But in practice such a boiler would not _Modern Machine-Shop Practice, Volumes I and II

about 75 lbs. per square inch, hence the shell plate thickness is still

largely in excess of the requirements, and it may be remarked that

plates less than 1/4 inch thick are not used on account of the

difficulty of caulking them and keeping them steam tight.

On account therefore of the excessive strength of the shell plates in

boilers of small diameter, butt straps are rarely used in stationary

boilers, while punching the rivet holes and other inferior modes of

construction are employed. We may now consider the circumferential seams

of the boilers for stationary engines, such boilers sometimes being of

great length in proportion to the diameter.

In proportion as the length of a boiler (in proportion to its diameter)

is increased, the construction of the circumferential or transverse

seams, as they are sometimes called, becomes of more importance.

The strength of the circumferential seams is so much greater than that

of the longitudinal seams that it is often taken for granted that they

are sufficiently strong if made with a lap joint and single riveted, but

that such is not always the case will be shown presently.

[Illustration: Fig. 3255.]

In Fig. 3255 is represented a boiler composed of three strakes (_i. e._,

three rings or sections), and it is clear that as the thickness of the

shell is doubled at the circumferential seams where the ends of the

middle strake pass within the end strakes, therefore the strength of the

lapped joint of the shell to resist rupture in a transverse direction,

as denoted by the arrows A, B, is actually increased by reason of the

lap of the riveted joint. But suppose this boiler to be supported at the

ends only, and the weight of the shell and of the water within it will

be in a direction to cause the middle of the boiler to sag down, and

therefore places a shearing strain on the rivets of the circumferential

seams.

Moreover, the temperature of the outside of the boiler cannot be made or

maintained uniform, because the fire passing beneath the bottom of the

boiler first will keep it hotter, causing it to expand more, and this

expansion acts to shear the rivets of the circumferential seams. In

proportion as the heat of the fire varies in intensity, the amount of

the expansion will vary, and the consequence is that the circumferential

seams may get leaky or the joint may work, especially in boilers that

are long in proportion to their diameters. It is clear, therefore, that

for the very best construction at least a double riveted circumferential

joint should be employed.

Leaving these considerations out of the question, however, we may find

the amount of stress on the circumferential seams by multiplying the

area of the end of the boiler by the working pressure, and dividing by

the cross-sectional area of all the rivets in one circumferential seam.

Suppose, for example, that the diameter of the boiler is 36 inches, the

working pressure 60 lbs. per square inch, and that there are in each

circumferential seam 50 rivets, each 3/4 inch in diameter, and we

proceed as follows:

The area of a circle 36 inches in diameter = 1017.87 square inches.

The area of a rivet 3/4 inches in diameter = .4417 square inch.

Then

Area of Working

boiler end. pressure.

1017.87 × 60

------------------ = 2765 lbs. per cross-sectional square inch of rivet.

50 × .4417

Number Area of

of rivets. each rivet.

By multiplying the area of the boiler end by the working pressure, we

get the total steam pressure acting to shear the rivets, and by

multiplying the number of rivets by the area of one rivet, we get the

total area resisting the steam pressure, and then by dividing the one

quantity into the other, we get the shearing stress per square inch of

rivet section.

In the case of longitudinal seams, we have as follows, the pitch being

say 2-1/8 and the rivets 3/4.

Diameter Steam Pitch.

of boiler pressure.

in inches.

36 × 60 × 2.125

------------------------------ = 5196 lbs. per square inch of rivet

2 × .4417 area.

Rivets in Area of

one pitch. rivet.

It is seen, therefore, that the stress placed by the steam pressure on

the transverse seam is about one-half of that it places on the

longitudinal seam. But, as before remarked, the transverse seam is

subject to racking strains, from which the longitudinal seams are

exempt; thus, for example, the expansion of the boiler diameter, whether

uniform or not, does not strain the longitudinal seam, whereas it may

severely strain the transverse seam.

The English Board of Trade rules, in assigning values to the various

constructions and qualities of workmanship, assign a certain value, in

the form of an addition to the factor of safety, which takes into

account the difference in the stress upon the transverse and

longitudinal seams, the quantities in each case having been determined

both from experiment and from experience. A comparison of the different

values may be made as follows:

The rules take a boiler shell made of the best material, with all the

rivet holes drilled after the strakes are rolled into shape and put

together, with all the seams (both longitudinal and transverse) fitted

with double butt straps each at least five-eighths of the thickness of

the shell plates they cover, and with all the seams at least double

riveted, with rivets having an allowance of not more than 75 per cent.

over the single shear, and provided that the boilers have been open to

the inspection of their surveyors during the whole period of

construction, and say that such a boiler shell shall be allowed a factor

of safety (divisor of seam strength) of 5.

But for every departure from this, which they deem the best mode of

construction, a penalty in the shape of an addition to the factor of

safety is made. These additions to the factors of safety with reference

to the longitudinal as compared to the transverse seams, are given in

the following table:

-------------------------------------+----------------+---------------

|Addition to the |Addition to the

|factor 5 if the |factor 5 if the

Nature of the deviation in the |deviation is in |deviation is in

construction or workmanship. |the longitudinal|the transverse

| seam. | seam.

-------------------------------------+----------------+---------------

The holes not fair and good | .75 | .2

Holes drilled out of place after | |

bending | .15 | .1

Holes drilled before bending | .3 | .15

Holes punched after bending | .3 | .15

Holes punched before bending | .5 | .2

Joints lapped and double riveted | |

instead of having double butt straps | .2 | .1

Joints double riveted but have single| |

butt straps | .3 | .1

Joints single riveted and have a | |

single butt strap | 1.0 | .2

Joints lapped and single riveted | 1.0 | .2

-------------------------------------+----------------+---------------

[Illustration: Fig. 3256.]

An addition of .25 is also made to the factor of safety, when the

strakes are not entirely under or over. In Fig. 3256 for example, strake

_b_ is within or under strake _a_ at one end and strake _c_ at the other

end, hence _b_ is entirely under; strake _c_ is over _b_ and _d_, and

therefore entirely over; while strake _d_ is under _c_, and over _e_,

and therefore not entirely under nor entirely over.

When the rivet holes are punched they do not match properly, and unless

the holes are punched somewhat smaller than the required size and reamed

out afterwards, some rivets receive more stress than others, and may

consequently shear in detail. It is customary, however, to punch the

holes for ordinary stationary boilers, and it is with seams having

punched holes therefore that we have at present to deal.

In the United States the rivet diameter and plate percentages are, in

the boilers of stationary engines, usually made equal, and the reasons

advanced both for and against this are as follows:

First, in favor of a greater plate percentage than rivet section, it is

advanced that the plate gets thinner by wear, whereas the rivet does

not, hence the wear reduces the plate section; that the plate is

weakened by the punching process, and requires a greater percentage to

make up its strength as compared to the rivet; that the rivets are

usually of better material than the plates.

In favor of a greater rivet section than plate section, it is advanced

that the shearing strength of iron is but about four-fifths of the

tensile strength, and that with equal plate and rivet sections the rivet

is therefore the weakest; that with punched holes the rivets may be

sheared in detail, and that the rivets may be sheared gradually by the

working of the joint from varying expansion and contraction.

From these premises the assumption is drawn that the weakening of the

plate from being punched and from corrosion about offsets the excess of

the tensile over the shearing strength, and that it is best therefore to

employ such a pitch that the area of the rivet and of the metal left

between the rivet holes shall be equal.

In order to do this the diameter of the rivet must be determined, and

the following are the proportions given by the various authorities

named:

TABLE OF THE DIAMETERS OF RIVETS FOR VARIOUS THICKNESSES OF PLATES WITH

SINGLE RIVETED LAP JOINT.

---------+-------------------------------------------------------

| DIAMETER OF RIVETS.

+-------+---------+----------+----------+-------+-------

---------+-------+---------+----------+----------+-------+-------

in. | in. | in. | in. | in. | in. | in.

5/16 | 5/8 | 5/8 | 1/2 | 5/8 | 11/16 | 5/8

3/8 | 5/8 | 5/8 | 5/8 | 3/4 | 3/4 | 11/16

7/16 | 5/8 | 3/4 | 3/4 | 21/32 | 13/16 | 3/4

1/2 | 3/4 | 13/16 | 3/4 | 3/4 | 7/8 | 3/4

---------+-------+---------+----------+----------+-------+-------

9/16 | 3/4 | 13/16 | 7/8 | 27/32 | 7/8 | 7/8

5/8 | 3/4 | 7/8 | 7/8 | 15/16 | 15/16 | 7/8

11/16 | 7/8 | 7/8 | 7/8 | 1-1/32 |1 | 7/8

3/4 | 7/8 | 15/16 | 1 | 1-1/8 |1-1/16 | 1

---------+-------+---------+----------+----------+-------+-------

13/16 | 7/8 | 1 | 1 | 1-7/32 |1-3/32 | 1

7/8 |1 | 1-1/8 | 1-1/8 | ... |1-1/8 | 1

15/16 |1 | 1-3/16 | 1-1/8 | ... |1-3/16 | 1-1/8

1 |1 | 1-1/4 | 1-1/8 | ... |1-1/4 | 1-1/8

---------+-------+---------+----------+----------+-------+-------

From the above it is seen that with thin plates the diameter of rivet

employed is about twice the thickness of the plate, whereas as the

thickness of plate increases the proportion of rivet diameter decreases,

and the reasons for this are, first, that with rivets twice the

thickness of thick plates and pitched so as to equalize the rivet and

plate sections the pitch would be too great to permit of the seams being

caulked steam tight.

The diameter of the rivet having been determined, the rivet area and

area of plate left between the rivet holes may be made equal by

determining the pitch by the following rule:

_Rule._--To the area of the rivet divided by the plate thickness add the

diameter of the rivet, and the sum so obtained is the pitch. The

correctness of this rule may be shown as follows:

Suppose the rivet diameter to be 7/8 inch = decimal equivalent .875, and

its area will be .6013 square inch. Suppose the thickness of the plate

to be 9/16 = decimal equivalent .5625, then by the rule:

Rivet area.

Plate thickness = .5625 ) .6013 ( 1.0689

5625

----

38800

33750

-----

50500

45000

-----

55000

50625

To this 1.0689 we are to add the rivet diameter, thus:

1.0689

.8750 = rivet diameter.

------

1.9439 = pitch of the rivets.

We have thus found the required pitch to be 1.9439 inches, and as the

joint is single riveted there are two half rivets or one whole one to

one pitch, and if we subtract the diameter of the rivet from the pitch

we shall get the width of the metal or plate left between the rivets,

thus:

1.9439 = pitch of rivets.

.8750 = diameter of rivet.

------

1.0689 = distance in inches between the rivets.

If now we multiply this distance between the rivets by the thickness of

the plate, we shall get the area of the plate that is left between the

rivet holes, thus:

1.0689 = width of plate between rivets.

.5625 = thickness of plate.

------

53445

21378

64134

53445

---------

Area of plate = .60125625

between rivets

Here then we find the area of plate left between the rivet holes to be

6.01 square inches, and as the area of the rivet is 6.01 square inches,

the two are shown to be equal.

We may now place the various rivet diameters and the pitches that will

make the rivet area and plate area in a single riveted joint equal in a

table as follows:

TABLE OF RIVET DIAMETERS AND PITCHES FOR SINGLE RIVETED LAP JOINTS.

-------------------+------------------+--------

Thickness of Plate.|Diameter of Rivet.| Pitch.

-------------------+------------------+--------

1/4 | 1/2 | 1-1/4

5/16 | 5/8 | 1-5/8

3/8 | 11/16 | 1-11/16

7/16 | 3/4 | 1-3/4

1/2 | 3/4 | 1-5/8

9/16 | 7/8 | 2

5/8 | 7/8 | 1-7/8

11/16 | 7/8 | 1-3/4

3/4 | 1 | ..

13/16 | 1 | 2

7/8 | 1 | 1-1/8

15/16 | 1-1/8 | 2-1/8

1 | 1-1/8 | 2-1/8

1-1/16 | 1-1/8 | 2-1/8

1-1/8 | 1-3/16 | 2-1/4

1-1/4 | 1-3/16 | 2-1/8

-------------------+------------------+--------

The rivets in double riveted lap joints, and in butt strap joints having

a single cover, are spaced alike, because in both cases there are two

rivets in one pitch, and the rivets are in single shear.

As there are two rivets in one pitch (instead of only one as in a single

riveted joint), therefore the percentage of rivet section is doubled,

and the plate section must therefore be doubled if the plate and rivet

sections are to be made equal, and the rule for finding the required

pitch is as follows:

_Rule._--To the amount of rivet area in one pitch, divided by the

thickness of the plate, add the diameter of the rivet.

_Example._--Let the plate thickness be as in the last example 9/16,

decimal equivalent = .5625, and the rivet diameter be 7/8 inch = decimal

equivalent .875, the area of one rivet being .6013 square inch, and the

pitch is calculated as follows:

.6013 = area of one rivet.

2 = the rivets in one pitch.

