A boiler explosion is a catastrophic failure of a boiler. 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 firebox containing the burning fuel, 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.
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A boiler explosion is a kind of boiling liquid expanding vapor explosion.
Boiler explosions are particularly devastating because of the huge amount of energy stored in the heated liquid water. A basic fire-tube boiler operating at 50 psi (340 kPa) contains water at a temperature of roughly 150 °C (300 °F). Checking against a steam table, we see the water will release 2.74 MJ/kg as it flashes to steam and condenses again. This is 66% of the energy per kilogram which TNT releases when detonated. A typical locomotive boiler holds over 10,000 liters of pressurized water under these conditions. Based on the data above, if such a boiler were to catastrophically fail, it would release approximately 1.6 times the energy of a RAF Blockbuster bomb - the largest conventional munition used during World War II.
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.
These boilers are of a fire-tube type with coke, wood, coal or oil used as fuel. The water feed is by means of steam-powered injectors or boiler feedwater pumps. 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 aka water gauge are provided for corrective action by the locomotive crew. Should that gauge fail, the crew cabin will fill with scalding steam, making it near to impossible for the crew to take corrective action. For this reason the glass gauges are provided with a thick outer glass screen as additional protection. It is also common practice to provide the gauges with steam valves so they may be isolated, and to fit them in pairs, allowing one to be replaced in the event of wear or failure while the boiler remains operational.
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 longitudinal joints, along the length of the boiler, 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, along the length of the joint. The cracks offered a starting point for internal corrosion, which could hasten failure.[1] 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.[2][3]
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.[4][2] 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.[5]
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;[6] a later report mentions that 27 were killed and 78 wounded.[7] Fox's Regimental Losses reports 29 killed.[8][9]
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 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.
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.
Hewison (1983)[10] 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 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 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. 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.
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