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American Technical Society.

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

. (page 21 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 21 of 30)
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The steam arm is keyed to the valve spindle and carries at its
lower end a steel die which is free to slip up and down a small
amount. The part of this gear corresponding to the Reynolds
bell crank becomes a straight rocker pivoted at its middle:



VALVE GEARS



69



and the part corresponding to the Reynolds hook has at one end
a die which engages the die of the steam arm, and at its other
end a roller running in the curved cam slot. This hook is really
a bell-crank lever with arms that are not in the same plane. The
hearing on which it turns is carried on the lower end of the
rocker, and therefore is equivalent to a movable pivot similar to
the hook stud of the Reynolds gear.




STEAM ROD



GOVERNOR CAM ROD



Pig. 61.

In the position shown the dies are engaged. Motion of the
steam rod toward the right will move the lower end of the rocker
toward the left, and consequently turn the valve spindle in a right-
handed direction. This .will open the valve and at the same time
raise the dashpot rod. Meanwhile, the roller is moving toward
the left in a circular part of the cam slot, the center of which is
at the center of the valve spindle. This causes the steam arm and
the bell-crank lever, which has the roller at one end, to move in
such a way that there is no relative motion between them. As
soon, however, as the roller comes to the point where it is forced
to move out of this circular path and move farther from the valve
spindle, both arms of the bell-crank lever are turned downward,



309



70



VALVE GEARS



the dies become disengaged, and the daslipot closes the valve.
The slight up-and-down motion of the steam-arm die allows it to
rise while the hook die passes underneath when returning to re-
engage for the next stroke. The makers claim that this gear per-
mits a much higher speed than is possible with other Corliss gears.
Greene Gear. Another well-known drop cut-off gear is the
Greene, shown in Fig. 62. The valves are of the gridiron type,
sliding on horizontal seats, the admission valves parallel to, and
the exhaust valves at right angles to the axis of the cylinder and
just below it. AA are rock shafts turning in fixed bearings.




Fig. 62.

BB are the admission valve stems. C is a slide bar, receiving a
reciprocating motion from an eccentric. TT are tappets connected
to the slide bar. They move to and fro with the slide bar and ca
also move independently up and down. They are made fast av
their lower end to the gauge plate I) which slides through the
guide E. The guide E is made fast to the governor rod F and
through this means can be raised or lowered, thus regulating the
height of the tappets.

As the slide bar moves toward the right, the right-han^
tappet is brought into contact with the toe of the rocker, causing
it to turn on its bearings and move the rock lever and the valve
stem B toward the right, thus opening the admission valve. Since



310



VALVE GEARS



71



the tappet moves in a horizontal direction while the toe of the
rocker moves in an arc, it will be seen at once that they will soon
become disengaged and release the valve which is at once closed by
a dashpot not shown in the figure. If the governor raises the
tappets, cut-off will be later. A nut at the bottom of the governor
rod allows a proper adjustment of the guide and gauge plate. As
the slide plate C moves toward the right, the left-hand tappet comes
in contact with the heel of the left-hand rocker, both being beveled,
it rises in its socket allowing the tappet to pass under. It then
falls by its own weight and is ready to engage the tappet on its




return and open the valve. In this gear the disengagement of the
valve throws no load whatever on the governor which is a distinct
advantao-e over the Corliss gear. The action of the exhaust valves

D O

is not shown in the cut.

The Sulzer Gear is a drop cut-off widely used in Europe.
The valves are of the poppet type, lifting straight from conical
seats, so that there is no friction. They are usually placed verti-
cally above and below the cylinder axis and are operated by eccen-
trics from a shaft geared to the main shaft. The admission valves
are lifted from their seats by suitable levers, then released by a



311



72



VALVE GEAKS



device equivalent in action to the' Reynolds hook and are quickly
closed by the action of springs,

The exhaust valves of all drop cut-off gears are comparatively
simple in their operation and both in opening and closing they are
moved by the direct action of the exhaust rods.

A common form of vacuum dashpot for closing admission
valves is shown in Fig. (58. The rod coming down from the steam
arm makes a ball-and-socket joint with the dashpot piston. The
dashpot is often let down into the engine frame as shown. When
lifted, the piston produces a partial vacuum underneath it so that
it tends to drop quickly as soon as the valve gear is released. On
some of the largest modern engines where the valves are very
heavv, steam-loaded dashpots are used; that is, the dashpot piston
has steam pressure on one side, and an air cushion on the other
prevents it from striking the bottom of the dashpot.