------

Plate thickness = .5625 ) 1.2026 ( 2.1377

1.1250

------

7760 2.137

5625 .875 = rivet diameter.

---- -----

21350 3.012 = pitch.

16875

-----

43750

39375

-----

43750

39375

-----

4375

We find, therefore, that the pitch is 3.012, or 3 inches (which is near

enough for practical purposes), and we may now make it clear that this

is correct.

[Illustration: Fig. 3257.]

In Fig. 3257 the joint is shown drawn one-half full size, and the length

a of plate left between the rivet holes measures (as nearly as it is

necessary to measure it) 2-5/32 inches, or 2.156, and if we multiply

this by the thickness of the plate = .5625 inch, we get 1.2 square

inches as the area of the plate left between the rivet holes.

Now there are two rivets in a pitch (as one-half of B, one-half of C,

and the whole of F), and as the area of each rivet is .6, therefore the

area of the two will be 1.2, and the plate section and rivet section are

shown to be equal.

The area at _a_ is obviously the same as that at A, because the pitches

of both rows of rivets are equal, this being an ordinary zigzag riveted

joint.

We may now consider the diagonal pitch of the rivets, using the rule

below.

The pitch × 6, + 4 times the rivet diameter

------------------------------------------- = the diagonal

divided by 10 pitch _p__{D}.

In this example the pitch has been found to be 3 inches, hence we have

.875 = diameter of rivet.

4 = constant.

-----

3.500

3 = pitch of the rivets.

6 = constant.

3.5 = rivet diameter multiplied by 4.

----

10 ) 21.5 (2.15 = the diagonal pitch.

The diagonal pitch, that is, the distance _p__{D}, Fig. 3257, is

therefore found to be 2.15, or 2-1/8 inch full.

The amount of metal left between the rivets, measured on the diagonal

pitch, is twice the dimension H multiplied by the thickness of the

plate, and as this (with the diagonal pitch determined as above) always

exceeds the pitch A or _a_, therefore if the plate fails, it will be

along the line _a_, and not through the diagonal pitch.

We may now consider the total amount that the plates overlap in a double

riveted lap joint zigzag riveted, this amount being twice the distance

E, added to the distance V between the rows of rivets.

The distance E, Fig. 3257, is usually made one and a half times the

diameter of the rivet, this being found to give sufficient strength to

prevent the edge of the plate from tearing out and to prevent the rivet

from shearing the plate out to the edge, rupture not being found to

occur in either of these directions.

The rule for finding the distance V, when the diagonal pitch has been

determined by the rules already explained, is as follows:

_Rule._--To the pitch multiplied by 11, add 4 times the rivet diameter,

then multiply by the pitch, plus 4 times the rivet diameter. Then

extract the square root and divide by 10.

Placed in formula, the rule appears as follows, _d_ representing the

rivet diameter, and _p_ the pitch.

__________________________

\/(11_p_ + 4_d_)(_p_ + 4_d_)

---------------------------- = distance V between the rows of rivets.

As this rule involves the extraction of the square root of the sum of

quantities above the line, and as in determining the diagonal pitch, we

have already determined the distance V, it is unnecessary to our purpose

to carry out this latter calculation, as it is easier to find the

diagonal pitch, and then, after drawing the joint, the distance between

the rows of rivets can be measured if it is required, as it might be in

finding the length of plate required to roll into a strake for a boiler

of a given diameter and having a double riveted lap joint.

We may now consider chain riveted joints in comparison with zigzag

riveted joints, which is especially necessary, because it has been

assumed by some that the second row of rivets in a chain riveted joint

added nothing to the strength of the joint.

[Illustration: Fig. 3258.]

Fig. 3258 represents a chain riveted joint, having the same thickness of

plate, rivet diameter and pitch as the zigzag riveted joint in Fig.

3257, and it will be seen that the plate sections at a and at _a_ are

the same in the two figures, and as there are four half rivets, which

are equal to two rivets, in one pitch, therefore the strength of the two

joints is equal.

Each joint can be as efficiently caulked as the other, as the rivet

spacing is the same and the edge of the plate is the same distance from

the rivets in both cases.

The pitch of the rivets is obtained by the same rule as for zigzag

riveted joints, and all we have now to consider is the distance apart of

the two rows of rivets or distance V in the Fig. 3258, and for this

there are two rules, the first being that it shall not be less than

twice the diameter of the rivet, which would leave a dimension at H in

the figure equal to the diameter of the rivet. The second rule is that a

better proportion than the above is to multiply the diameter of the

rivet by 3. This makes the dimension at H equal to twice the rivet

diameter.

When the joints have double buttstraps, the rivets may be spaced as wide

as the necessity for tight caulking will admit, because, on account of

the rivets being in double shear, the rivet percentage exceeds the plate

percentage.

[Illustration: Fig. 3259.]

The allowance for the rivets being in double shear is 75 per cent., or

in other words, a rivet in double shear is allowed 1.75 times the area

of the same size rivet in single shear.

STATIONARY ENGINE BOILERS.

The simplest form of horizontal boiler is the plain cylinder boiler, an

example of which is given in Fig. 3259, and which is largely used in

iron works and coal mines.

Boilers of this class are easily cleaned, because the whole interior can

be readily got at to clean.

As the bottom of this boiler gets thinned from wear, the boiler is

turned upside down, thus prolonging its life.

[Illustration: Fig. 3260.]

Fig. 3260 represents an internally fired flue boiler, known as the

Cornish or Lancashire boiler. The furnace is at one end of the flues,

the fire passing through them to the chimney. There is here obviously

more heating surface than in the plain cylinder boiler, but somewhat

less facility for cleaning.

The Galloway boiler is of this class, but has vertical water tubes

placed at intervals in the flues. These water tubes are wider at the top

than at the bottom. They serve to break up the body of heat that passes

through the flues, and increase the heating surface while extracting

more of the heat and promoting the circulation of the water in the

boiler.

A water tube is one in which the water is inside and the fire outside,

as distinguished from a fire tube, in which the fire passes through the

tube and the water is outside. A water tube is stronger than a fire

tube, because the former is subject to bursting pressure and the latter

to collapsing pressure.

Vertical boilers are internally fired, and in the ordinary forms have no

return tubes or flues, examples of those used for small stationary

engines being given as follows.

[Illustration: Fig. 3261.]

Fig. 3261 represents an ordinary form with vertical tubes. The upper

ends of the tubes here pass through the steam space--a condition that

under the moderate pressures and firing that this class of boiler is

subjected to is of less importance than it is in boilers having higher

chimneys and therefore a more rapid draught, and using higher pressures

of steam. Furthermore, the small diameters and lengths or heights in

which these boilers are made give them ample strength with shells and

tubes of less thickness, while the condition of tube ends with steam on

one side and fire on the other is permissible without the injurious

effects that ensue under rapid combustion and high pressures.

[Illustration: Fig. 3262.]

The crown sheet of the fire boxes or furnaces of this class of boiler is

very effective heating surface, first, because of the great depth (and

therefore weight) of water resting upon it insuring constant contact

between the water and the plate, while there is no danger of the crown

sheet burning from shortness of water.

A similar boiler, but with the upper ends of the tubes below the water

level, is shown in Fig. 3262.

From the small diameters of these boilers, the flat surfaces are not

stayed except to the extent that the holding power of the tubes serves

that end.

[Illustration: Fig. 3263.]

[Illustration: Fig. 3264.]

A return flue vertical boiler is shown in Figs. 3263 and 3264. The whole

of the surfaces having contact with the fire also have contact with the

water, and the height of the crown sheet removes it from the intense

heat of the fire. It is stayed to the top of the boiler. The fire box or

combustion chamber being taper increases the effectiveness of its sides

as heating surface, since the heat in its vertical passage impinges

against it.

The products of combustion pass from the top of the combustion chamber

through short horizontal flues, which enter an annular space surrounding

the lower section of the boiler, and from this space vertical flues pass

to a corresponding space at the bottom of the boiler.

The passage of the steam generated at the sides of the combustion

chamber is facilitated by the taper of the chamber, which gives

increased room for the steam as it gathers in ascending.

Vertical boilers for high pressures, as from 60 to 120 lbs. per inch,

are represented in the figures from 3265 to 3269.

In boilers of this class, a majority contain water tubes, which, when

properly arranged, promote rapid evaporation and circulation.

A boiler with _Field_ tubes is shown in Fig. 3265. It consists of an

outer shell and a cylindrical fire box, from the crown sheet of which a

number of Field tubes are suspended in the fire box or combustion

chamber.

Fig. 3266 is a sectional view of a Field tube, the construction being as

follows:

The outer tube, which is expanded into the tube plate, is enclosed at

its lower end, and has at its upper end in the water space of the boiler

a perforated mouth piece, from which is suspended an inner tube that

extends nearly to the bottom of the outer tube.

As the outer tube is bathed in the fire, steam is generated very

rapidly, and a thorough and rapid circulation is kept up, the water

passing down the inner and up the outer tubes, as denoted by the arrows.

The outer tube is spread out at the upper end to a slight cone, so that

it cannot be forced out of the tube sheet by the pressure, and as it

hangs free, there is no liability for it to loosen or get leaky from

expansion and contraction.

From the great amount of heating surface obtained with these tubes, the

fire box may be kept at a minimum diameter for the duty, while still

leaving a wide space for the water leg, which facilitates the

circulation.

[Illustration: Fig. 3265.]

[Illustration: Fig. 3266.]

The damper, which is suspended in the uptake, spreads the fire sideways.

[Illustration: Fig. 3267.]

Fig. 3267 represents the arrangement of Field tubes in a boiler.

A boiler of this form may for a given capacity be made lighter and

smaller than in any other of the ordinary forms, while the rapid

circulation acts to keep the tubes clean.

The inner tubes may be thin, because they are under pressure both inside

and out, while the outer tubes may be thin, because they are under a

bursting strain, whereas a fire tube is under collapsing pressure.

[Illustration: Fig. 3268.]

A design of high rate boilers, in which the uptake does not come into

contact with the water, and water tubes are employed, is shown in Fig.

3268. In the fire box is an inclined tube which promotes the

circulation, and is very effective heating surface, and in the

combustion chamber are a number of vertical water tubes.

Two manholes give access for cleaning purposes.

The efficiency of the heating surface in this class of boiler is

increased from the fact that, as the heat does not pass direct through

the boiler, it impinges against the surface. In Fig. 3269, for example,

the exit from the spherical fire box is on one side of the boiler, and

the uptake on the other, the heat passing from the fire box into a

combustion chamber, and thence through the horizontal fire tubes to the

uptake.

The crown sheet is here stayed by gusset stays, but if made spherical,

as in Fig. 3270, the stays may be omitted.

[Illustration: _VOL. II._ =BOILER FOR STATIONARY ENGINES.= _PLATE

XXVIII._

Fig. 3271.]

Figs. 3271, 3272, and 3273 illustrate a 60-inch horizontal return

tubular boiler constructed by the Hartford Steam Boiler Inspection and

Insurance Company. This class of boiler has found much favor in the

United States. It is an externally fired, return tube boiler, the fire

passing beneath the boiler and returning through the tubes to the front

end of the boiler, whence it passes through the drum to the chimney.

[Illustration: Fig. 3269.]

[Illustration: Fig. 3270.]

[Illustration: Fig. 3272.]

The boiler is supported on the brackets B, B´, the front one, B, resting

on an iron plate imbedded in the brickwork, and the back ones on rollers

which rest on the plates P´ imbedded in the brickwork. This allows the

boiler to expand and contract endways under variations of temperature

without racking the brickwork.

[Illustration: Fig. 3273.]

A, A, etc., are for holding the brickwork together. The blow-off pipe C

is for emptying or blowing down the boiler. The feed-pipe F enters the

front end of the boiler, passes along it, and then crosses over. A pipe

H from the steam space of the boiler supplies steam to the steam gauge

G, and to the upper end of the gauge glass, which is on the casting K.

The lower end of the gauge glass receives water from a pipe which passes

into the water space of the boiler; at J are the three gauge cocks for

testing the height of the water in the boiler.

The manhole affords ingress into the boiler for inspecting and for

scaling or cleaning it, the nozzles being for a safety valve. At E is a

hand-hole for washing out and cleaning the boiler. P is a damper in the

fire door for admitting air above the fire bars, and R is a damper for

regulating the draught.

In the brick walls that support the boiler there are air spaces to

prevent the conduction of the heat through and prevent cracking of the

brickwork. The tubes are arranged in vertical and horizontal rows and

are equally spaced throughout.

Fig. 3274 represents the front end, and Fig. 3275 a longitudinal

sectional view of the front end of a boiler of this class. In this case,

however, the pipes for the water gauge pass direct into the boiler.

In some practice the tubes are arranged as in Fig. 3276, being wider

pitched or spaced in the middle of the boiler to increase the

circulation of the water in the boiler.

Another arrangement is shown in Fig. 3277, the tubes being _staggered_

or arranged zigzag. This permits of the employment of a greater number

of tubes, but does not afford such free circulation of the water.

