A Magnesium Story - The Importance of Proper Process Control
Anyone who has ever seen the bright, blinding light of burning magnesium will never forget it. Here is a case of where a combination of process and equipment decisions resulted in catastrophic consequences – and one that will serve as a valuable reminder of how important our safety systems are. Fortunately, no one was injured. The drying oven involved, however, was a complete loss. Let’s learn more.
The Drying Process
The oven in question was a typical truck oven produced by a reputable company. The unit was designed to process a 19,090-kg (44,000-pound) load of magnesium ingots (Fig. 1). The objective was to dry the ingots prior to melting. Otherwise, rapid vaporization of the latent moisture in the ingots will cause an explosive reaction when the damp metal meets the molten magnesium at 600°C (1100°F).
The process to ensure dryness was to heat the ingots to 204°C (400°F), well above the boiling temperature of any latent water in the ingots. In order to accelerate the heating process, however, the oven was set at 426°C (800°F). This “heat head” greatly reduced the overall cycle time, especially when bringing the ingots up to the process temperature.
Some facts about magnesium are important here. In addition to being highly flammable, magnesium burns at a temperature of approximately 3100°C (5610°F), and the auto-ignition temperature of magnesium is 473°C (883°F).
Oven Heating/Recirculation System
The drying oven used a common combination airflow. The ingots were loaded on top of a traveling load car that rolled inside the oven, after which the doors were closed, and the heating/recirculation system was turned on. The heated air was then delivered from supply ducts located on both sides of the heating chamber, passed through the load of ingots and then returned to the heating/recirculation system on the oven roof (Fig. 2).
Control System and Thermocouple Locations
As in many industrial ovens, a type-J control thermocouple located in the recirculated air stream was used. The thermocouple was wired to a control instrument that sensed the air temperature and adjusted the burner output higher or lower to maintain the desired setpoint temperature of the heated air. It also includes a separate high-limit thermocouple and instrument set at 482°C (900°F), per NFPA standards.
The temperature-control thermocouple was located in the oven, just outside the supply duct, where the heated air first enters the oven chamber after leaving the duct (Fig. 2). This location allows precise control of the process temperature because the air temperature is being sensed just prior to impinging on the load. The burner output is adjusted via a 4-20 mA signal from the controller to maintain the exact burner output required to accurately sustain the temperature of heated air as it enters the load of magnesium.
On the ill-fated magnesium drying oven, the high-limit thermocouple was also located in the heating chamber just outside the supply duct. This “convenient” location near the front of the furnace was less than ideal because the air temperature was not being sensed at the hottest location in the oven. It also left the high-limit thermocouple exposed to the influence of the load itself and also vulnerable to entrainment of cold air entering through the oven doors should they not seal properly or be left open. This location of the thermocouples near the over door turned out to be a critical error.
The Fatal Scenario
During one particular heating cycle, the oven doors were inadvertently left partially open (Fig. 2). This allowed cold air from the factory to begin to enter the oven during heating. At first, this was not a problem because it took several hours for the oven to heat up due to the large mass of magnesium, the load cart, the oven interior sheet metal and structure.
As the oven continued to heat, however, the area inside the front of the oven, where the control thermocouple was measuring, remained colder than everywhere else in the oven. This caused the temperature controller to continue to demand heat from the burner in an unsuccessful effort to bring the control thermocouple up to the setpoint temperature.
The burner, located in a housing above the workload area, was rated at 0.6 million kilocalories (2.4 million BTU/hour), and it continued firing at 100% output. With the doors still partially open, the control thermocouple continued to mistakenly believe the oven interior was below setpoint, and the oven temperature continued to rise. Since the high-limit thermocouple was also located near the front of the oven, it did not sense the over-temperature condition and failed to turn off the heat.
The oven interior temperature continued to climb and reached 593°C (1100°F). This caused the supply-duct sheet metal to fail and split open right above the load of magnesium near the center of the oven (Fig. 3). As a result, extremely hot air exited the heater house (where the blower and burner are located) and directly impinged on the magnesium. The combination of overheated air and radiant heat from the burner caused the magnesium to reach its ignition temperature of 664°C (1227°F). An irreversible reaction began at this point, and the 19,090-kg (44,000-pound) load of magnesium ignited and burned with an intense, white heat.
As the magnesium continued to burn, ultimately reaching a temperature of 2200°C (3992°F), the oven was entirely engulfed in flames. Considering that magnesium has a heat of combustion of 25.1 MJ/kg, the 19,090-kg load contained over 47,000 MJ (44.6 million BTU) of energy, the equivalent of 1,545 kg (3,400 pounds) of coal. The fire burned so intensely, it was impossible to put out. The factory was evacuated, and the fire was allowed to burn until its magnesium fuel was entirely exhausted, which took over 12 hours!
If closed-loop control is used in a heating process, one must pay special attention to the location of both the control and high-limit (excess temperature) thermocouples. During an overheating situation, the high-limit thermocouple must be able to sense a runaway-temperature condition and signal the stand-alone high-limit instrument to shut off heat to the unit. In this case, the high-limit thermocouple location combined with the use of a heat head to reduce cycle time, prevented the over-temperature system from realizing the temperature was becoming dangerously high. As such, it was unable to protect the oven or the work.
The recommended location for a high-limit thermocouple is in the hottest spot inside the unit. Considering the design of this oven, this would have been either near the burner or directly at the immediate outlet of the recirculation fan. Either location would have sensed an overheating condition regardless of the oven doors being open and would have prevented a magnesium oven fire.
- Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014
- Herring, Daniel H., “How to Control Process and Equipment Induced Variability,” Furnaces North America, 2016
- SunnForest Enterprises (www.sunnforest.com)