A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, it occurs when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten metals (as in a fuel–coolant interaction, or FCI, of molten nuclear-reactor fuel rods with water in a nuclear reactor core following a core-meltdown). Steam explosions are instances of explosive boiling. Pressure vessels, such as pressurized water (nuclear) reactors, that operate above atmospheric pressure can also provide the conditions for a steam explosion. The water changes from a solid or liquid to a gas with extreme speed, increasing dramatically in volume. A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions (if not otherwise confined, e.g. by the walls of a container), creating a danger of scalding and burning. Steam explosions are not normally chemical explosions, although a number of substances react chemically with steam (for example, zirconium and superheated graphite (impure carbon, C) react with steam and air respectively to give off hydrogen (H2), which may explode violently in air (O2) to form water or H2O) so that chemical explosions and fires may follow. Some steam explosions appear to be special kinds of boiling liquid expanding vapor explosion (BLEVE), and rely on the release of stored superheat. But many large-scale events, including foundry accidents, show evidence of an energy-release front propagating through the material (see description of FCI below), where the forces create fragments and mix the hot phase into the cold volatile one; and the rapid heat transfer at the front sustains the propagation. Mechanisms of Steam Explosions The initiation of steam explosions while not fully understood and many areas remain a subject of ongoing research (unknown areas usually consist of incidents at large scales).
Examples
High steam generation rates can occur under other circumstances, such as boiler-drum failure, or at a quench front (for example when water re-enters a hot dry boiler). Though potentially damaging, they are usually less energetic than events in which the hot ("fuel") phase is molten and so can be finely fragmented within the volatile ("coolant") phase. Some examples follow:
Natural Steam explosions are naturally produced by certain volcanoes, especially stratovolcanoes, and are a major cause of human fatalities in volcanic eruptions. They are often encountered where hot lava meets sea water or ice. Such an occurrence is also called a littoral explosion. A dangerous steam explosion can also be created when liquid water or ice encounters hot, molten metal. As the water explodes into steam, it splashes the burning hot liquid metal along with it, causing an extreme risk of severe burns to anyone located nearby and creating a fire hazard.
Boiler explosions
When a pressurized container such as the waterside of a steam boiler ruptures, it is always followed by some degree of steam explosion. A common operating temperature and pressure for a marine boiler is around 950 psi (6,600 kPa) and 850 °F (454 °C) at the outlet of the superheater. A steam boiler has an interface of steam and water in the steam drum, which is where the water is finally evaporating due to the heat input, usually oil-fired burners. When a water tube fails due to any of a variety of reasons, it causes the water in the boiler to expand out of the opening into the furnace area that is only a few psi above atmospheric pressure. This will likely extinguish all fires and expands over the large surface area on the sides of the boiler. To decrease the likelihood of a devastating explosion, boilers have gone from the "fire-tube" designs, where the heat was added by passing hot gases through tubes in a body of water, to "water-tube" boilers that have the water inside of the tubes and the furnace area is around the tubes. Old "fire-tube" boilers often failed due to poor build quality or lack of maintenance (such as corrosion of the fire tubes, or fatigue of the boiler shell due to constant expansion and contraction). A failure of fire tubes forces large volumes of high pressure, high temperature steam back down the fire tubes in a fraction of a second and often blows the burners off the front of the boiler, whereas a failure of the pressure vessel surrounding the water would lead to a full and entire evacuation of the boiler's contents in a large steam explosion. On a marine boiler, this would certainly destroy the ship's propulsion plant and possibly the corresponding end of the ship. Tanks containing crude oil and certain commercial oil cuts, such as some diesel oils and kerosene, may be subject to boilover, an extremely hazardous situation in which a water layer under an open-top tank pool fire starts boiling, which results in a significant increase in fire intensity accompanied by violent expulsion of burning fluid to the surrounding areas. In many cases, the underlying water layer is superheated, in which case part of it goes through explosive boiling. When this happens, the abruptness of the expansion further enhances the expulsion of blazing fuel. Boil over overview Although the general concept of boil over is known, it is essential to understand the particularities of heat transfer involved to ensure proper safety engineering. According to Broeckmann and Schecker 1995, the phenomenon does not merely consist in a phase change. The boil over process entails a heat layer penetrating the fuel layer progressively from above due to distillation phenomena. Once it comes into contact with the layer of water below, an explosive flash can take place. This is amplified by the viscosity of the oil, which hinders the growth of steam bubbles. As a result, the pressure inside the system steadily rises, leading to a sudden release of accumulated energy. Another finding that the authors present is that the explosiveness of the event is positively related to the degree of "superheating" of the water layer. In other words, the temperature in the layer of water could be elevated to a significantly higher temperature than that of ordinary boiling water, thus resulting in even more catastrophic consequences than in conventional steam boiler explosions.
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