Fig. 3278 represents an arrangement where the tubes are in rows both

vertically and horizontally.

Fig. 3279 represents a boiler by the Erie Iron Works, the details of the

setting being as follows:

[Illustration: Fig. 3274.]

[Illustration: Fig. 3275.]

[Illustration: Fig. 3276.]

[Illustration: Fig. 3277.]

[Illustration: Fig. 3278.]

[Illustration: Fig. 3279.]

[Illustration: Fig. 3280.]

[Illustration: Fig. 3281.]

Fig. 3280 is an end view of the setting with the brickwork in section.

Fig. 3281 side view of the boiler and setting.

[Illustration: Fig. 3282.]

[Illustration: Fig. 3283.]

Fig. 3282 a front end view of the boiler, and Fig. 3283 a ground plan of

the brickwork. When the front plate of the boiler setting extends above

the middle of the boiler, as in Fig. 3279, it is said to have a "full

arch front." Whereas when this plate or casting extends to the middle

only of the boiler, it is said to have a half arch front.

[Illustration: Figs. 3284, 3285, 3286, 3287.]

Figs. 3284, 3285, 3286, and 3287 show the setting for a half arch front

boiler, the dimensions of the settings of both these boilers being given

in the following tables:

MEASUREMENTS FOR SETTING TUBULAR STATIONARY BOILERS WITH FULL ARCH

FRONTS.

REFERENCE LETTERS ON DIAGRAMS.

------+---+----+----+----+----+----+------+----+----+----+----+----

No. | A | B | C | D | E | F | G | H | I | J | K | L

------+---+----+----+----+----+----+------+----+----+----+----+----

|Ft.|Ins.|Ins.|Ins.|Ins.|Ins.| Ins. |Ins.|Ins.|Ins.|Ins.|Ins.

1 | 7| 32 | 12 | 20 | 16 | 45 |44 | 7 | 32 | 64 | 85 | 26

2 | 7| 34 | 12 | 20 | 16 | 48 |47 | 8 | 34 | 66 | 90 | 26

3 | 8| 36 | 12 | 20 | 16 | 48 |47 | 8 | 36 | 68 | 92 | 26

3-1/2| 10| 36 | 12 | 20 | 16 | 48 |47 | 8 | 36 | 68 | 92 | 26

4 | 8| 42 | 14 | 20 | 16 | 48 |47 | 8 | 42 | 74 | 98 | 27

5 | 10| 42 | 14 | 20 | 16 | 48 |47 | 8 | 42 | 74 | 98 | 27

6 | 10| 44 | 14 | 24 | 16 | 48 |47 | 10 | 44 | 76 |100 | 27

7 | 12| 44 | 14 | 24 | 16 | 48 |46-1/2| 10 | 44 | 76 |100 | 27

7-1/2| 14| 44 | 14 | 24 | 16 | 47 |45-1/2| 10 | 44 | 76 | 99 | 26

8 | 12| 48 | 16 | 24 | 16 | 47 |45-1/2| 10 | 48 | 88 |103 | 26

9 | 14| 48 | 16 | 24 | 16 | 47 |45-1/2| 10 | 48 | 88 |103 | 26

10 | 12| 54 | 16 | 24 | 20 | 50 |48-1/2| 10 | 54 | 94 |112 | 26

10-1/2| 15| 54 | 16 | 24 | 20 | 50 |48-1/2| 10 | 54 | 94 |112 | 26

11 | 12| 60 | 18 | 24 | 20 | 50 |48-1/2| 12 | 60 |108 |118 | 26

12 | 14| 60 | 18 | 24 | 20 | 50 |48-1/2| 12 | 60 |108 |118 | 26

13 | 16| 60 | 18 | 26 | 20 | 50 |48 | 12 | 60 |108 |118 | 26

14 | 15| 66 | 18 | 28 | 20 | 50 |48-1/2| 12 | 66 |114 |124 | 26

15 | 16| 66 | 18 | 28 | 20 | 50 |48 | 12 | 66 |114 |124 | 26

16 | 16| 72 | 20 | 30 | 20 | 50 |48 | 12 | 72 |120 |130 | 26

------+---+----+----+----+----+----+------+----+----+----+----+----

------+----+-------+----+----+----+----+----+----+----+------+------

| | | | | | | | | | | NO.

No. | M | N | O | P | Q | R | S | T | U | FIRE |FLOOR

------+----+-------+----+----+----+----+----+----+----+------+------

|Ins.|Ft.Ins.|Ins.|Ins.|Ins.|Ins.|Ins.|Ins.|Ins.| |

1 | 19 | 11-6 | 20 | 40 | 12 | 16 | 36 | 34 | 4 | 600 | 6800

2 | 22 | 11-6 | 20 | 40 | 12 | 16 | 36 | 34 | 4 | 600 | 7500

3 | 22 | 12-6 | 24 | 40 | 12 | 16 | 36 | 34 | 4 | 650 | 7700

3-1/2| 22 | 14-6 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 720 | 8500

4 | 21 | 12-8 | 24 | 40 | 12 | 16 | 36 | 34 | 4 | 730 | 8500

5 | 21 | 14-8 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 770 | 9600

6 | 21 | 15-0 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 880 | 10500

7 | 21 | 17-0 | 32 | 52 | 12 | 16 | 48 | 49 | 4 | 940 | 10800

7-1/2| 21 | 19-0 | 36 | 58 | 12 | 16 | 54 | 84 | 4 | 1120 | 11500

8 | 21 | 17-2 | 32 | 52 | 12 | 20 | 48 | 49 | 4 | 1120 | 13600

9 | 21 | 19-2 | 36 | 58 | 12 | 20 | 54 | 84 | 4 | 1140 | 15700

10 | 24 | 17-6 | 32 | 52 | 12 | 20 | 48 | 49 | 4 | 1160 | 16200

10-1/2| 24 | 20-8 | 36 | 56 | 16 | 20 | 54 | 90 | 4 | 1270 | 17500

11 | 24 | 17-10 | 32 | 50 | 16 | 24 | 48 | 49 | 4 | 1400 | 20500

12 | 24 | 19-10 | 36 | 56 | 16 | 24 | 54 | 84 | 4 | 1500 | 23000

13 | 24 | 22-0 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1540 | 25300

14 | 24 | 21-2 | 36 | 56 | 16 | 24 | 54 | 90 | 4 | 1590 | 26000

15 | 24 | 22-2 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1620 | 27000

16 | 24 | 22-6 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1750 | 30000

------+----+-------+----+----+----+----+----+----+----+------+------

NOTE.--In setting "Standard" boilers, the side walls should be so built

that the longitudinal seams of the shell will be protected from the

fire.

MEASUREMENTS FOR SETTING TUBULAR STATIONARY BOILERS WITH HALF ARCH

FRONTS.

REFERENCE LETTERS ON DIAGRAMS.

------+---+----+----+----+----+------+------+----+----+----+-------

No. | A | B | C | D | E | F | G | H | I | J | K

------+---+----+----+----+----+------+------+----+----+----+-------

|Ft.|Ins.|Ins.|Ins.|Ins.|Ins. | Ins. |Ins.|Ins.|Ins.| Ins.

1 | 7 | 32 | 14 | 20 | 16 |46 |45 | 7 | 32 | 64 | 73

2 | 7 | 34 | 14 | 20 | 16 |46 |45 | 8 | 34 | 66 | 75

3 | 8 | 36 | 14 | 20 | 16 |46 |45 | 8 | 36 | 68 | 77

3-1/2|10 | 36 | 14 | 20 | 16 |46 |45 | 8 | 36 | 68 | 77

4 | 8 | 42 | 18 | 20 | 16 |46 |45 | 8 | 42 | 74 | 83

5 |10 | 42 | 18 | 20 | 16 |46 |45 | 8 | 42 | 74 | 83

6 |10 | 44 | 18 | 24 | 16 |46 |45 | 10 | 44 | 76 | 85

7 |12 | 44 | 18 | 24 | 16 |46 |44-1/2| 10 | 44 | 76 | 85

7-1/2|14 | 44 | 18 | 24 | 16 |46 |44-1/2| 10 | 44 | 76 | 85

8 |12 | 48 | 19 | 24 | 16 |50 |48-1/2| 10 | 48 | 88 | 93

9 |14 | 48 | 19 | 24 | 16 |50 |48-1/2| 10 | 48 | 88 | 93

10 |12 | 54 | 19 | 24 | 20 |50 |48-1/2| 10 | 54 | 94 | 99

10-1/2|15 | 54 | 19 | 24 | 20 |50 |48-1/2| 10 | 54 | 94 | 99

11 |12 | 60 | 21 | 24 | 20 |46-3/4|45-1/2| 12 | 60 |108 |101-3/4

12 |14 | 60 | 21 | 24 | 20 |46-3/4|45 | 12 | 60 |108 |101-3/4

13 |16 | 60 | 21 | 26 | 20 |46-3/4|45 | 12 | 60 |108 |101-3/4

14 |15 | 66 | 24 | 28 | 20 |47 |45-1/2| 12 | 66 |114 |108

15 |16 | 66 | 24 | 28 | 20 |47 |45-1/2| 12 | 66 |114 |108

16 |16 | 72 | 24 | 30 | 20 |48 |46-1/2| 12 | 72 |120 |115

------+---+----+----+----+----+------+------+----+----+----+-------

------+------+------+--------+----+------+----+----+----+----+----

No. | L | M | N | O | P | Q | R | S | T | U

------+------+------+--------+----+------+----+----+----+----+----

| Ins. | Ins. |Ft. Ins.|Ins.| Ins. |Ins.|Ins.|Ins.|Ins.|Ins.

1 |26 |20 | 10-3 | 20 |33 | 12 | 16 | 36 | 34 | 4

2 |26 |20 | 10-3 | 20 |33 | 12 | 16 | 36 | 34 | 4

3 |26 |20 | 11-3 | 24 |33 | 12 | 16 | 36 | 34 | 4

3-1/2|26 |20 | 13-3 | 28 |39 | 12 | 16 | 42 | 42 | 4

4 |27 |19 | 11-3 | 24 |32-1/2| 12 | 16 | 36 | 34 | 4

5 |27 |19 | 13-3 | 28 |38-1/2| 12 | 16 | 42 | 42 | 4

6 |27 |19 | 13-7 | 28 |38-1/2| 12 | 16 | 42 | 42 | 4

7 |27 |19 | 15-7 | 32 |44-1/2| 12 | 16 | 48 | 49 | 4

7-1/2|27 |19 | 17-7 | 36 |50-1/2| 12 | 16 | 54 | 84 | 4

8 |26 |24 | 15-7 | 32 |48 | 12 | 20 | 48 | 49 | 4

9 |26 |24 | 17-7 | 36 |54 | 12 | 20 | 54 | 84 | 4

10 |26 |24 | 15-11 | 32 |48-1/2| 12 | 20 | 48 | 49 | 4

10-1/2|26 |24 | 19-1 | 36 |52-1/2| 16 | 20 | 54 | 90 | 4

11 |26 |20-3/4| 16-1 | 32 |47 | 16 | 24 | 48 | 49 | 4

12 |26 |20-3/4| 18-1 | 36 |53 | 16 | 24 | 54 | 84 | 4

13 |26 |20-3/4| 20-3 | 40 |53 | 16 | 24 | 54 | 96 | 4

14 |26 |21 | 19-5 | 36 |52-1/2| 16 | 24 | 54 | 90 | 4

15 |26 |21 | 20-5 | 40 |52-1/2| 16 | 24 | 54 | 96 | 4

16 |28-1/4|19-3/4| 20-7 | 40 |52-1/2| 16 | 24 | 54 | 96 | 4

------+------+------+--------+----+------+----+----+----+----+----

------+------+------+----+----+----+------+------

| | | | | | | NO.

No. | V | W | X | Y | Z | FIRE |FLOOR

------+------+------+----+----+----+------+------

| Ins. | Ins. |Ins.|Ins.|Ins.| |

1 |36 | 9 | 24 | 12 | 7 | 600 | 6150

2 |36 | 9 | 28 | 12 | 7 | 600 | 6200

3 |36 | 9 | 28 | 12 | 15 | 650 | 6700

3-1/2|36 | 9 | 28 | 12 | 25 | 720 | 7050

4 |32-3/4|12-1/4| 32 | 16 | 11 | 730 | 7700

5 |32-3/4|12-1/4| 32 | 16 | 25 | 770 | 8700

6 |32-3/4|12-1/4| 36 | 16 | 25 | 880 | 8800

7 |32-1/4|12-1/4| 36 | 20 | 35 | 940 | 9300

7-1/2|32-1/4|12-1/4| 36 | 24 | 45 | 1120 | 9500

8 |36-1/4|12-1/4| 36 | 20 | 35 | 1120 | 11100

9 |36-1/4|12-1/4| 36 | 24 | 45 | 1140 | 12900

10 |34 |14-1/2| 42 | 20 | 35 | 1160 | 13200

10-1/2|34 |14-1/2| 42 | 24 | 57 | 1270 | 14200

11 |31 |14-1/2| 48 | 20 | 37 | 1400 | 16700

12 |30-1/2|14-1/2| 48 | 24 | 45 | 1500 | 19200

13 |30-1/2|14-1/2| 48 | 24 | 65 | 1540 | 21500

14 |31 |14-1/2| 54 | 24 | 57 | 1590 | 22100

15 |31 |14-1/2| 54 | 24 | 65 | 1620 | 23100

16 |27-1/2|19 | 54 | 24 | 65 | 1750 | 26000

------+------+------+----+----+----+------+------

NOTE.--In setting "Standard" boilers, the side walls should be so built

that the longitudinal seams of the shell will be protected from the

fire.

