[A transcriber's note follows the text.]
RECORDS
OF
STEAM BOILER EXPLOSIONS,
BY
EDWARD BINDON MARTEN,
MEM. INST. OF MECHANICAL ENGINEERS; ASSOCIATE OF INSTITUTION OF
CIVIL ENGINEERS, AND CHIEF ENGINEER TO THE
MIDLAND STEAM BOILER INSPECTION AND ASSURANCE CO.
LONDON;
E. & F. N. SPON, 48, CHARING CROSS.
STOURBRIDGE:
R. BROOMHALL, 148, HIGH STREET.
1872.
PREFACE.
Accurate information as to Boiler Explosions must always be useful to
those who are interested in the safe working of Steam Boilers.
The following pages contain very brief abstracts of records obtained
for the Midland Steam Boiler Inspection and Assurance Company, by whose
permission they are now republished in a compact and convenient form.
By permission of the Council of the Institution of Mechanical
Engineers, the records are prefaced by a Paper on Steam Boiler
Explosions and their records, and on Inspection as a means of
prevention, read before that Institution at Manchester, August 1st,
1866, and a further Paper on the "Conclusions derived from the
experience of recent Steam Boiler Explosions," read before the same
Institution at Nottingham, August 3rd, 1870.
All names of Works or Firms are omitted from the records as
unnecessary.
ON STEAM BOILER EXPLOSIONS AND THEIR RECORDS, AND ON INSPECTION AS A
MEANS OF PREVENTION, BY EDWARD B. MARTEN, MEM. INST. M.E. A.I.C.E.,
EXCERPT MINUTES OF PROCEEDINGS OF THE MEETING OF THE INSTITUTION
OF MECHANICAL ENGINEERS, AT MANCHESTER, 1ST AUGUST, 1866, JOSEPH
WHITWORTH, ESQ., PRESIDENT, IN THE CHAIR. BY PERMISSION OF THE COUNCIL.
The subject of Steam Boiler Explosions, which was brought before this
Institution in June, 1848, in a paper by the late Mr. William Smith
of Dudley in reference to an explosion near that place, and again in
1859 in a paper by Mr. Longridge on the economy and durability of
stationary boilers, is one of great importance and is now attracting
increased attention. The first public notice of the subject was by a
parliamentary committee in 1817, which was appointed in consequence
of a very fatal boiler explosion in London in 1815; evidence was then
collected as to steamboats, and many boiler explosions were referred
to. That committee recommended among other things that boilers should
be made of wrought iron, instead of cast iron or copper, which had been
the materials mainly used previously; that they should be inspected
and tested; and that there should be two safety valves, each loaded
to one third of the test pressure, under penalties for any excess. A
great part of the information now existing upon the subject, especially
in regard to the earlier explosions, is to be found in the records of
inquests after fatal cases; and some of the careful reports of eminent
engineers on those occasions have materially assisted in the formation
of correct views as to the causes of explosion. Latterly also the
printed reports of the inspectors of mines, and more especially the
reports of the explosions of locomotives, illustrated by diagrams by
the inspectors of railways, have furnished very valuable information.
Since the subject has been taken up by private associations for the
prevention of explosions, many more records have been published,
although their usefulness is much impaired by their not containing the
names of the places whereby the explosions could be identified.
When the writer's attention was first directed to this subject, he
met with great difficulty in obtaining correct records of boiler
explosions, from which to arrive at the results of past experience;
and wishing to base his own opinion on facts, rather than on the
inferences of others however reliable, he followed the example of the
Franklin Institute in their elaborate investigation of the subject, and
collected all the records he could find; and by way of facilitating
reference, arranged an index, a manuscript copy of which is presented
with the present paper to the Library of this Institution. All must
be agreed as to the importance of reliable information on such
accidents as boiler explosions; and the writer would suggest that
this Institution may materially aid in obtaining the desired records
and placing them within easy access, by becoming the depository of
reports on explosions, and by inducing those who have the opportunity
to forward copies of reports, that these may be arranged so as to be
easily found and consulted. It is very desirable that these reports
should as far as possible be illustrated by sketches, as aids to
the description; and also by slight models like those now shown to
the meeting, by which the whole matter may be seen at a glance. So
few persons comparatively have the opportunity of examining boilers
after explosion, that the most erroneous ideas have prevailed, and
theories have been advanced which would soon be dissipated by practical
experience or by reading accurate reports. It would also very much
aid in the understanding of published matter on the subject, if full
descriptions of each case alluded to in illustration could be obtained.
These records are as useful to the engineer as the "precedents" or
"cases" to the lawyer or the surgeon. After any serious explosion,
the newspapers of the neighbourhood in which it has occurred contain
voluminous articles describing the disastrous result and the damage
done, which, although useful as far as they go, do not in the least
assist in arriving at the cause of explosion. The really important
particulars, such as the description and construction of the boiler,
its dimensions, and the pressure at which it worked, are in most cases
omitted altogether.
