(83b) Overfill Protection Systems - Complex Analysis, Simple Solution

Overfills have resulted in significant process safety incidents. Longford (Australia, 1998), Texas City (United States, 2005), and Buncefield (UK, 2005) can all be traced back to loss of level control resulting in high level. The Longford and Texas City incidents were caused when fractionating columns were overfilled and liquid passed to downstream equipment that was not designed to receive the liquid. The Buncefield incident occurred when a terminal tank was overfilled resulting in its rupture and the release of hydrocarbons to the atmosphere.

The causes of overfill are easy to identify; however, the risk analysis rapidly becomes complicated by the combination of manual and automated actions often required to control level and to respond to abnormal level events. This paper provides a series of examples involving storage tanks and fractionating columns to illustrate some of the complexities involved in analyzing overfill hazards. Fortunately, while the analysis is complicated, the risk reduction strategy is surprisingly simple.