and workmanship to secure strength in a boiler, however perfect the
design. If the plates are of weak and brittle iron, or imperfectly
manufactured, they will never make a good boiler. Apart from the strain
upon the boiler when at work, the iron has to undergo the strain of the
necessary manipulation, shaping, and punching, during the construction
of the boiler. If the plates forming the boiler are not well fitted to
their places before the rivet holes are made, the errors have to be
partially rectified by using the drift in the holes to an unwarrantable
extent, and then using imperfect rivets to fill up the holes that do
not correspond with each other; and the mischief is too frequently
increased afterwards by excessive caulking, in the endeavour to stop
the leaking which is sure to show itself. In this way a boiler is often
exposed to most unequal internal strain between its several parts
before it is set to work at all; and when the heat is applied to it,
the mere expansion causes undue contortion, and leads to seam rips,
and ultimately to disaster. Several specimens of faulty rivetting and
caulking were exhibited to the meeting, and a sketch of one of them is
shown in Fig. 52.
[Illustration: _Fig. 52._]
The strength of a boiler is often very much lessened by the injudicious
manner in which the mountings are fixed upon the boiler, and many
explosions are the consequence of this defect. Not only are a great
many holes for fittings cut out of the boiler in one line, but these
holes are made needlessly large. Steam domes are often so placed as
greatly to weaken the shell of the boiler, the hole cut out of the
plate being made the full diameter of the dome; and in some cases the
domes or steam chests have been made square or rectangular, so as to
weaken the shell still more, as shown in Fig. 53.
Manholes are often a source of danger, if not properly arranged and
duly strengthened. Even in very small boilers they are often placed
with the longest diameter in the longitudinal direction of the boiler,
so that the shell is greatly weakened, as in the sketch, Fig. 54, of
an exploded boiler at Walsall in 1865. This boiler was 5 ft. 3 ins.
long and 2 ft. 6 ins. diameter, and yet the manhole was 18 inches by 13
inches, and placed within a few inches of one end. The end was fastened
in by angle iron, which was not welded, and consequently there was so
little strength at the small portion of the shell between the end and
the manhole that it gave way and liberated the end and the manhole lid,
after which the main body of the boiler was thrown by the reaction
across several streets to a great distance.
[Illustration: _Fig. 53._]
[Illustration: _Fig. 54._]
A somewhat similar injudicious arrangement of the manhole is shown
in Fig. 55, where a manhole 17 inches by 14 inches was cut out of
the flat top of a steam dome only 2 ft. 6 ins. diameter, without
any strengthening ring to compensate for it. The repeated strain of
screwing up the manhole lid, combined with the pressure of the steam,
caused the lid to force its way out through the plate and blow away.
This explosion occurred at Birmingham in 1865.
[Illustration: _Fig. 55._]
The preceding examples have shown how explosions often result from
faults in the construction of boilers; and the following instances
illustrate the explosions caused by mischief arising during working.
A boiler perhaps more than any other structure is subject to wear
and tear; and let it be worked ever so carefully, it will seriously
deteriorate. The wonder is, considering the work they have to perform,
that so many boilers are found which have worked twenty, thirty, or
even fifty years without explosion. The terms wear and tear however
are too vague for this subject, and the mischief met with must be
considered under distinct heads.
There is no doubt that the thing most to be dreaded for boilers is
corrosion; because when the plate is once thinned, it cannot be
strengthened again, but must remain permanently weakened. Corrosion
the more deserves attention because it is easily detected by moderate
vigilance, and can generally be prevented by moderate care, or by the
boilers being so arranged that they can be readily examined in every
part. Corrosion has been the direct and unmistakeable cause of a very
large proportion of the explosions that have happened: it occurs both
inside and outside the boiler, according to circumstances, and attacks
the iron in various ways and in different places.