Corliss Valve Setting. The setting of a Corliss valve gear
is a much longer process than
the setting of a plain slide valve,
but is nevertheless a compar-
atively simple matter, for the
various adjustments are nearly
all independent of one another.
In gears like that shown in Fig.
58 the length of both the eccen-
tric rod and carrier rod are
unusually adjustable, and the
former should be of such length

O

that the carrier arm swings equai distances on each side of a verti-
cal line through its pivot, and the carrier rod should be adjusted
until the wrist plate oscillates symmetrically about a vertical line
through its pivot Nearly all Corliss engines have one mark on
the wrist plate hub and three on the wrist plate stand, as shown
in Fig. 04, and the wrist plate should swing so that A, the mark
it carries, moves from C to D, but not beyond either one. When
A is in line with 1>, the wrist plate is in mid-position. The valves
a re then not in their exact mid-position,' but it is customary to regard
them as being in mid-position, and to speak of the lap as the amount
the valve covers the port when the wrist plate is in mid-position.



WRIST PLATE
STAND




Fig 64.



312



VALVE GEARS



To set the valves, remove the bonnets or covers of the^valve
chambers on the side opposite the gear. The ends of the valves
are circular, but inside their cross-section is as shown in Fig. 65.
On the end, in line with the finished edge of the valve, and on
the seat in line with the edge of the steam port, are marks as
shown in Fig. 05. When these marks coincide, the valve is either
just opening or just closing, and when in any other position, the
amount of opening or the amount by which the port is closed is
shown directly by the distance between the marks. I'lock the
wrist plate in mid-position, hook up the admission valves and
adjust the length of the steam rods by means of the right and left
threads provided for the purpose, until the ports are covered by
the amount of lap indicated in the following table opposite the
given size of engine.



Dia. of Cyl.
in inches.

12

U to 16
IS to 22
24 to 28
30 to 30
36 to 42



Steam "Lap
in inches.



Kxhaust Clearance
in Inches.



V

5



3V

rV



r



Next adjust the exhaust rods until the exhaust ports are open
an amount equal to the clear-
ance given in the above table.
Set the engine on its head-
end dead point, hook the car-
rier rod onto the wrist plate
and in the direction in which
the engine is to run," turn the
eccentric enough to open the MARKS
head-end admission valve by
a proper amount of lead ;
then the eccentric will be 00"
plus the angular advance
ahead of the crank. The

proper amount of lead will depend upon both the design of the gear
and the speed at which the engine is to run ; and may vary from
J 2 " for small engines to as much as -^" or T ? 6 " for large and higher-




Fig, fio.



313



74 VALVE GEAKS



speed engines. When the proper amount of lead has been obtained,
fasten the eccentric on the shaft by means of the set screw and
make sure by trial that the wrist plate moves to its extremes of
travel. The dash pot rods must be adjusted so that when the dash-
pot piston is at its lowest position, the hooks (see Fig. 59) descend
just far enough for the hook dies to snap over the stud dies with
about J. r " to -^" to spare, depending on the size of the gear.

To adjust and equalize the cut-off, lift the governor to about
the position that it will occupy when running at normal speed,
and put a block under the collar to hold it in this position. First
set the double lever at the right of the governor cam rods so that
it makes approximately equal angles with each rod, and then turn
the engine over by hand until the piston has moved to the desired
point of cut-off. Adjust the proper cam rod until the knock-off
cam strikes the hook and allows the valve to close, then turn
the engine to the point of cut-off on the other stroke and adjust the
other cam rod in a similar manner. Xow set the governor in the
lowest position to which it could fall if there were no load on
the engine, and set the safety cams so that in this position the hook
cannot open the valve. A latch is provided on which the governor
can be supported slightly above its lowest position so that the valves
can be opened by the hook when starting the engine. As soon as
the engine speeds up this latch must be moved aside, so that if the
governor fails to act, it can drop to its lowest point, otherwise this
latch would hold it just high enough so that the safety cams
could not act.

