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Craig M. Carroll, Magnetrol Ametek, looks at the steps that have been taken to help prevent overfill spills, and how level instrumentation can help to reduce the risk of a catastrophic event.
n the early morning hours of December 2005, the largest fire in Europe since World War II engulfed the Buncefield Oil Depot in Hemel Hempstead, England. Tank 912 located within the complex had overflowed, which allowed the rapid formation of a vapour cloud; rich with a mixture of fuel and oxygen. Condensing and then flowing outward from tank 912, the vaporous cloud spread in all directions. While the source of the ignition is not clear, something ignited the rich mix and the first explosion rattled the oil storage terminal at 6:01 a.m. local time. Eventually, 20 large storage tanks ignited, which required a massive firefighting effort. The fire burned for five days, destroying much of the depot before the blaze was finally extinguished. Thankfully, there were no fatalities from the explosion, but the scale, damage and potential for loss brought a focus to overfill prevention on a global scale. The investigation ultimately revealed the cause to be the failure of an overfill safety system (a combination of electro-mechanical servo gauges and a failure of the high-level switch for tank 912 in bund A combined to allow
the overfill event). This safety system failed to operate and shut off the supply of petrol to the tank. The petrol would not easily explode in a liquid state, but when a large amount spilled out and mixed with oxygen, a concentration was reached which supported combustion. The transition of the petrol to a vapour state was rapid and exponentially increased the reach of the cloud. Since that time and with a renewed focus on preventing occurrences such as Buncefield, guidelines, best practices and standards have been developed to provide operations with a clear direction for overfill prevention. The final Buncefield report was released in 2008 and helped outline recommended practices for primary, secondary and tertiary containment of a potential overfill situation. These standards cover a wide range of overfill prevention areas. Even with the best practices, in recent years overfill incidents have resulted in loss of life and billions of dollars in damages to petroleum facilities worldwide. Some of these incidents have been traced to the failure of the level control 3 35 Autumn 2021
