Abstract: On September 15, 1992, during the day shift, the steam distillate was found to contain high levels of ACN. The source of the ACN was the reboiler (DE-708A). Because the other reboiler (DE-708B) had already been taken out of service, operations management decided to shut down the BEU at approximately 10:30 a.m. The first step of the shut down involved redirecting the butadiene product to the "off-spec" tank and placing the solvent loop in a recycle mode. At approximately 12:30 p.m. the compressor serving the stripper (T705) was being isolated. Before completing these two processes, the employees were directed by the supervisor to isolate the reboiler (DE-708A). Two operators, Employees #1 and #2, and the shift foreman, Employee #3, climbed up onto the platforms and began to isolate the reboiler. Employee #1 had closed the steam control valve to the reboiler and then stopped the lower level and began to close the block valve on the liquid leg. Employee #2 was attempting to close the steam block valve but had to wait for help from Employee #1 because it was too hard to turn. Employees #1 and #2 heard a hammering sound but simply attributed the sound to maintenance men in the area, Employees #4 , #5, and #6, knocking on something. Hearing the same hammering sound, Employees #4 through #6 thought the same of Employees #1 through #3. One of the operators and Employee #3 closed the vapor line block valve. Again a hammering sound was heard. The operator, thinking a PSV was releasing, reopened the vapor line valve. There was a loud noise and a cloud of yellow smoke appeared. Hot liquid from the reboiler sprayed Employees #1 through #6, knocking one employee over the rail to the lower level. There was no fire, only a lot of smoke. All six employees were rescued, and then transported and admitted to the hospital.