When Corliss gears are set as here described, the position of
the eccentric may not be quite right, due to an incorrect estimate
of the amount of lead required. The error is likely to produce
faulty release and compression as well as poor admission, but it
cannot be very serious, and the engine will turn over with its own
steam, so that indicator diagrams may be taken. The final adjust-
ments can then be determined from an examination of the diagrams.



214




II
l!



STEAM TURBINES

PART I



Introduction. The steam turbine is one of the most recent
engineering developments, and perhaps the most talked of, at the
present time. During the past ten years the most marked improve-
ments in its development have been made, and this has given a great
impetus to engineering, especially steam engineering, although the
very high speeds of rotation have driven the electrical engineer to
work out new ideas in designing generators suitable for these higher
speeds. The turbine has forged rapidly to the front, and, in spite of
an early and serious handicap in the way of steam economy, has
taken its place beside the best reciprocating engines of the present
time. Many claim it to be superior in the matter of steam economy,
but this will be discussed more fully later on. The turbine evidently
possesses many advantages over the reciprocating engine, and, in its
field of greatest usefulness, is likely to find in the near future a mors
severe competitor in the gas engine than in the reciprocating engine.
For some classes of work, the steam turbine in' its present state of
development is entirely unadapted.

The steam turbine consists essentially of nozzles or guide pas-
sages which direct the steam onto vanes or buckets attached to the
periphery of rotating wheels, the essential elements of which
are shown in Fig. 1. The simplest form of turbine is perhaps
one of the type in which a jet of steam impinges upon the buckets of
a wheel, in much the same manner that a stream of water impinges
upon the buckets of a Pelton water-wheel; there is, in fact, a great
similarity between water turbines and steam turbines. The under-
lying principles are the same in either case, but the application of
those principles is different. Steam flowing through a properly
designed nozzle, with 150 pounds boiler pressure on one side, and
the usual turbine vacuum on the other, will attain a velocity of about
4,000 feet per second, or about twice the muzzle velocity of a rifle

Copyright, 1'^Oy, by American School of Correspondence, Chicago.



317



STEAM TURBINES



ball. Water, to attain this enormous velocity, would have to flow
from a head of about 234,000 feet. When this is compared with
the ordinary water head of 150 feet or less, or even with such an ex-
ceptionally high head as 3,000 feet, which is sometimes met with in
water powers on the Pacific Slope, a glimpse will be had of the
magnitude of the problem confronting the steam turbine engineer. To
put this in other words, the steam turbine designer has to deal with
a velocity equivalent to that produced by a head of water nearly




Fig. 1. Elements of DeLaval Turbine.

1 ,500 times as high as Niagara Falls. It will at once be seen, then,
that the velocity of rotation of a simple turbine wheel to attain the
best efficiency must be enormous. If this total head is to be used in
one wheel, the peripheral speed must be nearly 2,000 feet per
second, and at such speeds, the centrifugal force is so great that it is
no easy matter to design a wheel that will not burst, even were there
available some material stronger than any we now know. As it is,
about 1,200 feet per second is considered the practical limit of
peripheral velocity for a wheel built of the best nickel steel.

A little mathematical calculation will show that wheels of five
feet in diameter will revolve 4,600 times per minute to attain a velocity
of even 1,200 feet per second at the periphery. It is the problem



318



STEAM TURBINES 3



of the steam turbine designer to reduce these speeds to more manage-
able rates without at the same time making too great a sacrifice of
efficiency.

From a thermodynamic standpoint, the turbine and reciprocating
engine are not unlike, but the force of the steam acts differently in
them. In both, it is the heat energy of the steam that does the work.
In the one, the steam slowly expands, exerting pressure on a piston ;
in the other, it expands in narrow passages, pushing the particles
ahead faster and faster and thus obtaining velocity which is then
imparted to the vanes of a rotating wheel. In the one, the steam
acts by virtue of statical pressure; in the other, by virtue of its high
velocity. In either case, it is the internal heat of the steam that
causes the expansion and does the work. If heat is lost in any way,
by condensation, radiation, etc., the work will be proportionally less.
In the turbine, a difference in pressure from inlet to outlet acts as
a motive force indirectly, and then only in so far as it causes a rapid
flow of steam.