The record of explosions presented to the Institution contains a list
of the boiler explosions in each year of the present century, as far as
known to the writer, with the names of the places, and the description
and sizes of the boilers, and the supposed cause of explosion, together
with references to the books or papers from which further information
may be obtained. Of course many of the explosions have to be put down
as uncertain in some of the particulars; but every year improves the
record, as fresh information is obtained, and with the assistance of
the members of this Institution it might be made far more perfect and
extensive.
* * * * *
The total number of explosions here recorded is 1046, and they caused
the death of 4076 persons and the injury of 2903. The causes assigned
for the several explosions are very numerous, and are no doubt
incorrect in many cases; but they may be generally stated as follows:
397 are too uncertain to place under any heading; but of the rest
145 were from the boilers being worn out, or from corrosion, or
from deteriorated plates or rivets.
137 from over pressure, from safety valves being wedged or
overweighted, in some cases intentionally, or from other
acts of carelessness.
125 from faulty construction of boiler or fittings, want of stays, or
neglect of timely repair.
119 from collapse of internal tubes, generally from insufficient
strength.
114 from shortness of water, or from scurf preventing the proper
contact of the water with the plates; or from improper
setting so as to expose the sides of the boiler to the flame
above the water line.
9 from extraneous causes, such as effect of lightning striking
down the stacks upon the boilers, or from fire in the building
or explosion of gas in the flues.
----
1046 total number of explosions.
====
The exploded boilers were of the following descriptions:--
232 are not sufficiently described to place under any head; but of
the rest
320 were Marine boilers of various forms.
141 were Cornish, Lancashire, or other boilers internally fired.
120 were Locomotive, or other multitubular boilers.
116 were plain Cylindrical boilers, externally fired.
64 were Balloon or haystack, wagon, Butterley, British-tube,
elephant, or Trevithick boilers.
29 were Portable, agricultural, upright, or crane boilers.
14 were Heating apparatus or kitchen boilers.
10 were Upright boilers attached to puddling or mill furnaces at
ironworks.
----
1046 total number of explosions.
====
[Illustration: _Fig. 1._]
The theories as to the causes of explosion have been numerous. In
the early days of the steam engine, when the steam was used only as
a condensing medium and the pressure in the boiler was frequently
allowed to get below atmospheric pressure, many boilers were destroyed
by the access of the external atmospheric pressure becoming too great,
causing them to be collapsed or crumpled up; and this led to the use
of the atmospheric valve still found on old boilers. Even so lately as
last year, 1865, a boiler in the neighbourhood of Bury, Lancashire,
has suffered in this way by collapse from external pressure; its
appearance after the accident is shown in Fig. 1, which is copied
from a photograph. The early explosions were so palpably due to the
weakness of the boilers, which compared with those of the present day
were most ill constructed, that no one thought of any other cause than
the insufficient strength of the vessel to bear the expansive force
of the steam contained in it. When the advantages of high-pressure
steam became recognized, and the boilers were improved so as to bear
the increased strain, the tremendous havoc caused by an explosion led
many to think that something more must be required than the expansive
force of the steam to produce such an effect; and they appear to have
attributed to steam under certain conditions a detonating force, or
a sudden access of expansive power that overcame all resistance.
To support this somewhat natural supposition, it was asserted that
the steam became partially decomposed into its constituent gases,
forming an explosive mixture within the boiler. That this belief is
still sometimes entertained is seen from the verdict of a jury even
in the present year, 1866, in the case of the explosion of a plain
cylindrical boiler at Leicester, shown in Fig. 2., the real cause of
which appears to have been that the shell of the boiler was weakened
by the manhole. It seems hardly necessary to point out the fallacy of
imagining decomposition and recomposition of the steam to take place
in succession in the same vessel without the introduction of any
new element for causing a change of chemical combination; but it is
necessary to refer to this supposition, as the idea is shown to be not
yet extinct.
[Illustration: _Fig. 2._]
Again it has been asserted that the steam when remaining quite still
in the boiler becomes heated much beyond the temperature due to the
pressure; and that therefore when it is stirred or mixed or brought
more in contact with the water by the opening of a valve or other
cause, the water evaporates so rapidly as to produce an excessive
pressure by accumulation of steam. In support of this view the
frequency of explosions upon the starting of the engine after a short
stand is adduced; but it is very doubtful whether by this means a
sufficient extra pressure could be produced to cause an explosion,
unless the boiler had been previously working up to within a very
small margin of its strength. Explosions are seldom caused by a sudden
increase of pressure, but rather by the pressure gradually mounting to
the bursting point, when of course the effect is sudden enough. Nor
is it necessary in many cases to look for much increase of pressure
as the cause of explosion; for it is far more often the case that the
strength of the boiler has gradually degenerated by wear or corrosion,
until unable to bear even the ordinary working pressure. It is so very
easy, when examining the scene of an explosion, for the first cause of
rupture to be confounded with the causes of the subsequent mischief,
that in many cases erroneous conclusions have been arrived at in this
way.
