Ernest Victor Lallier.

An elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer online

. (page 11 of 17)
Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 11 of 17)
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or a safety valve.

Where the steam flows into a pressure more than
two-thirds the pressure in the boiler,

W = 1.9 ak V{p - O) O,



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PIPES AND FITTINGS. 195

in which O = difference in pressure between the two
sides, in pounds per square inch, and a, i», and h are
as above.

To reduce to horse power multiply by 2.

Where a given horse power is required to flow through
a given opening, to determine the necessary difference
in pressure:



2 V 4



140^ k



PACKING.

Where two parts of a boiler, engine, or other steam
apparatus are to be joined and are exposed to steam
pressure, it is necessary that they be made steam-tight.
In some instances this may be done by very careful
facing of the parts, having them perfectly clean and
bolted together absolutely true to each other. In the
majority of cases, however, it is necessary to interpose
some other material between the two surfaces in order
to make an absolutely tight joint. Possibly the simplest
form of pa^cMng is that employed where two pipes of
ordinary size are screwed together, the threads having
been coated by the fitter with a quantity of red lead
which serves to fill any small openings that may exist
between the thread surfaces when brought in contact
with each other. In some instances where flat surfaces
of considerable size are in contact, such as flanged
couplings, small cylinder heads, etc., the surfaces having
been carefully machined, a washer or ring of strong
paper inserted between the two is sufficient to produce
the desired effect.



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196 ELEMENTARY STEilM ENGINEERING.

Whenever a ring of this character, of considerable
size and flexible material, is used for this purpose in
steam work, it is called a gasket. Instead of paper a
gasket of corrugated copper is often used. The copper
is soft and when the bolts, are tightened, it forces the
corrugations down and closes up all crevices. In case
of extremely high pressure, a groove is turned in each
of the opposing faces; in this is placed a wire of soft
material, such as lead. For manholes in boilers, large
cylinder heads, and in many cases for valve stems,
piston-rods, etc., packing is made either in sheet form,
to be cut as desired, or made to fit the stuffing box accu-
rately. This may be of various materials, depending
upon the conditions to which it is to be exposed. Tex-
tile packings are also largely used. These are made of
flax or hemp, or, with these as a base, in combination
with asbestos and rubber. These will not, however,
serve for high-pressure packings except in combination
with asbestos on account of its higher resistance to
heat. It is necessary to keep asbestos as dry as possible,
as moisture will readily disintegrate and destroy it.
One method of overcoming this trouble is to mix in
rubber or to give it a coating of graphite and grease.
One method of making packing is by alternating layers
of rubber and asbestos thread, braided around a stiitable
core. After thoroughly coating this with rubber, the
operation is continued until the required size is ob-
tained. Soapstone is sometimes employed to prevent
moisture in asbestos packing; the principal objection
to its use is that it is not a fibrous material. Metallic
packings have been largely used, particularly on piston-



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PIPES AND FITTINGS. 197

rods, in recent years. They are usually composed of
babbitt or of similar anti-friction metals and are made
in rings, so formed that when placed in the stuffing box
and tightened they slip on and form closely around the
piston-rod allowing it to move freely and yet prevent
the passage of steam. In another case fibres or pellets
of material are placed in small bags. These are coiled
around the rod in position, the gland is screwed down,
friction rapidly wears away the cloth covering and the
material beds itself closely around the piston-rod. For
hydraulic work packings are also made of rawhide and
similar material not readily affected by water. When
placing a packing in position, such as the gasket on an
engine, it is necessary first to see that all metallic sur-
faces are carefully scraped clean, that all bolt holes are
carefully and cleanly cut, and that a coat of hard grease
and graphite is rubbed on each side of the gasket. It
is then placed in position and the various nuts tight-
ened evenly by drawing up a little at a time on each one.
Never draw up one nut or bolt tightly, leaving the others
entirely loose. In the case of the packing of a ptunp, or
cylinder head of a gas engine or any machine where,
in addition to bolt holes, openings must be left for
passage of water or Other liquids or gases, care must be
taken to see that all of these are opened before attempt-
ing to put the gasket into position. For gas engines,
where the packing is exposed to extra heat and high
pressure, a packing composed of asbestos fibres in
combination with a wire gauze is now largely used.



