usually be guarded against where a separately fired superheater
is used, by keeping the fire low until the load comes on, or, in the
case where the superheater is part of the boiler, by either not starting
up the superheater until after load comes on, or else keeping the
superheat down by mixing saturated steam with that which has been
superheated. After the plant has been started up there is little danger
from this source, but such precautions should be taken as seem best in
the particular cases.
Taking up the exhaust end of the turbine, we have a much more striking
departure from the conditions familiar in the reciprocating engine. Due
to the limits imposed upon the volume of the cylinder of the engine, any
increase in the vacuum over 23 or 24 inches, in the case, for instance,
of a compound-condensing engine, has very little, if any, effect on the
economy of the engine. With the turbine, on the other hand, any increase
of vacuum, even up to the highest limits, increases the economy to a
very considerable extent and, moreover, the higher the vacuum the
greater will be the increase in the economy for a given addition to the
vacuum. Thus, raising the vacuum from 27 to 28 inches has a greater
effect than from 23 to 24 inches. For this reason the engineer will
readily perceive the great desirability of maintaining the vacuum at the
highest possible point consistent with the satisfactory and economical
operation of the condenser.
The exhaust pipe should always be carried downward to the condenser when
possible, to keep the water from backing up from the condenser into the
turbine. If the condenser must be located above the turbine, then the
pipe should be carried first downward and then upward in the U form, in
the manner of the familiar "entrainer," which will be found effectively
to prevent water getting back when the turbine is operating.
Condensers
As has been previously pointed out, the successful and satisfactory
operation of the turbine depends very largely on the condenser. With the
reciprocating engine, if the condenser will give 25 inches vacuum, it is
considered fairly good, and it is allowed to run along by itself until
the vacuum drops to somewhere below 20 inches, when it is completely
gone over, and in many cases practically rebuilt and the vacuum brought
back to the original 25 inches. It has been seen that this sort of
practice will never do in the case of the turbine condenser and, unless
the vacuum can be regularly maintained at 27 or 28 inches, the condenser
is not doing as well as it ought to do, or it is not of the proper type,
unless perhaps the temperature and the quantity of cooling water
available render a higher vacuum unattainable.
On account of the great purity of the condensed steam from the turbine
and its peculiar availability for boiler feed (there being no oil of any
kind mixed with it to injure the boilers), the surface condenser is very
desirable in connection with the turbine. It further recommends itself
by reason of the high vacuum obtainable.
Where a condenser system capable of the highest vacuum is installed,
the need of utilizing it to its utmost capacity can hardly be emphasized
too strongly. A high vacuum will, of course, mean special care and
attention, and continual vigilance for air leaks in the exhaust piping,
which will, however, be fully paid for by the great increase in economy.
It must not be inferred that a high vacuum is essential to successful
operation of this type of turbine, for excellent performance both in the
matter of steam consumption and operation is obtained with inferior
vacuum. The choice of a condenser, however, is a matter of special
engineering, and is hardly within the province of this article.
Oils
There are several oils on the market that are suitable for the purpose
of the turbine oiling system, but great care must be exercised in their
selection. In the first place, the oil must be pure mineral,
unadulterated with either animal or vegetable oils, and must have been
washed free from acid. Certain brands of oil require the use of
sulphuric acid in their manufacture and are very apt to contain varying
degrees of free acid in the finished product. A sample from one lot may
have almost no acid, while that from another lot may contain a dangerous
amount.
Mineral oils that have been adulterated, when heated up, will partially
decompose, forming acid. These oils may be very good lubricants when
first put into use, but after awhile they lose all their good qualities
and become very harmful to the machine by eating the journals in which
they are used. These oils must be very carefully avoided in the turbine,
as the cheapness of their first cost will in no way pay for the damage
they may do. A very good and simple way to test for such adulterations
is to take up a quantity of the oil in a test tube with a solution of
borax and water. If there is any animal or vegetable adulterant present
it will appear as a white milk-like emulsion which will separate out
when allowed to stand. The pure mineral oil will appear at the top as a
clear liquid and the excess of the borax solution at the bottom, the
emulsion being in between. A number of oils also contains a considerable
amount of paraffin which is deposited in the oil-cooling coil,
preventing the oil from being cooled properly, and in the pipes and
bearings, choking the oil passages and preventing the proper circulation
of the oil and cushioning effect in the bearing tubes. This is not
entirely a prohibitive drawback, the chief objection being that it
necessitates quite frequently cleaning the cooling coil, and the oil
piping and bearings.
Some high-class mineral oils of high viscosity are inclined to emulsify
with water, which emulsion appears as a jelly-like substance. It might
be added that high-grade oils having a high viscosity might not be the
most suitable for turbine use.
Since the consumption of oil in a turbine is so very small, being
practically due only to leakage or spilling, the price paid for it
should therefore be of secondary importance, the prime consideration
being its suitability for the purpose.
In some cases a central gravity system will be employed, instead of the
oil system furnished with the turbine, which, of course, will be a
special consideration.
For large installations a central gravity oiling system has much to
recommend it, but as it performs such an important function in the power
plant, and its failure would be the cause of so much damage, every
detail in connection with it should be most carefully thought out, and
designed with a view that under no combination of circumstances would it
be possible for the system to become inoperative. One of the great
advantages of such a system is that it can be designed to contain very
large quantities of oil in the settling tanks; thus the oil will have
quite a long rest between the times of its being used in the turbine,
which seems to be very helpful in extending the life of the oil. Where
the oil can have a long rest for settling, an inferior grade of oil may
be used, providing, however, that it is absolutely free of acid.
V. PROPER METHOD OF TESTING A STEAM TURBINE[3]
[3] Contributed to _Power_ by Thomas Franklin.
The condensing arrangements of a turbine are perhaps mainly instrumental
in determining the method of test. The condensed steam alone, issuing
from a turbine having, for example, a barometric or jet condenser,
cannot be directly measured or weighed, unless by meter, and these at
present are not sufficiently accurate to warrant their use for test
purposes, if anything more than approximate results are desired. The
steam consumed can, in such a case, only be arrived at by measuring the
amount of condensing water (which ultimately mingles with the condensed
steam), and subtracting this quantity from the condenser's total
outflow. Consequently, in the case of turbines equipped with barometric
or jet condensers, it is often thought sufficient to rely upon the
measurement taken of the boiler feed, and the boiler's initial and final
contents. Turbines equipped with surface-condensing plants offer better
facilities for accurate steam-consumption calculations than those plants
in which the condensed exhaust steam and the circulating water come into
actual contact, it being necessary with this type simply to pump the
condensed steam into a weighing or measuring tank.
In the case of a single-flow turbine of the Parsons type, the covers
should be taken off and every row of blades carefully examined for
deposits, mechanical irregularities, deflection from the true radial and
vertical positions, etc. The blade clearances also should be gaged all
around the circumference, to insure this clearance being an average
working minimum. On no account should a test be proceeded with when any
doubt exists as to the clearance dimensions.
[Illustration: FIG. 60]
The dummy rings of a turbine, namely, those rings which prevent
excessive leakage past the balancing pistons at the high-pressure end,
should have especial attention before a test. A diagrammatic sketch of a
turbine cylinder and spindle is shown in Fig. 60, for the benefit of
those unfamiliar with the subject. In this A is the cylinder or casing,
B the spindle or rotor, and C the blades. The balancing pistons, D, E,
and F, the pressure upon which counterbalances the axial thrust upon the
three-bladed stages, are grooved, the brass dummy rings G G in the
cylinder being alined within a few thousandths of an inch of the grooved
walls, as indicated. After these rings have been turned (the turning
being done after the rings have been calked in the cylinder), it is
necessary to insure that each ring is perfectly bedded to its respective
grooved wall so that when running the several small clearances between
the groove walls and rings are equal. A capital method of thus bedding
the dummy rings is to grind them down with a flour of emery or
carborundum, while the turbine spindle is slowly revolving under steam.
Under these conditions the operation is performed under a high
temperature, and any slight permanent warp the rings may take is thus
accounted for. The turbine thrust-block, which maintains the spindle in
correct position relatively to the spindle, may also be ground with
advantage in a similar manner.
The dummy rings are shown on a large scale in Fig. 61, and their
preliminary inspection may be made in the following manner:
The spindle has been set and the dummy rings C are consequently within a
few thousandths of an inch of the walls _d_ of the spindle dummy grooves
D. The clearances allowed can be gaged by a feeler placed between a ring
and the groove wall. Before a test the spindle should be turned slowly
around, the feelers being kept in position. By this means any mechanical
flaws or irregularities in the groove walls may be detected.
[Illustration: FIG. 61]
It has sometimes been found that the groove walls, under the combined
action of superheated steam and friction, in cases where actual running
contact has occurred, have worn very considerably, the wear taking the
form of a rapid crumbling away. It is possible, however, that such
deterioration may be due solely to the quality of the steel from which
the spindle is forged. Good low-percentage carbon-annealed steel ought
to withstand considerable friction; at all events the wear under any
conditions should be uniform. If the surfaces of both rings and grooves
be found in bad condition, they should be re-ground, if not sufficiently
worn to warrant skimming up with a tool.
As the question of dummy leakage is of very considerable importance
during a test, it may not be inadvisable to describe the manner of
setting the spindle and cylinder relatively to one another to insure
minimum leakage, and the methods of noting their conduct during a
prolonged run. In Fig. 62, showing the spindle, B is the thrust (made in
halves), the rings O of which fit into the grooved thrust-rings C in the
spindle. Two lugs D are cast on each half of the thrust-block. The
inside faces of these lugs are machined, and in them fit the ball ends
of the levers E, the latter being fulcrumed at F in the thrust-bearing
cover. The screws G, working in bushes, also fit into the thrust-bearing
cover, and are capable of pushing against the ends of the levers E and
thus adjusting the separate halves of the block in opposite directions.
[Illustration: FIG. 62]
The top half of the turbine cylinder having been lifted off, the spindle
is set relatively to the bottom half by means of the lower thrust-block
screw G. This screw is then locked in position and the top half of the
cover then lowered into place. With this method great care must
necessarily be exercised when lowering the top cover; otherwise the
brass dummy rings may be damaged.
