of water, and this is not unfrequently coupled with turning on the
feed suddenly into an overheated boiler. Many explosions have been
attributed to this cause, when closer investigation would have
revealed some far more probable reason. For instance, shortness of
water was stated as the cause of the explosion, at Abercarn in 1865,
of a single-tube boiler with a very large flue tube, which collapsed
upwards from the bottom. The top of the tube and the sides of the shell
had not the slightest mark of overheating, although exposed to the
flame of three furnaces, one of which worked through the tube, and the
others on each side of the shell. In this case the cause of explosion
was clearly the weakness of the tube, and not shortness of water. It
is erroneous to suppose that if a boiler runs dry, or if the feed is
turned into a red-hot boiler, there must necessarily be an explosion.
If a boiler unconnected with any other runs rapidly empty, from the
breaking of the blow-off pipe or any such cause, it will simply get
red-hot and sink out of shape upon the fire, as may often be seen, but
no explosion would happen. If the water only falls gradually, as it
would if the feed were turned off and evaporation continued, the parts
exposed to the fire would get overheated as the water left them. If the
subsidence of the water were very slow, those parts might get red-hot,
and so much softened and weakened as to be incapable of bearing the
pressure, when an explosion would take place, as at Smethwick, in the
present year, where the flues were set above the water line, as shown
in Fig. 74.
[Illustration: _Fig. 74._]
If however the water were turned on again before the overheating had
gone so far, and the feed pipe were, as usual, carried down to nearly
the bottom of the boiler, the water would gradually creep up the heated
sides and cool the plates, the heat of which would not be sufficient
to cause greater evaporation than the ordinary safety valves would
carry off. The danger would not arise so much from the excess of steam
generated by the heat accumulated in the heated plates of the boiler,
as from the injury and strain that would be caused to the plates by
the undue expansion and sudden contraction, especially as this action
would take place on only a portion of the boiler. A singular case,
bearing on this point, may be mentioned. A four-furnace upright boiler,
like that shown in Fig. 44, happened to run so nearly empty, through
the accidental sticking of the self-acting feed apparatus, that the
level of the water sank to the top of the hemispherical end forming
the bottom of the boiler. The feed apparatus then became released of
itself, and, the feed being turned full on, the water gradually rose
until the whole occurrence was only discovered by the leaking at the
seams that had been sprung, which caused so much steam in the flues
as to stop the working of the furnaces. The overheating had been
sufficient to buckle the plates, and in one place a rupture had almost
commenced; but there was no explosion. By way of direct experiment upon
this point, boilers have been purposely made red-hot and then filled
with cold water, without causing explosion.
It has been supposed that boilers sometimes explode from overheating
without the water level being below the usual point, or without
the accumulation of scurf previously alluded to, but simply by the
rapidity of the evaporation from an intensely heated surface causing
such a continuous current of steam as to prevent the proper contact
of the water with the heated plate. Such has been the cause assigned
for the explosion of a three-furnace upright boiler at Birmingham in
1865, shown in Fig. 75. A piece of plate about 3 ft. by 1-1/2 ft. was
blown out of the side, at a place where an enormous flame impinged
continually. The plates had first bulged out, and then given way in the
centre of the bulge, each edge being doubled back and broken off. There
was no positive evidence as to the water supply; but the crown of the
centre tube, which was much above the bottom of the part blown out,
remained uninjured.
A somewhat similar case was that of a large horizontal boiler at
Kidderminster, the tube of which collapsed in 1865, as shown in Fig.
76. It was heated by four furnaces, one of which worked into the tube,
one under the bottom, and one on each side; and all the furnaces worked
into the same end of the boiler. The tube was found to have partially
collapsed at that end, and the top had dropped 11 inches. This was
repaired in the first instance, but was afterwards again found injured
by overheating, although not so seriously. It is very probable that the
extremely rapid ebullition from the sides and bottom, from which the
steam had to pass up the narrow space between the tube and the shell,
produced such a foaming that very little solid water could reach the
top of the tube where it was exposed to extreme heat.