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198 ELEMENTARY STEilM ENGINEERING.

HEAT AND COLD INSULATOBS.

Since to insulate means to separate, heat and cold
insulators are obviously intended to separate such
bodies. This is a matter of vital importance in the
plant, not only for convenience but from considerations
of economy. If fuel is purchased and burned with the
idea of producing heat which is in turn to produce work,
it is not good policy to allow this heat to escape before
its work has been done.

For this reason it is necessary to cover all boilers,
pipes, etc., that conduct steam, with such materials as
will, to a considerable extent, prevent the condensing
action due to the cooling effect of the atmosphere.

Pipe coverings, like other things in the engineering
lines, are of many kinds and a detailed consideration
of each is not necessary. It may be mentioned, how-
ever, that one of the principal methods of insulating
pipes, in order to prevent condensation, is by covering
them with an asbestos mixture. Asbestos is a rock of
fibrous nature, found all over the world in various
grades, the best for our purpose being obtained in
Canada. It is readily separated into silky fibres.
These are thoroughly mixed with a binding material,
magnesia or other non-conductor of heat and placed
upon the market in the form of sheets, bricks and
tubes, and in pulverized form.

The bricks are largely used to cover upper portions
of boilers exposed to the air. The tubes, called pipe
covering^ are placed over steam piping and held in posi-
tion by a canvas coating pasted on the surface.

The pulverized material, mixed with water, forms an



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PIPES AND FITTINGS. 199

adhesive plaster, readily applied to surfaces of any
shape. The sheet material, and sometimes paper as
well, may be used by forming it into corrugations, or
crumpling it in such manner that, when placed around
the pipe, small quantities of air are caught in the corru-
gations and, being unable to escape, form a non-con-
ductor around the pipe. If these coverings will prevent
the cold from condensing the steam they will also pre-
vent the heat from being radiated throughout the sur-
rounding space*

Where pipes pass through the open air they are
sometimes protected by building a wooden box around
them which is filled with shavings and sawdust. The
pipes may also be covered with some kind of felt. All
of these substances serve similar purposes when prop-
erly employed.

There is a wide difference in the value of different
substances for protection from radiation, their values
vaiyiag nearly in the inverse ratio of their conducting
power for heat, up to their ability to transmit as much
heat as the surface of the pipe will radiate, after which
they become detrimental^ rather than useful, as cover-
ing. This point is reached nearly at the heat of baked
clay or brick.

Air space alone is one of the poorest of non-conductors,
though the best owe their efficiency to the numerous
minute air cells in their structure. This is best seen
in the value of different forms of carbon, from cork
charcoal to anthracite dust, the former being three times
as valuable as the latter for this purpose, though in
chemical constitution they are practically identical.



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200 ELEMENTARY STEAM ENGmEERING.

Any suitable substance used to prevent the escape of
steam heat should not be less than one inch thick.

The following table of the relative values of various
substances for protection against radiation has been
compiled from a variety of sources, principally from the
experiments at the Massachusetts Institute of Tech-
nology, and those of C. E. Emery, M. E.

TABLE OF RELATIVE VALUES OF NON-CONDUCTING

MATERIALS.
Substance. Value.

Mineral wool 68 to .83

Carbonate magnesia 67 to .76

Paper 50 to .74

Sawdust 61 to .68

Asbestos, paper 47

Asbestos, fibrous .36 •

Air space, undivided 14 to .22

Baked day, brick .07

A smooth or polished surface is of itself a good pro-
tection, polished tin or Russia iron having a ratio, for
radiationi of from 53 to 100 for cast iron.

Hair or wool felt and most of the better non-conductors
have the disadvantage of soon becoming charred from
the heat of steam at high pressure, and sometimes of
taking fire therefrom.

Mineral wool, a fibrous material made from blast-
furnace slag, is the best non-combustible covering, but
it is quite brittle and liable to fall to powder where much
jarring occurs*



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PIPES AND FITTINGS. 20l



QUESTIONS.

z. What are pipes and tubing?

2. How are they measured?

3. What is a fitting?

4. Sketch and describe several fittings.

5. Describe a union.

6. What is a valve?

7. Sketch and describe a globe valve.

8. Sketch and describe a check valve.

9. Sketch and describe a gate valve.

zo. Sketch and describe a reducing valve.