The most important points to find out in connection with any explosion
are the condition of the boiler and all belonging to it immediately
before the explosion, together with the locality of the first rent, the
direction of the line of rupture, and the nature of the fracture; as
everything occurring after the instant of the first rent is an effect
and not a cause of explosion. As soon as the first rent has taken
place, the balance of strain in the fabric is disturbed, and therefore
the internal pressure has greatly increased power in continuing the
rupture; and also the pressure being then removed from the surface of
the water, which is already heated to the temperature of the steam,
the whole body of the water gives out its heat in the form of steam
at a considerable pressure, and thus supplies the volume of steam for
carrying on the work of destruction. When thus quickly generated, the
steam perhaps carries part of the water with it in the same way that
it does in ordinary priming; and it has been thought by some that the
impact of the water is thus added to that of the steam, to aid in the
shock given to all surrounding obstacles.
It is seldom that one out of a bed of boilers explodes without more
or less injury to the others on either side of it; but sometimes
two boilers in one bed, or three, or even five, have exploded
simultaneously.
* * * * *
The causes of boiler explosions may be considered under the two general
heads of--
Firstly, faults in the fabric of the boiler itself as originally
constructed, such as bad shape, want of stays, bad material, defective
workmanship, or injudicious setting:--and
Secondly, mischief arising during working, either from wear and tear,
or from overheating through shortness of water or accumulation of
scurf; or from corrosion, in its several forms of general thinning,
pitting, furrowing, or channelling of the plates; or from flaws or
fractures in the material, or injury by the effect of repeated strain;
or from undue pressure through want of adequate arrangements for escape
of surplus steam.
[Illustration: _Fig. 3._]
[Illustration: _Fig. 4._]
[Illustration: _Fig. 5._]
There is no doubt that many of the early explosions were from faults
of construction. The stronger materials now used were then found so
difficult to manipulate that others easier to work were chosen, and
often the shape of the boiler was only selected as the one easiest to
make. The early boilers were made of copper or cast iron, with leaden
or even wooden tops, and of the weakest possible shape. Such was the
boiler used by Savery, shown in Fig. 3., and the Tun Boiler and Flange
boiler, Figs. 4 and 5. The very fatal explosion in London in 1815,
referred to by the parliamentary commission previously named, was of a
cast-iron boiler, which failed because one side was too thin to bear
the pressure, as the casting was of irregular thickness. The steam
being at that time used only at or below atmospheric pressure as a
means of obtaining a vacuum by condensation for working by the external
pressure of the atmosphere, so little was pressure of the steam thought
of, that boilers were proposed and it is believed were actually
constructed with hooped wooden shells, like barrels, and internal
fireplaces and flues of copper; and even a stone chamber was named
as being a suitable shell for a boiler, with internal fireplace and
copper flue passing three times the length of the inside and out at the
top, like an ordinary stove and piping. These boilers must have been
something like the sketches given in Figs. 6 and 7, and were intended
to be exposed only to the external pressure of the atmosphere.
[Illustration: _Fig. 6._]
[Illustration: _Fig. 7._]
[Illustration: _Fig. 8._]
[Illustration: _Fig. 9._]
[Illustration: _Fig. 10._]
Cast iron was frequently used for the shell of boilers, with an
internal fireplace and tubes of wrought iron, as shown in Fig. 8, and
boilers of this construction are still to be found in use at some of
the older works at the present day. As the outside shell and front
plate are 1-1/2 inch thick and are not exposed to any wear at all,
these boilers are sufficiently strong. A duplicate front plate with
set of tubes attached is always kept on hand in case of need. Another
form of cast-iron boiler is shown in Fig. 9., made in several parts
put together with flange joints, with an internal fireplace and flue
also made of cast iron. When cast iron was used for the parts exposed
to the fire in boilers intended for high pressure, it was sometimes
employed in the form of tubes of small diameter and proportionately
thinner; as in Woolf's boiler, so much spoken of in the evidence
before the parliamentary committee of 1817. This boiler, shown in Fig.
10., consisted of nine cast-iron pipes, about 1 foot diameter and 9
feet long, set in brickwork so that the flame played all round them.
These small tubes were connected with another of larger size placed
transversely above them, forming a steam receiver, and this again with
a still larger one, which formed a steam chamber. No details of any
explosions of the three last mentioned boilers have been obtained; but
it is known that the cast iron was found a most treacherous material,
especially when exposed to the action of the fire; and that the effect
of explosion was very disastrous, because the boiler burst at once into
many pieces, each of which was driven out with great velocity, and the
danger was not mitigated by the circumstance of large masses holding
together, as is found to be the case with wrought-iron boilers when
exploded.
