Boiler explosion
boiler explosion is a catastrophic failure of a boiler. As seen
today, boiler explosions are of two kinds. One kind is a failure of the
pressure parts of the steam
and water
sides. There can be many different causes, such as failure of the safety valve,
corrosion
of critical parts of the boiler, or low water level. Corrosion along the edges
of lap joints
was a common cause of early boiler explosions.
The second kind
is a fuel/air explosion in the furnace, which would more properly be termed a firebox
explosion. Firebox explosions in solid-fuel-fired boilers are rare, but firebox
explosions in gas or oil-fired boilers are still a potential hazard.
Causes of boiler explosions
"The
principal causes of explosions, in fact the only causes, are deficiency of
strength in the shell or other parts of the boilers, over-pressure and
over-heating. Deficiency of strength in steam boilers may be due to original
defects, bad workmanship, deterioration from use or mismanagement. "[1]
"Cause.-Boiler
explosions are always due to the fact that some part of the boiler is, for some
reason, too weak to withstand the pressure to which it is subjected. This may
be due to one of two causes: Either the boiler is not strong enough to safely
carry its proper working pressure, or else the pressure has been allowed to
rise above the usual point by the sticking of the safety valves, or some
similar cause" [2]
Locomotive-type boiler explosions
Boiler
explosions are of a particular danger in (locomotive-type) fire tube
boilers because the top of the firebox (crown sheet) must be covered
with some amount of water at all times; or the heat of the fire can weaken the
crown sheet or crown stays to the point of failure, even at normal working
pressure. Locomotive-type boilers have been used not only for locomotives, but
also traction engines, portable engines, skid engines used for mining or
logging, stationary engines for sawmills and factories, for heating, and as
package boilers providing steam for other processes. In all applications,
maintaining the proper water level is essential for safe operation.
Principle
Many shell-type
boilers carry a large bath of liquid water which is heated beyond the boiling
point of water at atmospheric pressure. During normal operation, the liquid
water remains in the bottom of the boiler due to gravity, steam bubbles rise
through the liquid water and collect at the top for use.
If this boiler
opens up to the atmosphere as a result of a break from over pressure or other
such failure the contents are allowed to expand suddenly into the atmosphere.
The rapid release of steam and water can provide a very potent blast, and cause
great damage to surrounding property or personnel. Since the water in the boiler
is at a higher temperature and pressure (enthalpy)
than boiling water would be at atmospheric pressure, some of this liquid will
flash into vapor as the pressure drops by the rapid formation of steam bubbles
throughout the water.
The energy of
this expanding steam and water is now performing work just as it would have
done in the engine, with a force that can peel back the material around the
break, severely distorting the shape of the plate which was formerly held in
place by stays, or self-supported by its original cylindrical shape.
The action of
the rapidly expanding steam bubbles will also perform work by throwing large
"slugs" of water inside the boiler. A fast-moving mass of water
carries a great deal of energy (from the expanding steam), and in collision
with the shell of the boiler results in a violent destructive effect. This can
greatly enlarge the original rupture, or tear the shell in two.[3]
Many plumbers
and steamfitters are aware of the phenomenon called "water hammer". A
few ounce "slug" of water passing through a steam line and striking a
90 degree elbow can instantly fracture a fitting that is otherwise capable of
handling several times the normal static pressure. It can then be understood
that a few hundred, or even a few thousand pounds of water moving at the
same velocity inside a boiler shell can easily blow out a tube sheet,
collapse a firebox, even toss the entire boiler a surprising distance through
reaction as the water exits the boiler, like the recoil of a heavy cannon
firing a ball.
A steam
locomotive operating at 350 psi (2.4 MPa) would have a temperature of
about 225 °C, and a specific enthalpy of 963.7 kJ/kg[4].
Since standard pressure saturated water has a specific enthalpy of just
418.91 kJ/kg[5],
the difference between the two specific enthalpies, 544.8 kJ/kg, is the
total energy expended in the explosion.
So in the case
of a large locomotive which can hold as much as 10,000 kg of water at a
high pressure and temperature state, this explosion would have an energy
release equal to about 1160 kg of TNT.
Firebox explosions
In the case of
a firebox explosion, these typically occur after a burner flameout. Oil fumes,
natural gas, propane, coal, or any other fuel can build up inside the
combustion chamber. This is especially of concern when the vessel is hot; the
fuels will rapidly volatize due to the temperature. Once the lower explosive
limit (LEL) is reached, any source of ignition will cause an explosion of the
vapors.
A fuel
explosion within the confines of the firebox may damage the pressurized boiler
tubes and interior shell, potentially triggering structural failure, steam or
water leakage, and/or a secondary boiler shell failure and steam
explosion.