Advantages. The well-known expression, work = force X
space, embodies the idea that a given amount of work may be
accomplished in a certain time by increasing the total force of the
steam on the piston at the expense of the number of revolutions of
the fly-wheel per minute, or vice versd. For example, a Corliss
engine running at 180 revolutions per minute requires a mighty
effort behind the piston to develop 1,000 horse-power, and this tre-
mendous force demands a large cylinder, a heavy frame, and an
immense fly-wheel. If, now, an engine were built to run at 800
revolutions per minute, much less push behind the piston would be
necessary to develop the same horse-power, and, therefore, the parts
could all be made smaller, and the whole weight very much reduced.

To go a step further, and consider the steam turbine, which
must run at 2,000 to 3,000 revolutions per minute, it is clearly seen
that this enormous speed reduces the mass of the parts even more.
The heavy fly-wheel is no longer necessary, as the rotating parts are
moving at a sufficiently high speed to acquire an immense inertia,
and there is always a constant effort exerted on the vanes by the
team, thereby producing an absolutely steady turning moment.
Furthermore, the motive parts of the turbine revolve, which is in
direct contrast to the reciprocating engine, in which the piston is



319



STEAM TURBINES



moving backward and forward, and the turning moment is continu-
ally changing from a maximum to a minimum. It is clear, therefore,
that for a given horse-power, the steam turbines produce smaller
machines, lighter foundations, and consequently smaller power
houses. A fair idea of the relative space occupied may be gained
from Fig. 2.

Again, the generator, on account of this high speed, will be
smaller and less expensive. The turbine requires oil in its bearings




'Fig. 2. Comparative Sizes of 5000-K. W. Corliss Engine and Generator and Curtis
Turbo-Generator of Same Power.

only; hence there is no oil to go over in the condensed steam, and
the condensation may be used for boiler feed without any danger of
carrying oil into the boilers. The turbine requires somewhat less
attendance than the reciprocating engine, and the whole machine is
compact and simple. To do its best, the turbine requires a higher
vacuum than is ordinarily obtained for the reciprocating engine,
and hence needs very much larger condensers, more cooling water,
and additional air-pump capacity. All this in a measure offsets some



320



STEAM TURBINES



of its advantages, and frequently more trouble arises from the air
pumps and condensers than from the turbine itself. The turbine
may of course operate at the usual vacuum with a somewhat greater
steam consumption and a slightly lower efficiency.

The reciprocating engine has its own advantages, and in certain
classes of work will doubtless hold its own, but for all such apparatus
as blowers, centrifugal pumps, generators, etc., which may be direct
connected to a turbine, the reciprocating engine is rapidly becoming
a thing of the past, and even for factories where belt drives are
used, the steam turbine has been suggested.

History. The steam turbine is not only one of the most recent
engineering developments, but is, at the same time, perhaps, one of
the most ancient forms of
prime mover. In a book
written by Hero of Alexan-
dria, over 100 years before
the beginning of the Chris-
tian era, a very simple form
of steam turbine is described.
It consisted of a hollow sphere
mounted on hollow trunions,
through which the steam
passed into the sphere. On
opposite sides of the sphere
were outlets consisting of
pipes bent at right angles in
lines tangent to the equator
of the sphere, in such a manner that the reaction of steam escaping
through these pipes caused the sphere to revolve on its trunions,
in much the same way that the water escaping from the arms of
a lawn sprinkler causes it to revolve. This turbine, which is illus-
trated in Fig. 3, is the simplest form of the pure reaction motor.

In 1629, Branca, an Italian, invented a turbine much like a
miniature water wheel, which was driven by a jet of steam from a
nozzle directed against the buckets of the wheel. This is the simplest
form of an impulse turbine, and is illustrated by Fig. 4.

In 1784, Wolfgang de Kempelen designed a turbine of the lawn
sprinkler type, similar in principle to Hero's engine, the chief dif-




Hero's Steam Turbine.



321



6



STEAM TURBINES



ference being the substitution of a horizontal revolving tube for the
hollow sphere which Hero used. Steam, escaping from the outlets
in opposite ends of the tube, caused it to revolve by reaction, just
as the escaping water causes the lawn sprinkler to revolve.