[Illustration: _Fig. 56._]
[Illustration: _Fig. 57._]
[Illustration: _Fig. 58._]
[Illustration: _Fig. 59._]
_Internal corrosion_ sometimes takes place from bad feed water, and
its effects are different in extent in the different parts of the same
boiler. It very seldom thins the plate over a large surface regularly,
but attacks the iron in spots, pitting it in a number of holes. These
are sometimes large, as if gradually increasing from a centre of
action; and sometimes small, but so close together as to leave very
little more space whole than that which is attacked. A very curious
example of the latter was exhibited to the meeting, and shown in Figs.
56 and 57, cut from the lower part of the shell of a large tubular
boiler externally fired. The corrosion was greatest along that part of
the shell most exposed to heat, and was so extensive that two boilers
exploded simultaneously. The boilers had been at work sixteen years,
but the corrosion commenced about eight years before the explosion,
when the feed water was rendered corrosive by being obtained from
some iron mines. This explosion occurred at Aberaman in 1864. The
corrosion had been seen going on for years, and was not considered
sufficient to cause danger; but the depth to which it extended through
the thickness of the metal is seen in the half size section, Fig.
57. Another sample equally curious was exhibited to the meeting, and
shown in Figs. 58 and 59, taken from the sweep plate over the fire
in a plain cylindrical boiler which had worked about ten years. The
feed water was occasionally bad, and attacked the iron over the area
DDD, where unprotected by scale. The protection afforded by scale
against occasional corrosive feed water is worthy of notice. In the two
specimens exhibited it is seen that the protection has been perfect
where the scale has not been chipped off; and the edge of the sound
part projects over the hollow, as seen in the half size sections,
Figs. 57 and 59, the corrosive water having eaten away a larger area
beneath than that through which it first entered the surface of the
iron.
Internal corrosion is frequently observed where boilers are fed
from canals or streams in the neighbourhood of chemical works from
which corrosive matter is discharged at intervals into the water.
The corrosion takes place in isolated spots, but causes deep holes;
which seems to be accounted for on the supposition that the scale
previously upon the plate cracks during the cooling of the boiler for
cleaning, and forms a blister, so that a piece of about 2 inches area
is raised slightly from the iron. When the boiler is again put to
work, this blister becomes filled with the corrosive water, which is
held there without circulation and causes corrosion. When the boiler
is again emptied these blisters may be seen, and if broken show the
blackened water and the injured surface. In future working each of
these blisters forms a constant unprotected point for attack. It is
frequently seen further that such corrosion is arrested if water be
used which deposits scurf; but fresh blisters and renewed corrosion
will result from a return to the use of the bad water.
The internal corrosion called furrowing has proved a frequent cause
of explosion, especially in locomotive boilers. It differs from other
corrosion by being in deep narrow continuous lines with abrupt edges.
It will sometimes go completely through a plate; and is found where a
sudden change of thickness occurs, either along the lines of the seams,
or opposite the edge of angle-iron attachments. This effect is supposed
to be due to the alternate springing of the plates under each variation
of the pressure or temperature, causing the line of least resistance to
receive a strain somewhat similar to that produced by bending a piece
of iron backwards and forwards for the purpose of breaking it. This
line of injury is exposed to constant attack from corrosion, because
the scurf is always thrown off from it.
* * * * *
_External corrosion_ is a far more frequent cause of explosion in
stationary boilers; and it arises from many causes. The most frequent
cause, although the most easily detected, is leakage from the joints
of the fittings on the top of the boiler, which are too frequently
attached by bolts instead of rivets. This evil is much increased when
the boilers are covered with brickwork, which holds the water against
the plates, and hides the mischief from observation. It is astonishing
to find how much damage is allowed in this way to go on without
attention, until the tops of boilers are corroded so thin that little
holes burst through. These are sometimes found stopped with wooden
pegs or covered by screwed patches of plate, either of which cause
leakage that hastens the mischief, as shown by the sample exhibited.
Boilers exposed to the weather will of course become corroded like
anything else made of iron and not painted; and yet so much mischief
is sometimes caused by leakage beneath improper covering that exposure
may almost be said to be the smaller evil of the two, as it is better
to see what is going on than to rest in false security. No covering
will be found cheaper, or better, in the long run, than a roof, which
prevents the loss of heat by exposure, and yet allows free access to
all the fittings and joints on the top of the boiler.