A safer method is to set the dummy rings in the center of the grooves of
the spindle, and then to lower the cover, with less possibility of
contact. There being usually plenty of side clearance between the blades
of a turbine, it may be deemed quite safe to lock the thrust-block in
its position, by screwing the screws G up lightly, and then to turn on
steam and begin running slowly.
Next, the spindle may be very carefully and gradually worked in the
required direction, namely, in that direction which will tend to bring
the dummy rings and groove walls into contact, until actual but very
light contact takes place. The slightest noise made by the rubbing parts
inside the turbine can be detected by placing one end of a metal rod
onto the casing in vicinity of the dummy pistons, and letting the other
end press hard against the ear. Contact between the dummy rings and
spindle being thus demonstrated, the spindle must be moved back by the
screws, but only by the slightest amount possible. The merest fraction
of a turn is enough to break the contact, which is all that is required.
In performing this operation it is important, during the axial movements
of the spindle, to adjust the halves of the thrust-block so that there
can exist no possible play which would leave the spindle free to move
axially and probably vibrate badly.
After ascertaining the condition of the dummy rings, attention might
next be turned to the thrust-block, which must not on any account be
tightened up too much. It is sufficient to say that the actual
requirements are such as will enable a very thin film of oil to
circulate between each wall of the spindle thrust-grooves and the brass
thrust-blocks ring. In other words, there should be no actual pressure,
irrespective of that exerted by the spindle when running, upon the
thrust-block rings, due to the separate halves having been nipped too
tightly. The results upon a test of considerable friction between the
spindle and thrust-rings are obvious.
The considerations outlined regarding balancing pistons and dummy rings
can be dispensed with in connection with impulse turbines of the De
Laval and Rateau types, and also with double-flow turbines of a type
which does not possess any dummies. The same general considerations
respecting blade conditions and thrust-blocks are applicable, especially
to the latter type. With pure so-called impulse turbines, where the
blade clearances are comparatively large, the preliminary blade
inspection should be devoted to the mechanical condition of the blade
edges and passages. As the steam velocities of these types are usually
higher, the importance of minimizing the skin friction and eliminating
the possibility of eddies is great.
Although steam leakage through the valves of a turbine may not
materially affect its steam consumption, unless it be the leakage
through the overload valve during a run on normal full load, a thorough
examination of all valves is advocated for many reasons. In a turbine
the main steam-inlet valve is usually operated automatically from the
governor; and whether it be of the pulsating type, admitting the steam
in blasts, or of the non-pulsating throttling type, it is equally
essential to obtain the least possible friction between all moving and
stationary parts. Similar remarks apply to the main governor, and any
sensitive transmitting mechanism connecting it with any of the turbine
valves. If a safety or "runaway" governor is possessed by the machine to
be tested, this should invariably be tried under the requisite
conditions before proceeding farther. The object of this governor being
automatically to shut off all steam from the turbine, should the latter
through any cause rise above the normal speed, it is often set to
operate at about 12 to 15 per cent. above the normal. Thus, a turbine
revolving at about 3000 revolutions per minute would be closed down at,
say, 3500, which would be within the limit of "safe" speed.
Importance of Oiling System and Water Service
The oil question, being important, should be solved in the early stages
previously, if possible, to any official or unofficial consumption
tests. Whether the oil be supplied to the turbine bearings by a
self-contained system having the oil stored in the turbine bedplate or
by gravity from a separate oil source, does not affect the question in
its present aspect. The necessary points to investigate are four in
number, and may be headed as follows:
(a) Examination of pipes and partitions for oil leakage.
(b) Determination of volume of oil flowing through each bearing per unit
of time.
(c) Examination for signs of water in oil.
(d) Determination of temperature rise between inlet and outlet of oil
bearings.
The turbine supplied with oil by the gravity or any other separate
system holds an advantage over the ordinary self-contained machine,
inasmuch as the oil pipes conveying oil into and from the bearings can
be easily approached and, if necessary, repaired. On the other hand, the
machine possessing its own oil tank, cooling chamber and pump is
somewhat at a disadvantage in this respect, as a part of the system is
necessarily hidden from view, and, further, it is not easily accessible.
The leakage taking place in any system, if there be any, must, however,
be detected and stopped.
Fig. 63 is given to illustrate a danger peculiar to the self-contained
oil system, in which the oil and oil-cooling chambers are situated
adjacently in the turbine bedplate. One end of the bedplate only is
shown; B is a cast-iron partition dividing the oil chamber C from the
oil-cooling chamber D. Castings of this kind have sometimes a tendency
to sponginess and the trouble consequent upon this weakness would take
the form of leakage between the two chambers. Of course this is only a
special case, and the conditions named are hardly likely to exist in
every similarly designed plant. The capacity of oil, and especially of
hot oil, to percolate through the most minute pores is well known.
Consequently, in advocating extreme caution when dealing with oil
leakage, no apology is needed.
[Illustration: FIG. 63]
It may be stated without fear of contradiction that the oil in a
self-contained system, namely, a system in which the oil, stored in a
reservoir near or underneath the turbine, passes only through that one
turbine's bearings, and immediately back to the storage compartment,
deteriorates more rapidly than when circulating around an "entire"
system, such as the gravity or other analogous system. In the latter,
the oil tanks are usually placed a considerable distance from the
turbine or turbines, with the oil-cooling arrangements in fairly close
proximity. The total length of the oil circuit is thus considerably
increased, incidentally increasing the relative cooling capacity of the
whole plant, and thereby reducing the loss of oil by vaporization.
The amount of oil passing through the bearings can be ascertained
accurately by measurement. With a system such as the gravity it is only
necessary to run the turbine up to speed, turn on the oil, and then,
over a period, calculate the volume of oil used by measuring the fall of
level in the storage tank and multiplying by its known cross-sectional
area. In those cases where the return oil, after passing through the
bearings, is delivered back into the same tank from which it is
extracted, it is of course necessary, during the period of test, to
divert this return into a separate temporary receptacle. Where the
system possesses two tanks, one delivery and one return (a superior
arrangement), this additional work is unnecessary. The same method can
be applied to individual turbines pumping their own oil from a tank in
the bedplate; the return oil, as previously described, being temporarily
prevented from running back to the supply.
The causes of excessive oil consumption by bearings are many. There is
an economical mean velocity at which the oil must flow along the
revolving spindle; also an economical mean pressure, the latter
diminishing from the center of the bearing toward the ends. The aim of
the economist must therefore be in the direction of adjusting these
quantities correctly in relation to a minimum supply of oil per bearing;
and the principal factors capable of variation to attain certain
requirements are the several bearing clearances measured as annular
orifices, and the bearing diameters.
It is not always an easy matter to detect the presence of water in an
oil system, and this difficulty is increased in large circuits, as the
water, when the oil is not flowing, generally filters to the lowest
members and pipes of the system, where it cannot usually be seen. A
considerable quantity of water in any system, however, indicates its
presence by small globular deposits on bearings and spindles, and in the
worst cases the water can clearly be seen in a small sample tapped from
the oil mains. There is only one effective method of ridding the oil of
this water, and this is by allowing the whole mass of oil in the system
to remain quiescent for a few days, after which the water, which falls
to the lowest parts, can be drained off. A simple method of clearing out
the system is to pump all the oil the whole circuit contains through the
filters, and thence to a tank from which all water can be taken off. One
of the ordinary supply tanks used in the gravity system will serve this
purpose, should a temporary tank not be at hand. If necessary, the
headers and auxiliary pipes of the system can be cleaned out before
circulating the oil again, but as this is rather a large undertaking, it
need only be resorted to in serious cases.
[Illustration: FIG. 64]
It is seldom possible to discover the correct and permanent temperature
rise of the circulating oil in a turbine within the limited time usually
alloted for a test. After a continuous run of one hundred hours it is
possible that the temperature at the bearing outlets may be lower than
it was after the machine had run for, say, only twenty hours. As a
matter of fact an oil-temperature curve plotted from periodical readings
taken over a continuous run of considerable length usually reaches a
maximum early, afterward falling to a temperature about which the
fluctuations are only slight during the remainder of the run. Fig. 64
illustrates an oil-temperature curve plotted from readings taken over a
period of twenty-four hours. In this case the oil system was of the
gravity description, the capacity of the turbine being about 6000
kilowatts. The bearings were of the ordinary white-metal spherical type.
Over extended runs of hundreds and even thousands of hours, the above
deductions may be scarcely applicable. Running without break for so
long, a small turbine circulating its own lubricant would possibly
require a renewal of the oil before the run was completed, in the main
owing to excessive temperature rise and consequent deterioration of the
quality of the oil. Under these conditions the probabilities are that
several temperature fluctuations might occur before the final maximum,
and more or less constant, temperature was reached. In this connection,
however, the results obtained are to a very large extent determined by
the general mechanical design and construction of the oiling system and
turbine. A reference to Fig. 63 again reveals at once a weakness in that
design, namely, the unnecessarily close proximity in which the oil and
water tanks are placed.
[Illustration: FIG. 65]
A design of thermometer cup suitable for oil thermometers is given in
Fig. 65 in which A is an end view of the turbine bedplate, B is a
turbine bearing and C and D are the inlet and outlet pipes,
respectively. The thermometer fittings, which are placed as near the
bearing as is practicable, are made in the form of an angular tee
fitting, the oil pipes being screwed into its ends. The construction of
the oil cup and tee piece is shown in the detail at the left where A is
the steel tee piece, into which is screwed the brass thermometer cup B.
The hollow bottom portion of this cup is less than 1/16 of an inch in
thickness. The top portion of the bored hole is enlarged as shown, and
into this, around the thermometer, is placed a non-conducting material.
The cup itself is generally filled with a thin oil of good conductance.
Allied to the oil system of a turbine plant is the water service, of
comparatively little importance in connection with single self-contained
units of small capacity, where the entire service simply consists of a
few coils and pipes, but of the first consideration in large
installations having numerous separate units supplied by oil and water
from an exterior source. The largest turbine units are often supplied
with water for cooling the bearings and other parts liable to attain
high temperature. Although the water used for cooling the bearings
indirectly supplements the action taking place in the separate oil
coolers, it is of necessity a separate auxiliary service in itself, and
the complexity of the system is thus added to. A carefully constructed
water service, however, is hardly likely to give trouble of a mechanical
nature. The more serious deficiencies usually arise from conditions
inherent to the design, and as such must be approached.