* * * * *
[Illustration: _Fig. 11._]
[Illustration: _Fig. 12._]
When wrought-iron boilers came into use the shapes were most varied,
and the dimensions much larger than before. One of the earliest was the
Wagon boiler, shown in Fig. 11, with round top and plain flat sides,
which could only be made to bear even the smallest pressure by being
strengthened with numerous stays. In most cases of explosion of this
class of boiler the bottom was torn off, owing to the angle iron round
it being weakened by the alternate bending backwards and forwards under
each variation of pressure, as all the sides and the bottom must be
constantly springing when at work. Such was the explosion at Chester
in 1822, and many others. This shape was soon improved in its steam
generating powers by making the sides concave instead of flat, as shown
in Fig. 12, so that the heating surface was greater and also in a
better position to receive the heat from the flame in the flues. This
shape was further elaborated by rounding the ends as in Fig 13, and
in some cases making the bottom convex to correspond with the top, as
in Fig. 14. All these forms however still required numerous stays to
retain them in shape, the safety of the boiler being dependent upon
the stays; and numerous explosions show the weakness of these boilers.
They generally gave way at the bottom, as in an explosion that occurred
at Manchester in 1842, where the boiler had been weakened by frequent
patching; they also sometimes exploded through the failure of the stays.
[Illustration: _Fig. 13._]
[Illustration: _Fig. 14._]
[Illustration: _Fig. 15._]
[Illustration: _Fig. 16._]
[Illustration: _Fig. 17._]
[Illustration: _Fig. 18._]
A very early improvement in the right direction consisted in making
the shell circular; and some few large boilers still exist that were
made completely spherical, as show in Fig. 15, so that the whole of
the iron was exposed to tension only, and required no assistance from
stays, and the boiler had no tendency to alter its shape under varying
pressure. This shape however had the great disadvantage of possessing
the least amount of heating surface for its size or cubic contents; and
also it was very liable to injury from sediment on the bottom, which
accumulated on the most central spot. The spherical form was therefore
soon modified into the shape shown in Fig. 16, by making the bottom
more shallow, although still convex; and afterwards by putting flat or
concave sides and a flat or concave bottom, with the angle constructed
either of bent plates or angle iron, as in Figs. 17 and 18, which
represent the forms known so well in the Staffordshire district as the
common Balloon or Haystack boiler. Many of these have been made of very
great size, measuring as much as 20 feet diameter, and containing so
much water and steam as to be most formidable magazines for explosion.
Perhaps no form of boiler has exploded more than this, partly because
of the great number that have been used, but chiefly because of the
inherent weakness of the shape. The records however have not been
obtained of the great majority of these explosions, because they seldom
caused sufficient damage or loss of life to attract much attention, as
these boilers generally worked in isolated positions at collieries.
The bottom is only prevented from blowing down into the fireplace by
numerous stays from the top, and the angle iron round the bottom of the
sides is much tried by the constant springing of the plates under every
alternation of pressure; and the weakness thus occasioned is increased
by the angle resting on the brickwork and being exposed to corrosion.
The effect of this continued alternation of strain is well shown by the
elastic model exhibited.
[Illustration: _Fig. 19._]
[Illustration: _Fig. 20._]
[Illustration: _Fig. 21._]
[Illustration: _Fig. 22._]
Notwithstanding the dependence of these boilers upon stays for their
strength, many have been made as large as 12 and 15 feet diameter
without stays; and explosion sooner or later has been the consequence.
Such was an explosion that took place at Smethwick in 1862, which
is shown in Fig. 19. As the force of the explosion was only slight,
the effect of the bottom giving way, and the consequent rolling over
caused by the reaction of the issuing steam and water, is clearly seen.
Another example that occurred at Wednesbury in 1862 is shown in Fig.
20, where the explosion was rather more violent, the bottom of the
boiler being torn off all round and left upon the firegrate, and rent
nearly into two pieces; while the top and sides were thrown some height
in one mass, and were only put out of shape by the fall. The weakness
of this boiler had been further increased by making the bottom angle of
angle iron, as shown enlarged in Fig. 21, with a ring of flat plate
A interposed between the angle-iron ring and the concave bottom of
the boiler; so that all the effect of the springing of the bottom, as
shown by the dotted lines, was thrown upon the angle iron, which was
accordingly found cut off all round. Had the concave bottom been made
to rise direct from the angle iron, as in Fig. 22, the springing could
not have been so great, and the angle iron would only have had to stand
the shearing strain of retaining in its place the rigid bottom; but as
about one foot all round the bottom was flat, and the concavity was
only in the central part, the angle-iron ring had to bear an up and
down strain, as shown by the dotted lines in Fig. 21, and the bending
action was more severe than it would have been if the bottom had even
been made quite flat all over.
[Illustration: _Fig. 23._]
A further form of the Balloon boiler is shown in Fig. 23, where the
heating surface of the bottom is increased by an internal central
dome-shaped fireplace, with an arched and curved flue conducting the
flame through one revolution within the boiler before passing again
round the outside. This construction however must necessarily have
diminished the strength of the boiler greatly. In the drawing the top
of the boiler, as indicated by the dotted lines, is removed to show the
interior.
The desire to add to the strength of boilers by lessening the diameter
of the shell led to the construction of the Plain Cylindrical boilers.
