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 6 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 6 of 17)
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tioned that each supports an equal amount of the flat
surface.

When in use the water partially fills the space above
the upper row of tubes, the remainder being the steam
space. As the steam in contact with the surface of the
water still retains a quantity of water, it is necessary, in
order to take the steam out as dry as possible, to make
the connection for this purpose as far as possible from



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FIRE-TUBE BOILERS.



95



the surface of the water. To facilitate the production
of dry steam, some boilers have a dome, A, Fig. 42. In






~^y/^^.




?^<^^


-m


>^


t 1


y:y


t


j=fci~J


t


t


,




h



Fig. 42.

some cases the steam connection is made at the upper
part of the dome; in others there is a short section of
heavy pipe, called the dry pipe, so connected as to take
its place. To provide access to the interior, an oval



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96



ELEMENTARY STEAM ENGINEERING.



Opening m, called the manhole, about lo inches by 14
inches, is cut in the upper part of the boiler shell. This
is closed by a plate placed in the boiler and held against
it by a bolt fastened to the plate and passing through a
casting called a spider, all being drawn tightly together
by a nut on the bolt. Where the surface of the plate and





Fig. 43.

the edge of the opening touch, a gasket of sheet packing
is placed in order to make the joint steam-tight.

At the bottom of the front and back heads, similar
but smaller openings and plates, ^, called handholes, are
provided.

On each side of the boiler, lugs or brackets, XX, are
placed. These are for the purpose of supporting it on
the brickwork. Some boilers are hung on chains from
a framework of iron.

Locomotive or marine boilers are self-contained; that



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FIRE-TUBE BOILERS.



97



Mf y'*'



[



I f I i f f n



^






«^






^



u u



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98 ELEMENTARY STEAM ENGINEERING.

is, the fire box and grates are all part of and in the boiler
itself; for stationary work, however, the fire box and
grates are in the brickwork of the boiler setting. The
side walls of this setting are built double, and as air at
rest is a bad conductor of heat, the body of air between
the two walls serves to prevent an excessive loss of heat,
due to radiation from the furnace. Across the front is
placed an iron casting, called the boiler front. Fig. 43.
This contains doors giving access to the head of the
boiler for cleaning the tubes, and through the lower doors
to the fire, and the ash pit. The grates gr. Fig. 44, are
supported by the brickwork around the fire door and by
the bridge wall B. This wall also causes the hot gases
to flow up against the bottom part of the boiler, called
the crown sheet, and affords a convenient surface against
which to bank the fire. The plate on the brickwork at
the bottom of the fire door is called the dead plate, and
the arch above it the fire arch.

The inner side of the brickwork is lined with fire brick,
and when the boiler is placed on the setting the front
lugs are bricked in. The rear ones are placed on rollers
resting on iron plates to allow for the expansion and
contraction due to changes of temperature.

The rear end of the boiler will be placed an inch or so
lower than the front, in order that it may drain readily
when it is desired to empty it; at the under side of this
end is placed the flange for connecting the blow-off pipe.



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CHAPTER Xm.
CALCULATIONS FOB FUE TUBULAB BOELEBS.

There are a number of calculations which it is neces-
sary that one should be familiar with in connection with
the construction and operation of fire-tube boilers.
Those ordinarily required are to find the bursting pres-
sure, safe working pressure, the heating surface, and
strength of braces.

We will make these calculations for a boiler the diam-
eter of which we will assume to be s feet, length 12 feet,
shell g inch thick, heads § inch thick, having double-
riveted longitudinal seams, also single-riveted vertical
seams, and containing sixty tubes 3 inches in diameter.
The first calculation will be to obtain the bursting
pressure of the boiler. It will be necessary to re-
member that the strength of a boiler or any other piece
of mechanism is equal only to that of its weakest point.
Therefore, if we find the strength of the weakest point
we will be on the safe side.

In such a boiler of cylindrical shape, and having, of
necessity, a seam along its length, this will be the point
where breakage is most likely to occur, both because of
the presence of the seam and because the greatest stress
occurs in this direction. Let us imagine ourselves look-
ing at the end of the boiler. There will then be pre-
sented to us a circle in which is exerted a force due to
the steam pressure.

99



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lOO



ELEMENTARY STEAM ENGINEERING.