THE EVAPORATIVE EFFICIENCIES OF BOILERS.

[56]"Many tests have been undertaken to ascertain the evaporative power

of different classes of boilers in actual work; but few of these are of

any value, owing to the unreliable means usually employed to measure the

quantity of water evaporated. The easiest method, and consequently the

one most frequently adopted, is to measure the quantity by the

difference of its height in the water-gauge glass at the beginning and

end of the trial, and also at intermediate stages. This method is very

rude and uncertain, since there can be little doubt that in many boilers

at work the surface of the water is not level, but is usually higher

over the furnace, or where the greatest ebullition occurs. The

difference in height at any moment will greatly depend upon the

intensity of the ebullition, which is ever varying during the intervals

between firing. With mechanical firing the difference of height is

probably reduced to a minimum.

[56] From "_A Treatise on Steam Boilers_," by Robert Wilson.

"The meters employed for measuring the water are sometimes not

trustworthy. The only sure method of ascertaining the quantity of water

evaporated is by actual measurement with a cistern or vessel whose cubic

contents are accurately known. The quantity of water in the boiler

before and after the trial should be measured at the same temperature,

which should not exceed 212° to insure accuracy. But even when the

amount of water introduced and the quantity passed off from the boiler

are accurately ascertained, there yet remains a doubt as to how much has

been actually evaporated, and how much may have passed off in priming,

unless the trial has been conducted with the boiler open to the

atmosphere, which appears to be the only condition under which accuracy

can be insured, unless a suitable apparatus can be provided for

accurately measuring the weight and temperature of all the steam and

water given off, when the boiler is working above atmospheric pressure.

"There are very few boilers that do not prime more or less, and the

quantity of water passed off in this manner is sometimes very

considerable, and has led to the impossible results of 16 and 17 lbs. of

water evaporated per lb. of ordinary coal in locomotive and water-tube

boilers being seriously recorded. Externally fired boilers, that have

given the moderate result of 5 lbs. of water per lb. of coal at

atmospheric pressure, have shown the unexpected result of 10 and 12 lbs.

of water evaporated at 40 lbs. pressure. In fact, unless the amount of

water passed over with the steam by priming or foaming, when working

under pressure, can be accurately ascertained, the evaporative results

are not to be relied upon, however careful in other respects the trial

may have been conducted. It is customary to give the quantity of water

evaporated from and at a temperature of 212°, or the boiling point at

atmospheric pressure, to which the results of evaporation are reduced."

The quantity corresponding to any temperature of feed water and working

pressure can readily be found with the aid of the annexed table, taken

from _The Encyclopædia Britannica_, wherein are presented the relations

of the properties of steam, as now accepted by the best authorities.

TABLE GIVING THE PRESSURE, TEMPERATURE, AND VOLUME OF STEAM.

-----------+-----------+-----------+----------+---------+-------------

lbs. | lbs. | | heit. | | evaporated.