Special Turbine Features to be Inquired into
Before leaving the prime mover itself, and proceeding to the auxiliary
plant inspection, it may be well to instance a few special features
relating to the general conduct of a turbine, which it is the duty of a
tester to inquire into. There are certain specified qualifications which
a machine must hold when running under its commercial conditions, among
these being lack of vibration of both turbine and machinery driven, be
it generator or fan, the satisfactory running of auxiliary turbine parts
directly driven from the turbine spindle, minimum friction between the
driving mediums, such as worm-wheels, pumps, fans, etc., slight
irregularities of construction, often resulting in heated parts and
excessive friction and wear, and must therefore be detected and righted
before the final test. Furthermore, those features of design--and they
are not infrequent in many machines of recent development--which, in
practice, do not fulfil theoretical expectations, must be re-designed
upon lines of practical consistency. The experienced tester's opinion is
often at this point invaluable. To illustrate the foregoing, Figs. 66,
67, and 68 are given, representing, respectively, three distinct phases
in the evolution of a turbine part, namely, the coupling. Briefly, an
ordinary coupling connecting a driving and a driven shaft becomes
obstinate when the two separate spindles which it connects are not truly
alined. The desire of turbine manufacturers has consequently been to
design a flexible coupling, capable of accommodating a certain want of
alinement between the two spindles without in any way affecting the
smooth running of the whole unit.
[Illustration: FIG. 66]
[Illustration: FIG. 67]
In Fig. 66 A is the turbine spindle end and B the generator spindle end,
which it is required to drive. It will be seen from the cross-sectional
end view that both spindle ends are squared, the coupling C, with a
square hole running through it, fitting accurately over both spindle
ends as shown. Obviously the fit between the coupling and spindle in
this case must be close, otherwise considerable wear would take place;
and equally obvious is the fact than any want of alinement between the
two spindles A and B will be accompanied by a severe strain upon the
coupling, and incidentally by many other troubles of operation of which
this inability of the coupling to accommodate itself to a little want
of alinement is the inherent cause.
Looking at the coupling illustrated in Fig. 67, it will be seen that
something here is much better adapted to dealing with troubles of
alinement. The turbine and generator spindles A and B, respectively, are
coned at the ends, and upon these tapered portions are shrunk circular
heads C and D having teeth upon their outer circumferences. Made in
halves, and fitting over the heads, is a sleeve-piece, with teeth cut
into its inner bored face. The teeth of the heads and sleeve are
proportioned correctly to withstand, without strain, the greatest
pressure liable to be thrown upon them. There is practically no play
between the teeth, but there exists a small annular clearance between
the periphery of the heads and the inside bore of the sleeve, which
allows a slight lack of alinement to exist between the two spindles,
without any strain whatever being felt by the coupling sleeve E. The
nuts F and G prevent any lateral movement of the coupling heads C and D.
For all practical requirements this type of coupling is satisfactory, as
the clearances allowed between sliding sleeve and coupling heads can
always be made sufficient to accommodate a considerable want of
alinement, far beyond anything which is likely to occur in actual
practice. Perhaps the only feature against it is its lack of simplicity
of construction and corresponding costliness.
[Illustration: FIG. 68]
The type illustrated in Fig. 68 is a distinct advance upon either of the
two previous examples, because, theoretically at least, it is capable of
successfully accommodating almost any amount of spindle movement. The
turbine and generator spindle ends, A and B, have toothed heads C and D
shrunk upon them, the heads being secured by the nuts E and F. The
teeth in this case are cut in the enlarged ends as shown. A sleeve G,
made in halves, fits over the heads, and the teeth cut in each half
engage with those of their respective heads. All the teeth and teeth
faces are cut radially, and a little side play is allowed.
The Condenser
To some extent, as previously remarked, the condenser and condensing
arrangements are instrumental in determining the lines upon which a test
ought to be carried out. In general, the local features of a plant
restrict the tester more or less in the application of his general
methods. A thorough inspection, including some preliminary tests if
necessary, is as essential to the good conduct of the condensing plant
as to the turbine above it. It may be interesting to outline the usual
course this inspection takes, and to draw attention to a few of the
special features of different plants. For this purpose a type of
vertical condenser is depicted in Fig. 69. Its general principle will be
gathered from the following description:
Exhaust steam from the turbine flows down the pipe T and enters the
condenser at the top as shown, where it at once comes into contact with
the water tubes in W. These tubes fill an annular area, the central
un-tubed portion below the baffle cap B forming the vapor chamber. The
condensed steam falls upon the bottom tube-plate P and is carried away
by the pipe S leading to the water pump H. The Y pipe E terminating
above the level of the water in the condenser enters the dry-air pump
section pipe A. Cold circulating water enters the condenser at the
bottom, through the pipe I, and entering the water chamber X proceeds
upward through the tubes into the top-water chamber Y, and from there
out of the condenser through the exit pipe. It will be observed that the
vapor extracted through the plate P passes on its journey out of the
condenser through the cooling chamber D surrounded by the cold
circulating water. This, of course, is a very advantageous feature. At R
is the condenser relief, at U the relief valve for the water chambers.
[Illustration: FIG. 69]
A new condenser, especially if it embody new and untried features,
generally requires a little time and patience ere the best results can
be obtained from it. Perhaps the quickest and most satisfactory method
of getting at the weak points of this portion of a plant is to test the
various elements individually before applying a strict load test. Thus,
in dealing with a condenser similar to that illustrated in Fig. 69, the
careful tester would probably make, in addition to a thorough mechanical
examination, three or four individual vacuum and water tests. A brief
description of these will be given. The water test, the purpose of which
is to discover any leakage from the tubes, tube-plates, water pipes,
etc., into portions of the steam or air chambers, should be made first.
Water Tests of Condenser
The condenser is first thoroughly dried out, particular care being given
to the outside of the tubes and the bottom tube-plate P. Water is then
circulated through the tubes and chambers for an hour or two, after
which the pumps are stopped, all water is allowed to drain out and a
careful examination is made inside. Any water leaking from the tubes
above the bottom baffle-plate will ultimately be deposited upon that
plate. It is essential to stop this leakage if there be any, otherwise
the condensed steam measured during the consumption test will be
increased to the extent of the leakage. A slight leakage in a large
condenser will obviously not affect the results to any serious extent.
The safest course to adopt when a leak is discovered and it is found
inopportune to effect immediate repair is to measure the actual volume
of leakage over a specified period, and the quantity then being known it
can be subtracted from the volume of the condensed steam at the end of
the consumption test.
It is equally essential that no leakage shall occur between the bottom
tube-plate P and the tube ends. The soundness of the tube joints, and
the joint at the periphery of the tube-plate can be tested by well
covering the plate with water, the water chamber W and cooling chamber
having been previously emptied, and observing the under side of the
plate. It must be admitted that the practice of measuring the extent of
a water leak over a period, and afterward with this knowledge adjusting
the obtained quantities, is not always satisfactory. On no account
should any test be made with considerable water leakage inside the
condenser. The above method, however, is perhaps the most reliable to be
followed, if during its conduct the conditions of temperature in the
condenser are made as near to the normal test temperature as possible.
There are many condensers using salt water in their tubes, and in these
cases it would seem natural to turn to some analytical method of
detecting the amount of saline and foreign matter leaking into the
condensed steam. Unless, however, only approximate results are required,
such methods are not advocated. There are many reasons why they cannot
be relied upon for accurate results, among these being the variation in
the percentage of saline matter in the sea-water, the varying
temperature of the condenser tubes through which the water flows, and
the uncertainty of such analysis, especially where the percentage
leakage of pure saline matter is comparatively small.
The Vacuum Test
Having convinced himself of the satisfactory conduct of the condenser
under the foregoing simple preparatory water tests, the tester may
safely pass to considerations of vacuum. There exists a good
old-fashioned method of discovering the points of leakage in a vacuum
chamber, namely, that of applying the flame of a candle to all seams and
other vulnerable spots, which in the location of big leaks is extremely
valuable. Assuming that the turbine joints and glands have been found
capable of preventing any inleak of air, with only a small absolute
pressure of steam or air inside it, and, further, an extremely important
condition, with the turbine casing at high and low temperatures,
separately, a vacuum test can be conducted on the condenser alone.
This test consists of three operations. In the first place a high vacuum
is obtained by means of the air pump, upon the attainment of which
communication with everything else is closed, and results noted. The
second operation consists in repeating the above with the water
circulating through the condenser tubes, the results in this case also
being carefully tabulated. Before conducting the third test, the
condensers must be thoroughly warmed throughout, by running the turbine
for a short time if necessary, and after closing communication with
everything, allowing the vacuum to slowly fall.
A careful consideration and comparison of the foregoing tests will
reveal the capabilities of the condenser in the aspect in which it is
being considered, and will suggest where necessary the desirable steps
to be taken.
VI. TESTING A STEAM TURBINE[4]
[4] Contributed to _Power_ by Thomas Franklin.
Special Auxiliary Plant for Consumption Test
There are one or two points of importance in the conduct of a test on a
turbine and these will be briefly touched upon. Fig. 70 illustrates the
general arrangement of the special auxiliary plant necessary for
carrying through a consumption test, when the turbine exhaust passes
through a surface condenser. The condensed steam, after leaving the
condenser, passes along the pipe A to the pump, and is then forced along
the pipe B (leading under ordinary circumstances to the hot-well),
through the main water valve C directly to the measuring tanks. To enter
these the water has to pass through the valves D and E, while the valves
F and G are for quickly emptying the tanks when necessary, being of a
larger bore than the inlet valves. The inlet pipes H I are placed
directly above the outlet valves, and thus, when required, before any
measurements are taken, the water can flow directly through the outlet
valves, the pipes terminating only a short distance above them, away to
an auxiliary tank or directly to the hot-well. Levers K and L fulcrumed
at J and J are connected to the valve spindles by auxiliary levers. The
valve arrangement is such that by pulling down the lever K the inlet
valve D is opened and the inlet valve E is closed. Again, by pulling
down the lever L the outlet valve F is closed, while the outlet valve G
is also simultaneously closed.