A common form
of minor firebox "explosion" is known as "drumming" and can
occur with any type of fuel. Instead of the normal "roar" of the
fire, a rhythmic series of "thumps" and flashes of fire below the
grate and through the firedoor indicate that the combustion of the fuel is
proceeding through a rapid series of detonations, caused by an inappropriate
air/fuel mixture with regard to the level of draft available. Usually causes no
damage in locomotive type boilers, but can cause cracks in masonry boiler
settings if allowed to continue.
Grooving
The plates of
early locomotive boilers were joined by simple overlapping
joints. This practice was satisfactory for the annular joints,
running around the boiler, but in longitudinal joints, along the length of the
boiler, the overlap of the plates diverted the boiler cross-section from its
ideal circular shape. Under pressure the boiler strained to reach, as nearly as
possible, the circular cross-section. Because the double-thickness overlap was
stronger than the surrounding metal, the repeated bending and release caused by
the variations in boiler pressure caused internal cracks, or grooves (deep
pitting), along the length of the joint. The cracks offered a starting point
for internal corrosion, which could hasten failure.[6]
It was eventually found that this internal corrosion could be reduced by using
plates of sufficient size so that no joins were situated below the water level.[7][8]
Eventually the simple lap seam was replaced by the single or double butt-strap
seams, which do not suffer from this defect.
Due to the
constant expansion and contraction of the firebox a similar form of
"stress corrosion" can take place at the ends of staybolts where they
enter the firebox plates, and is accelerated by poor water quality. Often
referred to as "necking", this type of corrosion can reduce the
strength of the staybolts until they are incapable of supporting the firebox at
normal pressure.
Grooving (deep,
localized pitting) also occurs near the waterline, particularly in boilers that
are fed with water that has not been de-aerated or treated with oxygen
scavenging agents. All "natural" sources of water contain dissolved
air, which is released as a gas when the water is heated. The air (which
contains oxygen) collects in a layer near the surface of the water and greatly
accelerates corrosion of the boiler plates in that area.[9]
Firebox
The intricate
shape of a locomotive firebox, whether made of soft copper or of steel, can
only resist the steam pressure on its internal walls if these are supported by stays attached to internal girders and the
outer walls. They are liable to fail through fatigue (because the inner and outer walls
expand at different rates under the heat of the fire), from corrosion, or from
wasting as the heads of the stays exposed to the fire are burned away. If the
stays fail the firebox will explode inwards. Regular visual inspection,
internally and externally, is employed to prevent this.[10][7]
Even a well-maintained firebox will fail explosively if the water level in the
boiler is allowed to fall far enough to leave the top plate of the firebox
uncovered.[11]
Steamboat boilers
Steamboat
explodes in Memphis, Tennessee in 1830
SS Ada
Hancock, a small steamboat used to transfer passengers and cargo to and from
the large coastal steamships that stopped in San Pedro
Harbor in the early 1860s, suffered disaster when its boiler
exploded violently in San Pedro Bay, the port of Los Angeles,
near Wilmington, California on April 27, 1863 killing
twenty-six people and injuring many others of the fifty-three or more
passengers on board.
The steamboat Sultana was destroyed in an explosion
on 27 April 1865, resulting in the greatest maritime disaster in United States
history. An estimated 1,700 passengers were killed when one of the ship's four
boilers exploded and the Sultana sank not far from Memphis, Tennessee.
Another US
Civil War Steamboat explosion was the Steamer Eclipse on January 27,
1865, which was carrying members of the 9th Indiana Artillery.
One official Records report mentions the disaster reports 10 killed and 68
injured;[12]
a later report mentions that 27 were killed and 78 wounded.[13]
Fox's Regimental Losses reports 29 killed.[14][15]
Use of boilers
The stationary steam engines used to power
machinery first came to prominence during the industrial revolution, and in the early
days there were many boiler explosions from a variety of causes. One of the
first investigators of the problem was William
Fairbairn, who helped establish the first insurance company dealing
with the losses such explosions could cause. He also established experimentally
that the hoop stress
in a cylindrical pressure vessel like a boiler was twice the longitudinal stress.[notes 1]
Such investigations helped him and others explain the importance of stress concentrations in weakening
boilers.
Modern boilers
Modern boilers
are designed with redundant pumps, valves, water level monitors, fuel cutoffs,
automated controls, and pressure relief valves.
In addition, the construction must adhere to strict engineering guidelines set
by the relevant authorities. The NBIC,
ASME, and others attempt
to ensure safe boiler designs by publishing detailed standards. The result is a
boiler unit which is less prone to catastrophic accidents.
