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A boiler explosion is a catastrophic failure of a boiler.
There are two types of boiler explosions. One type 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 was a common cause of early boiler 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 Safety provided, and because of regular inspections compelled by governmental and industry requirements.
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.
If steam is released normally, say by opening a throttle valve, the bubbling action of the water remains moderate and relatively dry steam can be drawn from the highest point in the vessel.
If steam is released more quickly, the more vigorous boiling action that results can throw a fine spray of droplets up as "wet steam" which can cause damage to piping, engines, turbines and other equipment downstream.
If a large crack or other opening in the boiler vessel allows the internal pressure to drop very suddenly, the heat energy remaining in the water will cause even more of the liquid to flash into steam bubbles, which then rapidly displace the remaining liquid. The potential energy of the escaping steam and water are now transformed into work, just as they would have done in an engine; with enough force to 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 rapid release of steam and water can provide a very potent blast, and cause great damage to surrounding property or personnel. A failure of this type qualifies as a boiling liquid expanding vapor explosion (BLEVE).
The rapidly expanding steam bubbles can also perform work by throwing large "slugs" of water inside the boiler in the direction of the opening, and at astonishing velocities. A fast-moving mass of water carries a great deal of kinetic energy, 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.The Colliery Engineers Company (1900) Locomotive Boilers (I.C.S. Reference Library #59) Stationer's Hall, London: International Textbook Company (sec.12, p. 76)
Many plumbers, firefighters, and are aware of this phenomenon, which is called "water hammer". A several-ounce "slug" of water passing through a steam line at high velocity 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.
Several accounts of the SL-1 experimental reactor accident vividly describe the incredibly powerful effect of water hammer on a pressure vessel:
A steam locomotive operating at would have a temperature of about , and a specific enthalpy of . Since standard pressure saturated water has a specific enthalpy of just , the difference between the two specific enthalpies, , is the total energy expended in the explosion. So in the case of a large locomotive which can hold as much as of water at a high pressure and temperature state, this explosion would have a theoretical energy release equal to about of TNT equivalent.
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. This usually causes no damage in locomotive type boilers, but can cause cracks in masonry boiler settings if allowed to continue.
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.
Before materials science, inspection standards, and quality control caught up with the rapidly growing boiler manufacturing industry, a significant number of boiler explosions were directly traceable to poor design, workmanship, and undetected flaws in poor quality materials. The alarming frequency of boiler failures in the U.S. due to defects in materials and design were attracting the attention of international engineering standards organizations, such as the ASME, which established their first Boiler Testing Code in 1884. The boiler explosion that caused the Grover Shoe Factory disaster in Brockton, Massachusetts, on 10 March 1905, resulted in 58 deaths and 150 injuries, and inspired the state of Massachusetts to publish its first boiler laws in 1908.
Several written sources provide a concise description of the causes of boiler explosions:
And:
While deterioration and mishandling are probably the most common causes of boiler explosions, the actual mechanism of a catastrophic boiler failure was not well documented until extensive experimentation was undertaken by U.S. boiler inspectors in the early 20th century. Several different attempts were made to cause a boiler to explode by various means, but one of the most interesting experiments demonstrated that in certain circumstances, if a sudden opening in the boiler allowed steam to escape too rapidly, water hammer could cause destruction of the entire pressure vessel:
But the highly destructive mechanism of water hammer in boiler explosions was understood long before then, as D. K. Clark wrote on 10 February 1860, in a letter to the editors of Mechanics Magazine:
Boiler explosions are common in sinking ships once the hot boiler touches cold sea water, as the sudden cooling of the hot metal causes it to crack; for instance, when the was torpedoed by a U-boat, the torpedoes and resulting boiler explosion caused the ship to go down in two minutes, leaving Poon Lim as the only survivor in a complement of 53 crew.
This was the cause of the Gettysburg Railroad firebox explosion near Gardners, Pennsylvania, in 1995, where low water allowed the front of the crown sheet to overheat until the regular crown stays pulled through the sheet, releasing a great deal of steam and water under full boiler pressure into the firebox. The crown sheet design included several alternating rows of button-head safety stays, which limited the failure of the crown sheet to the first five or six rows of conventional stays, preventing a collapse of the entire crown sheet.