[Illustration: FIG. 70]
During a consumption test the valves are operated in the following
manner: The lever K is pulled down, which opens the inlet valve to the
first tank and closes that to the second. The bottom lever L, however,
is lifted, which for the time being opens the outlet valve F, and
incidentally opens the valve G; the latter valve can; however, for the
moment be neglected. When the turbine is started, and the condensed
steam begins to accumulate in the condenser, the water is pumped along
the pipes and, both the inlet and outlet valves on the first tank being
open, passes through, without any being deposited in the tank, to the
drain. This may be continued until all conditions are right for a
consumption test and, the time being carefully noted, lever L is quickly
pulled down and the valves F and G closed. The first tank now gradually
fills, and after a definite period, say fifteen minutes, the lever K is
pushed up, thus diverting the flow into the second tank. While the
latter is filling, the water in the first tank is measured, and the tank
emptied by a large sluice valve, not shown.
The operation of alternately filling, measuring, and emptying the two
measuring tanks is thus carried on until the predetermined time of
duration of test has expired, when the total water as measured in the
tanks, and representing the amount of steam condensed during that time,
is easily found by adding together the quantities given at each
individual measurement.
All that are necessary to insure successful results from a plant similar
to this are care and accuracy in its operation and construction.
Undoubtedly in most cases it is preferable to weigh the condensed steam
instead of measuring the volume passed, and from that to calculate the
weight. If dependence is being placed upon the volumetric method, it is
advisable to lengthen the duration of the test considerably, and if
possible to measure the feed-water evaporated at the same time. Such a
course, however, would necessitate little change, and none of a radical
nature, from the arrangement described. Where, however, the measuring
method is adopted, the all-important feature, requiring on the tester's
part careful personal investigation, is the graduation of the tanks. It
facilitates this operation very considerably when the receptacles are
graduated upon a weight scale. That is to say, whether or not a vertical
scale showing the actual hight of water be placed inside the tank, it is
advisable to have a separate scale indicating at once to the attendant
the actual contents, by weight, of the tank at any time. It is the
tester's duty to himself to check the graduation of this latter scale by
weighing the water with which he performs the operation of checking.
Apart from the foregoing, there is little to be said about the measuring
apparatus. As has been stated, accuracy of result depends in this
connection, as in all others, upon careful supervision and sound and
accurate construction, and this the tester can only positively insure by
exhaustive inspection in the one case and careful deliberation in the
conduct of the other.
It will be readily understood that the procedure--and this implies some
limitations--of a test is to an extent controlled by the conditions, or
particular environment of the moment. This is strictly true, and as a
consequence it is often impossible, in a maker's works, for example, to
obtain every condition, coinciding with those specified, which are to be
had on the site of final operation only. For this reason it would
appear best to reserve the final and crucial test of a machine, which
test usually in the operating sense restricts a prime mover in certain
directions with regard to its auxiliary plant, etc., until the machine
has been finally erected on its site. Obviously, unless a machine had
become more or less standardized, a preliminary consumption test would
be necessary, but once this primary qualification respecting consumption
had been satisfactorily settled, there appears to be no reason why
exhaustive tests in other directions should not all be carried out upon
the site, where the conditions for them are so much more favorable.
When the steam consumption of a steam turbine is so much higher than the
guaranteed quantity, it usually takes little less than a reconstruction
to put things right. The minor qualifications of a machine, however,
which can be examined into and tested with greater ease, and usually at
considerably less expense, upon the site, and consequently under
specified conditions, may be advantageously left over until that site is
reached, where it is obvious that any shortcomings and general
deficiency in performance will be more quickly detected and diagnosed.
Test Loads from the Tester's View-point
Before proceeding to describe the points of actual interest in the
consumption test, a few considerations respecting test loads will be
dealt with from the tester's point of view. Here again we often find
ourselves restricted, to an extent, by the surrounding conditions. The
very first considerations, when undertaking to carry out a consumption
test, should be devoted to obtaining the steadiest possible lead
[Transcriber: load?]. It may be, and is in many cases, that
circumstances are such as to allow a steady electrical load to be
obtained at almost any time. On the other hand an electrical load of any
description is sometimes not procurable at all, without the installation
of a special plant for the purpose. In such cases a mechanical friction
load, as, for example, that obtained by the water brake, is sometimes
available, or can easily be procured. Whereas, however, this type of
load may be satisfactory for small machines, it is usually quite
impossible for use with large units, of, say, 5000 kilowatts and upward.
It is seldom, however, that turbines are made in large sizes for
directly driving anything but electrical plants, although there is every
possibility of direct mechanical driving between large steam turbines
and plants of various descriptions, shortly coming into vogue, so that
usually there exist some facilities for obtaining an electrical load at
both the maker's works and upon the site of operation.
One consideration of importance is worth inquiring into, and this has
relation to the largest turbo-generators supplied for power-station and
like purposes. Obviously, the testing of, say, a 7000-kilowatt
alternator by any standard electrical-testing method must entail
considerable expense, if such a test is to be carried out in the maker's
works. Nor would this expense be materially decreased by transferring
the operations to the power-station, and there erecting the necessary
electrical plant for obtaining a water load, or any other installation
of sufficient capacity to carry the required load according to the rated
full capacity of the machine.
Assuming, then, that there exist no permanent facilities at either end,
namely the maker's works and the power station, for adequately procuring
a steady electrical-testing load of sufficient capacity, there still
remains, in this instance, an alternative source of power which is
usually sufficiently elastic to serve all purposes, and this is of
course the total variable load procurable from the station bus-bars. It
is conceivable that one out of a number of machines running in parallel
might carry a perfectly steady load, the latter being a fraction of a
total varying quantity, leaving the remaining machines to receive and
deal with all fluctuations which might occur. Even in the event of there
being only two machines, it is possible to maintain the load on one of
them comparatively steady, though the percentage variation in load on
either side of the normal would in the latter case be greater than in
the previous one. This is accomplished by governor regulation after the
machines have been paralleled. For example, assuming three
turbo-alternators of similar make and capacity to be running in
parallel, each machine carrying exactly one-third of the total
distributed load, it is fair to regard the governor condition, allowing
for slight mechanical disparities of construction, of all three machines
as being similar; and even in the case of three machines of different
capacity and construction, the governor conditions when the machines
are paralleled are more or less relatively and permanently fixed in
relation to one another. In other words, while the variation in load on
each machine is the same, the relative variation in the governor
condition must be constant.
By a previously mentioned system of governor regulation, however, it is
possible, considering again for a moment the case of three machines in
parallel, by decreasing the sensitiveness of one governor only, to
accommodate nearly all the total variation in load by means of the two
remaining machines, the unresponsiveness of the one governor to change
in speed maintaining the load on that machine fairly constant. By this
method, at any rate, the variation in load on any one machine can be
minimized down to, say, 3 per cent, either side of the normal full load.
There is another and more positive method by which a perfectly steady
load can be maintained upon one machine of several running in parallel.
This may be carried out as follows: Suppose, in a station having a total
capacity of 20,000 kilowatts, there are three machines, two of 6000
kilowatts each, and one of 8000 kilowatts, and it is desired to carry
out a steady full-load test upon one of the 6000 kilowatts units.
Assuming that the test is to be of six hours' duration, and that the
conditions of load fluctuations upon the station are well known, the
first step to take is to select a period for the test during which the
total load upon all machines is not likely to fall below, say, 8000
kilowatts. The tension upon the governor spring of the turbine to be
tested must then be adjusted so that the machine on each peak load is
taxed to its utmost normal capacity; and even when the station load
falls to its minimum, the load from the particular machine shall not be
released sufficiently to allow it to fall below 6000 kilowatts. Under
these conditions, then, it may be assumed that although the load on the
test machine will vary, it cannot fall below 6000 kilowatts. Therefore,
all that remains to be done to insure a perfectly steady load equal to
the normal full load of the machine, or 6000 kilowatts, is to fix the
main throttle or governing valve in such a position that the steam
passing through at constant pressure is just capable of sustaining full
speed under the load required. When this method is adopted, it is
desirable to fix a simple hight-adjusting and locking mechanism to the
governing-valve spindle. The load as read on the indicating wattmeter
can then be very accurately varied until correct, and farther varied, if
necessary, should any change occur in the general conditions which might
either directly or indirectly bring about a change of load.
Preparing the Turbine for Testing
All preliminary labors connected with a test being satisfactorily
disposed of, it only remains to place the turbines under the required
conditions, and to then proceed with the test. For the benefit of those
inexperienced in the operation of large turbines, we will assume that
such a machine is about to be started for the purpose outlined.
It is always advisable to make a strict practice of getting all the
auxiliary plant under way before starting up the turbine. In handling a
turbine plant the several operations might be carried through in the
following order:
(1) Circulating oil through all bearings and oil chambers.[5]
(2) Starting of condenser circulating-water pumps, and continuous
circulation of circulating water through the tubes of condenser.
(3) Starting of pump delivering condensed steam from the condenser
hot-well to weighing tanks.
(4) Starting of air pump, vacuum being raised as high as possible within
condenser.
(5) Sealing of turbine glands, whether of liquid or steam type, no
adjustment of the quantity of sealing fluid being necessary, however, at
this point.
(6) Adjustment of valves on and leading to the water-weighing tanks.
(7) Opening of main exhaust valve or valves between turbine and
condenser.
(8) Starting up of turbine and slowly running to speed.
(9) Application of load, and adjustment of gland-sealing steam.
[5] In a self-contained system, where the oil pump is usually driven
from the turbine spindle, this would of course be impossible. In the
gravity and allied systems, however, it should always be the first
operation performed. The tests for oil consumption, described
previously, having been carried out, it is assumed that suitable means
have been adopted to restrict the total oil flow through the bearings to
a minimum quantity.
