The formation of a Grignard reagent is highly exothermic. In addition, the Grignard reagent reacts violently with many compounds, such as water, to generate heat and possibly result in the generation of volatile and flammable products. As a result, two potential upset scenarios during the manufacturing of a Grignard reagent were identified through a hazard and operability study. Bench-scale studies of these scenarios using the vent-sizing package (VSP) were employed to evaluate the consequences of these scenarios. Based on the VSP test results, two additional upset scenarios were identified and further evaluated by the VSP. Sizing of the emergency relief device for these upset scenarios was then conducted by utilizing pseudo-one-component tempered system analysis. The recommended emergency relief system was installed prior to start-up and was found to provide adequate protection after one of the identified scenarios (runaway reaction) occurred. The details and potential causes of this incident are also discussed.