during the
2007 New York City steam explosion 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 and 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. Reactor meltdowns typically occur by blowing up the fuel rods, and the fuel rods are blown up in one of two ways. The first is by blowing up the water in the bottom of the vessel with a steam explosion and the other is by blowing up the water above the fuel. The second is called "alpha" mode failure because when the bottom of the reactor vessel blows off, it will not only destroy the reactor core, but it may also destroy the containment structure. A steam explosion is generally classified as a fuel coolant interaction. According to Theofanous et al. 1987 they developed an initial probabilistic assessment by studying this specific scenario. In this study they found that while this type of event can occur physically the chance of it occurring would be nearly zero because the molten fuel and water would have to be nearly totally mixed together., Magallon 2009 which examined experimental records that determined that small steam explosions are expected during a meltdown, but large scale explosions that could destroy modern containment structures are unlikely in a realistic situation. As a result of these types of research, modern reactor safety systems are designed to manage the pressure and cooling conditions and do not consider a massive blast as a worst case blast any longer. The study found that molten aluminum can react with water by reducing water molecules, this reaction forms aluminum oxide and releases both hydrogen gas and energy in the form of additional heat. In conclusion, explosions involving molten aluminum may not be purely steam driven events, and instead, they could be an involved combination of rapid vaporization followed by hydrogen gas production, which may increase the overall severity if ignition conditions are present. This means that the use of water, such as from sprinkler systems, may in some cases worsen the hazard when significant quantities of molten aluminum are present instead of removing the hazard. In a more domestic setting, steam explosions can be a result of trying to extinguish burning oil with water, in a process called
slopover. When oil in a pan is on fire, the natural impulse may be to extinguish it with water; however, doing so will cause the hot oil to superheat the water. The resulting steam will disperse upwards and outwards rapidly and violently in a spray also containing the ignited oil. The correct method to extinguish such fires is to use either a damp cloth or a tight lid on the pan; both methods deprive the fire of
oxygen, and the cloth also cools it down. Alternatively, a non-volatile purpose designed
fire retardant agent or simply a
fire blanket can be used. == Practical uses==