The running to speed of large turbo-alternators requires considerable
care, and should always be done slowly; that is to say the rate of
acceleration should be slow. It is well known that the vibration of a
heavy unit is accompanied by a synchronous or non-synchronous vibration
of the foundation upon which it rests. The nearest approach to perfect
synchronism between unit and foundation is obtained by a gradual rise in
speed. A machine run up to speed too quickly might, after passing the
critical speed, settle down with little visible vibration, but at a
later time, even hours after, suddenly begin vibrating violently from no
apparent cause. The chances of this occurring are minimized by slow and
careful running to speed.
Whether the machine being tested is one of a number running in parallel,
or a single unit running on a steady water load, the latter should in
all cases be thrown on gradually until full load is reached. A
preliminary run of two or three hours--whenever possible--should then be
made, during which ample opportunity is afforded for regulating the
conditions in accordance with test requirements. The tester will do well
during the last hour of this trial run to station his recorders at their
several posts and, for a short time at least, to have a complete set of
readings taken at the correct test intervals. This more particularly
applies to the electrical water, superheat and vacuum readings. In the
case of a turbo-alternator the steadiness obtainable in the electrical
load may determine the frequency of readings taken, both electrical and
otherwise. On a perfectly steady water-tank load, for example, it may
be sufficiently adequate to read all wattmeters, voltmeters, and
ammeters from standard instruments at from one- to two-minute intervals.
Readings at half-minute intervals, however, should be taken with a
varying load, even when the variation is only slight.
The water-measurement readings may of course be taken at any suitable
intervals, the time being to an extent determined by the size of the
measuring tanks or the capacity of the weighing machine or machines.
When designing the measuring apparatus, the object should be to
minimize, within economical and practical range, the total number of
weighings or measurements necessary. Consequently, no strict time of
interval between individual weighings or measurements can be given in
this case. It may be said, however, that it is not desirable to take
these at anything less than five-minute intervals. Under ordinary
circumstances a three- to five-minute interval is sufficient in the case
of all steam-pressure, vacuum--including mercurial columns and
barometer--superheat and temperature readings.
Gland and Hot-Well Regulation
There are two highly important features requiring more or less constant
attention throughout a test, namely the gland and hot-well regulation.
For the present purpose we may assume that the glands are supplied with
either steam or water for sealing them. All steam supplied to the
turbine obviously goes to swell the hot-well contents, and to thus
increase the total steam consumption. The ordinary steam gland is in
reality a pressure gland. At both ends of the turbine casing is an
annular chamber, surrounding the turbine spindle at the point where it
projects through the casing. A number of brass rings on either side of
this chamber encircle the spindle, with only a very fine running
clearance between the latter and themselves. Steam enters the gland
chamber at a slight pressure, and, when a vacuum exists inside the
turbine casing, tends to flow inward. The pressure, however, inside the
gland is increased until it exceeds that of the atmosphere outside, and
by maintaining it at this pressure it is obvious that no air can
possibly enter the turbine through the glands, to destroy the vacuum.
The above principle must be borne in mind during a test upon a turbine
having steam-fed glands. Perhaps the best course to follow--in view of
the economy of gland steam consumption necessary--is as follows:
During the preliminary non-test run, full steam is turned into both
glands while the vacuum is being raised, and maintained until full load
has been on the turbine for some little time. The vacuum will by this
time have probably reached its maximum, and perhaps fallen to a point
slightly lower, at which hight it may be expected to remain, other
conditions also remaining constant. The gland steam must now be
gradually turned off until the amount of steam vapor issuing from the
glands is almost imperceptible. This should not lower the vacuum in the
slightest degree. By gradual degrees the gland steam can be still
farther cut down, until no steam vapor at all can be discerned issuing
from the gland boxes. This reduction should be continued until a point
is reached at which the vacuum is affected, when it must be stopped and
the amount of steam flowing to the gland again increased very slightly,
just enough to bring the vacuum again to its original hight. The steam
now passing into the glands is the minimum required under the
conditions, and should be maintained as nearly constant as possible
throughout the test. Practically all steam entering the glands is drawn
into the turbine, and thence to the condenser, and under the
circumstances it may be assumed the increase in steam consumption
arising from this source is also a minimum.
There is one mechanical feature which has an important bearing upon the
foregoing question, and which it is one of the tester's duties to
investigate. This is illustrated in Fig. 71, which shows a turbine
spindle projecting through the casing. The gland box is let into the
casing as shown. Brass rings A calked into the gland box encircle the
shaft on either side of the annular steam space S. As the clearance
between the turbine spindle and the rings A is in a measure instrumental
in determining the amount of steam required to maintain a required
pressure inside the chamber, it is obvious that this clearance should be
minimum. An unnecessarily large clearance means a proportionally large
increase in gland steam consumption and _vice versa_.
[Illustration: FIG. 71]
When the turbine glands are sealed with water, all water leakage which
takes place into the turbine, and ultimately to the condenser hot-well,
must be measured and subtracted from the hot-well contents at the end of
a test.
The foregoing remarks would not apply to those cases in which the gland
supply is drawn from and returned to the hot-well, or a pipe leading
from the hot-well. Then no correction would be necessary, as all water
used for gland purposes might be assumed as being taken from the
measuring tanks and returned again in time for same or next weighing or
measurement.
General Considerations
There are a few principal elementary points which it is necessary always
to keep in mind during the conduct of a test. Among these are the
effects of variation in vacuum, superheat, initial steam pressure, and,
as already indicated, in load. There exist many rules for determining
the corrections necessitated by this variation. For example, it is often
assumed that 9 degrees Fahrenheit, excess or otherwise, above or below
that specified, represents an increase or reduction in efficiency of
about 1 per cent. It is probable that the percentage increase or
decrease in steam consumption, in the case of superheat, can be more
reliably calculated than in other cases, as, for example, vacuum; but
the increase cannot be said to be due solely to the variation in
superheat. In other words, the individuality of the particular turbine
being tested always contributes something, however small this something
may be, to the results obtained.
These remarks are particularly applicable where vacuum is concerned.
Here again rules exist, one of these being that every additional inch of
vacuum increases the economy of the turbine by something slightly under
half a pound of steam per kilowatt-hour. But a moment's consideration
convinces one of the utter unreliability of such rules for general
application. It is, for instance, well known that many machines, when
under test, have demonstrated that the total increase in the water rate
is very far from constant. A machine tested, for example, gave
approximately the following results, the object of the test being to
discover the total increase in the water rate per inch decrease in
vacuum:
From 27 inches to 26 inches, 4.5 per cent.
From 26.2 inches to 24.5 inches, 2.5 per cent.
This illustrates to what an extent the ratio of increase can vary, and
it must be borne in mind that it is very probable that the variation is
different in different types and sizes of machines.
There can exist, therefore, no empirical rules of a reliable nature upon
which the tester can base his deductions. The only way calculated to
give satisfaction is to conduct a series of preliminary tests upon the
turbine undergoing observation, and from these to deduce all information
of the nature required, which can be permanently recorded in a set of
curves for reference during the final official tests.
In conclusion, it must be admitted that many published tests outlining
the performances of certain makes of turbine are unreliable. To
determine honestly the capabilities of any machine in the direction of
steam economy is an operation requiring time, and unbiased and accurate
supervision. By means of such assets as "floating quantities," short
tests during exceptionally favorable conditions, and disregard of the
vital necessity of running a test under the proper specified conditions,
it is comparatively easy to obtain results apparently highly
satisfactory, but which under other conditions might be just the
reverse. These considerations are, however, unworthy of the tester
proper.
VII. AUXILIARIES FOR STEAM TURBINES[6]
[6] Contributed to _Power_ by Thomas Franklin.
The Jet Condenser
The jet condenser illustrated in Fig. 72 is singularly well adapted for
the turbine installation. As the type has not been so widely adopted as
the more common forms of jet condenser and the surface types, it may
prove of interest to describe briefly its general construction and a few
of its special features in relation to tests.
[Illustration: FIG. 72]
Referring to the figure, C is the main condenser body. Exhaust steam
enters at the left-hand side through the pipe E, condensing water
issuing through the pipe D at the opposite side. Passing through the
short conical pipe P, the condensing water enters the cylindrical
chamber W and falls directly upon the spraying cone S. The hight of this
spraying cone is determined by the tension upon the spring T, below the
piston R, the latter being connected to the cone by a spindle L. An
increase of the water pressure inside the chamber W will thus compress
the spring, and the spraying cone being consequently lowered increases
the aperture between it and the sloping lower wall of the chamber W,
allowing a greater volume of water to be sprayed. The piston R
incidentally prevents water entering the top vapor chamber V. From the
foregoing it can be seen that this condenser is of the contra-flow type,
the entering steam coming immediately into contact with the sprayed
water. The perforated diaphragm plate F allows the vapor to rise into
the chamber V, from which it is drawn through the pipe A to the air
pump. A relief valve U prevents an excessive accumulation of pressure
in the vapor chamber, this valve being obviously of delicate
construction, capable of opening upon a very slight increase of the
internal pressure over that of the atmosphere. Condensed steam and
circulating water are together carried down the pipe B to the well Z,
from which a portion may be carried off as feed water, and the remainder
cooled and passed through the condenser again. Under any circumstances,
whether the air pump is working or not, a certain percentage of the
vapor in the condenser is always carried down the pipe B, and this
action alone creates a partial vacuum, thus rendering the work of the
air pump easier. As a matter of fact, a fairly high vacuum can be
maintained with the air pump closed down, and only the indirect pumping
action of the falling water operating to rarify the contents of the
condenser body. It is customary to place the condenser forty or more
feet above the circulating-water pump, the latter usually being a few
feet below the turbine.
Features Demanding Attention
When operating a condenser of this type, the most important features
requiring preliminary inspection and regulation while running are:
(a) Circulating-water regulation.
(b) Freedom of all mechanical parts of spraying mechanism.
(c) Relief-valve regulation.
(d) Water-cooling arrangements.
The tester will, however, devote his attention to a practical survey of
the condenser and its auxiliaries, before running operations commence.
A preliminary vacuum test ought to be conducted upon the condenser body,
and the exhaust piping between the condenser and turbine. To accomplish
this the circulating-water pipe D can be filled with water to the
condenser level. The relief valve should also be water-sealed. Any
existing leakage can thus be located and stopped.
