Abstract: Employees #1, #2, #3, #4 and a coworker were engaged in installing a new motor and support bracket system onto a process vessel. The vessel had been "cleaned" the day before by Employee #1. The cleaning included flushing the vessel with Methyl Ethyl Ketone. The chemical vapors were not removed or purged from the tank. When the welder struck an arc to weld the motor support bracket onto the tank, a flash fire occurred, blowing Employee's #1, #2 and #3 off of an unguarded mezzanine, approximately nine feet three inches above the concrete floor. Employee #4, although burned, was able to self-rescue out the exit door from the mezzanine.