11. Sketch a steam trap and describe its operation.

12. What is a steam separator?

13. What is a grease extractor and what is its use?

14. How may the flow of steam through pipes be determined?

15. What effect has friction on the flow?

16. Give the rule for finding the flow of steam through a given
opening.

17. What is paddng and its use?

z8. How may condensation of steam be prevented?



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CHAPTER ZVm.
BOTABT ENGINES.

Heretofore^ we have considered only the construction
of the reciprocating engine. However, there is now
being very largely used the rotary engine or turbine.

Strange as it may seem, the turbine was the original
form of steam engine, one of this kind having been
operated as far back as 120 B.C., when Hero of Alexan-
dria described such an apparatus, utilizing heat energy.
But this type of engine was neglected, as far as com-
mercial purposes were concerned, until recently, when
the turbine was again taken up and put to practical use.
The essential principle of the turbine consists of a
wheel containing vanes or plates against which a column
of steam is projected, causing it to rotate. We have
already seen, in the reciprocating engine, how advantage
is taken of the expansive power of steam. In the steam
turbine the process is quite similar except that there
is a constant, instead of an intermittent, flow and the
rotation being continuous and in one direction there is
no variation in the amounts of power developed at any
point. The steam continuously forced into the nozzle
by a pressure at the point of inlet and during its passage
through the turbine expands continuously, on account of
its internal energy, and hence a particle of steam pushes
those ahead of it at a faster rate, thus increasing the
velocity of the flow. In the turbine the steam is caused



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ROTARY ENGINES. 203

to impinge upon the vanes by means of nozzles, com-
pelling the wheel to rotate as a direct result of the im-
pulse and reaction of the impinging jet.

Impulse^ as generally understood, is a force acting
in a forward direction, and reaction is the equal and
opposite force. For example, in the diagrams in Fig. 81,
at a, a jet of water impinges 09 a fiat plate and the force
tending to push the plate away is entirely due to the
impulse of the jet.

Should we arrange the surface in a curved manner.



•III %v



I 111

a iMi




Fig. 81.

as shown at &, the water on striking the surface will be
ttuned backwards and the force acting on plate h will be
greater than that on a, depending on the amount of
backward inclination which occurs.

Should we have a complete reversal of the original
direction as at c, the force on the plate is practically
twice as great as that at a, due to the combined effect
of impulse and reaction.

The principal distinction between the types of turbines
is the fact of the expansion being completed within the
nozzle in one case, and after passing into the wheel in
the other.



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204 ELEMENTARY STEAM ENGINEERING.

As the expansion of the steam in an impulse turbine
is completed within the nozzle, no expansion takes place
in the wheel passages.

• )))))




Fig. 82.

The pressure of steam between the vanes is the same
as the pressure within the casing in which the wheel
runs. The moving element is thus driven first by the
pressure due to the impulse and then by the reaction



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ROTARY ENGmBS. 205

of the steam. In the reaction turbine the expansion in
the nozzle is only partial, therefore it expands still more
in the wheel when it leaves the vanes as the result of
the energy acquired in the wheel itself.

In Fig. 82, at a, are illustrated the guide vanes and
moving parts of an impulse turbine, while h illustrates
similar diagrams of a reaction turbine. The moving and
stationary parts are indicated by r and 8 respectively.
These merely illustrate the principle and not the actual
method of fastening in a turbine wheel. The two
general classes, impulse and reaction, may be again
subdivided into simple and compound.

In a simple turbine the steam from the nozzle is
directed against the vanes of a single wheel. It is often
necessary, however, to avoid such high speed of rotation
as would be obtained with this design and also to do
away with the reducing gears. This is accomplished by
using several wheels alternating with the stationary
guide, as shown in Fig. 83. With this arrangement only
part of the energy from the steam is imparted to each
wheel, and a much lower speed is the result. In addi-
tion, some compound turbines are divided into stages,
that is, two or more wheels and guides are arranged in
separate compartments. Each group is called a stage
and the number of stages in any turbine will depend
upon the subdivisions made. The advantages usually
claimed for steam turbines are small floor space, close
speed regulation, freedom from vibration, high economy
under variable loads, small cost for maintenance and
attendance.