Having made the condenser as tight as possible within practical limits,
vacuum might be again raised and, with the same parts sealed, allowed to
fall slowly for, say, ten minutes. A similar test over an equal period
may then be conducted with the relief valve not water-sealed. A
comparison of the times taken for an equal fall of vacuum in inches,
under the different conditions, during the above two tests, will reveal
the extent of the leakage taking place through the relief valve. It
seems superfluous to add that the fall of vacuum in both the foregoing
tests must not be accelerated in any way, but must be a result simply of
the slight inevitable leakage which is to be found in every system.
On a comparatively steady load, and with consequently only small
fluctuation in the volume of steam to be condensed, the conditions are
most favorable for regulating the amount of circulating water necessary.
Naturally, an excess of water above the required minimum will not affect
the pressure conditions inside the condenser. It does, however, increase
the quantity of water to be handled from the hot-well, and incidentally
lowers the temperature there, which, whether the feed-water pass through
economizers or otherwise, is not advisable from an economical
standpoint. Thus there is an economical minimum of circulating water to
be aimed at, and, as previously stated, it can best be arrived at by
running the turbine under normal load and adjusting the flow of the
circulating water by regulating the main valve and the tension upon the
spring T. Under abnormal conditions, the breakdown of an air pump, or
the sudden springing of a bad leak, for instance, the amount of
circulating water can be increased by a farther opening of the main
valve if necessary, and a relaxation of the spring tension by hand; or,
the spring tension might be automatically changed immediately upon the
vacuum falling.
The absolute freedom of all moving parts of the spraying mechanism
should be one of the tester's first assurances. To facilitate this, it
is customary to construct the parts, with the exception of the springs,
of brass or some other non-corrosive metal. The spraying cone must be
thoroughly clean in every channel, to insure a well-distributed stream
of water. Nor is it less important that careful attention be given to
the setting and operation of the relief valve, as will be seen later.
The obvious object of such a valve is to prevent the internal condenser
pressure ever being maintained much higher than the atmospheric
pressure. A number of carefully designed rubber flap valves, or one
large one, have been found to act successfully for this purpose,
although a balanced valve of more substantial construction would appear
to be more desirable.
Importance of Relief Valves
The question of relief valves in turbine installations is an important
one, and it seems desirable at this point to draw attention to another
necessary relief valve and its function, namely the turbine atmospheric
valve. As generally understood, this is placed between the turbine and
condenser, and, should the pressure in the latter, owing to any cause,
rise above that of the atmosphere, it opens automatically and allows the
exhaust steam to flow through it into the atmosphere, or into another
condenser.
A general diagrammatic arrangement of a steam turbine, condenser, and
exhaust piping is shown in Fig. 73. Connected to the exhaust pipe B,
near to the condenser, is the automatic atmospheric valve D, from which
leads the exhaust piping E to the atmosphere. The turbine relief valve
is shown at F, and the condenser relief valve at G. The main exhaust
valve between turbine and condenser is seen at H. We have here three
separate relief valves: one, F, to prevent excessive pressure in the
turbine: the second, D, an atmospheric valve opening a path to the air,
and, in addition to preventing excessive pressure accumulating, also
helping to keep the temperature of the condenser body and tubes low; the
third, the condenser relief valve G, which in itself ought to be capable
of exhausting all steam from the turbine, should occasion demand it.
[Illustration: FIG. 73]
Assuming a plant of this description to be operating favorably, the
conditions would of necessity be as follows: The valves F, D, and G, all
closed; the valve H open. Suppose that, owing to sudden loss of
circulating water, the vacuum fell to zero. The condenser would at once
fill with steam, a slight pressure would be set up, and whichever of the
three valves happened to be set to blow off at the lowest pressure would
do so. Now it is desirable that the first valve to open under such
circumstances should be the atmospheric valve D. This being so, the
condenser would remain full of steam at atmospheric pressure until the
attendant had had time to close the main hand-or motor-operated exhaust
valve H, which he would naturally do before attempting to regain the
circulation of the condensing water. Again, assume the installation to
be running under the initial conditions, with the atmospheric valve D
and all remaining valves except H closed.
Suppose the vacuum again fell to zero from a similar cause, and,
further, suppose the atmospheric valve D failed to operate
automatically. The only valves now capable of passing the exhaust steam
are the turbine and condenser relief valves F and G. Inasmuch as the
pressures at exhaust in the turbine proper, on varying load, vary over a
considerably greater range than the small fairly constant absolute
pressures inside the condenser, it is obviously necessary to allow for
this factor in the respective setting of these two relief valves. In
other words, the obvious deduction is to set the turbine relief valve to
blow off at a higher pressure than the condenser relief valve, even
when considering the question with respect to condensing conditions
only. In this second hypothetical case, then, with a closed and disabled
atmospheric valve, the exhaust must take place through the condenser,
until the turbine can be shut down, or the circulating water regained
without the former course being found necessary.
There is one other remote case which may be assumed, namely, the
simultaneous refusal of both atmospheric and condenser relief valves to
open, upon the vacuum inside the condenser being entirely lost. The
exhaust would then be blown through the turbine relief valve F, until
the plant could be closed down.
Although the conditions just cited are highly improbable in actual
practice, it can at once be seen that to insure the safety of the
condenser, absolutely, the turbine relief valve must be set to open at a
comparatively low pressure, say 40 pounds by gage, or thereabouts. To
set it much lower than this would create a possibility of its leaking
when the turbine was making a non-condensing run, and when the pressure
at the turbine exhaust end is often above that of the atmosphere. From
every point of view, therefore, it is advisable to make a minute
examination of all relief valves in a system, and before a test to
insure that these valves are all set to open at their correct relative
pressures.
It must be admitted that the practice of placing a large relief valve
upon a condenser in addition to the atmospheric exhausting valve is by
no means common. The latter valve, where surface condensing is adopted,
is often thought sufficient, working in conjunction with a quickly
operated main exhaust valve. Similarly, with a barometric condenser as
that illustrated in Fig. 72, the atmospheric exhaust valve D (seen in
Fig. 73) is sometimes dispensed with. This course is, however,
objectionable, for upon a loss of vacuum in the turbine, all exhaust
steam must pass through the condenser body, or the entire plant be
closed down until the vacuum is regained. The simple construction of the
barometric condenser, however, is in such an event much to its
advantage, and the passage of the hot steam right through it is not
likely to seriously warp or strain any of its parts, as might probably
happen in the case of a surface condenser.
The question of the advisability of thus adding to a plant can only be
fairly decided when all conditions, operating and otherwise, are fully
known. For example, if we assume a large turbine to be operating on a
greatly varying load, and exhausting into a condenser, as that in Fig.
72, and, further, having an adequate stand-by to back it up, one's
obvious recommendation would be to equip the installation with both a
condenser relief valve and an atmospheric valve, in addition, of course,
to the main exhaust valve, which is always placed between the
atmospheric valve and condenser. There are still other considerations,
such as water supply, condition of circulating water, style of pump,
etc., which must all necessarily have an obvious bearing upon the
settlement of this question; so that generalization is somewhat out of
place, the final design in all cases depending solely upon general
principles and local conditions.
Other Necessary Features of a Test
In connection with the condenser, of any type, and its auxiliaries,
there remain a few necessary examinations and operations to be
conducted, if it is desired to obtain the very best results during the
test. It will be sufficient to just outline them, the method of
procedure being well known, and the requirement of any strict routine
being unnecessary. These include:
(1) A thorough examination of the air-pump, and, if possible, an equally
careful examination of diagrams taken from it when running on full
load. Also careful examination of the piping, and of any other
connections between the air pump and condenser, or other auxiliaries. It
will be well in this examination to note the general "lay" of the air
pipes, length, hight to which they rise above condenser and air pump,
facilities for drainage, etc., as this information may prove valuable in
determining the course necessary to rectify deficiencies which may later
be found to exist.
(2) In a surface condenser, inspection of the pumps delivering condensed
steam to the measuring tanks or hot-well; inspection of piping between
the condenser and the pump, and also between the pump and measuring
tanks. If these pumps are of the centrifugal type it is essential to
insure, for the purposes of a steam-consumption test, as much regularity
of delivery as possible.
(3) In the case of a consumption test upon a turbine exhausting into a
barometric condenser, and where the steam consumed is being measured by
the evaporation in the boiler over the test period, time must be devoted
to the feed-pipes between the feed-water measuring meter or tank and the
boilers. Under conditions similar to those operating in a plant such as
that shown in Fig. 72, the necessary boiler feed might be drawn from the
hot-well, the remainder of the hot-well contents probably being pumped
through water coolers, or towers, for circulating through the condenser.
With the very best system, it is possible for a slight quantity of oil
to leak into the exhaust steam, and thence to the hot-well. In its
passage, say along wooden conduits, to the measuring tank or meter, this
water would probably pass through a number of filters. The efficiency of
these must be thoroughly insured. It is unusual, in those cases where a
simple turbine steam-consumption test is being carried out, and not an
efficiency test of a complete plant, to pass the measured feed-water
through economizers. Should the latter course, owing to special
conditions, become necessary, a careful examination of all economizer
pipes would be necessary.
(4) The very careful examination of all thermometer pockets, steam- and
temperature-gage holes, etc., as to cleanliness, non-accumulation of
scale, etc.
Special Auxiliaries Necessary
Having outlined the points of interest and importance in connection with
the more permanent features of a plant, we arrive at the preparation and
fitting of those special auxiliaries necessary to carry on the test.
[Illustration: FIG. 74]
It is customary, when carrying out a first test, upon both prime mover
and auxiliaries, to place every important stage in the expansion in
communication with a gage, so that the various pressures may be recorded
and later compared with the figures of actual requirement. To do this,
in the case of the turbine, it is necessary to bore holes in the cover
leading to the various expansion chambers, and into each of these holes
to screw a short length of steam pipe, having preferably a loop in its
length, to the other end of which the gage is attached. Fig. 74
illustrates, diagrammatically, a complete turbine installation, and
shows the various points along the course taken by the steam at which it
is desirable to place pressure gages. The figure does not show the
high-pressure steam pipe, nor any of the turbine valves. With regard to
these, it will be desirable to place a steam gage in the pipe,
immediately before the main stop-valve, and another immediately after
it. Any fall of pressure between the two sides of the valve can thus be
detected. To illustrate this clearly, Fig. 75 is given, showing the
valves of a turbine, and the position of the gages connected to them.