-----------+-----------+-----------+----------+---------+-------------

1 | --- | 102.1 | 1144.5 | .0030 | 20582

2 | --- | 126.3 | 1151.7 | .0058 | 10721

3 | --- | 141.6 | 1156.6 | .0085 | 7322

4 | --- | 153.1 | 1160.1 | .0112 | 5583

5 | --- | 162.3 | 1162.9 | .0138 | 4527

6 | --- | 170.2 | 1165.3 | .0163 | 3813

7 | --- | 176.9 | 1167.3 | .0189 | 3298

8 | --- | 182.9 | 1169.2 | .0214 | 2909

9 | --- | 188.3 | 1170.8 | .0239 | 2604

10 | --- | 193.3 | 1172.3 | .0264 | 2358

11 | --- | 197.8 | 1173.7 | .0289 | 2157

12 | --- | 202.0 | 1175.0 | .0314 | 1986

13 | --- | 205.9 | 1176.2 | .0338 | 1842

14 | --- | 209.6 | 1177.3 | .0362 | 1720

14.7 | 0 | 212.0 | 1178.1 | .0380 | 1642

15 | .3 | 213.1 | 1178.4 | .0387 | 1610

16 | 1.3 | 216.3 | 1179.4 | .0411 | 1515

17 | 2.3 | 219.6 | 1180.3 | .0435 | 1431

18 | 3.3 | 222.4 | 1181.2 | .0459 | 1357

19 | 4.3 | 225.3 | 1182.1 | .0483 | 1290

20 | 5.3 | 228.0 | 1182.9 | .0507 | 1229

21 | 6.3 | 230.6 | 1183.7 | .0531 | 1174

22 | 7.3 | 233.1 | 1184.5 | .0555 | 1123

23 | 8.3 | 235.3 | 1185.2 | .0580 | 1075

24 | 9.3 | 237.8 | 1185.9 | .0601 | 1036

25 | 10.3 | 240.1 | 1186.6 | .0625 | 996

26 | 11.3 | 242.3 | 1187.3 | .0650 | 958

27 | 12.3 | 244.4 | 1187.8 | .0673 | 926

28 | 13.3 | 246.4 | 1188.4 | .0696 | 895

29 | 14.3 | 248.4 | 1189.1 | .0719 | 866

30 | 15.3 | 250.4 | 1189.8 | .0743 | 838

31 | 16.3 | 252.2 | 1190.4 | .0766 | 813

32 | 17.3 | 254.1 | 1190.9 | .0779 | 789

33 | 18.3 | 255.9 | 1191.5 | .0812 | 767

34 | 19.3 | 257.6 | 1192.0 | .0835 | 746

35 | 20.3 | 259.3 | 1192.5 | .0858 | 726

36 | 21.3 | 260.9 | 1193.0 | .0881 | 707

37 | 22.3 | 262.6 | 1193.5 | .0905 | 688

38 | 23.3 | 264.2 | 1194.0 | .0929 | 671

39 | 24.3 | 265.8 | 1194.5 | .0952 | 655

40 | 25.3 | 267.3 | 1194.9 | .0974 | 640

41 | 26.3 | 268.7 | 1195.4 | .0996 | 625

42 | 27.3 | 270.2 | 1195.8 | .1020 | 611

43 | 28.3 | 271.6 | 1196.2 | .1042 | 598

44 | 29.3 | 273.0 | 1196.6 | .1065 | 595

45 | 30.3 | 274.4 | 1197.1 | .1089 | 572

46 | 31.3 | 275.8 | 1197.5 | .1111 | 561

47 | 32.3 | 277.1 | 1197.9 | .1133 | 550

48 | 33.3 | 278.4 | 1198.3 | .1156 | 539

49 | 34.3 | 279.7 | 1198.7 | .1179 | 529

50 | 35.3 | 281.0 | 1199.1 | .1202 | 518

51 | 36.3 | 282.3 | 1199.5 | .1224 | 509

52 | 37.3 | 283.5 | 1199.9 | .1246 | 500

53 | 38.3 | 284.7 | 1200.3 | .1269 | 491

54 | 39.3 | 285.9 | 1200.6 | .1291 | 482

55 | 40.3 | 287.1 | 1201.0 | .1314 | 474

56 | 41.3 | 288.2 | 1201.3 | .1336 | 466

57 | 42.3 | 289.3 | 1201.7 | .1364 | 458

58 | 43.3 | 290.4 | 1202.0 | .1380 | 451

59 | 44.3 | 291.6 | 1202.4 | .1403 | 444

60 | 45.3 | 292.7 | 1202.7 | .1425 | 437

61 | 46.3 | 293.8 | 1203.1 | .1447 | 403

62 | 47.3 | 294.8 | 1203.4 | .1469 | 424

63 | 48.3 | 295.9 | 1203.7 | .1493 | 417

64 | 49.3 | 296.9 | 1204.0 | .1516 | 411

65 | 50.3 | 298.0 | 1204.3 | .1538 | 405

66 | 51.3 | 299.0 | 1204.6 | .1560 | 399

67 | 52.3 | 300.0 | 1204.9 | .1583 | 393

68 | 53.3 | 300.9 | 1205.2 | .1605 | 388

69 | 54.3 | 301.9 | 1205.5 | .1627 | 383

70 | 55.3 | 302.9 | 1205.8 | .1648 | 378

71 | 56.3 | 303.9 | 1206.1 | .1670 | 373

72 | 57.3 | 304.8 | 1206.3 | .1692 | 368

73 | 58.3 | 305.7 | 1206.6 | .1714 | 363

74 | 59.3 | 306.6 | 1206.9 | .1736 | 359

75 | 60.3 | 307.5 | 1207.2 | .1759 | 353

76 | 61.3 | 308.4 | 1207.4 | .1782 | 349

77 | 62.3 | 309.3 | 1207.7 | .1804 | 345

78 | 63.3 | 310.2 | 1208.0 | .1826 | 341

79 | 64.3 | 311.1 | 1208.3 | .1848 | 337

80 | 65.3 | 312.0 | 1208.5 | .1869 | 333

81 | 66.3 | 312.8 | 1208.8 | .1891 | 329

82 | 67.3 | 313.6 | 1209.1 | .1913 | 325

83 | 68.3 | 314.5 | 1209.4 | .1935 | 321

84 | 69.3 | 315.3 | 1209.6 | .1957 | 318

85 | 70.3 | 316.1 | 1209.9 | .1980 | 314

86 | 71.3 | 316.9 | 1210.1 | .2002 | 311

87 | 72.3 | 317.8 | 1210.4 | .2024 | 308

88 | 73.3 | 318.6 | 1210.6 | .2044 | 305

89 | 74.3 | 319.4 | 1210.9 | .2067 | 301

90 | 75.3 | 320.2 | 1211.1 | .2089 | 298

91 | 76.3 | 321.0 | 1211.3 | .2111 | 295

92 | 77.3 | 321.7 | 1211.5 | .2133 | 292

93 | 78.3 | 322.5 | 1211.8 | .2155 | 289

94 | 79.3 | 323.3 | 1212.0 | .2176 | 286

95 | 80.3 | 324.1 | 1212.3 | .2198 | 283

96 | 81.3 | 324.8 | 1212.5 | .2219 | 281

97 | 82.3 | 325.6 | 1212.8 | .2241 | 278

98 | 83.3 | 326.3 | 1213.0 | .2263 | 275

99 | 84.3 | 327.1 | 1213.2 | .2285 | 272

100 | 85.3 | 327.9 | 1213.4 | .2307 | 270

101 | 86.3 | 328.5 | 1213.6 | .2329 | 267

102 | 87.3 | 329.1 | 1213.8 | .2351 | 265

103 | 88.3 | 329.9 | 1214.0 | .2373 | 262

104 | 89.3 | 330.6 | 1214.2 | .2393 | 260

105 | 90.3 | 331.3 | 1214.4 | .2414 | 257

106 | 91.3 | 331.9 | 1214.6 | .2435 | 255

107 | 92.3 | 332.6 | 1214.8 | .2456 | 253

108 | 93.3 | 333.3 | 1215.0 | .2477 | 251

109 | 94.3 | 334.0 | 1215.3 | .2499 | 249

110 | 95.3 | 334.6 | 1215.5 | .2521 | 247

111 | 96.3 | 335.3 | 1215.7 | .2543 | 245

112 | 97.3 | 336.0 | 1215.9 | .2564 | 243

113 | 98.3 | 336.7 | 1216.1 | .2586 | 241

114 | 99.3 | 337.4 | 1216.3 | .2607 | 239

115 | 100.3 | 338.0 | 1216.5 | .2628 | 237

116 | 101.3 | 338.6 | 1216.7 | .2649 | 235

117 | 102.3 | 339.3 | 1216.9 | .2674 | 233

118 | 103.3 | 339.9 | 1217.1 | .2696 | 231

119 | 104.3 | 340.5 | 1217.3 | .2738 | 229

120 | 105.3 | 341.1 | 1217.4 | .2759 | 227

121 | 106.3 | 341.8 | 1217.6 | .2780 | 225

122 | 107.3 | 342.4 | 1217.8 | .2801 | 224

123 | 108.3 | 343.0 | 1218.0 | .2822 | 222

124 | 109.3 | 343.6 | 1218.2 | .2845 | 221

125 | 110.3 | 344.2 | 1218.4 | .2867 | 219

126 | 111.3 | 344.8 | 1218.6 | .2889 | 217

127 | 112.3 | 345.4 | 1218.8 | .2911 | 215

128 | 113.3 | 346.0 | 1218.9 | .2933 | 214

129 | 114.3 | 346.6 | 1219.1 | .2955 | 212

130 | 115.3 | 347.2 | 1219.3 | .2977 | 211

131 | 116.3 | 347.8 | 1219.5 | .2999 | 209

132 | 117.3 | 348.3 | 1219.6 | .3020 | 208

133 | 118.3 | 348.9 | 1219.8 | .3040 | 206

134 | 119.3 | 349.5 | 1220.0 | .3060 | 205

135 | 120.3 | 350.1 | 1220.2 | .3080 | 203

136 | 121.3 | 350.6 | 1220.3 | .3101 | 202

137 | 122.3 | 351.2 | 1220.5 | .3121 | 200

138 | 123.3 | 351.8 | 1220.7 | .3142 | 199

139 | 124.3 | 352.4 | 1220.9 | .3162 | 198

140 | 125.3 | 352.9 | 1221.0 | .3184 | 197

141 | 126.3 | 353.5 | 1221.2 | .3206 | 195

142 | 127.3 | 354.0 | 1221.4 | .3228 | 194

143 | 128.3 | 354.5 | 1221.6 | .3250 | 193

144 | 129.3 | 355.0 | 1221.7 | .3273 | 192

145 | 130.3 | 355.6 | 1221.9 | .3294 | 190

146 | 131.3 | 356.1 | 1222.0 | .3315 | 189

147 | 132.3 | 356.7 | 1222.2 | .3336 | 188

148 | 133.3 | 357.2 | 1222.3 | .3357 | 187

149 | 134.3 | 357.8 | 1222.5 | .3377 | 186

150 | 135.3 | 358.3 | 1222.7 | .3397 | 184

155 | 140.3 | 361.0 | 1223.5 | .3500 | 179

160 | 145.3 | 363.4 | 1224.2 | .3607 | 174

165 | 150.3 | 366.0 | 1224.9 | .3714 | 169

170 | 155.3 | 368.2 | 1225.7 | .3821 | 164

175 | 160.3 | 370.8 | 1226.4 | .3928 | 159

180 | 165.3 | 372.9 | 1227.1 | .4035 | 155

185 | 170.3 | 375.3 | 1227.8 | .4142 | 151

190 | 175.3 | 377.5 | 1228.5 | .4250 | 148

195 | 180.3 | 379.7 | 1229.2 | .4357 | 144

200 | 185.3 | 381.7 | 1229.8 | .4464 | 141

210 | 195.3 | 386.0 | 1231.1 | .4668 | 135

220 | 205.3 | 389.9 | 1232.3 | .4872 | 129

230 | 215.3 | 393.8 | 1233.5 | .5072 | 123

240 | 225.3 | 397.5 | 1234.6 | .5270 | 119

250 | 235.3 | 401.1 | 1235.7 | .5471 | 114

260 | 245.3 | 404.5 | 1236.8 | .5670 | 110

270 | 255.3 | 407.9 | 1237.8 | .5871 | 106

280 | 265.3 | 411.2 | 1238.8 | .6070 | 102

290 | 275.3 | 414.4 | 1239.8 | .6268 | 99

300 | 285.3 | 417.5 | 1240.7 | .6469 | 96

-----------+-----------+-----------+----------+---------+-------------

Here we see that at 212° the total quantity of heat in the steam is

1178.1°, which gives a difference of 966.1°. This heat, usually termed

latent, is absorbed in performing the work of expanding the particles of

water from the liquid to the gaseous state. Now, suppose the water is

evaporated at 60 lbs. pressure, the steam will have a temperature of

307°, and a total heat of 1207°. If the feed has been introduced at 60°,

it is evident that 1147° of heat have been imparted. As the amount

evaporated is inversely proportional to the quantity of heat required,

we have 1147 ÷ 966 = 1.2. Multiplying by this factor, the quantity

evaporated at 60 lbs. pressure from 60°, we obtain the amount that would

be evaporated at 212° by the same quantity of fuel.

By the same table can be ascertained the comparatively small increase of

heat required to evaporate water at higher pressures. Suppose we take

water evaporated at 45 lbs. pressure from a feed temperature of 60°,

then each lb. of water will require 1202.7-60 = 1142.7 for its

conversion into steam. If we take the pressure at 100 lbs., we shall

have 1216.9-60 = 1156.9° as the quantity required. The difference

between these two total quantities is only 14.2°, and is so small as to

be scarcely worth considering. Leaving out of account the loss due to

the slight reduction of the conducting power of the material, the

increased amount of heat required for the higher pressure will be only

1/36 of the total heat required at 60 lbs. With an evaporation of 7 lbs.

of water from 1 lb. of coal, it will be obtained by using 1/563 more

fuel, or about 1 lb. in about 556 lbs., a quantity not appreciable to

the ordinary modes of weighing coal. The economy is then manifest of

using steam of high pressures when at the same time advantage is taken

of the facilities it offers for working the steam more expansively to

the engine cylinders.

The saving that may be effected by heating the feed water may be shown

as follows:

If we take the normal temperature of the feed water at 60°, the

temperature of the heated water at 212°, and the boiler pressure at 20

lbs., the total heat imparted to the steam in one case is

1192.5° - 60° = 1132.5°

and in the other case 1192.5° - 212° = 980.5°

152°

the difference being 152°, or a saving of -------

1132.5°

which is 13.4 per cent. If the pressure be taken at 120 lbs., instead of

20 lbs., the saving will be 13.1 per cent, showing a slight diminution

in the economy effected by heating the feed water when a high boiler

pressure is employed.

THE CARE AND MANAGEMENT OF STATIONARY ENGINE BOILERS.

The first thing to do in taking charge of a stationary engine boiler is

to know from personal inspection that the safety fittings and the

boiler-feeding apparatus are in good order.

The safety valve is the first thing to inspect, as it is liable to stick

in its seat, especially in cases in which it is set at a greater

pressure than is got up in the boiler, because in that case it is not

lifted from the seat and in time sticks fast there.

In such cases it is proper to lift the valve at least once a day while

steam is on. For this purpose a cord may be attached to the lever,

passing over a pulley directly above the lever, and thence to some

convenient place near the boiler, but where it is not liable to get

caught and pulled accidentally.

Before lighting the fire, see that there is sufficient water in the

boiler. If there is a gauge glass on the boiler, it should show

three-quarters full, or three-quarters of a glass, as it is called.

The gauge glass may show a false water level, and to be sure that such

is not the case, open the top gauge cock and the cock at the bottom of

the gauge glass, letting the water run through the gauge glass. Open and

close the cock below the gauge glass two or three times to see that the

water comes to the same level each time.

If the steam pressure has been allowed to fall in the boiler without any

of the cocks being opened, there will be a partial vacuum in the boiler,

and air must be let in before the true water level will be shown either

by the gauge glass or by the gauge cocks.

Opening the upper gauge cock will let in the air, and it should not be

closed again until enough steam has been got up in the boiler to expel

the air again, or in other words, until steam begins to issue from it.

The grate bars and ash pit should be cleaned of clinker, ashes, etc.,

and it should be seen that the tubes are clear of ashes, etc., before

the fire is laid; if the grate is a shaking one, the lever should be

applied to see that the grate will shake properly.

TO LIGHT THE FIRE--In the case of anthracite or hard coal, as it is

sometimes termed, first cover the bars with a thin layer of coal and

then put in pieces of lighted greasy waste (if it is at hand)

distributed about the furnace, taking especial care to light the fire at

the fire-door end and in the corners, because the fire will spread from

the front end towards the back easier than it will from the back end

towards the front.

The fire should light from the bottom and not from the top, hence the

thinnest pieces of the wood should be put in first.

If there is any soft coal at hand, a small quantity of it will

accelerate lighting the fire, as it burns easier and quicker than hard

coal.

Before putting on the coal the wood should be well lighted, the bottom

portion of it having ceased flaming.

This causes the lighted wood to spread over the bars and the fire to

light evenly.

Charge the coal lightly, first covering the places that have burned up

the most.

FIRING.--The fire door should be kept open as little as possible, as it

admits cold air that is detrimental to the combustion, as well as to the

draught, hence firing should be done quickly.

A good fireman will maintain as even a temperature as possible in the

fire box by charging the coal lightly and quickly.

Some firemen will, after the fire is at its proper depth all over the

grates, charge the fire in the front end, that is, at the fire-door end,

and push it back as it burns up, to keep up the thickness of the fire at

the back.

The thickness of the fire depends upon the size and kind of coal.

With small coal a fire from 4 to 6 inches deep will answer, while, if

the lumps are five or six inches in diameter, a fire from a foot to 15

inches deep may be maintained, as is done in some locomotives.

The object is to have the fire thick enough to prevent it from burning

through in spots or letting cold draughts of air pass through it.

The sides of the furnace require particular attention, not only because

cold air is more likely to get through there, but also in boilers

having fire boxes the cool sides of the box keep the temperature of the

fuel down, hence a thicker fire is necessary around the sides than in

the middle of the furnace or fire box.

Three things are to be considered in cleaning a fire--first, that the

boiler pressure will fall during, and for a short time after, the

cleaning; second, that the depth of fire will be diminished by the

cleaning; and third, that the temperature of the fire will fall during

the cleaning.

SHAKING GRATE BARS.

When a furnace has shaking grate bars, the cleaning of the fire is

greatly facilitated, and with bars that shake singly (and good coal) the

fire is often not disturbed during the day, except to shake the bars

occasionally, passing the poker through it and using the hoe to keep it

evenly spread.

If the grate shakes in sections, more cleaning will be required to break

up the clinker, while, if the bars do not shake, the cleaning assumes

greater importance.

Before cleaning, therefore, see that there is sufficient water in the

boiler, that it need not be fed while cleaning, nor just after cleaning

the fire.

Prepare for cleaning by having a thick fire on the grate, so that after

cleaning it will burn up quickly, and let the cleaning be done as

quickly as possible.

[Illustration: Fig. 3288.]

The tools used for cleaning the fire are the slice bar, Fig. 3288, which

is pushed along the top of the fire bars to loosen up the fire, and let

the ashes fall through the bars.

[Illustration: Fig. 3289.]

[Illustration: Fig. 3290.]

The hoe, Fig. 3289, which is used to push the fire to the back of the

furnace and to pull it forward. The poker, Fig. 3290, which dislodges

any clinker that may be between the bars, and lets the ashes fall

through.

[Illustration: Fig. 3291.]

[Illustration: Fig. 3292.]

The clinker hook or devil's claw, Fig. 3291, which is used to haul

clinker out of the fire, and the rake, Fig. 3292, which is used to

spread the fire evenly over the bars after it is cleaned.

In cleaning a fire, first use the slice bar to loosen up the fire and

let the ashes fall through, and also dislodge clinkers from the surface

of the bars. Then push the fire to the back of the furnace. Next use the

poker to clean out clinker from between the exposed part of the bars.

Then with the hoe pull a part of the fire forward and pull out the

clinker that may be in this part, doing so with the hoe as far as

possible, as that will save time, but if it should be necessary, use the

clinker hook.

Then pull forward a second portion of the fire, and spread it on the

bars, removing the clinker as before. When all the fire has thus been

cleaned, use the rake to spread it evenly over the bars, and put on a

light charge of coal, covering the brightest parts of the fire first,

and taking care that no part of the fire bars is left uncovered.

The cleaning should be done quickly.

DRAUGHT.--The draught should be decreased while the fire is being

cleaned, but the damper should never be entirely closed, as this might

cause an explosion in the fire box and tubes.

During a temporary interruption, as in the case of the engine stopping,

partly close all the dampers, as it is wasteful to make steam and blow

it off through the safety valve.

COMBUSTION.--A blue flame is evidence of incomplete combustion, but

there may be a blue flame and imperfect combustion at the back end of

the furnace, and a white flame and perfect combustion at the other end.

This is likely to occur with heavy firing near the fire door, and a thin

fire at the tube sheet end of the fire box. In this case the unconsumed

gases produced near the fire door (as evidenced by the blue flame) are

consumed in passing over the bright fire at the tube plate end of the

furnace.

AT NIGHT.--Always leave plenty of water in the boiler when leaving it

for the night, not only to allow for any leak, but also because it gives

a fair start in the morning and more time to remedy any defect in the

feed pump if it arise.

By plenty of water, very nearly a full gauge is meant, or if there is no

gauge glass to the boiler, let the water stand above the second or

middle cock.

The usual method of leaving the fire for the night is to bank it. There

is an element of danger, however, in banking a fire, unless it is done

to suit the circumstances, because steam may generate very rapidly, and

perhaps more rapidly than the safety valve can carry it off.

A safe method is to clean the fire, leaving the clinker and ashes

covering the front half of the grate and the fire piled up on the back

half.

The damper and ash pit door should be closed tight, the fire door open,

and the fire well covered with fresh coal, choosing small rather than

large coal.