[Illustration: _Fig. 75._]
[Illustration: _Fig. 76._]
Many explosions have been attributed to deterioration of the iron
through long use, as in an explosion at Durham in 1864, and another at
Haswell, near Sunderland, 1865, where the boilers had worked constantly
for 25 and 30 years respectively. When an explosion arises from the
failure of a plate which has not been properly welded in rolling,
there is no question that it was unsound when put in, and escaped
notice; but when the plate that fails is found to be brittle and of
bad iron, the fault is rather attributed to the effect of working than
to original bad quality. Of course this is not always the case, as
the injury done to plates by overheating has been already explained.
Pieces of plate have in some cases been erroneously pronounced to be
deteriorated by work, which have been taken from situations in the
boilers where they were not exposed to any action of fire that could
cause overheating; and therefore in reality the injury could only have
taken place when the boiler was being made, by burning the iron in
bending it to the required shape. A frequent cause of fatal injury to
boilers is injudicious repair, whereby the crossing of the seams is
destroyed, as in the explosion at Wolverhampton in 1865, previously
referred to and shown in Fig. 36. Moreover the edges of the old plates,
already tried by the first rivetting and the subsequent cutting out
of the rivets, are frequently strained again by the use of the drift
to draw them up to the strong new plates; and many a seam rip is thus
started which ultimately causes explosion.
Many explosions have been caused by the want of proper apparatus for
enabling the attendant to tell the height of the water and the pressure
of the steam, and also by the want of sufficient apparatus for supply
of feed water and escape of steam, or by the failure of one or other
of these; but such explosions can only be referred to generally in
the present paper. The mountings on a boiler are usually so open to
observation, and the importance of having them good and efficient is
so universally acknowledged, that much remark is not needed. Mention
has already been made of the sticking of self-acting feed apparatus as
a cause of mischief, and similar failures of floats and gauges have
constantly happened; but this should by no means be considered to
condemn self-acting apparatus, either for assisting in the steadiness
of working, or for giving warning of danger. The apparatus however
should be relied on for assistance only; and an attendant cannot be
called careful who leaves a boiler dependent on such apparatus without
watching. The self-acting principle has been seen by the writer applied
in a novel and useful way in a recording pressure gauge, which proved
the more interesting as it had shown the actual pressure of steam
at the time of the explosion of one of the boilers with which it was
connected.
Among the numerous boiler explosions that have been attributed to
over-pressure through deficient arrangements for escape of steam, in
many cases the safety valves have been placed on the steam pipes in
such a manner that the communication with them was cut off whenever
the steam stop-valve was shut, which is just the time when the safety
valves are most wanted. Safety valves are too often found needlessly
overweighted; and it is believed that many boilers are constantly
worked with safety valves so imprudently arranged and weighted, that
they could not carry off all the steam the boilers would generate
without a very great increase of pressure.
* * * * *
It is concluded that enough has now been said to show that boiler
explosions do not arise from mysterious causes, but generally from some
defect which could have been remedied if it had been known to exist. It
only remains therefore to consider what is the most ready and efficient
way to discover the true condition of a boiler. It has been maintained
that this end is best accomplished by what is called the hydraulic
test, in which a pressure of water is maintained in the boiler for a
given time at a certain excess above the working pressure. This test
is undoubtedly useful so far as it goes, and is perhaps the only one
that can be applied to boilers with small internal spaces, such as
locomotive boilers, not admitting of personal inspection over the whole
of the interior; and it is also admirable for testing the workmanship
of a new boiler. But on the other hand the conditions of a boiler at
work are so different from those which exist during the hydraulic test,
that this alone cannot be depended on; for old boilers have been known
to stand this test to double their working pressure without apparent
injury, although known to be dangerously corroded. The difficulty also
of seeing or measuring the effect of the hydraulic test upon large
boilers set in elaborate brickwork is so great that little practical
benefit has resulted in many cases.
It is believed by the writer that the surest way to ascertain the true
condition of a boiler is to examine it at frequent intervals in every
part, both inside and outside; and as this can only be done when both
the boilers and the flues can be readily entered, it is specially
important that facility for examination should be made a consideration
in selecting a construction of boiler. Permanent safety should be
considered as an element of economy, in addition to its still higher
importance in reference to the preservation of life.