While the pressure acts equally on all portions of the
boiler, it may be resolved into two forces, one a vertical
force tending to rupture the shell lengthwise, and the
other a horizontal force acting against the heads and
tending to cause rupture at the seam connecting the
heads to the shell.



A




Fig. 45.

As the former can exert pressure only in such a direc-
tion as to produce rupture at a point on the lengthwise
seam, we may consider the force as acting along a plane
equal in length to the diameter of the boiler and tending
to burst the shell at the two points A and A\ Fig. 45,



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CALCULATIONS FOR FIRE TUBULAR BOILERS. lOl

where the diameter joins the circumference of the
circle.

It may further be considered that only one of these
points is liable to be fractured as there is only one seam.
Then the forces tending to cause rupture at this point
are those acting on the plane from the point A! to the
center as indicated by the vertical lines in Fig. 45.
Bearing in mind that tensile strength and pressure are
both expressed as a certain number of pounds per square
inch, we need not consider the entire length of the boiler,
but only a strip of it one inch long. We are, therefore,
calculating the strain required to break at A! the hoop
5 feet in diameter, i inch wide, \ inch thick, exposed at
the given point to a pressure exerted along the plane
from the center to A!. The result of this calculation
will equal the breaking pressure per square inch for
this strip, which pressure is representative of that which
may be carried in the entire boiler. If the material of
which the boiler is composed is steel of the usual stand-
ard tensile strength of 60,000 pounds per square inch,
and if the section at ^', according to the previous de-
scription, is I inch thick and i inch long, or \ square inch
in area, its total strength will be 60,000 pounds multi-
plied by o.s which will equal 30,000 pounds.

Now, as the distance to this point from the center, or
the radius of the boiler, equals 30 inches, then



30,000
30



= 1000 lb., or 1000 pounds per square inch.



So, then, if on each of the 30 square inches represented
by the radius of the hoop i inch wide and 60 inches



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102 ELEMENTARY STEAM ENGINEERING.

in diameter, there is exerted a pressure of looo pounds,
the total pressure will equal 30,000 pounds, just equal
to the calculated strength at the point A\

The boiler is, however, weaker than this because in
order to rivet the seam it was necessary to drill holes in
the material, in which to insert the rivets, and as the
cutting away of material for this purpose naturally re-
duced the effective area of material present, the strength
of the material has been correspondingly reduced at
this point. Careful investigation has shown that if the
original strength of the plate is considered as 100 per
cent, then, the comparative strength of the seams are as
follows:

Single riveted 56 per cent

Double riveted 72 per cent

Triple riveted 85 per cent

Now, taking our previous pressure of 1000 pounds and
multiplying it by 0.72, the strength of our longitudinal
seam, the result is 720 pounds, or the bursting pressure
of the boiler at this point. Whence we have the rule :

To find the bursting pressure* Multiply the tensile
strength of the material by its thickness in hundredths
of an inch, divide this product by the radius of the
boiler in inches, and multiply by the percentage of
strength of the seam*

We must not, however, allow such a boiler to be
operated under a pressure of 720 pounds per square
inch, as this would not be safe; for the pressure may
occasionally rise above that point, and natural deteriora-
tion will gradually weaken the boiler. Furthermore,



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CALCULATIONS FOR FIKB TUBULAR BOILERS. 103

latent defects may exist which will render it unsafe to
operate the boiler at or near the bursting pressure.
The safe working pressure of a boiler is determined by
dividing the bursting pressure by the factor of safety,
which we will assume to be 6. We then have the rule
(see factor of safety) :

To find the safe working pressure: Divide the
bursting pressure by the factor of safety^ the result
equals the safe working pressure.

We have specified in this boiler that the vertical seams
shall be single riveted, and the longitudinal seams double
riveted. Because greater strain occurs on the seams
along the boiler's length, as shown in the following:

As the strain = , if we let

area

JP = pressure per square inch
D = diameter in inches
L = length
T = thickness
S = strain,
we have the formulas,

^ load rnz - ^- - .^ j. t
S = -^ — = — =7=- for the longitudinal seams,
area 2 TX ^

S = -2^ = -^T-TT — TT^TT^foT the vertical seams,
area D X 3«i4i6 X T

Solving the equation, we have,

S = = for longitudinal seams.