One of the early representative turbines was that of



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2o6



ELEMENTARY STEAM ENGINEERING.




uccu



Fig. 83.



Fig. 84.



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ROTARY ENOmES. 207



Fig. 85.

De Laval, originally designed in 1883 with the idea of
operating a cream separator. It has since developed
into engines used for greater powers. By use of the
diverging nozzle, Fig. 84, he secured a complete adiabatic



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2o8 ELEMENTARY STEAM ENGmEERING.

ezpansion of the steam and a complete conversion of
static into kinetic energy.

The general construction of the De Laval steam
turbine will be clearly understood from the sectional
plan and elevation, Fig. 85. The construction of the
turbine presents no extraordinary departure from every-
day engineering practice. However, the workmanship
and material used, owing to the high speed employed,
must be of the very highest quality. In the figure, B is
the turbine wheel mounted upon the slender flexible
shaft A, and in such position relative to the wheel case
as to revolve entirely free, liberal space being allowed
on each side, as shown. The wheel case and the wheel-
case cover are so shaped as to form safety hearings
around the hub of the wheel for the purpose of catching
and checking its speed in case of an accident to the shaft.

The steam after passing through the governor valve.
Fig. 86, enters the steam chamber D, Fig. 84, where it
is distributed to the various nozzles. These, according
to the size of the machine, number from i to 15. They
are generally fitted with shutting off valves, E, Fig. 84,
by which one or more nozzles can be cut out when the
turbine is not loaded to its full capacity. This allows
steam of boiler pressure to be almost always used, and
adds to the economy on light loads.

After passing through the nozzle, the steam, as else-
where explained, is now completely expanded, and in
blowing through the buckets F, Fig. 84, its kinetic energy
is transferred to the turbine wheel. After performing
its work the steam passes into the chamber H, Fig. 85,
and out to the exhaust opening.



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ROTARY ENGmBS. 209

The velocity of the turbine wheel and shaft, in most
cases too great for direct practical utilization, is con«
siderably reduced by means of a pair of spiral gears,
usually made ten to one. These gears are mounted and



Fig. 86.

enclosed in a gear case. In Fig. 85 J is the pinion
made solid with the flexible shaft; 8 is the spiral gear
which, with couplings M, connects with the dynamo, or
is extended for pulleys. At O is placed the governor,
held with a taper-shank in the end of the shaft L, and
by means of the bell-crank L, Fig. 86, operates the
governor valve.



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2IO ELEMENTARY STEAM ENGINEERING.

The governor is shown in detail in Fig. 86. The two
weights B, B are pivoted on the knife edges A with
hardened pins bearing on the spring seat D. The
governor body E fits the end of the shaft K. It is re-
duced in diameter at its outer end and threaded for an
adjusting nut.

When the normal speed is exceeded, the weights
spread apart and, pressing on the seat D, push the gover-
nor pin G forward, cutting off part of the flow of steam.

The flexible shaft is supported on three bearings.
Fig. 85. Q and R are the pinion bearings and Z is the
main shaft bearing which carries the greater part of
the weight of the wheel. This latter bearing is self-
aligning and is held to its seat by the spring and cap
shown. T is the flexible bearing which is entirely free
to oscillate with the shaft, and its only purpose is to
prevent escape of steam when running non-condensing
or to prevent air from entering the wheel case when
the turbine is running condensing. All the bearings of
the flexible shaft, as well as the gear wheel, are lubri-
cated from the central oil reservoir, mounted upon the
gear case; all other bearings are self-oiling.

The gear wheels are made of solid cast steel, or of
cast iron with steel rims pressed on. The teeth in two
rows are set at an angle of 90 degrees to each other.
This, while insuring smooth running, at the same time
checks any tendency of the wheel and shaft to move
lengthwise. Owing to the high speed of the gears and
their perfect alignment the stress on the teeth is ex-
tremely small.

The flexible shaft is mainly supported on each side



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ROTARY ENGINES. 211

of the pinion by the main bearings, Q and R, the shaft
being at the same time made very slender^ which gives
it a certain amount of flexibility and allows the turbine
wheel, when the so-called critical speed is reached, to
revolve around its true center of gravity. This critical
speed, dependent upon the flexibility of the shaft,
occurs well below the normal speed of the turbine and
marks the disappearance of all vibrations.