The two gages E and F on either side of the main stop-valve A are also
shown. The steam after passing through the valve, which, in the case of
small turbines, is hand-operated, goes in turn through the automatic
stop-valve B, the function of which is to automatically shut steam off
should the turbine attain a predetermined speed above the normal, the
steam strainer C, and finally through the governing valve D into the
turbine. As shown, gages G and H are also fitted on either side of the
strainer, and these, in conjunction with gages E and F, will enable any
fall in pressure between the first two valves and the governing valve to
be found. Up to the governing-valve inlet no throttling of the steam
ought to take place under normal conditions, i.e., with all valves open,
and consequently any fall in pressure between the steam inlet and this
point must be the result of internal wire-drawing. By placing the gages
as shown, the extent to which this wire-drawing affects the pressures
obtainable can be discovered.
[Illustration: FIG. 75]
On varying and even on normal and steady full load, the steam is more or
less reduced in pressure after passing through the governing valve D; a
gage I must consequently be placed between the valve, preferably on the
valve itself, and the turbine. Returning to Fig. 74, the gages shown are
A, B, C, D, and E, connected to the first, second, third, fourth, and
fifth expansions; also F in the turbine and exhaust space, where there
are no blades, G in the exhaust pipe immediately before the main exhaust
valve E (see Fig. 73), and H connected to the condenser. On condensing
full load it is probable that A, B, and C will all register pressures
above the atmosphere, while gages D, E, F, and G will register pressures
below the atmosphere, being for this purpose vacuum gages. On the other
hand, with a varying load, and consequently varying initial pressures,
one or two of the gages may register pressure at one moment and vacuum
at another. It will therefore be necessary to place at these points
compound gages capable of registering both pressure and vacuum. With the
pressures in the various stages constantly varying, however, a gage is
not by any means the most reliable instrument for recording such
variations. The constant swinging of the finger not only renders
accurate reading at any particular moment both difficult and, to an
extent, unreliable, but, in addition, the accompanying sudden changes of
condition, both of temperature and pressure, occurring inside the gage
tube, in a comparatively short time permanently warp this part, and thus
altogether destroy the accuracy of the gage. It is well known that even
with the best steel-tube gages, registering comparatively steady
pressures, this warping of the tube inevitably takes place. The quicker
deterioration of such gage tubes, when the gage is registering quickly
changing pressures, can therefore readily be conceived, and for this
reason alone it is desirable to have all gages, whatever the conditions
under which they work, carefully tested and adjusted at short intervals.
If it is desired to obtain reliable registration of the several
pressures in the different expansions of a turbine running on a varying
load, it would therefore seem advisable to obtain these by some type of
external spring gage (an ordinary indicator has been found to serve well
for this purpose) which the sudden internal variations in pressure and
temperature cannot deleteriously affect.
In view of the great importance he must attach to his gage readings, the
tester would do well to test and calibrate and adjust where necessary
all the gages he intends using during a test. This he can do with a
standard gage-testing outfit. By this means only can he have full
confidence in the accuracy of his results.
In like manner it is his duty personally to supervise the connecting and
arrangement of the gages, and the preliminary testing for leakage which
can be carried out simultaneously with the vacuum test made upon the
turbine casing.
Where Thermometers are Required
Equally important with the foregoing is the necessity of calibrating and
testing of all thermometers used during a test. Where possible it is
advisable to place new thermometers which have been previously tested
at all points of high temperature. Briefly running them over, the points
at which it is necessary to place thermometers in the entire system of
the steam and condensing plant are as follows:
(1) A thermometer in the steam pipe on the boiler, where the pipe leaves
the superheater.
(2) In the steam pipe immediately in front of the main stop-valve, near
point E in Fig. 75.
(3) In the main governing valve body (see I, Fig. 75) on the inlet side.
(4) In the main governing valve body on the turbine side, which will
register temperatures of steam after it has passed through the valve.
(5) In the steam-turbine high-pressure chamber, giving the temperature
of the steam before it has passed through any blades.
(6) In the exhaust chamber, giving the temperature of steam on leaving
the last row of blades.
(7) In the exhaust pipe near the condenser.
(8) In the condenser body.
(9) In the circulating-water inlet pipe close to the condenser.
(10) In the circulating-water outlet pipe close to the condenser.
(11) In the air-pump suction pipe close to the condenser.
(12) In the air-pump suction pipe close to the air pump.
It is not advisable to place at those vital points, the readings at
which directly or indirectly affect the consumption, two thermometers,
say one ordinary chemical thermometer and one thermometer of the gage
type, thus eliminating the possibility of any doubt which might exist
were only one thermometer placed there.
There is no apparent reason why one should attempt to take a series of
temperature readings during a consumption test on varying load. The
temperatures registered under a steady load test can be obtained with
great reliability, but on a varying load, with constantly changing
temperatures at all points, this is impossible. This is, of course,
owing to the natural sluggishness of the temperature-recording
instruments, of whatever class they belong to, in responding to changes
of condition. As a matter of fact, the possibility of obtaining
correctly the entire conditions in a system running under greatly
varying loads is very doubtful indeed, and consequently great reliance
cannot be placed upon figures obtained under such conditions.
A few simple calculations will reveal to the tester his special
requirements in the direction of measuring tanks, piping, etc., for his
steam consumption test. Thus, assuming the turbine to be tested to be of
3000 kilowatt capacity normal load, with a guaranteed steam consumption
of, say, 14.5 pounds per kilowatt-hour, he calculates the total water
rate per hour, which in this case would be 43,500 pounds, and designs
his weighing or measuring tanks to cope with that amount, allowing, of
course, a marginal tank volume for overload requirements.
VIII. TROUBLES WITH STEAM TURBINE AUXILIARIES[7]
[7] Contributed to _Power_ by Walter B. Gump.
The case about to be described concerns a steam plant in which there
were seven cross-compound condensing Corliss engines, and two Curtis
steam turbines. The latter were each of 1500-kilowatt capacity, and were
connected to surface condensers, dry-vacuum pumps, centrifugal, hot-well
and circulating pumps, respectively. In the illustration (Fig. 76), the
original lay-out of piping is shown in full lines. Being originally a
reciprocating plant it was difficult to make the allotted space for the
turbines suitable for their proper installation. The trouble which
followed was a perfectly natural result of the failure to meet the
requirements of a turbine plant, and the description herein given is but
one example of a great many where the executive head of a concern
insists upon controlling the situation without regard to engineering
advice or common sense.
[Illustration: FIG. 76. TURBINE AUXILIARIES AND PIPING]
Circulating Pump Fails to Meet Guarantee
Observing the plan view, it will be seen that the condensers for both
turbines receive their supply of cooling water from the same supply
pipe; that is, the pipes, both suction and discharge, leading to No. 1
condenser are simply branches from No. 2, which was installed first
without consideration for a second unit. When No. 1 was installed there
was a row of columns from the basement floor to the main floor extending
in a plane which came directly in front of the condenser. The column P
shown in the plan was so located as to prevent a direct connection
between the centrifugal circulating pump and the condenser inlet. The
centrifugal pump was direct-connected to a vertical high-speed engine,
and the coupling is shown at E in the elevation.
Every possible plan was contemplated to accommodate the engine and pump
without removing any of the columns, and the arrangement shown was
finally adopted, leaving the column P in its former place by employing
an S-connection from the pump to the condenser. It should be stated that
the pump was purchased under a guarantee to deliver 6000 gallons per
minute under a head of 50 feet, with an impeller velocity of 285
revolutions per minute. The vertical engine to which the pump was
connected proved to be utterly unfit for running at a speed beyond 225
to 230 revolutions per minute, and in addition the S-bend would
obviously reduce the capacity, even at the proper speed of the impeller.
Besides these factors there was another feature even more serious. It
was found that when No. 2 unit was operating No. 1 could not get as
great a quantity of circulating water as when No. 2 was shut down. This
was because No. 2 was drawing most of the water, and No. 1 received only
that which No. 2 could not pull from the suction pipe A. This will be
clear from the fact that the suction and discharge pipes for No. 1 were
only 16 inches, while those of No. 2 were 20 inches and 16 inches,
respectively. The condenser for No. 2 had 1000 square feet less cooling
surface than No. 1, which had 6000 square feet and was supplied with
cooling water by means of two centrifugal pumps of smaller capacity
than for No. 1 and arranged in parallel. These were each driven by an
electric motor, and were termed "The Siamese Twins," due to the way in
which they were connected.
The load factor of the plant ranged from 0.22 to 0.30, the load being
almost entirely lighting, so that for the winter season the load factor
reached the latter figure. The day load was, therefore, light and not
sufficient to give one turbine more than from one-fourth to one-third
its rated capacity. Under these conditions No. 1 unit was able to
operate much more satisfactorily than when fully loaded, because of the
fact that the cooling water was more effective. This was, of course, all
used by No. 1 unit when No. 2 was not operating. At best, however, it
was found that the vacuum could not be made to exceed 24 inches, and
during the peak, with the two turbines running, the vacuum would often
drop to 12 inches. A vacuum of 16 inches or 18 inches on the peak was
considered good.
An Investigation
Severe criticism "rained" heavily upon the engineer in charge, and
complaints were made in reference to the high oil consumption. An
investigation on the company's part followed, and the firm which
furnished the centrifugal pump and engine was next in order to receive
complaints. Repeated efforts were made to increase the speed of the
vertical engine to 285 revolutions per minute, but such a speed proved
detrimental to the engine, and a lower speed of about 225 revolutions
per minute had to be adopted.
A thorough test on the pump to ascertain its delivery at various speeds
was the next move, and a notched weir, such as is shown in the
elevation, was employed. The test was made on No. 2 cooling tower, not
shown in the sketch, and showed that barely 3000 gallons per minute were
being delivered to the cooling tower. While the firm furnishing the pump
was willing to concede that the pump might not be doing all it should,
attention was called to the fact that there might be some other
conditions in connection with the system which were responsible for the
losses. Notable among these was the hydraulic friction, and when this
feature of the case was presented, the company did not seem at all
anxious to investigate the matter further; obviously on account of
facing a possible necessity for new piping or other apparatus which
might cost something.