They were made first with flat ends of cast iron, which frequently
cracked and gave way when exposed to the fire, as described in many
of the early American explosions. The flat ends when made of wrought
iron, as shown in Fig. 24, are exposed to the same strain as the bottom
of the balloon and wagon boilers, and are constantly springing with
variation of pressure like drum heads, causing injury to the angle-iron
joint. They also require long stays through them to hold in the ends,
and these are subject to so much vibration that they seldom continue
sound for long together, especially when joined with forked ends and
cotters.
[Illustration: _Fig. 24._]
* * * * *
As the flat ends of such boilers are always being sprung by each
alternation of pressure into a more or less spherical shape, as shown
by the elastic model exhibited, this consideration no doubt led to
the ends being made hemispherical, as shown in Fig. 25; and plain
cylindrical boilers with these hemispherical ends are now so commonly
used that they far outnumber any other form of boiler. Their shape
renders them very strong, as the whole of the iron is in simple
tension, and internal pressure has no tendency to alter the shape, as
is shown by the elastic model exhibited. There is one circumstance very
much in favour of the plain cylindrical boilers, and that is that they
can be so easily cleaned and repaired, as a man can stand properly at
his work at every part and the whole of the interior surface is exposed
equally to view. They are of course exposed to all the evils of boilers
externally fired, the part under greatest strain being weakened by
the action of the fire; and the bottom is also exposed to injury from
accumulation of mud and chips of scurf, which cannot be prevented from
falling there, and lying upon the part exposed to the direct action of
the fire. When made of great length, such as 70 or 80 feet, as is the
practice for applying the waste gas from blast furnaces, these boilers
are also liable to seam-rips or "broken backs," owing to the greater
expansion of the bottom exposed to the fierce flame for its whole
length, than of the top which is kept cooler by exposure to the air;
and it would therefore be better to have a succession of short boilers,
rather than only a single one, where great length is required.
[Illustration: _Fig. 25._]
One boiler has been seen by the writer where extreme length was avoided
by curling the boiler round until the ends met forming a Ring or
Annular boiler. This boiler is shown in Fig. 26, and is 5 feet diameter
with 25 feet external diameter of the ring, or a mean length of about
63 feet; it has been found to work well for some years, although
exposed to the heat of six puddling furnaces.
[Illustration: _Fig. 26._]
[Illustration: _Fig. 27._]
Explosions of plain cylindrical boilers have been very frequent indeed,
although they have not caused a proportionate number of deaths, because
they work usually in isolated positions at colliery and mine engines.
The sketch shown in Fig. 27, represents an explosion that occurred
at Darlaston in 1863, and illustrates the way in which these boilers
usually explode. They generally open first at a longitudinal seam over
the fire, which has become deteriorated by accumulations of scurf
preventing proper contact of the water, so that the plates become
overheated, their quality injured, their edges cracked or burnt, and
the rivets drawn or loosened. The rent generally continues in the
longitudinal direction to the sound seam beyond the bridge at the one
end, and at the other end to the seam joining the front end to the
shell; and then runs up each of the transverse seams, allowing the rent
part of the shell to open out flat on both sides, and liberating both
ends of the boiler, which fly off in opposite directions. Of course
it is seldom that an explosion is quite so simple as this, as the
direction of the flight of the various pieces is so much influenced by
the last part that held in contact with the main body of the boiler.
The want of due observation of this point has often led to erroneous
conclusions.
[Illustration: _Fig. 28._]
In the explosion shown in Fig. 28, and in the model exhibited, which
occurred at Westbromwich in 1864, the lower part of the side of an
upright boiler was blown out; and the liberated part was also divided
into two pieces, each of which fell some distance behind the boiler, in
an opposite direction to the side from which they came. The explanation
of this became obvious on examination, as the cause of the rupture
had been the corrosion of the bottom, and the rent had run up the
seams until it met the angle iron of the side tubes, round which it
ran to the first seam above. This seam acted as a hinge on which the
ruptured pieces turned, and they swung round so violently that they
were wrenched off, but not before they had pulled the boiler over and
received the diverting force which gave them their direction, for they
flew off at a tangent, to the circle in which they had swung round on
the sound upper seam as upon a hinge.
[Illustration: _Fig. 29._]
[Illustration: _Fig. 30._]
In order to avoid having a large diameter for plain cylindrical
boilers, especially where exposed to the fire, boilers have been used
that have supplied the required steam power by a combination of several
cylinders of small diameter. One of these known as the Elephant boiler,
has been so much used in France that it is sometimes called the French
boiler; it is shown in Fig. 29, and consists of two cylinders of small
diameter connected by upright conical tubes to a large cylinder above.