2TL 2 T ^



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I04 ELEMENTARY STEAM ENGINEERING.

Substituting tlie given values for the letters of the for-
mula we have,

^ loool b. X6o'' X i" 60,000 ^ ,^ ^ ^^

^ ^ — 2X0.SXI" " ~i 60,000 lb. for the

longitudinal seam, and

^ nr 60 X 1000 60,000 ,^ ^

*=7t=Tx^1 r- = 30,ooo lb. for the

vertical seam.

It will be noted that, in this calculation for strength of
longitudinal seams, both ends of the diameter were
considered, giving us a total strain on both sides of
60,000 pounds per inch, and that as the vertical seam
supports the total pressure exerted on the entire surface
of the boiler head it is exposed only to a strain of one-
half as much as the longitudinal seams were. It, there-
fore, does not require so strong a seam to do the work.

As previously mentioned, the heads or flat portions
of the boiler, being structurally weaker than the curved
portions, require some additional strengthening device
beyond that of making them of thicker material.

Where, in a boiler, two flat surfaces are exposed to
pressure and are strengthened by bolts passing from
one to the other, they are called stay bolts, or through
bolts, depending on their length. Where they are sup-
ported by pieces of material extending diagonally from
the surface at right angles to it, the pieces are called
braces. These are made in several ways, the principal
ones being those known as the crow-foot braces, on
account of the form of the ends where they are riveted
to the surface. Sometimes they are called gusset stays.
These are simple flat plates joined to the surfaces they



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CALCULATIONS FOR FIRE TUBULAR BOILERS. 105

are designed to support, by right angle pieces or brackets,
Fig. 46.
The U. S. Marine Rules allow a strain on steel braces




Fig. 47.

of 8000 pounds per square inch in the case of diagonally
placed braces. Their strength is calculated according
to the following formula (Fig. 47).



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io6 ELEMENTARY STEAM ENQmEERING.

8 = ^XJPXA,
A.

where A = height of triangle.

B = hypotenuse, or diagonal length.
JP = pressure per square inch.
A = area to be supported.

Example: — What strain will there be on a brace supporting a
surface 8 inches square, height 23 inches, diagonal length 24 inches,
pressure 100 pounds?

Solution: — M X 100 = 104 (lb.) X 64 (sq. in.) = 6656 lb.

It must be borne in mind that each brace supports an
area extending to the limits of the area supported by
the other braces. Were the braces spaced 8 inches
apart each one would support an area of 64 square
inches.

It is not necessary to put in braces for the entire head
of such boilers, for the reason that the tubes themselves
serve to support a large portion of the head, and the
stiffening due to bending at right angles at its edges
makes a certain portion of the head of sufficient strength
to resist the strain. The amount to be braced will then
be approximately the area of the shaded segment, shown
in Fig. 48.

In order to find the sizes of the segment we have the
following rule:

To find the height of the segment requiring bracing,
subtract 5 inches from the distance between the tubes
and the highest part of the shell; and to find the di-
ameter of the circle to which the segment to be braced
belongs, subtract 6 inches from the diameter of the
boiler.



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CALCULATIONS FOR FIRE TUBULAR BOlLERS. 107

In order to calculate the area of the segment, several
convenient methods may be employed which do not




Fig. 48.



Fig. 49.

require the use of higher mathematics, for example,
Fig. 49.
Divide the base of the segment into halves, and divide



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io8



ELEMENTARY STEAM ENGINEERINa



one of these halves into four equal parts. Draw per-
pendiculars through each point of division until they
meet the circle and measure each one of the perpendicu-
lars. Then multiply the one at the middle of the seg-
ment by I, the next by 4, the next by 2, and the last by
4; add these products, multiply the sum by the base of
the segment, and divide by 12. (The error in this rule,
due to the imperfection of the rule itself, and taking no
account of error due to imperfection in the measure-




Fig. 50.

ments, is never greater than i per cent. In the case of
the segment shown the rule is in error by about two-
thirds of I per cent.)

Or we have as follows, Fig. 50.

