With the recent fatal accident involving methyl mercaptan, I thought it would be a good time to remind us of another significant methyl mercaptan accident that claimed the lives of three workers. The recent accident claimed four workers via inhalation/asphyxiation and the one in 2001 involved ignition and the workers died as a result of the flash fire. So obviously methyl mercaptan’s toxic and flammable properties are SIGNIFICANT and must be respected. It is still too soon to know all the facts of the recent methyl mercaptan accident, but we have a LOT TO LEARN from the 2001 accident. One such lesson is the IMPROPER RELIANCE on EXCESS FLOW VALVES as a “safeguard” during unloading of hazardous materials from transport containers. About 10 years ago I began restricting clients from claiming these devices as “safeguards” in PHAs based on the 2001 incident in MI and personal experience with these devices not functioning as we had hoped. It turns out, excess flow valves are designed to close and stop the release of product from the tank car in the event a tank car valve or fitting is broken or sheared off during transit. Attaching cargo transfer apparatus to a tank car can change product release rates and flow rate characteristics and can prevent the excess flow valve from closing in the event of an emergency. As noted by the Chlorine Institute in its Chlorine Manual and by the Safety Board in its investigation of a July 30, 1983, accident at the Formosa Plastics plant in Baton Rouge, Louisiana,tank car excess flow valves are NOT designed to act as an emergency shutoff device during cargo transfer.