This type of failure is not limited to railway engines, as locomotive-type boilers have been used for traction engines, portable engines, skid engines used for mining or logging, stationary engines for sawmills and factories, for heating, and as providing steam for other processes. In all applications, maintaining the proper water level is essential for safe operation.
Hewison (1983)Hewison, Locomotive Boiler Explosions 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 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 driver 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 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.
, a small steamboat used to transfer passengers and cargo to and from the large coastal 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 27 April 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 1,549 passengers were killed when three of the ship's four boilers exploded and the Sultana burned and sank not far from Memphis, Tennessee. The cause was traced to a poorly executed repair to the shell of one boiler; the patch failed, and debris from that boiler ruptured two more.
Another US Civil War steamboat explosion was the steamer Eclipse on 27 January 1865, which was carrying members of the 9th Indiana Artillery. One official record reports 10 killed and 68 injured; a later report mentions that 27 were killed and 78 wounded. Fox's Regimental Losses reports 29 killed.
The boiler of Canada's PS Waubuno may have exploded on the ship's final voyage in 1879, though the cause of the sinking remains unknown. An explosion could have occurred due to negligent upkeep or to contact with the cold water of Georgian Bay while foundering in a storm.
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 issues 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.
| 1840 | Henry R. Worthington invents boiler feed water pump | Equipment | Automatic boiler feed water system enabling adding water to a boiler while at operating pressure. | |
| 1847 | Institution of Mechanical Engineers | Technical society | IMechE formed, emphasizing the importance of specialized mechanical knowledge, particularly with respect to steam power (see also Institution of Civil Engineers). | |
| 1855 | Steam Users' Association | Technical society | In Manchester, the Association for the Prevention of Steam Boiler Explosions, and for effecting Economy in the Raising and Use of Steam is formed and, eschewing direct regulation, advocates creation of trained inspectors. Later adds the prefix "Manchester Steam Users' ..." to the name. | |
| 1855 | Ramsbottom safety valve | Equipment | John Ramsbottom invented a tamperproof safety valve. | |
| 1864 | Bengal Act VI of 1864 | Legislation | Provided for the inspection of steam boilers in and around Kolkata. | |
| 1866 | The Hartford Steam Boiler Inspection and Insurance Company | Commercial | The first boiler insurance company in the U.S. is established in Hartford, Connecticut. | |
| 1866 | Gesellschaft zur Überwachung und Versicherung von Dampfkesseln | Technical society | In response to a brewery explosion, a private society is founded to offer boiler inspections to its members. It is highly successful and later becomes the modern TÜV. | |
| 1880 | American Society of Mechanical Engineers | Technical society | ASME formed, largely in response to calls for improvements in boiler safety | |
| 1882 | Boiler Explosions Act 1882 | Legislation | Required notice of a boiler explosion to be sent to the Board of Trade within 24 hours of occurrence and established inquiry authorizations. | |
| 1884 | ASME Boiler Testing Code | Safety standard | The "Code for the Conduct of Trials of Steam Boilers", the first U.S. code for conducting boiler tests, is issued. | |
| 1887 | Robert Henry Thurston's book Steam Boiler Explosions in Theory, and in Practice | Book | ||
| 1890 | Boiler Explosions Act 1890 | Legislation | Extended 1882 requirements to marine vessels. | |
| 1911 | Uniform Boiler Rules, Massachusetts | Legislation | The Massachusetts adopts uniform boiler rules, the first statewide boiler code to apply in the U.S. Equivalent rules are quickly adopted by other states (e.g., Ohio). | |
| 1915 | ASME Boiler Code | Safety standard | The ASME Boiler Code Committee issues "Standards for Specifications and Construction of Boilers and Other Containing Vessels in Which High Pressure is Contained". | |
| 1919 | The National Board of Boiler and Pressure Vessel Inspectors | Safety standard | Formed to "promote greater safety to life and property through uniformity in the construction, installation, repair, maintenance, and inspection of pressure equipment". |
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