During plant start-up, a critical interface exists at block valves located at battery limits of process units, A relatively, small leak at a battery-limit isolation point was established as the source of gas that caused an explosion in a 26-foot diameter fractionator on a Fluid Coker nearing the end of a maintenance turnaround. This incident could easily, have occurred in any modern oil refinery or chemical plant. This paper shares the technology, and learning associated with small release scenarios that are often overlooked in large-scale plant operations. The explosion event did not damage the fractionator shell, but displaced all but two of the sieve trays in the column, and produced minor damage in the overhead condensers and receiver drum. Importantly, the consequent down-time resulted in a major production loss. Damage analysis supports the proposition that a deflagration occurred in the top portion of the fractionator involving under 50 lbs of fuel. It is believed that relatively, small leaks, persisting over a period of a few hours, supplied the required amount of fuel. The accident investigation identified multiple potential pathways whereby natural gas leaking past battery limit block valves could have reached the fractionator. This incident highlights the importance of securing leak tight connections between active and inactive sections Of process systems. As a result of this experience, several procedural and organizational reforms have been instituted.