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Bridging Situations Require Immediate Emergency Action
When material charged into the top portion of the furnace is not in contact with the molten metal in the bottom of the furnace, the condition known as “bridging” exists. When bridging occurs, unmelted charge material is no longer serving to moderate the temperature of the bath during the melting cycle. Also, the air gap between the molten metal and the bridge acts as an insulator. The molten metal in the bottom of the furnace, under the impact of melting power, will superheat. This superheating in an induction furnace will occur very rapidly and will soon raise the temperature of the bath above the maximum temperature rating of the refractory. Also, excessive stirring in the bottom of the furnace, due to the small metal mass and high power density, will combine with the high metal temperatures to cause rapid lining erosion or possibly complete refractory failure. Without immediate attention to a “bridging” condition, a run-out or explosion will occur. If the run-out is through the bottom of the furnace, it can cause a fire under the furnace and in the pit area with loss of hydraulics and water cooling. If the molten metal melts through the furnace coil and water comes in contact with the molten metal, the water instantaneously turns into steam with an expansion rate of 1600 to 1. If the water gets under the molten metal, this instantaneous expansion will produce an explosion which could cause injury or death and extensive damage to equipment. Be sure to keep the furnace pit area clean and dry because it is designed to hold molten metal in case of an emergency. Do not allow water or hydraulic fluid to accumulate in the pit. Bridging can occur in any induction furnace and all furnace operators must be trained to be able to recognize bridging and its dangers. All operators must be trained how to solve a bridging problem. ________________________________________________ Warning Signs of Bridging Bridging may reveal itself with one of several warning signs. The clearest warning sign that bridging has occurred is that the melt is taking longer than expected. Rather than increase the power, the operator must switch off power and evacuate all personnel from the area immediately. Under no circumstances should the operator increase power. If ferrous metal is being melted, the chemical reaction which it creates on contact with the furnace lining will, under superheated conditions, produce carbon monoxide. This gas may reveal itself as small blue flames on or in the bridge. The appearance of these flames indicates the bridge may be pressurized, and it must not be breached. In the case of a nonferrous charge, gas production will also occur, but there are no flames or other visible indications. ________________________________________________ In The Event of a Bridge Bridging can be minimized by using proper charge material and by making sure the different sizes of charge material are added correctly. If a bridge occurs, power must be turned off immediately. All personnel must be evacuated from the furnace area until enough time has elapsed to allow the molten metal to solidify.
A. Bridging occurs when a “cap” forms over the top of the furnace, allowing a buildup of superheated gases in the void below. If a bridge develops, power must be turned off immediately. B. The void separating molten metal from the bridge of solid charge material acts as an insulator. C. The molten metal will superheat and the temperature will rise rapidly. ________________________________________________ IMPORTANT: |