Approximately 34 feet was the static head of water to be pumped over No.
2 cooling tower. Pressure gages were connected to the suction,
discharge, and condenser inlet, as shown at G, G' and G'' respectively.
When No. 1 unit was operating alone the gage G showed practically zero,
indicating no vacuum in the suction pipe. Observing the same gage when
No. 2 unit was running, a vacuum as high as 2 pounds was indicated,
showing that No. 2 was drawing more than its share of cooling water from
the main A and hence the circulating pump for No. 1 was fighting for all
it received. Gage G' indicated a pressure of 21 pounds, while G''
indicated 18.5 pounds, showing a difference of 2.5 pounds pressure lost
in the S-bend. This is equivalent to a loss of head of nearly 6 feet,
0.43 pound per foot head being the constant employed. The total head
against which the pump worked was therefore
G' + G = 21 + 2,
or
23
---- = 53
0.43
feet approximately. Since the static head was 34 feet, the head lost in
friction was evidently
53-34 = 19
feet, or
1900
---- = 36
53
per cent., approximately.
Supply of Cooling Water Limited
In addition to this the supply of cooling water was limited, the vacuum
being extremely low at just the time when efficient operation should be
had. The natural result occurred, which was this: As the load on the
turbine increased, the amount of steam issuing into the condenser
increased, beating [Transcriber: heating?] the circulating water to a
temperature which the cooling tower (not in the best condition) was
unable to decrease to any great extent. The vacuum gradually dropped
off, which indicated that the condenser was being filled with vapor, and
in a short time the small centrifugal tail-pump lost its prime,
becoming "vapor bound," and the vacuum further decreased. The steam
which had condensed would not go into the tail-pump because of the
tendency of the dry-pump to maintain a vacuum. When a certain point was
reached the dry-vacuum pump started to draw water in its cylinder, and
the unit had to be shut down immediately.
Vapor-bound Pumps
As the circulating water gradually rose in temperature the circulating
pump also became "vapor bound," so that the unit would be tied up for
the rest of the night, as this pump could not be made to draw hot water.
The reason for this condition may be explained in the following way.
When the circulating pump was operating and there was a suction of 2
pounds indicated at G, the water was not flowing to the pump of its own
accord, but was being pulled through by force. This water would flow
through the pump until a point was reached when the water became hot
enough to be converted into vapor, this occurring at a point where the
pressure was sufficiently reduced to cause the water to boil. Naturally
this point was in the suction pipe and vapor was thus maintained behind
the pump as long as it was operating. In this case the pump was merely
maintaining a partial vacuum, but not drawing water. After the vacuum
was once lost, by reason of the facts given, it could not be regained,
as the circulating water, piping and condenser required a considerable
period of time in which to cool.
Before any radical changes were made it was decided that a man should
crawl in the suction pipe A, and remove such sand, dirt, or any other
obstacles as were believed to cause the friction. After this had been
done and considerable sand had been removed, tests were resumed with
practically the same results as before. The investigation was continued
and the dry-vacuum pumps were overhauled, as they had been damaged by
water in the cylinders, and furthermore needed re-boring. In short, the
auxiliaries were restored to the best condition that could be brought
about by the individual improvement of each piece of apparatus. As this
was not the seat of the trouble, however, the remedy failed to effect a
"cure." It was demonstrated that the steam consumption of the turbines
was greatly increased due to priming of the boilers, as well as
condensation in the turbine casing; hence, the ills above mentioned were
aggravated.
Changes in Piping
After a great deal of argument from the chief engineer, and the firm
which furnished the pump, both making a strong plea for a change in the
piping, the company accepted the inevitable, and the dotted portion
shows the present layout. The elbow M was removed, and a tee put in its
place to which the piping D was connected. The circulating pump was
removed to the position shown, and a direct connection substituted for
the S-bend. The discharge pipe C was carried from No. 1 unit separately,
as shown in the elevation, and terminated at No. 1 cooling tower
instead of No. 2, which shortened the distance about 60 feet, the total
length of pipe (one way) from No. 1 unit being originally 250 feet. In
this way the condensing equipment was made practically separate for each
turbine, as it should have been in the first place.
With the new piping a vacuum of 24 inches on the peak could be reached.
While this is far from an efficient value, yet it is better than the
former figure. The failure to reach a vacuum of 28 inches or better is
due primarily to a lack of cooling water, but an improvement in this
regard could be made by reconstructing the cooling towers, which at
present do not offer the proper amount of cooling surface. The screens
used were heavy galvanized wire of about 3/16-inch mesh, which became
coated in a short time, and must be thoroughly cleaned to permit the
water to drop through them. The supply of cooling water was taken from a
30-inch pipe line several miles long and fed from a spring. The amount
of water varied considerably and was at times quite insufficient for the
load on the plant. Instead of meeting this condition with the best
apparatus possible, a chain of difficulties were added to it, with the
results given.
INDEX
Acceleration, rate of, 147
Adjustment, axial, 65
making, 66
Air-pump, examining, 163
Allis-Chalmers Co. steam turbine, 41
Auxiliaries, 2, 154
special, 165
Auxiliary plant for consumption test, 137
spring on governor dome, 28
Axial adjustment, 65
Baffler, 36
functions, 39
Bearings, main, 69
Blades, construction details, 44
inspecting, 104
Blading, Allis-Chalmers turbine, 48
Westinghouse-Parsons turbine, 59, 92
Blueprints, studying, 11
Buckets, moving, 14
stationary, 14
Bushings, 36
Carbon packing, 19
ring, 20
Central gravity oiling system, 111
Circulating pump fails to meet guarantee, 172
Clearance, 15, 150
adjusting, 18
between moving and stationary buckets, 4
gages, 17
measuring, 18
radial, 63
Comma lashing, 95
Condensers, 108, 131
jet, 154
Conditions for successful operation, 105
Cooling water supply limited, 177
Coupling, 127
Cover-plate, 4
-plate, lowering, 9
Curtis turbine, 11
turbine in practice, 1
setting valves, 31, 32
De Laval turbines, 118
Draining system, 105
Dummy leakage, 115
pistons, 63, 65
rings, 43, 113, 114
Equalizing pipes, 64
Exhaust end of turbine, 107
pipe, 107
Expanding nozzles, 14
Feed-pipes, 164
Flow, rate, 38
Foundation drawings, 2
rings, 44, 46
Fourth-stage wheel, 14
Franklin, Thomas, 112, 137, 154
Gages, calibrating and adjusting, 169
clearance, 17
for test work, 165
Generator, 53
Glands, examination for scale, 104
packing, 71, 77
regulation, 148
Governor, Allis-Chalmers turbine, 48
Curtis turbine, 27, 31
improved, Westinghouse-Parsons turbine, 83
-rods, adjusting, 35
safety-stop, 86
Westinghouse-Parsons turbine, 80
Grinding, 38
Guide-bearing, lower, 9
Gump, Walter B., 172
Holly draining system, 106
Horseshoe shim, 8
Hot-well regulation, 148
Inspection, 103
Intermediate, 14
Jacking ring, 8
Jet condenser, 154
Johnson, Fred L., 1, 31
Leakage, 118
Load variation, 144
Lower guide-bearing, 9
Lubrication, 51
Measuring tanks, 171
Mechanical valve-gear, 32
Nozzles, expanding, 14
Oil, 57, 103, 109
amount passing through bearings, 122
consumption, high, 175
detecting water in, 122
pressure, 122
-temperature curve, 123
Oil, testing, 110
velocity of flow, 122
Oiling, 87
system, importance, 119
Operation, Allis-Chalmers turbine, 54, 55
successful, 105
Operations in handling turbine plant, 146
Overload valve, 28
Packing, carbon, 19
glands, 71
ring, self-centering, 14
Parsons type of turbine, 41
Passage in foundation, 2
Peep-holes, 15, 18
Piping, 171
changing, 179
inspection, 164
Pressure, 63
gages, 166
in glands, 57
Pump, circulating, fails to meet guarantee, 172
inspection, 164
Radial clearance, 63
Rateau turbines, 118
Relief valves, 31
valves, importance, 159
Ring, carbon, 20
Rotor, Westinghouse-Parsons turbine, 59
Running, 99
Safety-stop, 22
-stop governor, 86
Saucer steps, 39
Screw, step-bearing, 18
step-supporting, 4
Separators, 105
Setting spindle and cylinder for minimum leakage, 115
valves in Curtis turbine, 31, 32
Shaft, holding up while removing support, 8
Shield-plate, 26, 36
Shim, horseshoe, 8
Shroud rings, 44, 46
Shrouding on buckets and intermediates, 18
Shutting down, 101
Special turbine features, 127
Spindle, lifting, 96
removing, 104
Spraying mechanism, 158
Stage valves, 28, 31
Starting up, 54, 95
Step-bearing, lowering to examine, 8
-bearing screw, 18
-blocks, 4
-lubricant, 4
-pressure, 38
-supporting screw, 4
-water, flow, 38
Stopping turbine, 56
Sub-base, 8
Superheated steam, 105
Test loads, 141
necessary features, 163
Testing oil, 110
preparing turbine for, 145
steam turbine, 112, 137, 152
Thermometer, calibrating and testing, 169
oil, 125
Thrust-block, 118
Top block, 4
Troubles with steam turbine auxiliaries, 172
Turbine features, special, 127
Vacuum, 152
raising, 107
test, 135
Valve-gear, 83
-gear, mechanical, 22, 32
operation during consumption test, 138
overload, 28
relief, 31
importance, 159
setting in Curtis turbine, 31, 32
stage, 28,31
Vapor bound pumps, 178
Water, cooling, limited, 177
in oil, detecting, 122
-measurement readings, 148
pressure, 101
service, 126
importance, 119
tests of condenser, 133
used in glands, 57, 76
Westinghouse-Parsons steam turbine, 58
Wheels, 14
lower or fourth-stage, 14
position, 18
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