Another form called the Retort Boiler, shown in Fig. 30, has been
described at a previous meeting of this Institution (see Proceedings
Inst. M. E. 1855 page 191). The disadvantages of these two combinations
of plain cylinders are that they are not easy to clean or examine
internally, and also there is not free exit for the steam, which has
to find its way along small channels, and carries the water away with
it, causing priming, and also retarding the generation of steam and
endangering the boiler plates. With a view to strengthen the plain
cylinder made of wrought-iron plates, the seams are sometimes made to
run diagonally, as shown in Fig. 31, on the principle that, as the
longitudinal is the weakest seam and the transverse the strongest, a
diagonal between them gives the greatest amount of strength to the
boiler as a whole.
[Illustration: _Fig. 31._]
* * * * *
[Illustration: _Fig. 32._]
[Illustration: _Fig. 33._]
[Illustration: _Fig. 34._]
[Illustration: _Fig. 35._]
Plain cylindrical and wagon boilers have for many years been made with
internal tubes of various shapes and arrangement, through which the
flame passes to add to the heating surface. These are shown in dotted
lines on the previous drawings of wagon boilers, Figs. 11 and 12. They
are also shown in Fig. 32, where a tube passes from over the fire to
the front of a plain cylindrical boiler; in Fig. 33 two tubes pass
from the sides to the front: in Fig. 34 the tube passes from the back,
but returns over the fire and passes again to the back: and in Fig. 35
a tube from the back passes out through a cross tube in each side. The
boilers in all these cases are fired externally. This addition of tubes
has tended very much to increase the size of these boilers in order to
make room for the tubes. These boilers are now found of 9, 10, and
even 11 feet diameter; and this large shell being fired externally is
exposed to the same dangers as those described in the plain cylindrical
boiler, while it is not so easy to keep clean on account of the
obstruction offered by the internal flues. When the flame has passed
under the whole length of the bottom of these large boilers before
going through any tube, it is doubtful whether the heating surface of
the tube helps much in the generation of steam; but the tube is of
use in reducing the quantity of water in the boiler, as it occupies a
considerable space.
[Illustration: _Fig. 36._]
Explosions of these boilers have sometimes taken place by collapse of
the tubes, but much more generally by the failure of the shell over the
fire, as shown in the sketch Fig. 36, representing an explosion that
occurred at Wolverhampton in 1865, in which the first rent took place
in a seam over the fire where frequent repair had led to a considerable
length of longitudinal seam being in one continuous line. The four
plates over the fire parted and opened out until they had ripped two
seams completely round the boiler; and the plates were thrown in one
flat piece, as shown, upon a bank behind. The main body of the boiler
with the tubes was turned over, and the front end blown away.
[Illustration: _Fig. 37._]
A modification or amalgamation of several of the forms of boilers
already mentioned led to the construction known as the Butterley
boiler, shown in Fig. 37, with a wagon-shaped end over the fire,
continued in a single tube within a plain cylindrical shell beyond.
This boiler has been found to generate steam very rapidly; but the
extreme weakness of the construction over the fire and along the tube,
especially at the part where the front end of the tube widens out in
a bell mouth to meet the wagon-topped fireplace, has led to so many
explosions that few boilers are now made of this form. A very early
explosion that occurred at Edinburgh in 1821 was of a boiler somewhat
of this shape, only that the wagon-topped fireplace was much longer.
Other explosions of this form of boiler occurred at Ashton-under-Lyne
in 1845, at Wolverhampton in 1854, and at Tipton in 1856.
[Illustration: _Fig. 38._]
[Illustration: _Fig. 39._]
[Illustration: _Fig. 40._]
[Illustration: _Fig. 41._]
The desire to economise fuel led to placing the fire inside the boiler,
in a tube running from end to end, as shown in Fig. 38, and the great
number of boilers of this form used in Cornwall gave it the name of the
Cornish boiler. The exceedingly good duty performed by these boilers
led many to believe them the most perfect for economy and durability;
but the great number of explosions, or more properly of collapsed
flues, that have happened, have altered this opinion, and led to the
double-flue boiler shown in Fig. 39, in which not only is the heating
surface increased but the strength also, by having two tubes of smaller
diameter in the same shell. There are a great many varieties of the
two-tube boiler, which have been made for the purpose of obtaining
various particular results. In some cases the two tubes have been made
to unite into a single tube immediately behind the fires, forming what
is known as the Breeches-tube boiler, as shown in Fig. 40, and in other
instances the outside shell of the boiler has been made oval, as shown
in Fig. 41, with the two tubes continued through from end to end. The
heating surface has also been increased, and the strength of the main
tubes, by placing smaller transverse tubes across them at right angles;
but these advantages are gained by increased complication, leading of
course to greater difficulty in examination and repair.
* * * * *
The frequent failure of tubes by collapse when used for high pressures,
and also the results of careful experiments, led to the simple addition
of strengthening rings of different makes around the exterior of these
tubes, by which the shell and the tubes are rendered of equal strength.