ON THE CONCLUSIONS DERIVED FROM THE EXPERIENCE OF RECENT STEAM BOILER
EXPLOSIONS, BY EDWARD B. MARTEN, MEM. INST. M.E., EXCERPT MINUTES OF
PROCEEDINGS OF THE MEETING OF THE INSTITUTION OF MECHANICAL ENGINEERS,
AT NOTTINGHAM, 3RD AUGUST, 1870, THOMAS HAWKSLEY, ESQ., VICE-PRESIDENT,
IN THE CHAIR. BY PERMISSION OF THE COUNCIL.
The records of Steam Boiler Explosions in recent years are very
numerous, as the increased attention drawn to the subject in this
and other countries has placed far more information at disposal; and
the experience of the last four years, since a former paper was read
by the writer on the subject of boiler explosions, has confirmed the
opinion then expressed, that all boilers, however good in original
construction, are liable in the course of time to get into bad order
and explode. The particulars of the explosions during this period
are given in the Tables appended to the present paper, which show
the number of explosions due to each cause in each class of boiler,
distinguishing those of the United Kingdom from those in foreign
countries. An analysis is also given of the explosions in the last four
years, showing the causes of explosion of each form of boiler; and also
a summary of the causes of explosion under the three general heads
of--(1) faults in construction or repair: (2) faults in working which
creep on insidiously and unnoticed: (3) faults which might be seen and
guarded against by careful attendants. Nearly all of the faults in
these three classes would have been detected by periodical examination.
* * * * *
In the case of Cornish, Lancashire, and other boilers with internal
flues, the faults of construction which have caused explosions have
been weakness in the tubes, combustion chambers, ends, domes, or
manholes; and explosions in these, as in other classes of boilers,
have also resulted from external or internal corrosion, shortness of
water, undue pressure of steam, and scale or mud on the boiler plates.
In plain cylindrical boilers, and others without internal flues,
explosions have resulted from the boiler ends being made flat, and also
from frequent repairs producing seam rips, especially in boilers having
the plates arranged lengthways instead of in rings.
In marine boilers, weak flues and weak ends have also led to explosion,
in addition to the other causes mentioned above.
Locomotive boilers have in two cases exploded in consequence of the
strains thrown upon them by their being used as a frame for the engine.
Other explosions have resulted from want of stays, and from too much
heat impinging on some particular part; and in domestic boilers from
freezing of pipes under pressure.
* * * * *
Altogether the total number of explosions in this country that have
been recorded during the past four years has amounted to 219, which may
be classed under the following heads:--
Faults of construction or repair 95
Faults to be detected only by periodical examination 62
Faults which should be prevented by careful attendants 54
Extraneous or uncertain causes 8
---
Total 219
---
By these 219 explosions 315 persons were killed and 450 injured.
The following are the particulars of the construction of the 219
exploded boilers:--
Cornish, Lancashire, or other boilers with internal flues 84
Plain Cylindrical boilers or others without internal flues 54
Marine boilers 12
Agricultural boilers 11
Locomotive boilers 10
Furnace-upright boilers 8
Crane boilers 6
Rag steamers, &c. 6
Balloon and Elephant boilers 5
Domestic boilers, &c. 16
Not sufficiently described 7
---
Total 219
---
The causes of these 219 explosions may also be classed as follows:--
Worn out, corroded, or burnt plates 89
Undue pressure, overloaded valves, intentional or from carelessness 25
Bad construction, defective fittings or stays, or want of repair 69
Shortness of water, formation of scale or mud, or external flues
set too high 28
Extraneous or uncertain causes 8
---
Total 219
---
Sketches are given of the most instructive examples of boiler
explosions during the last four years, which are sufficient to explain
themselves, with a brief reference to their special features.