[Illustration: _Fig. 60._]
Some examples of the evils of covering can be given that have come
under the writer's observation. A set of boilers had been well covered
by arches of brickwork, so built as to keep out all water, and also
set so as to touch the boilers only at intervals, leaving a space
generally of a few inches. After about seven years' working, the whole
of the tops of the boilers were discovered to be dangerously thin, and
had to be renewed. The cause was leakage from the joints of fittings
and seams of the boilers, and the issuing steam had been drawn along
the space between the boilers and the arches, and had escaped at a
place where it had not attracted notice. In another case, a somewhat
similar set of boilers were covered with ashes, to prevent the loss
of heat by radiation; and the rain and the leakage beneath the ashes,
in conjunction with the corrosive matter from the ashes themselves,
thinned the tops of the boilers to a dangerous extent in less than two
years. A sketch of the corrosion caused in this instance by covering
with ashes is shown in Figs. 60 and 61.
[Illustration: _Fig. 61._]
[Illustration: _Fig. 62._]
[Illustration: _Fig. 63._]
Similar mischief has been noticed in boilers covered with sand, as
shown in the sketches Figs. 62 and 63, which represent an instance of
corrosion after eight years' working; although nothing forms a better
covering than sand for preventing loss of heat by radiation. In both
these examples it will be seen that the corrosion has continued until
the thickness of the plate has been so eaten away that a hole has been
burst out at SS. A very good covering is formed by brickwork in cement;
or various cements made for the purpose, which adhere to the surface of
the plate and yet show leakage; or such materials as sacking or felt;
or sheet-iron casing, leaving about 6 inches of air space all round
the boiler. But all these have the great objection that they hide the
boiler from inspection, except by the expensive process of removing
the covering; and in this way dangers that have caused explosion have
remained hidden from observation.
Explosions have also taken place from general corrosion of the surface
of the boilers in the flues. A new boiler which was set on sidewalls
built upon a foundation of porous rock was found to have become
corroded all along the bottom in less than two years, owing to the
dampness which rose from the foundations causing a constant presence
of vapour. The corrosion was peculiar, and more like that found on old
iron left for a long time in a damp place; for the iron plate fell
to pieces when touched, and large flakes could be raised from the
surface, and the greater part of the thickness of the plate could be
removed with the fingers. Somewhat similar corrosion had taken place
in a boiler which exploded at Loughborough in 1863; the bottom of the
shell became rent at the corroded part, and as the fracture continued
spirally round the boiler several times, nearly all the shell was
peeled off in the curious manner shown in Fig. 64. The explosion shown
in Fig. 65, which occurred at Leeds in 1866, also arose from corrosion
of the bottom of the boiler.
[Illustration: _Fig. 64._]
[Illustration: _Fig. 65._]
[Illustration: _Fig. 66._]
The greater part of the corrosion found in the side flues of boilers
is caused by the leakage of seams. Many boilers are emptied for
cleaning as soon as work is over on Saturday night, and long before the
brickwork of the fireplaces and flues has cooled; and consequently,
the boiler, having no water in it, is made much hotter than it ever
is in working, and the seams are injured and sprung and the rivets
loosened by the extra expansion so caused. This is sometimes done
intentionally, in order to loosen the scale by the greater expansion
of the iron than of the scurf. When the boiler is again set to work,
the seams and rivets leak and cause that corrosion which is called
channelling. This has been observed to occur to such an extent that
all the seams in a boiler have been seen thus corroded; and the same
has sometimes been found in all the boilers in a large manufactory.
Specimens of this channelling were exhibited to the meeting. One in
particular, shown in Figs. 66 and 67, deserves attention, as it
shows the effect of a jet of steam and water from the leaking rivet
R, in cutting a series of channels into the plate along the course of
the dotted lines EEE, and producing a hole in the plate at S. This
corrosion had been going on for about four years, but was in a part of
the boiler seldom seen in ordinary examination. Many explosions have
resulted from this form of corrosion; for when a rent is once made, the
fracture continues along the thinned channel of the plate.
