Abstract: At approximately 10:30 p.m. on April 22, 1987, Employees #1 through #3 were working in the sulfur recovery unit (SRU) trying to restore the flow of hydrogen to the SCOT unit. The SRU had several components, and Employees #1 through #3 were trying to isolate the cause of the problem. Hydrogen was entering the regulators at 200 psi, but the pressure dropped to 18 to 20 psi after it passed through them. The employees blocked a valve preceding the regulators and bled the bleeders before removing a flange located beyond the valve. Employee #1 then opened the blocked valve to check the flow. A hiss was heard, followed by a pop, and the area became engulfed in flames. Employees #1 through #3 suffered first-, second-, and third-degree burns over their bodies. All three employees were hospitalized.