Boiler explosion

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Aftermath of a boiler explosion on a railway locomotive circa 1850.

Boiler explosions are catastrophic failures of boilers. As seen today, boiler explosions are of two kinds. One kind is over-pressure in the pressure parts of the steam and water sides. The second kind is explosion in the furnace. Boiler explosions of pressure parts are particularly associated with steam locomotives. Locomotive boilers are of a construction with a very small hand-fed furnace, a boiler barrel containing boiling water under pressure, and tubes containing hot gases from the fire (a fire tube boiler). In these, the latter type of explosion from the furnace side is practically unknown. There can be many different causes, such as failure of the safety valve or corrosion of critical parts of the boiler. Corrosion at the edges of lap joints was a common cause of early boiler explosions.

Principle

A boiler explosion is a kind of boiling liquid expanding vapor explosion. It is particularly devastating because of the energy stored up in the heated liquid water. As an example, a basic fire-tube boiler steaming at 50 psi (340 kPa) contains water at a temperature of roughly 150 °C (300 °F). Were a catastrophic failure to occur and the vessel depressurized, most of the water would instantly flash into steam. Steam takes up 1,600 times more space than liquid water, meaning that each cubic metre of heated boiler water will expand into 1,600 cubic metres of steam and it will do so in a fraction of a second. That enormous volume of steam will expand outwardly to equalize its pressure with the atmosphere and thereby produce an explosion.

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.

Locomotive boilers

These boilers are of a smoke tube type with wood, coal or oil used as fuel. The water feed is by means of steam-powered injectors or boiler feedwater pumps. For storage of fuel and water a separate tender is often provided, adding to the length of the locomotive. This tender usually has a sloping floor for easy flow of fuel toward the locomotive cab and firebox therein. There is a safety valve included on the steam side and also one or more gauges to warn of low water levels. Any failure of these would result in an explosion of the pressure parts with consequent injury to operating personnel, apart from the damage to equipment. The consequences are more severe due to the restricted working space and constant movement of the locomotives.

Safety valves are provided to operate the pressure parts within safe limits. The water level alarms are provided for corrective action by the locomotive crew.

Steamboat boilers

SS Ada Hancock, a little steamboat used to transfer passengers and cargo to and from the large coastal steamships that stopped in San Pedro Harbor in the early 1860's, 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. The boiler was thought to be the victim of bad construction. Sometimes known as 'the leaky woes'.

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;^[1] a later report mentions that 27 were killed and 78 wounded.^[2] Fox's Regimental Losses reports 29 killed.^[3][4]

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 wth 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. Such investigations helped him and others explain the importance of stress concentrations in weakening boilers.

Modern boilers

Modern boilers are more sophisticated and larger, constructed for stationary use. These boilers are both water tube and high-pressure types. These types are more sophisticated with all necessary protections, primarily used in land based industries and thermal power stations, particularly of power-generating utilities. The installation in these provides sufficient space for operator movement for them to be in a safe place in case of emergency, such as boiler or furnace explosions, as compared to early locomotive boilers.

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 also because of regular inspections compelled by governmental and industry requirements. Furnace side explosions do happen occasionally, in spite of provisions requiring furnace side explosion doors, wrecking the whole boiler mostly due to operators bypassing the operating instructions.

Locomotive boiler explosions in the UK

Hewison (1983) 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 fall 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. 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 traincrew 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 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.

Of the 20th century accidents, just 2 were boiler barrel failures (at Cardiff 1909 and Buxton 1921), and both were caused by misassembly of the safety valves so that the boiler reached a pressure far higher than designed. Of the 13 firebox collapses, 4 were due to broken stays, 1 to scale buildup on the firebox and the remaining 8 were low water level.

See also

• Boiler safety
• Fusible plug
• Grover Shoe Factory disaster
• List of rail accidents (pre-1950)
• William Fairbairn

Bibliography

• Hewison, Christian H. (1983). Locomotive Boiler Explosions. David and Charles. ISBN 0-7153-8305-1. 
• Rolt, L.T.C. (1956 (and later editions)). Red for Danger. Bodley Head / David and Charles / Pan Books. 
• McEwen, Alan (2009). Historic Steam Boiler Explosions. Sledgehammer Engineering Press. ISBN 978-0-9532725-2-5. 

References

1. ^ http://cdl.library.cornell.edu/cgi-bin/moa/pageviewer?frames=1&coll=moa&view=50&root=%2Fmoa%2Fwaro%2Fwaro0103%2F&tif=00622.TIF&cite=http%3A%2F%2Fcdl.library.cornell.edu%2Fcgi-bin%2Fmoa%2Fmoa-cgi%3Fnotisid%3DANU4519-0103
2. ^ http://cdl.library.cornell.edu/cgi-bin/moa/pageviewer?frames=1&coll=moa&view=50&root=%2Fmoa%2Fwaro%2Fwaro0109%2F&tif=00722.TIF&cite=http%3A%2F%2Fcdl.library.cornell.edu%2Fcgi-bin%2Fmoa%2Fmoa-cgi%3Fnotisid%3DANU4519-0109
3. ^ http://www.civilwarhome.com/chapt12.htm
4. ^ 9th Indiana Artillery History, http://www.civilwararchive.com/Unreghst/uninarty.htm#9th 

External links

• http://www.steamlocomotive.com/