* * * * *
[Illustration: _Fig. 1._]
Although the importance of periodical examination as the best safeguard
against explosion is generally admitted, a great number of those who
make or use boilers have not at present sufficient belief in its
importance to adopt this course. Boilers are still constructed or set
in such a manner as to render examination next to impossible; and are
continued to be worked without making it the duty of those who mind
them, or of any one else, to examine every part at frequent intervals;
and hence such explosions have occurred as shown in Fig. 1, No. 12,
1870, in which the original position of the boiler before explosion is
indicated by the dotted lines. It is thought by many steam users that
all has been done which is possible, if their boilers are the best that
can be procured, and are set in the most approved way; and it is taken
for granted that such boilers should last for many years, under the
idea that a good boiler can never explode unless the feed is neglected.
Similar boilers are often referred to as having worked safely for ten
or twenty years, but it is forgotten that they may be exposed to the
insidious action of furrowing on the inside or channelling on the
outside, such as caused the explosions of the originally good boilers
shown in Figs. 2, 3, 4, and 5, Nos. 35, 1870; 50, 1866; 46, 1869; and
25, 1870.
[Illustration: _Fig. 2._]
[Illustration: _Fig. 3._]
Much mischief arises from special classes of boilers, fittings, or
apparatus, being looked upon as promising permanent safety from
explosion; while the inevitable circumstance is overlooked that it is
only so long as everything is maintained in good condition that safety
is insured. An apparatus, for instance, for preventing explosion from
shortness of water or over-pressure, however perfect for any such
object, would be quite inefficient as a safeguard against explosion
from corrosion, furrowing, channelling, or weak construction. It is
curious to note how often it is the case that every other part of an
establishment is subject to severe and perpetual scrutiny, the engines
especially being overhauled with the most scrupulous regularity; while
the boilers, the very source of the power and the heart of the whole
business, are left to themselves for long periods, even for years,
without examination; and it is too often only after bitter experience
that owners have understood the need of this examination. In this, as
in many other matters, experience has shown that there is no royal road
to safety, and that immunity is only secured by unremitting care and
constant watchfulness. It should never be forgotten that even a good
boiler can explode; for however good at the outset, sooner or later
the time must eventually arrive, when such wear and tear will have
taken place as will result in dangerous weakness, unless the boiler is
carefully and systematically attended to. Although a boiler may even
last safely for ten to thirty or more years if worked slowly and with
care, no confidence can be placed in a boiler which has worked so long,
unless it is examined in every part.
[Illustration: _Fig. 4._]
[Illustration: _Fig. 5._]
[Illustration: _Fig. 6._]
[Illustration: _Fig. 7._]
The opinion is more general than many are aware of, that explosions
as a rule are caused by shortness of water and the sudden turning on
of the feed water upon red-hot plates; and the appearances of injury
in the plates from fire, arising in the ordinary course of working,
have been frequently mistaken for signs of overheating from shortness
of water at the time of explosion, as illustrated in Figs. 6 and 7,
No. 24, 1867, and No. 59, 1866. Although boilers do explode from the
softening of the plates by overheating in consequence of shortness
of water, yet it is very doubtful whether the turning on of the cold
water at such a time is ever the cause of explosion. The feed water
being always introduced at the bottom of the boiler, as in Figs. 8, 9,
and 10, cannot be scattered suddenly near the overheated parts, but
must rise gradually up the sides; and the boiler would have gone to
pieces from the giving way of the softened parts long before the water
reached them, as was the case in the explosions shown in Figs. 11 and
12, end of 1868. The experiment of injecting cold water into red-hot
boilers has been carefully tried more than once, without producing any
explosion.
[Illustration: _Fig. 8._]
[Illustration: _Fig. 9._]
[Illustration: _Fig. 10._]
Although it may be too much to suppose that boiler explosions will ever
be entirely prevented, it is important that those who have the care of
boilers should understand better what are the true causes of explosion,
in order that they may know what to guard against in addition to
shortness of water. This better understanding of the subject has been
much retarded by the supposition that the causes of boiler explosions
are beyond the comprehension of the boiler minders; and still more by
the important differences of opinion among those under whom they work.