The turbine wheel is by far the most important part,
and is made of forged nickel steel. In the smaller
sizes the turbine wheels have a hole through the center
and are forced upon a tapered sleeve shrunk on to the
shaft. The larger wheels are made solid, with the
shaft in two pieces screwed to the flanges of the wheels.
The buckets are drop forged and made with a bulb
shank fitted in slots milled in the rim of the wheel. By
this method the buckets can easily be taken out and
new ones inserted, should occasion require, without
damage to the wheel.

The vacuum valve is only necessary when running
condensing, as in this case it has been found that the
governor valve alone is unable to hold the speed of the
turbine within the desirable narrow limit during sudden
and great changes in the load. The function of the
vacuum valve is as follows:

The governor pin G, Fig. 86, actuates the plunger H
screwed into the bell-crank L, however, without moving
the plunger relative to said crank. This is on account
of the spring M being stiffer than the spring N, whose
purpose it is to keep the governor valve open and the
plunger H in contact with the governor pin. When the



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312 ELEMENTARY STEAM ENGmEERING.

large part of the load is suddenly thrown off, the governor
opens, pushing the bell-crank in the direction of the
vacuum valve T. This closes the governor valve, which
is completely shut off when the bell-crank is pushed so
far forward that the screw O barely touches the valve
stem J. If this is not sufficient to check the speed, the
plunger H is pushed forward in the now stationary bell-
crank and opens the vacutun valve. This allows the
air to rush into the space P, where the turbine wheel
revolves, effectually checking its speed.

The Curtis steam turbine is interesting, due to the
fact of being a vertical turbine, that is, one in which the
wheel revolves horizontally, the central shaft being
placed on end. Steam is admitted to the vanes or
blades in a similar manner to the previous description,
expanding to some extent, and passing through a set of
blades, placed on a support, rotating on its center.

It then passes through a set of stationary blades,
after which it enters another set of moving vanes, in
which more of its energy is given up. In Fig. 87 is
shown the diagrammatic arrangement of the blades and
nozzles. Steam is admitted through the valves at a,
controlled by the governor, and passes into the nozzle &.
Partial expansion takes place here, and the velocity is
increased. Then striking the moving blades e it passes
through them to the stationary blades <;, after leaving the
last set of moving blades c, it exhausts into the atmos-
phere if running non-condensing. Should the turbine
be operated condensing, the steam will pass into another
set of nozzles 6', through another ntmiber of fixed and
movable blades, and then exhaust into a condenser at e.



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ROTARY ENGINS&



2»3




jmmmmw




Mmmm



^ u«iii(m



»»»»»»



»»»»»))>



e 4 i i "4'

Kg. 87.

The sets of stationary blades serve to change the
direction of the steam, in order that it shall always strike
the moving vanes in the same direction. Each of these



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214 ELEMENTART STEAM ENGINEERING.

two sets of nozzles, moving vanes and stationary ones,
forms a pressure stage as it is called.

The turbines are made in from one to four pressure
stages.

The Parsons turbine is possibly one of the best ex-
amples of the pressure type of turbines. In Fig. 88 is
shown a section illustrating the principal parts of the
machine. The main shaft A, resting in proper bearings,
carries all of the revolving parts. At B, B', and B'' are
shown three sets of blades of different sizes. The re-
volving or rotating portion is called the rotor. The steam
enters the turbine at boiler pressure through the valve
H, and fills the chamber I, which extends around the
rotor. From this chamber the steam expands through
a set of stationary blades, so placed that the steam is
made to strike the moving blades and cause them to
rotate. The stationary and moving blades alternate
in sets, forming a ring around the rotor. The velocity
of the steam increases while passing through the sta-
tionary blades and decreases on account of the energy
which it gives up while passing through the moving
blades. These blades are set in a radial position similar
to the De Laval, but instead of only one set being used,
a very large number is employed. The length of the
blades in any one set is the same. In the first series
they are comparatively short and close together, but
their length and distance increase in the next series.
This is to accommodate the expansion of steam, the
volume of which gradually increases as it passes onward.


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Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 11 of 17)