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Molten Metal Splash: The Most Visible Foundry Hazard
Wet charge materials are a serious safety hazard in all foundries. When molten metal comes in contact with any water, moisture or liquid-bearing material, the water instantaneously turns to steam, expanding to 1600 times its original volume and producing a violent explosion. This occurs without warning and throws molten metal and possibly high-temperature solids out of the furnace and puts workers, the plant, furnace and related equipment at risk. A water/molten metal explosion can occur in any type of furnace. For an induction furnace, the aftereffects may be more serious and include the possibility of additional explosions caused by liquid in a ruptured cooling system lines coming in contact with molten metal. Molten metal need not be present in the furnace for an explosion to occur. Explosions also can occur if sealed drums or containers are charged into an empty but hot furnace. In this case, the force of the explosion can eject the newly charged material and quite likely damage the refractory lining as well.
This image was taken from a video of a real furnace eruption, probably caused by wet scrap dropped into the furnace.
________________________________________________ Eliminating Wet Scrap In foundries where most of the charge originates as scrap, wet charge materials pose the greatest cause for concern. Some foundries reduce the possibility of water/molten metal explosions by storing scrap undercover for at least one day and then carefully inspecting the charge for any residual moisture. A more reliable solution being used by an increasing number of foundries today is to use remote charging systems with charge dryers or preheaters. Remote charging systems permit the operator to be away from the furnace or behind protective screens during charging. Dryers and preheaters maximize the removal of water and moisture before the scrap enters the bath.
________________________________________________ Sealed Containers An easily overlooked danger is posed by sealed containers and sections of tubing or piping that are sheared, closing the ends. Containers holding combustible liquids or their fumes will explode long before the scrap itself melts. Preheating sealed materials will not prevent this hazard. Aerosol cans, oxygen cylinders, propane tanks, acetylene tanks and shock absorbers must never be used as charge material. In fact, there is a risk that a sealed container will explode inside the preheating systems. Operator vigilance is the only preventive measure. Even though they do not contain combustible liquids, the air inside them can rapidly expand in the heat. In extreme cases, the pressure buildup will be sufficient to breach the container wall or escape through a sheared-closed end. If this occurs, the forceful expulsion of gas can propel the hot scrap out of the furnace or smash it into the furnace lining, causing damage.
________________________________________________ Other Hazards Cold charges, tools, cold aluminum “sow” molds and easily fragmented materials pose a special hazard for induction furnaces and their operating personnel because they may contain a thin layer of surface or absorbed moisture. On contact with the bath, the moisture turns to steam, causing spitting or splashing. Proper protective clothing and face and eye protection normally will protect the operator. Preheating the charge and tools helps prevent many splashing injuries.
During normal pours, sparks can ignite flammable clothing causing serious injury if workers are not properly protected. In ferrous metal foundries the greatest splashing risk occurs toward the end of the melt, when a foundry worker adds ferro-alloys or introduces tools into the melt. Ferro-alloy materials can absorb moisture from their surroundings. Sampling spoons and slag rakes collect moisture as a thin film of condensation. Following manufacturers’ instructions for storing alloying materials and preheating tools minimizes moisture accumulation, reducing the risk of splashing. In a nonferrous foundry, spitting or splashing can accompany the introduction of ingots into the melt, as surface condensation comes in contact with molten metal. Items such as ingots, pigs, sows, etc., must be placed in an empty furnace or on top of foundry returns. When added to an already molten pool, they must be preheated first. Since it is impossible to wring every bit of humidity from the open air, there is always a potential for moisture condensation and splashing. Moisture condensation and absorption tend to increase with time between melts. The greatest splashing hazards, therefore, are likely to occur at the beginning of the work week or workday, or after a furnace has been taken out of service for maintenance. Allowing more time for the initial melt during these start-up periods can help to reduce the potential for splashing hazards.
________________________________________________ Centrifugally-Cast Scrap Rolls Special steps need to be taken when charging a furnace with centrifugally-cast scrap rolls. This type of scrap should not be melted in an induction furnace. The hazard stems from the possibility a roll may contain a ductile inner core surrounded by a brittle outer layer. The different rates of expansion can cause the surface material to explosively separate from the roll, damaging equipment and injuring personnel. If scrap rolls are to be melted, the fragmenting hazard can be minimized by breaking up the scrap before charging. ________________________________________________ Furnace Cooling
For those unfamiliar with induction melting, it may seem unusual for a high-temperature furnace to be equipped with a cooling system that operates by circulating water inside electrical conductors carrying thousands of amps of electric current. Yet without continuous cooling, induction furnaces cannot operate. The furnace coil, which produces the electromagnetic field, is not designed to get hot. Although some heat is conducted from the molten bath through the lining to the coil, most of the heat load on the coil is caused by current flowing through it. This requires that it be continually cooled, not only to increase its electrical efficiency but to prevent it from melting. Typically, the cooling system is built into the coil itself which is made of hollow copper tubing in which the cooling water flows. The water picks up the heat caused by the current as well as heat conducted from the metal through the refractory and carries it to a heat exchanger for removal. If an electrical or mechanical failure damages the pump that circulates the water, a dangerous heat buildup could lead to coil insulation damage, coil arcing, steam buildup and water leaks. These could then lead to a major explosion that could occur within minutes. Therefore, induction furnaces must have a backup cooling system, such as a battery-powered or engine-powered water pump or a city water connection that can be engaged if normal pump operation fails.
________________________________________________ Open Water Systems Require Careful Maintenance
Modern induction systems are typically equipped with closed water cooling systems. Totally closed systems offer the best protection against low water flow caused by scaling or the accumulation of contaminants in the water passages. Open water systems, however, were common in systems built before 1980. For safe operation, open water systems require frequent cleaning, treatment and maintenance as specified in their operating manuals. Without careful maintenance, an accumulation of minerals, dirt, scale and other contaminants will block the cooling water passages, causing components within the power supply and/or furnace to overheat. In the power supply, this overheating could cause the decomposition of insulating materials and produce flammable hydrogen, methane and propane gases. This could produce an explosion resulting in injury or death. In the furnace, loss or restricted flow of cooling water could lead to overheating the copper coil resulting in failure of the tubing. This could produce water leaks leading to a water/molten metal explosion causing injury or death. Because water cooling is crucial to the safe operation of induction furnaces and power supplies, no induction system should be operated without functioning water temperature and flow interlocks. These interlocks must not be bypassed. ________________________________________________ IMPORTANT: |