[Illustration: _Fig. 67._]
The corrosion most to be dreaded, because most difficult to detect, is
that which takes place where the boiler is in contact with brickwork;
and it is found alike in all forms of boilers set in brickwork. When
found at the part where the side flues are gathered in at the top
against the boiler, it is usually occasioned by the leaking of fittings
or feed pipes, or by rain being allowed to run between the boiler and
the brickwork. More than one explosion has been caused by the droppings
from a roof being allowed to fall upon the tops of the flues. When the
corrosion is found at the point where the bottom flue walls touch the
boiler, it is frequently caused by the leaking of seams that have been
strained by the weight of the boiler; and this often arises from want
of care to replace the brickwork, after repair of the boiler or flues,
in such a position as to take again its proper proportion of the weight
of the boiler. Cases have been met with where the shape of the bottom
of large boilers has been quite altered by such means. The brackets
on the sides of heavy boilers have not only been strained so that the
rivets or bolts have leaked and caused corrosion, but they have also
bent or cracked the side plates of the boiler. The bracket shown at B
in Fig. 53, page 40, made of only an angle iron with a piece of plate
attached, is especially liable to cause injury if the brickwork is not
rebuilt close up to the angle iron, as the leverage is so great. This
is avoided by the better form of bracket shown at C, consisting of an
elbow of flat bar-iron rivetted at top and bottom to the boiler.
[Illustration: _Fig. 68._]
In the old balloon and wagon boilers, the angle where the bottom joined
the sides scarcely ever remained sound for long when in contact with
the brickwork, and many of those that exploded have been found almost
corroded through where they stood upon the brickwork. The explosion
before alluded to and shown in Fig. 7, was caused by corrosion of the
bottom of the boiler where it was set on the brickwork. Many boilers
are so set that the brickwork of the flues is made to follow the shape
of the boiler, with as little space between as possible; but the slight
advantage gained in increased heating effect is far outweighed by the
impossibility of getting into the flues for examination. It is only
by having the flues sufficiently roomy that proper examination can be
made, and that the indications on the brickwork of leaking can be seen
and remedied, and corrosion arrested. A remarkable case of corrosion
occurred in a boiler with an oval shell, set upon a middle wall. The
flues were too narrow for a man to enter, and a leak in the bottom was
only discovered by the boiler nearly running empty while the engine
pumps were standing for a short time. It was subsequently found that
the whole bottom where it rested on the wall was extensively corroded
in a continuous line, and that explosion was only prevented by the
numerous stays across the bottom to compensate for the oval shape. Fig.
68, shows the position and extent of the corrosion, and the plate was
completely in holes at the parts indicated by the black marks. This
corrosion was supposed to have been going on for about three years.
[Illustration: _Fig. 69._]
[Illustration: _Fig. 70._]
It is sometimes asserted that corrosion cannot be the cause of an
explosion, because the corroded place would simply give way and let off
the steam harmlessly, or at least the boiler would not be displaced
from its seating. When the corrosion is only local, and surrounded by
sound plates of sufficient strength to arrest the extension of the
fracture, this may be the case, as in an explosion at Sheffield in
1865, shown in Fig. 69, where a piece of plate was blown out on one
side of the boiler, allowing the steam and water to escape without
displacing the boiler; the thickness of the plate at that part had
been reduced to 1/8th inch by corrosion in about 1-1/2 years, which
had been caused by leakage at the seams from inefficient repair with
bolts instead of rivets, and also from the moisture having been allowed
to be kept against the plate by the brickwork. But even under such
circumstances, if the piece blown out should be from the bottom, the
whole boiler may be thrown a great distance by the reaction of the
issuing steam, as in an explosion at Leeds in 1865, shown in Fig. 70.