Much evil has resulted from the promulgation of strongly expressed
views, which have been founded upon facts but of too limited extent,
and such as must become modified by consideration of the facts of
a large number and variety of explosions. Mysterious theories to
account for explosions have been resorted to only from want of clearer
explanations.
[Illustration: _Fig. 11._]
[Illustration: _Fig. 12._]
[Illustration: _Fig. 13._]
[Illustration: _Fig. 14._]
Before considering in detail the causes of explosion, it is necessary
to recall to mind that beyond question there is sufficient accumulated
force in any working boiler to cause all the violent effects of an
explosion, if this force be suddenly liberated. In Figs. 13 and 14,
No. 18, 1869, and No. 63, 1866, are shown the violent effects of the
rupture of vessels employed for steaming rags, which were filled with
steam only. In ordinary boilers however there is present, besides the
steam, a quantity of water heated much beyond the atmospheric boiling
point; and when rupture takes place and the pressure is suddenly
relieved, part of this water evaporates, and keeps up the supply of
steam to continue the rupture and destruction. The explosion of a
boiler differs from the discharge of electricity or lightning, which
cleaves the air and instantly leaves a vacuum; it also differs from
the discharge of detonating compounds which act suddenly and leave a
vacuum; but it more nearly resembles the discharge of gunpowder, which
burns sufficiently slowly to keep up a continuous pressure behind a
projectile until it leaves a gun; and each cubic foot of water in a
boiler working at 60 lbs. pressure has been shown to produce in steam
an explosive effect equal to one pound of gunpowder. None of the
elaborate but unlikely theories of decomposed steam, or of electric
accumulations, suppose a force so fitted to cause destruction as that
contained in the highly heated water existing in all working boilers.
* * * * *
The following appear to be the general results to be derived from the
experience of the explosions in this country during the last four years.
[Illustration: _Fig. 15._]
[Illustration: _Fig. 16._]
First as to faults of construction which fall under the department
of the boiler maker or repairer. One of the most apparent causes of
explosion in stationery boilers is the loss of strength occasioned by
frequent repair, not only from the injury done to the old plates by
removing rivets, but from the want of bond in the new work. This has
lead to many of the explosions of the Plain Cylindrical boilers, such
as are shown in Figs. 15, 16, and 17, No. 45, 1869, No. 32, 1870, and
No. 20, 1870. Where the plates are arranged longitudinally instead
of in rings, the danger is increased, as there is less chance of a
dangerous rip being arrested by a crossed joint. So great a number of
boilers with continuous longitudinal seams, especially in the North,
have worked for twenty or thirty years, that it can hardly be supposed
they are any weaker than the boilers made in rings; but they are more
liable to explode, for if a seam rip occurs, it more easily extends
along the seam, and leads to the general break up of the boiler, shown
in Fig. 18, No. 59, 1869.
[Illustration: _Fig. 17._]
[Illustration: _Fig. 18._]
Perhaps no boilers have worked for a greater number of years than
the Plain Cylindrical boilers, many specimens being in existence and
apparently in good order which were put to work fifty or sixty years
ago. When such boilers have been too much or injudiciously repaired,
they are treacherous and uncertain; but their rupture and explosion
occur not so much on account of fault of shape, as from the simple
reason that like willing horses they are easily overworked. The grates
are usually twice as large as the fair proportion to the heating
surface, producing the double evil of forcing more heat through the
iron plates over the fire than they can transmit without injury, and
allowing a great amount of heat to pass away to the chimney without
useful effect. Careful experiment shows that nearly as good duty can
be obtained with the plain cylindrical boiler as with any other form,
provided the rate of combustion is in fair proportion to the extent
of heating surface in the boiler. The circumstance that many plain
cylindrical boilers have exploded is not sufficient to condemn this
make of boiler, which is the cheapest, simplest, and most easily set.
If the number of explosions alone were to be taken as the guide, it
would lead to the condemnation of the Cornish and Lancashire boilers,
from the experience of the past four years. But in case of both plain
cylindrical and other forms of boilers, most of the dangers admit of
remedy, and can be guarded against by frequent examination.
[Illustration: _Fig. 19._]
[Illustration: _Fig. 20._]