It has taken considerable time for the belief in the weakness of large
tubes when exposed to external pressure to become general, and a great
many boilers are still made and used having even large tubes without
the strengthening rings; and in some districts such boilers are used
in great numbers and at far higher pressures than can be considered
judicious. In more than one bed of boilers, one boiler after another
has exploded by the collapse of the tube from the want of strengthening
rings, and yet these have still been believed unnecessary; and the
cases of isolated boilers of this construction where the large tubes
have collapsed are extremely numerous, yet any other reason than the
weakness of the tube has been considered more probable as the cause of
explosion. A sketch of a boiler with collapsed flue is given in Fig.
42, which exploded at Burton-on-Trent in 1865; and it is selected from
many others because it was a new boiler, well made and mounted, and was
a good example of the weakness of a large tube to resist high external
pressure when made of great length without the support of strengthening
rings.
[Illustration: _Fig. 42._]
There are a great many advantages in the tubular boiler internally
fired. The shell which is exposed to the greatest tension is not also
exposed to the first action of the fire. The fire is in the midst of
the water, so that the greatest effect is obtained from it; and the
heating surface immediately over the fire, from which most steam is
generated, has not so great a depth of water above it for the steam to
pass through as in the externally fired boilers heated from the bottom.
The tubes also act as stays to the ends; and the mud in the water falls
off the tubes, where it would do mischief, and settles on the bottom,
where it is comparatively harmless.
These tubular boilers are however subject to disadvantages peculiarly
their own. It is not so easy to move about within them for cleaning
and examination as in the plain cylindrical boiler, as the tubes fill
up the space so much. The difference of expansion between the highly
heated tube and the comparatively cool shell produces a strain, which
causes the ends to bulge out; or if the ends are made rigid, the strain
sets up a contortion in the tube, which causes furrowing of the plates
by making the iron softer or more susceptible of corrosion in certain
lines of strain. Notwithstanding these drawbacks however this form of
boiler is an excellent one.
Many modifications in the forms of boilers have been made to enable the
manufacturers to use the waste heat from various processes, especially
from the making of iron. The plain cylindrical boiler has been used
in this way, with sometimes as many as eight puddling furnaces made to
work upon one boiler. One of the earliest special arrangements for this
purpose was the Upright boiler with central tube, shown in Fig. 43,
which was originally made for two furnaces; and about 7 feet diameter
and 16 feet high. The size has since been increased to 10 feet diameter
and 28 feet high, as shown in Fig. 44. These boilers are made for one,
two, three, or four puddling furnaces; and consist of a cylinder with
spherical ends, standing upright, with a central tube from the bottom
to about half the height, into which the side tubes join. The heat from
each furnace plays over a portion of the shell, and then passes through
the side tubes and down the centre tube into the underground flue to
the chimney.
[Illustration: _Fig. 43._]
[Illustration: _Fig. 44._]
These boilers have many good points: there is great heating surface;
and the shell being heated all round does not strain the plates
and seams by unequal expansion so much as in the horizontal plain
cylindrical boiler heated only at the bottom; and as both ends are
spherical there is no alteration of shape under internal pressure.
Moreover in consequence of the upright position of the boiler a safe
depth of water can easily be maintained, and yet the steam is taken off
so high above its surface that there is little priming; and every part
can most easily be cleaned and examined, as a man can stand upright
both in the boiler and in the flues. But the great drawback to this
class of boilers is that they must stand in the midst of the workmen;
so that, although they are not more liable to explode than any other
form of boiler, yet when they do burst they necessarily endanger more
lives than is usually the case with other boilers that can be placed
more away from the men employed at the works. Should anything arise
with the boiler to make it desirable to withdraw the fire, this cannot
be done without much delay, as the furnaces have to be stopped and the
iron run out. Also an explosion can hardly happen without some of the
melted iron being scattered among the men at work.
[Illustration: _Fig. 45._]
[Illustration: _Fig. 46._]
Some of the most fatal explosions of these boilers have arisen from
careless construction. Such was the case in an explosion at Dudley in
1862, shown in Fig. 45, where the crown plate forming the top of the
centre tube was attached to the sides of the tube by so slight an angle
iron, as shown enlarged in Fig. 46, that the pressure of steam on the
flat crown plate fairly sheared the angle iron through, and allowed the
plate to be blown down the centre tube into the chimney flue, whereupon
the boiler was violently thrown off its seating by the reaction of the
issuing steam and water thus liberated.
* * * * *
The double-tube horizontal boiler is also used in connection with
iron-making furnaces in many places, one furnace working into each
tube. Although by this arrangement the boiler can be placed a little
further from the workmen, some very fatal explosions have happened to
such boilers, as at Masborough in 1862.
* * * * *
[Illustration: _Fig. 47._]
Single-furnace boilers have been much used in the form of a single-tube
boiler standing on end, as shown in Fig. 47, with the flame passing up
the tube, which is continued in the form of a chimney on the top of the
boiler. The tube passes through the steam at the top, so that the plate
is not protected from overheating by contact with water; and this has
caused explosion in some instances, although the tube has been lined on
the inside with firebrick to shield the plate from the flame. Another
great disadvantage of this Chimney boiler is that the space between the
tube and the shell is so narrow that it is almost impossible to examine
or clean it internally.