If this method is found not to keep up the fire sufficiently, the same

plan may be employed, except that the ashes and clinker may be removed,

and if this still leaves too cold a boiler, and too poor a fire in the

morning, the fire may be left spread over the grate, but heavily covered

with fresh coal, the draught being stopped as much as possible by

closing the dampers and opening the furnace door.

To further insure safety, the weight on the safety valve lever should be

pushed towards the valve, so as to cause the safety valve to blow off at

a less pressure than during the day.

IN THE MORNING.--In starting up a banked fire in the morning, first

close the fire door and open the damper, so as to give the fire all the

draught possible, and let it burn up a little; then, if it has been

piled up at the back of the furnace, clean out the ashes by passing the

T bar beneath the fire, and spread it over the grate, letting it burn up

a little before making up a fire.

BOILER-FEED.--The fireman should endeavor, if possible, to so regulate

the boiler feed that it is kept going as nearly continuously as possible

while maintaining a uniform quantity of water in the boiler, and this,

with uniform firing, will give the greatest economy.

When pumps are used to feed with, the amount of the lift of the valves

can be regulated by a screw, so as to vary the amount of water the pump

will deliver, and in this case it is comparatively easy to set them so

that the pump may be kept going without putting too much water in the

boiler.

When injectors are used, however, the feed will be intermittent, and a

uniform quantity of water in the boiler is best obtained by feeding at

short intervals, stopping the feed when the fire door is opened much, as

when cleaning the fire.

If the feed water is dirty, the gauge glass should be kept clean by

first shutting off the upper cock and opening the lower one, so as to

let the water blow through the lower cock, and then shutting off the

lower cock from the boiler, and opening the upper one, which will let

the steam blow all the water out of the glass. This should be done two

or three times a day, so as to keep the holes in the boiler and those in

the cocks from closing up with fur or scale.

If the water falls in the glass, or if the gauge cocks show the water to

be falling, notwithstanding that the feed pump has been started, it is

evident that the pump is not working.

This may occur from a stuck valve, a leak in the suction pipe, from the

feed water being too hot, or from the pump failing to start in action

from leaky or choked valves.

A stuck valve may generally be relieved by striking a few blows on the

outside of the pump with a hammer and a block of wood, or if this does

not answer, with the hammer only. Check valves are the ones most likely

to stick.

If a pump fails to work by reason of the feed water being too hot, the

remedy is to open the pet cock to let the steam out of the pump, but if

this does not succeed, cold water may be poured on the outside of the

pump, which will start it, after which, in most cases, the pump will

keep going and the pet cock may be closed.

If the suction pipe has a joint, a leak there will impair the action of

the pump, and, if the leak is great enough, will stop it; the remedy is

to make the joint tight.

Plunger pumps sometimes fail to act because the plunger has worn so

small in diameter that there is sufficient air between the plunger and

the pump barrel to expand and compress without lifting the valve; the

remedy is obviously a new plunger of as large diameter as the pump gland

will admit of, boring the gland out to admit the new plunger.

All the impurities in the water are left in the boiler when the water

has evaporated, and it is obvious these impurities must be blown off or

they will form scale on the internal surface of the boiler and the

external surface of the tubes or flues.

This scale obstructs the passage of the heat from the iron to the water,

and if let get thick enough will cause the iron to rapidly burn out.

To prevent the formation of scale, two principal methods are employed,

one being to purify the feed water, and the other to occasionally blow

the impurities out of the boiler.

Feed-water heaters generally serve also as purifiers, and their

effectiveness is increased in proportion as the water can pass quietly

through them, and has a large area on which the impurities can settle.

Horizontal heaters have the advantage that they have a large settling

area, and a less distance for the impurities to fall through. The

water-gauge glass and the lower gauge cock are usually set so as to have

a margin of about three inches of water above the tubes or crown sheet

of the fire box, hence if it is known that the water is but just below

the bottom of the gauge glass or gauge cock, there is no positive

danger, although it is improper to let it get so low.

If the water is out of sight, and it is not known exactly how low it is,

then it is dangerously low, and every minute is of vital importance.

Should the water get dangerously low in the boiler, the most dangerous

thing to do is to lift the safety valve or pump in cold water,

especially if it is not known how much water there is in the boiler.

As quickly as possible cover the fire with ashes, coal, earth, sand, or

anything that is at hand that will smother the fire, then close the

draught to the fire, leaving the fire door and the chimney damper open.

Leave all the steam outlets just as they are, and also the feed.

PRIMING.--Priming, which is also called "foaming," is that the steam

carries up water into the steam space. This may arise from several

causes, but it is well known that what will stop priming in some cases

will cause it in others.

The known causes of priming are--first, too little room for the steam in

the boiler, and it follows that a high water level may cause priming;

second, it may be caused by a difference of temperature between the

water and the steam in the boiler. Suppose, for example, that the

pressure of the steam and water in the boiler is 160 lbs. by gauge, and

its sensible temperature will be 370 degrees. Suppose then that enough

steam is permitted to escape from the boiler to reduce the steam

pressure to 140 lbs., and its temperature will be reduced to 361

degrees. But the water will remain at 270 degrees, and the result will

be that it will pass into steam so rapidly that it will carry up the

water and hold it in suspension among the steam. The water will pass

with the steam into the engine cylinder, and the boiler will be said to

"prime," "foam," or "work water." The same thing may happen if the water

is heated very rapidly.

Priming is wasteful because it rapidly empties the boiler of its water,

and dangerous because it may cause the piston to knock out the cylinder

head or cover.

When the safety valve blows off, priming may be induced, especially if

the engine is at work, because in this case the boiler is being forced,

or, in other words, is making steam more rapidly than it is designed to

do, and the passage of so large a body of steam through the water is apt

to lift it.

Muddy water will sometimes cause foaming or priming, as will also

insufficient circulation of the water in the boiler or sometimes the

presence of grease or oil.

Priming may be detected from the discharge of water with the steam when

the gauge cock is opened, the steam looking white and fluttering as it

escapes, and also by violent motion of the water in the gauge glass, or

by a thump or pound at the ends of the piston stroke.

To stop priming, the steam from the boiler should be decreased by

slackening the speed of the engine, or if necessary, by stopping it. The

true water level can then be seen, and if there is too much water in the

boiler some of it may be blown off, while if the quantity of water in

the boiler will permit it, the feed may be put on.

If the boiler has a surface blow-off cock, or a mechanical boiler

cleaner, it is best to blow off from that, as it carries off the scum at

the same time as relieving the boiler.

To prevent priming, a steady and uniform rate of boiler feed, the use of

pure water, a clean boiler, and steady firing are the best means,

turning on the steam slowly so as not to violently disturb the water in

the boiler.

The engine as well as the boiler requires attention when the boiler

primes. Thus the cylinder cocks should be opened to let out the water

from the cylinder and prevent breakage of the cylinder cover.

SCALE IN BOILERS.--The steam leaves behind it all the impurities that

the water contained, and these impurities deposit in form of mud and

scale, which must be got rid of because it causes a loss of fuel, and if

allowed to get thick enough will cause the boiler to burn.

The use of boiler compounds or scale preventatives may be resorted to

with advantage, providing they are of a nature to suit the water, but

mechanical cleaning must also be resorted to at periods determined by

the nature of water.

Boilers are cleaned in two ways--first, by blowing off the impurities

before they have formed into scale; and second, by removing at certain

intervals whatever scale has formed.

Blowing down may be done in two ways--first, from the surface of the

water by means of mechanical cleaners; and second, by blowing out from

the bottom of the boiler.

The first draws off the impurities as they are thrown to the surface,

the second draws them off after they have become more condensed and sink

to the bottom.

How often a boiler should be blown down depends upon the kind of water

fed to the boiler; where purifiers are used, less blowing down is

obviously needed.

It is best to blow off from the bottom of the boiler when no steam is

being used, as during dinner time, letting the water blow down about a

quarter of the glass, or from the upper to the middle gauge cock.

As no steam is being used, the feed can then be put on to restore the

quantity of water without reducing the temperature of the boiler so

much. The feed should be gradual and the fire regulated to keep the

steam pressure even.

How often a boiler should be washed out and cleaned depends upon the

quality of the water it uses, and varies from about once a week to once

a month, according to whether bad and unpurified water or purified water

is used.

The first thing to do is to draw the fire, leaving the chimney damper

open and closing all the other dampers so that as little cold air as

possible can get into the boiler, while the heat can pass away up the

chimney.

Let the steam and water all remain in the boiler until there is a gauge

pressure of about 5 lbs. in the boiler.

Then open the blow-off cock and let out the water. If the water is blown

off under a high pressure, then after the waste is all out the iron is

hot enough to dry up the scale, making it hard and very difficult to

remove.

After all the water is blown off, take out all the mud plugs and the

man-hole and hand-hole covers, and wash out the boiler under as much

water pressure as can be had, directing the hose so to reach all parts

of the boiler and tubes, and continuing the washing until the water

leaves the boiler clean.

Then with a wooden hoe on a piece of gas-pipe of small diameter for a

handle, and small enough to pass through the hand-hole, draw all the

loose scale to the hand-hole and remove it, letting the water run

slowly, so as to carry the small pieces of scale towards the hand-hole

as fast as the hoe disturbs it.

Then get inside the boiler, and a few blows with a light ball-pened

hammer will loosen the scale, and a steel scraper will remove more,

which must be washed down and drawn out with a hoe.

After the cleaning and scaling are complete, the engineer, with lamp in

hand, should carefully examine the interior of the boiler and of the

fire box, paying especial attention to the stays to see that they are

not broken.

The hammer test should also be applied. It consists of sounding the

boiler by light blows given by a light ball-pened hand hammer, the sound

indicating defective places.

CHAPTER XXXVII.--THE STEAM ENGINE.

The high pressure steam engine, in whatever form it exists, consists of

a frame or bed plate carrying two distinct mechanisms, first, the

driving or power-transmitting mechanism, and second, the valve gear or

valve motion, and to these are added such other mechanisms as the nature

of the duty the engine is to perform may require.

The most prominent of these additional mechanisms is a governor for

regulating the speed at which the engine is to run; nearly all steam

engines require a governor in some form or other, while for electric

lighting and some other purposes it constitutes the main feature in the

design of the engine.

In a locomotive the air brake and the sand box are elements not found in

other engines.

In a jet condensing engine, the condenser and injection water, or

condensing water mechanism, is a part of the engine.

In a surface condensing engine, the air pumps and circulating pumps are

a part of the engine.

In marine engines there are mechanisms for turning the engine around

when no steam is up; for moving the reversing gear quickly, and for

varying the point of cut off, and therefore the amount of expansion, and

various other and minor mechanisms.

[Illustration: Fig. 3293.]

Referring now to the simplest form of high pressure stationary steam

engine, such as represented in Figs. 3293, 3294, and 3295, its valve

gear or valve motion consists of the eccentric and its strap, the

eccentric rod, the valve rod guide A, the valve rod or valve spindle,

and the valve _v_, these parts controlling the admission of steam to one

side of the piston, and the exhaust from the other.

The piston, piston rod, cross head, connecting rod, crank, crank shaft,

main shaft or driving shaft, and the fly wheel constitute the driving or

power-transmitting mechanism.

The steam side of the piston is that against which the steam is

pressing, as side S in Fig. 3295. The exhaust side, E, of the piston is

that on which the steam is passing out or exhausting.

The governor for a common D valve engine regulates the engine speed by

varying the opening in the bore of the pipe through which the steam

passes from the boiler to the steam chest, leaving a wider opening in

proportion as the engine runs slower, and reducing the opening when the

engine runs faster. Assuming the engine to be running at its slowest, or

its load to be so great that a full supply of steam is required in order

to keep the engine up to its proper speed, and the governor will be open

at its widest, so that all the further action the governor can have is

to reduce the steam pipe opening, and thus cause the pressure in the

steam chest to be less than that in the steam pipe.

This action is called wire-drawing the steam, and the governor is called

a throttling governor.

An engine bed or bed plate is a frame that is seated or bedded to its

foundation along its whole length.

An engine frame is seated to its foundations at two or more places, but

not continuously throughout its length.

THE CYLINDER.

Cylinders are secured to the engine frames in three principal ways, as

follows: by bolting them down to the bed plate; by bolting them to one

end of the bed plate, so that they may expand and contract without

springing the bed plate; and in vertical engines, by bolting them to the

top of the frames.

The bores of cylinders require to be parallel, so that the piston rings

may fit to the bore without requiring to expand and contract in diameter

at different parts of the stroke.

Cylinders are designated for size by the diameter of the cylinder bore

and the length of the stroke; thus, a 10 × 12 cylinder has a piston of

ten inches diameter and 12 inches stroke.

The wear of a cylinder bore is (if the engine is kept in proper line and

the piston rings, or packing rings as they are sometimes termed, fit to

the bore with an equal pressure throughout the stroke) greatest near the

middle of the length and least at the ends of the stroke. But when the

piston rings are set out by the steam pressure, and the point of cut off

occurs early in the stroke, the wear may be greatest at the ends of the

cylinder bore, because of the pressure of the steam diminishing during

the expansion.