Subtract the height of the given segment from the
radius of the circle, and multiply the result by the
diameter of the circle ; this product is to be subtracted
from the area of the semi-circle of which the segment
forms a part, and the remainder is the approximate area
of the segment. (This rule gives an approximate result
only when the shaded strip shown in Fig. 50 is approxi-
mately a rectangle. The error of the rule amounts to
5 per cent when the height of the segment is 0.272 times



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CALCULATIONS FOR FIRE TUBULAR BOILERS. 109

fhe diameter of the circle. Roughly, we may say that
when the height of the segment is greater than one-
fourth of the diameter of the circle, the rule may be
trusted to give a result within 5 per cent of the truth.)

Instead of taking the shaded area of Fig. 50 as equal
to a rectangle having a length equal to the diameter of
the circle, we may take it as having a length equal to
the base of the unshaded segment, but it is easily shown
that the rule obtained in this manner is not quite so close
as the one just given.

To find the heating surface of a boiler we have the
following rule : Find § the area of the boiler shell plus
the area of one head; to this, add the product obtained
by multiplying the area of one tube by the number of
tubes. The result in square feet will equal the heating
surface.

It was formerly the custom of various boiler makers
to figure the H.P. of boilers by the number of square
feet of heating surface. This is an incorrect method
of determining such results, because very many condi-
tions of operation, material, etc., may serve to produce
var3ring results with the same boiler. Therefore, the
H.P. of boilers is now determined according to certain
definite facts, and we have the following rule :

The bailer JBT.P. is equal to the evaporation of 34,5
pounds of water per hour into steam at 212° F., or
the evaporation of 30 pounds of water per hour from
a feed temperature of 100° F. into steam of 70
pounds gage pressure. This is equal to 33,305
B.T.U. per hour.



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no ELEMENTART STEAM ENGINEEIUNG.

QUBSTIONS.

z. What is a steam boiler?

2. Of what materials are boilers made?

3. Describe a horizontal fire tubular boiler.

4. Describe a water-tube boUer.

5. Give some advantages of each.

6. How are boilers fastened?

7. What is meant by the pitch line of riveted work?

8. What is the strength of a seam compared to that of the
original plate?

9. Why are the heads of a boiler thicker than the shell ?

10. What is the standard tensile strain of boUer material?

11. Give the rule for finding the safe working pressure of a
boiler.

12. Find the bursting pressure of a boiler, four feet in diameter,
if^7 inch shell, with double-riveted seams.

13. What is the safe working pressure of this boiler?

14. If braces are placed 3 in. apart and a pressure of 150 lb. is
carried, how much will each brace support?

15. If the braces are 30 in. long, one end being riveted to the
boiler shell 25 in. from the head, what area should each brace have ?

16. Find the heating surface of a fire tubular boiler 4 ft in
diameter, 12 ft. long, containing 65 two-inch tubes.

17. Of what use is the blow-off and where is it placed ?

18. Describe a method of setting boilers.

19. How would you find the area of the shell of a cylindrical
boiler 5 ft in diameter and 15 ft. in length.

20. What would be the area of surface of a 3-inch tube 12 feet
in length?

21. What is the area of the head of a boiler 5 ft in diameter?



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CHAPTER XIV.
WATEB-TUBE BOILERS.

In the water-tube boiler we reverse to some extent
the conditions of handling the fire and the water.

In this boiler the water contained inside the tubes
is exposed to the heat of the fire which surrounds the
tubes. The immediate result is that small bodies or
portions of water are exposed individually to the intense
heat of the fire. These smaller quantities are heated
to the steaming point more rapidly than the larger
amount, in the former case. On this account these
boilers are perhaps preferable for power stations subject
to sudden demand for increased power, as for instance
those supplying street railways, public electric light
plants and similar service. While this type of boiler
has usually a reservoir or supply outside of the tubes
themselves, such as the water drum placed on the upper
part of the boiler, yet the entire amount of water in the
boiler at any one time is comparatively small, and due
to its construction it responds more rapidly to changes
of temperature. Consequently if the fire is not con-
stantly maintained in good condition the steam will
drop more rapidly than in the fire-tube boiler type of
construction. As water is rapidly changed into steam
by heating, in the fairly restricted passages formed by
the water tubes, scale is more likely to collect in such
quantities in the tubes as to produce serious loss in



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112 ELEMENTARY STEAM ENGINEERING.

heating and also to prevent largely the flow of water
through them. It is also difficult to remove scale from
these tubes.