* * * * *
A further arrangement for a single-furnace boiler is the Elbow boiler,
shown in Fig. 48, where the two difficulties mentioned in the previous
boiler are avoided.
* * * * *
Many internally fired upright boilers of various shapes have been
constructed to suit various purposes. One of a large size that has been
at work many years is shown in Fig. 49, with an internal fireplace and
a suspended cone and cross tube for increasing the heating surface.
This boiler is set in brickwork in such a way that the heat passes
through the side tubes and round the exterior shell before going off to
the chimney.
[Illustration: _Fig. 48._]
[Illustration: _Fig. 49._]
A very fatal explosion at Stoke-upon-Trent, in 1863, resulted from an
attempt to work a boiler of somewhat the same general form, but without
the same careful attention to the details of construction. This boiler
is shown in Fig. 50; the internal fireplace is of conical shape, 4
ft. 6 ins. diameter on the top and 6 ft. 10 ins. at bottom, and was
joined to the external shell by a flat annular bottom. Almost the first
time it was worked at high pressure the conical fireplace collapsed,
breaking off at the seam at the top of the cone, and blowing down upon
the grate, as shown in Fig. 51. The flat bottom was then left without
the support of the cone and side tubes, and gave way all round the
outside angle iron; and the top flew up a great height into the air,
and fell a crumpled heap, as shown in the sketch. In this case the only
wonder is that a boiler of such weak construction worked at all without
explosion.
[Illustration: _Fig. 50._]
[Illustration: _Fig. 51._]
* * * * *
There yet remains to be noticed a very large and varied class of
boilers that have been designed with the express object of avoiding
explosion. Some of these, made of cast-iron pipes of small diameter,
have already been referred to. When steam carriages were first
constructed, boilers were tried made of a cluster of small pipes,
set both upright and horizontally, connected with a general receiver
and with each other by still smaller pipes. These were found to
have such small circulation of water that they very soon burnt out,
and also led to much priming. Afterwards, narrow chambers made of
corrugated plates set like the cells of a battery were tried, but
without much success. The multitubular boilers of the locomotive type
soon superseded all others as quick steam generators, and until lately
they have been considered as almost absolutely safe from explosion.
It is found however that the barrel of these boilers is peculiarly
liable to furrowing, owing to the strain weakening the iron in certain
lines. Perhaps no boiler shows more clearly than the locomotive how
necessary it is that every part should be open to examination; and also
how unwise it would be to use for stationary purposes small cramped
up boilers, only intended to meet the necessities of locomotion. Many
explosions of locomotive boilers have taken place; but it is not
necessary to give details in this paper, as they are fully given in the
published official reports of the government inspectors.
Among the form of boilers designed to obtain very rapid generation of
steam, combined with increased safety from explosion, may be specially
named that consisting of a system of small pipes within a shell with
an artificial circulation of water, and also the boiler consisting
of a cluster of cast-iron spheres, both of which have been described
at previous meetings of the Institution (see Proceedings Inst. M. E.
1861 page 30, and 1864 page 61); but neither has been much used in
this country at present. The boilers also which consist chiefly of
small tubes hanging down into the fire, with smaller tubes or other
arrangements within them for securing a natural circulation, deserve
mention, as they appear successfully to accomplish that end.
The principle of all these small boilers appears to be that only a
small quantity of water should be contained in them, so that there
should not be a reservoir of danger in the shape of a mass of highly
heated water ready to be converted into steam if a rupture takes place:
and it cannot be denied that this is an advantage. But on the other
hand these boilers of small capacity, which evaporate their whole
contents in a few minutes, are subject to new dangers from that very
cause; and although admirably adapted for purposes where steam is
wanted quickly on a sudden emergency, as in the case of fire engines,
or where the generating power required varies each moment, as in
the locomotive, they are for the most part ill adapted for ordinary
stationary purposes, such as the mill or the colliery. They require
constant firing and vigilant attention to the feed, and cannot be left
for a time with safety like the ordinary stationary boilers. It has to
be borne in mind also that the very reservoir of danger so much dreaded
is also a reservoir of power, which assists in the steady maintenance
of the machinery in motion. The large mass of water heated to the
evaporating point, the heated brickwork of the flues, and the large
fireplace, are so many assistances to regularity, and enable the man
in charge to attend to his other duties without the risk of spoiling
the boiler or letting down the steam by a few minutes' absence from the
stoke hole. Steam employers are found at present to prefer the known
dangers of the large boilers to the supposed safety of small boilers,
which they fear are troublesome in practice.
* * * * *
Many of the early boilers were rendered weak by the injudicious
manner of arranging the seams. The longitudinal seams were made in a
continuous line from end to end, as shown in Fig. 24, page 20, with
the transverse seams also continued completely round the boiler, so
that at the corner of each plate there were four thicknesses of iron.
The crossing of the seams, as in Fig. 25, page 21, adds much to the
strength, and also often prevents a rent from continuing forward to a
dangerous extent.