If the corrosion extends for any length, the first rent is almost
sure to continue until a complete explosion is the result. Several of
the small models exhibited to the meeting showed the line of fracture
in various cases of explosion. One showed the appearance of a plain
cylindrical boiler after explosion caused by corrosion along the whole
length where it rested on brickwork; this explosion occurred at Wigan
in 1865, and a sketch of it is given in Fig. 71.
[Illustration: _Fig. 71._]
Many explosions of boilers have been caused by accumulation of scurf.
The mischief is not so much from scurf being gradually deposited all
over the interior of the boiler to a dangerous thickness as from the
chips off the sides falling in heaps on the bottom. The plate beneath
this accumulation becomes overheated, because not in contact with the
water, and softens and sinks down into a "pocket," which if unnoticed
will soon burn quite through. If the scurf that has caused the mischief
is thick and hard enough to resist the pressure for a little time,
the hole enlarges, until the scurf suddenly gives way and allows the
contents to issue so violently as to disturb the boiler, or at least
to blow the fire out of the grate. Such was an explosion at Bilston
in 1863, where a large plain cylindrical boiler, 9 ft. diameter, was
heated by three large fires placed side by side along the bottom;
and a large "pocket" burst out over the third grate, and scalded the
attendant to death. A similar pocket in a boiler, 4 ft. 6 ins. diameter,
which exploded at Dudley in 1864, after having been at work six weeks
without cleaning, is shown in the transverse section, Fig. 72. In this
case the scurf had filled up the circle of the boiler to a depth of 3
inches at the bottom, as shown in the drawing, and was of a very hard
description; and the boiler plate was bent out in a gradual curve, and
thinned to about 1/16 inch, the original thickness being 1/2 inch.
[Illustration: _Fig. 72._]
[Illustration: _Fig. 73._]
The whole bottom of a boiler is sometimes injured, and the plates
buckled and the seams sprung, from the accumulation of mud. One case
may be mentioned where the water was very full of mud, and the boilers
were worked day and night during the week but stopped for several hours
on Sunday, during which time the deposit of mud was so thick that it
did not get thoroughly disengaged again from the bottom when the boiler
was set to work, but hardened into a mass. Although many of these
pockets and injuries to the plates may occur without serious damage,
they sometimes cause that first rent which destroys the equilibrium of
the structure and leads to explosion. Some of the specimens of scurf
exhibited to the Meeting show that their thickness is made up of small
chips, carelessly left after cleaning or fallen from the sides of the
boiler, as seen in Fig. 73, or from cotton waste or other matter left
in the boiler and forming a nucleus for the scurf to accumulate upon.
Other specimens show that foreign matter must have been put into the
boiler to stop leaking.
Accumulations of scurf in the feed pipes at the point of entrance into
the boiler have also caused explosion by stopping the supply of water.
The same result is caused by the freezing of the water in the pipes
which are exposed, and each winter one or two boilers are injured or
exploded from this cause, especially small household boilers placed
behind kitchen grates. Scurf cannot be considered so great an evil as
corrosion, since it can be removed, and if this is done in time, the
boiler is restored to its original condition.
The advantage of a pure water, which does not deposit scurf, is so
great for the supply of boilers that it is always worth while to
go to considerable expense for obtaining it; or to take some steps
for purifying the feed water as much as possible. If it is only mud
mechanically suspended, which would deposit by gravity on the bottom
of the boiler, frequent use should be made of the blow-off apparatus.
If the impurity is light enough to be carried to the surface in the
form of scum, the blow-off apparatus should discharge from the surface
of the water as well as from the bottom. If the impurity is chemically
suspended in the water, some one of the many substances which form
the refuse from various manufactures, and which may contain suitable
ingredients, should be used to counteract the effect of the impurity.
Common soda will answer the purpose perhaps better than anything else.
It must not be forgotten however that the blow-off apparatus must
afterwards be used more frequently, to rid the boiler of the foreign
matter, or the mischief will be increased. In marine boilers, constant
attention is necessary to get rid of the saline deposit; and in
stationary boilers using impure water an equally systematic attention
is needed to get rid of the earthy deposit.
* * * * *