A number of boilers of this type are of standard con-
struction. Probably among the best known is the
Babcock and Wilcox boiler, which is illustrated in Fig. 51.

Reference to the illustration will show us that water
entering the boiler at or near the rear and lower end
will, being colder, drop down to the lowest portion of
the boiler tubes.

The heated gases surround the tubes as shown in
Fig. 56. The furnace is generally similar in construc-
tion to those previously illustrated, but, in addition,
partitions of fire brick are so placed among the tubes, as
shown at F, Fig. 51, that the heated gases are caused to
travel downward and upward again, thus traversing the
tubes several times before making their final escape up
the smoke stack. Water thus heated and expanded
rises in the direction of the inclination of the tubes until,
reaching the front end, it enters the passage called a
header, connecting all of the tubes at h. From this
point it rises into the reservoir shown at the upper
portion of the boiler. The particles of water not form-
ing steam flow back along the bottom of this reservoir,
and down through the vertical pipes shown, to their
original starting point, thus producing a constant flow
or circulation in the boiler.

At the lower portion of the tubes and connected to
the blow-off pipe is shown a cylinder or mud drum, c.
The function of this is to collect as much of the scale or
sediment as possible while still in a soft state so that it



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WATER-TUBE BOILERS.



"3




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114 ELEMENTARY STEAM ENGINEERING.

may be readily gotten rid of by '' blowing down '' the
boiler.

Steam is taken to supply the engine from the upper
cylinder or reservoir. As the steam pipes must neces-
sarily connect at a point near the surface of the water
and as there would be a tendency to carry water over
into the pipe communicating with the engine, the steam
connection is, therefore, made at the rear end of the
reservoir or drum, for by the time the steam reaches
this point the water will have fallen by gravity back into
the main body. Also the baffle plate &, Fig. 52, serves
to prevent the rush of steam from the header throw-
ing a spray of water into the steam space, by spreading
the body of steam and water, as it rises, over the entire
surface.

The bolt heads and caps seen on the header opposite
the ends of the tubes represent handholes so placed
that the tubes may be cleaned by removing the hand-
hole plates in the usual manner and inserting a tube
scraper through the opening thus formed. On account
of the comparatively small diameter of the reservoir
it is not required that they be braced for strength as in
the case of the large head of the fire-tube boiler. Ad-
ditional and sufficient strength is gained in this case by
forming them to a hemispherical shape as is clearly
shown in Fig. 51.

In the calculation for bursting pressure of the water-
tube boiler, the same rule may be applied as was used
for the fire-tube boiler, except that, in this case, each
tube and the reservoir would be considered as an indi-
vidual cylinder and its strength would be separately



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WATER-TUBE BOILERS. 115



Fig. 52.



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Ii6 ELEMENTARY STEAM ENGINEERING.

calculated. It may be borne in mind, however, that
the results of the calculations for bursting pressure of
one tube may apply to all.

CARE AND OPERATION OF STEAM BOILERS.

Care should always be taken to supply the boiler with
water at as high a temperature as possible. This means
that, as the water enters the boiler in an already heated
condition, less coal will have to be used to bring it to the
steam temperature; also there will be less variation
between the temperature in the boiler and that of the
incoming feed water. In consequence of this, less
strain will be put on the boiler plates and there will be
less liability to damage from this source.

The proper point at which the boiler should receive
the feed is still a debated question.

Many boilers of the fire-tube type receive the water
in the lower part of the front head; some receive it
through a pipe passing through the upper part of the
boiler for a distance and emptying into the central por-
tion of the main body of water.

In some water-tube boilers the water is taken first
into a drum-shaped receptacle where it is partially
warmed before entering the boiler proper. One ad-
vantage of such a method is that a large amount of the
sediment and solid material contained in the water is
deposited in one readily accessible place from which it
may be removed with facility. This solid material or
scale is the source of one of the greatest annoyances to
the engineer. Even in apparently clear drinking water
large quantities of solid matter are held in solution.



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WATER-TUBE BOILERS. 117

When the water is changed into steam this solid matter
is deposited in the boiler, in the form of scale, and forms
an incrustation extending over large portions of the
boiler, first as a soft mud-like mass and then changmg
to a hard rock-like formation. If allowed to remain
there, this scale is productive of ill results for, being a


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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 6 of 17)