Also improving
safety is the increasing use of "package boilers." These are boilers
which are built at a factory then shipped out as a complete unit to the job
site. These typically have better quality and fewer problems than boilers which
are site assembled tube-by-tube. A package boiler only needs the final connections
to be made (electrical, breaching, condensate lines, etc.) to complete the
installation.
Explosions
In steam
locomotive boilers, as knowledge was gained by trial and
error in early days, the explosive situations and consequent damage
due to explosions were inevitable. However, improved design and maintenance markedly reduced the number of
boiler explosions by the end of the 19th century. Further improvements
continued in the 20th century.
On land-based
boilers, explosions of the pressure systems happened regularly in stationary
steam boilers in the Victorian era, but are now very rare because of
the various protections
provided, and because of regular inspections compelled by governmental
and industry requirements.
See also: List of Boiler Explosions
Locomotive boiler explosions in the UK
Aftermath of a
boiler explosion on a railway locomotive circa 1850.
Hewison (1983)[16]
gives a comprehensive account of British boiler explosions, listing 137 between
1815 and 1962. It is noteworthy that 122 of these were in the 19th century and
only 15 in the 20th century.
Boiler
explosions generally fell into two categories. The first is the breakage of the
boiler barrel itself, through weakness/damage or excessive internal pressure,
resulting in sudden discharge of steam over a wide area. Stress corrosion cracking at the lap joints
was a common cause of early boiler explosions, probably caused by caustic embrittlement. The water used in
boilers was not often closely controlled, and if acidic, could corrode the wrought iron
boiler plates. Galvanic corrosion was an additional problem
where copper
and iron were in contact. Boiler plates have been thrown up to a quarter of a
mile (Hewison, Rolt). The second type is the collapse of the firebox under
steam pressure from the adjoining boiler, releasing flames and hot gases into
the cab. Improved design and maintenance almost totally eliminated the first
type, but the second type is always possible if the engineer and fireman do not
maintain the water level in the boiler.
Boiler barrels
could explode if the internal pressure became too high. To prevent this, safety
valves were installed to release the pressure at a set level. Early examples
were spring-loaded, but John Ramsbottom invented a tamper-proof
valve which was universally adopted. The other common cause of explosions was
internal corrosion
which weakened the boiler barrel so that it could not withstand normal
operating pressure. In particular, grooves could occur along horizontal seams (lap
joints) below water level. Dozens of explosions resulted, but were eliminated
by 1900 by the adoption of butt joints, plus improved maintenance schedules and
regular hydraulic testing.
Fireboxes were
generally made of copper,
though later locomotives had steel fireboxes. They were held to the outer part of the
boiler by stays (numerous small supports). Parts of the firebox in contact with
full steam pressure have to be kept covered with water, to stop them
overheating and weakening. The usual cause of firebox collapses is that the
boiler water level falls too low and the top of the firebox (crown sheet)
becomes uncovered and overheats. This occurs if the fireman has failed to
maintain water level or the level indicator (gauge glass) is faulty. A less
common reason is breakage of large numbers of stays, due to corrosion or
unsuitable material.
Throughout the
20th century, two boiler barrel failures and thirteen firebox collapses
occurred in the UK. The boiler barrel failures occurred at Cardiff in 1909 and
Buxton in 1921; both were caused by misassembly of the safety valves
causing the boilers to exceed their design pressures. Of the 13 firebox
collapses, four were due to broken stays, one to scale buildup on the firebox,
and the rest were due to low water level.
