![]() ![]() These materials had to withstand the operating temperatures of 1000 ☏, provide an airtight seal for the negative 12 inch water gage static pressure during normal operating conditions and not be blown off the door during a deflagration event. Among the custom made items that had to be selected were a gasketing material for the doors and the seal material for the joints in the walls. The erector later decided to preassemble the wall panels into boxes for each of the 4 tiers, to be erected like the layers of a wedding cake during the shutdown. Door size is important because the larger the size, the greater the rotational moment of inertia resisting the door opening rapidly when a deflagration event occurs and consequently the higher the corresponding release pressure, Pred, that had to be sustained by the roof and walls of the box as well as the doors and their hinges. ![]() To maintain appropriate door sizes, the box was designed in 4 tiers of about 10 ft height with the walls panelized in approximately 20 ft in lengths. To facilitate the speed of reconstruction it was decided to repair and reuse the existing hoopers which were not severely damaged. Because of the large uncertainties associated with the design parameters, it was agreed that the box would be a structurally robust design with strength in excess of the calculated demand and that all available surface area on the walls of the Spark Box would be used to locate pressure relief doors, thus providing a relief area in excess of the NFPA requirements and providing the maximum opportunity for successful performance.įor functional considerations, the new design was to duplicate the original Spark Box in size, 60 ft wide x 20 ft deep x 40 ft tall with a triple hopper bottom and internal baffle wall. There was full understanding with the client that the system would not work if the combustible gas was hydrogen. ![]() The decision was made to base the design on the most likely culprit, which was CO, as mentioned above. In order to meet the schedule, the permissible time for each phase was worked backwards to allow time for fabrication and shop drawing preparation and design drawings. The installation of the new Spark Box was to be made during the regular August shut down. The client purchased a new gas sampling system that could sample fast enough that it might capture the fluctuations in the gas content when a deflagration occurred, but it would not be operational for several months. This is a different phenomenon than an "explosion", which is the ignition of an explosive material, which in turn is a material that already contains both the combustible substance and the oxygenating agent within itself.ĭue to the time constraints, the design had to proceed without definitive input information. The phenomena that was occurring was a "deflagration", which is a rapidly burning flame front passing through a mixture of a combustible material, either gas or dust, and air. The methodology to be used was the procedures contained in NFPA 68, "Guide for Venting Deflagrations". The proposed solution, which was acceptable to the client, was to design a new Spark Arrestor Box with doors which would open to relieve the overpressure before the box would be damaged. ![]() There was a possibility that the ingredient responsible for producing the explosive gases was Direct Reduced Iron.The ingredients for a given grade can also vary depending on the current cost of the raw material. The composition of the gases varies depending on the ingredients used to produce the desired grade of steel.Two possible candidates for the combustible gas responsible for the explosions were carbon monoxide (CO) and hydrogen, both of which are by-products of the steel making process.There are many impurities on the scrap steel which produce combustible gases.The constituents of the gas stream at the time of the events were not known.The client knew that the problem had never occurred in the previous 8 years of operation since the plant was commissioned.The 1000 ☏ hot gases from the Electric Arc Furnaces, which melt the scrap steel, are pulled from the furnaces through 10 ft diameter ducts, then through the Spark Arrestor Box where any larger pieces of slag entrained in the airstream are dropped out before the gasses pass through the exhaust fans and then are pushed out through the bag house to the atmosphere. At a local Recycling Steel Mill, a "Mini-Mill", two explosions occurred within a two week period in the Spark Arrestor Box causing severe damage and shutting down the mill for several days each time while repairs were made. ![]()
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