The counterbore of a cylinder is a short length at each end of the

cylinder, that is made of larger diameter than the rest of the bore, so

that the piston head may travel completely over the working bore, and

thus prevent the formation of a shoulder at each end of the cylinder.

Such a shoulder forms when there is a part of the bore over which the

piston does not pass. The length of the counterbore should exceed the

amount of the taper on the connecting rod key, so that as the connecting

rod length alters from the wear, the piston shall not strike the

cylinder head.

The clearance of a cylinder is the amount of space that exists between

the face of the piston when it is at the end of its stroke and that of

the valve when it covers the port, the piston being at the end of the

stroke, and as this space exists at each end of the cylinder, the total

clearance for a revolution is twice the above amount.

The clearance at the crank end of the cylinder is reduced by the piston

rod passing through it.

The amount of clearance may be measured by the following method, which

has been given by Professor John E. Sweet:

[Illustration: Fig. 3294.]

[Illustration: Fig. 3295.]

See that the piston and valves are made tight, and the valves

disconnected; arrange to fill the clearance spaces with water through

the indicator holes, or holes drilled for the purpose. Turn the engine

on the dead centre; make marks on the cross-head and guide that

correspond; weigh a pail of water, and from it fill all the clearance

space. Weigh the remaining water, so as to determine how much is used.

Then weigh out exactly the same amount of water, turn the engine off the

centre, pour in the second charge of water, and turn back until the

water comes to the same point that it did in the first case. Make

another mark on the cross-head, and the distance between these marks is

exactly what you really wish to know; that is, it is just what piston

travel equals the clearance. This gives the proportion that the

clearance space bears to the space in the cylinder occupied by the steam

at the end of the piston stroke. Thus, if it takes one pound of water to

fill this space, and to admit the one pound of water the piston must be

moved one inch, then the clearance bears the same relation to the

capacity of the engine as one inch bears to the stroke of the piston.

Thus, under these circumstances, in an engine of ten-inch stroke, it

would be said the engine had ten per cent. clearance.

When a cylinder is to be rebored, the boring bar should be set true or

central to the circumference of the counterbore, so that the bore of the

cylinder may be brought to its original position with reference to the

bore of the stuffing box.

Cylinders require lubricating, both to avoid friction and wear of the

cylinder bore, as well as of the valve and valve seat. The amount of

lubrication required depends upon the degree of tightness of the piston

rings, upon the speed of the piston, upon the amount of pressure of the

valve to its seat, and upon the method of operating the side valve.

Cylinders with releasing valve gears require freely lubricating, because

the closure of the valve depends upon the dash pot, and undue friction

retards the closing motion.

The less the movement of the valve at the moment of its release, the

easier it is to move it, because the friction is less, and less

lubrication is required.

Cylinders are lubricated by automatic oilers placed on the steam pipe of

the engine, the oil being distributed over the surfaces by the steam.

Cylinder oilers sometimes have a pump to force the oil in, and in others

the steam in the oiler condenses, and the water thus formed floats the

oil over the top of a tube, or up to an orifice through which the oil

gradually feeds as the condensation proceeds.

In other oil feeders, the feed is regulated by increasing or diminishing

the opening through which the steam passes from the cup to the steam

pipe.

Sight oil feeders are those in which there is a glass tube or body, in

which the passage of the oil can be seen as it drops.

Cylinder cocks are employed at each end of the cylinder to let out the

water that condenses from the steam when admitted to a cold or partly

cooled cylinder. The two cocks are usually connected together by a rod,

so that both may operate together.

Cylinder relief valves are valves at each end of the cylinder to relieve

the cylinder from the charges of water that sometimes enter from the

boiler with the live steam.

Steam ports give a quicker admission in proportion as their length is

increased, and this reduces the amount of valve travel, and are

sometimes given a length equal to the diameter of the cylinder bore.

The bottoms of the steam ports are sometimes so placed as to be below

the level of the cylinder bore, so as to drain off the water of

condensation of the steam.

Rule to find the required area of steam port.

Multiply the area in square inches of the piston, by the number opposite

to the given piston speed in the following table:

Speed of piston in Number by which to

feet per minute. multiply the piston area.

100 0.02

200 0.04

300 0.06

400 0.07

500 0.09

600 0.1

700 0.12

800 0.14

900 0.15

1,000 0.17

The cylinder exhaust port must be open when the valve is at the end of

its travel, to an amount equal to the width of the steam port, but what

this width will be in any given case depends upon the width of the

bridges, the amount of the steam lap and the travel of the valve, as

will be explained with reference to the slide valve.

Jacketed cylinders are those in which there is a space around the

cylinder that is filled with live steam.

The object of jacketing is to prevent the loss of heat from the steam

within the cylinder by radiation. The steam in the jacket should be

received direct from the boiler, and should not be drawn from the jacket

into the steam chest because the jacket reduces its temperature and

condenses it.

The water of condensation of a steam jacket should not be allowed to

accumulate in any part of the jacket, but should drain off and pass back

to the boiler. To render the jacket as effective as possible, it should

extend from end to end of the cylinder, the exhaust steam pipe leading

directly away, so as to have as little communication with both the

cylinder and the jacket as possible.

The jacket should have open communication with the boiler at all times,

so as to have the pressure in the jacket at the same pressure as that in

the steam chest, while the cylinder being kept hot, it will be

unnecessary to blow steam through in order to warm the cylinder when

starting the engine. The steam should enter the jacket at the highest

point, so as to prevent the accumulation of air in the jacket. Or, if

the steam is admitted at some other point, it should be so arranged as

to permit its thorough circulation in the jacket. When a jacket is used,

the metal of the cylinder body should be as thin as possible, because

the transmission of heat through the metal is, both in time and

quantity, inversely as the distance or thickness passed through.

The steam in the jacket should be as dry as possible, so that all wet

steam admitted during the live steam period may be evaporated by the

heat received from the steam in the jacket. The outside of the jacket

should be thoroughly protected from cooling by being lagged or clothed

with felt or some other material that is a non-conductor of heat.

From experiments made by Mr. Charles A. Smith, of St. Louis, it was

found that the amount of variation of temperature that occurred during

the stroke in a locomotive cylinder was inversely proportional to the

speed of engine revolution, which shows the advantages of jacketing

cylinders and of lagging them, as well as the advantage of a high

rotative speed.

A lagged cylinder is one clothed, which is sometimes done with wood or

metal strips, leaving an air space around the cylinder, while in others

this space is filled with felt or some non-conducting material.

Experiments made by Charles E. Emery gave the following general results:

The thickness of the pipes and of the non-conducting materials was kept

constant.

Hair felt was the best non-conducting material of all those tested, and

the value of a thickness of two inches of hair felt was taken as unity

and the maximum.

The value of two inches of mineral wool as a non-conductor was 0.832 of

hair felt; two inches of mineral wool and tar was 0.715. Two inches of

sawdust, 0.68; two inches of a cheaper grade of mineral wool, 0.676;

charcoal, 0.632; two inches of pine wood, across the grain, 0.553; two

inches of loam, 0.55. This was from the Jersey flats, and almost all

vegetable fibre not yet become compact. Slaked lime from the gas works,

expressed decimally, with hair felt as unity, 0.48; coal ashes, 0.345;

coke, only 0.277, the same as used for fuel; two inches of air space,

only 0.136, which dashes a great many people's hopes, and is as

interesting as any part of the data; two inches of asbestos, 0.363; two

inches of Western coke, about the same as the other coke; two inches of

gas house charcoal, 0.47.

These are very interesting, particularly so this matter of an air space.

It has been supposed that an air space around a pipe is as good as

anything we can have. The fact is, convection or circulation takes

place; the air is cooled on one side of the space, descends, and rises

on the other, and it is necessary to break up the air space, and that

undoubtedly accounts for the efficiency of these different materials. It

is the air probably that is the non-conductor; but it should be kept

quiescent instead of being allowed to circulate. The air space itself is

of very little value until the circulation is prevented.

THE PISTON.

In calculating the power of an engine it is the piston speed that is

taken into account, and not the length of the stroke, the latter being

used merely in order to obtain the piston speed.

Long strokes are usually employed upon engines running at moderate

piston speeds, as from 300 to 500 feet per minute, and short strokes for

piston speeds from 400 to 800 feet per minute.

The Porter Allen engine has been run noiselessly at 1,100 feet per

minute.

In determining the stroke of an engine the nature of the valve-operating

mechanism is taken into account.

In releasing mechanisms, or those in which connection between the

eccentric rod and valve spindle is broken in order to permit the valve

to close quickly, too high a speed of revolution may cause the tripping

mechanism to fail to act, hence a high piston speed is obtained by means

of employing a comparatively long stroke.

In positive valve gears, or those in which the valve is controlled

throughout the whole of its movement by the eccentric, the valve

mechanism may operate quicker without danger of missing, hence the

piston speed may be greater.

When the stroke equals the diameter of the cylinder bore, the cylinder

presents the least amount of exposed surface in proportion to its

cubical contents.

To obtain the same amount of expansion in a short as in a long stroke

engine, the steam must be expanded through an equal proportion of the

stroke; thus, if the steam is cut off at half stroke in both cases, the

amount of this expansion will be equal.

Pistons are made an easy fit to the cylinder bore, a steam-tight fit

between the two being obtained by means of the piston rings.

Solid pistons are provided with snap piston rings.

A snap piston ring is one that is larger in diameter than the cylinder

bore, and is closed in to get it into the cylinder, while it depends on

its own spring outwards for its fit to the cylinder bore, having no

supplementary rings or springs to force it out.

Piston rings that are expanded by supplementary springs should be

tapering in thickness, the thickest part being opposite to the split,

and the thinnest at the split. This causes the ring to conform itself to

the cylinder bore, and makes it sit more evenly around its whole

circumference. These rings are made larger in diameter than the cylinder

bore, in proportion of about 1/8 inch per foot of diameter, the split

being closed when the ring is sprung into place in the cylinder. But if

made of brass, the split must be left open enough to allow for the

expansion, or otherwise the ring expanding more than the cylinder will

seize and cut single.

The split of a piston ring should be placed on the bottom of the piston

(in a horizontal engine), so that the piston head, in resting on the

cylinder bore, will cover up the opening of the ring.

When two or more rings are employed, the splits may be placed on the

lower half of the cylinder, so as to cover up their splits as much as

possible.

The follower of a piston is a plate or cover that is employed to hold

the piston rings in place, and the piston rings should be so fitted that

the follower should be bolted firmly up, or otherwise the bolts may come

loose and work out, and getting between the piston and the cylinder

cover, may cause the piston to knock the cylinder cover out.

Piston followers are necessary when the rings are set out by springs or

other parts adjustable within the piston head. Snap piston rings,

however, permit the use of a solid piston, dispensing with the need for

a follower.

The effectiveness of a piston ring may be tested, when the construction

of the engine will permit it, by disconnecting the valve for the head

end, setting it so that it covers the port, and then taking off the

cylinder cover at the head end and admitting steam through the crank

end steam port, when any leak in the piston rings will be seen by the

escape of the steam.

THE PISTON ROD.

Piston rods should be of slightly diminishing diameter at the ends, so

that the wear shall not leave a shoulder at each end of the rod.

In determining the diameter of the piston rod, allowance is made for

turning it occasionally in the lathe to restore its parallelism, the

wear reducing its diameter more in the middle than at the ends. The

diameter of a piston rod is found in practice to range between one-sixth

and one-tenth the diameter of the cylinder bore.

Steel piston rods wear better than those of wrought iron, being free

from scaly seams which are apt to cut the packing and cause the rod to

wear in grooves.

The best method of securing a piston rod to a piston head and to the

cross head is by a taper seat and a key, so that no nut is needed, and

the cylinder cover need not have a recess to receive the nut when the

piston is at the end of the stroke, and the amount of clearance is

correspondingly reduced.

Piston head key ways are sometimes given so little clearance that the

key completely fills the keyway when driven fully home. This prevents

the edges of the keys from bulging into the clearance space in the

keyway, which action is apt to cause the key to loosen in time. The key

should have a safety pin at its small end.

When piston rods are threaded into the cross head, or into the piston,

the threads are made an easy fit, and taper seats or split hubs secured

by clamping screws are relied upon to keep the rod true to the cross

head or piston, it being found that the screw alone cannot be relied

upon for this purpose.

PISTON ROD PACKING.

Piston rod packing, of fibrous or similar material, should be cut in

rings that will not quite fully envelop the piston rod, and the first

ring should be placed with its split upwards. After two or three rings

have been inserted, each having its split at a different part of the

bore, so as to "break joints," the gland should be screwed up enough so

as to carry the packing home to the back of the stuffing box. This

process should be continued until the stuffing box is filled for about

two-thirds of its depth, when the gland may be screwed home.

The gland should be screwed up quite evenly, so that the packing in the

stuffing box shall be compressed equally all around the rod, and will

not cause the gland to bind on the rod or in the stuffing box bore.

