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 lap joints 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 protections 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.
Many shell-type boilers carry a large bath of liquid water which is heated to a higher temperature and pressure (enthalpy) than boiling water would be 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 until saturation pressure is reached, then the boiling stops. If some pressure is released, boiling begins again, and so on.
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.
Many plumbers, firefighters, and steamfitters 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:
The expansion caused by this heating process caused water hammer as water was accelerated upwards toward the reactor vessel head, producing approximately 10,000 pounds per square inch (69,000 kPa) of pressure on the head of the reactor vessel when water struck the head at 160 feet per second (50 m/s) ... This extreme form of water hammer propelled control rods, shield plugs, and the entire reactor vessel upward. A later investigation concluded that the 26,000-pound (12,000 kg) vessel had jumped 9 feet 1 inch (2.77 m) and the upper control rod drive mechanisms had struck the ceiling of the reactor building prior to settling back into its original location.
A steam locomotive operating at 350 psi (2,400 kPa) would have a temperature of about 220 °C (400 °F), and a specific enthalpy of 960 kJ/kg (440 kJ/lb). Since standard pressure saturated water has a specific enthalpy of just 420 kJ/kg (190 kJ/lb), the difference between the two specific enthalpies, 540 kJ/kg (240 kJ/lb), 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 (22,000 lb) of water at a high pressure and temperature state, this explosion would have a theoretical energy release equal to about 1,200 kilograms (2,600 lb) of TNT.
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 volatilize 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.
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. 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. 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.