Event | Date | Type | Nation | City/Locale | Killed | Injured | Missing |
---|---|---|---|---|---|---|---|
Savery Explosion[1] | 1716 | Industrial | United Kingdom | 1 | |||
Brunton's Mechanical Traveller | 1815 | Locomotive | United Kingdom | Philadelphia, Tyne and Wear | 16 | ||
The Washington[2] | 1816 | Marine (Civilian) | United States | Marietta, Ohio | 12 | 5 | |
The Aetna[2] | May 15, 1824 | Marine (Civilian) | United States | New York City, New York | 10+ | 40~ | |
Locomotion No 1 | 1828 | Locomotive | United Kingdom | 1 | |||
Best Friend of Charleston | June 17, 1831 | Locomotive | United States | Charleston, South Carolina | 3 | ||
Explosion of the PS Union | 1837 | Packet steamer | Hull, UK | 20+[3] | more | ||
SS Moselle [4] | April 28, 1838 | Marine (Civilian) | United States | Cincinnati, Ohio | 81 | 13 | 55 |
SS Henry Eckford | April 27, 1841 | Marine (Civilian) | United States | New York City, New York | 1 | 2 | |
Firth Woolen Mill[5] | 1850 | Industrial | United Kingdom | Halifax, West Yorkshire | 10 | ||
The Hague Street Explosion[6] | February 4, 1850 | Industrial | United States | New York City, New York | 67 | 50~ | |
SS Canemah | August 8, 1853 | Marine (Civilian) | United States | Champoeg, Oregon | 1 | ||
SS Shoalwater | May 1853 | Marine (Civilian) | United States | Rock Island, Oregon | |||
SS Gazelle | April 8, 1854 | Marine (Civilian) | United States | Oregon City, Oregon | 1 | ||
Fieldhouse Mills[7] | July 15, 1855 | Industrial | United Kingdom | Rochdale, Greater Manchester | 10 | 13 | |
SS Pennsylvania | June 13, 1858 | Marine (Civilian) | United States | Memphis, Tennessee | 250+ | ||
Hembrigg Mills[8] | June 27, 1863 | Industrial | United Kingdom | Morley, West Yorkshire | 9 | ||
SS Ada Hancock | April 23, 1863 | Marine (Civilian) | United States | San Pedro Bay, California | 26+ | 23 | |
Calcutta Explosion | 1863 | India | Kolkata, West Bengal | ||||
USS Chenango | April 15, 1864 | Marine (Military) | United States | New York City New York | 33 | ||
SS Sultana | April 27, 1865 | Marine (Civilian) | United States | Memphis, Tennessee | 1800~ | ||
Town & Son Factory[9] | June 9, 1869 | Industrial | United Kingdom | Bingley, West Yorkshire | 15 | ||
SS Senator | May 6, 1875 | Marine (Civilian) | United States | Portland, Oregon | |||
HMS Thunderer | July 14, 1876 | Marine (Military) | United Kingdom | Portsmouth Harbour | 45 | 70~ | |
Railway Explosion | December 21, 1881 | Locomotive | Australia | New South Wales | |||
Queensland Railways | 1890 | Locomotive | Queensland | ||||
North East Dundas Tramway | June 18, 1892 | Locomotive | Australia | Tasmania | |||
SS Annie Faxon | April 14, 1893 | Marine (Civilian) | United States | Columbia River | 8 | ||
Zeehan | May 1, 1899 | Locomotive | Australia | Tasmania | |||
Grover Shoe Factory disaster | March 20, 1905 | Industrial | United States | Brockton, Massachusetts | 58 | 150 | |
HMS Implacable | July 12, 1905 | Marine (Military) | United Kingdom | 2 | |||
USS Bennington | July 21, 1905 | Marine (Military) | United States | San Diego, California | 66 | ||
HMS Implacable | August 16, 1906 | Marine (Military) | United Kingdom | 2 | |||
SS Sarah Dixon | January 18, 1912 | Marine (Civilian) | United States | Kalama, Washington | 3 | ||
Southern Pacific Roundhouse | March 18, 1912 | Locomotive | United States | San Antonio, Texas | 26 | ||
Rumney Shed | April 21, 1912 | Locomotive | United Kingdom | Cardiff, Wales | |||
SS City of Liverpool[10] | February 24, 1913 | Marine (Civilian) | United Kingdom | Manchester Ship Canal, Nr Runcorn | 1 | ||
Destroyer Ikazuchi | October 9, 1913 | Marine (Military) | Japan | Ominato | |||
Buxton | November 11, 1921 | Locomotive | United Kingdom | Buxton, Derbyshire | |||
LMS 6399 Fury | February 10, 1930 | Locomotive | United Kingdom | 1 | |||
Canadian National 242 | August 9, 1941 | Locomotive | Canada | Montreal, Quebec | 1 | 1 | |
PRR 520 | November 14, 1942 | Locomotive | United States | Cresson, Pennsylvania | 2 | 4 | |
SS Cascades | 1943 | Marine (Civilian) | United States | Portland, Oregon | |||
USATC S160 Class Failures | 1943, 1944 | Locomotive | United Kingdom | ||||
Union Pacific 9000 Class | October 20, 1948 | Locomotive | United States | 3 | |||
British Railways | January 24, 1962 | Locomotive | United Kingdom | Bletchley, Buckinghamshire | 0 | 2 | |
Bitterfeld Railway | November 27, 1977 | Locomotive | East Germany | Bitterfeld | 9 | 45 | |
USS Willamette | June 29, 1995 | Marine (Civilian) | United States | 0 | 7 | ||
Medina County Fair Ground [11][12] | July 29, 2001 | Antique Steam Tractor | United States | Medina County | 5 | ~40 | |
SS Norway | May 25, 2003 | Marine (Civilian) | United States | Miami, Florida | 8 | 17 |
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