Create account Log in Article Talk Read Edit ViewMore historySearch Wiki Loves Earth in focus during May 2015 Discover nature, make it visible, take photos, help Wikipedia! Main page Contents Boiler explosion Featured content From Wikipedia, the free encyclopedia Current events Random article Donate to Wikipedia This article may need to be rewritten entirely to comply with Wikipedia's quality standards. You Wikipedia store can help. The discussion page may contain suggestions. (May 2009) Interaction A 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 Help 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 About Wikipedia Community portal water level. Corrosion along the edges of lap joints was a common cause of early boiler explosions. Recent changes 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- Contact page fired boilers are rare, but firebox explosions in gas or oil-fired boilers are still a potential hazard. Tools What links here Contents [hide] Related changes 1 Causes of boiler explosions Upload file 2 Locomotive-type boiler explosions Special pages 3 Principle Permanent link 4 Firebox explosions Page information 4.1 Grooving Wikidata item 4.2 Firebox Cite this page 5 Steamboat boilers Print/export 6 Use of boilers Create a book 7 Modern boilers Download as PDF 8 Steam explosions Printable version 8.1 Reactor explosions 9 Locomotive boiler explosions in the UK Languages 10 See also Deutsch 11 Notes Ελληνικά 12 Bibliography Русский 13 References Українська Edit links 14 Further reading 15 External links Causes of boiler explosions [edit] "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] Boiler explosions are common in sinking ships once the superheated boiler touches cold sea water, as the sudden cooling of the superheated metal causes it to crack; for instance, when the SS Ben Lomond 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 54 crew.[3][4] Locomotive-type boiler explosions [edit] 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 [edit] 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.[5] 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.[6] Since standard pressure saturated water has a specific enthalpy of just 418.91 kJ/kg,[7] 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 [edit] 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. This usually causes no damage in locomotive type boilers, but can cause cracks in masonry boiler settings if allowed to continue. Grooving [edit] 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.[8] It was eventually found that this internal corrosion could be reduced by using plates of sufficient size so that no joints were situated below the water level.[9][10] 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.[11] Firebox [edit] 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.