The wrench should be applied first to one nut, giving it a turn or two,

and then to the other, and after the gland is firmly home the nuts

should be eased back about two turns.

When a gland requires packing, it is proper to take out all the old

packing that has become hard and set.

A leak in piston rod packing may sometimes be remedied by taking out

three or four rings of the packing and reversing it.

If the packing is tightened up while the engine is running, it should be

done very gently and evenly, as a very little screwing up may stop the

leak, while excessive screwing produces undue friction.

Piston rods are in some of the most advanced practice packed with

metallic packing, or packing composed of soft metal. In some forms of

metallic packing the construction is such that the gland and packing do

not attempt to restrain the line of motion of the piston rod, this duty

being left to the guide blocks and guide bars, where it properly

belongs.

THE CROSS HEAD.

In engines having Corliss frames, the cross head is provided with shoes

and adjusting screws, to take up the wear.

When guide bars are shaped thus __|¯¯ the cross head is provided with

gibs (usually of brass composition) to take up the wear.

In either case care must be taken to make the adjustment correct, and

thus keep the piston rod in line. The shoes or gibs should not bear hard

upon the guides, but be an easy sliding fit without lost motion.

Cross head pins should be kept eased away on the two parts of their

circumference which are within the connecting rod brasses or boxes and

near the joint faces of the same. This is necessary because the wear is

greatest on the crowns of the boxes, and the pins are apt to wear oval.

In some engines, the surface of the pin is cut away, but if it is not,

and the pin can be revolved in the cross head, it is a good plan to give

it half a turn occasionally, which will keep it round.

THE GUIDE BARS.

The guide bars of an engine require to be set exactly in line with the

axis of the cylinder bore, so that they may guide the piston to travel

in a straight line. They should be an easy sliding fit to the cross-head

guide.

The top bar is more difficult to lubricate than the bottom one,

especially when it receives the most pressure, as is the case when the

top of the fly-wheel runs towards the cylinder.

Cast iron guide bars wear better than either brass, iron, or steel ones,

so long as they are properly lubricated. The face of each guide bar

should be cut away, so that the ends of the cross head guides will

travel past it. This will prevent a shoulder forming at the ends of the

bar as the face wears away. Such shoulders are apt to cause a knock as

the connecting rods are lined up, because in the lining the connecting

rod is restored to its original length, and the path of the cross-head

guides along the bars may be altered.

THE CONNECTING ROD.

There are two principal kinds of connecting rods, the "strap ended" and

the "solid ended." The solid ended wear the best, but are more difficult

to get on and off the engine.

Connecting rod straps are secured to the stub ends (as the ends of the

rod are called), either by bolts or by one or two gibs, and the brasses

are set up by a taper key or wedge.

The taper for connecting rod keys is about an inch per foot.

The angularity of a connecting rod is a term that applies to its path of

motion, which is (during all parts of the stroke except on the dead

centre) at an angle to the line of engine centres. The effect of this

angularity is to cause the piston motion to be accelerated at one part

of the stroke and retarded at another, thus causing the point of cut-off

to occur at different points of the two strokes.

The direction of the variation is to cause the point of cut-off to occur

later on the stroke when the piston is moving from the head end of the

cylinder towards the crank.

The amount of variation caused in the two points of cut off by the

connecting rod depends upon the proportion that exists between the

length of the crank and that of the connecting rod, and is less in

proportion as the length of the connecting rod is greater than that of

the crank.

An ordinary length of connecting rod is six times the length of the

crank, or _six cranks_, as it is commonly termed.

Fig. 3296 represents a cylinder, piston and rod, cross head, connecting

rod, and crank.

The piston _b_ is shown in the middle of the cylinder, the cross head at

E, and the crank pin at B, instead of being at G´, as it would but for

the connecting rod, or if the connecting rod was infinitely long.

Now take a pair of compasses and set it from _b_ to E, and then try it

from _a_ to D, and from _c_ to F, and it will be seen that the three

cross head positions D, E, and F correspond correctly to the three

piston positions _a_, _b_, _c_. Then take a pair of compasses and set

them to the length of the connecting rod (from E to B) and try them from

D to A, from B to E, and from C to F, and it will be seen that crank pin

positions A, B, and C correspond to cross head positions D, E and F, and

therefore that the crank is not at half stroke when the piston is in

the middle of the cylinder. Take these same compasses, and resting one

point at (G´) mark the arc H, and that is where the cross head would be

when the crank was at (G´). Now then we see that the connecting rod

causes the piston to move slower while running from _a_ to _b_ than it

does while running from _b_ to _c_.

[Illustration: Fig. 3296.]

THE D SLIDE VALVE.

The various events which are governed by the D slide valve of a steam

engine are as follows:

The live steam period is that during which the steam is admitted from

the steam chest into the cylinder and the steam admitted during this

period is termed _live_ steam.

The point of cut off is that at which the valve closes the steam port,

and the admission of steam into the cylinder is stopped, hence the point

of cut off is at the end of the live steam period.

The period of expansion is that during which the steam is allowed to

expand in the cylinder, and therefore begins at the point of cut off,

and ends at the point of release.

The point of release is that at which the valve opens the port and

permits the steam to escape.

The point of compression is that at which the exhaust port is closed,

which occurs before the piston has reached the end of its stroke; the

steam that has not passed out of the cylinder is therefore compressed,

the compression continuing until the valve opens for the lead.

The lead of the valve is the amount the port is open to the live steam

when the crank is on the dead centre.

The point of admission is that at which the port opens for the live

steam to enter, and it follows that the lead and compression both act as

a cushion, arresting the motion of the piston when it reaches the end of

the stroke.

Cushioning begins, however, at the time the exhaust port is closed

enough to arrest the escape of the steam, while compression begins when

the valve has closed the exhaust port.

[Illustration: Fig. 3297.]

The construction of a common slide valve is shown in Fig. 3297, in which

the valve is shown in its mid-position. P P are the cylinder steam ports

(as the openings through which the steam passes from the steam chest to

the cylinder are termed), and at X is the cylinder exhaust port, through

which the steam escapes from the cylinder. Z is the valve exhaust port

or exhaust cavity.

The lip of a valve is the width of its flange face, or the distance L,

which is measured from the steam edge A to the exhaust cavity Z. At the

other end of the valve, H is the lip extending from the steam edge B to

the exhaust cavity.

Steam lap is the distance the steam ends (or the steam edges as they are

called) A, B overlap the steam ports, this distance being shown on the

ends of the valve at _a_ C. If the valve had no steam lap, its steam

edges would just cover the ports, as denoted by the dimension W.

Exhaust lap is the amount the exhaust cavity Z overlaps the bridges _q

q´_, as at _p_, _r_.

Unequal steam lap is given to cause the point of cut off to occur at

equal points in the piston stroke; thus in the figure there is more

steam lap at the head end than at the crank end of the valve. But

unequal lap could also be given in order to greatly vary the points of

cut off for the two piston strokes, if such was desired.

Unequal exhaust lap may be given to equalize the point of release, or to

equalize the points of compression.

The head end of the valve (or of the cylinder) is that which is furthest

from the crank shaft, the other end, or that nearest to the crank shaft,

being termed the crank end.

THE ACTION OF A COMMON SLIDE VALVE.

The action of a common slide valve may be traced as follows:

[Illustration: Fig. 3298.

Port _a_, open to the amount of the lead.]

[Illustration: Fig. 3299.

Port _a_, full open for the admission.]

[Illustration: Fig. 3300.

Port _a_, closed off for cut.]

[Illustration: Fig. 3301.

Valve opening port _a_, for the exhaust.]

[Illustration: Fig. 3302.

Port _a_, full open for the exhaust.]

Suppose the port _a_ to be at the head end of the cylinder and open to

the amount of the lead with the crank on the corresponding dead centre,

and if the valve travel be made equal to twice the lap and the lead, the

various positions of the valve will be as marked in Figs. from 3298 to

3302; the event corresponding to each valve position being stated in the

figures.

DOUBLE PORTED VALVES.

The term _port_ applies strictly to the area of opening of the steam

passage where it emerges upon the valve seat. The term _steam passage_

includes the full length of the opening from the cylinder bore to the

face upon which the valve is seated.

A double ported steam port is one in which there are _two_ openings or

steam ports, leading into _one steam passage_.

A double ported valve is one in which there are two ports _at each end_

of the valve. These two ports in some cases admit steam to a single

cylinder port, and in others to two steam ports, terminating in one

steam passage.

A griddle valve is one that has two or more ports at each end upon a

seat that has two or more ports for each steam passage.

Double ported valves are employed in some cases to increase the

admission of live steam to the cylinder, and in others to increase the

exhaust openings also. The effectiveness of a double ported valve is

mainly valuable at the beginning of the stroke, and is especially

valuable in cases when the travel of the valve is diminished to hasten

the point of cut off, because in such cases the outer edges of the valve

do not open the steam port to its full width, and a single port is apt

to wire draw the steam. By the employment of more than one port, or

several ports, a sufficient admission may be obtained with less valve

travel.

[Illustration: Fig. 3303.]

The Allen double ported valve is one in which the second port increases

the port opening for the admission only, as shown in Fig. 3303, in which

the valve is moving in the direction of the arrow; the port K will

receive steam through the opening at _g_, and from a port passing

through the valve, the steam entering it as shown by the arrow. The

second port forms part of the lap of the valve, and enables the travel

to be short enough to be cut off at early points in the stroke, without

employing so much steam lap as to widely distort the points of cut off,

this latter being a defect of the D valve.

Webb's patent slide valve is circular, and is so arranged as to be free

to revolve in the hoop of the valve rod, the effect being that the valve

moves around, or to and fro in the hoop, without any special mechanism

to produce such movement, and the result is, that the valve and port

facings wear smooth and even without any tendency to become grooved.

BALANCED VALVES.

A balanced valve is one in which means are employed to relieve the back

of the valve of the steam pressure, and thus prevent its being forced to

its seat with unnecessary pressure.

In some of the most successful balanced valves this is accomplished by

providing a cover plate, which may be set up to exclude the steam from

the back of the valve which works (a sliding fit) between the valve face

and the face of the cover plate. Such a method of balancing is

sufficiently effective for all practical purposes, if the following

conditions are observed: The valve rod must be accurately guided so as

to avoid side strains; the valve must fit accurately to its seat and to

the cover plate, and the adjustment so made that the valve slides freely

at first, being steam tight, and yet allowing room for lubrication to

enter. When the travel of a valve, balanced by a cover plate, is varied

to alter the point of cut off, the construction must be such that the

ends of the valve at the shortest stroke pass over the ends of the seat

and cover plate faces, or otherwise the middle of the seat and cover

plate faces will wear hollow.

The Buckeye, Porter-Allen, and Straight-Line Engines are examples of

practically balanced valves. The first of these has a balancing device

that follows up the wear; the second has an adjustment whereby the cover

plate may be set up to take up the wear; and in the third the wear is

reduced to a minimum, by accurately fitting and guiding the parts.

[Illustration: Fig. 3304.]

The construction of the valve in the Straight-Line Engine is shown in

Fig. 3304, in which B represents the cylinder bore; the valve _v_ rests

on a parallel strip _n_, and on its top rests the parallel strip _m_,

the pressure relieving plate P is set up firmly against the pieces _m_

_n_, whose thicknesses are such as to leave the valve a working fit

between the faces of R R and of P.

[Illustration: Fig. 3304 _a_.]

Instead of the valve sliding on a flat face, it may work upon a shaft or

spindle as a centre, its face moving in an arc of a circle, and its

action will be the same as a flat valve having the same proportions.

Fig. 3304_a_ represents a valve V of this construction, whose shaft is

at S, A being an arm fast on S, and driven by the eccentric rod R. To

find the necessary amount of travel for such a valve, we draw lines, as

_f_, _g_, from the inner edges of the steam ports, through the centre of

the shaft S, and also draw an arc through the centre of the eye of arm

A, and where lines _f_ _g_ cut the arc, as at _d_ and _e_, are the

extremes of motion of A.

PISTON VALVES.

[Illustration: Fig. 3305.]

A piston valve acts the same as a flat or plain (D) valve, having the

same amount of lap lead and travel. In Fig. 3305 we have a cylinder with

a flat valve on one side and a piston valve on the other, the head end

ports being about to take steam, and it is seen that the eccentrics

occupy the same positions for the two valves. The steam ports are, for

the piston valve, annular grooves provided in the bore in which the

valve fits. The piston valve is balanced because it receives its steam

pressure on the ends, but it will not follow up its wear as the flat

valve does, hence it is liable to leak.

SEPARATE CUT OFF VALVES.

[Illustration: Fig. 3306.]

Meyer's cut off valve is constructed as shown in Fig. 3306, M being the

main valve, and _v_ _v_ the two cut off valves, whose sole duty is to

cut off the steam at an earlier point than the main valve would do. If

the engine is to have a fixed point of cut off, or, in other words, if

the cut off is always to occur at some one particular point in the

stroke, the valves may be set to do so, and equalize the points of cut

off.

But in practice such a boiler would not be used for pressures above