In 1784 it is
said that James
Watt took out a
patent on a tur-
bine, but, as we
all so well know,
devoted his gen-
ius to the devel-
opment of the
reciprocating
engine. At this
time, both types
of engine were
in about the

same crude form, and it is possible that had Watt devoted his
energies to the turbine instead of to the reciprocating engine we
might not have had the ordinary form of steam engine in its present

-Knife dqe /slue i






Fig. 5. Reaction Wheel of Avery & Foster.

high state of efficiency, for the turbine possesses so many advantages.
This is proved by the fact that it has at last come to the front in spite
of the great commercial success of the reciprocating engine.



STEAM TURBINES



In 1831 , Avery & Foster took out the first patent granted for a
turbine by the United States Patent Office. This was on the Hero
lines, and was really an improvement on the Wolfgang de Kempelen
turbine of 1784. This turbine appears to be the first to attain com-
mercial success. Several were built under the Avery patent and
were used to run sawmills near Syracuse, N. Y.

Steam entered a hollow shaft, Fig. 5, through a stuffing box,
passed through to the hollow arms, and escaped through plain open-
ings in opposite ends

of the arms. The <* ^^< * ^-n C-

speed of rotation was
enormous, the periph- H-
ery of a 7 ft. wheel
traveling at the rate of
about 14 miles per
minute. The wear
was excessive, and
this, combined with
inability to get proper
packing for the stuf-
fing boxes, rather than
the lack of steam
economy, doubtless
caused its failure, for
the reciprocating en-
gine of those days had
not reached its highest
state of economy. Had
Avery used the present




Fig. 6. Real & Pichon Compound Turbine.



expanding nozzles instead of plain openings for his steam outlets,
his steam consumption would doubtless have been less, but the
speed of rotation more. Diverging nozzles were used as early as
1838, but as they were not correctly proportioned, they were a hin-
drance rather than a help, and the idea seems to have been given up
for a time.

As early as 1827, a compound turbine was patented by Real
& Pichon, the idea being to reduce the velocity of rotation by passing



8



STEAM TURBINES



the steam through successive wheels G, Fig. 6, separated by disks
B B containing outlets C to permit the passage of the steam from
one chamber to another. H is one of the blades, F the shaft, and
M the steam exhaust. This is the principle on which the present
Rateau turbine works.

The chief cause for early failure in turbine work was lack of
comprehensive knowledge of the flow of steam. It was not until
1840 or thereabouts that anyone seemed to get at the real facts or
appreciate the true significance of the situation. In this year Pil-
brow patented a machine that was a distinct advance in the right
direction, and his patent claims show that he, at least, understood
some of the fundamental principles. In 1842, he attempted to re-
duce the speed of rotation by compounding, passing steam through




Outlet
Fig. 7. Wilson's Compound Turbine.

successive wheels revolving in opposite directions. There are, of
course, grave objections to such an arrangement. He later invented
a turbine with several nozzles that could be successively cut out of
action as the load on the turbine varied. This, in its crude form, is
the fundamental idea of the arrangement of the nozzles used in the
DeLaval and Curtis turbines, but Pilbrow used a converging, instead
of a diverging nozzle, and his wheel was unlike others of the impulse

type-
In 1840, Wilson patented the forerunner of the Parsons type of
turbine. He passed steam successively through rows of running and
stationary vanes, gradually expanding it until the exhaust pressure
was reached. A view of Wilson's invention is shown in Fig. 7; a, b,



STEAM TURBINES



c'



and c, are vanes which are attached to and rotate with the drum D,
while d, e, and / are stationary guide vanes. Steam enters at the
left, passes through the turbine longitudinally, and exhausts at the
right. Wilson appears to have been among the first to realize that
the volume of steam increases as it expands to lower pressures, to
provide for the same by increased size of passages, and, what is per-
haps most important, to claim
this in his patent.

In 1858, Hartman Bros, pat-
ented a turbine consisting of two
revolving disks c and c' fixed to a
shaft D t as shown in Fig. 8.
Between them was a segment of
stationary reversing blades dd.
Steam entered from a nozzle F
and was exhausted at H; G is the
casing. This turbine embodied
the essential element of the one-
stage Curtis turbine of the present
day.

Perrigault & Farcot, about
1870, patented a compound tur-
bine in which the steam, as it left
the buckets, passed through suc-
cessive passages and again and
again impinged upon the face of
the same wheel. This is the prin-
ciple adopted in the Riedler-



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 21 of 30)