Abstract: At approximately 10:15 a.m., Employees #1 through #3 were engaged in semiannual maintenance operations in the #6 evaporator, and Employees #4 and #5, both watchmen, were on the platform directly outside the manhole opening. The manhole had been checked for gas, and the electrical power and valves had been locked out. Employees #6 and #7 were approximately 15 feet above the group, performing a separate evaluation. There was a problem with the welder (located at ground level), and Employee #4 went down and made adjustments. There was a discharge of noncondensable gases (hydrogen sulfide, methyl mercaptan, dimethyl sulfate, and other organics) caused by the back pressurization of a line. A kiln was down at the time of the blowout so the gases were diverted and traveled down the line. They should have been vented into the atmosphere, but a valve malfunction resulted in the gases being released in the evaporator building, approximately 20 feet above the heads of workers in the vicinity of evaporator #6. Employees #1 through #7 inhaled the hydrogen sulfide and other gases, and Employees #1 through #5 were hospitalized. Since the incident, a check was placed on the valves. It was also determined that the counterweight for the valves was broken and a second, computer-operated valve was not operating properly.